Abstract
TiO2 has been synthesized within the pores of carbon molecular sieve fibers (CMSF) in order to grow particles of quantum size. TiO2/CMSF characteristics were followed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and UV–vis diffuse reflectance. XPS showed that all Ti cations are in a +4 oxidation state. The reduction profile of Ti cations (made by preferential O anion removal due to Ar+ sputtering), as evidenced by Ti+x/Ti+4 cations, is very similar to that already observed for well-defined TiO2 surfaces. The absence of XRD pattern indicated that TiO2 particles are in an amorphous form. UV–vis diffuse reflectance showed a considerable blue shift (ΔE=0.6–0.7 eV) of the band gap of TiO2/CMSF when compared to TiO2 (anatase). This shift translates an average particle radius of 15±2 Å. Larger TiO2 particles, outside the CMSF nanopores, are, however, observed by TEM. Dark- and photoreactions of ethanol and acetaldehyde have been investigated over TiO2/CMSF by steady state kinetics and temperature programmed desorption in UHV conditions, as well as in batch conditions at atmospheric pressure. UHV-steady state ethanol reactions have shown eightfold increase in the reaction rate at 573 K in the presence of UV when compared to dark reactions at the same temperatures. The rate constants ratio k2K2/k1K1, for the photoreactions of ethanol, is ca. 40 times higher for TiO2/CMSF than for TiO2 (powder) indicating the high selectivity of the former toward total conversion of ethanol to CO2 with minor accumulation of acetaldehyde (k1K1 and k2K2 are the rate constants for ethanol to acetaldehyde and acetaldehyde to CO2, respectively). Evidence of C–C bond dissociation is given by formaldehyde desorption during UV-acetaldehyde-TPD over TiO2/CMSF under UHV conditions. Moreover, UV-acetaldehyde-TPD indicated a twofold increase of the reductive coupling product (butadiene). The latter requiring Ti+x (x<4) to be formed translates an increase of Ti+x populations under UV illumination.