Gels are elastomers with network-like structures, the voids of which are filled with either solvents (hydrogels) or air (airgels) (aerogels).
“Aerogels are extremely porous, with pores accounting for up to 99.98 percent of the total volume. Aerogels also have a fractal structure with a strong dendritic structure. In the book chapter “Gels: Hydrogels and Aerogels,” Günter Jakob Lauth and Jürgen Kowalczyk explain that the pore size can be more than 1000 m 2 per gram “.. Other molecules, or even whole cells, might be housed in these airy frameworks, and the Springer authors predict that the technical utility of these gels will continue to grow in the future.
Markus Niederberger, a materials scientist at the ETH Zurich, has been working on aerogels for a long time; after all, these world’s lightest solids have exceptional properties. His multifunctional materials laboratory focuses in crystalline semiconductor nanoparticle aerogels. Niederberger hopes to turn nanoparticle-based aerogels into photocatalysts, allowing or speeding up chemical reactions with the help of sunlight. The creation of hydrogen is an appealing goal of this growth path.
Titanium dioxide (TiO 2), a semiconductor, has long been the material of choice for photocatalysts. According to Michael Tausch in the book chapter, it is used as a photocatalyst in technical systems for the decontamination of wastewater from industry and hospitals “”They must be removed before they get into rivers and lakes, because otherwise they accumulate in food chains and endanger the health of animals and humans,” according to “Conceptual Basics of Photochemistry,” because these are frequently contaminated with organic halogen compounds from solvents or drugs, which are very difficult to break down: “They must be removed before they get into rivers and lakes, because otherwise they accumulate in food chains and endanger the health of animals and humans.” This is accomplished by photocatalytic oxidation of organic halogen compounds using titanium dioxide (anatase) as a photocatalyst and air or oxygen as a source of energy. Even long-lasting chlorine compounds like tetrachlorethylene are mineralized to create carbon dioxide and chloride ions in this fashion.”
However, TiO 2 has one significant drawback: it can only absorb around 5% of the UV component of the daylight spectrum. However, if photocatalysis is to be effective and commercially viable, the catalyst must be able to operate over a wider range of wavelengths. Photocatalytic splitting of water is another prospective application, according to Michael Tausch: “Water photolysis would be a sustainable technique of manufacturing hydrogen, the most environmentally benign of all energy sources, with the help of sunshine.” In principle, this is achievable since the free enthalpy of reaction when a water molecule decomposes equals to the energy of a light quantum, i.e. a visible-range photon.
Junggou Kwon, a PhD student in Niederberger’s group, was looking for a novel technique to improve an airgel formed of TiO 2 nanoparticles so that it could use a wider range of light. And she had a fantastic idea: she started with TiO 2 nanoparticles and added modest amounts of the noble metal palladium to make the airgel. Individual nitrogen atoms were incorporated in the crystal structure of the TiO 2 nanoparticles as a result of passing ammonia gas through the airgel in a reactor, allowing the airgel to absorb extra sections of the visible spectrum. The Zurich researchers’ study was just published in the ACS journal “Applied Materials & Interfaces.”
Kwon created a customized reactor in which she employed the modified airgel directly as a monolith to evaluate if the modified airgel makes a desired chemical reaction – in this case, the creation of hydrogen from methanol and water – more efficient. She then blew a gas combination of water and methanol vapor through an airgel that was illuminated by two LED lights. Indeed, the gas combination diffused through the airgel’s pore space, releasing the required hydrogen on the surface of the TiO 2- and palladium nanoparticles. The reaction in this test system, according to the researcher, was stable and constant before the experiment was stopped after five days. “The process would have stayed constant for a longer period of time,” adds Niederberger. “In particular, for industrial applications, it is critical that the process be steady for as long as feasible.” The yield is also satisfactory; the noble metal palladium greatly improved the conversion efficiency: in palladium-containing aerogels, up to 70 times more hydrogen was created than in non-palladium-containing aerogels.
There is still additional development work to be done before the airgel created by the research group can manufacture hydrogen on an industrial scale. The topic of how to speed up gas flow through the airgel, for example, remains unanswered; the airgel’s narrow pores obstruct gas flow too much. According to Niederberger, the aerogels’ irradiation needs to be improved as well. Despite the uncertainties, the study demonstrated that photocatalysts built from aerogels, with their distinctive three-dimensional structures, are viable and represent a novel class of photocatalysts that might be employed for a variety of gas-phase reactions other than hydrogen production.
