Surface Science 401 (1998) 282–295 Interactions between Cs and noble metals (Cu, Ag, Au) on ZnO surfaces: electronic perturbations and alloy formation S. Chaturvedi, J.A. Rodriguez * Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973, USA Received 14 August 1997; accepted for publication 9 December 1997 Abstract The adsorption of cesium on polycrystalline ZnO and the coadsorption of the alkali with noble metals (NM=Cu, Ag or Au) were investigated using X-ray and synchrotron-based photoelectron spectroscopies. At small coverages, Cs forms very strong adsorption bonds on polycrystalline ZnO with desorption temperatures that are higher than 700 K. The strength of the Cs–ZnO bond depends on the coverage of Cs, and for a saturated adlayer, the desorption of the alkali starts around 300 K. In the Cs/ZnO systems, there is a migration of oxygen toward the alkali–oxide interface, but the Cs does not become fully oxidized. When Cu or Au is deposited on Cs/ZnO, Cs moves from the alkali–oxide interface to the top of the noble metal overlayer. This migration substantially increases the thermal stability of Cs on the surface. In NM/ZnO surfaces, the metal overlayers exhibit valence bands that are significantly different from those of the corresponding bulk metals. These electronic perturbations do not facilitate the formation of Cs–NM alloys, and the Cs/NM/ZnO systems show a pattern of reactivity identical to that found for bulk samples: one can see the formation of Cs–Au alloys in Cs/Au/ZnO, whereas no alloying is found in Cs/Cu/ZnO and Cs/Ag/ZnO. The supported Cs–Au alloys are very reactive toward O 2 . They decompose at temperatures between 400 and 500 K, with a fraction of the Cs desorbing into gas phase and the rest remaining chemisorbed on Au or ZnO. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Alkali metals; Alloys; Cesium; Copper; Gold; Oxygen; Silver; Surface chemical reaction; X-ray photoelectron spectroscopy; Zinc oxide 1. Introduction tions over Cu/ZnO [5–10]. Over a cesium-pro- moted Cu/ZnO catalyst, methanol is readily formed from CO and H 2 even in the absence of Cesium is used as a promoter of catalytic activity CO 2 in the feedstream. In addition, the cesium or selectivity in several industrial processes carried promoted catalyst is very effective for the synthesis out over metal/oxide catalysts [1–7]. Among of higher order alcohols [5–7]. In order to gain the most important are ethylene epoxidation insight into these catalytic phenomena, one needs (2C 2 H 4 +O 2 2C 2 H 4 O) over Ag/Al 2 O 3 [3,4] and to understand how cesium interacts with a noble the water–gas shift (CO+H 2 OCO 2 +H 2 ) and metal when both are supported on an oxide. In methanol synthesis (CO+2H 2 CH 3 OH) reac- this article, we investigate the interactions between cesium and group 11 metals (Cu, Ag, Au) on zinc oxide using photoemission and O 2 chemisorption. * Corresponding author. Fax: ( +1) 516 344 5815; e-mail: rodrigez@bnl.gov The adsorption of cesium on the O-terminated 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0039-6028(97)01095-9