CNTs catalyst: Effects of metallic cobalt particle size

Journal of Natural Gas Science and Engineering 21 (2014) 772e778

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Intrinsic kinetics of FischereTropsch synthesis over Co/CNTs catalyst: Effects of metallic cobalt particle size Ali Nakhaei Pour a, *, Elham Hosaini a, Ahmad Tavasoli b, Alireza Behroozsarand c, Fatemeh Dolati a a b c

Department of Chemistry, Ferdowsi University of Mashhad, P.O.Box: 9177948974, Mashhad, Iran School of Chemistry, College of Science, University of Tehran, Tehran, Iran Department of Chemical Engineering, Urmia University of Technology, Urmia, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 July 2014 Received in revised form 7 October 2014 Accepted 8 October 2014 Available online 25 October 2014

The influence of the average particle size of cobalt nanoparticles on the kinetics parameters of Fischer eTropsch synthesis (FTS) were investigated over carbon nanotubes (CNTs) supported cobalt catalysts. Five Co/CNTs catalysts with different particle sizes were prepared by microemulsion and impregnation methods. To achieve a series of catalysts with different Co particle size, water-to-surfactant molar ratio (W/S) in microemulsion system varied from 2 to 12. A useful kinetic model was used to fit the experimental results and the size-dependent of kinetic parameters were evaluated. The experimental results suggest that the average Co particle size influenced on kinetic parameters such as rate constant, and adsorption parameter. Experimental results show that by decreasing the average Co particle size both the intrinsic reaction rate constant (kFTS), and adsorption parameter are increased. © 2014 Elsevier B.V. All rights reserved.

Keywords: FischereTropsch synthesis Cobalt catalyst Carbon nanotubes Particle size Kinetic

1. Introduction FischereTropsch synthesis (FTS), the production of liquid hydrocarbons from synthesis gas (CO and H2), is a promising developing option for the production of chemicals and fuels from coal and natural gas (Dry, 2004). Supported cobalt catalysts have received renewed attention FTS reaction, due to their high activity and high selectivity toward heavier hydrocarbons (Davis, 2007). Support plays a fundamental role in the synthesis of small metallic cobalt crystallites and the use of supports or alloys is increases the rate per surface Co (turnover rate) (Design, 1997; Karimi et al., 2010). However, the interaction between support and metallic cobalt nanoparticles leads to a decrease of the catalyst reduction efficiency (Borg Ø et al., 2008; Borg Ø et al., 2007). Carbon nanotubes (CNTs) as cobalt catalyst support allow a better metal dispersion control and minimize the metal phase interaction (formation of mixed compounds) with the support (Tavasoli et al., 2008; panier et al., 2010). Thus, CNTs as a new type of carbon mateTre rial have shown interesting properties, favoring catalytic activity for FTS cobalt catalysts.

* Corresponding author. Tel./fax: þ98 5118795457. E-mail addresses: [email protected], [email protected] (A. Nakhaei Pour). http://dx.doi.org/10.1016/j.jngse.2014.10.008 1875-5100/© 2014 Elsevier B.V. All rights reserved.

Nanometer-sized cobalt particles embedded on the surface of different supports are promising materials for high-density recording. In addition, metallic cobalt particle size in supported cobalt catalysts may be affected on catalyst activity in the FTS reaction (den Breejen et al., 2009; Ma et al., 2011). However, the interaction between cobalt and support for smaller cobalt particle size is very high. Thus, the CNTs supported cobalt catalysts are the best candidate for the evaluation of particle size effects in supported cobalt catalysts because of low interaction between support panier et al., and cobalt nanoparticles (Bezemer et al., 2006; Tre 2009). The kinetic description of the FTS reaction is a very important task for industrial process design, optimization, and simulation (Van Der Laan and Beenackers, 1999). Many kinetic equations have been proposed in the literature for cobalt catalysts supported on various metal oxides. These equations have been obtained either empirically (using a power law rate equation) or to fit a proposed mechanism. Eric van Steen and Hans Schulz (van Steen and Schulz, 1999), Nakhaei Pour et al. (Nakhaei Pour et al., 2014; Pour et al., 2012; Mansouri et al., 2013; Todic et al., 2013 and Visconti et al., 2007) have displayed the explicit rate equations based on the Langmuir-Hinshelwood- Hougen-Watson (LHHW) adsorption theory that has been developed for the cobalt and iron-based FTS catalysts.

A. Nakhaei Pour et al. / Journal of Natural Gas Science and Engineering 21 (2014) 772e778

In our previous work, we extensively studied the activity and product selectivity of carbon nanotubes (CNTs) supported cobalt catalysts with different cobalt particle size. The catalysts were fully characterized with deferent methods and some activity results like as cobalt-time yields, C5þ selectivity and chain growth probability for hydrocarbons were evaluated (Nakhaei Pour and Housaindokht, 2013). To study the intrinsic cobalt particle size effects on an inert support material, we made use of carbon nanotubes (CNTs), a pure and structured material with a large surface area. Five Co/CNTs model catalysts with Co particle sizes in the range of 4.9e12.4 nm were prepared to examine the possible intrinsic particle size effect in FTS synthesis. Transmission electron microscopy (TEM) and Xray diffraction (XRD) were used to characterize the particle size distribution. The kinetic experimental study was carried out over a wide range of reaction conditions using Eric van Steen and Hans Schulz kinetic model. These kinetic investigations have shown that our Co/CNTs catalysts are an ideal model for the study of the intrinsic cobalt particle size effects on FTS reaction. 2. Experimental 2.1. Catalyst preparation A series of cobalt catalysts were prepared with 15 wt% cobalt on carbon nanotubes (CNTs) support via incipient wetness impregnation and microemulsion methods, as reported in previous work (Nakhaei Pour and Housaindokht, 2013). For achieve a series of catalysts displaying different Co particle size, the water-tosurfactant molar ratio (W/S) varied from 2 to 12 at the surfactantto-oil (S/O) weight ratio equal to 0.3 in the microemulsion system. Table 1 listed the catalyst nomenclatures. 2.2. Catalyst characterization Powder X-Ray diffraction (XRD) measurements were conducted on a Philips PW1840 X-ray diffractometer with monochromatized Cu/Ka radiation. The phase identification has been made using JCPDS database and the Co3O4 particle size in the fresh samples were calculated from Scherer equation using the half-width peak located at 2q ¼ 36.9 and K factor of 0.89. Using the densities of the Co3O4 and cubic Co crystal structures as a basis, the particle size of Co metal is estimated as 75% of the Co3O4 particle size as:




nm ¼ 0:75  d Co3 O4 d Co0

Catalyst preparation method

W/Sa ratio

dpb Co (nm) XRD results

TEM average

Impregnation Microemulsion Microemulsion Microemulsion Microemulsion

e 2 4 8 12

12.1 4.6 7.6 8.3 9.4

12.4 4.9 7.9 8.6 9.8

Water-to-surfactant. Average metallic cobalt particle sizes.

2.3. Catalyst activity Steady-state FTS reaction rates were measured in a continuous spinning basket reactor (stainless steel, H ¼ 0.122 m, D0 ¼ 0.052 m, Di ¼ 0.046 m) with temperature controllers (WEST series 3800), mass flow meters (Brooks) for hydrogen and carbon monoxide and back pressure valve for pressure controlling. A detailed description of the experimental setup and procedures has been provided in our previous work (Nakhaei Pour et al., 2014; Nakhaei Pour et al., 2013). For loading of catalyst into reactor, 2.5 g of the catalysts diluted by 30 cm3 inert silica (with the same mesh size range) and charged into basket of the reactor. The external mass transfer limitation was investigated by comparing the CO conversions under different stirring speeds of the reactor. Apparently, the stirring speed needed to eliminate the external mass transfer limitation. Therefore, the corresponding stirring speed should ensure that the measured experimental data are in the kinetically limited regime. In these experiments, all the experiments were carried out at 1800 rpm that is safe to eliminate the external mass transfer limitations in all kinetic conditions. The fresh catalysts reduced in situ with H2 for 12 h (at 673 K, 1 bar, and 3.6 Nl/gcat/h). After pretreatment the FTS tests were carried out at T ¼ 493 K, 20 bar, H2/CO ¼ 2 and 2.4 Nl/gcat/h, feed rate for 12 h (for stabilization of catalyst activity). During kinetic studies, the reaction pressure was kept constant at 20 bars, the reactor temperature varied between 493 and 508 K, and the space velocity of the synthesis gas varied between 2.4 and 12 Nl/gcat/h. The H2/CO ratio of the feed was kept constant in all space velocities. Table 2, listed the reaction conditions for catalysts activities tests. For each operation condition, it took at least 4 h to ensure the steady-state behavior of the catalyst after a change in the reaction conditions. Periodically during the run, the catalyst activity was measured under a preset “standard” condition (a space velocity equal to 2.4 Nl/gcat/h) to check the catalyst deactivation. CO conversion and different product selectivity were calculated based on the GC analyses and carbon balance. The products and un-reacted feeds were analyzed by means of three gas chromatographs and a detailed description of the analyzing setup and procedures has been provided in our previous work (Nakhaei Pour et al., 2014; Nakhaei Pour et al., 2013). 3. Results and discussion

Table 1 Average Co particle size of the fresh catalysts.

a

AB, Germany). For TEM observation, an appropriate amount of catalyst suspension was dropped onto the carbon-coated copper grids.

(1)

XRD measurements of the used samples were made after passivation with a 1 vol% O2/He mixture at room temperature for 1 h, according to a standard procedure described elsewhere (Nakhaei Pour et al., 2010a,b). The morphology of the calcined cobalt nanoparticles was observed with a transmission electron microscope (TEM, LEO 912

b

773

3.1. Catalyst characterization XRD patterns of the fresh catalysts after calcination are shown in Fig. 1. In the XRD spectra the peaks at 2q values of 25 and 43 correspond to the CNT support, the peak at 2q value of 36.8 corresponds to the (311), and minor peaks at 44 (400), 59 (511) and 65 (440), correlate with a cubic spinel structure of Co3O4, based on JCPD: panier et al., 2010). The calculated average Co3O4 crys78-1970 (Tre tallite sizes (using Scherrer equation) and the corresponding cobalt metal particle size (according to Eq. (1)) are listed in Table 1. As shown in Table 1, the average particle size of cobalt nanoparticles depended on water-to-surfactant (W/S) ratio in microemulsion system. Fig. 2 shows the XRD pattern of the catalysts reduced at 673 K after passivation. As shown in Fig. 2, the XRD results confirm a complex composition of nanoparticles contains cobalt oxides and metallic cobalt with hexagonal close-packed (hcp) and facecentered-cubic (fcc) phases. In the XRD spectra of passivated samples the peaks at 2q values of 42 , 48 , 76 and 92 correspond

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Table 2 Reaction conditions, CO conversions and FTS reaction rates for all the catalysts. Pressure (bar) Catalyst Temperature GHSV (Nl/gcat/h) particle (K) H2 CO H2O size (nm) 12.4

508

498

9.8

508

498

8.6

508

498

7.9

508

498

4.9

508

498

2.4 4.8 7.2 9.6 12.0 2.4 4.8 7.2 9.6 12.0 2.4 4.8 7.2 9.6 12.0 2.4 4.8 7.2 9.6 12.0 2.4 4.8 7.2 9.6 12.0 2.4 4.8 7.2 9.6 12.0 2.4 4.8 7.2 9.6 12.0 2.4 4.8 7.2 9.6 12.0 2.4 4.8 7.2 9.6 12.0 2.4 4.8 7.2 9.6 12.0

7.21 9.51 9.93 10.40 10.56 7.82 9.64 10.20 10.80 11.07 6.16 9.23 9.50 9.90 10.41 6.86 9.11 9.73 10.43 10.75 5.65 8.35 9.09 10.10 10.81 6.31 8.81 9.45 10.19 10.57 5.06 8.02 8.81 9.72 10.13 5.74 8.45 9.17 10.00 10.38 5.73 8.37 9.10 9.91 10.75 6.37 8.79 9.44 10.20 10.85

RFTS CO conversion mol/gcat/h (%)

2.33 8.10 78.1 3.83 5.49 62.6 4.20 4.69 54.1 4.43 4.02 46.2 4.79 3.54 40.6 2.64 7.77 75.1 3.71 5.45 60.0 4.29 4.56 52.1 4.80 3.69 44.2 5.12 3.21 39.0 1.74 9.14 83.3 4.11 5.77 67.7 4.27 4.88 58.3 4.26 4.35 50.1 4.72 3.71 43.7 2.10 8.74 81.1 3.32 6.02 65.2 4.04 5.02 56.0 4.52 4.07 48.1 4.95 3.50 42.3 1.33 9.67 88.2 2.80 6.62 70.8 3.63 5.50 60.9 4.20 4.48 52.3 4.47 4.09 45.7 1.69 9.33 85.0 3.07 6.35 68.3 3.80 5.33 58.7 4.36 4.31 50.4 4.81 3.69 44.1 1.04 10.18 91.0 2.63 6.91 73.2 3.49 5.76 62.9 4.09 4.65 53.8 4.60 3.99 47.1 1.26 9.93 88.2 2.78 6.74 71.4 3.61 5.61 61.5 4.18 4.54 52.7 4.68 3.90 46.1 1.43 9.33 87.2 2.86 6.43 69.8 3.65 5.38 60.2 4.11 4.57 51.6 4.29 4.16 45.1 1.98 8.76 82.1 3.23 6.04 65.8 3.96 5.05 56.7 4.46 4.11 48.6 4.95 3.53 42.5

0.0273 0.0438 0.0567 0.0647 0.0711 0.0263 0.0420 0.0546 0.0616 0.0683 0.0294 0.0473 0.0611 0.0699 0.0763 0.0283 0.0454 0.0587 0.0671 0.0734 0.0308 0.0494 0.0637 0.0730 0.0797 0.0297 0.0475 0.0618 0.0703 0.0769 0.0317 0.0509 0.0658 0.0752 0.0822 0.0310 0.0498 0.0644 0.0736 0.0804 0.0303 0.0487 0.0630 0.0720 0.0787 0.0295 0.0459 0.0594 0.0678 0.0715

to the metallic cobalt with hcp phase based on JCPDS: 05-0727 and the peaks at 2q values of 45 , 52 and 98 correspond to the metallic cobalt with fcc phase based on JCPDS: 01-1259 (Matveev et al., 2006; Enache et al., 2002). As shown in Fig. 2, the samples with significantly larger particle size exhibit more fcc and less hcp metallic cobalt. The TEM image of the fresh catalyst sample prepared by W/ S ¼ 4, is shown in Fig. 3. The average particle size (dTEM) was determined by counting more than 100 particles in TEM images using average Feret diameter, as reported in the previous work, listed in Table 1, and used for further calculations (Nakhaei Pour and Housaindokht, 2013). 3.2. FischereTropsch activity The series of Co/CNT catalysts are excellent candidates to study particle size effects in FTS reaction. Their catalytic activity in the FTS

Fig. 1. XRD patterns of fresh catalysts after calcinations. a, catalyst prepared by impregnation method, b, catalyst prepared by microemulsion method and W/S ¼ 12, c, catalyst prepared by microemulsion method and W/S ¼ 8, d, catalyst prepared by microemulsion method and W/S ¼ 4, e, catalyst prepared by microemulsion method and W/S ¼ 2.

at T ¼ 493 and 508 K, P ¼ 20 bar, H2/CO ¼ 2 and the space velocity of the synthesis gas between 2.4 and 12 Nl/h/gcat has been examined. Recently Bezemer et al. reported that the catalytic activity of cobalt catalyst increases with decreasing of cobalt particle size and passed from a maximum at 6 nm (Bezemer et al., 2006). The results for FTS reaction rates as function of temperature and cobalt particle size have been shown in Fig. 4. In Fig. 4, the catalyst particle size is the radius of metallic cobalt nanoparticles and rFTS is the rate of CO conversion to organic products. As shown in Fig. 4, the FTS reaction rate (mol/gcat./h) increases by increasing the reaction temperature (from 493 to 508 K). In addition, Fig. 4 shows that the FTS reaction rates passes from a maximum (7.6 nm) by decreasing the catalyst particle size for supported Co/CNT catalyst. These complexities (a maximum in FTS activities against catalyst particle size) make it difficult to establish a simple correlation between catalyst activity and particle size. As reported in the previous section, larger particle size exhibits more fcc and less hcp metallic cobalt. As reported in literatures, cobalt phase composition in cobalt nanoparticles could also affect FTS catalytic performance. The recent experimental results for structure determinations show that the hcp phase of metallic cobalt shows higher activity in FTS reaction than fcc phase and the cobalt catalysts with higher fractions of hcp phase exhibited higher conversion in FTS reaction (Khodakov, 2009). The origin of this difference in FTS activities is not clear and may be

A. Nakhaei Pour et al. / Journal of Natural Gas Science and Engineering 21 (2014) 772e778

775

3.3. Kinetic model parameters Many mechanistic LangmuireHinshelwoodeHougeneWatson (LHHW) and empirical equations are used to describe the kinetic behavior of synthesis gas conversion to hydrocarbons (van Steen and Schulz, 1999; Nakhaei Pour et al., 2014; Pour et al., 2012; Mansouri et al., 2013; Todic et al., 2013; Visconti et al., 2007). Eric van Steen and Hans Schulz (van Steen and Schulz, 1999) developed a useful LHHW kinetic equation for FTS reaction on iron and cobalt based catalyst by considering the rate of monomer formation and polymerization steps. Their cobalt based kinetic equation was developed by assuming a state of equilibrium between the surface species H, CO, O, and OH, in elementary reactions and obtained equation is:

rFTS

0 1 3=2 P P kFTS K12 K2 K3 K4 B H2 CO C ¼ A  2 @ PH2 O P P H CO 1 þ K12 K2 K3 K4 P2H O

(2)

2

Table 3 lists the elementary reactions for driving the Eq. (1). As shown in Table 3, K1, K2, K3 and K4 is thermodynamic constant for proposed elementary reactions. The FTS reaction rate expression given in Eq. (2) is linearized by rearrangement as: Fig. 2. XRD pattern of the catalysts reduced at 673 K after passivation. a, catalyst prepared by microemulsion method and W/S ¼ 4, b, catalyst prepared by microemulsion method and W/S ¼ 8 and c, catalyst prepared by impregnation method.

explained according to the FTS mechanism in future studies. Thus, higher activities of smaller cobalt particle size are related to higher fractions of hcp metallic cobalt phase, but the origin of lower activity of the catalyst with cobalt particle size of 4.8 nm will requires more attention. The reduction of the catalyst activity for small Co nanoparticles is related to several reasons like as deposition of inactive carbons on smaller cobalt nanoparticles. The effect of the blocking of the sites of small Co nanoparticles by strong adsorption of carbon as the reason of the low activity has been already shown earlier by SSITKA experiments (den Breejen et al., 2009).

0

3=2

PH2 PCO

11=2 ¼

where, b ¼

K21K2K3K4.

ðkFTS :bÞ

 þ 1=2

b

1=2 ð

kFTS

PH2 PCO Þ PH2 O

0

(3)

11=2 P

3=2

P

CO H Hence, a plot of @rFTS2PH O A 2

versus

PCO PH2 PH2 O

1 should give a straight line with intercept of and slope ðkFTS :bÞ1=2  1=2 of k b . Figs. 5 to 9, show the linearised plots for the model FTS

proposed by Eric van Steen and Hans Schulz (van Steen and Schulz, 1999) for the our Co/CNT catalysts in present work. As shown in Figs. 5 to 9, the experimental results obtained from various cobalt catalysts in our laboratory shows good agreement with the rate equation proposed by Eric van Steen and Hans Schulz. A small deviation from the model in our results can be related to different support used for cobalt catalysts. Eric van Steen and Hans Schulz are developed or model for precipitated cobalt catalyst and cobalt supported on silica support. Nevertheless, our cobalt catalyst supported on carbon nanotubes. It can be concluded that the deposition of the catalytic sites inside the nanotube pores results some mass transfer limitations in catalyst activities. These mass transfer limitations are the most important factor in deviation from the Eric van Steen and Hans Schulz model. The mean absolute relative residual (MARR) was used to implement the experimental results with the model parameters:

MARR ¼ 100

Fig. 3. TEM image of W/S ratio of 4.

1

@ A rFTS PH2 O

 n  X   Rexp  Rmod  1  n  R exp 1

(4)

where n is the number of data points included. The calculated rate constant (kFTS), the adsorption parameter (b) and related MARR at various temperatures are listed in Table 4. The experimental result listed in Table 4 showed that both the FTS rate constant (k) and the adsorption parameter (b) are increases by decreasing the cobalt particle size. These results could explain the complex behavior of FTS rate as function of the catalyst particle size. In the rate equation proposed by Eric van Steen and Hans Schulz, the FTS rate constant (kFTS) is located in

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A. Nakhaei Pour et al. / Journal of Natural Gas Science and Engineering 21 (2014) 772e778

Fig. 4. The effects of temperature and catalyst particle size on FTS reaction rates.

Table 3 Elementary reactions for proposed model (van Steen and Schulz, 1999). Model

Reaction steps

Equilibrium constants

Elementary reactions

Eric van Steen and Hans Schulz

1 2 3 4 5

K1 K2 K3 K4 kFTS

H2 þ 2s#2Hs CO þ s# COs COs þ s#Cs þ Os Os þ 2Hs#H2 O þ 3s Cs þ Hs/CHs þ s

the numerator and adsorption parameter (b) are located in numerator and denominator, but adsorption parameter (b) in the denominator is a power of two. Thus this kinetic equation predicted that FTS reaction rate over the cobalt catalysts increases

by increasing of FTS rate constant (k) and decreases by increasing of the adsorption parameter (b), because the adsorption parameter (b) appearing in the denominator a power of 2. Therefore the maximum of the curve in Fig. 4 can be explained as follows, before the maximum of FTS reaction rate, the rate constant parameters (kFTS) will prevail and FTS reaction rate is increased by increasing the rate constant (kFTS) (Ma et al., 2011; Nakhaei Pour et al., 2010a,b). Nevertheless, after this point, the adsorption parameter (b) to overcome and FTS reaction rate is decreased by increasing the adsorption parameter (b). This interpretation is in fact a simple statement of Sabatier principle. Our results confirm with Sabatier principle and the complex behavior of catalyst against particle size can be justified with Sabatier principle.

11=2

0 P 3=2 P

CO H Fig. 5. Linearised plot of @rFTS2PH O A

versus

2

PCO PH2 PH2 O

for the model proposed by Eric van Steen and Hans Schulz for the 12.4 (nm) size cobalt catalyst.

11=2

0 P 3=2 P

CO H Fig. 6. Linearised plot of @rFTS2PH O A 2

versus

PCO PH2 PH2 O

for the model proposed by Eric van Steen and Hans Schulz for the 9.8 (nm) size cobalt catalyst.

A. Nakhaei Pour et al. / Journal of Natural Gas Science and Engineering 21 (2014) 772e778

777

11=2

0 P

3=2

P

CO H Fig. 7. Linearised plot of @rFTS2PH O A

versus

PCO PH2 PH 2 O

for the model proposed by Eric van Steen and Hans Schulz for the 8.6 (nm) size cobalt catalyst.

versus

PCO PH2 PH2 O

for the model proposed by Eric van Steen and Hans Schulz for the 7.9 (nm) size cobalt catalyst.

versus

PCO PH2 PH2 O

for the model proposed by Eric van Steen and Hans Schulz for the 4.8 (nm) size cobalt catalyst.

2

11=2

0 P

3=2

P

CO H Fig. 8. Linearised plot of @rFTS2PH O A 2

11=2

0 P

3=2

P

CO H Fig. 9. Linearised plot of @rFTS2PH O A 2

The activation energy of FTS reaction determined from calculated rate constant (k) at two deferent temperatures (493 and 508 K), using the Arrhenius equation as: Table 4 The calculated rate constant (k), the adsorption parameter (b) activation energies and adsorption enthalpies for cobalt catalysts at 493 and 508 K. 1 Catalyst particle T (K) K (mol g1 b (bar2) Ea (kJ) DH (kJ) MARR cat. h size (nm) bar3/2)

12.4 9.8 8.6 7.9 4.8

493 508 493 508 493 508 493 508 493 508

0.052 0.105 0.060 0.121 0.070 0.142 0.081 0.156 0.085 0.162

0.019 0.030 0.045 0.073 0.074 0.122 0.111 0.186 0.155 0.255

98

63

96

67

94

69

91

71

89

74

8.9 7.8 10.2 5.6 4.6 8.7 9.2 11.7 9.5 10.6

  Ea k ¼ k0 exp RT

(5)

The calculated activation energy for the FTS reaction using Eq. (5) is listed in Table 4. As shown in Table 4, the activation energies of catalysts increase from 89 to 98 kJ/mol by increasing the catalyst particle size (r) from 4.8 to 12.4 nm. These activation energies are in excellent agreement with those reported for equivalent values reported in previous works (Van Der Laan and Beenackers, 1999; van Steen and Schulz, 1999; Teng et al., 2006; van der Laan and Beenackers, 2000; Yang et al., 2003; Zhang et al., 2009). Adsorption enthalpy DHads is determined with adsorption parameter (b) via van't Hoff equation:

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A. Nakhaei Pour et al. / Journal of Natural Gas Science and Engineering 21 (2014) 772e778

dlnb DHads ¼ dT RT2

(6)

The calculated heats of adsorption for the FTS catalytic process are listed in Table 4. The experimental results for heats of catalyst adsorption calculated about 63e74 kJ/mol, which are in agreement with those reported for equivalent values reported in literatures (Van Der Laan and Beenackers, 1999; van Steen and Schulz, 1999; Teng et al., 2006; van der Laan and Beenackers, 2000; Yang et al., 2003; Wang et al., 2003). The adsorption parameter (b) in kinetic model proposed by Eric van Steen and Hans Schulz (van Steen and Schulz, 1999) equals to b ¼ K21K2K3K4, where K1 is equilibrium constant for dissociative adsorption of hydrogen, K2 is equilibrium constant for adsorption of carbon monoxide, K3 is equilibrium constant for surface dissociation of carbon monoxide, and K4 is equilibrium constant for water adsorption. Using Eq. (6), the heat of adsorption calculated for the b parameter can be related as:

DHads ¼ 2DHads; H2 þ




 DHads;CO þ DHdis;CO  DHads; H2 O

(7)

where, DHads; H2 is the heat of dissociative adsorption of hydrogen, DHads;CO is the heat of adsorption of carbon monoxide, DHdis;CO is the heat of surface dissociation of carbon monoxide and DHads; H2 O is the heat of adsorption of water. As shown in Table 4, the total heat of adsorption increases with decreasing the catalyst particle size. The thermodynamic analysis of ideal models by Parmon (2007) shows that dispersion of the nanoparticles of the catalyst active phase can affect the adsorption equilibrium. These data shows that in most cases, the adsorption equilibrium constant and the heat of adsorption increases with decreasing the nanosize of the catalyst active phase. Based on this information it can be concluded that by reducing the size of the catalyst, the hydrogen, carbon monoxide and water adsorption heats are increase. 4. Conclusions The series of Co/CNT catalysts with various particle sizes were prepared and used for a structured sensitivity of the FTS reaction. The catalyst activity results show that the FTS reaction rate (mol/gcat./h) increases by increasing the reaction temperature (from 493 to 508 K). In addition, the FTS reaction rates passes from a maximum (7.6 nm) by decreasing the catalyst particle size for supported Co/CNT catalyst. The higher activity of catalyst with small particle size may be related to more hcp metallic cobalt in the catalyst active phase (XRD results). The kinetic studies of prepared catalyst are performed using Eric van Steen and Hans Schulz equation. The calculated kinetic results showed that both the rate constant (kFTS) and the adsorption parameter (b) for FTS reaction are increases by decreasing the catalyst particle size. The maximum of FTS reaction against the catalyst particle size can be explained as follows, before the maximum of FTS reaction rate, the rate constant parameters (kFTS) will prevail and FTS reaction rate is increased by increasing the rate constant (kFTS). Nevertheless, after this point, the adsorption parameter (b) to overcome and FTS reaction rate is decreased by increasing the adsorption parameter (b). This interpretation is in fact a simple statement of Sabatier principle. Our results confirm with Sabatier principle and the complex behavior of catalyst against particle size can be justified with Sabatier principle. References Bezemer, G.L., Bitter, J.H., Kuipers, H.P.C.E., Oosterbeek, H., Holewijn, J.E., Xu, X., et al., 2006. Cobalt particle size effects in the FischereTropsch reaction studied with carbon nanofiber supported catalysts. J. Am. Chem. Soc. 128, 3956e3964. Borg Ø, Eri S., Blekkan, E.A., Storsæter, S., Wigum, H., Rytter, E., et al., 2007. FischereTropsch synthesis over g-alumina-supported cobalt catalysts: effect of support variables. J. Cataly. 248, 89e100.

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