The characterization of Ti and Al oxide overlayers on rhodium and gold by XPS

Abstract

X-ray photoelectron spectroscopy (XPS) has been used to characterize alumina and titania overlayers deposited on a polycrystalline Rh foil. Only the 3+ state of Al is observed for submonolayer deposits of alumina. Titanium is found, though, in both the 4+ and 3+ states, the proportion of Ti 3+ increasing with the extent of sample reduction. The proportion of Ti 3+ is also found to decrease with increasing coverage of the Rh foil by titania. This trend is associated with preferential reduction of Ti 4+ cations located at the perimeter of the TiO x islands comprising the overlayer. XPS studies of 0.15 ML of TiO x deposited on Au substrates revealed that all the Ti is in the 4+ oxidation state independent of CO and H 2 treatments. These results indicate that reduction of Ti to the 3+ state proceeds only by reaction with CO and H 2 chemisorbed on the metal. XPS characterization of TiO x /Rh following CO hydrogenation indicates that the proportion of Ti 3+ is nearly independent of the sample pretreatment conditions.