Carbon monoxide is a common contaminant in hydrogen produced by the reformation of natural gas and acts as a poison for platinum-based catalysts in hydrogen anodes in polymer electrolyte fuel cells (PEFCs). CO poisoning can be alleviated through the use of Pt skin-covered Pt alloys including first row transition metals, e.g., Pt-Fe, Pt-Co and Pt-Ni, as well as second row transition metals, e.g., Pt-Ru, on which CO is known to adsorb more weakly than on pure Pt.¹ Thus, hydrogen would be able to compete more successfully for adsorption sites. The weakened CO adsorption has usually been explained on the basis of the d-band center being lowered and antibonding orbitals becoming populated. However, our recent density functional theory (DFT) results, together with a range of experimental results, as reported by our group (H. Yano, et al.) separately at this Meeting, suggest that the situation is somewhat more complicated.^2-5 In addition, little attention has been paid to the hydrogen oxidation reaction (HOR) itself, which we have found to be enhanced on these surfaces in parallel with experimental work.^2,3 Our calculations indicate that the adsorption of H atoms is also weakened, so that the latter can become more mobile on the surface. The largest degree of weakening was found for the Pt-Fe alloy.³ Making use of realistic models for the surfaces, with (110) steps and (111) terraces, we found evidence for a new type of hydrogen spillover mechanism, in which H₂ adsorbs at steps, dissociates, and the dissociated H atoms diffuse to (111) terraces.^3,4 Finally, the calculations indicate that the Pt skin/Pt alloy structure is more rigid than that of pure Pt, which might be a factor in the enhanced durability of the catalyst under fuel cell operating conditions.⁵

This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cells (SPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References

1. H. Igarashi, T. Fujino, Y. Zhu, H. Uchida, and M. Watanabe, Phys. Chem. Chem. Phys. 3, 306 (2001).
2. G. Y. Shi, H. Yano, D. A. Tryk, M. Watanabe, A. Iiyama, and H. Uchida, Nanoscale 8, 13893 (2016).
3. G. Y. Shi, H. Yano, D. A. Tryk, A. Iiyama, and H. Uchida, ACS Catal. 7, 267 (2017).
4. Y. Ogihara, H. Yano, T. Matsumoto, D. A. Tryk, A. Iiyama, and H. Uchida, Catalysts 7, 8 (2017).
5. G. Y. Shi, et al., to be submitted.

Fig. 1. Adsorption energies for (A) H₂ or 2H, (B) CO, and (C) H₂O at step edges (triangles) and terraces (squares). In (A), the solid symbols denote dissociated 2H, while the open symbols denote undissociated H₂. In all cases except for PtRu, undissociated H₂ does not adsorb on the surface, either at step edges or terraces. On pure Pt(221), H₂ dissociates spontaneously when close to the surface, while on Pt_1AL–PtFe, Pt_1AL–PtCo and Pt_1AL–PtNi, it “floats” away from the surface.³