The use of abundant solar energy and semiconductor photocatalysts to produce hydrogen has great potential for the production of clean and sustainable energy carriers.

Researchers at ETH Zurich have developed a new type of photocatalyst made of aerogel that can produce hydrogen more efficiently. Aerogels increase the efficiency of converting light into hydrogen energy, producing 70 times more hydrogen than competing methods.

Aerogel is an extraordinary material that has set Guinness World Records many times, including the honorary title of being one of the world's lightest solids. Professor Markus Niederberger of the Multifunctional Materials Laboratory of ETH Zurich has been studying these special materials.

Nanoparticle-based aerogels can be used as photocatalysts. They are used whenever there is a need to activate or accelerate chemical reactions with the help of sunlight to produce very useful products in the modern world (including hydrogen).

The material of choice for photocatalysts is titanium dioxide (TiO2), which is also a semiconductor. But titanium dioxide has a major shortcoming-it can only absorb the ultraviolet spectrum of sunlight, which is only about 5% of the total amount of sunlight. If photocatalysis is to be efficient and industrially useful, the catalyst must be able to utilize a wider wavelength range.

Niederberger's doctoral student Junggou Kwon has been looking for a new alternative method to optimize aerogels made of TiO2 nanoparticles. She found that if the TiO2 nanoparticle aerogel is "doped" with nitrogen so that nitrogen atoms replace individual oxygen atoms in the material, the aerogel can absorb more of the visible part of the solar spectrum. The doping process leaves the porous structure of the aerogel intact.

Initially, Kwon used TiO2 nanoparticles and a small amount of precious metal palladium to produce aerogels. Palladium plays a key role in photocatalytic hydrogen production. Then she put the aerogel into the reactor and injected ammonia gas. This results in a single nitrogen atom being embedded in the crystal structure of the TiO2 nanoparticles.

In order to test whether the aerogel modified in this way actually improves the efficiency of the required chemical reaction-in this case, the production of hydrogen from methanol and water-Quan developed a special reactor, she Put the aerogel directly into it. Then she introduced water vapor and methanol into the aerogel in the reactor, and then irradiated it with two LED lights.

Five days later, Kwon stopped the experiment, but until then, the reaction stabilized and continued in the test system. "This process may be stable for a longer time," Niederberger said. "Especially in industrial applications, it is important that it remains stable for as long as possible." The researchers are also satisfied with the results of the reaction. The addition of the precious metal palladium significantly improves the conversion efficiency: the use of palladium-containing aerogels produces 70 times more hydrogen than the aerogels without palladium.

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