Collection of Czechoslovak Chemical Communications - digital archive 

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.

Keywords: Model metal catalysts; Angle resolved photoemission; Depth profiles; Supported gold; Au/NbO_x interface; Niobium; XPS spectroscopy; Heterogeneous catalysis.

References: 34 live references.