(552d) Effect of Pre- and Post-Reaction Measurement of Cobalt Particle Size and Metal Fraction On the Turnover Frequency of Silica Supported Fischer-Tropsch Catalysts

Ghampson, I. T. - Presenter, University of Maine
Hurley, K. D. - Presenter, University of Maine
Pollock, R. - Presenter, University of Maine
Walsh, B. - Presenter, University of Maine
Frederick, B. G. - Presenter, University of Maine
Pier, E. - Presenter, Bates College
Austin, R. - Presenter, Bates College
van Heiningen, A. - Presenter, University of Maine
DeSisto, W. J. - Presenter, University of Maine

Fischer-Tropsch synthesis (FTS) is receiving renewed attention, driven by the global need to convert non-petroleum-based energy resources into fuels and chemicals. Cobalt catalysts are known to favor higher molecular weight hydrocarbons in FTS. The influence of support pore size on the cobalt dispersion and intrinsic activity of the resultant catalysts has been examined by several groups recently.[1-4] In addition to influencing the catalyst dispersion, narrow pore size supports may also shift product distribution through the restriction of molecular diffusion. With the wide variety of supports available as well as new synthetic methods it is important to understand the relationships between support pore size and catalytic performance. Despite the recent advances, there remains much to learn regarding the effect of cobalt dispersion and support on FTS. In this work, we characterized cobalt catalysts impregnated onto silica supports with different pore sizes. Cobalt catalysts were prepared by wet impregnation of aqueous cobalt nitrate (10 wt% cobalt) onto four silica supports: CoSi1, wide pore commercial silica gel (dpore=22.3nm; Sarea=307m2/g); CoSi2, mesoporous beads (dpore=13.0nm; Sarea=464m2/g); CoSi3, ordered mesoporous, SBA-15 (dpore=10.0nm; Sarea=862m2/g); and CoSi4, ordered mesoporous MCM-41 (dpore=3.2nm; Sarea=978m2/g).  The nitrate precursor was thermally converted to the oxide by calcination in air. The catalyst was then reduced in hydrogen for activation followed by Fischer-Tropsch synthesis which was performed at 543 K and 10 bar in an Altamira AMI-200 R-HP characterization instrument. The catalyst samples were calcined in air at 723K for one hour and reduced in 10% H2 in Ar at 773K for 5 hrs prior to the FT reactions. During the FT synthesis, two gases, 10% CO in He and 10 % H2 in Ar (with a 1:2.1 mole ratio), were fed to the catalyst. The reactor was heated 543 K at 10 K/min and held at temperature for 10 hours. Reaction products were analyzed with a SRS RGA-300 Mass Spectrometer. Inert gases were used as internal standards. Conversion was calculated from the change in CO/He and H2/Ar ratios. We characterized the catalyst properties, Co0 particle diameter and extent of reduction, at three different stages in catalyst history: (1) pre-reduction/pre-reaction; (2) post-reduction/pre-reaction; and (3) post-reaction. Catalyst properties were characterized by nitrogen porosimetry, temperature programmed reduction and x-ray diffraction (phase identification and particle size. Table 1 shows the Co0 particle size at all three stages measured by XRD. In stage one, Co0 size was determined from XRD data of Co3O4 particle size reduced by 75%. Turnover frequencies (TOF) calculated from specific reaction rates were significantly different depending on the measurements used to determine dispersion at shown in Figure 1. We also observed the oxidation of Co0 to Co(II)O during FTS of the catalyst supported on MCM-41, the silica support with the smallest pore size.

Table 1. Mole fraction (χ) Co0 as determined by TPR and XRD and the Co0 particle diameters (nm) in parentheses, determined by XRD.


Stage 1

Stage 2

Stage 3



0.78 (21.4)

0.77 (15.7)



0.52 (9.8)

0.66 (13.1)



0.88 (8.4)

0.89 (7.8)



1 (10.2)

0.52 (3.0)

Figure 1. The effect of cobalt metal particle size on turnover frequency.


[1]       O. Borg, P.D.C. Dietzel, A.I. Spjelkavik, E.Z. Tveten, J.C. Walmsley, S. Diplas, S. Eri, A. Holmen, and E. Ryttera, Journal of Catalysis 259 (2008) 161-164.

[2]       A.Y. Khodakov, R. Bechara, and A. Griboval-Constant, Applied Catalysis a-General 254 (2003) 273-288.

[3]       G.L. Bezemer, J.H. Bitter, H. Kuipers, H. Oosterbeek, J.E. Holewijn, X.D. Xu, F. Kapteijn, A.J. van Dillen, and K.P. de Jong, Journal of the American Chemical Society 128 (2006) 3956-3964.

[4]       A. Martinez, and G. Prieto, Journal of Catalysis 245 (2007) 470-476.


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