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 4f_7/2 and 4f_5/2 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 O₂. 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.