The chemisorption of CO on clean and oxidized Ir(110)

Abstract

The chemisorption of CO on clean and oxidized Ir(110) has been investigated under ultra-high vacuum conditions with thermal desorption mass spectrometry, contact potential difference measurements, Auger electron spectroscopy, and LEED. On both surfaces, adsorption is nondissociative under all conditions examined, i.e., temperatures between 90 and 1300 K and pressures below 10−6 torr. Two LEED patterns, p (2×2) and (4×2) disordered ones in the [001] direction, form on the clean surface. A (2×1) p1g1 structure forms on the oxidized surface. Thermal desorption spectra from the clean and oxidized surfaces exhibit three and two features, respectively. The desorption energy and pre-exponential factor in the Arrhenius equation describing the desorption kinetics have been measured as functions of surface coverage. The compensation effect occurs on both surfaces. In the limit of zero coverage, the desorption energies are 37 and 31 kcal/mole for clean and oxidized Ir(110), respectively. The adsorption kinetics are identical for both surfaces. At 90 K, the sticking probability is unity until the surface coverage nears saturation. At 300 K, the adsorption kinetics display second-order behavior with an initial sticking probability of unity. The saturation coverage is about 1015 molecules/cm2 for both surfaces at 90 and 300 K. Upon adsorption, the work function increases for both surfaces, but is larger for the clean surface and at lower temperatures. A model developed to fit the change in work function with coverage and temperature indicates that binding sites between the topmost rows of surface atoms are preferred energetically.