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Adsorption/Reaction of CO on Fe Fischer-Tropsch Catalysts

Calvin H. Bartholomew, Hu Zou, and Uchenna Paul. Chemical Engineering, Brigham Young University, 350 CB, Provo, UT 84602

Significant previous work has focused on CO adsorption/dissociation over Fe FT catalysts for the purpose of better understanding the mechanism of Fischer-Tropsch Synthesis (FTS). The most widely-accepted FTS mechanism is the carbene model, involving CO adsorption and dissociation to adsorbed C and O atoms, hydrogenation of C atoms to CHx species, and insertion of CHx monomers into the metal-carbon bond of an adsorbed alkyl chain. Nevertheless, further quantitative study of CO adsorption and dissociation and of carbon hydrogenation on Fe surfaces is needed for the development of microkinetic models for FTS. Unsupported Fe FT catalysts with/without K and/or Pt were prepared by a nonaqueous evaporative deposition method. Temperature programmed desorption (TPD) was used to study CO adsorption and dissociation. Isothermal hydrogenation and temperature programmed hydrogenation (TPH) were used to measure rates of hydrogenation of the carbonaceous adsorbed species after CO/H2 treatment. Experiments were carried out at different CO adsorption temperatures. Two desorption peaks, associated with molecularly-adsorbed CO and dissociated CO respectively, were typically observed in CO-TPD spectra. Rates of desorption decreased for dissociated CO with increasing adsorption temperature. Quantitative analysis of the data is expected to provide information about the heat of adsorption and the kinetics of carbon hydrogenation on polycrystalline Fe.