Charge separation in CO oxidation involving supported gold clusters
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TOMS, MOLECULES, OPTICS
Charge Separation in CO Oxidation Involving Supported Gold Clusters1 R. S. Berrya and B. M. Smirnovb a
Department of Chemistry, University of Chicago 60637 IL, Chicago, USA Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow, 125412 Russia email: [email protected]
b
Received March 29, 2011
Abstract—The character of the catalytic oxidation of CO by supported gold cluster catalysts is analyzed with emphasis on the unique characteristics of this process. The scheme of this process used here has the reagent CO molecule captured in the interface between the cluster and support, with oxygen molecules or atoms located on the support surface to react with the CO. (Other models have also been presented.) The experi mental data indicate that, together with configurational transitions that lead to the CO molecule joining an oxygen atom to form the CO2 molecule, the charge separation due to capture of the CO molecule by the sup ported gold cluster is important. The process of release of the CO2 molecule results in charge exchange; the time for this process is relatively long because of the large distance separating positive and negative charges, a distance exceeding the cluster radius. This provides a high efficiency of the oxidation of CO with this catalyst despite the relatively high activation energy for the configurational transition. DOI: 10.1134/S1063776111140019 1
1. INTRODUCTION
The gold cluster is a particularly interesting physi cal object. Due to competition of interaction between 5d and 6s shells of valence electrons and relativistic effects, this cluster admits an unusual variety of struc tures [1]. Clusters of small sizes exhibit linear, zigzag, planar, and 3D structures; the transition between the planar and 3D groundstate structures of the nega – tively charged cluster Au n occurs for n = 12, 13, 14, as shown by both experimental studies [2, 3] and calcu lations [4, 5]. For positively charged gold clusters + Au n , this transition occurs at n = 7 (see [4, 5]}. Larger clusters, in addition to the icosahedral structure, can have tetrahedral, cagelike, and tubular structures [3, 6, 7]. Next, in contrast to other metal clusters with the icosahedral structure, the gold cluster consisting of 55 atoms does not have such a structure in the ground state [8]. The same rich behavior appears in the melt ing of gold clusters. The energy gap separating the solid and liquid aggregate states is significantly smaller for 13atom gold clusters than for such clusters of other metals [9–13], if we express these parameters in reduced units where the measure is the binding energy of cluster atoms. Moreover, the melting points of gold clusters are anomalously low. These properties of gold clusters make them espe cially good catalysts. To function, the catalyst must form bonds with substrate reagent molecules, and the 1
The article is published in the original.
molecules so attached may react with a lower activa tion energy than the uncatalyzed substrate, which of cour
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