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)), pre viously 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 stu dies have cast doubt on this explanation. We have investigated the int eraction of oxygen on Pd(110) by temperature programmed desorption (TP D), ultraviolet photoemission spectroscopy (UPS) and x-ray photoelectr on spectroscopy (XPS). The XPS combines conventional UHV with in-situ surface analysis in controlled atmospheres of up to 1 mbar. UPS work f unction measurements show a decrease with oxygen adsorption, coincidin g with beta(1) formation observed in TPD as reported previously. XPS m easurements of the Pd 3d(5/2) line show only oxygen induced surface co re-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 me asurements and TPD for higher oxygen exposures (p(O2) = 4.0 x 10(-2) m bar for 20 min at T = 400 K) show oxide formation coinciding with a co rresponding high increase in beta(1) formation. These results for oxyg en pressures and temperatures in the oscillation regime, suggest that the oscillatory behavior on this surface may be due to an oxidation an d reduction mechanism.