A theoretical study of the effects of the charge state and size of gold clusters on the adsorption and dissociation of H2

摘要

The past few years witnessed a great increase in interest in gold clusters and particles.1,2,3,4,5 This phenomenon is understandable because recently the conventionally regarded “catalytically inert” gold is found to possess unexpected performance for a number of reactions, such as CO oxidation,6 water gas shift reaction,7 C–C bond forming reactions,8,9 and hydrogenation of α,β-unsaturated aldehydes.10,11,12,13 Gold particles or clusters are believed to play a key role in these reactions. We are interested in the hydrogenation of α,β-unsaturated aldehydes. This reaction is believed to follow the Houriti–Polanyi mechanism14 in which H2 molecules adsorbed on catalysts first dissociate into H atoms that then attack the double bonds. Therefore, knowledge of the adsorption and dissociation of molecular hydrogen is a prerequisite for a comprehensive understanding of the hydrogenation reaction.nIt is known that bulk gold and flat gold surfaces interact with H2 very weakly and show no activity for H2 dissociation.15,16 On the other hand, H atoms are detected on TiO2 supported gold nanoparticles exposed to H2 gas, indicating that H2 dissociation takes place.12,17 Theoretically, some questions concerning H2 dissociation remain unclear. The ab initio calculations at CASSCF/MRCI(SD) level produced a much higher barrier of 1.95 eV on the Au7 cluster;18 the density functional theory (DFT) B3LYP predicted a low dissociation barrier of 0.32 eV on the rigid Au13 cluster.19 Recently Corma et al.16 investigated H2 dissociation on gold using three different models. They found that on four-coordinated Au atoms, H2 dissociation is facile while on five-coordinated atoms or surfaces, the dissociation is difficult. They concluded that existence of low coordinated Au atoms would be the sufficient and necessary condition for H2 dissociation. However, Strømsnes et al.18 reported a quite high dissociation barrier of 1.95 eV on a four-coordinated Au atom. Hence the relationship between the coordination number of the Au atom and the feasibility of H2 dissociation is still inconclusive. Another motivation for us to conduct this study is that the active sites in practical gold catalysts may be charged. A very recent theoretical study of Au clusters on TiO2 shows that different size clusters are charged differently.20 Unfortunately less attention has been paid to the effect of the cluster charge state on H2 dissociation. As far as we know, only one paper dealing with H2 dissociation on gold cluster anion is reported.21nIn order to examine the influence of the charge state and cluster size on the H2 activation, we carried out a DFT study of H2 adsorption and dissociation on a series of neutral and charged Au clusters, Aunm(m = 0,±1;  n = 1–6). The present paper reports these new results. This paper is arranged as follows. Computational methods are presented in Sec. 2. The favorable adsorption structures and energetics relevant to H2 dissociation are discussed in detail in Sec. 3. Finally, the conclusions are given in Sec. 4.