QnAs with Helmut Schwarz

In the early 1900s, German chemists Fritz Haber and Carl Bosch developed a way to synthesize ammonia from atmospheric nitrogen and methane-derived hydrogen. The method became a vital step in manufacturing not only fertilizers but also explosives. Haber likened the process to making “bread from air” (Haber and Bosch are both winners of Nobel Prizes in Chemistry). In his Inaugural Article (1), the chemist Helmut Schwarz has himself devised a method to “make bread from air,” synthesizing ammonia from nitrogen and hydrogen using the metal tantalum as a catalyst. A professor of chemistry at the Technische Universität Berlin, Schwarz has uncovered the mechanisms of catalysis at an atomic level. He has identified so-called “aristocratic atoms,” which act as the workhorses of chemical reactions and has deepened our understanding of organometallic processes. Schwarz has also served as President of the Alexander von Humboldt Foundation, which supports international partnerships between scientists from Germany and around the world. PNAS recently spoke to Schwarz, who was elected as a Foreign Associate of the National Academy of Sciences in 2018, about his current research.

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Helmut Schwarz. Image courtesy of the Alexander von Humboldt Foundation/David Ausserhofer.

PNAS:In your Inaugural Article (1), you replicate the Haber–Bosch process. What does your method demonstrate?

Schwarz:About 1% of energy consumption on Earth goes into making ammonia, because the process is extremely energy-demanding and the product is used in so many ways. The challenge in ammonia synthesis is breaking the triple bond in atmospheric nitrogen so that it will accept hydrogen atoms. I have been looking at the Haber–Bosch process for several years now. It relies on high temperature and high pressure to activate the hydrogen molecule and to cleave the nitrogen triple bond.

Our experiment may not be more energy efficient, but it is a new attempt at an entrenched problem. We combined experimental and computational methods to understand how to form ammonia at ambient temperature, in the gas phase. The benefit is that in the gas phase we have removed complicating factors that exist with reactions in condensed phases. Our approach has enabled us to identify the elementary steps and the structure of reaction intermediates, both in the activation of nitrogen and in the decomposition of ammonia, at a strictly atomistic level.

The widely available transition metal tantalum acts as a diatomic cluster and breaks the dinitrogen triple bond to form a four-membered ring structure of alternating tantalum and nitrogen atoms. Only tantalum does this type of chemistry at ambient temperature in the gas phase. In the forward reaction, the catalyst serves as both an electron acceptor and donor during the cleavage of dinitrogen, and also splits the dihydrogen single bond. We find the very same intermediate in the reverse reaction. We hope that [our method] might improve the ammonia synthesis process and in the future help conserve ecological resources.

PNAS:How does this work fit within the broader context of your research?

Schwarz:For at least the past 20 years, I have been interested in activating small, inert molecules like nitrogen, methane, and carbon dioxide at ambient temperatures to provide an understanding of how they can react. For example, the oxidation of methane to methanol is a huge challenge —a sort of a “Holy Grail”—and the mechanism was an ongoing controversy for decades among biochemists. By paring it all down to just the few atoms at the active centers of the molecules, we learned with electronic structure calculations that various types of reactions can take place. We developed a new concept, looking at the role of spin states, which is now broadly accepted (2, 3).

The commonality of these various problems is identifying pathways that are only accessible by using particular transition metal elements as catalysts.

Gas-phase experiments, however, do not lend themselves to upscaling; they exclude everything which matters in real life. However, as we’ve shown from our work, a mechanistic understanding can provide suggestions for improving existing technologies. In this process, hydrogen cyanide is synthesized from methane and ammonia at 1,500 Kelvin with a platinum catalyst (4).

When we first started combining a theoretical/computational approach with an experimental one in the 1980s, we were criticized. At that time, computational chemistry was not where it is today, and using it was more the exception than the rule. It’s very standard now, and we use theory to guide our experiments. The computational side really has opened the doors to test reactions that were unthinkable years ago.

PNAS:How did you decide to focus on computational chemistry, and where do you see your work headed in the future?

Schwarz:Back before I went to university, I wasn’t sure what to study. I was interested in physics and chemistry, but also law and theater. I am extremely fortunate to do research in Germany where by and large there is sufficient funding for sometimes seemingly “useless studies.” I could pick problems to look at from the standpoint of curiosity and not utility or “what good does this do for society?” (5).

I’m still interested in some very basic questions in chemistry, like: How do you transition from a small system of molecules to a larger system? How many molecules does it take to make a solution and to see the effects of the solution on the chemistry of a reaction? What difference do minor electric fields play at the active center of a small catalyst on a molecular level?

Personally, I value the so-called “usefulness of useless knowledge” (5), which was a phrase coined by Abraham Flexner, who founded the Institute for Advanced Study at Princeton. As long ago as 1939, he argued against a need for utility in promoting research and allocating funding. It is a cause I remain dedicated to today.