(569fo) Mitigation of Methane from Low Concentration Sources Using Transition Metal Based Oxidation Catalysts

Methane (CH4), as a non-CO₂ greenhouse gas in the atmosphere, poses a significant challenge to the global objective of limiting Earth's temperature rise to below 2°C. [1]. Despite the fact that methane is being emitted in smaller quantities than CO₂ [2], its ability to trap heat in the atmosphere, known as its "global warming potential," is 28 times greater than that of CO₂ on 100-year timescale [3]. Therefore, by catalytically converting CH₄ to CO₂, it is possible to avoid CO₂ equivalents to the atmosphere [4].

Various methods for methane abatement have been studied, with noble metal-based (Pt/Pd) thermos-catalytic catalytic oxidation standing out as the most investigated technique [5]. However, elevated cost and susceptibility to poisoning have prompted investigations for alternative solutions. Transition metal oxide (TMO) catalysts have emerged as promising alternatives, given their abundance in the Earth's crust, cost-effectiveness, diverse oxidation states, and abundant acidic and basic sites [6]. While many TMO have demonstrated promising catalytic performance, their underlying reaction mechanisms have been relatively underexplored and remain subject to debate. This knowledge gap impedes the further optimization of TMO catalytic systems. Therefore, in this work, efforts are directed towards understanding the reaction mechanisms of TMO catalysts to facilitate their enhanced utilization in catalytic lean methane oxidation. Subsequently, various preparation methods are analyzed to investigate their impact on the catalyst's structure, morphology, and exposed crystal planes. Additionally, the synergistic effects of binary metal oxides mixtures are studied to obtain highly active and stable catalysts capable of accelerating methane combustion at low temperatures.

Methodology

In this study, different TMOs and mixtures thereof were synthesized, namely: Co₃O₄, NiO, CeO₂ using various precursors. Different preparation methods such as coprecipitation, mechanochemical reaction and thermal decomposition were employed to prepare the catalysts, and their effects on catalytic activity are assessed.

X-ray diffraction (XRD) analysis is used to determine the crystal structures of the calcined samples while N₂ physisorption is used to determine the textural properties of the prepared catalysts. The morphology and particle size distribution are assessed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Additionally, the surface oxidation state of the catalysts prior and after the reaction are analyzed via X-ray photoelectron spectroscopy (XPS).

The methane oxidation performance of various TMO catalysts is evaluated in a fixed-bed stainless steel reactor. The temperature ranges from 250 to 600°C, with a feed composition of 1 vol% CH₄ in air at a space velocity of 18000 h^-1 (NTP). The concentration of CH₄ in the inlet and outlet gases is measured using an online flame ionization detector (FID). Catalyst activity is assessed based on CH₄ conversion, calculated as (C_in−C_out)/C_in × 100%, where C_in and C_out represent the methane concentrations at the inlet and outlet, respectively. Activity data is recorded at each measurement point after stabilization.

Preliminary results:

Both bulk Co₃O₄ and 1:5 Co₃O₄-CeO₂ mixture were synthesized using the co-precipitation method. Catalytic performances of catalysts are presented in Figure 1. Co₃O₄ catalysts exhibited superior activity than mixed metal oxides in the whole temperature range: the onset of methane conversion is observed at a temperature of 225°C, with an approximate conversion of 90% being attained at 370°C. Respectively, for Co₃O₄-CeO₂, the methane conversion started at a temperature of 325°C and reached a conversion rate of 90% at 550°C.

Figure 1. CH₄ conversion (%) over bulk Co3O4 and Co₃O₄-CeO2. (1vol.%-CH4 in the air) (18000 1/h (NTP))

Implication

This study aims to explore catalytic conversion methods for reducing non-CO₂ greenhouse gas emissions, particularly methane (CH₄) at low concentrations and proposes using transition metal oxides as catalysts to convert CH_4. Additionally, the study intends to unravel conversion mechanisms on different transition metals and investigate the effects of synthesis methods on catalytic functions of the different catalysts. These findings hold promise for the development of efficient technology aimed at mitigating non-CO₂ greenhouse gas emissions, particularly from agricultural and waste sectors, which could aid in limiting global warming.

Acknowledgment

The work is part of the project “Removing non-CO2 greenhouse gas emissions to support ambitious climate transitions (REPAIR)” (Project number: 101069905) funded by the European Commission via the European Climate, Infrastructure and Environment Executive Agency (CINEA) within the Horizon Europe framework.

References

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3. Lan, X., K.W. Thoning, and E.J. Dlugokencky: Trends in globally-averaged CH4, N2O, and SF6 determined from NOAA Global Monitoring Laboratory measurements. Version 2024-01, https://doi.org/10.15138/P8XG-AA10.
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