Oxygen Interaction with Pd(110): Surface Oxide Formation
Physics of Low-Dimensional Structures (up to 2006)
Boreskov Institute of Catalysis SB RAS
Oxygen lateraction with Pd(110) has been studied in a wide range of pressures (Po-2 =5x10(-7) - 10 Pa) at T = 400 K by X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). When the 02 exposure is enhanced by increasing the treatment time and oxygen pressure from 5x10(-) to 10Pa, the amount of oxygen absorbed by Pd(110) sing e crystal increases up to 4.5 ML. The oxygen coverage increase up to 0.5 ML leads to the formation of the c(2x4) structure on Pd(110). When the amount of absorbed oxygen increases from 0.5 to 2 ML, oxygen penetrates into subsurface Pd layers to the depth of more than 15-20 angstrom whereas the c(2x4) structure is preserved. When the amount of oxygen increases from 2 ML to 4.5 ML, a new surface phase is formed on Pd(110) surface. XPS characteristics of this phase were attributed to a surface oxide. It was found that two nonequivalent types of oxygen and two nonequivalent types of palladium atoms appear. Oxygen species (O-i) and (O-so) are characterized by the position of the O1s line at 528.8 and 529.6 eV, respectively. The first oxygen type O-i is located above the surface layer and is similar to ionic species of adsorbed oxygen. Oxygen O-so is located under the surface layer or within it and seems to be characterized by a charge similar to the charge of oxygen in PdO. Nonequivalent states of Pd atoms in the surface oxide are due to the different coordination by oxygen. The interaction of different oxygen species with CO was investigated by the isothermal titration method. Kinetic behavior of the CO reaction was shown to depend on oxygen coverage which can be explained by the CO interaction with adsorbed oxygen accompanied by the surface phase transitions.