One great goal for many is the sunlight driven design of efficient systems for splitting water into hydrogen and oxygen. A new innovation developed by Prof. David Milstein and colleagues of the Weizmann Institute’s Organic Chemistry Department, describes the steps in new process that rises to the challenge.
The Institute’s team is demonstrating a new understanding of bond generation between oxygen atoms and even defined the mechanism by which it takes place. The fact they’re showing is the generation of oxygen gas by the formation of a bond between two oxygen atoms originating from splitting water molecules that proves to be the bottleneck in the water splitting process.
This brings us to a discovery of an efficient artificial catalyst for the sunlight-driven splitting of water into dioxygen and dihydrogen. In the April 3, 2009 issue of Science the team reports a solution-phase reaction scheme that leads to a measurable liberation of dihydrogen and dioxygen in consecutive thermal and light driven steps mediated by mononuclear, well-defined ruthenium complexes.
The new process is a set of distinct sequences of reaction that leads to liberating hydrogen and oxygen. The reactions are describes as thermal and light powered steps. The unique special metal complex made from ruthenium mediates the whole process series. The team is showing the ruthenium complex is “smart” such that the metal center and the organic part attached cooperate in the splitting of water molecules.
Milstein’s team designed the ruthenium complex in previous work. Then they found that 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. That sets up the next step, a heat or thermal step where the whole kit is warmed to 100º C and half of the available hydrogen releases from the complex with the remaining OH added to the metal center.
Mistein says, “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.”
It’s a bit mysterious in view that the generation of a bond between two oxygen atoms promoted by a man-made metal complex is a very rare event, and it remains unexplained how it can take place. But the research works. Milstein’s team has also succeeded in identifying an unprecedented mechanism for such a process. Further research 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.
Milstein points out, “Because hydrogen peroxide is considered a relatively unstable molecule, scientists have always disregarded this step, deeming it implausible; but we have shown otherwise.” The paper shows that the bond between the two oxygen atoms is generated within a single molecule – not between oxygen atoms residing on separate molecules, as previously thought – and it comes from the single metal center.
The result is 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 low inputs of heat and light. The team is using the word catalyst to cover the activity of the ruthenium and organic complex, which for some may be a stretch, but not really, it’s a reacting recycling catalyst, one more step than usually considered for catalyst.
The next study is planned to combine these stages to create an efficient catalytic system, bringing those in the field of alternative energy an important step closer to realizing a simple low cost low input solution to isolating hydrogen for fuel use.
Its important to note that these people are on the team – 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.
Richard Eisenberg, at the University of Rochester also considers splitting water using metal catalysts in the same issue of Science. This field is on the move.
The team’s work was supported by the Mary and Tom Beck-Canadian Center for Alternative Energy Research; and the Helen and Martin Kimmel Center for Molecular Design. Now the question is – will this be low cost enough to substitute at the home and small business level? The needed energy inputs, heat and light suggest small-scale widely disbursed units that could be personal or family sized fuel makers. Wouldn’t that be something? People wouldn’t need great storage breakthroughs with a steady supply.
What’s missing though is the the energy in to energy out formula. It doesn’t have to be, nor does one reasonably expect, a ‘net’ gain, but an idea of the energy needed to drive the process compared to the energy made available from the hydrogen would help assess and perhaps motivate further or competitive research.