THE ROLE OF BULK H-SPECIES AND C-SPECIES IN THE CHAIN LENGTHENING OF FISCHER-TROPSCH SYNTHESIS OVER NICKEL

This work documents the possible role of bulk species (dissolved H and C atoms) in the Fischer-Tropsch reaction. This has led us to study th e sorption (adsorption or absorption) of hydrogen when increasing the pressure from 0.1 to 5 MPa, the selectivity toward C2+ hydrocarbons in carbon monoxide hydrogenation at 5 MPa total pressure, and the amount of nickel carbide formed in the course of the reaction over unsupport ed nickel, Ni/SiO2, Ni/TiO2 and Ni/Cr2O3 catalysts. Increasing the hyd rogen pressure results in a subsequent sorption which is found to be p roportional to the nickel dispersion, and can be attributed either to a completion of the surface hydrogen adlayer (8%) or to a formation of a hydrogen sublayer. No relationship could be found between the selec tivity towards C2+ hydrocarbons and the quantity of sorbed hydrogen. A correlation between this selectivity and the amount of nickel carbide formation was demonstrated, suggesting that the active phase which le ads to C2+ hydrocarbons is nickel carbide, or that carbon atoms of sub surface sites participate in the homologation reaction.