Probing the Site-Specific Reactivity and Catalytic Activity of Ag n ( n = 15-20) Silver Clusters.

Density functional theory calculations within the framework of generalized gradient approximation (GGA), meta-GGA, and local functionals were carried out to investigate the reactivity and catalytic activity of Ag n ( n = 15-20) clusters. Our results reveal that all the Ag n clusters in this size range, except Ag 20 , adsorb O 2 preferably in the bridged mode with enhanced binding energy as compared to the atop mode. The O 2 binding energies range from 0.77 to 0.29 in the bridged mode and from 0.36 to 0.15 eV in the atop mode of O 2 adsorption. The strong binding in the case of the bridged mode of O 2 adsorption is also reflected in the increase in O-O bond distance. Natural bond orbital charge analysis and vibrational frequency calculations reveal that enhanced charge transfer occurs to the O 2 molecule and there is significant red shift in the stretching frequency of O-O bond in the case of the bridged mode of O 2 adsorption on the clusters, thereby confirming the above results. Moreover, the simulated CO oxidation reaction pathways show that the oxidation of the CO molecule is highly facile on Ag 16 and Ag 18 clusters involving small kinetic barriers and higher heats toward CO 2 formation.