The roles of chloride ions in the direct formation of H2O2 from H2 and O2 over a Pd/SiO2 catalyst in a H2SO4/ethanol system

Hydrogen peroxide was produced directly from H2 and O2 over a Pd/SiO2 catalyst in a slurry of ethanol that had been acidified with H2SO4. All reactions were carried out at atmospheric pressure and at 10 °C. The effects of chloride ions on the reactions involving the formation of H2O2, as well as the formation of water via the combustion of H2 and the reduction or decomposition of H2O2, were investigated by adding small amounts of HCl to the system. In the absence of chloride ions, the net rate of H2O2 formation was very small, because the combustion reaction was dominant. At a Cl− concentration of image, the formation of H2O2 proceeded at a reasonable rate, with a selectivity of ca. 45% based on H2 conversion. During the early stages of the reaction, hydrogen was consumed both in the formation of H2O2 and in the combustion reaction, but as more peroxide was formed, the reduction of H2O2 became a significant reaction. The positive effect of Cl− on the surface Pd appears to result from the inhibition of Osingle bondO bond breaking in both dioxygen and hydrogen peroxide. The former effect limits the combustion reaction, and the latter limits the decomposition and reduction reactions. Chloride ions also inhibit the dissolution of palladium in H2SO4; in the presence of small amounts of Cl−, the loss of Pd was generally <20% of the total amount in the catalyst.