Revealing the surface atomic arrangement of noble metal alkane dehydrogenation catalysts by a stepwise reduction-oxidation approach

Surface characterization of metal nanoparticles is a critical need in nanocatalysis for in-depth understanding of the structure-function relationships. The surface structure of nanoparticles is often different from the subsurface, and it is challenging to separately characterize the surface and the subsurface. In this work, theoretical calculations and extended X-ray absorption fine structure (EXAFS) analysis illustrate that the surface atoms of noble metals (Pt and Pd) are oxidized in the air, while the subsurface atoms are not easily oxidized. Taking advantage of the oxidation properties, we suggest a stepwise reduction-oxidation approach to determine the surface atomic arrangement of noble metal nanoparticles, and confirm the rationality of this approach by identifying the surface structure of typical 2–3 nm Pt and Pd nanoparticles. The reduction-oxidation approach is applied to characterize the surface structure of model Pd-Sb bimetallic catalyst, which illustrates that the surface Pd is well isolated by Sb atoms with short bond distance at 2.70 A, while there are still Pd-Pd bonds in the subsurface. Density functional theory (DFT) calculations and Pd L edge X-ray absorption near edge structure (XANES) indicate that the isolation of surface Pd significantly decreases the adsorption energies of Pd-hydrocarbon, which leads to the high propylene selectivity and turnover frequency Pd-Sb bimetallic catalyst for propane dehydrogenation.