Surface Structure of Pd 3 Fe(111) and Effects of Oxygen Adsorption
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1217-Y08-43
Surface Structure of Pd3Fe(111) and Effects of Oxygen Adsorption Xiaofang Yang, Lindsey A. Welch, Jie Fu and Bruce E. Koel* Department of Chemistry, and Center for Advanced Materials and Nanotechnology, Lehigh University, Bethlehem, PA 18015 USA ABSTRACT 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. In this study, the surface morphology of a Pd3Fe(111) single-crystal sample and oxygen reaction on the surface were investigated by low energy electron diffraction (LEED), low energy ion scattering (LEIS), x-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM) under ultra-high vacuum (UHV) conditions. Strong segregation of Pd atoms was observed after annealing in UHV. Particularly, Pd single adatoms and dimers were found on the surface after high temperature annealing, which differs from most other well-studied binary alloy systems. Low free energy of Pd, strain relaxation, and interaction between Pd and Fe, are potentially responsible for the formation of this unusual surface. Adsorption of oxygen reversed the segregation trend and oxidized surface Fe. Ordered surface phases were observed after oxygen exposures at elevated temperatures. The reducing activity of Fe atoms in the alloy inhibited Pd oxidation, and weakened Pd-O interactions on Pd3Fe(111) are consistent with enhanced ORR kinetics. INTRODUCTION Pd-based bimetallic catalysts (Pd/M, M=Au, Ag, Fe, etc.) have been widely studied in heterogeneous reactions such as selective hydrogenation of unsaturated hydrocarbons [1], selective oxidation of alcohols to aldehydes [2] and as electrocatalysts in fuel cells [3]. Enhanced catalytic properties in bimetallic catalysts can arise from two different effects, an electronic effect and a geometric effect [1], and so, studying segregation in Pd-based alloys, including adsorption, is important [4]. Bimetallic catalysts are widely studied in fuel cells in terms of their performance as cathodes and anodes [5]. Recently, the study of non-Pt based electrocatalysts has become an increasing research focus. The cost is reduced when inexpensive metals such as Fe, Co and Ni are alloyed in the catalysts. Much higher electrocatalytic activity is also found when these bimetallic nanoparticles act as the support of a Pt or Pd monolayer [6]. To achieve fast oxygen reduction reaction (ORR) kinetics, the binding between oxygen and the active sites should be neither too strong nor too weak. Optimally, oxygen would bond to the surface strongly enough to enhance electron transfer, but weakly enough to allow facile desorption so that site-blocking does not hinder ORR kinetics. To provide a better understanding of these newly developed Pd-based catalysts, it is
critical to fully characterize the surface composition and structure of these materials and to study their interactions with oxygen by applying modern experimental surface science techniques. In research, a well
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