Adsorption of Ethylene on Neutral, Anionic and Cationic Gold Clusters
📝 Abstract
The adsorption of ethylene molecule on neutral, anionic and cationic gold clusters consisting of up to 10 atoms has been investigated using density-functional theory. It is demonstrated that C2H4 can be adsorbed on small gold clusters in two different configurations, corresponding to the pi- and di-sigma-bonded species. Adsorption in the pi-bonded mode dominates over the di-sigma mode over all considered cluster sizes n, with the exception of the neutral C2H4-Au5 system. A striking difference is found in the size-dependence of the adsorption energy of C2H4 bonded to the neutral gold clusters in the pi and di-sigma configurations. The important role of the electronic shell effects in the di-sigma mode of ethylene adsorption on neutral gold clusters is demonstrated. It is shown that the interaction of C2H4 with small gold clusters strongly depends on their charge. The typical shift in the vibrational frequencies of C2H4 adsorbed in the pi- and the di-sigma configurations gives a guidance to experimentally distinguish between the two modes of adsorption.
💡 Analysis
The adsorption of ethylene molecule on neutral, anionic and cationic gold clusters consisting of up to 10 atoms has been investigated using density-functional theory. It is demonstrated that C2H4 can be adsorbed on small gold clusters in two different configurations, corresponding to the pi- and di-sigma-bonded species. Adsorption in the pi-bonded mode dominates over the di-sigma mode over all considered cluster sizes n, with the exception of the neutral C2H4-Au5 system. A striking difference is found in the size-dependence of the adsorption energy of C2H4 bonded to the neutral gold clusters in the pi and di-sigma configurations. The important role of the electronic shell effects in the di-sigma mode of ethylene adsorption on neutral gold clusters is demonstrated. It is shown that the interaction of C2H4 with small gold clusters strongly depends on their charge. The typical shift in the vibrational frequencies of C2H4 adsorbed in the pi- and the di-sigma configurations gives a guidance to experimentally distinguish between the two modes of adsorption.
📄 Content
arXiv:0909.5272v1 [physics.atm-clus] 29 Sep 2009 Adsorption of Ethylene on Neutral, Anionic and Cationic Gold Clusters Andrey Lyalin,∗,†,‡ and Tetsuya Taketsugu† Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan E-mail: lyalin@mail.sci.hokudai.ac.jp Abstract The adsorption of ethylene molecule on neutral, anionic and cationic gold clusters consist- ing of up to 10 atoms has been investigated using density-functional theory. It is demonstrated that C2H4 can be adsorbed on small gold clusters in two different configurations, correspond- ing to the π- and di-σ-bonded species. Adsorption in the π-bonded mode dominates over the di-σ mode over all considered cluster sizes n, with the exception of the neutral C2H4−Au5 sys- tem. A striking difference is found in the size-dependence of the adsorption energy of C2H4 bonded to the neutral gold clusters in the π and di-σ configurations. The important role of the electronic shell effects in the di-σ mode of ethylene adsorption on neutral gold clusters is demonstrated. It is shown that the interaction of C2H4 with small gold clusters strongly de- pends on their charge. The typical shift in the vibrational frequencies of C2H4 adsorbed in the π- and the di-σ configurations gives a guidance to experimentally distinguish between the two modes of adsorption. ∗To whom correspondence should be addressed †Hokkaido University ‡On leave from: Institute of Physics, St Petersburg State University, 198504 St Petersburg, Petrodvorez, Russia 1 Introduction The adsorption of unsaturated hydrocarbons on transition metal surfaces has been studied exten- sively in order to understand the nature of hydrocarbon – metal interaction and chemical processes on solid surfaces; see, e.g., Refs.1,2,3,4,5,6 and references therein. The most significant attention was paid to investigation of ethylene adsorption,7,8,9,10,11,12,13,14,15,16,17 because it is the simplest alkene containing an isolated carbon–carbon double bond. Hence it can be treated as a prototype to study the interaction and reactivity of different alkenes on metal surfaces. Moreover, the ethylene epoxidation is one of the most important processes in the chemical industry, because the product of such a reaction – ethylene oxide is widely used in various applications; see, e.g., Refs.1,2,18,19,20,21 and references therein. Ethylene can adsorb on metal surfaces in two different configurations.1,11,12,14 The first one is the π mode where one metal atom on the surface is involved in the adsorption of ethylene via a π-bonding. The second one is the di-σ-bonded mode, when two metal atoms are involved in the adsorption via a σ bonding. It was found that the di-σ mode of adsorption is characterized by the increasingly important role of sp3 hybridization in ethylene, while sp2 hybridization (typical for free molecules) remains unchanged for π-bonded species.11,12,14 The bonding of ethylene with transition metals involves electron transfer from the filled bond- ing π orbital of the ethylene to the metal, alongside a back-donation from the d-orbital of transi- tion metal to the empty π∗anti-bonding orbital of ethylene in accordance with the Dewar-Chatt- Duncanson model.22 Thus the appearance of the sp3 character of hybridization in the di-σ-bonded ethylene can be explained by the increasing role of the electron back-donation from the metal to the π∗anti-bonding orbital of ethylene. Hence the di-σ-bonded ethylene is activated more strongly in comparison with the π-bonded one, thus it can be more reactive.11,12 Therefore, an understanding of the specific mechanisms of ethylene adsorption on metal surfaces is important in order to gain a better insight of the catalytic processes and reactivity of adsorbed hydrocarbons. Surprisingly, not many works have been devoted to the investigation of ethylene adsorption on metal clusters and nanoparticles, despite the fact that chemical and physical properties of matter 2 at nanoscale are very different from those of the corresponding bulk solids. These properties are often controlled by quantum size effects and significantly depend on the size and structure of atomic clusters.23 A remarkable example is gold. It is well known that gold in its bulk form is a catalytically in- active and inert metal. However gold at nanoscale manifests extraordinary catalytic activity which increases with a decrease in the cluster size of up to 1-5 nm.24,25,26,27 Moreover, recent studies demonstrate that catalytic activity of gold clusters adsorbed on an iron oxide support correlates with the presence of very small clusters of ∼10 atoms.28 It was also reported that gold clusters with the number of atoms n = 3 −11 possess extraordinarily high electrocatalytic activity toward the O2 reduction reaction in acid solutions.29 The origin of such size-dependent catalytic activity of gold remains highly debated and has yet to be fully understood. A comprehensive survey of the field can be found in revi
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