PCCP Perspective Revisiting the Fundamental Physical Chemistry in Heterogeneous Photocatalysis: Its Thermodynamics and Kinetics

Although the history of photocatalysis research is not so long, many researchers have studied photocatalysis and a large number of papers on photocatalysis have been published. The objectives of this review paper are to revisit the fundamentals of photocatalysis, especially its thermodynamics and kinetics, which have not been reexamined in recent studies, to clarify the problems, if any, that prevent developments in the field of photocatalysis, and to present insights for future progress. What we know and we don't know about photocatalysis: an introduction Heterogeneous photocatalysis has been believed, at least in early studies in 1980's and 1990's, to be a dream technology for solving energy and/or environmental problems that we are facing. However, photolysis of water into hydrogen, a fuel, and oxygen 1 has not yet been realized as an industrial process, and photocatalysts working under visible-light irradiation with activity higher than that of titanium(IV) oxide (titania) powders under ultraviolet-light irradiation 2 have not yet been developed despite extensive studies for more than a quarter of a century. The principle, a basic mechanism of heterogeneous photocatalysis, seems not to have changed, reading original and review papers on heterogeneous photocatalysis. Then, why have we not succeeded in practical applications of photocatalysis? Are there any hidden secrets preventing progress in photocatalysis studies? This review paper aims at clarifying what we know, what we don't know and what we should know for heterogeneous photocatalysis in a strict scientific sense and at suggesting insights for future progress. The author thanks the readers for their understanding that some of the concepts discussed in this paper have already been shown in his recent review papers 3,4 and book sections. 5,6 Heterogeneous photocatalysis is categorized as physical chemistry and there are two fundamental concepts in physical chemistry, thermodynamics and kinetics. The thermodynamics of heterogeneous photocatalysis is first discussed, followed by discussion on its kinetics. Thermodynamics Thermodynamics predicts whether a chemical reaction proceeds or not by showing the equilibrium of the reaction. According to text books on thermodynamics (or text books on physical chemistry), a forward reaction proceeds when Gibbs energy change (G) is negative and a reversed reaction proceeds when G is positive. Then, how can we evaluate G and predict the reaction equilibrium in photocatalysis? <2> Thermodynamic requirements for photocatalysis The mechanism of heterogeneous photocatalysis 7 is always explained with a band structure of electronic energy in photocatalysts, as Fig. 1 shows. An electron in …