We present kinetics data of O2, n/iso-butane, CO2, and CO adsorbed at ultrahigh vacuum conditions on TiO2 nanotube (TiNTs) arrays produced by electrochemical anodization; amorphous and polycrystalline (anatase and mixed anatase/rutile) TiNTs have been studied addressing structure-activity relationships. Oxygen distinctly interacts with the TiNTs, whereas this process is not observed on fully oxidized TiO2 single crystals. Both molecularly and atomically bonded oxygen have been observed. Variations in the binding energies of alkanes were also detected.

Presented are initial, S(0) and coverage, Theta, dependent S(Theta), adsorption probability measurements of CO(2) as a function of impact energy, E(i) = 0.12-1.3 eV, adsorption temperature, T(s) = 85-300 K, hydrogen and oxygen pre-exposure, as well as density of defects, Gamma, as varied by annealing (T = 600-900 K) and Ar(+) ion sputtering (dose chi(Ar) at 600 eV at 85 K) of a rutile (1 x 1) TiO(2)(110) surface. The defect densities were qualitatively characterized by thermal desorption spectroscopy (TDS) of CO(2). The CO(2) TDS curves consisted of two structures that can be assigned to adsorption on pristine and oxygen vacancy sites, in agreement with earlier studies. S(0) decreased linearly with E(i) and was independent of T(s). The adsorption dynamics were dominated by the effect of precursor states leading to Kisliuk-like shapes over the E(i) and T(s) range studied. Oxygen vacancy sites reduced S(0) of CO(2). Preadsorbed oxygen blocked preferentially defect sites, which led to an increase in S(0). Hydrogen preadsorption results in physical site blocking with decreased S(0) as H-preexposure increased, while the shape of S(Theta) curves was conserved. In contrast to oxygen, hydrogen does not adsorb preferentially on defect sites. The adsorption probability data were parameterized by analytic functions (Kisliuk model) and by Monte Carlo simulations (MCSs).

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The adsorption dynamics of n-/isobutane on "closed"-end (as-prepared, c-CNTs) and open-end (vacuumannealed) carbon nanotubes (o-CNTs) have been studied by molecular beam scattering adsorption probability measurements. Thermal desorption spectroscopy (TDS) and scanning electron microscopy have been used to characterize the CNTs. Evident from TDS data, o-CNTs allow internal adsorption sites to be populated, which is correlated with an increase in the initial adsorption probabilities as compared with c-CNTs, consistent with the enhancement in the surface area by opening the tube ends. Furthermore, precursor-mediated adsorption dynamics were observed.