A mechanistic study of H2O2 and H2O formation from H2 and O2 catalyzed by palladium in an aqueous medium

The simultaneous determination of H2 conversion, selectivity for H2O2, and concentration of H2O2 has provided insight into the pathways for the reactions of H2 and O2 over colloidal palladium in an aqueous medium. Reactions typically were carried out at an O2/H2 ratio of 4 and at 10 °C by introducing the gases at 1 atm into a solution that was 0.1 N in HCl and 0.01 M in Br− ions. Palladium was added to the system as the metal supported on microspheroidal silica. Concentrations of H2O2 approaching 2 wt% have been achieved with selectivities, based on H2 reacted, in excess of 90%. Water is formed both by a parallel reaction between H2 and O2 and by a secondary reduction of H2O2 by H2. The parallel reaction is inhibited by the presence of bromine. Hydronium ions can serve as the source of hydrogen and may be involved in a catalytic cycle that includes their reaction with peroxide species to form H2O2.