Importance of Pd monomer pairs in enhancing the oxygen reduction reaction activity of the AuPd(1 0 0) surface: A first principles study

• ORR strongly depends on the distribution of Pd atoms in the AuPd(1 0 0) surface.
• Pd monomers pairs can greatly enhance the kinetics of ORR.
• A single Pd monomer slow down the ORR kinetics by suppressing the O-O scission.
• Pd monomers pairs allows O2 adsorption in a bidentate configuration.

Based on density functional theory calculations, we present that pairs of 1st nearest Pd monomers play an important role in significantly enhancing the oxygen reduction reaction (ORR) on the AuPd(1 0 0) surface. While the catalytic ORR activity tends to be sensitive to the surface atomic ordering, we find that the Pd monomer pairs lead to a substantial reduction in the activation barrier for O/OH hydrogenation with no significant suppression of O–O bond scission, thereby considerably lowering the overall activation energy for the ORR as compared to the case of pure Pd(1 0 0). On the other hand, an isolated Pd monomer tends to greatly suppress the O–O bond cleavage reaction, which in turn slows down the ORR kinetics. Unlike the monodentate adsorption of O2 on an isolated Pd monomer, the pairing of Pd monomers allows O2 adsorption in a bidentate configuration and consequently facilitating O–O bond scission. However, the barrier for OH hydrogenation at each Pd site shows no significant change between the isolated and paired cases, while it is noticeably lower than the pure Pd case.

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