Exploring the bounds of methane catalysis in the context of atmospheric methane removal

Methane, a potent greenhouse gas, is a significant contributor to global warming, with future increases in its abundance potentially leading to an increase of more than 1∘C by 2050 beyond other greenhouse gases if left unaddressed. To remain within the crucial target of limiting global warming to 1.5 ∘C, it is imperative to evaluate the potential of methane removal techniques. This study presents a scoping analysis of different catalytic technologies (thermal, photochemical and electrochemical) and materials to evaluate potential limitations and energy requirements. An analysis of mass transport and reaction rates is conducted for atmospheric methane conversion system configurations. For the vast majority of catalytic technologies, the reaction rates limit the conversion which motivates future efforts for catalyst development. An analysis of energy requirements for atmospheric methane conversion shows minimum energy configurations for various catalytic technologies within classic tube or parallel plate architectures that have analogs to ventilation and industrial fins. Methane concentrations ranging from 2 ppm (ambient) to 1000 ppm (sources, such as wetlands, fossil-fuel extraction sites, landfills etc) are examined. The study finds that electrocatalysis offers the most energy efficient approach (∼0.2 GJ tonne−1 CO2e) for new installations in turbulent ducts, with a total energy intensity < 1 GJ tonne−1 CO2e. Photocatalytic methane removal catalysts are moderately more energy intensive (∼2 GJ tonne−1 CO2e), but could derive much of their energy input from ‘free’ solar energy sources. Thermal systems are shown to be excessively energy intensive ( > 100 GJ tonne−1), while combining photovoltaics with electrochemical catalysts (∼1 GJ tonne−1 CO2e) have comparable energy intensity to photocatalytic methane removal catalysts.


Introduction
Methane is the fastest growing greenhouse gas (GHG) and has approximately 80.8-82.5 (natural, fossil, respectively) times the global warming potential of carbon dioxide over a 20 year time-span [1].A commitment to reduce methane emissions was made by 111 countries subsequent to the COP26 summit, with a reduction target of 30% by 2030.This commitment has initiated growth of emissions abatement and methane removal technologies.Studies have estimated that current technologies can achieve significant methane emissions reductions (∼50%), but there remains a need for more approaches that enable ambient methane removal [2].Methane is released from a wide variety of agricultural, industrial, and natural sources, making its abatement quite challenging.The oil and gas industry, contributing to around 30% of total emissions, could primarily eliminate leaks and off gassing by implementing monitoring programs rather than relying on catalysis.Conversely, a significant and increasing fraction of methane, around 40%, comes from wetlands and other natural sources, and about 39% from farming.These emissions are more difficult to control through leak detection and mitigation [3].
Reducing methane emissions and extracting methane from the atmosphere may both contribute to lowering the concentration of atmospheric methane.One methane abatement method that has been proposed, based on natural analogues [4], is the use of Cl radicals in air.According to models, such an approach would effectively remove methane and may also reduce harmful tropospheric ozone air pollution [5].Jackson et al [6] reported that removing approximately three years' worth of human-caused methane emissions could decrease global warming by 0.21 • C. Zosia et al [7] showed that removing all anthropogenic methane emissions from 2020 would result in not only 1 • C reduction in surface temperature, but air quality improvements that would lead to a reduction of 690 000 premature deaths per year by 2050.The temporary temperature effect of removing one year's worth of human-caused methane emissions would be roughly four times greater than that of removing the equivalent amount of carbon dioxide emissions.The main technological pathways for removing methane are the application of catalytic chemical oxidation (thermal, photochemical, electrochemical), the enhancement of atmospheric sinks, and the conversion of methane using microbes [6,8,9].Herein, we focus on methane conversion by means of thermal, photochemical and electrochemical catalysis.
The challenge for atmospheric methane is that although oxidation releases energy, the energy release is insufficient to sustain a reaction.The methane C-H bond is strong, requiring 439 kJ mol −1 to break [10], but in the presence of oxygen results in an exothermic release of 890 kJ mol −1 for fully oxidized products.However, the enthalpy of oxidation for 100 ppm in air of CH 4 increases the kinetic energy of the gas by only ∼1%, which is insignificant at diluted concentrations, resulting in a ∼3 K temperature increase in the surrounding gas.Thus, a primary constraint in implementing large-scale methane conversion is providing the energy to overcome the activation energy for successive methane oxidation reactions.Heterogeneous catalysts offer the potential to lower the activation energy, by means of providing a surface that adsorbs, converts and releases the methane molecule and its product(s).
The viability of heterogeneous catalyst technologies for methane oxidation depends on a variety of factors, such as rates of reaction, required energy inputs, mass transport to the catalyst surface, cost of implementation, technology longevity etc.To evaluate the potential for heterogeneous catalyst approaches to atmospheric methane oxidation, it is useful to explore fundamental limits, such as mass transfer to surfaces, as well as catalyst activation energies to bound the potential for such approaches.Therefore, our purpose is to explore catalytic rates (e.g.activities, turn over frequencies and limitations) to mass transport in classical engineered configurations for a variety of heterogeneous catalytic approaches.Our objective is to evaluate the potential for methane conversion technologies employed in thermal, electrochemical and photochemical processes.
Figure 1(a) presents the macroscopic descriptors of heterogeneous catalyst technologies of interest herein: mass flux to the surface (j CH4 ), reaction rate (r CH4 ), turn-over frequency (ν TOF ) and activation energy (E a ).These processes are dictated by microscale processes such as adsorption, catalytic oxidation, desorption and Lagrangian regime (grey) that most studies do not report.In particular, adsorption and desorption rates are dictated by binding energies, often depicted in volcano plots [11,12], and play a critical role in catalyst performance per the Sabatier Principle [13].However binding energies, as well as other properties (e.g.pore sizes and surface structure), are not consistently reported in literature.As our purpose is to investigate the viability of methane catalysis across a range of technologies, we have homogenized the range of results commonly reported in literature into the parameters indicated by red arrows crossing the system bounds (dashed black lines).

Methane catalysis
In the context of this work, methane heterogeneous catalysts are considered that increase the oxidation state of the methane molecules through chemical reactions facilitated at material surfaces.Thus, the partial oxidation of methane may result in methanol (CH 3 OH), formaldehyde (CH 2 O), or other oxygenated compounds.Another research focus lies in heterogeneous catalysis aimed at generating reactant species for subsequent gas-phase reactions with methane, such as chlorine [14].As an example the Methane Eradication Photochemical System relies on gas-phase radicals for methane degradation [15] and is not subject to surface rate limitations.While those approaches exhibit promise and are poised for further development, our study considers solely heterogeneous catalysis for direct surface conversion of methane.
Normally, thermo-catalytic methane oxidation requires high reaction temperatures (>300 • C) to occur, where platinum group metals (PGMs), e.g.Pt, Pd and Rh, have proved the most efficient catalysts.However, some transition metal oxides e.g.CuO, V 2 O 5 , TiO 2 have also been used as methane oxidation catalysts, due to their lower cost.
Catalytic processes that operate at low temperatures, such as photocatalysis, electrocatalysis, and photoelectrocatalysis, have been developed to promote reactions under mild conditions.These methods serve as alternatives to traditional thermocatalysis, which requires high temperatures.The most common photocatalysts applied are TiO 2 [16,17] and ZnO [18,19] offering a potential low-temperature route to achieve methane conversion.The efficiency of photocatalysts is known to change with temperature and humidity.Some approaches are only effective at completely dry conditions or for relative humidities less than 5% [20], and thus their application to ambient methane removal is not plausible.While there is promise in the early laboratory results, there remains significant challenges in progressing to higher technology readiness levels.
The potential integration of catalytic methane conversion into diverse contexts is being explored as a means to facilitate in−situ mitigation of methane and reduce emissions.Methane catalysts are available for compressed natural gas vehicles and are being explored for applications in the ventilation infrastructure of coal mines and gas fields, as well as domestic heating systems.

Thermal, electrical and photocatalysis
To enhance the reaction rate at low methane concentrations in thermal catalysis, strategies need to be implemented to reduce the activation energy while maintaining a constant temperature.The turn-over frequencies (TOFs), activation energies, temperature, reactant concentrations, catalyst surface area and stoichiometry of the reaction are typically required in order to determine the reaction rates for catalyst technologies.Data from all known studies on thermal gas-phase methane oxidation have been collected and presented in section 3 for different metals and substrates enabling for direct methane reaction rate comparisons.
Photocatalytic processes involve the use of a photocatalyst to initiate chemical reactions under light irradiation.Photocatalysts are typically semiconducting materials that can absorb light energy and generate electron-hole pairs.These materials can facilitate methane conversion by activating methane molecules under light irradiation and are often paired with PGMs.Direct cleavage of the C-H bond (∼415 kJ mol −1 ) [10] requires photon energies in the UV range (λ < 300 nm), thus guiding the selection of semiconducting materials.The current leading photocatalyst material is Ag @ZnO with 8% quantum yield at wavelengths < 400 nm [18].Photocatalytic reactions can induce reactions to proceed at lower (or room) temperatures.This capability offers several benefits, including reduced energy consumption and the preservation of catalyst stability.Herein, photocatalytic data has been surveyed from all known academic methane sources also shown in section 3.
Electrocatalysts facilitate redox reactions, where electrons are transferred between the reactants.In these reactions, the electron charge (e − ) is transferred between the catalyst and the reacting species.The catalytic sites often have tailored electronic configurations that allow methane adsorbtion and are typically metals or metal oxides.The primary engineering parameters that govern electrochemical catalysis are current (charge transfer rate), voltage (oxidation potential), electrode area (adsorption sites) as well as temperature, methane concentration, and loading among others.
Table 1 presents catalytic methods for extracting methane from the atmosphere.While thermal catalysis is energy-intensive and requires controlled heating environments, it can still find applications in specific internal scenarios, especially within ducting systems.Photochemical catalysis primarily relies on external light sources, while UV light can be applied internally, e.g.inside ducts.Electrocatalysis shows promise for internal applications due to its energy efficiency and versatility.[10], respectively.In thermal catalysis, the activation energy, E a , required is generally higher than that in photocatalysis and electrocatalysis.As is shown, this observation underscores the potential advantages of photocatalytic and electrocatalytic processes, as they may enable more energy-efficient pathways for catalytic reactions.
Well-designed catalyst systems often provide molecular fluxes in excess of the catalytic rate when mass transport is 'cheap' relative to the catalyst 'cost' .More specifically, to maximize the cost-effectiveness of system components, it is essential to employ welloptimized processes that ensure an abundant supply of the more economical component.For instance, in situations where mass transport is economically favorable compared to catalyst cost, the molecular flux to surfaces will exceed the conversion rate, resulting in catalytically rate-limited processes.Here we may consider 'cost' in either energy, carbon intensity or financial terms.To facilitate comparisons of molecular fluxes to molecular conversion, reaction data was compiled from a variety of different methane conversion technologies.Reaction rate information appears in a variety of reported values in literature including TOFs and molar conversion per projected surface area, catalyst surface area and catalyst mass with assumed first and second order reactions (see tables SI1-3 and figures SI1-6).We have homogenized this data to comply with the first order reaction rate equation with respect to methane concentration, as shown in equation (1).
For thermal systems, data related to catalyst and support materials, reaction temperature, metal loading (often PGMs), dispersion, TOFs, activation energies, etc have been classified for different methods of methane conversion (e.g.oxidation, steam reforming) allowing for the final evaluation of the thermal reaction rates.For photochemical systems additional data such as power, light intensity, wavelength, and photoirradiated area have been gathered.Similarly, the voltage, current and electrode area have been extracted from different electrocatalytic reactions and studies for the calculation of the corresponding reaction rates.Raw data on reaction rates were provided for the various studies enabling a direct evaluation (tables SI1-3).While the vast majority of thermocatalytic studies [21][22][23][24][25][26] report reaction rates per catalyst support surface area, here we normalize results per projected area.

Mass transport formulation
The mass transport of gas molecules towards reaction sites occurs at two length scales; firstly via the bulk transport of fluid to the vicinity of the solid interface, and secondly via molecular diffusion within the boundary layer at the interface.The former depends on the fluid embodiment of the system while the latter on the composition of the gas-phase, and surface structure.
Mass transport limitations related to a disconnect in catalyst performance evaluation can be overcome via the use of gas diffusion electrode (GDE) technology [27].This technology involves the use of specialized electrodes designed with a porous structure to enhance the mass transport of gases, e.g.methane, to the electrochemical reaction sites.The GDE innovation ensures improved contact between the reactant and the catalyst, overcoming concerns about the effectiveness of electrocatalysis in low-solubility scenarios [27,28].
Testing systems (e.g.ducts, monoliths and parallel plates) can accommodate thermal, electrical and/or photochemical reactions in realistic fluid geometries and flow regimes, which reflect use cases of proposed catalytic systems.In order to compare methane conversion rates to surface fluxes, we evaluate mass transport of atmospheric-level methane at a gas-solid interface, for realistic flow geometries and conditions.This enables us to provide a framework for evaluating the mass transport energy and material intensity of emerging methane abatement technologies.

Forced convective flow over parallel flat plates and ducts
In forced convection, fluid movement is induced by an external pressure source (e.g fan) to overcome the pressure drop caused by viscous dissipation of energy because of stationary walls.Figure 2 illustrates the mass transport in laminar and turbulent flow as characterized by the Reynolds number Re = ρVd/µ for parallel flat plates and ducts for a given density ρ, velocity V and viscosity µ.Both configurations have been examined, while the calculated rate of mass where Sc = ν/D is the ratio of kinematic viscosity ν to the diffusivity of methane at given temperatures.The Sherwood number for parallel flat plates in the turbulent regime is given by The Sherwood expressions for a duct in laminar and turbulent flow, are described via equations ( 4) and ( 5), respectively.
The molecular flux, j, can be evaluated via the Sherwood numbers obtained from the above expressions, for the different configurations at varying methane concentration (C at 2 ppm, 100 ppm, 1000 ppm) and plate gap thicknesses t or duct diameters d according to equation (6).By evaluating the integral from 0 to d or t, the corresponding molecular fluxes are given as, The length of the ducts is evaluated (see equation (13), SI) as an exponential expression considering an optimal conversion rate at 63%.A higher conversion rate at 99% has been also examined (see figure 10, SI) resulting in diminishing returns of longer piping, and thus higher energy requirements.Further analytical development is provided in the supplementary information (SI).
In only the laminar duct configuration do a few catalyst molecular rates exceed the molecular fluxes at the lower methane concentrations of 2 ppm, indicating rare instances of transport limited kinetics [21,22,26].The empty (unfilled) symbols correspond to reaction rates expressed in terms of catalytic area to show an extreme upper bound of catalyst loading where the entire surface is covered with the active materials, e.g.PGMs, where the reaction rates approach the laminar duct configuration molecular flux at 100 ppm.In all other instances the molecular fluxes vastly exceed the reaction rates.
Photocatalytic methane conversion is shown in figure 4 with reaction rates ranging from 10 −9 to 10 −3 mol (m 2 • s) −1 .The photocatalysts are grouped based on the spectral range of light used (e.g.UV, visible) and there is a positive correlation between rate and light intensity.Higher intensities (⩾10 3 W m −2 ) have often been tested for the visible and solar spectrums, where air mass (AM) 1.5G sunlight corresponds to 1000 W m −2 .The Pt/TiO 2 [16] material, operating with solar spectrum (AM 1.5G), has the highest measured reaction rate (3.4 •10 −4 mol (m 2 • s) −1 ).This rate is higher than the molecular flux at low laminar concentrations (2 and 100 ppm).However, all photocatalysts experiments in literature are tested at high methane concentrations, and almost none of them are for less than 1000 ppm.Similar to thermal catalysts, all configurations except laminar ducts are severely reaction rate limited, indicating that methane conversion systems will benefit from enhanced catalytic rates.Nonetheless, photocatalysts operating in the visible and solar spectrum achieve reasonably high rates (>10 −2 mol (m 2 • s) −1 ) and offer the potential to minimize the operational energy requirements using a 'free' energy source for methane decomposition.
The electrocatalytic methane systems are shown in figure 5 where molecular conversion rates are depicted versus input power densities for a range of applied voltages (groupings).The electrocatalytic systems achieve higher reaction rates (10 −7 -10 −1 mol (m 2 • s) −1 ) than the photocatalytic and thermocatalytic systems.However, many of the measured rates in literature were also tested at elevated temperatures, thus combining effects of thermal and electrocatalysis.While quantum chemistry indicates that  The duct laminar configuration is also evaluated for 100 ppm and 1000 ppm (further details in SI figure 5) [38][39][40][41][42][43].
individual molecular reactions require input energies that exceed a threshold activation, there appears to be no consistent trend between rates and applied voltages as may be obscured by different testing temperatures.The upper bound of electrocatalytic reaction rates is ∼2 orders of magnitude higher than solely photocatalytic or thermal systems.However, the majority of configurations are still reaction rate limited and would benefit from further catalyst improvements.The V 2 O 5 /SnO 2 [38] electrochemical catalyst operating at 373 K achieves a reaction rate of 10 −4 mol (m 2 • s) −1 at 0.79 V and 32 W m −2 and is the highest rate of those tested near ambient temperatures.Although higher reaction rates have been measured [41], they were tested at high reaction temperatures (1073 K) that are not viable for large-scale methane removal.The typical methane concentrations of electrocatalytic experiments in literature are ⩾10 5 ppm [38,[40][41][42][43]. Currently, ambient temperature electrocatalysts have comparable (but lower) rates than photocatalysts which would benefit from further development as increased rates could be readily matched with sufficient molecular fluxes.

Estimation of energy requirements
The central task of this work is to determine whether atmospheric methane removal has any viability at scale and provide insights into potential technologies.Thus, estimations of methane removal energy requirements (figure 6) are assessed for the different catalytic technologies in forced convection technologies with catalysts applied to various ducting configurations, i.e. representative of monoliths, piping or ventilation.The first order energy considerations indicate that a target of <1 GJ tonne −1 CO 2 e may be possible when considering first order contributions of convection energy E conv (energy to move air), the capital energy E cap (energy intensity of bulk materials), and the input energy for the different types of catalysts, i.e. thermal E Thermal , photochemical E Photo and electrochemical E Elec .The representative assessment is made for fixed convection (Re = 1.7•10 5 ), indicative of standard ventilation ducts in turbulent flow with a variety of duct diameters (∼5 mm to 1 m, see SI for Re variation and further analyses).
The required scale of airflow and number of units depends on air methane concentrations, thus E conv and E cap are evaluated at 2 ppm, 100 ppm and 1000 ppm.E cap is the sum of the ducting structure, E structure , considering representative metals, e.g.ducting aluminium (200 MJ kg −1 ) [44] and steel (20 MJ kg −1 ) [45] and catalyst energy, E cat, cap intensities (further details in SI section 4).
The energy intensity for methane removal normalized to CO 2 (GJ tonne −1 CO 2 e) for a 20 year time horizon (GWP20 CH4 = 86) [1] and annual energy requirement for each technology are evaluated for the best currently available catalyst system scaled to removal rates of anthropogenic annual methane emissions (0.36 Gt CH 4 yr −1 ) [6] at current methane concentrations (∼2 ppm).The 20 years GWP is mainly considered allowing for a more immediate response to the climate effects of methane emissions.However, the results are also evaluated for 100 years GWP (GWP100 CH4 = Thermal catalysts had an energy intensity of E Thermal ∼180 GJ tonne −1 CO 2 e (455 GJ tonne −1 CO 2 e GWP100) , where the energy required for the required mass of air is calculated by E Thermal = m air • C p • ∆T.Recycled heat is considered for incoming/outgoing air heat exchange at an efficiency of η ∼ 0.9.The calculated photocatalytic energy intensity was E Photo ∼2 GJ tonne −1 CO 2 e (5 GJ tonne −1 CO 2 e GWP100) as evaluated for a Pt/TiO 2 [16] system exposed to the solar spectrum (1000 W m −2 ).Electrocatalytic systems had the lowest energy intensities at E Elec = 0.2 GJ tonne −1 CO 2 e (0.5 GJ tonne −1 CO 2 e GWP100) as determined for V 2 O 5 /SnO 2 [38] (see figure 5) at measured powers (product of reported current and voltage) [38].The V 2 O 5 /SnO 2 [38] catalyst is selected as the optimal choice among the other catalysts shown in figure 5 where a temperature of 373 K is applied.
Figure 6 shows that the estimated methane removal energy intensity (left y-axis) achieves a minimum when the convective E conv and material capital E cap are balanced (red circle) at ∼0.15 GJ tonne −1 CO 2 e (0.38 GJ tonne −1 CO 2 e GWP100).The equivalence E conv and E cap occurs for a duct diameter of d = 0.17 m, corresponding to individual volumetric flow rate, V i = 0.35 m 3 s −1 (equation (15), SI) which balances the increased convective energy due to pressure drop at small diameters with increased mass transport to the catalyzed walls.Critically, at this optimum E conv and E cap have energy intensities comparable to electrocatalytic intensities E Elec ∼0.15 GJ tonne −1 CO 2 e (0.38 GJ tonne −1 CO 2 e GWP100).
These results suggest that electrocatalytic coatings paired with an optimal forced-flow ducting configuration could result in a total energy intensity <1 GJ tonne −1 CO 2 e as E tot = E cap + E conv + E Elec ≈0.5 GJ tonne −1 CO 2 e (1.26 GJ tonne −1 CO 2 e GWP100).The resulting total energy required for removal of all annual anthropogenic methane emissions (right y-axis) would be of the order ∼ 10 10 GJ yr −1 which represents ∼2% of the total world energy production (∼6•10 11 GJ yr −1 ) [46].Conversely, thermocatalytic technologies are substantially unviable as E Thermal ∼ 10 13 GJ yr −1 is 100 times the global energy production.
Photocatalysts have an intensity of E Photo ∼ 2.1 GJ tonne −1  CO 2 e (5.3 GJ tonne −1 CO 2 e GWP100), but could derive this energy directly from solar irradiation [16,35].Solar irradiation could also be coupled to electrochemical via photovoltaic (PV) systems where electron-hole harvesting has been optimized for decades.Thus, it could be instructive to compare co-optimized PV with electrocatalysis to photocatalysts.PV-electrocatalytic systems would require a solar energy intensity of E Elec-PV ≈ E Elec /η PV ≈1 GJ tonne −1 where current commercial PV is η PV ∼ 0.2 [47].Thus, electrochemical and photochemical have comparable energy intensities when solar powered.Therefore their adoption is likely dictated by other factors such as presence of 'free' forced or natural advection.It is likely that surface coatings on buildings using natural advection can be more easily realized by photocatalysts, whereas internal coatings in ducts may be realized with PV-electrocatalysts. Finally, this study does not delve into the long-term trajectories of various technologies to make informed comments on research paths over decades, however our analysis highlights that electrochemical catalysts, based on the reviewed information, are most promising compared to photocatalysts and thermal catalysts, as depicted in figure 6.
Abernethy et al [48] conducted a study on the viability of methane oxidation technologies, focusing on concentration thresholds and energy efficiency optimization.While their energy intensity metrics, addressing climate and cost neutrality, differ from the ones in the current study, both works highlight the impracticality of heat-based technologies requiring high temperatures for atmospheric methane concentrations.Abernethy et al [48] found that achieving cost neutrality at 2 ppm CH 4 in photocatalytic systems necessitates significant advancements in catalytic efficiency, suggesting that using visible wavelengths of light could alleviate concentration constraints.This aligns with the current study's argument that the energy intensity of photocatalysts could directly come from solar irradiation.

Environmental evolution of oxidized species
It is important to consider any potential environmental impacts of engineering interventions for methane and the timescales involved.It is well known that methane is an important precursor to tropospheric ozone [49] through the reaction of the methyl peroxy radical (CH 3 OO) with nitrogen monoxide (NO) and the subsequent oxidation of CH 3 OO to HO 2 and CO 2 .The lifetime of the CH 3 OO is estimated as only a few seconds, up to a few minutes depending on the concentrations of NO and other reactants [50].Whilst we lack a full understanding of the likely chemical conditions that would occur under all of the different methane catalysis environments discussed above, following the initial (energy intensive) oxidation step for methane (removal of H-atom), the subsequent radical driven oxidation steps through to the conversion to CO 2 would occur on a time scale of seconds-to-days and could have consequences for the generation of radicals in the vicinity of where the methane oxidation is taking place.An exception in the context of observable presence may be attributed to carbon monoxide with a lifespan of approximately two months [51].The main implications, however, for global air quality is that a reduction in methane levels will result in a reduction of ozone levels [7].

Conclusions
To achieve efficient conversion rates in both atmospheric and source methane abatement systems, it is crucial to consider a broader spectrum of environmental and economic factors.Balancing mass transport and reaction rates is integral to optimizing these systems, where the cost and intensity of processes play a pivotal role in determining the rate-limiting steps for the technology.
As in any optimized system, the process or species that is expensive (e.g.high financial, energy, or CO 2 intensity) will determine the rate limiting step for the technology.Thus, processes or components that are 'cheap' will be provided in excess to ensure effective use of the expensive process or component.For methane catalysis our analysis shows that for the vast majority of catalytic technologies, the reaction rates limit conversion.Thus, efforts to improve catalytic rates are warranted and can be readily incorporated into systems with sufficiently high molecular fluxes even at atmospheric methane concentrations.
Mass transport of species to the catalyst surface is typically less energy intensive, especially for optimized convective systems paired with photochemical or electrochemical catalysis.This study suggests that optimizing electrocatalytic systems with specific duct configurations can make methane removal more energy-efficient and cost-effective where technologies may be viable at <1 GJ tonne −1 CO 2 e. Optimized energy intensities were found for advection and ducting material intensity (both ∼0.15 GJ tonne −1 CO 2 e), as well as electrochemical (∼0.2 GJ tonne −1 ), photochemical (∼2 GJ tonne −1 ) and thermal (∼180 GJ tonne −1 ) catalysis.Thus, a total energy intensity for a new installation could be achieved at ∼0.5 GJ tonne −1 .
Further reductions in energy intensities may be occured if a component of either mass transport or energy input is nominally 'free' .For example, in systems where photon energy is provided freely by the Sun, as in photocatalytic surface coatings on buildings, the primary energy intensities will be associated with the installed photocatalytic material energy intensity.Further, natural advection to the built environment can likely be matched to the catalyst conversion rate by means of optimizing catalyst density within the applied photocatalytic surface coatings.Additional analysis is needed to explore the magnitudes of natural advection to the built environment to accurately determine the magnitude of this potential on a global scale.Likewise, in existing buildings with HVAC ducts where advection already occurs, the additional energy requirement will predominantly be for the electrocatalytic chemical conversion.Collectively, these results show configurations of catalytic methane removal that may prove viable, but significant challenges remain.Efficient methane abatement systems must not only balance mass transport and reaction rates but also strategically prioritize cost-effective and environmentally sustainable components, ensuring optimization for energy efficiency, robustness, safety, cost-effectiveness, and scalability to effectively address climate change.

Figure 1 .
Figure 1.(a) Schematic representation of methane catalysis technologies, including bulk flux of methane, (jCH4), reaction rates, (rCH 4 ), turn over frequency, (ν TOF ), energy (E).Further processes shown in grey that are not reported in all literature include adsorption, oxidation processes, desorption and Lagrangian transport processes.The black dashed lines bound our system.(b) Reaction pathways indicating activation energy required for catalyst strategies.

Figure 5 .
Figure 5. Reaction and molecular fluxes as a function of power density in electrocatalytic systems.The molecular fluxes for the different configurations are calculated at 2 ppm.The duct laminar configuration is also evaluated for 100 ppm and 1000 ppm (further details in SI figure5)[38][39][40][41][42][43].

Figure 6 .
Figure 6.Estimated energy requirements at varying duct diameter, considering a 63% conversion rate in turbulent flow.The values given in the left y-axis are scaled by the annual methane emissions, and further considers the 86 times higher global warming potential of methane over CO2, on a 20 year timescale [1].The right hand y-axis represents the Econv that reflects the annual methane anthropogenic emissions.Ecap and Econv are calculated at three different methane concentrations (2 ppm, 100 ppm and 1000 ppm) for aluminium and steel.E Thermal (calculated at 2ppm and 600 K), E Photo and E Elec represent the catalyst input energy of the corresponding optimal catalytic performances.

Table 1 .
Summary table of catalytic methods for extracting methane from atmosphere.Internal applications pertain to utilization within ducting systems where convective flow occurs, whereas external applications involve surfaces exposed to the environment, such as building surfaces.Internal application of photocatalysis is achievable solely through UV light exposure.)provides a conceptual illustration of the activation energy required for catalyst strategies, for the first oxidation step from methane conversion to methanol.The heats of formation for CH 4 , CH 3 and CO 2 , are −74.8kJmol −1 , 145.7 kJ mol −1 and −393.5 kJ mol−1