Low temperature methane oxidation without the use of noble metals is a critical challenge for efficient utilization of natural gas. In recent times, spinel oxides have shown promising activity.[1, 2] Methane activation, or more fundamentally the C-H bond activation of methane, is closely related to the intrinsic surface characteristics of the oxides. Depending on the availability of surface oxygen, methane oxidation can either happen via the Langmuir-Hinshelwood mechanism or Mars-van-Krevelen mechanism, or a combination of both. Regardless, the two major factors that govern methane activation are oxygen vacancy formation and hydrogen affinity of the catalyst. While the oxygen vacancy formation energy of spinel oxides is a good descriptor for the reducibility of the metal-oxygen bonds (allowing for creation of oxygen vacancies), hydrogen binding energies over these spinel oxides demonstrate their tendency towards methane binding and subsequently methane activation.[3] Using these two DFT-calculated fundamental descriptors, we investigated the methane oxidation activity for a vast material composition space for spinel oxides. We observed a systematic trend of methane oxidation activity of spinels as a function of simple material properties such as oxidation states, number of ‘d’ electrons in the transition metal, and electronegativity. Prediction of high-performance methane oxidation was corroborated by experiments. Moreover, in the vast range of redox conditions of methane oxidation, spinel oxides can decompose to simple binary oxides and metals (under reducing scenario) and can revert to their original stoichiometric forms (under oxidizing conditions). This contributes to the superior oxygen storage capacity of these spinel oxides. Through ab-initio thermodynamic calculations, we have ranked these spinel oxides in terms of their oxygen release characteristics. Based on the catalysts features related to oxygen vacancy formation, hydrogen affinity, and oxygen storage capacities, we have identified several high-performance spinel oxides for low temperature methane activation.

Methane oxidation over spinels: High-performance material prediction though fundamental material insights

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.