No abstract

Ammonia is one of the most important feedstocks for the production of fertilizer and as a potential energy carrier. Nitride compounds such as LaN have recently attracted considerable attention due to their nitrogen vacancy sites that can activate N 2 for ammonia synthesis. Here, we propose a general rule for the design of nitride-based catalysts for ammonia synthesis, in which the nitrogen vacancy formation energy (E NV ) dominates the catalytic performance. The relatively low E NV (ca. 1.3 eV) of CeN means it can serve as an efficient and stable catalyst upon Ni loading. The catalytic activity of Ni/CeN reached 6.5 mmol•g −1 •h −1 with an effluent NH 3 concentration (E NH3 ) of 0.45 vol %, reaching the thermodynamic equilibrium (E NH3 = 0.45 vol %) at 400 °C and 0.1 MPa, thereby circumventing the bottleneck for N 2 activation on Ni metal with an extremely weak nitrogen binding energy. The activity far exceeds those for other Co-and Ni-based catalysts, and is even comparable to those for Ru-based catalysts. It was determined that CeN itself can produce ammonia without Ni-loading at almost the same activation energy. Kinetic analysis and isotope experiments combined with density functional theory (DFT) calculations indicate that the nitrogen vacancies in CeN can activate both N 2 and H 2 during the reaction, which accounts for the much higher catalytic performance than other reported nonloaded catalysts for ammonia synthesis.

We report the synthesis, structure, and electromagnetic properties of Cr-based layered oxyarsenides LnCrAsO (Ln = La, Ce, Pr, and Nd) with a ZrCuSiAs-type structure. All LnCrAsO samples showed metallic electronic conduction. Electron doping in LaCrAsO by Mn-substitution for the Cr sites gave rise to a metal-insulator transition. Analysis of powder neutron diffraction data revealed that LaCrAsO had G-type antiferromagnetic (AFM) ordering, i.e., a checkerboard-type AFM ordering in the CrAs plane and antiparallel spin coupling between the adjacent CrAs planes, at 300 K with a large spin moment of 1.57 μB along the c axis. The magnetic susceptibility of LaCrAsO was very small (on the order of 10(-3) emu/mol) and showed a broad hump at ∼550 K. First-principles density functional theory calculations of LaCrAsO explained its crystal structure and metallic nature well, but could not replicate the antiparallel spin coupling between the CrAs layers. The electronic structure of LaCrAsO is discussed with regard to those of related compounds LaFeAsO and LaMnAsO.

The current catalytic reaction mechanism for ammonia synthesis relies on either dissociative or associative routes, in which adsorbed N2 dissociates directly or is hydrogenated step-by-step until it is broken upon the release of NH3 through associative adsorption. Here, we propose a concerted mechanism of associative and dissociative routes for ammonia synthesis over a cobalt-loaded nitride catalyst. Isotope exchange experiments reveal that the adsorbed N2 can be activated on both Co metal and the nitride support, which leads to superior low-temperature catalytic performance. The cooperation of the surface low work function (2.6 eV) feature and the formation of surface nitrogen vacancies on the CeN support gives rise to a dual pathway for N2 activation with much reduced activation energy (45 kJ·mol–1) over that of Co-based catalysts reported so far, which results in efficient ammonia synthesis under mild conditions.