Controlling factors in the direct formation of H2O2 from H2 and O2 over a Pd/SiO2 catalyst in ethanol

The direct formation of H2O2 in ethanol over a Pd/SiO2 catalyst has been studied under conditions that yield good selectivities for the peroxide, moderate rates of formation, and concentrations of H2O2 that approach 2 wt.%. Several factors, including the partial pressures of H-2 and O-2, the presence of protons along with chloride and/or bromide ions, and the reaction temperature significantly affect the rate of H2O2 formation, the selectivity of the reaction, and the loss of palladium from the support. Mass transport, which is a function of a particular system, may also limit the rate of peroxide formation. Halide ions (Cl- or Br-) and protons (derived from H2SO4 in this study) are essential for limiting the combustion reaction; i.e., in the absence of these ions almost no peroxide was formed. A selectivity for H2O2 approaching 80% was achieved using 2 x 10(-5) M Br- and 0.12 M H2SO4 with an O-2/H-2 ratio of 15. Bromide is particularly useful as the halide because it inhibits the loss of Pd from the support. The net formation rate for H2O2 was found to be first order with respect to H-2 and zero order with respect to O-2. The effects of changing the temperature from 5 to 15 degrees C were evaluated, and it was observed that the system was most stable at the lowest temperature. As might be expected, the initial rate of peroxide formation and the conversion of H-2 Was the largest at 15 degrees C, but as the reaction proceeded the rate of peroxide formation and the selectivity decreased. These results further establish the complexity of this three-phase system that involves a network of reactions and the possibility that the catalyst undergoes change with time. (c) 2006 Elsevier B.V. All rights reserved.