First Principles Calculations of Palladium Nanoparticle XANES Spectra

Abstract

X-ray absorption spectroscopy is a common technique for in situ studies of catalysts. The interpretation of the near edge structure is, however, often hampered by lack of information of how structural and electronic contributions affect the spectra. Here, first principles calculations were used to explore effects of particle size, structural motif and oxidation state on the X-ray absorption near edge structure (XANES) for palladium nanoparticles (PdNP). A range of PdNP were structurally optimized within the density function theory and Pd K edge XANES spectra were calculated using the real-space multiple-scattering formalism. The results show that the Pd-Pd distances are compressed for small NP which yields shifts in the XANES peak positions as compared to bulk Pd. Moreover, the amplitude of the fine structure oscillations is found to increase with the average coordination number. The spectra are only to a minor extent influenced by the structural motif of the PdNP. Oxidation of the Pd surface increases the intensity in the XANES spectrum between the first and the second absorption feature, which correspond to the initial development of a whiteline peak. It is found that a strong whiteline peak only is developed for Pd-atoms with complete oxidation, which corresponds to coordination to four oxygen atoms.