Weizmann Institute Scientists Demonstrate Novel Approach for Splitting Water into Hydrogen and Oxygen
A team from the Weizmann Institute’s Organic Chemistry Department has demonstrated a novel approach for splitting water into hydrogen and oxygen, driven by sunlight.
The approach developed by Prof. David Milstein and colleagues of the Organic Chemistry Department is divided into a sequence of reactions, which leads to the liberation of hydrogen and oxygen in consecutive thermal- and light-driven steps, mediated by a special ruthenium metal complex that Milstein’s team designed in previous studies. A report on their work was published in the 3 April issue of the journal Science.
The metal complex of the element ruthenium is a ‘smart’ complex in which the metal center and the organic part attached to it cooperate in the cleavage of the water molecule.
The team found that upon 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 center.
This modified version of the complex provides the basis for the next stage of the process: the ‘heat stage.’ When the water solution is heated to 100 °C, hydrogen gas is released from the complex, and another OH group is added to the metal center.
But the most interesting part is the third ‘light stage’. 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.—David Milstein
The generation of a bond between two oxygen atoms promoted by a man-made metal complex is a rare event, and it has been unclear how it can take place. Milstein and his team have also succeeded in identifying a new mechanism for such a process. Additional experiments have indicated 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 quickly breaks up into oxygen and water.
Because hydrogen peroxide is considered a relatively unstable molecule, scientists have always disregarded this step, deeming it implausible; but we have shown otherwise.—David Milstein
Moreover, the team has provided evidence showing that the bond between the two oxygen atoms is generated within a single molecule—not between oxygen atoms residing on separate molecules, as commonly believed— and it comes from a single metal center.
So far, Milstein’s team has demonstrated 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.
In a Perspectives piece on “Rethinking Water Splitting” published in the same issue of Science, Dr. Richard Eisenberg from the University of Rochester notes of Milstein’s work that:
The water-splitting scheme that Kohl et al. describe presents challenges. It is not yet catalytic, and it has an uncertain energy balance depending on how the thermal steps are driven. Also, the system is not very durable because the phosphine arm of the pincer becomes oxygenated. However, the fact that a relatively simple molecular system can accomplish water splitting with steps not conceived of or brought together by other studies on water splitting is stimulating and thought-provoking.
Participating in the research were former postdoctoral student Stephan Kohl, Ph.D. student Leonid Schwartsburd and technician Yehoshoa Ben-David all of the Organic Chemistry Department, together with staff scientists Lev Weiner, Leonid Konstantinovski, Linda Shimon and Mark Iron of the Chemical Research Support Department.
Prof. David Milstein’s research is supported by the Mary and Tom Beck-Canadian Center for Alternative Energy Research; and the Helen and Martin Kimmel Center for Molecular Design. Prof. Milstein is the incumbent of the Israel Matz Professorial Chair of Organic Chemistry.
Stephan W. Kohl, Lev Weiner, Leonid Schwartsburd, Leonid Konstantinovski, Linda J. W. Shimon, Yehoshoa Ben-David, Mark A. Iron, David Milstein (2009) Consecutive Thermal H2 and Light-Induced O2 Evolution from Water Promoted by a Metal Complex. Science Vol. 324. no. 5923, pp. 74 - 77 doi: 10.1126/science.1168600
Richard Eisenberg (2009) Rethinking Water Splitting. Science 324 (5923), 44. doi: 10.1126/science.1172247