Photocatalytic degradation of oxalic acid on a semiconductive layer of n-TiO2 particles in a batch plate reactor. Part III: Rate determining steps and nonsteady diffusion model for oxygen transport

The effect of oxygen concentration on the photocatalytic degradation rate o f oxalic acid on a fixed layer of TiO2 particles in a batch mode plate phot oreactor was investigated at various light intensities. The regions where t he photocatalytical decomposition rate is controlled by the flux of oxygen, photons, or both, were identified. For low oxygen concentration (0-0.15 mo l m(-3)) and photon flux intensity in the range from 10 to 24 x 10(-5) eins tein m(-2) s(-1) the experimentally determined photocatalytical decompositi on rate was in agreement with that theoretically calculated assuming the pr ocess to be controlled by the limiting flux of oxygen to the TiO2 surface. At higher concentrations of oxygen (0.15-0.94 mol m(-3)) the rate of photoc atalysis was controlled simultaneously by both the flux of oxygen and photo ns. The influence of the oxygen concentration decreased with decreasing pho ton flux. For low photon flux intensities (less than or equal to 3.5 x 10(- 5) einstein m(-2) s(-1)), the reaction rate was controlled by the photon fl ux. The concentration profile of oxygen in the diffusion layer along the re actor plate was calculated and showed a significant decrease in oxygen conc entration on the TiO2 surface.