On the reaction mechanism of direct H2O2 formation over Pd catalysts

Hydrogen peroxide (H₂O₂) is an effective green oxidant, which is used in many industrial processes. Here, the reaction mechanism for direct formation of H₂O₂ from H₂ and O₂ over Pd catalysts is studied by using density functional theory calculations and mean-field kinetic modeling. The state of the catalyst under various conditions is determined from ab initio thermodynamics. It is found that Pd is in a hydride phase during typical reaction conditions. Reaction landscapes are constructed for the reaction over PdH(111) and PdH(211). Formation of H₂O₂ instead of H₂O requires that O₂ adsorbs and that the surface intermediates O₂, OOH and H₂O₂ do not dissociate. We find that these requirements are fullled on the stepped PdH(211) surface. A stepped surface is needed for O₂ chemisorption as the adsorption on PdH(111) is endothermic. The high H coverage on the surface of the hydride is important to slow down the unwanted scission of the O-O bond and promote the desorption of the products. Our findings demonstrate the importance of surface steps and high hydrogen coverage for direct synthesis of H₂O₂ from H₂ and O₂ over Pd catalysts. The results imply that the selectivity of the reaction towards H₂O₂ is enhanced by a high partial pressure of H₂, which agrees with experimental observations

Reaction cycle for direct H₂O₂ formation over the stepped PdH(211)