This paper focuses on the use of in situ and ex situ characterisation techniques to provide evidences of carbon species on a commercial iron-based Fischer-Tropsch synthesis catalyst as well as other indices of potential deactivation mechanisms. In situ XANES measurements demonstrate that re-oxidation or transformation of the active iron phase, i.e. the Hägg carbide phase, was not a significant deactivation mechanism at the studied conditions. Sintering of Hägg carbide nanoparticles is significant with increasing temperatures and time on stream. The sintering mechanism is proposed to be a hydrothermally-assisted process. In situ DRIFTS indicates the presence of different carbon species on the catalyst surface such as aliphatic hydrocarbons from wax products and oxygenate compounds such as alcohols, aldehydes/ketones and carboxylate species. Carboxylate species are resistant towards hydrogenation at 280°C. The presence of different carbon species on the surface after wax product extraction is evident from TPH-MS measurements. GC-MS analysis shows that the strongly adsorbed carbon species remaining on the catalyst surface from wax products are mainly α-olefins and branched carboxylic species. The interaction of oxygenate compounds, especially carboxylate species with iron oxide, may form stable complexes limiting further iron catalyst carburization. STEM-EDX analysis shows that carbon is preferentially located on iron particles.
The effect of copper as promoter on iron carbide formation and the nature of surface species on iron‐based catalyst during Fischer–Tropsch synthesis (FTS) was investigated. Iron‐based catalysts (15 wt % of Fe) supported on alumina promoted with copper (0, 0.6, 2, and 5 wt %) were characterised in situ at relevant FTS conditions. The catalysts promoted with 2 and 5 wt % of Cu showed higher catalytic activity due to the formation of Hägg carbide (Fe5C2) detected by in situ XANES and XRD. The carbide formation is attributed to a weakening of the iron–alumina mixed‐compound interactions, and hence increasing iron reducibility and dispersion. The in situ XANES measurements indicated a maximum carburization degree (ca. 20–22 %) even at high Cu loading. The catalyst promoted with 5 wt % of Cu exhibited higher water‐gas shift activity. Aliphatic hydrocarbons, formate, and carboxylate species were detected on the catalyst surface during FTS. After exposing the spent catalysts to hydrogenation conditions, the carboxylate species remained strongly adsorbed while aliphatic hydrocarbons and formate species disappeared. The accumulation of oxygenates species on the catalyst surface increased with Cu loading. The interaction of oxygenates with alumina and iron oxide particles (FexOy) were revealed, with the latter being a possible reason for inhibition of further iron carburization.
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