Mitigating low-concentration methane emissions via catalytic oxidation

Abstract

Methane (CH₄) is a potent greenhouse gas (GHG) with a significantly higher global warming potential (GWP) compared to carbon dioxide (CO₂). Over a century-long period, its GWP is estimated to be around 27-35 times greater than that of CO₂. However, due to its relatively short atmospheric lifetime, its GWP increases substantially when evaluated over a shorter timeframe. For example, its GWP can reach about 84 times that of CO₂ over a 20-year period. In most emissions (including agriculture, wetlands, fossil fuels, etc.), the concentration of CH₄ is below 2500 ppm. However, most existing methane removal technologies are designed for high CH₄ concentrations (greater than 2500 ppm), leaving a significant technology gap in addressing low-concentration emissions. This low concentration necessitates processing a large volume of gases for any potential solution, resulting in 1) significant energy penalty if high temperatures are required; 2) high demand of low-cost catalysts for large scale application Therefore, identifying a methane removal method that combines high efficiency with low temperature and low cost is crucial. Thermocatalytic CH₄ oxidation has been considered as a promising technology for removing low-concentration CH₄ and mitigating its GWP. Compared with other catalytic technologies (e.g., photocatalysis and electrocatalysis), thermocatalysis demonstrates advantages, including high CO₂ conversion efficiencies, limited by-product formation, and rapid reaction kinetics. However, significant research gaps remain. 1) How can we design and prepare efficient catalysts based on cheap materials? Since the initial investigation of thermo-catalytic CH₄ oxidation dates back to 1927, the catalysts developed by the previous studies in this field mostly relied on the noble metal composition including Pd, Pt, and Rh, which are expensive for the practical application. In light of economic considerations, certain transition metal oxides, including perovskites and Co₃O₄, have been explored as alternatives for CH₄ oxidation catalysts. While these alternative options exhibit improved cost-effectiveness compared to palladium-based catalysts, their performance still requires further enhancement. 2) How can we bring down the reaction temperature? Typically, achieving total oxidation of CH₄ to CO₂ through thermos-catalysis necessitates elevated reaction temperatures, such as those exceeding 400 °C. 3) Other gaps include stability and reusability of catalysts and their application for low concentration (i.e., hundreds of ppm) CH₄. This thesis starts with the investigation of the CuOx loaded mordenite zeolite (MOR) catalysts (Chapter 3). Copper is an earth-abundant and cost-effective material, making it an attractive alternative for noble metals. Zeolite materials, known for their high ion exchange capacities, excellent (hydro)thermal stability, adjustable acidity, large specific surface area and porous, have been extensively developed as promising supports for catalytic CH4 oxidation. As a result, Cu loaded zeolite could be promising catalysts for CH₄ oxidation. However, high activation temperatures are still required, which will lead to high energy demand, and low utilization of copper precursor will result in high materials cost. To address this challenge, a simple synthesising process for preparing CuOx loaded mordenite zeolite catalysts was proposed. We modified the acid sites and copper oxidation states by adjusting the pH environment during catalysts preparation. The generation of more Brønsted acid sites which facilitated the absorption and dissociation of CH₄. The presence of Al³⁺ as acid sites in the MOR supports played a crucial role in achieving high CuOx species dispersion, acting as anchoring sites to effectively stabilize and disperse CuOx species, which provides more active sites. Variation in preparation environments (e.g., pH) led to different oxidation states of the catalysts, with alkaline conditions facilitating the deoxidation of CuOx species, resulting in more Cu⁺ & Cu⁰ compared to CuO. The presence of Brønsted acid sites which mitigated coking at low temperatures and prevented the loss of structural stability at high temperatures. Despite this, Cu catalysts exhibit relatively lower catalytic activity at low temperatures, presenting a challenge for their practical use in methane oxidation processes. Therefore, combining the advantages of Pd and Cu is important for more efficient, durable, and low cost CH₄ oxidation. Chapter 4 aims to combine the advantages of Pd and Cu by partially replacing Pd with earth-abundant Cu and preparing Cu-Pd loaded MOR catalysts for more efficient and sustainable CH₄ oxidation. We synthesized low cost and highly dispersed Cu-Pd (Cu: 0.57 wt%; Pd: 0.10 wt%) loaded on MOR, thereby improving the catalytic performance for the thermal oxidation of low-concentration CH₄. The Cu-Pd-MOR catalysts exhibit excellent catalytic properties, including high catalytic activity with nearly 90 % CH₄ total oxidation to CO₂ at 350 °C, low reaction temperature with a T₁₀ at 260 °C and T₅₀ at 317 °C, as well as excellent long-term stability and reusability over a 100-hour reaction period. These characteristics position it as a promising candidate for large-scale CH₄ oxidation applications. To explain the mechanisms behind the improved catalytic performance of Cu-Pd-MOR catalysts, we explored the unique role of Cu loaded from the perspectives of metal dispersion, stability and Cu-Pd interaction. Chapter 5 investigates the influence of zeolite structure, specifically topology and Si/Al ratios, on the thermocatalytic oxidation of low-concentration CH₄. The Si/Al ratio in zeolites influences their surface acid sites, redox properties, metal-support interactions, oxidation states and accessibility of active component, which play crucial roles in CH₄ adsorption and activation. Additionally, variations in zeolite topology, including differences in pore size and connectivity, further impact these catalytic properties. By exploring these structural parameters, this study aims to optimize the catalytic efficiency of the zeolite, making it more effective for CH₄ removal at lower temperatures. This investigation is crucial for designing more efficient and effective catalysts for CH₄ removal in real-world applications. Overall, this thesis presented a detailed investigation about the development of low-cost and efficient thermocatalysts for low-concentration CH₄ oxidation. It is hoped that the design principles of thermocatalysts and the exploration of reaction mechanisms presented in this thesis could inspire future research on low-concentration CH₄ oxidation, contributing to efforts in addressing the challenges of climate change.