Thermocatalytic methane pyrolysis over iron-based catalysts for turquoise hydrogen production: activity and kinetic studies

• SCS co-synthesis technique was employed to prepare methane pyrolysis catalysts. • Structural properties and iron loading are related to the catalytic performances. • Activity, durability, and kinetics during methane pyrolysis were evaluated. • Spent catalysts were comprehensively investigated focusing on carbon nanotubes growth. Alumina-supported iron-based catalysts were synthesized with the aim of studying the thermocatalytic pyrolysis of methane. Two different synthesis techniques, i.e. Wet-Impregnation (WI) and Solution Combustion Synthesis (SCS), were employed to prepare samples with various formulations (40 % Fe 2 O 3 – 60 % Al 2 O 3 , 60 % Fe 2 O 3 – 40 % Al 2 O 3 ). The structure, composition, and morphology of these materials were investigated and compared with several techniques, including inductively coupled plasma-mass spectrometry (ICP-MS), N 2 -physisorption, X-ray powder diffraction (XRD), temperature programmed reduction (H 2 -TPR), Raman spectroscopy, transmission and field emission scanning electron microscopy (TEM and FESEM). Time on stream and kinetic tests were performed to evaluate the performance and kinetic parameters of the catalysts during methane decomposition. The results pointed out a higher activity and stability of the iron-based catalysts synthesized by the SCS technique and with a higher iron loading, in agreement with the calculated kinetic parameters (activation energy of 133 kJ mol −1 for SCS 60 % Fe 2 O 3 catalyst vs 160 kJ mol −1 for WI 60 % Fe 2 O 3 catalyst). The superior performance of the SCS catalysts can be attributed to an improved dispersion of iron on the alumina support. Although the SCS samples were characterized by a lower surface area, the intimate mixing of iron and alumina achieved through this technique effectively reduced the sintering of iron particles under the high temperatures encountered during both the reduction processes and the pyrolysis reaction. Furthermore, all the samples exhibited the formation of carbon nanotubes (CNTs) with varying degrees of structural order, depending on the synthesis technique and the iron loading of the catalyst