The issue of selectivity in the direct synthesis of H2O2 from H2 and O2:: the role of the catalyst in relation to the kinetics of reaction

The kinetic behavior in the direct synthesis of H2O2 with Pd-Me (Me = Ag, Pt) catalysts prepared by depositing the noble metals by electroless plating deposition (EPD) or deposition-precipitation (DP) methods on alpha-Al2O3 asymmetric ceramic membrane with or without a further surface coating by a carbon thin layer is reported. The effect of the second metal with respect to Pd-only catalysts considerably depends on the presence of the carbon layer on the membrane support. Several factors in the preparation of these membranes as well as the reaction conditions (temperature, concentration of Br-, pH) determine the selectivity in H2O2 formation, influencing the rate of the consecutive reduction of H2O2 (which is faster with respect to H2O2 decomposition on the metal surface) and/or of direct H-2 + O-2 conversion to H2O. Defective Pd sites are indicated to be responsible for the two unselective reactions leading to water formation (parallel and consecutive to H2O2 formation), but the rate constants of the two reactions are differently influenced from the catalytic membrane characteristics. Increasing the noble metal loading on the membrane not only increases the productivity to H2O2, but also the selectivity, due to the formation of larger, less defective, Pd particles.