Dynamics of direct H2O2 synthesis from H2 and O2 on a Pd nano-particle catalyst protected with polyvinylpyrrolidone

The kinetics of the H-2-O-2 reaction was studied on Pd-PVP (polyvinylpyrrolidone) nano-particles in water. With W and Br-, Pd-PVP catalyzed the H2O2 synthesis. H-2 and O-2 partial pressure dependencies revealed that the H2O2 formation followed the Langmuir-Hinshelwood mechanism and that the H2O2 selectivity varied greatly with p(H2), which was quite different from the results using Pd/C. In the absence of Br-, the H-2-O-2 reaction primarily yielded H2O, and the reaction rate was very high and proportional to p(H2). Br- adsorption measurements suggested that the proportion of highly unsaturated sites on Pd-PVP was much higher than on Pd/C. In the presence of H+ and Br-, the most unsaturated sites such as a corner site of the Pd particles would be blocked by the HBr adsorption and inert, and the major reaction would proceed on moderately unsaturated sites such as an edge site, which stabilize intermediates more than the least unsaturated sites such as a plane site. (C) 2011 Elsevier Inc. All rights reserved.