Promotional Effect of Carbon on Fe Catalysts for Ammonia Decomposition: A Density Functional Theory Study

Abstract

Density functional theory calculations have been performed to investigate the recombinative desorption of N on terraced Fe(100) and Fe(110) as well as stepped Fe(111) and Fe(211) surfaces. The results showed that the stepped surfaces, especially the Fe(111) surface, are more active than the terraced surfaces, which could be related to the presence of so-called C7 sites for the stepped surfaces. Carbon atoms were found to stabilize the thermodynamically unstable stepped surfaces to be the energetically favored facets and thus facilitate the preferential exposure. As these carbon atoms incorporated into the stepped surfaces in the form of surface or subsurface carbon, the activation energy for the recombinative desorption of N was lowered, which was mainly ascribed to the decrease in the d band center of surface Fe atoms. In addition, a relation between the electronic and structural properties of Fe catalysts in the absence or presence of carbon and ammonia decomposition activity was also correlated.