Thermal Stability of Au/NbOx/Nb and Au/Nb2O5/W Model Catalysts Studied by Angle-Resolved X-Ray Photoelectron Spectroscopy

Abstract

The thermal stability of gold deposited on Nb2O5/W, Nb2O5/Nb and NbO/Nb substrates was investigated by angle-resolved X-ray photoelectron spectroscopy. Gold in amounts 0.1-1.2 monolayer was vapour-deposited on an oxide substrate under ultrahigh vacuum. A shift of the Au (4f) core level towards higher binding energies and an increase in the line width with decreasing surface coverage by Au were observed for all supports and explained by final state effects in the photoemission process. The lower magnitude of the shift observed in the case of Au/NbO/Nb was attributed to better screening of the core hole by electrons of the metallic NbO phase. The results present evidence that on heating of the Au/Nb2O5/W system up to 550 °C only agglomeration occurred without significant diffusion of Au into the oxide substrate. Annealing of the Au/Nb2O5/Nb model catalyst above 350 °C resulted in reduction of Nb2O5 to suboxides and inward diffusion of Au. Diffusion into the substrate above 350 °C has also been observed for the Au/NbO/Nb sample. The distribution of Au within the surface layers of substrate has been estimated by analysis of Au (4f) and Nb (3d) angle-resolved spectra. The inward diffusion of Au on annealing observed for the Nb2O5/Nb and NbO/Nb substrates is facilitated by a high density of defects in NbO. Besides information on the particular Au/Nb2O5/W, Au/Nb2O5/Nb and Au/NbO/Nb systems, our results demonstrated potentials of angle-resolved photoelectron spectroscopy for determination of the distribution of a metal in annealed model catalysts and for study of extended interfaces.