(585d) Volcano Behavior of Pd3M@Pd3Pt Core-Shell Oxygen Reduction Electrocatalysts by Gradually Changing the Oxygen Binding Energies

First principle calculations were used to screen the oxygen reduction activity of a series of Pd₃M@Pd₃Pt core-shell electrocatalysts for direct methanol fuel cells. Density functional theory (DFT) calculations indicate that the subsurface substitution of Pt by a 3d transition metal M improves the activity of the Pd₃Pt catalysts by reducing the oxygen binding energy [1]. Carbon-supported Pd₃M@Pd₃Pt (M=Ni, Co, Fe and Cr) core-shell electrocatalysts with similar particle size, a Pd₃Pt rich surface, and a Pd₃M alloy core were prepared by a galvanic replacement reaction [2] between PdM alloy nanoparticles with a 70:30 Pt:M atomic ratio and an aqueous solution of PtCl₄^2-. The predicted change in the surface Pt electronic structure was confirmed by comparing the calculated shift in the Pt 4f_7/2 core level binding energies with XPS data, and by comparing the change in the calculated CO binding energies with the shift in the position of the CO stripping peak. Optimal activity close to the maximum of a volcano curve [3] is predicted for Pd₃Fe@Pd₃Pt and Pd₃Mn@Pd₃Pt core-shell catalysts. In agreement with the DFT calculations, optimal activity and high methanol tolerance were observed for Pd₃Fe@Pd₃Pt. A lower activity is observed for the Pd₃Cr@Pd₃Pt catalyst with a lower CO binding energy and for the Pd₃Co@Pd₃Pt and Pd₃Ni@Pd₃Pt catalysts with higher CO binding energies. In the presence of 0.1 M methanol, the current density per Pt atom for the optimal Pd₃Fe@Pd₃Pt/C catalyst is 10 times higher than for commercial Pt/C catalysts. DFT calculations further indicate the CO and OH covered Pd₃Fe@Pd₃Pt electrocatalyst is stable towards Fe surface segregation.

References

1. J. Xu, J.H. Yang, J.Y. Lee, M. Saeys, Ind. Eng. Chem. Res., 49, 10251 (2010)

2. J.H. Yang, W.J. Zhou, C.H. Cheng, J.Y. Lee, Z.L. Liu, ACS Appl. Mater. Interfaces 2, 119 (2010)

3. J.K. Norskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. Phys. Chem. B, 108, 17886 (2004)