Surface structure of Pd₃Fe(111): Anomolous surface morphology and implications for electrocatalysis

Pd-Fe alloys have attracted attention in PEM fuel cell research because they were found to be comparable to Pt electrocatalysts in oxygen reduction reaction (ORR) kinetics at the cathode. We report on studies of the surface morphology of a Pd₃Fe (111) single crystal and oxygen reaction on the surface using LEIS, XPS, LEED, and STM. Extensive segregation of Pd was observed after annealing in UHV, and in particular, Pd single adatoms and dimers were found, which differs from other well-studied binary alloy systems. The experimental observations of unusual Pd segregation are supported by DFT calculations. Adsorption of oxygen reversed the segregation trend and oxidized surface Fe, and ordered surface phases were observed at elevated temperatures. The formation of Pd adatoms, reducing activity of Fe atoms in the alloy which inhibits Pd oxidation, and weakened Pd-O interactions on Pd₃Fe(111) are consistent with enhanced ORR kinetics.