Catalytic Properties of Surface Sites on Pd Clusters for Direct H2O2 Synthesis from H2 and O2: A DFT Study

Density functional theory (DFT) calculations were applied for the H 2 –O 2 reaction on Pd catalysts to distinguish the reactivity of coordinatively more unsaturated sites such as corners and edges (site A) and of more saturated sites such as a (111) face (site B). The effect of the cluster size of Pd on the reactivity was examined first and a Pd 12+12+7 cluster was employed as a typical model catalyst. The adsorption energies of H 2, O 2, H 2 O 2, and HBr were greater on site A than on site B. The energy profiles of the H 2 + O 2 reaction suggested that H 2 O 2 would be smoothly produced on site B, whereas the formation of H 2 O and the decomposition of H 2 O 2 would be preferred on site A. The H + and Br – ions adsorbed in pairs more strongly on site A than H 2 and O 2, which would suppress the undesired side reactions on site A to result in improvement of the H 2 O 2 yield. The direct participation of H 2 O in the reaction through hydrogen bonding was also proposed to explain the high H 2 O 2 selectivity observed on Pd catalysts. Furthermore, sulfur poisoning effect on a (111) surface was studied. The findings are all consistent with the reaction mechanism suggested based on the kinetic studies in the previous papers.