(428d) Manipulating Structures and Chemical Bonding in Nitrides Using First-Principles Methods for Ammonia Production
AIChE Annual Meeting
2020
2020 Virtual AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Nitrogen Chemistry I: Nitrogen Reduction and Fixation
Wednesday, November 18, 2020 - 8:45am to 9:00am
On the nitrogen fixation side of the N conversion cycle, the nitridation of Mn exhibits strong temperature dependence. XRD analyses revealed a transition from the N-lean Mn4N phase to a N-rich Mn2N (and MnN) phase as temperature increases. Above 800°C, the N-rich nitride phase becomes unstable and decomposes into Mn4N and N2. This transition was also confirmed by the calculated free energies from Density Functional Theory (DFT) calculations. On the NH3 formation side of the cycle, an Eley Rideal-Mars-van Krevelan pathway is assumed to study the hydrogenation and extract of lattice N. The formation of NH3 tends to be strongly endothermic on most of the nitride models considered. The diffusion of sublayer lattice N onto the top layer is the kinetic controlling step. Based on DFT calculations, we showed that early hydrogenation NHx intermediates still follow linear scaling relationships a behavior similar to transition metal surfaces. From kinetic modeling, we showed that single-atom doping with Fe, Co, Ni will promote NH3 formation, while Cr, and Mo dopant hinder this process. These molecular-level insights based on well-defined sites and lattice structures are critical for the identification of promising materials, and ultimately, accelerate material discovery and synthesis.