CH4 Activation over Perovskite Catalysts: True Density and Reactivity of Active Sites

Abstract

The high thermal stability of perovskites has drawn attention toward their applications for catalytic CH4 activation and conversion, typically occurring at high temperatures. The reaction rates of perovskite catalysts for CH4 combustion, however, trail behind those of noble metal catalysts. Ways to optimize the performance of perovskite catalysts are destined to trial-and-error approaches unless their complex reconstructed surfaces are correlated with fundamental kinetic parameters. Discerning the intrinsic activity of surface catalytic sites and the density of those sites is crucial to rationally envision complex metal oxides with enhanced catalytic performance. The present work presents a detailed kinetic analysis of catalytic CH4 combustion over a set of seven perovskites (SrTiO3, SrZrO3, SrFeO3, LaFeO3, LaInO3, LaCoO3, LaMnO3) with various surface terminations. Steady-state isotopic transient kinetic analysis was employed to measure turnover frequency (TOF) and density of surface intermediates (N) under operando conditions. Top surface characterization elucidated performance-structure relationships between near-monolayer surface composition and intrinsic reactivity of the catalysts. By using a chemical etching procedure to expose Fe-sites at the top surface of LaFeO3 (LaFeO3,HNO3), its TOF was increased 4-fold, compared with the unmodified sample, although N on the surface of LaFeO3,HNO3 decreased. Density functional theory simulations corroborated that surface Fe-termination and La-Fe termination offer lower energetic barriers for CH4 activation when compared with La-termination. In general, surface reconstruction is shown as a tool to tune TOF and N to improve reaction rates. This work fills a gap in current kinetic studies of perovskites through a careful assessment and discussion of the density and intrinsic reactivity of active sites for methane combustion over well-characterized reconstructed perovskite surfaces.