C-Cl Bond Activation on Au/Pd Bimetallic Nanocatalysts Studied by Density Functional Theory and Genetic Algorithm Calculations

The C-Cl bond activation by Au/Pd bimetallic alloy nanocatalysts has been investigated with regard to the oxidative addition of chlorobenzene (PhCl). Fifteen stable structures of the Au10Pd10 nanocluster (NC) obtained by a genetic algorithm were examined by DFT calculations using the M06-L, TPSS, and B3LYP functionals. Triplet states of cage-like C-1 and C-s, structures are found to be relevant reflecting the quasi-degenerate nature of the Pd moiety, while several other low-lying structures and spin states may also contribute to the oxidative addition. For all examined cluster structures, the oxidative addition step is exothermic, and internal conversion and/or spin crossing are expected to occur as several states are close in energy and geometry. Based on an energetic analysis of a model system consisting of the Au10Pd10 NC surrounded by four poly(n-vinylpyrrolidone) (PVP) molecules, the PVP units activate the system as electron donors and stabilize it. While a neutral NC model overestimates the energy barrier slightly, the opposite holds for an anionic NC model. In the oxidative addition, the interaction between the phenyl group and the Pd atom on the NC surface as well as a dissociation taking place at the Pd site are found to be essential. This indicates the importance direct coordination effects in the Au/Pd bimetallic NC. NBO analysis shows that a pi back-donation of the M(d pi) to sigma*(C-Cl) orbital is relevant for the C-Cl bond activation and the interaction energy explains the favorable dissociation at the Pd site compared to the Au site.