Higher Activity of Ni/γ-Al 2 O 3 over Fe/γ-Al 2 O 3 and Ru/γ-Al 2 O 3 for Catalytic Ammonia Synthesis in Nonthermal Atmospheric-Pressure Plasma of N 2 and H 2

摘要

Developing a novel ammonia synthesis process from N 2 and H 2 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/Al 2 O 3 was quantitatively compared with that of Fe/Al 2 O 3 and Ru/Al 2 O 3 , which contained active metals for the conventional Haber–Bosch process. The activity sequence was Ni/Al 2 O 3 Al 2 O 3 Fe/Al 2 O 3 no additive Ru/Al 2 O 3 , surprisingly indicating that the loading of Fe and Ru decreased the activity of Al 2 O 3 . The catalytic activity of Ni/Al 2 O 3 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 NiAl 2 O 4 on Al 2 O 3 , 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 NiAl 2 O 4 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 P N2 /P H2 ratio dependence and rate constants of formation and decomposition of ammonia were finally determined for 5.0 wt% Ni/Al 2 O 3 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 P H2 /P N2 = 1.