Revisiting the fundamental physical chemistry in heterogeneous photocatalysis: its thermodynamics and kinetics

Abstract

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 introductionHeterogeneous 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. ThermodynamicsThermodynamics 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 photocatalysisThe mechanism of heterogeneous photocatalysis 7 is always explained with a band structure of electronic energy in photocatalysts, as Fig. 1 shows.( Fig. 1) An electron in a filled valence band (VB) is excited into a vacant conduction band (CB) state by a photon of energy greater than the energy of the gap between the VB and the CB, the band gap;8 CB bottom and VB top positions determine the ability of reduction and oxidation by a photoexcited electron (e -) and a positive hole (h + ), since those ch...

Titania Photocatalysis beyond Recombination: A Critical Review

“…When such an n-type semiconductor is immersed, or in contact in an electrolyte solution, electrons in the donor levels go out to the electrolyte since the donor level in an n-type semiconductor may be more cathodic than the electrode potential of electrolyte solutions. This flow of electrons in the donor level to an electrolyte results in the formation of a so-called Schottky-type barrier [10] at the interface to prohibit electron flow from an electrolyte to an electrode, i.e., potential slope due to an electron-deficient so-called depletion layer is built in a semiconductor. …”

“…The rate of photoinduced silver metal deposition was analyzed using a Stern-Volmer plot, a popular method in the field of photochemistry; almost linear double-reciprocal plots of the (apparent) quantum efficiency for the deposition (Ф) as a function of concentration of silver ion (Ag + ) in solutions (C Ag ) were obtained for both TiO 2 and ZnO crystals, as (9) where Ф lim , and C L are constants. In the first approximation for interpretation of the meaning of Equation 9, the authors assumed a reaction mechanism in which electrons photoexcited by the efficiency  react with surface-adsorbed Ag + (with the surface concentration N Ag ) to give deposits with a rate constant k 3 , otherwise e − is deactivated with a rate constant k 4 , as (10) This equation can be derived by "steady-state (stationary-state) approximation", which is often employed for reactions induced by short-lived active species such as the photoexcited state of a given material (e − and h + in a photocatalyst), based on the assumption of a constant concentration of e − that is produced by photoabsorption of a photocatalyst and consumed by (a) surface reaction and (b) recombination with holes. Thus, the rate of photocatalytic reaction is governed by the rate of recombination (k 4 ), though analysis of actual data using Equation 10 gives only the ratio of rate constants, k 3 /k 4 .…”

Hidden but Possibly Fatal Misconceptions in Photocatalysis Studies: A Short Critical Review

“…Thus, what is the source of the author's frustration? Possible reasons, mainly misunderstandings, misconceptions, and/or speculation in a scientific sense, have been pointed out in the present author's recent review papers [2][3][4][5]. This short review presents new misconceptions often found in discussions of photocatalysis studies that have not yet been recognized by such papers' authors.…”

Perovskite Sr1−x Ba x W1−y Ta y (O,N)3: synthesis by thermal ammonolysis and photocatalytic oxygen evolution under visible light

“…The turnover numbers (TONs) for 5 h photocatalytic O 2 gas evolution reactions were estimated as the total moles of substrates (H 2 O) consumed over the reaction per moles of active catalytic species (CoO x ) [17,18]:…”