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

Abstract

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 Pd 4Si. In particular, Si-stabilized oxides were detected via Auger spectroscopy for Pd(100) subjected to 800–1000°C in ∼10 −6 Torr O 2. 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 Pd 4Si are compared. It is speculated that both SiO 2- and SiO-like speies form at high and room temperature, respectively, for the O 2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.