(Invited) Electrochemical Properties for UHV-Prepared Pt/M Well-Defined Single Crystal Surfaces

Background

Discussion on oxygen reduction reaction (ORR) mechanisms¹⁾ of alloy catalysts is complicated because of uncertainties in outermost surface structures. To clarify the effect of catalyst’s outermost surface structures on ORR, Markovic and co-workers²⁾ evaluated ORR activities single-crystal planes of Pt₃Ni. Their pioneering work clearly demonstrates that design of atomic arrangements of the catalyst surfaces directly links to developments in superior PEFC cathode electrode catalysts. To date, EC properties for the Pt-based single crystal alloy surfaces have been reported^3-9): our approach is well-defined model-catalyst fabrications by using molecular beam epitaxy in ultra-high vacuum (UHV:~10^-8Pa) condition. In this study, ORR activities and structural stabilities for the UHV-prepared well-defined Pt-based bimetallic (Pt/M) surfaces are discussed.

Experimental

The MBE apparatus and UHV-EC sample transfer system have been described earlier¹⁰⁾. Pt-shell layers were prepared on fcc single crystal substrates through electron-beam vacuum depositions of another elements and thermal annealing in UHV. The resulting surface structures were verified with reflection high-energy electron diffraction (RHEED), IR reflection absorption (IRRAS) spectra for carbon monoxide (CO) adsorption, and scanning tunneling microsope (STM). The UHV-prepared samples were transferred to a N₂-purged glove box without air exposure. CV curves are recorded in N₂-purged 0.1 M HClO₄. After that, LSV measurements were conducted in O₂-saturated 0.1 M HClO₄: RHE is used as a reference electrode.

Results and discussion

Pt/Au(111): On the basis of RHEED and CO-IRRAS measurements, epitaxial growth of Pt on Au(111) was confirmed. On the basis of the hydrogen-adsorption charges (Q_Had), we deduce that the 0.3nm-thick Pt almost covers the Au(111) substrate. The half-wave potential for the Pt_0.3nm/Au(111) was ca. 10mV more positive than that of the Pt(111). In contrast, the potential for the Pt_0.15nm/Au(111) shifted 60mV negatively. The results show that extended Pt(111) domains might correlate with the ORR activity enhancement.

Ni/Pt(111)-skin: A UHV-STM image of the sample fabricated through 1ML-thick Ni deposition on ca. 800K-Pt(111) (Ni/Pt(111)-skin) showed 6-fold symmetric, honeycomb atomic arrangement. However, atomic-scale disorder with a corrugation of 0.06-0.12nm height is present at the topmost surface. XPS analysis showed positive shift of Pt 4f band. The LSV curve of the Ni/Pt(111)-skin shifted 70mV positively, suggesting that remarkable ORR activity enhancement is originated from the strains- and electronic-changes of the topmost Pt(111) lattice.

Pt/Pd(111): Atomically flat Pt_mML/Pd(111) surfaces were prepared through Pt depositions on Pd(111) at 673 K. The LSV curve for the Pt_2ML/Pd(111) showed 20mV positive shift relative to clean Pt(111). To evaluate EC stability of the Pt_mML/Pd(111), potential cycles (0.6~1.0V) were applied in O₂ saturated 0.1M HClO₄. Although the Pt layers less than 2MLs showed decrease in activity with increasing cycles, the activity of the Pt_4ML/Pd(111) remained nearly unchanged during the cycles.

EC Stability: Changes in ORR activities for the Pt-shell layers (Pt/Au(111), Ni/Pt(111)-skin, Pt/Pd(111)) during the potential cycles are summarized in Fig. 1. The results suggest that the Pt-shell layers thicker than 3MLs stabilize the topmost Pt(111) lattice in PEFC conditions. 

 

Acknowledgement

This work was supported by NEDO.

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