"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; G .PUNTE

Medellín

Encuentro; Latin American Conference of the Applications of the Mössbauer Effects, LACAME 2012; 2012

Comité Latinoamericano de Espectroscopía Mössbauer

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 outusing pre-synthesized gamma-Fe2O3 NPs with 2.5(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 0.1488 nm. The range of q detected was 0.3 l.t. q l.t. 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 gamma-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 gamma-Fe2O3 and the Fe3+ ions coordinatedto the oleic acid that coated the gamma-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 gamma-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 gamma-Fe2O3 of the precursor into a mixture of 84(9) per cent of Fe3O4 and 16(5) per cent of chi-Fe2C5 and epsilon prime-Fe2.2C carbides without noticeable change in the NPs size.The combination of the above mentioned results with those yielded by the other techniques shows that the gamma-Fe2O3 /SBA-15 system has a narrow distribution of NPs sizes and keeps its structural properties after impregnation and activation.