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.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films