Titania-based true heterogeneous photocatalysis.

INTRODUCTION: This article recalls and demonstrates that heterogeneous photocatalysis belongs to heterogeneous catalysis according to its initial history. There are criteria, which have to be imperatively taken into account to deal with true (photo-)catalytic reactions. The photocatalytic activity, chosen as the reaction rate, is governed by five physical parameters (mass of catalyst, wavelength, initial concentrations (or pressures) of the reactants, temperature (around room temperature), and radiant flux). DISCUSSION: Once performed in optimal conditions, the reaction has to be characterized with (a) its quantum yield <QY> (ratio of the reaction rate to the incident photon flux), (b) the turnover number <TON> (number of molecules converted per active site during a given time), and (c) the turnover frequency <TOF> (<TON> per second). The true catalytic nature demands that the conversion provides a number of converted molecules higher than the "stoichiometric threshold" defined as the number of the active sites initially present on the catalyst, i.e., that <TON> be much greater than 1, and possibly than at least two or three orders of magnitude. Eventually, a complete mass balance determination should be established for all elements. It is shown that the main reaction intermediates are radicals. All these concepts and principles are illustrated by various examples (selective mild oxidation reactions, water pollutants elimination, and air purification), leading to a list of recommendations for performing really true catalytic reactions.