Methane pyrolysis on fused Fe-Al2O3 catalysts: Characterization of catalyst structure, performance and carbon formation

In this work, an investigation of methane pyrolysis over Fe/Al-based catalysts has been carried out to optimize the formulation, that is to obtain structural features suitable to high C/Fe ratios before the catalyst is fully deactivated. At this scope, Fe-Al 2 O 3 catalysts at different Fe-Al ratios were prepared by the fusion-decomposition method and reduction in H 2 ; fresh and reduced samples were extensively characterized by N 2 adsorption/desorption, electron microscopies, X-Ray Diffraction and Raman spectroscopy. As a general feature, the reduced catalyst structures are characterized by mesoporous Fe aggregates dispersed in microporous alumina, with presence of FeAl 2 O 4 at the interface; however, several textural parameters (including specific surface area, pore size distribution, Fe crystallite size, FeAl 2 O 4 abundance) change importantly with increasing Fe/Al ratio. Close to the equimolar Fe-Al ratio, such textural properties appear optimal to guarantee high Fe dispersion and accessibility. All the formulations were first screened in methane pyrolysis in a thermobalance; a selected subset of formulations was then tested in a packed bed reactor. The study reveals that formulations with Fe-Al molar ratios from 50-50 to 75-25 are all characterized by high initial activity and lower deactivation rate, thus achieving the maximum C-accumulation capacity. Among them, the equimolar formulation achieved the highest C/Fe ratio, reaching 2.5 g C /g Fe after 1 hour testing with 40% CH 4 at 800°C. The spent catalysts were characterized by electron microscopies, X-Ray Diffraction and Raman spectroscopy to better comprehend the effects of extent of reaction and Fe-Al ratio on the characteristics of the solid. In all the samples, after reaction, the Fe 3 C was the main iron-containing phase and a progressive decrease of the residual Fe phase was observed at increasing C-accumulation. Concerning the nature of C-structures, a large variety of them was observed depending on Fe content and crystalline size. • Fe-Al 2 O 3 catalysts at varying Fe/Al molar ratios were synthesized for CH 4 pyrolysis • The balance of Fe content and textural properties controls the catalyst performances • 50-50 Fe-Al catalyst shows superior activity and lower deactivation rate • 50-50 Fe-Al is a viable candidate for scaling up the preparation and testing