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.