Ultrathin 2D Fe-Nanosheets Stabilized by 2D Mesoporous Silica: Synthesis and Application in Ammonia Synthesis

Developing high-performance Fe-based ammonia catalysts through simple and cost-efficient methods has received an increased level of attention. Herein, we report for the first time, the synthesis of two-dimensional (2D) FeOOH nanoflakes encapsulated by mesoporous SiO2 (mSiO(2)) via a simple solution-based method for ammonia synthesis. Due to the sticking of the mSiO(2) coating layers and the limited spaces in between, the Fe after reduction retains the 2D morphology, showing high resistance against the sintering in the harsh Haber-Bosch process. Compared to supported Fe particles dispersed on mSiO(2) spheres, the coated catalyst shows a significantly improved catalytic activity by 50% at 425 degrees C. Thermal desorption spectroscopy (TDS) reveals the existence of a higher density of reactive sites for N-2 activation in the 2D Fe catalyst, which is possibly coupled to a larger density of surface defect sites (kinks, steps, point defects) that are generally considered as active centers in ammonia synthesis. Besides the structural impact of the coating on the 2D Fe, the electronic one is elucidated by partially substituting Si with Al in the coating, confirmed by Si-29 and Al-27 magic-angle spinning nuclear magnetic resonance (MAS NMR). An increased apparent activation energy (E-a) of the Al-containing catalyst evidences an influence on the nature of the active site. The herein-developed stable 2D Fe nanostructures can serve as an example of a 2D material applied in catalysis, offering the chance of a rational catalyst design based on a stepwise introduction of various promoters, in the coating and on the metal, maintaining the spatial control of the active centers.

Automated summary

Developing high-performance Fe-based ammonia catalysts using cost-efficient methods has become a focus of research. By synthesizing 2D FeOOH nanoflakes encapsulated by mSiO2 through a simple solution-based method, the catalyst shows improved catalytic activity and resistance against sintering. The presence of reactive sites and surface defect sites contribute to the enhanced performance, with the potential for further rational catalyst design through introduction of various promoters.