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

The simultaneous determination of H-2 conversion, selectivity for H2O2, and concentration of H2O2 has provided insight into the pathways for the reactions of H-2 and O-2 over colloidal palladium in an aqueous medium. Reactions typically were carried out at an O-2/H-2 ratio of 4 and at 10 degreesC 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 H-2 reacted, in excess of 90%. Water is formed both by a parallel reaction between H-2 and O-2 and by a secondary reduction of H2O2 by H-2. 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. (C) 2004 Elsevier Inc. All rights reserved.