The effect of catalyst preparation method on the performance of supported Au-Pd catalysts for the direct synthesis of hydrogen peroxide

The direct synthesis of hydrogen peroxide from H-2 and O-2 offers the possibility of a new green production method for this important commodity chemical. Active catalysts for this reaction are typically prepared using an impregnation method and it is important to identify improvements in the preparation methodology that can result in more active catalysts that retain their stability. The effect of the precise procedure by which the metals are impregnated onto TiO2 and C supports during the preparation of supported Au-Pd catalysts has been investigated and it is shown that the two supports exhibit significant differences. The concentration of the solution of the mixed aqueous solution of HAuCl4 and PdCl2 immediately prior to the initial drying step has a profound effect on the structure and activity of the TiO2-supported catalysts. TiO2-supported catalysts prepared using impregnation with the minimal amount of added water whilst ensuring that the catalyst is not formed into a paste (i.e. still contains ca. 1.5-2 ml of H2O) prior to drying at 110 degrees C exhibit very high activity (ca. 120 mol H2O2 kg(cat)(-1) h(-1)) which is equivalent to the corresponding carbon-supported catalyst. The presence of more water (ca. 2-28 ml) in the catalyst impregnation step prior to drying leads to a significant change in the particle size distribution and a bimodal distribution is observed for the TiO2-supported catalysts. These catalysts also show a change in the nature of the Au and Pd nanoparticles. Unfortunately, TiO2-supported catalysts prepared in this manner are not stable on re-use. However, catalysts prepared using a similar method, but with the removal of ca. 75% of the initial H2O ensuring that a paste is formed prior to drying, are found to be fully re-usable. In contrast, for carbon-supported catalysts dilution of the Au and Pd compounds during the initial impregnation step, coupled with subsequent removal of water to form paste with varying water content, did not affect the activity and these catalysts could be re-used without loss of catalyst performance. The effect of the catalyst structure on activity and re-usability is discussed.