Implication of iron nitride species to enhance the catalytic activity and stability of carbon nanotubes supported Fe catalysts for carbon-free hydrogen production via low-temperature ammonia decomposition

Abstract

This study was aimed to boost the catalytic ability of carbon nanotubes (CNTs) supported Fe-based catalysts, prepared by using wet-impregnation and followed by nitrogenization, for carbon-free hydrogen production from NH₃ decomposition at a low temperature. The nitrogenization temperature and iron loading had significant effects on the size of the well-dispersed Fe₂N crystallites. The Fe₃O₄/CNTs catalysts at a higher nitrogenation temperature under NH₃ flow with a suitable Fe content led to the formation of the stable and uniformly distributed Fe₂N species, which played an active role in the enhanced catalytic ability of the Fe₃O₄/CNTs catalysts. However, the nitrogenization of the Fe₃O₄/CNTs catalyst under either H₂ or Ar led to the formation of the Fe₄N and Fe₂N species. In the presence of the Fe₄N phases, the Fe₃O₄/CNTs catalyst exhibited an enhanced catalytic activity. In brief, the collaborative interaction of the active site Fe₂N and carbon nanotubes in the Fe₂N/CNTs catalysts resulted in a significant increase in the catalytic activity and durability up to 40 h. The effective control of the density of the active sites Fe₂N and the synergism between the carrier and the crystallite composition of iron nitrides are the key aspects for the efficient design of the transition nitride catalysts for carbon-free hydrogen production via ammonia decomposition.