Philip Landon

The oxidation of alcohols to aldehydes with O2 in place of stoichiometric oxygen donors is a crucial process for the synthesis of fine chemicals. However, the catalysts that have been identified so far are relatively inactive with primary alkyl alcohols. We showed that Au/Pd-TiO2 catalysts give very high turnover frequencies (up to 270,000 turnovers per hour) for the oxidation of alcohols, including primary alkyl alcohols. The addition of Au to Pd nanocrystals improved the overall selectivity and, using scanning transmission electron microscopy combined with x-ray photoelectron spectroscopy, we showed that the Au-Pd nanocrystals were made up of a Au-rich core with a Pd-rich shell, indicating that the Au electronically influences the catalytic properties of Pd.

Gold nanocrystals absorbed on metal oxides have exceptional properties in oxidation catalysis, including the oxidation of carbon monoxide at ambient temperatures, but the identification of the active catalytic gold species among the many present on real catalysts is challenging. We have used aberration-corrected scanning transmission electron microscopy to analyze several iron oxide-supported catalyst samples, ranging from those with little or no activity to others with high activities. High catalytic activity for carbon monoxide oxidation is correlated with the presence of bilayer clusters that are approximately 0.5 nanometer in diameter and contain only approximately 10 gold atoms. The activity of these bilayer clusters is consistent with that demonstrated previously with the use of model catalyst systems.

Supported Au catalysts are very selective for the direct formation of hydrogen peroxide from H2/O2 mixtures at 2 degrees C; the rate of H2O2 synthesis is markedly increased if Au-Pd alloy nanoparticles are generated by the addition of Pd.

The direct synthesis of hydrogen peroxide from H2 and O2 using a range of supported metal catalysts is described and discussed. A detailed study of the factors influencing the formation and decomposition of hydrogen peroxide is presented for a Pd/sulfonated carbon catalyst in a methanol/water solvent. The use of low temperatures (1–2 °C) and short reaction (residence) time are identified as the key factors that favour high selectivity to hydrogen peroxide. Decomposition of hydrogen peroxide, mainly via further hydrogenation, prevents the formation of high concentrations of hydrogen peroxide. Combustion of hydrogen to water is a competing reaction that becomes significant at higher temperatures, but this can be partially inhibited by the addition of HBr. A second set of supported Pd and Au catalysts are evaluated for the direct synthesis of hydrogen peroxide using supercritical CO2 as a solvent. The use of supercritical CO2 is shown to be beneficial when compared with hydrogen peroxide formation at a temperature just below the critical temperature for CO2. However, at the critical temperature of CO2 (31.1 °C), the decomposition of hydrogen peroxide is rapid and only low rates of hydrogen peroxide formation are observed. At low temperature (2 °C) supported Au catalysts are shown to be very selective for the synthesis of hydrogen peroxide. The rate of hydrogen peroxide synthesis is enhanced markedly when Pd is present with Au and a detailed scanning transmission electron microscopy study shows that the 2–9 nm metal nanoparticles present in this supported catalyst are a Au∶Pd alloy.