Plants need nitrogen fertilizers, which are usually ammonia-based. Ammonia is therefore one of the most important chemical products of all. However, its production is currently extremely energy-intensive. An international team from TU Wien and Soochow University in China has now developed a novel catalyst that can convert nitrate from wastewater into ammonia in a much more efficient way than before – powered by the light of the sun and about 1.5 volts.
Crucially, not only is the absorbed sunlight used to stimulate the chemical reaction, but the heat generated in the process can also be used for the same process. In this way, the sunlight has a double effect and the efficiency is significantly increased.
"Converting sunlight into chemical energy by using suitable catalysts is a well-developed strategy," says Prof. Günther Rupprechter from the Institute of Materials Chemistry at TU Wien. "The problem is that most of the time, a large part of the sun's energy is lost immediately in the form of heat, and only a small part of the energy is actually used for the chemical reaction you want."
To change this, the team combined several effects in an unusual way: In a new material, light, heat and electricity interact optimally. The team combined MXene (pronounced "Maxeen"), a special material composed mainly of carbon and titanium, with nanoparticles of gold. The MXene consists of lamellae, of atomically thin layers parallel to each other, which act as "nanoantennas" and capture the light. The gold nanoparticles are the place where the actual chemical reaction takes place.
The MXene lamellae have very special electronic properties: "When sunlight falls on these lamellae, the electrons in the material oscillate collectively back and forth, like a swing," explains Günther Rupprechter. "These oscillations are called plasmons."
The energy of this rocking motion must now be transported to the gold nanoparticles. This happens through two different effects: First, individual electrons can be strongly accelerated, which then move at high speed. Secondly, an important temperature effect also comes into play: "The plasmons heat up the MXene, and when the material heats up, the heat spreads along the lamellar direction," explain Xingda An and Le He from Soochow University. The gold nanoparticles, on the other hand, are cooler – and additional energy can be obtained from this temperature difference. The so-called Seebeck effect ensures that the temperature difference between MXene and gold sets additional electrons in motion. An applied voltage in the order of an AA battery contributes as well.
This is new: Thermal energy normally plays a rather subordinate role in photocatalysts. However, due to the special coupling of MXene and gold, thermal energy is responsible for 57 percent of the reactivity in this case.
Alexander Genest from TU Wien has carried out computer simulations that can be used to explain the effect of this electron transport: "Nitrate molecules are polarized directly on the gold nanoparticles," he says. "And this is exactly what makes the production of ammonia much easier. The amount of energy required for this is thus significantly reduced."
"Our MXene-Gold catalyst achieved ammonia production of 2.1 mol per gram of catalyst per hour – this is a very high value that promises great potential for industrial applications," says Günther Rupprechter.
Advanced Functional Materials
Experimental study
Not applicable
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