Comparison between the Oxygen Reduction Reaction Activity of Pd5Ce and Pt5Ce: The Importance of Crystal Structure

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• 作者：Vladimir Tripkovic ; Jian Zheng ; Gian Andrea Rizzi ; Carla Marega ; Christian Durante ; Jan Rossmeisl ; Gaetano Granozzi
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：October 2, 2015
• 年：2015
• 卷：5
• 期：10
• 页码：6032-6040
• 全文大小：516K
• ISSN：2155-5435

文摘

A set of electrochemical and X-ray spectroscopy measurements have been used conjointly with density functional theory (DFT) simulations to study the activity and stability of Pd₅Ce for the oxygen reduction reaction. A polycrystalline Pd₅Ce rod has been selected as a model catalyst to test if results on a several-fold activity increase of a series of Pt/rare-earth alloys hold also for Pd rare-earth alloys. Pd₅Ce crystallizes in two phases, a so-called low-temperature phase, L-Pd₅Ce, which has a cubic symmetry, and a high-temperature phase, H-Pd₅Ce, with a hexagonal symmetry. In both cases, a several-layers-thick Pd skin forms on the surface. Preliminary DFT results show that Pd overlayers under 鈮?% compressive strain should be more active than Pt. In L-Pd₅Ce, the overlayer is under tensile strain, whereas in H-Pd₅Ce (a structure similar to Pt₅Ce), it is under compressive strain. We have confirmed that in our sample, L-Pd₅Ce is the dominant phase, both in the bulk and the outermost layers, while a H-Pd₅Ce-like phase is also present as a minor component far below the surface. Electrochemical ORR assessments show that the Pd overlayer in Pd₅Ce is less active than the polycrystalline Pd sample, in agreement with DFT results for the L-Pd₅Ce phase. Although we did not discover a new promising Pd-based catalyst, we have shown that the activity for oxygen reduction is strongly influenced by the alloy crystal structure. Furthermore, we have qualitatively demonstrated that transformation from H-Pd₅Ce to L-Pd₅Ce is more facile, requires less atom rearrangement, than transformation from Pt₅Ce to Pt₃Ce, which might explain the kinetic stability of Pt₅Ce at low temperatures.