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Kinetic study and modeling of homogeneous thermocatalytic decomposition of methane over Ni-Cu-Zn/Al2O3 catalyst for production of hydrogen and bamboo-shaped carbon nanotubes Sushil Kumar Saraswat, Bipul Sinha, Kamal K. Pant, and Ram B. Gupta Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03145 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 31, 2016

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Industrial & Engineering Chemistry Research

Kinetic study and modeling of homogeneous thermocatalytic decomposition of methane over NiCu-Zn/Al2O3 catalyst for production of hydrogen and bamboo-shaped carbon nanotubes Sushil Kumar Saraswat1*, Bipul Sinha2, K.K. Pant3, Ram B. Gupta4 1

Department of Chemical Engineering, MBM Engineering College, Jai Narain Vyas University, Jodhpur -342011, India

2,3 4

Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi – 110016, India

Department of Chemical & Life Science Engineering, Virginia Commonwealth University, Richmond,VA-23284, USA

*Corresponding Author, Email: [email protected], Tel: +91-7073467800

Abstract Kinetic study of methane decomposition reaction to hydrogen and carbon nanotubes over 60%Ni5%Cu-5%Zn/Al2O3 catalyst has been carried out in the temperature range of 873-1073 K at atmospheric pressure. A fixed-bed quartz reactor was employed to investigate the kinetics and evaluate the model parameters. The rate of methane decomposition was strongly dependent on temperature, flow rate and partial pressure of methane. The proposed kinetic model was based on molecular adsorption pathway and well explained the catalytic behavior in the range of operating conditions. The estimated value of activation energy for the overall reaction was 73.2 ±5.8 kJ/mol. The relation between catalytic activity and surface properties of calcined and spent catalysts was analyzed on the basis of the results of XRD, TGA, SEM and TEM analysis. TEM and HRTEM analysis confirmed the formation of bamboo shaped carbon nanotubes and the walls of these nanotubes consisted of oblique graphene planes with respect to the tube axis. The interlayer spacing between two graphitic layers was found to be 0.34 nm.

Keywords: Hydrogen; Methane decomposition; Bamboo-shaped carbon nanotubes; Ni/Al2O3 catalysts

1. Introduction Thermocatalytic decomposition of methane (TDM) is a greener single step process to produce COxfree hydrogen and carbon nanofiber. Extensive study has been conducted for the development of safe, economical and efficient hydrogen production to meet the global energy demand. Among the hydrogen production methods, steam methane reforming (SMR) is the most attractive and widely used

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method for hydrogen production due to high efficiency, low operating cost and low heating value process.1 However, this route is a source of significant CO2 emissions into the atmosphere (13.7 kg CO2/kg of hydrogen produce).2 The input energy requirement per mole of hydrogen for TDM is significantly less than that of SMR (37.8 and 63.3 kJ/mol H2, respectively). In addition to pure hydrogen, TDM produces high-quality carbon nanotubes (CNTs) which have potential in a variety of applications such as energy storage (supercapacitor) devices, polymer additives, catalyst support or direct catalyst, nanometer semiconductor transistors including drug delivery and nanoelectromechanical systems in biomedical fields.3-5 Several catalysts have been used for the catalytic decomposition of methane with an attempt to produce a high yield of hydrogen and CNTs. The mechanism and kinetics of methane decomposition on metal catalysts have been studied to understand the complex nature and involvement of many phases and steps in the reaction.6-8 Snoeck et al.9 illustrated a kinetic model for the formation of carbon nanofiber over a nickel catalyst by methane decomposition at different temperature and at a different partial pressure of methane. The mechanism suggested that the methane adsorbs either molecularly or dissociatively and the abstraction of the hydrogen is the rate limiting step. Demicheli et al.10 demonstrated the kinetic modeling of carbon formation from a mixture of CH4 - H2 on Ni/Al2O3-CaO catalyst and established that the adsorption of methane molecule on nickel atom is the slowest and the rate determining step. Alstrup et al.11,12 proposed kinetic models based on dissociative chemisorption of methane at atmospheric pressure. This mechanism was based on the stepwise dehydrogenation of the surface species subsequent to a direct dissociative adsorption of methane. The results at different temperatures depicted that the dissociative adsorption is more likely to be the rate-determining step. Hoogenraad

13

reported a model for

nucleation and growth of carbon nanofibers. This model concluded that carbon atoms dissolve into nickel and meta-stable nickel carbide is formed which further decomposes into metallic nickel and graphite form. This decomposition is probably due to the increase in the pressure between the graphite layers at the internal surface and the metal particles showing liquid-like behavior. Therefore, nanofibers squeezed out and a fresh surface is exposed to methane and then the steady state growth continues. Bamboo shaped carbon nanotubes (BCNTs) are synthesized by different methods such as high vapor pressure of the catalyst, surface diffusion with particles co-operates and movement of catalyst particles under the action of shell stress.14 An analysis of the current literature revealed that there are many variations in the growth mechanism of the BCNTs formation.

15,16

These suggest that

the formation of bamboo shape might be due to the shape of the catalyst particle, the bulk diffusion of


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carbon in the catalyst, or due to the slow movement of the catalyst compared to the growth rate of the CNT.17, 18 It was observed from literature that methane decomposition reaction showed wide ranges of activation energy (Ea) for the different catalysts. Holmen

19

reported the Ea for methane decomposition, using a

tubular reactor without a catalyst, of 370 kJ/mol over the temperature range of 1773-2273 K at 0.1 atm pressure of methane. Steinberg

20

reported Ea of 131 kJ/mol for methane decomposition over the

temperature range 973-1173 K at 28-56 atm. pressure, which was found substantially lower than the value for methane decomposition without a catalyst. Kuvshino et al.21 observed an Ea value 97 kJ/mol for methane decomposition with a mixture of hydrogen over nickel catalysts over the temperature range 803-863 K. The present study investigates the development of rate expressions of Langmuir-Hinshelwood type followed by parameter estimation of the kinetic models based on the reaction mechanism proposed. The Ea for hydrogen and bamboo-shaped carbon nanotubes (BCNTs) production over selected catalyst was estimated in the temperature range of 873-1073 K. The kinetic experiments were carried out over selected 60%Ni-5%Cu-5%Zn/Al2O3 catalyst. 2. Experimental Section 2.1 Experimental procedure The 60%Ni-5%Cu-5%Zn/Al2O3 catalyst was prepared by wet impregnation method. Prerequisite amounts of the metal nitrates (Ni:Cu:Zn, 60:5:5) were dissolved in distilled- deionized water with the required wt% of alumina. The final slurry was then dried overnight in an oven and followed by calcination at 823 K. Methane decomposition reaction was carried out in a fixed-bed quartz reactor (i.d. = 1.9 cm, and length= 60 cm). The schematic diagram of the experimental setup is shown in Fig. 1. The details of the experimental conditions have been discussed elsewhere and only briefly will be ˷

discussed here.22 A weigh amount of catalyst ( 1.0 mm size) was loaded in the reactor filled with similar size of ceramic beads in the test space of the reactor to minimize the dead volume of the reactor. A single zone vertical furnace with a temperature controller was used to maintain the reaction temperature. The inlet flow rates of methane, nitrogen and hydrogen were regulated using mass flow controllers (Bronkhorst, Netherlands). Preliminary experiments were carried out with different particle sizes and gas flow rates to eliminate the effect of external mass transfer and internal diffusion resistance. Experiments were carried out at various flow rates of CH4 and N2 to explore the



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relationship between methane conversion and reaction temperature varied in the range of 873-1073 K. Total inlet gas flow rate was kept in the range of 60-250 mL/min. Accordingly, gas hourly space velocity (GHSV) was varied from 3.6-15 L/h.gcat. The outlet gas flow was monitored by a gas flow meter. The reaction products were analyzed by an on-line gas chromatography (Nucone-5675), equipped with TCD to monitor product gas concentrations.

Fig. 1 Schematic diagram of the experimental set-up

2.2 Catalyst characterization The characteristics of the fresh calcined catalyst and CNTs were observed using a XRD, Philips X'Pert in the 2θ range of 20 to 97o, scanning electron microscope (SEM), ZEISS EVO-50 and transmission electron microscope (TEM), Philips CM12 operated at 100 KeV. The structural analysis of CNT was carried out using high-resolution transmission electron microscopy (TEM) on a JEOL-JEM 2100 (HRTEM) instrument. The thermogravimetric analysis was performed using a SDT Q600 instrument (TA Instruments, USA) to determine the stability and the amount of the carbon produced by the catalysts.

3. Results and discussions 3.1 Characterization of the calcined and spent catalyst XRD patterns of calcined and spent catalysts (Fig. 2a,b) were recorded to determine the crystallite structure and phases of the Ni-Cu-Zn/Al2O3 catalysts. The calcined catalyst showed the presence of reflections at 2θ = 37.4, 43.4, 63.0, 75.2, 79.4, 95o pertaining to the NiO phase (Fig. 2a). The addition of copper and zinc assigned two additional weak peaks at 2θ= 35.6 and 32.0o, attributed to the CuO and ZnO phases, respectively.23,24 Fig. 2b depicts the graphite with hexagonal structure in spent catalyst at 2θ = 26.28 and 77.0o. The reflections at 2θ = 44.4, 52.4, and 76.4o indicated the presence of metallic nickel phase. Copper and zinc metals showed less intense peaks in the spent catalyst at 2θ= 43.2 and 50.6o respectively, indicating the presence of pure Cu and Zn metal in spent catalyst. TGA measurement was carried out to get the quantitative information of the carbon distribution and the thermal stability of CNTs synthesized at 1023 K. Fig. 3 depicts a plot for % weight loss vs. oxidation temperature, measured by heating the catalyst in the air ambiance. Higher oxidation temperature (>773 K) depicts pure, less defective and higher degree of crystalline CNTs.25 The weight loss in the catalyst is due to the combustion of carbon in the air and the maximum carbon content on the catalyst was ACS Paragon4 Plus Environment

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83.5%. Fig. 4a presents SEM micrographs of calcined catalyst (at 823 K) whereas, Fig. 4b reveals the morphology of spent catalyst in which catalyst surface is completely covered with filamentous carbon. The carbon consisted of highly entangled, cross-linked and web-like CNTs with a length of several micrometers. The low-resolution TEM of the spent catalyst at 1023 K shows BCNTs as long tubular products with varying diameters (Fig. 5A). The CNTs consist of hollow compartments spaced by curved carbon domes fabricate from the carbon sheets joining the tubes inner walls. The nickel nanoparticles are present at the end of the bamboo compartments, indicating the tip-based growth mechanism.26 The Ni particle had a larger particle size than that of the pristine Ni particle, showing the coalescence and reshaping of Ni particles during the TDM process. The interface between the Ni particle and CNTs revealed that the graphene layers were well curved to match the lattice of Ni (1 1 1) planes, resulting in a quasi-coherent interface as indicated by the local darkness of the Ni lattice. The HRTEM image of the graphitic shells was well resolved and depicted in Fig. 5B. The inside of BCNT is hollowed and free of catalyst particles. It was observed that the BCNTs with internal domes are exhibit well-defined conical shape, which showed diameter of approximately 26 nm measured in the widest distant. Furthermore, we can also note that the produced bamboo structure CNTs is around 23 nm thick with an outer diameter of 63 nm. The particular dome shape of the compartments develop when the catalytic nickel particle is deformed or in a quasi-liquid state with the growth of BCNTs and the accumulated stress in the cap sheath reaches its critical value.27 Bamboo shaped nanotube compartments with an empty conical hole were possibly formed when surface tension of the catalyst Ni particle increases to such an extent that the capillary action by the nanotubes cannot maintain strong interactions with the Ni particle. The part of the Ni particle being pulled by the capillary action will experience opposing forces away from the nanotube due to a combination of surface tension on the Ni particle and the stress of the nanotube resulting from its increase in length.28 TEM images (Fig. 5A,C,D) depicted the changes in the morphology of the products with varying the reaction temperature for catalytic decomposition. Fig. 5C shows the carbon formation at 973 K on the catalyst which revealed some BCNTs like structure; however, the joints were not clear and regular. Additionally, some amorphous carbon was also found on the catalysts which may be due to the lower catalytic activity of the nickel particles in which the carbon formed poor interaction with the nickel particles. In contrast, at 1023 K, the bamboo-shape of the CNTs were fully developed (Fig. 5A). At this condition, amorphous carbon was also observed, but this time, the formation of amorphous carbon was probably due to the decomposition of carbon on the



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CNTs.29 At 1073 K, short length bamboo-shaped carbon tubes were produced with some carbon encapsulated metal particles (Fig. 5D). Further, it was also noticed that a larger diameter CNTs are synthesized at the higher reaction temperature. This was mainly due to the aggregation of the nickel particles at high temperatures, leading to the formation of larger catalyst particles and resulting CNTs of larger diameter. This diameter may be attributed to the fact that at lower reaction temperature the nickel nanoparticle served as the nucleation sites, however at elevated temperature grain boundaries and defective sites were the nucleation sites for CNTs growth.30 It can be deduced from Fig. 5A,C,D that the BCNTs produced consist of nanotubes with the average inner diameters of 23 and 27 nm, at 973 and 1023 K respectively, while at 1073 K, the average inner diameter CNTs increased to 38 nm. Similar findings have also been reported in published literature on different catalyst prepared by using different precursors.31,32

Fig. 2 XRD spectra of 60%Ni-5%Cu-5%Zn/Al2O3 (a) Calcined at 823 K and (b) Spent catalyst at 1023 K.

Fig. 3 TGA of the catalyst after methane decomposition reaction at 1023 K. Fig. 4 SEM image of 60%Ni-5%Cu-5%Zn/Al2O3 (a) Calcined catalyst at 823 K and (b) as grown BCNTs catalyst at 1023 K. Fig. 5 TEM image of growth BCNTs on 60%Ni-5%Cu-5%Zn/Al2O3 catalyst (A) at 1023 K (B) HRTEM image of wall and compartment of a BCNT formed at 1023 K (C) TEM image at 973 K and (D) TEM image at 1073 K.

3.2 Kinetic and mechanism of bamboo-shaped CNTs formation Experiments were performed to investigate the effect of reaction temperature, methane partial pressure and GHSV on methane conversion and type of BCNTs formed. The blank experiments were also carried out in the reactor filled with quartz particles with a feed consisting of methane and nitrogen in a ratio of 1: 1 at a GHSV of 3.6 L/h.gcat and in the temperature range of 873-1073 K. The conversion of methane was < 2% in all of these experiments, revealed that the methane decomposition was negligible without catalyst in the above temperature ranges.


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To explore the stability of the catalyst, methane conversion as a function of time on stream was measured at reaction temperatures of 973, 1023 and 1073 K and the results are reported in Fig. 6. It can be seen that the methane conversion was high at higher reaction temperature but the stability of the catalyst decreased at a higher temperature (1073 K). It was also observed that the catalyst was fairly stable at 973 and 1023 K, and 45 and 57% methane conversion could be obtained steadily for approximately 15 h, however at 1073K, catalytic activity declined just after 5 h on stream, indicating that the catalyst lost its activity and stability at higher temperatures due to sintering of Ni particles causing catalyst deactivation. These findings are in agreement with those reported by Saraswat et al.33, Chen et al. 34 and Villacampa et al.35 Carbon formation is always associated as the byproduct of methane decomposition reaction. For each run, after the reaction, final weight of catalyst was measured and the carbon yield of catalyst was plotted as a function of reaction time as shown in Fig. 7. Product yields were calculated based on the degree of methane conversion and defined as the total accumulated weight divided by the original mass of the catalyst. It was clearly revealed that the significant deactivation of catalyst occurred at 1073 K due to sintering of Ni particles and carbon yield was lower compared to a reaction temperature at 1023 K for a 20 h run.

Fig. 6 Methane conversion (%) as a function of time at different temperatures

Fig. 7 Carbon yield as a function of time on stream over 60%Ni-5%Cu-5%Zn/Al2O3 catalyst at 1023 and 1073 K.

3.3 Kinetics of the decomposition reaction Preliminary experiments were carried out at different GHSV and particle size to eliminate mass transfer and diffusional resistance. It was assumed that the isothermal and plug flow conditions were maintained in the reactor due to proper control of flow rates, heat and mass transfer limitations, and kinetic control of the reaction. The absence of diffusional resistance was also confirmed by WeiszPrater criterion.36 According to this criterion, in the absence of pore diffusion resistance, the following condition has to be satisfied:

φ=

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