(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 . 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  between PdM alloy nanoparticles with a 70:30 Pt:M atomic ratio and an aqueous solution of PtCl42-. The predicted change in the surface Pt electronic structure was confirmed by comparing the calculated shift in the Pt 4f7/2 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  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.
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)