Direct synthesis of hydrogen peroxide from H2 and O2 using Pd and Au catalysts

Abstract

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.