Recent progress of catalytic methane combustion over transition metal oxide catalysts

Abstract

Methane (CH4) is one of the cleanest fossil fuel resources and is playing an increasingly indispensable role in our way to carbon neutrality, by providing less carbon-intensive heat and electricity worldwide. On the other hand, the atmospheric concentration of CH4 has raced past 1,900 ppb in 2021, almost triple its pre-industrial levels. As a greenhouse gas at least 86 times as potent as carbon dioxide (CO2) over 20 years, CH4 is becoming a major threat to the global goal of deviating Earth temperature from the +2°C scenario. Consequently, all CH4-powered facilities must be strictly coupled with remediation plans for unburned CH4 in the exhaust to avoid further exacerbating the environmental stress, among which catalytic CH4 combustion (CMC) is one of the most effective strategies to solve this issue. Most current CMC catalysts are noble-metal-based owing to their outstanding C–H bond activation capability, while their high cost and poor thermal stability have driven the search for alternative options, among which transition metal oxide (TMO) catalysts have attracted extensive attention due to their Earth abundance, high thermal stability, variable oxidation states, rich acidic and basic sites, etc. To date, many TMO catalysts have shown comparable catalytic performance with that of noble metals, while their fundamental reaction mechanisms are explored to a much less extent and remain to be controversial, which hinders the further optimization of the TMO catalytic systems. Therefore, in this review, we provide a systematic compilation of the recent research advances in TMO-based CMC reactions, together with their detailed reaction mechanisms. We start with introducing the scientific fundamentals of the CMC reaction itself as well as the unique and desirable features of TMOs applied in CMC, followed by a detailed introduction of four different kinetic reaction models proposed for the reactions. Next, we categorize the TMOs of interests into single and hybrid systems, summarizing their specific morphology characterization, catalytic performance, kinetic properties, with special emphasis on the reaction mechanisms and interfacial properties. Finally, we conclude the review with a summary and outlook on the TMOs for practical CMC applications. In addition, we also further prospect the enormous potentials of TMOs in producing value-added chemicals beyond combustion, such as direct partial oxidation to methanol.

Advancement of modification engineering in lean methane combustion catalysts based on defect chemistry

“…32 They are mainly divided into the Langmuir–Hinshelwood (L–H) and the Eley–Rideal (E–R) mechanism dominated by surface-adsorbed oxygen, the Mars–van Krevelen (M–vK) mechanism dominated by lattice oxygen, and the Two–Term (T–T) mechanism co-participated by surface adsorbed oxygen and lattice oxygen. 64…”

“…62 With the development of characterization techniques and in-depth studies, a clearer understanding of the reaction mechanisms involved in the reaction of CMC has been gained. 63,64 In general, the catalytic reaction mechanism is determined by the catalyst material type (such as noble metal, non-noble metal, single metal oxide, and multimetallic oxide), structure, reaction conditions, and other factors. The four reaction models of CMC currently proposed by researchers are shown in Fig.…”

“…32 They are mainly divided into the Langmuir-Hinshelwood (L-H) and the Eley-Rideal (E-R) mechanism dominated by surface-adsorbed oxygen, the Mars-van Krevelen (M-vK) mechanism dominated by lattice oxygen, and the Two-Term (T-T) mechanism co-participated by surface adsorbed oxygen and lattice oxygen. 64 In the Mv-K mechanism, CH 4 and O 2 are first adsorbed onto the catalyst surface. Then, the adsorbed CH 4 molecules react with the surface lattice oxygen of the catalysts to generate CO 2 and H 2 O.…”

Plasma Regeneration of Spent Pd/Al2O3 Catalysts and Their Electrochemical Performance

“…As a member of the platinum group metals, Pd exhibits a unique 4d 10 5s 0 electron configuration compared to other metals (Ru, Rh, Os, Ir, and Pt), and has been extensively studied due to its high catalytic activity in chemical reactions. Palladium-based catalysts are not only widely used in petroleum cracking, methane catalytic combustion, − CO oxidation, alcohol oxidation, and dehydrogenation of alkanes but also are commonly applied to convert environmental pollution, such as vehicle exhaust and hydrochloro­fluorocarbons into less toxic substances . These applications indicate the important role of palladium catalysts in modern industry .…”

Boosting Methane Combustion over Pd/Y2O3–ZrO2 Catalyst by Inert Silicate Patches Tuning Both Palladium Chemistry and Support Hydrophobicity

“…Typically, lean-burn NGVs emit exhaust gases with low methane concentrations (500–2000 ppm) and temperatures (400–500 °C). − Catalytic combustion is necessary for the abatement of exhaust methane. Under such reaction conditions, 90% methane conversion over transition metal oxide catalysts tends to be reached in the temperature range of 400 to 600 °C. , Palladium-based catalysts supported on high-surface-area Al 2 O 3 , CeO 2 , and ZrO 2 -based oxides are commonly used. − About 10 vol % water vapor in the exhaust severely deactivates palladium (Pd) catalysts at temperatures below 450 °C . To meet increasingly stringent emission regulations, 90% methane conversion over palladium-based after-treatment catalysts is desired at about 400 °C.…”