An Investigation of the Effects of Water on Rate and Selectivity for the Fischer-Tropsch Synthesis on Cobalt-Based Catalysts

Water, the primary oxygen-containing product in Co-catalyzed Fischer–Tropsch synthesis (FTS), increases CO conversion rates and the selectivity to olefins and to C5+ products. These marked rate and selectivity enhancements can reflect changes in the density or reactivity of the active Co surface atoms available during FTS. Kinetic isotope effects and in situ infrared spectroscopy were used to probe possible mechanisms for these water effects. Kinetic isotope effects (rH/rD) measured from CO/H2 and CO/D2 were less than unity for both CO conversion and C5+ formation rates, suggesting that hydrogen adsorption–desorption thermodynamics and C–H bond formation are involved in kinetically relevant FTS steps. The presence of varying amounts of water (H2O or D2O) did not influence the measured kinetic isotope values, even though water in concentration range led to marked changes in FTS rate and selectivity. Thus, the isotopic identity of the water molecules does not influence the rate of kinetically relevant steps, the identity of which appears to remain unchanged by the presence of water. No new pathways are apparently introduced by the presence of water in H2/CO reactant streams. The intensity and vibrational frequency of adsorbed CO bands were also not influenced by the concentration of water in the reactant stream. These in situ infrared spectroscopic studies showed that water does not influence the density or structure of adsorbed CO intermediates or the number of exposed Co atoms, which bind CO at near-monolayer coverages during Fischer–Tropsch synthesis reactions. These spectroscopic studies also suggest that neither CO transport restrictions nor their removal via the formation of water-rich intrapellet liquids at high water concentrations are responsible for the observed effects of water. These studies are silent about a possible effect of water on the concentration of reactive carbon species, which may be present as minority but kinetically relevant intermediates during FTS.

[1]  Alexis T. Bell,et al.  Evidence for H2D2 isotope effects on fischer-tropsch synthesis over supported ruthenium catalysts , 1981 .

[2]  C. H. Bartholomew,et al.  The kinetics of H2 adsorption on and desorption from cobalt and the effects of support thereon , 1983 .

[3]  M. Shelef,et al.  Heterogeneous methanation: In situ infrared spectroscopic study of RuAl2O3 during the hydrogenation of CO , 1977 .

[4]  Hans Schulz,et al.  Polymerisation kinetics of the Fischer-Tropsch CO hydrogenation using iron and cobalt based catalysts , 1999 .

[5]  V. Ponec,et al.  Methanation of carbon monoxide on nickel and nickel-copper alloys , 1976 .

[6]  R. Anderson,et al.  The Fischer-Tropsch Synthesis , 1984 .

[7]  G. Kadinov,et al.  IR spectroscopy study of CO adsorption and of the interaction between CO and hydrogen on alumina-supported cobalt , 1998 .

[8]  Hans Schulz,et al.  Selectivity and mechanism of Fischer-Tropsch synthesis with iron and cobalt catalysts , 1994 .

[9]  D. King.  An in situ infrared study of CO hydrogenation over silica and alumina-supported ruthenium and silica-supported iron , 1980 .

[10]  K. Kobe.  The properties of gases and liquids , 1959 .

[11]  A. Holmen,et al.  Study of the deactivation mechanism of Al2O3-supported cobalt Fischer-Tropsch catalysts , 1995 .

[12]  S. C. Fung,et al.  Strong metal-support interactions: Occurrence among the binary oxides of groups IIA–VB , 1978 .

[13]  G. Bond,et al.  Catalysis, science and technology , 1983 .

[14]  Enrique Iglesia,et al.  Promoted iron-based catalysts for the Fischer-Tropsch synthesis: Design, synthesis, site densities, and catalytic properties , 2002 .

[15]  Enrique Iglesia,et al.  Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis catalysts , 1997 .

[16]  Alexis T. Bell,et al.  Studies of carbon- and hydrogen-containing adspecies present during CO hydrogenation over unsupported Ru, Ni and Rh , 1985 .

[17]  C. Mims,et al.  Surface carbon coverage and selectivity in FT synthesis: a simple model for selectivity correlations , 2001 .

[18]  E. Steen,et al.  On the effect of water during Fischer–Tropsch synthesis with a ruthenium catalyst , 2002 .

[19]  H. Storch.  The Fischer-Tropsch and related syntheses , 1951 .

[20]  H. Schulz,et al.  Effect of water partial pressure on steady state Fischer-Tropsch activity and selectivity of a promoted cobalt catalyst , 1997 .

[21]  A. Holmen,et al.  The effect of water on cobalt Fischer-Tropsch catalysts studied by steady-state isotopic transient kinetic analysis (SSITKA) , 1997 .

[22]  P. J. van Berge,et al.  Oxidation of cobalt based Fischer–Tropsch catalysts as a deactivation mechanism , 2000 .

[23]  John G. Ekerdt,et al.  Synthesis of hydrocarbons from CO and H2 over silica-supported Ru: Reaction rate measurements and infrared spectra of adsorbed species , 1979 .

[24]  C. H. Bartholomew,et al.  Activated adsorption of H2 on cobalt and effects of support thereon , 1983 .

[25]  A. Holmen,et al.  Transient and steady-state studies of the effect of water on cobalt Fischer-Tropsch catalysts , 1997 .

[26]  A. Poater,et al.  Catalysis Science and Technology , 2022 .