A comparison between the oxygen reduction reaction activity of Pd5Ce and Pt5Ce: the importance of crystal structure

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 crystalizes in two phases, a so-called low temperature phase, L-Pd5Ce, that 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 ≥ 2% compressive strain should be more active than Pt. In L-Pd5Ce, the overlayer is under tensile strain, while 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 kinetic stability of Pt5Ce at low temperatures.