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Atmospheric- and Low-Level Methane Abatement via an Earth-Abundant Catalyst

  • Rebecca J. Brenneis
    Rebecca J. Brenneis
    Ralph M. Parsons Laboratory, School of Engineering, Massachusetts Institute of Technology, 15 Vassar Street, Cambridge, Massachusetts 02139-4307, United States
    More by Rebecca J. Brenneis
    Orcidhttps://orcid.org/0000-0002-5743-2096
  • , 
  • Eric P. Johnson
    Eric P. Johnson
    Ralph M. Parsons Laboratory, School of Engineering, Massachusetts Institute of Technology, 15 Vassar Street, Cambridge, Massachusetts 02139-4307, United States
    School of Engineering and Applied Sciences, Yale University, 17 Hillhouse Avenue, New Haven, Connecticut 06520, United States
    More by Eric P. Johnson
    Orcidhttps://orcid.org/0000-0003-2214-2977
  • , 
  • Wenbo Shi
    Wenbo Shi
    Ralph M. Parsons Laboratory, School of Engineering, Massachusetts Institute of Technology, 15 Vassar Street, Cambridge, Massachusetts 02139-4307, United States
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  • , and 
  • Desiree L. Plata*
    Desiree L. Plata
    Ralph M. Parsons Laboratory, School of Engineering, Massachusetts Institute of Technology, 15 Vassar Street, Cambridge, Massachusetts 02139-4307, United States
    *Email: [email protected]
    More by Desiree L. Plata
    Orcidhttps://orcid.org/0000-0003-0657-7735
Cite this: ACS Environ. Au 2022, 2, 3, 223–231
Publication Date (Web):December 29, 2021
https://doi.org/10.1021/acsenvironau.1c00034

Copyright © 2021 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Climate action scenarios that limit changes in global temperature to less than 1.5 °C require methane controls, yet there are no abatement technologies effective for the treatment of low-level methane. Here, we describe the use of a biomimetic copper zeolite capable of converting atmospheric- and low-level methane at relatively low temperatures (e.g., 200–300 °C) in simulated air. Depending on the duty cycle, 40%, over 60%, or complete conversion could be achieved (via a two-step process at 450 °C activation and 200 °C reaction or a short and long activation under isothermal 310 °C conditions, respectively). Improved performance at longer activation was attributed to active site evolution, as determined by X-ray diffraction. The conversion rate increased over a range of methane concentrations (0.00019–2%), indicating the potential to abate methane from any sub-flammable stream. Finally, the uncompromised catalyst turnover for 300 h in simulated air illustrates the promise of using low-cost, earth-abundant materials to mitigate methane and slow the pace of climate change.

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Introduction

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The Intergovernmental Panel on Climate Change and the United Nations have recently published calls to action to reduce atmospheric methane emissions in order to slow the rate of global temperature change as quickly as possible. (1,2) The reason methane is uniquely well suited to bring urgently needed reductions in climate warming rates is because methane has a pronounced radiative forcing effect in the near term: instantaneously, methane is 120 times as powerful a warmer as CO2 on a per-mass basis and exhibits a 28–34-fold greater global warming potential even after 100 years. (3,4) By 2030, methane will do as much damage as CO2 in spite of methane’s much lower atmospheric abundance (1.85 ppmv CH4vs 417 ppmv CO2). (3,5,6) Considering these temporally dynamic effects, reducing atmospheric methane levels presents a unique opportunity for a rapid and large impact on climate. For example, restoring atmospheric concentrations to pre-industrial levels (ca. 0.76 ppmv) would reduce radiative forcing by as much as 16% within only 10–20 years. (7,8) However, methane in the atmosphere is steadily increasing, and the time frame for meeting warming reduction goals is narrowing. (5,7)
Historically, the approach to reducing methane emissions at concentrated sources (greater than 4%) has been to flare the stream to CO2. Emergent efforts to reduce primary emissions include finding and stopping leaks from fossil energy systems, (9) leveraging landfill and other anaerobic gas for energy, (10) or reducing enteric fermentation in animal husbandry practices. (11,12) However, the dominant sources of methane to the atmosphere are both distributed and diffuse (e.g., 465–607 Tg yr–1; 81–83% globally). (5) As these sources are spatially dispersed, temporally transient, and often below flammability limits, engineering solutions have been uneconomical or intractable. Strategies to reduce atmospheric methane should be able to act on atmospheric levels anywhere or be co-located with enrichments in the presence of oxygen at or near ambient conditions.
Biological systems have evolved to achieve a similar chemistry: to react low-level methane at relatively low temperatures in sub-oxic environments. In particular, methanotrophs utilize methane monooxygenase (MMO) enzymes to metabolize available methane to methanol. (13,14) Existing in multiple forms but with a highly conserved active site, MMOs feature high-spin reactive oxygen capable of activating C–H bonds and a redox-controlled “breathable” pore structure that directs reagents to prevent errant diffusion that could lead to “overoxidation” to CO2. (13−15) Over the last decade, attempts to replicate this chemistry via transition metals and zeolites have been promising. (15−24) The zeolite pore structures enhance catalytic reactions by placing metal oxides into unfavorably high energetic configurations that geometrically constrain gaseous reactants into contact through nanoscale channels. Most efforts have focused on methane-to-methanol conversion for fuel and chemical production under industrial conditions, (25,26) but these efforts have been stymied by a lack of selectivity, overoxidation, and poor economics of methanol separation and use. Further, in an attempt to reduce overoxidation to CO2, researchers have employed extreme oxygen levels (e.g., 100%) in a two-step process of catalyst oxidation followed by reaction with high levels of methane, which is irrelevant to deployment in the environment and also not representative of the evolutionary optimum of MMOs (i.e., low levels of methane and oxygen). However, it is precisely the acceleration of conversion of methane to CO2 that could dramatically reduce net radiative forcing and overcome the practical challenges associated with methanol production. Additionally, the ability to operate at low levels of methane and oxygen would expand the application space for the catalyst where it is most needed and across a spectrum of methane sources. Recognizing that this chemistry is desirable from the perspective of atmospheric methane abatement, we investigated copper-doped zeolites in methane oxidation at low temperatures and atmospheric gas compositions, seeking to provide an urgently needed tool to combat global climate warming.

Methods

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Copper Mordenite Synthesis

Ammonium mordenite zeolite powder (5 ± 0.1 g; Alpha Aesar) was stirred with 0.05 M copper nitrate solution (500 mL) for 22–26 h and then vacuum-filtered through a glass fiber filter (0.7 μm GFF). Filtered solids were dried at 130 °C for 10–14 h, transferred to a glass vial, and stored in a desiccator until use. Importantly, we note that this preparation route is benign and strives to meet Green Chemistry (27−29) principles: the ion exchange occurs at room temperature with minimized volumes and relies on earth-abundant, non-toxic materials, without acidic or organic solvents, with low energy requirements, and without the need for exotic or complex, multi-step syntheses. The process was highly reproducible.

Reactor Design and Analytical Measurement

Gases were pre-mixed using a custom-built mass flow control array with electronic control, customized for the delivery of trace gases, including ultra-high-purity (UHP) helium, UHP oxygen, and 25, 700, and 70,000 ppmv methane in helium (Airgas). These were delivered to a vertically oriented, quartz tube furnace [16 × 1/2 O.D. inch (length × diameter)] fitted with a quartz frit and placed inside an Applied Systems 3210 series vertical tube furnace, which provided thermal control via an 850 W power supply. The reactor effluent was delivered to an SRI Instruments 8160C gas chromatograph with a flame ionization detector by direct injection every 90 s via two calibrated loops (5 mL each) and a Valco Instruments eight-port valve. The GC was calibrated daily with authentic standards (Mesa Specialty Gases).

Catalyst Activation and Methane Conversion Reactions

In all cases, after loading 1 g of copper mordenite (approximately 3 cm height) into the vertical tube furnace, the reactor volume (34.5 mL) was flushed with helium while heating prior to introduction of activation gases. Then, experiments were conducted in one of two modes: a two-step activation-then-reaction or an isothermal reaction. For the “two-step” process, the catalyst was activated in an atmosphere of 20% oxygen and 80% helium (at 450 °C for 30 min unless otherwise noted) or 100% oxygen. Subsequently, the conversion reaction was carried out in the same atmosphere with additional methane (0.0002–2% methane; 30 min at 200 °C unless otherwise indicated). Note that traditional work on these types of materials for methane-to-methanol conversion relies on the two-step process to avoid overoxidation of methane to CO2. In the activation step, 100% O2 atmospheres are used to maximize efficacy, precluding simultaneous delivery of methane and necessitating segregated activation and reaction steps. In our experiments, we sought to simulate the operation in real air (e.g., 20% O2 with methane). In all cases, we held the total flow of gas constant to preserve an approximately 30 s residence time of gas in the reactor. For the “isothermal” process, the catalyst was activated for 30 min (or 8 h for a long-duration study) under methane-free, 20% oxygen, and 80% helium and then reacted in 0.0002% methane for 30 min (or 300 h for a long-duration study) at a constant temperature. In order to achieve a constant temperature in the reactor, a continuous supply of heat was provided through a power supply with temperature feedback. Isothermal processes were explored to investigate the possibility of catalytic function without a thermal cycle (i.e., duty cycle), which brings a net energy and operational expense savings during operation (e.g., energy savings are conferred by avoiding heating and cooling cycles).
At the beginning of the reaction step, directly following the introduction of methane to the feed stream (70 sccm in total), a 15 min flushing time was allowed to ensure adequate mixing within the catalyst bed volume. After this equilibration phase, injections were made every 90 s for an additional 30 min (21 injections), and an average of these values (21 injections for the two-step reactions; five injections for isothermal reactions) was used to establish the effluent concentration. Standard deviations were less than 2.6% of the mean. Conversion efficiency was calculated by comparing the effluent methane concentration with the influent methane concentration (no catalysts, same gas mixture) at the same temperature. There was no evidence of non-catalytic methane oxidation (i.e., direct combustion) at any temperature below 550 °C, and copper-free mordenite zeolite powder illustrated no potential to convert methane at the assayed temperatures (i.e., 200 °C).

Material Characterization

Copper-doped mordenite metal content was determined by inductively coupled plasma mass spectrometry (ICP–MS) using a NexION 300D. Briefly, samples were prepared by refluxing approximately 1 g of catalyst in 50 mL of 50% v/v HNO3 for 2 h until the solids were visibly bleached white. The leachate was extracted, concentrated to 10% of its initial volume, and then reconstituted in 2% HNO3. All copper loadings fell between 1 and 2 weight percent (see Supporting Information Figure S1). The crystal structure of the copper zeolite was studied by X-ray diffraction (XRD) using a PANalytical X’pert PRO diffractometer equipped with Bragg–Brentano geometry and Ni-filtered Cu Kα radiation (λ = 1.5418 Å, 45 kV, 40 mA); data were recorded in the range of 5–80 2θ. All results were consistent with the dominant crystal structure of the native zeolite. Scanning electron microscopy was conducted using a Zeiss Merlin Gemini field emission scanning electron microscope (5 kV accelerating voltage; 184 pA probe current) (Supporting Information Figure S2). A photograph of the catalyst before and after use shows a noticeable color change (Supporting Information Figure S3).

Results and Discussion

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Activation and Reuse Potential

The performance efficiency of catalysts for methane capture and conversion is typically evaluated under a two-step process, where the first step (“activation”) occurs in methane-free, oxygen-rich conditions, and the second step (“reaction”) occurs in the presence of the reagent methane. Conducting all reaction steps at 200 °C allowed the evaluation of the activation step parameters spanning a range of thermal, temporal, and gas composition conditions. After relatively short (30 min) activations, increasing temperature in 100% oxygen improved reaction conversion efficiencies, where modest conversion efficiencies of 40% were observed at 450 °C, and 80–100% conversion was shown at 500 and 550 °C, respectively (Figure 1). These same efficiencies were observed in 20% oxygen atmospheres, indicating that the oxygen level was not limiting to the activation, and thermal effects may be more important. This has promising implications for field deployment of the catalyst as ambient oxygen levels (e.g., from air) might be useful for catalyst activation, rather than requiring the presence of explosive levels of reactant oxygen. While the high end of observed activation temperatures and durations are within the range of previous studies (at or above 450 °C (15,16,18,20) and with durations at or above 1 h (18,20)), use of ambient levels of oxygen has not been demonstrated previously for copper zeolites. (17) Some novel “lean methane” catalysts based on cobalt, (30) nickel mixtures, (31,32) or platinum group metal (PGM) (33−38) structures have been shown to work at or below 10% oxygen, (30,33,34,36,38) but these require both high temperatures, expensive or complex synthetic processes, applied voltage, and/or a combination of activation gases far from ambient.
The impact of activation time was clear, where progressively longer 450 °C treatments conferred better methane conversion efficiency during the reaction. A short, 15 min activation resulted in only modest conversion, but this increased to over 60% by 2 h and nearly 80% after 8 h. While the conversion metrics of previous studies are not directly comparable, prior methane-to-methanol conversion attempts typically utilize oxidations of 1 h or longer. (16,18,20) Extremely short or implicit activation times in a unique reaction environment have shown little reactivity (e.g., heating to 270 °C in 1% oxygen, 3% water, and 18% methane; 0.03% conversion). (17) In our work, because copper oxidation is kinetically fast, the time dependence suggests (1) that either gas transport through the constrained zeolite pore structure is slow or (2) that there is a critical, thermally mediated structural rearrangement of the copper zeolite or atoms therein that occurs slowly. While the former could be addressed by constructing hierarchical materials with improved gas access without compromising the pore structure, the latter might require re-engineering of the catalyst nano-structure or strategic thermal pre-treatments as part of the catalyst preparation.
To explore the possibility that longer-duration thermal pretreatments were giving rise to uniquely active catalyst nano- or mesostructures, we utilized powder XRD to probe the existence of a thermally driven catalyst rearrangement (Figure 2). As a whole, the XRD spectra were consistent with mordenite powder structures, and no quantifiable copper or copper oxide phase was observed [likely due to the low loading of Cu; ca. 1% (see Supporting Information Figure S1 and associated discussion of anticipated Cu-oxidation state dynamics during the reaction (39−42))]. As thermal treatment time increased, deformations in peak patterns emerged at low diffraction angles (less than 10° 2θ). These peaks correspond to planes that bisect mordenite’s large channel (6.5 × 7 Å) pores lengthwise (110, 020, and 200), whereas smaller pocket pores (2.6 × 5.7 Å) were not affected. Specific evolutions found were that the integral breadth of the first and second observable peaks (corresponding to the 110 and 020 planes, respectively) increased and decreased in equal measure [approximately 0.01 rad. Note that the integral breadth (area of the peak divided by height) was employed because it is less susceptible to overinterpretations of changes in peak shape at low angles that would be associated with the extractable parameter full width at half-maximum]. As integral breadth is inversely proportional to the average crystallite thickness normal to the reflecting plane, (43) these observations are consistent with a crystal structure development in the mordenite pore channels over time, where the channel became more compressed normal to the 110 plane and more expanded normal to the 020 plane with longer duration thermal treatments. Thus, the longer activation times appear to be associated with a micro-structural change in the larger pore structure, which leads to better methane conversion.

Figure 1

Figure 1. Catalyst activation as a function of gas composition, temperature, duration, and repetition illustrates that activation can be achieved under ambient-to-moderate conditions with reuse potential. The carbon conversion efficiency during the reaction steps is shown for all activation trials, which were conducted for 30 min at 450 °C and with a freshly prepared catalyst unless otherwise noted and in 100% (black) or 20% oxygen (red) in helium mixtures. All methane conversion reactions were conducted at 200 °C in 2 ppmv methane; note that 200 °C is not the maximum conversion temperature but enables one to see a spread in the behavior as a function of the activation temperature. The effect of (a) activation temperature, (b) activation duration, and (c) multiple reactivations is shown.

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Figure 2

Figure 2. Powder XRD pattern of copper-exchanged ammonium zeolite powder (mordenite) after activations varying from 0 to 240 min in duration (unexchanged, 0, 30, 240 min from bottom to top).

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Such long in situ activation timescales would only be practical in field deployments if the catalyst could be recycled or activated and then reacted in series. Engineered strategies to achieve this type of cycling chemistry exist in the form of reactors that can be strategically cycled (in series or parallel), such as regenerative thermal oxidation, regenerative catalytic oxidation, or catalytic recuperative oxidizers, all developed for the conversion of volatile organic compounds (VOCs) in industrial waste streams. (44−49) With this in mind, catalyst re-activation was illustrated for at least six cycles, showing a general trend of improvement consistent with the effect of longer activation times. This repeatability suggests that the total methane conversion potential per unit of catalyst may be sufficiently high to reduce the cost of CO2 equivalent capture below critical thresholds of 15–50 USD/ton. Importantly, these reuse experiments were conducted by oscillating between 450 °C activation in 100% oxygen and 200 °C reaction in 20% oxygen, which would necessitate undesirable life cycle costs associated with the duty cycle and risk associated with the use of concentrated oxygen. As such, we explored a broader range of reaction conditions and operation strategies.

Methane Conversion Approach

Minimizing the temperature requirements of low-to-ambient level methane conversion reduces the energetic demand and infrastructural requirements of reactors, improves the overall operational lifecycle greenhouse gas benefit, and expands the possible deployment opportunities by reducing the flammability or explosion hazard (e.g., for ventilation air at coal mines). To evaluate the potential for use at low temperatures, we varied conversion reaction temperature from 50 to 350 °C over a range of activation temperatures between 250 and 550 °C. At high activation temperatures (above 450 °C), non-zero catalytic activity was achieved at reaction temperatures as low as 100 °C, and over 40% conversion was achieved at 200 °C and above (Figure 3). At lower activation temperatures (350 °C or below), conversion over 40% requires systematically higher reaction temperatures (above 250 °C). Limited but quantifiable conversion was observed at temperatures as low as 100 °C (Figure 3a), reduced from previous experiments carried out between 150 and 200 °C. Catalytic conversion of methane using PGMs, (33−38) cobalt, (30) or nickel–cobalt mixtures (31,32) has typically relied on higher activation temperatures (500–1000 °C) or long durations (e.g., 24 h at 350–400 °C (33,35)); that is, successful activations generally exceed the time and temperature ranges explored here. Using relatively aggressive activations, successful conversions of 1000–10,000 ppmv methane (0.1–1% CH4, with 10% or less oxygen) at temperatures well below the ignition point (600 °C (50)) have been recorded (31,33,34,36,38) with a few demonstrating conversion temperatures as low as 300 °C. (32,35) While the reaction temperatures we observed are moderately lower than previous demonstrations, the dramatically different activation procedures and simplified catalyst synthesis offer unique benefits. As a point of comparison, conversion efficiencies to CO2 are not often reported and cannot be directly compared to our results; nevertheless, methanol production rates in previous studies have been quite low (order 0.1–0.3 mol methanol per mol Cu or less; (26) methanol production was not systematically quantified in our study, but early spot checks in the two-step process revealed around 1–2 orders of magnitude lower methanol in the post-reaction, water-extracted catalyst, where the input methane was around 106-fold lower than in previous studies). While CO2, formaldehyde, formic acid, and methanol are the only anticipated products in the oxidation pathway, we have not evaluated the potential formation of other products to date. To our knowledge, no prior studies have shown complete conversion of methane at these temperatures in simulated air with an easy-to-produce, earth-abundant catalyst.

Figure 3

Figure 3. Methane conversion efficiency as a function of reaction temperature and operation mode. (a) In a two-step, activation followed by the conversion approach, both activation temperature and reaction conversion temperature influence methane removal. All activation steps were carried out for 30 min in 20% O2. (b) In a continuous reaction approach, 30 min activation in methane-free gas mixtures (20% O2) was followed by a 30 min conversion reaction in the presence of atmospheric levels of methane (2 ppmv) to simulate isothermal operations. Each data point was generated with a freshly prepared catalyst and represents the mean of at least 21 measurements.

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The achievement of harmonized catalyst activation and reaction temperatures confers important operational and life cycle advantages. Specifically, the isothermal operation minimizes the need to repeatedly deliver power for heating the thermal mass of a reactor and the associated catalyst, reducing the levelized cost, energy, and greenhouse emissions. Using the operational space as a roadmap (Figure 3a) for optimizing catalytic efficiency, we demonstrated that the isothermal reaction offers complete removal of atmospheric methane at 350 °C (Figure 3b). Minor conversion (approx. 7.1%) was initially detected at 270 °C. This is consistent with the novel continuous reactions conducted by Dinh et al., (17) who demonstrated methanol production at this temperature, albeit with efficiencies below 1% (by design, to minimize “overoxidation” to CO2 and in dramatically different conditions). At modestly higher temperatures (e.g., 300–310 °C), 42–67% of methane was oxidized following a 30 min activation. This is valuable for some remediation applications and could be pushed to higher conversion rates under different activation schema (see below). Levelized life cycle budgets that consider the potential trade-off between total methane abatement versus operational energy demands (both in CO2 equivalents) must be conducted to determine if the additional conversion merits the operational temperature. Theoretically speaking, the conversion of methane to CO2 is exothermic, and depending on the reactor design and flowrates, higher input concentrations of methane might be able to generate excess heat. For example, ventilation air in mines can contain between 0.1 and 2% methane. (51) A simple calculation of energy generation compared to energy requirements can be derived from the theoretical energy generated by the reaction relative to the energy needed to heat incoming air to the operating temperature (eq 1).
(1)
where mCH4 is the mass of methane in the incoming air stream, ΔHrxn is the enthalpy of methane oxidation to CO2 (890 kJ/mol), mair is the mass of incoming air with specific heat, Cp (700 J/kg K), and ΔTair is the temperature change required to get from the ambient temperature to the operating temperature (e.g., 310 °C). At these sources, a methane concentration of 1% gives a heat-generation-to-heat-demand ratio of 1.5; that is, the process generates excess energy. At the ventilation air flowrates used at mines (approximately 100,000–1,000,000 CFM), this could potentially yield electricity at the power-plant scale (order 5 kW, depending on specifics of the system). One could use this excess energy to pre-heat incoming air, offset the energy demands of the system (e.g., “parasitic power” associated with air movement across a pressure drop), and ultimately drive electricity-generating heat exchangers to meet other needs on-site. Notably, such a system could accelerate payback times for the device as well. Considering that low-level methane sources responsible for the majority of emissions to the atmosphere span 4 orders of magnitude in concentration, it is possible and somewhat remarkable that this catalyst could be deployed with a net energy yield in modestly elevated methane scenarios (over approximately 0.67% methane; see Supporting Information Figure S4).

Methane Abatement at All Sub-Flammable Thresholds

To explore the possibility of deploying the catalyst for methane abatement at any sub-flammable level, we evaluated incoming methane conversion rates between 2 ppmv and 2% v/v methane (e.g., from near-atmospheric to typical ventilation air levels observed in coal mines). Overall, higher incoming methane corresponded with higher rates of conversion (Figure 4; 4.28 × 10–9 to 2.28 × 10–5 mol min–1 gcatalyst–1 isothermally) but came at a cost to the total proportion of methane removed (Figure S5). Further, the conversion rate exhibited sensitivity to the activation time and temperature: isothermal reactions at 310 °C exhibited steady and monotonic increases in the methane conversion rate, whereas step-wise reactions at short and long (30 and 60 min at 450 °C) activations followed by lower temperature reactions (200 °C) showed a diminishing conversion rate with higher methane loadings. The influence of thermal history (i.e., duration of elevated temperature treatment in both continuous and non-continuous modes) implies that there is a physical limitation to catalyst activation, consistent with our earlier observation of the importance of activation time (Figure 1). Promisingly, there does not appear to be a definitive ceiling in the catalyst’s ability to convert methane at higher concentrations, implying that higher conversion rates could be achieved via optimization of activation parameters and/or reactor geometry. Hierarchical material construction, such as preserving the nanoconfinement of the Cu-aluminosilicate active sites but supporting those on a substrate to promote better gas-catalyst contact, improved heat transfer properties by varying the material choice, and novel reactor design are all viable routes to enhance conversion rates. This potential for improvement aside, for the first time, these results demonstrate that copper mordenite can convert methane at low-level concentrations previously untested by either other zeolites, lean combustion, or ventilation air methane catalysts. Considering that these conversions can be achieved at any methane concentration of concern well below flammability limits (less than 5%) or ignition temperatures, strategic deployment at or near elevated methane sources would bring rapid benefit to reduce or reverse the exponential accumulation of atmospheric methane observed since the early 1800s. (5,52−54)

Figure 4

Figure 4. Methane conversion rate increases with the input methane concentration over a range of sub-flarable levels. Methane conversion was tested from 2 ppmv to 2% v/v methane in the presence of 20% oxygen in isothermal operation at 310 °C (30 min initial activation in methane-free gas; asterisks), and following 30 min (filled red symbol) and 60 min (open symbol) activations (450 °C) and reaction (200 °C) in 20% oxygen. Each data point collected with the freshly prepared catalyst represents at least 20 methane conversion measurements.

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Catalyst Conversion Capacity and Lifetime

Viability and adoption of this catalytic technology will depend on both the opportunity for levelized greenhouse gas reductions and the total cost of the abatement strategy. Both scale with the reusability of the catalysts and requisite strategy for regeneration (e.g., thermal cycling or chemical recharge). First, a traditional two-step activation followed by a reaction at differential temperatures illustrated a relatively rapid deterioration in the methane conversion potential (30 min, 450 °C activation in 20% oxygen followed by continuous exposure to 200 °C, with 2 ppmv methane added; Figure 5). In contrast, a pre-activation for 8 h followed by isothermal reaction at 310 °C showed prolonged, near complete methane removal for up to 300 h (12 days). The high reactivity initially was a consequence of the long activation time and consistent with earlier observations (Figure 1b) that activation time promotes formation of reactive pore structures observed by XRD (Figure 2). Maintenance of a strong catalytic activity over this time suggested that either the catalyst capacity was never reached or that the catalyst was reactivating and performing in a continuous manner (i.e., regenerating itself, as in the definition of a catalyst). The dramatically different behavior observed between a two-step activation and an isothermal operation is the outcome of both thermal and kinetic effects. In a two-step process, the short, high-temperature (450 °C) activation is followed by operation at a relatively low temperature (200 °C). The continuous decay in performance implies that 200 °C is insufficient to reactivate and fails to maintain the activity of the catalyst. The low conversion efficiencies (approaching 0% methane conversion) are consistent with low-temperature activation experiments (see Figure 3a for 250 °C activation and reaction). In contrast, a long activation step (8 h) at 310 °C gave a high and consistent methane conversion performance. The near complete conversion of methane (100%) exceeded the conversion at 300 or 350 °C for only 30 min (40–50% methane conversion; Figure 3a), reinforcing the sensitivity of catalyst performance to activation time (see Figure 1b) associated with catalyst pore re-structuring (Figure 2). While the long-duration, 8 h activation could initially produce a very reactive catalyst structure that is maintained or regenerated, it is also possible that the isothermal reaction condition is functionally serving as a very long “activation” in and of itself, conferring sustained catalytic activity.

Figure 5

Figure 5. Long-term activity of the catalyst. Low-level methane (2 ppmv methane in 20% oxygen) was catalytically reacted over 300 h under continuous, isothermal operation at 310 °C (asterisks, following 8 h activation) and a traditional two-step process (red circles; 450 °C, 30 min activation followed by 200 °C continuous reaction).

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Continuous reactivity under isothermal operations increased the overall quantity of converted methane more than fivefold compared to the two-step reaction (0.3 vs 1.7 mg CH4 gcatalyst–1 total when integrating over the duration of the long-term trials). Here, we note that these conversion ratios are only relative to one another and cannot be used to determine the total potential of the catalyst for methane drawdown from the atmosphere or near more concentrated sources. Indeed, evidence suggests that the total conversion capacity may be much higher if larger input methane concentrations were delivered (Figure 4).

Implications, Novelty, and Feasibility

The application of zeolites for biomimetic methane conversion has been explored for decades, but several key limitations existed in prior work that obfuscated the utility of these catalysts for low-level methane abatement. First, there had been a focus on stopping the reaction at methanol, so that methanol could be used as a chemical feedstock. However, technical challenges existed that prevented the arrest of the reaction at methanol (which proceeds exergonically to CO2) and separation of the liquid stream from the gaseous input. Further, current methanol markets (75 MMT in 2018) would only abate about 37.5 MMT of methane, enough to reverse atmospheric accumulation but insufficient to offset major atmospheric source terms and restore atmospheric methane levels quickly. Second, methanol separation is only technically feasible at the most concentrated methane input streams but remains uneconomic due to the relatively low methanol yields. Methanol is often recovered and measured via water extraction of the catalyst, which would create major technological hurdles in and of itself. Third, in an effort to avoid “over oxidation” to CO2, a practice emerged in which zeolites were cycled between oxygen-rich and methane-rich environments (i.e., the activation and reaction steps, respectively). The key novelty of our work is (1) the demonstration that catalyst turnover can be achieved in air at modestly low temperatures, in a continuous fashion, and at exceedingly low levels of methane in the sub-flammable region and (2) the recognition CO2 is the desired product, overcoming separation challenges while conferring major climate-impact reduction. Any concern for CO2 generation as a pollutant is misguided: methane-to-CO2 conversion brings an instantaneous radiative forcing reduction and converting 60% of the atmospheric methane reservoir (i.e., restoration to pre-industrial levels) would result in a modest 1.1 ppmv increase in CO2 (e.g., from 415 ppmv CO2 today to 416 ppmv CO2).
The ability to mitigate low-to-ambient levels of methane in simulated atmosphere indicates promise to deploy a stand-alone system anywhere where fugitive methane emissions exist. However, maximum environmental impact and economic yield (i.e., greenhouse gas (GHG) equivalents removed per $) would come with strategic deployment at relatively enriched methane sources (e.g., dairy and meat barns or coal mines). We described the potential for low or no energy needs on-site depending on the input methane concentration and the associated thermal yield. At high methane sources (e.g., coal and mineral mines, where ventilation air can be up to 2%), excess heat can meet the demands of heating incoming air and the catalyst and even offer excess electricity generation (see Supporting Information Figure S4). This energy generation potential could improve the net GHG benefit of the technology. If modest carbon taxes are instated, the energy generation coupled with the sale of superfluous electricity could potentially pay back capital equipment expenses and operating costs over a relatively short time (order decade). Here, we note that the catalyst is synthesized from earth-abundant Cu and clay aluminosilicates. We estimate the copper-zeolite catalyst costs to be on the order of cents per pound- $0.15–0.82/lb, many orders of magnitude lower than those of competing technologies. Thus, there is great potential for this technology to be developed at low cost and with minimum environmental impact, potentially reducing operating costs below critical thresholds of proposed CO2 pricing strategies (e.g., below $15–50/ton of CO2 equivalents).
Several technical achievements remain on the path to feasible deployment. Catalyst poisoning regimes need to be tested with typical atmospheric interferents (primarily water) and other light VOCs that might prematurely saturate or spoil the catalyst. Pre-filtration strategies may be needed to overcome any emergent complications, and real-world transformation products should be monitored on initial deployment. Ideally, control systems and in situ monitoring capability would ensure the continuous function and efficacy of any commercial device. Finally, the catalyst material should be supported or structured in a way such as to maximize air flow through the reactor system. Then, reactors could be interfaced downstream of extant air handling capacity on-site and further lower the levelized GHG impact and cost.
One such integration would be with ventilation air systems at coal mines. Coal mining disasters prompted the creation of the 1977 Mine Safety and Health Act and 2006 MINER Act in the US. Associated Mine Safety and Health Administration (MSHA) standards, while necessary to protect miners, present technological challenges to methane abatement in ventilation air, as explosive or hazardous reagents must be avoided. As such, state-of-the-art mine ventilation air systems offer zero methane conversion. Here, we demonstrate low operation temperatures and alleviation of the need for oxidative reagents, improving compatibility with MSHA guidelines. Incorporation of our catalyst to ventilation air systems could mitigate nearly 39 MMT of CO2 equivalents (CO2,e) in the US, (55) translating to nearly 410 MMT of CO2,e if deployed globally. (48,55,56) Atmospheric methane levels are growing on the order of 16.8 MMT methane annually (5) (470–571 MMT CO2,e using methane’s 100-year global warming potential range of 28–34 (3,4)). Thus, if scaling challenges are addressed, the proposed technology could nearly offset the trend of accumulating atmospheric methane, offering critically needed near-term climatic forcing benefits.
Jackson and colleagues highlighted atmospheric restoration to pre-industrial methane levels (750 ppbv vs today’s 1860 ppbv and steadily climbing CH4) as a mechanism for saving 16% of global climate forcing. (7) This near-term impact would be felt within decades, whereas CO2 mitigation strategies would not exert influence for 50 years or more. (11) While both are necessary and urgently needed, efforts to address near-term warming GHGs in the atmosphere provide a unique route to stall the effects of anthropogenic warming, providing much needed time to adapt and respond to the changing climate.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenvironau.1c00034.

  • Additional material characterization details, including ICP analysis, SEM images, and photographs of the catalyst (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

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  • Corresponding Author
  • Authors
    • Rebecca J. Brenneis - Ralph M. Parsons Laboratory, School of Engineering, Massachusetts Institute of Technology, 15 Vassar Street, Cambridge, Massachusetts 02139-4307, United States;  Orcidhttps://orcid.org/0000-0002-5743-2096
    • Eric P. Johnson - Ralph M. Parsons Laboratory, School of Engineering, Massachusetts Institute of Technology, 15 Vassar Street, Cambridge, Massachusetts 02139-4307, United States;  School of Engineering and Applied Sciences, Yale University, 17 Hillhouse Avenue, New Haven, Connecticut 06520, United States;  Orcidhttps://orcid.org/0000-0003-2214-2977
    • Wenbo Shi - Ralph M. Parsons Laboratory, School of Engineering, Massachusetts Institute of Technology, 15 Vassar Street, Cambridge, Massachusetts 02139-4307, United States
  • Author Contributions

    D.L.P. conceived the concept, D.L.P. and R.J.B. planned the experiments, R.J.B. performed the experiments, E.P.J. and W.S. assisted with equipment automation and experimentation, and R.J.B. and D.L.P. analyzed the data and wrote the manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors thank Gerstner Philanthropies, Vanguard Charitable Trust, the Gordon and Betty Moore Inventor Fellows Program, and MIT’s Research Support Committee.

References

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    Petrov, A. W.; Ferri, D.; Krumeich, F.; Nachtegaal, M.; Van Bokhoven, J. A.; Kröcher, O. Stable Complete Methane Oxidation over Palladium Based Zeolite Catalysts. Nat. Commun. 2018, 9, 2545,  DOI: 10.1038/s41467-018-04748-x
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    Stable complete methane oxidation over palladium based zeolite catalysts
    Petrov Andrey W; Ferri Davide; Nachtegaal Maarten; van Bokhoven Jeroen A; Krocher Oliver; Petrov Andrey W; Krumeich Frank; van Bokhoven Jeroen A; Krocher Oliver
    Nature communications (2018), 9 (1), 2545 ISSN:.
    Increasing the use of natural gas engines is an important step to reduce the carbon footprint of mobility and power generation sectors. To avoid emissions of unburnt methane and the associated severe greenhouse effect of lean-burn engines, the stability of methane oxidation catalysts against steam-induced sintering at low temperatures (<500 °C) needs to be improved. Here we demonstrate how the combination of catalyst development and improved process control yields a highly efficient solution for complete methane oxidation. We design a material based on palladium and hierarchical zeolite with fully sodium-exchanged acid sites, which improves the support stability and prevents steam-induced palladium sintering under reaction conditions by confining the metal within the zeolite. Repeated short reducing pulses enable the use of a highly active transient state of the catalyst, which in combination with its high stability provides excellent performance without deactivation for over 90 h in the presence of steam.
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  23. 23
    Hou, Y.; Nagamatsu, S.; Asakura, K.; Fukuoka, A.; Kobayashi, H. Trace Mono-Atomically Dispersed Rhodium on Zeolite-Supported Cobalt Catalyst for the Efficient Methane Oxidation. Commun. Chem. 2018, 1, 41,  DOI: 10.1038/s42004-018-0044-9
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    Kim, J.; Maiti, A.; Lin, L.-C.; Stolaroff, J. K.; Smit, B.; Aines, R. D. New Materials for Methane Capture from Dilute and Medium-Concentration Sources. Nat. Commun. 2013, 4, 1– 7,  DOI: 10.1038/ncomms2697
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    Sushkevich, V. L.; Palagin, D.; Ranocchiari, M.; van Bokhoven, J. A. Synthesis of Methanol. Science 2017, 356, 523– 527,  DOI: 10.1126/science.aam9035
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    Selective anaerobic oxidation of methane enables direct synthesis of methanol
    Sushkevich, Vitaly L.; Palagin, Dennis; Ranocchiari, Marco; van Bokhoven, Jeroen A.
    Science (Washington, DC, United States) (2017), 356 (6337), 523-527CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Direct functionalization of methane in natural gas remains a key challenge. We present a direct stepwise method for converting methane into methanol with high selectivity (∼97%) over a copper-contg. zeolite, based on partial oxidn. with water. The activation in helium at 673 K (K), followed by consecutive catalyst exposures to 7 bars of methane and then water at 473 K, consistently produced 0.204 mol of CH3OH per mol of copper in zeolite. Isotopic labeling confirmed water as the source of oxygen to regenerate the zeolite active centers and renders methanol desorption energetically favorable. On the basis of in situ x-ray absorption spectroscopy, IR spectroscopy, and d. functional theory calcns., we propose a mechanism involving methane oxidn. at CuII oxide active centers, followed by CuI reoxidn. by water with concurrent formation of hydrogen.
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  26. 26
    Kulkarni, A. R.; Zhao, Z.-J.; Siahrostami, S.; Nørskov, J. K.; Studt, F. Cation-Exchanged Zeolites for the Selective Oxidation of Methane to Methanol. Catal. Sci. Technol. 2018, 8, 114– 123,  DOI: 10.1039/c7cy01229b
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    Cation-exchanged zeolites for the selective oxidation of methane to methanol
    Kulkarni, Ambarish R.; Zhao, Zhi-Jian; Siahrostami, Samira; Noerskov, Jens K.; Studt, Felix
    Catalysis Science & Technology (2018), 8 (1), 114-123CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    Motivated by the increasing availability of cheap natural gas resources, considerable exptl. and computational research efforts have focused on identifying selective catalysts for the direct conversion of methane to methanol. One promising class of catalysts are cation-exchanged zeolites, which have steadily increased in popularity over the past decade. In this article, we first present a broad overview of this field from a conceptual perspective, and highlight the role of theory in developing a mol.-level understanding of the reaction. Next, by performing and analyzing a large database of d. functional theory (DFT) calcns. for a wide range of transition metal cations, zeolite topologies and active site motifs, we present a unifying picture of the methane activation process in terms of active site stability, C-H bond activation and methanol extn. Based on the trade-offs of active site stability and reactivity, we propose a framework for identifying new, promising active site motifs in these systems. Further, we show that the high methanol selectivity arises due to the strong binding nature of the C-H activation products. Finally, using the atomistic and mechanistic insight obtained from these analyses, we summarize the key challenges and future strategies for improving the performance of cation-exchanged zeolites for this industrially relevant conversion.
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    Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998.
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    Gilbertson, L. M.; Zimmerman, J. B.; Plata, D. L.; Hutchison, J. E.; Anastas, P. T. Designing Nanomaterials to Maximize Performance and Minimize Undesirable Implications Guided by the Principles of Green Chemistry. Chem. Soc. Rev. 2015, 44, 5758– 5777,  DOI: 10.1039/C4CS00445K
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    Designing nanomaterials to maximize performance and minimize undesirable implications guided by the Principles of Green Chemistry
    Gilbertson, Leanne M.; Zimmerman, Julie B.; Plata, Desiree L.; Hutchison, James E.; Anastas, Paul T.
    Chemical Society Reviews (2015), 44 (16), 5758-5777CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    The Twelve Principles of Green Chem. were first published in 1998 and provide a framework that has been adopted not only by chemists, but also by design practitioners and decision-makers (e.g., materials scientists and regulators). The development of the Principles was initially motivated by the need to address decades of unintended environmental pollution and human health impacts from the prodn. and use of hazardous chems. Yet, for over a decade now, the Principles have been applied to the synthesis and prodn. of engineered nanomaterials (ENMs) and the products they enable. While the combined efforts of the global scientific community have led to promising advances in the field of nanotechnol., there remain significant research gaps and the opportunity to leverage the potential global economic, societal and environmental benefits of ENMs safely and sustainably. As such, this tutorial review benchmarks the successes to date and identifies crit. research gaps to be considered as future opportunities for the community to address. A sustainable material design framework is proposed that emphasizes the importance of establishing structure-property-function (SPF) and structure-property-hazard (SPH) relationships to guide the rational design of ENMs. The goal is to achieve or exceed the functional performance of current materials and the technologies they enable, while minimizing inherent hazard to avoid risk to human health and the environment at all stages of the life cycle.
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  29. 29
    Erythropel, H. C.; Zimmerman, J. B.; de Winter, T. M.; Petitjean, L.; Melnikov, F.; Lam, C. H.; Lounsbury, A. W.; Mellor, K. E.; Janković, N. Z.; Tu, Q.; Pincus, L. N.; Falinski, M. M.; Shi, W.; Coish, P.; Plata, D. L.; Anastas, P. T. The Green ChemisTREE: 20 Years after Taking Root with the 12 Principles. Green Chem 2018, 20, 1929– 1961,  DOI: 10.1039/C8GC00482J
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    The Green ChemisTREE: 20 years after taking root with the 12 principles
    Erythropel, Hanno C.; Zimmerman, Julie B.; de Winter, Tamara M.; Petitjean, Laurene; Melnikov, Fjodor; Lam, Chun Ho; Lounsbury, Amanda W.; Mellor, Karolina E.; Jankovic, Nina Z.; Tu, Qingshi; Pincus, Lauren N.; Falinski, Mark M.; Shi, Wenbo; Coish, Philip; Plata, Desiree L.; Anastas, Paul T.
    Green Chemistry (2018), 20 (9), 1929-1961CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    The field of Green Chem. has seen many scientific discoveries and inventions during the 20 years since the 12 Principles were first published. Inspired by tree diagrams that illustrate diversity of products stemming from raw materials, we present here the Green ChemisTREE as a showcase for the diversity of research and achievements stemming from Green Chem. Each branch of the Green ChemisTREE represents one of the 12 Principles, and the leaves represent areas of inquiry and development relevant to that Principle (branch). As such, in this 'meta-review', we aim to describe the history and current status of the field of Green Chem.: by exploring activity within each Principle, by summarizing the benefits of Green Chem. through robust examples, by discussing tools and metrics available to measure progress towards Green Chem., and by outlining knowledge gaps, opportunities, and future challenges for the field.
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  30. 30
    Dou, J.; Tang, Y.; Nie, L.; Andolina, C. M.; Zhang, X.; House, S.; Li, Y.; Yang, J.; Tao, F. F. Complete Oxidation of Methane on Co3O4/CeO2 Nanocomposite: A Synergic Effect. Catal. Today 2018, 311, 48– 55,  DOI: 10.1016/j.cattod.2017.12.027
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    Complete Oxidation of Methane on Co3O4/CeO2 Nanocomposite: A Synergic Effect
    Dou, Jian; Tang, Yu; Nie, Longhui; Andolina, Christopher M.; Zhang, Xiaoyan; House, Stephen; Li, Yuting; Yang, Judith; Tao, Franklin
    Catalysis Today (2018), 311 (), 48-55CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    Development of nonprecious metal-based catalysts highly active for complete oxidn. of CH4 at a temp. ≤600°C is significant for removing unburned CH4 at exhaust of engines of vehicles using natural gas or liquefied petroleum gas. CeO2 is active for complete oxidn. of CH4. Co3O4 has been identified as a promising catalyst for this reaction. A Co3O4/CeO2 nanocomposite catalyst consisting of ceria nanorods supported on Co3O4 nanoparticles was prepd. through a modified deposition pptn. method. The Co3O4/CeO2 nanocomposite exhibits high activity for complete oxidn. of methane with an apparent activation energy of 43.9kJ/mol, which is obviously lower than 95.1kJ/mol of pure CeO2 and 89.7kJ/mol of pure Co3O4, suggesting a synergetic effect between Co3O4 and CeO2. Surface of the Co3O4/CeO2 nanocomposite during complete oxidn. of CH4 at of 200-500°C and potential stable intermediate of this catalysis were identified with ambient pressure XPS (AP-XPS).
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  31. 31
    Lim, T. H.; Cho, S. J.; Yang, H. S.; Engelhard, M. H.; Kim, D. H. Effect of Co/Ni Ratios in Cobalt Nickel Mixed Oxide Catalysts on Methane Combustion. Appl. Catal. A Gen. 2015, 505, 62– 69,  DOI: 10.1016/j.apcata.2015.07.040
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    31
    Effect of Co/Ni ratios in cobalt nickel mixed oxide catalysts on methane combustion
    Lim, Tae Hwan; Cho, Sung June; Yang, Hee Sung; Engelhard, M. H.; Kim, Do Heui
    Applied Catalysis, A: General (2015), 505 (), 62-69CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
    A series of cobalt nickel mixed oxide catalysts with the varying ratios of Co to Ni, prepd. by co-pptn. method, were applied to methane combustion. Among the various ratios, cobalt nickel mixed oxides having the ratios of Co to Ni of (50:50) and (67:33) demonstrate the highest activity for methane combustion. Structural anal. obtained from X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) evidently demonstrates that CoNi (50:50) and (67:33) samples consist of NiCo2O4 and NiO phase and, more importantly, NiCo2O4 spinel structure is largely distorted, which is attributed to the insertion of Ni2+ ions into octahedral sites in Co3O4 spinel structure. Such structural disorder results in the enhanced portion of surface oxygen species, thus leading to the improved reducibility of the catalysts in the low temp. region as evidenced by temp. programmed redn. by hydrogen (H2 TPR) and XPS O 1s results. They prove that structural disorder in cobalt nickel mixed oxides enhances the catalytic performance for methane combustion. Thus, it is concluded that a strong relationship between structural property and activity in cobalt nickel mixed oxide for methane combustion exists and, more importantly, distorted NiCo2O4 spinel structure is found to be an active site for methane combustion.
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  32. 32
    Tao, F. F.; Shan, J.-j.; Nguyen, L.; Wang, Z.; Zhang, S.; Zhang, L.; Wu, Z.; Huang, W.; Zeng, S.; Hu, P. Understanding Complete Oxidation of Methane on Spinel Oxides at a Molecular Level. Nat. Commun. 2015, 6, 7798,  DOI: 10.1038/ncomms8798
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    Understanding complete oxidation of methane on spinel oxides at a molecular level
    Tao, Franklin Feng; Shan, Jun-jun; Nguyen, Luan; Wang, Ziyun; Zhang, Shiran; Zhang, Li; Wu, Zili; Huang, Weixin; Zeng, Shibi; Hu, P.
    Nature Communications (2015), 6 (), 7798CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    It is crucial to develop a catalyst made of earth-abundant elements highly active for a complete oxidn. of methane at a relatively low temp. NiCo2O4 consisting of earth-abundant elements which can completely oxidize methane in the temp. range of 350-550 °C. Being a cost-effective catalyst, NiCo2O4 exhibits activity higher than precious-metal-based catalysts. Here we report that the higher catalytic activity at the relatively low temp. results from the integration of nickel cations, cobalt cations and surface lattice oxygen atoms/oxygen vacancies at the at. scale. In situ studies of complete oxidn. of methane on NiCo2O4 and theor. simulations show that methane dissocs. to Me on nickel cations and then couple with surface lattice oxygen atoms to form -CH3O with a following dehydrogenation to -CH2O; a following oxidative dehydrogenation forms CHO; CHO is transformed to product mols. through two different sub-pathways including dehydrogenation of OCHO and CO oxidn.
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  33. 33
    Wu, M.; Li, W.; Zhang, X.; Xue, F.; Yang, T.; Yuan, L. Penta-coordinated Al3+ Stabilized Defect-Rich Ceria on Al2O3 Supported Palladium Catalysts for Lean Methane Oxidation. ChemCatChem 2021, 13, 3490– 3500,  DOI: 10.1002/cctc.202100668
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    33
    Penta-coordinated Al3+ Stabilized Defect-Rich Ceria on Al2O3 Supported Palladium Catalysts for Lean Methane Oxidation
    Wu, Mingwei; Li, Wenzhi; Zhang, Xia; Xue, Fengyang; Yang, Tao; Yuan, Liang
    ChemCatChem (2021), 13 (15), 3490-3500CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In heterogeneous catalysis, the strong interaction between metal and support can significantly modulate the electronic properties of the metals and play a crucial role in metal particle dispersion and morphol. Herein, a facile strategy was utilized to fabricate highly dispersive palladium catalysts supported on defective Al2O3-CeO2 for lean methane oxidn. Ceria was immobilized on the coordinatively unsatd. Al3+penta sites of γ-alumina activated by pre-redn. to fabricate hybrid-oxide support (abbreviated as RAl2O3-CeO2), and then Pd precursor was uniformly deposited on the defective surface. Such a RAl2O3-CeO2 interface can effectively upgrade the dispersion of deposited palladium species and improve the concn. of reactive oxygen species owing to strong electronic interactions, invoking a superior catalytic activity for lean methane oxidn. After loading 1.0 wt% palladium, Pd/RAl2O3-CeO2 exhibited a 90% methane conversion at 328°C under the space velocity of 60,000 mL g-1 h-1, far lower than that of bare 1.0 wt% Pd/RAl2O3(400°C). In addn., Pd/RAl2O3-CeO2 catalyst showed an excellent hydrothermal stability under the harsh conditions (600°C, water vapor)for 120 h without obvious deactivation. The reaction pathway of total methane combustion was elucidated by in situ DRIFTS. The crucial intermediate compns. (carbon oxygenates and carbonate)on Pd/RAl2O3-CeO2 surface are more readily oxidized into CO2 and H2O. This work provides an effective interface-promoted strategy to develop efficient and durable palladium catalysts for many challenging reactions.
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  34. 34
    Cao, P.; Yan, B.; Chu, Y.; Wang, S.; Yu, H.; Li, T.; Xiong, C.; Yin, H. Synthesis of Highly Dispersed Palladium Nanoparticles Supported on Silica for Catalytic Combustion of Methane. Ind. Eng. Chem. Res. 2021, 60, 7545– 7557,  DOI: 10.1021/acs.iecr.1c00175
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    34
    Synthesis of Highly Dispersed Palladium Nanoparticles Supported on Silica for Catalytic Combustion of Methane
    Cao, Peng; Yan, Bo; Chu, Yuting; Wang, Shiwei; Yu, Hongbo; Li, Tong; Xiong, Chunrong; Yin, Hongfeng
    Industrial & Engineering Chemistry Research (2021), 60 (20), 7545-7557CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    Pd nanoparticles supported on SiO2 catalysts (Pd/SiO2) were prepd. by wet impregnation (WI), dry impregnation (DI), strong electrostatic adsorption (SEA), and charge-enhanced dry impregnation (CEDI) methods. The Pd/SiO2 samples with highly dispersed and tight size-distributed Pd nanoparticles are obtained via SEA and CEDI methods based on strong electrostatic interactions between the dissolved metal precursor ([Pd(NH3)4]2+) and pos. charged SiO2 support in an alkali-impregnating soln. (initial pH = 12). The Pd/SiO2-SEA samples prepd. by the SEA method usually showed higher Pd dispersions (>50%) than those prepd. by CEDI (Pd dispersion = 32-45%). The surface loading (support surface area per L of prepn. soln.), pH regulator (NaOH or NH4OH), Pd loading, and redn. temp. are key factors affecting the dispersion of Pd in the Pd/SiO2-SEA samples, as well as the leaching/dissoln. of SiO2 and Pd in the alkali soln. The Pd/SiO2-SEA samples prepd. with proper SLs of 30,000-100,000 m2 L-1 using NH4OH as the pH regulator exhibited not only very high Pd dispersions (64-97%) but also negligible losses of SiO2 and Pd in the impregnating soln. The Pd/SiO2-SEA samples also exhibited better catalytic performance in methane combustion based on both the T10 and T50 temps. and the intrinsic activities (mass-specific activity and/or turnover frequency (TOFs)). The TOFs generally decreased from 130 h-1to 6.2 h-1 as Pd dispersion increased from 32% to 97% for the Pd/SiO2-SEA(NH4OH) catalysts. Also, the reaction activity of Pd/SiO2-SEA catalysts was significantly improved by increasing the fraction of Pd0 at 70-85%, indicating that this size-sensitive catalysis would be related to the redox properties of the supported Pd nanoparticles.
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  35. 35
    Li, K.; Xu, D.; Liu, K.; Ni, H.; Shen, F.; Chen, T.; Guan, B.; Zhan, R.; Huang, Z.; Lin, H. Catalytic Combustion of Lean Methane Assisted by an Electric Field over Mn XCo y Catalysts at Low Temperature. J. Phys. Chem. C 2019, 123, 10377– 10388,  DOI: 10.1021/acs.jpcc.9b00496
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    Catalytic Combustion of Lean Methane Assisted by an Electric Field over MnxCoy Catalysts at Low Temperature
    Li, Ke; Xu, Dejun; Liu, Ke; Ni, Hong; Shen, Feixiang; Chen, Ting; Guan, Bin; Zhan, Reggie; Huang, Zhen; Lin, He
    Journal of Physical Chemistry C (2019), 123 (16), 10377-10388CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    An elec. field was introduced into the catalytic oxidn. of lean methane at low temp. over MnxCoy catalysts. Mn1Co5 exhibited the best catalytic performance with the light off temp. (T50) as low as 271 °C in the elec. field, nearly 60 °C lower than that in a conventional reaction system. The elec. field promoted the formation of octahedrally coordinated Mn3+ with active oxygen species released from the redn. of octahedrally coordinated Co3+ in Co3O4 spinel. Also, octahedrally coordinated Mn3+ sites were proven to be the main active sites for methane catalytic oxidn. With an in situ FTIR technique, it was found that the oxygen species from the catalyst bulk instead of gaseous oxygen will adsorb on the octahedrally coordinated Mn3+ sites in the elec. field, promoting the activation of CH4 at low temp. The dehydroxylation process will be accelerated through the formation of CoO(OH) species that will quickly convert due to the enhanced reducibility of Co3+ in the elec. field, weakening the inhibition of produced hydroxyl species on active sites. Based on the exptl. results, the mechanism of catalytic oxidn. of CH4 over MnxCoy catalysts in an elec. field was proposed.
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  36. 36
    Auvinen, P.; Kinnunen, N. M.; Hirvi, J. T.; Maunula, T.; Kallinen, K.; Keenan, M.; Baert, R.; van den Tillaart, E.; Suvanto, M. Development of a Rapid Ageing Technique for Modern Methane Combustion Catalysts in the Laboratory: Why Does SO2 Concentration Play an Essential Role?. Appl. Catal. B Environ. 2019, 258, 117976,  DOI: 10.1016/j.apcatb.2019.117976
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    Development of a rapid ageing technique for modern methane combustion catalysts in the laboratory: Why does SO2 concentration play an essential role?
    Auvinen, Paavo; Kinnunen, Niko M.; Hirvi, Janne T.; Maunula, Teuvo; Kallinen, Kauko; Keenan, Matthew; Baert, Rik; van den Tillaart, Erik; Suvanto, Mika
    Applied Catalysis, B: Environmental (2019), 258 (), 117976CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    In pursuance to decrease emissions of transportation sector, the durability and longevity of modern catalysts has become a topical issue. Understanding the deactivation of catalysts is esp. urgent. Our aim in this study was to clarify the roles of reaction temp. and SO2 concn. in methane combustion catalyst poisoning. Information about this process can help the research community perform more realistic lab. simulations and provide new insights for catalyst development. With the collected exptl. data, the likelihood that poisoning would be influenced by different sulfur species and mechanisms was evaluated. Our results suggested that both reaction temp. and SO2 concn. influenced the stability of the resulting sulfates. Low SO2 concns. lead to formation of stabler sulfates and lower total amt. of sulfur in the catalyst. In turn, high SO2 concns. formed less stable sulfates but accumulated more sulfur. Lesser stability was attributed to formation of Al2(SO4)3 through spillover.
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  37. 37
    Losch, P.; Huang, W.; Vozniuk, O.; Goodman, E. D.; Schmidt, W.; Cargnello, M. Modular Pd/Zeolite Composites Demonstrating the Key Role of Support Hydrophobic/Hydrophilic Character in Methane Catalytic Combustion. ACS Catal 2019, 9, 4742– 4753,  DOI: 10.1021/acscatal.9b00596
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    37
    Modular Pd/Zeolite Composites Demonstrating the Key Role of Support Hydrophobic/Hydrophilic Character in Methane Catalytic Combustion
    Losch, Pit; Huang, Weixin; Vozniuk, Olena; Goodman, Emmett D.; Schmidt, Wolfgang; Cargnello, Matteo
    ACS Catalysis (2019), 9 (6), 4742-4753CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Complete catalytic oxidn. of methane in the presence of steam at low temps. (T < 400 °C) is a crucial reaction for emission control, yet it presents profound challenges. The activation of the strong C-H bond of methane at low temp. is difficult, and the water present in any realistic application poisons the active surface and promotes sintering of Pd particles during the reaction. Finding materials that can deliver high reaction rates while being more resistant to the presence of water is imperative for advancing several technol. applications of natural gas-based systems. However, methods to fairly compare the activity of Pd catalysts (the most active metal for methane combustion) are needed in order to perform useful structure-property relationship studies. Here, we report a method to study how zeolite hydrophobicity affects the activity of Pd nanoparticles in the reaction, which led to a significant improvement in the water resistance. Mesoporous zeolites were synthesized starting from com. available microporous zeolites. In this way, a variety of hierarchically porous zeolites, with different hydrophobic/hydrophilic character, were prepd. Preformed colloidal Pd nanoparticles could be deposited within mesostructured zeolites. This approach enabled the systematic study of key parameters such as zeolite framework, Al content, and the Pd loading while maintaining the same Pd particle size and structure for all the samples. Detailed catalytic studies revealed an optimum hydrophobic/hydrophilic character, and a promising steam-resistant catalyst, namely, 3.2 nm Pd particles supported on mesoporous zeolite beta or USY with a Si/Al ratio of 40, emerged from this multiparametric study with a T50 of 355 °C and T90 of 375 °C (where T50 and T90 are temp. values at which the samples reach 50% and 90% methane conversion, resp.) in steam-contg. reaction conditions. Finally, we verified that the designed catalysts were stable by in-depth postcatalysis characterization and operando diffuse-reflectance IR Fourier-transform spectroscopy (DRIFTS) analyses confirming that water adsorbs less strongly on the active PdO surface due to interaction with the zeolite acid sites. This method can be of general use to study how zeolite supports affect the reactivity of supported metals in several catalytic applications.
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    Rare earth and specialty elements (RESE) are functionally integral to several clean energy technologies, but there is no domestic source of virgin RESE in the United States. Manufg. waste streams, which are relatively simple compositionally, and electronic wastes, which are chem. complex, could both serve as viable sources of secondary RESE if efficient methods existed to recover and sep. these metals for reuse. Leveraging differences in RESE redn. potentials, high surface area, high cond. carbon nanotubes (CNTs) could enable space- and solvent-efficient, selective recovery of RESE from mixed metal wastes. In this study, unaligned CNTs encapsulated in polyvinyl alc. were used to develop an electrochem. active filter and tested for recovery of six metals or metalloids (Cu, As, Eu, Nd, Ga, and Sc) as a function of flow rate (1-5 mL min-1), pH (2-10), and voltage (0.1-3.0 V), with max. recoveries of 86-96%, except for As, which was unretained. All metals were recovered as oxides, rather than their zero valent or reduced forms (except for Cu, which was partially reduced at low pHs). Deaeration expts. suggested electrochem. redn. of dissolved O2 and O2 derived from water splitting were jointly responsible for metal capture, where metal oxides were first formed via metal hydroxide intermediates, and this mechanism was enhanced at higher pHs. A synthetic, multi-metal waste stream of Cu and Eu was successfully sepd. on multiple stages with increasing voltages (97 ± 0.1% Cu and 65 ± 0.3% Eu recovery), indicating the approach might be useful for the treatment of electronic end-of-life and manufg. derived wastes.
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    Figure 1. Catalyst activation as a function of gas composition, temperature, duration, and repetition illustrates that activation can be achieved under ambient-to-moderate conditions with reuse potential. The carbon conversion efficiency during the reaction steps is shown for all activation trials, which were conducted for 30 min at 450 °C and with a freshly prepared catalyst unless otherwise noted and in 100% (black) or 20% oxygen (red) in helium mixtures. All methane conversion reactions were conducted at 200 °C in 2 ppmv methane; note that 200 °C is not the maximum conversion temperature but enables one to see a spread in the behavior as a function of the activation temperature. The effect of (a) activation temperature, (b) activation duration, and (c) multiple reactivations is shown.

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    Figure 2

    Figure 2. Powder XRD pattern of copper-exchanged ammonium zeolite powder (mordenite) after activations varying from 0 to 240 min in duration (unexchanged, 0, 30, 240 min from bottom to top).

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    Figure 3

    Figure 3. Methane conversion efficiency as a function of reaction temperature and operation mode. (a) In a two-step, activation followed by the conversion approach, both activation temperature and reaction conversion temperature influence methane removal. All activation steps were carried out for 30 min in 20% O2. (b) In a continuous reaction approach, 30 min activation in methane-free gas mixtures (20% O2) was followed by a 30 min conversion reaction in the presence of atmospheric levels of methane (2 ppmv) to simulate isothermal operations. Each data point was generated with a freshly prepared catalyst and represents the mean of at least 21 measurements.

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    Figure 4

    Figure 4. Methane conversion rate increases with the input methane concentration over a range of sub-flarable levels. Methane conversion was tested from 2 ppmv to 2% v/v methane in the presence of 20% oxygen in isothermal operation at 310 °C (30 min initial activation in methane-free gas; asterisks), and following 30 min (filled red symbol) and 60 min (open symbol) activations (450 °C) and reaction (200 °C) in 20% oxygen. Each data point collected with the freshly prepared catalyst represents at least 20 methane conversion measurements.

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    Figure 5

    Figure 5. Long-term activity of the catalyst. Low-level methane (2 ppmv methane in 20% oxygen) was catalytically reacted over 300 h under continuous, isothermal operation at 310 °C (asterisks, following 8 h activation) and a traditional two-step process (red circles; 450 °C, 30 min activation followed by 200 °C continuous reaction).

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  • References

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      Duren, R. M.; Thorpe, A. K.; Foster, K. T.; Rafiq, T.; Hopkins, F. M.; Yadav, V.; Bue, B. D.; Thompson, D. R.; Conley, S.; Colombi, N. K.; Frankenberg, C.; McCubbin, I. B.; Eastwood, M. L.; Falk, M.; Herner, J. D.; Croes, B. E.; Green, R. O.; Miller, C. E. California’s Methane Super-Emitters. Nature 2019, 575, 180– 184,  DOI: 10.1038/s41586-019-1720-3
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      California's methane super-emitters
      Duren, Riley M.; Thorpe, Andrew K.; Foster, Kelsey T.; Rafiq, Talha; Hopkins, Francesca M.; Yadav, Vineet; Bue, Brian D.; Thompson, David R.; Conley, Stephen; Colombi, Nadia K.; Frankenberg, Christian; McCubbin, Ian B.; Eastwood, Michael L.; Falk, Matthias; Herner, Jorn D.; Croes, Bart E.; Green, Robert O.; Miller, Charles E.
      Nature (London, United Kingdom) (2019), 575 (7781), 180-184CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Methane, a powerful greenhouse gas, is targeted for emissions mitigation by California state and other jurisdictions worldwide (California Senate Bill 1383, 2016; Global Methane Initiative, 2019). Unique mitigation opportunities are presented by point-source emitters surface features or infrastructure components which are typically <10 m diam. and emit highly concd. CH4 plumes. Point-source emissions data are sparse and typically lack sufficient spatiotemporal resoln. to guide their mitigation and accurately assess their magnitude (National Academies of Sciences, Engineering, and Medicine, 2018). This work surveyed >272,000 infrastructure elements in California using an airborne imaging spectrometer which can rapidly map CH4 plumes (Hamlin, L. et al., 2011; Thorpe, A.K., et al., 2016; Thompson, D.R., et al., 2015). Five campaigns were conducted for several months (2016-2018), spanning oil and gas, manure management, waste management sectors, resulting in detection, geo-location and quantification of emissions from 564 strong CH4 point sources. A remote sensing approach enables rapid, repeated assessment of large areas at high spatial resoln. for a poorly characterized population of CH4 emitters which often appear intermittently and stochastically. The authors estd. net CH4 point-source emissions in California to be 0.618 Tg/yr (95% confidence interval, 0.523-0.725), equiv. to 34-46% of the state CH4 inventory (California Greenhouse Gas Emission Inventory, 2018) for 2016. Methane super-emitter activity occurred in every surveyed sector, with 10% of point sources contributing roughly 60% of point-source emissions, consistent with a study of the US Four Corners region which had a different sectoral mix (Frankenberg, C., et al., 2016). Largest California CH4 emitters were a subset of landfills which exhibited persistent anomalous activity. California CH4 point-source emissions are dominated by landfills (41%), followed by dairies (26%) and the oil and gas sector (26%). Data enabled an identification of the 0.2% of California infrastructure responsible for these emissions. Sharing these data with collaborating infrastructure operators led to mitigation of anomalous CH4-emission activity (Photojournal, 2018).
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      Nisbet, E. G.; Fisher, R. E.; Lowry, D.; France, J. L.; Allen, G.; Bakkaloglu, S.; Broderick, T. J.; Cain, M.; Coleman, M.; Fernandez, J.; Forster, G.; Griffiths, P. T.; Iverach, C. P.; Kelly, B. F. J.; Manning, M. R.; Nisbet-Jones, P. B. R.; Pyle, J. A.; Townsend-Small, A.; al-Shalaan, A.; Warwick, N.; Zazzeri, G. Methane Mitigation: Methods to Reduce Emissions, on the Path to the Paris Agreement. Rev. Geophys. 2020, 58, 1– 51,  DOI: 10.1029/2019RG000675
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      Haque, M. N. Dietary Manipulation: A Sustainable Way to Mitigate Methane Emissions from Ruminants. J. Anim. Sci. Technol. 2018, 60, 15,  DOI: 10.1186/s40781-018-0175-7
      12
      Dietary manipulation: a sustainable way to mitigate methane emissions from ruminants
      Haque, Najmul
      Journal of Animal Science and Technology (2018), 60 (), 15/1-15/10CODEN: JASTCC; ISSN:2055-0391. (BioMed Central Ltd.)
      Methane emission from the enteric fermn. of ruminant livestock is a main source of greenhouse gas (GHG) emission and a major concern for global warming. Methane emission is also assocd. with dietary energy lose; hence, reduce feed efficiency. Due to the neg. environmental impacts, methane mitigation has come forward in last few decades. To date numerous efforts were made in order to reduce methane emission from ruminants. No table mitigation approaches are rumen manipulation, alteration of rumen fermn., modification of rumen microbial biodiversity by different means and rarely by animal manipulations. However, a comprehensive exploration for a sustainable methane mitigation approach is still lacking. Dietary modification is directly linked to changes in the rumen fermn. pattern and types of end products. Studies showed that changing fermn. pattern is one of the most effective ways of methane abatement. Desirable dietary changes provide two fold benefits i.e. improve prodn. and reduce GHG emissions. Therefore, the aim of this review is to discuss biol. of methane emission from ruminants and its mitigation through dietary manipulation.
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      Smith, T. J.; Murrell, J. C. Methanotrophy/methane oxidation. In Encyclopedia of Microbiology, 3rd ed.; Schaechter, M., Ed.; Academic Press: Oxford, 2009; pp 293– 298.
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      Khmelenina, V. N.; Murrell, J. C.; Smith, T. J.; Trotsenko, Y. A. Physiology and Biochemistry of the Aerobic Methanotrophs. In BT─Aerobic Utilization of Hydrocarbons, Oils and Lipids; Rojo, F., Ed.; Springer International Publishing: Cham, 2018; pp 1– 25.
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    15. 15
      Dinh, K. T.; Sullivan, M. M.; Serna, P.; Meyer, R. J.; Dincă, M.; Román-Leshkov, Y. Viewpoint on the Partial Oxidation of Methane to Methanol Using Cu- and Fe-Exchanged Zeolites. ACS Catal 2018, 8, 8306– 8313,  DOI: 10.1021/acscatal.8b01180
      15
      Viewpoint on the Partial Oxidation of Methane to Methanol Using Cu- and Fe-Exchanged Zeolites
      Dinh, Kimberly T.; Sullivan, Mark M.; Serna, Pedro; Meyer, Randall J.; Dinca, Mircea; Roman-Leshkov, Yuriy
      ACS Catalysis (2018), 8 (9), 8306-8313CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Abundant and inexpensive reserves of CH4, obtained from increased prodn. of natural gas, can effortlessly be incorporated into the current petrochem. infrastructure via conversion to methanol, a staple of the petrochem. industry. Although oxidative C-H bond activation of CH4 is thermodynamically and kinetically accessible at low temps., few catalysts are capable of preventing overoxidn. to carbon dioxide. This lack in selectivity arises from the high C-H bond energy (415 kJ mol-1) of the methane mol. in comparison to the partially oxidized products, which results in further oxidn. through consecutive reactions. Currently, the catalytic prodn. of methanol from methane, accomplished through the two-step process of high temp. (∼1170 K) steam reforming to syngas and its subsequent conversion over Cu-based methanol synthesis catalysts, is effective only at large scale. To date, no synthetic catalyst exists that can convert methane to methanol in high yields using oxygen as a terminal oxidant in a single step.
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    16. 16
      Kulkarni, A. R.; Zhao, Z.-J.; Siahrostami, S.; Nørskov, J. K.; Studt, F. Cation-Exchanged Zeolites for the Selective Oxidation of Methane to Methanol. Catal. Sci. Technol. 2018, 8, 114– 123,  DOI: 10.1039/C7CY01229B
      16
      Cation-exchanged zeolites for the selective oxidation of methane to methanol
      Kulkarni, Ambarish R.; Zhao, Zhi-Jian; Siahrostami, Samira; Noerskov, Jens K.; Studt, Felix
      Catalysis Science & Technology (2018), 8 (1), 114-123CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      Motivated by the increasing availability of cheap natural gas resources, considerable exptl. and computational research efforts have focused on identifying selective catalysts for the direct conversion of methane to methanol. One promising class of catalysts are cation-exchanged zeolites, which have steadily increased in popularity over the past decade. In this article, we first present a broad overview of this field from a conceptual perspective, and highlight the role of theory in developing a mol.-level understanding of the reaction. Next, by performing and analyzing a large database of d. functional theory (DFT) calcns. for a wide range of transition metal cations, zeolite topologies and active site motifs, we present a unifying picture of the methane activation process in terms of active site stability, C-H bond activation and methanol extn. Based on the trade-offs of active site stability and reactivity, we propose a framework for identifying new, promising active site motifs in these systems. Further, we show that the high methanol selectivity arises due to the strong binding nature of the C-H activation products. Finally, using the atomistic and mechanistic insight obtained from these analyses, we summarize the key challenges and future strategies for improving the performance of cation-exchanged zeolites for this industrially relevant conversion.
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    17. 17
      Dinh, K. T.; Sullivan, M. M.; Narsimhan, K.; Serna, P.; Meyer, R. J.; Dincă, M.; Román-Leshkov, Y. Continuous Partial Oxidation of Methane to Methanol Catalyzed by Diffusion-Paired Copper Dimers in Copper-Exchanged Zeolites. J. Am. Chem. Soc. 2019, 141, 11641– 11650,  DOI: 10.1021/jacs.9b04906
      17
      Continuous Partial Oxidation of Methane to Methanol Catalyzed by Diffusion-Paired Copper Dimers in Copper-Exchanged Zeolites
      Dinh, Kimberly T.; Sullivan, Mark M.; Narsimhan, Karthik; Serna, Pedro; Meyer, Randall J.; Dinca, Mircea; Roman-Leshkov, Yuriy
      Journal of the American Chemical Society (2019), 141 (29), 11641-11650CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Copper-exchanged zeolites can continuously and selectively catalyze the partial oxidn. of methane to methanol using only oxygen and water at low temps., but the genesis and nature of the active sites are currently unknown. Herein, we demonstrate that this reaction is catalyzed by a [Cu-O-Cu]2+ motif that forms via a hypothesized proton-aided diffusion of hydrated Cu ions within the cages of SSZ-13 zeolites. While various Cu configurations may be present and active for methane oxidn., a dimeric Cu motif is the primary active site for selective partial methane oxidn. Mechanistically, CH4 activation proceeds via rate-detg. C-H scission to form a surface-bound C1 intermediate that can either be desorbed as methanol in the presence of H2O/H+ or completely oxidized to CO2 by gas-phase O2. High partial oxidn. selectivity can be obtained with (i) high methane and water partial pressures and (ii) maximizing Cu dimer formation by using zeolites with high Al content and low Cu loadings.
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    18. 18
      Narsimhan, K.; Iyoki, K.; Dinh, K.; Román-Leshkov, Y. Catalytic Oxidation of Methane into Methanol over Copper-Exchanged Zeolites with Oxygen at Low Temperature. ACS Cent. Sci. 2016, 2, 424– 429,  DOI: 10.1021/acscentsci.6b00139
      18
      Catalytic Oxidation of Methane into Methanol over Copper-Exchanged Zeolites with Oxygen at Low Temperature
      Narsimhan, Karthik; Iyoki, Kenta; Dinh, Kimberly; Roman-Leshkov, Yuriy
      ACS Central Science (2016), 2 (6), 424-429CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
      The direct catalytic conversion of methane to liq. oxygenated compds., such as methanol or di-Me ether, at low temp. using mol. oxygen is a grand challenge in C-H activation that has never been met with synthetic, heterogeneous catalysts. We report the first demonstration of direct, catalytic oxidn. of methane into methanol with mol. oxygen over copper-exchanged zeolites at low reaction temps. (483-498 K). Reaction kinetics studies show sustained catalytic activity and high selectivity for a variety of com. available zeolite topologies under mild conditions (e.g., 483 K and atm. pressure). Transient and steady state measurements with isotopically labeled mols. confirm catalytic turnover. The catalytic rates and apparent activation energies are affected by the zeolite topol., with caged-based zeolites (e.g., Cu-SSZ-13) showing the highest rates. Although the reaction rates are low, the discovery of catalytic sites in copper-exchanged zeolites will accelerate the development of strategies to directly oxidize methane into methanol under mild conditions.
      >> More from SciFinder ®
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    19. 19
      Shi, Y.; Liu, S.; Liu, Y.; Huang, W.; Guan, G.; Zuo, Z. Quasicatalytic and Catalytic Selective Oxidation of Methane to Methanol over Solid Materials: A Review on the Roles of Water. Catal. Rev. - Sci. Eng. 2020, 62, 313– 345,  DOI: 10.1080/01614940.2019.1674475
      19
      Quasicatalytic and catalytic selective oxidation of methane to methanol over solid materials: a review on the roles of water
      Shi, Yayun; Liu, Shizhong; Liu, Yiming; Huang, Wei; Guan, Guoqing; Zuo, Zhijun
      Catalysis Reviews: Science and Engineering (2020), 62 (3), 313-345CODEN: CRSEC9; ISSN:0161-4940. (Taylor & Francis, Inc.)
      To date, although no com. process for the selective oxidn. of methane has been realized, various novel processes with effective solid materials operated at low temp. have been proposed. It is found that the addn. of water in any processes not only influences the activity, selectivity, and stability of the solid materials but also affects the extn. efficiency of methanol from the product. Herein, the published results on the roles of water in the methanol prodn. via the quasicatalytic and catalytic selective methane oxidn. process using various solid materials in gas and liq. phases at low temps. are critically reviewed.
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      Grundner, S.; Luo, W.; Sanchez-Sanchez, M.; Lercher, J. A. Synthesis of Single-Site Copper Catalysts for Methane Partial Oxidation. Chem. Commun. 2016, 52, 2553– 2556,  DOI: 10.1039/c5cc08371k
      20
      Synthesis of single-site copper catalysts for methane partial oxidation
      Grundner, S.; Luo, W.; Sanchez-Sanchez, M.; Lercher, J. A.
      Chemical Communications (Cambridge, United Kingdom) (2016), 52 (12), 2553-2556CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Cu-Exchanged zeolites are known as active materials for methane oxidn. to MeOH. However, understanding of the formation of Cu active species during synthesis, dehydration and activation is fragmented and rudimentary. The authors show here how a synthesis protocol guided by insight in the ion exchange elementary steps leads to highly uniform Cu species in mordenite (MOR).
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      Tabor, E.; Lemishka, M.; Sobalik, Z.; Mlekodaj, K.; Andrikopoulos, P. C.; Dedecek, J.; Sklenak, S. Low-Temperature Selective Oxidation of Methane over Distant Binuclear Cationic Centers in Zeolites. Commun. Chem. 2019, 2, 1– 9,  DOI: 10.1038/s42004-019-0173-9
      21
      Low-temperature selective oxidation of methane over distant binuclear cationic centers in zeolites
      Tabor, Edyta; Lemishka, Mariia; Sobalik, Zdenek; Mlekodaj, Kinga; Andrikopoulos, Prokopis C.; Dedecek, Jiri; Sklenak, Stepan
      Communications Chemistry (2019), 2 (1), 1-9CODEN: CCOHCT; ISSN:2399-3669. (Nature Research)
      Highly active oxygen capable to selectively oxidize methane to methanol at low temp. can be prepd. in transition-metal cation exchanged zeolites. Here we show that the α-oxygen stabilized by the neg. charges of two framework aluminum atoms can be prepd. by the dissocn. of nitrous oxide over distant binuclear cation structures (M(II)...M(II), M = cobalt, nickel, and iron) accommodated in two adjacent 6-rings forming cationic sites in the ferrierite zeolite. This α-oxygen species is analogous to that known only for iron exchanged zeolites. In contrast to divalent iron cations, only binuclear divalent cobalt cationic structures and not isolated divalent cobalt cations are active. Created methoxy moieties are easily protonated to yield methanol, formaldehyde, and formic acid which are desorbed to the gas phase without the aid of water vapor while previous studies showed that highly stable methoxy groups were formed on isolated iron cations in iron exchanged ZSM-5 zeolites.
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    22. 22
      Petrov, A. W.; Ferri, D.; Krumeich, F.; Nachtegaal, M.; Van Bokhoven, J. A.; Kröcher, O. Stable Complete Methane Oxidation over Palladium Based Zeolite Catalysts. Nat. Commun. 2018, 9, 2545,  DOI: 10.1038/s41467-018-04748-x
      22
      Stable complete methane oxidation over palladium based zeolite catalysts
      Petrov Andrey W; Ferri Davide; Nachtegaal Maarten; van Bokhoven Jeroen A; Krocher Oliver; Petrov Andrey W; Krumeich Frank; van Bokhoven Jeroen A; Krocher Oliver
      Nature communications (2018), 9 (1), 2545 ISSN:.
      Increasing the use of natural gas engines is an important step to reduce the carbon footprint of mobility and power generation sectors. To avoid emissions of unburnt methane and the associated severe greenhouse effect of lean-burn engines, the stability of methane oxidation catalysts against steam-induced sintering at low temperatures (<500 °C) needs to be improved. Here we demonstrate how the combination of catalyst development and improved process control yields a highly efficient solution for complete methane oxidation. We design a material based on palladium and hierarchical zeolite with fully sodium-exchanged acid sites, which improves the support stability and prevents steam-induced palladium sintering under reaction conditions by confining the metal within the zeolite. Repeated short reducing pulses enable the use of a highly active transient state of the catalyst, which in combination with its high stability provides excellent performance without deactivation for over 90 h in the presence of steam.
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      Hou, Y.; Nagamatsu, S.; Asakura, K.; Fukuoka, A.; Kobayashi, H. Trace Mono-Atomically Dispersed Rhodium on Zeolite-Supported Cobalt Catalyst for the Efficient Methane Oxidation. Commun. Chem. 2018, 1, 41,  DOI: 10.1038/s42004-018-0044-9
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      Kim, J.; Maiti, A.; Lin, L.-C.; Stolaroff, J. K.; Smit, B.; Aines, R. D. New Materials for Methane Capture from Dilute and Medium-Concentration Sources. Nat. Commun. 2013, 4, 1– 7,  DOI: 10.1038/ncomms2697
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      Sushkevich, V. L.; Palagin, D.; Ranocchiari, M.; van Bokhoven, J. A. Synthesis of Methanol. Science 2017, 356, 523– 527,  DOI: 10.1126/science.aam9035
      25
      Selective anaerobic oxidation of methane enables direct synthesis of methanol
      Sushkevich, Vitaly L.; Palagin, Dennis; Ranocchiari, Marco; van Bokhoven, Jeroen A.
      Science (Washington, DC, United States) (2017), 356 (6337), 523-527CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Direct functionalization of methane in natural gas remains a key challenge. We present a direct stepwise method for converting methane into methanol with high selectivity (∼97%) over a copper-contg. zeolite, based on partial oxidn. with water. The activation in helium at 673 K (K), followed by consecutive catalyst exposures to 7 bars of methane and then water at 473 K, consistently produced 0.204 mol of CH3OH per mol of copper in zeolite. Isotopic labeling confirmed water as the source of oxygen to regenerate the zeolite active centers and renders methanol desorption energetically favorable. On the basis of in situ x-ray absorption spectroscopy, IR spectroscopy, and d. functional theory calcns., we propose a mechanism involving methane oxidn. at CuII oxide active centers, followed by CuI reoxidn. by water with concurrent formation of hydrogen.
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    26. 26
      Kulkarni, A. R.; Zhao, Z.-J.; Siahrostami, S.; Nørskov, J. K.; Studt, F. Cation-Exchanged Zeolites for the Selective Oxidation of Methane to Methanol. Catal. Sci. Technol. 2018, 8, 114– 123,  DOI: 10.1039/c7cy01229b
      26
      Cation-exchanged zeolites for the selective oxidation of methane to methanol
      Kulkarni, Ambarish R.; Zhao, Zhi-Jian; Siahrostami, Samira; Noerskov, Jens K.; Studt, Felix
      Catalysis Science & Technology (2018), 8 (1), 114-123CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      Motivated by the increasing availability of cheap natural gas resources, considerable exptl. and computational research efforts have focused on identifying selective catalysts for the direct conversion of methane to methanol. One promising class of catalysts are cation-exchanged zeolites, which have steadily increased in popularity over the past decade. In this article, we first present a broad overview of this field from a conceptual perspective, and highlight the role of theory in developing a mol.-level understanding of the reaction. Next, by performing and analyzing a large database of d. functional theory (DFT) calcns. for a wide range of transition metal cations, zeolite topologies and active site motifs, we present a unifying picture of the methane activation process in terms of active site stability, C-H bond activation and methanol extn. Based on the trade-offs of active site stability and reactivity, we propose a framework for identifying new, promising active site motifs in these systems. Further, we show that the high methanol selectivity arises due to the strong binding nature of the C-H activation products. Finally, using the atomistic and mechanistic insight obtained from these analyses, we summarize the key challenges and future strategies for improving the performance of cation-exchanged zeolites for this industrially relevant conversion.
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    27. 27
      Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998.
      There is no corresponding record for this reference.
    28. 28
      Gilbertson, L. M.; Zimmerman, J. B.; Plata, D. L.; Hutchison, J. E.; Anastas, P. T. Designing Nanomaterials to Maximize Performance and Minimize Undesirable Implications Guided by the Principles of Green Chemistry. Chem. Soc. Rev. 2015, 44, 5758– 5777,  DOI: 10.1039/C4CS00445K
      28
      Designing nanomaterials to maximize performance and minimize undesirable implications guided by the Principles of Green Chemistry
      Gilbertson, Leanne M.; Zimmerman, Julie B.; Plata, Desiree L.; Hutchison, James E.; Anastas, Paul T.
      Chemical Society Reviews (2015), 44 (16), 5758-5777CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      The Twelve Principles of Green Chem. were first published in 1998 and provide a framework that has been adopted not only by chemists, but also by design practitioners and decision-makers (e.g., materials scientists and regulators). The development of the Principles was initially motivated by the need to address decades of unintended environmental pollution and human health impacts from the prodn. and use of hazardous chems. Yet, for over a decade now, the Principles have been applied to the synthesis and prodn. of engineered nanomaterials (ENMs) and the products they enable. While the combined efforts of the global scientific community have led to promising advances in the field of nanotechnol., there remain significant research gaps and the opportunity to leverage the potential global economic, societal and environmental benefits of ENMs safely and sustainably. As such, this tutorial review benchmarks the successes to date and identifies crit. research gaps to be considered as future opportunities for the community to address. A sustainable material design framework is proposed that emphasizes the importance of establishing structure-property-function (SPF) and structure-property-hazard (SPH) relationships to guide the rational design of ENMs. The goal is to achieve or exceed the functional performance of current materials and the technologies they enable, while minimizing inherent hazard to avoid risk to human health and the environment at all stages of the life cycle.
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    29. 29
      Erythropel, H. C.; Zimmerman, J. B.; de Winter, T. M.; Petitjean, L.; Melnikov, F.; Lam, C. H.; Lounsbury, A. W.; Mellor, K. E.; Janković, N. Z.; Tu, Q.; Pincus, L. N.; Falinski, M. M.; Shi, W.; Coish, P.; Plata, D. L.; Anastas, P. T. The Green ChemisTREE: 20 Years after Taking Root with the 12 Principles. Green Chem 2018, 20, 1929– 1961,  DOI: 10.1039/C8GC00482J
      29
      The Green ChemisTREE: 20 years after taking root with the 12 principles
      Erythropel, Hanno C.; Zimmerman, Julie B.; de Winter, Tamara M.; Petitjean, Laurene; Melnikov, Fjodor; Lam, Chun Ho; Lounsbury, Amanda W.; Mellor, Karolina E.; Jankovic, Nina Z.; Tu, Qingshi; Pincus, Lauren N.; Falinski, Mark M.; Shi, Wenbo; Coish, Philip; Plata, Desiree L.; Anastas, Paul T.
      Green Chemistry (2018), 20 (9), 1929-1961CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      The field of Green Chem. has seen many scientific discoveries and inventions during the 20 years since the 12 Principles were first published. Inspired by tree diagrams that illustrate diversity of products stemming from raw materials, we present here the Green ChemisTREE as a showcase for the diversity of research and achievements stemming from Green Chem. Each branch of the Green ChemisTREE represents one of the 12 Principles, and the leaves represent areas of inquiry and development relevant to that Principle (branch). As such, in this 'meta-review', we aim to describe the history and current status of the field of Green Chem.: by exploring activity within each Principle, by summarizing the benefits of Green Chem. through robust examples, by discussing tools and metrics available to measure progress towards Green Chem., and by outlining knowledge gaps, opportunities, and future challenges for the field.
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    30. 30
      Dou, J.; Tang, Y.; Nie, L.; Andolina, C. M.; Zhang, X.; House, S.; Li, Y.; Yang, J.; Tao, F. F. Complete Oxidation of Methane on Co3O4/CeO2 Nanocomposite: A Synergic Effect. Catal. Today 2018, 311, 48– 55,  DOI: 10.1016/j.cattod.2017.12.027
      30
      Complete Oxidation of Methane on Co3O4/CeO2 Nanocomposite: A Synergic Effect
      Dou, Jian; Tang, Yu; Nie, Longhui; Andolina, Christopher M.; Zhang, Xiaoyan; House, Stephen; Li, Yuting; Yang, Judith; Tao, Franklin
      Catalysis Today (2018), 311 (), 48-55CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      Development of nonprecious metal-based catalysts highly active for complete oxidn. of CH4 at a temp. ≤600°C is significant for removing unburned CH4 at exhaust of engines of vehicles using natural gas or liquefied petroleum gas. CeO2 is active for complete oxidn. of CH4. Co3O4 has been identified as a promising catalyst for this reaction. A Co3O4/CeO2 nanocomposite catalyst consisting of ceria nanorods supported on Co3O4 nanoparticles was prepd. through a modified deposition pptn. method. The Co3O4/CeO2 nanocomposite exhibits high activity for complete oxidn. of methane with an apparent activation energy of 43.9kJ/mol, which is obviously lower than 95.1kJ/mol of pure CeO2 and 89.7kJ/mol of pure Co3O4, suggesting a synergetic effect between Co3O4 and CeO2. Surface of the Co3O4/CeO2 nanocomposite during complete oxidn. of CH4 at of 200-500°C and potential stable intermediate of this catalysis were identified with ambient pressure XPS (AP-XPS).
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    31. 31
      Lim, T. H.; Cho, S. J.; Yang, H. S.; Engelhard, M. H.; Kim, D. H. Effect of Co/Ni Ratios in Cobalt Nickel Mixed Oxide Catalysts on Methane Combustion. Appl. Catal. A Gen. 2015, 505, 62– 69,  DOI: 10.1016/j.apcata.2015.07.040
      31
      Effect of Co/Ni ratios in cobalt nickel mixed oxide catalysts on methane combustion
      Lim, Tae Hwan; Cho, Sung June; Yang, Hee Sung; Engelhard, M. H.; Kim, Do Heui
      Applied Catalysis, A: General (2015), 505 (), 62-69CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
      A series of cobalt nickel mixed oxide catalysts with the varying ratios of Co to Ni, prepd. by co-pptn. method, were applied to methane combustion. Among the various ratios, cobalt nickel mixed oxides having the ratios of Co to Ni of (50:50) and (67:33) demonstrate the highest activity for methane combustion. Structural anal. obtained from X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) evidently demonstrates that CoNi (50:50) and (67:33) samples consist of NiCo2O4 and NiO phase and, more importantly, NiCo2O4 spinel structure is largely distorted, which is attributed to the insertion of Ni2+ ions into octahedral sites in Co3O4 spinel structure. Such structural disorder results in the enhanced portion of surface oxygen species, thus leading to the improved reducibility of the catalysts in the low temp. region as evidenced by temp. programmed redn. by hydrogen (H2 TPR) and XPS O 1s results. They prove that structural disorder in cobalt nickel mixed oxides enhances the catalytic performance for methane combustion. Thus, it is concluded that a strong relationship between structural property and activity in cobalt nickel mixed oxide for methane combustion exists and, more importantly, distorted NiCo2O4 spinel structure is found to be an active site for methane combustion.
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    32. 32
      Tao, F. F.; Shan, J.-j.; Nguyen, L.; Wang, Z.; Zhang, S.; Zhang, L.; Wu, Z.; Huang, W.; Zeng, S.; Hu, P. Understanding Complete Oxidation of Methane on Spinel Oxides at a Molecular Level. Nat. Commun. 2015, 6, 7798,  DOI: 10.1038/ncomms8798
      32
      Understanding complete oxidation of methane on spinel oxides at a molecular level
      Tao, Franklin Feng; Shan, Jun-jun; Nguyen, Luan; Wang, Ziyun; Zhang, Shiran; Zhang, Li; Wu, Zili; Huang, Weixin; Zeng, Shibi; Hu, P.
      Nature Communications (2015), 6 (), 7798CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      It is crucial to develop a catalyst made of earth-abundant elements highly active for a complete oxidn. of methane at a relatively low temp. NiCo2O4 consisting of earth-abundant elements which can completely oxidize methane in the temp. range of 350-550 °C. Being a cost-effective catalyst, NiCo2O4 exhibits activity higher than precious-metal-based catalysts. Here we report that the higher catalytic activity at the relatively low temp. results from the integration of nickel cations, cobalt cations and surface lattice oxygen atoms/oxygen vacancies at the at. scale. In situ studies of complete oxidn. of methane on NiCo2O4 and theor. simulations show that methane dissocs. to Me on nickel cations and then couple with surface lattice oxygen atoms to form -CH3O with a following dehydrogenation to -CH2O; a following oxidative dehydrogenation forms CHO; CHO is transformed to product mols. through two different sub-pathways including dehydrogenation of OCHO and CO oxidn.
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    33. 33
      Wu, M.; Li, W.; Zhang, X.; Xue, F.; Yang, T.; Yuan, L. Penta-coordinated Al3+ Stabilized Defect-Rich Ceria on Al2O3 Supported Palladium Catalysts for Lean Methane Oxidation. ChemCatChem 2021, 13, 3490– 3500,  DOI: 10.1002/cctc.202100668
      33
      Penta-coordinated Al3+ Stabilized Defect-Rich Ceria on Al2O3 Supported Palladium Catalysts for Lean Methane Oxidation
      Wu, Mingwei; Li, Wenzhi; Zhang, Xia; Xue, Fengyang; Yang, Tao; Yuan, Liang
      ChemCatChem (2021), 13 (15), 3490-3500CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In heterogeneous catalysis, the strong interaction between metal and support can significantly modulate the electronic properties of the metals and play a crucial role in metal particle dispersion and morphol. Herein, a facile strategy was utilized to fabricate highly dispersive palladium catalysts supported on defective Al2O3-CeO2 for lean methane oxidn. Ceria was immobilized on the coordinatively unsatd. Al3+penta sites of γ-alumina activated by pre-redn. to fabricate hybrid-oxide support (abbreviated as RAl2O3-CeO2), and then Pd precursor was uniformly deposited on the defective surface. Such a RAl2O3-CeO2 interface can effectively upgrade the dispersion of deposited palladium species and improve the concn. of reactive oxygen species owing to strong electronic interactions, invoking a superior catalytic activity for lean methane oxidn. After loading 1.0 wt% palladium, Pd/RAl2O3-CeO2 exhibited a 90% methane conversion at 328°C under the space velocity of 60,000 mL g-1 h-1, far lower than that of bare 1.0 wt% Pd/RAl2O3(400°C). In addn., Pd/RAl2O3-CeO2 catalyst showed an excellent hydrothermal stability under the harsh conditions (600°C, water vapor)for 120 h without obvious deactivation. The reaction pathway of total methane combustion was elucidated by in situ DRIFTS. The crucial intermediate compns. (carbon oxygenates and carbonate)on Pd/RAl2O3-CeO2 surface are more readily oxidized into CO2 and H2O. This work provides an effective interface-promoted strategy to develop efficient and durable palladium catalysts for many challenging reactions.
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    34. 34
      Cao, P.; Yan, B.; Chu, Y.; Wang, S.; Yu, H.; Li, T.; Xiong, C.; Yin, H. Synthesis of Highly Dispersed Palladium Nanoparticles Supported on Silica for Catalytic Combustion of Methane. Ind. Eng. Chem. Res. 2021, 60, 7545– 7557,  DOI: 10.1021/acs.iecr.1c00175
      34
      Synthesis of Highly Dispersed Palladium Nanoparticles Supported on Silica for Catalytic Combustion of Methane
      Cao, Peng; Yan, Bo; Chu, Yuting; Wang, Shiwei; Yu, Hongbo; Li, Tong; Xiong, Chunrong; Yin, Hongfeng
      Industrial & Engineering Chemistry Research (2021), 60 (20), 7545-7557CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      Pd nanoparticles supported on SiO2 catalysts (Pd/SiO2) were prepd. by wet impregnation (WI), dry impregnation (DI), strong electrostatic adsorption (SEA), and charge-enhanced dry impregnation (CEDI) methods. The Pd/SiO2 samples with highly dispersed and tight size-distributed Pd nanoparticles are obtained via SEA and CEDI methods based on strong electrostatic interactions between the dissolved metal precursor ([Pd(NH3)4]2+) and pos. charged SiO2 support in an alkali-impregnating soln. (initial pH = 12). The Pd/SiO2-SEA samples prepd. by the SEA method usually showed higher Pd dispersions (>50%) than those prepd. by CEDI (Pd dispersion = 32-45%). The surface loading (support surface area per L of prepn. soln.), pH regulator (NaOH or NH4OH), Pd loading, and redn. temp. are key factors affecting the dispersion of Pd in the Pd/SiO2-SEA samples, as well as the leaching/dissoln. of SiO2 and Pd in the alkali soln. The Pd/SiO2-SEA samples prepd. with proper SLs of 30,000-100,000 m2 L-1 using NH4OH as the pH regulator exhibited not only very high Pd dispersions (64-97%) but also negligible losses of SiO2 and Pd in the impregnating soln. The Pd/SiO2-SEA samples also exhibited better catalytic performance in methane combustion based on both the T10 and T50 temps. and the intrinsic activities (mass-specific activity and/or turnover frequency (TOFs)). The TOFs generally decreased from 130 h-1to 6.2 h-1 as Pd dispersion increased from 32% to 97% for the Pd/SiO2-SEA(NH4OH) catalysts. Also, the reaction activity of Pd/SiO2-SEA catalysts was significantly improved by increasing the fraction of Pd0 at 70-85%, indicating that this size-sensitive catalysis would be related to the redox properties of the supported Pd nanoparticles.
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    35. 35
      Li, K.; Xu, D.; Liu, K.; Ni, H.; Shen, F.; Chen, T.; Guan, B.; Zhan, R.; Huang, Z.; Lin, H. Catalytic Combustion of Lean Methane Assisted by an Electric Field over Mn XCo y Catalysts at Low Temperature. J. Phys. Chem. C 2019, 123, 10377– 10388,  DOI: 10.1021/acs.jpcc.9b00496
      35
      Catalytic Combustion of Lean Methane Assisted by an Electric Field over MnxCoy Catalysts at Low Temperature
      Li, Ke; Xu, Dejun; Liu, Ke; Ni, Hong; Shen, Feixiang; Chen, Ting; Guan, Bin; Zhan, Reggie; Huang, Zhen; Lin, He
      Journal of Physical Chemistry C (2019), 123 (16), 10377-10388CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      An elec. field was introduced into the catalytic oxidn. of lean methane at low temp. over MnxCoy catalysts. Mn1Co5 exhibited the best catalytic performance with the light off temp. (T50) as low as 271 °C in the elec. field, nearly 60 °C lower than that in a conventional reaction system. The elec. field promoted the formation of octahedrally coordinated Mn3+ with active oxygen species released from the redn. of octahedrally coordinated Co3+ in Co3O4 spinel. Also, octahedrally coordinated Mn3+ sites were proven to be the main active sites for methane catalytic oxidn. With an in situ FTIR technique, it was found that the oxygen species from the catalyst bulk instead of gaseous oxygen will adsorb on the octahedrally coordinated Mn3+ sites in the elec. field, promoting the activation of CH4 at low temp. The dehydroxylation process will be accelerated through the formation of CoO(OH) species that will quickly convert due to the enhanced reducibility of Co3+ in the elec. field, weakening the inhibition of produced hydroxyl species on active sites. Based on the exptl. results, the mechanism of catalytic oxidn. of CH4 over MnxCoy catalysts in an elec. field was proposed.
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    36. 36
      Auvinen, P.; Kinnunen, N. M.; Hirvi, J. T.; Maunula, T.; Kallinen, K.; Keenan, M.; Baert, R.; van den Tillaart, E.; Suvanto, M. Development of a Rapid Ageing Technique for Modern Methane Combustion Catalysts in the Laboratory: Why Does SO2 Concentration Play an Essential Role?. Appl. Catal. B Environ. 2019, 258, 117976,  DOI: 10.1016/j.apcatb.2019.117976
      36
      Development of a rapid ageing technique for modern methane combustion catalysts in the laboratory: Why does SO2 concentration play an essential role?
      Auvinen, Paavo; Kinnunen, Niko M.; Hirvi, Janne T.; Maunula, Teuvo; Kallinen, Kauko; Keenan, Matthew; Baert, Rik; van den Tillaart, Erik; Suvanto, Mika
      Applied Catalysis, B: Environmental (2019), 258 (), 117976CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      In pursuance to decrease emissions of transportation sector, the durability and longevity of modern catalysts has become a topical issue. Understanding the deactivation of catalysts is esp. urgent. Our aim in this study was to clarify the roles of reaction temp. and SO2 concn. in methane combustion catalyst poisoning. Information about this process can help the research community perform more realistic lab. simulations and provide new insights for catalyst development. With the collected exptl. data, the likelihood that poisoning would be influenced by different sulfur species and mechanisms was evaluated. Our results suggested that both reaction temp. and SO2 concn. influenced the stability of the resulting sulfates. Low SO2 concns. lead to formation of stabler sulfates and lower total amt. of sulfur in the catalyst. In turn, high SO2 concns. formed less stable sulfates but accumulated more sulfur. Lesser stability was attributed to formation of Al2(SO4)3 through spillover.
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    37. 37
      Losch, P.; Huang, W.; Vozniuk, O.; Goodman, E. D.; Schmidt, W.; Cargnello, M. Modular Pd/Zeolite Composites Demonstrating the Key Role of Support Hydrophobic/Hydrophilic Character in Methane Catalytic Combustion. ACS Catal 2019, 9, 4742– 4753,  DOI: 10.1021/acscatal.9b00596
      37
      Modular Pd/Zeolite Composites Demonstrating the Key Role of Support Hydrophobic/Hydrophilic Character in Methane Catalytic Combustion
      Losch, Pit; Huang, Weixin; Vozniuk, Olena; Goodman, Emmett D.; Schmidt, Wolfgang; Cargnello, Matteo
      ACS Catalysis (2019), 9 (6), 4742-4753CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Complete catalytic oxidn. of methane in the presence of steam at low temps. (T < 400 °C) is a crucial reaction for emission control, yet it presents profound challenges. The activation of the strong C-H bond of methane at low temp. is difficult, and the water present in any realistic application poisons the active surface and promotes sintering of Pd particles during the reaction. Finding materials that can deliver high reaction rates while being more resistant to the presence of water is imperative for advancing several technol. applications of natural gas-based systems. However, methods to fairly compare the activity of Pd catalysts (the most active metal for methane combustion) are needed in order to perform useful structure-property relationship studies. Here, we report a method to study how zeolite hydrophobicity affects the activity of Pd nanoparticles in the reaction, which led to a significant improvement in the water resistance. Mesoporous zeolites were synthesized starting from com. available microporous zeolites. In this way, a variety of hierarchically porous zeolites, with different hydrophobic/hydrophilic character, were prepd. Preformed colloidal Pd nanoparticles could be deposited within mesostructured zeolites. This approach enabled the systematic study of key parameters such as zeolite framework, Al content, and the Pd loading while maintaining the same Pd particle size and structure for all the samples. Detailed catalytic studies revealed an optimum hydrophobic/hydrophilic character, and a promising steam-resistant catalyst, namely, 3.2 nm Pd particles supported on mesoporous zeolite beta or USY with a Si/Al ratio of 40, emerged from this multiparametric study with a T50 of 355 °C and T90 of 375 °C (where T50 and T90 are temp. values at which the samples reach 50% and 90% methane conversion, resp.) in steam-contg. reaction conditions. Finally, we verified that the designed catalysts were stable by in-depth postcatalysis characterization and operando diffuse-reflectance IR Fourier-transform spectroscopy (DRIFTS) analyses confirming that water adsorbs less strongly on the active PdO surface due to interaction with the zeolite acid sites. This method can be of general use to study how zeolite supports affect the reactivity of supported metals in several catalytic applications.
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    38. 38
      Kinnunen, N. M.; Hirvi, J. T.; Kallinen, K.; Maunula, T.; Keenan, M.; Suvanto, M. Case Study of a Modern Lean-Burn Methane Combustion Catalyst for Automotive Applications: What Are the Deactivation and Regeneration Mechanisms?. Appl. Catal. B Environ. 2017, 207, 114– 119,  DOI: 10.1016/j.apcatb.2017.02.018
      38
      Case study of a modern lean-burn methane combustion catalyst for automotive applications: What are the deactivation and regeneration mechanisms?
      Kinnunen, Niko M.; Hirvi, Janne T.; Kallinen, Kauko; Maunula, Teuvo; Keenan, Matthew; Suvanto, Mika
      Applied Catalysis, B: Environmental (2017), 207 (), 114-119CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      One way to lower CO2 and other harmful emissions of the transportation sector is the development of natural gas fueled vehicles. Availability of natural gas is good, and it is easy to apply to stoichiometric and lean-burn engines, which makes it ready-to-use technol. The main concern in the field is a sulfur poisoning of the exhaust gas after treatment system. We aim to clarify mechanisms of sulfur poisoning and regeneration of a lean-burn methane oxidn. catalyst. Overall, sulfur itself is not the only reason for the deactivation of methane oxidn. catalyst, but it is a joint effect of water vapor and sulfur species. The irreversible sulfur poisoning deteriorates oxygen mobility and hinders water desorption, which inhibits low temp. methane oxidn. activity. The regeneration of sulfur poisoned catalyst takes place stepwise: PdSO4 → PdSO3 + 0.5O2 → Pd + SO2 + 0.5O2. The formation of metallic palladium makes the catalyst vulnerable for sintering, which leads to deactivation during long-term regeneration.
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    39. 39
      O’Connor, M. P.; Coulthard, R. M.; Plata, D. L. Electrochemical Deposition for the Separation and Recovery of Metals Using Carbon Nanotube-Enabled Filters. Environ. Sci. Water Res. Technol. 2018, 4, 58– 66,  DOI: 10.1039/C7EW00187H
      39
      Electrochemical deposition for the separation and recovery of metals using carbon nanotube-enabled filters
      O'Connor, Megan P.; Coulthard, Riley M.; Plata, Desiree L.
      Environmental Science: Water Research & Technology (2018), 4 (1), 58-66CODEN: ESWRAR; ISSN:2053-1419. (Royal Society of Chemistry)
      Rare earth and specialty elements (RESE) are functionally integral to several clean energy technologies, but there is no domestic source of virgin RESE in the United States. Manufg. waste streams, which are relatively simple compositionally, and electronic wastes, which are chem. complex, could both serve as viable sources of secondary RESE if efficient methods existed to recover and sep. these metals for reuse. Leveraging differences in RESE redn. potentials, high surface area, high cond. carbon nanotubes (CNTs) could enable space- and solvent-efficient, selective recovery of RESE from mixed metal wastes. In this study, unaligned CNTs encapsulated in polyvinyl alc. were used to develop an electrochem. active filter and tested for recovery of six metals or metalloids (Cu, As, Eu, Nd, Ga, and Sc) as a function of flow rate (1-5 mL min-1), pH (2-10), and voltage (0.1-3.0 V), with max. recoveries of 86-96%, except for As, which was unretained. All metals were recovered as oxides, rather than their zero valent or reduced forms (except for Cu, which was partially reduced at low pHs). Deaeration expts. suggested electrochem. redn. of dissolved O2 and O2 derived from water splitting were jointly responsible for metal capture, where metal oxides were first formed via metal hydroxide intermediates, and this mechanism was enhanced at higher pHs. A synthetic, multi-metal waste stream of Cu and Eu was successfully sepd. on multiple stages with increasing voltages (97 ± 0.1% Cu and 65 ± 0.3% Eu recovery), indicating the approach might be useful for the treatment of electronic end-of-life and manufg. derived wastes.
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    40. 40
      Steiner, S. A.; Baumann, T. F.; Bayer, B. C.; Blume, R.; Worsley, M. A.; MoberlyChan, W. J.; Shaw, E. L.; Schlögl, R.; Hart, A. J.; Hofmann, S.; Wardle, B. L. Nanoscale Zirconia as a Nonmetallic Catalyst for Graphitization of Carbon and Growth of Single- and Multiwall Carbon Nanotubes. J. Am. Chem. Soc. 2009, 131, 12144– 12154,  DOI: 10.1021/ja902913r
      40
      Nanoscale Zirconia as a Nonmetallic Catalyst for Graphitization of Carbon and Growth of Single- and Multiwall Carbon Nanotubes
      Steiner, Stephen A.; Baumann, Theodore F.; Bayer, Bernhard C.; Blume, Raoul; Worsley, Marcus A.; MoberlyChan, Warren J.; Shaw, Elisabeth L.; Schlogl, Robert; Hart, A. John; Hofmann, Stephan; Wardle, Brian L.
      Journal of the American Chemical Society (2009), 131 (34), 12144-12154CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We report that nanoparticulate zirconia (ZrO2) catalyzes both growth of single-wall and multiwall carbon nanotubes (CNTs) by thermal chem. vapor deposition (CVD) and graphitization of solid amorphous carbon. We observe that silica-, silicon nitride-, and alumina-supported zirconia on silicon nucleates single- and multiwall carbon nanotubes upon exposure to hydrocarbons at moderate temps. (750 °C). High-pressure, time-resolved XPS of these substrates during carbon nanotube nucleation and growth shows that the zirconia catalyst neither reduces to a metal nor forms a carbide. Point-localized energy-dispersive X-ray spectroscopy (EDAX) using scanning transmission electron microscopy (STEM) confirms catalyst nanoparticles attached to CNTs are zirconia. We also observe that carbon aerogels prepd. through pyrolysis of a Zr(IV)-contg. resorcinol-formaldehyde polymer aerogel precursor at 800 °C contain fullerenic cage structures absent in undoped carbon aerogels. Zirconia nanoparticles embedded in these carbon aerogels are further obsd. to act as nucleation sites for multiwall carbon nanotube growth upon exposure to hydrocarbons at CVD growth temps. Our study unambiguously demonstrates that a nonmetallic catalyst can catalyze CNT growth by thermal CVD while remaining in an oxidized state and provides new insight into the interactions between nanoparticulate metal oxides and carbon at elevated temps.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXpsFyisLk%253D&md5=5d1d15552ad49f55828963ce2975d786
    41. 41
      Hofmann, S.; Sharma, R.; Ducati, C.; Du, G.; Mattevi, C.; Cepek, C.; Cantoro, M.; Pisana, S.; Parvez, A.; Cervantes-Sodi, F.; Ferrari, A. C.; Dunin-Borkowski, R.; Lizzit, S.; Petaccia, L.; Goldoni, A.; Robertson, J. In Situ Observations of Catalyst Dynamics during Surface-Bound Carbon Nanotube Nucleation. Nano Lett 2007, 7, 602– 608,  DOI: 10.1021/nl0624824
      41
      In situ Observations of Catalyst Dynamics during Surface-Bound Carbon Nanotube Nucleation
      Hofmann, Stephan; Sharma, Renu; Ducati, Caterina; Du, Gaohui; Mattevi, Cecilia; Cepek, Cinzia; Cantoro, Mirco; Pisana, Simone; Parvez, Atlus; Cervantes-Sodi, Felipe; Ferrari, Andrea C.; Dunin-Borkowski, Rafal; Lizzit, Silvano; Petaccia, Luca; Goldoni, Andrea; Robertson, John
      Nano Letters (2007), 7 (3), 602-608CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We present at.-scale, video-rate environmental transmission electron microscopy and in situ time-resolved XPS of surface-bound catalytic chem. vapor deposition of single-walled carbon nanotubes and nanofibers. We observe that transition metal catalyst nanoparticles on SiOx support show cryst. lattice fringe contrast and high deformability before and during nanotube formation. A single-walled carbon nanotube nucleates by lift-off of a carbon cap. Cap stabilization and nanotube growth involve the dynamic reshaping of the catalyst nanocrystal itself. For a carbon nanofiber, the graphene layer stacking is detd. by the successive elongation and contraction of the catalyst nanoparticle at its tip.
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    42. 42
      Kim, S. J.; Kim, S.; Lee, J.; Jo, Y.; Seo, Y. S.; Lee, M.; Lee, Y.; Cho, C. R.; Kim, J. p.; Cheon, M.; Hwang, J.; Kim, Y. I.; Kim, Y. H.; Kim, Y. M.; Soon, A.; Choi, M.; Choi, W. S.; Jeong, S. Y.; Lee, Y. H. Color of Copper/Copper Oxide. Adv. Mater. 2021, 33, 2007345,  DOI: 10.1002/adma.202007345
      42
      Color of Copper/Copper Oxide
      Kim, Su Jae; Kim, Seonghoon; Lee, Jegon; Jo, Yongjae; Seo, Yu-Seong; Lee, Myounghoon; Lee, Yousil; Cho, Chae Ryong; Kim, Jong-pil; Cheon, Miyeon; Hwang, Jungseek; Kim, Yong In; Kim, Young-Hoon; Kim, Young-Min; Soon, Aloysius; Choi, Myunghwan; Choi, Woo Seok; Jeong, Se-Young; Lee, Young Hee
      Advanced Materials (Weinheim, Germany) (2021), 33 (15), 2007345CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Stochastic inhomogeneous oxidn. is an inherent characteristic of copper (Cu), often hindering color tuning and bandgap engineering of oxides. Coherent control of the interface between metal and metal oxide remains unresolved. Coherent propagation of an oxidn. front in single-crystal Cu thin film is demonstrated to achieve a full-color spectrum for Cu by precisely controlling its oxide-layer thickness. Grain-boundary-free and atomically flat films prepd. by at.-sputtering epitaxy allow tailoring of the oxide layer with an abrupt interface via heat treatment with a suppressed temp. gradient. Color tuning of nearly full-color red/green/blue indexes is realized by precise control of the oxide-layer thickness; the samples cover ≈50.4% of the std. red/green/blue color space. The color of copper/copper oxide is realized by the reconstruction of the quant. yield color from the oxide "pigment" (complex dielec. functions of Cu2O) and light-layer interference (reflectance spectra obtained from the Fresnel equations) to produce structural color. Furthermore, laser-oxide lithog. is demonstrated with micrometer-scale linewidth and depth through local phase transformation to oxides embedded in the metal, providing spacing necessary for semiconducting transport and optoelectronics functionality.
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    43. 43
      Langford, J. I.; Wilson, A. J. C. Scherrer after Sixty Years: A Survey and Some New Results in the Determination of Crystallite Size. J. Appl. Crystallogr. 1978, 11, 102– 113,  DOI: 10.1107/S0021889878012844
      43
      Scherrer after sixty years: a survey and some new results in the determination of crystallite size
      Langford, J. I.; Wilson, A. J. C.
      Journal of Applied Crystallography (1978), 11 (2), 102-13CODEN: JACGAR; ISSN:0021-8898.
      The interpretation of the broadening arising from small crystallites is summarized. Studies of the half-width as a measure of breadth were completed Scherrer consts. of simple regular shapes were detd. for all low-angle reflections (h2 + k2 + l2 ≤ 100) for 4 measures of breadth. The systematic variation of Scherrer consts. with hkl is discussed, and a convenient representation in the form of contour maps is applied to simple shapes. The relation between the apparent and true size is considered for crystallites with the same shape. If they are of the same size, then the normal Scherrer const. applies, but if there is a distribution of sizes, a modified Scherrer const. must be used.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE1cXhvVSlsLo%253D&md5=f4237494ce4ba498cdff2598abe95c13
    44. 44
      Fernández, J.; Marín, P.; Díez, F. V.; Ordóñez, S. Combustion of Coal Mine Ventilation Air Methane in a Regenerative Combustor with Integrated Adsorption: Reactor Design and Optimization. Appl. Therm. Eng. 2016, 102, 167– 175,  DOI: 10.1016/j.applthermaleng.2016.03.171
      44
      Combustion of coal mine ventilation air methane in a regenerative combustor with integrated adsorption: Reactor design and optimization
      Fernandez, Javier; Marin, Pablo; Diez, Fernando V.; Ordonez, Salvador
      Applied Thermal Engineering (2016), 102 (), 167-175CODEN: ATENFT; ISSN:1359-4311. (Elsevier Ltd.)
      Coal mine ventilation air methane is an important environmental concern due to its contribution to global warming. Catalytic combustion in reverse flow reactors is an efficient treatment technique, but high emission moistures lead to catalyst inhibition. To overcome this issue a novel reverse flow reactor with integrated water adsorption has been proposed. In this work, the design of a reverse flow reactor adequate to treat a typical real coal ventilation stream, 45 m3/s with 0.30% (mol) methane and 5% (mol) water, has been studied. The performance of the reactor design has been simulated using a 1D heterogeneous dynamic model, previously validated with exptl. results. Particular attention has been paid to reactor stability when water and methane feed concn. change upon time. Real coal mine ventilation air data have been used to produce realistic simulations. The optimization of the operating conditions (surface velocity and switching time) has been carried out based on the total cost of the reactor (considering fixed capital and 10-yr variable cost).
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XlvV2gu7o%253D&md5=1ed77232d0dc20d9ffcd86b024aefb68
    45. 45
      Zhou, F.-x.; Zhao, J.-t.; Zhang, L.; Wu, Z.-w.; Wang, J.-g.; Fang, Y.-t.; Qin, Z.-f. Catalytic Deoxygenating Characteristics of Oxygen-Bearing Coal Mine Methane in the Fluidized Bed Reactor. J. Fuel Chem. Technol. 2013, 41, 523– 529,  DOI: 10.1016/s1872-5813(13)60028-6
      There is no corresponding record for this reference.
    46. 46
      Lan, B.; Li, Y.-R. Numerical Study on Thermal Oxidation of Lean Coal Mine Methane in a Thermal Flow-Reversal Reactor. Chem. Eng. J. 2018, 351, 922– 929,  DOI: 10.1016/j.cej.2018.06.153
      46
      Numerical study on thermal oxidation of lean coal mine methane in a thermal flow-reversal reactor
      Lan, Bo; Li, You-Rong
      Chemical Engineering Journal (Amsterdam, Netherlands) (2018), 351 (), 922-929CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      This paper presents a three-dimensional numerical investigation on thermal oxidn. of lean coal mine methane in a thermal flow-reversal reactor by the finite vol. method. The effects of channel length, feed methane concn., inlet velocity and cycle time on the reactor behavior were analyzed. Results show that the temp. distributions and methane concn. profiles in the reverse-flow semicycle are mirror images of the ones in the forward-flow semicycle. Thus the cycle for the cyclic steady state is sym. The max. temp. of the reactor rises significantly with the increases of methane concn. and inlet velocity, and it is nearly unchanged with the increases of the channel length and cycle time. Long channel length, high feed methane concn., low inlet velocity and short cycle time could achieve a wider high temp. zone in the reactor. The min. feed methane concn. for self-maintained running rises dramatically with the decrease of channel length and the increase of inlet velocity, and it is almost not affected by the change in cycle time. For a desired min. feed methane concn. of 0.18 vol.% when vin = 1 m/s, the required channel length should not be less than 1.8 m.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXht1Gms7bF&md5=f67f07e4f710ff0212ed7d7d5f40716f
    47. 47
      Marín, P.; Vega, A.; Díez, F. V.; Ordóñez, S. Control of Regenerative Catalytic Oxidizers Used in Coal Mine Ventilation Air Methane Exploitation. Process Saf. Environ. Prot. 2020, 134, 333– 342,  DOI: 10.1016/j.psep.2019.12.011
      47
      Control of regenerative catalytic oxidizers used in coal mine ventilation air methane exploitation
      Marin, Pablo; Vega, Aurelio; Diez, Fernando V.; Ordonez, Salvador
      Process Safety and Environmental Protection (2020), 134 (), 333-342CODEN: PSEPEM; ISSN:0957-5820. (Elsevier B.V.)
      Ventilation air methane in coal mining has an important environmental impact, since methane is a strong greenhouse gas (1 kg of methane is equiv. to 28 kg of carbon dioxide). The oxidn. of methane in regenerative oxidizers can be an attractive technique to exploit this resource. Thus, part of the heat released by the reaction can potentially be recovered, in addn. to decreasing methane environmental impact. However, the concn. of methane in the mine ventilation air may change considerably with respect to the oxidizer design value, which have neg. consequences. An increase in concn. can produce overheating (with possible damage to the unit), while a decrease in concn. may cause the extinction of the reaction. In this work, three control systems have been considered in order to deal with these issues: proportional-integral-deriv. (PID) and proportional-integral (PI) feedback controllers, and model predictive controller (MPC). The control action is based on regulating the heat extd. from the oxidizer by adjusting a hot gas purge from the center of the reactor. First, the control systems have been designed (i.e. the tuning parameters of the controller have been calcd.). To carry out the design of the controllers, a simplified dynamic model was obtained from a complex model of the oxidizer. Then, the performance of the controlled oxidizer has been simulated for different types of disturbances. In these simulations, the simple PID controller performed well, and the MPC exhibited the fastest response.
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    48. 48
      Hinde, P.; Mitchell, I.; Riddell, M. COMET TM – A New Ventilation Air Methane (VAM) Abatement Technology. Johnson Matthey Technol. Rev. 2016, 60, 211– 221,  DOI: 10.1595/205651316x692059
      48
      COMET - a new ventilation air methane (VAM) abatement technology: reducing greenhouse gas potential from the mining industry
      Hinde, Peter; Mitchell, Ian; Riddell, Martin
      Johnson Matthey Technology Review (2016), 60 (3), 211-221CODEN: JMTRAP; ISSN:2056-5135. (Johnson Matthey Plc)
      Ventilation air methane (VAM) found in coal mines is a huge and global problem because it acts as a greenhouse gas (GHG) contributing to climate change. Methods for removing this methane and reducing its impact have to date been limited due to a lack of legislative drivers and a technol. focus on reducing the emissions of higher hydrocarbons. Now a new technol., known as COMET, has been developed at Johnson Matthey in collaboration with Anglo Coal for abating this methane emission source. This article describes the development of the catalytic system and its engineering aspects to the point where the technol. is ready for com. launch.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXot1Sgu7Y%253D&md5=df90ffb04ad58434786bd701a048aafc
    49. 49
      Somers, J. Coal Mine Methane Developments in the United States. 31st Annual International Pittsburgh Coal Conference: Coal─Energy, Environment and Sustainable Development, PCC 2014; International Pittsburgh Coal Conference, 2014.
      There is no corresponding record for this reference.
    50. 50
      Robinson, C.; Smith, D. B. The Auto-Ignition Temperature of Methane. J. Hazard. Mater. 1984, 8, 199– 203,  DOI: 10.1016/0304-3894(84)85001-3
      50
      The auto-ignition temperature of methane
      Robinson, C.; Smith, D. B.
      Journal of Hazardous Materials (1984), 8 (3), 199-203CODEN: JHMAD9; ISSN:0304-3894.
      An improved method for measuring auto-ignition temps. of gaseous fuels was developed. Results are reported for CH4 [74-82-8]-air mixts. and compared with previous work. It appears that the normally accepted min. auto-ignition temp. for methane of 537 (or 540°) was theor. rather than exptl. derived. No values with a firm exptl. basis lie below 600°. The min. auto-ignition temp. of 600° reported here is the lowest available. This could be considered as the new std. value.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL2cXhsVWltrs%253D&md5=2e6455e65be1f775cdb9916da2fe87c6
    51. 51
      United States Environmental Protection Agency. US Coalbed Methane Outreach Program (CMOP). US Underground Coal Ventilation Air Methane Exhaust Characterization , 2010.
      There is no corresponding record for this reference.
    52. 52
      Etheridge, D. M.; Steele, L. P.; Francey, R. J.; Langenfelds, R. L. Atmospheric Methane between 1000 A. D. and Present: Evidence of Anthropogenic Emissions and Climatic Variability. J. Geophys. Res. 1998, 103, 15979– 15993,  DOI: 10.1029/98jd00923
      52
      Atmospheric methane between 1000 A.D. and present. Evidence of anthropogenic emissions and climatic variability
      Etheridge, D. M.; Steele, L. P.; Francey, R. J.; Langenfelds, R. L.
      Journal of Geophysical Research, [Atmospheres] (1998), 103 (D13), 15979-15993CODEN: JGRDE3 ISSN:. (American Geophysical Union)
      Atm. methane mixing ratios from 1000 A.D. to present were measured in three Antarctic ice cores, two Greenland ice cores, the Antarctic firn layer, and archived air from Tasmania, Australia. The record is unified by using the same measurement procedure and calibration scale for all samples and by ensuring high age resoln. and accuracy of the ice core and firn air. In this way, methane mixing ratios, growth rates, and interpolar differences are accurately detd. From 1000 to 1800 A.D. the global mean methane mixing ratio averaged 695 ppb and varied about 40 ppb, contemporaneous with climatic variations. Interpolar (N-S) differences varied between 24 and 58 ppb. The industrial period is marked by high methane growth rates from 1945 to 1990, peaking at about 17 ppb yr-1 in 1981 and decreasing significantly since. We calcd. an av. total methane source of 250 Tg yr-1 for 1000-1800 A.D., reaching near stabilization at about 560 Tg yr-1 in the 1980s and 1990s. The isotopic ratio, δ13CH4, measured in the archived air and firn air, increased since 1978 but the rate of increase slowed in the mid-1980s. The combined CH4 and δ13CH4 trends support the stabilization of the total CH4 source.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1cXltFGlt7Y%253D&md5=4b289a7b5e0b116bd97e2a99044d619c
    53. 53
      MacFarling Meure, C.; Etheridge, D.; Trudinger, C.; Steele, P.; Langenfelds, R.; Van Ommen, T.; Smith, A.; Elkins, J. Law Dome CO2, CH4 and N2O Ice Core Records Extended to 2000 Years BP. Geophys. Res. Lett. 2006, 33, 2000– 2003,  DOI: 10.1029/2006GL026152
      There is no corresponding record for this reference.
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      Rubino, M.; Etheridge, D. M.; Thornton, D. P.; Howden, R.; Allison, C. E.; Francey, R. J.; Langenfelds, R. L.; Steele, L. P.; Trudinger, C. M.; Spencer, D. A.; Curran, M. A. J.; van Ommen, T. D.; Smith, A. M. Revised Records of Atmospheric Trace Gases CO2, CH4, N2O, and Delta13C-CO2 over the Last 2000 Years from Law Dome, Antarctica. Earth Syst. Sci. Data 2019, 11, 473– 492,  DOI: 10.5194/essd-11-473-2019
      There is no corresponding record for this reference.
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      United States Environmental Protection Agency. Inventory of US Greenhouse Gas Emissions and Sinks. 2019, pp 1990– 2018. EPA Report. https://www.epa.gov/sites/production/files/2020-04/documents/us-ghg-inventory-2020-main-text.pdf (accessed April 2020).
      There is no corresponding record for this reference.
    56. 56
      BP Statistical Review of World Energy Globally Consistent Data on World Energy Markets; Review of World Energy Data, 2020; Vol. 66.
      There is no corresponding record for this reference.

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