XPS identification of the chemical state of subsurface oxygen in the O/Pd(110) system

Adsorbed oxygen at room temperatures exists on Pd(110) in two states, a surface state (beta(2)) and a more weakly bound state (beta(1)), previously designated as ''subsurface'' oxygen. The ''subsurface'' state was postulated as responsible for the oscillations in the kinetics of catalytic CO oxidation observed on this surface. However, previous studies have cast doubt on this explanation. We have investigated the interaction of oxygen on Pd(110) by temperature programmed desorption (TPD), ultraviolet photoemission spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). The XPS combines conventional UHV with in-situ surface analysis in controlled atmospheres of up to 1 mbar. UPS work function measurements show a decrease with oxygen adsorption, coinciding with beta(1) formation observed in TPD as reported previously. XPS measurements of the Pd 3d(5/2) line show only oxygen induced surface core-level shifts (SCLS) for oxygen exposures as high as 22 800 L (p(O2) = 1.0 x 10(-4) mbar and T = 400 K). However, surface-sensitive XPS measurements and TPD for higher oxygen exposures (p(O2) = 4.0 x 10(-2) mbar for 20 min at T = 400 K) show oxide formation coinciding with a corresponding high increase in beta(1) formation. These results for oxygen pressures and temperatures in the oscillation regime, suggest that the oscillatory behavior on this surface may be due to an oxidation and reduction mechanism.