Effects of particle dispersion on the measurement of semi-conductor photocatalytic activity

Abstract Milling changes the particle size distribution of TiO 2 suspensions and this is reflected in the changed optical characteristics. Typically, the visible transmission is increased, as large TiO 2 agglomerates are broken down, and the UV absorption is increased. It has been shown that the rates of dye decolouration, alcohol oxidation, and carboxylic acid degradation are all changed by milling. For all the TiO 2 's tested (anatase and rutile, surface areas from ∼8 to 140 m 2  g −1 ) photocatalytic oxidation of propan-2-ol is reduced by milling. It is suggested that this is a consequence of the I 0.5 dependence of reaction rate on UV intensity. This argument was supported by demonstrating that the rate of salicylic acid degradation (which also exhibits I 0.5 dependence), is reduced by milling. By contrast there was no significant decrease in the rate of degradation of dichloroacetate (which exhibits I 1 dependence). Fourfold changes in the rate of salicylic acid degradation were induced when the same catalyst was milled for increasing times. This implies that two catalysts which differ in activity by a factor of four may do so because of differences in their dispersion rather than because they have different intrinsic activities. It is also shown the relative performance of two catalysts which differ in their surface properties changes when they are milled. However, the relative activities of a series of iron doped catalysts with no deliberate differences in surface properties were not changed by milling.