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 H-2 and O-2 over Pd catalysts is studied using density functional theory calculations and mean-field kinetic modeling. The state of the catalyst as a function of reaction 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 O-2 adsorbs and that the surface intermediates O-2, OOH, and H2O2 do not dissociate. We find that these requirements are fulfilled on the stepped PdH(211) surface. Surface steps are needed for O-2 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. Comparative calculations for the Pd(111) surface show that this surface is inactive for both H2O2 and H2O formation below room temperature for typical reaction mixtures. Our findings demonstrate the importance of surface steps and high hydrogen coverage for direct synthesis of H2O2 from H-2 and O-2 over Pd catalysts. The results imply that the selectivity of the reaction toward H2O2 is enhanced by a high partial pressure of H-2, which is in agreement with experimental observations.

Automated summary

Hydrogen peroxide (H2O2) is an effective green oxidant used in various industrial processes. This study focused on the reaction mechanism for direct formation of H2O2 from H2 and O2 over Pd catalysts, finding that Pd is in a hydride phase under typical reaction conditions. The importance of surface steps and high hydrogen coverage for enhancing the selectivity of the reaction towards H2O2 was demonstrated in the results.