LEED and ELS study of the initial oxidation of Pd(100)

Oxygen-Pd(100) interactions were studied by means of LEED, AES and ELS to characterize the initial oxidation and elucidate the relationship between surface-oxide reconstructions and bulk oxidation. Five oxygen-related LEED patterns were identified and classified into three types. First, the chemisorption structures included the usual p(2 × 2) which was replaced by a c(2 × 2) upon increased exposure to oxygen at room temperature. Second, surface-oxide reconstructions included a p(5 × 5) which formed above 200°C and which was replaced by a (√5 × √5) R27° above 300°. These two structures are attributed to PdO(110) and PdO(001) coincidence lattices, respectively. Above approximately 500°C the oxygen was removed due to thermal desorption and possibly absorption, but a multilayer oxide, distinct from PdO, could form upon oxygen exposure above 800°C. The ELS study indicated that this last species had significantly different electronic properties than that of the other structures. It is quite plausibly a Si-stabilized oxide as has been suggested recently by Niehus and Comsa for the Pt-oxygen system.