Early stages of Fischer–Tropsch activity on Tho2 and Th0.9Ce0.1O2

Abstract

The early stages of CO hydrogenation catalysed by high-purity high surface area thorium oxide and ceriumdoped thorium oxide have been examined in the 15–20 Torr range in an attempt to find the intrinsic factors that influence the Fischer–Tropsch (FT) activity of these robust materials. The surfaces of the clean oxide powders were characterized by gas–solid oxygen isotope exchange over an O₂ pressure range of 1–4 Torr. Doping with cerium significantly enhanced the exchange rate, causing a lowering of 40 kJ mol^–1 of the activation energy for both atomic (single O atom) and molecular (double O atom) exchange mechanisms relative to the same exchange processes on pure thoria.

The initial product distribution obtained at very low conversions of CO at 685 K and 15–20 Torr total pressure over the two materials indicates that the cerium-doped material makes alkenes preferentially over alkanes. Neither catalyst was found to generate oxygenated hydrocarbons as earlier reported. The shift in product distribution obtained over doped thoria (Th_0.9Ce_0.1O₂) compared with that obtained over pure thoria was consistent with the redox chemistry of cerium and an assumed enediolate intermediate. The product distribution obtained over doped oxide was accompanied by a threefold increase in the rate of consumption of CO with respect to the rate obtained over pure thoria. The rate increase correlated positively with the change in rate of oxygen exchange. A significant induction period was observed over both catalysts. Such early-stage delays have not previously been reported for this system.