Vibrational Spectroscopy Signatures of Catalytically Relevant Configurations for N₂ Reduction to NH₃ on Fe Surfaces via Density Functional Theory

Abstract

Measuring and predicting accurate spectroscopic signatures of catalytic systems is essential to monitor and validate reaction mechanisms to provide a basis for rational catalyst design. Here, we apply Density Functional Theory (DFT) vibrational analysis techniques to predict the binding and vibrational features of adsorbate species (H, N, NH₂, NH₃, and N₂) important for ammonia synthesis on pure and doped Fe-bcc(111), Fe-bcc(211), and reconstructed (Fe-bcc(211)R) surfaces. We focus on configurations predicted to be dominant under realistic reaction conditions. For each configuration, the vibrational modes with expected high IR intensity (motions perpendicular to the surface or signature modes of complex species such as NH₂ or NH₃) are singled out and discussed in terms of the following: (a) a comparison with previous experiments to validate the accuracy of our results, and (b) a perspective use in operando/in situ monitoring of catalytic processes. We find that calculated frequencies and intensities are in good agreement with available experimental data, thus, validating our predictions. We then show that changes in the frequencies of characteristic modes as a function of doping and reaction conditions (giving different dominant configurations) could be observable via vibrational spectroscopy. Monitoring these signatures could enable determination of the catalytically active configurations in the complex catalytic process as well as the surface orientations most influential on reaction rates. This could allow unambiguous identification of reaction mechanisms.

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