The high thermal stability of perovskites has drawn attention toward their applications for catalytic CH4 activation and conversion; however, 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. In this work, detailed characterization of 7 perovskite structures (SrTiO3, SrZrO3, SrFeO3, LaFeO3, LaInO3, LaCoO3, LaMnO3) showed that surface termination is non-stoichiometric (i.e., A/B≠1 for ABO3 perovskite), and that tuning the composition of the top-surface monolayer greatly influences the density of surface intermediates (N) and turnover frequency (TOF) for CH4 combustion, and in turn, macroscopic reaction rates. The density of adsorption sites for the reaction product, CO2, is suggested as a descriptor for N, whereas the surface composition influences N and TOF. Next, intrigued by the impact of surface termination of perovskites on CH4 combustion, we studied the interplay between reconstructed surface termination and the “intelligent behavior” of doped perovskites, since dopants are typically used to enhance catalytic properties. In doped perovskites (AB1-xMxO3, M=metal dopant), the “intelligent behavior” refers to the ability of the metal dopant to migrate in and out of the perovskite lattice. Using Ni and Cu as the metal dopant in SrTiO3, we found that during CH4 combustion, the diffusion of Ni and Cu into the lattice due to the “intelligent behavior” is accompanied by Sr enrichment on the surface of the perovskite. This Sr-enrichment process is reversible when Cu or Ni species exsolute as clusters/nanoparticles upon H2 treatment. Such a surface reconstruction is found to greatly impact the catalytic activity of doped perovskites towards methane combustion. This presentation highlights the need to fundamentally understand reconstruction of perovskites with and without dopants, in order to finely manipulate the properties of the catalytic sites for methane activation.

Impact of top-surface reconstruction of perovskite catalysts on methane conversion

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