(211e) 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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