Effect of Temperature and Pressure in the Photocatalytic Oxidation of n-Octanol on Partially Desilanized Hydrophobic TiO2 Suspended in Aerated Supercritical CO2

Abstract

Reaction temperature and CO2 pressure affect the rate of reaction in photocatalytic oxidations in supercritical (sc) CO2 on partially silanized TiO2. In supercritical CO2, the photocatalytic oxidation of octanol appeared to follow pseudo-first-order kinetics, with octanal being produced as the initial product. At 309 K, the pseudo-first-order rate constant kdis decreased upon increasing CO2 pressure from 8 to 20 MPa, giving rise to an increased yield of octanal. When the temperature was raised within the range from 299 to 319 K at constant CO2 pressure (10 MPa), kdis increased while the net yield of octanal decreased. These changes are rationalized as being caused by changes in near-surface CO2 density and, hence, in efficiency of mass transport of octanal to and from the catalytically active site. The observed dependence of the photoactivity of hydrophobic T805 TiO2 in scCO2 on temperature and pressure likely varies with surface conditions promoted by irradiation, which effects partial desilanization photocatalytically active sites. When scCO2 density was very low, T805 TiO2 did not suspend well into the scCO2 fluid; that is, almost all of the T805 TiO2 precipitated on a window or inner wall of the cell. At scCO2 densities between 8 and 20 MPa, T805 TiO2 was efficiently dispersed under stirring. At very low pressures near the critical point, the kinetic analysis was complicated by incident light scattering caused by suspended particles which act as inhibitory inner filters.