On the Reaction Mechanism of Direct H2O2Formation over Pd Catalysts

Journal article, 2021

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 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 O2 adsorbs and that the surface intermediates O2, OOH, and H2O2 do not dissociate. We find that these requirements are fulfilled on the stepped PdH(211) surface. Surface steps are 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. 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 H2 and O2 over Pd catalysts. The results imply that the selectivity of the reaction toward H2O2 is enhanced by a high partial pressure of H2, which is in agreement with experimental observations.