Researchers Move Ahead in Understanding Mechanisms of Water Oxidation Using Novel Catalyst
10 March 2008
Scientists at the US Department of Energy’s Brookhaven National Laboratory (BNL) and the Institute for Molecular Science (IMS) in Japan report on their progress toward understanding the electronic and structural mechanisms of water oxidation using a novel ruthenium catalyst first developed by the IMS researchers. Their paper appears online in the 10 March 2008 edition of the journal Inorganic Chemistry.
Water oxidation is one part of the process of splitting water into hydrogen and oxygen. Water-splitting requires a large amount of energy from sunlight and metal catalysts to activate the very stable water molecules. It occurs as two separate half reactions: water oxidation produces the oxygen, along with protons and electrons; these protons and electrons are then combined to make molecular hydrogen.
The water oxidation reaction is generally believed to be the limiting process, meaning that if it is not catalyzed efficiently, it limits hydrogen production. You can’t sustain hydrogen production without the protons and electrons generated by water oxidation. So, to make hydrogen from water for use in fuel cells, we must meet the challenge of performing efficient and inexpensive water oxidation.
—James Muckerman, Brookhaven
Japanese scientists Koji Tanaka and Tohru Wada at the Institute for Molecular Science in 2001 discovered a novel dinuclear Ru complex that contains redox active quinone ligands and has an excellent electrocatalytic activity for water oxidation when immobilized on an indium-tin-oxide electrode.
To accomplish the water-oxidation reaction, Tanaka and Wada immobilized the ruthenium catalyst on the electrode, placed it in an aqueous solution, and applied a voltage, resulting in a rapid turnover for oxidizing water to oxygen. The research team continues to collaborate on further studies to understand the details of how the catalyst works.
We are combining theoretical and experimental studies to determine how this ruthenium complex with bound quinone molecules efficiently catalyzes water oxidation to form oxygen.
—Etsuko Fujita, Brookhaven
The scientists have discovered that when the protons from two water molecules are removed due to acid-base reactions in solution, four electrons are transferred to electron receptor sites in the catalyst. Once all the protons are removed, the theoretical calculations predict that an oxygen-oxygen bond is formed.
What makes their catalyst novel is that in most metal-based compound catalysts these electron receptor sites are located on the metal atoms, but in this ruthenium complex the receptor sites are on the quinone molecules. More theoretical and experimental studies will be needed to fully understand and improve the mechanisms of quinone-containing catalysts.
The ruthenium in our catalyst is somewhat expensive, so we plan to continue our studies with more economical catalysts incorporating less-expensive metals.
—James Muckerman
The US Department of Energy’s Office of Basic Energy Sciences (BES) within its Office of Science funded this basic research at Brookhaven Lab. The research is part of the BES Hydrogen Fuel Initiative program.
Resources
James T. Muckerman, Dmitry E. Polyansky, Tohru Wada, Koji Tanaka, and Etsuko Fujita. Water Oxidation by a Ruthenium Complex with Noninnocent Quinone Ligands: Possible Formation of an O-O Bond at a Low Oxidation State of the Metal. Inorg. Chem., 47 (6), 1787–1802, 2008. 10.1021/ic701892v
There's no such thing as water oxidation. This describes the chemical reduction of hydrogen oxide to molecular hydrogen, an inorganic analogue to what happens during photosynthesis.
Posted by: Rafael Seidl | 10 March 2008 at 08:46 AM
That seemed like an odd phrase to me also, but it did seem like a significant discovery of how the process they described works.
Posted by: sjc | 10 March 2008 at 11:42 AM
It seems strange to discuss the catalyst mechanism,call the process oxidation,when its reduction. The main point,however,is regardless of the catalyst,the huge energy to break up the water molecule is still the problem.
Posted by: earl rogak | 10 March 2008 at 12:25 PM
Water oxidation never get Hydrogen. The real problem is on the other electrode where Hydrogen formation occurs. A good oxidation catalyst can just improve energy efficiency by reducing electrolysis voltage. When you say: "these protons and electrons are then combined to make molecular hydrogen" unfortunately these electrons are coming from an other electrode! Here is the problem.
Posted by: Raymond | 11 March 2008 at 12:10 AM
The Second Law of Thermodynamics says that anything you do has a cost. There are no perpetual motion machines.
Converting hydrogen dioxide to its constituent components and then re-oxidizing them requires an input of unrecoverable energy. Unless you have a surplus of such energy that you can dedicate to be consumed for this purpose. You can never come out ahead.
The present situation is that there is NOT a surplus of energy available. Somehow this is supposed to provide such. No such luck.
When controlled Fusion Energy is available on a large scale, that solar-like energy source may provide a surplus of energy to allow such energy consuming conversion.
Posted by: stan peterson | 12 March 2008 at 11:51 AM
The surplus energy is from that fusion reactor in the sky. Get it straight.
And there is such a thing as water oxidation.
Posted by: Mark | 12 March 2008 at 12:30 PM
It's not hydrogen dioxide, but dihydrogen oxide.
Oxidizing means taking away electrons. So you take away electrons of the water. The result is two protons, that disolve in the water, and an oxygen atom that craves for another electron. two such oxygen atoms combine to O2, that bubbels away.
The electrons taken from the water molecule are pumped to the other electrode (this requires an external energy source) and are given to the dissolved protons. there, H2 forms, whitch bubbels up. So you produce O2 gas at one electrode and H2 gas at the other. you do this by taking taking away electrons from the H2O, thus by oxydizing water.
If you use nuclear or other CO2-neutral energy, this can be a very clean and abundant H2 source.
Posted by: Alain | 14 March 2008 at 03:12 AM
That makes sense now. As I understand it, heat can make the electrolysis more efficient. If you can get the heat cheap, like concentrated solar, it might work. There was an article about using an SOFC to do electrolysis. You heat the stack with concentrated solar, put in water, add an electric potential and it produces H2 and O2.
Posted by: sjc | 15 March 2008 at 06:53 PM
I would like to confer with someone on this topic; the goal being a functional prototype.
Any one who is willing please contact me via E-mail: [email protected] or by phone (575)914-0301 I have voice mail on while at work please leave a massage.
Posted by: jerry r tice | 16 May 2008 at 03:11 PM