Enhancement of cobalt catalyst stability in Fischer–Tropsch synthesis using graphene nanosheets as catalyst support

chemical engineering research and design 1 0 4 ( 2 0 1 5 ) 713–722

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Enhancement of cobalt catalyst stability in Fischer–Tropsch synthesis using graphene nanosheets as catalyst support Saba Karimi a , Ahmad Tavasoli a,∗ , Yadollah Mortazavi b , Ali Karimi c a b c

School of Chemistry, College of Science, University of Tehran, Tehran, Iran School of Chemical engineering, University of Tehran, Tehran, Iran Research Institute of Petroleum Industry, PO Box 18745 4163, Tehran, Iran

a r t i c l e

i n f o

a b s t r a c t

Article history:

The effect of morphology and structure of graphene nanosheets (GNS) and carbon nanotubes

Received 22 June 2015

(CNT) as support on the stability of cobalt catalyst for Fischer–Tropsch synthesis (FTS) was

Received in revised form 9 October

investigated using a fixed bed micro-reactor.15.0 wt.% of cobalt was loaded on the supports

2015

by impregnation method. The deactivation of the two catalysts was studied at 220 ◦ C, 1.8 MPa

Accepted 12 October 2015

and 45 STP mL/min feed flow rate. Characterization of the supports, calcined fresh and used

Available online 19 October 2015

catalysts were studied by Raman spectroscopy, BET, XRD, TEM, TPR and H2 chemisorption techniques. XRD confirmed formation of cobalt oxides on the used catalysts. According to

Keywords:

the TEM and H2 chemisorption tests, 480 h continuous FT synthesis increased the average

Fischer–Tropsch synthesis

cobalt particle size from about 6 to 6.8 nm for Co/GNS catalyst and from about 7.5 to 9.5 nm

Cobalt

for Co/CNT catalyst. The initial CO conversion of the Co/GNS catalyst was 12% higher than

Graphene

that of the Co/CNT. For the Co/GNS, 480 h continuous FT synthesis decreased the CO conver-

Activity

sion by 3.9%, whereas, under the same reaction conditions the CO conversion for Co/CNT

Deactivation

decreased by 20.7%. Regeneration of the Co/GNS and Co/CNT recovered 99.3 and 92.8% of

Stability

the initial activities of the catalysts, respectively. Significant stability of Co/GNS catalyst in FT synthesis, introduces graphene as an excellent support for the cobalt catalysts. © 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

Fischer–Tropsch synthesis (FTS) is the heart of Gas-to-Liquid (GTL) technologies and a tool for utilization of synthesis gas derived from different sources, including natural gas, coal, and biomass (Tsakoumis et al., 2012). Cobalt is considered as the most suitable metal for the low-temperature FTS due to its higher activity and selectivity to linear paraffins and low water–gas shift (WGS) activity (Yao et al., 2012). In addition to catalytic activity and products selectivity, catalyst deactivation is an important matter for industrial catalyst development. The activity of cobalt-based FTS catalysts changes during their operation because of (a) oxidation of the

∗

surface cobalt atoms, (b) cobalt–support interactions and formation of mixed compounds that are reducible only at high temperatures, (c) sintering, (d) refractory coke formation, (e) loss of metal cobalt because of attrition and (f) hetero atoms poisoning (e.g. sulphur) (Das et al., 2005; Tsakoumis et al., 2010; Argyle et al., 2014; Dalai and Davis, 2008). The recent emergence of graphene nanosheets (GNS) has opened a new avenue for utilizing 2D carbon material as support for catalysts. It has shown unique properties and remarkable tunability in supporting a variety of metallic and bimetallic heterogeneous catalysts (Machadoab and Serp, 2012; Avouris and Dimitrakopoulos, 2012; Haag and Kung, 2014; Johns et al., 2013). Graphene possesses

Corresponding author. Tel.: +98 2161113643. E-mail address: [email protected] (A. Tavasoli). http://dx.doi.org/10.1016/j.cherd.2015.10.016 0263-8762/© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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List of symbols VL VC GA H2 ATPD AC D Aw S %wt Ns Nt O2 AP M mcat rCO t nC  d

loop volume calibration value % analytical gas H2 uptake (mole/gcat ) analytical area from TPD mean calibration area dispersion atomic weight stoichiometry % metal number of Co atoms on surface number of Co atoms in sample O2 uptake consumed pulses area mole of gas from calibration mass [gram] of catalyst rate of CO conversion [mol/min/gcat ] period of reaction total number of moles of carbon contained in a product fraction density particle size

interesting electronic, optical, thermal, and mechanical properties. Furthermore, it has large specific surface area, good bio-compatibility and high adsorption capacity. Thus, it not only maximizes the availability of nano sized catalyst surface area for the reaction but also provides better mass transfer of reactants and products to the catalyst surface (Guo and Sun, 2012; Kou et al., 2009; Chen et al., 2011; Subrahmanyam et al., 2008). It has been shown that using graphene nano sheets increases the catalyst activity and selectivity as compared to carbon nano tubes supported cobalt FTS (Co/CNT) catalyst (Karimi et al., 2015). In the present study, the deactivation behavior of the Co/CNT and Co/GNS catalysts during 480 h continuous FTS in a fixed bed micro reactor was monitored. The aim of the study is to investigate how the structure and properties of support affect the deactivation trends of cobalt catalyst. Furthermore, a number of characterization techniques were also employed to find out the main causes of Co/CNT and Co/GNS catalysts deactivation.

2.

Experimental

2.1.

Catalyst preparation

CNT and GNS were used as support materials for preparation of cobalt catalysts. The supports were prepared in Research Institute of Petroleum Industry (RIPI), Nano Technology Research Center. The chemical vapor deposition (CVD) technique was used to prepare GNS; the method was carried out in an electric furnace consisting of a quartz tube with a diameter of 50 mm and 120 mm length. The furnace provided programmable heating up to 900–1100 ◦ C for 5–30 min. The reaction was carried out using methane as the carbon source and hydrogen as the carrier gas in a ratio of 4:1. Prior to impregnation, the supports were treated with 30% HNO3 reflux at 120 ◦ C overnight, this approach creates oxygen containing functional groups on both the edges and basal plane of

GNS which can be used as chemically active anchoring sites for metal particles, then washed with distilled water, dried at 120 ◦ C for 6 h and treated in air at 400 ◦ C for 12 h to oxidize the probable amorphous carbons. The purified GNS and CNT were loaded with 15.0 wt.% cobalt using impregnation of cobalt nitrate (Co (NO3 )2 ·6H2 O, 99.0%, Merck) aqueous solution. Following the impregnation, the catalysts were dried at 120 ◦ C for 6 h and then calcined in air at 350 ◦ C for 3 h with a heating rate of 10 ◦ C/min under argon flow.

2.2.

Raman spectroscopy

The Raman shift of the support materials was measured on a Confocal Raman Microscope Systems with a laser source of 785 nm.

2.3.

TEM

The morphology of the supports, calcined fresh and used catalysts were characterized by transmission electron microscopy (TEM). The sample specimens for TEM studies were prepared by ultrasonic dispersion of the catalysts in ethanol, followed by locating a couple of suspension drops onto a carbon-coated copper grid. The TEM images were taken by a Philips CM20 (100 kV) transmission electron microscope equipped with an NARON energy-dispersive spectrometer with a germanium detector.

2.4.

ICP

The cobalt loadings of the calcined fresh and used catalysts were verified by an Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) system.

2.5. BET surface area/pore size distribution measurements The measurement of surface area, pore volume, and average pore radius of the supports, fresh and used catalysts were conducted by an ASAP-2010 Micromeritics system. The samples were evacuated at 200 ◦ C for 4 h to 50 m Torr and their surface properties were determined.

2.6.

X-ray diffraction

XRD patterns of the calcined fresh and used catalysts were conducted with a Philips PW1840 X-ray diffractometer with mono chromatized Cu K␣ radiation. Phase identification was done using X’Pert High-score Plus and the average size of the cobalt oxide crystallites in the calcined fresh and used catalysts, was estimated using the Scherrer equation and from the line broadening of the cobalt oxide peaks.

2.7.

Temperature-programmed reduction

Temperature-programmed reduction (TPR) profiles of the fresh and used catalysts were recorded using a Micromeritics TPD-TPR 290 system, equipped with a thermal conductivity detector. The catalyst samples were first purged in a flow of argon at 100 ◦ C to remove traces of water and other adsorbed species and then cooled to 40 ◦ C. The TPR of 50 mg of each sample was performed using 5.1% hydrogen in argon gas mixture with a flow rate of 40 cm3 /min. The samples were heated from 40 to 900 ◦ C with a rate of 10 ◦ C/min.

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2.8.

Hydrogen chemisorption and reoxidation

The amount of chemisorbed hydrogen on the fresh and used catalysts was measured using Micromeritics TPD-TPR 290 system. 0.25 g of the sample was reduced under hydrogen flow at 400 ◦ C for 12 h and then cooled to 100 ◦ C under hydrogen flow. The flow of hydrogen was then switched to argon at the same temperature, which lasted about 30 min in order to remove the weakly adsorbed hydrogen. Afterwards, the temperatureprogrammed desorption (TPD) of H2 over the samples was obtained by increasing the temperature of the samples, with a ramp rate of 10 ◦ C/min, to 400 ◦ C under the argon flow. H2 -TPD measurement was used to determine the cobalt dispersion and its surface average crystallite size. After H2 -TPD, the sample was reoxidized at 400 ◦ C by pulses of 10% oxygen in helium to determine the extent of catalyst reduction. It is noted that during reoxidation of the catalysts no CO2 peak is observed indicating that CNT and GNS as support of the catalyst have not reacted with oxygen in the oxygen titration test. It is assumed that Coo is oxidized to Co3 O4 . The calculations are summarized as follows (Tavasoli et al., 2008a,b). VC =

H2 =

VL ∗ %GA AC ∗ 100

(1)

ATPD ∗ MH2 AC

(2)



d=

2.9.

Ap ∗ MO2 AC

Sel() =

[nC] −rCO ∗ t ∗ mcat

The carbon balance was checked as follows: [nC]gas,

product

+ [nC]wax,

+ [nC]liquid,

product

product

= −rCO ∗ t ∗ mcat

The error on the carbon balance was calculated as:

%error

H2 ∗ Aw ∗ S NS ∗ 100 = %D = %wt Nt O2 =

The reaction products were collected in two traps, one maintained at 100 ◦ C (hot trap) and the other at 0 ◦ C (cold trap). Accumulated reactor wax was removed every 12 h through a tube fitted with a porous metal filter. The contents of hot and cold traps removed every 12 h, the hydrocarbon and water fractions separated, and then analyzed by GC. The analyses of C5 C30 hydrocarbons and reactor wax were performed on a HP 5890 gas chromatograph equipped with a capillary column, flame ionized detector (FID). The carrier gas was He and operated with temperature programming from 35 to 325 ◦ C at 4 ◦ C/min. The product data were handled using data analysis software. The product selectivity was calculated on moles of carbon basis, as follows:

∗ 100

6000  ∗ AM ∗ D



(3)

(4)

(5)

Reaction setup and experimental outline

The FTS experiments were conducted in a fixed bed down-flow micro-reactor and the catalysts were evaluated in terms of their FTS activity (g produced hydrocarbon/(gcat h)) and selectivity (the percentage of the converted CO that appears as the hydrocarbon products). The reactor temperature was maintained constant (±1 ◦ C) by a PID temperature controller. Brooks 5850 mass flow controllers were used to add H2 and CO at a desired rate into the reactor. Prior to the activity tests, the catalyst activation was conducted according to the following procedure. 0.6 g of catalyst powder (400–500 ␮m) which was diluted with 2 g of 90 mesh SiC to eliminate the temperature gradient was placed in the reactor and pure hydrogen was introduced at a flow rate of 45 mL/min. The reactor temperature was raised to 400 ◦ C at a heating rate of 10 ◦ C/min. It was activated for 24 h (first treatment step). At the end of the activation period, the reactor temperature was cooled down to 180 ◦ C under flowing hydrogen. Afterwards, the FTS reaction was started by adding the synthesis gas mixture to the reactor at a flow rate of 45 mL/min and a H2 /CO ratio of 2 and the reactor pressure was increased to 1.8 MPa. The reactor temperature was increased to 220 ◦ C at a rate of 10 ◦ C/min. The uncondensed vapor stream was reduced to atmospheric pressure through a pressure letdown valve. The flow was measured with a bubble-meter and in order to determination of the percentage conversion of CO and product selectivity the composition quantified using an on line gas chromatograph.

=

−rCO ∗ t ∗ mcat − [nC]gas,

product

− [nC]liquid,

product

− [nC]wax,

 product

−rCO ∗ t ∗ mcat

The carbon balance was considered satisfactory when the % error was <5%. After 480 h, the first FT reaction step, the flow of synthesis gas was switched to hydrogen and catalyst was re-reduced at the same condition similar to the first reduction step (second treatment step). The second FT synthesis step was carried out and the activity and selectivity of the system were measured. After that, the flow of synthesis gas was switched off and the catalytic bed was washed by helium flow for 3 h at 270 ◦ C to remove the heavy waxes inside the catalyst pores. The reactor was cooled to 20 ◦ C and the catalyst was passivated with pulses of dry air (Tavasoli et al., 2010). The used catalyst was discharged and characterized.

3.

Results and discussion

3.1.

Characterization of fresh and used catalysts

Raman spectroscopy is a powerful, nondestructive tool to identify the nature of materials. Raman spectra of CNT, GNS, Co/CNT and Co/GNS are shown in Fig. 1. There are two peaks in Raman spectra, the peaks at 1348 and 1585 could be ascribed to the D and G bands of GNS and CNT (Guo et al., 2010). The G band is related to the vibration of the sp2 bonded carbon atoms, and the D band corresponds to structural disorder in the curved GNS, with the D/G ratio usually taken as a measure of the quality of the graphitic structures. For highly ordered pyrolytic graphite, this ratio (D/G) approaches zero (Zhao et al., 2013). For the GNS the D/G ratio is about 1.18 which indicates a significant increase in the degree of disorder and defect sites in the GNS. The functional groups take place in defect sites and act as favorable nucleation sites for formation of the Co nano particles which can be anchored to the surface, and these sites

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Fig. 1 – Raman spectra of the supports and catalysts. reduce the mobility of these cobalt particles and decrease their tendency to agglomerate. For CNT, the ratio equals to 1.03. Increasing in the intensity of D/G ratio in the Co/GNS (1.23) and Co/CNTs (1.07) catalysts could indicate more structural defects in the supports lattice induced by Co nano particles (Moussa et al., 2014). The results of ICP, BET and pore size distribution for the supports, the fresh calcined and used catalysts are shown in Table 1. According to ICP analyses, the metal contents of the calcined fresh and used catalysts were close to the nominal Co metal content of 15.0 wt.% and the metal loss for catalysts during FTS is insignificant. According to BET results (Table 1), BET surface area for functionalized GNS is significantly higher than that of CNT (848 vs. 497 m2 /g). The difference may be attributed to the nature and textural morphology of the functionalized GNS. Pore volume for the CNT supports 1.034 cm3 /g and pore volume

of GNS is 2.2 cm3 /g. Higher porosity for GNS can be attributed to interlayer spacing of them. The BET surface areas for the fresh and used catalysts are also shown on Table 1. As is shown the textural properties of the used catalysts have changed as compared to that of the fresh catalysts. 480 h continuous FT synthesis decreased the surface area of Co/CNT and Co/GNS catalysts from 372 to 298 m2 /g and 602 to 586 m2 /g, correspond to 20 and 3% reduction in BET surface area, respectively. Also, FT synthesis decreased the pore volume of Co/CNT and Co/GNS catalysts from 0.765 to 0.428 cm3 /g and 1.46 to 1.23 cm3 /g, respectively. These values indicate that pore blockage in Co/CNT catalyst is higher than that of the Co/GNS and this is likely due to higher rates of sintering and clusters growth. For the Co/CNT catalyst, in comparison to pure CNT, the average pore size shifted to higher values, suggesting that the smaller pores have been partially blocked or destroyed. To ensure that hydrocarbon buildup in

Table 1 – ICP, BET, and porosity and XRD data. Support/Catalyst

CNT Fresh Co/CNT Used Co/CNT GNS Fresh Co/GNS Used Co/GNS

Amount of Co (wt.%)

SBET (m2 /g)

Pore volume (single point) (cm3 /g)

Average pore radius (nm)

dXRD (nm)

– 14.85 14.79 – 14.88 14.87

497 372 298 848 602 586

1.034 0.765 0.428 2.2 1.46 1.23

4.46 4.32 4.39 0.71 0.44 0.41

– 8.6 10.08 – 7.8 8.8

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717

Fig. 2 – (a) TEM image of the graphene (b) TEM image of the fresh Co/GNS catalyst (c)TEM image of the used Co/GNS catalyst (d) TEM image of the CNT (e) TEM image of the fresh Co/CNT catalyst (f) TEM image of the used Co/CNT catalyst. narrow pores was not a significant factor, the used catalyst, after wax extraction, was oxidized in 1% O2 in He at 500 ◦ C and the BET surface area and porosity were again measured. Only a minor increase of BET area to 596 and 310 (respectively for Co/GNS and Co/CNT) was observed (Dalai et al., 2005).

Fig. 2a–c shows the TEM images of the purified GNS, fresh calcined Co/GNS catalyst and used Co/GNS catalysts. The dark spots represent the cobalt oxides which are attached to the GNS surface. The TEM images show cobalt nano particles, which are uniformly dispersed on the GNS (Fig. 2b). As shown, the metal nano particles are well-dispersed on GNS

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surface and the sizes of cobalt particles are between about 3–8 nm. Fig. 2d–f, shows the TEM images of the purified CNT, fresh Co/CNT and used Co/CNT catalysts. The purified CNT consists of an interwoven matrix of tubes, which are mainly multi wall. Carbon nanotubes tubular morphology induces capillary forces during the impregnation process which can disperse catalyst particles inside the tubes and also on the perimeter of the tube walls (Fig. 2e) (Jacobs et al., 2002a). Dark spots represent the cobalt oxides which are attached inside and outside of the nanotubes. It can be resulted the cobalt oxide particles inside the CNTs are fairly uniform, and the most abounded ones have sizes in the range of 3–10 nm. This is in accordance with the average inner diameter of the CNTs (12 nm), whereas those on the outer surface have grown to about 16 nm (Fig. 2e). Obviously, the CNTs channels have restricted the growth of the particles inside the tubes. The data suggest that the inner pore of the nanotubes physically restricted the particle size growth located inside these nanotubes (Tavasoli et al., 2010). For the used catalysts (Fig. 2c and f) images reveal an increase in particle size, which proves sintering and agglomeration of cobalt particles. In the case of Co/CNT, Fig. 2f shows that, agglomeration of the cobalt particles which are located on the outer surface of CNT is much higher than that of the cobalt particles located on the inner surface of CNT. Fig. 2 also depicts the size distribution of the cobalt particles, which determined using the population of cobalt particles on 5 different TEM pictures for each catalyst (about one hundred particles). This figure shows that cobalt nanoparticle size distributions for Co/GNS catalyst are better than Co/CNTs catalyst. The surface functional groups on functionalized GNS support act as sites of interaction with metal precursor and create a narrow cobalt nanoparticle size distribution (Trepanier et al., 2010). According to Raman results there are more defects on GNS compare to Co/CNTs; these anchoring sites prevent metal particles sintering, which is in agreement with the histogram of the used catalysts (c and f). XRD patterns for the catalysts are shown in Fig. 3. For the calcined fresh Co/CNT nano catalyst, the peaks at 2 of 25.0◦ and 43.0◦ correspond to graphite layers (multi wall carbon nanotubes), while the other peaks are related to different crystal planes of Co3 O4 and CoO. The peak at 36.8◦ is the most intense peak of Co3 O4 . No peak corresponding to the formation of cobalt–support compounds was observed. On the pattern of the fresh Co/GNS nano catalyst the peaks at 25.0◦ and 43.0◦ correspond to GNS. The peak at 36.8◦ is the most intense peak of Co3 O4 nano particles. Several studies suggest that Co3 O4 is the active phase and other peaks, which correlate with a cubic cobalt structure has no influence on the product selectivity (Dalai et al., 2005; Jacobs et al., 2002a). As is shown, for Co/GNS nano catalyst, no peak is appeared indicating bulk crystalline cobalt carbide. Table 1 shows the average cobalt oxide particle size of the fresh catalysts calculated with Scherrer equation at 2 value of 36.8◦ . XRD patterns of the used Co/CNT and Co/GNS nano catalysts are also shown in Fig. 3. As is observed, the resulting patterns for the used catalysts are very complex. In the XRD pattern of the used Co/CNT, support peaks appeared at 2 values of 25.0◦ and 43.0◦ . The peaks at 2 values of 51.1◦ and 75.8◦ correspond to the metallic cobalt (Coo ) (Jacobs et al., 2002b). Also the peak at 2 value of 46.9◦ correlates well with Co2 C. It is known that an elementary step in the Fischer–Tropsch reaction is the dissociation of CO to form surface carbidic carbon and adsorbed atomic oxygen, this surface carbidic carbon

Fig. 3 – XRD patterns of the fresh and used Co/GNS and Co/CNT catalysts. may be converted to other less reactive forms, i.e. polymeric or graphitic carbon, interacting with the metal, which may build-up over time and possibly have a negative influence on catalyst activity (Moodley et al., 2009). The peaks at 2 values of 36.8◦ and 42.5◦ correspond to Co3 O4 and CoO. Although a fraction of cobalt clusters may be oxidized in presence of significant amount of water formed during FT synthesis with high conversions, some amount of the cobalt oxide in the used sample probably is formed during the discharge and passivation step at room temperature. In the XRD pattern of the used Co/GNS, similar to the corresponding pattern of the used Co/CNT, the peaks of support, metallic cobalt, cobalt oxides and cobalt carbide have been appeared at the same 2 values but the intensities of the peaks are different. Table 1 shows that 480 h continuous FT synthesis increased the average crystal sizes for both catalysts. However, the crystal growth was more significant in the case of Co/CNT catalyst. The reducibility of the catalysts in H2 atmosphere was determined by TPR experiments. The TPR spectra of the fresh calcined catalysts and the reduction peak temperatures are presented in Fig. 4. Both profiles show two apparent reduction peaks. The first peak is attributed to the reduction of Co3 O4 to CoO, and a fraction of the peak could comprise the reduction of the larger, bulk-like CoO species to Co0 . The second peaks involve the reduction of CoO to Coo . This figure shows that, using GNS as cobalt catalyst support will shift both TPR peaks significantly to the lower temperatures. It results in a decrease

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Fig. 4 – TPR patterns of different catalysts from 30 to 900 ◦ C. in the peak temperature of the first TPR peak from 408 to 368 ◦ C and the peak temperature of the second TPR peak from 539 to 523 ◦ C, suggesting an easier reduction process (Fig. 4). The unpaired electrons on GNS surface, which are ready to be shared with other electrons, makes reduction of cobalt oxides easier and shifts the reduction peak temperatures to lower temperatures (Chen et al., 2011). Also the high accessible surface area offered by GNS could favor the reduction of the Co/GNS. TEM and XRD tests showed that the average particles size of Co/CNT catalyst is greater than that of Co/GNS catalysts. Large particles can be reduced easier than small particles. A large amount of functional groups on Co/GNS may be considered as the cause of the peak shift to lower temperatures, due to spillover of hydrogen from the functional groups which facilitates the cobalt reduction (Davari et al., 2014). The broader shoulder on the TPR profile of Co/GNS catalyst is likely due to varying degrees of interaction of cobalt particles with support that depends on the size. The degree of interaction is higher for smaller cobalt particles (Jacobs et al., 2002b). It should be noted that, the area under the TPR peaks is proportional to the amount of H2 consumed during the reduction process. The area under the TPR peaks for Co/GNS is 1.23 times larger than that for Co/CNT catalyst, indicating the higher degree of reduction for Co/GNS catalyst. Fig. 4 shows that, 480 h continuous FT synthesis with Co/CNT and Co/GNS catalysts shifted both TPR peak temperatures to higher temperatures, indicating an increase in the degree of interaction of cobalt with the supports. Also, this figure shows that 480 h continuous FT synthesis with both catalysts increased the tailing of the second TPR peak, which confirms the larger extent of cobalt interaction with support during FT synthesis. Furthermore, the area under the TPR

peaks for the used Co/GNS is 1.274 times higher than that for the used Co/CNT catalyst. Percentage of dispersion and sizes of the cobalt particles determined by H2 -TPD and pulse reoxidation of the catalysts are given in Table 2. For each sample, dispersion and particle size were calculated based on the total amount of cobalt in the catalyst samples. The percentage of reduction was measured from the oxygen titration after H2 -TPD, assuming Coo is reoxidized to Co3 O4 . Comparing the results of TPD and oxygen titrations of the calcined fresh catalysts in Table 2, the hydrogen uptake increases significantly using GNS as cobalt catalyst support. In agreement with the results of TPR, results indicate that a remarkable improvement in the percentage reduction is obtained by switching to GNS support with the same Co loading. While the dispersion of the cobalt crystallites calculated based on the total amount of cobalt increases significantly, the average cobalt particle size decreases, which is due to the lower degree of agglomeration of the cobalt crystallites in Co/GNS supported catalyst. These results are in agreement with the results of XRD patterns and TEM images. Higher dispersion and lower cobalt cluster size will increase the number of sites available for FT reaction in the Co/GNS catalysts in comparison with the Co/CNT with the same cobalt loading. Also, the results of H2 chemisorption and oxygen titration tests for the used catalysts are shown in Table 2. As is evident, hydrogen consumption for the used catalysts are lower than that of their corresponding fresh calcined catalysts. Both the reduction in percentage and dispersion calculated based on the total cobalt decreased significantly. Continues FT synthesis for 480 h decreased the dispersion of Co/GNS nano catalyst form 21.6 to 19.1% and that of Co/CNT nano catalyst from 19.5 to 15.6%. According to results in Table 2 which are in agreement with the data obtained from XRD patterns, the average diameter of cobalt nano particles increased especially in the case of Co/CNT nano catalyst. For Co/CNT nano catalyst, the average diameter increased from about 7.5 to 9.5 nm. At the same time, that of Co/GNS nano catalyst increased from about 6 to 6.8 nm. The data shows that the rate of sintering or cluster growth for the Co/CNT nano catalyst is higher than that for the Co/GNS nano catalyst.

3.2.

Activity and product selectivity results

Table 3 and Fig. 5 present the FT synthesis rate (g hydrocarbon/(gcat h)), CO conversion, and product selectivity during the first 24 h of FT synthesis over the Co/GNS and Co/CNT nano catalysts. As is shown the rate (0.330 g hydrocarbon/(gcat h)) of the Co/GNS catalyst is significantly higher than that of the Co/CNT nano catalyst (0.258 g hydrocarbon/(gcat h)). The CO conversion for Co/GNS is 73% in comparison to 61% for the Co/CNT catalyst. Thus, in industrial scale, using Co/GNS nano catalyst, with higher volumetric productivity, will decrease reactor volume requirements and

Table 2 – %Dispersion and crystallite sizes of cobalt particles determined by H2 TPD and pulse reoxidation of fresh calcined and used catalysts. Catalyst

Fresh Co/CNT Used Co/CNT Fresh Co/GNS Used Co/GNS

␮ mole H2 desorbed/gcat 198 148.5 246 214

␮ mole O2 consumed/gcat 1223 1087 1382 1271

%Red.

64 56.8 72.3 66.6

%Dispersion

19.5 15.6 21.6 19.1

dp (nm)

7.5 9.5 6 6.8

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Table 3 – FT synthesis rate (gCH/(gcat h)) and CO conversion (%) after 24 and 480 h FT synthesis and after regeneration at 400 ◦ C over the Co/GNS and Co/CNT nano catalysts (T = 220 ◦ C, P = 1.8 MPa, H2 /CO = 2). Catalyst

%CO conversion after 24 h

O/P

Co/GNS Co/CNT

73 61

0.80 0.75

%CO conversion after 480 h

69.1 40.3

%CO conversion after 480 h and regeneration at 400 ◦ C (%)

FT synthesis rate (gCH/(gcat h)) after 24 h

FT synthesis rate (gCH/(gcat h)) after 480 h

0.330 0.258

0.312 0.171

72.5 56.6

as a result the capital cost of the plant as compared to Co/CNT catalyst. The honeycomb lattice of GNS is porous, electrically conductive and provides large surface area for cobalt atoms to be dispersed and so generates numerous catalytic active sites as was confirmed by TEM and H2 chemisorption tests and also GNS facilitate the reduction of cobalt oxides and increase the surface density of cobalt active sites. In the case of Co/CNT catalyst there is a deviation of the graphene layers from planarity and which in turn causes the shift of ␲-electron density from the concave inner surface to the convex outer surface, leading to a ␲-electron deficient interior surface and an electron-enriched exterior surface. Different catalytic activity of these surfaces leads to lower activity in comparison to Co/GNS catalyst (Pan and Bao, 2008). The distribution of hydrocarbon products for the Co/GNS catalyst shows a slight shift to the higher molecular weight hydrocarbons as indicated in Fig. 5. This figure shows the selectivity of products during the first 24 h FT synthesis for the catalysts. GNS has electron rich ␲ sites and electron donations to the vacant d orbital of the transition metal (Co) would enhance the dissociative adsorption of CO, while suppressing H2 adsorption (Pendyala et al., 2014). According to the carbide mechanism, CO bond cleavage generates CHx intermediates on the catalytically reactive surface that are incorporated in consecutive steps into the growing hydrocarbon chain (van Santen and Markvoort, 2013). However, a high surface coverage of C or CH2 groups is required to ensure a fast chain growth relative to chain termination with surface hydrogen, and therefore, a sufficiently fast CO dissociation rate is required (Zhuo et al., 2009). This effect could explain higher selectivity to higher molecular weight hydrocarbons. On the other hand, the higher mass transfer rate for reactants and products over

FT synthesis rate (gCH/(gcat h)) after 480 h and regeneration at 400 ◦ C (%) 0.327 0.240

the GNS can enhance the activity (Liu et al., 2012). It should be mentioned that the ␣-olefin/n-paraffin ratios were measured for C2 C5 hydrocarbons in this work. The olefin/paraffin ratio is higher for the Co/GNS catalyst. On Co/CNT catalyst, higher H*/CO* ratios leads to preferential termination of surface hydrocarbon chains to alkanes for all carbon numbers (Shi and Davis, 2005). It is worthy to note that, decreasing the amount of methane and light gaseous hydrocarbons selectivities in the case of Co/GNS nano catalyst will decrease the amount of recycle gas (light gaseous hydrocarbons) to the entrance of the synthesis gas production unit and as a result decreases the size of this unit and improve the process economy.

3.3.

Catalyst stability

Fig. 6 presents the percentage CO conversion (XCO ) variations in the duration of FT synthesis for the catalysts. As it can be seen, for both catalysts, especially for Co/CNT, the percentage CO conversion decreases in the first days, and then levels off. The declining trend of activity for both catalysts has two different slopes. In the case of Co/CNT nano catalyst, the slope of the deactivation curve has steep slope at first and then changes to a more sluggish slope. Fig. 6 shows that, for Co/CNT, the plateau region was reached after about 120 h which indicates that the loss of active sites decreases significantly during this period. For this catalyst, the CO conversion drops by 15.8% in the first 120 h while during the remaining period of the test, i.e. between 120 and 480 h, it only drops by 4.9%. For the Co/GNS, it is difficult to observe two distinguished regions. However, the CO conversion decline is almost rapid during the first days and is equal to about 2.7% in the first 120 h while during the

Fig. 5 – Product selectivity for Co/GNS and Co/CNT catalysts for first 24 h (T = 220 ◦ C, P = 1.8 MPa, H2 /CO = 2).

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Fig. 6 – CO conversion against time-on-stream for Co/GNS and Co/CNT catalysts (T = 220 ◦ C, P = 1.8 MPa, H2 /CO = 2). 120–480 h FT synthesis it drops only by 1.2%. The loss of activity during the first deactivation step for the catalysts can be described with the following linear correlations. Co/GNS catalyst :

XCO = −0.0218 T(h) + 73.005

(6)

Co/CNTs catalyst :

XCO = −0.1304 T(h) + 60.57

(7)

According to linear deactivation equations, the deactivation rate of catalysts is zero order with respect to CO conversion, which implies that the deactivation is not related to the number of the catalyst active sites and is caused mainly by other parameters (Tavasoli et al., 2012). It has been suggested that in FT synthesis on cobalt-based catalysts at high conversions, the loss of activity is caused by oxidation of cobalt induced by water. However, the extent of oxidation is affected by the ratio of PH2 O /PH2 and the cobalt crystallite size (Bezemer et al., 2006). According to XRD, TEM and H2 Chemisorption tests, Co/GNS catalyst with a higher fraction of smaller clusters should be more susceptible to reoxidation. However, Eqs. (6) and (7) and also the results in Fig. 6 show that the rate of deactivation during the first 120 h reaction is higher for the Co/CNT catalyst. In addition, higher conversions in the case of Co/GNS catalyst can increase the PH2 O /PH2 that must result in higher catalyst deactivation rates. This behavior may be explained by the highly hydrophobic nature of GNS (Zhao et al., 2013; Leenaerts et al., 2009). The hydrophobic GNS surface can reduce water deposition on the catalyst surface and prevent reoxidation of Co to a large extent. For the second deactivation step (120–480 h FT synthesis), the catalyst deactivation can be described with the following power law expressions: Co/GNS :

XCO = 77.177 Th−0.047

(8)

Co/CNT :

XCO = 88.813 Th−0.141

(9)

Assuming the deactivation rate is dX = kXn dt

(10)

721

Fig. 7 – Liquid C5+ hydrocarbon and CH4 selectivity variations with time on stream for the Co/GNS and Co/CNT catalysts (T = 220 ◦ C, P = 1.8 MPa, H2 /CO = 2). After integration and data reduction by the least square fit, the power order (n) can be determined as 22.2 and 8.1 for the Co/GNS and Co/CNT catalysts, respectively. These values are in the range that ordinary metal catalysts would experience during sintering (Tavasoli et al., 2010). However, the lower power order of 8.1 for the Co/CNT catalyst demonstrates the higher rate of sintering in comparison to that for the Co/GNS catalyst. The results of XRD tests (Table 1 and Fig. 3), the results of H2 chemisorption and re-oxidation tests (Table 3) and TEM images (Fig. 2) confirm the higher cluster growth rate during 480 h on stream for the Co/CNT catalyst in comparison with Co/GNS catalyst. One hypothesis of agglomeration states that linkages between molecules of water on the surfaces of particles cause the particles to agglomerate (Feng et al., 1993). Hydrophobic surface of Co/GNS causes lower PH2 O on the surface and makes it difficult to form larger agglomerates. Second, more functional groups and defects in Co/GNS (according to Raman spectra on Fig. 1), which can act as anchoring sites for the cobalt particles leads to stronger interactions between active metal and support, which in turn decreases cobalt agglomeration and enhances catalyst stability (Karimi et al., 2014). Fig. 7 shows the selectivity variations during 480 h continuous FT synthesis at 220 ◦ C and 1.8 MPa. As is shown, the CH4 selectivity decreases and the liquid C5 + selectivity increases with time-on stream. It has been shown (Borg et al., 2008) that the larger cobalt particles are more selective to higher molecular weight hydrocarbons and smaller cobalt particles are more selective to methane and light gaseous hydrocarbons. It may be concluded that the sintering of smaller Co particles leads to enhancement of C5 + selectivity and suppression of CH4 production with time on stream. Higher rate of sintering of the cobalt particles in the case of Co/CNT catalyst is believed to be the main reason for greater enhancement of C5 + selectivity and suppression of CH4 . In order to determine the contribution of each deactivation aspect on the overall deactivation of the catalyst, the used catalysts were again regenerated at 400 ◦ C. Subsequently, the reactor was cooled to 220 ◦ C and the second FT synthesis step was carried out under similar conditions as the previous

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synthesis step and CO conversions and FTS rate were measured. Table 3 presents CO conversions and FTS rates for both catalysts after regeneration and 24 h FT synthesis. The results show that the regeneration of the used Co/GNS catalyst at 400 ◦ C increased CO conversion from 69 to 72.5%. In the case of Co/CNT catalyst regeneration at 400 ◦ C increased CO conversion from 40.3% to 56.6%. Thus, for Co/GNS about 0.5% of activity loss and for the Co/CNT about 4.4% of activity loss is not recoverable which may be attributed to the sintering of cobalt particles. Therefore, it is conceivable that the extent of irreversible deactivation in Co/CNT is larger than that of the Co/GNS catalyst.

4.

Conclusion

This research has been carried out using functionalized CNT and GNS as catalyst support, to compare the effects of their morphology and structure on activity, selectivity and stability of 15 wt.% cobalt catalyst. TPR results showed that, deposition of cobalt nano particles on the functionalized GNS, shifts the reduction steps to lower temperatures and the reducibility of the catalysts improved significantly. According to TEM images and H2 chemisorption, the catalyst prepared on GNS had a narrow particle size distribution and consequently larger dispersion in comparison with the CNT-Supported cobalt catalyst. Improvements on the uniformity of the catalyst particles in the case of the catalyst prepared on functionalized GNS, leads to a higher CO conversion. Using GNS as catalyst support also, decreased the sintering of cobalt nano particles and affected stability of catalyst in the case of FTS reaction. This new support may offer an attractive alternative for synthesis of nanoparticles with narrow size distributions for fundamental catalytic studies especially for structure-sensitive reactions such as FTS catalysts.

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