Influence of Polyoxometalate Protecting Ligands on Catalytic Aerobic Oxidation at the Surfaces of Gold Nanoparticles in Water.

Metal oxide cluster-anion (polyoxometalate, or POM) protecting ligands, [α-PWO] (1), modify the rates at which 14 nm gold nanoparticles (Au NPs) catalyze an important model reaction, the aerobic (O) oxidation of CO to CO in water. At 20 °C and pH 6.2, the following stoichiometry was observed: CO + O + HO = CO + HO. After control experiments verified that the HO product was sufficiently stable and did not react with 1 under turnover conditions, quantitative analysis of HO was used to monitor the rates of CO oxidation, which increased linearly with the percent coverage of the Au NPs by 1 (0-64% coverage, with the latter value corresponding to 211 ± 19 surface-bound molecules of 1). X-ray photoelectron spectroscopy of Au NPs protected by a series of POM ligands (K salts): 1, the Wells-Dawson ion [α-PWO] (2) and the monodefect Keggin anion [α-SiWO] (3) revealed that binding energies of electrons in the Au 4f and 4f atomic orbitals decreased as a linear function of the POM charge and percent coverage of Au NPs, providing a direct correlation between the electronic effects of the POMs bound to the surfaces of the Au NPs and the rates of CO oxidation by O. Additional data show that this effect is not limited to POMs but occurs, albeit to a lesser extent, when common anions capable of binding to Au-NP surfaces, such as citrate or phosphate, are present.