NH3 Decomposition for H2 Generation: Effects of Cheap Metals and Supports on Plasma–Catalyst Synergy

NH3 decomposition is important because of its potential use in generating CO-free H2. In this study, several cheap metals (Fe, Co, Ni, and Cu) and a series of supports (zeolite materials: TS-1 um, TS-1 nm, HZSM-5 nm, and NaZSM-5 nm; SiO2-based materials: fumed SiO2 and SiO2-ball; and metal oxide materials: r′-Al2O3 and TiO2) were used to prepare supported catalysts. X-ray fluorescence, N2 physisorption, X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, temperature-programmed desorption, mass spectrometry, temperature-programmed reduction, and electrical property analysis were used to investigate the effect of the catalyst on the synergy between a plasma (produced by dielectric barrier discharge) and the catalyst in NH3 decomposition. The results show that the synergy depends strongly on the strength of the metal–nitrogen (M–N) bond, and the relative dielectric constant (εd) of the support. When Fe, Co, Ni, and Cu were supported on fumed SiO2, the order of the strengths of the M–N bonds was Cu–N < Ni–N < Co–N < Fe–N. Among the catalysts, Co/fumed SiO2 showed a stronger synergy with the plasma and gave higher NH3 conversion in plasma catalysis. Co catalysts supported on fumed SiO2, SiO2-ball, and r′-Al2O3, which have small εd values, had stronger synergies with plasma and therefore gave higher NH3 conversions. The relative dielectric constant of the support correlated well with NH3 conversion in plasma catalysis. These results show that the relative dielectric constant is an essential parameter in developing catalyst supports for plasma conditions. This study provides direct proof that the recombinative desorption of adsorbed N atoms is the rate-limiting step in the catalytic decomposition of NH3 over cheap metal catalysts such as Fe, Co, and Ni and that there is synergy between plasma and cheap metal catalysts in plasma-catalytic NH3 decomposition.