Promotion effects of plasma treatment on silica supports and catalyst precursors for cobalt Fischer–Tropsch catalysts

Abstract

The glow discharge plasma technique was applied to decorate the silica support and to decompose the precursors of cobalt-based catalysts. The catalysts were characterized by N₂-physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H₂-TPR), temperature programmed desorption of H₂ (H₂-TPD) and O₂-titration. The results confirmed that plasma decoration would modify both the SiO₂ pore structure and surface properties. Compared with the calcined catalysts, plasma treated samples had a much higher cobalt dispersion and good cobalt reducibility, leading to an apparently improved FTS activity.

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1 Introduction

Fischer–Tropsch synthesis (FTS) is recognized as an alternative way to use coal, natural gas and biomass, and can be used to produce clean fuels and chemicals.^1–3 Supported cobalt catalysts are the preferred catalysts for a low-temperature FTS process because of their high reactivity, selectivity for linear C₅₊ hydrocarbons, and low reactivity for the water–gas shift reaction.^4–7 For the economic viability of FTS plants, catalytic performance in terms of high activity, selectivity and stability with affordable costs is essential for a good catalyst, which makes the improvements in these aspects become the target of research in industrial catalysis.

Conventional cobalt-based FTS catalysts were usually prepared by impregnation of common supports with aqueous solution of cobalt salts. The deposited samples were dried and calcined in an oxidizing atmosphere at certain temperature. Calcination at lower temperature cannot guarantee the complete decomposition of cobalt precursor, while do that at high temperature usually causes aggregation of cobalt oxide and lead to a decrease of the number of active sites.⁸ Meanwhile, the traditional calcination process is rather energy consuming; therefore, development of efficient precursor decomposition techniques is with high potential.

Compared with other chemical techniques, plasma discharges (glow discharge plasma, plasma jet, dielectric-barrier discharge (DBD) plasma, etc.) offer an unique advantage because non-equilibrium reactions can be performed at low temperature, and requires much less amount of compressed gases and energy consumption.⁹ The energetic species (electrons, ions and radicals) produced in plasma field would bombard and sputter the surface of the materials, thus, catalyst treated with plasma could lead to some specific catalytic properties.^10–14 Liu's group^15–19 has used glow discharge plasma to synthesize a series of catalysts and nano-materials, for example, they used argon glow-discharge plasma to decompose and reduce Ni-based catalysts, and a special metal-support interface was observed, better low-temperature activity as well as enhanced stability in plasma treated samples were exhibited for methane conversion;^16,19 as for CO oxidation, the DBD plasma treated WO₃ supported Pt catalyst had higher Pt dispersion, improved metal-support interaction and weaker adsorption of CO, hence, a remarkable enhanced catalytic activity was observed.¹⁷

The primary applications of plasmas in FTS were also performed. Li's group has treated cobalt FTS catalysts by DBD plasma;^20,21 Khodakov's group has prepared alumina and silica supported cobalt catalysts using glow discharge plasma,^22,23 both results showed that plasma techniques could be efficiently used to decompose cobalt precursor at low temperature, improve the metal dispersion and enhance the catalytic activity. However, the results so far focused on the application of plasma to decompose catalyst precursors, there is currently few investigation on using plasma to treat support and studying this effect on catalytic performance.

Further investigation and analysis of plasma effects is helpful for better understanding the effect of plasma treatment on cobalt surface structure in cobalt-based catalysts. In present work, glow discharge plasma was introduced to decorate the silica support; traditional thermal calcination and glow discharge plasma treatment were applied to decompose the catalyst precursors, and CoPt/SiO₂ catalyst system was used as research target. The objective of this work is to understand the effects of glow discharge plasma on SiO₂ support and/or catalyst precursors, and their catalytic performance in Fischer–Tropsch synthesis.

2 Experimental

2.1 Preparation of the supports

A commercial silica gel (Qingdao Haiyang Chemical Co., Ltd, China) was used as a support material. The silica gel was crushed and sieved through a mesh to reach a diameter of 150–250 μm. The glow discharge plasma decorated support was symbolled as “Sup.-p”, the detailed treating procedure was as follows. Under air atmosphere, glow discharge plasma was initiated by a high-voltage AC pulse generator with the output power of 420 W, and a duty factor of 20%. During the treating process, the temperature measured by thermocouple was indicated as 400 °C and the pressure of system was maintained at 100 Pa through the adjustment of air flow rate. To obtain an uniform decomposition, the sample was treated with glow discharge plasma for 2 h. The apparatus of glow discharge plasma treatment is shown in Fig. 1.

Fig. 1 Schematic representation of the glow discharge plasma treatment apparatus.

2.2 Preparation of the catalysts

CoPt/SiO₂ and CoPt/Sup.-p catalysts with cobalt loading of 15 wt% and platinum loading of 0.5 wt% were prepared by incipient wetness impregnation using aqueous solution of cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, AR, Sinopharm Chemical Reagent Co., Ltd, China) and dinitro-diamine platinum ammoniacal (Pt(NO₂)₂(NH₃)₂, AR, Sino-platinum metals Co., Ltd, China). After being dried in an oven at 100 °C for 10 h, the catalyst precursors were divided into two parts. One part was calcined under air atmosphere at 400 °C for 5 h using a muffle roaster with a ramping rate of 3 °C min⁻¹ to obtain the thermal calcined Co catalysts, labelled as ‘‘Catal.-c’’ and “Sup.-p-Catal.-c”. The other part of the samples was treated by the glow discharge plasma to decompose cobalt nitrate using the same procedures and parameters as those to silica support, the plasma treated catalysts were signed as ‘‘Catal.-p’’ and “Sup.-p-Catal.-p”, respectively. In order to distinguish the description of support and catalyst plasma treatments, in the following text, “plasma decoration” or “plasma decorated support” were used to represent the support with plasma treatment, and “plasma treatment” or “plasma treated catalysts” were used to describe the catalyst precursors decomposed by plasma technique.

2.3 Characterization of the catalysts

N₂ physisorption of supports and the catalysts was conducted on automatic physical/chemical adsorption analyzer (Autosorb-1-C-MS, Quantachrome Instruments, USA) to obtain the adsorption–desorption isotherms. To remove impurities, the samples were degassed at 200 °C for 6 h under vacuum before any measurements were collected. The specific surface area of samples were calculated with Brunauer–Emmett–Teller (BET) model, while the diameter and pore volume of samples were evaluated with Barrett–Joyner–Halenda (BJH) model.

The transmission infrared spectroscopy studies of supports were performed on a Nicolet NEXUS-470 Fourier Transform Infrared (FTIR) Spectrometer. Before the measurements, mixture of sample and KBr with a weight ratio of 1 : 100 was used for tabletting. Spectral range was 4000–500 cm⁻¹ and resolution was 4 cm⁻¹ for each spectrum.

The crystalline structure of the catalysts was characterized by powder X-ray diffraction (XRD) (D8 Advance, Bruker Co., Ltd, Germany) with a Ni-filtered Cu-Kα radiation source (λ = 1.54056 Å). The XRD pattern was recorded with a scanning angle that ranged from 10° to 80° with a scanning rate of 0.05° s⁻¹. The phase identification was determined by comparison with the Joint Committee on Powder Diffraction Standards (JCPDSs). An average particle size of the Co₃O₄ crystallites in the catalysts was calculated with the Scherrer equation from the most intense Co₃O₄ peak at 2θ = 36.9°.

The morphologies of the catalysts were characterized by TEM using a transmission electron microscope (Tecnai G²F20, FEI Co., Holland) operated at accelerating voltage of 200 kV. The sample was prepared by dispersing the powder in ethanol and dropping a drop of very dilute suspension onto a carbon film-coated micromesh copper grid.

X-ray photoelectron spectroscopy (XPS) measurements were conducted on a X-ray photoelectron spectroscope (VG Multilab 2000, Thermal Electron, Inc., USA) photoelectron spectrometer using Al (Kα) radiation as the excitation source. All the binding energy (BE) values were calibrated by the C 1s peak.

The H₂-TPR profiles of the prepared catalysts were obtained using a catalyst multifunctional characterization instruments (AMI-200, Altamira Instruments, Inc., USA). Before the TPR measurements, the catalyst (ca. 50 mg) was flushed with high purity argon at 150 °C for 1 h to remove traces of water and was cooled to 50 °C. A gas mixture of 10% v/v of hydrogen in argon was used at a flow rate of 25 mL min⁻¹, and the temperature was increased with a rate of 10 °C min⁻¹ from 50 to 800 °C and maintained at 800 °C for 0.5 h.

H₂-TPD and O₂-titration experiments were performed in a quartz tube with the same apparatus as TPR. The degree of dispersion and reducibility of cobalt on the supports were evaluated on the basis of TPD files and O₂-titration files. The sample (100 mg) was reduced in situ under hydrogen gas at 400 °C for 10 h and then was cooled under hydrogen flow to 100 °C. The flow of hydrogen was switched to argon at a flow rate of 25 mL min⁻¹ to remove physisorbed hydrogen and/or weakly bound species. Subsequently, the temperature of the samples was increased to 450 °C at a rate of 10 °C min⁻¹ and maintained 2 h under argon gas flow. Based on the assumption that the H : Co stoichiometric ratio was 1 : 1, Co dispersion was calculated according to the TPD spectra.

The O₂-titration experiments was proceeding after the TPD process was finished. Argon was regard as carrier gas and O₂ as reaction gas, the temperature of samples was increased up to 450 °C. Then high purity oxygen gas was injected into samples in the form of a pulse until no further consumption of oxygen was observed. Reduction degree of Co was calculated according to the O₂ consumption.

2.4 Activity evaluation of the catalysts

Catalysts were tested at 1.0 MPa in a fixed-bed reactor. Typically, the catalyst (0.5 g) was mixed with carborundum (5 g) and then placed in a fixed-bed stainless steel tubular microreactor (i.d. 2.5 cm) inserting into a vertical furnace, and the sample was reduced by high purity H₂ with GHSV of 2 NL h⁻¹ g⁻¹ at 400 °C for 10 h. Subsequently, the reactor was cooled down to 100 °C, then the gas flow was switched to syngas (H₂/CO = 2, GHSV = 2 SL h⁻¹ g⁻¹) and the pressure was increased to 1.0 MPa. The reaction was conducted at 210 °C and sustained for 100 h. The solid products were collected in a hot trap (100 °C), and the liquid mixture was collected in a cold trap (−2 °C). The effluent gaseous products were taken at 3 h intervals and analyzed online by an Agilent MicroGC 3000A gas chromatograph with a TCD detector. The liquid oil and solid wax products were analyzed with an Agilent 6890N GC and an Agilent 7890A GC, respectively. The catalytic performance was expressed as CO conversion, FT reaction rate and the selectivities of hydrocarbons, which were measured at steady state and calculated based on the carbon conservation. The FT reaction rate is in moles of converted CO per second divided by the total amount of cobalt (in moles) loaded into the reactor. And the α-value was the probability of chain growth, which was determined based on the Anderson–Schultz–Flory chain-length statistics equation.

3 Results and discussion

3.1 N₂ physisorption–desorption of supports

The textural properties of supports were determined by nitrogen physisorption measurement. The results are shown in Fig. 2 and Table 1. Both the supports with and without plasma decoration exhibited typical type IV isotherms of mesoporous materials with narrow pore size distribution,²⁴ suggesting that the porous structure of supports is preserved after the treatment of plasma. Compared with raw silica support, the lower intensity and similar position of hysteresis loop in plasma decorated support illustrated its smaller surface area, smaller pore volume and similar pore size, as indicated in Table 1. Indeed, after plasma decoration, the surface area of silica was decreased from 449.3 to 390.1 m² g⁻¹, pore volume was reduced from 1.30 to 1.14 cm³ g⁻¹, while the average pore diameter was almost unchanged, remained at 12.7 nm in both supports.

Fig. 2 N₂ adsorption–desorption isotherms and pore size distribution of the supports.

Table 1 Textural properties of SiO₂ and supported Co catalysts

Samples	Surface area^a (m^2 g^−1)	Pore volume^b (cm^3 g^−1)	Pore diameter^c (nm)	Co3O4 size^d (nm)
a Determined using the standard BET method for adsorption branch.b Calculated from the amount of N2 adsorbed at a relative pressure of 0.99.c Determined using the standard BJH model for adsorption branch.d Calculated using the Scherrer equation for peak at 36.9°.
SiO2	449.3	1.30	12.7	—
Plasma-SiO2	390.1	1.14	12.7	—
Catal.-c	287.3	0.81	12.4	13.2
Catal.-p	264.1	0.80	12.3	7.5
Sup.-p-Catal.-c	294.2	0.80	12.4	15.7
Sup.-p-Catal.-p	280.6	0.83	12.3	9.4

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3.2 Fourier transform infrared spectra (FTIR) of the supports

Characteristic chemical bonds in the SiO₂ and plasma decorated SiO₂ supports were identified from the FTIR spectra and presented in Fig. 3. The band at 974 cm⁻¹ was assigned to the stretching vibration of Si–OH groups.^25–27 Much lower intensity of this band in plasma decorated sample indicated less number of surface Si–OH species in plasma-SiO₂. The major band at approximately 1103 cm⁻¹ was assigned to the stretch vibration of Si–O–Si.^25,26 The broad band situated around 3449 cm⁻¹ and 1628 cm⁻¹ were assigned to O–H stretching and O–H bending vibrations originated from absorbed water, respectively.^25,27 These two bands appeared to indicate the hydrophilic nature of the supports.^25,27 It was clearly showed that plasma decoration of silica resulted in partial dehydroxylation of the support surface and led to weaker hydrophilicity.

Fig. 3 FTIR spectra of the supports.

3.3 N₂ physisorption–desorption of the catalysts

N₂ physisorption–desorption results of cobalt catalysts are shown in Fig. 4 and listed in Table 1. The shape and magnitude of hysteresis loop in nitrogen physisorption–desorption isotherms of all catalysts were almost the same, indicating their similar mesoporous structure, the pore volume and average pore diameter of all catalysts were approximately 0.80 cm³ g⁻¹ and 12.3 nm, respectively. After incipient wetness impregnation with supports, the BET specific surface area, pore volume and pore size of the cobalt catalysts were all decreased in contrast with corresponding supports, indicating that in both calcined and plasma treated catalysts, cobalt species was located inside the pores.

Fig. 4 N₂ adsorption–desorption isotherms and pore size distribution of the catalysts.

3.4 X-ray diffraction (XRD) of the catalysts

Fig. 5 shows XRD patterns of supported catalysts. Characteristic diffraction peaks assigned to Co₃O₄ crystallites at 2θ of 19°, 31.3°, 36.9°, 44.8°, 59.4°and 65.2° were detected in all samples, no other cobalt species was observed. The average size of Co₃O₄ which determined by the Scherrer equation from peaks at 2θ of 36.9° is listed in Table 1.

Fig. 5 XRD patterns of different catalysts.

Glow discharge plasma is a very effective technology to decompose catalyst precursors, cobalt nitrate could be almost completely transferred to Co₃O₄ crystallites, meanwhile, compared with calcined samples, the cobalt dispersion was significantly improved, i.e. much smaller Co₃O₄ particles was found in the catalysts with plasma treatment. In regular silica supported catalysts, the Co₃O₄ particle size was decreased from 13.2 nm of Catal.-c to 7.5 nm of Catal.-p, while in plasma decorated silica supported catalysts, slightly larger Co₃O₄ crystallites were presented in corresponding counterparts, their particle size was reduced from 15.7 nm of Sup.-p-Catal.-c to 9.4 nm of Sup.-p-Catal.-p.

3.5 Transmission electron microscopy (TEM) of the catalysts

Fig. 6 displays the transmission electron microscopy (TEM) images of the catalysts prepared by different methods. Co₃O₄ particles with various dispersion can be found in all four catalysts. Comparing with calcined samples (Fig. 6a and c), the dispersion of the Co₃O₄ particles on the plasma treated catalysts (Fig. 6b and d) was remarkably improved. The homogeneous dispersion of cobalt species in Catal.-p and Sup.-p-Catal.-p catalysts indicated that glow discharge plasma could prevent the aggregation of Co₃O₄ particles at higher temperature during thermal calcination.⁵ Meanwhile, it can be seen that in plasma decorated SiO₂ supported catalysts (Fig. 6c and d), the Co₃O₄ particles were better crystallized than regular silica supported ones (Fig. 6a and b), consistent with XRD findings.

Fig. 6 TEM micrographs of the catalysts: (a) Catal.-c; (b) Catal.-p; (c) Sup.-p-Catal.-c; (d) Sup.-p-Catal.-p.

3.6 X-ray photoelectron spectroscopy (XPS) of the catalysts

XPS is an effective technique to measure the surface element compositions of solid materials. Co 2p_3/2 XPS spectra of the catalysts supported on the silicas with and without plasma decoration were compared in Fig. 7, both Co²⁺ and Co³⁺ were detected as the main cobalt phases on the catalysts surface. The binding energies (BEs) for Co 2p_3/2 of the plasma treated catalysts Catal.-p and Sup.-p-Catal.-p were almost 1 eV higher than those of calcined catalysts Catal.-c and Sup.-p-Catal.-c, meanwhile, apparent satellite peaks assigned to Co²⁺ were presented in two plasma-treated catalysts, indicating that the cobalt–silica interaction was significantly increased when decomposing catalyst precursors by plasma technique.

Fig. 7 Co 2p_3/2 XPS spectra of calcined and plasma treated CoPt/SiO₂ catalysts.

To better understand the surface properties of the catalysts, the asymmetrical Co 2p_3/2 signal of each sample was fitted and can be decomposed into two peaks at binding energy (BE) around 779.9 eV and 781.7 eV, corresponding to Co³⁺ and Co²⁺, respectively.^28,29 The presence of a weak satellite signal at 786.2 eV in plasma treated catalysts indicated the abundant presence of Co²⁺. Surface elements such as cobalt and silica composition were obtained from quantitative analyses of XPS peaks, the data are listed in Table 2. An interesting observation in the XPS data is that the surface Co/Si molar ratios in catalysts either with plasma decorated support or precursor plasma decomposition (0.097–0.139) were much higher than that of the calcined Catal.-c catalyst supported on regular silica (0.022), indicating a significant enhancement of surface cobalt dispersion by plasma. Meanwhile, lower number of Co³⁺/Co²⁺ molar ratio in plasma participant samples (0.35–0.97) as compared with calcined Catal.-c catalyst (2.42), illustrated that less surface cobalt species were in the form of Co₃O₄, amount of CoO or Co₂SiO₄ like compounds was increased. The above findings showed that plasma could not only decompose the catalyst precursor and improve the dispersion of active metal, but also modify the surface properties of silica support, and finally influence the surface structure of cobalt species.

Table 2 Surface properties of 15%Co0.5 Pt/SiO₂ catalysts

Samples	Surface Co/Si	BE Co 2p3/2 (eV)	Surface Co^3+/Co^2+
Catal.-c	0.022	779.8	2.42
Catal.-p	0.097	780.8	0.54
Sup.-p-Catal.-c	0.139	779.7	0.97
Sup.-p-Catal.-p	0.085	780.9	0.35

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3.7 H₂-temperature programmed reduction (TPR) of the catalysts

The reducibility of the catalysts was measured by H₂-TPR, and the results are shown in Fig. 8. It was found that the TPR profiles of the calcined catalysts Catal.-c and Sup.-p-Catal.-c were similar, the three peaks in the region of 110–175 °C, 175–210 °C and 210–410 °C were attributed to the hydrogen spillover produced by Pt and the two-step reduction of Co₃O₄ (Co₃O₄ → CoO → Co⁰),³⁰ respectively. After plasma treatment, the reduction peaks of Catal.-p and Sup.-p-Catal.-p catalysts were shifted to much higher temperatures, the final reduction temperature of CoO to Co⁰ was 40–50 °C higher than the calcined counterpart. Combined with the XRD, TEM and XPS results, compared with calcined samples, the lower reducibility of plasma treated catalysts Catal.-p and Sup.-p-Catal.-p were attributed to their much higher cobalt dispersion and stronger interaction between cobalt and silica support.^22,23 However, profit from the addition of Pt, the reduction temperature of CoO to Co⁰ in all four catalysts was below 400 °C (reduction temperature for FTS tests), most of the cobalt species could be reduced into the active Co⁰ phase under reaction conditions.

Fig. 8 H₂-TPR profiles of the catalysts.

In addition, a reduction peak located at higher than 450 °C was observed on Catal.-p catalyst, which was assigned to the hardly reducible species of cobalt silicate, as to the plasma treated catalyst supported on plasma decorated silica Sup.-p-Catal.-p, this peak was not detected, indicating that less hydrophilic property caused by plasma decoration could alter the structure of cobalt species, and modify the reducibility of corresponding catalyst.

3.8 H₂-temperature programmed desorption (TPD) and O₂-titration of the catalysts

To better understand the reductive behaviour and cobalt dispersion of the catalysts, H₂-TPD and O₂-titration were also performed; the results are shown in Table 3. Consistent with TPR and XRD results, the reducibility of the catalysts were related to the cobalt dispersion as well as the strength of cobalt–silica interaction. The calcined catalysts Catal.-c and Sup.-p-Catal.-c, which had larger cobalt particles, showed quite high cobalt reducibilities; after reducing in hydrogen flow at 400 °C for 10 h, almost all cobalt species could be reduced (>95%). As to the Catal.-p and Sup.-p-Catal.-p catalysts, in which catalyst precursors were decomposed by plasma technique, since cobalt dispersion was remarkably increased and stronger cobalt–silica interaction was presented, the reducibility of cobalt species was apparently decreased. Benefit by the promotion of 0.5% platinum, the reduction degree of plasma treated catalysts maintained at a relatively high level (around 79%).

Table 3 H₂-TPD and O₂ titration results for various catalysts

Catalyst	H2 uptake (μmol g^−1)	Duncorr^a (%)	duncorr^b (nm)	O2 uptake (μmol g^−1)	R^c (%)	Dcorr^d (%)	dcorr^e (nm)
a Uncorrected dispersion of metallic Co, assuming H/Co = 1.b Uncorrected particle diameter of metallic Co.c Reduction degree of Co calculated by O2 titration.d Corrected dispersion of metallic cobalt.e Corrected particle size of metallic cobalt.
Catal.-c	98.6	7.8	13.3	1621.8	95.1	8.2	12.7
Catal.-p	172.1	13.5	7.6	1347.2	79.0	17.1	6.0
Sup.-p-Catal.-c	91.1	7.2	14.4	1694.7	99.4	7.2	14.3
Sup.-p-Catal.-p	113.0	8.9	11.6	1350.4	79.2	11.2	9.2

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3.9 Catalytic performance of the catalysts

Relationships between the structure of plasma treated supports and the catalytic performance of 15%Co0.5%Pt/SiO₂ catalysts were examined in Fischer–Tropsch synthesis. The FTS activities as a function of time on stream of the catalysts were shown in Fig. 9, the distribution of hydrocarbons products was presented in Fig. 10, and the activity and selectivity data shown in Table 4 were calculated within 75-80 h quasi-steady state after initial 20 h of catalytic induction period. As described previously, using plasma technique instead of thermal calcination method resulted in a better dispersion of cobalt species and a higher FTS activity, the average CO conversion was increased from 41.4% (Catal.-c) and 44.1% (Sup.-p-Catal.-c) of calcined catalysts to 64.8% (Catal.-p) and 65.6% (Sup.-p-Catal.-p) of plasma treated catalysts, separately. Plasma proved to be an efficient alternative approach for Co-based catalyst preparation for FTS.²¹ It should be noted that the slightly higher average CO conversion of Sup.-p-Catal.-p was due to its higher initial activity, the followed significant deactivation behavior in this catalyst caused that it was even less active than Catal.-p after 45 h time on stream.

Fig. 9 CO conversion as a function of time on stream (TOS) of the catalysts.

Fig. 10 Selectivities of FTS products in different catalysts.^a Calculated within 75 h stability period after 20 h of catalytic induction period. Reaction conditions: P = 1.0 MPa, T = 210 °C, GHSV = 2SL h⁻¹ g⁻¹, H₂/CO = 2.

Table 4 Activity testing of the supported Co catalysts

Samples	Average CO conversion^a (%)	FT reaction rate^b (10^−3 s^−1)	Hydrocarbon selectivity^a (mol%)						α^c
			CH4	C2–4	C5+	Petrol C5–12	Diesel C13–20	Paraffin C21–59
a Calculated from time on stream of 20–100 h.b The FT reaction rate is in moles of converted CO per second divided by the total amount of cobalt (in moles) loaded into the reactor.c The α-value calculated in the range of C3–C60.
Catal.-c	41.4	1.34	12.5	9.2	78.3	33.6	26.5	18.3	0.88
Catal.-p	64.8	2.10	10.4	9.8	79.8	41.5	25.5	12.8	0.86
Sup.-p-Catal.-c	44.1	1.43	10.7	10.5	78.8	39.1	27.7	11.9	0.87
Sup.-p-Catal.-p	65.6	2.13	11.5	12.7	75.8	40.4	24.7	10.7	0.85

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The effect of support plasma decoration was also investigated. Compared with the counterparts of Catal.-c and Sup.-p-Catal.-c, and of Catal.-p and Sup.-p-Catal.-p, the catalysts supported on the silica with plasma decoration showed higher FTS activity (Fig. 9), correlating with the modified support properties by plasma. Another interesting finding is the selectivity of hydrocarbons was altered by plasma technique (Fig. 10 and Table 4). The product distributions of the four catalysts were all followed the Anderson–Schulz–Flory distribution. Not only in the plasma treated catalysts (Catal.-p and Sup.-p-Catal.-p), but also in the catalyst supported on plasma decorated silica (Sup.-p-Catal.-c), the chain growth probability was slightly decreased, the selectivity of paraffin (C₂₁–C₅₉) was apparent decreased (from ∼18% to ∼12%) and that of petrol section (C₅–C₁₂) was increased (from ∼33 to ∼40%), as comparison with traditional calcined Catal.-c catalyst. Decoration of SiO₂ via glow discharge plasma technique gave a contribution on both FTS activity and selectivity of hydrocarbons.

The promotion effect of plasma treatment could be explained by the origin of glow discharge plasma technique, a large mass of high energy electrons generated in the plasma zone would bombard both to the cobalt nitrate precursor^5,15,31 and surface of SiO₂ support, leading to the split of the bonds of the precursors as well as the modification of surface properties of support. The reactions between the active plasma species and catalyst precursors are much faster than those in thermal calcination, which results in rapid nucleation of the cobalt crystals in plasma field, and helps to produce more homogeneously dispersed small cobalt particles.³¹

4 Conclusions

Characterization and catalytic results showed that glow discharge plasma was an effective technique to decompose the catalyst precursors. Compared with traditional calcination method, the plasma technique could not only reduce the decomposition time, but also improve the cobalt dispersion, more homogeneously dispersed small Co₃O₄ particles were observed in plasma treated catalysts. The high cobalt dispersion as well as good cobalt reducibility in Catal.-p and Sup.-p-Catal.-p catalysts resulted in a much higher catalytic activity in FTS. Meanwhile, plasma could also modify the surface properties of silica support, and influence the structure of supported cobalt species, finally the catalytic performance, especially the selectivities of petrol and paraffin were altered. Glow discharge plasma technique can be a potential method to prepare FT cobalt catalysts with high catalytic performance.

Acknowledgements

This work was supported by National Natural Science foundation of China (21203255), the Key Program of Technology Innovation of Hubei Province (2013AGB002), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (BZY14037) and the Fund for Basic Scientific Research of South-Central University for Nationalities (YZZ12001).

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