Structural stability and chemisorption properties of a stepped palladium surface: O 2 and no on Pd(331)
The Pd(331) surface is found to thermally facet at temperatures >900 K to a surface containing a mixture of (320), (230) and (111) planes. The original surface may be restorted by treatment with oxygen at 300 K, and is also stabilized in the presence of sulphur contamination. A consideration of the thermodynamic and kinetic aspects of these processes indicates that the energetics of the two surface forms are finely balanced and that the surface diffusion coefficient for Pd is greatly lowered in the presence of adsorbed oxygen. The chemisorption of NO and O 2 on the (1 × 1)-(331) surface is studied by LEED, AES and thermal desorption techniques, and the results are compared with those obtained by previous workers on the low index planes. During thermal desorption both N 2 and N 2O as well as NO are observed. The degree of dissociation in the NO adlayer is shown to depend both on the substrate temperature and also the NO coverage. The presence of surface steps seems to offer a low energy pathway for nitrogen atom recombination. The reduction of the NO adlayer in the presence of CO is discussed and appears to proceed via a direct molecular pathway, i.e. without prior surface dissociation of the NO. Oxygen adsorbs on the Pd(331) surface with an initial energy of 228 ± 10 kJ mol -1. During high temperature treatment with oxygen a "surface oxide" species is formed, which may be identified by a 3 V chemical shift in its Auger transition relative to the chemisorbed oxygen.