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Auger-UPS study of intrinsic and Si-stabilized oxides of palladium

1982; Elsevier BV; Volume: 115; Issue: 3 Linguagem: Inglês

10.1016/0039-6028(82)90383-1

1879-2758

S. D. Bader, Lee J. Richter, T. W. Orent,

Catalytic Processes in Materials Science

Three types of oxide were identified that could persist in the subsurface region of Pd and be responsible for the known surface-property modifications that oxygen pretreatments induce in Pt-group metals. The identifications are based on Auger and HeI UPS studies of Pd(100), PdO and Pd4Si. In particular, Si-stabilized oxides were detected via Auger spectroscopy for Pd(100) subjected to 800–1000°C in ∼10−6 Torr O2. These observations are similar to those recently reported by Niehus and Comsa, and Bonzel et al. for Pt single crystals. The Si, a bulk impurity, and oxygen were characterized by Auger signals at 78 and 506 eV, respectively, while O bonded to Pd has its Auger signal at 511 eV. Polycrystalline PdO exhibited a 2 eV crystal-field splitting of the O 2p emission, in UPS studies below 600°C, due to the tetragonal structure. Above 600°C this splitting collapsed, due to thermal decomposition and a metastable O-trapped-in-Pd species was observed via UPS below 700°C. At 900°C a residual Si-stabilized oxide strikingly similar to that observed on Pd(100) was isolated. The UPS results for these Si-stabilized oxides and that formed by the room-temperature oxidation of Pd4Si are compared. It is speculated that both SiO2- and SiO-like speies form at high and room temperature, respectively, for the O2Pd(Si) system. The role of oxygen pretreatments in modifying the chemisorption and catalytic properties of Pd are briefly discussed with respect to the three oxide species observed in this study: (i) PdO, (ii) decomposed or “collapsed” PdO, and (iii) Si-stabilized oxides.