Take a metal complex. Add water and heat to 100°C for three days, stirring occasionally. Then add a generous amount of light and continue to “simmer” at room temperature for a further two days. The resulting hydrogen and oxygen are now ready to be “served.”
This is the gist of a unique new strategy devised by Prof. David Milstein and his colleagues in the Weizmann Institute’s Organic Chemistry Department; and it represents the first steps toward obtaining a clean, sustainable source of hydrogen for fuel. While today’s methods of producing hydrogen using sunlight are inefficient and often discharge chemical waste, the new system relies on a metal complex that is “reset” for reuse at the end of the procedure. In the process, the team demonstrated a new mode of bond generation between oxygen atoms and they even defined the mechanisms by which this takes place. In fact, says Milstein, the production of oxygen gas through the pairing of oxygen atoms that have been split off from water molecules – a crucial step in the process – has proven to be a bottleneck. Their results have recently been published in Science.
Nature has taken a very different path to producing free oxygen: It’s a byproduct of the photosynthesis carried out by plants. Spurred on by plants’ “green” example, vast worldwide efforts have been devoted to the creation of artificial photosynthetic systems. The ones Milstein develops are based on metal complexes that serve as catalysts (substances that increase the rate of a chemical reaction without getting used up themselves).
The new approach devised by the Weizmann team is divided into a stepwise sequence of reactions, beginning with water splitting. Milstein’s “secret ingredient” is a complex of the element ruthenium designed by his group in previous studies. This is a “smart” synthetic complex composed of a metal center and an organic (carbon-based) component; the two cooperate in cleaving the water molecule. This complex not only breaks the chemical bond between hydrogen and oxygen, but prevents them from getting back together by binding one hydrogen atom to its organic part and the remaining hydrogen and oxygen atoms (an OH group) to its metal part, creating a new metal complex.
The second stage – the heat stage – involves heating the resulting complex in water to 100°C, leading to the release of hydrogen gas – a potential source of clean fuel – and creating another chemical structure on the metal complex, this one containing two OH groups.
“But the most interesting part is the third, light-driven stage,” says Milstein. “When we exposed the third version of the complex to light at room temperature, not only was oxygen gas produced but the metal complex also reverted back to its original state, and this could be recycled for use in further reactions.”
These results have garnered a fair amount of interest in their field, as bonding between two oxygen atoms promoted by a man-made metal complex was previously a very rare event and its mechanism had been a mystery. Milstein and his team succeeded, for the first time, in identifying an unprecedented mechanism for this process. Their experiments indicated that during the third stage, the energy provided by the light causes the two OH groups to get together and form hydrogen peroxide (H2O2), which then quickly breaks up into oxygen and water. “Because hydrogen peroxide is considered a relatively unstable molecule, scientists have generally deemed this step implausible; but we have shown otherwise,” says Milstein. The team also challenged another misconception, providing evidence that the bond between the two oxygen atoms is generated within a single molecule, involving just one metal center, and not between oxygen atoms residing on separate molecules as was commonly thought.
So far, Milstein’s team has demonstrated a three-step mechanism for the formation of hydrogen and oxygen from water using light, without the production of chemical waste. For their next study, they plan to combine these stages to create an efficient catalytic system, bringing those in the field of alternative energy one step closer to realizing the goal of a clean, efficient method for producing hydrogen fuel from water using sunlight.
Participating in the research were former postdoctoral fellow Dr. Stephan Kohl, research student Leonid Schwartsburd and Yehoshoa Ben-David, all of the Organic Chemistry Department, together with Drs. Lev Weiner, Leonid Konstantinovski, Linda Shimon and Mark Iron of Chemical Research Support.
Prof. David Milstein’s research is supported by the Mary and Tom Beck-Canadian Center for Alternative Energy Research; the Helen and Martin Kimmel Center for Molecular Design; and the Bernice and Peter Cohn Catalysis Research Fund. Prof. Milstein is the incumbent of the Israel Matz Professorial Chair of Organic Chemistry.