Splitting Water with Ease

Harvesting solar energy with a specially designed metal complex that splits water into hydrogen and oxygen with novel chemistry and regenerates itself. Dr. Mae-Wan Ho

Splitting water is one of the ways to harvest sustainable energy from the sun, especially in creating hydrogen which can burn as clean fuel [1] ( Harvesting Energy from Sunlight with Artificial Photosynthesis , SiS 43). This imitates the natural process of photosynthesis in green plants that use sunlight to abstract hydrogen from water (to fix carbon dioxide into carbohydrates and ultimately biomass), liberating oxygen. Despite a great deal of effort, nothing practical has emerged so far.

David Milstein and his research group at Weiszmann Institute's Organic Chemistry Department in Israel have now found a new way of splitting water altogether [2]. This involves the liberation of hydrogen and oxygen in consecutive heat and light driven steps, catalyzed by a single organic metal complex of the element ruthenium that the team has designed, in which the metal centre and the organic part attached to it cooperate in splitting the water molecules much more easily, it seems.

An organic ruthenium complex does the trick

The team found that on mixing the ruthenium complex with water, the bonds between the hydrogen and oxygen in water break, with one hydrogen atom ending up binding to the organic part, while the hydroxyl group (OH) binds to the metal centre. When the water is heated to 100 C, hydrogen gas is released from the complex, and another OH group attaches to the metal centre.

The most interesting part is the third light stage, says Milstein. “When we exposed this third complex to light at room temperature, not only was oxygen gas produced, but the metal complex also reverted back to its original state, which could be recycled for use in further reactions.”

Most remarkable is the generation of a bond between two oxygen atoms (of the two OH groups) promoted by the metal complex, and it has been unclear how it could take place. But Milstein and his team have worked out the new mechanism. Using a combination of heavy isotopes especially of H and O in water, they were able to track the actually molecular reactions involved [3]. These experiments showed that during the third stage, light provides the energy required to cause the two OH groups to get together to form hydrogen peroxide (H₂O₂ ), which then quickly breaks up into oxygen and water. The team provided evidence that the bond between the two oxygen atoms required in forming H₂O₂ is generated within a single molecule; that is, from a the two OH groups attached to a single metal centre, and not between oxygen atoms residing on separate molecules from different metal centres.

The new process departs from the photosynthesis model

A simplified version of the usual photosynthesis-based water splitting scheme (Figure 1, upper half) involves the absorption of a photon (h n ) by the light-sensitive pigment, or chromophore (C), exciting an electron (e-) from its ground state to initiate the process of charge separation and charge transfer. The excited electron travels to an electron acceptor (A), leaving behind a positive hole that is filled by the electron donor (D). Proton reduction and hydrogen evolution occurs at the catalyst Cat_red , which accepts electrons, and water oxidation and oxygen evolution occurs at the other Cat _ox , which accepts holes [4]. Because the electrons and holes separate, water splitting is really two half reactions that either consume or generate electrons. Each half-reaction can be investigated and optimized independently, and research successes have been reported for both. But inevitably, a ‘sacrificial' electron donor or acceptor is consumed in the reaction that is not optimised. Although H 2 has been generated by light induced chemical reactions, the efficiency and durability of the systems are still far from practical. The formation of O₂ is even more difficult (but see [5] Making Fuel from Water , SiS 43, for the latest in getting oxygen from water)..

Figure 1. Comparison of photosynthesis-based water splitting with that of Milstein and coworkers (redrawn from [3])

The organic ruthenium complex designed by Milstein's group works differently altogether. It depends on the reversible addition and subtraction of a proton (H ⁺ ) to an arm of the ‘pincer' ligand holding the ruthenium ion. This usually difficult process is made possible through aromatization and dearomatization of the central ring of the pincer ligand. (This refers to the six-member ring in the aromatic form with three alternating double bonds inter-converting with one that has only two double bonds.) In the presence of water, protonation of the ligand arm, and binding of OH to the Ru metal centre takes place (see Fig. 2).

Figure 2. Ruthenium complex 1 is protonated by H₂O to complex 2 , in which the OH group is coordinated to the ruthenium centre

The added proton becomes a captive source of H⁺ that can react with the ruthenium bound hydride (H^- ) to give H2 gas, which is also an unusual reaction.

H + + H - ? H₂                    (1)

Ru complex 2 , with hydride and hydroxide ions bound (RuH(OH) can react with another H₂O molecule (see lower half of Fig.1). This reaction proceeds on heating to liberate H₂ .(as in reaction (1) above) and subsequent addition of water to form the dihydroxide complex 3 (Ru(OH)₂ .

On exposure to light, Ru(OH)₂eliminates the hydrogen peroxide. Rapid reaction of 2H₂O₂into O₂and 2H₂O follows, completing the process of splitting water. This reaction also regenerates the ruthenium complex 1 which reacts with water to give complex 2 , and so on to start another cycle.

The water-splitting scheme of Milstein and his team still has some way to go before it can be practical. “The project is directed, in the long run, for efficiently producing hydrogen from water using visible light, and without using sacrificial chemicals.” David Milstein told me. But the fact that a relatively simple molecular system can split water with mechanisms not previously conceived has excited the water-splitting community.

Article first published 06/07/09

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References

1. Ho MW. Harvesting energy from sunlight with artificial photosynthesis. Science in Society 43 (to appear).
2. “New way to split water into hydrogen and oxygen developed”. ScienceDaily 8 April 2009, http://www.sciencedaily.com/releases/2009/04/090406102555.htm
3. Eisenberg R. Rethinking water splitting, Science 2009, 324, 44-5.
4. Kohl SW, Weiner L, Schwartsburd L, Konstaninovski L, Shimon LJW, Ben-David Y, Iron MA and Milstein D. Consecutive thermal H₂ and light-induced O₂ evolution from water promoted by a metal complex. Science 2009, 324, 74-7.
5. Ho MW. Making fuel from water. Science in Society 43 (to appear).