Active Agents in Heterogeneous Photocatalysis: Atomic Oxygen Species vs. OH. Radicals: Related Quantum Yields

After a general survey of the fundamental characteristics of heterogeneous photocatalysis, the present article classifies the ensemble of reactions into three major categories: i) mild oxidations, ii) total oxidations, and iii) reactions involving hydrogen. Depending on the presence or absence of H2O, the active oxidizing species will be either a dissociated neutral oxygen species, denoted as O*, which is present in anhydrous systems and responsible for selective mild oxidation reactions, or an OH. radical, formed in the presence of H2O and responsible for totally degradative oxidation reactions. The existence of O* species is substantiated by photoconductivity measurements, oxygen-isotope exchange, and reactions in which oxygen-free NO is the oxidizing agent. The influence of the five main parameters that govern kinetics experiments i) the mass of the catalysts, ii) the wavelength, iii) the concentrations or partial pressures of reactants, iv) the temperature, and v) the radiant flux is examined to determine the best conditions for obtaining the optimum photocatalytic quantum yield (PQY), the definition of which is based on the quantum yield given in [1] for photochemistry.