Ambient-Pressure and Low-Temperature Hydrogenation of Nitrogen to Ammonia in A Synergetic Catalytic System

Abstract

Catalytic hydrogenation of nitrogen is an important step for synthesis of ammonia. However, hydrogenation of nitrogen usually requires a high hydrogen pressure and high temperature due to the good stability of the N≡N bonds. Herein, a multiphase hydrogenation of nitrogen at temperatures lower than 100oC and hydrogen pressures lower than 1atm was reported using a synergetic catalytic system which consists of silicotungstic acid H4SiW12O40 (SiW12) aqueous solution suspended with Pt/TiO2 and Ru/C catalysts particles. It is suggested that the low redox potential SiW12 plays four vital roles in hydrogenation of nitrogen to ammonia. First, it quickly oxidizes and stores the H2 gas to form reduced SiW12 in the presence of Pt/TiO2. Second, it transfers both electrons and H+ ions from the gas-liquid interface to the bulk solution. Third, it releases the electrons and H+ ions to form active H* or H2 which converts nitrogen N2 to ammonia NH3 on the Ru/C surface. Fourth, the active H* released by SiW12 to be added on nitrogen from bulk phase reduces the hydrogenation energy barriers, which results in elimination of the hydrogen poisoning. The SiW12-induced hydrogenation mechanisms are supported by reaction kinetic analysis and density functional theory computations. By these properties of SiW12, the hydrogenation of nitrogen to ammonia can be completed at relative low temperature and low hydrogen pressure.

Keywords: Hydrogenation, Nitrogen, Ammonia, Synergetic catalytic system, Silicotungstic acid