A first principles study of water dissociation on small copper clusters Lei Chen, a Qingfan Zhang, a Yunfeng Zhang, a Winston Z. Li,w a Bo Han, a Chenggang Zhou, a Jinping Wu,* a Robert C. Forrey, b Diwakar Garg c and Hansong Cheng* ad Received 15th January 2010, Accepted 2nd June 2010 DOI: 10.1039/c001006e Water dissociation on copper is one of the rate-limiting steps in the water-gas-shift (WGS) reaction. Copper atoms dispersed evenly from freshly made catalyst segregate to form clusters under the WGS operating conditions. Using density functional theory, we have examined water adsorption and dissociation on the smallest stable 3-dimensional copper cluster, Cu 7 . Water molecules are adsorbed on the cluster sequentially until full saturation at which no direct water–copper contact is sterically possible. The adsorption is driven mainly by the overlap between the p-orbital of O atom occupied by the lone pair and the 3d-orbitals of copper, from which a fractional charge is promoted to the 4s-orbital to accommodate the charge transfer from water. Water dissociation on the Cu 7 cluster was investigated at both low and high water coverage. It was found that water dissociation into OH and H is exothermic but is inherently a high temperature process at low coverage. At high coverage, the reaction becomes more exothermic with fast kinetics. In both cases, water can catalyze the reaction. It was found that direct dissociation of the OH species is endothermic with a significantly higher barrier at both low and high coverage. However, the OH species can readily react with another adjacent hydroxyl group to form an O adatom and water molecule. Our studies indicate that the basic chemical properties of water dissociative chemisorption may not change significantly with the size of small copper clusters. Similarities between water dissociation on copper clusters and on copper crystalline surfaces are discussed. 1. Introduction Water gas shift (WGS) reaction (CO + H 2 O = CO 2 +H 2 ) is an important industry process for hydrogen production and CO removal and has been widely used for ammonia 1,2 and methanol syntheses, 3–5 and for PEM fuel cell applications. 6–8 For high temperature WGS reaction, Fe 3 O 4 catalyst is promoted with Cu to improve the catalytic activity. 9,10 It has been found in several recent experiments that Cu atoms are evenly dispersed in solid solutions in the freshly prepared catalyst but segre- gated out from the solid to form Cu clusters on the catalyst surfaces at the operating conditions of WGS reaction. 11,12 Water dissociation has been identified to be the rate limiting step of the WGS reaction process. 13–15 Detailed mechanistic understanding of the dissociation process on copper clusters is essential for design and development of novel catalyst to achieve high H 2 production efficiency. Interactions between small molecules and Cu clusters have been widely studied in the last few years. For example, O 2 adsorption was largely attributed to the electron transfer from the HOMO of Cu clusters to the O 2 antibonding p*-orbital but the O 2 molecular nature remains essentially intact. 13 Strong CO chemisorption on small Cu clusters was also reported to be driven mostly by s donation, which was found to be twice as important as the p back donation. 14 H 2 dissociative chemisorption on Cu clusters has been studied by Guvelioglu and co-workers. 15–17 The overlap between the s*-orbital of H 2 and the 4s orbital of the sharp-corner Cu atoms was found to be the main driving force for the adsorp- tion, leading to hydride formation. Of particular interest is methanol dissociative chemisorption on small cationic copper clusters reported by Ichihashi, et al. 18 They found that methanol dissociation is cluster size-dependent. For n = 4 and 5, the dissociation leads to the formation of H, OH and CH 2 species on the cluster, while on larger size clusters (n = 6–8), it gives rise to demethanation (Cu n O + ). Water adsorption on copper surfaces has been a subject of active research. 19–21 Experimentally, the heat of adsorption of a water molecule on Cu(110) surface was reported to be 0.42 eV. 19 The water molecule on the surface then undergoes dissociative chemisorption to form surface OH and H species. The corresponding activation barrier was found to be coverage dependent: for an isolated water molecule the dissociation barrier was measured to be 0.8–0.9 eV, while at the coverage of a water monolayer the reported experimental value was 0.53–0.56 eV. 20,21 Density functional theory calculations 22 confirmed that the dissociation barrier indeed decreases as water coverage increases. Gokhale et al. recently showed that a Institute of Theoretical Chemistry and Computational Materials Science, China University of Geosciences, Wuhan 430074, China. E-mail: wujp@cug.edu.cn b Department of Physics, Penn State University, Berks Campus, Reading, PA 19610-6009 c Air Products and Chemicals, Inc. 7201 Hamilton Boulevard, Allentown, PA 18195-1501 d Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543. E-mail: chmch@nus.edu.sg w Summer intern from Parkland High School, 2700 N Cedar Crest Blvd, Allentown, PA 18104-9643, USA. This journal is  c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 9845–9851 | 9845 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics