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C catalysts
Applied Catalysis A: General 325 (2007) 68–75 www.elsevier.com/locate/apcata
Influence of intermediate iron reduced species in Fischer-Tropsch synthesis using Fe/C catalysts J.F. Bengoa, A.M. Alvarez, M.V. Cagnoli, N.G. Gallegos, S.G. Marchetti * Cindeca, Facultad Cs. Exactas, Facultad de Ingenierı´a, UNLP, CONICET, CICPBA, Calle 47 N8 257, 1900 La Plata, Argentina Received 1 December 2006; received in revised form 26 February 2007; accepted 11 March 2007 Available online 15 March 2007
Abstract Two Fe/carbon catalysts were prepared to study the influence of the intermediate iron species on the activity and selectivity in the FischerTropsch synthesis (FTS). The solids were characterized by XRD, CO chemisorption, Atomic Absorption, Nitrogen Adsorption and Mo¨ssbauer spectroscopy. Using two loading of consolidant agent, two supports with different specific surface areas were obtained. The resulting catalysts showed different iron species. The catalyst with lower reduction degree produces a non-stoichiometric iron carbide during FTS, which has a higher activity and light olefins selectivity. # 2007 Elsevier B.V. All rights reserved. Keywords: Fischer-Tropsch synthesis; Fe/C catalysts; Mo¨ssbauer spectroscopy; Iron carbides
1. Introduction The Fischer-Tropsch synthesis (FTS) has been previously studied using catalysts of Fe supported on different substrates, such as SiO2, Al2O3, MgO, zeolites. It is well known that in these systems it is very hard to reach a complete metal reduction when the iron loading is lower than 10% (w/w) [1,2]. In these cases, intermediates iron species are present on the fresh reduced catalyst jointly with the Fe0. Notwithstanding, the role played by these intermediate species, in the synthesis, is still unknown. To carry out the study above mentioned it is necessary to have a support that allow a complete iron reduction in order to be used as a reference. In this way, we have chosen a carbon support due to its low metal-support interaction. Since some reactions involved in FTS are ‘‘sensitive to structure’’, that is the activity and selectivity depend on the metal crystal size [3], it is necessary to prepare catalysts with similar Fe0 crystal diameters in order to avoid the influence of this parameter. With the aim to determine the effect of the intermediate iron reduced species on the activity and selectivity on the FTS, two Fe/C catalysts with different reduction degree were prepared,
* Corresponding author. Tel.: +54 221 4210711; fax: +54 221 4211353. E-mail address: [email protected] (S.G. Marchetti). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.03.012
catalytically tested and the results were correlated with its structural properties. 2. Experimental The catalyst supports were two glassy carbons prepared in the laboratory using the method outlined in the Hucke patent [4]. In a similar way as Moreno-Castilla et al. [5], we used furfuryl alcohol, as a binder (20 cm3), polyethylene glycol having an average molecular weight of 400 as pore former and dispersant (15 cm3), Triton X-100 (iso-octil phenoxy polyethoxi ethanol) as dispersing agent (15 cm3) and oxalic acid as consolidating agent or polimerization catalyst, in two different amounts: 6.00 and 8.42 g to obtain two carbons with low and high specific surface area: C(l.s) and C(h.s), respectively. The following procedure was used to prepare the carbons. The oxalic acid was dissolved in a mix composed of polyethylene glycol and Triton at 348 K. After cooling the resultant solution at 288 K, furfuryl alcohol was added drop by drop with gently constant stirring, in order to preserve bubble formation. After addition, the temperature was kept at 318 K during 2 h, to allow the beginning of the polymerization. After this step, the stirring was stopped and the following thermal program was setted: 323 K, 24 h, 343 K, 48 h, and 368 K, 72 h. The solids were then pyrolized in a N2 flow (25 cm3/min) using the following heating cycle: 373–573 K at 4 K/h,
J.F. Bengoa et al. / Applied Catalysis A: General 325 (2007) 68–75
573–598 K at 2 K/h, 598–698 K at 5 K/h and from 698 to 873 K at 10 K/min, and kept at this temperature during 30 min. The iron was introduced into these systems by standard incipient wetness technique, using an alcoholic solution of Fe(NO3)39H2O at a high enough concentration to yield a catalyst with 5% (w/w) of Fe. The impregnated solids were calcined in dry N2 stream (60 cm3/min) from 298 to 698 K at 10 K/min and kept at this temperature during 8 h. The precursors with low and high specific surface area were called: p-Fe/C(l.s) and p-Fe/C(h.s), respectively. The precursors were reduced in H2 stream (60 cm3/min) following the thermal program described in Ref. [2]. The catalysts obtained were called c-Fe/C(l.s) and c-Fe/C(h.s). Mo¨ssbauer spectroscopy at 298 K (RT) and low temperatures, X-ray diffraction (XRD), CO chemisorption, atomic absorption and N2 adsorption (BET) were used to characterize the solids. The Mo¨ssbauer spectra were obtained in transmission geometry with a 512-channel constant acceleration spectrometer. A source of 57Co in Rh matrix of nominally 50 mCi was used. Velocity calibration was performed against a 6 mm-thick a-Fe foil. All isomer shifts (d) mentioned in this paper are referred to this standard. The temperature between 15 and 298 K was varied using a Displex DE-202 Closed Cycle Cryogenic System. The Mo¨ssbauer spectra were evaluated using a fitting commercial program named Recoil. Lorentzian lines were considered with equal widths for each spectrum component. The spectra were folded to minimize geometric effects. The precursors were characterized by XRD using Cu Ka radiation. CO chemisorption experiments were carried out in conventional volumetric static equipment [2]. Activity and selectivity measurements were carried out during 48 h in a fixed bed reactor at 543 K at atmospheric pressure, H2/CO ratio of 3/1, mass catalysts of 800 mg and a space velocity of 0.28 s1. The products were analyzed ‘‘on line’’ in a Konik chromatograph having a Flame Ionization Detector, using GS-Alumina PLOT (JW Scientific) column.
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sample. This ratio have remains nearly constant after impregnation with iron salt and calcination. The total iron contents determined by atomic absorption are also shown in Table 1. The XRD patterns of both precursors (not shown) display three peaks at 2u = 30.28, 35.88 and 57.28. All of them are characteristics of Fe3O4 and g-Fe2O3. Therefore, this technique does not allow us to distinguish between both species. The Mo¨ssbauer spectra at 298 and 15 K of the precursors are shown in Fig. 1. In the Mo¨ssbauer spectra of p-Fe/C(l.s), two sextets can be seen at both temperatures. At RT the spectrum was fitted with two interactions which hyperfine parameters (Table 2a) can be assigned to Fe3+ in octahedral sites and Fe‘‘2.5+’’ in tetrahedral sites of Fe3O4 [6]. From the relative areas of both species it can be seen that the Fe3+/Fe2+ ratio is higher than that found for stoichiometric Fe3O4. This result indicates that this magnetite is partially oxidized. The magnetite Mo¨ssbauer spectra below the Verwey temperature (Tv) are very complex. Thus, below Tv (120 K), Fe3O4 exists in a multiple twinned monoclinic state. As a result the lines in the Mo¨ssbauer spectrum are broadened because of the distribution of angles between the magnetic field and the electric field gradient. The broadening can be removed by cooling a singlecrystals specimen through Tv in a magnetic field applied along [1 0 0] direction [7]. The spectrum obtained for such sample can be resolved into five components corresponding to one Asite Fe3+ ion, two B-site Fe3+ ions and two B-site Fe2+ ions. Berry et al. [8] reported that in presence and absence of applied fields at low temperature the spectrum of magnetite monocrystals is best fitted to five magnetic components. However, Vandenberghe and De Grave [6] considered that although the fittings give a reasonable agreement with the experimental line shape, it is difficult to assign the subspectra to particular Fe2+ and Fe3+ states. The situation is more complex with Fe3O4 polycrystalline. In this way some authors have used three components to fit its spectrum below Tv [9]. Taking into account this description, and considering that it is not the purpose of this investigation to discuss the properties of Fe3O4, the present spectrum at 15 K has been fitted with three interactions following the ideas of Vandenberghe and De Grave [6] without to assign the tetrahedral and octahedral sites to specific iron ions. The other two components resulted null in fitting in our catalyst (Table 2b). In the Mo¨ssbauer spectrum of p-Fe/C(h.s), two sextets partially overlapped and a central signal can be seen at 298 K. Besides, the background is markedly curved, suggesting the
3. Results and discussion 3.1. Characterization Table 1 shows the structural properties of the solids. The carbon prepared with lower amount of consolidating agent presents a specific surface area 90% lower than the other Table 1 Structural properties of the solids Properties 2
Sg (m /g) %Fe (w/w) CO chemisorption (mmol/g) Reduction to Fe0 (%) DVA (nm)
C(l.s)
C(h.s)
p-Fe/C(l.s)
p-Fe/C(h.s)
c-Fe/C(l.s)
c-Fe/C(h.s)
22 – 77
292 – 59
37 5.5 –
225 5.3 –
– 5.6 119
– 5.4 70
– –
– –
– –
– –
100 9
31 10
Sg: specific surface area; DVA: average diameter of de Fe0 crystals.
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Fig. 1. Mo¨ssbauer spectra at 298 and 15 K of p-Fe/C(h.s) and p-Fe/C(l.s).
presence of superparamagnetic relaxing species. The magnetic signal was fitted with two sextuplets assigned in a similar way that in p-Fe/C(l.s), to Fe3O4 (Table 2a). The magnetic hyperfine fields are lower than that shown by p-Fe/ C(l.s) indicating the existence of crystallites with lower size. The central signal was simulated with a relaxing sextuplet. Considering that its isomer shift is slightly higher than that corresponding to Fe3+, this signal could be assigned to superparamagnetic Fe3O4.
At 15 K the spectrum shows an asymmetric sextuplet that was fitted in the same way that p-Fe/C(l.s) at 15 K (Table 2b). This magnetic blocking confirms the existence of superparamagnetic Fe3O4 at room temperature and a lower crystal size in comparison with p-Fe/C(l.s). The thermal decomposition of Fe(NO3)39H2O generally produces a-Fe2O3 instead of magnetite. However, the use of nitrogen stream free of oxygen would produce vacancies in the oxide crystal framework, yielding Fe3O4. After the end of the
Table 2a Mo¨ssbauer parameters of p-Fe/C(l.s) and p-Fe/C(h.s) at 298 K Species
Parameters
p-Fe/C(l.s) at 298 K
p-Fe/C(h.s) at 298 K
3+
H (T) d (mm/s) 2e (mm/s) %
49.4 0.1 0.29 0.01 0.02 0.01 55 1
48.2 0.1 0.32 0.01 0.03 0.01 25 2
Fe‘‘2.5+’’ in B sites of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
46.1 0.1 0.62 0.01 0.01 0.01 45 1
45.1 0.2 0.50 0.01 0.01 0.01 38 3
Relaxing signal
H (T) d (mm/s) 2e (mm/s) %
Fe
in A sites of Fe3O4
– – – –
H: hyperfine magnetic field; d: isomer shift (all the isomer shifts are referred to a-Fe at 298 K); 2e: quadrupole shift. a Parameters held fixed in fitting.
45 a 0.4 0.1 0.1 0.2 37 3
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Table 2b Mo¨ssbauer parameters of p-Fe/C(l.s) and p-Fe/C(h.s) at 15 K Species
Parameters
p-Fe/C(l.s) at 15 K
p-Fe/C(h.s) at 15 K
First interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
51.3 0.1 0.34 0.01 0.01 0.01 41 7
51.1 0.1 0.28 0.01 0.01 0.01 37 2
Second interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
51.9 0.4 0.50 0.06 0.01 0.05 29 7
51.8 0.1 0.55 0.01 0.01 0.01 46 2
Third interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
48.1 0.5 0.79 0.03 0.01 0.05 30 5
48a 0.8a 0.03 0.04 17 1
H: hyperfine magnetic field; d: isomer shift (all the isomer shifts are referred to a-Fe at 298 K); 2e: quadrupole shift. a Parameters held fixed in fitting.
thermal treatment, the solid is exposed to air and a fraction of Fe3O4 is partially oxidized. Magnetite could be also produced by reduction of a-Fe2O3 with CO originated from the partial carbon oxidation during the iron salt decomposition. According with the Collective Magnetic Excitations Model (CMEM) [10], the magnetic hyperfine fields appear diminished with respect to the bulk value, when the crystal sizes are lower than a critical diameter. Assuming that a magnetic hyperfine field decrease lower than 1% from the bulk value (H0) is not significant, a critical diameter could be estimated. To carry out this estimation, we calculated a weighted average H0, assuming a relative population of Fe3+/Fe2+ = 2 for an ideal bulk magnetite. Using the anisotropy energy constant for uniaxial symmetry provides by Mørup and Topsøe [10] K = 0.90 0.3 106 erg/cm3, a critical size of 28 nm was estimated. In p-Fe/C(l.s) the weighted average H is not diminished, therefore we assumed that the Fe3O4 crystals are higher than 28 nm diameter. Instead, in p-Fe/C(h.s) the weighted average field is markedly lower than that corresponding to the bulk value. Applying CMEM, an average size of Fe3O4 crystals of 13.5 nm was obtained. Fig. 2 displays the Mo¨ssbauer spectrum at 298 K of c-Fe/ C(l.s). This shows a magnetic sextet corresponding to a-Fe0, according with its hyperfine parameters (Table 3a). The hydrogen treatment led to the total reduction of Fe3O4 to a-Fe0. A spectrum at low temperature was not necessary because the RT spectrum allowed us to determine the species presents without doubt. Fig. 2 also shows the Mo¨ssbauer spectra at 298 and 15 K of c-Fe/C(h.s). At RT the spectrum displays a magnetic sextet and several central signals intense and highly overlapped. The spectrum was interpreted in terms of a superposition of two sextets, one of them relaxing. The hyperfine parameters of the last one would correspond to superparamagnetic (sp) Fe3O4. The other sextet was assigned to a-Fe0 (Table 3a). When the temperature decreases to 15 K, the spectrum shows the presence of, at least, 10 peaks. This complex spectrum was fitted with six sextets. One of them corresponds to a-Fe0. The five remaining sextets belong to Fe3O4 as it was described for
Fig. 2. Mo¨ssbauer spectra at 298 and 15 K of c-Fe/C(h.s) and c-Fe/C(l.s).
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Table 3a Mo¨ssbauer parameters of c-Fe/C(h.s) and c-Fe/C(l.s) at 298 K Species Fe
0
Relaxing signal
Parameters
c-Fe/C(h.s)
c-Fe/C(l.s)
H (T) d (mm/s) 2e (mm/s) %
32.9 0.1 0.01 0.01 0.01 0.01 35 1
32.9 0.1 0.01 0.01 0.01 0.01 100
H (T) d (mm/s) 2e (mm/s) %
45 a 0.61 0.01 0.01 0.01 65 1
– – – –
H: hyperfine magnetic field; d: isomer shift (all the isomer shifts are referred to a-Fe at 298 K); 2e: quadrupole shift. a Parameters held fixed in fitting.
the precursors [6]. In this solid none component resulted null in fitting (Table 3b). This behavior could be attributed to a higher Fe2+ quantity as a consequence of the reduction treatment. Analyzing the above results, it is possible to conclude that we obtained two catalysts with different iron species and reduction degrees, starting from two precursors with the same iron species but with different crystal sizes. In order to obtain the metallic crystal sizes and the amount of Fe0 surface atoms, CO chemisorption at 193 K in 200 to 600 Torr pressure range, using the double-isotherm method, was carried out. The CO chemisorption at 193 K generally leds to crystal diameter values concordant with those obtained by
other methods [11]. The CO chemisorbed by the supports was determined and subtracted from the total uptake of the catalysts obtaining a net CO uptake of 42 mmol/g for c-Fe/C(l.s) and 11 mmol/g for c-Fe/C(h.s) (Table 1). Assuming spherical shape and no size distribution for the Fe0 crystals, the metallic crystal diameters were calculated. Similar sizes for both catalysts were obtained (Table 1). Taking into account that the oxide crystal size for p-Fe/C(l.s) is higher than that of the other precursor, the reduction treatment would be responsible of the crystal breaking, leading to a similar metallic crystal size for both catalysts. 3.2. Activity and selectivity measurements The total hydrocarbon productions per gram of catalyst were calculated at the steady state. Values of 10.8 1016 molecules/ (g s) for c-Fe/C(h.s) and 2.1 1016 molecules/(g s) for c-Fe/ C(l.s) were obtained. It can be seen that the former is five times more active than the last. We can discard any difference due to transport effects on these results since the pore diameter and pore volume of both precursors are very similar: 1.8 nm and 0.20 cm3/g for p-Fe/C(l.s) and 2.1 nm and 0.24 cm3/g for p-Fe/ C(h.s). Besides, any textural change during FTS, produced by carbon deposition, would result negligible due to the use of differential reactor.
Table 3b Mo¨ssbauer parameters of c-Fe/C(h.s) at 15 K Species
Parameters
c-Fe/C(h.s)
First interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
50.3 0.1 0.28 0.02 0.01 0.03 18 3
Second interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
50.8 0.1 0.55 0.03 0.04 0.02 27 3
Third interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
51.0a 0.80a 0.17a 31
Fourth interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
44.2 0.8 1.01 0.06 0.5 0.1 11 2
Fifth interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
33.3 0.3 1.46 0.04 0.45 0.07 10 1
Fe 0
H (T) d (mm/s) 2e (mm/s) %
34.0 0.1 0.12 0.01 0.02 0.01 31 2
H: hyperfine magnetic field; d: isomer shift (all the isomer shifts are referred to a-Fe at 298 K); 2e: quadrupole shift. a Parameters held fixed in fitting.
Fig. 3. Mo¨ssbauer spectra at 25 K of used c-Fe/C(h.s) and c-Fe/C(l.s).
J.F. Bengoa et al. / Applied Catalysis A: General 325 (2007) 68–75
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Table 4 Mo¨ssbauer parameters of used c-Fe/C(l.s) and c-Fe/C(h.s) at 25 K Species
Parameters
c-Fe/C(l.s)
c-Fe/C(h.s)
I Site of x-Fe5C2
H (T) d (mm/s) 2e (mm/s) %
22 1 0.3 0.1 0.1 0.1 18 10
22 1 0.37a 0.12a 15 1
II Site of x-Fe5C2
H (T) d (mm/s) 2e (mm/s) %
25.2 0.6 0.38 0.06 0.3 0.1 38 9
III Site of x-Fe5C2
H (T) d (mm/s) 2e (mm/s) %
12.0 0.8 0.3 0.1 0.1 0.2 31 8
12.3 0.6 0.36 0.07 0.05 (a) 10 2
I Site of ‘‘O’’ carbide + II site of x-Fe5C2
H (T) d (mm/s) 2e (mm/s) %
– – – –
25.2 0.3 0.38 0.03 0.12a 19 3
II Site of ‘‘O’’ carbide
H (T) d (mm/s) 2e (mm/s) %
– – – –
17 1 0.4 0.1 0.05a 11 3
Site of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
– – – –
50.7 0.8 0.47 0.07 0a 18 4
Site of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
– – – –
37 1 0.6 0.1 0.6 0.2 19 5
Superparamagnetic carbide
D d %
0.3 0.2 0.1 0.1 13 4
– – – –
0.36 0.08 0.27 0.05 81
H: hyperfine magnetic field; d: isomer shift (all the isomer shifts are referred to a-Fe at 298 K); 2e: quadrupole shift; D: quadrupole splitting. a Parameters held fixed in fitting.
Mo¨ssbauer spectra in controlled atmosphere of the used catalysts were taken at 25 K (Fig. 3), using a cell specially built by us [12], in order to explain why the catalyst with higher number of surface metallic iron atoms is less active. The Mo¨ssbauer spectra of the used catalysts at room temperature were not shown since the superparamagnetic relaxation inhibits to obtain any conclusion. The spectra at 25 K were very complex with several overlapped and broad lines. The used cFe/C(l.s) spectrum at 25 K was fitted with one doublet and three sextets. The former interaction was assigned to superparamagnetic iron carbide since its d and quadrupole splitting (D) values were very low to correspond to a sp oxide. The remainder three sextets correspond to the three different sites of stoichiometric carbide x-Fe5C2 according to their hyperfine parameters (Table 4) [13]. The used c-Fe/C(h.s) spectrum at 25 K was fitted with one doublet and six sextets. The former component was also assigned to sp iron carbide. The two sextets with the higher hyperfine fields and isomer shifts can be assigned to iron oxide (probably Fe3O4) [6]. The remainder four sextets have hyperfine parameters typical of x-carbide and a non-stoichio-
metric ‘‘O’’ carbide (Fe2+xC) [14]. The component with H 25 T is an overlapping of site I of ‘‘O’’ carbide and site II of x-carbide. We estimated the pxII population using the expression deduced by Brauer-Grosse and Le Cae¨r [15]: pxII ¼ 2ð pxI pxIII Þ where pxI , pxII and pxIII are the relative populations of I, II, and III sites of x-carbide. This value was subtracted from the sextet area with H 25 T and we obtained the site I population of ‘‘O’’ carbide. Finally, we calculated the ‘‘O’’ carbide stoichiometry (Fe2+xC), from the equation deduced by Pijolat et al. [14]: 6 O ¼ 3 pO I þ 2 pII 2þx O where pO I and pII are the relative populations of sites I and II of ‘‘O’’ carbide. The stoichiometry resulted close to: Fe2.4C, therefore this carbide is highly non-stoichiometric. Fig. 4 displays a speculative schematic representation of the different suggested changes and the steps followed by each
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Fig. 4. Schematic representation of the different steps followed by each solid to reach the structure of the ‘‘working’’ catalyst at the steady state.
solid to reach the structure of the ‘‘working’’ catalyst at the steady state. As it was described above the only species present in p-Fe/ C(l.s) and p-Fe/C(h.s) was Fe3O4. In p-Fe/C(l.s) there is only one fraction of ferrimagnetic Fe3O4, larger than 28 nm of diameter. Instead, in p-Fe/C(h.s) there are two fractions: one of them ferrimagnetic, of about 13.5 nm diameter and another one, extremely small, with superparamagnetic behavior. The different crystal oxide sizes produce distinct metal-support interaction degrees which were symbolized by a sphere (low interaction) and an hemisphere (high interaction).
After reduction, in p-Fe/C(l.s) all the Fe3O4 was reduced to a-Fe0 due to the low metal-support interaction as a consequence of the big oxide particle sizes. On the other hand, in p-Fe/C(h.s) the lower size of the ferrimagnetic fraction and the presence of a sp fraction of Fe3O4 lead to a lower reduction degree. Perhaps, the sp fraction did not experiment any change and the ‘‘core’’ of the bigger, but interacting particles, remains as Fe3O4(sp) with a ‘‘shell’’ of a-Fe0. Another structural change is the diminution of the metallic crystal size in comparison with that of the original oxide. In cFe/C(h.s) this effect is compatible with that computed from
J.F. Bengoa et al. / Applied Catalysis A: General 325 (2007) 68–75
bulk density change for Fe3O4 to a-Fe0 of 26%. Instead, in cFe/C(l.s) this decrease is larger than 70%, therefore, another effect is present. Probably, the combination of an important density change and water evolve during the reduction step, with a low oxide-support interaction would lead to a breaking of the crystals. During FTS all a-Fe0 is carburized to x-Fe5C2 and sp carbides in c-Fe/C(l.s). Considering that the Fe0 crystals in the fresh reduced catalyst have a size of about 9 nm, the presence of carbides with two different sizes (magnetic and superparamagnetic fractions) would indicate that carburization has followed a sequence similar to that proposed by Xu and Bartholomew [16] for iron phase transformations during FTS. In this model, carburization begins with a nucleation step on the iron crystal surface leading to the sp carbide. In a similar way to c-Fe/C(l.s) we can suppose that the a-Fe0 ‘‘shell’’ in c-Fe/C(h.s) produces x-Fe5C2 and sp carbides. Besides, the Fe3O4(sp) ‘‘core’’ remains without any change considering that the syngas accessibility is hard. The ‘‘O’’ carbide, only present in used c-Fe/C(h.s), must become from the partial carburization of the Fe3O4(sp) located on the carbon support and accessible to syngas, since the presence of this oxide was only detected in fresh c-Fe/C(h.s). Taking into account the total species percentages obtained from the Mo¨ssbauer spectroscopy and the existence of a Fe3O4(sp) distribution sizes, it could be considered that: - a minor quantity of the smaller crystallites could not be carburized, remaining as Fe3O4(sp). The carburization suppression could be attributed to that there are fewer places for the carbon accessibility [17]; - the fraction with the bigger size would be transformed to xFe5C2; - the fraction with intermediate size leads to ‘‘O’’ carbide, as it was mentioned above, with a high non-stoichiometric degree. This carbide could not localize more C atoms due to its small size. It has a large number of carbon vacancies, therefore, it presents a high catalytic activity in agreement with the results obtained by Pijolat et al. [14]. Considering that Fe3O4 has very low activity in FTS [18] and the sp carbide amount is the same in both catalysts, the activity differences between both solids can be attributed to the ‘‘O’’ carbide presence. Therefore, we can conclude that this last phase must be more active than x-Fe5C2 in the present system. When the olefins/paraffins ratio was calculated as ðC2¼ þ C3¼ þ C4¼ Þ=ðC 2 þ C 3 þ C 4 Þ, no differences between both catalysts can be observed (2.02 for c-Fe/C(l.s) versus 1.94 for c-Fe/C(h.s)). On the other hand, the CO conversion to hydrocarbons was 1.6% for c-Fe/C(l.s) and 7.4% for c-Fe/ C(h.s), analyzed at the same space velocity and catalyst mass. It is well known that when conversion increases, the selectivity to olefins decreases [19], consequently, c-Fe/C(h.s) might produce lower olefins/paraffins ratio. Since both catalysts selectivities are similar, one can infer that c-Fe/C(h.s) is more selective towards light olefins. This result can be explained considering that incompletely carburized iron has a lower hydrogenation
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potential (higher olefins/paraffins ratio) than the fully carburized material [20]. 4. Conclusions Using glassy carbons as support of iron we have obtained different reduction degrees maintaining constant: -
the the the the
metallic crystal diameter; iron loading; chemical nature of the support; thermal pre-treatment and reduction.
Therefore, it has been possible to compare the activity differences only changing reduction degree variable. In this conditions, it has been demonstrated that the presence of an intermediate reduction species in the reduced fresh catalyst, Fe3O4(sp), leads to the formation of a non-stoichiometric carbide (‘‘O’’ carbide), less stable, more active and more selective towards olefins than x-Fe5C2 carbide. Therefore, we can conclude that it is not convenient to reduce completely the iron to metallic state, since this species leads only to x-Fe5C2 carbide during the FTS. Acknowledgements The authors acknowledge support of this work by Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Comisio´n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires, Universidad Nacional de La Plata, and Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (PICT No. 14-11267). References [1] G.B. Raupp, W.N. Delgass, J. Catal. 58 (1979) 337. [2] M.V. Cagnoli, S.G. Marchett, N.G. Gallegos, A.M. Alvarez, A.A. Yeramia´n, R.C. Mercader, J. Catal. 123 (1990) 21. [3] V.K. Jones, L.R. Neubauer, C.H. Bartholomew, J. Phys. Chem. 90 (1986) 4832. [4] E.E. Hucke, U.S. Patent 3,859,421 (1975). [5] C. Moreno-Castilla, O.P. Mahajan, P.L. Walker Jr., Carbon 18 (1980) 271. [6] R.E. Vandenberghe, E. De Grave, in: G.J. Long, F. Grandjean (Eds.), Mo¨ssbauer Spectroscopy Applied to Inorganic Chemistry, Plenun Press, New York, 1989 , p. 59 (Chapter 3). [7] R.S. Hardrove, W. Ku¨mdig, Solid State Commun. 8 (1970) 303. [8] F.J. Berry, S. Skinner, M.F. Thomas, J. Phys.: Condens. Matter 10 (1998) 215. [9] P.A. Dickof, P.J. Schurer, A.H. Morrish, Phys. Rev. B 22 (1980) 1. [10] S. Mørup, H. Topsøe, Appl. Phys. 11 (1976) 63. [11] D. Bianchi, S. Borcar, F. Teule-Gay, C.O. Bennet, J. Catal. 82 (1983) 442. [12] S.G. Marchetti, J.F. Bengoa, M.V. Cagnoli, A.M. Alvarez, N.G. Gallegos, A.A. Yeramia´n, R.C. Mercader, Meas. Sci. Technol. 7 (1996) 758. [13] S.Ch. Lin, J. Phillips, J. Appl. Phys. 58 (5) (1985) 1943. [14] M. Pijolat, V. Perrichon, P. Bussie`re, J. Catal. 107 (1987) 82. [15] E. Brauer-Grosse, G. Le Cae¨r, J. Phys. F: Met. Phys. 16 (1986) 399. [16] J. Xu, C.H. Bartholomew, J. Phys. Chem. B 109 (2005) 2392. [17] J.B. Butt, Catal. Lett. 7 (1990) 61. [18] R.J. O’Brien, L. Xu, R.L. Spicer, B.H. Davis, Energy Fuels 10 (1996) 921. [19] M. Ro¨per, in: W. Keim (Ed.), Catalysis in C1 Chemistry, Reidel Publishing Co., Dordrecht/Boston/Lancaster, 1983, pp. 41–88. [20] J.B. Butt, Catal. Lett. 7 (1990) 83.
Influence of intermediate iron reduced species in Fischer-Tropsch synthesis using Fe/C catalysts J.F. Bengoa, A.M. Alvarez, M.V. Cagnoli, N.G. Gallegos, S.G. Marchetti * Cindeca, Facultad Cs. Exactas, Facultad de Ingenierı´a, UNLP, CONICET, CICPBA, Calle 47 N8 257, 1900 La Plata, Argentina Received 1 December 2006; received in revised form 26 February 2007; accepted 11 March 2007 Available online 15 March 2007
Abstract Two Fe/carbon catalysts were prepared to study the influence of the intermediate iron species on the activity and selectivity in the FischerTropsch synthesis (FTS). The solids were characterized by XRD, CO chemisorption, Atomic Absorption, Nitrogen Adsorption and Mo¨ssbauer spectroscopy. Using two loading of consolidant agent, two supports with different specific surface areas were obtained. The resulting catalysts showed different iron species. The catalyst with lower reduction degree produces a non-stoichiometric iron carbide during FTS, which has a higher activity and light olefins selectivity. # 2007 Elsevier B.V. All rights reserved. Keywords: Fischer-Tropsch synthesis; Fe/C catalysts; Mo¨ssbauer spectroscopy; Iron carbides
1. Introduction The Fischer-Tropsch synthesis (FTS) has been previously studied using catalysts of Fe supported on different substrates, such as SiO2, Al2O3, MgO, zeolites. It is well known that in these systems it is very hard to reach a complete metal reduction when the iron loading is lower than 10% (w/w) [1,2]. In these cases, intermediates iron species are present on the fresh reduced catalyst jointly with the Fe0. Notwithstanding, the role played by these intermediate species, in the synthesis, is still unknown. To carry out the study above mentioned it is necessary to have a support that allow a complete iron reduction in order to be used as a reference. In this way, we have chosen a carbon support due to its low metal-support interaction. Since some reactions involved in FTS are ‘‘sensitive to structure’’, that is the activity and selectivity depend on the metal crystal size [3], it is necessary to prepare catalysts with similar Fe0 crystal diameters in order to avoid the influence of this parameter. With the aim to determine the effect of the intermediate iron reduced species on the activity and selectivity on the FTS, two Fe/C catalysts with different reduction degree were prepared,
* Corresponding author. Tel.: +54 221 4210711; fax: +54 221 4211353. E-mail address: [email protected] (S.G. Marchetti). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.03.012
catalytically tested and the results were correlated with its structural properties. 2. Experimental The catalyst supports were two glassy carbons prepared in the laboratory using the method outlined in the Hucke patent [4]. In a similar way as Moreno-Castilla et al. [5], we used furfuryl alcohol, as a binder (20 cm3), polyethylene glycol having an average molecular weight of 400 as pore former and dispersant (15 cm3), Triton X-100 (iso-octil phenoxy polyethoxi ethanol) as dispersing agent (15 cm3) and oxalic acid as consolidating agent or polimerization catalyst, in two different amounts: 6.00 and 8.42 g to obtain two carbons with low and high specific surface area: C(l.s) and C(h.s), respectively. The following procedure was used to prepare the carbons. The oxalic acid was dissolved in a mix composed of polyethylene glycol and Triton at 348 K. After cooling the resultant solution at 288 K, furfuryl alcohol was added drop by drop with gently constant stirring, in order to preserve bubble formation. After addition, the temperature was kept at 318 K during 2 h, to allow the beginning of the polymerization. After this step, the stirring was stopped and the following thermal program was setted: 323 K, 24 h, 343 K, 48 h, and 368 K, 72 h. The solids were then pyrolized in a N2 flow (25 cm3/min) using the following heating cycle: 373–573 K at 4 K/h,
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573–598 K at 2 K/h, 598–698 K at 5 K/h and from 698 to 873 K at 10 K/min, and kept at this temperature during 30 min. The iron was introduced into these systems by standard incipient wetness technique, using an alcoholic solution of Fe(NO3)39H2O at a high enough concentration to yield a catalyst with 5% (w/w) of Fe. The impregnated solids were calcined in dry N2 stream (60 cm3/min) from 298 to 698 K at 10 K/min and kept at this temperature during 8 h. The precursors with low and high specific surface area were called: p-Fe/C(l.s) and p-Fe/C(h.s), respectively. The precursors were reduced in H2 stream (60 cm3/min) following the thermal program described in Ref. [2]. The catalysts obtained were called c-Fe/C(l.s) and c-Fe/C(h.s). Mo¨ssbauer spectroscopy at 298 K (RT) and low temperatures, X-ray diffraction (XRD), CO chemisorption, atomic absorption and N2 adsorption (BET) were used to characterize the solids. The Mo¨ssbauer spectra were obtained in transmission geometry with a 512-channel constant acceleration spectrometer. A source of 57Co in Rh matrix of nominally 50 mCi was used. Velocity calibration was performed against a 6 mm-thick a-Fe foil. All isomer shifts (d) mentioned in this paper are referred to this standard. The temperature between 15 and 298 K was varied using a Displex DE-202 Closed Cycle Cryogenic System. The Mo¨ssbauer spectra were evaluated using a fitting commercial program named Recoil. Lorentzian lines were considered with equal widths for each spectrum component. The spectra were folded to minimize geometric effects. The precursors were characterized by XRD using Cu Ka radiation. CO chemisorption experiments were carried out in conventional volumetric static equipment [2]. Activity and selectivity measurements were carried out during 48 h in a fixed bed reactor at 543 K at atmospheric pressure, H2/CO ratio of 3/1, mass catalysts of 800 mg and a space velocity of 0.28 s1. The products were analyzed ‘‘on line’’ in a Konik chromatograph having a Flame Ionization Detector, using GS-Alumina PLOT (JW Scientific) column.
69
sample. This ratio have remains nearly constant after impregnation with iron salt and calcination. The total iron contents determined by atomic absorption are also shown in Table 1. The XRD patterns of both precursors (not shown) display three peaks at 2u = 30.28, 35.88 and 57.28. All of them are characteristics of Fe3O4 and g-Fe2O3. Therefore, this technique does not allow us to distinguish between both species. The Mo¨ssbauer spectra at 298 and 15 K of the precursors are shown in Fig. 1. In the Mo¨ssbauer spectra of p-Fe/C(l.s), two sextets can be seen at both temperatures. At RT the spectrum was fitted with two interactions which hyperfine parameters (Table 2a) can be assigned to Fe3+ in octahedral sites and Fe‘‘2.5+’’ in tetrahedral sites of Fe3O4 [6]. From the relative areas of both species it can be seen that the Fe3+/Fe2+ ratio is higher than that found for stoichiometric Fe3O4. This result indicates that this magnetite is partially oxidized. The magnetite Mo¨ssbauer spectra below the Verwey temperature (Tv) are very complex. Thus, below Tv (120 K), Fe3O4 exists in a multiple twinned monoclinic state. As a result the lines in the Mo¨ssbauer spectrum are broadened because of the distribution of angles between the magnetic field and the electric field gradient. The broadening can be removed by cooling a singlecrystals specimen through Tv in a magnetic field applied along [1 0 0] direction [7]. The spectrum obtained for such sample can be resolved into five components corresponding to one Asite Fe3+ ion, two B-site Fe3+ ions and two B-site Fe2+ ions. Berry et al. [8] reported that in presence and absence of applied fields at low temperature the spectrum of magnetite monocrystals is best fitted to five magnetic components. However, Vandenberghe and De Grave [6] considered that although the fittings give a reasonable agreement with the experimental line shape, it is difficult to assign the subspectra to particular Fe2+ and Fe3+ states. The situation is more complex with Fe3O4 polycrystalline. In this way some authors have used three components to fit its spectrum below Tv [9]. Taking into account this description, and considering that it is not the purpose of this investigation to discuss the properties of Fe3O4, the present spectrum at 15 K has been fitted with three interactions following the ideas of Vandenberghe and De Grave [6] without to assign the tetrahedral and octahedral sites to specific iron ions. The other two components resulted null in fitting in our catalyst (Table 2b). In the Mo¨ssbauer spectrum of p-Fe/C(h.s), two sextets partially overlapped and a central signal can be seen at 298 K. Besides, the background is markedly curved, suggesting the
3. Results and discussion 3.1. Characterization Table 1 shows the structural properties of the solids. The carbon prepared with lower amount of consolidating agent presents a specific surface area 90% lower than the other Table 1 Structural properties of the solids Properties 2
Sg (m /g) %Fe (w/w) CO chemisorption (mmol/g) Reduction to Fe0 (%) DVA (nm)
C(l.s)
C(h.s)
p-Fe/C(l.s)
p-Fe/C(h.s)
c-Fe/C(l.s)
c-Fe/C(h.s)
22 – 77
292 – 59
37 5.5 –
225 5.3 –
– 5.6 119
– 5.4 70
– –
– –
– –
– –
100 9
31 10
Sg: specific surface area; DVA: average diameter of de Fe0 crystals.
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Fig. 1. Mo¨ssbauer spectra at 298 and 15 K of p-Fe/C(h.s) and p-Fe/C(l.s).
presence of superparamagnetic relaxing species. The magnetic signal was fitted with two sextuplets assigned in a similar way that in p-Fe/C(l.s), to Fe3O4 (Table 2a). The magnetic hyperfine fields are lower than that shown by p-Fe/ C(l.s) indicating the existence of crystallites with lower size. The central signal was simulated with a relaxing sextuplet. Considering that its isomer shift is slightly higher than that corresponding to Fe3+, this signal could be assigned to superparamagnetic Fe3O4.
At 15 K the spectrum shows an asymmetric sextuplet that was fitted in the same way that p-Fe/C(l.s) at 15 K (Table 2b). This magnetic blocking confirms the existence of superparamagnetic Fe3O4 at room temperature and a lower crystal size in comparison with p-Fe/C(l.s). The thermal decomposition of Fe(NO3)39H2O generally produces a-Fe2O3 instead of magnetite. However, the use of nitrogen stream free of oxygen would produce vacancies in the oxide crystal framework, yielding Fe3O4. After the end of the
Table 2a Mo¨ssbauer parameters of p-Fe/C(l.s) and p-Fe/C(h.s) at 298 K Species
Parameters
p-Fe/C(l.s) at 298 K
p-Fe/C(h.s) at 298 K
3+
H (T) d (mm/s) 2e (mm/s) %
49.4 0.1 0.29 0.01 0.02 0.01 55 1
48.2 0.1 0.32 0.01 0.03 0.01 25 2
Fe‘‘2.5+’’ in B sites of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
46.1 0.1 0.62 0.01 0.01 0.01 45 1
45.1 0.2 0.50 0.01 0.01 0.01 38 3
Relaxing signal
H (T) d (mm/s) 2e (mm/s) %
Fe
in A sites of Fe3O4
– – – –
H: hyperfine magnetic field; d: isomer shift (all the isomer shifts are referred to a-Fe at 298 K); 2e: quadrupole shift. a Parameters held fixed in fitting.
45 a 0.4 0.1 0.1 0.2 37 3
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Table 2b Mo¨ssbauer parameters of p-Fe/C(l.s) and p-Fe/C(h.s) at 15 K Species
Parameters
p-Fe/C(l.s) at 15 K
p-Fe/C(h.s) at 15 K
First interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
51.3 0.1 0.34 0.01 0.01 0.01 41 7
51.1 0.1 0.28 0.01 0.01 0.01 37 2
Second interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
51.9 0.4 0.50 0.06 0.01 0.05 29 7
51.8 0.1 0.55 0.01 0.01 0.01 46 2
Third interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
48.1 0.5 0.79 0.03 0.01 0.05 30 5
48a 0.8a 0.03 0.04 17 1
H: hyperfine magnetic field; d: isomer shift (all the isomer shifts are referred to a-Fe at 298 K); 2e: quadrupole shift. a Parameters held fixed in fitting.
thermal treatment, the solid is exposed to air and a fraction of Fe3O4 is partially oxidized. Magnetite could be also produced by reduction of a-Fe2O3 with CO originated from the partial carbon oxidation during the iron salt decomposition. According with the Collective Magnetic Excitations Model (CMEM) [10], the magnetic hyperfine fields appear diminished with respect to the bulk value, when the crystal sizes are lower than a critical diameter. Assuming that a magnetic hyperfine field decrease lower than 1% from the bulk value (H0) is not significant, a critical diameter could be estimated. To carry out this estimation, we calculated a weighted average H0, assuming a relative population of Fe3+/Fe2+ = 2 for an ideal bulk magnetite. Using the anisotropy energy constant for uniaxial symmetry provides by Mørup and Topsøe [10] K = 0.90 0.3 106 erg/cm3, a critical size of 28 nm was estimated. In p-Fe/C(l.s) the weighted average H is not diminished, therefore we assumed that the Fe3O4 crystals are higher than 28 nm diameter. Instead, in p-Fe/C(h.s) the weighted average field is markedly lower than that corresponding to the bulk value. Applying CMEM, an average size of Fe3O4 crystals of 13.5 nm was obtained. Fig. 2 displays the Mo¨ssbauer spectrum at 298 K of c-Fe/ C(l.s). This shows a magnetic sextet corresponding to a-Fe0, according with its hyperfine parameters (Table 3a). The hydrogen treatment led to the total reduction of Fe3O4 to a-Fe0. A spectrum at low temperature was not necessary because the RT spectrum allowed us to determine the species presents without doubt. Fig. 2 also shows the Mo¨ssbauer spectra at 298 and 15 K of c-Fe/C(h.s). At RT the spectrum displays a magnetic sextet and several central signals intense and highly overlapped. The spectrum was interpreted in terms of a superposition of two sextets, one of them relaxing. The hyperfine parameters of the last one would correspond to superparamagnetic (sp) Fe3O4. The other sextet was assigned to a-Fe0 (Table 3a). When the temperature decreases to 15 K, the spectrum shows the presence of, at least, 10 peaks. This complex spectrum was fitted with six sextets. One of them corresponds to a-Fe0. The five remaining sextets belong to Fe3O4 as it was described for
Fig. 2. Mo¨ssbauer spectra at 298 and 15 K of c-Fe/C(h.s) and c-Fe/C(l.s).
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Table 3a Mo¨ssbauer parameters of c-Fe/C(h.s) and c-Fe/C(l.s) at 298 K Species Fe
0
Relaxing signal
Parameters
c-Fe/C(h.s)
c-Fe/C(l.s)
H (T) d (mm/s) 2e (mm/s) %
32.9 0.1 0.01 0.01 0.01 0.01 35 1
32.9 0.1 0.01 0.01 0.01 0.01 100
H (T) d (mm/s) 2e (mm/s) %
45 a 0.61 0.01 0.01 0.01 65 1
– – – –
H: hyperfine magnetic field; d: isomer shift (all the isomer shifts are referred to a-Fe at 298 K); 2e: quadrupole shift. a Parameters held fixed in fitting.
the precursors [6]. In this solid none component resulted null in fitting (Table 3b). This behavior could be attributed to a higher Fe2+ quantity as a consequence of the reduction treatment. Analyzing the above results, it is possible to conclude that we obtained two catalysts with different iron species and reduction degrees, starting from two precursors with the same iron species but with different crystal sizes. In order to obtain the metallic crystal sizes and the amount of Fe0 surface atoms, CO chemisorption at 193 K in 200 to 600 Torr pressure range, using the double-isotherm method, was carried out. The CO chemisorption at 193 K generally leds to crystal diameter values concordant with those obtained by
other methods [11]. The CO chemisorbed by the supports was determined and subtracted from the total uptake of the catalysts obtaining a net CO uptake of 42 mmol/g for c-Fe/C(l.s) and 11 mmol/g for c-Fe/C(h.s) (Table 1). Assuming spherical shape and no size distribution for the Fe0 crystals, the metallic crystal diameters were calculated. Similar sizes for both catalysts were obtained (Table 1). Taking into account that the oxide crystal size for p-Fe/C(l.s) is higher than that of the other precursor, the reduction treatment would be responsible of the crystal breaking, leading to a similar metallic crystal size for both catalysts. 3.2. Activity and selectivity measurements The total hydrocarbon productions per gram of catalyst were calculated at the steady state. Values of 10.8 1016 molecules/ (g s) for c-Fe/C(h.s) and 2.1 1016 molecules/(g s) for c-Fe/ C(l.s) were obtained. It can be seen that the former is five times more active than the last. We can discard any difference due to transport effects on these results since the pore diameter and pore volume of both precursors are very similar: 1.8 nm and 0.20 cm3/g for p-Fe/C(l.s) and 2.1 nm and 0.24 cm3/g for p-Fe/ C(h.s). Besides, any textural change during FTS, produced by carbon deposition, would result negligible due to the use of differential reactor.
Table 3b Mo¨ssbauer parameters of c-Fe/C(h.s) at 15 K Species
Parameters
c-Fe/C(h.s)
First interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
50.3 0.1 0.28 0.02 0.01 0.03 18 3
Second interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
50.8 0.1 0.55 0.03 0.04 0.02 27 3
Third interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
51.0a 0.80a 0.17a 31
Fourth interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
44.2 0.8 1.01 0.06 0.5 0.1 11 2
Fifth interaction of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
33.3 0.3 1.46 0.04 0.45 0.07 10 1
Fe 0
H (T) d (mm/s) 2e (mm/s) %
34.0 0.1 0.12 0.01 0.02 0.01 31 2
H: hyperfine magnetic field; d: isomer shift (all the isomer shifts are referred to a-Fe at 298 K); 2e: quadrupole shift. a Parameters held fixed in fitting.
Fig. 3. Mo¨ssbauer spectra at 25 K of used c-Fe/C(h.s) and c-Fe/C(l.s).
J.F. Bengoa et al. / Applied Catalysis A: General 325 (2007) 68–75
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Table 4 Mo¨ssbauer parameters of used c-Fe/C(l.s) and c-Fe/C(h.s) at 25 K Species
Parameters
c-Fe/C(l.s)
c-Fe/C(h.s)
I Site of x-Fe5C2
H (T) d (mm/s) 2e (mm/s) %
22 1 0.3 0.1 0.1 0.1 18 10
22 1 0.37a 0.12a 15 1
II Site of x-Fe5C2
H (T) d (mm/s) 2e (mm/s) %
25.2 0.6 0.38 0.06 0.3 0.1 38 9
III Site of x-Fe5C2
H (T) d (mm/s) 2e (mm/s) %
12.0 0.8 0.3 0.1 0.1 0.2 31 8
12.3 0.6 0.36 0.07 0.05 (a) 10 2
I Site of ‘‘O’’ carbide + II site of x-Fe5C2
H (T) d (mm/s) 2e (mm/s) %
– – – –
25.2 0.3 0.38 0.03 0.12a 19 3
II Site of ‘‘O’’ carbide
H (T) d (mm/s) 2e (mm/s) %
– – – –
17 1 0.4 0.1 0.05a 11 3
Site of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
– – – –
50.7 0.8 0.47 0.07 0a 18 4
Site of Fe3O4
H (T) d (mm/s) 2e (mm/s) %
– – – –
37 1 0.6 0.1 0.6 0.2 19 5
Superparamagnetic carbide
D d %
0.3 0.2 0.1 0.1 13 4
– – – –
0.36 0.08 0.27 0.05 81
H: hyperfine magnetic field; d: isomer shift (all the isomer shifts are referred to a-Fe at 298 K); 2e: quadrupole shift; D: quadrupole splitting. a Parameters held fixed in fitting.
Mo¨ssbauer spectra in controlled atmosphere of the used catalysts were taken at 25 K (Fig. 3), using a cell specially built by us [12], in order to explain why the catalyst with higher number of surface metallic iron atoms is less active. The Mo¨ssbauer spectra of the used catalysts at room temperature were not shown since the superparamagnetic relaxation inhibits to obtain any conclusion. The spectra at 25 K were very complex with several overlapped and broad lines. The used cFe/C(l.s) spectrum at 25 K was fitted with one doublet and three sextets. The former interaction was assigned to superparamagnetic iron carbide since its d and quadrupole splitting (D) values were very low to correspond to a sp oxide. The remainder three sextets correspond to the three different sites of stoichiometric carbide x-Fe5C2 according to their hyperfine parameters (Table 4) [13]. The used c-Fe/C(h.s) spectrum at 25 K was fitted with one doublet and six sextets. The former component was also assigned to sp iron carbide. The two sextets with the higher hyperfine fields and isomer shifts can be assigned to iron oxide (probably Fe3O4) [6]. The remainder four sextets have hyperfine parameters typical of x-carbide and a non-stoichio-
metric ‘‘O’’ carbide (Fe2+xC) [14]. The component with H 25 T is an overlapping of site I of ‘‘O’’ carbide and site II of x-carbide. We estimated the pxII population using the expression deduced by Brauer-Grosse and Le Cae¨r [15]: pxII ¼ 2ð pxI pxIII Þ where pxI , pxII and pxIII are the relative populations of I, II, and III sites of x-carbide. This value was subtracted from the sextet area with H 25 T and we obtained the site I population of ‘‘O’’ carbide. Finally, we calculated the ‘‘O’’ carbide stoichiometry (Fe2+xC), from the equation deduced by Pijolat et al. [14]: 6 O ¼ 3 pO I þ 2 pII 2þx O where pO I and pII are the relative populations of sites I and II of ‘‘O’’ carbide. The stoichiometry resulted close to: Fe2.4C, therefore this carbide is highly non-stoichiometric. Fig. 4 displays a speculative schematic representation of the different suggested changes and the steps followed by each
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Fig. 4. Schematic representation of the different steps followed by each solid to reach the structure of the ‘‘working’’ catalyst at the steady state.
solid to reach the structure of the ‘‘working’’ catalyst at the steady state. As it was described above the only species present in p-Fe/ C(l.s) and p-Fe/C(h.s) was Fe3O4. In p-Fe/C(l.s) there is only one fraction of ferrimagnetic Fe3O4, larger than 28 nm of diameter. Instead, in p-Fe/C(h.s) there are two fractions: one of them ferrimagnetic, of about 13.5 nm diameter and another one, extremely small, with superparamagnetic behavior. The different crystal oxide sizes produce distinct metal-support interaction degrees which were symbolized by a sphere (low interaction) and an hemisphere (high interaction).
After reduction, in p-Fe/C(l.s) all the Fe3O4 was reduced to a-Fe0 due to the low metal-support interaction as a consequence of the big oxide particle sizes. On the other hand, in p-Fe/C(h.s) the lower size of the ferrimagnetic fraction and the presence of a sp fraction of Fe3O4 lead to a lower reduction degree. Perhaps, the sp fraction did not experiment any change and the ‘‘core’’ of the bigger, but interacting particles, remains as Fe3O4(sp) with a ‘‘shell’’ of a-Fe0. Another structural change is the diminution of the metallic crystal size in comparison with that of the original oxide. In cFe/C(h.s) this effect is compatible with that computed from
J.F. Bengoa et al. / Applied Catalysis A: General 325 (2007) 68–75
bulk density change for Fe3O4 to a-Fe0 of 26%. Instead, in cFe/C(l.s) this decrease is larger than 70%, therefore, another effect is present. Probably, the combination of an important density change and water evolve during the reduction step, with a low oxide-support interaction would lead to a breaking of the crystals. During FTS all a-Fe0 is carburized to x-Fe5C2 and sp carbides in c-Fe/C(l.s). Considering that the Fe0 crystals in the fresh reduced catalyst have a size of about 9 nm, the presence of carbides with two different sizes (magnetic and superparamagnetic fractions) would indicate that carburization has followed a sequence similar to that proposed by Xu and Bartholomew [16] for iron phase transformations during FTS. In this model, carburization begins with a nucleation step on the iron crystal surface leading to the sp carbide. In a similar way to c-Fe/C(l.s) we can suppose that the a-Fe0 ‘‘shell’’ in c-Fe/C(h.s) produces x-Fe5C2 and sp carbides. Besides, the Fe3O4(sp) ‘‘core’’ remains without any change considering that the syngas accessibility is hard. The ‘‘O’’ carbide, only present in used c-Fe/C(h.s), must become from the partial carburization of the Fe3O4(sp) located on the carbon support and accessible to syngas, since the presence of this oxide was only detected in fresh c-Fe/C(h.s). Taking into account the total species percentages obtained from the Mo¨ssbauer spectroscopy and the existence of a Fe3O4(sp) distribution sizes, it could be considered that: - a minor quantity of the smaller crystallites could not be carburized, remaining as Fe3O4(sp). The carburization suppression could be attributed to that there are fewer places for the carbon accessibility [17]; - the fraction with the bigger size would be transformed to xFe5C2; - the fraction with intermediate size leads to ‘‘O’’ carbide, as it was mentioned above, with a high non-stoichiometric degree. This carbide could not localize more C atoms due to its small size. It has a large number of carbon vacancies, therefore, it presents a high catalytic activity in agreement with the results obtained by Pijolat et al. [14]. Considering that Fe3O4 has very low activity in FTS [18] and the sp carbide amount is the same in both catalysts, the activity differences between both solids can be attributed to the ‘‘O’’ carbide presence. Therefore, we can conclude that this last phase must be more active than x-Fe5C2 in the present system. When the olefins/paraffins ratio was calculated as ðC2¼ þ C3¼ þ C4¼ Þ=ðC 2 þ C 3 þ C 4 Þ, no differences between both catalysts can be observed (2.02 for c-Fe/C(l.s) versus 1.94 for c-Fe/C(h.s)). On the other hand, the CO conversion to hydrocarbons was 1.6% for c-Fe/C(l.s) and 7.4% for c-Fe/ C(h.s), analyzed at the same space velocity and catalyst mass. It is well known that when conversion increases, the selectivity to olefins decreases [19], consequently, c-Fe/C(h.s) might produce lower olefins/paraffins ratio. Since both catalysts selectivities are similar, one can infer that c-Fe/C(h.s) is more selective towards light olefins. This result can be explained considering that incompletely carburized iron has a lower hydrogenation
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potential (higher olefins/paraffins ratio) than the fully carburized material [20]. 4. Conclusions Using glassy carbons as support of iron we have obtained different reduction degrees maintaining constant: -
the the the the
metallic crystal diameter; iron loading; chemical nature of the support; thermal pre-treatment and reduction.
Therefore, it has been possible to compare the activity differences only changing reduction degree variable. In this conditions, it has been demonstrated that the presence of an intermediate reduction species in the reduced fresh catalyst, Fe3O4(sp), leads to the formation of a non-stoichiometric carbide (‘‘O’’ carbide), less stable, more active and more selective towards olefins than x-Fe5C2 carbide. Therefore, we can conclude that it is not convenient to reduce completely the iron to metallic state, since this species leads only to x-Fe5C2 carbide during the FTS. Acknowledgements The authors acknowledge support of this work by Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Comisio´n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires, Universidad Nacional de La Plata, and Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (PICT No. 14-11267). References [1] G.B. Raupp, W.N. Delgass, J. Catal. 58 (1979) 337. [2] M.V. Cagnoli, S.G. Marchett, N.G. Gallegos, A.M. Alvarez, A.A. Yeramia´n, R.C. Mercader, J. Catal. 123 (1990) 21. [3] V.K. Jones, L.R. Neubauer, C.H. Bartholomew, J. Phys. Chem. 90 (1986) 4832. [4] E.E. Hucke, U.S. Patent 3,859,421 (1975). [5] C. Moreno-Castilla, O.P. Mahajan, P.L. Walker Jr., Carbon 18 (1980) 271. [6] R.E. Vandenberghe, E. De Grave, in: G.J. Long, F. Grandjean (Eds.), Mo¨ssbauer Spectroscopy Applied to Inorganic Chemistry, Plenun Press, New York, 1989 , p. 59 (Chapter 3). [7] R.S. Hardrove, W. Ku¨mdig, Solid State Commun. 8 (1970) 303. [8] F.J. Berry, S. Skinner, M.F. Thomas, J. Phys.: Condens. Matter 10 (1998) 215. [9] P.A. Dickof, P.J. Schurer, A.H. Morrish, Phys. Rev. B 22 (1980) 1. [10] S. Mørup, H. Topsøe, Appl. Phys. 11 (1976) 63. [11] D. Bianchi, S. Borcar, F. Teule-Gay, C.O. Bennet, J. Catal. 82 (1983) 442. [12] S.G. Marchetti, J.F. Bengoa, M.V. Cagnoli, A.M. Alvarez, N.G. Gallegos, A.A. Yeramia´n, R.C. Mercader, Meas. Sci. Technol. 7 (1996) 758. [13] S.Ch. Lin, J. Phillips, J. Appl. Phys. 58 (5) (1985) 1943. [14] M. Pijolat, V. Perrichon, P. Bussie`re, J. Catal. 107 (1987) 82. [15] E. Brauer-Grosse, G. Le Cae¨r, J. Phys. F: Met. Phys. 16 (1986) 399. [16] J. Xu, C.H. Bartholomew, J. Phys. Chem. B 109 (2005) 2392. [17] J.B. Butt, Catal. Lett. 7 (1990) 61. [18] R.J. O’Brien, L. Xu, R.L. Spicer, B.H. Davis, Energy Fuels 10 (1996) 921. [19] M. Ro¨per, in: W. Keim (Ed.), Catalysis in C1 Chemistry, Reidel Publishing Co., Dordrecht/Boston/Lancaster, 1983, pp. 41–88. [20] J.B. Butt, Catal. Lett. 7 (1990) 83.