Alternative reaction mechanisms of ammonia reduction of iron oxide examined using density functional theory modelling

Abstract

Several conceivable mechanisms of NH₃ reduction of Fe₃O₄, represented by the Fe₃O₄ (001)-B surface, have been hypothesised and examined using density functional theory (DFT) modelling. The proposed reaction mechanisms include NH₃ adsorption, NH₃ reduction of Fe₃O₄ to H₂O, NH₃ dissociation to H₂, and NH₃ oxidation to NO, with the energy barrier of each reaction calculated. The most likely pathway of NH₃ reduction of Fe₃O₄ to form H₂O involves NH₃ first adsorbing on an Fe site of the Fe₃O₄ (001)-B surface. Subsequently, two H atoms are transferred to the adjacent O sites. One H atom from the resulting NH then combines with a surface hydroxyl group to form H₂O. For NH₃ dissociation into H₂, the most probable pathway involves the combination of the H atom from the resulting NH with an H atom from NH₃ dehydrogenation adsorbed on the Fe₃O₄ (001)-B surface to form H₂. Fe₃O₄ reduction creates oxygen vacancies on the surface, which leads to nitrogen migration from the Fe site to these vacancies, forming Fe₃N. The adsorbed N atom also bonds with the O atom on the Fe₃O₄ (001)-B surface to form NO. The activation energy of H₂O formation is lower than that of H₂ formation on the Fe₃O₄ (001)-B surface.