Ammonia, which is one of the most important chemicals for the synthesis of dyes, pharmaceuticals, and fertilizers, is produced by the reaction of molecular hydrogen with nitrogen, over an iron-based catalyst at 400-500 °C under pressure of over 100 bar. Decreasing the operating temperature and pressure of this highly energy-intensive process, developed by Haber and Bosch over 100 years ago, would decrease energy consumption in the world. In this work, we used two-dimensional Mo₂CT_x MXene as a support for a cobalt-based catalyst. The MXene functionalized by Co showed catalytic activity for ammonia synthesis from H₂ and N₂ at temperatures as low as 250 °C, without any pretreatment. The developed catalyst was highly active for ammonia synthesis, demonstrating a high rate of up to 9500 μmol g^-1_{active phase} h^-1 at 400 °C under ambient pressure in steady-state conditions, and did not suffer from any deactivation after 15 days of reaction. The apparent activation energy (E_a) was found to be in the range of 68-74 kJ mol^-1, which is in line with values reported for highly active catalysts. This improved catalyst may decrease the energy consumption in the synthesis of ammonia and its derivatives, as well as facilitate the use of ammonia as a hydrogen carrier for renewable energy storage.

In this perspective we present recent experimental and computational progress in catalytic ammonia synthesis research on metal nitrides involving a combined approach. On this basis, it suggested that the consideration of nitrogen vacancies in the synthesis of ammonia can offer new low energy pathways that were previously unknown. We have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies on their surfaces. These vacancies adsorb dinitrogen much more strongly than the defect-free surfaces and can efficiently activate the strong N–N triple bond. These fundamental studies suggest that heterogeneously catalysed ammonia synthesis over metal nitrides is strongly affected by bulk and surface defects and that further progress in the discovery of low temperature catalysts relies on more careful consideration of nitrogen vacancies. The potential occurrence of an associative pathway in the case of the Co₃Mo₃N catalytic system provides a possible link with enzymatic catalysis, which will be of importance in the design of heterogeneous catalytic systems operational under process conditions of reduced severity which are necessary for the development of localised facilities for the production of more sustainable “green” ammonia.

The performance of Co₃Mo₃N, Co₃Mo₃C, and Co₆Mo₆C for ammonia synthesis has been compared. In contrast to Co₃Mo₃N, which is active at 400 °C, a reaction temperature of 500 °C, which was preceded by an induction period, was necessary for the establishment of steady state activity for Co₃Mo₃C. Co₆Mo₆C was found to be inactive under the conditions tested. During the induction period, nitridation of the Co₃Mo₃C lattice was found to occur, and this continued throughout the period of steady state reaction with the material transforming in composition toward Co₃Mo₃N. Taken together, these observations demonstrate that ammonia synthesis activity in ternary cobalt molybdenum systems is associated with the presence of N in the 16c Wyckoff lattice site.

Ta3-xMxNy (M=Re, Fe, Co; x=0, 0.25, 0.5, 1) materials with different microstructural features (e.g. surface area) were successfully prepared using different synthesis techniques. The dependence of nitrogen transfer properties upon tantalum nitride microstructure and its chemical composition was evaluated using the ammonia synthesis with a H2/Ar feedstream (a reaction involving lattice nitrogen transfer). It was shown that nitrogen reactivity for tantalum nitride is more dominated by lattice nitrogen stability rather than microstructural properties. In the case of non-doped tantalum nitride, only a limited improvement of reactivity with enhanced surface area was observed which demonstrates the limited impact of microstructure upon reactivity. However, the nature of the transition metal dopant as well as its content was observed to play a key role in the nitrogen transfer properties of tantalum nitride and to impact strongly upon its reactivity. In fact, doping tantalum nitride with low levels of Co resulted in enhanced reactivity at lower temperature