Strontium Iron Hexaaluminates for CO _<i>x</i> -Free Hydrogen and Carbon Nanotubes via Catalytic Decomposition of Methane

In this study, a set of iron-substituted strontium hexaaluminate materials (SrFexAl12–xO19) were synthesized and evaluated for catalytic methane decomposition (CDM) using a concentrated methane feed (PCH4 = 0.9 atm). Despite their lower surface areas, it is showcased that the SrFexAl12–xO19 materials can be quite active for CDM, reaching overall carbon yields up to 8.08 gC·gcat–1 or 15.68 gC·gFe–1 at GHSV = 5 L·g–1·h–1. In situ XRD under a reducing environment indicates that catalytic activity for high iron-containing samples (x ≥ 6) originates from the collapse of the parent SrFexAl12–xO19 structure to α-Fe supported on residual SrAl2O4 and FeAl2O4. Further in situ XRD studies on bulk Fe3C under the presence of CH4 show that iron carbide is metastable and will transform to BCC α-Fe in the range between 600 and 800 °C and subsequently to FCC γ-Fe and Fe3C ≥900 °C. Analogous in situ XRD experiments on SrFe9Al3O19 under CH4 show a clear sequential phase transformation of α-Fe → γ-Fe → Fe3C and the evolution of a small amount of graphite after testing, which suggests that Fe3C is catalytically active for CDM. Density functional theory (DFT) calculations further probed the energetics of surface carbon diffusion on Fe3C and methane dehydrogenation on low-index facets of BCC α-Fe, FCC γ-Fe, and Fe3C, respectively. These results, based on in situ measurements coupled with detailed ab initio calculations, give nuanced perspectives on the active phases for iron-based CDM catalysts and CNT growth.