(428d) Manipulating Structures and Chemical Bonding in Nitrides Using First-Principles Methods for Ammonia Production

In this work, an alternative approach to the Haber-Bosch process based on a cyclic process is investigated for smaller-scale, intermittent NH₃production at atmospheric pressure. Manganese (Mn) and Mn nitrides are uniquely advantageous for the proposed cyclic step catalysis scheme. The elemental N is incorporated into Mn substrate, as lattice N, which will be reduced subsequently by H₂ to yield NH₃. Currently, the lattice N utilization (< 8%) and NH₃ yield remain too low.

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 NH_x 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.