Superior Catalytic Properties of Iron and Iron Carbide for Hydrogen Production Via Methane Dissociation: First Principles Calculation and Experimental Verification

Abstract

The catalytic decomposition of methane (CH4) into hydrogen (H2) and solid carbon represents a promising approach for sustainable H2 production. In this work, density functional theory (DFT) simulations and experimental investigations to Fe-based catalysts for CH4 pyrolysis are combined and compared with Ni-based counterparts. The experiments show that the formation of the Fe3C (cementite) active phase over the oxidized Fe-based catalyst under reaction conditions improves performance by preventing the rapid sintering observed when the catalyst is pre-reduced in H2, thereby enhancing productivity and extending catalyst lifetime. Despite the higher temperatures required to run the reaction with Fe (700° C) than with Ni catalysts (550° C), a well-stabilized Fe3C phase, in comparison with an ex situ formed Fe3C, promotes the base-growth mechanism of 9.6 nm average diameter CNTs, whereas Ni-based catalysts favor tip-growth and larger 17.5 nm average diameter CNTs. In agreement with the experimental results, DFT simulations provide evidence that Fe3C exhibits the more energy gain, on average, in all the dissociation steps that bring from CH4 to H2 and solid C, and lower activation barriers than Ni0 and Fe0. However, the calculations show a stronger interaction, on average, between Fe3C and adsorbates, might limit the molecular diffusivity on the catalytic surface at lower temperatures, potentially making Fe3C less effective than Ni under these conditions.

Keywords: Methane decomposition, Fe-based catalyst, Iron carbide, hydrogen production, carbon nanotube, Density functional theory