Valence-to-core X-ray emission spectroscopy to resolve the size-dependent valence electronic structure of Pt nanoparticles

Abstract

X-ray characterization of catalyst materials using synchrotron radiation has become more widely available to the scientific community in recent decades. Techniques such as X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) have enabled in situ and in operando studies of dilute catalyst sites for heterogeneous catalyst materials in order to obtain information about local geometric and electronic structure and how it impacts chemical transformations. Non-resonant or valence-to-core X-ray emission spectroscopy (NR-XES or VtC-XES) is an emerging technique used to probe changes to the d-electron density of states (DOS) for various factors such as ligand environment or alloy formation. In this study, VtC-XES provides insights into the electronic structure of Pt nanoparticles of different sizes dispersed in a typical heterogeneous catalyst support of SiO₂. The results experimentally verify that the d-band center of the Pt catalysts systematically increases with decreasing nanoparticle size. Transitioning from a fully coordinated Pt foil to Pt nanoparticles of 2 nm in diameter, the shift in the d-band center scales linearly with the proportion of surface Pt atoms to bulk Pt atoms. While VtC-XES shows that the filled Pt 5d states shift systematically to higher energy, XANES shows that the unfilled Pt 5d states also shift systematically to higher energy. These findings align with previous computational and experimental studies and confirm that in situ VtC-XES is a capable technique for assessing the d-DOS of catalyst materials with dilute metal loadings (<2 wt%). Moreover, a beamline with an in situ XAS capability can be coupled with VtC-XES to holistically assess both the geometric (EXAFS) and electronic structure (XANES, VtC-XES) of a catalyst under realistic reaction conditions. This capability is accessible to others to apply to their catalytic problems to help drive the design of future catalyst materials.