Characterisation 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 investigated on a clean as well as on two differently oxidized Ir(110)surfaces in the pressure range 10(-6)-10(-1) mbar. X-ray photoelectron spectroscopy( XPS), work-function (WF) measurements via a Kelvin probe, LEED 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 surface 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 the first state corresponding to the oxide species and chemisorbed oxygen while the second state correlates with the chemisorbed state which is responsible for the large WF increase. Rate measurements during catalytic CO oxidation, which were conducted by varying p(CO) and p(O2) and keeping T fixed, showed the usual clockwise rate hysteresis up to 10(-1) mbar with no indication of oscillatory behavior. (C) 1997 Elsevier Science B,V.