Identification of oxidized platinum single atoms on chlorinated γ–alumina support by density functional theory calculations and X-ray absorption spectroscopy

Abstract

The existence and nature of oxidized Pt single atoms (SA) are unraveled on two γ-Al2O3 supported Pt catalysts (0.3 wt%) containing two chlorine contents (0.1 and 1.4 wt%) characterized by HR-HAADF-STEM and HERFD-XANES/EXAFS after a calcination treatment at 520 °C. Pt SA are revealed as PtOxCly complexes where the number of oxygen (x) and chlorine (y) ligands directly depends on the chlorine content. In order to interpret these experimental observations and provide coherent atomic structures for these PtOxCly complexes, the thermodynamic stability of the oxidized Pt SA species supported on the (100) surface model of γ-Al2O3 is determined by density functional theory (DFT) calculations as a function of the calcination conditions determined by the temperature, and partial pressures of H2O, HCl and O2. It is shown that various ligands such as Cl, OH, O (from the alumina surface or as extra-ligands) and H2O are coordinated to Pt depending on the conditions and chlorine content. Various square planar and square-based pyramidal PtOxCly species such as PtO2, PtOCl2, PtCl4 are found to be stable in calcination conditions, while octahedral Pt(OH)xCl4-x(H2O) are stabilized in XAS recording conditions. The features of these DFT simulated octahedral Pt SA species are successfully compared to XAS data. On one hand, based on thermodynamics arguments, we proposed models of supported complexes in agreement with EXAFS coordination numbers and distances (Pt-O, Pt-Cl and Pt-Al). On the other hand, the simulated XANES spectra at the Pt L3-edge for these complexes match well with the experimental HERFD-XANES and reveal that the bump feature observed between 11575 eV and 11585 eV (post-edge signal) is a tracer of the amount of Cl coordinated to Pt SA.