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

First principles electronic structure calculations based on periodic, self-consistent density functional theory (DFT-GGA) were utilized to study the mechanism of the vapor phase reaction between hydrogen and oxygen on the PdAg(110) alloy surface. The hydrogen–oxygen reaction is an important reaction in the direct synthesis of hydrogen peroxide (H2O2) and at the cathode in proton exchange membrane fuel cells (PEMFCs). Our results demonstrate that the minimum energy path involves the initial formation of a peroxyl (OOH) intermediate followed by O–O bond scission, consistent with the minimum energy path shown on the (111) facet of monometallic Pd and Ag surfaces. The lower activation energy barrier for O–O bond scission in OOH versus hydrogenation of OOH to form HOOH, and the low barrier for HOOH decomposition, suggest that PdAg(110) may not be an effective catalyst for the direct synthesis of H2O2. 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 Pd(111) and Ag(111). Based on the potential energy surfaces, Ag(111) is tentatively predicted to be more selective toward H2O2 production than PdAg(110) and Pd(111). The calculated d-band center of the Pd and Ag surface atoms in PdAg(110) reveals that alloying Pd and Ag increases the reactivity of the Ag atoms more than that of the Pd atoms, compared to the respective monometallic close-packed (111) surfaces, and that Ag atoms in PdAg(110) are more reactive than Ag atoms at the step-edge of Ag(211). Still, the overall similarity between the energetics on PdAg(110) and Pd(111) is demonstrated. The Pd surface atoms in PdAg(110) behave as 1D arrays of more active surface sites and essentially dominate surface chemistry.