Theoretical Analysis of the CO Oxidation Process on Neutral and Single Ionized Au 18 Clusters
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Theoretical Analysis of the CO Oxidation Process on Neutral and Single Ionized Au18 Clusters J. J. Castro1, 2, J. R. Soto2, B. Molina2 and A. Calles2 1 Dep. Física, CINVESTAV del IPN, Apdo. Post. 14-740, 07000 Del. Gustavo A. Madero México D.F., México. 2 Dep. Física, Fac. Ciencias, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, Apdo. Post. 70-646, 04510 México D.F., México.
ABSTRACT Experiments on the reactivity of CO for Au nanoclusters have shown a local maximum in the adsorption of the first molecule for Au18 and its cation, whereas O2 adsorption has been observed in Au18-1. In this work we present a theoretical analysis of the preferential sites for the adsorption of the CO and O2 molecules on neutral and single ionized Au18 clusters with C2v symmetry, which has been shown both theoretical and experimentally, to be the most stable isomer of the Au18 cluster. We report the results of the calculation for the binding energies of CO and O2 for non-equivalent sites and compare with the available experimental values. The oxidation mechanism is studied in first instance by the subsequent adsorption of the CO on the O2 molecule, which was previously adsorbed on the respective gold cluster. The study is based on a DFT-GGA calculation with the PW91 functional.
INTRODUCTION In recent years, the catalytic properties of gold nanoparticles in gas phase and supported on oxide substrates have been extensively studied. Among other chemical reactions, the carbon monoxide combustion, catalyzed by gold nanoparticles at low temperature, is probably one of the most studied. Some of the main characteristics of the chemisorption processes of carbon monoxide and dioxygen molecule on free gold clusters can be summarized as follow: i) the charge state of gold cluster plays an essential role in the interaction between gold cluster and CO and O2; ii) the reaction rate between gas molecules and gold nanoparticles is strongly dependent on the cluster size; iii) the adsorption activity of molecular O2 on anionic gold clusters shows an even/odd behavior with the cluster size; iv) the adsorption activity of CO on cationic and neutral clusters shows particular local maxima; v) preadsorption of one molecule seems to facilitate the subsequent adsorption. Whereas, studies on gold clusters dispersed on metal oxide supports revealed, that the charge state could be more important than the particle size in determining a high catalytic activity [1-6]. Although important and significant advances have been made to understand the mechanism of carbon monoxide oxidation on gold clusters, the reaction mechanism of the CO oxidation is still unclear. In particular, some of the topics that need a better understanding are: the nature of
the CO/O2 binding interaction with the gold cluster, the preferred adsorption sites and the changes in the equilibrium structure of the metal cluster core after adsorption [3]. Experiments on the reactivity of CO for Au nanoclusters have shown a local maxima in the adsorption on the first
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