Study of the methane reaction mechanism with a LaNiO3 oxygen carrier in chemical looping reforming

Abstract

This study employs density functional theory (DFT) to investigate the microscopic reaction mechanism of methane on the NiO₂-terminated (001) surface of the perovskite-type oxygen carrier LaNiO₃ in chemical looping reforming, including the processes of CH₄ sequential dehydrogenation, H₂ and CO/CO₂ formation, and the migration of lattice oxygen. And the methane reforming experiment was also conducted. The results demonstrate that under ideal LaNiO₃ surface conditions and low H coverage, CH₄ dehydrogenation occurs through two steps, involving the migration of methyl groups from the Ni top site to the O top site. CH₂ dehydrogenation is the rate-limiting step, with an activation energy of 1.24 eV. H₂ is mainly formed through the migration of H atoms from the O top site to the adjacent Ni top site and their subsequent combination. However, this process is relatively unfavorable in terms of both kinetics and thermodynamics. Preliminary calculations indicate that surface reduction will lower the activation energy barrier for H₂ formation, suggesting that surface reduction is the key to activating H₂ generation. The energy barrier for lattice oxygen migration is relatively low (0.70 eV), resulting in a mismatch between its kinetics and the dehydrogenation of CH₄. This mismatch, combined with the lower energy barrier for CO₂ formation (0.32 eV) compared to the CO desorption energy barrier (1.14 eV), leads to an overoxidation tendency of CO in the initial stage. Experimental results confirm the dominance of CO₂ in the early stage and the low selectivity of CO. The fundamental insights and theoretical framework provided by this study pave the way for future exploration of realistic reaction environments, guiding the rational design of advanced LaNiO₃-based oxygen carriers.