Revealing the Facet Dependence of Carbon Monoxide Oxidation Catalyzed by Pd Nanoparticles via Single-Molecule Nanocatalysis

It is a challenge for ensemble measurements to understand the effect of the nanocatalyst structure on the reactivity and stability. Using single-molecule techniques, the facet-dependent catalytic properties and dynamics of nanocatalyst were revealed for the CO oxidation on individual palladium nanoparticles (Pd NPs) in two types (Pd-100 with high proportion of (100) facets and Pd-111 with low proportion of (100) facets) at the single nanoparticle level. It is discovered that Pd-100 possesses higher intrinsic catalytic activity per site than Pd-111, indicating remarkable activity of Pd-100 for catalyzing the oxidation of CO. Based on the Langmuir–Hinshelwood (L-H) mechanism with competitive adsorption on two adjacent sites, the facet-dependent adsorption properties of substrate molecules on Pd (111) and (100) facets are revealed by a single-molecule fluorescence spectrum. For these two types of Pd NPs, it was found that the catalytic kinetics or heterogeneities in the reaction of CO oxidation could be mainly attributed to the different activity heterogeneity in product formation and the desorption process on different crystal faces. Furthermore, density functional theory calculations confirmed the L-H mechanism, which indicates that the obtained unique crystal-plane-dependent properties of Pd NPs could be attributed to the stronger binding of the reactant on (100) facets than those on (111) facets. Such results provide fundamental insights into the structure–activity relationship of nanocatalysts and guide the design of highly efficient catalysts.