Crude oil still represents the world's major energy supply. Depleting resources and geopolitical factors are currently pushing oil prices to historic highs. With this background, alternative energy sources are being pressed ahead. Large deposits in natural gas are playing a major role in developing the gas-to-liquid process (GTL), at the heart of which is Fischer-Tropsch synthesis (FT).
Catalysts are prepared by coprecipitation of Co along with a support (or an additive) using the oxalate route (Buess, P.; Caers, R. F. I.; Frennet, A.; Ghenne, E.; Hubert, C.; Kruse, N. In US Patent 20030036573, 2003). (CoxX1-x)C2O4 precipitates are subsequently characterised and activated using Temperature Programmed Decomposition (TPDec) or Temperature Programmed Reduction (TPR).
BET measurements and hydrogen chemisorption are performed in the same set-up as are activation and kinetic studies. No sample transfer is needed and the characterisation can be done at any time it is considered useful. On-line quadrupole mass spectrometry is used as an analytical tool along with gas chromatography.
Chemical Transient Kinetics (CTK) are used to elucidate mechanistic details of the FT synthesis. Imposing partial pressure steps, e.g. replacing a H2/He flow by H2/CO (build up), enables the following of the time response of various species desorbing into the gas phase until the steady-state of the reaction is reached. Reversing the procedure (back transient) provides information on the relaxation behaviour back to non-catalytic conditions. In this manner, we have demonstrated that the active surface is not metallic but oxidised during the CO+H2 reaction over Co and Co-Cu based catalysts. In an attempt to provide clues on the composition of the "most abundant surface intermediate(s)"(MASI), a formate type species is being observed to form. Moreover, it has been proven that CO is the monomer for chain lenghtening and not CHx.
Current research in our laboratory focuses on the FT reaction over pure metallic cobalt, Co/MgO and Co/carbon catalysts (cooperation with the Inorganic Chemistry and Catalysis Department of Utrecht University, NL).
A surface science approach to FT catalysis is hampered by the fact that low reactant pressures have to be applied under net vacuum conditions. Nevertheless, a PFDMS analysis of a single metallic Co particle (short field pulses rupture species during the ongoing reaction) while interacting with CO/H2 has provided a spectrum of hydrocarbons. Co-subcarbonyls are also present at the surface.