ORR Activity for Pt/Pd(111) Bimetallic Surface Prepared By Molecular Beam Epitaxy

Tuesday, 7 October 2014
Expo Center, 1st Floor, Center and Right Foyers (Moon Palace Resort)
Y. Bando, Y. Takahashi, E. Ueta, N. Todoroki, and T. Wadayama (Graduate School of Environmental Studies,Tohoku University)

  From the view-points of reduction in usage of expensive Pt and enhancement in oxygen reduction reaction (ORR) activity [1], Pt-shell and Pd-core, core-shell type nano-particles attract much interest as cathode-electrode materials for polymer electrolyte fuel cells. Relation between topmost surface structures of the core-shell nano-particles and ORR activities is a key for developing highly-active, low-Pt-content catalysts. However, disscussion of ORR mechanism is complicated. In this study, we prepared well-defined Pt/Pd(111) bimetallic surfaces by molecular beam epitaxy (MBE) in ultra-high vacuum (UHV) and performed EC measurements in an inert gas atomosphere.


  The MBE apparatus and UHV-EC sample transfer system have been described earlier [2]. All sample fabrication processes were conducted in UHV. Pd(111) single crystal substrate was cleaned by Ar+ sputtering and annealing. Various-thick Pt was deposited onto the clean Pd(111) by an electron-beam evaporation method at 673K. The resulting surface structures were verified with low energy electron diffraction (LEED),   scanning tunelling microscope in UHV (UHV-STM). Then, the MBE-prepared Ptxnm/Pd(111) surfaces were transferred without being exposed to air to the EC system set in an N2-purged glove box. Cyclic voltammogram (CV) of the samples were recorded in N2-purged 0.1M HClO4, and, then, linear sweep voltammetry (LSV) was conducted by a rotating electrode (RDE) method at 1600 rpm after saturating the solution with O2. EC degradation of the Ptxnm/Pd(111)  was evaluated by applying potential cycles between 0.6V (3s) and 1.0V (3s) vs. RHE. The potential-cycled Ptxnm/Pd(111) were re-transferred to the UHV-STM system to evaluate the potential-cycle-induced topmost surface structural changes.

Results and discussion

  From the results obtained by LEED and IRRAS for carbon monoxide adsorption, we deduce that 0.6nm-thick Pt almost covers the Pd(111) substrate surface. A UHV-STM image of the Pt0.6nm/Pd(111) (inset in Fig. 1 (a)) clearly shows atomically-flat, wide terraces.

  Fig. 1 (a) shows CV curves of the Pt0.6nm/Pd(111) (blue). CV curves for clean Pt(111) (black) and Pd(111) (grey) are also presented as references. EC charge of the H-related features (0.15V-0.3V ; Qab&de) for the Pt0.6nm/Pd(111) markedly shirinks in comparison to that of clean Pd(111), suggesting that the topmost surface of the Pt0.6nm/Pd(111) is epitaxial Pt(111).

  Fig. 1 (b) shows LSV curves for the respective surfaces. A half-wave potential of the as-prepared Pt0.6nm/Pd(111) (blue) shifts positively relative to clean Pt(111), indicating ORR activity enhancement. The LSV curve of the as-prepared Pt0.6nm/Pd(111) shifts to lower potentials after the 1000 potential cycles (red), revealing potential-cycle induced surface degradation. One might notice that the red LSV curve is almost identical to that for 0.15nm-thick-Pd deposited Pt(111) (Pd0.15nm/Pt(111);green). The results obtained in this study suggest that the topmost surface of MBE-prepared Pt0.6nm/Pd(111) is degraded through dissolution and precipitation of the substrate Pd atoms during the potential cycles.


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


[1] J. Zhang et al. J. Phys. Chem. B, 108, 10955 (2004).

[2] Y. Iijima, T. Wadayama et al., J. Electroanal. Chem., 685, 79 (2012).