Co-doping facilitated plasma-catalytic ammonia synthesis over Mo₂N-Co catalysts

Abstract

Ammonia is a promising hydrogen storage carrier due to its high hydrogen density (17.8 wt %) and mild liquefaction conditions. Plasma-catalytic ammonia synthesis is an alternative synthesis route regarding green ammonia generation at ambient conditions. In this study, Co-doped Mo₂N-Co catalysts were developed to enhance plasma-catalytic ammonia synthesis, with a focus on the effects of Co/Mo molar ratios and operating parameters. Among the catalysts tested, Mo₂N-Co₁ possessed the highest ammonia synthesis rate and energy efficiency. Optimal operating conditions including a feed ratio of N₂:H₂ = 1:1 and a higher discharge power is favored. An ammonia synthesis rate of 11925 μmol·g^–1·h^–1 and an energy efficiency of 3.6 g-NH₃·kWh^–1 were achieved over Mo₂N-Co₁ at a feed ratio of N₂:H₂ = 1:1 and a discharge power of 57 W. Comprehensive characterizations, including X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance, hydrogen temperature-programmed reduction, and ammonia temperature-programmed desorption, demonstrated that Co doping introduced abundant nitrogen vacancies and weak acidic surface, both of which facilitated ammonia desorption and electron transfer. Key reactive intermediates were identified using optical emission spectroscopy, providing insight into the proposed reaction mechanism for this synergistic plasma-catalytic ammonia synthesis over Mo₂N-Co catalysts.

Graphical abstract

Keywords

ammonia synthesis / plasma catalysis / Co-based catalysts / nitrogen vacancy / synergistic effect

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