Ammonia is an attractive storage medium for renewable energy. It can be produced from nitrogen and hydrogen, which themselves may be obtained from water and air using renewable energy. It is a liquid at room temperature, and so has high energy density and is easily transportable. It can be converted back to hydrogen and nitrogen when required, using, for example, membrane technology. The standard Haber–Bosch process for ammonia production from hydrogen and nitrogen is better suited to large-scale chemical manufacture than the small-scale intermittent operation required by most renewable energy sources. In particular, the high pressures (typically 200 bar) needed for acceptable yields and high temperatures (typically 800 K) required for catalyst activation mean that the process cannot be rapidly turned on and off, and that small plants are less economic. Combining a non-equilibrium plasma with catalytic surfaces allows production of ammonia at atmospheric pressure and ambient temperatures. The plasma produces excited species (e.g. excited molecules, radicals and atoms) that are not present in conventional gas-phase processes, which allows ammonia to be produced by new chemical pathways. Several groups around the world have developed combined plasma-catalytic reactors to convert nitrogen and hydrogen to ammonia. Single-pass conversion of up to around 10% has been demonstrated, but the energy efficiency remains much lower than that of the Haber–Bosch process. This paper will consider the status of and outlook for plasma-catalytic production of ammonia, and possible approaches to improving the yield and energy efficiency will be proposed. Some of these will be based on the predictions of a chemical kinetic model that combines detailed gas-phase and plasma chemistry with surface reactions, taking into account different surface properties. The model indicates the importance of the surface temperature, electron energy and surface composition, pointing the way to tailoring the plasma reactor to minimize energy use and maximize hydrogen-to-ammonia conversion. For example, the model indicates that the main surface-adsorbed species under standard operating conditions is hydrogen (rather than nitrogen as commonly assumed) and that this places strong limits on ammonia production. However, it is predicted that by increasing the surface temperature, this problem can be overcome.

Plasma-enhanced catalysis for ammonia production

2019 Spring National Meeting

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