Alumina Coated Silica Nanosprings (NS) Support Based Cobalt Catalysts for Liquid Hydrocarbon Fuel Production From Syngas

Abstract

The effects of Al₂O₃ coating on the performance of silica nanospring (NS) supported Co catalysts for Fischer–Tropsch synthesis (FTS) were evaluated in a quartz fixed-bed microreactor. The Co/NS-Al₂O₃ catalysts were synthesized by coating the Co/NS and NS with Al₂O₃ by an alkoxide-based sol-gel method (NS-Al-A and NS-Al-B, respectively) and then by decorating them with Co. Co deposition was via an impregnation method. Catalysts were characterized before the FTS reaction by the Brunauer–Emmett–Teller (BET) method, X-ray diffraction, transmission electron microscopy, temperature programmed reduction, X-ray photoelectron spectroscopy, differential thermal analysis and thermogravimetric analysis in order to find correlations between physico-chemical properties of catalysts and catalytic performance. The products of the FTS were trapped and analyzed by GC-TCD and GC-MS to determine the CO conversion and reaction selectivity. The Al₂O₃ coated NS catalyst had a significant affect in FTS activity and selectivity in both Co/NS-Al₂O₃ catalysts. A high CO conversion (82.4%) and Σ > C₆ (86.3%) yield were obtained on the Co/NS-Al-B catalyst, whereas the CO conversion was 62.8% and Σ > C₆ was 58.5% on the Co/NS-Al-A catalyst under the same FTS experimental condition. The Co/NS-Al-A catalyst yielded the aromatic selectivity of 10.2% and oxygenated compounds.

1. Introduction

Fischer–Tropsch Synthesis (FTS) has been recognized as a promising route to produce environmentally clean liquid fuels and industrial chemicals (heavy and light hydrocarbons) from renewable feed-stocks [1]. FTS is a heterogeneous catalyzed polymerization reaction of syngas (mixture of CO and H₂), which is derived from the gasification of a variety of feed-stocks (natural gas, coal, and biomass), into a wide range of molecular weight hydrocarbon chains. The stoichiometry of FTS reactions can be written as Equations (1 and 2):

nCO + (2n +1)H₂ → C_nH_2n+2 + nH₂O

(1)

nCO + 2nH₂ → C_nH_2n + nH₂O

(2)

For this reaction, several Group VIII transition metals, such as iron (Fe), cobalt (Co) and ruthenium (Ru), are all active in FTS, but only Fe and Co are used for industrial application because of their high activity, low methane selectivity, low cost and high water gas shift (WGS) activity [1,2,3]. The FTS efficiency and hydrocarbon product distribution depend on variables, such as catalyst specifications (e.g., nature and composition of the catalyst, promoters, support, etc.) and processes conditions (e.g., reactor type, temperature and reaction pressure, activation and preparation methods, etc.) [1,4]. The type of inorganic support (e.g., SiO₂, Al₂O₃, TiO₂, activated carbon, zeolite, etc.) plays a key role in the control of catalytic activity, reducibility and the product distribution for FTS [5,6]. It is known that Co has a low dispersion on SiO₂ support because of the weak interaction between Co nanoparticles and silica, which leads to agglomeration of Co particles. This problem can be overcome by modifying or coating the silica support with an oxide promoter such as Al₂O₃. The use of these metal oxides supports can improve the surface properties such as dispersion of Co on the support [3,6,7].

Coating the silica support can enhance the catalytic activity of the Co catalysts by changing properties like the surface acidity [5,8]. A number of studies have shown improved activity and/or selectivity of Co-based FTS catalysts with silica supports by mixing with Al₂O₃ [6,7]. For example, Zhang et al. [6] and Savost’yanov et al. [5] studied the effect of Al₂O₃ promoter on the FTS performances of the Co/SiO₂ catalyst. They found that the addition of Al₂O₃ onto the Co/SiO₂ catalyst significantly improved the activity and selectivity to >C₅ hydrocarbons by adjusting surface properties such as Co dispersion and reduction temperature. To date, we are unaware of FTS studies using Al₂O₃ to modify 1 dimensional (1D) nanostructured supports, such as silica nanosprings (NS). Silica NS are a new 1D support materials for catalysts, and have been demonstrated to meet the criteria of supports for FTS applications [1,2,9].

Therefore, the objective of the present study is to investigate the effects of the Al₂O₃ coating on the physico-chemical properties and the catalytic performance of Co/NS catalysts during FTS. The properties of prepared catalysts were comparatively characterized by various analytical techniques such as surface area, 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 (FTIR) and thermogravimetric analysis (TGA). In addition, the CO conversion and hydrocarbon selectivity of the FTS catalysts were determined by gas chromatography (GC) and GC-mass spectrometry (GC-MS) analyses.

2. Experimental Methods

2.1. Preparation of Catalysts

The silica NS were synthesized in 0.5 g batches by using a gold-catalyzed vapor–liquid–solid growth technique, then heated at 600 °C for 5 h in order to remove any residual precursors on the support according to previous reports [2] and Wang et al. [1]. The NS were dried over night at 110 °C before use. The 15 wt% Co/NS catalyst was prepared by the conventional incipient wetness impregnation (IWI) method. To the prepared 15 wt% Co/NS catalyst, the dried NS support (79.5 mg in 15 mL of ethanol) was impregnated at 70 °C to incipient wetness with an aqueous solution containing Co(NO₃)₂·6H₂O (70 mg in 15 mL of water).

The Al₂O₃ nanoparticle support (control) was prepared by an alkoxide based sol-gel method [10]. Aluminum tri-sec-butoxide (Al (O-s-Bu)₃) as an aluminum precursor (12 mmol) was dissolved in isopropanol (C₃H₈O, 150 mmol) with acetylacetone (C₅H₈O₂, 10 mmol) at 20 °C, and then ultra-sonicated for 1 h at 50 °C. Nitric acid (0.4 mL) and ethanol (100 mmol) were added drop-wise to the aluminum precursor solution (0.5 mL). After cooling, a viscous gel was obtained. The Al₂O₃ gel solution was vigorously stirred for 30 min, and then aged for 8 d at room temperature. For the synthesis of 15 wt% Co/Al₂O₃ catalyst, the aqueous solution containing Co(NO₃)₂·6H₂O (75 mg in 15 mL of water) was added dropwise under stirring at 70 °C onto the solution of Al₂O₃ support precursor (85.3 mg).

For the synthesis of 15 wt% Co/NS-Al₂O₃ catalyst, two types of catalysts were prepared in this study (Figure 1). The process for preparing the first type (A) of Co/NS-Al₂O₃ catalyst was by impregnating an aqueous solution of Co(NO₃)₂·6H₂O (88 mg in 15 mL of water) onto the NS (100 mg in 15 mL of ethanol) support at room temperature. Then, the solution of Al₂O₃ gel (47 mg) was added dropwise to the Co/NS and labeled Co/NS-Al-A. The second type (B) Co/NS-Al₂O₃ catalyst was made by adding a solution of Al₂O₃ gel (47 mg) onto the NS (100 mg in 15 mL of ethanol) support at room temperature, as described by the chemical Equation (3) as follows:

3Al₂O₃ + 2SiO₂ (NS) → 3Al₂O₃.2SiO₂ (NS)

(3)

Then, Co(NO₃)₂·6H₂O (88 mg in 15 mL of water) solution was added to the NS-Al₂O₃ suspension, and the catalyst was named Co/NS-Al-B. All prepared (Co/NS-Al-A, Co/NS-Al-B and Co/Al₂O₃) catalyst suspensions were stirred at 70 °C for 12 h, dried at 110 °C overnight and then the obtained catalysts were calcined immediately in the air at 550 °C for 5 h.

2.2. Characterization of As-Prepared Catalysts

The reducibility of the calcined catalysts was studied by H₂-TPR using a ChemiSorb 2720 chemisorption analyzer (Micromeritics, Norcross, GA, USA) equipped with a thermal conductivity detector (TCD). For TCD calibration, CuO (20 mg, 99.99%) was reduced between 25 and 500 °C. Prior to the H₂-TPR measurement, the catalyst was pretreated in a N₂ flow of 30 mL min⁻¹ at 150 °C for 1 h to remove surface impurities, and then cooled down to room temperature. Subsequently, 5 vol.% H₂ in N₂ atmosphere (30 mL min⁻¹) was passed through the catalyst then ramped from room temperature to 1000 °C at a heating rate of 10 °C min⁻¹. The Brunauer−Emmett−Teller surface area (S_BET) measurements of all degassed (220 °C for 30 min) catalysts (60 mg) were determined by an N₂ adsorption-desorption isotherm at −196 °C using a FlowSorb II 2300 instrument (Micromeritics, Norcross, GA, USA).

FTIR measurement of the calcined catalysts (10% in KBr powder) was recorded in the 500–3500 cm⁻¹ range using a diffuse reflectance (5% in KBr) accessory on an iS10 spectrometer (ThermoNicolet, Madison, WI, USA). The TGA and DTA analysis of the calcined catalysts (5 mg) were performed, respectively, on a Perkin Elmer TGA-7 instrument and a DTA-7 instrument from room temperature to 900 °C at a rate of 20 °C min⁻¹ in a flow of N₂ (30 mL min⁻¹).

The XRD pattern of the calcined catalysts was obtained with a Siemens Diffractometer D5000 using monochromatic Cu/kα radiation at an X-ray wavelength (λ) of 0.1540 nm. The diffraction intensities were collected within 2θ range of 10–80° with 0.01° step and a 1 s acquisition time per step. To calculate the average crystallite size of Co₃O₄, the Debye–Scherrer’s equation was employed as [11]:

$$d_{XRD}=\frac{\mathrm{k}\lambda }{{\beta }_{hkl}\cos \theta }$$

(4)

where d_XRD is the average crystallite in nm; K is a constant related to crystallite shape, normally taken as 0.9; λ is the X-ray wavelength (λ = 1.54 Å); β is line broadening at half the maximum intensity (FWHM) in radians and θ is the angular position of the peak of interest. The average particle size of d(Co⁰) was calculated from the d_XRD(Co₃O₄) according to following formula [12]:

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

(5)

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 estimated by assuming a spherical geometry of the metal particles with a uniform site density of 14.6 atoms/nm² using the following formula [13]:

D = [96/d_XRD (Co⁰)]

(6)

where D is the % Co⁰ dispersion and d_XRD (Co⁰) is the mean particle size of Co⁰ in (nm).

The microstructures and morphology of prepared catalysts (dispersed in ethanol and applied to a copper grid coated with carbon support film) was characterized by TEM (JEOL JEM-2100 or JEM-2010, JEOL USA Inc., Peabody, MA, USA), operated at 200 kV. The cobalt particle size (d_TEM) from TEM micrographs was measured using ImageJ software (version 1.52).

The XPS scans of powder samples were acquired with a custom built ultrahigh vacuum (UHV) chamber using the monochromatic Al K-α radiation (1486.6 eV) of a dual anode X-ray source, XR 04-548 from PHYSICAL ELECTRONICS, and the kinetic energy of the photoelectrons was measured with an EA 125 hemispherical electron energy analyzer (Scienta Omicron, Taunusstein, Germany) with a resolution of 25 meV.

2.3. Catalytic Activation and FTS Evaluation

FTS experiments were of the calcined Co/NS, Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts were carried out in a quartz fix-bed micro-reactor (10 mm Ø × 300 mm with a “0” quartz frit connected 180 mm from the top to support the catalyst) housed in a tube furnace (25 mm Ø × 150 mm). In each FTS run, 20 mg of calcined catalyst was placed in the reactor and mixed with 40 mg quartz sand. The catalysts were activated in a flow of (H₂/N₂ = 40/60 mL min⁻¹) gas mixture metered with mass flow controllers (CG1, Dakota Instruments). The reactor temperature was increased from 25 to 700 °C and held there for 24 h. The activated catalyst was cooled to 150 °C and subsequently used for in-situ FTS reaction. After catalyst activation, the gases (H₂/CO/ N₂ = 60/30/10 mL min⁻¹) were metered with mass flow controllers (CG1, Dakota Instruments) at atmospheric pressure and fed to the reactor at 230 °C. The liquid products were collected in a three-stage impinger trap placed in a liquid nitrogen bath. During the reaction, the tail gas was collected in a Tedlar^TM PVF (300 × 300 mm²) gas-sampling bag. FTS reaction was operated for 34 h under reaction temperature of 230 °C. The outlet gas composition was analyzed by GC-TCD (Series 350, GOW-MAC Instrument Co., Bethlehem, PA, USA) with (i) a packed HaySep DB stainless steel column (3.3 mm Ø × 9.1 m) at 25 °C for CO, CO₂, H₂, N₂ and CH₄, and (ii) a packed PoraPakQ stainless steel column (3.3 mm Ø × 1.8 m) at 60 °C for C_xH_y (x ≤ C4) with He elution (30 mL min⁻¹). The liquid products C_xH_y (x ≥ C5) collected were then identified by GC-MS (Focus-ISQ, ThermoElectron, West Palm Beach, FL, USA). Separation was achieved with a ZB5ms (0.25 mm Ø × 30 m, Phenomenex) capillary column with a temperature program of 40 °C (1 min) ramped to 250 °C at 5 °C min⁻¹. Data was analyzed using the Xcalibur v4 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. H₂, CO conversions (%) and product selectivities (%) were calculated based on previous studies [1,4,14_ENREF_19].

3. Results and Discussion

3.1. Catalyst Preparation and Characterization

Four different catalysts (Co-NS, Co/Al₂O₃, Co-NS-Al-A and Co-NS-Al-B catalysts) were prepared in order to examine the effect of the addition of alumina to the NS supports for FTS. TEM micrographs of prepared silica NS are shown in Supplementary Figure S1. The S_BET of calcined catalysts were determined and given in

Table 1. Surface area (S_BET) of calcined catalysts.

Catalysts	Al (wt.%)	Co (wt.%)	SBET (m2 g−1)	Size of Co3O4 Particles (nm)		dXRD (Co0) (nm)	Co Dispersion (%)
				dXRD	dTEM
NS	-	-	314	-	-	-	-
Al2O3	-	-	227	-	-	-	-
Co/NS	-	15	193	9.8	6.6	7.3	13.1
Co/Al2O3	85	15	108	12.2	10.8	9.1	10.5
Co/NS-Al-A Co/NS-Al-B	8 8	15 15	199 260	10.6 8.1	8.2 5.7	7.8 6.0	12.3 16.0

. The S_BET for the Co/NS and Co/Al₂O₃ catalysts were 193 and 108 m² g⁻¹, respectively, which were lower than their supports of 314 and 227 m² g⁻¹, respectively, while the calcined Co/NS-Al-A and Co/NS-Al-B catalysts were higher at 199 and 260 m² g⁻¹, respectively. The addition of Al₂O₃ to the Co/NS catalyst did not noticeably change the surface area of the Co/NS-Al-A catalyst, whereas, the surface area was higher by applying Al₂O₃ to the NS support in the Co/NS-Al-B catalyst. This seems to indicate that the addition of Al₂O₃ species to the Co/NS-Al-B catalyst improved the dispersion of Co oxides particles on the surface, as well as the porous structure (space limitation inside the pores) of these oxides and, thus, increased the SBET value of the Co/NS-Al-B catalyst [4,7]. Furthermore, the Co/NS-Al-B catalyst shows slightly higher surface dispersion (16%) than other catalysts due to the strong Co−NS interaction with modification of Al₂O₃ species, which lead to the small particle size of Co₃O₄ species and higher supported Co dispersion. Very similar results were obtained using the Co/SiO₂ catalysts promoted with an Al₂O₃ support [6]. This result is probably attributable to the fact that the interactions of Co oxides with Al₂O₃ are stronger that those between Co oxides and SiO₂ NS [6].

Thermogravimetric analysis (TGA) was carried out to determine the thermal stability of the calcined Co/NS, Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts. The TGA thermograms of the calcined Co/NS, Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts (Figure 2a) indicates that the overall weight loss up to 900 °C were 6.9%, 6.3%, 5.4% and 4.0%, respectively. The first stage mass loss (about 3%) of all catalysts occurred between the temperature range of 30–150 °C, which is likely due to evaporation of adsorbed water, solvent and organic compounds on the catalysts; this is supported by an exothermic peak in the DTA thermograms (Figure 2b) [15,16]. Weight loss was observed above 600 °C and can be attributed to complete removal of material during calcination. This weight loss corresponds to an exothermic peak between 600–800 °C observed by DTA for the Co/NS-Al-A and Co/NS-Al-B catalysts (Figure 2b). Both the TGA and DTA analysis of all the catalysts exhibited some minor decomposition up to 900 °C, which clearly implies that all of the catalysts in this study have good thermal stabilities.

The XRD diffractograms of the calcined Co/NS, Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts are displayed in Figure 3. The crystalline phase of the Co/NS-Al-A and of the Co/NS-Al-B catalysts were identified by comparisons with the Co/NS and Co/Al₂O₃ catalysts. The XRD patterns of the calcined Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts show several diffraction peaks. The 2θ diffraction peaks of the Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts are approximately 31.7°, 36.9°, 45.0°, 60.1°, 66.0° and 77.8°, and can be attributed to the characteristic diffraction peaks of the Co₃O₄ and CoAl₂O₃ species [17]. All calcined catalysts show the characteristic reflection peak at around 2θ = 36.9° that corresponds to Co₃O₄. Furthermore, all catalysts also showed peaks at 2θ = 38.3° and 77.3°, which were attributed to CoO with cubical structure [18]. The diffraction peaks of Co₃O₄ are very close to that of CoAl₂O₄ (2θ = 37.0°, 65.0°). The peaks assigned to either Co₃O₄ or CoAl₂O₄ were observed in the Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts. Similar findings were reported by Saraswat et al. [18] and Lee et al. [19]. A broad diffraction peak assigned to amorphous silica NS was present (2θ ~ 23°) in the diffractograms of catalysts containing NS (Figure 3) [14]. The size of the Co₃O₄ particles was calculated using Equation (4) of the Co₃O₄ diffraction peak (311) at 2θ = 36.9°. The average Co₃O₄ crystallite size of in the calcined Co/NS, Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts were found to be 9.8, 12.2, 10.6 and 8.6 nm, respectively, and are summarized in Table 1. The Co₃O₄ crystalline size of the Co/NS-Al-B catalyst was the smallest for the NS modified with Al₂O₃ support. We postulate that this is due to the stronger interaction between the cobalt oxide phase, Al₂O₃ modified NS support and the high surface area, resulting in higher catalytic activity [6].

TEM was employed to determine the particle sizes and morphologies of calcined catalysts (Figure 4). The dark spots in the micrographs are Co particles dispersed on the NS surface (Figure 4). The nano-helical structure of the NS support is clearly observed in the micrographs of the Co/NS, Co/NS-Al-A and Co/NS-Al-B catalysts. The Co nanoparticles are well dispersed on the NS for the Co/NS (Figure 4a), Co/NS-AL-A (Figure 4c) and Co/NS-AL-B (Figure 4d) catalysts, whereas for Co/Al₂O₃ (Figure 4b) the Co particles were not well dispersed with some agglomerates. Moreover, it was found that the average Co particle size in the Co/NS, Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts were 6.6 nm, 10.8 nm, 8.2 nm and 5.3 nm, respectively (Table 1). Thus, the addition of NS as support during the preparation process decreased the average Co particle size in the Co/NS-Al-B catalyst, consistent with the results obtained by XRD.

The chemistry of the catalysts was examined by FTIR spectroscopy (Figure 5). Although the FTIR spectra of the Co/NS, Co/NS-Al-A and Co/NS-Al-B catalysts were similar, some differences in intensity were observed with the addition of Al₂O₃. The FTIR spectra of the Co/NS, Co/NS-Al-A and Co/NS-Al-B catalysts all showed characteristic Si–O–Si antisymmetric stretching and Si–O symmetric stretching vibrations at approximately 1085 cm⁻¹ and 802 cm⁻¹, respectively [1,4]. An additional band at 457 cm⁻¹ is assigned to Si–O–Si or O–Si–O bending vibrations. The two absorption bands at approximately 586 cm⁻¹ and 664 cm⁻¹ in all catalysts have been assigned to Co–O [14]. Moreover, all catalysts exhibit a broad band centered at 3443 cm⁻¹ associated with O–H stretching, and a relatively weak band at 1633 cm⁻¹ of hydrogen bonded surface silanol groups and physically adsorbed water [16,20,21]. The spectra of the Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts showed two bands at 567 and 663 cm⁻¹, which have been attributed to CoAl₂O₄ [22].

The H₂-TPR was used to investigate the reduction behavior of the as-prepared catalysts. It is well known that the reduction of Co₃O₄ species with H₂ follows a three stage process:

3CoO(OH) + 0.5 H₂→ Co₃O₄ + 2H₂O

(7)

Co₃O₄ + H₂→3CoO+ H₂O

(8)

3CoO + 3H₂→3Co + 3H₂O

(9)

The reducibility of metal oxides is known to play a major role in determining FTS activity and product selectivity [1]. Displayed in Figure 6 are the H₂-TPR profiles of the calcined Co/NS, Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts. It can be seen from the H₂-TPR profiles of the Co/NS and Co/Al₂O₃ catalysts that two main peaks occur at 394 and 553 °C, and at 393 and 547 °C, respectively. The low temperature reduction peaks (394 and 393 °C) are attributed to the reduction of Co³⁺ to Co²⁺, whereas the peak areas at the high reduction temperature (553 and 547 °C) are assigned to the reduction of Co²⁺ to Co⁰ metal. In the TPR profiles, peaks were observed at 319 °C, 373 °C and 492 °C for the Co/NS-Al-A catalyst, and for the Co/NS-Al-B catalysts, they were at 311 °C, 395 °C and 557 °C. The low temperature peak in both catalysts is assigned to either the reductive decomposition of residual nitrate species or the reduction of cobalt-oxyhydroxide (CoOOH) species [23]. The two higher temperature peaks at 373 °C and 492 °C for the Co/NS-Al-A catalyst and 395 °C and 557 °C for the Co/NS-Al-B catalyst are due to the reduction of Co₃O₄ to CoO and CoO to Co⁰, respectively. The results obtained herein are in general agreement with data previously reported [1,14]. These results demonstrate that the preparation method for the surface coating of the NS with Al₂O₃ impacted their reduction temperatures. That is to say, the Co/NS-Al-A catalyst had lower reduction temperatures than the Co/NS-Al-B catalyst, which could be due to weaker interfacial interactions between the Co and silica of the NS. Nevertheless, the TPR profiles for all calcined catalysts clearly show that the activation temperature occurs between 550 and 700 °C and is sufficient to obtain metallic Co.

XPS measurements were carried out to investigate the surface chemical nature of the catalysts. The XPS spectra of the Co/NS, Co/NS-Al₂O₃, Co/Al-A and Co/Al-B catalysts are displayed in Figure 7. The XPS survey scan for all catalysts presents photoelectron lines corresponding to C 1s, O1s, Co 2p, Si 2p and Si 2s plus Al2p for the Co/Al₂O₃, Co/Al-A and Co/Al-B catalysts. The Co 2p spectra of all the calcined catalysts are shown in Figure 8. All spectra exhibit two main Co2p_3/2 and Co2p_1/2 peaks and two satellites peaks. Characteristic Co 2p_3/2 and Co2p_1/2 peaks of the calcined Co/NS, Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts were observed at binding energies of 780.0 eV and 795.3 eV, 780.1 eV and 795.3eV, 780.1 eV and 795.4eV and 780.1 eV and 795.1 eV, respectively. Moreover, the spin-orbit coupling (Δ_SOC) between the Co 2p_3/2 and Co 2p_1/2 in Co/NS, Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts were 15.3 and 15.2 eV, 15.3 eV and 15.0 eV, respectively. Therefore, the weak satellite peaks observed for all the catalysts indicate that at the oxidation state of the catalysts are Co₃O₄ phase, and this is in accordance with the literature [14,22]. The peak in the Co/NS-Al-A and Co/NS-Al-B catalysts at 782.2eV and 782.3 eV is tentatively assigned to the component of CoAl₂O₄ [22]. The XPS results suggest that the surface structure of Co/NS is significantly influenced by the modification of NS with Al₂O₃ support, and the result of these values are in good agreement with the XRD analysis. Overall, by using XRD, XPS and FTIR data, nanoparticles of Co₃O₄ are found to be the dominant phase of the prepared catalysts, where the presence of CoAl₂O₄ was observed in both the Co/NS-Al-A and Co/NS-Al-B catalysts.

3.2. Catalytic Activity Testing

FTS testing (condensable liquid products) of the catalysts was analyzed by GC-MS (Figure 9 and Supplementary Tables S1–S4), while the non-condensable gases (CO, CO₂, H₂, N₂, CH₄ and C₂-C₄) 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. Catalytic performance and major components of synthesized liquid FTS fuel over the calcined Co/NS, Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts at 230 °C, H₂/CO = 2 and at atmospheric pressure.

Catalyst	Co/NS	Co/Al2O3	Co/NS-Al-A	Co/NS-Al-B
CO Conversion (%)	65.5	46.8	62.8	82.4
H2 Conversion (%)	61.2	39.7	56.2	73.3
Products Selectivity (%)
CO2 select. (%)	5.3	17.4	8.7	0.6
CH4 select. (%)	6.7	20.4	10.6	7.7
Σ < C5	17.1	29.6	21.0	5.4
Product distribution (Mol. %)
Σ > C6	70.9	32.6	58.5	86.3
Paraffins	18.4	5.8	8.9	16.2
Olefins	26.6	13.6	23.7	55.1
Naphthenes	17.3	8.6	16.9	12.6
Oxygenates	8.6	4.6	-	2.4
Aromatics	-	-	10.2	-
Olefins /Paraffins (O/P)	1.44	2.34	2.66	3.40

). Based on GC and GC-MS analysis, the FTS activity of the Co/NS, Co/Al₂O₃, Co/NS-Al-A and Co/NS-Al-B catalysts were 65.5%, 46.8%, 62.8% and 82.4%, respectively. The Co/Al₂O₃ catalyst exhibited the lowest activity of 46.8% CO conversion, which was possibly attributable to having the lowest S_BET (108 m²/g), lowest Co dispersion (10.5%) and largest Co particle size (12.2 nm), relative to the other catalysts (Table 1). It is well known that decreasing the active phase particle size can improve the FTS catalytic performance, control their selectivity and stability [4]. It was noted that no difference in activity was observed between the Co/NS and when it was coated with Al₂O₃ to form the Co/NS-Al-A catalyst. However, there was a distinct change in hydrocarbon product distribution Σ < C₅ (light weight hydrocarbons), CO₂ and CH₄ undesirable selectivity going from the Co/NS to the Co/NS-AL-A catalysts (Table 2). However, the Co/NS-Al-B catalyst (NS coated with Al₂O₃ and then decorated with Co) showed significantly higher FTS activities (Σ < C₅, CO₂ and CH₄ selectivities) compared to the Co/NS-Al-A catalyst (Table 2). For the Co/NS-Al-B catalyst, the total CO₂ (0.6%) and Σ < C₅ selectivity (5.4%) was the lowest of the catalysts in this study. This suggests that the improved Co dispersion (16%), largest S_BET (260 m²/g) and smaller Co particles (5.7 nm) all contributed to its overall improved performance [3]. Several research groups have studied the effect of the addition of Al₂O₃ on the catalytic performance of the Co/SiO₂ catalysts. For instance, Rathousky et al. [24] examined the influence of Al₂O₃ on catalytic performance of the Co/SiO₂ catalyst, and found that the catalytic behavior of 10% Co/SiO₂-Al₂O₃ was more similar to Co/SiO₂ than to Co/Al₂O₃.

The condensable liquid fuel products were characterized by GC-MS (Figure 9), where the Co/NS and Co/Al₂O₃ catalysts have a hydrocarbon product distribution in the carbon number range of C₆–C₁₄ (Figure 10), while the Co/NS-Al-A and Co/NS-Al-B catalysts had hydrocarbons ranges of C₆–C₁₅ (naphtha fraction) and C₆–C₁₇, respectively (Figure 10)_. The FTS hydrocarbons products obtained from the Co/NS-Al-A catalyst were found to be qualitatively and quantitatively different from those produced by the Co/NS-Al-B catalyst (Table 2). The main hydrocarbon products in the C₆–C₁₅ range for the Co/NS-Al-A catalyst were olefins (23.7%), paraffins (8.9%), naphthenes (cycloalkanes) (16.9%) and aromatics (10.2%), giving it a total hydrocarbon selectivity of 58.5%. In addition, there were no oxygenated products detected for the Co/NS-Al-A catalyst. In contrast, the Co/NS-Al-B catalyst produced paraffins (16.2%), olefins (55.1%), naphthenes (12.6%) and some oxygenated products (2.4%), giving it a total hydrocarbons selectivity of 86.3%. In general, the amount of produced olefins in all catalysts was higher than paraffins. Oxygenated products were observed for the Co/NS, Co/Al₂O₃ and Co/NS-Al-B catalysts.

Surprisingly, the Co/NS catalyst coated with Al₂O₃ (Co/NS-Al-A) yielded C₆–C₁₅ aromatic compounds (10.2%), mainly comprised of mono- and di-nuclear aromatics that include o-xylenes, alkyl-benzenes, naphthalene and alkyl-naphthalene isomers. It is believed that coating the Co/NS catalyst with Al₂O₃ (Co/NS-Al-A) may convert the primary products, including olefins and oxygenates, to aromatics via secondary reactions. The olefin/paraffin (O/P) ratio was the least for the Co/NS catalyst (1.44) and increased with the presence of Al₂O₃ (Table 2). The Co/NS-Al-B had the highest O/P of 3.40. This increase in olefin content is likely attributable to a reduced hydrogenation rate [14]. The data reported above clearly shows that the addition of Al₂O₃ to the NS support, in both the Co/NS-Al-A and Co/NS-Al-B catalysts, has an impact on the product selectivity and distribution of C₆–C₁₄ aromatics. This could be attributable to changes in the surface coverage and morphology rather than a change in the intrinsic activity of the active Co sites [25]. The addition of a small amount of Al₂O₃ to the silica-supported Co catalyst significantly improved the dispersion of Co and led to an increase in the Fischer−Tropsch synthesis (FTS) activity, which has been observed by others [5,6]. Furthermore, the Co/NS-Al-B catalyst was superior to the traditional Co-Al₂O₃, Co/SiO₂ and Co/SiO₂-Al₂O₃ catalysts in terms of hydrocarbon distribution Σ > C₆ and resistant to coke formation.

The variation of the CO conversion with reaction time can be used as an indicator of catalyst stability (Figure 11). The CO conversion in all catalysts was relatively stable over a 34 h period, with the exception of the Co/Al₂O₃ catalyst, which showed a slight decrease from 50% to 47% CO conversion. These results demonstrate that FTS catalysts were stable.

4. Conclusions

Catalytic performance of two kinds of Co/NS-Al₂O₃ (Co/NS-Al-A and Co/NS-Al-B) catalysts were evaluated in a quartz fix-bed micro-reactor and compared with Co/NS and Co/Al₂O₃ catalysts. It was found that the combination of Al₂O₃ onto the Co/NS catalyst had a remarkable change in the hydrocarbon product selectivity in both the Co/NS-Al-A and Co/NS-Al-B catalysts. The highest CO conversion was achieved using the Co/NS-Al-B catalyst, which also had the highest surface area and Co dispersion. The product distribution towards the formation of aromatic compounds was formed by adding Al₂O₃ to the NS support; also, light hydrocarbon products, CO₂ and CH₄, selectivity increased. However, the addition of Al₂O₃ in the Co/NS-Al-A catalyst does not appear to have any significant influence on the physical properties of catalysts, such as dispersion, reducibility, average particle size and surface area. From these results, we conclude that the effect of coating Al₂O₃ onto the NS catalyst and then impregnating with cobalt (Co/NS-Al-B catalyst) can be attributed to structural rearrangement of the Co surface, not to a change in the intrinsic activity. In general, our findings have implications for designing modified catalysts that can decrease the oxygenated compound and increase aromatic content, and this represents a fertile area for further research.