CHARACTERIZATION OF THE OXYGEN-ADSORPTION STATES ON CLEAN AND OXIDIZED IR(110) SURFACES

The adsorption of O-2 and the reaction of CO with O-2 have been invest igated on a clean as well as on two differently oxidized Ir(110)surfac es in the pressure range 10(-6)-10(-1) mbar. X-ray photoelectron spect roscopy( XPS), work-function (WF) measurements via a Kelvin probe, LEE D and rate measurements were employed as experimental methods. Oxygen adsorption causes a total WF change up to 1.2 eV on the clean surface and up to 1.6 eV on the weakly oxidized surface using the clean surfac e as a reference level in both cases. On the strongly oxidized surface , the maximum WF change due to chemisorbed oxygen is about 1.8 eV with respect to the WF level of the oxidized surface, XPS measurements of the strongly oxidized, oxygen-saturated surface show two oxygen states characterized by O Is binding energies of 530.6 and 528.9 eV, with th e first state corresponding to the oxide species and chemisorbed oxyge n while the second state correlates with the chemisorbed state which i s responsible for the large WF increase. Rate measurements during cata lytic CO oxidation, which were conducted by varying p(CO) and p(O2) an d keeping T fixed, showed the usual clockwise rate hysteresis up to 10 (-1) mbar with no indication of oscillatory behavior. (C) 1997 Elsevie r Science B,V.