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Marcus Niederberg/ ETH Zurich

Tweezers hold a sheet-like aerogel composed of TiO2 nanoparticles doped with palladium and nitrogen.

Researchers at ETH Zurich have developed a new photocatalyst made of aerogel that can produce hydrogen more efficiently. The key is to carry out precise pretreatment of the material.

Aerogel is an extraordinary material that has set Guinness World Records many times, including being the lightest solid in the world.

Professor Markus Niederberger of the Multifunctional Materials Laboratory of ETH Zurich has been studying these special materials. His laboratory specializes in aerogels composed of crystalline semiconductor nanoparticles. "We are the only group in the world that can produce such a high-quality aerogel," he said.

One use of nanoparticle-based aerogels is as a photocatalyst. These methods are used when a chemical reaction needs to be activated or accelerated with the help of sunlight-an example is the production of hydrogen.

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

This is why Junggou Kwon, a doctoral student at Niederberger, has been looking for a new way to optimize aerogels made of TiO2 nanoparticles. She has a brilliant idea: if the TiO2 nanoparticle aerogel is "doped" (in technical terms) with nitrogen, the individual oxygen atoms in the material will be replaced by nitrogen atoms, and then the aerogel can absorb more visible Part of the spectrum. The doping process leaves the porous structure of the aerogel intact. Research on this method was recently published in the journal Applied Materials and Interface.

Kwon first 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 and methanol vapor into the aerogel in the reactor, and then irradiated it with two LED lights. The gaseous mixture diffuses through the pores of the aerogel, where it is converted into hydrogen required for the surface of titanium dioxide and palladium nanoparticles.

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.

The experiment mainly provides feasibility studies for researchers. As a new class of photocatalysts, aerogels provide a special three-dimensional structure. In addition to generating hydrogen, it also provides potential for many other interesting gas-phase reactions. Compared with electrolysis commonly used today, the advantage of photocatalysts is that they can use only light instead of electricity to produce hydrogen.

Whether the aerogel developed by the Niederberger team will be used on a large scale is still uncertain. For example, there is still the problem of how to accelerate the flow of gas through the aerogel; at this moment, the extremely small pores obstruct the gas flow too much. "To run such a system on an industrial scale, we must first increase the airflow and improve the aerogel irradiation," Niederberger said. He and his team are already studying these issues.

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