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Iron ore as precursor for preparation of highly active χ-Fe5C2 core-shell catalyst for Fischer-Tropsch synthesis
Applied Catalysis A, General 587 (2019) 117264
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Iron ore as precursor for preparation of highly active χ-Fe5C2 core-shell catalyst for Fischer-Tropsch synthesis
T
Sebastián Pérez, Fanor Mondragón, Andrés Moreno
⁎
Química de Recursos Energéticos y Medio Ambiente, Instituto de Química, Facultad de Ciencias Exactas y Naturales, Universidad de Antioquia, UdeA, Calle 70 No. 52 - 21, Medellín, Antioquia, 050010, Colombia
ARTICLE INFO
ABSTRACT
Keywords: Catalyst Core-shell Fischer-Tropsch Iron ore Oxalic acid
This work describes a novel method for the preparation of active and stable Fe-based catalyst from an iron ore, which was chemically treated with oxalic acid to obtain a mixture of iron oxides and oxalate. It was found that under CO atmosphere at 350 °C, this mixture can be effectively transformed into active iron carbide phase (χFe5C2), which is highly desirable for FTS. The catalysts were characterized by SEM, TEM, TGA, Raman, XRD, BET, and CO-TPR-MS. The catalytic performance was evaluated under FTS conditions (280 °C, H2 / CO = 1, P =20 bar) and GHSV = 12 N L gcat−1 h−1 for 100 h. The results showed that the CO conversion (66.8%) was higher on the oxalic acid-treated iron ore than in the untreated iron ore (46.5%), obtaining a high C5+ hydrocarbon selectivity (> 60.0%) in all cases. The proposed method significantly reduced the catalyst preparation steps using an inexpensive and abundant raw material for the sustainable preparation of very active and stable Fe-based catalysts for the FTS.
1. Introduction Nowadays the Fischer-Tropsch synthesis (FTS) has gained importance because it transforms syngas (H2 + CO) derived from biomass, natural gas, or coal into a wide variety of hydrocarbons [1]. Industrially iron and cobalt-based catalysts are mainly used as the active transition metals in the FTS catalysts [2], because of their cost, activity, and selectivity. However, Fe-based catalysts have some superior properties over Co-based catalysts [3], associated to its higher activity for the water-gas-shift reaction [4], and its improved selectivity toward light olefins at high-temperature FTS (HT-FTS) [5]. In the case of Fe-based catalysts, it is accepted that the Hägg ironcarbide (χ-Fe5C2) phase presents the highest activity [6–10]. Additionally the Hägg carbide phase has shown excellent performance when supported or even better embedded in carbonaceous materials [11]; indeed, according to literature, when χ-Fe5C2 is coated with carbonaceous material (@C) it showed higher activity and selectivity than carbon-supported systems [12]. In agreement with literature data, Santos et al. [13], prepared highly dispersed iron carbides with a high percentage of χ-Fe5C2 carbide (> 60%) embedded in a porous carbonaceous material. They found that the catalytic activity, in terms of iron-time yield (FTY) at 340 °C and 20 bar, was higher (4.9 × 10−4 molCO gFe-1 s-1) than that obtained on carbon-supported catalysts Fe/ CNT (1.4 × 10−4 molCO gFe-1 s-1) [13]. On the other hand, Park et al ⁎
[7]. have reported a method for obtaining a carbon encapsulated χFe5C2 catalyst obtained from iron (II) oxalate precursor, with high stability and good performance in the FTS (1.5 × 10−4 molCO gFe-1 s-1, at 320 °C and 15 bar). Nevertheless, after the FTS reaction, the χ-Fe5C2 carbide contains many impurities like metallic iron, hematite, magnetite, and other kinds of iron carbides (ε-Fe2C, ε’-Fe2.2C, Fe7C3, θ-Fe3C) phases [6], which are formed under the FTS conditions. Due to their complicated structure, with several oxides and carbides phases, Fe-based FTS catalysts are usually disposed since they are not easy to regenerate after being used. For this reason, simple, inexpensive and environmentally friendly methodologies are needed in the synthesis of the Fe-based catalyst. Natural iron ores have been often used as raw materials in the form of disposable Fe-based catalysts due to their low cost [14]. In previous studies in FTS [15–17], an iron ore treatment using a wet-milling process has been proposed followed by impregnation with precursors of Cu and K. This catalyst showed higher activity and stability than the iron ore and good selectivity to C5+ products (56.7%). However, to the best of our knowledge, there are not previous studies that use iron ore as raw material to obtain carbon-encapsulated Fe-based catalysts with a high content of the χ-Fe5C2 phase. For that reason, in this work, a new methodology to obtain a χ-Fe5C2@C FTS catalyst from a Colombian iron ore is proposed.
Corresponding author. E-mail address: [email protected] (A. Moreno).
https://doi.org/10.1016/j.apcata.2019.117264 Received 5 April 2019; Received in revised form 15 September 2019; Accepted 18 September 2019 Available online 18 September 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
Applied Catalysis A, General 587 (2019) 117264
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This work demonstrates that the chemical treatment of an iron ore with oxalic acid caused a partial dissolution of iron species, obtaining a mixture of iron(II) oxalate and iron oxides, an adequate catalyst precursor, which after drying were transformed into an active and stable χFe5C2@C catalyst for FTS using a thermal treatment under CO atmosphere [7]. The synthesis of the Fe-based catalysts involved a simple, inexpensive and environmentally friendly method.
of wavelength. The specific surface area was determined using the Brunauer–Emmett–Teller (BET) model for the nitrogen adsorption–desorption isotherms obtained with a Micromeritics ASAP 2020 analyzer at −196 °C. The catalysts were degassed at 300 °C for 10 h before each measurement. 2.3. Catalytic evaluation
2. Experimental
The catalytic performance was evaluated in a fixed bed reactor. About 100 mg (pellets size: 200–300 μm) of catalyst was diluted with 1.0 g of quartz sand (100–300 μm). The catalysts were used in FTS without any further in-situ activation process because the active phase χ-Fe5C2 had already been generated during the catalyst preparation step before FTS reaction. The reactions were carried out at 280 °C and 20 bars in the presence of synthesis gas (H2/CO/N2 = 45:45:10, GHSV = 12 N L gcat−1 h−1). The procedure for attaining the reaction conditions was as follows: First, the system was heated to 2 °C / min until 270 °C, then the pressure was increased to 20 bars. Between 270 °C and 280 °C the temperature was increased at1 °C / min, this slow heating rate allows reaching the reaction conditions in a controlled manner. Diffusional problems were evaluated using a different amount of catalyst (50 and 100 mg) and different gas hours space velocity (GHSV:12 – 36 N L gcat−1 h−1. Possible temperature gradients were evaluated using different weight ratios of catalyst/quartz sand (1/10 and 1/20). Repeatability of the proposed methodology was evaluated and reported in the supporting information.
2.1. Catalyst preparation In a typical procedure, 10 g of oxalic acid dihydrate (H2C2O4•2H2O 98%, Alfa Aesar), was dissolved in deionized water, the solution was heated up to 80 °C and stirred at 200 rpm for 15 min. Then 5 g of iron ore with particle size between 25–100 μm (goethite (FeO(OH): 37.9%), siderite (FeCO3: 18.1%), hematite (Fe2O3: 31.1%) and aluminosilicate (12.9%)) was added to the solution for a H2C2O4/Fe molar ratio approximately of 1.5, the mixture was left in agitation for 30 min. After vacuum was applied to remove water, the sample was dried at 90 °C for 24 h. Subsequently, to eliminate the excess of oxalic acid, the precursor was heated at 150 °C under Ar flow (30 mL/min) for 2 h in a tube furnace. The solid thus obtained was treated thermally (2 °C/min at temperatures between 300 °C and 380 °C) under CO atmosphere (90% CO, 40 mL.min−1 g−1) for 4 h, followed by passivation at room temperature under O2 (2%) for 1 h. The activation procedure was followed by mass spectrometry, QMS 200 (PFEIFFER). For comparison purposes two blanc samples were prepared, one by treating the iron ore with distilled water only (without oxalic acid), and the other one by activating a commercial sample of FeC2O4•2H2O (Alfa Aesar 99%) under CO atmosphere. The catalyst precursor obtained after the treatment of the iron ore with oxalic acid was labeled as PRE-OA (H2C2O4/Fe ratio equal to 1.5). The iron ore that was treated with only water and the commercial sample of FeC2O4 •2H2O were labeled as PRE-H2O and PRE-Ox respectively. The samples that were treated under CO atmosphere were labeled as CAT-OA, CAT-H2O, and CAT-Ox.
3. Results and discution 3.1. Precursor characterization The XRD patterns of the precursors are shown in Fig. 1. The crystal structure of the PRE-H2O shows a combination of siderite (ICSD # 71808), goethite (ICSD # 100678), and hematite (ICSD # 161286), showing that the treatment with only water did not change the iron ore composition. In the PRE-OA, the peaks at 2θ = 21.3° and 2θ = 33.2° corresponding to the (101) plane of goethite and (104) plane of hematite respectively, show a lower intensity, while the peak at 2θ = 32° of the (104) plane of siderite completely disappeared. The XRD pattern of the PRE-OA sample shows new peaks at about 2θ = 18.5°, corresponding to the (200) plane of humboldtine (FeC2O4 •2H2O, ICSD # 161344) and another type of oxalate as Fe2(C2O4)3 •xH2O. These results show that the initial composition of the iron ore was transformed
2.2. Catalysts characterization Phase structure of the samples was determined by X-ray diffraction (XRD) using a Panalytical X´pert PRO MPD diffractometer with Cu Kα1 = 1.5406 Ǻ, in the 2θ angle range of 10° to 90° with a step size of 0.026° and period of 50 s. Thermal stability analysis was carried out in a Thermogravimetric Analyser Q500 (TA Instruments). For the precursors, an amount of sample between 10 and 15 mg was used and heated at 5 °C / min from room temperature to 600 °C under N2 atmosphere. In the case of the catalysts, the thermal analysis was performed heating at 10 °C / min from room temperature to 700 °C in air atmosphere. Temperature programmed reduction under CO atmosphere (COTPR-MS, 10% CO in Ar) was performed in a down-flow fixed-bed quartz reactor (30 cm long x9 mm i.d.) using 100 mg of precursor and flow of 50 mL / min. The temperature was increased at 5 °C / min from room temperature to 800 °C. A mass spectrometer QMS 200 (PFEIFFER) was used to follow CO2 evolution. Morphology was analyzed by scanning electron microscopy (SEM) in a JEOL JSM-6490LV instrument with a LaB6 filament, at 20 kV accelerating voltage. TEM micrographs were obtained using a Tecnai F20 Super Twin TMP instrument; samples were dispersed in acetone and sonicated for 15 min before being dropped on a carbon-coated nickel grid. Raman spectra were recorded using a Raman Microscope spectrometer (Renishaw, Ltd) from 50 to 3000 cm−1 using a laser with 632 nm
Fig. 1. XRD patterns of PRE-H2O and PRE-OA and PRE-Ox. (Blue) goethite (ICSD # 100678), (pink) siderite (ICSD # 71808), and (green) hematite (ICSD # 161286). 2
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Fig. 3. Reduction profiles of samples treated with CO-TPR-MS (10 % CO), signal of 44 amu for CO2. E1 to E5 are the main thermal events.
Fig. 2. TGA profiles under N2 atmosphere, PRE-H2O (black), PRE-OA (green) and PRE-Ox (red).
FeCO3(s)
mainly into a mixture of hematite and iron oxalates. To determine the thermal stability of the phases that are present in the precursors, a TGA analysis was carried out under a N2 flow, the results are shown in Fig. 2. As it is observed, the PRE-H2O exhibits two weight losses, the first between 200 °C and 400 °C, which corresponds to 4.6% and is attributed to the decomposition of goethite [18] as shown in Eq. 1.
2 FeO(OH)(s) Fe2 O3(s) + H2 O(g)
x Fe3O4(s) + (4x+6) CO(g) x FeO(s) +(2+x) CO(g)
FeCO3(s) FeO(s) + CO2(g)
(3)
The weight losses for the PRE-H2O are in agreement with the content of the phases in the iron ore that were observed in the XRD pattern (Figure S1 and Table S1). Regarding the PRE-OA, two large weight losses were observed, the first between room temperature until about 200 °C, attributed to the dehydration of iron oxalates like FeC2O4 ·2H2O [20,21], as described in Reaction 4. The second weight loss, observed at temperatures higher than 300 °C, is associated with the decomposition of FeC2O4 which produces iron oxides, according to Reaction 5 [21,22]. It is important to note that the losses in the PRE-OA are quite similar to those observed in the commercial sample PRE-Ox, however, the PREOA shows a slightly higher residual weight (45% instead of 42%) which can be due to the presence of other phases such as Fe2O3 and an aluminosilicate.
FeC2 O4 •2H2 O(s) 3 FeC2 O4(s)
FeC2 O4(s) +2 H2 O(g)
Fe3 O4(s) + 4 CO(g) +2 CO2(g)
The reduction behavior of the precursors under CO atmosphere was investigated employing CO-TPR, the signal of the CO2 was followed using mass spectrometry. The CO-TPR profiles show different events (E1 to E5, see Fig. 3). First, the event E1 can correspond to the decomposition of traces of H2C2O4 in the PRE-OA [23], then the FeO(OH) and Fe2O3 of the PRE-H2O material react with the CO, forming Fe3O4 at temperatures below 400 °C (see Reaction 6 and Reaction 7 for E2), and then CO2 is released by the decomposition of the siderite (see Reaction 8 for E3). On the other hand, the signal at higher temperature is due to the reduction of Fe3O4 and FeO into iron carbides [24] (Reaction 9 and Reaction10 for E4).
6 FeO(OH) (s) + CO(g)
(7)
2 Fe3O4(s) + CO2(g) + 3 H2 O(g)
(11)
The objective of this work is to propose a methodology for the preparation of a highly active catalyst for FTS. For this purpose, an active iron carbide phase such as the χ-Fe5C2 phase is indispensable [26], it has been reported that the reaction temperature is an key parameter to get this active phase [27,28]. For that reason, taking into account the TGA and CO-TPR results (Fig. 2 and 3), different temperatures in the range from 300 °C to 380 °C were evaluated in the carburization of the PRE-OA sample under CO atmosphere for 4 h. Fig. 5 shows the XRD patterns for the PRE-OA after the carburization process at different temperatures. It is possible to observe that at low temperatures (< 340 °C) the iron oxalate was not completely converted to carbides, which could also be appreciated by TGA analysis (Figure S2. Supporting information), whereas at higher temperatures (> 360 °C) the formation of a carbonaceous material was favored. As shown in Fig. 5, after carburization at 380 °C, the broad peak of the XRD pattern at about 26.5° is indexed as the (002) planes of the graphitic carbon, the other diffraction peaks were characteristic of -Fe3C phase (ICSD # 99018). It is worth noting that XRD patterns showed that the main phase in the PRE-OA after carburization at 350 °C and 360 °C is χ-Fe5C2 (ICSD # 181367). These results showed that using the
(5)
(6)
(10)
3.2. Carburization of PRE-OA at different temperatures
(4)
3 Fe2 O3(s) + CO(g) 2 Fe3 O4(s) + CO2(g)
Fe x C(s) + (x+1) CO2(g)
(9)
The CO-TPR profile of the PRE-OA material shows a maximum of CO2 signal around 380 °C, whereas the PRE-Ox material exhibits a maximum at higher temperature, about 420 °C, both signals are associated with the decomposition of the FeC2O4 [21], (see Reaction 5 for E5) these results are in accordance with the observed TGA profile (Fig. 2). At higher temperatures, the formation of CO2 is related with the reduction of iron oxides (Reactions 9 and 10 for E4) and the formation of carbonaceous material by the Boudouard reaction (See Reaction 11) [22,25]. According to the characterization results, PRE-OA and PRE-Ox samples have similar thermal stability under CO atmosphere, which indicates that the proposed method causes a significant change in the iron ore properties. On the other hand, scanning electron microscopy (SEM) was used to observe the effect of the oxalic acid in the iron ore morphology as shown in Fig. 4, Then of the oxalic acid treatment, the PRE-OA show particles without defined size or morphology different from PRE-H2O, which indicates the formation of new structures, such as oxalates (observed by XRD, Fig. 1), whereas well defined and smaller (size about 5 μm) particles are observed for PRE-Ox solid.
The second weight loss of 5.6% that occurs between 400 °C–600 °C can be attributed to the siderite decomposition where two possible routes can be considered [18,19] (Eqs. 2 and 3): (2)
3 Fe x C (s) +(4x+3) CO2(g)
2 CO(g) C(s) + CO2(g)
(1)
3 FeCO3(s) Fe3O4(s) + 2 CO2(g) + CO(g)
(8)
FeO + CO2(g)
3
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Fig. 4. SEM Micrographs for PRE-H2O, PRE-OA and PRE-OX. Table 1 Iron content, carbonaceous material and textural properties. Sample
Fe (wt %)
CAT-H2O CAT-OA CAT-Ox
54.5 57.3 72.8
a
Total carbon (wt %)
Others
17.3 23.7 25.4
28.2 19 1.8
c
BET area (m2/g) 42 84 70
a
determined by atomic absorption spectroscopy, b determined by CHN elemental analysis. b The sum of the composition is not 100% for the presence of other elements as Al, Si, O for CAT-H2O and CAT-OA, and O for CAT-Ox.
defined for CAT-H2O, which additionally presents the peak at 2θ = 32° of the (104) plane of siderite phase (ICSD # 71808) and another broad peak at peak at 2θ = 36.1° of the (113) plane corresponding to magnetite (ICSD # 158505). According to the XRD patterns of the precursors (see Fig. 1), upon treatment with oxalic acid, the siderite phase present in the iron ore readily reacts forming iron oxalates (Table 1) Hong et al. [7] reported that the formation of carbonaceous material at 350 °C during the carburization process of iron(II) oxalate had a good effect on the activity and stability in the FTS; for that reason, Raman spectroscopy was used in this work to evaluate the nature of the carbonaceous material. As shown in Fig. 7, all samples show two broad peaks at ∼1330 cm−1 (D1 band) and ∼1595 cm−1 (G band) respectively, which are observed in many disordered carbonaceous materials [29], additionally many disordered and amorphous carbons also indicate peaks appearing at ˜1150 (D4) and ˜1500 (D3), which are related to sp3 carbon impurities, and amorphous carbon [30], respectively. The values of the integrated intensity ratio of the D-band to the Gband (AD1/AG), are 4.5, 3.9 and 3.5 for CAT-H2O, CAT-OA and CAT-Ox respectively, indicating a greater proportion of disordered carbon for CAT-H2O; additionally, all samples show the D3 band for amorphous carbon, which is lower for CAT-OA (AD3/AG = 0.77) compared the CAT-Ox (AD3/AG = 0.94) and CAT-H2O (AD3/AG = 1.05). On the other
Fig. 5. XRD pattern of the precursor CAT-OA treated at different temperatures under CO atmosphere for 4 h. Reference patterns (Blue) χ-Fe5C2 (ICSD # 181367), (pink) θ-Fe3C (ICSD # 99018).
proposed methodology it was possible to obtain a solid material with the χ-Fe5C2 active phase as the main component. In the published scientific literature 350 °C has been reported as an adequate temperature for catalysts formation with a high content of the χ-Fe5C2 phase [7,10]. 3.3. Carburization at 350 °C under CO atmosphere The PRE-OA solid treated under CO at 350 °C was labeled as CATOA, and for comparison, the precursors PRE-H2O and PRE-Ox were also carburized at the same temperature and labeled as CAT-H2O and CATOx, respectively. According to the CO2 signal, the formation of iron carbides and carbonaceous material occurs during the four hours of carburization (Figure S3, Supporting information). The XRD patterns for the catalysts are shown in Fig. 6. The three diffraction patterns displayed peaks corresponding to the χ-Fe5C2 phase although not well
Fig. 6. XRD patterns of the catalysts obtained after carburization at 350 °C.
Fig. 7. Raman spectra of the catalysts active at 350 °C under CO atmosphere. 4
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Fig. 8. SEM and TEM images for the precursors carburized at 350 °C under CO atmosphere for 4 h; (a, d, g) SEM images for CAT-H2O, CAT-OA and CAT-Ox respectively; (b, c) TEM images for CAT-H2O; (e, f) TEM images for CAT-OA; and (h, i) TEM images for CAT-Ox.
hand, below 800 cm−1 the Raman spectra shows signals corresponding to the Fe2O3 at 223, 293 y 407 cm−1 while the broad peak at 670 cm−1 corresponds to magnetite [31], these oxides were possibly formed during the passivation process of the carbides [32]. Upon treatment with diluted oxygen the most reactive iron carbide particles are surface passivated to avoid a violent reaction when the solid is exposed to the atmosphere; however, due to the low content of these oxides it was not possible to identify them by XRD. The morphological and structural features of the catalysts were analyzed using SEM and TEM. According to SEM images, (Fig. 8a, d and g), the particles of CAT-H2O and CAT-OA retain a shape similar to that of the precursors whereas the commercial iron oxalate appears to have been fragmented. The TEM images indicate that CAT-OA and CAT-Ox catalysts mainly consist of particles in the range of 10 nm–60 nm that could be described as a core-shell system, where the core is constituted by iron carbide (i.e., χ-Fe5C2) and the shell corresponding to the carbonaceous material (about 3 nm–5 nm), this kind of particles was reported before using iron oxalate (II) with a defined cubic morphology [7]. In the case of CAT-H2O, some particles do not show carbonaceous material, which could correspond to siderite and magnetite particles, as detected by XRD. The magnetite in CAT-H2O possibly needs more time or temperature to be transformed into iron carbides; on the other hand, the siderite requires a higher temperature to decompose under CO atmosphere according to the CO-TPR (Fig. 3). Table 2 shows the textural properties of the catalysts. The specific surface areas were 42 m2 g−1, 84 m2 g−1 and 70 m2 g−1 for CAT-H2O, CAT-OA and CAT-Ox respectively, indicating an improvement in the apparent surface area of the iron ore with the oxalic acid treatment,
Table 2 Summary of the overall catalytic performance during 50 h–100 h of reaction. T=280 °C, H2/CO = 1, P =.20 bar. Sample
CAT-H2O CAT-OA CAT-Ox
Conversion (%) XCO
XH2
46.5 66.8 88.8
64.7 73.6 79.6
FTY*
0.36 0.42 0.42
CO2 (Cmol %)
HC distribution (C-mol %) CH4
C2-C4
C5+
30.0 36.7 42.3
11.7 11.8 9.2
25.9 26.5 24.8
62.4 61.7 66.0
* FTY: molCO gFe−1 s−1 x10−4.
possibly this is due to the porosity associated with the higher amount of carbonaceous material in CAT-OA (23.7%) compared to that in CATH2O (17.3%). Moreover, according to chemical analysis by atomic absorption spectroscopy, the iron content in the CAT-Ox is 72.8%, which is higher than those found in CAT-OA (57.3%) and CAT-H2O (54.5%). The lower iron content on the catalysts derived from the iron ore is consistent with the aluminosilicate impurities present in the parent mineral. 3.4. Catalytic performance of the iron carbides The catalytic activity of CAT-OA, CAT-H2O, CAT-Ox was evaluated at 280 °C in the Fischer-Tropsch reaction. Fig. 10a shows the CO conversion as a function of the reaction time. For CAT-OA and CAT-Ox, CO conversion gradually increased during the first 20 h of reaction reaching about 71% and 93% respectively. This increase could be partly due to the transformation of iron oxides formed in the passivation 5
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+ H2). The amount of CO2 depends on the CO and H2 conversion (XCO and XH2). At high XCO and XH2 the partial pressure of the H2O increases and H2 pressure decrease, these conditions favor the WGS [4], for this reason the CO2 selectivity is higher for the CAT-Ox catalyst (42.3%) than that of CAT-OA (36.7%) and CAT-H2O (30.0%), which have a lower conversion. In the present work the iron ore without treatment (CAT-H2O) showed a high C5+ hydrocarbons selectivity (62.4%) and best stability than that a similar iron ore without modifications employed in others works [15,17] using comparable reaction conditions; however, a lower stability, and lower C5+ hydrocarbons selectivity (37.3%) could be due to the non-identical conditions in the carburization process; in fact, in the cited works [15,17] the iron ore was carburized 280 °C instead of 350 °C as done in the present case. Moreover, it is noteworthy that in the procedure reported here a CAT-OA catalyst with high activity and stability was obtained without the need to add other metals or promoters such as Cu and K [15–17]. In addition, by comparison with literature, our results indicate that the performance of the catalyst is better than that for other catalysts prepared by more complex procedures (Table S5). To determine the effect of temperature on catalyst stability, conversion, selectivity and activity at two additional temperatures 300 °C and 320 °C were evaluated for CAT-OA catalyst, (20 bar, GHSV = 24 N L gcat−1 h−1, TOS =25 h). The CAT-OA catalyst showed good stability and a high activity of 0.87 × 10-4 molCO gFe−1 s−1 at 300 °C (XCO = 71.0%), and 0.93 × 10-4 molCO gFe−1 s−1 at 320 °C (XCO = 78.3%) (Figure S6, Supporting information). On the other hand, the selectivity (CO2 free) towards CH4 and C5+ was 17.0% and 50.4% at 300 °C and 22.1% and 45.1% at 320 °C respectively. (Table S5, Supporting information). The CO conversion, FTY, and the CO2 selectivity increase whit the temperature (see Table S5). Note however, that the iron-time-yield (FTY) which indicates the number of CO molecules converted to hydrocarbons per gram of iron per second, explicitly excluding carbon dioxide selectivity, does not consider the contribution of the water-gas shift reaction (WGS). The mole percent of CO converted to CO2 is an indicator of the WGS activity. Then, the increase of CO2 selectivity with temperature for CAT-OA catalyst suggests that the activation energy for the WGS reaction is larger than that for the FTS reaction. At the present conditions, the activity data should not be used to estimate an apparent activation energy (see Supporting Information) because the WGS and other secondary reactions as readsorption of olefins play an important role, thus, the overall FTS reaction rate is highly affected by the rate/ extent of the WGS reaction at high FTS conversions. The apparent activation energy should be better estimated at lower conversions where the FTS reaction rate is unaffected by the WGS reaction [34]. The recovered CAT-OA catalyst after 100 h of reaction was further analyzed by TEM and XRD (Fig. 10). In the TEM image of CAT-OA, the active particles mostly maintained their carbon-encapsulated structure which is consistent with the long time conversion stability, and indicates that the proposed methodology allows preparing catalysts as stable as other methodologies reported for carbon-encapsulated iron nanoparticles [36,37]. The high stability of the catalyst particles can be partly related to the core-shell structure, which protects the particle and prevents catalyst fragmentation [5,38]. According to XRD pattern, after 100 h reaction the main crystalline phase after reaction is still the carbide χ-Fe5C2, with a minor fraction of Fe3O4, this later phase being frequently found in used Fe-based FTS catalysts [8,26]. Nevertheless, more efforts are necessary to optimize the conditions for the formation of carbon-encapsulated iron nanostructures that can be used for the catalytic process of syngas conversion [39]. Finally, it is important to point out that the results of the present work let us to suggest that the proposed methodology for the preparation of Fe-based FTS catalyst from a Colombian iron ore could be also applied to iron ores of different origin and using fewer steps than the conventional methodologies from reagent-grade iron precursors
Fig. 9. (a) CO conversion, (b) activity: molCO gFe−1 s−1 (CO2 free), for 100 h of time on stream. 280 °C, 20 bars, H2/CO = 1, GHSV = 12 N L gcat−1 h−1.
process (observed by Raman, Fig. 7) into actives iron carbides, then the CO conversion slightly decreased to about 67% for CAT-OA and 89% for CAT-Ox at 100 h of time on stream, this shows that the catalysts continue to change over time. The higher CO conversion of the reference catalyst, CAT-Ox, could be due to the higher Fe content of 72.8% as compared to 57.3% for the CAT-OA. On the other hand, among the catalysts, the induction period was longer for CAT-H2O, which could be associated to the progressive carburization of the higher amount of iron oxide species present in this solid, as evidenced by the XRD results in Fig. 6. The higher CO conversion achieved with the CAT-OA, as compared to CAT-H2O, shows that the treatment with oxalic acid of the iron ore has positive effects on the activity. The higher activity of iron ore treated with oxalic acid is certainly related with the transformation of some of the iron phases present in the iron ore (e.g., siderite, Fig. 1) to iron oxalate, which was further transformed by the carburization process into the active χ-Fe5C2 carbide phase (Fig. 6). Fig. 9 b show the catalytic performance expressed as iron-time-yield (FTY, the number of CO molecules converted to hydrocarbons per gram of iron per second, explicitly excluding carbon dioxide selectivity). The FTY value between 50 h–100 h of time on stream for CAT-OA and CATOx was the same and approximately 0.42 × 10−4 molCO gFe-1 s-1, in contrast to the CAT-H2O which display an activity of 0.36 × 10−4 molCO gFe-1 s-1. Table 2 summarizes the overall catalytic performance of the catalysts evaluated. The overall CO conversion for CAT-OA was higher than that of CAT-H2O. Because of the different conversion degree, the selectivity data can only be compared qualitatively; however, the hydrocarbon distribution (CO2 free) shows that the CH4 selectivity data for all catalysts is relatively low (< 12 %) and the C5+ selectivity is higher than 60%. The hydrocarbon distributions are showed in detail in the supporting information Figure S5, the sum of C1 - C30 products were 96%, 92% and 84% for CAT-H2O CAT-OA and CAT-Ox respectively. Therefore, these results confirm that it is possible to prepare active and stable catalysts using this iron ore, with high selectivity to liquid fuel production in the FTS. The CO2 selectivity is indicative of the WGS activity of the catalysts, which also involves the additional formation of H2 (H2O + CO CO2 6
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Fig. 10. (a, b) TEM images, (c) particle size distribution, and (d) XRD pattern of CAT-OA catalyst after 100 h of reaction.
[40].
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In this work, a simple methodology for the preparation of carbonencapsulated iron catalyst using an iron ore as a precursor is proposed. With the catalyst thus prepared, it was possible to achieve a high selectivity to C5+ hydrocarbons (61.7%) at 280 °C of reaction. The high activity (Fe-time yield: 0.42 × 10−4 molCO gFe-1 s-1 at 280 °C and 0.93 × 10−4 molCO gFe-1 s-1 at 320 °C) of the catalyst is mainly attributed to two factors: the proper formation of the active χ-Fe5C2 phase, and the formation of a carbonaceous layer that 'protects' the active phase, which result in the formation of a 'core-shell' catalytic system. The preparation of the catalyst was achieved by a simple strategy of modifying an iron ore, due to the high reactivity of the siderite and goethite phases upon oxalic acid treatment, forming iron oxalates. Our results showed that the activation temperature is a determining factor in the obtaining the FTS active iron carbide phase (i.e., χ-Fe5C2) from the iron ore as a raw material, being 350 °C the optimum temperature for the preparation of this phase. At higher temperatures the formation of a less active iron carbide (θ-Fe3C) and carbonaceous materials are favored. In the core-shell particles (χ-Fe5C2@C), the shell was formed by the Boudouard reaction, giving stability to the catalyst up to 100 h of time on stream. It is worth noting that the structure χ-Fe5C2@C was stable after the reaction according to TEM images. Finally, the proposed method significantly reduced the catalyst preparation steps while using an inexpensive raw material for the sustainable preparation of highly active and stable Fe-based catalysts for the FTS. Acknowledgment The authors thank to the University of Antioquia for the financial support (Project 2014-1088). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2019.117264. 7
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Iron ore as precursor for preparation of highly active χ-Fe5C2 core-shell catalyst for Fischer-Tropsch synthesis
T
Sebastián Pérez, Fanor Mondragón, Andrés Moreno
⁎
Química de Recursos Energéticos y Medio Ambiente, Instituto de Química, Facultad de Ciencias Exactas y Naturales, Universidad de Antioquia, UdeA, Calle 70 No. 52 - 21, Medellín, Antioquia, 050010, Colombia
ARTICLE INFO
ABSTRACT
Keywords: Catalyst Core-shell Fischer-Tropsch Iron ore Oxalic acid
This work describes a novel method for the preparation of active and stable Fe-based catalyst from an iron ore, which was chemically treated with oxalic acid to obtain a mixture of iron oxides and oxalate. It was found that under CO atmosphere at 350 °C, this mixture can be effectively transformed into active iron carbide phase (χFe5C2), which is highly desirable for FTS. The catalysts were characterized by SEM, TEM, TGA, Raman, XRD, BET, and CO-TPR-MS. The catalytic performance was evaluated under FTS conditions (280 °C, H2 / CO = 1, P =20 bar) and GHSV = 12 N L gcat−1 h−1 for 100 h. The results showed that the CO conversion (66.8%) was higher on the oxalic acid-treated iron ore than in the untreated iron ore (46.5%), obtaining a high C5+ hydrocarbon selectivity (> 60.0%) in all cases. The proposed method significantly reduced the catalyst preparation steps using an inexpensive and abundant raw material for the sustainable preparation of very active and stable Fe-based catalysts for the FTS.
1. Introduction Nowadays the Fischer-Tropsch synthesis (FTS) has gained importance because it transforms syngas (H2 + CO) derived from biomass, natural gas, or coal into a wide variety of hydrocarbons [1]. Industrially iron and cobalt-based catalysts are mainly used as the active transition metals in the FTS catalysts [2], because of their cost, activity, and selectivity. However, Fe-based catalysts have some superior properties over Co-based catalysts [3], associated to its higher activity for the water-gas-shift reaction [4], and its improved selectivity toward light olefins at high-temperature FTS (HT-FTS) [5]. In the case of Fe-based catalysts, it is accepted that the Hägg ironcarbide (χ-Fe5C2) phase presents the highest activity [6–10]. Additionally the Hägg carbide phase has shown excellent performance when supported or even better embedded in carbonaceous materials [11]; indeed, according to literature, when χ-Fe5C2 is coated with carbonaceous material (@C) it showed higher activity and selectivity than carbon-supported systems [12]. In agreement with literature data, Santos et al. [13], prepared highly dispersed iron carbides with a high percentage of χ-Fe5C2 carbide (> 60%) embedded in a porous carbonaceous material. They found that the catalytic activity, in terms of iron-time yield (FTY) at 340 °C and 20 bar, was higher (4.9 × 10−4 molCO gFe-1 s-1) than that obtained on carbon-supported catalysts Fe/ CNT (1.4 × 10−4 molCO gFe-1 s-1) [13]. On the other hand, Park et al ⁎
[7]. have reported a method for obtaining a carbon encapsulated χFe5C2 catalyst obtained from iron (II) oxalate precursor, with high stability and good performance in the FTS (1.5 × 10−4 molCO gFe-1 s-1, at 320 °C and 15 bar). Nevertheless, after the FTS reaction, the χ-Fe5C2 carbide contains many impurities like metallic iron, hematite, magnetite, and other kinds of iron carbides (ε-Fe2C, ε’-Fe2.2C, Fe7C3, θ-Fe3C) phases [6], which are formed under the FTS conditions. Due to their complicated structure, with several oxides and carbides phases, Fe-based FTS catalysts are usually disposed since they are not easy to regenerate after being used. For this reason, simple, inexpensive and environmentally friendly methodologies are needed in the synthesis of the Fe-based catalyst. Natural iron ores have been often used as raw materials in the form of disposable Fe-based catalysts due to their low cost [14]. In previous studies in FTS [15–17], an iron ore treatment using a wet-milling process has been proposed followed by impregnation with precursors of Cu and K. This catalyst showed higher activity and stability than the iron ore and good selectivity to C5+ products (56.7%). However, to the best of our knowledge, there are not previous studies that use iron ore as raw material to obtain carbon-encapsulated Fe-based catalysts with a high content of the χ-Fe5C2 phase. For that reason, in this work, a new methodology to obtain a χ-Fe5C2@C FTS catalyst from a Colombian iron ore is proposed.
Corresponding author. E-mail address: [email protected] (A. Moreno).
https://doi.org/10.1016/j.apcata.2019.117264 Received 5 April 2019; Received in revised form 15 September 2019; Accepted 18 September 2019 Available online 18 September 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
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This work demonstrates that the chemical treatment of an iron ore with oxalic acid caused a partial dissolution of iron species, obtaining a mixture of iron(II) oxalate and iron oxides, an adequate catalyst precursor, which after drying were transformed into an active and stable χFe5C2@C catalyst for FTS using a thermal treatment under CO atmosphere [7]. The synthesis of the Fe-based catalysts involved a simple, inexpensive and environmentally friendly method.
of wavelength. The specific surface area was determined using the Brunauer–Emmett–Teller (BET) model for the nitrogen adsorption–desorption isotherms obtained with a Micromeritics ASAP 2020 analyzer at −196 °C. The catalysts were degassed at 300 °C for 10 h before each measurement. 2.3. Catalytic evaluation
2. Experimental
The catalytic performance was evaluated in a fixed bed reactor. About 100 mg (pellets size: 200–300 μm) of catalyst was diluted with 1.0 g of quartz sand (100–300 μm). The catalysts were used in FTS without any further in-situ activation process because the active phase χ-Fe5C2 had already been generated during the catalyst preparation step before FTS reaction. The reactions were carried out at 280 °C and 20 bars in the presence of synthesis gas (H2/CO/N2 = 45:45:10, GHSV = 12 N L gcat−1 h−1). The procedure for attaining the reaction conditions was as follows: First, the system was heated to 2 °C / min until 270 °C, then the pressure was increased to 20 bars. Between 270 °C and 280 °C the temperature was increased at1 °C / min, this slow heating rate allows reaching the reaction conditions in a controlled manner. Diffusional problems were evaluated using a different amount of catalyst (50 and 100 mg) and different gas hours space velocity (GHSV:12 – 36 N L gcat−1 h−1. Possible temperature gradients were evaluated using different weight ratios of catalyst/quartz sand (1/10 and 1/20). Repeatability of the proposed methodology was evaluated and reported in the supporting information.
2.1. Catalyst preparation In a typical procedure, 10 g of oxalic acid dihydrate (H2C2O4•2H2O 98%, Alfa Aesar), was dissolved in deionized water, the solution was heated up to 80 °C and stirred at 200 rpm for 15 min. Then 5 g of iron ore with particle size between 25–100 μm (goethite (FeO(OH): 37.9%), siderite (FeCO3: 18.1%), hematite (Fe2O3: 31.1%) and aluminosilicate (12.9%)) was added to the solution for a H2C2O4/Fe molar ratio approximately of 1.5, the mixture was left in agitation for 30 min. After vacuum was applied to remove water, the sample was dried at 90 °C for 24 h. Subsequently, to eliminate the excess of oxalic acid, the precursor was heated at 150 °C under Ar flow (30 mL/min) for 2 h in a tube furnace. The solid thus obtained was treated thermally (2 °C/min at temperatures between 300 °C and 380 °C) under CO atmosphere (90% CO, 40 mL.min−1 g−1) for 4 h, followed by passivation at room temperature under O2 (2%) for 1 h. The activation procedure was followed by mass spectrometry, QMS 200 (PFEIFFER). For comparison purposes two blanc samples were prepared, one by treating the iron ore with distilled water only (without oxalic acid), and the other one by activating a commercial sample of FeC2O4•2H2O (Alfa Aesar 99%) under CO atmosphere. The catalyst precursor obtained after the treatment of the iron ore with oxalic acid was labeled as PRE-OA (H2C2O4/Fe ratio equal to 1.5). The iron ore that was treated with only water and the commercial sample of FeC2O4 •2H2O were labeled as PRE-H2O and PRE-Ox respectively. The samples that were treated under CO atmosphere were labeled as CAT-OA, CAT-H2O, and CAT-Ox.
3. Results and discution 3.1. Precursor characterization The XRD patterns of the precursors are shown in Fig. 1. The crystal structure of the PRE-H2O shows a combination of siderite (ICSD # 71808), goethite (ICSD # 100678), and hematite (ICSD # 161286), showing that the treatment with only water did not change the iron ore composition. In the PRE-OA, the peaks at 2θ = 21.3° and 2θ = 33.2° corresponding to the (101) plane of goethite and (104) plane of hematite respectively, show a lower intensity, while the peak at 2θ = 32° of the (104) plane of siderite completely disappeared. The XRD pattern of the PRE-OA sample shows new peaks at about 2θ = 18.5°, corresponding to the (200) plane of humboldtine (FeC2O4 •2H2O, ICSD # 161344) and another type of oxalate as Fe2(C2O4)3 •xH2O. These results show that the initial composition of the iron ore was transformed
2.2. Catalysts characterization Phase structure of the samples was determined by X-ray diffraction (XRD) using a Panalytical X´pert PRO MPD diffractometer with Cu Kα1 = 1.5406 Ǻ, in the 2θ angle range of 10° to 90° with a step size of 0.026° and period of 50 s. Thermal stability analysis was carried out in a Thermogravimetric Analyser Q500 (TA Instruments). For the precursors, an amount of sample between 10 and 15 mg was used and heated at 5 °C / min from room temperature to 600 °C under N2 atmosphere. In the case of the catalysts, the thermal analysis was performed heating at 10 °C / min from room temperature to 700 °C in air atmosphere. Temperature programmed reduction under CO atmosphere (COTPR-MS, 10% CO in Ar) was performed in a down-flow fixed-bed quartz reactor (30 cm long x9 mm i.d.) using 100 mg of precursor and flow of 50 mL / min. The temperature was increased at 5 °C / min from room temperature to 800 °C. A mass spectrometer QMS 200 (PFEIFFER) was used to follow CO2 evolution. Morphology was analyzed by scanning electron microscopy (SEM) in a JEOL JSM-6490LV instrument with a LaB6 filament, at 20 kV accelerating voltage. TEM micrographs were obtained using a Tecnai F20 Super Twin TMP instrument; samples were dispersed in acetone and sonicated for 15 min before being dropped on a carbon-coated nickel grid. Raman spectra were recorded using a Raman Microscope spectrometer (Renishaw, Ltd) from 50 to 3000 cm−1 using a laser with 632 nm
Fig. 1. XRD patterns of PRE-H2O and PRE-OA and PRE-Ox. (Blue) goethite (ICSD # 100678), (pink) siderite (ICSD # 71808), and (green) hematite (ICSD # 161286). 2
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Fig. 3. Reduction profiles of samples treated with CO-TPR-MS (10 % CO), signal of 44 amu for CO2. E1 to E5 are the main thermal events.
Fig. 2. TGA profiles under N2 atmosphere, PRE-H2O (black), PRE-OA (green) and PRE-Ox (red).
FeCO3(s)
mainly into a mixture of hematite and iron oxalates. To determine the thermal stability of the phases that are present in the precursors, a TGA analysis was carried out under a N2 flow, the results are shown in Fig. 2. As it is observed, the PRE-H2O exhibits two weight losses, the first between 200 °C and 400 °C, which corresponds to 4.6% and is attributed to the decomposition of goethite [18] as shown in Eq. 1.
2 FeO(OH)(s) Fe2 O3(s) + H2 O(g)
x Fe3O4(s) + (4x+6) CO(g) x FeO(s) +(2+x) CO(g)
FeCO3(s) FeO(s) + CO2(g)
(3)
The weight losses for the PRE-H2O are in agreement with the content of the phases in the iron ore that were observed in the XRD pattern (Figure S1 and Table S1). Regarding the PRE-OA, two large weight losses were observed, the first between room temperature until about 200 °C, attributed to the dehydration of iron oxalates like FeC2O4 ·2H2O [20,21], as described in Reaction 4. The second weight loss, observed at temperatures higher than 300 °C, is associated with the decomposition of FeC2O4 which produces iron oxides, according to Reaction 5 [21,22]. It is important to note that the losses in the PRE-OA are quite similar to those observed in the commercial sample PRE-Ox, however, the PREOA shows a slightly higher residual weight (45% instead of 42%) which can be due to the presence of other phases such as Fe2O3 and an aluminosilicate.
FeC2 O4 •2H2 O(s) 3 FeC2 O4(s)
FeC2 O4(s) +2 H2 O(g)
Fe3 O4(s) + 4 CO(g) +2 CO2(g)
The reduction behavior of the precursors under CO atmosphere was investigated employing CO-TPR, the signal of the CO2 was followed using mass spectrometry. The CO-TPR profiles show different events (E1 to E5, see Fig. 3). First, the event E1 can correspond to the decomposition of traces of H2C2O4 in the PRE-OA [23], then the FeO(OH) and Fe2O3 of the PRE-H2O material react with the CO, forming Fe3O4 at temperatures below 400 °C (see Reaction 6 and Reaction 7 for E2), and then CO2 is released by the decomposition of the siderite (see Reaction 8 for E3). On the other hand, the signal at higher temperature is due to the reduction of Fe3O4 and FeO into iron carbides [24] (Reaction 9 and Reaction10 for E4).
6 FeO(OH) (s) + CO(g)
(7)
2 Fe3O4(s) + CO2(g) + 3 H2 O(g)
(11)
The objective of this work is to propose a methodology for the preparation of a highly active catalyst for FTS. For this purpose, an active iron carbide phase such as the χ-Fe5C2 phase is indispensable [26], it has been reported that the reaction temperature is an key parameter to get this active phase [27,28]. For that reason, taking into account the TGA and CO-TPR results (Fig. 2 and 3), different temperatures in the range from 300 °C to 380 °C were evaluated in the carburization of the PRE-OA sample under CO atmosphere for 4 h. Fig. 5 shows the XRD patterns for the PRE-OA after the carburization process at different temperatures. It is possible to observe that at low temperatures (< 340 °C) the iron oxalate was not completely converted to carbides, which could also be appreciated by TGA analysis (Figure S2. Supporting information), whereas at higher temperatures (> 360 °C) the formation of a carbonaceous material was favored. As shown in Fig. 5, after carburization at 380 °C, the broad peak of the XRD pattern at about 26.5° is indexed as the (002) planes of the graphitic carbon, the other diffraction peaks were characteristic of -Fe3C phase (ICSD # 99018). It is worth noting that XRD patterns showed that the main phase in the PRE-OA after carburization at 350 °C and 360 °C is χ-Fe5C2 (ICSD # 181367). These results showed that using the
(5)
(6)
(10)
3.2. Carburization of PRE-OA at different temperatures
(4)
3 Fe2 O3(s) + CO(g) 2 Fe3 O4(s) + CO2(g)
Fe x C(s) + (x+1) CO2(g)
(9)
The CO-TPR profile of the PRE-OA material shows a maximum of CO2 signal around 380 °C, whereas the PRE-Ox material exhibits a maximum at higher temperature, about 420 °C, both signals are associated with the decomposition of the FeC2O4 [21], (see Reaction 5 for E5) these results are in accordance with the observed TGA profile (Fig. 2). At higher temperatures, the formation of CO2 is related with the reduction of iron oxides (Reactions 9 and 10 for E4) and the formation of carbonaceous material by the Boudouard reaction (See Reaction 11) [22,25]. According to the characterization results, PRE-OA and PRE-Ox samples have similar thermal stability under CO atmosphere, which indicates that the proposed method causes a significant change in the iron ore properties. On the other hand, scanning electron microscopy (SEM) was used to observe the effect of the oxalic acid in the iron ore morphology as shown in Fig. 4, Then of the oxalic acid treatment, the PRE-OA show particles without defined size or morphology different from PRE-H2O, which indicates the formation of new structures, such as oxalates (observed by XRD, Fig. 1), whereas well defined and smaller (size about 5 μm) particles are observed for PRE-Ox solid.
The second weight loss of 5.6% that occurs between 400 °C–600 °C can be attributed to the siderite decomposition where two possible routes can be considered [18,19] (Eqs. 2 and 3): (2)
3 Fe x C (s) +(4x+3) CO2(g)
2 CO(g) C(s) + CO2(g)
(1)
3 FeCO3(s) Fe3O4(s) + 2 CO2(g) + CO(g)
(8)
FeO + CO2(g)
3
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Fig. 4. SEM Micrographs for PRE-H2O, PRE-OA and PRE-OX. Table 1 Iron content, carbonaceous material and textural properties. Sample
Fe (wt %)
CAT-H2O CAT-OA CAT-Ox
54.5 57.3 72.8
a
Total carbon (wt %)
Others
17.3 23.7 25.4
28.2 19 1.8
c
BET area (m2/g) 42 84 70
a
determined by atomic absorption spectroscopy, b determined by CHN elemental analysis. b The sum of the composition is not 100% for the presence of other elements as Al, Si, O for CAT-H2O and CAT-OA, and O for CAT-Ox.
defined for CAT-H2O, which additionally presents the peak at 2θ = 32° of the (104) plane of siderite phase (ICSD # 71808) and another broad peak at peak at 2θ = 36.1° of the (113) plane corresponding to magnetite (ICSD # 158505). According to the XRD patterns of the precursors (see Fig. 1), upon treatment with oxalic acid, the siderite phase present in the iron ore readily reacts forming iron oxalates (Table 1) Hong et al. [7] reported that the formation of carbonaceous material at 350 °C during the carburization process of iron(II) oxalate had a good effect on the activity and stability in the FTS; for that reason, Raman spectroscopy was used in this work to evaluate the nature of the carbonaceous material. As shown in Fig. 7, all samples show two broad peaks at ∼1330 cm−1 (D1 band) and ∼1595 cm−1 (G band) respectively, which are observed in many disordered carbonaceous materials [29], additionally many disordered and amorphous carbons also indicate peaks appearing at ˜1150 (D4) and ˜1500 (D3), which are related to sp3 carbon impurities, and amorphous carbon [30], respectively. The values of the integrated intensity ratio of the D-band to the Gband (AD1/AG), are 4.5, 3.9 and 3.5 for CAT-H2O, CAT-OA and CAT-Ox respectively, indicating a greater proportion of disordered carbon for CAT-H2O; additionally, all samples show the D3 band for amorphous carbon, which is lower for CAT-OA (AD3/AG = 0.77) compared the CAT-Ox (AD3/AG = 0.94) and CAT-H2O (AD3/AG = 1.05). On the other
Fig. 5. XRD pattern of the precursor CAT-OA treated at different temperatures under CO atmosphere for 4 h. Reference patterns (Blue) χ-Fe5C2 (ICSD # 181367), (pink) θ-Fe3C (ICSD # 99018).
proposed methodology it was possible to obtain a solid material with the χ-Fe5C2 active phase as the main component. In the published scientific literature 350 °C has been reported as an adequate temperature for catalysts formation with a high content of the χ-Fe5C2 phase [7,10]. 3.3. Carburization at 350 °C under CO atmosphere The PRE-OA solid treated under CO at 350 °C was labeled as CATOA, and for comparison, the precursors PRE-H2O and PRE-Ox were also carburized at the same temperature and labeled as CAT-H2O and CATOx, respectively. According to the CO2 signal, the formation of iron carbides and carbonaceous material occurs during the four hours of carburization (Figure S3, Supporting information). The XRD patterns for the catalysts are shown in Fig. 6. The three diffraction patterns displayed peaks corresponding to the χ-Fe5C2 phase although not well
Fig. 6. XRD patterns of the catalysts obtained after carburization at 350 °C.
Fig. 7. Raman spectra of the catalysts active at 350 °C under CO atmosphere. 4
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Fig. 8. SEM and TEM images for the precursors carburized at 350 °C under CO atmosphere for 4 h; (a, d, g) SEM images for CAT-H2O, CAT-OA and CAT-Ox respectively; (b, c) TEM images for CAT-H2O; (e, f) TEM images for CAT-OA; and (h, i) TEM images for CAT-Ox.
hand, below 800 cm−1 the Raman spectra shows signals corresponding to the Fe2O3 at 223, 293 y 407 cm−1 while the broad peak at 670 cm−1 corresponds to magnetite [31], these oxides were possibly formed during the passivation process of the carbides [32]. Upon treatment with diluted oxygen the most reactive iron carbide particles are surface passivated to avoid a violent reaction when the solid is exposed to the atmosphere; however, due to the low content of these oxides it was not possible to identify them by XRD. The morphological and structural features of the catalysts were analyzed using SEM and TEM. According to SEM images, (Fig. 8a, d and g), the particles of CAT-H2O and CAT-OA retain a shape similar to that of the precursors whereas the commercial iron oxalate appears to have been fragmented. The TEM images indicate that CAT-OA and CAT-Ox catalysts mainly consist of particles in the range of 10 nm–60 nm that could be described as a core-shell system, where the core is constituted by iron carbide (i.e., χ-Fe5C2) and the shell corresponding to the carbonaceous material (about 3 nm–5 nm), this kind of particles was reported before using iron oxalate (II) with a defined cubic morphology [7]. In the case of CAT-H2O, some particles do not show carbonaceous material, which could correspond to siderite and magnetite particles, as detected by XRD. The magnetite in CAT-H2O possibly needs more time or temperature to be transformed into iron carbides; on the other hand, the siderite requires a higher temperature to decompose under CO atmosphere according to the CO-TPR (Fig. 3). Table 2 shows the textural properties of the catalysts. The specific surface areas were 42 m2 g−1, 84 m2 g−1 and 70 m2 g−1 for CAT-H2O, CAT-OA and CAT-Ox respectively, indicating an improvement in the apparent surface area of the iron ore with the oxalic acid treatment,
Table 2 Summary of the overall catalytic performance during 50 h–100 h of reaction. T=280 °C, H2/CO = 1, P =.20 bar. Sample
CAT-H2O CAT-OA CAT-Ox
Conversion (%) XCO
XH2
46.5 66.8 88.8
64.7 73.6 79.6
FTY*
0.36 0.42 0.42
CO2 (Cmol %)
HC distribution (C-mol %) CH4
C2-C4
C5+
30.0 36.7 42.3
11.7 11.8 9.2
25.9 26.5 24.8
62.4 61.7 66.0
* FTY: molCO gFe−1 s−1 x10−4.
possibly this is due to the porosity associated with the higher amount of carbonaceous material in CAT-OA (23.7%) compared to that in CATH2O (17.3%). Moreover, according to chemical analysis by atomic absorption spectroscopy, the iron content in the CAT-Ox is 72.8%, which is higher than those found in CAT-OA (57.3%) and CAT-H2O (54.5%). The lower iron content on the catalysts derived from the iron ore is consistent with the aluminosilicate impurities present in the parent mineral. 3.4. Catalytic performance of the iron carbides The catalytic activity of CAT-OA, CAT-H2O, CAT-Ox was evaluated at 280 °C in the Fischer-Tropsch reaction. Fig. 10a shows the CO conversion as a function of the reaction time. For CAT-OA and CAT-Ox, CO conversion gradually increased during the first 20 h of reaction reaching about 71% and 93% respectively. This increase could be partly due to the transformation of iron oxides formed in the passivation 5
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+ H2). The amount of CO2 depends on the CO and H2 conversion (XCO and XH2). At high XCO and XH2 the partial pressure of the H2O increases and H2 pressure decrease, these conditions favor the WGS [4], for this reason the CO2 selectivity is higher for the CAT-Ox catalyst (42.3%) than that of CAT-OA (36.7%) and CAT-H2O (30.0%), which have a lower conversion. In the present work the iron ore without treatment (CAT-H2O) showed a high C5+ hydrocarbons selectivity (62.4%) and best stability than that a similar iron ore without modifications employed in others works [15,17] using comparable reaction conditions; however, a lower stability, and lower C5+ hydrocarbons selectivity (37.3%) could be due to the non-identical conditions in the carburization process; in fact, in the cited works [15,17] the iron ore was carburized 280 °C instead of 350 °C as done in the present case. Moreover, it is noteworthy that in the procedure reported here a CAT-OA catalyst with high activity and stability was obtained without the need to add other metals or promoters such as Cu and K [15–17]. In addition, by comparison with literature, our results indicate that the performance of the catalyst is better than that for other catalysts prepared by more complex procedures (Table S5). To determine the effect of temperature on catalyst stability, conversion, selectivity and activity at two additional temperatures 300 °C and 320 °C were evaluated for CAT-OA catalyst, (20 bar, GHSV = 24 N L gcat−1 h−1, TOS =25 h). The CAT-OA catalyst showed good stability and a high activity of 0.87 × 10-4 molCO gFe−1 s−1 at 300 °C (XCO = 71.0%), and 0.93 × 10-4 molCO gFe−1 s−1 at 320 °C (XCO = 78.3%) (Figure S6, Supporting information). On the other hand, the selectivity (CO2 free) towards CH4 and C5+ was 17.0% and 50.4% at 300 °C and 22.1% and 45.1% at 320 °C respectively. (Table S5, Supporting information). The CO conversion, FTY, and the CO2 selectivity increase whit the temperature (see Table S5). Note however, that the iron-time-yield (FTY) which indicates the number of CO molecules converted to hydrocarbons per gram of iron per second, explicitly excluding carbon dioxide selectivity, does not consider the contribution of the water-gas shift reaction (WGS). The mole percent of CO converted to CO2 is an indicator of the WGS activity. Then, the increase of CO2 selectivity with temperature for CAT-OA catalyst suggests that the activation energy for the WGS reaction is larger than that for the FTS reaction. At the present conditions, the activity data should not be used to estimate an apparent activation energy (see Supporting Information) because the WGS and other secondary reactions as readsorption of olefins play an important role, thus, the overall FTS reaction rate is highly affected by the rate/ extent of the WGS reaction at high FTS conversions. The apparent activation energy should be better estimated at lower conversions where the FTS reaction rate is unaffected by the WGS reaction [34]. The recovered CAT-OA catalyst after 100 h of reaction was further analyzed by TEM and XRD (Fig. 10). In the TEM image of CAT-OA, the active particles mostly maintained their carbon-encapsulated structure which is consistent with the long time conversion stability, and indicates that the proposed methodology allows preparing catalysts as stable as other methodologies reported for carbon-encapsulated iron nanoparticles [36,37]. The high stability of the catalyst particles can be partly related to the core-shell structure, which protects the particle and prevents catalyst fragmentation [5,38]. According to XRD pattern, after 100 h reaction the main crystalline phase after reaction is still the carbide χ-Fe5C2, with a minor fraction of Fe3O4, this later phase being frequently found in used Fe-based FTS catalysts [8,26]. Nevertheless, more efforts are necessary to optimize the conditions for the formation of carbon-encapsulated iron nanostructures that can be used for the catalytic process of syngas conversion [39]. Finally, it is important to point out that the results of the present work let us to suggest that the proposed methodology for the preparation of Fe-based FTS catalyst from a Colombian iron ore could be also applied to iron ores of different origin and using fewer steps than the conventional methodologies from reagent-grade iron precursors
Fig. 9. (a) CO conversion, (b) activity: molCO gFe−1 s−1 (CO2 free), for 100 h of time on stream. 280 °C, 20 bars, H2/CO = 1, GHSV = 12 N L gcat−1 h−1.
process (observed by Raman, Fig. 7) into actives iron carbides, then the CO conversion slightly decreased to about 67% for CAT-OA and 89% for CAT-Ox at 100 h of time on stream, this shows that the catalysts continue to change over time. The higher CO conversion of the reference catalyst, CAT-Ox, could be due to the higher Fe content of 72.8% as compared to 57.3% for the CAT-OA. On the other hand, among the catalysts, the induction period was longer for CAT-H2O, which could be associated to the progressive carburization of the higher amount of iron oxide species present in this solid, as evidenced by the XRD results in Fig. 6. The higher CO conversion achieved with the CAT-OA, as compared to CAT-H2O, shows that the treatment with oxalic acid of the iron ore has positive effects on the activity. The higher activity of iron ore treated with oxalic acid is certainly related with the transformation of some of the iron phases present in the iron ore (e.g., siderite, Fig. 1) to iron oxalate, which was further transformed by the carburization process into the active χ-Fe5C2 carbide phase (Fig. 6). Fig. 9 b show the catalytic performance expressed as iron-time-yield (FTY, the number of CO molecules converted to hydrocarbons per gram of iron per second, explicitly excluding carbon dioxide selectivity). The FTY value between 50 h–100 h of time on stream for CAT-OA and CATOx was the same and approximately 0.42 × 10−4 molCO gFe-1 s-1, in contrast to the CAT-H2O which display an activity of 0.36 × 10−4 molCO gFe-1 s-1. Table 2 summarizes the overall catalytic performance of the catalysts evaluated. The overall CO conversion for CAT-OA was higher than that of CAT-H2O. Because of the different conversion degree, the selectivity data can only be compared qualitatively; however, the hydrocarbon distribution (CO2 free) shows that the CH4 selectivity data for all catalysts is relatively low (< 12 %) and the C5+ selectivity is higher than 60%. The hydrocarbon distributions are showed in detail in the supporting information Figure S5, the sum of C1 - C30 products were 96%, 92% and 84% for CAT-H2O CAT-OA and CAT-Ox respectively. Therefore, these results confirm that it is possible to prepare active and stable catalysts using this iron ore, with high selectivity to liquid fuel production in the FTS. The CO2 selectivity is indicative of the WGS activity of the catalysts, which also involves the additional formation of H2 (H2O + CO CO2 6
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Fig. 10. (a, b) TEM images, (c) particle size distribution, and (d) XRD pattern of CAT-OA catalyst after 100 h of reaction.
[40].
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4. Conclusions
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In this work, a simple methodology for the preparation of carbonencapsulated iron catalyst using an iron ore as a precursor is proposed. With the catalyst thus prepared, it was possible to achieve a high selectivity to C5+ hydrocarbons (61.7%) at 280 °C of reaction. The high activity (Fe-time yield: 0.42 × 10−4 molCO gFe-1 s-1 at 280 °C and 0.93 × 10−4 molCO gFe-1 s-1 at 320 °C) of the catalyst is mainly attributed to two factors: the proper formation of the active χ-Fe5C2 phase, and the formation of a carbonaceous layer that 'protects' the active phase, which result in the formation of a 'core-shell' catalytic system. The preparation of the catalyst was achieved by a simple strategy of modifying an iron ore, due to the high reactivity of the siderite and goethite phases upon oxalic acid treatment, forming iron oxalates. Our results showed that the activation temperature is a determining factor in the obtaining the FTS active iron carbide phase (i.e., χ-Fe5C2) from the iron ore as a raw material, being 350 °C the optimum temperature for the preparation of this phase. At higher temperatures the formation of a less active iron carbide (θ-Fe3C) and carbonaceous materials are favored. In the core-shell particles (χ-Fe5C2@C), the shell was formed by the Boudouard reaction, giving stability to the catalyst up to 100 h of time on stream. It is worth noting that the structure χ-Fe5C2@C was stable after the reaction according to TEM images. Finally, the proposed method significantly reduced the catalyst preparation steps while using an inexpensive raw material for the sustainable preparation of highly active and stable Fe-based catalysts for the FTS. Acknowledgment The authors thank to the University of Antioquia for the financial support (Project 2014-1088). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2019.117264. 7
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