Substrate-Specific Photocatalytic Activities of TiO2 and Multiactivity Test for Water Treatment Application

The multidimensional aspects of the photocatalytic activity were investigated in a systematic way. The photocatalytic activities of eight commercial TiO2 samples were quantified by employing 19 test substrates (phenols, organic acids, amines, chlorohydrocarbons, dyes, inorganic ions, etc.) in terms of their degradation or conversion rates in water. The measured activities exhibited a complex behavior that depends on the test substrate. The photocatalytic activities are roughly correlated only among structurally related compounds. The photocatalytic activities can be represented in many different ways, and even the relative activity order among the tested photocatalysts depends on what substrate is used. Each TiO2 (among eight samples) showed the best activity for at least one test-substrate. This highly substrate-specific activity of TiO2 photocatalysts hinders the straightforward comparison of which catalyst is better than others. Even the common belief that anatase is more photoactive than rutile cannot be fully supported on the basis of the present data set. Although there seems to be no simple correlation between the activity and the common physicochemical parameters of photocatalysts, the substrate-specific activity was analyzed and discussed in terms of various parameters such as surface area, crystallinity, surface charge, and substrate adsorption. Finally, the multivalue photocatalytic activity test in relation with water treatment application was proposed to take the substrate-specificity into account.