An isotopic tracer study of the deactivation of Ru/Ti0 2 catalysts during Fischer-Tropsch synthesis

Abstract

An investigation of the causes of catalyst deactivation during Fischer-Tropsch synthesis over Ru/Ti0 2 catalysts has been carried out. The effects of Ru dispersion, TiO 2 phase, and TiO 2 surface area on catalyst activity and selectivity were examined. CO chemisorption capacity and carbon species accumulation were determined as a function of reaction time using isotopic tracer techniques in conjunction with temperature-programmed surface reaction (TPSR). The results of this investigation show that all of the catalysts undergo deactivation with time, with no change in product selectivity. Initially, deactivation is very rapid followed by a slower activity loss. The long-term rate of deactivation is proportional to the initial CO turnover frequency, obtained by extrapolation of the long-term activity data. Deactivation is accompanied by a progressive loss in CO chemisorption capacity as well as an accumulation of various types of carbon species. Carbidic carbon, Cα, and alkyl carbon chains, C β , were observed to accumulate as reaction proceeds. C β consists of two species, C′ β which is the precursor to C 2+ hydrocarbon products and C″ β which consists of longer alkyl chains and does not participate in the production of gas phase products. The rapid initial loss in activity in the first 10 min correlates with the C α + C β accumulation; the long-term loss in CO uptake and catalyst activity are probably due to C″ β . The alkyl chains comprising C″ β do not undergo hydrogenolysis under reaction conditions, probably due to inaccessibility to hydrogen.