Real Surface Structures of the High Index Planes of Pt3Co in Electrochemical Environments
Authors: Hoshi Nagahiro , Kihara Yohei , Nakamura Masashi 
Published: February 2020

Pt3Co and Pt3Ni alloys have extremely higher activity for the oxygen reduction reaction (ORR) than Pt electrodes [1,2]. Structural effects on the ORR were studied on the high index planes of Pt3Co [3] and Pt3Ni [4]. The ORR activity increases with the increase of the step atom density d
S on n(111)-(111) series of Pt3Co, whereas the activity of the same series decreases with increasing d
S on Pt3Ni. The ORR activities plotted against d
S gives volcano shape on n(111)-(100) series of Pt3Co and Pt3Ni. The ORR activity depends on Pt/M ratio in the inner layers [5] and atomic distance. Pt oxides also affect the ORR activity [6，7]. However, no paper has reported real surface structure and adsorbates of the high index planes of Pt alloys. In this paper, we have studied the real surface structures of Pt3Co(331) = 3(111)-(111) and Pt3Co(544) = 9(111)-(100) that have the highest activity for the ORR [3] using surface X-ray scattering (SXS) and infrared reflection absorption spectroscopy (IRAS).

SXS was measured at 0.5 (double layer region) and 0.9 V(RHE), at which the ORR activity is estimated, in 0.1 M HClO4at BL13XU of SPring-8. IRAS spectra were measured between 0.2 and 1.2 V(RHE) using SNIFTIRS in 0.1 M HF. Reference spectra were collected at 0.1 V(RHE). All the potentials were referred to RHE.

SXS shows that Pt skins are formed on Pt3Co(331) and Pt3Co(544) as is the case of the low index planes of Pt3Ni [2]. Both surfaces have unreconstructed (1×1) structure. Ratios of Co between 2nd and 5th layers are higher than that of the bulk on Pt3Co(331) at 0.9 V(RHE). On Pt3Co(544), however, ratios of Co at 2nd and 3rd layers are as high as that of the bulk, whereas the rations below the 4th layers are higher than that of the bulk. Interlayer spacing between the 1st and the 2nd layers is contracted at 0.9 V(RHE) compared with the bulk on Pt3Co(331). On Pt3Co(544), atomic distance shrinks at the step.

IRAS spectra give a band around 1100 cm-1 above 0.4 and 0.7 V(RHE) on Pt3Co(331) and Pt3Co(544), respectively. This band is assigned to bending vibration of PtO-H δ(OH) [7]. We measured the IRAS spectra on n(111)-(111) (n=2, 3, 5, 9, ∞) and n(111)-(100) series (n=2, 3, 9, ∞), and plotted the ORR activity against the integrated band intensity of δ(OH). No correlation was found between the ORR activity and the coverage of PtOH on n(111)-(111) (n=2, 3, 5, 9, ∞) and n(111)-(100) series Pt3Co.

These results suggest that high activity of Pt3Co(331) is attributed to the high ratio of Co below the 2nd layer and compression of the interlayer spacing between 1st and 2nd layers. The activity of Pt3Co(544) may originate from the shrink of atomic distance at the step.

Acknowledgement

This work was supported by New Energy Development Organization (NEDO).

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