Activation and deactivation scenarios in a plasma‐synthesized Co/C catalyst for Fischer‐Tropsch synthesis

Abstract

A carbon‐supported cobalt nano‐catalyst (Co/C) synthesized through plasma was tested for Fischer‐Tropsch activity. Catalyst deactivation and activation protocols studied included in situ sample pre‐treatment at 673 K in gas flowing at the rate of 250 cm3 · min−1 in: (a) H2 only, (b) CO only, and (c) CO followed by H2, with each cycle lasting 24 h. The so‐treated catalyst samples were then tested for FTS activity for over 50 h of time‐on‐stream (TOS), at 2.2 MPa pressure and 493 or 518 K temperature in a 3‐phase continuously‐stirred tank slurry reactor (3‐φ‐CSTSR) using squalane (C30H62) as the carrier liquid phase. The feed gas composition of molar H2:CO = 1.7 comprising 50 % H2, 30 % CO, and 10 % CO2 balanced in Ar for mass balance determination, flowing at a gas hourly space velocity (GHSV) of 3360 cm3 · h−1 · g−1 of catalyst. Fresh catalysts and those reduced by CO‐only were completely inactive at 493 K and initial inactivity was attributed to the excessive C matrix formed around the metal nanoparticles during catalyst synthesis. The sample that was reduced by H2 only was the most active, although it showed the fastest declining activity due to cumulative high H2O vapour pressure in the FTS reactor, while the sample pre‐treated in both CO and H2 demonstrated a higher degree of stability with TOS.

Hydrogen Spillover in the Fischer‐Tropsch Synthesis on Carbon‐supported Cobalt Catalysts

“…for Co/CNT and Co/C catalysts . Small average Co particle size and high availability of surface cobalt is known to enhance CO dissociation . In our work, the turnover frequency (TOF) (Figure b and Table ) decreased with the increase of the average Co particle size (measured on spent catalysts) in the following order 15 %Co/CNF (17 nm) <15 %Co/CNT (14 nm) <15 %Co/FM (11 nm).…”

Use of Plasma-Synthesized Nano-Catalysts for CO Hydrogenation in Low-Temperature Fischer–Tropsch Synthesis: Effect of Catalyst Pre-Treatment

“…The porosity witnessed here could be associated with the packing of the nano-carbon powder particles since the superimposed isotherms (Type II) as indicated in Figure 10 a shows that the samples are indeed non-porous [ 52 ]. Figure 10 b provides the pore distribution plots proving that the samples are nanometric with limited micro-porosity (below 2 nm), while the augmented meso-porosity in the Co-Fe/C could be due to the larger metal nanoparticle size in the carbon support that subsequently creates sizeable interstitial voids in the nanomaterials.Some authors have found that the catalyst with BET specific surface areas of 40–60 m 2 g −1 had similar average pore volumes in the range of 0.19–0.22 cm 3 g −1 , with the average pore diameter of about 14–22 nm [ 53 ].…”

“…Moreover, when samples were pre-treated in pure H 2 alone (e.g., Co/C at 673 K), the catalyst was observed to deactivate more rapidly with TOS than the samples reduced in CO followed by a H 2 reduction [ 52 ]. Furthermore, the samples that were pre-treated in CO followed by a H 2 reduction were observed to be comparatively more effective in CO hydrogenation than those reduced in H 2 alone since a CO-reduction step limited the production of CO 2 as a side FTS product.…”

“…Furthermore, the samples that were pre-treated in CO followed by a H 2 reduction were observed to be comparatively more effective in CO hydrogenation than those reduced in H 2 alone since a CO-reduction step limited the production of CO 2 as a side FTS product. The CO-reduction step in catalyst pre-treatment has been shown to produce a more stable catalyst over TOS [ 52 ], accompanied by a lower H 2 O production [ 56 ].…”

“…Earlier work with the 50%Co-50%Fe/C formulation indicated that the catalyst produces less H 2 O in FTS (at 533 K, 2 MPa, and H 2 :CO = 2.0) when reduced in CO only than when reduced in H 2 only [ 56 ]. Recent work with the Co/C catalyst showed that successive reductions in CO followed by H 2 improves the catalyst performance (at 518 K, 2.2 MPa, and H 2 :CO = 1.7), producing relatively less H 2 O when compared to the sample that was reduced in H 2 only [ 52 ]. In this work, both the above-named catalysts (Co/C, Co-Fe/C) in addition to the Fe/C one have been reduced successively, first in pure H 2 , then in pure CO and finally in pure H 2 again.…”