Abstract
Developing a novel ammonia synthesis process from N2 and H2 is of interest to the catalysis and hydrogen research communities. γ-Alumina-supported nickel was determined capable of serving as an efficient catalyst for ammonia synthesis using nonthermal plasma under atmospheric pressure without heating. The catalytic activity was almost unrelated to the crystal structure and the surface area of the alumina carrier. The activity of Ni/Al2O3 was quantitatively compared with that of Fe/Al2O3 and Ru/Al2O3, which contained active metals for the conventional Haber–Bosch process. The activity sequence was Ni/Al2O3 > Al2O3 > Fe/Al2O3 > no additive > Ru/Al2O3, surprisingly indicating that the loading of Fe and Ru decreased the activity of Al2O3. The catalytic activity of Ni/Al2O3 was dependent on the amount of loaded Ni, the calcination temperature, and the reaction time. XRD, visual, and XPS observations of the catalysts before the plasma reaction indicated the generation of NiO and NiAl2O4 on Al2O3, the latter of which was generated upon high-temperature calcination. The NiO species was readily reduced to Ni metal in the plasma reaction, whereas the NiAl2O4 species was difficult to reduce. The catalytic behavior could be attributed to the production of fine Ni metal particles that served as active sites. The PN2/PH2 ratio dependence and rate constants of formation and decomposition of ammonia were finally determined for 5.0 wt% Ni/Al2O3 calcined at 773 K. The ammonia yield was 6.3% at an applied voltage of 6.0 kV, a residence time of reactant gases of 0.12 min, and PH2/PN2 = 1.
Highlights
New methods for the synthesis of ammonia have been widely investigated in many catalysis fields to improve or displace the current Haber–Bosch process due to its extreme reaction conditions and high consumption of energy supply
It has been reported that alumina, used as a carrier in this study, shows catalytic activity for the plasma synthesis of ammonia
We first compared the catalytic activity of alumina as a function of surface area and crystal structure
Summary
New methods for the synthesis of ammonia have been widely investigated in many catalysis fields to improve or displace the current Haber–Bosch process due to its extreme reaction conditions and high consumption of energy supply. Several nickel-containing nitride compounds have been reported as active for ammonia synthesis [47,48,49,50,51], very severe reaction conditions, such as high reaction temperature and high pressure, were often required. The new reaction system effectively produced ammonia with yields of 3.0% and 6.3% on the catalysts at a residence time of reactant gases of 0.12 min and PH2 /PN2 = 1. This method provides new insights into ammonia synthesis that will be significant for the future hydrogen economy
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