Weizmann Institute scientists have developed a new approach to break down water into oxygen and hydrogen

Hydrogen may be used as an efficient and environmentally friendly fuel

Developing efficient systems for separating water into its components - hydrogen and oxygen - using sunlight, is a scientific challenge to which many efforts are focused, in different parts of the world. Such systems may allow the use of hydrogen gas as a clean and environmentally friendly fuel. However, most of the artificial systems that exist today fail to "deliver the goods", because they require the use of chemical substances that cannot be recycled. A new approach developed by Prof. David Milstein and his research partners from the Department of Organic Chemistry at the Weizmann Institute of Science takes an important step in dealing with this challenge - by developing a new approach to "crack" water molecules. During the research work, published in the scientific journal Science, the scientists also discovered a new and unknown mechanism for the formation of chemical bonds between oxygen atoms, and clarified its stages.

Linking two oxygen atoms originating from the water molecules to form oxygen gas is the process that is the bottleneck in water splitting. Nature has already chosen its own path to deal with the problem: photosynthesis, carried out by plants, is the source of all the oxygen in the Earth's atmosphere. Despite the significant progress that has taken place in the understanding of photosynthesis, the question of how exactly the system works is not fully clear. Many research groups around the world are trying to develop artificial photosynthetic systems - but so far with rather limited success.

The new approach developed by Weizmann Institute of Science scientists includes a sequence of successive reactions, driven by light and heat, which lead to the release of hydrogen and oxygen. Also participating in the process is a "smart" metallic compound that was designed and developed in previous research by the team. It is a collection of organic molecules, in the center of which is an atom of the metal ruthenium. At each stage of the process, the smart aggregate undergoes chemical changes - which enable the existence of the next stage. "When we expose the aggregate created in the third and last step to light, at room temperature, the catalyst returns to its original state, so that it can be used again for a new cycle of reactions," says Prof. Milstein.

In the first stage, the joint activity of the metallic center and the organic part enable the breakdown of the water molecules: the presence of the smart aggregate in the water causes the chemical bonds between the oxygen and hydrogen to break. One hydrogen atom binds to the organic part of the aggregate, and another hydrogen atom and an oxygen atom (hydroxide group) bind to the metallic center. In the next stage of the process, the heat stage, water is added to the mixture and heated to a temperature of 100 degrees Celsius. This step causes the release of hydrogen gas from the accumulator - a possible source of clean fuel - while binding an additional hydroxide group to the metal.

In the third stage of the process - the light stage - oxygen gas is created, and at the end the metallic aggregate returns to its original state. This is a surprising finding, since until now it was not known that it was possible to form a bond between two oxygen atoms in this way. Prof. Milstein and his team members also discovered the unknown mechanism by which the process takes place. It turns out that during the third stage, the light provides the energy needed for the coalescence of two hydroxide groups, and the creation of hydrogen peroxide (H2O2). This is a short-lived bond, as it is an unstable substance that quickly breaks down into water and a single atom of oxygen. Later, two oxygen atoms combine to form a molecule of oxygen gas. "The relative instability of hydrogen peroxide caused scientists to ignore the possibility of the existence of this phase, considering it improbable. We have shown that this phase is indeed taking place", says Prof. Milstein. The scientists were able to prove that the bond between the two oxygen atoms was formed within a single molecule, and not between oxygen atoms originating from different molecules - as was commonly thought - and that they originated from a single metallic cluster.

The development of effective artificial methods for breaking down water molecules into hydrogen and oxygen using sunlight is a main goal in the field of clean and sustainable energy research. So far, the team of researchers led by Prof. Milstein has managed to find a three-step mechanism to create hydrogen and oxygen from water, which is based on the use of sunlight and does not require the use of biodegradable chemicals. Now the scientists plan to unite the three steps into one sequence, and create an efficient system, which will advance the energy researchers another and important step on their way to achieving their goal.

The post-doctoral researcher (at that time) Dr. Stefan Kohl, research student Leonid Schwarzbard and laboratory technician Yehoshua Ben-David from Prof. Milstein's group in the organic chemistry department participated in the study, along with Dr. Lev Weiner, Dr. Leonid Konstantinovsky, Dr. R. Linda Shimon and Dr. Mark Iron from the Department of Chemical Research Infrastructures.