Currently the energy infrastructure in our society largely depends on fossil fuels. This caused the release of huge amounts of carbon dioxide into earth's atmosphere, resulting in a significant increase of the greenhouse effect and consequently global climate change. It is therefore of huge importance that renewable energy systems are implemented in our society. Solar energy is the most attractive renewable energy source that is available in sufficiently large amounts, yet it is not available 24 hours per day. Therefore, the key for a society that fully relies on renewable energy lies with the storage of solar energy. One way to convert solar energy into a chemical fuel is the light-driven splitting of water. Within the water splitting process, the water oxidation reaction is a major bottleneck. For future industrial processes to produce hydrogen from renewable energy, we rely on new and improved water oxidation catalysts. This awareness has led to a significant increase in scientific research in this area, and, in recent years, quite some progress has been made in our understanding of the catalytic water oxidation processes. In order to further boost the momentum of the field and to provide tools for new researchers entering the field of catalytic water oxidation, we have guest-edited this Cluster Issue of EurJIC on water oxidation.
It is very exciting to see contributions that focus on the catalytic understanding of reaction mechanisms, as well as on the development of new catalysts and photoelectrochemical systems, both in the fields of heterogeneous and homogeneous chemistry. Historically, heterogeneous and homogeneous catalysts are studied by different scientific communities, yet these fields sometimes show huge overlaps. Under strong oxidative conditions, degradation of homogeneous catalysts is often a problem, while the resulting heterogeneous products may actually be the true active species. Fukuzumi and Hong contribute a Microreview precisely addressing this problem. Pinpointing which species is the active species is not always easy and requires complicated experimental techniques. Hill and co-workers contribute with a case study on polyoxometalate catalysts dealing with this issue. Degradation of the ligands employed may also be used advantageously to obtain superior catalysts. Berlinguette and co-workers describe the formation of an amorphous lanthanum metal oxide catalyst upon decomposition of spin-coated molecular complexes with UV light irradiation.
Kurz, Plieth, Dau and co-workers studied the water oxidation activity of biogenic Ca–Mn oxide clusters that have deposited on the cell walls of algae. These clusters are interesting because of their resemblance to the Ca–Mn oxide core of the oxygen-evolving system of Photosystem II, the natural water oxidation catalysts. For the same reason, manganese complexes have received a wide interest in water oxidation chemistry. These are reviewed by Yagi and co-workers in their contribution to this Cluster Issue. Successful artificial catalysts typically are based on ruthenium. Nagao and co-workers and Webster, Zhao and co-workers describe new ruthenium water oxidation catalysts in their contributions. Grotjahn, Paul, Papish and co-workers studied the effect of pendant proton–donor and proton–acceptor groups on the catalytic activity of ruthenium water oxidation catalysts. Lately, iridium catalysts also are popular. The contributions of Bernhard, Albrecht and co-workers, Tubaro, Bonchio and co-workers, Lin and co-workers and Macchioni and co-workers all describe new molecular iridium water oxidation catalysts. Differences between dinuclear and related mononuclear systems, photochemically driven water oxidation catalysis, evolution of dioxygen versus ligand oxidation and catalyst modification reactions are discussed, respectively, in these contributions as well.
Understanding the functions of water oxidation catalysts is very important in order to design better catalysts. In their contribution Tanaka and Kikuchi review the mechanism of several molecular water oxidation catalysts. Lymar and co-workers review the use of pulse radiolysis in catalytic water oxidation. With this technique, high oxidation state radicals can be generated in situ under conditions wherein other chemical oxidants are not available, and therefore may be a very useful technique in catalytic water oxidation. Another useful technique to study how water oxidation catalysis takes place is density functional theory. Liao and co-workers studied the reaction mechanism of a molecular water oxidation catalyst. Such first-row transition-metal catalysts are rare, yet because of the high abundance of iron, they are very interesting for future applications. The authors also considered non-innocent pathways by the chemical oxidant in their studies on how dioxygen is formed by these iron catalysts. In line with these oxidant involving pathways, Hetterscheid and Reek investigated the role of periodate as a chemical oxidant in iridium-catalyzed dioxygen production by DFT calculations.
Eventually water oxidation catalysts should lead to the formation of dioxygen in combination with the capture of light and the formation of hydrogen. Spiccia and co-workers investigated photoassisted water oxidation by electrodeposited aluminum-modified zinc oxide nanorod arrays in a photoelectrochemical cell, and Liu and co-workers studied cadmium–tellurium quantum-dot-sensitized zinc oxide nanowires for photoelectrochemical water splitting. Li et al. describe the temperature dependence of the hydrogenation pretreatment of titanium dioxide on the performance of titanium dioxide photoanodes. Domen and co-workers studied mixed gallium nitride and zinc oxide systems, on which both water oxidation and proton reduction catalysts were deposited. The authors show superior photocatalytic activity for systems with both catalysts present.
We thank all authors for their contributions to this Cluster Issue and the reviewers for their expert advice. We hope that you enjoy reading this Cluster Issue on catalytic water oxidation.
Received: November 4, 2013