The Effect of Copper Loading on Iron Carbide Formation and Surface Species in Iron‐Based Fischer–Tropsch Synthesis Catalysts

Abstract

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.

Identification of efficient promoters and selectivity trends in high temperature Fischer-Tropsch synthesis over supported iron catalysts

“…Numerous studies have shown [7,27,50] that the addition of copper can result in an enhancement of the activity of Fe-based FT catalysts. The major copper function is to decrease the temperature required for the reduction of iron oxides, while the reports about the effect of copper on the selectivity are still controversial.…”

“…Several elements have been considered previously for promotion of iron catalysts. Most of earlier publications have been focused on the promotion of iron catalysts with alkali metals [6,[19][20][21][22][23][24][25] and copper [6,24,26,27]. More recently, the group of de Jong [28][29][30] and Sasol researchers [31] reported that simultaneous addition of sodium and sulfur improves selectivity to olefins in their synthesis from syngas over iron catalysts.…”

Iron and copper nanoparticles inside and outside carbon nanotubes: Nanoconfinement, migration, interaction and catalytic performance in Fischer-Tropsch synthesis

“…In reality, iron catalysts for FT synthesis contain [1] several components. Iron catalysts are most frequently promoted with copper [17][18][19][20][21][22]. Copper has very low solubility in metallic iron (2.7 at.…”

Preparation of Iron Carbides Formed by Iron Oxalate Carburization for Fischer–Tropsch Synthesis

“…It is a challenge to make sure the phase of iron carbide before and after the FTS reaction is consistent, and the phases of the used IO-350 • C and IO-450 • C catalysts were characterized by XRD (Figure 7a). It can be seen that in addition to the respective iron carbide (Fe 5 C 2 and Fe 3 C) phases, the used catalysts also have a Fe 3 O 4 phase, that is, internal oxidation occurred [30,31], which is the active phase of the WGS reaction. It can be said that the iron carbide phases of the used catalyst have been maintained almost the same as the fresh catalysts.…”