Numerical Modeling and Mechanism of Gas Phase-surface Coupling for Plasma Catalytic Ammonia Synthesis

Abstract

Plasma catalytic ammonia synthesis is a new sustainable nitrogen fixation technology which possesses the characteristics of low power consumption, miniaturization, simple process and not requiring high temperatures and high pressures. In this paper, the surface reaction on Fe/γ-Al2O3 catalyst is embedded in the ZDPlaskin solver, the gas phase-surface coupling kinetics model of the plasma catalytic synthesis ammonia is reasonably established, and the calculation methods of reaction rate coefficients of the vibrational excitation, the wall relaxation and the surface reaction are specifically given. Effects of molar fraction ratio of the reaction gas nitrogen-to-hydrogen r(N2):r(H2) on the electron energy deposition, the contribution rate and loss rate of key reaction paths, and the particle mole fraction in ammonia synthesis are investigated. The gas phase-surface coupling kinetics mechanism of the plasma catalytic ammonia synthesis is expounded by the cyclic reaction path diagram and system diagram, in addition, the particle number density temporal evolution characteristics are discussed when r(N2):r(H2)= 3:1. The results show that the density of NH3 increases first and then decreases with the increase of N2 content, and reaches the maximum when r(N2):r(H2)= 3:1. At this time, the vibrationally excited particles obtain more sufficient electron energy deposition, producing higher number density of adsorbed particles N(s) and H(s) by dissociation and adsorption. Compared with the rate when r(N2):r(H2)=1:3, the reaction production rate of NH2+H2→NH3+H improves by nearly 103 times and the ammonia contribution rate of NH2(s)+H(s)→NH3+Surf+Surf (Surf is the active site on catalyst surface) increases by about 24%. Furthermore, the circulating reactions system formed between NH, NH2 and other intermediates and NH3 has a greater difference in forward-and-reverse yields, so that the NH3 number density is maximum under the more obvious plasma-catalyst synergism.