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-PW11O39]7– (1), modify the rates at which 14 nm gold nanoparticles (Au NPs) catalyze an important model reaction, the aerobic (O2) oxidation of CO to CO2 in water. At 20 °C and pH 6.2, the following stoichiometry was observed: CO + O2 + H2O = CO2 + H2O2. After control experiments verified that the H2O2 product was sufficiently stable and did not react with 1 under turnover conditions, quantitative analysis of H2O2 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-P2W18O62]6– (2) and the monodefect Keggin anion [α-SiW11O39]8– (3) revealed that binding energies of electrons in the Au 4f7/2 and 4f5/2 atomic orbitals decreased as a linear function of the POM ...