Particle Size Effects in Stoichiometric Methane Combustion: Structure-Activity Relationship of Pd Catalyst Supported on Gamma-Alumina

It is an urgent desire to shed insight into the structure-activity relationship of catalysts for stoichiometric methane combustion, a very important reaction in energy utilization and environmental governance. Here, we report variedly sized Pd nanoparticles (NPs) (2.1-10.4 nm) on gamma-alumina by a one-step colloid synthesis method for stoichiometric methane combustion. The results of structural analysis based on transmission electron microscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy show that, with increasing size, Pd NPs evolve their shape from irregular to spherical-like structure along with a progressively decreased fraction of Pd corner sites. Moreover, the intrinsic catalytic activity (turnover frequency) of Pd NPs for methane combustion monotonously decreases by about 6-fold when the size of Pd NPs increases from 2.1 to 10.4 nm, suggestive of a protuberant size-dependent activity of Pd catalyst in this reaction. Further confirmed by the potential energy profiles of the rate-determining step in the reaction from density functional theory calculations, the methane molecule is much easier to be activated on corner sites than on the other sites (i.e., edge and terrace sites) over Pd NPs. This work elucidates the origin of size-dependent activity of Pd catalysts via an investigation on their surface structure and could help to engineer highly efficient catalysts for this reaction.