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The effects of Co particle size on the deactivation of Co/Al₂O₃ and Re-Co/Al₂O₃ catalysts in the Fischer-Tropsch synthesis Ghasvareh, Pooneh

Abstract

To assess the effect of Co particle size on the deactivation of Co catalysts during Fischer-Tropsch (FT) synthesis, a series of Co/Al₂O₃ and Re-Co/Al₂O₃ FT catalysts with varying Co particle size, were tested in a continuous flow, stirred tank reactor operated at 220ºC, 2.1MPa and a H₂/CO = 2/1 synthesis gas for periods up to 190 h time-on-stream (TOS). At the chosen operating conditions, carbon deposition was the main cause of catalyst deactivation and the initial rate of carbon deposition per active Co site increased with increased Co particle size (dC₀=2– 22 nm) when measured at approximately the same CO conversion level. Results showed that catalyst stability was dependent upon the Co particle size, degree-of-reduction (DOR) of the catalyst precursor and the CO conversion. The initial rate of carbon deposition increased with increase in CO conversion, when CO conversion ≤ 40%, for a particular catalyst, whereas at high concentrations of H₂O and CO₂ in the reactor (CO conversions> 60%), the initial rate of carbon deposition decreased with increased CO conversion. Furthermore, Co/Al₂O₃ catalysts with small Co particles (dC₀ = 2 nm and dC₀ = 1 nm) were activated with TOS during the FT synthesis. These catalysts had a low DOR (≤3%) and were reduced further when exposed to the synthesis gas. Comparison between the extent of catalyst deactivation for the Co/Al₂O₃ and Re-Co/Al₂O₃ model catalysts and a commercial Co/P-Al₂O₃ catalyst (Co catalyst on a P modified Al₂O₃ support), showed that the Co/P-Al₂O₃ catalyst was more stable during ~160 TOS due to a reduced carbon deposition rate. However, when the catalyst was operated over a range of process conditions (i.e, temperature, pressure and H₂/CO ratio) for extended operating periods (up to 1200 h), the CH₄ selectivity increased at TOS > 400 h and when the catalyst was exposed to high temperature (T≥ 230 ºC) and a PH₂O/PH₂ ratio > 0.5. The change in CH₄ selectivity was shown to be dependent on the high 𝑃𝐻₂𝑂 in the reactor which resulted in Co oxidation and hence a change in product selectivity.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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