Physisorption of N2, O2, and CO on fully oxidized TiO2(110)

Physisorption of N-2, O-2, and CO was studied on fully oxidized TiO2 (110) using beam reflection and temperature-programmed desorption (TPD) techniques. Sticking coefficients for all three molecules are nearly equal (0.75 +/- 0.05) and approximately independent of coverage suggesting that adsorption occurs via a precursor-mediated mechanism. Excluding multilayer coverages, the TPD spectra for all three adsorbates exhibit three distinct coverage regimes that can be interpreted in accord with previous theoretical studies of N, adsorption. At low coverages (0-0.5 N-2/Ti4+), N-2 molecules bind head-on to five-coordinated Ti4+ ions. The adsorption occurs preferentially on the Till sites that do not have neighboring adsorbates. This arrangement minimizes the repulsive interactions between the adsorbed molecules along the Till rows resulting in a relatively small shift of the TPD peak (105 -> 90 K) with increasing coverage. At higher N-2 coverages (0.5-1.0 N-2/Ti4+) the nearest-neighbor Till sites become occupied. The close proximity of the adsorbates results in strong repulsion thus giving rise to a significant shift of the TPD leading edges (90 -> 45 K) with increasing coverage. For N-2/Ti4+ > 1, an additional low-temperature peak (similar to 43 K) is present and is ascribed to N-2 adsorption on bridge-bonded oxygen rows. The results for O-2 and CO are qualitatively similar. The repulsive adsorbate-adsorbate interactions are largest for CO, most likely due to alignment of CO dipole moments. The coverage-dependent binding energies of O-2, N-2, and CO are determined by inverting TPD profiles.