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

Abstract

The direct synthesis of H₂O₂ at low temperature (2 °C) from H₂ and O₂ using carbon-supported Au, Pd and Au–Pd catalysts is described and contrasted with data for TiO₂, Al₂O₃ and Fe₂O₃ as supports. The Au–Pd catalysts all perform significantly better than the pure Pd/TiO₂ and Au/TiO₂ materials. The Au–Pd/carbon catalysts gave the highest rate of H₂O₂ production, and the order of reactivity observed is: carbon > TiO₂ > Al₂O₃. Catalysts were prepared by co-impregnation of the supports using incipient wetness with aqueous solutions of PdCl₂ and HAuCl₄, and following calcination at 400 °C the catalysts were stable and could be re-used several time without loss of metal. The method of preparation is critical, however, to achieve stable catalysts. No promoters are required (e.g. halides) to achieve the high rates of hydrogen peroxide synthesis. The surface and bulk composition of the gold palladium nanoparticles was investigated by STEM-XEDS spectrum imaging. For TiO₂ and Al₂O₃ as supports the Au–Pd particles were found to exhibit a core-shell structure, Pd being concentrated on the surface. In contrast, the Au–Pd/carbon catalyst exhibited Au–Pd nanoparticles which were homogeneous alloys and X-ray photoelectron studies were consistent with these observations. The origin of the enhanced activity for the carbon supported catalysts is a result of higher H₂ selectivity for the formation of hydrogen peroxide which is due to the surface composition and size distribution of the nanoparticles. The key problem remaining is the sequential hydrogenation of hydrogen peroxide which limits the utilisation of the direct synthesis methodology and this is discussed in detail.