Concerning the Role of Sacrificial Reagents in Photocatalytic Water Splitting Systems: A Critical Assessment

One of the most important limitations for the application of photocatalytic techniques for water decomposition is that the process employing pure water is usually found to be rather inefficient. This is generally related to the fact that the simultaneous reduction and oxidation of water is a complex multistep reaction involving (at least!) four electrons. Using sacrificial molecules as electron donors can remarkably improve the H₂ production with the holes being scavenged by these molecules thus reducing the charge carrier recombination significantly. Furthermore, as O₂ is not produced, the back reaction to produce water is suppressed, increasing the H₂ yield and avoiding a subsequent gas separation stage. However, it should be noted here that the yield of the H₂ formation will eventually be reduced by competing reduction reactions with the products formed upon the oxidation of the sacrificial reagents.

Organic compounds such as alcohols, organic acids, and hydrocarbons can act as efficient hole scavengers (i.e., as electron donors) for the photocatalytic H₂ generation. In particular, methanol is frequently used as sacrificial reagent. For practical applications, the utilization of methanol will only be environmentally sensible provided that it is derived from biomass or from toxic residues that must be disposed of. Adding methanol as electron donor to react irreversibly with the photo-generated holes can enhance the photocatalytic electron/hole separation efficiency resulting in higher quantum yields. Since electron donors are consumed in the photocatalytic reaction, their continuous addition is required to sustain H₂ production. A sequential oxidation mechanism has been proposed for the overall methanol decomposition resulting in a lower energy that can be stored as compared with the cyclic splitting of water.

While the first two reactions have a positive Gibbs free energy and are thus thermodynamically unfavorable at room temperature, the third reaction has a large negative Gibbs energy, thus it intrinsically provides a barrier for the undesired reverse consumption of the generated H₂ gas. Two possible mechanisms are proposed for photocatalytic oxidation of methanol: (1) the direct oxidation by photogenerated holes and (2) the indirect oxidation via interfacial by formed ^•OH radicals that are products of trapping valence band holes by surface −OH groups or adsorbed water molecules. It is still a challenge to distinguish between the two mechanisms in practice due to the lack of suitable probe techniques.

While the choice of sacrificial electron donors for studies concerning the photocatalytic formation of molecular H₂ appears to be rather large, the sacrificial photocatalytic oxidation of water is only reported for a rather limited variety of electron acceptors. By far the vast majority of the research groups working on this topic employs silver cations, Ag⁺, as electron acceptors with the involved reactions being proposed just suggest that silver cations act solely as electron acceptors. Hence, the photocatalytic formation of molecular oxygen is accompanied by the deposition of metallic silver nano-contacts on the semiconductor’s surface. Obviously, this will lead to irreversible optical changes of these systems due to the plasmonic absorption band of the silver nanoparticles in the visible spectral region. Moreover, noble metal nanoparticles are known for their catalytic activity resulting most likely in changes in the chemical and photochemical properties of these systems.

It is interesting to note that these rather drastic changes are hardly ever discussed in the respective literature nor is any experimental work conducted to ensure that the photocatalytic properties remain unchanged upon the formation of the silver particles. Even though no exact count exists here, it seems fair to say that Ag⁺ is employed in at least 95 % of the published papers dealing with the sacrificial photocatalytic water oxidation. However, no O₂ is formed once electron acceptors such as carbon tetrachloride or tetranitromethane are used even though their irreversible one-electron reduction is readily observed.

To explain these at first sight contradictory results it is suggested here that the role of suitable sacrificial electron acceptors such as  (and also ) is highly underestimated. In particular, their possible involvement in the actual water oxidation mechanism has so far not been discussed at all. We are convinced that it is most certainly highly indicated to study the role of such metal cations in the photocatalytic water oxidation in detail.  Their catalytic role is proposed here for the first time, however, it could be part of a much more general mechanism thus opening up new design features for photocatalytic and photoelectrochemical energy-to-fuel conversion systems.

Reference:

J. Schneider, D. W. Bahnemann, “Undesired Role of Sacrificial Reagents in Photocatalysis”, J. Phys. Chem. Letters 4 (2013) 3479-3483