CHARACTERIZATION OF A FISCHER-TROPSCH CATALYST OF MAGHEMITE NANOPARTICLES EMBEDDED IN SBA-15 PORES

I. O. PÉREZ DE BERTI, J. F. BENGOA, S. G. MARCHETTI, A. E. BIANCHI, R. C. MERCADER, AND G. PUNTE

Congreso; XIII Latin American Conference on the Applications of the Mössbauer Effect - LACAME 2012; 2012

The Fischer-Tropsch synthesis is a process meant to produce hydrocarbons out of a mixture of H2 and CO: (2n+1)H2+ nCO → CnH2n+2+ nH2O Its main disadvantage is its low selectivity. E.g., in certain conditions one can find hydrocarbons from C1 up to more than C100, with mixtures of olefins and paraffins, linear and branched in addition to oxygenated compounds. The use of promoters and the control of the operative variables are the two methods mainly used to increase the selectivity. However, the optimum procedure is still an open question. In this study we show that it is possible to synthesize a Fe-supported catalyst with an average size of Fe nanoparticles (NPs) and with a controlled width of the NPs size distribution to be used in studies of "structure sensitivity" effect in the Fischer-Tropsch synthesis. The catalyst preparation was carried out using pre-synthesized γ-Fe2O3 NPs with 2.5 ± 0.6 nm diameters. Afterwards, these NPs were supported on a modified SBA-15 matrix. The solid was characterized using N2 adsorption, transmission electron microscopy, small-angle X-ray scattering (SAXS), dynamic light scattering, thermo-gravimetric analysis and Mössbauer spectroscopy. The transmission Mössbauer spectra were taken at different temperatures from 17 to 298 K using He closed-cycle cryogenic systems. The spectra were fitted taking into account hyperfine field distributions and relaxing magnetic signals. SAXS data for a pure SBA-15 matrix and a loaded one were obtained at room temperature at the D11A- SAXS1 line workstation of the Brazilian Synchrotron Light Source, Campinas, Brazil, using a wavelength of 1.488 Å. The range of q detected was 0.3 ≤ q ≤ 1.95 nm-1. The SAXS results allowed identifying four diffraction lines that could be indexed as (100), (110), (200), and (210) reflections with a cell parameter a =12.65(5) nm. This result indicates that the sample has a well-ordered two-dimensional (2D) mesostructure (p6mm). The loaded SBA-15 SAXS curve shows a larger slope and a smoother background that might be related to a decrease in the surface roughness. We could index reflections (100), (110) and (200) with a cell parameter a = 12.84(5) nm. These results point to the conservation of the SBA-15 mesoporous structure after the γ-Fe2O3 NPs loading. The Mössbauer spectra of the NPs alone exhibited parameters belonging only to Fe3+ ions. At low temperatures, the spectra were fitted with magnetic relaxing signals that proved to originate in the γ-Fe2O3 and the Fe3+ ions coordinated to the oleic acid that coated the γ-Fe2O3 NPs. The Fe species in the catalyst precursor were identified by their Mössbauer signals that showed again that the parameters could be assigned only to Fe3+ ions coordinated to the oleic acid and to γ-Fe2O3. For the reduced catalyst, however, the fitting of the complex spectra required one doublet and eight sextuplets. Mössbauer spectroscopy allowed us assessing that the reduction treatment transformed the existing γ-Fe2O3 of the precursor into a mixture of 84 ± 9 % of Fe3O4 and 16 ± 5 % of χ-Fe2C5 and ε`-Fe2.2C carbides without noticeable change in the NPs size. The combination of the above mentioned results with those yielded by the other techniques show that the γ-Fe2O3/SBA-15 system has a narrow distribution of NPs sizes and keeps its structural properties after impregnation and activation.