Effects of Co Particle Size on the Stability of Co/Al₂O₃ and Re–Co/Al₂O₃ Catalysts in a Slurry-Phase Fischer-Tropsch Reactor

Abstract

The stability of a series of Co/Al2O3 and Re–Co/Al2O3 Fischer–Tropsch (FT) catalysts, with varying Co particle size, was measured in a continuous flow, stirred tank reactor operated at 220 °C, 2.1 MPa and with a H2/CO = 2/1 synthesis gas for periods up to 190 h time-on-stream (TOS). Results showed that catalyst stability was dependent upon the Co particle size, the degree-of-reduction (DOR) of the catalyst precursor, and the CO conversion. 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 (d Co = 2–22 nm) when measured at approximately the same CO conversion level. On the 15 wt % Co/Al2O3 catalyst the initial rate of carbon deposition increased with CO conversion (CO conversion ≤40%) whereas, on the 1.2 wt %Re-12 wt %Co/Al2O3 catalyst, the initial rate of carbon deposition decreased with increased CO conversion (CO conversions >60%) due to high concentrations of H2O and CO2 in the reactor.

Selectivity loss in Fischer-Tropsch synthesis: The effect of cobalt carbide formation

“…As for the activity, the selectivity for the two reduced catalysts is quite similar. This observation may seem contrary to the well-known tendency of the selectivity to increase with cobalt particle size [60][61][62][63], but is in line with the conclusions of Bezemer et al [61] who found no significant effect of particle size above 8 nm. The effect of the contact time (i.e.…”

Particle Size Effects of Cobalt Carbide for Fischer–Tropsch to Olefins

“…However, for the catalysts supported on carbon nanofibers (CNF) with cobalt particle sizes in a range of 2.6∼27 nm, the TOF was found to be independent of cobalt particle size with sizes larger than 6∼8 nm, but the activity and product selectivity varied greatly when the cobalt particle sizes ranged from 3 to 8 nm . Similar phenomena were also observed for other cobalt-supported catalysts. − Iron carbides as the active phase of Fe-based FT catalysts are easily formed under FT reaction conditions. ,,, The effect of particle size of iron carbide on the catalytic behavior for syngas conversion was also studied in recent years. ,, Galvis et al found that the TOF increased with decreasing iron carbide size from 7 to 2 nm for Na-promoted catalysts, whereas methane selectivity increased and lower olefins selectivity decreased.…”

Chemical and Thermal Sintering of Supported Metals with Emphasis on Cobalt Catalysts During Fischer–Tropsch Synthesis

“…The Scherrer equation (eq ) is commonly used to roughly estimate NC size from XRD spectra for powders as follows: a single XRD peak of width β, at position θ, created by scattering of radiation of wavelength λ is translated directly into an estimate of the volume-weighted average NC size, d NC . ,,, …”

“…Instrumental factors come from the properties of the particular combination of source, detector, and optics which are largely accounted for through an XRD of a standard sample of known line-broadening. Sample characteristics which can influence broadening, line width, and position include crystalline strain, compositional variation, stacking faults, NC size distribution, sample thickness, and defect concencentration. ,, Consequently, in measuring Co 3 O 4 , CoO, or Co 0 NC size from XRD spectra, the broadening due to sample characteristics and instrumental factors must be considered in detail to accurately measure NC size, using more sophisticated methods.…”