Reactivity of Gold Clusters in the Regime of Structural Fluxionality

Abstract

This work present results of a systematic investigation of adsorption and dissociation of H 2 on the neutral, positively, and negatively charged gold clusters Au n q (n = 2−11; q = 0, ±1) using the global reaction route mapping (GRRM) technique combined with the anharmonic downward distortion following (ADDF) and the artificial forceinduced reaction (AFIR) methods. An exhaustive search for H 2 dissociation pathways is performed not only on the most stable cluster structures but also on the large number of low-energy isomers, allowing structural transformations between them. The present strategy can automatically identify the structure-dependent lowest transition states (TS) for H 2 dissociation with a systematic procedure in the regime of the structural fluxionality of gold clusters at finite temperature. Temperature effects, cluster isomerization, and influence of the charge state of gold clusters on H 2 adsorption and dissociation are studied. It is demonstrated that the most stable structures of the gold clusters are not always highly reactive, and an ensemble of isomeric structures must be taken into account for adequate description of the reaction rates at finite temperatures. The proposed approach can serve as a promising tool for a systematic analysis and prediction of reactivity of small metal clusters. ■ INTRODUCTIONGold is one of the most intensively studied elements in nanocatalysis due to its unique catalytic activity and selectivity emerging at nanoscale even at room temperatures. 1,2 This feature makes gold a very promising catalyst for the chemical industry and environmental applications. 3,4 The most explored type of catalytic reactions with gold nanoparticles are reactions of oxidation and epoxidation by molecular oxygen, 1,2,5−15 with a strong focus on oxidation of carbon monoxide 1,2,4,9,16−24 as a most simple model case, allowing us to test various approaches and theories. Heterogeneously catalyzed hydrogenation is another type of reaction where gold nanoparticles have shown their great potential as catalysts. 11,25,26 It has been shown that gold nanoparticles supported on the surfaces of some metal oxides can effectively catalyze selective hydrogenation of several classes of organic molecules, including unsaturated aldehydes, ketones, hydrocarbons, and nitro compounds. 26−36 Moreover, gold nanoparticles are very selective for the direct formation of hydrogen peroxide from H 2 and O 2 mixtures. 8,37−40 Dissociative adsorption of molecular hydrogen on gold nanoparticles is the first and one of the most important steps in heterogeneously catalyzed hydrogenation reactions because often it determines the reaction rate. It has been demonstrated that H 2 does not bind to the clean gold bulk surface 41 but adsorbs and dissociates on the supported 11,26,42−49 and free 50−60 gold nanoparticles.Theoretical calculations performed by Corma and coworkers demonstrate that the active sites for H 2 dissociation are usually located at low-coordinated corner or edge positions on a cluster surface and do not dir...

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