Catalytic Activities of Subnanometer Gold Clusters (Au16-Au18, Au20, and Au27-Au35) for CO Oxidation

Using the CO oxidation as a chemical probe, we perform a comprehensive ab initio study of catalytic activities of subnanometer gold clusters. Particular attention Is placed on 12 different clusters in the size range of Au-16-Au-35, whose atomic structures In the anionic state have been resolved from previous experiments. Adsorption energies of a single CO or O-2 molecule as well as coadsorption energies of both CO and O-2 molecules on various distinctive surface sites of each anionic duster and their neutral counterpart are computed. In general, the anionic clusters can adsorb CO and O-2 more strongly than their neutral counterparts. The coadsorption energies of both CO and O-2 molecules decrease as the size of gold clusters increases with the exception of Au-34 (an electronic "magic-number" cluster). Besides the known factor of low coordination site, we find that a relatively small cone angle (<110 degrees) associated with each surface site is another key geometric factor that can enhance the binding strength of CO and O-2. For the subnanometer dusters, although the size effect can be important to the strength of CO adsorption, it Is less important to the activation energy. Using Au34 as a prototype model, we show that strong CO and O-2 adsorption sites tend to yield a lower reaction barrier for the CO oxidation, but they have little effect on the stability of the reaction intermediate. Our calculations support the notion that CO and O-2 adsorption energies on the gold clusters can be an effective indicator to assess catalytic activities of subnanometer gold dusters. This systematic study of the site- and size-dependent adsorption energies and reaction pathways enables a quantitative assessment of the site-size-activity relationship for the CO oxidation on subnanometer gold clusters.