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

The direct synthesis of H2O2 at low temperature (2 degrees 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 degrees C the catalysts were stable and could be reused 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.