Preparation and Properties of Small Silica-Supported Iron Catalyst Particles

Mössbauer cell for low-temperature studies of catalysts under reaction conditions

Mössbauer spectroscopy is an essential tool to investigate the structure of Fe supported catalysts and their changes, when they are used in the Fischer-Tropsch synthesis. A cell, that allows keeping the samples in the same atmosphere of the reduction treatment, was designed in order to characterize the Fe species without changing the working atmosphere avoiding the oxidation. It allows to measure at low temperatures in a helium closed-cycle refrigerator. Besides, this cell is useful to perform Mössbauer measurements on the used catalysts, preserving the oxidation of its species, using an inert atmosphere. In this work, we describe the details of this new cell and, as an example of its utility, we present the results obtained with a system of 12 nm iron oxide nanoparticles supported on a mesoporous silica matrix.

Temperature-Programmed Hydrogenation (TPH) and in Situ Mössbauer Spectroscopy Studies of Carbonaceous Species on Silica-Supported Iron Fischer−Tropsch Catalysts

Carbonaceous surface species and bulk iron carbides formed under realistic Fischer-Tropsch synthesis (FTS) conditions on moderately dispersed, active silica-supported iron catalysts (Fe/SiO2, FePt/SiO2, and FePtK/SiO2) were characterized. Bulk iron phase compositions were determined by Mössbauer spectroscopy and phase transformations of carbonaceous species during pretreatment with CO, H2, or H2/CO and following reaction were characterized using temperature-programmed hydrogenation (TPH). Isothermal transient rates of FTS were also measured for catalysts after different pretreatments. Six surface and bulk carbonaceous species were quantitatively identified from combined TPH and Mössbauer spectra of the FePtK catalyst. They include, in order of decreasing reactivity, (a) adsorbed, atomic carbon; (b) amorphous, lightly polymerized hydrocarbon or carbon surface species; (c) bulk epsilon' and chi carbides (Fe(2.2)C and Fe(2.5)C); and (d) disordered and moderately ordered graphitic surface carbons. A correlation between the amount of reactive alpha-carbon (C(alpha)) and initial catalytic activity was established. The method of Li et al. for measuring irreversible chemisorption of CO does not appear to provide quantitative measurements of active site densities on silica-supported iron. Models, based on this and previous work, are proposed for iron phase and carbon phase transformations in silica-supported iron during pretreatment, FTS, and postreaction passivation/oxidation.