Nitrides, Hydrides and Carbides as Alternative Heterogeneous Catalysts for Ammonia Synthesis: A Brief Overview : Recent approaches to nitrogen activation

Driven by the desire to develop novel catalyst formulations which are applicable for localised, more sustainable routes, the area of heterogeneously catalysed ammonia synthesis has attracted much attention in the academic literature in recent times. One of the key incentives for this has been the idea that ammonia synthesis for the production of synthetic fertiliser can be achieved on, for example, a farm close to its point of application with the required hydrogen feedstream being derived from sustainable sources such as electrolysis of water accomplished using electricity produced using wind turbines or solar energy sources. Further drivers are the possible application of ammonia as a non-fossil based fuel and also as a means to indirectly store intermittent over-supply of sustainably derived electricity. In the literature, the energy intensive nature of the Haber Bosch Process, frequently quoted to be 1-2% of global energy demand, and its CO2 footprint, stated to comprise 2.5% of fossil fuel based emissions, are statistics that are frequently quoted in justification for the search for new routes to ammonia production [1,2]. However, due recognition has to be given to the highly efficient integration of the Haber Bosch Process as currently operated. In relation to this, large scale synthesis of ammonia is highly optimised and it can be credited with the sustenance of ca 40% of the global population. These considerations, coupled to the recently reported UK CO2 supply chain shortage, related to a reduction in commercial fertiliser production [3], underline the importance of the highly integrated nature of the process.