Theoretical Revisit of the Direct Synthesis of H2O2 on Pd and Au@Pd Surfaces: A Comprehensive Mechanistic Study

The direct synthesis of H2O2 from H2 and O2 on Pd(111) and Au@Pd(111) surfaces is studied with periodic density functional theory calculations. Ten possible reactions and processes involved in the H2O2 synthesis steps are considered: For O2, (1) O2* + H* → OOH*, (2) O2* → 2O*, and (3) O2* → O2; for OOH, (4) OOH* + H* → H2O2*, (5) OOH* + H* → H2O* + O*, (6) OOH* + H* → 2OH*, (7) OOH* → O* + OH*, and (8) OOH* → OOH; for H2O2, (9) H2O2* → 2OH* and (10) H2O2* → H2O2, 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, O2 dissociation is suppressed; OOH dissociation is more favorable than all OOH hydrogenation reactions; three OOH hydrogenation reactions have comparable activation barriers; the barrier for H2O2 dissociation is also comparable to that for H2O2 desorption. However, on the H atoms coadsorbed Au@Pd(111) surface, the main reactions for H2O2 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 H2O2; however, the O–Au bond is weaker than the O–O bonds. Consequently, the final product H2O2 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 H2O2 from H2 and O2 is to provide H atoms as the feedstock for the hydrogenation of O2.