In-Situ Formation of Fe Nanoparticles from FeOOH Nanosheets on γ-Al2O3 as Efficient Catalysts for Ammonia Synthesis

Abstract

The preparation process of industrial Fe-based catalysts for ammonia synthesis involves energy-intense and sophisticated treatments. It is thus highly desirable to develop new catalysts with sufficient catalytic performance within smart and more economic approaches. Herein, γ-Al2O3-supported FeOOH nanosheets are employed for the first time as ammonia synthesis catalysts. To monitor the potential of these new materials, unsupported FeOOH and K-promoted catalysts are additionally investigated for comparison. While no activity is observed from the unsupported catalyst, the activity of γ-Al2O3-supported catalysts shows an inverse relationship with respect to the amount of Fe. Upon correlation of the catalytic performance with the final state of catalysts, the activity is closely related to the particle size of metallic Fe. Higher activity and lower apparent activation energy can be reached for smaller nanoparticles that are likely to contain more step-kink structures serving as active sites. The best catalytic performance is obtained for a K-promoted catalyst which shows an activity about 18 times higher compared to that of the corresponding unpromoted one (at 425 °C). We conclude that, in addition to the nanostructured character of the Fe, the modifications of its electronic and surface structures induced by the K-promoter are responsible for this enhanced activity.