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Dreamers, Doers, and Catalytic Converters

If you can dream it, you can do it.” It worked for Walt Disney. Over the years he pioneered much of the technology needed to achieve his dreams. What he, and others pursuing their goals have found, is that technology is a key factor to success, and sometimes it needs to catch up with what scientists and engineers can dream. Computer simulations of chemical reactions provides one excellent example of this. As far back as 1929, Nobel Prize winner Paul Dirac pointed out that the all of chemistry could be predicted using a mathematical theory known as quantum mechanics, but that the complexity of the quantum mechanical equations made practical solutions impossible.

Computer technology and the density functional theory (DFT) approach to solving the equations of quantum mechanics have since overturned Dirac’s idea. Thus, it is now possible for researchers like William F. Schneider, professor of chemical and biomolecular engineering, to predict chemistry on the computer. He and others are using this capability to design, at the molecular scale, catalysts that are able to control and guide the course of chemical reactions in desired directions.

Chemical engineering is fundamentally about controlling chemical reactions, in particular chemical reactions that transform the raw stuff of nature into useful products. Catalysts are the knobs that give engineers that control. For many years, however, the process of discovering and applying catalysts has been largely trial and error. These catalysts are solid materials, often made up of many chemical elements themselves, and in many cases the precise way they function remains a mystery. Advances in computer power, theory, and algorithms now allow Schneider and his group to apply quantum mechanical models to these catalysts, to understand how known ones actually work and to predict new, more robust, durable, and cost effective ones.  

DFT calculations reveal electronic character of metal-oxide interface in WGS.
One example of the work in Schneider’s group relates to water-gas shift (WGS) catalysis, a major industrial process and primary way that hydrogen is produced. Current water-gas shift catalysts are notoriously unstable and must be handled with great care. Using the tools of quantum mechanics and in collaboration with colleagues at Purdue University, Schneider is working to understand the molecular processes that control water-gas shift, to predict performance and discover new, more reliable materials. Water-gas shift catalysts always start as mixtures of metals and metal oxides. While researchers in the past have focused only on the metal part of this combination, Schneider and his students have shown that the metal-metal oxide interface is the key to the catalysis. Further, they have discovered that this boundary is not simply a physical interface; rather the chemical properties of the metal and, most surprisingly, the metal oxide are tuned by their neighbor. This new insight leads the catalyst discovery effort in new directions and gives new hope of finding superior catalytic materials.

DFT models reveal key steps in catalytic conversion of NOx to nitrogen.
The hydrogen produced by water-gas shift is used to make many other materials, including ammonia. Another project in the Schneider group, carried out in collaboration with researchers at Purdue University and Cummins Inc. in Columbus, Ind., explores the use of this ammonia to clean the exhaust produced by diesel engines. Nitrogen oxides, or “NOx,” are one component of diesel exhaust that is both particularly harmful to the environment and difficult to control. The team is examining catalysts that promote a reaction between NOx and ammonia to produce harmless water and nitrogen. The key to making this catalytic conversion work is ensuring that the ammonia reacts only with NOx, not with the other components in the exhaust. Recent work from Schneider and co-workers have uncovered the molecular basis of this selective catalysis; they are using this knowledge to guide the synthesis of catalytic materials that are effective over a wider range of operating conditions. These superior catalysts will free diesel engine designers to create engines that are cleaner and more efficient over a wide range of operating conditions.

From fertilizers to pharmaceuticals to the conversion of crude oil to gasoline, catalysis is key to the production of all the materials that make modern life possible. While produced on many ton scales, catalysts work atom-by-atom, and quantum mechanical models are revealing the secrets of those atomic processes. For more information on projects in computational catalysis at Notre Dame, visit the Computational Environmental Catalysis Laboratory.