(707a) Portraying the Mars-Van Krevelen Mechanism on Nitride Mxenes through Operando Spectroelectrochemical and Computational Techniques

The electrochemical nitrogen reduction reaction (NRR) provides an opportunity to convert nitrogen (N₂) to ammonia (NH₃) at ambient conditions. Protons (H⁺), which stem from water oxidation, are necessary for this process, but result in the hydrogen evolution reaction (HER) reducing overall selectivity. Another cause for low selectivity in the NRR is the high energy required to activate the N≡N bond during typical associative/dissociative mechanisms. An avenue to mitigate these challenges altogether is available on transition metal nitride materials through the Mars-van Krevelen (MvK) mechanism, where sub-lattice nitrogen atoms are protonated to form NH₃, which then desorb to form nitrogen vacancies that require less energy to weaken the N≡N. Recently, we have provided preliminary evidence that the Ti₂NT_x MXene operates through this MvK mechanism. To further corroborate this mechanism, a series of electrochemical and operando spectroscopic techniques, including X-ray absorption spectroscopy (XAS) and Fourier-transform infrared (FTIR) spectroscopy, were utilized. We use XAS to track the Ti oxidation state shift as a function of applied potential to understand the role of the Ti in the MvK mechanism and to understand the dynamic change in the Ti-N-Ti bond network. Furthermore, we use FTIR to elucidate in real-time the adsorption of NRR reactive species and intermediates on the MXene surface, especially with regards to the periodicity of the -NH₂ and N-N spectroscopic intensities, which allows for the MvK mechanism to be differentiated from the typical associative/dissociative mechanisms. Finally, we use DFT and AIMD to corroborate the experimental work and further enhance our understanding of the mechanisms of NRR. These findings serve as the foundation to produce green NH₃ using earth abundant resources.