CO2 Hydrogenation to Methanol over Mesoporous SiO2-Coated Cu-Based Catalysts

Although chemical promotion led to essential improvements in Cu-based catalysts for CO2 hydrogenation to methanol, surpassing structural limitations such as active phase aggregation under reaction conditions remains challenging. In this report, we improved the textural properties of Cu/In2O3/CeO2 and Cu/In2O3/ZrO2 catalysts by coating the nanoparticles with a mesoporous SiO2 shell. This strategy limited particle size up to 3.5 nm, increasing metal dispersion and widening the metal–metal oxide interface region. Chemometric analysis revealed that these structures could maintain high activity and selectivity in a wide range of reaction conditions, with methanol space-time yields up to 4 times higher than those of the uncoated catalysts.


Synthesis of In-promoted catalysts
The catalysts were synthesized using the surfactant-assisted co-precipitation method.Initially, 60 mmol of CTAB were added to 250 mL of H 2 O.The solution was kept under constant stirring until complete homogenization.Then, 15 mmol of Cu(NO 3 ) 2 , 13.5 mmol of ZrO(NO 3 ) 2 or Ce(NO 3 ) 3 , and 1.5 mmol of In(NO 3 ) 3 were added and the stirring was maintained for another 2 hours.The precipitation occurred by the addition of a 0.5 M solution of NaOH, keeping the pH at a constant value of 10 throughout the procedure.After completing the precipitation, the solution was kept under stirring for 12 hours at room temperature.The precipitate was separated by vacuum filtration and washed in H 2 O until a pH ~7 was obtained in the washing solution.The solid was dried in an oven at 333 K for 48 hours and calcined at 873 K for 2 hours under an oxidizing atmosphere.The catalyst prepared using the ZrO(NO 3 ) 2 precursor was named CuZrIn and those prepared using Ce(NO 3 ) 3 were named CuCeIn.

Synthesis of SiO 2 -coated catalysts
For the synthesis of the SiO 2 -coated catalysts, 0.89 g of the polymer polyvinylpyrrolidone (PVP10, average molar mass equal to 10000 g.mol -1 ) were dissolved in 100 mL of an H 2 O and CH 3 CH 2 OH solution 50% (v/v) along with 0.36 g of Cu(NO 3 ) 2 , 0.05 g of In 2 (NO 3 ) 3 , and 0.31 g of ZrO(NO 3 ) 2 (or 0.59 g of Ce(NO 3 ) 3 ).The mixture was stirred for 4 h at room temperature to ensure the complete solubilization of the salts and the chains of the PVP polymer.After that, the resulting solution was transferred to a stainless-steel autoclave (filling ~80% of its volumetric capacity) and kept in an oven for 12 hours at 453 K.The mixture containing the oxide particles was transferred to a 1L beaker.After this, 312.8 mL of ethanol, 93.9 mL of water, 14.5 mL of 27% (v/v) NH 4 OH solution, and 0.168g of the surfactant CTAB were introduced into the same beaker.This suspension was stirred for 2 hours at room temperature followed by the addition of 2.175 mL of TEOS at a rate of ~1 drop per minute.The resulting suspension was left under vigorous stirring for 24 hours at room temperature.After this step, the suspension was centrifuged, and the obtained gel was dried 353 K for 24 hours.Finally, the resulting solid was ground and calcined at 923 K for 6 hours.The materials were named CuZrIn@mSiO 2 and CuCeIn@mSiO 2 .The molar proportions between the cations were the same as the best catalysts obtained in previous stages.50% Cu, 5% In, and 45% Zr/Ce.PVP was employed as a control agent for the size and homogeneity of the particles that would be formed and as a directing agent, along with CTAB, for the formation of the silica coating around the oxide particles formed during the solvothermal treatment.

Catalyst characterization 1.3.1. X-ray diffraction
X-ray diffraction (XRD) measurements were conducted using a Brucker Da Vinci D8 Advance diffractometer equipped with CuKα radiation (wavelength of 0.15418 nm) and a curved graphite monochromator.Data were collected over the 2θ range from 10° to 80° with a scan step of 0.02° and a counting time of 1 s.The crystalline phases were identified using Crystallographica Search Match software.

X-ray fluorescence
The chemical composition of the synthesized materials was analyzed using a PANalytical X-ray fluorescence spectrometer, model MiniPaI4.

Transmission electron microscopy
The transmission electron microscopy analyses were performed using a JEOL transmission electron microscope (model 2100).The instrument was operated with a hexaboron lanthanum (LaB 6 ) source and an acceleration voltage of 200 kV.The samples were previously dispersed in isopropanol using an ultrasound bath and deposited on Ni grids for analysis.

Temperature-programmed reduction in H 2
The temperature-programmed reduction (TPR) analysis was conducted using a Micromeritics Pulse ChemSorb 2750 instrument equipped with a thermal conductivity detector (TCD).A 50 mg sample of the catalyst was loaded into the reactor and subjected to reduction while heating in a 10% H 2 /Ar gas mixture at a flow rate of 30 mL/min.The temperature ramp ranged from room temperature to 1173 K at a rate of 10 K/min.

N 2 physisorption
The specific surface area and pore diameter distribution of the samples were characterized using N 2 adsorption/desorption measurements conducted at liquid nitrogen temperature (77 K).The relative pressure intervals ranged from 0.001 to 0.998.The experimental setup employed a Quantachrome Nova 100 system.Prior to analysis, the samples underwent vacuum pretreatment at approximately 10 × 10 −6 Pa for 2 hours at 493 K.The BET method was utilized to determine the specific surface area (S BET ), while the BJH method was employed to assess mesopore volume and pore diameter distribution.

N 2 O chemisorption
N 2 O chemisorption experiments were conducted using an analytical Multipurpose system equipped with a thermal conductivity detector (TCD).
Approximately 200 mg of the catalysts were loaded into a U-tube reactor.The catalyst surfaces underwent cleaning at 523 K for 1 hour under an inert atmosphere, with a flow rate of 30 mL/min.After cooling to room temperature, the samples were heated to 503 K in a 10% H 2 /Ar mixture with a flow rate of 30 mL/min, ramping up at 5 K/min to completely reduce the copper.Subsequently, the samples were cooled to 298 K and exposed to a 10% N 2 O/He mixture for 30 minutes to oxidize the surface copper layer.Finally, a second reduction was performed at 503 K in a 10% H 2 /Ar mixture with a flow rate of 30 mL/min at 5 K/min to reduce the surface copper atoms.Between each gas mixture exchange, the system was purged using N 2 at a flow rate of 30 mL/min for 30 minutes to remove the physically adsorbed molecules remaining on the surface of the catalysts.Dispersion was calculated by the ratio between the amount of copper in the surface and total amount in the catalyst.
Metallic surface area was calculated by relating the number of copper atoms in the surface and the atom density in a monolayer ( = 1.469 x 10 19 atoms.m - ).

Temperature-programmed desorption of CO 2
The CO 2 temperature-programmed desorption (TPD) experiments were performed using a Micromeritics Pulse ChemSorb 2750 instrument equipped with a thermal conductivity detector (TCD).200 mg of the catalysts were loaded into a Utube reactor for analysis.The catalyst surfaces underwent cleaning at 523 K for 1 hour under an inert atmosphere, with a flow rate of 30 mL/min.After cooling to room temperature, the samples were heated to 503 K in a 10% H 2 /Ar mixture with a flow rate of 30 mL/min, ramping up at 5 K/min.Subsequently, the system was purged with N 2 at a flow rate of 30 mL/min for 30 minutes and cooled to 298 K.The catalysts were then exposed to CO 2 at a flow rate of 30 mL/min.To remove physically adsorbed CO 2 molecules, the system was purged with N 2 once more.
Finally, the catalysts were heated from room temperature to 473 K at a rate of 10 K/min and maintained at this temperature until the signal in the TCD detector returned to the baseline.

Catalytic tests
The CO 2 hydrogenation reactions were carried out in a fixed-bed stainless-steel tubular reactor with dimensions of 304.3 mm in length and an internal diameter of 9.1 mm.Prior to each test, the catalyst underwent in situ pre-reduction at 503 K for 1 hour using a pure H 2 flow (at a rate of 30 mL/min) under atmospheric pressure.
The reduction temperature was chosen based on the maximum high-temperature peak observed in TPR analysis to ensure that all available surface Cu sites were effectively reduced and to prevent unnecessary use of higher temperatures.Subsequently, the reactor was cooled down to the designated reaction temperature, and the gas flow was switched to an H 2 /CO 2 mixture with a molar ratio of 3:1.The system was then pressurized to specified levels.Gaseous products were analyzed using a 7890A gas chromatograph (Agilent Technologies) equipped with thermal conductivity (TCD) and flame ionization (FID) detectors.The CO 2 conversion and product selectivity were calculated using the following equations: and  2, represent the total number of mols of CO 2 that entered the reaction and the total number of mols of CO 2 that exited, respectively.
The space-time yield of methanol (g MeOH .kgcat -1 .h - ) was calculated based on conversion and selectivity to methanol, using the following equation: where  2 represents the conversion of CO 2 ,  3 is the selectivity to CH 3 OH,  2,  is the volumetric flow rate of CO 2 (mL.h -1 ),  3 is the molecular weight of methanol (32 g.mol -1 ),   is the catalyst weight used (kg), and   is the ideal gas molar volume at standard pressure and temperature (mL.mol -1 ).
The turnover frequency (TOF) of methanol was calculated by the following equation: where  represents methanol activity in mol.g -1 .h - ,  is Avogadro´s number (6.023 x 10 23 ),   denotes Cu metallic area in m 2 .g - and  designates the number of Cu atoms in a monolayer ( = 1.469 x 10 19 atoms.m - ).The TOF of CO was similarly calculated, based on number of CO molecules produced.

Chemometric analysis
The impact of pressure, temperature, and space velocity parameters on the CO 2 hydrogenation process was investigated using chemometric tools.The experiments followed the Central Composite Design (CCD) methodology, which included upper, lower, center, and axial points.These points were defined for each of the variables (pressure, temperature, and space velocity) and are summarized in Table S3.We examined the main effects of each variable on methanol selectivity, as well as their interactions.The results were visually represented through Pareto charts (Figures S2-S4).To predict methanol selectivity values for specific combinations of pressure, temperature, and space velocity that were not experimentally tested within the analysis range defined by the central composite design, we constructed response surfaces using a quadratic model.This model involved fitting the experimental data to a second-degree polynomial equation.The statistical data processing was carried out using Chemoface software version 1.6.1.

Figure S2 .
Figure S2.Pareto charts of the standardized effects caused by the variation of pressure, temperature, and space velocity on the CH 3 OH selectivity for (a) CuCeIn@mSiO 2 , (b) CuCeIn, (c) CuZrIn@mSiO 2 and (d) CuZrIn catalysts.The analysis was carried out with 95% confidence level.

Figure S3 .
Figure S3.Pareto charts of the standardized effects caused by the variation of pressure, temperature, and space velocity on the CO 2 conversion for (a) CuCeIn@mSiO 2 , (b) CuCeIn, (c) CuZrIn@mSiO 2 and (d) CuZrIn catalysts.The analysis was carried out with 95% confidence level.

Figure S4 .
Figure S4.Pareto charts of the standardized effects caused by the variation of pressure, temperature, and space velocity on the CH 3 OH space-time yield (STY) for (a) CuCeIn@mSiO 2 , (b) CuCeIn, (c) CuZrIn@mSiO 2 and (d) CuZrIn catalysts.The analysis was carried out with 95% confidence level.

Figure S5 .
Figure S5.Comparison between predicted and measured values of CH 3 OH selectivity, CO 2 conversion and CH 3 OH space-time yield for (a) CuCeIn@mSiO 2 , (b) CuCeIn, (c) CuZrIn@mSiO 2 and (d) CuZrIn catalysts.The predicted values were obtained by adjusting a regression equation based on the quadratic model.The experimental conditions applied are the same as described in Figure 2.

Figure S6 .
Figure S6.Catalytic activity in terms of CO 2 conversion, CH 3 OH selectivity and productivity during reuse tests.Before each use catalysts were reduced in H 2 (30 mL.min -1 ) for 1 h at 573 K. WHSV values in the figure are in L.g -1 .h - .
, and   represent the chromatogram peak area and the molar calibration factor, respectively, for each component  identified at the output. 2,

Table S1 .
Metallic surface area and basicity normalized by the amount of active phases in catalysts.

Table S2 .
Values defined for the minimum, maximum, average, minimum axial and maximum axial points of the pressure, temperature, and space velocity parameters.

Table S3 .
Matrix of experiments defined by the Central Composite experimental planning methodology based on combinations between minimum, maximum, average, and axial points of each parameter.

Table S4 .
Catalytic results for CuZrIn@mSiO 2 catalyst based on the experimental matrix of TableS4.

Table S5 .
Catalytic results for CuZrIn catalyst based on the experimental matrix of TableS4.

Table S6 .
Catalytic results for CuCeIn@mSiO 2 catalyst based on the experimental matrix of TableS4.

Table S7 .
Catalytic results for CuCeIn catalyst based on the experimental matrix of TableS4.

Table S8 .
Catalytic performance of Cu-based materials during CO 2 hydrogenation to methanol reported in the literature.

Table S21 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuZrIn@mSiO 2 and the CH 3 OH selectivity results from central composite experimental design.

Table S22 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuZrIn and the CH 3 OH selectivity results from central composite experimental design.

Table S23 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuCeIn@mSiO 2 and the CH 3 OH selectivity results from central composite experimental design.

Table S24 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuCeIn and the CH 3 OH selectivity results from central composite experimental design.

Table S25 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuZrIn@mSiO 2 and the CO 2 conversion results from central composite experimental design.

Table S26 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuZrIn and the CO 2 conversion results from central composite experimental design.

Table S27 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuCeIn@mSiO 2 and the CO 2 conversion results from central composite experimental design.

Table S28 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuCeIn and the CO 2 conversion results from central composite experimental design.

Table S29 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuZrIn@mSiO 2 and the CH 3 OH productivity results from central composite experimental design.

Table S30 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuZrIn and the CH 3 OH productivity results from central composite experimental design.

Table S31 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuCeIn@mSiO 2 and the CH 3 OH productivity results from central composite experimental design.

Table S32 .
Analysis of variance (ANOVA) for the quadratic model applied using the catalyst CuCeIn and the CH 3 OH productivity results from central composite experimental design.