CNTs catalysts: Functionalized

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Kinetics, Catalysis, and Reaction Engineering

Fischer –Tropsch synthesis over cobalt/CNTs catalysts: Functionalized support and catalyst preparation effect on activity and kinetic parameters Ali Nakhaei Pour, Mohammad Reza Housaindokht, and Seyed Mehdi Kamali Shahri Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02485 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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Fischer –Tropsch synthesis over cobalt/CNTs catalysts: Functionalized support and catalyst preparation effect on activity and kinetic parameters Ali Nakhaei Pour a*, Mohammad Reza Housaindokht a, Seyed Mehdi Kamali Shahri b a: Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, P.O. Box: 9177948974, Mashhad, Iran. b: Department of Chemical Engineering, Pennsylvania State University, State College, PA, USA

*

To whom correspondence should be addressed: [email protected]; [email protected];

Tel & Fax: (+98) 513 8795457 1


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Abstract The effect of carbon nanotubes (CNTs) functionalization as a support on cobalt catalyst activity in Fischer–Tropsch synthesis (FTS) was investigated. The CNTs functionalized with hydrogen peroxide and sonicated via a pulsing method with different pulse time. The cobalt catalysts synthesized by impregnation method. Confinement of concise Co particle inside the functionalized CNTs at a shorter time of sonication (Co/FCNTs-10 with 10-second pulses of sonication) was confirmed by TEM analysis. The Co/FCNTs-10 catalyst show higher FTS activity than non-sonicated Co/CNTs and Co/FCNTs-20 with 20-second pulses of sonication. The activation energies obtained through the kinetic results for FTS reaction over Co/FCNTs10, Co/FCNTs-20, and Co/CNTs are 92.1, 98.8, and 102.7 kJ/mol. The heat of hydrogen adsorption is calculated -40.8, -34.2 and -21.4 (kJ/mol) for Co/CNTs, Co/FCNTs-20 and Co/FCNTs-10 catalysts, respectively. Co/FCNTs-10 catalyst shows higher hydrogen spillover than non-sonicated Co/CNTs catalyst.

Keywords: Nanoparticles dispersion, Carbon nanotubes, Fischer–Tropsch synthesis, Functionalization

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1. Introduction Fischer-Tropsch synthesis (FTS) process consists of an exothermic process in which converts syngas into a broad mixture of hydrocarbons, such as paraffins, olefins, and alcohols compounds1-7. As the world population is increasing, substantial endeavors have been implemented in FTS process not only in academia but also in the industry to supply cleaner energy and filling the gap of absence in renewable energy production for future transportation. However an array of studied catalysts can be utilized, transition metals such as cobalt, iron, and ruthenium considerably attracted the catalyst community for the FTS process 8-13

. The tendency of nickel for methane formation prohibits the usage in this process

14, 15

.

Cobalt-based catalysts indicated a superior activity, despite the fact that iron-based catalysts perhaps showed more suitable applications, like as coal-derived syngas with low H2/CO ratio 16-22

. Cobalt-based catalysts previously demonstrated numerous advantages in key selecting

parameters such as superior chain growth probability, subordinate deactivation rates, suppressed water-gas shift activity, and more costs that are favorable over other candidates2326

.

A high dispersion of cobalt particles for superlative catalytic activity is essential and achievable through depositing the particles on a support that comprise the high surface area, such as alumina or silica

27-29

. Several critical factors such as size, dispersion, loading, and

reduction degree of cobalt particles directly impact the number of active sites and consequently on the FTS activity

17, 30, 31

. The catalyst performance in terms of activity and

product selectivity presented a high dependence to the support physicochemical properties. Therefore, any changes in the support surface chemistry influence not only metal-support interaction (MSI), but also metal reducibility and dispersion, which ultimately form new cobalt species

17, 32, 33

. These cobalt-support mixed compounds, which primarily formed at

relatively high temperature during the reduction process, cause loss of cobalt active phase and

3


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are formed at very high reduction temperatures. As a result, support materials similar to Carbon nanotubes (CNTs) that previously showed weak interaction with metal can circumvent the issue as an alternative approach

28, 34-36

. CNTs support originally consist of

numerous unique structural properties and therefore, attracted enormous attentions as a support media for heterogeneous catalysis

12, 28, 37

. But, the interaction between cobalt and

CNTs as a support for the catalyst reduced considerably. This reduction prevented the Co nanoparticles from proper stabilization and well-dispersion throughout the support material 38-40

. However, a change in the CNTs surface chemistry such as functionalization (FCNTs)

process may significantly alter the cobalt-CNTs interaction

41, 42

. Thus, functionalization of

the CNTs supports resulted in a narrow particle size distribution through controlling in the size of metal particles. In addition, functionalization process interfered in the hydrophobic to hydrophilic balance in CNTs structure, so that the CNTs surface converts into a more reactive surface

43, 44

. It appears that functionalization of CNTs changes the FTS reaction kinetic

parameters. Instantaneous use of strong acids and ultrasonication in pretreatment process considerably modified CNTs support in functionalization process. These chemical processes generated tremendous quantities of defects and dangling bonds on the tube wall by reacting with the surface compounds 45. Sonication procedure may result in a deleterious effect on the physicochemical properties of the functionalized CNTs. Prolonged sonication fractured the CNTs structure and stripped the external graphitic films that are changed their charge-carrier mobility

46

. Therefore, it is better to use sonication in a short time. Pulsed sonication

techniques are used for short periods in functionalization of CNTs. To achieve the best time of sonication, it must be used different pulse times in the functionalization process. Thus, the pulse time of sonication was considered as a variable on functionalization of CNTs and catalytic activity of the final catalyst.

4



In our recent previous endeavors, we studied the kinetic of the higher hydrocarbons production by iron and cobalt catalysts6,

19, 22, 26, 47-50

. In present work, CNTs support is

functionalized prior to the impregnation of cobalt precursors by pulse sonication. The pulse time of sonication was considered as a variable on functionalization of CNTs for evaluating the catalytic activity of final catalyst in the desired reaction. Then the prepared cobalt catalyst supported on functionalized CNTs (FCNTs) evaluated and compared with common Co/CNTs catalyst. In addition, the FTS activity and products selectivity of prepared functionalized CNTs support compared with common Co/CNTs catalyst. The kinetic parameters of the Co/FCNTs and Co/CNTs catalysts evaluated by using a kinetic model named Van Steen and Schulz 51.

2. Experimental 2.1. Catalyst preparation Multiwall CNTs as support were utilized to prepare the desired cobalt catalyst. The CNTs support were initially purified by HNO3 treatment process, filtered to achieve the purified cake, and then dried at 120 oC for 8 h. At this stage, the undesirable formed amorphous carbons were removed by raising the temperature to 400 oC in the presence of airflow with a 10 oC /min ramp rate. Then, the synthesized samples were maintained at this temperature for 20 min. The purity of the primitive untreated CNTs as received was approximately 95%, and diameters and lengths varied between 10-20 nm and 5–15 µm, respectively. The purified CNTs were functionalized with hydrogen peroxide (30%) through 10 min stirring in solution. Then, the slurry sonicated by means of a pulsing technique (10s-on/10s-off and 20s-on/20soff for five cycles). The obtained black slurry samples were entitled as FCNTs-10 and FCNTs-20. The process was continued by passing a stream of ozone gaseous through the

5


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sample at a rate of 300 ml/min for 30 min. The precursor filtered and washed with methanol to remove the excess H2O2. Finally, the functionalized CNTs dehydrated at 120°C for 5 h. We used the pure and functionalized CNTs as a support for preparation of catalysts to compare the effect of CNTs functionalization process. Catalysts with 15 wt% cobalt loadings of supported purified CNTs prepared using wet impregnation of cobalt nitrate (Co(NO3)2·6H2O 99.0%, Merck). Impregnated catalyst precursors filtered and arid overnight at 120 °C. Subsequently, the dehydrated catalysts ramped up to 360 °C with 2 oC/min heating rate and hold for 3.5 h at the presence of N2 flow. After that, the sample cooled down along with gradual exposing under the air flow. The catalysts marked as Co/FCNTs for catalyst impregnated on functionalized CNTs and Co/CNTs for catalyst impregnated on nontreatment CNTs.

2.2. Catalyst Characterization Formation of functional groups on the surface of CNTs was explored by FT-IR absorption technique (Bruker ISS-88). In this technique, we pelletized a mixture of 2.5% of CNTs samples and 97.5% potassium bromide (KBr), which was already prepared and transparent pellet. Then, the infrared beam was conducted to pass through the pellet. Surface area based on Brunauer–Emmett–Teller (BET) equation and pore characteristics of the finalized catalysts were inspected by a volumetric adsorption method (ASAP-2010 Micrometrics apparatus). In this physical measurement method, we primarily degassed the prepared catalysts at 200˚C and hold for 4h under a partial vacuum condition (i.e. 50 mTorr). Afterward, the adsorption isotherms were measured on the prepared samples. We expanded our research on the morphology of the synthesized catalyst via utilizing transmission electron microscopy (TEM). The instrument was Philips CM20 (100 kV). The

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sample preparation and method for the TEM measurements has already been explained elsewhere in detail 15. We evaluated the crystal size of cobalt oxide (Co3O4) nanoparticles using X-ray diffraction (XRD) measurements (Philips PW1840 X-ray diffractometer) and subsequently Scherrer equation. Then the particle size of metal is attained following the molar volumes of the cobalt in metallic state and Co3O4, as equation (1): dCo0 (nm) = 0.75×dCo3O4

(1)

The reduction pattern of the calcined catalysts (0.05 g) was evaluated by temperatureprogrammed reduction (TPR) technique. We executed the TPR on the samples at the presence of diluted hydrogen (5% H2 /Ar) flow stream in Micrometrics TPD-TPR 2900 analyzer. The heating ramp rate and the final temperature were 10˚C/min and 850˚C, respectively. The Raman spectroscopy measurements were performed on an instrument titled Bomem MB154 Raman spectrometer to study the degree of graphitization via sonication process. The details about the instrument and experimental procedure were explained elsewhere 15. The hydrogen chemisorption via Micromeritics TPD-TPR 2900 apparatus was utilized to collect the information about cobalt dispersion and average surface crystallite size. In this experiment, we had some assumptions, such as adsorbing one H2 molecule on two cobalt surface atoms and calculating the metal particle size according to the presence of cobalt metal particles in spherical and uniform conditions from metal dispersion

52

. The assumptions

concluded the following equation: dCo0 (nm) = 96/D (%)

(2)

here, D and dCo0 are the dispersion and the diameter of cobalt particle, respectively. The procedure of the experimental measurement explained elsewhere 15.

7


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2.3. Catalyst activity test We used a continuous spinning basket reactor (stainless steel, H= 122 cm, D0 = 5.2 cm, Di = 4.6 cm) to measure FTS reaction rates for kinetic evaluations. The inlet flows (i.e. H2 and CO), and temperature of the reactor were controlled by the mass flow controller (MFC, Brooks Co.), and the temperature controllers (WEST series 3800) along with back pressure valve for pressure controlling. In each experiment, the desired amount of synthesized catalyst (3 g) diluted with silica (30 cm3) containing analogous bulk particle size was loaded into the reactor. Carbon monoxide conversion as a function of mixing speeds was used to remove the external mass transfer issue. This approach guarantees that the obtained data are collected in the kinetic scope. As a result, we achieved 1800 rpm as an appropriate stirring speed to satisfy the kinetic measurement criteria. Additionally, the catalyst deactivation was checked during the FTS reaction test. The experiments for kinetic evaluations followed three steps on any individual fresh catalyst inside the reactor as, 1) Reduction: the new synthesized sample was reduced by H2 flow (3.6 Nl/gcat/h) at 400 oC and atmospheric pressure, 2) Stabilization: the actual reaction conditions such as temperature (230 oC), pressure (20 bar), H2/CO ratio (2), inlet flow rate (2.4 Nl/gcat/h), and time (12 h) were selected to prepare the new catalyst for the actual kinetic measurements, 3) Kinetic measurement: the reaction conditions such as pressure and inlet H2/CO ratio were kept constant at 20 bar and 2, respectively. However, temperature and space velocity were changed in the range 215-245 oC and 2.4-12 Nl/gcat/h during the kinetic investigation, respectively. The details about the reaction conditions are shown in Table 1. Then, we allowed at least 4 h to reach a steady-state condition for each kinetic measurement. We analyzed the products in the outlet through a series of three Gas Chromatographs (GC). Produced hydrogen was analyzed by a Shimadzu 4C GC equipped with columns of Porapak

8



Q and Molecular Sieve 5A along with a thermal conductivity detector (TCD). A Varian CP 3800 GC equipped with TCD detector and a chromosorb pack column analyzed permanent gases. The organic liquid products were analyzed through another GC (Varian CP 3800) instrument loaded with a capillary column (PetrocolTm DH100 fused silica) and a Flame Ionization Detector (FID). We also applied total carbon mass balance to account for the possible accumulated carbon inside the reactor in different formats. The results achieved from combined carbon balance and GCs were employed to compute CO conversion and product selectivity. The water collected in the cold trap was precisely weighted and then converted to water partial pressure. The water partial pressure was determined by collecting the water in the trap and determining its amount by weighing. The weight of water converted to partial pressure based on the ideal gas law. The partial pressures of carbon monoxide (CO) and hydrogen (H2) were computed based on Dalton's law using total pressure and GC results of the reactor.

3. Results and Discussions 3.1. CNTs characterization Figure1 showed FTIR spectrum of the purified CNTs and functionalized ones. The purified and functionalized CNTs showed the absorption band around 1580 cm-1, which is recognized by the C=C stretching of the graphitic structure of CNTs 53. An array of vibrational bands at 3300-3600 cm-1 (OH) were observed that are attributed to the attachment of functional groups to CNTs. Detecting a peak around 1200 cm-1 for FCNTs is attributed to the C–O stretching of COOH group. In addition, the peak at 1730 cm−1 is assigned to the C=O− stretching vibration within the COOH group

44

. Furthermore, the peaks around 1410 and 3215 cm-1 match to

bending and stretching vibration of O–H bond, respectively 54. The evidence of anchoring the carboxylic groups through acid treatment on the FCNTs surface were disclosed via these new

9


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hydroxyl groups. The C-H symmetric and anti-symmetric stretch for CH3 and CH2was observed at peaks 2850-2950 cm-1 and 597 cm-1 (Figure 1). Based on FT-IR results, we concluded that the CNTs were effectively functionalized by the H2O2 solution. The corrosive action of the H2O2 can be attributed to its strong oxidizing nature. The microwave heating process substantially increases the CNTs temperature and therefore, facilitates the reaction condition. As shown in Figure 1, no distinct changes were detected in the structure of FCNTs-10 and FCNTs-20. Table 2 indicates a summary of the textural properties of CNTs before-and-after acid pretreatment. The results showed that the properties such as BET surface area, pore volume, and average CNTs internal diameter improved significantly through acid-wash treatment on the support. Enhancement in the catalyst activity and metal dispersion are the most important results of such support treatment. The extent of metal for acid treated and untreated CNTs presented about 0.6 and 0.0 wt%, respectively (Table 2). Table 2 listed the textural properties of the fresh and functionalized CNTs. As illustrated, the surface area along with the total pore volume of the FCNTs are increased after 10s sonication (FCNTs-10) but decreased at higher sonication process (FCNTs-20).

Fig. 2 (a) demonstrations the liquid N2 adsorption-

desorption isotherms of the fresh CNTs support after acid treatment. An initial rapid increase by the activated CNTs in the nitrogen adsorbed that observed below of P/P0 = 0.05. The adsorption-desorption patterns of the functionalized CNTs (FCNTs-10 and FCNTs-20) are similar to acid-treated CNTs and there are a few variations. As shown in Table 2, higher sonication of CNTs in FCNTs-20 sample decreased the pore volume of support due to high production of the defects in the CNTs structure. However, this indicates that the fundamental change in the structure of CNTs does not occur after functionalization. BJH method was employed to inspect the pore size distribution (PSD) of the purified CNTs as shown in Fig. 2 (b). The PSD of the purified CNTs demonstrated one main peak in the

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mesopores region at 2–10, and 15-25 nm that is related the pores inside the tubes and the space between the tubes, respectively. Consequently, the acid-washed CNTs were primarily composed of mesopores. Raman spectrum of CNTs, FCNTs-10 and FCNTs-20 supports are shown in Figure 3. As shown in this Figure, the peaks at 1345 and1595 cm−1 are allocated to the disordered graphite (D-line), and the sp2-hybridized carbon atom of graphite (G-line), respectively. The intensity ratio of the D- and G-lines is practically utilized as an indicator of the structural integrity in the sp2-hybridized carbon atoms 55. According to the Raman spectrum shown in Figure 3, the intensities of the D- and G-line ratio attained 0.79, 0.84, and 0.91 for CNTs, FCNTs-10, and FCNTs-20, respectively. As a result, the FCNTs-20 displayed the lowest degree of graphitization and is consequently attributed to higher defects produced during sonication.

3.2. Catalyst characterization Textural property results for the fresh calcined catalysts are shown in Table 2. However, the measured BET surface areas of prepared final samples are lower than CNTs and FCNTs supports. This outcome could be due to a few pore blockages caused by cobalt oxide clusters. As shown in Table 2, the quantities of BET surface area, total pore volume, and average pore size for Co/FCNTs catalysts are essentially larger than the Co/CNTs catalyst due to the lower prepared cobalt particle in the functionalized catalyst and lower pore obstruction of support during metal impregnation. Functionalization of CNTs with treatment by H2O2, increased the level of mesopores and defects in carbon nanotubes structure

56

. In addition, produced

functional groups reinforced the cobalt species for a better interaction with CNTs support on both the surface and the interior of the mesopores. Cobalt particles prefer to interact with these functionalized centers on the surface and into the pores of CNTs support 56. Pore filling

11


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of the CNTs support is affected by interactions between cobalt particles and support, and preparation of defects during of functionalization by hydrogen peroxide. In this study, we employed the sonication treatment to disperse the CNTs into a liquid medium before functionalization process

45

. However, the outer graphitic layers of CNTs

support may strip away at this stage and destructively influence the desirable properties of the functionalized CNTs. As shown in Table 2, higher sonication of CNTs in FCNTs-20 sample decreased the pore volume of support due to high production of the defects in the CNTs structure. XRD pattern of the catalysts at the final phase are shown in Figure 4. In these XRD patterns, the 25o and 43o peaks match with the CNTs structure, whereas the further peaks correspond to various crystal planes of Co3O4

57, 58

. The peaks at 2θ value of 36.8, 44 and 65o for (311),

(400) and (440) planes of Co3O4 correlate with a face-centered-cubic (fcc) cubic cobalt structure16, 33. XRD results show that the structure of prepared cobalt oxides on the CNTs and FCNTs are the same and functionalizing the support do not change the cobalt crystal shape. The most intense peak at 2θ value appeared at 36.8o (311) in the XRD pattern and was selected for computing the crystal size of cobalt considering the Scherrer equation. The average cobalt crystal size that is calculated using Eq. (1) for Co/FCNTs-10, Co/FCNTs-20 and Co/CNTs is 8.9, 9.8, and 11.0 nm, respectively (Table 2). Differences in cobalt dimensions may be related to the mesopores and the defects caused an increase in the contacts between the cobalt nanoparticles and the support. As a result, the agglomeration of cobalt particles was primarily prevented and the diffusion of cobalt nitrate solution into CNTs mesopores was increased. Thus, the Co/CNTs have the biggest crystal size. Conversely, further H2O2 treatment precluded the beneficial effects of mesopores and defects. Therefore, the Co crystal size on FCNTs-20 is larger than that of FCNTs-10.

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The TEM images of the prepared Co/CNTs and Co/FCNTs-10 catalysts are shown in Fig.5. This Fig. demonstrates the ubiquitous presence of the cobalt nanoparticles on the CNT support. The cobalt nanoparticles inside the CNTs channels papered due to diffusion of solvent into channels and capillary forces confined the cobalt particles inside the CNTs channels

36, 40

. The functional groups on the CNTs surface increased the interaction between

the cobalt species with particularly the interior surface of CNTs and increased the capillary force for injection of cobalt nanoparticles into the CNTs channels

59, 60

. The cobalt particles

of Co/FCNTs-10 that are located inside the tubes, as shown in Fig. 5, are higher than Co/CNTs catalyst. The TEM images demonstrated a small inner tube diameter (10 nm). Therefore, the insertion of metal particles is controlled by sizes close to the channels diameter and all larger particle sizes precipitate on the outer surface of the CNTs walls. Furthermore, Fig. 5 illustrates the size distribution of cobalt particles. The functionalization of CNTs support can systematically assist to manipulate the cobalt particle size distribution, due to the enhanced interaction of functional groups with cobalt precursor. The hydrogen chemisorption on the reduced cobalt catalysts was employed by H2-TPD to estimate the cobalt particle size. As indicated in Table 2, the metal particle size varied from 9.4 to 11.3 nm. This data showed larger values than the XRD and TEM measurements, which may depend on the sintering of cobalt particles during of process. The TPR pattern of the prepared catalysts is presented in Fig. 6. The first peak located at low temperature is assigned to Co3O4 to CoO reduction and the second peak obtained at high temperature is attributed to the reduction of prepared CoO to Coo species. The peak at approximately 650 oC is related to the CNTs gasification. As shown in Fig.5, cobalt catalysts supported on functionalized CNTs reduced at lower temperatures compared to the common Co/CNTs catalysts. Thus, functionalization of CNTs support will increase the reduction capability of the cobalt particles located on the surface and into the tubes due to the

13


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confinement and hydrogen spill-over the functional groups35-37. As shown in Fig.5, the cobalt nanoparticles located into the tubes for the Co/FCNTs catalysts is higher than Co/CNTs catalyst. As discussed in the literature, the external CNTs surfaces consist of more electron than internal surfaces 37, 43. Therefore, the cobalt oxide particles locating inside the CNTs are reduced easier than the particles locating outside the CNTs tube. The difference in electron population on the interior and exterior CNTs surfaces promotes the dissimilar reducibility and therefore facilitates the Co/FCNTs reduction via weakening the Co-O bond strength. High production of the defects in the functionalized CNTs structure due to higher sonications in Co/FCNTs-20, changed the electron density in CNTs support. Thus, the reduction peaks in TPR pattern of Co/FCNTs-20 catalyst shift to a higher temperature.

3.3. Activity and product selectivity The comparative results of catalyst activities (as CO conversion %), products selectivity, FTS rate (gHC produced/gcat/h) and chain growth probabilities for prepared catalysts are presented in Table 3. According to this Table, FTS rate, C1-C4 selectivity of products and CO conversion increased on cobalt supported on FCNTs support, however, the chain growth probabilities and C5+ selectivity decreased. TEM results show that the Co/FCNTs-10 is characterized by a narrow and uniform particle size distribution in comparison with Co/FCNTs-20 and Co/CNT catalysts. In addition, cobalt particles located mostly on the exterior tube surface of Co/CNTs catalyst. Because of uniformity and being trapped inside the tubes in the functionalized CNTs support, the cobalt sites are more active on the FCNTs than the untreated CNTs 24, 29, 30, 34. Hence, enhancement in the particle uniformity in Co/FCNTs-10 leads to increasing the CO conversion and FTS activity of the Co/FCNTs-20 and Co/FCNTs catalysts 61.

14



FTS products distribution are given in Fig.7 of catalysts after 25-30 h time-on-stream (Reaction condition: T = 225 oC, P = 20 bar, H2/CO = 2.0, and GHSV = 2.4 Nl/gcat/h). The average carbon number of products increased from Co/CNTs to Co/FCNTs-10 as shown in Figure 7. Characterizing of the chain growth probabilities of such distributions are given in Table 3. The chain growth probabilities of Co/FCNTs-10, Co/FCNTs-20 and Co/CNTs catalysts are 0.81, 0.83 and 0.86, respectively. It seems that the steric hindrance limited the chain growth probability in Co/FCNTs-10 and Co/FCNTs-20 catalysts, because of the cobalt active sites being trapped inside the tubes 43, 44. Thus, the selectivity of heavy hydrocarbons is decreased in Co/FCNTs catalysts. Accordingly, the Co/CNTs catalyst shows higher selectivity toward heavier hydrocarbon molecules (i.e. C10+) due to the existence of larger cobalt particle sizes (10+ nm) than the samples prepared on functionalized CNTs (Co/FCNTs10 catalyst). The C1 species prepared on the cobalt FTS catalyst can form several degrees of hydrogenation, such as CO, HCO, HCOH, CH, and CH2

10, 62

. The monomers with high and

low degrees of hydrogenation promote the production of light and heavy hydrocarbon, respectively

10, 62

. Our previous studies show that the concentration of hydrogen on the

catalyst surface decreased as the cobalt particle size increased

16

. The concentration of C1

monomers with a higher degree of hydrogenation increases with increasing hydrogen concentration on the catalyst surface and therefore, the selectivity to lower hydrocarbons enhanced in lower cobalt particle size. In addition, increasing the defects in the CNTs support structure of FCNTs-20 in comparison of FCNTs-10 will affect the absorption of CO and H2 on the surface and consequently have a negative effect on the activity of the Co/FCNTs-20.

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3.4. Kinetic evaluation Kinetic evaluation of functionalized CNTs support catalyst done by using of Van Steen and Schulz kinetic model 51. Van Steen and Schulz established a useful Langmuir-HinshelwoodHougen-Watson (LHHW) kinetic model specifically for FTS reaction based on cobalt catalyst by rate consideration of formation of the monomer and polymerization steps as below: rFTS

PH32/ 2 PCO k FTS b ( ) = PH 2O PH 2 PCO 2 [ 1 + c( )] PH 2O

(3)

where, b = K13K2K3K4 and c = K12K2K3K4. In which K1, K2, K3, and K4 are thermodynamic constants for the assumed elementary reactions (Table 4). In addition, K1, K2, K3, and K4 represent equilibrium constant for dissociative adsorption of hydrogen, adsorption of carbon monoxide, surface dissociation of carbon monoxide, and water adsorption. The parameters of the proposed kinetic model were computed by the Levenberg–Marquardt (LM) algorithm 63. The advantage of fit was using the statistical test as well as mean absolute relative residual (MARR). The MARR between experimental and calculated consumption rate of CO is defined as56, 63: N exp

MARR = ∑ i =1

rexp − rcal rexp

×

1 × 100 N exp

(4)

Where rexp, rcal, and Nexp are experimental reaction rate, model reaction rate, and the number of data points included, respectively. The distinction between competing models was performed through comparing the MARR and statistical F-test on the residual sum of squares at the 95% confidence level, together with the t-test are used as a criterion of parameters in the discriminations. The calculated results of kFTS, b, c and MARR for Co/CNTs, Co/FCNTs-10 and Co/FCNTs20 catalysts are listed in Table 5. As shown in this Table, the kinetic model fit the data well 16



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and the calculated MARR fall within the narrow range of 7.4 and 8.5 (%) of catalysts. The calculated results by the developed model for each catalyst are shown in Tables S1 to S3 in Supporting Information for Publication. Fig.8 represents the comparison between the experimental and predicted FTS reaction rates of the model. As shown in Fig.8, the calculated reaction rates show a very good fit with experimental results. Therefore, it can be concluded that the Van Steen and Schulz kinetic model well predicts the behavior of the reaction over cobalt supported on CNTs under our experimental conditions. Therefore, the obtained kinetic parameters show the behavior of the reaction as well. The activation energies of the FTS reaction rate constant in Table 5 are calculated based on the Arrhenius-type equations:

k FTS = k FTS 0 exp(

− E a ,i RT

)

(5)

where, Ea is overall activation energy for FTS reaction rate over each catalyst. As shown in Table 5, the calculated values for FTS reaction over prepared catalysts fitted within the narrow range of 92–102 kJ/mol, which are match with the reports in the literature64-66. The legitimacy of the experimental approaches and numerical parameter assessment were confirmed via this method. The apparent activation energies were obtained as a large value. This suggests that the interparticle mass transport limitations are important. In addition, the kinetic experiments are in the kinetically limited regime 65, 67. As demonstrated in Table 5, the activation energies for overall FTS reaction rate (Ea) over Co/FCNTs-10, Co/FCNTs-20 and Co/CNTs are 92.1, 98.8, and 102.7 kJ/mol, which is consistent with the results of the catalysts activities (Table 3). The activation of FTS reaction decreases as the catalyst activity increased in this reaction. As reported in previous sections, the cobalt nanoparticles over the FCNTs supports are uniform and high reducibility. As shown in Fig.6, the reducibility of Co/FCNTs-10 is higher than Co/FCNTs-20 and Co/CNTs catalysts. Thus, we can expect that the Co/FCNTs-10 catalyst show higher FTS reaction rate. 17


Page 19 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Adsorption enthalpy of carbon monoxide and hydrogen (∆Hads), is calculated through equilibrium constant and using the Van’t Hoff equation: b = b0 exp (

− ∆H b ) RT

(6)

c = c 0 exp (

− ∆H c ) RT

(7)

where, b = K13K2K3K4 and c = K12K2K3K4. Using Eq. (6) and (7), the heat of adsorption obtained for b parameter can be linked as:

∆Η b = 3∆Η ads , H 2 + (∆Η ads ,CO + ∆Η dis ,CO ) - ∆Η ads , H 2O

(8)

∆Η c = 2∆Η ads , H 2 + (∆Η ads ,CO + ∆Η dis ,CO ) - ∆Η ads , H 2O

(9)

Where, ∆Η ads , H 2 , ∆Η ads ,CO , ∆Η dis,CO , and ∆Η ads , H 2O are the heat of dissociative adsorption for hydrogen, the heat of adsorption for carbon monoxide, the heat of surface dissociation of carbon monoxide, and heat of adsorption for water. ∆Η ads , H 2 can be calculated from Eq. (8) and (9) as ∆Hads,H2=∆Hb - ∆Hc. The heat of hydrogen adsorption is calculated -40.8, -34.2 and -21.4 (kJ/mol) for Co/CNTs Co/FCNTs-20 and Co/FCNTs-10 catalysts, respectively. The heat of adsorption of H2 on a fresh cobalt film is calculated about -100 kJ/mol

68

. We know that the heat of adsorption

decreases as the occupancy levels and hydrogen spillover increases. In the spillover process, H2 molecule dissociates into two hydrogen atoms on the metal part of the catalyst, then the atoms supposedly diffuse on the surface to the support, while some atoms persist attaching to the metal. Therefore, the support of catalyst acts as a reservoir for hydrogen atom69, 70. Lower heat of adsorption of hydrogen on Co/FCNTs-10 and Co/FCNTs-20 catalysts can be attributed to the functional group and metal particle size on CNT surface. These functional groups enhanced the effect of H2 spillover and assisted our understanding of higher dispersion of metal on the catalyst support simultaneously. In addition, the higher number of

18



cobalt particles trapped in the CNTs tubes on FCNTS-10 support increased the hydrogen spillover in Co/CNTs-10 catalyst. We eventually concluded that the electron density is principally responsible as increased from the interior (i.e. electron poor) to exterior (i.e. electron rich) CNTs surface.

4. Conclusions The results demonstrate that the CNTs support was effectively functionalized by the H2O2 solution. The adsorption-desorption patterns of the functionalized CNTs (FCNTs-10 and FCNTs-20) are similar to acid-treated CNTs and there are a few variations. Higher sonication time of CNTs in FCNTs-20 sample decreased the pore volume of support due to high production of the defects in the CNTs structure. FCNTs-10 support with lower sonication time shows low defects, which affects the catalyst FTS activity. The average crystal size of cobalt particles for Co/FCNTs-10, Co/FCNTs-20, and Co/CNTs is 9.8, 8.9, and 11.0 nm, respectively. From TEM results, small Co nanoparticles synthesized by functionalized CNTs resulted in a remarkable narrow particle size distribution and were typically confined inside the functionalized CNTs for a shorter time of sonication (Co/FCNTs-10 with 10-second pulses of sonication). The proposed FCNTs-10 supported catalyst increased the FTS rate and C5+ selectivity, compared to that prepared on common CNTs and higher time of sonication (Co/FCNTs-20 with 20-second pulses of sonication). Kinetic results showed that the kinetic model that developed by van Steen and Schulz fit the data well that the values of activation energies for FTS reaction over Co/FCNTs-10, Co/FCNTs-20, and Co/CNTs is 92.1, 98.8 and 102.7 kJ/mol. The heat of hydrogen adsorption is calculated -40.8, -34.2 and -21.4 (kJ/mol) for Co/CNTs Co/FCNTs-20 and Co/FCNTs-10 catalysts, respectively. Functionalized CNTs with a lower time of sonication maintained the effect of higher hydrogen spillover that event will unlikely occur normally in catalyst prepared on conventional CNTs. Finally, the electron

19


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density, which increases from interior to exterior CNTs surface, is mainly responsible for changes in kinetic parameters.

Acknowledgments Financial support of the Ferdowsi University of Mashhad, Iran (2/45199-2/8/96) is gratefully acknowledged. We are also indebted to Dr. Ahmad Tavassoli, Tehran University, for assistance in the preparation and interpretation of TEM images and useful comment about the results.

Supporting Information for Publication Results for syngas conversion, partial pressures and calculated FTS experimental rates results are presented in Tables S1, S2 and S3 in supporting information for publication.

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26



Table 1. The FTS conditions for kinetic evaluation. Table 2. Chemical composition and textural properties of the fresh and functionalized CNTs and calcined catalysts.

Table 3. CO conversion, chain growth probability, and hydrocarbons selectivity calculated after 25-30 h time on stream (T = 225 oC, P = 20 bar, H2/CO = 2.0, and GHSV = 2.4 Nl/gcat/h)).

Table 4. Elementary reactions for van Steen and Schulz kinetic model [51]. Table 5. The calculated rate constant (kFTS), thermodynamic parameters (b) and (c), activation energies and adsorption enthalpies for cobalt catalysts.

Figure 1. The FTIR spectrum of the purified carbon nanotubes (CNTs) and functionalized ones (FCNTs). The purified and functionalized carbon nanotubes showed the same absorption band around 1580 cm-1 which can be ascribed by the C=C stretching of the graphitic structure of carbon nanotubes

Figure 2. The liquid N2 adsorption-desorption isotherm of the activated CNTs (a); the pore size distribution (PSD) of the purified CNTs from the adsorption branch of the isotherm (b). BJH method (for 2–50 nm regions) was used to obtain PSD of the purified CNTs from the adsorption branch of the isotherm;

Figure 3. Raman spectroscopy of prepared CNTs supports. Figure 4. XRD patterns of calcined catalysts. In the XRD pattern, the peaks at 2θ values of 25o and 43o correspond to the CNTs, while the other peaks in the XRD pattern of the catalyst are related to different crystal planes of Co3O4.

Figure 5. TEM images and particles size distributions for calcined catalysts. TEM image demonstrate the presence of the cobalt nanoparticles inside and outside of the CNTs channels. The cobalt nanoparticles inside the CNTs channels papered due to diffusion of solvent into channels and capillary forces led to confinement of cobalt particles inside the CNTs channels

Figure 6. TPR patterns of calcined catalysts. Figure 7. FTS products distribution of catalysts after 25-30 h time on stream (Reaction condition: T = 225 oC, P = 20 bar, H2/CO = 2.0, and GHSV = 2.4 Nl/gcat/h)) (●Co/CNTs, ▲Co/FCNTs-20, ■ Co/FCNTs-20).

Figure 8. Comparison between the experimental and predicted FTS reaction rates of model.

27


Page 28 of 41

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Table 1. The FTS conditions for kinetic evaluation. Run

PTotal (bar)

T(oC )

H2/CO

GHSV (NL/gcat/h)

1

20

215

2

2.4

2

20

215

2

4.8

3

20

215

2

7.2

4

20

215

2

9.6

5

20

215

2

12.0

6

20

225

2

2.4

7

20

225

2

4.8

8

20

225

2

7.2

9

20

225

2

9.6

10

20

225

2

12.0

11

20

235

2

2.4

12

20

235

2

4.8

13

20

235

2

7.2

14

20

235

2

9.6

15

20

235

2

12.0

16

20

245

2

2.4

17

20

245

2

4.8

18

20

245

2

7.2

19

20

245

2

9.6

20

20

245

2

12.0

28



Page 30 of 41

Table 2. Chemical composition and textural properties of the fresh and functionalized CNTs and calcined catalysts.

Catalyst

Support

BET surface area (m2/g)

Fresh CNTs

--------

209

0.53

94

30% HNO3 Treated CNTs

-------

252

0.62

FCNTs-10

-------

267

FCNTs-20

-------

Co/CNTs

dCo0 (nm)

Total pore volume (ml/g)

Average pore diameter (´˚A)

% metals in CNTs

TEM average

XRD results

H2 TPD

0.6%

-------

-------

-------

108

0.0%

------

-------

-------

0.65

112

0.0%

------

------

-------

258

0.63

114

0.0%

------

------

-------

CNTs

102

0.35

138

------

10.9

11.0

11.3

Co/FCNTs-10

Functional CNTs-10

141

0.42

149

------

9.1

8.9

9.4

Co/FCNTs-20

Functional CNTs-20

132

0.40

129

------

9.6

9.8

10.1

29


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Table 3. CO conversion, chain growth probability, and hydrocarbons selectivity % calculated after 25-30 h time on stream (T = 225 oC, P = 20 bar, H2/CO = 2.0, and GHSV = 2.4 Nl/gcat/h)). Catalyst

CO Conversion (%)

Selectivity%

FTS rate

α

(gHC/gcat/h)

CH4

C2-C4

C5-C12

C13-C19

C20+

Co/FCNTs-10

53.8

0.81

0.2690

15.4

32.5

36.1

10.2

5.8

Co/FCNTs-20

52.3

0.83

0.2615

13.8

30.5

37.3

11.6

6.7

Co/CNTs

48.3

0.86

0.2415

11.5

26.8

38.1

14.2

9.4

30



Page 32 of 41

Table 4. Elementary reactions for van Steen and Schulz kinetic model. Reaction step

Equilibrium Constants

Elementary reaction

1

K1

H2 + 2s

2Hs

2

K2

CO + s

COs

3

K3

COs + s

Cs + Os

4

K4

Os + 2Hs

5

kFTS

Cs + Hs → CHs + s

31


H2O + 3s

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Table 5. The calculated rate constant (kFTS), thermodynamic parameters (b) and (c), activation energies and adsorption enthalpies for cobalt catalysts. Catalysts

kFTS

T (oC)

(mol.bar-1/2/gcat/.h)

215

225

b (bar-3)

c (bar-2)

0.042

0.008

0.010

0.069

0.017

0.024

Co/CNTs 235

0.113

0.034

0.058

245

0.184

0.061

0.124

215

0.064

0.010

0.016

225

0.100

0.019

0.034

Ea (kJ/mol)

∆Hb (kJ/mol)

∆Hc (kJ/mol)

∆HH2 (kJ/mol)

MARR (%)

F-Test

102.7

139.4

180.2

-40.8

7.4

91.2

92.1

165.5

186.9

-21.4

8.5

62.4

98.8

149.9

184.1

-34.2

7.6

69.6

Co/FCNTs10

235

0.155

0.045

0.089

245

0.240

0.11

0.22

215

0.057

0.01

0.014

225

0.092

0.017

0.029

235

0.147

0.039

0.077

245

0.236

0.082

0.182

Co/FCNTs20

t0.05(19)=1.729, and the T-value is the range 56 to 125 for kinetic parameters.

32



Figure 1. FT-IR Spectra of purified CNTs and Functionalized CNTs supports.

33


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Figure 2. The liquid N2 adsorption-desorption isotherm (a); and the pore size distribution

(PSD) of the purified CNTs from the adsorption branch of the isotherm (b). .

34



Figure 3. Raman spectroscopy of prepared CNTs supports.

35


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Figure 4. XRD patterns of calcined catalysts (a: Co/FCNT-10, b: Co/FCNT-20, c: Co/CNT)

36



Figure 5. TEM images and particles size distributions for calcined catalysts a: Co-CNT, b: Co-FCNT-10 and c: Co-FCNT-20.

37


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Figure 6. TPR patterns of calcined catalysts

38



Figure 7. FTS products distribution of catalysts after 25-30 h time on stream (Reaction condition: T = 225 oC, P = 20 bar, H2/CO = 2.0, and GHSV = 2.4 Nl/gcat/h)) (●Co/CNTs, ▲Co/FCNTs-20, ■ Co/FCNTs-20).

39


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Figure 8. Comparison between the experimental and predicted FTS reaction rates of model.

40


CNTs catalysts: Functionalized

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