(6i) A Unique Example of Thermokinetic Oscillations in Heterogeneous Catalysis: The Fischer-Tropsch Reaction over Co/Ce-Oxide Catalysts

Catalytic reactions occur far from equilibrium and may therefore exhibit multiple steady states leading to hysteresis or even oscillatory behavior and chaos. Different levels of complexity may be encountered depending on the catalyst formulation and the choice of the control parameters. By far the most detailed insight into oscillatory reaction behavior was so far obtained by using single crystals under low pressure conditions. Oscillatory behaviors in studies of catalyst powders at atmospheric pressure conditions usually imply a considerably higher level of complexity because heat and mass transport limitations may come into play and mask the possible influence of surface chemistry effects.

In the present contribution we demonstrate rate-and-selectivity oscillations of the Fischer-Tropsch reaction (FT) over non-supported Co/CeO_x powder catalysts. These oscillations are non-isothermal and may extend self-sustained over many hours time-on-stream depending on the catalyst composition and choice of vector gas (Ar or He) in H₂/CO flows. Oscillations in temperature have well-defined periods of several hundred seconds and occur with amplitudes of several °C, depending on the temperature (200 to 240 °C) and the H₂/CO ratio (1 to 12). Reactants (CO) and products (hydrocarbons and CO₂) appear with a phase lag of Ï€.

It seems clear from these observations that a simple C-C coupling mechanism of CH_x species cannot explain oscillatory product formation. Our efforts are presently based on examining a reaction mechanism which involves formate/carboxylate-derived species to explain both CO₂ and chain-lengthened HC product formation for a periodically changing Co-to-Co₂C catalyst surface composition.

To demonstrate the level of complexity in oscillatory behaviors, data of the present study over a powder Co/CeO_x sample will be compared with previous single crystal work in which rate oscillations were strictly isothermal and the coupling of local oscillators was clearly defined by surface diffusion.