You have to walk past a number of laboratories to reach Vera Krewald’s office. As is the case in all chemical research institutes, you are greeted by a subtle smell – a mix of solvents and other chemicals. “I can offer you the aroma of chemicals”, says Krewald with a laugh. However, she doesn’t contribute to the aroma herself any more as she packed away her lab coat over ten years ago. She moved to TU Darmstadt as an assistant professor for theoretical chemistry at the end of 2018 and has since established her own research group here. Instead of carrying out research using chemicals, flasks and pipettes, Krewald and her team work with the tools of quantum chemistry to decode chemical reaction mechanisms on a computer. “ In a sense, our calculations enable us to look inside molecules and into their electronic structures”, explains Krewald. “We can switch certain effects on and off and then see the impact this has on the electronic structure and thus on chemical reactions.”
New processes for the energy and raw materials transition
Krewald carries out basic research but her findings are certainly relevant for the chemical industry. “I am particularly interested in processes related to the energy and raw materials transition”, emphasises Krewald. Her working group carries out research, for example, on the splitting of dinitrogen – chemical formula N2 – to make this element available for the production of fertilisers or basic chemicals. This reaction is currently carried out using the Haber-Bosch process, which was developed more than 100 years ago and requires temperatures of several hundred degrees Celsius, high pressure and a catalyst. The process is far from sustainable and in fact counts as one of the largest industrial consumers of energy. This is why Krewald is looking for an alternative solution. Recent research results show that the extremely stable nitrogen-nitrogen bond can also be split with light – in principle even with sunlight – if the dinitrogen is bound with certain chemical compounds. In simple terms, the two nitrogen atoms are given attachments that pull at the bond and weaken their cohesion.
“We are currently aware of about ten compounds that facilitate the photolytic splitting of nitrogen”, explains Krewald but then also highlights a problem: all ten molecules contain relatively expensive metals such as rhenium, tungsten or osmium. “To develop a sustainable process suitable for industrial application, we need to transfer this principle to other compounds that contain, for example, iron instead of these expensive metals.”, says Krewald. Before it is possible to design these types of substances, a more detailed understanding of nitrogen splitting needs to be developed first. This is where the quantum chemical calculations come into play because, as with many chemical reactions, the light-induced processes happen so fast that it is not possible to observe the mechanism using experiments. However, quantum chemical methods can be used to calculate the molecules and intermediate stages that only exist for a very short period of time. In addition, the molecules can be changed virtually to observe the effects on the electronic structure. As Krewald explains: “This enables us to understand how bonds are formed or split.”
To develop a sustainable process suitable for industrial application, we need to transfer the principle to other compounds that don’t contain any expensive metals.
Good technical equipment as a basic requirement
Quantum chemistry calculations are complex and require a lot of memory. Krewald’s team uses both the Lichtenberg high-performance computer at TU Darmstadt and also the group’s own computer cluster for the calculations. “In principle, we make use of the same quantum mechanical methods found in physics but adapt the approaches for our purposes”, emphasises Krewald. This is because things start to get complicated in quantum mechanics as soon as you want to include three or more particles in the calculations. As a quantum chemist, she investigates N particles, where N stands for the number of electrons in the molecules – and this number can be in the hundreds: “It is thus necessary for us to use a few approximations but they do work quite well.” She knows this because she has compared her calculations to results found in the laboratory. Krewald’s group works closely together with other experimental chemists in their research into the splitting of nitrogen and also in other projects: “As a theoretical chemist, I need to work together with people who can try out our suggestions in the laboratory and check our interpretations.” The process is usually based on the interplay of computer calculations and laboratory experiments until they identify the ideal molecule for a particular chemical process or the explanation for a reaction mechanism.
Social relevance of quantum chemistry
The splitting of nitrogen became her main field of research five years ago as Krewald was a postdoc at the University of Vienna. Her group also carries out research into the bonding and splitting reactions of other small, extremely stable molecules that will play a key role in the energy transition: hydrogen (H2), oxygen (O2) and water (H2O). For example, splitting water with the sun’s energy produces the regenerative energy source hydrogen, while the splitting of oxygen is a key reaction in fuel cells. Krewald decodes their catalytic processes together with her colleague Ulrike Kramm, who is also a professor in the Department of Chemistry.
Krewald’s research is a good example for the social relevance of quantum chemistry. She received the Young Scientist Award from the Working Group of German University Professors for Chemistry (ADUC) at the beginning of the year for the establishment of her specialist field. In November, she was also awarded the Dr. Hans Messer Foundation Prize, which she shares with Meike Saul from the Department of Biology at TU Darmstadt. Krewald will invest her prize money of 25,000 euros in hardware to provide additional computing power and in organising an international meeting on the photolytic splitting of nitrogen. She knows that it will not only require experimental and theoretical chemists but also other researchers across the world to work hand in hand to achieve the energy transition and overcome other challenges of our time.
Bastian Schluschaß, Jan-Henrik Borter et al.: Cyanate Formation via Photolytic Splitting of Dinitrogen, Journal of the American Chemical Society Au, May 2021, https://doi.org/10.1021/jacsau.1c00117