New Method for Splitting Water Passively with Heat and Light

Light and water are both involved in the process of green plants’ photosynthesis. Scientists have been trying to find out the deep truths of how plants acquire their energy from the Sun, split the water, generate oxygen, and create hydrocarbons, eating CO2 out of the air.

Prof. David Milstein, and his colleagues from the Weizmann Institute, the Organic Chemistry Department, demonstrated a new way of generating hydrogen and oxygen, through a series of steps that involve applying heat, light, and a special catalyst.

Milstein’s team discovered that upon mixing a ruthenium “smart” complex they designed in previous studies, with water, the bonds between the hydrogen and oxygen from the water break, having one hydrogen atom binding to the complex’ organic part, while the remaining OH group binds to the metal ruthenium center.

Releasing the hydrogen from this complex is pretty simple: they heat the water to 100°C. While this happens, another OH group adds itself to the metal center. This would be the second stage.

In the third stage, something interesting happens, and I’ll let Milstein tell it: “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”

The generation of a bond between two O2 atoms, promoted by a man-made metal complex is a very rare event, unexplained until now. Milstein and his colleagues discovered that light provides the complex with the energy that causes the two OH groups to get together and form hydrogen peroxide (H2O2), that quickly breaks up into hydrogen 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”. Moreover, they showed evidence that the bond between the two oxygen atoms is generated within a single molecule, and not between oxygen atoms residing on separate molecules, as it was previously believed, and all of these come from a single metal center.

Generating hydrogen is good for the energy industry, while generating oxygen is good for the environment. If we use solar power to heat and light the metal complex, all that remains to be seen is if this method is more energy efficient than the traditional electrolysis. It probably isn’t at this stage, but, like all new inventions, this one may have time to evolve into something more elaborated.