Cobalt and iron supported on carbon nanofibers as catalysts for Fischer–Tropsch synthesis

Fuel Processing Technology 128 (2014) 417–424

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Cobalt and iron supported on carbon nanofibers as catalysts for Fischer–Tropsch synthesis José Antonio Díaz a,⁎, Hasti Akhavan b,c, Amaya Romero d, Alba María Garcia-Minguillan a, Rubí Romero e, A. Giroir-Fendler b,c, Jose Luis Valverde a a

Facultad de Ciencias y Tecnologías Químicas, Departamento de Ingeniería Química, Universidad de Castilla la Mancha, Avenida de Camilo José Cela 12, 13071 Ciudad Real, Spain Université Claude Bernard Lyon 1, Villeurbanne F-69622, France CNRS, UMR 5256, IRCELYON, 2 Avenue Albert Einstein, Villeurbanne F-69622, France d Escuela de Ingenieros Agrónomos, Departamento de Ingeniería Química, Universidad de Castilla la Mancha, Avenida de Camilo José Cela 12, 13071 Ciudad Real, Spain e Centro Conjunto de Investigación en Química Sustentable, UAEMex-UNAM, km 14.5 Toluca–Altacomulco Road, 50200 Toluca, Mexico b c

a r t i c l e

i n f o

Article history: Received 14 May 2014 Received in revised form 14 July 2014 Accepted 4 August 2014 Available online xxxx Keywords: Fischer–Tropsch Carbon nanofibers Cobalt Iron Bimetallic catalysts

a b s t r a c t Cobalt and/or iron supported on carbon nanofibers were prepared and used as monometallic or bimetallic catalysts for Fischer–Tropsch synthesis at 523 K and 20 bar. Catalysts were characterized by ICP, N 2 adsorption–desorption, TPR, XRD and XPS. Characterization results revealed that cobalt and iron had a synergetic effect: cobalt particles were better dispersed in presence of iron, and the latter was reduced to Fe0 in a higher extent due to the presence of the former. Catalytic results revealed that cobalt content played an important role in the catalytic conversion of CO. This way, the higher the content in cobalt, the higher the CO conversions were observed. Thus, sample 15Co/CNF presented the highest CO conversion. However, the presence of iron in bimetallic catalysts avoided an excessive production of CH4. The bimetallic sample with the highest Co loading (10Co5Fe/CNF) was the most active catalyst for the FTS reaction, because it led to the highest yield of longchained hydrocarbons. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The Fischer–Tropsch synthesis (FTS) is a heterogeneously catalyzed polymerization process that converts syngas (CO and H2) into a wide variety of hydrocarbons, which constitutes a promising route for the production of clean liquid fuels. The product distribution in the FTS is very broad. Consequently, many studies have been carried out with the aim of controlling and limiting the product selectivity. Nickel-, ruthenium-, iron- and cobalt-based catalysts have been actually used in this process, but the latter (Co- and Fe-based catalysts) are the most widely studied. Although Ni is relatively inexpensive, it produces short-chain alkanes. Ru shows good catalytic properties but its annual world supply cannot even fulfill the requirements of an average plant [1]. Under the same experimental conditions, Fe-based catalysts lead to the formation of light hydrocarbons and small amounts of CH4 in comparison to Co-based ones. On the other hand, Co-based catalysts show high catalytic activity and are suitable for the production of middle distillates and waxes, but they are more expensive. In recent years, the preparation of bimetallic Co:Fe catalysts to be used in FTS has gained increased interest. It has been reported that the

⁎ Corresponding author. E-mail address: [email protected] (J.A. Díaz).

http://dx.doi.org/10.1016/j.fuproc.2014.08.005 0378-3820/© 2014 Elsevier B.V. All rights reserved.

simultaneous use of Fe and Co gives rise to a synergistic effect between the two active phases [2–4]. Co:Fe metal mixtures have traditionally been supported on typical FTS supports such as silica [3,5,6], titania [7,8] and alumina [4]. However, several studies related to the preparation of bimetallic catalysts supported on carbonaceous materials and their use in FTS can be found in the literature [9]. Carbon materials have shown special properties (high mechanical strength, chemical inertness, and possibility of being used with both acidic and basic solutions) that allow them to be used as catalyst supports [10–13]. Among these materials, amorphous ones have proven to be ill-defined and thus they are not appropriate as FTS catalysts [14]. Among carbon materials, structured ones such as carbon nanofibers (CNF), which are based on ordered parallel graphene layers arranged in a specific conformation, could be good candidates to be used as catalyst supports in this process. CNF are believed to be less prone than inorganic supports to coke formation and, in case of deactivation, the active phase would be relatively easy to recover [15]. Moreover, CNF presented some defects in their structure, leading to higher porosity than that observed in other structured carbon materials such as carbon nanotubes (CNT). Very few studies related to the use of CNF as a catalyst support in the FTS processes have been reported in the literature. Den Breejen et al. [16] and Bezemer et al. [14] compared CNF and SiO2 as cobalt catalyst

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supports in the FTS at 493 K and 1 bar. In both cases, Co/SiO2 showed a slightly lower FTS activity and higher selectivity for long-chain hydrocarbons (C5 +) than Co/CNF. Yu et al. [17] compared Platelet and Fishbone type CNF-based catalysts with Al2O3 systems at 493 K and 20 bar. It was confirmed that Platelet type CNF-based catalysts had high activity and selectivity, the catalytic activity being comparable to that of Al2O3-based ones. Finally, Bezemer et al. [18] studied the influence of different promoters present in CNF-supported catalysts. They found that the presence of MnO in the catalysts led to positive effects on both activity and selectivity. In this work, catalysts with different contents in Co and Fe supported on CNF were prepared. The influence of the Co:Fe ratio in the resulting catalysts on their catalytic activities was studied in order to maximize fuel production in the FTS. To the best of our knowledge, no study related to the use of bimetallic catalysts supported on CNF for the FTS has been reported until now. 2. Experimental CNF were prepared by the catalytic decomposition of ethylene over a Ni/SiO2 catalyst at 873 K according to the procedure described elsewhere [19]. Once CNF were synthesized, they were subjected to a demineralization treatment with HF (48% v/v) for 15 h with vigorous stirring, to remove any residual metal that could contribute to the subsequent reaction. CNF-supported cobalt and iron catalysts with ca. 15 wt.% of metal (namely 15Co/CNF and 15Fe/CNF, respectively) were prepared by the incipient wetness impregnation method using aqueous solutions of Co(NO3)2·6H2O and Fe(NO3)3·9H2O (Merck), respectively. Three bimetallic Co–Fe catalysts, containing different amounts of both metals (namely 10Co5Fe/CNF, 7Co7Fe/CNF and 5Co10Fe/CNF) were also prepared using the same procedure. The total amount of metal in each catalyst was close to 15 wt.%. The catalyst support was placed in a glass vessel and kept under vacuum at room temperature for 2 h to remove water and other impurities adsorbed on the structure. A known volume of an aqueous solution (the minimum amount required to wet the solid) was then poured over the sample. In the case of bimetallic samples, two aqueous solutions, one for each metal precursor, were prepared and poured simultaneously over the support. After 2 h, the solvent was removed by evaporation under vacuum at 363 K. The final catalysts were dried at 403 K overnight and sieved into a batch with an average diameter of 254 μm. Surface area/porosity measurements were conducted using a Quantachrome Quadrasorb SI apparatus with N2 as the sorbate at 77 K. All samples were outgassed prior to analysis at 453 K under vacuum (1°10− 2 Torr) for 12 h. Total specific surface areas and mesoporosities were determined by the multipoint BET and the Barret–Joyner–Halenda (BJH) methods, respectively. XRD analyses were conducted with a Philips X'Pert instrument using nickel-filtered Cu-Kα radiation. Samples were scanned at a rate of 0.02° step− 1 over the range 5° ≤ 2θ ≤ 90° (scan time = 2 s∙step− 1) and the

corresponding diffractograms were compared with those of the PDFICDD references. Temperature-programmed reduction (TPR) experiments were conducted in a commercial Micromeritics AutoChem 2950 HP unit with TCD detection. Samples (ca. 0.15 g) were loaded in a U-shaped quartz reactor and ramped from room temperature to 1173 K (5 K min− 1) under a reducing atmosphere (17.5% v/v H2/Ar, 60 cm3 min− 1). X-ray photoelectron spectroscopy (XPS) analyses were performed in an AXIS Ultra DLD spectrometer with a monochromatized Al-Kα X-ray source (1486.6 eV) with pass energy of 40 eV and spot size aperture of 300 × 700 μm. C1s (binding energy of 284.6 eV) of adventitious carbon was used as the reference. In the case of reduced catalysts, samples were reduced in situ prior to analysis. Cobalt and/or iron content were measured by using an inductively coupled plasma spectrometer (ICP, model Liberty Sequential, Varian). Samples were diluted to 1:1 v/v using 4N HNO3, in order to ensure the total solubility of the metal. FTS catalytic activity was tested in a stainless steel fixed bed reactor (9 mm i.d. × 305 mm length) provided with a porous plate (2 μm pore size). Initially, the catalyst bed (2 g) was activated at 623 K (heating ramp of 5 K min−1) for 5 h with a flow rate of 100 Nml min− 1 of ultrapure hydrogen. The reactor was then cooled and pressurized up to reaction conditions (523 K and 20 bar, respectively) under a N2 atmosphere (100 Nml min−1). A flow of a mixture of CO, H2 and N2 (CO:H2:N2 volume ratio of 3:6:1, N2 used as the internal standard) was established through the reactor during 18 h, with a constant gas hourly space velocity (GHSV) of 3000 Nml g−1 h−1. The product stream was cooled in a wax trap (T ≈ 393 K) to retain the waxes and then in a liquid–liquid–gas separator, which consisted of a Peltier cell (T ≈ 278 K). Liquid hydrocarbon samples were analyzed off-line by capillary GC (VARIAN 430) equipped with a flame ionization detector (FID). C1–C3 hydrocarbons, unreacted CO and H2, N2 and CO2 were analyzed on-line by GC (VARIAN 4900). Calibrations were performed with standard samples for data quantification. The CO reaction rate and the yield of long-chain hydrocarbons (C5+) were calculated using the following equations: rate ¼

NCO;converted ðmol= minÞ
mcobaltþiron ðmolÞ

yieldC 5þ ¼

NCO;converted ðmol= minÞ  SC 5þ : NCO;feed ðmol= minÞ

3. Results and discussion 3.1. Textural properties The physicochemical properties of the supports and catalysts used in this work are listed in Table 1. Nitrogen adsorption–desorption

Table 1 Physicochemical properties of the carbonaceous support and prepared catalysts. Sample

CNF

15Co/CNF

10Co5Fe/CNF

7Co7Fe/CNF

5Co10Fe/CNF

15Fe/CNF

Metal loading (% wt.) (Co/Fe) BET surface area (m2 g−1) Mesopore surface area (m2 g−1) Total pore volume (cm3 g−1) Mesopore volume (cm3 g−1) Average pore diameter (nm) XRD-average metal particle size (nm)a TPR-Tmax (K)b

−/− 275 219 0.46 0.43 6.7 – C: 864

15.0/− 231 134 0.30 0.26 5.3 25 Co: 573 Fe: – C: 794

8.9/5.5 217 143 0.29 0.25 5.3 – Co: 605 Fe: – C: 834

6.5/6.7 260 158 0.30 0.25 4.7 – Co: 612 Fe: 764 C: 859

5.2/9.4 257 155 0.30 0.25 4.7 – Co: 619 Fe: 762 C: 877

−/15.8 300 176 0.37 0.31 4.8 60 Co: – Fe: 783 C: 894

a b

Calculated from the XRD main peak in each case, only possible in monometallic samples. Maximum reduction temperatures of the following phases: Co: Co3O4 + CoO → Co0. Fe: Fe3O4 → Fe0. C: CNF gasification.

J.A. Díaz et al. / Fuel Processing Technology 128 (2014) 417–424

isotherms corresponding to both the support (CNF) and the prepared catalysts are plotted in Fig. 1. It can be observed that all isotherms correspond to type IV according to the IUPAC classification, which is characteristic of mesoporous materials, with a significant volume increase in the middle relative pressure range [20]. The observed H3 type hysteresis loop (IUPAC classification) is associated with nitrogen capillary condensation [20]. It is worth noting that the shape of the catalyst isotherms remained almost identical to that of the CNF support, indicating that the carbon support structure did not suffer any significant change after metal incorporation. The CNF support was shown to have pores of different sizes (see Fig. 1). The largest mesopores (N 4 nm radius) were accompanied by smaller ones (around 2–4 nm radius). The larger pores were related to interstices between the interlaced filaments accumulated optionally and chaotically, whereas the smaller pores might be associated with the surface roughness of CNF [21,22].

200

After the metal incorporation, the total pore volumes of all catalysts decreased, which could be a consequence of partial pore blockage by metal particles [23–25]. However, there were marked differences between the monometallic catalysts. Sample 15Co/CNF experienced a strong decrease in BET surface area (16%) and pore volume (35%) if compared to parent CNF. On the contrary, in sample 15Fe/CNF, pore volume appeared to be only 20% lower and BET surface area higher than those of parent CNF. Therefore, it can be concluded that blockage by Co particles would be predominant in smaller pores, whereas that of Fe ones would take place in larger pores. Average pore diameter values obtained confirmed this fact. Thus, pore diameter in sample 15Co/CNF was higher than that observed for sample 15Fe/CNF. Regarding the textural properties of bimetallic catalysts, it is important to note that the results generally matched with those obtained for monometallic catalysts, i.e., the higher the cobalt content, the higher the blockage of small pores was. This can be confirmed by

0.6

0.6

0.5

0.5

0.4

0.4

dV/d(log r)

250

dV/d(log r)

300

0.3 0.2 0.1

150

0.0

10

100

100

Adsorption Desorption

0.6

0.6

0.5

0.5

0.4

0.4

0.3 0.2 0.1

150

10

CNF

dV/d(log r)

dV/d(log r)

Adsorption Desorption

7Co7Fe/CNF

0.3 0.2 0.1

0.0 1

10

0.0

100

1

Pore radius (nm)

10

100

Pore radius (nm)

100 50

200

Adsorption Desorption 5Co10Fe/CNF

15Co/CNF

0.6

0.6

0.5

0.5

0.4

0.4

dV/d(log r)

250

Adsorption Desorption

dV/d(log r)

3

1

Pore radius (nm)

50

Volume N 2 adsorbed (cm /g)

0.2

Pore radius (nm)

100

200

0.3

0.1

0.0 1

250

419

0.3 0.2 0.1

150

0.3 0.2 0.1

0.0

0.0 1

10

100

1

Pore radius (nm)

10

100

Pore radius (nm)

100 50 0 0.0

Adsorption Desorption

Adsorption Desorption 10Co5Fe/CNF 0.2

0.4

0.6

0.8

0.0 1.0

0.2

0.4

0.6

Relative pressure (P/P0) Fig. 1. Nitrogen adsorption–desorption isotherms and mesopore distributions of the support and prepared catalysts.

15Fe/CNF 0.8

1.0

420

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comparing the textural properties of catalysts 5Co10Fe/CNF and 10Co5Fe/CNF.

reflection of the Co and Fe alloy. However, in the present study it was not possible to confirm the presence of any Co–Fe alloy by XRD.

3.2. Crystalline structure and active metal phases

3.3. Metal surface structures before reduction

The crystalline character of the carbonaceous support and the final reduced catalysts (623 K, 5 h) was evaluated by XRD (see Fig. 2). All samples can be considered as graphitized carbon materials since (002) and, in a lesser extent, (100) diffraction peaks of graphite at 26° and 42°, respectively, are clearly recognized (PDF 056-0156). These results demonstrate that the crystallinity of the CNF support was retained after metal incorporation. Regarding the active phases of the catalysts, it was observed that the monometallic catalysts, 15Co/CNF and 15Fe/CNF, showed the corresponding metallic phases: cobalt (Co0, reflections at 2θ ≈ 44°, 52° and 77°, PDF 015-0806) and iron (Fe0, reflections at 2θ ≈ 44° and 65°, PDF 006-0696), respectively. Furthermore, sample 15Fe/CNF presented a small peak at 2θ ≈ 35°, which was attributed to the Fe2O3 hematite reflection (PDF 033-0664). The average metal particle size of samples 15Co/CNF and 15Fe/CNF was evaluated from their 44° peak using the Debye–Scherrer formula (Table 1). Calculations revealed that iron particles in sample 15Fe/CNF were larger than cobalt particles in sample 15Co/CNF. These results fit well N2 adsorption–desorption analyses. On the other hand, bimetallic samples presented similar XRD profiles as monometallic ones. It was impossible to differentiate between metallic Co and Fe because they reflect at the same angle (2θ ≈ 44°). However, it can be observed that the higher the iron content, the narrower and higher 44° peak was. According to previous studies [3,5,9], bimetallic catalysts would present two independent peaks corresponding to Co0 and Fe0 along with a third peak assigned to the

The chemical state and the relative abundance of cobalt and iron oxides on the catalyst surface were evaluated by X-ray photoelectron spectroscopy (XPS). Co and Fe spectra of the monometallic catalysts are shown in Fig. 3. The Co 2p3/2 and Fe 2p3/2 core levels were close to those of Co3O4 (BE = 780.4 eV) and Fe2O3 (BE = 710.4 eV) phases, respectively. It is important to note that, although other phases (such as CoO and FeO) could be present in a lesser extent, it was not possible to apply any other curve-fitting procedure to evaluate the proportions of other phases. The results of the quantitative surface analysis of the catalysts are given in Table 2. It can be seen that the Fe atomic concentration in the surface of sample 15Fe/CNF was slightly higher than the Co atomic concentration in the surface of sample 15Co/CNF. Nevertheless, significant differences were observed between the bimetallic catalysts. For example, the sample with the highest Fe loading (5Co10Fe/CNF) showed a low concentration of surface active sites whereas the sample with the highest Co content (10Co5Fe/CNF) showed the highest concentration of such sites. It is clearly seen that the higher the amount of Fe in the bimetallic catalyst, the lower the concentration of surface active sites in the resulting catalyst was.

(b) (0 0 2)C

(1 1 1)Co (1 1 0)Fe (1 1 0)Fe2O3

(2 0 0)Co

(2 0 0)Fe (2 2 0)Co

10Co5Fe/CNF

Intensity (a.u.)

7Co7Fe/CNF

5Co10Fe/CNF

(a) 15Co/CNF

15Fe/CNF

CNF 20

40

60

80

2 Fig. 2. XRD patterns of the support and reduced monometallic samples (a) and reduced bimetallic samples (b).

3.4. Metal reducibility TPR profiles for each catalyst are shown in Fig. 4a (monometallic catalysts) and b (bimetallic catalysts). Catalyst 15Fe/CNF showed four different hydrogen consumption peaks. The first three peaks, which appeared at 597, 688 and 783 K, can be assigned to the following consecutive reduction of the iron precursor salt: Fe(NO3)3 → Fe2O3 → Fe3O4 → Fe0 [7]. Catalyst 15Co/CNF showed three hydrogen consumption peaks. The first peak, which appears at around 509 K, was attributed to the decomposition of a proportion of the cobalt nitrate precursor (which remained in the catalyst after the drying step) into CoO. Note that the majority of the salt precursor was transformed into CoO and Co3O4 during the drying step at 403 K. The second peak, at around 573 K, was associated with the reduction of the above mentioned Co oxides to Co0 [26]. Finally, samples 15Fe/CNF and 15Co/CNF showed an additional peak, which was also observed in the support, which was related to the gasification of the carbonaceous substrate [27]. Note that the temperature at which the gasification of the support in catalyst 15Fe/CNF took place was higher than that in sample 15Co/CNF. This effect has also been observed in other carbonaceous supports [9], and can be attributed to the presence of metallic Fe, which did not favor the support gasification. Finally, CNF support presented a negative TPR peak at 1000 K, which can be attributed to carbon gasification to methane, as reported elsewhere [27]. In addition, it is worth mentioning that Fe oxide reduction steps were overlapped by that of the Co oxide ones. The first hydrogen consumption peak, which is related to the reduction of Co oxides, was located at a temperature slightly higher (605–619 K) than that observed for the monometallic catalyst (573 K). In other words, the presence of Fe (in Co:Fe samples) would led to the Co reduction to be more difficult, as reported elsewhere [7,9]. This fact was attributed to the presence of iron, which could promote the dispersion of cobalt particles in bimetallic catalysts, leading to smaller particles that strongly interacted with the carbonaceous support. On the other hand, the Fe oxide reduction peak only became significant in samples with higher Fe loadings (catalysts 7Co7Fe/CNF and 5Co10Fe/CNF). This peak appeared at lower temperatures (762–764 K) if compared to that observed for the monometallic Fe catalyst (783 K). These results are in good agreement with those reported by Duvenhage and Coville [7]. They demonstrated that the presence of Co assisted the reduction of Fe.

J.A. Díaz et al. / Fuel Processing Technology 128 (2014) 417–424

780.5 eV = Co3O4

421

711 eV = Fe2O3 15Fe/CNF

Intensity (cps)

15Co/CNF

10Co5Fe/CNF

10Co5Fe/CNF

5Co10Fe/CNF

7Co7Fe/CNF

7Co7Fe/CNF

5Co10Fe/CNF

810 805 800 795 790 785 780 775 740 770 735 730 725 720 715 710 705 700

Binding energy (eV) Fig. 3. XPS patterns of the prepared catalysts before reduction.

According to the results showed in this section, 623 K was chosen as a suitable reduction temperature to ensure the maximum metal activation without affecting the surface properties of the support. 3.5. Metal surface structures after reduction The chemical state and the relative abundance of cobalt and iron species over the catalyst surface, after reduction at 623 K, were also evaluated by X-ray photoelectron spectroscopy (XPS). Co and Fe spectra of the catalysts are shown in Fig. 5. The binding energy values show that in all cases the Co precursors/oxides were reduced to Co0 (BE = 778.3 eV). Two different situations were observed in the reduction of the Fe precursor. In catalysts 15Fe/CNF 5Co10Fe/CNF, the Fe precursor was mainly reduced to Fe2O3 (BE = 710.4 eV) but also to Fe0. On the other hand, in catalysts with high Co loading (7Co7Fe/CNF and 10Co5Fe/CNF), the iron precursor was essentially reduced to Fe0 (BE = 707 eV). These results are in agreement with those obtained by

TPR, which would confirm that in the bimetallic catalysts the presence of Co assisted the Fe reduction. Anyway, Fe was not completely reduced after the activation procedure at 623 K. It can be seen from the results in Table 2 that regardless of the catalyst the surface atomic Co concentration clearly decreased after reduction (in fact, surface cobalt was not clearly observed in sample 5Co10Fe/CNF). In general, the surface concentration of atomic Fe increased after reduction in all samples except 5Co10Fe/CNF. As summarized, during the reduction step a re-dispersion and a subsequent migration of the Co metal particles into the catalyst pores should occur. On the contrary, during the same step Fe metal particles would migrate to the catalyst external surface. Consequently with XPS analyses of the catalysts before their reduction, catalyst 10Co5Fe/CNF contained the highest amounts of surface cobalt and iron. Finally, it is important to note that Co–Fe alloys were not observed in the XPS profiles after reduction. This means that Co–Fe alloys were not formed during the reduction step. 3.6. Catalytic activity

Table 2 XPS results for the catalysts before and after reduction. Sample

Quantitative analysis (% atomic)

Binding energies (eV)

Co

Fe

Co2p3/2

Fe2p3/2

Before reduction 15Co/CNF 10Co5Fe/CNF 7Co7Fe/CNF 5Co10Fe/CNF 15Fe/CNF

1.76 3.31 0.75 0.47 –

– 2.77 1.16 1.20 2.00

780.50 780.50 780.60 780.60 –

– 711.30 711.20 711.20 711.10

After reductiona 15Co/CNF 10Co5Fe/CNF 7Co7Fe/CNF 5Co10Fe/CNF 15Fe/CNF

1.22 2.70 0.58 – –

– 3.76 1.30 0.72 2.82

778.40 778.60 778.60 – –

– 707.10 707.20 710.50 710.70

a

(Reduction conditions: 623 K, 5 h, 5 K min−1).

Fischer–Tropsch tests were performed at 523 K and 20 bar. The catalytic results, including the catalytic activity and product selectivity obtained with the different catalysts, are shown in Table 3. The main products obtained were CH4, CO2, C2H6, C3H8 and long-chain hydrocarbons (C5 +). These latter products included liquid hydrocarbons and waxes. All the catalytic tests were performed twice, and the relative errors obtained in all cases were less than 10%. Experiments without cobalt/iron-impregnated CNF did not show any relevant catalytic activity. It can be observed that the higher the amount of Co in the catalysts, the higher the reaction rate was. Thus, the CO catalytic activities were much higher for samples 15Co/CNF and 10Co5Fe/CNF than those obtained with other catalysts. On the other hand, other two factors should be considered: – Fe oxide phase (mostly Fe2O3) in samples 5Co10Fe/CNF and 15Fe/CNF appeared to be less active for the FTS process than Fe0

422

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(a)

Activation

TCD Signal (a.u.)

CoO to Co

(b)

0

10Co5Fe/CNF

Co(NO3)2 to CoO

15Co/CNF

Activation

0

Fe2O3 to Fe3O4

7Co7Fe/CNF

Fe3O4 to Fe

Fe(NO3)3 to Fe2O3

5Co10Fe/CNF

15Fe/CNF 300

600

1200 300

900

600

900

1200

Temperature (K) Fig. 4. TPR profiles of the support (dash curve) and monometallic (a) and bimetallic catalysts (b).

present in samples 10Co5Fe/CNF and 7Co7Fe/CNF. It is generally accepted that in Fe-based catalysts, Fe carbides, which are produced under FTS conditions, are the active phases. In addition, it has been also reported [28,29] that the carbides obtained from Fe0 were more active than those obtained from Fe oxides. However, the lack of agreement on this point does not make possible a general conclusion to be drawn. – The high surface concentration of active phases observed in sample 10Co5Fe/CNF is to be related to its high catalytic conversion. This way, the active sites would be more accessible to the reactants that could be more easily activated to form the different products. Fig. 6 provides information about the catalytic activity of all samples at different times on stream. After the replacement of the reactor dead volume, only samples 15Co/CNF and 10Co5Fe/CNF were slightly deactivated. Indeed, this deactivation was related to their high catalytic

activity that could lead to some secondary processes affecting the catalytic performance. Regarding the product selectivity, it was observed that higher Fe contents gave rise to higher selectivity to lighter hydrocarbons (C2–C3). This behavior was related to the properties of iron and cobalt as active phases in FTS. Iron catalysts favor the formation of shortchain hydrocarbons, whereas cobalt ones are selective to middle distillates and waxes [30,31]. On the other hand, the monometallic catalyst 15Co/CNF led to the highest value of CH4 selectivity, whereas the other catalysts led to similar values of CH4 selectivity. Therefore, a significant part of the activity of catalyst 15Co/CNF corresponded to the undesired methanation reaction. Although cobalt is not a CH 4-selective active phase, methanation reaction is favored at high temperatures like those used in the present work (523 K) [32]. On the other hand, although the catalytic activity of catalyst 10Co5Fe/CNF was lower than that obtained with 15Co/CNF, almost half of methane

0

778 eV = Co

711 eV = Fe2O3 0

707 eV = Fe 15Fe/CNF

Intensity (cps)

15Co/CNF

10Co5Fe/CNF 10Co5Fe/CNF 7Co7Fe/CNF

7Co7Fe/CNF

5Co10Fe/CNF

5Co10Fe/CNF

810 805 800 795 790 785 780 775 770 740 735 730 725 720 715 710 705 700

Binding energy (eV) Fig. 5. XPS patterns of the prepared catalysts after reduction (reduction conditions: 623 K, 5 h, 5 K min−1).

J.A. Díaz et al. / Fuel Processing Technology 128 (2014) 417–424

423

Table 3 Catalytic results (FTS at 523 K, 20 bar, 3000 Nml g−1 h−1, TOS 40 h). Catalyst

2

-1

-1

Catalytic activity 10 (mol COmolCo+Femin )

15Co/CNF 10Co5Fe/CNF 7Co7Fe/CNF 5Co10Fe/CNF 15Fe/CNF

Catalytic activity · 102 (molCO · mol(Co + Fe)−1 min−1)

Selectivity (%) CO2

CH4

C2H6

C3H8

C5+

C5+

29.9 15.4 4.4 4.1 1.7

24.67 15.14 3.71 11.14 5.66

60.18 27.74 27.12 28.22 24.74

0.53 5.61 4.53 8.93 10.19

0.12 1.69 3.04 2.05 4.75

14.49 49.80 61.60 49.65 54.66

11.18 21.15 7.67 5.63 2.83

40 15Co/CNF

30

10Co5Fe/CNF

20

7Co7Fe/CNF

10

5Co10Fe/CNF 15Fe/CNF

0 0

5

10

15

20

25

30

35

40

Time (h) Fig. 6. Catalytic activity vs time-on-stream (TOS).

was produced. As reported previously, Fe could increase its catalytic activity without a strong influence on undesired reactions/products [33]. Therefore, iron could be used with cobalt to avoid an excessive CH4 production at certain experimental conditions. In the FTS, CO2 selectivity could be used to indirectly measure the rate of the undesired Water–Gas-Shift (WGS) reaction. In this study, CO2 production was higher in samples 15Co/CNF and 10Co5Fe/CNF, on the contrary that expected in cobalt-based catalysts. Therefore, the increase of CO2 selectivity is related to the high catalytic activity values obtained with the catalysts mentioned above. It has been reported [34] that CO2 selectivity increased with increasing CO conversion due to the promotion of WGS reaction at high water partial pressures. On the other hand, the degradation of the carbonaceous support, an effect that has

20

15Fe/CNF 5Co10Fe/CNF 7Co7Fe/CNF 10Co5Fe/CNF 15Co/CNF

% mol

15

10

Yield (%)

been predicted elsewhere [26], could also contribute to the CO2 production. Nevertheless, this effect could not be demonstrated in this work. Liquid hydrocarbon distributions obtained with the catalysts here considered are shown in Fig. 7 and listed in Table 4. All distributions showed hydrocarbons in the range C7–C20. It can be seen that quite different distributions were obtained when the monometallic catalysts were used: 15Fe/CNF yielded gasoline (C7–C10) and kerosene (C11–C14) fractions, whereas 15Co/CNF favored the formation of kerosene and diesel (C15–C18) fractions. These results are related to the properties of the two different metals as active phases in FTS, as explained above. Regarding bimetallic catalysts, if catalysts 5Co10Fe/CNF and 7Co7Fe/CNF are considered: the higher the cobalt content, the higher the production of longer hydrocarbons was. However, catalyst 10Co5Fe/CNF favored the formation of shorter hydrocarbons. In this catalyst, the high surface concentration of both active phases (Co and Fe) respect to the rest of bimetallic catalysts would lead to both the activation of a higher proportion of reactants and an easier desorption of the reaction intermediates, favoring the formation of shorter hydrocarbons. Finally, Table 3 lists the obtained C5+ yields of the catalysts studied. It can be observed that sample 10Co5Fe/CNF presented the highest value. As mentioned above, the presence of iron in this catalyst avoided an excessive production of CH4 whereas the selectivity towards CO2 was hindered due to the lower catalytic conversion of CO in comparison with that observed in sample 15Co/CNF. These facts caused that sample 10Co5Fe/CNF presented the highest C5+ yield. 4. Conclusions This work describes the behavior in the Fischer–Tropsch process of different mono- and bimetallic catalysts with different contents of cobalt and iron supported on carbon nanofibers. It was observed that cobalt particles were well-dispersed over the support and reduced to metallic cobalt under the conditions used in this work. On the contrary, iron particles occupied larger pores and were not completely reduced to metallic iron. Regarding the bimetallic catalysts, cobalt particles were better dispersed in presence of iron which was in turn reduced to Fe0 in a higher extent due to the presence of the cobalt. Catalytic results revealed that the higher the content in cobalt, the higher the CO conversions were observed. However, the presence of iron in these catalysts avoided an excessive production of CH4. The bimetallic sample with the highest Co loading (10Co5Fe/CNF) yielded the highest production of C5 whereas its selectivity towards CH4 and CO2 was considerably lower than those obtained with sample 15Co/CNF. Table 4 Liquid hydrocarbon distribution.

5

0 7

8

9

10 11 12 13 14 15 16 17 18 19 20

Carbon number Fig. 7. Liquid hydrocarbon distributions of the prepared catalysts.

Catalyst

Gasoline (C7–C10)

Kerosene (C11–C14)

Diesel (C15–C18)

Lubricants (C19–C20)

15Co/CNF 10Co5Fe/CNF 7Co7Fe/CNF 5Co10Fe/CNF 15Fe/CNF

4.1 64.1 40.5 52.2 32.1

42.9 25.6 44.7 32.7 53.0

29.4 3.5 7.2 8.7 11.3

4.9 0 0.4 0 0

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