Direct synthesis of hydrogen peroxide from H2 and O2 using Al2O3 supported Au-Pd catalysts

The direct synthesis of hydrogen peroxide from H-2 and O-2 using a range of alumina supported Au, Pd, and Au-Pd metal catalysts is described and discussed in detail for both fresh catalysts and materials that have been aged in storage. The conditions previously identified as being optimal for hydrogen peroxide synthesis (i.e., low temperatures (2 degrees C) and short reaction residence times) are employed, and the effect of catalyst composition and preparation was investigated in detail for the fresh catalysts. The addition of Pd to the Au catalyst increases the rate of hydrogen peroxide synthesis as well as the concentration of hydrogen peroxide formed. Interestingly, the addition of relatively small amounts of Pd to the Au/Al2O3 catalyst, replacing up to 20% of the Au, significantly increases the formation of hydrogen peroxide. The microstructure of the catalysts was studied using transmission electron microscopy and X-ray photoelectron spectroscopy for both the fresh and the aged catalyst, and the supported bimetallic particles were found to be of core-shell morphology with a Pd-rich surface. Hence, relatively little Pd is needed to induce the enhanced catalytic formation of hydrogen peroxide. Calcination at 400 degrees C leads to the formation of stable reusable catalysts, whereas lower temperature pretreatments generate unstable catalysts that leach both Au and Pd into solution. Catalysts that have been aged in storage give an increase in activity for hydrogen peroxide synthesis by a factor of ca 3, and the relationship between the catalyst structure and this activity is discussed. The catalysts were also investigated for CO oxidation at 25 degrees C, and all were found to be inactive or almost inactive; similarly, catalysts that are very effective for low-temperature CO oxidation were found to be totally inactive for H-2 oxidation to H2O2. This suggests an inverse correlation between catalysts that are active for either CO or H-2 activation, and this is commented on with respect to the mechanism.