A first principles study of water dissociation on small copper clusters

Abstract

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.

Computational investigation of Cu7 as a model biomimetic CO2 capture catalyst

“…Since nanomaterials have demonstrated effects on catalysis, we have chosen Cu 7 for our analysis and DFT as the technique for mechanistic insights. Cu 7 was found to stabilize in pentagonal bipyramidal structure, similar to the observation of Chen et al (2010). The optimized geometries in vacuum and in water as the implicit solvent are shown in Fig.…”

“…Adsorption of multiple H 2 O molecules can lead to the formation of Cu 7 (OH) 2 -type complex. Multi-H 2 O as well as multi-OH complexes have been observed by Chen et al (2010) in their study with Cu 7 clusters. In such a scenario, OH-OH recombination reaction, observed over Cu surfaces (Yamamoto et al, 2008), can take place leading to the formation of oxygenated species.…”

“…DFT has been successfully used for the analysis of sub-nanometer transition metal clusters (Serapian et al, 2013;Hauser et al, 2013;Gao et al, 2011). Since nanomaterials have demonstrated effects on catalysis, we have chosen Cu 7 for our analysis and DFT as the technique for mechanistic insights.…”

There and Back Again: The Unique Nature of Copper in Ambient Pressure Dried-Silica Aerogels

“…63,64 This phenomenon is detectable by EXAFS in very small metal nanoclusters, due to the large fraction of surface atoms. Chen et al reported on Cu−Cu shell expansion of hydrogen-reduced clusters (2−4 nm) coinciding with water being added to the feed.…”

Thermodynamics of H2O Splitting and H2 Formation at the Cu(110)–Water Interface

“…Because copper is one of the materials used in heterogeneous catalysis for the industrial water-gas shift reaction-in which CO is oxidized by H 2 O to form CO 2 and H 2 -the reactions of water with copper surfaces have been extensively studied. [20][21][24][25][26][27][28][29][30][31][32][33][34][35][36] However, most computational investigations have been limited to the reactivity of single water molecules with perfect copper surfaces, 17,21,27,[37][38][39][40][41][42] which in many aspects are limited models of the real system. Recently, a few studies have gone further and considered a monolayer (ML) of water adsorbed on a perfect copper surface.…”