Reaction of water with Ce-Au(111) and CeO_x/Au(111) surfaces: Photoemission and STM studies

摘要

This article reports photoemission and STM studies for the adsorption and dissociation of water on Ce-Au(111) alloys and CeO_x/Au(111) surfaces. In general, the adsorption of water at 300 K on disordered Ce-Au(111) 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 H_2 and H_2O into gas phase and the formation of Ce_2O_3 islands on the gold substrate. The intrinsic Ce reversible H_2O interactions were explored by depositing Ce atoms on water multilayers supported on Au(111). After adsorbing Ce on ice layers at 100 K, the admetal was oxidized immediately to yield Ce~(3+). 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 Ce_2O_3 particles over the surface. On these systems, water was not able to fully oxidize Ce into CeO_2 under UHV conditions. A complete Ce_2O_3 → CeO_2 transformation was seen upon reaction with O_2. The particles of CeO_2 dispersed on Au(111) did not interact with water at 300 K or higher temperatures. In this respect, they exhibited the same reactivity as does a periodic CeO_2(111) surface. On the other hand, the Ce_2O_3/Au(111) and CeO_(2-x)/Au(111) surfaces readily dissociated H_2O at 300-500 K. These systems showed an interesting reactivity for H_2O decomposition. Water decomposed into OH groups on Ce_2O_3/Au(111) or CeO_(2-x)/Au(111) without completely oxidizing Ce~(3+) into Ce~(4+). Annealing over 500 K removed the hydroxyl groups leaving behind CeO_(2-x)/Au(11 1) surfaces. In other words, the activity of CeO_x/Au(111) 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.