Reaction of water with Ce–Au(1 1 1) and CeOx/Au(1 1 1) surfaces: Photoemission and STM studies

This article reports photoemission and STM studies for the adsorption and dissociation of water on Ce–Au(1 1 1) alloys and CeOx/Au(1 1 1) surfaces. In general, the adsorption of water at 300 K on disordered Ce–Au(1 1 1) alloys led to O–H bond breaking and the formation of Ce(OH)x species. Heating to 500–600 K induced the decomposition or disproportionation of the adsorbed OH groups, with the evolution of H2 and H2O into gas phase and the formation of Ce2O3 islands on the gold substrate. The intrinsic Ce ↔ H2O interactions were explored by depositing Ce atoms on water multilayers supported on Au(1 1 1). After adsorbing Ce on ice layers at 100 K, the admetal was oxidized immediately to yield Ce3+. Heating to room temperature produced finger-like islands of Ce(OH)x on the gold substrate. The hydroxyl groups dissociated upon additional heating to 500–600 K, leaving Ce2O3 particles over the surface. On these systems, water was not able to fully oxidize Ce into CeO2 under UHV conditions. A complete Ce2O3 → CeO2 transformation was seen upon reaction with O2. The particles of CeO2 dispersed on Au(1 1 1) did not interact with water at 300 K or higher temperatures. In this respect, they exhibited the same reactivity as does a periodic CeO2(1 1 1) surface. On the other hand, the Ce2O3/Au(1 1 1) and CeO2−x/Au(1 1 1) surfaces readily dissociated H2O at 300–500 K. These systems showed an interesting reactivity for H2O decomposition. Water decomposed into OH groups on Ce2O3/Au(1 1 1) or CeO2−x/Au(1 1 1) without completely oxidizing Ce3+ into Ce4+. Annealing over 500 K removed the hydroxyl groups leaving behind CeO2−x/Au(1 1 1) surfaces. In other words, the activity of CeOx/Au(1 1 1) for water dissociation can be easily recovered. The behavior of gold–ceria catalysts during the water–gas shift reaction is discussed in light of these results.