Methane splitting to hydrogen and base growth carbon nanotubes over Fe-based catalysts

Catalytic methane splitting produces CO2-free H2 from renewable CH4, since the C is captured as a solid. The feasibility of the process heavily relies on the catalyst stability and final application of the carbon. Carbon nanotubes (CNTs) are high value products, decreasing the H2 cost, and can be grown on non-toxic and easily available Fe catalysts. Simultaneously achieving high H2 productivity and controlling the quality of CNTs, which will mark a major advancement in catalytic methane splitting research, is, however, still challenging. The aim of this work is to develop Fe-based catalysts with active species able to grow CNTs from the base to avoid the loss of active phase during the reaction. To achieve this, hydrotalcite-derived catalysts were employed to exploit the metal-support interaction and dispersion of Fe species in an oxidic matrix. A moderately loaded, 20 wt% Fe-based catalyst (FeMgAl), containing a MgAl2O4 spinel with Fe3+ species, obtained by calcination at 800 degrees C is preferred to its counterpart calcined at 700 degrees C with a mixed oxide (MgFeAlOx) structure and performs better than conventional Fe-MgO and Fe-Al2O3 catalysts. Furthermore, reaction conditions have been identified as important for tailoring the catalyst properties and to control CNT growth. The simultaneous production of H2 at ca 5 LH2/ gcat h and base-growth multi-walled CNTs is achieved by the adoption of relatively low temperatures (700 degrees C-750 degrees C) and activation of the catalyst under reaction conditions, without the need for H2 pre-reduction. As revealed by in situ XRD, Fe3C forms as the active species, which plays a key role in the activation of CH4, consistent with Density Functional Theory (DFT) results. DFT calculations reveal that the Fe3C(010) surface lowers the energy barrier of the first dehydrogenation step of CH4 to 0.66 eV-notably smaller with respect to reported values for well-studied Ni catalysts and the analysis of the adhesion energy between carbon and Fe3C (010) suggests that, at reaction temperatures, carbon atoms are moderately bound to the surface, which may explain the particles' resistance to encapsulation. These dual descriptors, corroborated by our experimental data, demonstrates why cementite, and not metallic Fe, emerges as the kinetically dominant phase during base-growth CNT nucleation.