The role of heterogeneous catalysts in the plasma-catalytic ammonia synthesis

Ammonia, being the second largest produced industrial chemical, is used as a raw material for many chemicals. Besides, there is a growing interest in the applications of ammonia as electrical energy storage chemical, as fuel, and in selective catalytic reduction of NOx. These applications demand on-site distributed ammonia production under mild process conditions. In this paper, we investigated 16 different transition metal and oxide catalysts supported on gamma-Al2O3 for plasma-catalytic ammonia production in a dielectric barrier discharge (DBD) reactor. This paper discusses the influence of the feed ratio (N-2/H-2), specific energy input, reaction temperature, metal loading, and gas flow rates on the yield and energy efficiency of ammonia production. The optimum N-2/H-2 feed flow ratio was either 1 or 2 depending on the catalyst - substantially above ammonia stoichiometry of 0.33. The concentration of ammonia formed was proportional to the specific energy input. Increasing the reaction temperature or decreasing gas flow rates resulted in a lower specific production due to ammonia decomposition. The most efficient catalysts were found to be 2 wt% Rh/Al2O3 among platinum-group metals and 5 wt% Ni/Al2O3 among transitional metals. With the 2 wt% Rh catalyst, 1.43 vol% ammonia was produced with an energy efficiency of 0.94 g kWh(-1). The observed behaviour was explained by a combination of gas-phase and catalytic ammonia formation reactions with plasma-activated nitrogen species. Plasma catalysts provide a synergetic effect by activation of hydrogen on the surface requiring lower-energy nitrogen species.