Physisorption of N(2), O(2), and CO was studied on fully oxidized TiO(2)(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(2) adsorption. At low coverages (0-0.5 N(2)/Ti(4+)), N(2) molecules bind head-on to five-coordinated Ti(4+) ions. The adsorption occurs preferentially on the Ti(4+) sites that do not have neighboring adsorbates. This arrangement minimizes the repulsive interactions between the adsorbed molecules along the Ti(4+) 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)/Ti(4+)) the nearest-neighbor Ti(4+) 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)/Ti(4+) > 1, an additional low-temperature peak (approximately 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.

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Dosed on rutile TiO 2 (110) at 100 K, the thermal chemistry of 2-propanol in three formssC 3 H 7 OH, C 3 D 7 OD, and C 3 H 7 ODswas characterized using temperature-programmed desorption. Only 2-propanol, propene, and water desorb with no evidence for acetone. The propene forms and desorbs by two paths, a heretofore unreported low-temperature path extending from 300 to 450 K and, concurring with prior work, a high-temperature path peaking between 570 and 580 K. Both paths exhibit isotope effects. The high-temperature path is interpreted in terms of decomposition of 2-propoxy species located on bridging oxygen atom rows. The low-temperature path is attributed to 2-propanol dehydration on undercoordinated Ti 4+ ions of the Ti 4+ rows. The low-temperature path characteristics vary with the long-range order and bridge-bonded oxygen atom vacancy concentration.

Polymerization of formaldehyde, H2CO, was studied under ultrahigh vacuum conditions on a model catalyst consisting of monodispersed (WO3)3 clusters anchored on TiO2(110). Formaldehyde oligomers, (H2CO) n , desorbing from the polymer that formed on the catalyst surface are detected between 250 and 325 K in temperature-programmed desorption experiments. At least two monolayers (ML) of H2CO are required on the surface to observe (H2CO) n , desorption and the amount saturates for H2CO coverages in excess of ∼40 ML. The presence of H2CO multilayers is required for the polymerization to take place indicating that it had to occur below 100 K. The saturation amount increases with increasing coverage of (WO3)3 clusters with the highest amount of ∼13 ML observed on 1.2 (WO3)3/nm2. No (H2CO) n desorption was observed on the bare TiO2(110) surface.