Surface Science 458 (2000) 71–79 www.elsevier.nl/locate/susc Adsorption of chlorine on ZnO(0001)–Zn and coadsorption with HCOOH Ann W. Grant, Andrew Jamieson, Charles T. Campbell * Department of Chemistry, University of Washington, Seattle,WA 98195-1700, USA Received 10 December 1999; accepted for publication 2 March 2000 Abstract The adsorption of Cl 2 on the zinc-terminated ZnO(0001) surface at 300 K was studied with low-energy ion- scattering spectroscopy (ISS ), X-ray photoelectron spectroscopy ( XPS), angle-resolved XPS (ARXPS ), and work- function and band-bending measurements. The surface saturated with ~0.30 chlorine adatoms [Cl(a)] per zinc site, sitting above the zinc layer in no apparent registry with the substrate. Combined work-function and band-bending measurements indicate anionic Cl(a). Using formic acid to produce surface formate, the effect of Cl(a) on formate decomposition was studied also. On the chlorine-free ZnO(0001) surface, formate decomposes through two different pathways: dehydration and dehydrogenation. Pre-adsorbed chlorine suppresses the amount of adsorbed formate produced, and enhances the selectivity for its dehydrogenation. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Chemisorption; Chlorine; Ion scattering spectroscopy; Surface electronic phenomena (work function, surface potential, surface states, etc.); X-ray photoelectron spectroscopy 1. Introduction few pm of ethylene dichloride suppresses the unwanted production of CO 2 [5]. Therefore, the Chlorine is a common impurity in oxide-sup- interactions of chlorine with oxide surfaces are of ported metal catalysts. It is known to influence the fundamental interest. Here, we examine the reac- dispersion of supported metal particles [1,2]. For tivity of Cl 2 with well-defined ZnO(0001)–Zn instance, in low-temperature water gas shift cata- surfaces. lysts, primarily Cu/ZnO, it is found that chlorine, There are a few prior studies of chlorine adsorp- in the form of HCl gas, acts to increase the tion on single-crystal ZnO surfaces examining sintering rates of the copper particles [2]. Chlorine, structural and electronic properties. Changes in as a strong electron acceptor, can act also as a the conductivity of several metal oxide surfaces in bonding modifier [3,4] by either strengthening or response to exposure to Cl 2 gas have been studied weakening the adsorbate–substrate bond. Chlorine [6]. ZnO was a main focus of study since it was is added to some catalysts as a modifier to increase well known to readily change in conductivity with selectivity, for instance in ethylene epoxidation on adsorbed gases, and a potential candidate for Cl 2 alumina-supported silver, where the addition of a gas sensors. Hopkins and Taylor [7] focused on the struc- * Corresponding author. Fax: +1-206-685-8665. tural properties of Cl/ZnO(0001) by utilizing E-mail address: campbell@chem.washington.edu (C.T. Campbell ) Auger electron spectroscopy (AES ), low-energy 0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0039-6028(00)00419-2