Finding an effective way of splitting water into hydrogen and oxygen, driven by sunlight, is one of the most important challenges facing science today.
Water hydrolysis has been known about for a long time and there have been claims of dramatic breakthroughs in the past – none of which could be substantiated.
Systems developed so far are very inefficient and often require additional use of sacrificial chemical agents.
So the hunt goes on for the “holy grail” that could finally unlock the promise of limitless supplies of clean, green hydrogen to fuel our lives.
Among the hunters are Professor David Milstein and colleagues, of the Weizmann Institute’s organic chemistry department, in the US.
Apparently, it is the generation of oxygen gas by the formation of a bond between two oxygen atoms originating from water molecules that proves to be the bottleneck in the water-splitting process.
During their work, the Weizmann team have been able to demonstrate a new mode of bond generation between oxygen atoms. They have also worked out how it takes place.
Nature, by taking a different path, has evolved a very efficient process – photosynthesis – carried out by plants and the source of all oxygen on Earth. Although there has been significant progress towards the understanding of photosynthesis, just how this system functions remains unclear. Vast worldwide efforts have been devoted to the development of artificial photosynthetic systems based on metal complexes that serve as catalysts, but with little success.
The approach the Weizmann team has come up with comprises a sequence of reactions that lead to the liberation of hydrogen and oxygen in consecutive thermal and light-driven steps, mediated by a unique ingredient – a special metal “complex” based on the element, ruthenium.
This “smart” complex has a metal centre with an organic part attached to it and they “co-operate” in the splitting of the water molecule.
The team found that, on mixing this complex with water, the bonds between the hydrogen and oxygen atoms break, with one hydrogen atom ending up binding to its organic part, while the remaining hydrogen and oxygen atoms (OH group) bind to its metal centre.
The next step is the “heat stage”: when the water solution is heated to 100C (boiling point of water), hydrogen gas is released from the complex – a potential source of clean fuel – and another OH group is added to the metal centre.
“‘But 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.”
Finding an efficient artificial catalyst for the sunlight-driven splitting of water into oxygen and hydrogen is a major goal of renewable clean energy research.
What Milstein’s team has demonstrated is a mechanism for the formation of hydrogen and oxygen from water, without the need for sacrificial chemical agents, through individual steps, using light.
For their next study, they plan to combine these stages to create an efficient catalytic system, bringing those in the field of alternative energy an important step closer to realising this goal.
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