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

Authors: 
Liu, B. - Presenter, Kansas State University
Huang, C., Kansas State University
Shan, N., Kansas State University
H. Pfromm, P., Kansas State University
Chikan, V., Kansas State University
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 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.

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