Theoretical revisit of the direct synthesis of H ₂O ₂ on Pd and Au@Pd surfaces: A comprehensive mechanistic study

The direct synthesis of H ₂O ₂ from H ₂ and O ₂ on Pd(111) and Au@Pd(111) surfaces is studied with periodic density functional theory calculations. Ten possible reactions and processes involved in the H ₂O ₂ synthesis steps are considered: For O ₂, (1) O ₂* + H* → OOH*, (2) O ₂* → 2O*, and (3) O ₂* → O ₂; for OOH, (4) OOH* + H* → H ₂O ₂*, (5) OOH* + H* → H ₂O* + O*, (6) OOH* + H* → 2OH*, (7) OOH* → O* + OH*, and (8) OOH* → OOH; for H ₂O _2, (9) H ₂O ₂* → 2OH* and (10) H ₂O ₂* → H ₂O ₂, where the asterisks indicate these species to be surface species. All side reactions involve O-O bond dissociation. On the Pd(111) surface with H atoms coadsorbed, O ₂ dissociation is suppressed; OOH dissociation is more favorable than all OOH hydrogenation reactions; three OOH hydrogenation reactions have comparable activation barriers; the barrier for H ₂O ₂ dissociation is also comparable to that for H ₂O ₂ desorption. However, on the H atoms coadsorbed Au@Pd(111) surface, the main reactions for H ₂O ₂ production exceed all side reactions. The competition between the main reactions and the side reactions is actually the competition between the O-O bond and the O-M bond, where M is Pd in the case of the Pd(111) surface and Au in the case of the Au@Pd(111) surface. The O-Pd bond is usually stronger than the O-O bonds in the OOH intermediate and H ₂O ₂; however, the O-Au bond is weaker than the O-O bonds. Consequently, the final product H ₂O ₂ is easily produced and released from the Au@Pd(111) surface, and the side reactions involving O-O bond dissociation are suppressed. The role of the metal surface in the direct synthesis of H ₂O ₂ from H ₂ and O ₂ is to provide H atoms as the feedstock for the hydrogenation of O ₂.