Al2O3 Catalysts in Carbon Nanotube

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Effects of nickel species on Ni/Al2O3 catalysts in CNTs and hydrogen production by waste plastics gasification: Bench-scale and pilot-scale tests Ren-Xuan Yang, Kui-Hao Chuang, and Ming-Yen Wey Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01866 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 19, 2015

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Effects of nickel species on Ni/Al2O3 catalysts in CNTs and hydrogen production by waste plastics gasification: Bench-scale and pilot-scale tests Ren-Xuan Yanga, Kui-Hao Chuangb, Ming-Yen Weya,* a

Department of Environmental Engineering, National Chung Hsing University, Taichung, 402, Taiwan, ROC

b

Department of Safety, Health and Environmental Engineering, Central Taiwan University of Science and Technology, Taichung, 406, Taiwan, ROC

*

Corresponding author.

Tel: +886 4 22840441x533; Fax: +886 4 22862587. E-mail address: [email protected] (M.-Y Wey).

Keywords: waste plastics; nickel species; carbon nanotubes; hydrogen; gasification 1

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Abstract Upcycling waste plastics into carbon nanotubes (CNTs) and hydrogen is attractive for its efficient disposal. Although Ni-based catalysts are typically used in both hydrogen production and CNTs synthesis, few studies have investigated the catalytic active site for the co-production of CNTs and hydrogen by waste plastics gasification. To evaluate the effect of nickel species distribution of the Ni/Al2O3 catalyst, it was prepared by an impregnation method using different calcination atmospheres to determine their feasibility for the co-production of CNTs and hydrogen. For comparison, various Ni/Al2O3 catalysts for CNT growth were examined by CH4-thermal chemical vapor deposition (CVD). Ni/Al2O3 calcined under a reductive H2 atmosphere (H-Ni/Al2O3) gave smaller nickel nanoparticles containing metallic nickel species, which showed optimal performance for CNT and hydrogen co-production by waste plastics gasification. In addition, the quality of the CNTs was higher using this process compared to the CNTs synthesized by CH4-thermal CVD. Further examination of the catalysis temperature found that the H-Ni/Al2O3 catalyst gave higher quality CNTs in a 24.3 % yield, along with a hydrogen production rate of 325.4 mmol/h-g catalyst at 680 °C. The produced H-Ni/Al2O3 contained metallic nickel, demonstrating an improved catalytic activity for CNT and hydrogen production from waste plastics. 2


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Introduction The worldwide demand for plastic increased following the first industrial scale production of manufactured plastic in the 1940s.1 In 2013, 299 million tons of plastics were produced, accompanied by a considerable increase in waste plastics.2 Currently, large amounts of waste plastics are deposited in landfill sites and incinerated, which is costly, harmful to environment, and results in wastage of materials.3,4 Plastics are mainly composed of carbon and hydrogen, and so upcycling plastic wastes into carbon nanomaterials and hydrogen can not only solve the problem of waste plastics management, but can also give high value added products. Liu et al. have demonstrated the potential of simultaneously converting waste plastics into carbon nanotubes (CNTs) and hydrogen.3 Wu et al. later applied catalytic gasification to process waste polypropylene (PP), yielding hydrogen-rich synthesis gas and high quality CNTs.5 In addition, Acomb et al. used Ni/Al2O3 catalysts to produce CNTs and hydrogen from waste plastics by a two stage pyrolysis-gasification process.6

The choice of catalyst is key in achieving good levels of CNT and hydrogen production from waste plastic pyrolysis or gasification. Transition metals, such as Ni, Fe, and Co, are often used to decompose hydrocarbon feedstocks and solid waste for CNT synthesis. Zhang et al. compared different metal/Al2O3 catalysts for CNTs and H2 production from waste tires via catalytic reforming/gasification. Ni/Al2O3 catalysts 3


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gave higher quality multi-walled CNTs (MWCNTs) and higher yields of H2.7 Overall, Ni-based catalysts were extremely efficient in converting waste plastics and hydrocarbons into hydrogen, due to efficient C-C bond breakage. In addition, Ni-based catalysts were also more economically viable than comparable noble metal catalysts.5,8,9 Moreover, the support used also influenced the catalytic performance in terms of H2 production. Deshmane et al. indicated that the loading of metals on the high specific surface area support significantly improved metal dispersion on the support, thus inhibiting metal sintering and hindering crystal growth. This in turn enhanced the stability, giving high H2 selectivity via methanol steam reforming.10,11 Furthermore, due to its high thermal stability, mechanical strength, and adsorption capacity, Al2O3 has been used in industrial applications.12,13 Thus, Ni/Al2O3 catalysts have been widely applied in hydrogen production from fossil fuel steam reforming, biomass gasification, and solid waste gasification.7,14-16 However, few studies have focused on investigating the true active site of Ni-based catalysts in the production of CNTs and hydrogen from waste plastics.

CNTs, nanostructured carbon materials, have attracted growing attention since their discovery by Iijima.17 This is mainly due to their favorable properties, including hydrothermal stability, electrical conductivity, and mechanical resistance.4,18-20 Many methods have therefore been proposed for CNT production, including arc-discharge, 4


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laser vaporization, and chemical vapor deposition.4,19,21 However, the majority of these methods require fossil fuels as a carbon source, and are thus environment unfriendly and resource intensive processes. In contrast, the use of waste plastic as a feedstock for CNT production could be a promising environmentally preferable technology, but has attracted little attention to date. Song and Ji reported that combustion of PP in the presence of Ni/Mo/MgO catalysts in a crucible reactor gave efficient and rapid growth of multi-walled carbon nanotubes (MWCNTs) with both straight and double helical structures.22 Gong et al. later synthesized cup-stacked CNTs

through

the

carbonization

of

PP

using

organically-modified

montmorillonite/NiO catalysts in a crucible reactor.23 Furthermore, Nahil et al. investigated the effect of metal addition to Ni-based catalysts for CNT and hydrogen production by waste plastics reforming in a two stage fixed bed reactor.9 Present studies on CNT production by waste plastic gasification, decomposition, or reforming were operated in a fixed bed reactor and crucible reactor due to its ease of control and relatively low cost. Indeed, recently, a two-stage process has been applied, giving high yields of both CNTs and hydrogen from plastics.6,9,23,24 In this process, the plastics were first pyrolyzed or decomposed in an initial reactor, followed by upgrading of the gaseous products through a second reactor to give CNTs and hydrogen.

While several research groups have utilized waste plastics as carbon sources for 5


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CNT growth, few studies have been carried out in a fluidized bed reactor. This type of reactor is a heterogeneous gas-solid phase reactor, exhibiting high heat transfer efficiency, good mixing, and high turbulence, and can thus be utilized in a continuous feed operation in large-scale production.25,26

We therefore chose to investigate the fluidized bed reactor for use in waste plastics gasification, followed by treatment of the resulting gases in a catalytic reactor to product CNTs and H2 gas. Ni/Al2O3 catalysts were prepared via an impregnation method under different calcination atmospheres for use in both a CH4-thermal CVD bench-scale system and a waste plastics gasification pilot-scale system. In addition, the

prepared

catalysts

and

carbon

products

were

examined

by

Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction (XRD) spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and Raman spectroscopy. We aimed to investigate the effect of the Ni/Al2O3 catalyst nickel species by calcination under different atmospheres, for the co-production of CNTs and H2 from waste plastics gasification in a fluidized bed pilot-scale system.

EXPERIMENTAL SECTION Feedstock and catalyst preparation

6


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The waste plastic feedstock was composed of PP and polyethylene (PE). Virgin PP pellets, obtained from the Formosa Chemicals and Fibre Corporation, were employed to prepare the feedstock for the waste plastics gasification experiments. The PP pellets (3.0 g) were packed into a PE bag (0.3 g) to give the plastic feedstock. The ultimate elemental and proximate analyses of the feedstock are summarized in Table 1. The feeding rate was adjusted by altering the equivalence ratio (ER). Silica sand with a near constant density (ρp = 2600 kg/m3) was used as the bed material and sieved to a size range of 425-500 µm.

The Ni/Al2O3 catalyst was prepared by an impregnation method from a Ni(NO3)2·6H2O precursor (Aldrich) to give a Ni content of 10 wt.%. During impregnation, the Al2O3 support (EIKME Co.) was mixed with the precursor in distilled water. The solutions were stirred continuously at 70 °C overnight until the liquids had evaporated, after which time they were dried at 105 °C for 4 h, and then the catalysts calcined at 500 °C for 3 h in the presence of air, N2, or 5% H2/He. The three Ni/Al2O3 catalysts were assigned as A-Ni/Al2O3, N-Ni/Al2O3, and H-Ni/Al2O3, respectively.

Experimental procedure Preparation of the carbon nanotubes synthesis was carried out using two different

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systems, namely the CH4-thermal CVD bench-scale system and the waste plastics gasification pilot-scale system. These two systems are described in more detail below.

CH4-thermal CVD bench-scale system

The carbon nanotubes were synthesized using a CH4-thermal CVD apparatus consisting of a quartz tube reactor, a vacuum pump, and two mass flow controllers. The Ni/Al2O3 catalyst (1.5 g) was placed in a quartz boat (15 mm × 90 mm), which was placed horizontally in the central part of the quartz tube reactor. The reactor was then evacuated for 10 min using vacuum pump, after which argon gas was introduced into the reactor to create an inert environment. Subsequently, the furnace was heated to 680 °C at a rate of 10 °C/min under a flow of argon (60 mL/min). Methane was then introduced (60 mL/min) over 40 min at 680 °C to form the CNTs. After CNT growth was complete, the sample was cooled to 25 °C under argon. To ensure the reliability and reproducibility of the measurements, the activity tests of the bench-scale experiments were repeated in triplicate for each experimental run. The following equations were applied to calculate the CH4 conversion and CNT yield, where Mtotal is the mass of reacted catalysts (carbon product and catalyst), Mcata. is the catalyst mass before the reaction, and MCH4 is the mass of CH4 used in the reaction21: CH4 conversion (%) =

Mtotal -Mcata. MCH4

×100 %

8


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= CNT yield (%) =

Mtotal Mcata.

Flow rate (L⁄min)×Time (min) ÷22.4 (L⁄mol) ×16 (g⁄mol)

Mtotal Mcata. Mcata.

× 100 %

(1) (2)

Waste plastics gasification pilot-scale system Waste plastics gasification was carried out in a laboratory-scale fluidized bed reaction system, consisting of a gasifier chamber, a cyclone, two column filters, two cooling trapping tubes, and three parallel connection catalysis reactors (Fig. 1). The fluidized bed reactor (height = 67 cm) was composed of stainless steel (3 mm thick, 6 cm inner diameter, AISI 316SS) equipped with a stainless steel porous plate (15% open area) as a gas distributor. The reactor was enclosed by an electrically resistant material packed with 1 in ceramic fibers. A temperature indicator controller (TIC) was used to control and determine the temperatures of the free board chamber, and of the sand bed using a K-type thermocouple. The input air was used as the gasifying agent to determine the ER, which is defined as the ratio of actual airflow supply divided by the stoichiometric airflow required for complete combustion. The fluidized bed reactor initially gasified waste plastics to produce hydrocarbons. This was followed by upgrading the gases in a fixed bed catalysis reactor to produce CNTs and hydrogen.

When the system reached the target temperature, the plastic feedstock materials were fed into the reactor through a lock hopper. The effluent gas from the waste plastics gasification process was passed through the cyclone and column filter to 9


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remove particulates, and then flowed into the catalysis reactor. After 10 min of feedstock flow, the gasification system stabilized, and the gasification products were introduced into the fixed bed catalytic reactor (i.d = 10 mm) through a flowmeter at 200 mL/min (space velocity = 10185 h-1). A systematic approach for stability was tested in triplicate to evaluate the reproducibility of the measured data.

The fixed bed reactor temperature was also investigated in this study. The operating parameters of the fluidized bed were controlled at 600 °C with an ER of 0.1. In addition, the gaseous products (H2, CO, CO2, CH4, and C2-C5 gaseous hydrocarbons) in the inlet and outlet gas streams of the catalysis reactor were sampled during the experimental process. The concentration of permanent gases, namely H2, CO, CO2, and CH4, were determined by gas chromatography (Clarus 500 GC, and Carboxen 1000 column, Perkin Elmer) equipped with a thermal conductivity detector (TCD), and that of the C2-C5 gaseous hydrocarbons were obtained using a gas chromatography-flame ionization detector (GC/FID, 6890N with an alumina sulfate PLOT capillary column (50 m × 0.53 mm ID), Agilent). The H2 production rate and yield were calculated according to Eqs. (3) and (4): H2 production rate (mmol/h-g catalyst) = H2 yield (mmol/g plastic feedstock) =

H2 conc. × gas flow rate Mcata. H2 conc. × gas flow rate plastic feeding rate

(3) (4)

10


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Catalyst and product characterization

The N2 adsorption at 77 K measured using a BET-201-AEL (Porous Materials, Inc.) was used to determine the BET (Brunauer‐Emmett‐Teller) surface area and pore parameters.

Powder X-ray diffraction (XRD, BRUKER D8 SSS) using a Cu Kα radiation source (λ = 1.5418 Å) was conducted to determine the crystal structures of the catalysts. The XRD measurements were carried out with a step increment ratio of 0.02 °/s in the 2θ range 30-80°.

The chemical state of the prepared Ni/Al2O3 catalysts was identified through X-ray photoelectron spectroscopy (XPS, PHI-5000) with a monochromatic Al Kα radiation source.

Thermogravimetric analysis (TGA) was carried out to measure the thermal stability of the carbon products using a Perkin-Elmer STA 6000. The sample (~20 mg) was placed in a platinum specimen holder and heated to 850 °C under a flow of air (20 mL/min) at a heating rate of 20 °C/min.

The morphology of the synthesized CNTs was analyzed by TEM using a JEOL JEM-1200CX II operated at 120 keV. The samples were crushed and suspended in

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ethanol in an ultrasonic bath. A drop of the resulting suspension was then deposited on the lacey carbon film coated copper grid.

The graphitic quality of the synthesized CNTs was characterized by Raman spectroscopy (Horiba) under ambient conditions. Measurements were recorded between 500 and 3200 cm-1 with a 632.8 nm ion laser (JDS Uniphase Co.) equipped with a charge-coupled device (CCD) detector.

RESULTS AND DISCUSSION Catalyst characterization Table 2 lists the physical properties of the Al2O3 support and the as-prepared catalysts. By BET analysis, the specific surface areas were calculated as 205.9, 157.5, 169.9, and 184.0 m2/g for Al2O3 support, A-Ni/Al2O3, N-Ni/Al2O3, and H-Ni/Al2O3, respectively. Compared to the Al2O3 support, the active sites block the catalyst pores, resulting in smaller specific surface areas for the prepared catalysts. As a result, the H-Ni/Al2O3 catalyst shows little decrease in specific surface area, indicating its improved ability to induce the formation of highly dispersed metal nanoparticles compared to the other two catalysts. In agreement with our results, Ren et al. found that the particle size of nickel metal calcined in H2 was smaller than that calcined in air.27

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XRD analysis of the as-prepared catalysts showed that the crystalline phases were γ-Al2O3 (JCPDS File Card No. 47-1308) and cubic NiO (JCPDS File Card No. 47-1049), as shown in Figure 2. The diffraction peak of cubic NiO was observed at 2θ = 43.3° (2 0 0). The Scherrer equation, given in Eq. (4) was employed to estimate the mean NiO crystal size (D) of the Ni catalysts: D =

Kλ

(4)

B1/2 cos θ

where K is a constant (0.89), λ is the X-ray wavelength (1.54056 Å for Cu Kα radiation), B1/2 is the half-intensity width of the relevant diffraction peak, and θ is the peak position. The mean calculated NiO crystal sizes on the Al2O3 supports obtained from different calcination atmospheres are given in Table 2. The crystallite size of the Ni/Al2O3 catalysts was 6.4-11.5 nm, with H-Ni/Al2O3 exhibiting the smallest NiO crystallite size (6.4 nm) of the three catalysts. This indicates that the H-Ni/Al2O3 catalyst possessed improved nickel particle dispersion on the Al2O3 support, which is consistent with the BET results.

The chemical states of the calcined catalysts were obtained via XPS measurements. Figure 3 shows the XPS spectra of A-Ni/Al2O3, N-Ni/Al2O3, and H-Ni/Al2O3, while Table 3 summarizes the data for each sample. The Ni 2p3/2 spin-orbital photoelectron, which was assigned to the Ni component of Ni/Al2O3, was located at a binding energy of 855.3 eV with a shake-up satellite at 861.3 eV. 13


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Calcination under different atmospheres was found to affect the nickel species composition of the Ni/Al2O3 catalysts. The H-Ni/Al2O3 catalyst exhibited three deconvoluted Ni 2p3/2 peaks; the peak at 853.4 eV was assigned to nickel metal, while those at 855.3 and 856.4 eV were attributed to nickel oxide species (NiO and Ni2O3).28,29 However, the Ni 2p3/2 lines of both A-Ni/Al2O3 and N-Ni/Al2O3 expressed a convolution of two peaks, corresponding to the two different oxidation states of nickel. As Table 3 shows, only the H-Ni/Al2O3 catalyst contained the reduced nickel species Ni0, while the catalysts calcined under air possessed a higher ratio of the oxidized nickel species Ni2O3. These results infer that the chemical states of the active metal phase can be controlled by the calcination atmosphere. This is in agreement with previous studies.30,31 For example, Wang et al. found that a Pd/C catalyst pretreated under a H2 atmosphere facilitated formation of both oxidized and reduced metal species (i.e., Pd0 and Pd2+), while under air, only the oxidized metal species was produced.30 Wang et al. later reported that Pt-SnOx/C catalysts prepared under an inert (Ar) or reductive (H2) atmosphere at high temperature lead to a range of Sn oxidation states.31

CNTs synthesis in bench-scale tests We then investigated CNT synthesis in bench-scale tests. The influence of different calcination atmospheres on catalyst activity for CNT production by CH4-thermal 14


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CVD was investigated, and the results are shown in Table 4. H-Ni/Al2O3 gave the highest CNT yield (27.4%), while A-Ni/Al2O3 gave the lowest yield (16.4%). CH4 conversion showed a similar trend. Generally, the rate-determining step in carbon nanostructure growth is the diffusion of carbon in the catalyst metal particles.32,33 Chen et al. reported that with decreasing metal particle size, carbon atom absorption on the surface of the metal-gas interface was easier to diffuse to the growth surface of the metal-nanotube interface by bulk diffusion. Consequently, the catalyst possessing smaller metal particles gave a higher CNT yield through increased bulk diffusion of the adsorbed carbon atoms.21 Comparing the size of the NiO crystallite from XRD results with the CNT yield, H-Ni/Al2O3 was found to have the smallest NiO crystallite in combination with the highest CNT yield, thus supporting such a mechanism.

TEM, TGA, and Raman analysis were used to analyze the physical and chemical properties of the carbon products resulting from the CH4-thermal CVD process. Figure 4 shows the TEM images of the carbon products from the use of A-Ni/Al2O3, N-Ni/Al2O3, and H-Ni/Al2O3. The TEM images show that the carbon products are hollow, with outer-diameters of ~20-30 nm composed of reacted Ni/Al2O3 catalysts. In addition, the CNTs grown on the H-Ni/Al2O3 catalyst were relatively smooth, long, and exhibited a relatively uniform diameter.

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Figure 5 shows the TGA and derivative thermogravimetric (DTG) profiles of the Ni/Al2O3 catalysts following CH4-thermal CVD. A slight weight loss is evident between 50 and 200 °C for the reacted catalysts, corresponding to the loss of absorbed water from the catalyst. The weight loss at ~500 °C was due to the oxidation of amorphous carbon, while a further weight loss and oxidation at 600-700 °C was attributed to the MWCNTs.7,34 DTG curves also show that the oxidation of H-Ni/Al2O3 took place at 693 °C, which was higher than that of N-Ni/Al2O3 (667 °C) and A-Ni/Al2O3 (653 °C). Higher oxidation temperatures represent higher purity CNTs containing fewer defects, and CNT purity can be calculated from the weight loss between 500-700 °C. Thus, among the three reacted catalysts, H-Ni/Al2O3 gave the highest purity (25%), followed by A-Ni/Al2O3 (21%) and N-Ni/Al2O3 (20%).

Raman spectroscopy results are presented in Figure 6. Two strong peaks were observed in the profiles of each reacted catalyst. The signal at ~1320 cm-1 was assigned to the amorphous or disordered carbon vibration band (D-band), while the signal at ~1580 cm-1 indicated the presence of sp2 C-C vibrations (G-band). The relative intensity ratio (IG/ID) of the G-band to the D-band can be used to evaluate the degree of graphitization and the quality of the CNTs. CNTs growing on H-Ni/Al2O3 gave the highest IG/ID value of 0.27, indicating fewer defects and a smooth surface. In contrast, the lowest intensity ratio (IG/ID) of 0.07 was obtained for the carbon products 16


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prepared from the N-Ni/Al2O3 catalyst, which contained more defects, disorder, and amorphous carbon. Thus, the order of IG/ID for the catalysts is as follows: H-Ni/Al2O3 > A-Ni/Al2O3 > N-Ni/Al2O3. This is consistent with the TGA and TEM results.

CNTs and H2 production in pilot-scale tests Bench-scale experiments confirmed that calcination of the Ni/Al2O3 catalyst under different atmospheres affected the nickel chemical valence state ratio, thus influencing the yield and quality of CNT products from CH4-CVD. Further experiments were then carried out to demonstrate the feasibility of simultaneous CNT and H2 production from waste plastics gasification in a pilot-scale system consisting a fluidized bed, and equipped with three parallel fixed catalysis reactors. The waste plastics feedstock was gasified in the fluidized bed reactor, and the gasified products were subsequently passed to the catalytic fixed bed reactors for upgrading H2 and synthesis of CNT.

Table 5 shows the main compositions of the gaseous products and the H2 yields at 600 °C with an ER of 0.1 in a fluidized bed, and with upgrading through three catalytic fixed bed reactors. The gas composition, H2 production rate, and H2 yield of the gasified products in the fluidized bed reactor are shown in the row labelled “none”. Without catalytic reaction in the fixed bed reactors, the H2 yield from the feedstock in

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the fluidized bed reactor was 23.9 mmol/g plastic feedstock, which was higher than previously reported results using fixed bed reactors.6 This may be ascribed to better mixing behavior and higher mass and heat transfer efficiency of the fluidized bed reactor. Among the three catalysts, the highest H2 production rate and CNT yield for the waste plastics gasification process were obtained using H-Ni/Al2O3, giving the H2 production rate of 385.1 mmol/h-g catalyst and 22.0% CNT yield. In contrast, A-Ni/Al2O3 gave the lowest H2 production rate (350.4 mmol/h-g catalyst) and CNT yield (8.4%). This gives a catalytic activity sequence for H2 production and CNT yield in the waste plastics gasification process as follows: H-Ni/Al2O3 > N-Ni/Al2O3 > A-Ni/Al2O3. The variation in H2 production rate and CNT yield was related to metal particle size and to the chemical state of the nickel species, as determined by XRD and XPS measurements. Hofmann et al. indicated that the active state of the Ni/SiO2 catalyst for single-walled CNT CVD from C2H2 decomposition was the metallic Ni nanoparticle.35 Moreover, Gonzalez et al. found that Ni nanoparticles synthesized by an alternative emulsion-mediated method above 480 °C contained more fractions of metallic nickel than commercial Ni nanoparticles. Thus, the quality and quantity of CNT growth on Ni nanoparticles from catalytic methane decomposition prepared by the emulsion-mediated method were superior to the commercial Ni nanoparticles.36 In addition to the chemical state of the active site, metal particle size and dispersion on 18


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the support also affect the H2 production activity. Thus, H-Ni/Al2O3 exhibiting the smallest metal crystallite and improved metal dispersion, gave the highest H2 promotion ratio. The hydrogen production ability of the H-Ni/Al2O3 catalyst in waste plastics gasification was then compared with other hydrogen production methods, as presented in Table 6.37-39 Thus, compared to other methods with similar operating temperatures (500-700 °C), our waste plastics gasification procedure exhibits increased hydrogen production.

As shown in Table 5, in the presence of Ni/Al2O3 catalysts, the concentrations of H2 and CO increased, while the concentrations of CO2, CH4, and C2-C5 decreased. This can be attributed to the dry reforming of hydrocarbons (Eq. 5) and of methane (Eq. 6).

Dry reforming of hydrocarbons: Cn Hm + nCO2 →O2nCO + m2 H2

∆H>0

(5)

Dry reforming of methane: CH4 + CO2 → 2CO + 2H2

∆H = 247.0 kJ/mol

(6)

Although N-Ni/Al2O3 showed better catalytic activity for CH4 decomposition than H-Ni/Al2O3 (10. 97 vol% vs. 12.67 vol%, respectively), the H-Ni/Al2O3 catalyst exhibited higher CNT yields, likely resulting from the decrease in other hydrocarbon

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gases (C2-C5). Liu et al. indicated that the MWCNT yield was related to the hydrocarbon composition of the pyrolysis gas, suggesting that the use of propylene and butylene as the feedstock gave higher CNT yielsd than CH4.3 Furthermore, Wu and Williams reported that the decrease in H2 concentration may be ascribed to the higher conversion of PP to C2-C4 hydrocarbons instead of conversion to H2.40

TEM images of the reacted catalysts from the waste plastics gasification pilot-scale system are shown in Figure 7. These images show that carbon deposited on the Ni/Al2O3 catalysts was in the form of CNTs with a hollow structure. The outer diameters of the CNTs measured 12-20 nm, which are smaller than the CNTs produced from CH4-CVD (Fig. 4). The size of the Ni particles in the CNTs was also smaller than that those shown in Figure 4. This is likely due to the reduction of Ni oxide to Ni metal by the higher quantities of H2 and other hydrocarbons present during the reaction.

TGA and DTG plots of the different reacted catalysts displayed in Figure 8 show similarities to the TGA results given in Figure 5, with peaks associated with absorbed water (50-200 °C), amorphous carbon (500 °C), and CNTs (600-700 °C) being observed. However, one significant difference between these two figures is the position of the peak corresponding to CNT oxidation, which shifted from

20


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approximately 690 to 640 °C. This implies that CNTs grown on the Ni/Al2O3 catalysts in the waste plastics gasification pilot-scale system were more reactive than those in the CH4-thermal CVD bench-scale system. This in turn may be due to the smaller diameters of the CNTs generated from the waste plastics gasification process. Wang et al. reported that filamentous carbons species deposited on Ni/SiO2 catalysts with smaller diameters were more reactive towards oxidation than larger diameter filamentous carbon species.41 However, the purity of the CNTs deposited on the H-Ni/Al2O3 was higher (33%) than that of the CNTs (25%) from the thermal CVD process in the horizontal quartz tube reactor.

Figure 9 shows the Raman spectra recorded for the three different reacted catalysts. Two characteristic peaks, namely the D-band and the G-band, were observed at 1320 and 1580 cm-1, respectively, for each catalyst. The intensity ratios between the two bands (IG/ID) were 0.71, 0.66, and 0.43 for the carbon products on the H-Ni/Al2O3, A-Ni/Al2O3, and N-Ni/Al2O3 catalysts, respectively. In addition, the CNTs produced in the waste plastics gasification pilot-scale system gave higher IG/ID values than those produced in the CH4-thermal CVD bench-scale system (Fig. 6), thus confirming the higher quality of the CNTs produced by former. This difference was related to the source of carbon feedstock used in the CNT formation process. The carbon sources used in the CNT synthesis have a significant influence on the 21


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construction of carbon nanostructures, and thus on the quality of CNTs. Li et al. reported that light hydrocarbons from the acetylene/xylene mixture gave CNTs with high purities and more homogeneous morphologies.42 Gong et al. also found that using PP as the feedstock gave high quality cup-stacked CNTs due to the contribution from light hydrocarbons (C2-C4).23 Thus, the application of waste plastics as the feedstock for the gasification pilot-scale system not only recycles waste plastics, but also yields higher quality CNTs and H2.

To obtain the optimal operating conditions for CNT and hydrogen production in our waste plastics gasification system, the effect of the catalysis reaction temperature was investigated. The experiments were conducted at 600, 680, and 750 °C using the H-Ni/Al2O3 catalyst. Table 7 shows the H2 production rate, H2 yield, and gaseous product compositions at 600 °C and at an ER of 0.1 in a fluidized bed, upgrading through three catalytic fixed bed reactors at different reaction temperatures. When the reaction temperature was increased from 600 to 750 °C, the H2 concentration increased from 34.53 to 36.13 vol% and the CO concentration increased from 27.17 to 33.70 vol%. In addition, an increase in reaction temperature gave a decrease in both CH4 and C2-C5 hydrocarbons concentrations and an increase H2 production rate, H2 yield, and CNT yield. This can be explained by the higher reaction temperature enhancing the dry reforming of methane, dry reforming of hydrocarbons, and direct 22


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decomposition of hydrocarbons. Thus, the C2-C5 hydrocarbons were broken down for deposition on the catalyst and subsequent CNT formation. These results are consistent with previous studies.3,24

Figure 10 shows the TEM images of the reacted H-Ni/Al2O3 catalyst at different catalysis reactor temperatures. CNTs with larger diameters (~27 nm) and little amorphous carbon accumulation were found in the sample obtained at 600 °C. Figure 10(c) shows that the CNTs curve and tangle, and possess a smaller inner diameter of ~3 nm without amorphous carbon. CNTs with smoother structures were obtained at 680 °C, while less uniform CNTs were obtained at 600 °C. The TGA and DTG curves of the carbon products produced on H-Ni/Al2O3 at different temperatures are shown in Figure 11. As the catalysis temperature is raised, the weight loss ratio between the 500-700 °C TGA range increases, with 30% weight loss observed for CNTs produced at 750 °C, compared to 18% and 25% for those produced at 600 °C and 680 °C, respectively. However, the main oxidation peak temperature of the carbon products was at ~640 °C for the sample obtained at 680 °C, indicating that CNT growth at 680 °C possessed a higher degree of graphitization. In addition, the DTG profile of the carbon products obtained at 750 °C shows the oxidation peak of amorphous carbon at ~500 °C. Although a higher catalysis reaction temperature can upgrade the waste plastics gasification products to generate additional carbon products, CNT 23


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growth at higher temperatures results in lower quality carbon products. Therefore, the optimum catalysis temperature for CNT growth in our waste plastics gasification pilot-scale system was 680 °C.

CONCLUSIONS We report the evaluation of the effect of nickel species in Ni/Al2O3 catalysts on the simultaneous production of hydrogen and carbon nanotubes (CNTs) using a waste plastics gasification pilot-scale system, consisting of a fluidized bed and fixed bed catalysis reactors. The CH4-thermal chemical vapor deposition (CVD) bench-scale system was also used for CNT growth using Ni/Al2O3 catalysts. The Ni/Al2O3 catalysts were calcined under oxidative (air - A), inert (N2 - N), and reductive (H2 - H) atmospheres to give varied Ni species distribution. The CH4-thermal CVD bench-scale system gave higher CNT yields, but lower quality CNTs compared to the waste plastics gasification pilot-scale system. H-Ni/Al2O3 exhibited the highest activity for CNT and hydrogen production in the waste plastics gasification pilot-scale system, with a hydrogen promotion ratio of 35.6%. XPS analysis indicated that the calcined atmosphere was the main factor in influencing Ni0 formation, as the catalyst calcined under a H2 atmosphere had smaller nickel nanoparticles containing metallic nickel. Moreover, the catalytic performance for CNT and hydrogen production was associated with the metallic nickel species content due to the presence of smaller 24


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nickel nanoparticles and higher dispersion of metal on the Al2O3 support. In addition, the CNT yield and H2 production ability both improved with increasing catalysis reaction temperature. The highest quality CNTs were observed on H-Ni/Al2O3 at 680 °C, while a further increase in temperature to 750 °C gave CNTs with a lower degree of graphitization. Consequently, this work provides a potential means to not only treat waste plastics continuously, but also to co-produce CNTs and hydrogen using Ni/Al2O3 catalysts by waste plastics gasification.

ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology (MOST), Taiwan,

ROC

for

the

financial

support

under

Grant

No.

MOST

103-2221-E-005-001-MY3.

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Table 1. Ultimate elemental and proximate analyses of the feedstock by weight Materials

Elemental analysis C

H

Proximate analysis

O

N

Moisture

Volatile Fixed matter carbon

Ash

PP

85.18 14.39

0.43

−

0.4

99.6

Al2O3 Catalysts in Carbon Nanotube

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