An XPS study of interactions in thin films containing a noble metal with valence-invariant and reducible oxides

• Published in: Journal of catalysis 1992-09, Vol.137 (1), p.114-126
• Main Authors: Shelef, M., Haack, L.P., Soltis, R.E., Devries, J.E., Logothetis, E.M.
• Format: Article
• Language: English
• Subjects: 400201 - Chemical & Physicochemical Properties; ALUMINIUM COMPOUNDS; ALUMINIUM OXIDES; CATALYST SUPPORTS; CATALYSTS; CATALYTIC EFFECTS; CERIUM COMPOUNDS; CERIUM OXIDES; CHALCOGENIDES; CHEMICAL REACTIONS; Condensed matter: structure, mechanical and thermal properties; CONFIGURATION INTERACTION; CORUNDUM; DEPOSITION; ELECTRON SPECTROSCOPY; ELEMENTS; ENERGY BEAM DEPOSITION; Exact sciences and technology; HYDROGEN; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; LANTHANUM COMPOUNDS; LANTHANUM OXIDES; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; METALS; MINERALS; NIOBIUM COMPOUNDS; NIOBIUM OXIDES; NONMETALS; OXIDE MINERALS; OXIDES; OXYGEN COMPOUNDS; PALLADIUM; PHOTOELECTRON SPECTROSCOPY; Physics; PLATINUM METALS; RARE EARTH COMPOUNDS; REDUCTION; REFRACTORY METAL COMPOUNDS; SAPPHIRE; SPECTROSCOPY; SPUTTERING; SURFACE AREA; SURFACE COATING; SURFACE PROPERTIES; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); THIN FILMS; TITANIUM COMPOUNDS; TITANIUM OXIDES; TRANSITION ELEMENT COMPOUNDS; TRANSITION ELEMENTS; VALENCE; X-RAY SPECTROSCOPY
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/0021-9517(92)90143-6

X-ray photoelectron spectroscopy was employed to examine the reducibility of oxides possessing multiple oxidation states (titania, niobia, and ceria) in the presence of palladium and a valence-invariant oxide (lanthana). The materials consisted of sputter-deposited ultra-thin films containing palladium with either titania, niobia, or ceria, with and without lanthana, on a sapphire substrate. For lanthana/palladium films, no reduction of the lanthana surface was observed after in situ treatment in hydrogen up to 600°C. After similar treatment, in the absence of lanthana a partial reduction to the lower oxidation states of niobia and titania and a total reduction to the lower oxidation state of ceria was observed. The presence of lanthana in the films inhibits the reduction of the titania completely and that of the niobia and ceria partially. Also, in a complementing set of films containing ceria an overlayer of high-surface-area γ-Al 2O 3, from the decomposition of an aqueous film of aluminum propoxide, was put over the smooth sapphire surface before the deposition of the other film components. In this case, the inhibiting effect on ceria-reducibility was attenuated. This points to the redistribution of lanthana over the available oxide surfaces. The preponderant portion of the lanthana accommodated on the porous γ-Al 2O 3 has rendered a significant part of the ceria unprotected and, in the presence of Pd, susceptible to reduction by H 2 at 600°C. The results of these studies are a further manifestation of the known surface reactivity of lanthana, which while remaining itself irreducible, may strongly affect the behavior of other irreducible oxides such as alumina, or as presently shown, of reducible oxides such as titania, niobia, and ceria.