Fundamentals and misconceptions in photocatalysis

Abstract

Photocatalysis has presently become a major discipline owing to two factors: (i) the intuition of the pioneers of last 20th century and (ii) the mutual enrichment of scientists arising from different fields: photochemistry, electrochemistry, analytical chemistry, radiochemistry, material chemistry, surface science, electronics, and hopefully catalysis. However, heterogeneous photocatalysis belongs to catalysis, which means that all the bases of this discipline must be respected and consequently, it has become imperative to refocus the frame of photocatalysis to avoid misfits and misconceptions: (i) proportionality of the reaction rate to the mass of catalyst (below the plateau due to a full absorption of photons); (ii) implication of the Langmuir–Hinshelwood mechanism of kinetics with the initial rate being proportional to the coverages θ in reactants; (iii) obtention of conversions beyond the stoichiometric threshold defined as the number of potential active sites initially present at the surface. Photonics should be respected with the photocatalytic activity being (i) parallel to the absorbance of the photocatalyst and (ii) proportional to the radiant flux Φ, enabling one to determine the quantum yield defined as the ratio of the reaction rate r (in molecules converted per second) to the efficient photonic flux (in photons per second) received by the solid. True photocatalytic normalized tests should be established to prove the real catalytic activity of irradiated solids, independent of non-catalytic side-reactions. In particular, dye decolorization is a misleading test, which only provides a “visible” and apparent “disappearance” of the dye, purely photochemical but not photocatalytic. Thermodynamics have also to be respected. The decrease of photon energy to the visible may be thermodynamically detrimental for the generation of highly active species such as OH°. Concerning solid state chemistry, it is now eventually admitted that cationic doping is detrimental for photocatalysis. Anionic doping must be rapidly clarified or otherwise abandoned. In conclusion, all these recommendations have to be addressed and experiments have to be operated in suitable conditions before claiming that one deals with a true photocatalytic reaction, whose veracity can be proved by following a protocol suggested at the end.