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A new kind of “green hydrogen” that gives off light instead of electricity

Structural models of two clusters that enable water splitting into O2 and H2 by means of light energy. Credit: Vienna University of Technology

Hydrogen could be a big part of our energy needs in the future because it can be stored, moved, and burned as needed. But most of the hydrogen that is available now comes from the production of natural gas. This needs to change to protect the climate. So far, the best way to make “green hydrogen” that is good for the environment is to use electricity from renewable energy sources, like photovoltaic cells, to split water into hydrogen and oxygen.

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But it would be much easier if water could be split directly by sunlight. This is now possible thanks to new catalysts. The process is called “photocatalytic water splitting.” The idea is not used in industry yet. Important steps have been made in this direction at TU Wien. Scientists have found a new way to combine molecular and solid-state catalysts that can do the job while using materials that aren’t too expensive.

Atoms talking to each other

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Alexey Cherevan from the Institute for Materials Chemistry at TU Wien says, “Actually, to split water with light, you have to do two things at once.” “We need to think about both hydrogen and oxygen. The oxygen atoms in the water must be turned into O2 molecules, and the remaining hydrogen ions, which are just protons, must be turned into H2 molecules.”

Now, there are answers to both problems. Tiny groups of atoms that are not organic are held in place on the surface of light-absorbing support structures like titanium oxide. The behaviour we want is caused by the combination of clusters and carefully chosen semiconductor supports.

Cobalt, tungsten, and oxygen come together to make clusters that oxidise oxygen, while sulphur and molybdenum clusters are especially good at making hydrogen molecules. Researchers at TU Wien were the first to put these clusters on a titanium oxide surface, where they can speed up the process of splitting water.

“It was already known that titanium oxide is sensitive to light,” says Alexey Cherevan. “When light is taken in, its energy makes electrons and positive charges that can move around freely in the titanium oxide. The clusters of atoms that sit on this surface are then able to help split water into oxygen and hydrogen because of these charges.”

Control down to each atom

Alexey Cherevan says, “Other research groups that are trying to split water with light use nanoparticles that can change their shape and surface properties in a lot of different ways.” “It’s hard to control the sizes, and the atoms aren’t all in the same place. So, it is not possible to explain in detail how the catalysis process works in this case.” On the other hand, at TU Wien, the exact structure of the clusters is figured out down to the atomic level. This makes it possible to fully understand the catalytic cycle.

Alexey Cherevan says, “This is the only way to find out what makes the process work or not work well.” “We don’t want to just try different nanoparticles until we find the best one. Instead, we want to figure out at the atomic level what the best catalyst is.”

Now that it has been shown that the chosen materials can split water, the next step is to fine-tune their exact structures to make them even more effective.

Simple and hopeful

Alexey Cherevan says that the biggest advantage of his method over electrolysis is that it is easy to use. Before you can make electric hydrogen, you need a source of clean energy, like photovoltaic cells, an electric energy storage device, and an electrolysis cell. In the end, this leads to a fairly complicated system made up of many different raw materials. Photocatalytic water splitting, on the other hand, only needs a surface that has been coated in the right way, is covered in water, and is exposed to sunlight.

Using the idea of “artificial photosynthesis,” this method could also be used in the long run to make more complicated molecules. It might even be possible to use the energy from the sun to turn carbon dioxide from the air and water into hydrocarbons, which can then be used in other ways.

The related studies are published in ACS Materials Au and ACS Catalysis.

Further information: Samar Batool et al, Surface Anchoring and Active Sites of [Mo3S13]2– Clusters as Co-Catalysts for Photocatalytic Hydrogen Evolution, ACS Catalysis (2022). DOI: 10.1021/acscatal.2c00972

Sreejith P. Nandan et al, Immobilization of a [CoIIICoII(H2O)W11O39]7– Polyoxoanion for the Photocatalytic Oxygen Evolution Reaction, ACS Materials Au (2022). DOI: 10.1021/acsmaterialsau.2c00025

Journal information: ACS Catalysis

Source: Vienna University of Technology

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