The effect of boron on the catalyst reducibility and activity of Co/TiO2 Fischer-Tropsch catalysts

The effect of boron (as H3BO3) on the CO hydrogenation ability of Co/TiO2 c atalysts was investigated using XRD, LRS, TGA, DRIFTS and a fixed bed flow reactor. The introduction of boron (0.02-1.5%) into a 10 wt% Co/TiO2 cataly st decreased the Co3O4 crystallite size (26-16 nm) in the oxidic catalysts (calcined at 300 degrees C) and decreased the hydrogen uptake (0.35-0.9 ml/ g cat) in the reduced catalyst. Reduction of the Co/TiO2 catalyst was made more difficult by the presence of boron. The CO conversion and overall hydr ogenation rate decreased with decreasing ease of reducibility and decreasin g cobalt dispersion caused by boron. Turnover frequency (ca. 20 x 10(-3) s( -1)), however, remained constant throughout and was independent of the exte nt of reduction and dispersion of the catalysts. This provides further evid ence of the structure-insensitivity of supported Co Fischer-Tropsch catalys ts. Addition of small amounts of B (<0.1%) do, however, result in an increa se in alpha, less CH4 production and an increase in the olefin/paraffin rat io. This suggests an increase in the monomer propagation to termination rat io. At higher B loadings, product selectivity shifted to the lower molecula r weight hydrocarbons and CO2 selectivity increased (0-2.5%). (C) 1999 Else vier Science B.V. All rights reserved.