The effect of adding Pt to a Pd/SiO2 catalyst for the direct formation Of H2O2 in ethanol that contains H2SO4 and halide ions (Cl- or Br-) has been investigated. The addition of only 5 atom% Pt to a catalyst that contained 0.5 wt.% Pd resulted in a 2.5-fold increase in the rate of peroxide formation with only a small decrease in selectivity. The addition of substantially more Pt caused a decrease in the selectivity; whereas, the peroxide formation rate depended on the amount of Pt, as well as the type and amount of the halide. The Pt had no significant effect on the particle size distribution of the supported bimetallic catalysts. The positive role of Pt on the rate of H2O2 formation can be understood in terms of an electronic modification through which Pt partially reverses the negative influence that the halides have on hydrogen activation. (c) 2008 Elsevier B.V. All rights reserved.