The perfect replacement for petroleum and other polluting fuels would be using sunlight to split water into hydrogen and oxygen. Hydrogen clearly has a long-term potential as a clean, sustainable fuel. But so far, man-made systems require additional chemical agents. Now, a unique approach developed by Prof. David Milstein and colleagues at the Weizmann Institute of Science organic chemistry department takes important steps in overcoming this challenge.
During this work, the team demonstrated a new mode of bond generation between oxygen atoms, and even defined the mechanism by which it takes place. In fact, 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. Their results were recently published in Science.
Nature has evolved a very efficient process - photosynthesis, which is the source of all oxygen on Earth. Although there has been significant progress in understanding photosynthesis, just how this system functions remains unclear; 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 Rehovot institute's approach is divided into a sequence of reactions, which leads to the release of hydrogen and oxygen in consecutive thermal- and light-driven steps, mediated by a special metal complex that Milstein's team designed in previous studies. Moreover, the one they designed from the element ruthenium is a "smart" complex in which the metal center and the organic part attached to it cooperate in cleaving 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 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 100ºC, hydrogen 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," says Milstein. "When we exposed this third complex to light at room temperature, not only was oxygen produced, but the metal complex reverted to its original state, which could be recycled."
Milstein and his team have also succeeded in identifying an unprecedented 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 join and form hydrogen peroxide (H2O2), which quickly breaks 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," says 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 comes from a single metal center.
Discovery of an efficient artificial catalyst for the sunlight-driven splitting of water is a major goal of renewable clean energy research. 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, bringing those in the field of alternative energy an important step closer to realizing this goal.
