An explosion of interests in nanocatalysis is to be able to adjust the structure of catalytic materials to obtain the optimal electronic properties for desired chemical reactivity [1]. The mechanisms of surface reactions are very complex and any discover in recognize of these reactions increase the ability of design new active catalytic species. The study of the interaction of molecules with surfaces is the important step in surface reactions. Hydrogen adsorption on metal surfaces is very important subject of study, as it brings up the opportunity to understand many interesting problems such as catalytic process, and hydrogen storage [2]. In particular, the interaction of hydrogen with metal surfaces has served as a model system for the investigation of fundamental reaction steps at surfaces for many reactions like as hydrogenation reactions and Fischer-Tropsch synthesis (FTS) [3]. An enormous body of experimental data on the effect of the particle size of the active component of a catalyst on the specific catalytic activity. This parameter is not related to the total surface of the catalyst active phase or to the number of active sites; therefore, they characterize the properties of the active site itself. As a step toward understanding how the structure of catalyst particles affects these functions, we have studied the effect of the size of Cobalt clusters, including the infinite Co (111) surface, on the energy of adsorption of hydrogen atoms. These results indicate that small clusters may exhibit significant differences in reactivity among themselves and between themselves and large particles.

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