Unravelling Single-Atom Catalysis: A Surface Science Approach

Catalysis is a global industry underpinning the production of fuels, plastics, fine chemicals and pharmaceuticals. Unfortunately, the best catalysts are rare and expensive metals, which makes efficient usage of the active material an important consideration as we build the low-cost, energy efficient, environmentally friendly catalysts of the future. Recently it has emerged that catalysis can be performed using single metal atoms adsorbed on inexpensive metal oxide supports, the ultimate in efficiency. The development of this exciting, yet controversial concept is hampered by an inability to characterize a system based on single-atom active sites. The latest aberration-corrected transmission electron microscopes can confirm the presence of single metal adatoms, but can say nothing about the local environment of the active site. Such information is crucial to better understand adatom stability, identify deactivation processes, accurately model reaction mechanisms, and ultimately, to design better catalysts. Surface science has traditionally provided the mechanistic insights to understand heterogeneous catalysis. However, model systems in which single atoms remain stable on well characterised metal oxide surfaces are scarce, and work has largely been restricted to cryogenic temperatures where diffusion is frozen out, but most reactions are as well. My group at the TU Wien has made a game changing discovery; a model system (the Fe3O4(001) surface) in which dense arrays of metal adatoms remain isolated even when the system is heated to 700 K, easily high enough to study many important catalytic reactions. With the experimental knowhow and theoretical expertise at our disposal, we are uniquely placed to utilize this special system to unravel the atomic-scale mechanisms of catalysis using single metal atoms and very small clusters. The work plan builds our knowledge via a step-by-step approach. The beauty of the Fe3O4(001) adatom template is that the metal/adsorbate combination is easily switched, in both experiment and theory, allowing the best atom for a specific task to be quickly identified. Preliminary results for CO oxidation over a Pt1/Fe3O4 catalyst are encouraging; an entire Mars van Krevelen cycle can be followed at the atomic-scale. We will use our knowledge to tailor single-atom catalysts for a series of probe reactions, test performance in realistic high pressure environments, and attempt to recreate similar systems on Fe3O4 nanocubes. The comprehensive experimentalheory approach to be employed here reflects the broad expertise acquired in my career to date. Funding this proposal will support successful collaborations, and establish new, both within the TU Vienna and internationally. I firmly believe that the excellent infrastructure and world class research environment makes this the ideal venue to pursue this challenging line of research, and it is here that I want to build my academic career and make a lasting impression in the field of heterogeneous catalysis research.

Catalysts are employed throughout the modern economy to convert one material into another. The best catalysts tend to be rare and expensive metals, so much effort is expended on attempts to minimize the amount of metal required. In this FWF START project, we were investigating the potential of "single-atom" catalysts, which represent the ultimate limit in efficiency. To be useful, the single atoms must be anchored on a cheap support material, such as iron oxide. Our project was designed to determine how the anchoring works, and how the reactions occur. Real industrial catalysts are highly complex, which makes it impossible to know the atomic-scale structure. Our work was based on an idealized model system, in which the support was a well-defined iron-oxide single crystal, and the catalyst was studied in a highly controlled vacuum environment. Over the course of the project, we demonstrated that the way in which the "single-atom" binds to the support is critical to its reactivity and stability. This is because the metal atom becomes electrically charged when it forms bonds, and this directly affects the binding strength of reactant molecules. Moreover, we showed that it is possible to control the bonding, and thus the reactivity by correct treatments of the catalyst. All of these studies (38 papers were published in total) utilized a tight collaboration between experiments and simulations, and pushed the limits of what could be achieved in this type of work. One of the most difficult issues with experiments using "single atoms", is that the signal is usually very low. During the project, we realised that we needed to improve the capability of our experiments to get reliable data, and this motivated a push to improve our equipment. The START funding allowed us to dramatically improve the sensitivity of our equipment with state-of-the-art equipment. We also had to improve our methodologies, and a major success of the project was the development of a new setup for "infrared reflection absorption spectroscopy" that allows us to study model single atom catalysts for the first time. Finally, the project has been highly beneficial for both the PI, who is now full professor at the TU Wien, and the PhD students involved who are all pursuing successful careers in academic research. The results of this project have inspired new ideas, and allowed many of the team to apply successfully for further research funding totaling approximately 5M from the FWF and the European Union.