Plants produce oxygen from water, but the same chemical reaction is hard to achieve synthetically. A new family of catalysts could breathe fresh life into the quest for artificial photosynthesis.
Photosynthesis in plants underpins the existence of many life-forms on Earth. At its heart is a remarkable chemical reaction: the light-powered conversion of water and carbon dioxide into oxygen and carbohydrates. The development of an artificial version of this reaction, based on splitting water into oxygen and hydrogen, is highly desirable, not least because of hydrogen's attraction as a fuel. Reporting in the Journal of the American Chemical Society, Bernhard and colleagues1 describe the preparation of a new family of synthetic catalysts for the first part of this splitting reaction — water oxidation. The reactivity of the iridium-based catalysts that they have developed can be modified simply by varying the organic framework surrounding the metal.
Given the breakthroughs reported seemingly daily in materials science, nanotechnology and molecular biology, it might seem odd that we have made so little progress in designing catalysts for essential reactions such as water oxidation under conditions comparable to those of photosynthesis. The problem is that water oxidation is inherently difficult. It demands the loss of four electrons and four protons from two water molecules, all accompanied by the formation of an oxygen–oxygen bond (Fig. 1a). Removing one electron at a time doesn't work because it produces a high-energy hydroxyl radical and is much too slow (Fig. 1b).
Thankfully, water oxidation occurs all around us during photosynthesis in green plants and certain bacteria — at least when the sun is shining. In photosynthesis, light is absorbed by antenna arrays found in photosystem II, part of the photosynthetic apparatus within the chloroplasts of green leaves. The light-excited antenna transfers its energy to a chlorophyll molecule in a neighbouring protein complex, known as the reaction centre. The excitation of the chlorophyll is quenched by transferring electrons to acceptor molecules elsewhere in the photosystem. Electron-deficient molecules (oxidants) are thus created that can accept electrons from neighbours.
The source of these electrons is a part of the reaction centre known as the oxygen-evolving complex, or OEC. The OEC makes up its lost electrons by stealing some from water molecules, producing oxygen in the process2,3,4. Even plants find this task difficult: under ambient sunlight in the chloroplast, the OEC must be resynthesized every half an hour or so owing to the oxidation damage it suffers from the oxygen that it has produced.
A synthetic, non-protein catalyst that can oxidize water as effectively as the OEC does exist. This is the ruthenium 'blue dimer'5,6, a complex that must be activated electrochemically or by a strong oxidizing agent (usually a cerium salt; Fig. 1c). The blue dimer is a well-defined molecule that undergoes the stepwise loss of four electrons and four protons, producing a fleeting intermediate that rapidly oxidizes water. But although initially effective, the blue dimer can lose its catalytic efficiency after just a few cycles.
Compared with earlier catalysts such as the blue dimer, Bernhard and colleagues' iridium-based catalysts1 (Fig. 1d) are simple to make. Their oxidizing ability is tuned by changing the ligand molecules that bind to the iridium metal. As with the blue dimer, they need to be activated by a cerium oxidizing agent. The details of this mechanism are still lacking, but proton-coupled electron transfer is likely to be essential. This trick of the trade is used by both the OEC and the ruthenium blue dimer, and involves simultaneous transfers of both electrons and protons as the catalyst is activated. This allows many oxidative equivalents to accumulate at one site, one step at a time, without a catastrophic build-up of charge7.
Barring serendipitous discoveries, further progress in designing catalysts for water oxidation will require detailed knowledge of the mechanism by which these reactions occur. Bernard and colleagues' iridium compounds1 are promising. But we have limited understanding of how they work, and their catalysed reactions are slow — more than 100 million times slower than those in the OEC. Nevertheless, catalysts for water oxidation are so rare that the discovery of a new family is cause for celebration. Once the essential design details that govern their behaviour have been thrashed out, and their stability is improved, they offer real prospects as tunable catalysts for the essential process of water oxidation.
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