Mechanistic Studies of Oxygen Reduction by Hydrogen On PdAg(110)

The reduction of oxygen by hydrogen is a metal-catalyzed reaction important in the direct synthesis of hydrogen peroxide (HOOH) and at the cathode in proton exchange membrane fuel cells (PEMFCs).  Due to its technological applicability and widespread historical interest within the catalysis community, many detailed mechanistic studies of this reaction have been conducted on monometallic and alloy surfaces.  Most recently, bimetallic catalysts containing Pt^{1, 2} have garnered much attention for fuel cell applications and PdAu alloys³ have been the subject of research for hydrogen peroxide synthesis applications.  In this study, first principles electronic structure calculations based on periodic, self-consistent, density functional theory (DFT) were utilized to study the mechanism for oxygen reduction by hydrogen on an alternative Pd-noble metal alloy surface, namely PdAg(110).  Our results demonstrate that the minimum energy pathway involves the initial formation of a peroxyl (OOH) intermediate followed by O-O bond scission, consistent with the minimum energy pathway shown for oxygen reduction by hydrogen on monometallic Pd and Ag (111) surfaces.⁴ The lower activation energy barrier for O-O bond scission in OOH versus hydrogenation of OOH to form HOOH suggests that PdAg is not an effective catalyst for the direct synthesis of hydrogen peroxide.  The detailed thermochemistry and activation energy barriers of important elementary steps and intermediates in oxygen reduction by hydrogen on PdAg(110) are compared and contrasted with the analogous results recently reported for the monometallic surfaces.⁴

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