Paper SS1-TuM9
Structure and Segregation at Clean and Oxidized Pd₃Fe(111) Alloy Surfaces

Tuesday, October 19, 2010, 10:40 am, Room Picuris

Pd/Fe alloy catalysts have attracted attention in PEM fuel cell research because they have been found to be comparable to Pt electrocatalysts in oxygen reduction reaction (ORR) kinetics at the cathode. Higher electrocatalytic activity is also found when these bimetallic nanoparticles act as a support of Pt or Pd monolayers. The mechanism of enhancing ORR kinetics with this alloy is not well understood. We report here on the structure and segregation at a clean and oxidized Pd₃Fe(111) alloy using LEED, LEIS, XPS, and STM. Pd starts to segregate at the surface at 600 K and reaches a coverage of 0.9 monolayers at 1100 K. STM reveals a complex structure with 0.17 monolayers of Pd adatoms on top of the outmost alloy layer, which is different from other well-studied binary alloy systems. The experimental observation of this unusual Pd structure is supported by DFT calculations. Adsorption of oxygen reverses the segregation trend and causes Fe atoms to accumulate on the surface. Fe at the surface is oxidized by oxygen and initially protects Pd oxidation. Oxygen adsorption at different temperatures caused the formation of nanclusters at lower temperatures (300-500 K) and several well-organized surface structures at higher temperatures. Such studies of structure, segregation and oxidation of these model Pd/Fe alloy catalysts may help explain the origin of enhanced ORR kinetics using these alloys.