The hydrogen anode in polymer electrolyte fuel cells (PEFCs) is vulnerable to poisoning from traces of carbon monoxide in the reformed hydrogen fuel, due to strong CO adsorption on the platinum surface. Over the past several decades, there have been efforts to develop catalysts for the hydrogen oxidation reaction (HOR) that are CO-tolerant by alloying Pt with other metals, e.g., Fe, Co, Ni and Ru. The latter is a well-known commercial HOR catalyst, but Ru can be replaced with the other, less costly metals. The CO tolerance is expected to be greater for catalysts with decreased CO adsorption strength. Recently, we have found that Pt-Fe and Pt-Co alloys with specially stabilized Pt “skins” with x atomic layers, Pt_xAL–PtM/C have excellent CO tolerance, HOR activity and durability.^1,2 More specifically, the CO tolerance and HOR activity increase in the order Pt < Pt-Ru < Pt-Ni < Pt-Co < Pt-Fe.² This trend is the same as that for decreasing CO adsorption strength on (110) steps and (111) terraces, as calculated with density functional theory (DFT). It is also the same trend as that for atomic H adsorption on (111) terraces, while the trend is not followed for the (110) steps: the adsorption strengths for H on (110) steps on Pt_xAL–PtFe(221), Pt_xAL–PtCo(221), and Pt_xAL–PtNi(221) are all very similar. Thus, we have proposed a new type of metal-metal spillover mechanism, in which H₂ adsorbs dissociatively at the (110) steps and then diffuses to the (111) terraces, from which the 2H_ad desorb oxidatively to produce 2H⁺ (Fig. 1). Our understanding of these catalysts has been deepened by in situ Fourier transform infrared (FTIR) spectroscopy³ and X-ray absorption spectroscopy (XAS).⁴ The FTIR spectra can distinguish between step edge and terrace sites for CO adsorption and can monitor the diffusion of CO from edges to steps in real time. The XAS studies have led to a deep understanding of both the structural and electronic properties of the nanoparticulate Pt_xAL–PtCo/C catalyst in comparison with Pt/C. When the electrodes are held at low potentials, within the H adsorption region, the Pt particles undergo significant structural (as seen in the EXAFS) and electronic transformations (as seen in the XANES), whereas the Pt_xAL–PtCo particles are remarkably stable, due to the shorter Pt-Co bond lengths and rigid framework. The same is true of CO adsorption. All of these characteristics are consistent with the DFT calculations, including the weakened H and CO adsorption. We are now extending this work in order to develop even more active, CO tolerant and durable HOR catalysts.

References

1. Shi, G.; Yano, H.; Tryk, D. A.; Watanabe, M.; Iiyama, A.; Uchida, H. Nanoscale 2016, 8, 13893-13897.
2. Shi, G.; Yano, H.; Tryk, D. A.; Iiyama, A.; Uchida, H. ACS Catal. 2017, 7, 267-274.
3. Ogihara, Y.; Yano, H.; Matsumoto, T.; Tryk, D.; Iiyama, A.; Uchida, H. Catalysts 2017, 8 (1-13).
4. Shi, G.; Yano, H.; Tryk, D. A.; Matsumoto, M.; Tanida, H.; Arao, M.; Imai, H.; Inukai, J.; Iiyama, A.; Uchida, H. Catal. Sci. Tech. 2017, in press.

Figure 1. Atomic models of the Pt_xAL–PtCo(221) surface, showing (A) non-dissociative H₂ adsorption, (B) spontaneously dissociated 2H at the (110) step, and (C) 2H after diffusion to the (111) terraces.