English

• Fisher-Tropsch synthesis (FTS) has been recognized as one of the most promising alternative and sustainable production technologies of liquid transportation hydrocarbon fuels (gasoline, diesel, jet fuel, etc.) and industrial chemicals. The FT process involves a catalytic conversion of synthesis gas (syngas), derived from a variety of different feed-stocks that contain carbon (natural gas, coal, and biomass), into hydrocarbons of different molecular weights^[1]. This conversion process is known to be catalyzed by certain transition metals, such as iron (Fe), cobalt (Co) and ruthenium (Ru), which are deemed to be the most active metals for FTS. Both Fe and Co are the most common metal catalysts used in industrial scale FTS because of their high activity, low methane selectivity, low cost and high water gas shift (WGS) activity^{[2, 3]}.

  Hydrocarbon product selectivity control is the biggest research challenge in FTS. In order to develop excellent catalytic performance of the Co catalyst with control in hydrocarbon product selectivity, many attempts have been made to control the factors that affect FTS performance. Among these factors is maintaining the dispersion of the active phase on the support, which plays an important role in influencing catalytic activity^[4]. Therefore, the Co₃O₄ species has a much lower dispersion on SiO₂ support compared to Al₂O₃ support^[5]. The use of chelating agents to modify the support, such as nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA), have been studied due to their potential for improving the dispersion of Co₃O₄ on the support surface, thus enhancing the activity and selectivity of the catalyst^{[4, 6-8]}. More recently, the use of various chelating agents has been used in many industries for preparing catalysts, such as FTS, oxidation, hydrogenation, nuclear industry, pharmaceuticals and hydrodechlorination processes^{[9, 10]}. The main reason for using EDTA in FTS is to form a new chemical phase between EDTA and supporting material. For example, silanol groups (SiOH) on the SiO₂ surface support with EDTA are able to form a stable strong complex with the metal^[11]. In regard to the FTS industrial applications, it has been found that the Co/SiO₂ catalyst showed a high dispersion of Co₃O₄ species as well as a relatively high FTS activity after EDTA was modified onto catalyst^[4].

  As mentioned earlier, several studies have been conducted to verify the effects of different chelating agents, such as EDTA, on Co catalysts supported on conventional oxides (e.g., SiO₂, Al₂O₃, ZnO and MgO/Al₂O₃)^{[12, 13]}, but there have been no studies done using chelating agents (EDTA) to modify one dimensional (1D) nanostructured supports, such as silica nanosprings (NS), for FTS. Silica NS is a new 1D support material for catalysis and has been recognized as meeting the criteria of supports because they have high surface area (350 m²/g), high thermal stability (up to 900 ℃), easy to grow, and can be grown in various surfaces^[14]. NS supports have recently been used as a support material for FTS and offers great potential for FTS applications because of their unique properties^{[1, 15, 16]}.

  The objective of the present study is to investigate the effect of EDTA modification on the catalytic performance of 15% Co/NS catalyst for FTS and describe the dispersion of the catalyst particles and surface properties. The properties of prepared catalysts were comparatively characterized by various analytical techniques, such as surface area by BET, hydrogen temperature programmed reduction (H₂-TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), differential thermal analysis (DTA), Fourier transform-infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA). In addition, the CO conversion and hydrocarbon selectivity of the studied catalysts were determined by gas chromatography (GC) and GC-mass spectrometry (GC-MS) analyses.

  1.   Experimental

  1.1   Catalyst preparation

  The silica NSs were synthesized in 0.5 g batches and then heated at 600 ℃ for 5 h to remove any residual precursors^[1]. The NS were dried at 110 ℃ overnight before use. The unmodified 15% Co/NS and modified 15% Co/NS-EDTA catalysts were synthesized by aqueous incipient wetness impregnation method. To obtain the unmodified Co/NS catalyst, NS (100 mg in 15 mL of ethanol) was impregnated with an aqueous solution containing Co (NO₃)₂·6H₂O (88 mg in 15 mL of water). For the synthesis of modified Co/NS-EDTA catalyst, an aqueous solution containing EDTA (C₁₀H₁₆N₂O₈) (43 mg in 15 mL of water) was first impregnated into the NS (100 mg in 15 mL of ethanol) support at room temperature. The pH values of the solutions were adjusted to 9.0 by adding 28% aqueous NH₃. Then, the aqueous solution containing Co (NO₃)₂·6H₂O (90 mg in 15 mL of water) was added to the EDTA/NS. Both catalysts were stirred at 70 ℃ for 12 h, the solutions were evaporated at 110 ℃ for 12 h, and then the obtained catalysts were calcined immediately in an air atmosphere at 550 ℃ for 5 h.

  1.2   Catalyst characterization

  The reduction behavior of the prepared catalysts were studied by H₂-TPR using a ChemiSorb 2720 instrument (Micrometrics, USA) with a TCD detector. The TCD was calibrated by the reduction of CuO (20 mg, 99.99%) between 25 and 500 ℃. Prior to running each TPR experiment, the catalyst sample (30 mg) was flushed with N₂ (30 mL/min) at 150 ℃ for 1 h to remove the surface impurities and then cooled down to room temperature. H₂-TPR experiments were conducted in a 10% H₂ in N₂ atmosphere. The total flow was adjusted to 30 mL/min. The temperature was ramped from room temperature to 1000 ℃ at a heating rate of 10 ℃/min.

  The S_BET-specific surface area measurements of all degassed (220 ℃ for 30 min) catalysts (60-80 mg) were determined by an N₂ adsorption-desorption isotherm at -196 ℃ using a Micromeritics Flowsorb Ⅱ 2300 instrument.

  FT-IR spectra of the calcined catalysts (10% mixed with KBr) was recorded at room temperature in the range 480-3500 cm^-1 by diffuse reflectance (5% in KBr) using a ThermoScientific iS10 spectrometer.

  XRD measurements of the unmodified Co/NS and modified Co/NS-EDTA catalysts were recorded on a Siemens D500 powder diffractometer with Cu/Kα radiation (λ= 0.154 nm). The diffraction intensities were recorded from 10°-80° (2θ value) with 0.01° step using a 1 s acquisition time per step. The average crystallite size (d_XRD) in nm of Co₃O₄ was estimated using Scherrer′s equation^[17]:

  $ {d_{{\rm{XRD}}}} = \frac{{0.9\lambda }}{{\beta \cos \theta }} $

  (1)

  where, 0.9 is the shape factor (for spherical shape particles), λ is the wavelength of X-ray (λ = 0.154 nm), β is line broadening at half the maximum intensity (FWHM) in radians, and θ is the Bragg angle. The average particle size of d(Co⁰) was estimated from the d_XRD(Co₃O₄) according to the following equation^[18]:

  $ {d_{{\rm{XRD}}}}({\rm{C}}{{\rm{o}}^0}){\rm{ = }}0.75{d_{{\rm{XRD}}}}({\rm{C}}{{\rm{o}}_3}{{\rm{O}}_4}) $

  (2)

  where, d_XRD(Co⁰) is an average particle size in nm of Co metal and d_XRD(Co₃O₄) is the average particle size of Co oxide. The (Co⁰) metal dispersion (D_XRD) was determined from d(Co⁰) by assuming that the cobalt particles have a spherical geometry with a uniform site density of 14.6 atoms/nm² using the following equation^[2]:

  $ {D_{{\rm{XRD}}}} = \frac{{0.96}}{{{d_{{\rm{XRD}}}}({\rm{C}}{{\rm{o}}^0})}} $

  (3)

  The surface morphology of the catalysts (dispersed in ethanol and applied to a copper grid coated with carbon support film) was characterized by TEM (JEOL JEM-2010) operated at 200 kV. The average particle size (d_TEM) of Co₃O₄ was measured on TEM images using ImageJ software. The thermal stability of unmodified Co/NS, modified Co/NS-EDTA catalysts and virgin NS were tested using by TGA and DTA analyses respectively on Perkin Elmer TGA-7 and DTA-7 instruments from 30 to 900 ℃ at 20 ℃/min under N₂ (30 mL/min). X-ray photoelectron spectroscopy (XPS) measurements were recorded on a custom built ultrahigh vacuum (UHV) chamber using a dual anode X-ray lamp, XR 04-548 from PHYSICAL ELECTRONICS, and kinetic energy of the photoelectrons was measured with an Omicron EA 125 hemispherical energy analyzer with a net resolution of 25 meV. The X-ray source used was the Al Kα line at 1486.6 eV.

  1.3   Catalyst evaluation for the FT reaction

  Prior to FTS, the catalysts (~20 mg were calcined and mixed with 40 mg quartz sand) were loaded into a quartz fix-bed micro-reactor (Φ10 mm × 300 mm with a "0" quartz frit connected 180 mm from the top to support the catalyst) housed in a tube furnace (Φ25 mm × 150 mm), and then reduced in H₂ (40 mL/min) in N₂ (60 mL/min) gas mixture using mass flow controllers (CG1, Dakota Instruments). The reactor temperature was increased from room temperature to 700 ℃ for 24 h at atmospheric pressure. The activated catalyst was cooled to 175 ℃ and subsequently used for in-situ FTS reaction. After catalyst activation, H₂ (60 mL/min), CO (30 mL/min) and N₂ (10 mL/min) were fed to the reactor at 230 ℃ using mass flow controllers (CG1, Dakota Instruments) at atmospheric pressure. The FTS reaction products were collected in a three-stage impinger trap placed in a liquid nitrogen bath. The uncondensed vapor stream was collected in a Tedlar^TM PVF (300 × 300 mm²) gas-sampling bag. FTS reaction was operated for 13 h under reaction temperature of 230 ℃. Unreacted gases and gaseous reaction products were analyzed by GC-TCD (GOW-MAC, Series 350) with a packed HaySep DB stainless steel column (Φ3.3 mm× 9.1 m) at 25 ℃ for CO, CO₂, H₂, N₂ and CH₄ and a packed PoraPakQ stainless steel column (Φ3.3 mm × 1.8 m) at 60 ℃ for C_xH_y (x ≤ 4)) on elution with He. The liquid products C_xH_y (x≥ 5) collected were then identified by GC-MS (Focus-ISQ, ThermoScientific). Separation was achieved on a ZB5ms (Φ 0.25 mm × 30 m, Phenomenex) capillary column with a temperature program of 40 ℃ (1 min) ramped to 250 ℃ at 5 ℃ /min. Data was analyzed using the Xcalibur v2.2 software. The identity of the compounds was determined with n-alkane standards (C₆ to C₃₀) and mass spectral matching with the NIST 2017 mass spectral library. The results in terms of CO conversions (%) and selectivities product (%) were calculated according to Alayat et al^[1].

  2.   Results and discussion

  2.1   Catalyst preparation and characterization

  Surface area (S_BET) measurements, particle diameters and dispersions of unmodified Co/NS and modified Co/NS-EDTA catalysts are given in Table 1. The S_BET of virgin NS was 314 m²/g, while the Co/NS and Co/NS-EDTA catalysts had lower S_BET surface areas of 193 and 94.5 m²/g, respectively. The decrease in surface area is likely due to Co oxides being incorporated inside the NS pores after the impregnation and further by EDTA blocking of the NS pores^{[11, 19]}. The average crystallite size of Co particles in Co/NS and Co/NS-EDTA catalysts, as determined using equation (1), was found to be 12.4 and 11.8 nm, respectively. A positive correlation was observed between the average crystallite sizes of Co particles obtained by d_TEM and d_XRD (Table 1) and corroborated by TEM and XRD (discussed later).

  Table 1

  

  Table 1.  Physical characteristics of unmodified Co/NS, modified Co/NS-EDTA catalysts, and virgin NS

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  Catalyst	Co w/%	SBET /(m2·g-1)	Size of Co3O4 particles d/nm		dXRD(Co0) /nm	Co dispersion/%
  			dXRD	dTEM
  NS	-	314	-	-	-	-
  Co/NS	15	193	12.4	9.4	9.3	10.3
  Co/NS-EDTA	15	94.5	11.8	14.6	8.8	10.9

  The H₂-TPR experiments were performed to investigate the reducibility of the Co catalysts. Figure 1 shows the H₂-TPR profiles of the calcined Co/NS and Co/NS-EDTA catalysts. The H₂-TPR of calcined Co/NS and Co/NS-EDTA catalysts suggests a two-step reduction of Co₃O₄ according to the following equations:

  Figure 1

  

  Figure 1.  H₂-TPR profiles of calcined Co/NS and Co/NS-EDTA catalysts

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  $ {\rm{C}}{{\rm{o}}_{\rm{3}}}{{\rm{O}}_{\rm{4}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}} \to {\rm{3CoO + }}{{\rm{H}}_{\rm{2}}}{{\rm{O}}_{{\rm{(g)}}}} $

  (5)

  $ {\rm{3CoO + 3}}{{\rm{H}}_{\rm{2}}} \to {\rm{3Co + 3}}{{\rm{H}}_{\rm{2}}}{{\rm{O}}_{{\rm{(g)}}}} $

  (6)

  It can clearly be seen in Figure 1 that the Co/NS catalyst has two main peaks located at 396 and 545 ℃, corresponding to the reduction of Co₃O₄ to CoO and that of CoO to Co⁰, respectively. The H₂-TPR profile of the calcined Co/NS-EDTA catalyst was clearly different from that of the H₂-TPR profile of the Co/NS catalyst. For both catalysts, the two-step reduction to Co⁰ occurs at 368 ℃ and continues up to 700 ℃, consisting of two peaks centered at 447 and 522 ℃. The minor reduction peak occurring around 368 ℃ in Co/NS-EDTA catalyst can tentatively be assigned to the reductive decomposition of residual cobalt nitrate. Moreover, two minor peaks are observed around 700 ℃ in the H₂-TPR profiles of both catalysts. Due to the high temperature of their reduction, that are tentatively assigned to the reduction of cobalt silicate Co₂SiO₄, where the high reduction temperature is due to the strong interactions between Co particles and Si-OH of the NS support^{[8, 13, 18]}. Moreover, it is concluded from this result that the addition of EDTA shifts slightly the second reduction peak to a lower reduction temperature, as compared to Co/NS catalysts. The results obtained herein are in general agreement with data previously reported^[20].

  The phase structure and particle size of cobalt oxide in the calcined Co/NS and Co/NS-EDTA catalysts were determined by XRD analysis. The diffractograms are shown in Figure 2. The peak around 23° in the diffractogram of Co/NS catalyst is assigned to crystal planes of the silica NS^{[21, 22]}. For the Co/NS-EDTA catalyst, a lower intensity diffraction peak at around 23° is observed (Figure 2), where the lower intensity may be due to effects of EDTA on the structure of silica NS during functionalization. Both catalysts present characteristic diffraction peaks centered at 2θ= 22.8°, 31.4°, 36.9°, 38.6°, 44.8°, 59.6° and 65.5°, which correspond to the (111), (220), (311), (222), (400), (511), and (440) diffraction planes of Co₃O₄ (ICSD-36256), respectively^{[3, 23]}. The addition of EDTA to Co/NS catalyst displayed sharp and intense diffraction peaks indicating that the crystallinity of Co₃O₄ phase is improved. However, it is also found that no new Co phase forms when the EDTA is added to the Co/NS catalyst. The average size of Co₃O₄ crystallites was estimated by the Scherrer's equation using the Co₃O₄ diffraction peak (311) at 2θ= 36.9°. The Co₃O₄ crystallite size for Co/NS catalyst (12.4 nm) is similar after the addition of EDTA (11.8 nm). The Co/NS-EDTA catalyst also exhibits a slight metal dispersion increase of 6% relative to the Co/NS catalyst. It is also found that in comparison to the Co/NS catalyst, the size of metallic Co in the Co/NS-EDTA catalyst decreases by 5% to 8.8 nm.

  Figure 2

  

  Figure 2.  X-ray diffractograms of calcined Co/NS and Co/NS-EDTA catalysts

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  TEM was performed to provide additional information of the Co₃O₄ particle size, dispersion and morphology on the NS support (Figure 3(a)-3(f)). Figures 3(a)-3(c) of the Co/NS catalysts demonstrate that the nano-helical 1D structure of the support is retained and that the Co₃O₄ particles have a spherical morphology. The average particle size of the Co₃O₄ (d_TEM) in Co/NS catalyst is approximately 9.4 nm. However, with the addition of EDTA to the Co/NS catalyst, agglomeration of Co particles occurs and are randomly dispersed on the NS surface. Thus, the nano-helical structure of Co/NS catalyst transforms into a nanoparticle agglomeration/aggregate with an average Co₃O₄ particle size of 14.6 nm (Figure 3(d)-3(f)). TEM analysis shows that the addition of EDTA marginally improved the dispersion of Co₃O₄ from 10.3% to 10.9% (Table 1). The crystallite size of Co₃O₄ for Co/NS and Co/NS-EDTA catalysts measured by d_XRD matched reasonably well to that measured by d_TEM measurements (Table 1).

  Figure 3

  

  Figure 3.  Transmission electron micrographs of calcined catalysts (a)-(c) Co/NS and (d)-(f) Co/NS-EDTA

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  The surface chemical composition of the calcined catalysts was determined by XPS survey scans, which are shown in Figure 4. The XPS survey scan for Co/NS present photoelectron lines corresponding to C 1s, O 1s, Co 2p, Si 2p, and Si 2s, while the Co/NS-EDTS spectra showed peaks for C 1s, O 1s, and Co 2p. The high resolution XPS spectra of Co 2p for calcined Co/NS (a) and Co/NS-EDTA catalysts exhibit a characteristic doublet corresponding to the spin-orbit coupled 2p_3/2 and 2p_1/2 states with binding energies at approximately at 780.0 and 795.6 eV and (779.1 and 794.5 eV), respectively (Figure 5). The spin-orbit splitting between the Co 2p_3/2 and Co 2p_1/2 in Co/NS and Co/NS-EDTA catalysts are 15.6 and 15.4 eV, respectively. These findings indicate that the Co oxides in the Co/NS and Co/NS-EDTA catalysts are the Co₃O₄ phase and this is in accordance with the literature^{[13, 24]}. The binding energy of Co 2p_3/2 and Co 2p_1/2 in the Co/NS-EDTA catalyst are shifted to lower binding energy by 0.9 eV relative to those of the Co/NS catalyst. This binding energy shift of Co 2p states may be due to complex between the NS, EDTA and/or Co₃O₄ nanoparticles^[13]. The XPS results suggest that the surface structure of Co/NS is significantly influenced by the addition of EDTA.

  Figure 4

  

  Figure 4.  Survey XPS spectra of the calcined Co/NS and Co/NS-EDTA catalysts

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  Figure 5

  

  Figure 5.  High resolution XPS spectra of the Co 2p of calcined Co/NS and Co/NS-EDTA catalysts

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  The TGA and DTA analyses of the calcined catalysts were performed to help identify thermal changes that occur during the activation process and FT reaction. The TGA and DTA thermograms of calcined Co/NS, Co/NS-EDTA catalysts and virgin NS are shown in Figure 6(a) and 6(b), respectively. Figure 6(a) shows total weight loss (at 900 ℃) of approximately 6.6%, 10.9% and 17.6% for the Co/NS, Co/NS-EDTA and virgin NS, respectively. This moderate weight loss is probably due to evaporation of adsorbed material and NS precursor on the NS and both catalysts^{[1, 2]}.Additionally, the weight loss for > 500 ℃ is assigned to complete removal of volatile material during calcination, which corresponds to exothermic peaks between 500-700 ℃ in DTA thermograms for NS and Co/NS catalyst (Figure 6(b)). However, the initial exothermic peaks in both catalysts and NS at ~100 ℃ is likely to the evaporation of adsorbed water. Interestingly, the DTA thermograms for the Co/NS-EDTA catalyst exhibits a very weak/or broad exothermic peak at ~650 ℃, as compared with those for NS and Co/NS. This suggests that restructuring of the surface in Co/NS-EDTA catalyst results in the formation of a stable Co/NS-EDTA complex. In general, the TGA and DTA analyzes of the Co/NS and Co/NS-EDTA catalysts exhibit very little thermal decomposition up to 550 ℃, which clearly implies that both catalysts have good thermal stability in the FTS reaction temperature range.

  Figure 6

  

  Figure 6.  (a) TGA and (b) DTA thermograms of Co/NS, Co/NS-EDTA and virgin style="class:table_top_border2"

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  FT-IR spectroscopy was performed on each as-prepared catalyst to identify the presence of surface functional groups, as well as to examine the removal by calcination at 550 ℃ of organic species associated with the preparation of the catalysts (Figure 7). The FT-IR spectra of NS, Co/NS and Co/NS-EDTA all show characteristic Si-O-Si stretching and Si-O bands at around 1086 cm^-1 (with a shoulder at 1251 cm^-1) and 802 cm^-1, respectively, and are in good agreement with the literature^[25-27]. In addition, the absorption bands at approximately 2975 cm^-1 and between 1411 and 1460 cm^-1 are assigned to C-H stretching and bending vibration modes, respectively. The band at around 1637 cm^-1 is attributed to the H-O-H stretching and vibration of hydrogen bonded surface silanol groups and physically adsorbed water^[27-29]. Furthermore, the absorption bands of Co-O bond are observed in both catalysts at approximately 586 and 664 cm^-1, and are once again in good agreement with the literature^[30]. The band at around 943 cm^-1 is due to the C-C stretch of the carboxyl group^[20]. Additionally, the Co/NS-EDTA catalyst displays a slight increase in the intensity of the bands at 1452 and 1773 cm^-1, as compared to Co/NS catalyst. This may be attributed to the interactions of carboxyl groups (COO^-) of EDTA with inorganic hydroxyl groups (OH) present on the surface of the NS^[31]. The above results verify that the NS are functionalized with EDTA.

  Figure 7

  

  Figure 7.  FT-IR spectra of unmodified Co/NS, modified Co/NS-EDTA catalysts and virgin style="class:table_top_border2"

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  2.2   Catalyst evaluation

  The condensable liquid products of the FT process for the two catalysts were analyzed by GC-MS (Figure 8 and supplementary Tables 1 and 2) and the non-condensable gases (CO, CO₂, H₂, N₂, CH₄, and C_2-4) were analyzed by GC. The FTS activities, carbon selectivity to different product ranges and the paraffin to olefin ratios over calcined catalysts have been determined (Table 2). The FTS activity of the unmodified Co/NS catalyst increases from 65.5% to 70.3% with the addition of EDTA to the Co/NS catalyst.

  Figure 8

  

  Figure 8.  Total ion current chromatograms of condensable FTS products from the (a) Co/NS and (b) Co/NS-EDTA catalysts

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  Table 2

  

  Table 2.  Catalytic performance and major components of synthesized liquid F-T fuel over Co/NS and Co/NS-EDTA catalysts at 230 ℃, H₂/CO of 2:1 and at atmospheric pressure

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  Catalyst	Co/NS	Co/NS-EDTA
  CO conversion x/%	65.5	70.3
  H2 conversion x/%	61.2	73.2
  Products selectivity s/%
  CO2 select. s/%	5.3	3.4
  CH4 select. s/%	6.7	12.7
  ∑ < C5	17.1	16.6
  Product distribution wm/%
  ∑ C6-11 (naphtha range fuel)	59.3	34.4
  ∑ C12-15 (jet range fuel)	11.6	21.3
  ∑ > C15(diesel range fuel)	-	11.6
  ∑ > C6	70.9	67.3
  C6-18 paraffins	18.4	28.7
  C6-18 olefins	26.6	16.5
  C6-18 naphthenes	17.3	13.8
  C6-18 oxygenates	8.6	8.3
  Paraffins/olefins (P/O)	0.69	1.72

  The FT hydrocarbon product distribution is in the carbon number range of C_6-18 (jet A) with a CO conversion of 70.3%, while the carbon number range of C_6-14 (naphtha fraction) forms over the unmodified Co/NS catalyst with a CO conversion of 65.5% (Figure 9). The enhanced catalytic performance of Co/NS-EDTA catalyst may be due to Co particle dispersion and size by the addition EDTA^{[4, 13]}. Several research groups have reported that the addition of EDTA to Co catalyst gives higher CO conversion activity and high selectivity than the unmodified Co catalyst^{[7, 13, 32]}. For instance, Bambal et al^[7] has shown improvements (dispersion, CO and H₂ conversion, and selectivity towards diesel) using Co/SiO₂ catalyst that was prepared using Co nitrate with EDTA.

  Figure 9

  

  Figure 9.  Production distribution of FT hydrocarbons (C_6-18) from Co/NS and Co/NS-EDTA catalysts

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  The Co/NS and Co/NS-EDTA catalysts both have undesirable selectivities to C₁, <C₅ and CO₂ products as shown in Table 2. The selectivity towards CO₂ and <C₅ is slightly decreased, while the selectivities towards methane (C₁) increase with the addition of EDTA. Moreover, the addition of EDTA to Co/NS increased the product distribution towards C_6-18 (Jet A or JP-8 range fuels) (Table 2). The Co/NS catalyst favors the formation of hydrocarbons centered in C_6-14 with 59.3% and 11.6% selectivities of naphtha and jet fuel, respectively. Whereas, the 34.4%, 21.3% and 11.6% selectivities of naphtha range fuels (C_6-11), jet fuel (C_12-15) and diesel (C_16-18), respectively, are achieved over the Co/NS-EDTA catalyst.

  The FTS hydrocarbons products obtained from Co/NS catalyst are found to be qualitatively and quantitatively different from those produced by the Co/NS-EDTA catalyst (Table 2). The main hydrocarbon products in the C_6-18 range for Co/NS catalyst are paraffins (18.4%), olefins (26.6%), naphthenes (cycloalkanes) (17.3%) and a small amount of oxygenated products (8.6%) at 70.9% total hydrocarbons selectivity. While the Co/NS-EDTA FTS products are 28.7% paraffins, 16.5% olefins, 13.8% naphthenes and 8.3% oxygenated products with a 67.3% total hydrocarbons selectivity (Table 2). Unlike the Co/NS catalyst, which favors olefins, the Co/NS-EDTA catalyst favors paraffins over olefins and naphthenes. This is possibly due to the Co morphology when using EDTA. However, the ratio of olefin to paraffin (P/O) of the hydrocarbons >C₆ increases with the addition of EDTA to the catalyst. The paraffin to olefin (P/O) ratio for Co/NS and Co/NS-EDTA catalysts are 0.69 and 1.72, respectively. This increase in paraffin content is likely attributable to an enhanced hydrogenation rate^[33]. FTS with the Co/NS-EDTA catalyst promotes longer chain hydrocarbons. However, under the same FT reaction conditions, the distributions of naphthene selectivities are 17.3% and 13.8% for Co/NS and Co/NS-EDTA catalysts, respectively. While, the distributions of oxygenate byproducts selectivity is almost the same for both catalysts.

  The FTS CO conversion was monitored as a function of reaction time (up to 13 h) for both catalysts at 230 ℃ and shown in Figure 10. The CO conversion in both catalysts is relatively constant. The conclusion for both catalysts is that no significant deactivation in catalytic stability occurred during the 13 h run of the FT reaction.

  Figure 10

  

  Figure 10.  CO conversion with time on stream Co/NS and Co/NS-EDTA catalysts

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  3.   Conclusions

  In the present work the effects of EDTA on the physico-chemical properties and catalytic activity of NS supported Co catalyst was studied. Unmodified Co/NS and modified Co/NS-EDTA catalysts were successfully prepared using impregnation method, and investigated in a quartz fix-bed micro-reactor for FTS. The modified Co/NS by the addition of EDTA (complex formation with Co²⁺) significantly led to better Co₃O₄ dispersion of supported Co on NS support, decreased Co₃O₄ crystal size, shifted the reduction at lower reduction temperature, as well as improved the catalytic activity and product selectivity of FTS as compared to Co/NS catalyst. The selectivity to total C_6-18 hydrocarbons reaches about 67.3% with a ratio of paraffins to olefins of about 1.7. The improved FT performance of Co/NS-EDTA catalysts is ascribed to the formation of a stable complex with Co ions. The overall catalytic performance of the Co/NS-EDTA catalysts displays a promising industrially applicable catalyst in the direct synthesis of the jet fuel range and diesel from syngas, with total selectivity to the C_6-18 products. In general, the addition of EDTA to Co/NS catalyst had a significant enhancing effect on the physico-chemical properties, FT activity, and selectivity of the catalyst.

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  Figure 1  H₂-TPR profiles of calcined Co/NS and Co/NS-EDTA catalysts

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  Figure 2  X-ray diffractograms of calcined Co/NS and Co/NS-EDTA catalysts

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  Figure 3  Transmission electron micrographs of calcined catalysts (a)-(c) Co/NS and (d)-(f) Co/NS-EDTA

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  Figure 4  Survey XPS spectra of the calcined Co/NS and Co/NS-EDTA catalysts

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  Figure 5  High resolution XPS spectra of the Co 2p of calcined Co/NS and Co/NS-EDTA catalysts

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  Figure 6  (a) TGA and (b) DTA thermograms of Co/NS, Co/NS-EDTA and virgin style="class:table_top_border2"

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  Figure 7  FT-IR spectra of unmodified Co/NS, modified Co/NS-EDTA catalysts and virgin style="class:table_top_border2"

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  Figure 8  Total ion current chromatograms of condensable FTS products from the (a) Co/NS and (b) Co/NS-EDTA catalysts

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  Figure 9  Production distribution of FT hydrocarbons (C_6-18) from Co/NS and Co/NS-EDTA catalysts

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  Figure 10  CO conversion with time on stream Co/NS and Co/NS-EDTA catalysts

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  Table 1.  Physical characteristics of unmodified Co/NS, modified Co/NS-EDTA catalysts, and virgin NS

  Catalyst	Co w/%	SBET /(m2·g-1)	Size of Co3O4 particles d/nm		dXRD(Co0) /nm	Co dispersion/%
  			dXRD	dTEM
  NS	-	314	-	-	-	-
  Co/NS	15	193	12.4	9.4	9.3	10.3
  Co/NS-EDTA	15	94.5	11.8	14.6	8.8	10.9

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  Table 2.  Catalytic performance and major components of synthesized liquid F-T fuel over Co/NS and Co/NS-EDTA catalysts at 230 ℃, H₂/CO of 2:1 and at atmospheric pressure

  Catalyst	Co/NS	Co/NS-EDTA
  CO conversion x/%	65.5	70.3
  H2 conversion x/%	61.2	73.2
  Products selectivity s/%
  CO2 select. s/%	5.3	3.4
  CH4 select. s/%	6.7	12.7
  ∑ < C5	17.1	16.6
  Product distribution wm/%
  ∑ C6-11 (naphtha range fuel)	59.3	34.4
  ∑ C12-15 (jet range fuel)	11.6	21.3
  ∑ > C15(diesel range fuel)	-	11.6
  ∑ > C6	70.9	67.3
  C6-18 paraffins	18.4	28.7
  C6-18 olefins	26.6	16.5
  C6-18 naphthenes	17.3	13.8
  C6-18 oxygenates	8.6	8.3
  Paraffins/olefins (P/O)	0.69	1.72

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