Technische Universität Berlin

Keep Calm and Drink Tea in the Catalysis Lab

Long-term catalytic stability is of key importance in industrial processes. Professor Dr. Franziska Hess researches the predictability of catalyst degradation at the Institute of Chemistry’s Chemical Catalysis Group in Faculty II Mathematics and Natural Sciences at TU Berlin, where she was appointed to a junior professorship at the end of April 2020.

What are your main areas of focus and interest as a researcher? Are there any issues that particularly inspired you?

Franziska Hess: One of my main areas of research is the analysis and predictability of catalytic degradation. This is very important for me, as the lack of long-term catalytic stability is often a problem in technical applications. However, it is very difficult to study long-term stability using experiments and there are no established methods at the theoretical level either. In the long term, such methods are necessary to develop functional catalytic materials.

Can you provide a familiar example of the importance of long-term catalytic stability?

The best-known example is the catalytic converter used in car exhaust systems. Anyone who has driven a car for some time knows that it is only a matter of time before the catalytic convertor has to be replaced and at considerable cost. It’s the same with industrial processes, just on a larger scale.

What was your most interesting or most exciting research project?

Studying how impurity atoms accumulate or deplete at the surface when impurities are selectively introduced into a crystal to alter conductivity or oxygen vacancy concentration, a process known as segregation of dopants, in solid oxide fuel cell electrodes.

Solid oxide fuel cells are high-temperature fuel cells used at operating temperatures of 650 to 1000 °C. They have a higher efficiency than low-temperature fuel cells, making them suitable for longer continuous operation under stable load conditions. They have a number of applications, including replacing car engines. Doping is applied to fuel cell electrodes to create oxygen vacancies in the crystal lattice, allowing oxygen ion conductivity. Working in this project, I had to take account of many different aspects in order to understand the whole phenomenon.

How does segregation, i.e. the accumulation of deposits, occur? How long does such a process take?
In this context, we can understand segregation as a kind of de-mixing. In the fresh material, two different metal cations are homogeneously mixed in the electrode. As a result of segregation, one of these accumulates on the surface, rather like fat rising to the top of a bowl of soup. As the surface is the active component of the electrode, this enrichment can, under certain conditions, lead to a measurable reduction in catalytic activity within just a few hours.  

What important insights has this led to?

Based on my findings, I have developed possible strategies to improve the durability of electrodes. One option is to choose a material composition with a slight deficit of a metal. This inhibits segregation, enabling the electrode to be used for longer.

About Prof. Dr. Franziska Hess

Franziska Hess completed her studies (bachelor’s and master’s in chemistry) as well as her doctorate at Justus Liebig University Giessen before commencing postdoc work at Massachusetts Institute of Technology. Prior to her appointment as a junior professor at TU Berlin, she had been awarded a Liebig Fellowship to establish her own junior research group at RWTH Aachen.

Why are electrodes important for fuel cells?

The electrodes are used to activate the reactants, oxygen and hydrogen, which serve as fuel in the fuel cell, and to exchange electrons in the electrochemical reaction, from which electricity is then generated.

Do any of your findings mark a turning point?

I became interested in catalytic stability while studying electrochemical corrosion on the RuO2(100) surface. Ruthenium (chemical symbol Ru) serves as model catalyst for oxidation reactions as well as an active electrode material for splitting water. During my research, I discovered something new and unexpected every day. This project showed me that a catalyst can undergo side reactions resulting in its long-term destruction and that we need to find an approach to these reactions to improve catalytic materials.

Which side reactions can destroy a catalyst and how does this negatively impact their use, such as in the chemical industry?

If we take electrochemical water splitting as an example, a volatile molecule, RuO4, is formed, causing the electrode to dissolve. This process is called corrosion. In industrial processes, often only a thin layer of RuO2 is deposited on a substrate, as ruthenium is rare and expensive. Corrosion causes this layer to become thinner until it eventually disappears completely and has to be replaced.

What are you working on at the moment?

I am currently working on a screening model capable of predicting stable catalytic materials for HCI oxidation in the Deacon process. Named after the English chemist Henry Deacon (1822-1876), who patented it in 1868, this process produces chlorine by oxidizing hydrogen chloride (HCI) with oxygen. Better known is the aqueous solution of hydrogen chloride, hydrochloric acid.

What is the goal of the screening model?
The aim of the screening model is to identify a few promising candidates in terms of their stability and activity from the wide array of possible catalysts. Making this initial selection on a theoretical basis reduces the work involved in the subsequent experimental testing stage.

Do you have a favorite quotation or life motto?

“Keep calm and drink tee” in both the literal and metaphorical sense.

Is there a book you would recommend?

Sternstunden der frühen Chemie by Ernst F. Schwenk. What interests me is that early chemistry, also known as alchemy, was on the one hand concerned with the search for the Philosopher’s Stone, while also being a means for “magic” tricks and sleight of hand. Many of these tricks still are still featured today in our Christmas and show lectures. Chemistry as a science the way we know it today did not exist at the time. Alchemy followed known patters, but these could not be explained. Combining certain components, results in sparks or a change in color. But no one knew why. The book describes the development of chemistry from a mystica toward a modern science by drawing on selected biographies of the founding fathers of chemistry.

How would things have been different without the coronavirus?

The coronavirus pandemic has meant that I have very much been thrown blindly into my work as a junior professor. There are no classes on how to become a professor.  My appointment for the written certification of the professorship, for example, has been postponed, and it is not so easy to just strike up a conversation with colleagues. There is no “informal” exchange of information such as would normally take place over lunch or in the hallway.

And what have you learned: How do you become a professor?

There is the Young Academics Network for junior research group leaders at TU Berlin. The network is run by Professor Dr. Johannes Teichert, junior professor at the Institute of Chemistry, and used by junior professors, junior leaders of DFG-funded projects and fellows. Back in September 2020, we met up at a beer garden, which gave us a chance to discuss and share our experiences. We all face similar problems even if we work in different areas. The only other option is to keep on asking.

I particularly like my work at TU Berlin because …

… the students are very motivated and interested.

Interviewer: Christina Camier.

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