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Spectroscopic Studies of N2 Reduction: From Biological to Heterogeneous Catalysis

Final Report Summary - N2RED (Spectroscopic Studies of N2 Reduction: From Biological to Heterogeneous Catalysis)

The focus of N2RED was on the development of novel X-ray spectroscopic tools to enable a fundamental understanding of the process of dinitrogen reduction in both biological and heterogeneous catalysts. This is a process of great fundamental importance, as neither plants nor animals possess enzymes that are able to convert inert dinitrogen to a bioavailable form. However, this conversion is essential for the incorporation of nitrogen into our amino acids and nucleic acids. As a result, all life on earth relies on the nitrogenase family of enzymes or the industrial Haber-Bosch process to provide the source of bioavailable nitrogen, in the form of ammonia. The detailed mechanism of both the biological and industrial nitrogen reduction catalysts, however, are far from fully understood. Yet, this information is essential for optimizing conversion and rationally designing new catalysts. The goal of this project was to obtain a detailed understanding of the electronic structure factors that govern efficient catalysis in these systems. To this end, several new technological developments were made over the course of this grant. This included the development of a first of its kind dispersive X-ray emission spectroscopy (XES) instrument for applications in homogeneous and heterogeneous catalysis. In addition, we developed 1s-Valence resonant XES as a probe of polarization dependent information from non-crystalline samples. This thus allows, to some extent, the ability to obtain ligand selective information from complex systems. The information content of hard X-ray RIXS was further developed and utilized to obtain insights into the differences in the electronic structure between V- and Mo-dependent nitrogenases. In addition, XMCD was applied to obtain insight into the complex magnetic coupling in these systems. The combination of these methods allowed us to understand the role of the heterometal, the carbide and the protein environment in tuning the biological active site for optimal reactivity. We were also able to apply these methods to intermediates in biological nitrogen reduction, obtaining insight into the earliest steps in the mechanism. In addition, the methods first developed for the biological catalysts were extended to the heterogeneous catalysts. We were able to establish signatures for in situ nitriding during ammonia decomposition, however, signatures for nitriding during ammonia synthesis remain elusive. These challenges, however, motivated the development of a novel nanoreactor setup for studies of working catalysts, thus providing promise that we will also meet the final goal of understanding the heterogeneous mechanism in due course. Overall, this project has taken important steps towards understanding both biological and heterogeneous dinitrogen reduction on the atomic level, while providing new tools and methods to serve the broader catalysis community.