Colloidal Fe nanocrystals for CH4 decomposition: role of the support

Metal-supported nanoparticles play a crucial role in heterogeneous catalysis, with their activity governed by size, composition, and metal-support interactions. Conventional preparation methods often lack precise control of nanoparticle descriptors or introduce secondary metallic phases, limiting insights into intrinsic catalytic properties. In this study, colloidal iron nanoparticles (FeNPs) were used as pre-formed active sites to investigate CH4 decomposition, a reaction that yields high-purity H2 and carbon nanotubes (CNTs). FeNPs were supported on SiO2, MgO, Si3N4 and MgAl2O4, to systematically assess the influence of support materials on catalytic activity and the nature and morphology of carbon products. Characterization via ex situ TEM, STEM-EDS, SEM, XPS, Raman spectroscopy, and in situ XRD revealed distinct support-dependent behaviors. Fe/SiO2 exhibited deactivation due to severe sintering and encapsulation. Nevertheless, Fe3C species were identified as a stable phase under reaction conditions. Fe/MgO showed high stability against sintering but severe FeNPs encapsulation. Surprisingly, FeNPs on Si3N4 retained their nanometric size, probably due to strong anchoring in the support. Nevertheless, the support partially reacts with CH4 leading to C3N3 species. In contrast, Fe/MgAl2O4 demonstrated the highest activity towards carbon nanostructures formation, producing abundant CNTs via the tip growth and base growth mechanisms. Beam-enhanced in situ TEM confirmed a base growth process, with FeNPs remaining attached to the spinel support. The results evidenced the oxophylic nature of the support as an important descriptor of the stability of preformed FeNPs. This study provides a systematic comparison of colloidal FeNPs on various supports for CH4 decomposition to form CNTs and CO2-free H2, offering insights into active phase formation, CNTs growth mechanisms, and optimal support selection for efficient CNTs production.