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Catalytic Reduction of Carbon Dioxide on the (001), (011), and (111) Surfaces of TiC and ZrC: A Computational Study
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C: Chemical and Catalytic Reactivity at Interfaces

Catalytic Reduction of Carbon Dioxide on the (001), (011), and (111) Surfaces of TiC and ZrC: A Computational Study
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  • Fabrizio Silveri*
    Fabrizio Silveri
    School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.
    Departament de Ciència de Materials i Química Física and Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1-11, 08028 Barcelona, Spain
    Gemmate Technologies s.r.l., via Reano, 31, 10090 Buttigliera Alta, TO, Italy
    *Email: [email protected]. Tel: +393791822311.
    More by Fabrizio Silveri
    Orcidhttps://orcid.org/0000-0003-0875-7406
  • Matthew G. Quesne
    Matthew G. Quesne
    School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.
    UK Catalysis Hub, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0FA, U.K.
    More by Matthew G. Quesne
    Orcidhttps://orcid.org/0000-0001-5130-1266
  • Francesc Viñes
    Francesc Viñes
    Departament de Ciència de Materials i Química Física and Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1-11, 08028 Barcelona, Spain
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    Orcidhttps://orcid.org/0000-0001-9987-8654
  • Francesc Illas
    Francesc Illas
    Departament de Ciència de Materials i Química Física and Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1-11, 08028 Barcelona, Spain
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    Orcidhttps://orcid.org/0000-0003-2104-6123
  • C. Richard A. Catlow
    C. Richard A. Catlow
    School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.
    UK Catalysis Hub, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0FA, U.K.
    Department of Chemistry, University College London, 20 Gordon Street, London WC1 HOAJ, U.K.
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    Orcidhttps://orcid.org/0000-0002-1341-1541
  • Nora H. de Leeuw
    Nora H. de Leeuw
    School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.
    School of Chemistry, University of Leeds, Leeds LS2 9JT, U.K.
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    Orcidhttps://orcid.org/0000-0002-8271-0545
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2022, 126, 11, 5138–5150
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https://doi.org/10.1021/acs.jpcc.1c10180
Published March 13, 2022

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Abstract

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We present a computational study of the activity and selectivity of early transition-metal carbides as carbon dioxide reduction catalysts. We analyze the effects of the adsorption of CO2 and H2 on the (001), (011), and metal-terminated (111) surfaces of TiC and ZrC, as carbon dioxide undergoes either dissociation to CO or hydrogenation to COOH or HCOO. The relative stabilities of the three reduction intermediates and the activation energies for their formation allow the identification of favored pathways on each surface, which are examined as they lead to the release of CO, HCOOH, CH3OH, and CH4, thereby also characterizing the activity and selectivity of the two materials. Reaction energetics implicate HCO as the key common intermediate on all surfaces studied and rule out the release of formaldehyde. Surface hydroxylation is shown to be highly selective toward methane production as the formation of methanol is hindered on all surfaces by its barrierless conversion to CO.

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Introduction

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The growing awareness and impact of climate change have led to ambitious sustainable development goals in all sectors of the industry. Carbon capture and utilization schemes are one of the many routes that have been proposed to increase the environmental sustainability of the transport and chemical industries. The aim of this strategy is to close the cycle of fossil fuel burning by using renewable energy to recycle water and carbon dioxide into fuels and chemicals, thereby mimicking the outcome of natural photosynthesis. (1,2)
The synthetic fixation of CO2 into fuels and chemicals can be performed through two main strategies: by sequestering CO2 into existing molecular frameworks without changing its oxidation state or by reducing the CO2 into base chemicals that can be further processed. While the former strategy has already been implemented in several industrial processes as a means of reducing the environmental impact of an industrial process by recycling the emitted CO2, its use is limited by the demand for chemicals that can be produced through either exothermic or mildly endothermic reactions─typically urea, carbonates, and polymers (3)─which are several orders of magnitude lower in carbon density than the global carbon emissions. (4) In contrast, direct carbon dioxide reduction with water or other readily available natural chemicals is a highly endothermic process, requiring either large amounts of energy or the use of highly reductive chemicals to obtain the desired product. Several processes have been proposed for CO2 recycling using the oxidation of renewably generated H2 to provide the energy required for the reaction, which often makes use of technology that is already employed by the chemical industry. Target chemicals for CO2 reduction can be CO, HCOOH, CH3OH, and CH4, which can either be used as fuels or transformed into a variety of fuels and chemicals using the technology currently employed by the chemical industry. (1,5) The reduction reactions, however, often produce a mixture of these and other carbon-based products, making selectivity an issue that must be tackled as much as activity, since the separation of chemicals can be a complex and expensive process. (6) For this reason, several studies have investigated the product selectivity of CO2 reduction over various catalysts, and theoretical effort has often focused on this aspect of the catalytic reduction to provide insight into the design of suitable catalysts. (7−9)
The computational investigation of the catalytic activity of CO2 reduction over Mo2S was able to correlate its remarkable selectivity toward CO (with a ratio of 154:1 to CH3OH), with its computed oxygen binding energy. (10) In addition, adsorption studies were performed on copper catalysts with metal oxide support, predicting higher activity at the interface between the two materials as a result of the preferential binding of H2 on the metal, while CO2 favors adsorption on the oxide. (6) The effect of oxygen vacancies and surface morphology on carbon dioxide activation has been highlighted by similar studies on Cu2O, In2O3, and TiO2. (11) The reaction pathway has been investigated in several studies, identifying two recurring processes, which are selected by the site of first hydrogenation of CO2: if this is on the carbon atom, producing an HCOO species on the catalyst surface, the first step is rate-determining and the catalyst cannot produce CO, as the product will necessarily be hydrogenated; however, if the first hydrogenation involves the oxygen, the selectivity will depend on the relative rates of CO desorption and CO hydrogenation, as the latter will be the rate-determining step for methanol formation. (12) In some cases, the mechanism was reported to change depending on the surface stoichiometry of the material, e.g., molybdenum carbides, for which combined computational and experimental studies have highlighted how selectivity toward either CO or CH3OH is driven through a complex set of reactions by the carbon/metal ratio of the exposed surface. (13) Transition-metal carbides of groups 4–6 have been investigated both experimentally and computationally for CO2 reduction, giving promising results. However, less work has been dedicated to group 4 transition-metal carbides. Porosoff et al. (10) have shown low activity for TiC compared to that of MoC and Mo2C, linking that to a high oxygen binding energy, which hinders the catalytic cycle on TiC surfaces. Nevertheless, Quesne et al. (14,15) obtained interesting work function and d-band center values for TiC and ZrC surfaces, which can be correlated with the surface ability to catalyze reduction reactions. Moreover, CO2 shows significant bond elongation and charge transfer upon adsorption on these materials, suggesting the possibility of catalyzing a reduction reaction.
In this study, TiC and ZrC were therefore chosen for a thorough investigation of their catalytic activity toward CO2 dissociation and hydrogenation upon their three lowest Miller index surfaces. The investigation was carried out through the density functional theory (DFT) modeling of each of the (001), (011), and metal-terminated (111) surfaces of the two materials, with the aim of identifying the most energetically efficient pathways for the formation of CO, HCOOH, CH3OH, and CH4 from carbon dioxide and molecular hydrogen and to identify the critical factors controlling their activity and selectivity. The two carbides and their respective surfaces were identified following previous work on groups 4 and 5 transition metals, which gave promising results for surface properties as well as for CO2 and H2 adsorption, (14−16) despite the high surface energy of the (111) surfaces. (14,17)

Computational Methods

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All calculations reported in this work were performed within the framework of the periodic density functional theory using the VASP code (Vienna ab initio software package) version 5.4.1. (18) Core electrons were described using the projected-augmented wave (PAW) method, (19) whereas a plane-wave basis set has been used to expand the valence electron density. The Perdew–Burke–Ernzerhof (PBE) functional (20) was used to approximate exchange and correlation interactions in the framework of generalized gradient approximation (GGA). Additionally, long-range-dispersive interactions were modeled using the Grimme D3 dispersion method. (21) All energies are converged within a cutoff of 520 eV and an electronic self-consistent field (SCF) threshold of 10–5 eV. Convergence was determined using the tetrahedron method, implementing Blochl corrected smearing. (22) A dipole correction was enabled in all directions to avoid numerical problems with the perpendicular dipole inherent in the (111) facets. All parameters were benchmarked to optimize computational time and accuracy for the calculations.
The surfaces of the transition-metal carbides (TMCs) studied in the present work were simulated by six atomic layers within (2 × 2) supercell slab models, cutting the bulk along the (001), (011), and (111) planes, as previously reported by Quesne et al. (14) Hence, each slab surface model has 6 atomic layers and 16 atoms per layer. The (001) and (011) surfaces are created so that they preserve bulk stoichiometry, resulting in an equal number of carbon and metal atoms being exposed to the vacuum. Conversely, the (111) plane is parallel to carbon and metal atomic layers, resulting in two possible surface terminations that, respectively, expose carbon and metal atoms to the vacuum in such a way that the stoichiometry is preserved. Such a protocol has been previously applied to TMCs and has been shown to describe accurately the electronic and structural properties of the (111) surfaces and is consistent with previous reports of the same materials. (14) To avoid interactions along the axis perpendicular to the surface, the lattice parameter was increased by 12 Å in such a direction. A 5 × 5 × 1 k-points reciprocal lattice matrix was generated using the Monkhorst–Pack method. These parameters were benchmarked, ensuring that vacuum spacing, grid k-points, and cutoff energy values allowed convergence to a constant value of the energy. Spin polarization was allowed for the determination of the energy of all structures except for the H2 reference molecule. Atomic charges were calculated for specific systems through a Bader charge analysis. The energy of the system was minimized by keeping the cell parameters fixed at their bulk-optimized value and allowing relaxation of all atoms excepting those belonging to the two bottom layers of the model slab, which were kept fixed. The minimum energy structures were found using a built-in DIIS algorithm with a convergence force threshold of 10–2 eV/Å. The energies of molecular H2, CO2, CO, O2, HCOH, HCOOH, CH3OH, and CH4 have been used as references for the adsorption and desorption energies on transition-metal carbide slabs, using half of the energy of the H2 and O2 molecules as a reference for atomic H and O adsorption. These calculations have been performed using a single Γ k-point and the same cutoff energy as all other calculations within a suitable unit cell, optimized so as to minimize the interaction between neighboring images.
The electronic energies thus calculated were used to compute the reaction and activation energies of the reaction path from CO2 to its reduced products, which we consider sufficient to inform our understanding of the mechanistic aspects, which is the main objective of this study. Transition-state structures were primarily located using the built-in CI-NEB (climbing image nudged elastic band) algorithm of the VASP code; these calculations used a variable number of system images, in the range 8−13, connected by springs, and was usually performed in two steps: the first was to find an initial guess of the minimum energy path between reactants and products by relaxing the full string of connected images, while the second aimed at identifying the saddle point by turning on the climbing image algorithm on the highest energy image. In the few cases in which the saddle point of the potential energy surface (PES) proved difficult to isolate using this method, an additional third step was used, employing the built-in Dimer method algorithm on the closest NEB image to obtain the relevant transition-state structure. All presented transition-state structures were confirmed to be PES saddle points through a vibrational frequency calculation limited to the adsorbed species and their respective adsorption sites on the surface.
In analyzing the reduction path of CO2, the key quantities are the adsorption energy (Eads), overall reaction energy (Er), reduction step energy (Es), and the activation energy (Ea), which are defined as follows:
Adsorption energy (Eads):
(1)
The adsorption energy is defined in terms of Eslab+mol, the total electronic energy of an adsorbed chemical species on a surface, Eslab, the energy of the bare surface, and Emol, the electronic energy of the respective isolated molecule(s).
Overall reaction energy (Er):
(2)
The overall reaction energy of stage n is defined as the difference between its total electronic energy (Estage) and the sum of the electronic energies of its respective surface and gas-phase molecular reactants in terms of CO2 and H2.
Single-step energy (Es):
(3)
The reduction step energy of each stage is instead the energy difference between two subsequent intermediates or products along any reduction path.
Activation energy (Ea):
(4)
For a given elementary step, the activation energy is the total energy difference between the identified transition-state structure and the previous stable intermediate.
Each one of the reaction steps and transition states reported in this paper is the result of geometrical optimizations which included distinct initial geometries or reaction coordinates, so that the minimum energy products and reaction paths could be identified for each investigated step. In the case of hydrogenation reactions, molecular hydrogen was always considered to be dissociatively adsorbed on the catalytic surfaces prior to the reaction, consistently with a Langmuir–Hinshelwood mechanism. This choice is consistent with the results of previous works, (16,23) which highlighted small or nonexistent activation barriers for hydrogen chemisorption on the TiC(001) surface and even greater chemical driving force for dissociative adsorption on all other investigated surfaces. Other hydrogenation mechanisms were therefore not investigated.

Results and Discussion

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The investigation of the CO2 reduction on TiC and ZrC surfaces was carried out in two steps. Initially, the first CO2 reduction step is analyzed through the three reaction pathways leading to adsorbed CO + O, COOH, and HCOO; the reaction and activation energies of the three reactions are investigated on each of the six surfaces, and the most favorable reaction pathways are identified on each surface. Subsequently, the identified pathways are investigated until the final reduced products, highlighting the activity and selectivity of each surface for the conversion of CO2 to CO, HCOOH, CH3OH, and CH4.

Oxidized and Reduced Carbon on TiC and ZrC Surfaces

The electronic and geometrical effects of CO2 adsorption are very similar on TiC and ZrC. All surfaces of TiC and ZrC show a favorable chemisorption of CO2. The molecule is strongly bound to a surface carbon atom on (001) and (011) surfaces, on which all results are comparable to the previously reported computational work on the same surfaces (15) and show similar adsorption patterns as those highlighted on Ti2C MXenes. (24,25) The main CO2 adsorption geometries identified for these surfaces, Con-top for (001) surfaces and M2C and M4C on (011) surfaces, are displayed in Figure 1. On the two (111) surfaces, however, we find an adsorption geometry that has not been previously reported: upon adsorption, the CO2 molecular plane is almost parallel to the plane of the surface, with the two oxygen atoms coordinating 5 metal atoms in the M5 adsorption site. Longer bond lengths and more reduced CO2 molecules are found, as the bond elongates up to +0.3 Å compared to the gas-phase molecule and the CO2 carbon atom gains +1.8 e of charge, but the adsorption is less exothermic than on the (011) surfaces, as Eads is computed to be of −3.1 eV on TiC(111) and of −3.2 eV on ZrC(111). Previous studies (15) only reported vertical adsorption sites for these surfaces, which show significantly less exothermic adsorption. As can be seen in Figure 1, all four identified adsorption sites have a bent geometry.

Figure 1

Figure 1. CO2 adsorption sites on TiC. From left to right: Con-top on (001), M2C on (001), M4C on (011), and M5 on (111). The same adsorption sites are also found on ZrC(001). The color scheme is as follows: Ti, blue; C, brown; and O, red.

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In the initial step of the catalytic reduction process, CO2 may either dissociate yielding CO and O on the surface of the carbide or be hydrogenated at the C or O atoms, yielding, respectively, a carboxyl or a formate on the catalyst surface. In the latter case, the hydrogen atom is transferred from the surface of the catalyst since the dissociation of the H2 molecule has previously been shown to be spontaneous in all but the most unfavorable conditions. (16) These first reaction steps have been investigated on all surfaces, modeling the formation of the three first-step intermediates
(a)
(b)
(c)
The adsorption of each of the chemical species on every surface has therefore been studied, identifying the minimum energy structures shown in Figure 2 and highlighting the adsorption energies reported in Table 1. The adsorption of CO is generally very exothermic (Eads = −1.82 to −3.91 eV) on all surfaces. Con-top sites are dominant on both (001) surfaces, as shown by Figure 2a, in which the molecule stands almost perpendicular to the plane of the catalyst, only slightly leaning toward a metal atom; on these sites, the C–O bond shows significant elongation (increasing to 1.20 Å), as the distance between the adsorbed molecule and the nearest surface carbon atom also resembles that of a strong covalent C–C bond at 1.32 Å. The (011) surfaces lead to the most exothermic CO adsorption on both carbides, as was previously noted for the case of CO2; Figure 2b shows the most favorable adsorption site on TiC(001), where the CO molecule is roughly perpendicular to the surface and coordinated only through the carbon atom. While a similar adsorption is also possible on ZrC(011), a more exothermic adsorption site is present, in which the CO molecule is roughly parallel to the surface and coordinates a surface carbon with its carbon atom and two metal atoms with is oxygen atom; on both surfaces, the molecule appears to be activated, as shown by the elongation of the C–O bond, which reaches 1.27 Å on ZrC(011). Both adsorption sites are shown in Figure 2d.

Figure 2

Figure 2. Most favorable adsorption geometries of, from left to right, CO, O, COOH, and HCOO on (a) ZrC(001), (b) TiC(011), and (c) Zr-terminated ZrC(111). (d) Top view, added for clarity, of (from left to right) CO on TiC(011), CO on ZrC(011), HCOO on ZrC(001), and HCOO on TiC(011). Analogous geometries are found for all species not shown in this picture, except for HCOO on ZrC(011), which spontaneously dissociates upon adsorption. The adsorption geometries shown here are those with the minimum energy among those tested on each surface. The color scheme is as follows: Ti, blue; Zr, green; C, brown; O, red; and H, pink.

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Table 1. Adsorption Energies for CO, O, COOH, and HCOO on TiC and ZrC, Given in eVa
 COOCOOHHCOO
TiC
(001)–1.82 (Con-top)–2.54 (M2C)–1.25 (M2C)–2.08 (Con-top)
(011)–3.22 (Con-top)–4.40 (M-O-C)–1.82 (M4C)–5.35 (M4C)
(111)–3.22 (M2)–5.76 (M3)–1.20 (M4-vertical)–6.12 (M5-up)
ZrC
(001)–2.13 (Con-top)–2.79 (M2C)–1.70 (M2C)–3.29 (Con-top)
(011)–3.91 (C-Mbridge)–4.56 (M2-bridge)–3.74 (Con-top)N/A
(111)–3.13 (M2)–5.85 (M3)–2.46 (M4-vertical)–6.17 (M5-up)
a

Only the most favorable adsorption sites are considered in this table.

The (111) surfaces offer multiple coordination patterns, resulting in low energy structures where both atoms of CO maximize coordination by lying almost parallel to the surface, as shown in Figure 2c. Isolated oxygen atoms are bound very strongly to all surfaces (Eads = −2.5 to −5.9 eV), as expected from the well-known tendency of TMCs to form oxycarbides. (26) On the (001) surfaces, adsorbed O is coordinated to one carbon and two metal atoms. On the (011), oxygen interacts very strongly with carbon atoms, pulling them strongly out of the plane of the surface, whereas on the (111), O fits in the hexagonal sites coordinating three metal atoms, binding strongly to the surface.
The carboxyl group formed by the hydrogenation of CO2 on one of the oxygen atoms also has different optimized geometries on the different surfaces. On both (001) surfaces, two planar structures could be identified: the first, named as Con-top adsorption site, has its O–C–O plane perpendicular to the surface and is slightly bent to maximize the interaction between the nonhydrogenated oxygen and the surface; alternatively, the second structure, the M2C adsorption site, is bent toward the surface to maximize the interaction between the latter and the oxygen atoms. The two structures have very similar energies (ΔEads < 0.05 eV), with the latter at a slightly lower energy, which is shown in Figure 2a. On the (011), similar structures are found albeit with different relative stabilities, with Con-top being stable on ZrC, while the M2C adsorption site is favored on TiC. On the (111) surface of both carbides, the COOH group is most stable when aligned perpendicularly to the surface, with C═O coordinated to the surface and −OH pointing upwards.
Formate groups formed by the hydrogenation of a CO2 carbon atom are not stable on ZrC(011), giving spontaneous dissociation to an sp2-hybridized HCO on a Con-top site, with O coordinated to a metal, plus adsorbed oxygen. The same process was expected for TiC(011), but in this case, a surface formate intermediate could be isolated, as shown in Figure 2b. On the four other surfaces, HCOO has stable configurations similar to those for COOH, with Con-top and M5 being the most favored on (001) and (111) surfaces, respectively, on both carbides.
Across the three surfaces, adsorption shows similar characteristics for TiC and ZrC, as adsorption geometries and energies are similar on the two materials for the (001) and (111) surfaces, where adsorption geometries are always equivalent, with only minor differences due to the change in lattice parameters between the two cells. In contrast, all adsorption sites identified for the (011) surfaces differ between the two materials. Energetically, adsorption on ZrC is always shown to be more exothermic than the equivalent adsorption on TiC; the energy difference is usually small (0.02–0.35 eV) but exceeds 1 eV for HCOO/(001), COOH/(011), and COOH/(111).

CO2 Dissociation and Hydrogenation

As we have seen, the species discussed above are all adsorbed exothermically. However, when we consider the energetics of reactions 1–3 above, we find that the energy of the reaction step on the surface is only negative for the dissociation reactions, while it can be significantly positive for the two hydrogenation pathways.
Table 2 summarizes the reaction energies associated with the formation of CO, COOH, and HCOO, reported with respect to gas-phase CO2 (+ 1/2 H2 in the case of hydrogenation) and to the single reaction step from adsorbed CO2. As noted, the dissociation reactions are the only ones for which single-step energies are negative, due to the high adsorption energies found for CO and O, which bind strongly to the surfaces of TiC and ZrC. However, previous reports (8,10) have shown that when oxygen is bound too strongly, it can hinder the catalytic cycle. In contrast, single-step reaction energies for the formation of COOH and HCOO are generally endothermic, as a result of the relative instability of these intermediates. Nevertheless, the reaction energies from gas-phase reactants are generally exothermic, the formation of HCOO on the TiC(001) surface being the only exception, for which Er = +0.19 eV. As a result, it is possible that the adsorption of CO2 and H2 on the low-index surfaces of TiC and ZrC might provide the energy for the formation of these intermediates on the catalysts, especially if an Eley–Rideal type of mechanism is operative. (27) The endothermicity of CO2 hydrogenation can also be linked to the results of hydrogen adsorption on TMCs (16) since hydrogenation on TiC and ZrC surfaces is always exothermic but energy is required for the formation of HCOO and COOH, as one strongly bound H atom has to be removed from binding directly to the surface. On the (011) and (111) surfaces, because of their more exothermic adsorptions, this results in especially high single-step energies for COOH formation. HCOO formation, however, is less thermodynamically unfavorable on the (111) surfaces, owing to the very strong bond between this intermediate and the surface.
Table 2. Reaction Energies and Single-Step Energies (in eV) for the Dissociation to CO + O, Hydrogenation to COOH, and Hydrogenation to HCOOa
 CO + OCOOHHCOO
 ErEsErEsErEs
TiC
(001)–1.13–0.23–1.250.590.192.03
(011)–7.05–3.6–3.381.43–3.381.43
(111)–5.70–2.60–2.761.43–3.840.35
ZrC
(001)–1.72–0.04–1.700.95–1.021.63
(011)–5.03–0.84–3.731.71–5.92*–0.48*
(111)–5.67–2.49–2.471.91–3.90.48
a

For each reaction, two energies are reported, differing for the reference used: in the column on the left of each section, the reaction energy Er is reported, which is the energy of the intermediate on the surface of the catalyst minus the energy of the gas-phase reactants and pristine surface; in the column on the right of each section, the single-step energy Es is reported, which is the energy of the intermediate on the surface of the catalyst minus the energy of the previous intermediate on the same surface. The energies reported for HCOO on ZrC(011) are marked with an asterisk because they are referred to the formation of HCO + O since HCOO dissociation occurs spontaneously on this surface.

From the results of Table 2, it is also possible to make a preliminary assessment of how the three competitive reactions are balanced on each surface. In absolute terms, the dissociation is always thermodynamically favored, as it always shows Es < 0 for the single-step process. However, if only the hydrogenation reactions are considered, a strong preference toward the formation of COOH is found on (001) surfaces, while on (111) surfaces, the preference is toward the formation of HCOO, which on TiC(111) has the least positive single-step reaction energy of all hydrogenated intermediates. On the two (011) surfaces, the formate is unstable and can dissociate spontaneously to CHO + O. It was possible to identify a formate intermediate on TiC(011), for which Es = 1.43 eV, the same as for CO2 hydroxylation. On ZrC(011), however, HCOO is so unstable that it undergoes spontaneous dissociation and no intermediate could be found. Table 2 reports Es = −1.48 eV for this reaction, which refers to the formation of CHO + O.
To evaluate the selectivity of each surface, the simple prediction of reaction energies is insufficient. Nudged elastic band calculations (NEB and CI-NEB) and the dimer method have been employed to sample the region of the potential energy surface connecting each intermediate to their adsorbed reactants, identifying the transition states corresponding to the reactions leading to CO, COOH, and HCOO on (001), (011), and (111) surfaces. From this investigation, two reactions have been excluded: the formation of formate on TiC(001) and ZrC(001) has been considered too high in energy to take place, the former showing the only positive reaction energy from gas-phase reactants, the latter being strongly unfavorable compared to COOH formation. Figure 3 illustrates the initial state, transition state, and final state of the PES linking adsorbed CO2 and H to COOH on TiC(001), CO2 to CO and O on ZrC(011), and CO2 and H to HCOO on TiC(111). The calculated data for those and all other reactions are reported in Table 3.

Figure 3

Figure 3. Proposed reduction mechanism of (a) CO2 hydrogenation to COOH on TiC(001), (b) CO2 dissociation to CO + O on ZrC(011), and (c) CO2 hydrogenation to HCOO on TiC(111). From left to right: initial state, in which the reactants are adsorbed on the surface of the catalyst; transition state, identified through the presence of one imaginary frequency related to the formation of the desired product; and final state, in which the product is adsorbed on the surface of the reactant. The color scheme is as follows: Ti, blue; Zr, green; C, brown; O, red; and H, pink.

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Table 3. Activation and Reactive Step Energies of CO2 Reduction over TiC and ZrCa
 EadsEaEs
TiC
001─CO–0.90+2.13–0.23
011─CO–3.45+0.69–3.6
111─CO–3.11+0.99–2.60
001─COOH–1.84+1.58+0.59
011─COOH–4.80+2.15+1.43
111─COOH–4.19+2.15+1.43
001─HCOO–1.84N/A+2.03
011─HCOO–4.80+1.39+1.43
111─HCOO–4.19+1.44+0.35
ZrC
001─CO–1.68+1.86–0.04
011─CO–4.19+0.49–0.84
111─CO–3.18+1.29–2.49
001─COOH–2.65+1.48+0.95
011─COOH–5.44+1.97+1.71
111─COOH–4.38+2.44+1.91
001─HCOO–2.65N/A+1.63
011─HCOO–5.44>3 eV–0.48*
111─HCOO–4.38+1.58+0.48
a

All energies are referred to the adsorbed CO2, for dissociation reactions, or CO2 + H, for hydrogenation reactions, and are reported in eV. The first column refers to the adsorption energy; the second row is the activation energy, hence, the energy difference between the adsorbed reactant and the transition state; the final row reports the energy of the products referred to that of the adsorbed reactants.

Dissociation to CO on both (001) surfaces is mildly exothermic but shows very high barriers─over 2 eV for TiC(001). These results might seem inconsistent with the previous investigation of CO2 adsorption on the same surfaces, (15,28) which highlight the efficient activation of the CO2 molecule on the (001) surfaces of several transition-metal carbides through the investigation of their elongated bond distances, modified bond angle, and effective charge transfer from the surface to the carbon center. The calculations presented here suggest that this discrepancy is due to an inefficient coordination of the transition state correlated to the dissociation of the C–O bond, since the electronic and geometric characteristics of adsorbed CO2 that indicate activation of the molecule (28) do not translate into a sufficiently low activation barrier.
This result shows how, even if the (001) surface can chemisorb both reactants and products efficiently, CO2 is not sufficiently activated on these surfaces for appreciable dissociation to occur. CO2 adsorbed on TiC(001) and ZrC(001) shows longer bond lengths, a bent geometry, and a higher number of electrons than the gas-phase molecule, all characteristics that have been correlated with CO2 activation. (29) However, this activation is ineffective for this reaction since on both carbides, the transition state has a higher energy than the gas-phase reactants, effectively ruling out the direct dissociation of CO2 on (001) surfaces.
The dissociation reaction is considerably more exothermic on (011) and (111) surfaces, as the more exposed metal atoms can coordinate CO2 more efficiently while the C–O bond is elongated until full dissociation. Dissociation on ZrC(011) is, of all reactions, the one with the lowest energetic barrier (Ea = 0.49 eV over the adsorbed reactants), while TiC(011) shows the most exothermic reaction energy (Es = −3.60 eV) but has a slightly higher activation barrier, Ea = 0.69 eV. This can be correlated with the higher metal area exposed on ZrC, allowing for a better coordination of the dissociating oxygen but resulting in its less efficient coordination once the reaction is complete. However, this trend is inverted for the transition states on (111) surfaces: as a result of the larger cell parameters of ZrC, the dissociating oxygen cannot be coordinated as efficiently, resulting in a higher barrier than that on TiC(111).
Conversely, the hydrogenation reaction to carboxyl is most favorable on the (001) surfaces of TiC and ZrC. In all cases, the reaction step is endothermic, and the barrier is at least 1.5 eV above the adsorbed state of the reactants. However, this energy can be provided by the adsorption energies of CO2 and H2 on the surfaces of the catalysts, as both are significantly larger than the activation barriers. This reaction is thermodynamically least unfavorable on TiC(001), where it shows an Es = +0.59 eV, while kinetically ZrC shows a slightly lower barrier of Ea = +1.48 eV. The hydrogenation on both (011) and (111) surfaces is instead significantly endothermic and shows barriers of 2 eV and higher, hindering the formation of COOH on these four surfaces. As mentioned in the previous section, the strong bond formed by hydrogen with these surfaces can be considered as a reason for the high hydrogenation barriers found on this surface.
The hydrogenation to formate presents an interesting case study, as the results vary greatly depending on the surface. On the (001) surfaces, as mentioned in the previous section, the reaction is highly endothermic and no investigation of the potential energy surface has been performed. On the two (011) surfaces, formate can dissociate rapidly to HCO + O. The process seems to be spontaneous on ZrC, for which no HCOO intermediate could be isolated, while it requires a small activation energy on TiC. This reaction is more viable than the hydrogenation to carboxyl despite its identical reaction energy, as Ea(HCOO) = 1.47 eV < Ea(COOH) = 2.44 eV. On ZrC(011), however, the structure of the transition state leading to the simultaneous hydrogenation and dissociation is found to be very close to the unstable intermediate HCOO, which obviously leads to a high kinetic barrier (Ea > 3 eV) after which the intermediate spontaneously dissociates. The height of this barrier rules out the possibility of hydrogenation to formate on the latter surface, as it is significantly higher than Ea = 1.97 eV found for carboxyl production. Conversely, on both (111) surfaces the reduction to formate is mildly endothermic, with Es = 0.35 eV on TiC, probably due to the geometry of the surfaces, which allows for strong interaction of the carbon and oxygen atoms of HCOO with the metal atoms of the material while minimizing interaction with hydrogen. Activation barriers on the two surfaces are similar─Ea(TiC) = 1.44 eV, Ea(ZrC) = 1.58 eV. While the kinetic barrier to hydrogenation is high, it is still much lower than the adsorption energies of the reactants, CO2 and H2; it should therefore be possible to observe the COOH intermediate if the two reaction steps take place in such a way that the energy required to overcome the barrier to hydrogenation is provided by the adsorption onto the (111) surfaces.

Surface-Mediated Product Selectivity

After the first reduction step, multiple pathways are available for the conversion of CO2 into useful chemicals, as illustrated in Figure 4. The results highlighted in the first part of the present work allowed us to reduce the number of investigated surfaces for the subsequent reaction steps by excluding those involving high activation energies. The final part of our study will investigate, first, the reaction steps following the dissociation to CO on the (011) and (111) surfaces, and second, those following the hydrogenation to COOH on the (001) and HCOO on the (111) surfaces. All other reaction paths, as discussed above, have been discounted due to the high energies of their intermediate steps or transition states, which inhibit further reaction.

Figure 4

Figure 4. Proposed reduction pathways of CO2 on transition-metal carbide surfaces. The background chosen for each compound corresponds to its phase: dark blue for gas phase, and green and light blue for molecules on TiC and ZrC, respectively. The color code of the arrows between each compound is associated with the results of the present investigation: blue for reactions favored on (001) surfaces, black for (011), gray for (111), green for reactions favored on all surfaces, red for reactions hindered by either thermodynamic or kinetic impediments, and yellow for desorption reactions. As will be discussed in this paper, the main reduction path on all surfaces is that leading to CH4 through a HCO intermediate.

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The investigation of the second steps of the reaction has entailed the analysis of the thermodynamic reaction energies of the pathways for the conversion of COOH and HCOO to HCOOH on the (001) and (111) surfaces, respectively; COOH to CO on (001) surfaces; and CO to HCO and COH on (011) and (111) surfaces. On the (111) surface of TiC, all attempts at optimizing HCOOH led to its decomposition to HCO and OH. It was therefore concluded that all reactivity would proceed via HCO rather than via HCOOH. The alternative ZrC(111) facet does enable the formation of a stable HCOOH intermediate. However, despite the negative reaction energy of Er = −0.51 eV, this step is strongly endothermic, with Es = 3.4 eV with respect to HCOO. Such an endothermic reaction energy will be inaccessible, and therefore formic acid is also ruled out as a final product on the ZrC(111) surface.
To assess the viability of COOH hydrogenation on the (001) facets of TiC and ZrC, the reaction landscapes for the transfer of a surface hydrogen to either oxygen atom or to the carbon center were computed. Figure 5 shows the driving force for the production of dihydroxymethylidene and formic acid from COOH on the (001) surfaces of TiC and ZrC. The reaction energy for the formation of dihydroxymethylidene is far too high for it to be an important intermediate in the further reduction of COOH and was not considered further. Similarly, a second hydrogenation of the hydroxyl oxygen followed by the dissociation of the product into CO and H2O is shown to be highly endothermic, with Es = 1.67 eV on TiC(001) and Es = 1.99 eV on ZrC(001).

Figure 5

Figure 5. TiC (top) and ZrC (bottom) (001) surface-mediated formation of dihydroxymethylidene (left) and formic acid (right). All energies are given in eV.

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While still endothermic, the reaction energy for formic acid formation (∼1 eV) appears to be more accessible, so CI-NEB calculations were undertaken to determine the activation energies for such a reaction. Unfortunately, the results showed that these pathways were kinetically blocked by barriers larger than 2 eV. The direct formation of formic acid was therefore ruled out as a possible outcome of the CO2 reduction upon these catalysts. As a result, the investigation shifted toward the hydroxylation of surface-bound CO/HCO intermediates, as the most likely reaction paths for carbide-catalyzed CO2 reduction.
Our calculations suggest that the most likely route for the CO/HCO intermediates on both (001) facets was the surface-mediated decomposition of COOH to CO and OH. It was determined that such a decomposition corresponds to a decrease in energy of Es = −0.70 eV on the ZrC(001) facet but was slightly endothermic on the TiC(001) surface (Es = +0.21 eV). Nevertheless, both reaction steps can be considered accessible when considering both the potential gain in entropy as COOH is dissociated and the inherent margin of error of the calculated energies. Therefore, it might be expected that COOH readily decomposes on the (001) termination of ZrC and COOH/CO + OH forms an equilibrium on the (001) surface of TiC, which is then pushed toward the decomposed product as CO goes through further reduction. For completeness, the decomposition of formate (to HCO + O) was also considered, obtaining reactive step energies of Es = +0.46 eV and Es = +1.02 eV on the (001) surfaces of TiC and ZrC, respectively. Calculations discussed below examining CO hydrogenation will indicate that the decomposition of formate might be important as a shortcut to HCO formation on the (001) surface of TiC (see Figure 6). However, carboxylic acid decomposition is likely to be the dominant pathway for the alternative ZrC(001) facet.

Figure 6

Figure 6. Surface-mediated HCO formation by TiC by (top) (001), (middle) (011), and (bottom) (111)-metal-terminated facets. All energies are given in eV related to surface-bound carbon monoxide and hydrogen.

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Data from Figure 6 show that the most active surface for carbon monoxide hydrogenation to HCO on TiC is the metal-terminated (111) surface, followed closely by the (011) facet. While this ordering is reversed on ZrC, all barriers were assessed to be larger than 1 eV. As mentioned above, an activation barrier of almost 3 eV, such as that found for HCO formation on TiC(001), clearly implicates formate decomposition as a more likely mechanism for HCO formation than CO hydrogenation. Interestingly, these results indicate that surface geometry─and especially CO adsorption morphology─plays a far more significant factor in the activation energies for CO hydrogenation than does hydrogen adsorption, with no obvious correlation between barrier heights and the surface hydrogenation energies previously reported using comparable methodologies. (16) There are far more product-like and higher-energy transition states on the two (001) surfaces, and the fact that the preferred binding site for the hydrogen and carbon atoms are both located on top of a surface carbon site implies that these late transition states are far from optimal. The more favorable barrier seen on the (111) surface of TiC over the comparable facet of ZrC is also due to surface morphology, as the shorter TiC lattice constants enable a transition state where the hydrogen atom is close to a highly coordinated hollow site. The hydrogen position in the comparable ZrC(111) transition state, on the other hand, more closely resembles a Con-top low-coordinated position, as shown in Figure 7.

Figure 7

Figure 7. Surface-mediated HCO formation by ZrC by (top) (001), (middle) (011), and (bottom) (111)-metal-terminated facets. All energies are given in eV related to surface-bound carbon monoxide and hydrogen.

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Across all surfaces investigated, our calculations have indicated the importance of a common HCO intermediate, either as a result of direct hydrogenation of CO (as is the case for (011) and (111) facets) or via the (001) surface-mediated decomposition of formate or carboxylic acid. It was therefore decided to focus on the further reduction of HCO. Table 4 shows the reaction energies for HCO hydrogenation to formaldehyde. Crucially, the desorption energy for formaldehyde is very high for all surfaces except for TiC(111), where the value for H2CO release is actually slightly negative (−0.09 eV/mol). Additionally, when examining the transition-state energies of formaldehyde formation, we can rule out that this process takes place on the TiC(111) surface entirely. On this surface, the alternative and highly exothermic oxygen hydrogenation step appears far more likely, but no stable HCOH minima could be found on either (011) surface, with HCOH undergoing a barrierless decomposition to surface-bound HC and OH species.
Table 4. Activation and Reaction Energies of HCO Reduction over TiC and ZrCa
 Eads(HCO)EaEs
TiC
001─H2CO–1.81+1.67–1.49
011─H2CO–4.11+1.77–6.78
111─H2CO–3.88+5.60–0.49
001─HCOH–1.81+1.08–2.12
011─HC OHb–4.11 –5.80
111─HC OHb–3.88 –5.26
ZrC
001─H2CO–2.25+5.86–2.01
011─H2CO–4.57+1.70–4.51
111─H2CO–3.89+1.43–3.93
001─HCOH–2.25+1.80–1.89
011─HC OHb–4.57 –5.83
111─HCOH–3.89+2.63–2.85
a

The first column refers to the surface-bound HCO with reference to gas-phase CO and one-half of H2Eads(HCO); the second row is the activation energy Ea, and the third and final row reports the energy of the products referred to that of the reactants Es. All energies are reported in eV.

The decomposition of HCOH is likely to promote the production of various surface-bound C1 hydrocarbons and water, with no obvious route through to methanol formation. However, the strongly bound formaldehyde on the other surfaces does offer the potential for the production of the alcohol product. Therefore, the hydrogenation of H2CO was examined on all surfaces, with the resulting reaction energies given in Table 5. Hydrogenation of formaldehyde to CH3O is an essentially thermodynamically neutral process on both (001) surfaces and on the (011) facet of ZrC, while the reaction energy is highly exothermic on both (111) surfaces studied. Importantly, the adsorption energy for surface-bound formaldehyde is far too large to enable the desorption to occur on any of these facets. Interestingly, the reaction energy is highly endothermic on the (011) facet of TiC, which is due to the bidentate binding of formaldehyde causing an additional σ-bond not seen in the monodentate H3CO. Most importantly, and despite repeated attempts, no stable methanol species could be produced by hydrogenation of surface-bound H3CO, and in all cases, decomposition to surface CH3 and OH species was observed to be a barrierless process. This result opens a new route toward methane and water formation, with the alternative pathway to methanol being again blocked by the surface-mediated cleavage of the C–O bond.
Table 5. Reaction Energies Er for H2CO Reduction to H3CO and H3COH over TiC and ZrCa
 H2COH3COH3C + OH
TiC
001─H3C OH–1.49–1.74–2.92
011─H3C OH–6.78–3.98–8.11
111─H3C OH–0.49–4.78–6.63
ZrC
001─H3C OH–2.01–1.94–3.33
011─H3C OH–4.51–4.44–6.39
111─H3C OH–3.93–4.87–6.17
a

Due to the lack of a stable CH3OH intermediate, the last column refers to adsorbed CH3 and OH species on each surface. All energies are reported in eV.

Taken together, these results help to rationalize the observed selectivity toward methane production over methanol when CO2 conversion by TiC and ZrC is investigated. (10,30) The selectivity that these materials have demonstrated experimentally is here confirmed computationally on all low-index surfaces, as all paths leading to CO and other valuable products are blocked by either endothermic reactions or very high activation energies. Furthermore, the match between experimental and computational results suggests the importance of the HCO intermediate on all facets of ZrC and TiC as a fundamental step in the reduction path for CO2. The color scheme of Figure 4 shows the favorable and unfavorable reactions on each surface, rationalizing the product selectivity and proposing a few possible reduction paths. These results provide a clearer picture of the reactivity on each facet of the carbides. Figures 8 and 9 combine the reduction paths of CO2 on the three surfaces of TiC and ZrC, displaying the reaction energies (i.e., the energy of the adsorbed intermediate relative to the appropriate number of CO2 and H2 molecules in the gas phase) of each intermediate along the six paths. The reaction landscapes are similar for the two carbides, further revealing the similarity between the catalytic properties of the two materials, and in all cases, the HCO intermediate is central within the reduction path to CH4. Furthermore, on both materials, the (001) surface presents the least negative energies, suggesting an easier release of CH4 and a consequently better catalytic activity for these facets, in contrast to previous reports on the hydrogen evolution reaction on the same surfaces, (16) for which the (001) facet appeared to be too stable to catalyze the reaction adequately. On both carbides, however, reported conversion rates are low, probably due to oxidation of the surface under reaction conditions. (10,30) This behavior might be due to the highly negative reaction energies reported in Figures 8 and 9 (−3 to −8 eV), which slow the release of the reduced products, as well as to the strong adsorption of oxygen on the surface, reported in Table 1, which might create a passivating layer at the interface of the catalyst.

Figure 8

Figure 8. Thermodynamic profile of the reaction paths for CO2 reduction on TiC (001) (black), (011) (blue), and (111) (red) surfaces. The numbers displayed are the reaction energies of each intermediate (Es), in eV. All energies are referred to all reactants and products being simultaneously adsorbed on the relevant surfaces, without interaction between the adsorbed species, so that the reference energy level corresponds to two adsorbed hydrogen molecules and gas-phase CO2 for all steps. Only the carbon-containing molecule of each step is reported for clarity.

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Figure 9

Figure 9. Thermodynamic profile of the reduction paths for CO2 reduction on ZrC (001) (black), (011) (blue), and (111) (red) surfaces. The numbers displayed are the reaction energies of each intermediate (Es), in eV. All energies are referred to all reactants and products being simultaneously adsorbed on the relevant surfaces, without interaction between the adsorbed species, so that the reference energy level corresponds to two adsorbed hydrogen molecules and gas-phase CO2 for all steps. Only the carbon-containing molecule of each step is reported for clarity.

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Summary and Conclusions

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This work has reported a systematic study into the selective hydrogenation of carbon dioxide over the low-Miller index (001), (011), and (111) surfaces of TiC and ZrC. DFT calculations and periodic models have been used to assess the viability of reaction mechanisms toward formic acid, carbon monoxide, formaldehyde, methanol, and methane. Carbon dioxide adsorbs very exothermically on all surfaces and is highly reduced (in many cases by the transfer of more than one electron from the carbide), with the highly activated adsorbate readily undergoing further surface-mediated reactions. The C–O bond cleavage appears to be favored on both (011) surfaces, while COOH formation appears to be the initial step on the two (001) facets. The alternate metal-terminated (111) surfaces are capable of catalyzing both carbon monoxide and formate production as their primary reaction step. Whichever initial mechanism is promoted, all accessible reduction pathways go through a HCO intermediate and ultimately promote the formation of methane over any other reduction product. These results help to rationalize the experimentally observed selectivity toward methane over carbon monoxide when CO2 conversion by hydrogen is studied and further demonstrate the difference in chemical reactivity of the three facets of each carbide.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.1c10180.

  • Activation energy for CO2 adsorption reported in this work compared to previously reported values (15) (Table S1) (PDF)

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Author Information

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  • Corresponding Author
    • Fabrizio Silveri - School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.;  Departament de Ciència de Materials i Química Física and Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1-11, 08028 Barcelona, Spain;  Gemmate Technologies s.r.l., via Reano, 31, 10090 Buttigliera Alta, TO, Italy;  Orcidhttps://orcid.org/0000-0003-0875-7406;  Email: [email protected]
  • Authors
    • Matthew G. Quesne - School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.;  UK Catalysis Hub, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0FA, U.K.;  Orcidhttps://orcid.org/0000-0001-5130-1266
    • Francesc Viñes - Departament de Ciència de Materials i Química Física and Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1-11, 08028 Barcelona, Spain;  Orcidhttps://orcid.org/0000-0001-9987-8654
    • Francesc Illas - Departament de Ciència de Materials i Química Física and Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1-11, 08028 Barcelona, Spain;  Orcidhttps://orcid.org/0000-0003-2104-6123
    • C. Richard A. Catlow - School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.;  UK Catalysis Hub, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0FA, U.K.;  Department of Chemistry, University College London, 20 Gordon Street, London WC1 HOAJ, U.K.;  Orcidhttps://orcid.org/0000-0002-1341-1541
    • Nora H. de Leeuw - School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.;  School of Chemistry, University of Leeds, Leeds LS2 9JT, U.K.;  Orcidhttps://orcid.org/0000-0002-8271-0545
  • Author Contributions

    The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The underlying research was planned through the contribution of all authors and performed primarily by F.S. and M.G.Q. between Cardiff University and Universitat de Barcelona, however, with the supervision and contribution of all authors throughout the duration of the project.

  • Notes
    The authors declare no competing financial interest.

    All data created as part of this study are openly available at: http://doi.org/10.17035/d.2022.0162515110.

Acknowledgments

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This work was funded as part of an EPSRC low carbon fuel grant (EP/N009533/1). Computing facilities for this work were provided by ARCCA at Cardiff University, HPC Wales, and through our membership of the U.K.’s Materials Chemistry Consortium (MCC). The U.K. Catalysis Hub is thanked for resources and support provided via membership of the U.K. This work also used the Cirrus UK National Tier-2 HPC Service at EPCC (http://www.cirrus.ac.uk) funded by the University of Edinburgh and EPSRC (EP/P020267/1). Catalysis Hub Consortium is funded by EPSRC (grants EP/R026815/1, EP/K014854/1, and EP/M013219/1). The MCC is funded by EPSRC (EP/F067496). The research of F.V. and F.I. carried out the Universitat de Barcelona has been supported by the Spanish MCIN/AEI/10.13039/501100011033 funded RTI2018-095460-B-I00 and María de Maeztu MDM-2017-0767 grants, including funding from European Union. Further funding was provided by the HPC Europa-3 program, through which F.S. could access the computing facilities of the Barcelona Supercomputing Centre.

Abbreviations

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DFT

density functional theory

PBE

Perdew–Burke–Ernzerhof

PAW

projected-augmented wave

TMC

transition-metal carbide

References

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    Carbon dioxide/epoxide copolymn. is an efficient way to add value to waste CO2 and to reduce pollution in polymer manufg. Using this process to make low molar mass polycarbonate polyols is a com. relevant route to new thermosets and polyurethanes. In contrast, high molar mass polycarbonates, produced from CO2, generally under-deliver in terms of properties, and one of the most widely investigated, poly(cyclohexene carbonate), is limited by its low elongation at break and high brittleness. Here, a new catalytic polymn. process is reported that selectively and efficiently yields degradable ABA-block polymers, incorporating 6-23 wt. % CO2. The polymers are synthesized using a new, highly active organometallic heterodinuclear Zn(II)/Mg(II) catalyst applied in a one-pot procedure together with biobased ε-decalactone, cyclohexene oxide, and carbon dioxide to make a series of poly(cyclohexene carbonate-b-decalactone-b-cyclohexene carbonate) [PCHC-PDL-PCHC]. The process is highly selective (CO2 selectivity >99% of theor. value), allows for high monomer conversions (>90%), and yields polymers with predictable compns., molar mass (from 38-71 kg mol-1), and forms dihydroxyl telechelic chains. These new materials improve upon the properties of poly(cyclohexene carbonate) and, specifically, they show good thermal stability (Td,5 ∼ 280°C), high toughness (112 MJ m-3), and very high elongation at break (>900%). Materials properties are improved by precisely controlling both the quantity and location of carbon dioxide in the polymer chain. Preliminary studies show that polymers are stable in aq. environments at room temp. over months, but they are rapidly degraded upon gentle heating in an acidic environment (60°C, toluene, p-toluene sulfonic acid). The process is likely generally applicable to many other lactones, lactides, anhydrides, epoxides, and heterocumulenes and sets the scene for a host of new applications for CO2-derived polymers.
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    Quadrelli, E. A.; Centi, G.; Duplan, J. L.; Perathoner, S. Carbon Dioxide Recycling: Emerging Large-Scale Technologies with Industrial Potential. ChemSusChem 2011, 4, 1194– 1215,  DOI: 10.1002/cssc.201100473
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    Carbon Dioxide Recycling: Emerging Large-Scale Technologies with Industrial Potential
    Quadrelli, Elsje Alessandra; Centi, Gabriele; Duplan, Jean-Luc; Perathoner, Siglinda
    ChemSusChem (2011), 4 (9), 1194-1215CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. This Review introduces this special issue of ChemSusChem dedicated to CO2 recycling. Its aim is to offer an up-to-date overview of CO2 chem. utilization (inorg. mineralization, org. carboxylation, redn. reactions, and biochem. conversion), as a continuation and extension of earlier books and reviews on this topic, but with a specific focus on large-vol. routes and projects/pilot plants that are currently emerging at (pre-)industrial level. The review also highlights how some of these routes will offer a valuable opportunity to introduce renewable energy into the existing energy and chem. infrastructure (i.e., "drop-in" renewable energy) by synthesis of chems. from CO2 that are easy to transport and store. CO2 conversion therefore has the potential to become a key pillar of the sustainable and resource-efficient prodn. of chems. and energy from renewables.
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    Aresta, M.; Dibenedetto, A.; Angelini, A. Catalysis for the Valorization of Exhaust Carbon: From CO2 to Chemicals, Materials, and Fuels. Technological Use of CO2. Chem. Rev. 2014, 114, 1709– 1742,  DOI: 10.1021/cr4002758
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    Catalysis for the Valorization of Exhaust Carbon: from CO2 to Chemicals, Materials, and Fuels. Technological Use of CO2
    Aresta, Michele; Dibenedetto, Angela; Angelini, Antonella
    Chemical Reviews (Washington, DC, United States) (2014), 114 (3), 1709-1742CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review.
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    Ma, J.; Sun, N.; Zhang, X.; Zhao, N.; Xiao, F.; Wei, W.; Sun, Y. A Short Review of Catalysis for CO2 Conversion. Catal. Today 2009, 148, 221– 231,  DOI: 10.1016/j.cattod.2009.08.015
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    A short review of catalysis for CO2 conversion
    Ma, Jun; Sun, Nannan; Zhang, Xuelan; Zhao, Ning; Xiao, Fukui; Wei, Wei; Sun, Yuhan
    Catalysis Today (2009), 148 (3-4), 221-231CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    A review. To be the abundant natural feedstock, CO2 chem. utilization has attracted the great interest in recent years. The key point to CO2 conversion is the activation of either CO2 or co-reactant at different conditions. To bear this in mind, our strategy is to activate CO2 either with the presence of electron-rich chems. or by hydrogen, and to convert CO2 with a coupling reaction in some case. In this way, catalytic conversion of CO2 has been carried out by different methodol. at our lab, including CO2 reforming of methane to syngas prodn. over bifunctional catalysis, CO2 hydrogenation for methanol synthesis by nano-structured catalyst, and synthesis of carbonates from sub- or super-crit. CO2 with a coupling of in situ water removal reaction or bifunctional catalyst system. Herein, this brief review presents the recent progress of catalytic CO2 conversion and aims to shed a light into the chem. fixation of CO2.
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  7. 7
    Quesne, M. G.; Silveri, F.; de Leeuw, N. H.; Catlow, C. R. A. Advances in Sustainable Catalysis: A Computational Perspective. Front. Chem. 2019, 7, 182  DOI: 10.3389/fchem.2019.00182
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    Advances in sustainable catalysis: a computational perspective
    Quesne, Matthew G.; Silveri, Fabrizio; de Leeuw, Nora H.; Catlow, C. Richard A.
    Frontiers in Chemistry (Lausanne, Switzerland) (2019), 7 (), 182CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
    The enormous challenge of moving our societies to a more sustainable future offers several exciting opportunities for computational chemists. The first principles approach to "catalysis by design" will enable new and much greener chem. routes to produce vital fuels and fine chems. This prospective outlines a wide variety of case studies to underscore how the use of theor. techniques, from QM/MM to unrestricted DFT and periodic boundary conditions, can be applied to biocatalysis and to both homogeneous and heterogenous catalysts of all sizes and morphologies to provide invaluable insights into the reaction mechanisms they catalyze.
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  8. 8
    Posada-Pérez, S.; Gutiérrez, R. A.; Zuo, Z.; Ramírez, P. J.; Viñes, F.; Liu, P.; Illas, F.; Rodriguez, J. A. Highly Active Au/δ-MoC and Au/β-Mo2C Catalysts for the Low-Temperature Water Gas Shift Reaction: Effects of the Carbide Metal/Carbon Ratio on the Catalyst Performance. Catal. Sci. Technol. 2017, 7, 5332– 5342,  DOI: 10.1039/c7cy00639j
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    Highly active Au/δ-MoC and Au/β-Mo2C catalysts for the low-temperature water gas shift reaction: effects of the carbide metal/carbon ratio on the catalyst performance
    Posada-Perez, Sergio; Gutierrez, Ramon A.; Zuo, Zhijun; Ramirez, Pedro J.; Vines, Francesc; Liu, Ping; Illas, Francesc; Rodriguez, Jose A.
    Catalysis Science & Technology (2017), 7 (22), 5332-5342CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    The water gas shift (WGS) reaction catalyzed by orthorhombic β-Mo2C and cubic δ-MoC surfaces with and without Au clusters supported thereon has been studied by means of a combination of sophisticated expts. and state-of-the-art computational modeling. Expts. evidence the importance of the metal/carbon ratio on the performance of these systems, where Au/δ-MoC is presented as a suitable catalyst for WGS at low temps. owing to its high activity, selectivity (only CO2 and H2 are detected), and stability (oxycarbides are not obsd.). Periodic d. functional theory-based calcns. show that the supported Au clusters and the Au/δ-MoC interface do not take part directly in water dissocn. but their presence is crucial to switch the reaction mechanism, drastically decreasing the effect of the reverse WGS reaction and favoring the WGS products desorption, thus leading to an increase in CO2 and H2 prodn. The present results clearly display the importance of the Mo/C ratio and the synergy with the admetal clusters in tuning the activity and selectivity of the carbide substrate.
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  9. 9
    Kunkel, C.; Viñes, F.; Ramírez, P. J.; Rodriguez, J. A.; Illas, F. Combining Theory and Experiment for Multitechnique Characterization of Activated CO2 on Transition Metal Carbide (001) Surfaces. J. Phys. Chem. C 2019, 123, 7567– 7576,  DOI: 10.1021/acs.jpcc.7b12227
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    Characterization of activated CO2 on transition metal carbide (001) surfaces
    Kunkel, Christian; Vines, Francesc; Ramirez, Pedro J.; Rodriguez, Jose A.; Illas, Francesc
    Journal of Physical Chemistry C (2019), 123 (13), 7567-7576CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Early transition metal carbides (TMC; TM = Ti, Zr, Hf, V, Nb, Ta, Mo) with face-centered cubic crystallog. structure have emerged as promising materials for CO2 capture and activation. D. functional theory (DFT) calcns. using the Perdew-Burke-Ernzerhof exchange-correlation functional evidence charge transfer from the TMC surface to CO2 on the two possible adsorption sites, namely, MMC and TopC, and the electronic structure and binding strength differences are discussed. Further, the suitability of multiple exptl. techniques with respect to (1) adsorbed CO2 recognition and (2) MMC/TopC adsorption distinction is assessed from extensive DFT simulations. Results show that UV photoemission spectroscopies (UPS), work function changes, core level X-ray photoemission spectroscopy (XPS), and changes in linear optical properties could well allow for adsorbed CO2 detection. Only IR (IR) spectra and scanning tunneling microscopy (STM) seem to addnl. allow for MMC/TopC adsorption site distinction. These findings are confirmed with exptl. XPS measurements, demonstrating CO2 binding on single crystal (001) surfaces of TiC, ZrC, and VC. The expts. also help resolving ambiguities for VC, where CO2 activation was unexpected due to low adsorption energy, but could be related to kinetic trapping involving a desorption barrier. With a wealth of data reported and direct exptl. evidence provided, this study aims to motivate further basic surface science expts. on an interesting case of CO2 activating materials, allowing also for a benchmark of employed theor. models.
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  10. 10
    Porosoff, M. D.; Kattel, S.; Li, W.; Liu, P.; Chen, J. G. Identifying Trends and Descriptors for Selective CO2 Conversion to CO over Transition Metal Carbides. Chem. Commun. 2015, 51, 6988– 6991,  DOI: 10.1039/C5CC01545F
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    Identifying trends and descriptors for selective CO2 conversion to CO over transition metal carbides
    Porosoff, Marc D.; Kattel, Shyam; Li, Wenhui; Liu, Ping; Chen, Jingguang G.
    Chemical Communications (Cambridge, United Kingdom) (2015), 51 (32), 6988-6991CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Catalytic redn. of CO2 requires active, selective and low-cost catalysts. Results of this study show that transition metal carbides are a class of promising catalysts and their activity is correlated with oxygen binding energy and reducibility as shown by DFT calcns. and in situ measurements.
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  11. 11
    Lim, R. J.; Xie, M.; Sk, M. A.; Lee, J. M.; Fisher, A.; Wang, X.; Lim, K. H. A Review on the Electrochemical Reduction of CO2 in Fuel Cells, Metal Electrodes and Molecular Catalysts. Catal. Today 2014, 233, 169– 180,  DOI: 10.1016/j.cattod.2013.11.037
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    A review on the electrochemical reduction of CO2 in fuel cells, metal electrodes and molecular catalysts
    Lim, Rern Jern; Xie, Mingshi; Sk, Mahasin Alam; Lee, Jong-Min; Fisher, Adrian; Wang, Xin; Lim, Kok Hwa
    Catalysis Today (2014), 233 (), 169-180CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    A review; in this review article, we report the development and utilization of fuel cells, metal electrodes in aq. electrolyte and mol. catalysts in the electrochem. redn. of CO2. Fuel cells are able to function in both electrolyzer and fuel cell mode and could potentially reduce CO2 and produce energy at the same time. However, it requires considerably high temps. for efficient operation. Direct redn. using metal electrodes and mol. catalysts are possible at room temps. but require an addnl. applied potential and generally have low current densities. D. functional theory (DFT) studies have been used and have begun to unveil possible mechanisms involved which could lead to improvements and development of more efficient catalysts.
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  12. 12
    Wang, W.; Wang, S.; Ma, X.; Gong, J. Recent Advances in Catalytic Hydrogenation of Carbon Dioxide. Chem. Soc. Rev. 2011, 40, 3703– 3727,  DOI: 10.1039/c1cs15008a
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    Recent advances in catalytic hydrogenation of carbon dioxide
    Wang, Wei; Wang, Shengping; Ma, Xinbin; Gong, Jinlong
    Chemical Society Reviews (2011), 40 (7), 3703-3727CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. In this crit. review we discuss recent developments in catalytic hydrogenation of carbon dioxide , with emphasis on catalytic reactivity, reactor innovation, and reaction mechanism. We also provide an overview regarding the challenges and opportunities for future research in the field.
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  13. 13
    Posada-Pérez, S.; Ramírez, P. J.; Gutiérrez, R. A.; Stacchiola, D. J.; Viñes, F.; Liu, P.; Illas, F.; Rodriguez, J. A. The Conversion of CO2 to Methanol on Orthorhombic β-Mo2C and Cu/β-Mo2C Catalysts: Mechanism for Admetal Induced Change in the Selectivity and Activity. Catal. Sci. Technol. 2016, 6, 6766– 6777,  DOI: 10.1039/c5cy02143j
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    The conversion of CO2 to methanol on orthorhombic β-Mo2C and Cu/β-Mo2C catalysts: mechanism for admetal induced change in the selectivity and activity
    Posada-Perez, Sergio; Ramirez, Pedro J.; Gutierrez, Ramon A.; Stacchiola, Dario J.; Vines, Francesc; Liu, Ping; Illas, Francesc; Rodriguez, Jose A.
    Catalysis Science & Technology (2016), 6 (18), 6766-6777CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    The conversion of CO2 into methanol catalyzed by β-Mo2C and Cu/β-Mo2C surfaces has been investigated by means of a combined exptl. and theor. study. Expts. have shown the direct activation and dissocn. of the CO2 mol. on bare β-Mo2C, whereas on Cu/β-Mo2C, CO2 must be assisted by hydrogen for its conversion. Methane and CO are the main products on the clean surface and methanol prodn. is lower. However, the deposition of Cu clusters avoids methane formation and increases methanol prodn. even above that corresponding to a model of the tech. catalyst. DFT calcns. on surface models of both possible C- and Mo-terminations corroborate the exptl. observations. Calcns. for the clean Mo-terminated surface reveal the existence of two possible routes for methane prodn. (C + 4H → CH4; CH3O + 3H → CH4 + H2O) which are competitive with methanol synthesis, displaying slightly lower energy barriers. On the other hand, a model for Cu deposited clusters on the Mo-terminated surface points towards a new route for methanol and CO prodn. avoiding methane formation. The new route is a direct consequence of the generation of a Mo2C-Cu interface. The present exptl. and theor. results entail the interesting catalytic properties of Mo2C as an active support of metallic nanoparticles, and also illustrate how the deposition of a metal can drastically change the activity and selectivity of a carbide substrate for CO2 hydrogenation.
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    Quesne, M. G.; Roldan, A.; de Leeuw, N. H.; Catlow, C. R. A. Bulk and Surface Properties of Metal Carbides: Implications for Catalysis. Phys. Chem. Chem. Phys. 2018, 20, 6905– 6916,  DOI: 10.1039/c7cp06336a
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    Bulk and surface properties of metal carbides: implications for catalysis
    Quesne, Matthew G.; Roldan, Alberto; de Leeuw, Nora H.; Catlow, C. Richard A.
    Physical Chemistry Chemical Physics (2018), 20 (10), 6905-6916CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    We present a comprehensive study of the bulk and surface properties of transition metal carbides with rock salt structures and discuss their formation energies and electronic structures. The bonding character of the materials is shown to be dependent on the periodic position of the transition metal as well as the surface termination, which in turn tunes the densities of states and electronic surface properties. Specific focus is given to the possible catalytic implications of the surface properties on CO2 hydrogenation.
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    Quesne, M. G.; Roldan, A.; De Leeuw, N. H.; Catlow, C. R. A. Carbon Dioxide and Water Co-Adsorption on the Low-Index Surfaces of TiC, VC, ZrC and NbC: A DFT Study. Phys. Chem. Chem. Phys. 2019, 21, 10750– 10760,  DOI: 10.1039/c9cp00924h
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    Carbon dioxide and water co-adsorption on the low-index surfaces of TiC, VC, ZrC and NbC: a DFT study
    Quesne, Matthew G.; Roldan, Alberto; de Leeuw, Nora H.; Catlow, C. Richard A.
    Physical Chemistry Chemical Physics (2019), 21 (20), 10750-10760CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    We present a theor. DFT study into the activation of CO2 by TiC, VC, ZrC and NbC. Particular focus is given to the study of CO2/H2O co-adsorption and interaction on four carbide low-index surfaces: {001}, {011}, carbon-terminated {111} and metal-terminated {111}. The adsorption and activation of CO2 is shown to be most exothermic and indeed barrierless on the metal-terminated {111} surfaces, while adsorption on the {001} and {011} planes occurs via a small activation energy barrier. In contrast, the carbon-terminated {111} surface proves to be unstable in the presence of the adsorbates. Both water and carbon dioxide adsorb most strongly on TiC and most weakly on NbC, with the strongest co-adsorption interactions being seen in conformations that maximize hydrogen-bonding.
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    Silveri, F.; Quesne, M. G. M. G.; Roldan, A.; De Leeuw, N. H. N. H.; Catlow, C. R. A. R. A. Hydrogen Adsorption on Transition Metal Carbides: A DFT Study. Phys. Chem. Chem. Phys. 2019, 21, 5335– 5343,  DOI: 10.1039/c8cp05975f
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    Hydrogen adsorption on transition metal carbides: a DFT study
    Silveri, Fabrizio; Quesne, Matthew G.; Roldan, Alberto; de Leeuw, Nora H.; Catlow, C. Richard A.
    Physical Chemistry Chemical Physics (2019), 21 (10), 5335-5343CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Transition metal carbides are a class of materials widely known for both their interesting phys. properties and catalytic activity. In this work, we have used plane-wave DFT methods to study the interaction with increasing amts. of mol. hydrogen on the low-index surfaces of four major carbides - TiC, VC, ZrC and NbC. Adsorption is found to be generally exothermic and occurs predominantly on the surface carbon atoms. We identify trends over the carbides and their surfaces for the energetics of the adsorption as a function of their electronic and geometrical characteristics. An ab initio thermodn. formalism is used to study the properties of the slabs as the hydrogen coverage is increased.
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    Kunkel, C.; Viñes, F.; Illas, F. Transition Metal Carbides as Novel Materials for CO 2 Capture, Storage, and Activation. Energy Environ. Sci. 2016, 9, 141– 144,  DOI: 10.1039/C5EE03649F
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    Transition metal carbides as novel materials for CO2 capture, storage, and activation
    Kunkel, Christian; Vines, Francesc; Illas, Francesc
    Energy & Environmental Science (2016), 9 (1), 141-144CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    The capture and activation of the greenhouse gas carbon dioxide (CO2) is a prerequisite to its catalytic reforming or breakdown. Here we report, by means of d. functional theory calcns. including dispersive forces, that transition metal carbides (TMC; TM = Ti, Zr, Hf, Nb, Ta, Mo) are able to uptake and activate CO2 on their most-stable (001) surfaces with considerable adsorption strength. Estns. of adsorption and desorption rates predict a capture of CO2 at ambient temp. and even low partial pressures, suggesting TMCs as potential materials for CO2 abatement.
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    Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169– 11186,  DOI: 10.1103/PhysRevB.54.11169
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    Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
    Kresse, G.; Furthmueller, J.
    Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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    Projector augmented-wave method
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    Physical review. B, Condensed matter (1994), 50 (24), 17953-17979 ISSN:0163-1829.
    There is no expanded citation for this reference.
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    Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865– 3868,  DOI: 10.1103/PhysRevLett.77.3865
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    Generalized gradient approximation made simple
    Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
    Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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    Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104  DOI: 10.1063/1.3382344
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    A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu
    Grimme, Stefan; Antony, Jens; Ehrlich, Stephan; Krieg, Helge
    Journal of Chemical Physics (2010), 132 (15), 154104/1-154104/19CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The method of dispersion correction as an add-on to std. Kohn-Sham d. functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coeffs. and cutoff radii that are both computed from first principles. The coeffs. for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination nos. (CN). They are used to interpolate between dispersion coeffs. of atoms in different chem. environments. The method only requires adjustment of two global parameters for each d. functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of at. forces. Three-body nonadditivity terms are considered. The method has been assessed on std. benchmark sets for inter- and intramol. noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean abs. deviations for the S22 benchmark set of noncovalent interactions for 11 std. d. functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C6 coeffs. also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in mols. and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems. (c) 2010 American Institute of Physics.
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    Piñero, J. J.; Ramírez, P. J.; Bromley, S. T.; Illas, F.; Viñes, F.; Rodriguez, J. A. Diversity of Adsorbed Hydrogen on the TiC(001) Surface at High Coverages. J. Phys. Chem. C 2018, 122, 28013– 28020,  DOI: 10.1021/acs.jpcc.8b07340
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    Diversity of Adsorbed Hydrogen on the TiC(001) Surface at High Coverages
    Pinero, Juan Jose; Ramirez, Pedro J.; Bromley, Stefan T.; Illas, Francesc; Vines, Francesc; Rodriguez, Jose A.
    Journal of Physical Chemistry C (2018), 122 (49), 28013-28020CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The catalyzed dissocn. of mol. hydrogen on the surfaces of diverse materials is currently widely studied due to its importance in a broad range of hydrogenation reactions that convert noxious exhaust products and/or greenhouse gases into added-value greener products, such as methanol. In the search for viable replacements for expensive late-transition-metal catalysts, TiC has been increasingly investigated as a potential catalyst for H2 dissocn. Here, we report on a combination of expts. and d. functional theory calcns. on the well-defined TiC(001) surface, revealing that multiple H and H2 species are available on this substrate, with different binding configurations and adsorption energies. Our calcns. predict an initial occupancy of H atoms on the surface C atom sites, which then enables the subsequent stabilization of H atoms on top of the surface Ti atoms. Further H2 can be also molecularly adsorbed over Ti sites. These theor. predictions are in full accordance with information extd. from X-ray photoemission spectroscopy and temp.-programmed desorption expts. The exptl. results show that at high coverages of hydrogen, there is a reconstruction of the TiC(001) surface, which facilitates the binding of hydrogen.
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    Morales-García, Á.; Fernández-Fernández, A.; Viñes, F.; Illas, F. CO2 Abatement Using Two-Dimensional MXene Carbides. J. Mater. Chem. A 2018, 6, 3381– 3385,  DOI: 10.1039/c7ta11379j
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    CO2 abatement using two-dimensional MXene carbides
    Morales-Garcia, Angel; Fernandez-Fernandez, Adrian; Vines, Francesc; Illas, Francesc
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (8), 3381-3385CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Two-dimensional transition metal carbides with a formula of M2C (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) have been recently synthesized and isolated, and are here presented as very promising candidates for carbon dioxide (CO2) capture, storage, and activation. By means of d. functional theory investigations including dispersion we show the strong CO2 uptake and activation on M2C compds., where the ests. of adsorption and desorption rates indicate their CO2 adsorption capacity even at low CO2 partial pressures and high temps. The M2C compds. feature noteworthy CO2 loading capacities ranging from 2.34 to 8.25 mol CO2 kg-1, making them practical materials for CO2 abatement.
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    Morales-García, Á.; Mayans-Llorach, M.; Viñes, F.; Illas, F. Thickness Biased Capture of CO2 on Carbide MXenes. Phys. Chem. Chem. Phys. 2019, 21, 23136– 23142,  DOI: 10.1039/c9cp04833b
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    Thickness biased capture of CO2 on carbide MXenes
    Morales-Garcia, Angel; Mayans-Llorach, Marc; Vines, Francesc; Illas, Francesc
    Physical Chemistry Chemical Physics (2019), 21 (41), 23136-23142CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Synthesis of two-dimensional transition metal nano-sized carbides (MXenes) with a pre-defined no. of at. layers offers a possible way to tune their chem. activity. MXenes have been theor. predicted to be able to store CO2, even at high temps. and low CO2 partial pressures; a prediction which was exptl. confirmed. The no. of at. layers effect on CO2 adsorption was examd. by d. functional theory-based calcns., using suitable periodic models representing the (0001) surface of a series of these materials with the formula Mn+1Cn, where M = Ti, Zr, Hf, V, Nb, Ta, Mo, W, and n = 1-3. The CO2 interaction with MXene surfaces was always favorable: adsorption energy decreased as transition metal electronic configuration goes from d2 through d3 to d4, in agreement with previous work for n = 1. The thickness effect was rather small, yet noticeable and somewhat erratic. Nevertheless, adsorption energy seemed to converge to a defined clear limit for sufficiently thick MXenes. This value was close to that corresponding to the (111) surface of bulk transition metal carbides (TMC). The close structural similarity between the MXene (0001) and TMC (111) surfaces strongly suggested the former provides a practical way to approach this otherwise unstable surface. The possibility to tune CO2 interaction based on MXene thickness was further assessed using kinetic phase diagrams. These provided addnl. evidence that carbide MXene surfaces are promising materials for CO2 capture, even at low CO2 partial pressure, and that MXene thickness can be used to fine tune this appealing behavior.
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    Rodriguez, J. A.; Liu, P.; Gomes, J.; Nakamura, K.; Viñes, F.; Sousa, C.; Illas, F. Interaction of Oxygen with ZrC(001) and VC(001): Photoemission and First-Principles Studies. Phys. Rev. B 2005, 72, 075427  DOI: 10.1103/PhysRevB.72.075427
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    Interaction of oxygen with ZrC(001) and VC(001): Photoemission and first-principles studies
    Rodriguez, J. A.; Liu, P.; Gomes, J.; Nakamura, K.; Vines, F.; Sousa, C.; Illas, F.
    Physical Review B: Condensed Matter and Materials Physics (2005), 72 (7), 075427/1-075427/11CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    High-resoln. photoemission and 1st-principles d.-functional calcns. were used to study the interaction of oxygen with ZrC(001) and VC(001) surfaces. At. oxygen is present on the carbide substrates after small doses of O2 at room temp. At 500 K, the oxidn. of the surfaces is fast and clear features for ZrOx or VOx are seen in the O(1s), Zr(3d), and V(2p3/2) core levels spectra, with an increase in the metal/C ratio of the samples. A big pos. shift (1.3-1.6 eV) was detected for the C 1s core level in O/ZrC(001), indicating the existence of strong O↔C or C↔C interactions. A phenomenon corroborated by the results of 1st-principles calcns., which show a CZrZr hollow as the most stable site for the adsorption of O. The calcns. also show that a C↔O exchange is exothermic on ZrC(001), and the displaced C atoms bond to CZrZr sites. In the O/ZrC(001) interface, the surface C atoms play a major role in detg. the behavior of the system. But the adsorption of oxygen induces very minor changes in the C(1s) spectrum of VC(001). The O↔V interactions are stronger than the O↔Zr interactions, and O↔C interactions do not play a dominant role in the O/VC(001) interface. In this system, C↔O exchange is endothermic. VC(001) has a larger d. of metal d states near the Fermi level than ZrC(001), but the rate of oxidn. of VC(001) is slower. Therefore the O/ZrC(001) and O/VC(001) systems illustrate 2 different types of pathways for the oxidn. of carbide surfaces.
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    Weinberg, W. H. Eley-Rideal Surface Chemistry: Direct Reactivity of Gas Phase Atomic Hydrogen with Adsorbed Species. Acc. Chem. Res. 1996, 29, 479– 487,  DOI: 10.1021/ar9500980
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    Eley-Rideal Surface Chemistry: Direct Reactivity of Gas Phase Atomic Hydrogen with Adsorbed Species
    Weinberg, W. Henry
    Accounts of Chemical Research (1996), 29 (10), 479-487CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review with 80 refs. We will review here four specific examples, taken from our lab., which illustrate most of the important concepts of Eley-Rideal surface chem. The discussion will be limited to the chem. of the Ru(001) surface which was studied under ultrahigh vacuum (UHV) using std. surface characterization methods and at. hydrogen produced with either a hot filament source or a microwave discharge. These four specific examples are the abstraction of chemisorbed hydrogen, forming dihydrogen, which desorbs at least 150 K below the threshold temp. for recombinative desorption of two hydrogen adatoms; the hydrogenation of chemisorbed CO, forming η1- and η2-formyls and η2-formaldehyde; the selective hydrogenation of chemisorbed formate, forming a formyl and a hydroxyl; and the hydrogenation of oxygen adatoms to form water.
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    Posada-Pérez, S.; Viñes, F.; Ramirez, P. J.; Vidal, A. B.; Rodriguez, J. A.; Illas, F. The Bending Machine: CO2 Activation and Hydrogenation on δ-MoC(001) and β-Mo2C(001) Surfaces. Phys. Chem. Chem. Phys. 2014, 16, 14912– 14921,  DOI: 10.1039/c4cp01943a
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    28
    The bending machine: CO2 activation and hydrogenation on δ-MoC(001) and β-Mo2C(001) surfaces
    Posada-Perez, Sergio; Vines, Francesc; Ramirez, Pedro J.; Vidal, Alba B.; Rodriguez, Jose A.; Illas, Francesc
    Physical Chemistry Chemical Physics (2014), 16 (28), 14912-14921CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The adsorption and activation of a CO2 mol. on cubic δ-MoC(001) and orthorhombic β-Mo2C(001) surfaces have been investigated by means of periodic d. functional theory based calcns. using the Perdew-Burke-Ernzerhof exchange-correlation functional and explicitly accounting for (or neglecting) the dispersive force term description as proposed by Grimme. The DFT results indicate that an orthorhombic β-Mo2C(001) Mo-terminated polar surface provokes the spontaneous cleavage of a C-O bond in CO2 and carbon monoxide formation, whereas on a β-Mo2C(001) C-terminated polar surface or on a δ-MoC(001) nonpolar surface the CO2 mol. is activated yet the C-O bond prevails. Exptl. tests showed that Mo-terminated β-Mo2C(001) easily adsorbs and decomps. the CO2 mol. This surface is an active catalyst for the hydrogenation of CO2 to methanol and methane. Although MoC does not dissoc. C-O bonds on its own, it binds CO2 better than transition metal surfaces and is an active and selective catalyst for the CO2 + 3H2 → CH3OH + H2O reaction. Our theor. and exptl. results illustrate the tremendous impact that the carbon/metal ratio has on the chem. and catalytic properties of molybdenum carbides. This ratio must be taken into consideration when designing catalysts for the activation and conversion of CO2.
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    Posada-Pérez, S.; Viñes, F.; Ramirez, P. J.; Vidal, A. B.; Rodriguez, J. A.; Illas, F. The Bending Machine: CO2 Activation and Hydrogenation on δ-MoC(001) and β-Mo2C(001) Surfaces. Phys. Chem. Chem. Phys. 2014, 16, 14912– 14921,  DOI: 10.1039/C4CP01943A
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    29
    The bending machine: CO2 activation and hydrogenation on δ-MoC(001) and β-Mo2C(001) surfaces
    Posada-Perez, Sergio; Vines, Francesc; Ramirez, Pedro J.; Vidal, Alba B.; Rodriguez, Jose A.; Illas, Francesc
    Physical Chemistry Chemical Physics (2014), 16 (28), 14912-14921CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The adsorption and activation of a CO2 mol. on cubic δ-MoC(001) and orthorhombic β-Mo2C(001) surfaces have been investigated by means of periodic d. functional theory based calcns. using the Perdew-Burke-Ernzerhof exchange-correlation functional and explicitly accounting for (or neglecting) the dispersive force term description as proposed by Grimme. The DFT results indicate that an orthorhombic β-Mo2C(001) Mo-terminated polar surface provokes the spontaneous cleavage of a C-O bond in CO2 and carbon monoxide formation, whereas on a β-Mo2C(001) C-terminated polar surface or on a δ-MoC(001) nonpolar surface the CO2 mol. is activated yet the C-O bond prevails. Exptl. tests showed that Mo-terminated β-Mo2C(001) easily adsorbs and decomps. the CO2 mol. This surface is an active catalyst for the hydrogenation of CO2 to methanol and methane. Although MoC does not dissoc. C-O bonds on its own, it binds CO2 better than transition metal surfaces and is an active and selective catalyst for the CO2 + 3H2 → CH3OH + H2O reaction. Our theor. and exptl. results illustrate the tremendous impact that the carbon/metal ratio has on the chem. and catalytic properties of molybdenum carbides. This ratio must be taken into consideration when designing catalysts for the activation and conversion of CO2.
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    Wan, W.; Tackett, B. M.; Chen, J. G. Reactions of Water and C1 Molecules on Carbide and Metal-Modified Carbide Surfaces. Chem. Soc. Rev. 2017, 46, 1807– 1823,  DOI: 10.1039/c6cs00862c
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    Reactions of water and C1 molecules on carbide and metal-modified carbide surfaces
    Wan, Weiming; Tackett, Brian M.; Chen, Jingguang G.
    Chemical Society Reviews (2017), 46 (7), 1807-1823CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. The formation of carbides can significantly modify the phys. and chem. properties of the parent metals. In the current review, we summarize the general trends in the reactions of water and C1 mols. over transition metal carbide (TMC) and metal-modified TMC surfaces and thin films. Although the primary focus of the current review is on the theor. and exptl. studies of reactions of C1 mols. (CO, CO2, CH3OH, etc.), the reactions of water will also be reviewed because water plays an important role in many of the C1 transformation reactions. This review is organized by discussing sep. thermal reactions and electrochem. reactions, which provides insights into the application of TMCs in heterogeneous catalysis and electrocatalysis, resp. In thermal reactions, we discuss the thermal decompn. of water and methanol, as well as the reactions of CO and CO2 over TMC surfaces. In electrochem. reactions, we summarize recent studies in the hydrogen evolution reaction, electrooxidn. of methanol and CO, and electroredn. of CO2. Finally, future research opportunities and challenges assocd. with using TMCs as catalysts and electrocatalysts are also discussed.
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  2. Karim Harrath, Zhen Yao, Ya-Fei Jiang, Yang-Gang Wang, Jun Li. Activity Origin of the Nickel Cluster on TiC Support for Nonoxidative Methane Conversion. The Journal of Physical Chemistry Letters 2023, 14 (17) , 4033-4041. https://doi.org/10.1021/acs.jpclett.3c00375
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  • Abstract

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    Figure 1

    Figure 1. CO2 adsorption sites on TiC. From left to right: Con-top on (001), M2C on (001), M4C on (011), and M5 on (111). The same adsorption sites are also found on ZrC(001). The color scheme is as follows: Ti, blue; C, brown; and O, red.

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    Figure 2

    Figure 2. Most favorable adsorption geometries of, from left to right, CO, O, COOH, and HCOO on (a) ZrC(001), (b) TiC(011), and (c) Zr-terminated ZrC(111). (d) Top view, added for clarity, of (from left to right) CO on TiC(011), CO on ZrC(011), HCOO on ZrC(001), and HCOO on TiC(011). Analogous geometries are found for all species not shown in this picture, except for HCOO on ZrC(011), which spontaneously dissociates upon adsorption. The adsorption geometries shown here are those with the minimum energy among those tested on each surface. The color scheme is as follows: Ti, blue; Zr, green; C, brown; O, red; and H, pink.

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    Figure 3

    Figure 3. Proposed reduction mechanism of (a) CO2 hydrogenation to COOH on TiC(001), (b) CO2 dissociation to CO + O on ZrC(011), and (c) CO2 hydrogenation to HCOO on TiC(111). From left to right: initial state, in which the reactants are adsorbed on the surface of the catalyst; transition state, identified through the presence of one imaginary frequency related to the formation of the desired product; and final state, in which the product is adsorbed on the surface of the reactant. The color scheme is as follows: Ti, blue; Zr, green; C, brown; O, red; and H, pink.

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    Figure 4

    Figure 4. Proposed reduction pathways of CO2 on transition-metal carbide surfaces. The background chosen for each compound corresponds to its phase: dark blue for gas phase, and green and light blue for molecules on TiC and ZrC, respectively. The color code of the arrows between each compound is associated with the results of the present investigation: blue for reactions favored on (001) surfaces, black for (011), gray for (111), green for reactions favored on all surfaces, red for reactions hindered by either thermodynamic or kinetic impediments, and yellow for desorption reactions. As will be discussed in this paper, the main reduction path on all surfaces is that leading to CH4 through a HCO intermediate.

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    Figure 5

    Figure 5. TiC (top) and ZrC (bottom) (001) surface-mediated formation of dihydroxymethylidene (left) and formic acid (right). All energies are given in eV.

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    Figure 6

    Figure 6. Surface-mediated HCO formation by TiC by (top) (001), (middle) (011), and (bottom) (111)-metal-terminated facets. All energies are given in eV related to surface-bound carbon monoxide and hydrogen.

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    Figure 7

    Figure 7. Surface-mediated HCO formation by ZrC by (top) (001), (middle) (011), and (bottom) (111)-metal-terminated facets. All energies are given in eV related to surface-bound carbon monoxide and hydrogen.

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    Figure 8

    Figure 8. Thermodynamic profile of the reaction paths for CO2 reduction on TiC (001) (black), (011) (blue), and (111) (red) surfaces. The numbers displayed are the reaction energies of each intermediate (Es), in eV. All energies are referred to all reactants and products being simultaneously adsorbed on the relevant surfaces, without interaction between the adsorbed species, so that the reference energy level corresponds to two adsorbed hydrogen molecules and gas-phase CO2 for all steps. Only the carbon-containing molecule of each step is reported for clarity.

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    Figure 9

    Figure 9. Thermodynamic profile of the reduction paths for CO2 reduction on ZrC (001) (black), (011) (blue), and (111) (red) surfaces. The numbers displayed are the reaction energies of each intermediate (Es), in eV. All energies are referred to all reactants and products being simultaneously adsorbed on the relevant surfaces, without interaction between the adsorbed species, so that the reference energy level corresponds to two adsorbed hydrogen molecules and gas-phase CO2 for all steps. Only the carbon-containing molecule of each step is reported for clarity.

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  • References

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      Centi, Gabriele; Quadrelli, Elsje Alessandra; Perathoner, Siglinda
      Energy & Environmental Science (2013), 6 (6), 1711-1731CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      A review. Replacement of part of the fossil fuel consumption by renewable energy, in particular in the chem. industry, is a central strategy for resource and energy efficiency. This perspective will show that CO2 is the key mol. to proceed effectively in this direction. The routes, opportunities and barriers in increasing the share of renewable energy by using CO2 reaction and their impact on the chem. and energy value chains are discussed after introducing the general aspects of this topic evidencing the tight integration between the CO2 use and renewable energy insertion in the value chain of the process industry. The focus of this perspective article is on the catalytic aspects of the chemistries involved, with an anal. of the state-of-the-art, perspectives and targets to be developed. The reactions discussed are the prodn. of short-chain olefins (ethylene, propylene) from CO2, and the conversion of carbon dioxide to syngas, formic acid, methanol and di-Me ether, hydrocarbons via Fischer-Tropsch synthesis and methane. The relevance of availability, cost and environmental footprints of H2 prodn. routes using renewable energies is addressed. The final part discusses the possible scenario for CO2 as an intermediary for the incorporation of renewable energy in the process industry, with a concise road-map for catalysis needs and barriers to reach this goal.
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    2. 2
      Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729– 15735,  DOI: 10.1073/pnas.0603395103
      2
      Powering the planet: chemical challenges in solar energy utilization
      Lewis, Nathan S.; Nocera, Daniel G.
      Proceedings of the National Academy of Sciences of the United States of America (2006), 103 (43), 15729-15735CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      A review. Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, ≥2-fold by mid century relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO2 emissions in the atm. demands that holding atm. CO2 levels to even twice their pre-anthropogenic values by mid century will require invention, development, and deployment of schemes for C-neutral energy prodn. on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 h to the earth than all of the energy consumed by humans in an entire year. In view of the intermittence of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An esp. attractive approach is to store solar-converted energy as chem. bonds, i.e., in a photosynthetic process at a year-round av. efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy as chem. bonds, producing O from H2O and a reduced fuel such as H, methane, MeOH, or other hydrocarbon species.
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    3. 3
      Sulley, G. S.; Gregory, G. L.; Chen, T. T. D.; Peña Carrodeguas, L.; Trott, G.; Santmarti, A.; Lee, K. Y.; Terrill, N. J.; Williams, C. K. Switchable Catalysis Improves the Properties of CO2-Derived Polymers: Poly(Cyclohexene Carbonate- b-ϵ-Decalactone- b-Cyclohexene Carbonate) Adhesives, Elastomers, and Toughened Plastics. J. Am. Chem. Soc. 2020, 142, 4367– 4378,  DOI: 10.1021/jacs.9b13106
      3
      Switchable Catalysis Improves the Properties of CO2-Derived Polymers: Poly(cyclohexene carbonate-b-ε-decalactone-b-cyclohexene carbonate) Adhesives, Elastomers, and Toughened Plastics
      Sulley, Gregory S.; Gregory, Georgina L.; Chen, Thomas T. D.; Pena Carrodeguas, Leticia; Trott, Gemma; Santmarti, Alba; Lee, Koon-Yang; Terrill, Nicholas J.; Williams, Charlotte K.
      Journal of the American Chemical Society (2020), 142 (9), 4367-4378CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Carbon dioxide/epoxide copolymn. is an efficient way to add value to waste CO2 and to reduce pollution in polymer manufg. Using this process to make low molar mass polycarbonate polyols is a com. relevant route to new thermosets and polyurethanes. In contrast, high molar mass polycarbonates, produced from CO2, generally under-deliver in terms of properties, and one of the most widely investigated, poly(cyclohexene carbonate), is limited by its low elongation at break and high brittleness. Here, a new catalytic polymn. process is reported that selectively and efficiently yields degradable ABA-block polymers, incorporating 6-23 wt. % CO2. The polymers are synthesized using a new, highly active organometallic heterodinuclear Zn(II)/Mg(II) catalyst applied in a one-pot procedure together with biobased ε-decalactone, cyclohexene oxide, and carbon dioxide to make a series of poly(cyclohexene carbonate-b-decalactone-b-cyclohexene carbonate) [PCHC-PDL-PCHC]. The process is highly selective (CO2 selectivity >99% of theor. value), allows for high monomer conversions (>90%), and yields polymers with predictable compns., molar mass (from 38-71 kg mol-1), and forms dihydroxyl telechelic chains. These new materials improve upon the properties of poly(cyclohexene carbonate) and, specifically, they show good thermal stability (Td,5 ∼ 280°C), high toughness (112 MJ m-3), and very high elongation at break (>900%). Materials properties are improved by precisely controlling both the quantity and location of carbon dioxide in the polymer chain. Preliminary studies show that polymers are stable in aq. environments at room temp. over months, but they are rapidly degraded upon gentle heating in an acidic environment (60°C, toluene, p-toluene sulfonic acid). The process is likely generally applicable to many other lactones, lactides, anhydrides, epoxides, and heterocumulenes and sets the scene for a host of new applications for CO2-derived polymers.
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    4. 4
      Quadrelli, E. A.; Centi, G.; Duplan, J. L.; Perathoner, S. Carbon Dioxide Recycling: Emerging Large-Scale Technologies with Industrial Potential. ChemSusChem 2011, 4, 1194– 1215,  DOI: 10.1002/cssc.201100473
      4
      Carbon Dioxide Recycling: Emerging Large-Scale Technologies with Industrial Potential
      Quadrelli, Elsje Alessandra; Centi, Gabriele; Duplan, Jean-Luc; Perathoner, Siglinda
      ChemSusChem (2011), 4 (9), 1194-1215CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. This Review introduces this special issue of ChemSusChem dedicated to CO2 recycling. Its aim is to offer an up-to-date overview of CO2 chem. utilization (inorg. mineralization, org. carboxylation, redn. reactions, and biochem. conversion), as a continuation and extension of earlier books and reviews on this topic, but with a specific focus on large-vol. routes and projects/pilot plants that are currently emerging at (pre-)industrial level. The review also highlights how some of these routes will offer a valuable opportunity to introduce renewable energy into the existing energy and chem. infrastructure (i.e., "drop-in" renewable energy) by synthesis of chems. from CO2 that are easy to transport and store. CO2 conversion therefore has the potential to become a key pillar of the sustainable and resource-efficient prodn. of chems. and energy from renewables.
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    5. 5
      Aresta, M.; Dibenedetto, A.; Angelini, A. Catalysis for the Valorization of Exhaust Carbon: From CO2 to Chemicals, Materials, and Fuels. Technological Use of CO2. Chem. Rev. 2014, 114, 1709– 1742,  DOI: 10.1021/cr4002758
      5
      Catalysis for the Valorization of Exhaust Carbon: from CO2 to Chemicals, Materials, and Fuels. Technological Use of CO2
      Aresta, Michele; Dibenedetto, Angela; Angelini, Antonella
      Chemical Reviews (Washington, DC, United States) (2014), 114 (3), 1709-1742CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review.
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    6. 6
      Ma, J.; Sun, N.; Zhang, X.; Zhao, N.; Xiao, F.; Wei, W.; Sun, Y. A Short Review of Catalysis for CO2 Conversion. Catal. Today 2009, 148, 221– 231,  DOI: 10.1016/j.cattod.2009.08.015
      6
      A short review of catalysis for CO2 conversion
      Ma, Jun; Sun, Nannan; Zhang, Xuelan; Zhao, Ning; Xiao, Fukui; Wei, Wei; Sun, Yuhan
      Catalysis Today (2009), 148 (3-4), 221-231CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      A review. To be the abundant natural feedstock, CO2 chem. utilization has attracted the great interest in recent years. The key point to CO2 conversion is the activation of either CO2 or co-reactant at different conditions. To bear this in mind, our strategy is to activate CO2 either with the presence of electron-rich chems. or by hydrogen, and to convert CO2 with a coupling reaction in some case. In this way, catalytic conversion of CO2 has been carried out by different methodol. at our lab, including CO2 reforming of methane to syngas prodn. over bifunctional catalysis, CO2 hydrogenation for methanol synthesis by nano-structured catalyst, and synthesis of carbonates from sub- or super-crit. CO2 with a coupling of in situ water removal reaction or bifunctional catalyst system. Herein, this brief review presents the recent progress of catalytic CO2 conversion and aims to shed a light into the chem. fixation of CO2.
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    7. 7
      Quesne, M. G.; Silveri, F.; de Leeuw, N. H.; Catlow, C. R. A. Advances in Sustainable Catalysis: A Computational Perspective. Front. Chem. 2019, 7, 182  DOI: 10.3389/fchem.2019.00182
      7
      Advances in sustainable catalysis: a computational perspective
      Quesne, Matthew G.; Silveri, Fabrizio; de Leeuw, Nora H.; Catlow, C. Richard A.
      Frontiers in Chemistry (Lausanne, Switzerland) (2019), 7 (), 182CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
      The enormous challenge of moving our societies to a more sustainable future offers several exciting opportunities for computational chemists. The first principles approach to "catalysis by design" will enable new and much greener chem. routes to produce vital fuels and fine chems. This prospective outlines a wide variety of case studies to underscore how the use of theor. techniques, from QM/MM to unrestricted DFT and periodic boundary conditions, can be applied to biocatalysis and to both homogeneous and heterogenous catalysts of all sizes and morphologies to provide invaluable insights into the reaction mechanisms they catalyze.
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    8. 8
      Posada-Pérez, S.; Gutiérrez, R. A.; Zuo, Z.; Ramírez, P. J.; Viñes, F.; Liu, P.; Illas, F.; Rodriguez, J. A. Highly Active Au/δ-MoC and Au/β-Mo2C Catalysts for the Low-Temperature Water Gas Shift Reaction: Effects of the Carbide Metal/Carbon Ratio on the Catalyst Performance. Catal. Sci. Technol. 2017, 7, 5332– 5342,  DOI: 10.1039/c7cy00639j
      8
      Highly active Au/δ-MoC and Au/β-Mo2C catalysts for the low-temperature water gas shift reaction: effects of the carbide metal/carbon ratio on the catalyst performance
      Posada-Perez, Sergio; Gutierrez, Ramon A.; Zuo, Zhijun; Ramirez, Pedro J.; Vines, Francesc; Liu, Ping; Illas, Francesc; Rodriguez, Jose A.
      Catalysis Science & Technology (2017), 7 (22), 5332-5342CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      The water gas shift (WGS) reaction catalyzed by orthorhombic β-Mo2C and cubic δ-MoC surfaces with and without Au clusters supported thereon has been studied by means of a combination of sophisticated expts. and state-of-the-art computational modeling. Expts. evidence the importance of the metal/carbon ratio on the performance of these systems, where Au/δ-MoC is presented as a suitable catalyst for WGS at low temps. owing to its high activity, selectivity (only CO2 and H2 are detected), and stability (oxycarbides are not obsd.). Periodic d. functional theory-based calcns. show that the supported Au clusters and the Au/δ-MoC interface do not take part directly in water dissocn. but their presence is crucial to switch the reaction mechanism, drastically decreasing the effect of the reverse WGS reaction and favoring the WGS products desorption, thus leading to an increase in CO2 and H2 prodn. The present results clearly display the importance of the Mo/C ratio and the synergy with the admetal clusters in tuning the activity and selectivity of the carbide substrate.
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    9. 9
      Kunkel, C.; Viñes, F.; Ramírez, P. J.; Rodriguez, J. A.; Illas, F. Combining Theory and Experiment for Multitechnique Characterization of Activated CO2 on Transition Metal Carbide (001) Surfaces. J. Phys. Chem. C 2019, 123, 7567– 7576,  DOI: 10.1021/acs.jpcc.7b12227
      9
      Characterization of activated CO2 on transition metal carbide (001) surfaces
      Kunkel, Christian; Vines, Francesc; Ramirez, Pedro J.; Rodriguez, Jose A.; Illas, Francesc
      Journal of Physical Chemistry C (2019), 123 (13), 7567-7576CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Early transition metal carbides (TMC; TM = Ti, Zr, Hf, V, Nb, Ta, Mo) with face-centered cubic crystallog. structure have emerged as promising materials for CO2 capture and activation. D. functional theory (DFT) calcns. using the Perdew-Burke-Ernzerhof exchange-correlation functional evidence charge transfer from the TMC surface to CO2 on the two possible adsorption sites, namely, MMC and TopC, and the electronic structure and binding strength differences are discussed. Further, the suitability of multiple exptl. techniques with respect to (1) adsorbed CO2 recognition and (2) MMC/TopC adsorption distinction is assessed from extensive DFT simulations. Results show that UV photoemission spectroscopies (UPS), work function changes, core level X-ray photoemission spectroscopy (XPS), and changes in linear optical properties could well allow for adsorbed CO2 detection. Only IR (IR) spectra and scanning tunneling microscopy (STM) seem to addnl. allow for MMC/TopC adsorption site distinction. These findings are confirmed with exptl. XPS measurements, demonstrating CO2 binding on single crystal (001) surfaces of TiC, ZrC, and VC. The expts. also help resolving ambiguities for VC, where CO2 activation was unexpected due to low adsorption energy, but could be related to kinetic trapping involving a desorption barrier. With a wealth of data reported and direct exptl. evidence provided, this study aims to motivate further basic surface science expts. on an interesting case of CO2 activating materials, allowing also for a benchmark of employed theor. models.
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    10. 10
      Porosoff, M. D.; Kattel, S.; Li, W.; Liu, P.; Chen, J. G. Identifying Trends and Descriptors for Selective CO2 Conversion to CO over Transition Metal Carbides. Chem. Commun. 2015, 51, 6988– 6991,  DOI: 10.1039/C5CC01545F
      10
      Identifying trends and descriptors for selective CO2 conversion to CO over transition metal carbides
      Porosoff, Marc D.; Kattel, Shyam; Li, Wenhui; Liu, Ping; Chen, Jingguang G.
      Chemical Communications (Cambridge, United Kingdom) (2015), 51 (32), 6988-6991CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Catalytic redn. of CO2 requires active, selective and low-cost catalysts. Results of this study show that transition metal carbides are a class of promising catalysts and their activity is correlated with oxygen binding energy and reducibility as shown by DFT calcns. and in situ measurements.
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    11. 11
      Lim, R. J.; Xie, M.; Sk, M. A.; Lee, J. M.; Fisher, A.; Wang, X.; Lim, K. H. A Review on the Electrochemical Reduction of CO2 in Fuel Cells, Metal Electrodes and Molecular Catalysts. Catal. Today 2014, 233, 169– 180,  DOI: 10.1016/j.cattod.2013.11.037
      11
      A review on the electrochemical reduction of CO2 in fuel cells, metal electrodes and molecular catalysts
      Lim, Rern Jern; Xie, Mingshi; Sk, Mahasin Alam; Lee, Jong-Min; Fisher, Adrian; Wang, Xin; Lim, Kok Hwa
      Catalysis Today (2014), 233 (), 169-180CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      A review; in this review article, we report the development and utilization of fuel cells, metal electrodes in aq. electrolyte and mol. catalysts in the electrochem. redn. of CO2. Fuel cells are able to function in both electrolyzer and fuel cell mode and could potentially reduce CO2 and produce energy at the same time. However, it requires considerably high temps. for efficient operation. Direct redn. using metal electrodes and mol. catalysts are possible at room temps. but require an addnl. applied potential and generally have low current densities. D. functional theory (DFT) studies have been used and have begun to unveil possible mechanisms involved which could lead to improvements and development of more efficient catalysts.
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    12. 12
      Wang, W.; Wang, S.; Ma, X.; Gong, J. Recent Advances in Catalytic Hydrogenation of Carbon Dioxide. Chem. Soc. Rev. 2011, 40, 3703– 3727,  DOI: 10.1039/c1cs15008a
      12
      Recent advances in catalytic hydrogenation of carbon dioxide
      Wang, Wei; Wang, Shengping; Ma, Xinbin; Gong, Jinlong
      Chemical Society Reviews (2011), 40 (7), 3703-3727CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. In this crit. review we discuss recent developments in catalytic hydrogenation of carbon dioxide , with emphasis on catalytic reactivity, reactor innovation, and reaction mechanism. We also provide an overview regarding the challenges and opportunities for future research in the field.
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    13. 13
      Posada-Pérez, S.; Ramírez, P. J.; Gutiérrez, R. A.; Stacchiola, D. J.; Viñes, F.; Liu, P.; Illas, F.; Rodriguez, J. A. The Conversion of CO2 to Methanol on Orthorhombic β-Mo2C and Cu/β-Mo2C Catalysts: Mechanism for Admetal Induced Change in the Selectivity and Activity. Catal. Sci. Technol. 2016, 6, 6766– 6777,  DOI: 10.1039/c5cy02143j
      13
      The conversion of CO2 to methanol on orthorhombic β-Mo2C and Cu/β-Mo2C catalysts: mechanism for admetal induced change in the selectivity and activity
      Posada-Perez, Sergio; Ramirez, Pedro J.; Gutierrez, Ramon A.; Stacchiola, Dario J.; Vines, Francesc; Liu, Ping; Illas, Francesc; Rodriguez, Jose A.
      Catalysis Science & Technology (2016), 6 (18), 6766-6777CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      The conversion of CO2 into methanol catalyzed by β-Mo2C and Cu/β-Mo2C surfaces has been investigated by means of a combined exptl. and theor. study. Expts. have shown the direct activation and dissocn. of the CO2 mol. on bare β-Mo2C, whereas on Cu/β-Mo2C, CO2 must be assisted by hydrogen for its conversion. Methane and CO are the main products on the clean surface and methanol prodn. is lower. However, the deposition of Cu clusters avoids methane formation and increases methanol prodn. even above that corresponding to a model of the tech. catalyst. DFT calcns. on surface models of both possible C- and Mo-terminations corroborate the exptl. observations. Calcns. for the clean Mo-terminated surface reveal the existence of two possible routes for methane prodn. (C + 4H → CH4; CH3O + 3H → CH4 + H2O) which are competitive with methanol synthesis, displaying slightly lower energy barriers. On the other hand, a model for Cu deposited clusters on the Mo-terminated surface points towards a new route for methanol and CO prodn. avoiding methane formation. The new route is a direct consequence of the generation of a Mo2C-Cu interface. The present exptl. and theor. results entail the interesting catalytic properties of Mo2C as an active support of metallic nanoparticles, and also illustrate how the deposition of a metal can drastically change the activity and selectivity of a carbide substrate for CO2 hydrogenation.
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    14. 14
      Quesne, M. G.; Roldan, A.; de Leeuw, N. H.; Catlow, C. R. A. Bulk and Surface Properties of Metal Carbides: Implications for Catalysis. Phys. Chem. Chem. Phys. 2018, 20, 6905– 6916,  DOI: 10.1039/c7cp06336a
      14
      Bulk and surface properties of metal carbides: implications for catalysis
      Quesne, Matthew G.; Roldan, Alberto; de Leeuw, Nora H.; Catlow, C. Richard A.
      Physical Chemistry Chemical Physics (2018), 20 (10), 6905-6916CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      We present a comprehensive study of the bulk and surface properties of transition metal carbides with rock salt structures and discuss their formation energies and electronic structures. The bonding character of the materials is shown to be dependent on the periodic position of the transition metal as well as the surface termination, which in turn tunes the densities of states and electronic surface properties. Specific focus is given to the possible catalytic implications of the surface properties on CO2 hydrogenation.
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    15. 15
      Quesne, M. G.; Roldan, A.; De Leeuw, N. H.; Catlow, C. R. A. Carbon Dioxide and Water Co-Adsorption on the Low-Index Surfaces of TiC, VC, ZrC and NbC: A DFT Study. Phys. Chem. Chem. Phys. 2019, 21, 10750– 10760,  DOI: 10.1039/c9cp00924h
      15
      Carbon dioxide and water co-adsorption on the low-index surfaces of TiC, VC, ZrC and NbC: a DFT study
      Quesne, Matthew G.; Roldan, Alberto; de Leeuw, Nora H.; Catlow, C. Richard A.
      Physical Chemistry Chemical Physics (2019), 21 (20), 10750-10760CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      We present a theor. DFT study into the activation of CO2 by TiC, VC, ZrC and NbC. Particular focus is given to the study of CO2/H2O co-adsorption and interaction on four carbide low-index surfaces: {001}, {011}, carbon-terminated {111} and metal-terminated {111}. The adsorption and activation of CO2 is shown to be most exothermic and indeed barrierless on the metal-terminated {111} surfaces, while adsorption on the {001} and {011} planes occurs via a small activation energy barrier. In contrast, the carbon-terminated {111} surface proves to be unstable in the presence of the adsorbates. Both water and carbon dioxide adsorb most strongly on TiC and most weakly on NbC, with the strongest co-adsorption interactions being seen in conformations that maximize hydrogen-bonding.
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    16. 16
      Silveri, F.; Quesne, M. G. M. G.; Roldan, A.; De Leeuw, N. H. N. H.; Catlow, C. R. A. R. A. Hydrogen Adsorption on Transition Metal Carbides: A DFT Study. Phys. Chem. Chem. Phys. 2019, 21, 5335– 5343,  DOI: 10.1039/c8cp05975f
      16
      Hydrogen adsorption on transition metal carbides: a DFT study
      Silveri, Fabrizio; Quesne, Matthew G.; Roldan, Alberto; de Leeuw, Nora H.; Catlow, C. Richard A.
      Physical Chemistry Chemical Physics (2019), 21 (10), 5335-5343CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Transition metal carbides are a class of materials widely known for both their interesting phys. properties and catalytic activity. In this work, we have used plane-wave DFT methods to study the interaction with increasing amts. of mol. hydrogen on the low-index surfaces of four major carbides - TiC, VC, ZrC and NbC. Adsorption is found to be generally exothermic and occurs predominantly on the surface carbon atoms. We identify trends over the carbides and their surfaces for the energetics of the adsorption as a function of their electronic and geometrical characteristics. An ab initio thermodn. formalism is used to study the properties of the slabs as the hydrogen coverage is increased.
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    17. 17
      Kunkel, C.; Viñes, F.; Illas, F. Transition Metal Carbides as Novel Materials for CO 2 Capture, Storage, and Activation. Energy Environ. Sci. 2016, 9, 141– 144,  DOI: 10.1039/C5EE03649F
      17
      Transition metal carbides as novel materials for CO2 capture, storage, and activation
      Kunkel, Christian; Vines, Francesc; Illas, Francesc
      Energy & Environmental Science (2016), 9 (1), 141-144CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      The capture and activation of the greenhouse gas carbon dioxide (CO2) is a prerequisite to its catalytic reforming or breakdown. Here we report, by means of d. functional theory calcns. including dispersive forces, that transition metal carbides (TMC; TM = Ti, Zr, Hf, Nb, Ta, Mo) are able to uptake and activate CO2 on their most-stable (001) surfaces with considerable adsorption strength. Estns. of adsorption and desorption rates predict a capture of CO2 at ambient temp. and even low partial pressures, suggesting TMCs as potential materials for CO2 abatement.
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    18. 18
      Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169– 11186,  DOI: 10.1103/PhysRevB.54.11169
      18
      Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
      Kresse, G.; Furthmueller, J.
      Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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    19. 19
      Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953– 17979,  DOI: 10.1103/PhysRevB.50.17953
      19
      Projector augmented-wave method
      Blochl
      Physical review. B, Condensed matter (1994), 50 (24), 17953-17979 ISSN:0163-1829.
      There is no expanded citation for this reference.
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    20. 20
      Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865– 3868,  DOI: 10.1103/PhysRevLett.77.3865
      20
      Generalized gradient approximation made simple
      Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
      Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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    21. 21
      Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104  DOI: 10.1063/1.3382344
      21
      A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu
      Grimme, Stefan; Antony, Jens; Ehrlich, Stephan; Krieg, Helge
      Journal of Chemical Physics (2010), 132 (15), 154104/1-154104/19CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The method of dispersion correction as an add-on to std. Kohn-Sham d. functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coeffs. and cutoff radii that are both computed from first principles. The coeffs. for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination nos. (CN). They are used to interpolate between dispersion coeffs. of atoms in different chem. environments. The method only requires adjustment of two global parameters for each d. functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of at. forces. Three-body nonadditivity terms are considered. The method has been assessed on std. benchmark sets for inter- and intramol. noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean abs. deviations for the S22 benchmark set of noncovalent interactions for 11 std. d. functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C6 coeffs. also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in mols. and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems. (c) 2010 American Institute of Physics.
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    22. 22
      Blöchl, P. E.; Jepsen, O.; Andersen, O. K. Improved Tetrahedron Method for Brillouin-Zone Integrations. Phys. Rev. B 1994, 49, 16223,  DOI: 10.1103/PhysRevB.49.16223
      22
      Improved tetrahedron method for Brillouin-zone integrations
      Blochl, Peter E.; Jepsen, O.; Andersen, O. K.
      Physical Review B: Condensed Matter and Materials Physics (1994), 49 (23), 16223-33CODEN: PRBMDO; ISSN:0163-1829.
      Several improvements of the tetrahedron method for Brillouin-zone integrations are presented. (1) A translational grid of k points and tetrahedra is suggested that renders the results for insulators identical to those obtained with special-point methods with the same no. of k points. (2) A simple correction formula goes beyond the linear approxn. of matrix elements within the tetrahedra and also improves the results for metals significantly. For a required accuracy this reduces the no. of k points by orders of magnitude. (3) Irreducible k points and tetrahedra are selected by a fully automated procedure, requiring as input only the space-group operations. (4) The integration is formulated as a weighted sum over irreducible k points with integration wts. calcd. using the tetrahedron method once for a given band structure. This allows an efficient use of the tetrahedron method also in plane-wave-based electronic-structure methods.
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    23. 23
      Piñero, J. J.; Ramírez, P. J.; Bromley, S. T.; Illas, F.; Viñes, F.; Rodriguez, J. A. Diversity of Adsorbed Hydrogen on the TiC(001) Surface at High Coverages. J. Phys. Chem. C 2018, 122, 28013– 28020,  DOI: 10.1021/acs.jpcc.8b07340
      23
      Diversity of Adsorbed Hydrogen on the TiC(001) Surface at High Coverages
      Pinero, Juan Jose; Ramirez, Pedro J.; Bromley, Stefan T.; Illas, Francesc; Vines, Francesc; Rodriguez, Jose A.
      Journal of Physical Chemistry C (2018), 122 (49), 28013-28020CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The catalyzed dissocn. of mol. hydrogen on the surfaces of diverse materials is currently widely studied due to its importance in a broad range of hydrogenation reactions that convert noxious exhaust products and/or greenhouse gases into added-value greener products, such as methanol. In the search for viable replacements for expensive late-transition-metal catalysts, TiC has been increasingly investigated as a potential catalyst for H2 dissocn. Here, we report on a combination of expts. and d. functional theory calcns. on the well-defined TiC(001) surface, revealing that multiple H and H2 species are available on this substrate, with different binding configurations and adsorption energies. Our calcns. predict an initial occupancy of H atoms on the surface C atom sites, which then enables the subsequent stabilization of H atoms on top of the surface Ti atoms. Further H2 can be also molecularly adsorbed over Ti sites. These theor. predictions are in full accordance with information extd. from X-ray photoemission spectroscopy and temp.-programmed desorption expts. The exptl. results show that at high coverages of hydrogen, there is a reconstruction of the TiC(001) surface, which facilitates the binding of hydrogen.
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    24. 24
      Morales-García, Á.; Fernández-Fernández, A.; Viñes, F.; Illas, F. CO2 Abatement Using Two-Dimensional MXene Carbides. J. Mater. Chem. A 2018, 6, 3381– 3385,  DOI: 10.1039/c7ta11379j
      24
      CO2 abatement using two-dimensional MXene carbides
      Morales-Garcia, Angel; Fernandez-Fernandez, Adrian; Vines, Francesc; Illas, Francesc
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (8), 3381-3385CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Two-dimensional transition metal carbides with a formula of M2C (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) have been recently synthesized and isolated, and are here presented as very promising candidates for carbon dioxide (CO2) capture, storage, and activation. By means of d. functional theory investigations including dispersion we show the strong CO2 uptake and activation on M2C compds., where the ests. of adsorption and desorption rates indicate their CO2 adsorption capacity even at low CO2 partial pressures and high temps. The M2C compds. feature noteworthy CO2 loading capacities ranging from 2.34 to 8.25 mol CO2 kg-1, making them practical materials for CO2 abatement.
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    25. 25
      Morales-García, Á.; Mayans-Llorach, M.; Viñes, F.; Illas, F. Thickness Biased Capture of CO2 on Carbide MXenes. Phys. Chem. Chem. Phys. 2019, 21, 23136– 23142,  DOI: 10.1039/c9cp04833b
      25
      Thickness biased capture of CO2 on carbide MXenes
      Morales-Garcia, Angel; Mayans-Llorach, Marc; Vines, Francesc; Illas, Francesc
      Physical Chemistry Chemical Physics (2019), 21 (41), 23136-23142CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Synthesis of two-dimensional transition metal nano-sized carbides (MXenes) with a pre-defined no. of at. layers offers a possible way to tune their chem. activity. MXenes have been theor. predicted to be able to store CO2, even at high temps. and low CO2 partial pressures; a prediction which was exptl. confirmed. The no. of at. layers effect on CO2 adsorption was examd. by d. functional theory-based calcns., using suitable periodic models representing the (0001) surface of a series of these materials with the formula Mn+1Cn, where M = Ti, Zr, Hf, V, Nb, Ta, Mo, W, and n = 1-3. The CO2 interaction with MXene surfaces was always favorable: adsorption energy decreased as transition metal electronic configuration goes from d2 through d3 to d4, in agreement with previous work for n = 1. The thickness effect was rather small, yet noticeable and somewhat erratic. Nevertheless, adsorption energy seemed to converge to a defined clear limit for sufficiently thick MXenes. This value was close to that corresponding to the (111) surface of bulk transition metal carbides (TMC). The close structural similarity between the MXene (0001) and TMC (111) surfaces strongly suggested the former provides a practical way to approach this otherwise unstable surface. The possibility to tune CO2 interaction based on MXene thickness was further assessed using kinetic phase diagrams. These provided addnl. evidence that carbide MXene surfaces are promising materials for CO2 capture, even at low CO2 partial pressure, and that MXene thickness can be used to fine tune this appealing behavior.
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    26. 26
      Rodriguez, J. A.; Liu, P.; Gomes, J.; Nakamura, K.; Viñes, F.; Sousa, C.; Illas, F. Interaction of Oxygen with ZrC(001) and VC(001): Photoemission and First-Principles Studies. Phys. Rev. B 2005, 72, 075427  DOI: 10.1103/PhysRevB.72.075427
      26
      Interaction of oxygen with ZrC(001) and VC(001): Photoemission and first-principles studies
      Rodriguez, J. A.; Liu, P.; Gomes, J.; Nakamura, K.; Vines, F.; Sousa, C.; Illas, F.
      Physical Review B: Condensed Matter and Materials Physics (2005), 72 (7), 075427/1-075427/11CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      High-resoln. photoemission and 1st-principles d.-functional calcns. were used to study the interaction of oxygen with ZrC(001) and VC(001) surfaces. At. oxygen is present on the carbide substrates after small doses of O2 at room temp. At 500 K, the oxidn. of the surfaces is fast and clear features for ZrOx or VOx are seen in the O(1s), Zr(3d), and V(2p3/2) core levels spectra, with an increase in the metal/C ratio of the samples. A big pos. shift (1.3-1.6 eV) was detected for the C 1s core level in O/ZrC(001), indicating the existence of strong O↔C or C↔C interactions. A phenomenon corroborated by the results of 1st-principles calcns., which show a CZrZr hollow as the most stable site for the adsorption of O. The calcns. also show that a C↔O exchange is exothermic on ZrC(001), and the displaced C atoms bond to CZrZr sites. In the O/ZrC(001) interface, the surface C atoms play a major role in detg. the behavior of the system. But the adsorption of oxygen induces very minor changes in the C(1s) spectrum of VC(001). The O↔V interactions are stronger than the O↔Zr interactions, and O↔C interactions do not play a dominant role in the O/VC(001) interface. In this system, C↔O exchange is endothermic. VC(001) has a larger d. of metal d states near the Fermi level than ZrC(001), but the rate of oxidn. of VC(001) is slower. Therefore the O/ZrC(001) and O/VC(001) systems illustrate 2 different types of pathways for the oxidn. of carbide surfaces.
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    27. 27
      Weinberg, W. H. Eley-Rideal Surface Chemistry: Direct Reactivity of Gas Phase Atomic Hydrogen with Adsorbed Species. Acc. Chem. Res. 1996, 29, 479– 487,  DOI: 10.1021/ar9500980
      27
      Eley-Rideal Surface Chemistry: Direct Reactivity of Gas Phase Atomic Hydrogen with Adsorbed Species
      Weinberg, W. Henry
      Accounts of Chemical Research (1996), 29 (10), 479-487CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review with 80 refs. We will review here four specific examples, taken from our lab., which illustrate most of the important concepts of Eley-Rideal surface chem. The discussion will be limited to the chem. of the Ru(001) surface which was studied under ultrahigh vacuum (UHV) using std. surface characterization methods and at. hydrogen produced with either a hot filament source or a microwave discharge. These four specific examples are the abstraction of chemisorbed hydrogen, forming dihydrogen, which desorbs at least 150 K below the threshold temp. for recombinative desorption of two hydrogen adatoms; the hydrogenation of chemisorbed CO, forming η1- and η2-formyls and η2-formaldehyde; the selective hydrogenation of chemisorbed formate, forming a formyl and a hydroxyl; and the hydrogenation of oxygen adatoms to form water.
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    28. 28
      Posada-Pérez, S.; Viñes, F.; Ramirez, P. J.; Vidal, A. B.; Rodriguez, J. A.; Illas, F. The Bending Machine: CO2 Activation and Hydrogenation on δ-MoC(001) and β-Mo2C(001) Surfaces. Phys. Chem. Chem. Phys. 2014, 16, 14912– 14921,  DOI: 10.1039/c4cp01943a
      28
      The bending machine: CO2 activation and hydrogenation on δ-MoC(001) and β-Mo2C(001) surfaces
      Posada-Perez, Sergio; Vines, Francesc; Ramirez, Pedro J.; Vidal, Alba B.; Rodriguez, Jose A.; Illas, Francesc
      Physical Chemistry Chemical Physics (2014), 16 (28), 14912-14921CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The adsorption and activation of a CO2 mol. on cubic δ-MoC(001) and orthorhombic β-Mo2C(001) surfaces have been investigated by means of periodic d. functional theory based calcns. using the Perdew-Burke-Ernzerhof exchange-correlation functional and explicitly accounting for (or neglecting) the dispersive force term description as proposed by Grimme. The DFT results indicate that an orthorhombic β-Mo2C(001) Mo-terminated polar surface provokes the spontaneous cleavage of a C-O bond in CO2 and carbon monoxide formation, whereas on a β-Mo2C(001) C-terminated polar surface or on a δ-MoC(001) nonpolar surface the CO2 mol. is activated yet the C-O bond prevails. Exptl. tests showed that Mo-terminated β-Mo2C(001) easily adsorbs and decomps. the CO2 mol. This surface is an active catalyst for the hydrogenation of CO2 to methanol and methane. Although MoC does not dissoc. C-O bonds on its own, it binds CO2 better than transition metal surfaces and is an active and selective catalyst for the CO2 + 3H2 → CH3OH + H2O reaction. Our theor. and exptl. results illustrate the tremendous impact that the carbon/metal ratio has on the chem. and catalytic properties of molybdenum carbides. This ratio must be taken into consideration when designing catalysts for the activation and conversion of CO2.
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    29. 29
      Posada-Pérez, S.; Viñes, F.; Ramirez, P. J.; Vidal, A. B.; Rodriguez, J. A.; Illas, F. The Bending Machine: CO2 Activation and Hydrogenation on δ-MoC(001) and β-Mo2C(001) Surfaces. Phys. Chem. Chem. Phys. 2014, 16, 14912– 14921,  DOI: 10.1039/C4CP01943A
      29
      The bending machine: CO2 activation and hydrogenation on δ-MoC(001) and β-Mo2C(001) surfaces
      Posada-Perez, Sergio; Vines, Francesc; Ramirez, Pedro J.; Vidal, Alba B.; Rodriguez, Jose A.; Illas, Francesc
      Physical Chemistry Chemical Physics (2014), 16 (28), 14912-14921CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The adsorption and activation of a CO2 mol. on cubic δ-MoC(001) and orthorhombic β-Mo2C(001) surfaces have been investigated by means of periodic d. functional theory based calcns. using the Perdew-Burke-Ernzerhof exchange-correlation functional and explicitly accounting for (or neglecting) the dispersive force term description as proposed by Grimme. The DFT results indicate that an orthorhombic β-Mo2C(001) Mo-terminated polar surface provokes the spontaneous cleavage of a C-O bond in CO2 and carbon monoxide formation, whereas on a β-Mo2C(001) C-terminated polar surface or on a δ-MoC(001) nonpolar surface the CO2 mol. is activated yet the C-O bond prevails. Exptl. tests showed that Mo-terminated β-Mo2C(001) easily adsorbs and decomps. the CO2 mol. This surface is an active catalyst for the hydrogenation of CO2 to methanol and methane. Although MoC does not dissoc. C-O bonds on its own, it binds CO2 better than transition metal surfaces and is an active and selective catalyst for the CO2 + 3H2 → CH3OH + H2O reaction. Our theor. and exptl. results illustrate the tremendous impact that the carbon/metal ratio has on the chem. and catalytic properties of molybdenum carbides. This ratio must be taken into consideration when designing catalysts for the activation and conversion of CO2.
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    30. 30
      Wan, W.; Tackett, B. M.; Chen, J. G. Reactions of Water and C1 Molecules on Carbide and Metal-Modified Carbide Surfaces. Chem. Soc. Rev. 2017, 46, 1807– 1823,  DOI: 10.1039/c6cs00862c
      30
      Reactions of water and C1 molecules on carbide and metal-modified carbide surfaces
      Wan, Weiming; Tackett, Brian M.; Chen, Jingguang G.
      Chemical Society Reviews (2017), 46 (7), 1807-1823CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. The formation of carbides can significantly modify the phys. and chem. properties of the parent metals. In the current review, we summarize the general trends in the reactions of water and C1 mols. over transition metal carbide (TMC) and metal-modified TMC surfaces and thin films. Although the primary focus of the current review is on the theor. and exptl. studies of reactions of C1 mols. (CO, CO2, CH3OH, etc.), the reactions of water will also be reviewed because water plays an important role in many of the C1 transformation reactions. This review is organized by discussing sep. thermal reactions and electrochem. reactions, which provides insights into the application of TMCs in heterogeneous catalysis and electrocatalysis, resp. In thermal reactions, we discuss the thermal decompn. of water and methanol, as well as the reactions of CO and CO2 over TMC surfaces. In electrochem. reactions, we summarize recent studies in the hydrogen evolution reaction, electrooxidn. of methanol and CO, and electroredn. of CO2. Finally, future research opportunities and challenges assocd. with using TMCs as catalysts and electrocatalysts are also discussed.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.1c10180.

    • Activation energy for CO2 adsorption reported in this work compared to previously reported values (15) (Table S1) (PDF)


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