Thermocatalytic Conversion of Automotive ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Thermocatalytic Conversion of Automotive Shredder Waste and Formation of Nanocarbons as a Process Byproduct Mohannad Mayyas,*,† Farshid Pahlevani,† Martin Bucknall,‡ Samane Maroufi,† Yi You,† Zhao Liu,§ and Veena Sahajwalla† †

Centre for Sustainable Materials Research and Technology (SMaRT), School of Materials Science and Engineering, University of New South Wales, Kensington, Sydney 2052, Australia ‡ Mark Wainwright Analytical Centre, University of New South Wales, Kensington, Sydney 2052, Australia § School of Materials Science and Engineering, University of New South Wales, Kensington, Sydney 2052, Australia S Supporting Information *

ABSTRACT: Thermocatalytic conversion of plastic wastes using titanium dioxide (TiO2) is a novel and promising approach for waste management and sustainable production of materials. In this approach, the heat-treatment of a mixture of plastic waste and TiO2 at elevated temperatures helps to achieve fast and complete decomposition and offers several advanced byproducts. In this study, the different physicochemical interactions between TiO2 and an industrial plastic waste (i.e., automotive shredder residue, ASR) at elevated temperatures were investigated. The nonisothermal degradation kinetics of ASR, with and without TiO2, were calculated from the thermogravimetric analysis (TGA) and results confirmed that the TiO2 influences and catalyzes the degradation of ASR. The analysis of the resulting gas showed that the TiO2 limits the formation of CO2 gas, which is considered an unfavorable product of thermal processes, without changing the quality of the hydrocarbons (i.e., oils) generated during the heat treatment of ASR. While ASR decomposes, TiO2 transforms into value-added Ti-based ceramics; this transformation generates a high yield of CO that can be collected for commercial purposes. In addition, TiO2 led to the formation of considerable quantities of onion-like carbon nanoparticles (OLCNPs) from the gas phase; this product was characterized and its formation mechanism was discussed. KEYWORDS: Automotive shredder residue, Titanium dioxide interactions, Catalytic conversion, Carbon nanoparticles

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INTRODUCTION ASR, sometimes referred to as “auto fluff”, is the fraction of material that remains after the processing of scrap automobiles to recover metals. In this process, vehicle hulks are entirely shredded into small pieces of metal and nonmetal materials (i.e., plastics). The metal-containing fraction is separated out from plastics by a well-established technology called the postshredder process. The remaining fraction of plastics is the ASR which is commonly disposed of in landfills.1,2 ASR composition varies significantly based on the type of feed and the process technology adopted for recycling. ASR consists of several plastics such as polypropylene, polyethylene, polycarbonate, polyurethane, acrylonitrile butadiene styrene, and polystyrene with a very low content of ash. ASR may also contain some deleterious components that cause adverse impacts on human health and the environment if not managed properly.3−6 As a consequence of the increasing volume of ASR, many countries have imposed a number of regulations and directives regarding scrap vehicle management, and put more restrictions © 2017 American Chemical Society

on ASR disposal. Under this legislative pressure, a great deal of research work was undertaken to find an efficient and costeffective solution for ASR management. A number of solutions were suggested such as coincineration with other waste streams,7−9 pyrolysis at low and moderate temperatures to produce pyro-gas,10,11 use as a feed material in cement manufacturing,12,13 and incorporation in composite materials.12,14 Among these solutions, only thermal processes (i.e., coincineration and pyrolysis) seem really suitable, as ASR comprises plastics of high calorific value. However, coincineration results in an undesirable quantity of deleterious byproducts such as dioxins and furans, whereas pyrolysis generates lower concentrations of these compounds. Although a number of researchers have investigated ASR pyrolysis at low and moderate temperatures, this process remains environmentally and commercially unviable. Received: March 12, 2017 Revised: April 11, 2017 Published: May 7, 2017 5440

DOI: 10.1021/acssuschemeng.7b00774 ACS Sustainable Chem. Eng. 2017, 5, 5440−5448

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ACS Sustainable Chemistry & Engineering As the production of waste materials continues to increase at a high rate, the use of waste as a raw material for the production of advanced materials has gained great importance. Several advanced materials such as Ti-based ceramics (e.g., TiC and TiN) and OLC-NPs have received great attention in recent years. Ti-based ceramics have outstanding chemical and mechanical characteristics and they are used in a variety of applications such as coatings for cutting materials,15 catalyst support,16,17 and supercapacitor applications.18,19 OLC-NPs owe their name to their concentric layered architecture of fullerene-like spheres, which look like an onion. This type of carbon has been shown to be excellent as a conductive additive to supercapacitor electrodes with high capacitance retention at high current densities; it also has around 10 times the specific power of activated carbon. Several techniques can be used to synthesize OLC-NPs such as vacuum annealing of nanodiamonds (high yield, 5 to 10 nm particle size),20,21 high temperature evaporation of nanodiamonds (low yield, ∼5 nm particle size),22 laser excitation of ethylene (high yield),23 arc discharge (low yield and impure product),24 chemical vapor deposition of acetylene gas with the assistance of iron catalyst supported on sodium chloride (reasonable yield, ∼50 nm particle size),25 carbon ion implantation (3 to 30 nm particle size),26 solid state carbonization of a phenolic resin (phenolformaldehyde) with ferric nitrate catalyst (∼40 nm particle size),27 and thermolysis using hexachlorobenzene and sodium azide (impure product, 30 to 100 nm particle size).28 It is apparent that all above-mentioned techniques suffer from several drawbacks regarding the control of purity and particle size. Additionally, these techniques involve costly and hazardous precursor materials that may pose a serious threat to human and environmental health. In the current work, we introduce an alternative approach for ASR management where oils and advanced materials such as Ti-based nanoceramics and OLC-NPs can be generated as byproducts. In this approach, the high-temperature transformation of an intimate mixture of ASR and TiO2 has been adopted; TiO2 catalyzes some degradation reactions and helps to achieve a fast and complete decomposition of ASR. The TiO2 catalyst eventually turns into a titanium nitride (TiN) ceramic, which is considered a highly valuable material and has a wide range of applications. The formation mechanism, kinetics, and morphology of TiN resulting from this approach were discussed in previous work.29 The aim of this paper is to investigate the catalytic effect of TiO2 on ASR degradation by comparatively studying the nonisothermal degradation kinetics of ASR and ASR/TiO2 mixture. This paper also reports the formation of the hydrocarbons resulting from the fast-hightemperature pyrolysis of ASR and ASR/TiO2 mixtures. The fast-high-temperature pyrolysis of ASR/TiO2 mixture was found to also result in a secondary carbon product (i.e., OLC-NPs). This product was characterized and its probable formation mechanism elucidated.

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Figure 1. Proximate composition of ASR.

mixture were studied using a thermogravimetric analyzer (PerkinElmer Simultaneous Thermal Analyzer (STA) 8000, Waltham, MA). The interaction between ASR and TiO2 at nonisothermal conditions was studied by extrapolating TGA data and comparatively studying the degradation kinetics of both ASR and ASR/TiO2 mixture. The gas evolved during TGA analysis was characterized by Fourier transform infrared (FTIR) spectrometry (PerkinElmer Frontier, Waltham, MA) linked to the TGA heating chamber. The pyrolysis experiments at isothermal conditions were carried out by placing 3 g of sample in an alumina crucible covered with a lid and inserting it in a preheated tubular furnace (100 cm length × 5 cm diameter) under N2 purge (1 L min−1) for a given period of time. The furnace was coupled with an infrared gas analyzer (Advanced Optima AO2020, ABB Measurement and Analytics, Australia) for continuous measurement of noncondensable gases, e.g., CO, CO2, and CH4. Hydrocarbons resulting during pyrolysis were collected by trapping impingers (25 mL) filled with an organic solvent and linked to the furnace outlet. Several organic solvents were used including acetone, dichloromethane (DCM), tetrahydrofuran (THF), and n-hexane. Oilladen solvents were centrifuged to remove all suspended particulates, and subsequently analyzed by GC−MS analysis. The carbon product was characterized by X’pert PRO multipurpose XRD (MPD system, operating at 40 kV and 40 mA), FEI Nova NanoSEM 450 FE-SEM (all FE-SEM specimens were sputtered with thin layer platinum for better resolution), and Philips CM200 field emission transmission electron microscopy (TEM). GC−MS Analysis. A Thermo DSQ II mass spectrometer interfaced to a Thermo Trace gas chromatograph and Triplus autosampler (Thermo Fisher Scientific, Waltham, MA) was operated in electron impact mode for all analyses. A Thermo TR-50MS 60 m × 0.25 mm i.d., 0.25 μm film, GC column was installed in the split inlet of the GC with an SGE split Focus liner. The inlet was maintained at 305 °C and the helium carrier gas set to a constant flow rate of 1.5 mL/min. The ion source of the MS was maintained at 200 °C and the detector gain set to 3 × 105. The autosampler used heptane as a syringe wash. 1 μL of the sample phase was injected into the heated GC inlet at a split ratio of 50:1. The initial oven temperature was 45 °C, held for 0.5 min. The oven temperature was then increased at a rate of 5 °C/min to 310 °C, where it was held for 12 min. The GC transfer line was maintained at 315 °C throughout the run and the MS scanned m/z 45−550 at a rate of 3.5 scans per second. The MS was switched off during the first 6.5 min of the run to protect the filament. When THF samples were analyzed, the MS was additionally switched off from 27.8 to 29.8 min, to avoid detecting butylated hydroxyl toluene (BHT) that eluted at 28.66 min. BHT is added to THF preparations as a stabilizer during manufacture to inhibit the formation of explosive peroxides. Eluting compounds were identified by comparison of their baselinesubtracted mass spectra with those contained in the Wiley 9/NIST 2011 Mass Spectral Library. Identifications with a match score greater than 900/1000 were considered valid.

MATERIALS AND METHODOLOGY

The ASR sample was supplied by a steel recycling company in Australia. The composition of as-received ASR is given in Figure 1. For preparing the ASR/TiO2 mixture, ASR was first pulverized by a cryomill (Retsch, Haan, Germany) and subsequently ball-milled with titanium IV oxide powder (TiO2, anatase, 99.8% trace metals basis, supplied by Sigma-Aldrich) for 1 h. The powder mixture was hotpressed in a steel cylindrical die (∼15 mm diameter) using a uniaxial hydraulic press by applying moderate pressure (3 bar) at 180 °C for 10 min. The degradation characteristics of both ASR and ASR/TiO2 5441


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RESULTS AND DISCUSSION The physicochemical interactions between ASR and TiO2 at nonisothermal conditions were investigated by TGA. The TGA data of ASR/TiO2 mixture and its derived kinetic parameters were obtained and compared to those of ASR discussed in a previous work.30 TGA curves were first obtained by heating ASR/TiO2 mixture (ratio 5/1) from room temperature to 1300 °C under N2 purge (20 mL min−1) using several different heating profiles; 5, 10, 15, and 20 °C min−1. Curves of mass loss and Gaussian fit of derivative mass loss (DTG) are given in Figure 2a. Although mass loss and DTG curves of ASR/TiO2

isothermal degradation kinetics of ASR/TiO2 mixture. The Coats−Redfern model is described by eqs 1 and 2.31 when n ≠ 1: ⎛ AR ⎞ ⎧ 1 − (1 − α)1 − n ⎫ E ⎬ ln⎨ ln = ⎜ ⎟− a 2 ⎩ T ·(1 − n) ⎭ ⎝ βEa ⎠ RT

(1)

when n = 1: ⎛ AR ⎞ ⎧ ln(1 − α) ⎫ E ⎬ = ln⎜ ln⎨− ⎟− a 2 ⎩ ⎭ T ⎝ βEa ⎠ RT

(2)

where α is the conversion rate of a decomposed sample, A is the pre-exponential factor, T is the temperature (K), Ea is the activation energy (kJ mol−1), R is the universal gas constant (8.314 J mol−1 K−1), and β is the heating rate. The conversion rate (α) can be calculated from the following equation: α=

W0 − Wt W0 − Wf

(3)

where W0 is the initial mass of a sample, Wt is the actual mass of the sample at time t, and Wf is the final mass of the sample after pyrolysis. −ln(1 − α) 1 − (1 − α)1 − n or ln results T2 T 2·(1 − n) Ea slope − R for the proper value of n.

{

Plotting 1/T against ln

} {

}

in a straight line with the The proper value of n was determined using a least-squares technique, by trying different n values until the highest value of linear correlation coefficient is obtained, whereas A was

( ). Figure 2b shows the

obtained from the intercept ln

AR βEa

obtained lines from Coats−Redfern model when n = 1 and n ≠ 1 at different heating profiles. The kinetic parameters of ASR/ TiO2 mixture were calculated from the fitting data of Coats− Redfern’s lines, Table 1, and compared to those of ASR that were calculated in a previous study.30 The thermal degradation of a material should be theoretically similar at different heating profiles, but due to heat transfer limitations within the sample, variations in the observed kinetic parameters are usually seen. These variations can be minor or major depending on the physical characteristics of the degraded material. However, these variations can give valuable information about the physical interactions during the thermal degradation of a sample. As shown in Table 1, the ASR/TiO2 mixture results in lower Ea values with very small variations, when compared to ASR at different heating profiles when n = 1 and n ≠ 1. The small variations in the Ea values of ASR/TiO2 mixture are due to the relatively high thermal conductivity of TiO2 compared to ASR, which reduces the effect of the heat transfer limitation subsequently resulting in closer Ea values at different heating profiles. The catalysis is usually characterized by a lower Ea value of a reaction. The lower Ea values of ASR/TiO2 mixture at different heating profiles compared to ASR provides a clear indication of the catalytic effect of TiO2 on ASR in the ASR/ TiO2 mixture. It can also be noticed that A values of ASR/TiO2 mixture are lower than those of ASR when n = 1 and n ≠ 1 at all heating profiles. This observation is related to the fact that the TiO2 in ASR/TiO2 mixture remains stable (stationary reactant) without undergoing any phase transition in the main degradation stage at temperature below 500 °C. This subsequently results in lower molecular collision rates compared to ASR.

Figure 2. (a) Mass loss and Gaussian fit of DTG curves of ASR/TiO2 mixture obtained at heating rates of 5, 10, 15, and 20 °C min−1 under nitrogen purge of 20 mL min−1, (b) Coats−Redfern plots of ASR/ TiO2 mixture at different heating rates under nitrogen purge of 20 mL min−1, and (c) DHF curves of pure TiO2 and ASR/TiO2 at different heating rates.

mixture show only one degradation step, similar to that of ASR, small variations can be noticed in the Tonset, Tendset, and Tmax. These variations can be generally attributed to the physiochemical interactions between TiO2 and ASR. To indicate the catalytic interaction between the two components, a Coats−Redfern model was applied to extract the non5442


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ACS Sustainable Chemistry & Engineering Table 1. Thermal Degradation Kinetics of ASR/TiO2 Heating rate (°C/min) ASR30

ASR/TiO2 mixture Coats−Redfern, when n = 1 Slope Intercept Adj. R2 Ea (kJ mol−1) ln A (min−1) When n ≠ 1 Slope Intercept Adj. R2 Ea (kJ mol−1) ln A (min−1)

5

10

15

20

5

10

15

20

−10614.5 1.55 0.98891 88.25 12.42

−10487.2 1.18 0.98687 87.19 12.05

−10477.9 0.92 0.98206 87.11 11.79

−10364.4 0.66 0.98356 86.17 11.51

−12693.8 4.63 0.99938 105.53 15.69

−13080.0 4.87 0.99883 108.74 16.64

−14279.1 6.18 0.99909 118.71 11.54

−14694.9 7.08 0.99897 122.17 12.75

−8687.6 −2.48 0.99069 72.23 8.20

−8580.1 −2.26 0.99353 71.33 8.40

−8584.9 −2.14 0.98923 71.37 8.52

−8702.0 −1.98 0.99121 72.35 8.70

−12235.0 0.68 0.99932 101.72 11.70

−14133.2 3.23 0.99938 117.50 15.08

−14052.5 −0.15 0.99926 116.83 12.11

−14195.9 1.19 0.99928 118.02 13.75

Derivative heat flow (DHF) curves for the ASR/TiO2 mixture using different heating profiles and for TiO2 alone, Figure 2c, were used to illustrate the physical transition of reactants and chemical reactions that occur as a function of temperature and time. The DHF curve of TiO2 shows only two peaks; the first large and wide exothermic peak between 500 and 1000 °C is related to the phase transition of TiO2, which is postulated to be the transition from anatase to rutile that has smaller unit cell parameters (a = b = 4.584 Å and c = 2.953 Å). This process of phase transition from a phase of large unit cell parameters to another phase of smaller parameters yields energy and results in an exothermic peak in the DHF curve. The following endothermic peak that starts to appear at 1000 °C is postulated to be related to the crystal growth of TiO2.32 DHF curves of ASR/TiO2 mixture show an endothermic peak followed by an exothermic one in the main degradation stage. These two peaks indicate the polycondensation reactions of ASR polymers where they first decompose (endothermic process) forming radicals and intermediates that subsequently react with each other in several ways (i.e., cyclization and aromatization reactions, exothermic processes). Other two endothermic and exothermic peaks can be observed at around 600 °C. These two peaks are related to the dealkylation (endothermic) and polycondensation (exothermic) of the polycyclic aromatic hydrocarbons (PAHs) that form graphene-like nanosheets and eventually graphite-like structure. The wide and large exothermic peak noticed in all DHF curves of ASR/TiO2 mixture at temperature above 980 °C result from the reduction of TiO2 with the residual carbon. According to previous studies, the lowest temperature for the reduction of TiO2 using various carbon sources was reported around 1180 °C.33 However, this value was determined under isothermal conditions using techniques with limited accuracy. The volatiles evolved during the decomposition of ASR and ASR/TiO2 at nonisothermal conditions were characterized by FTIR gas analyzer. The FTIR spectra of volatiles as a function of temperature are given in Figure 3. The degradation of ASR results in several IR bands that become sharp in the main degradation stage between 300 and 500 °C. The largest band at 2928.5 cm−1 is assigned to the CH of alkyls in alkylated aromatics, whereas the small band at 3017.1 cm−1 is related to the =CH stretch in the phenyl moiety or benzene rings of these alkylated, or substituted, aromatics. The high transmittance difference between these two bands suggests that the main hydrocarbon evolved at this temperature is composed of

Figure 3. FTIR gas analysis of (a) ASR and (b) ASR/TiO2 mixture at nonisothermal conditions.

simple aromatic compounds (e.g., benzene) that are alkylated with long chain of aliphatic compounds and/or several simple aryls (e.g., methyl and ethyl moieties). The other two bands at 1512.8 and 1453.9 cm−1 are respectively assigned to the ring stretch (CC stretch) in aromatics and to the symmetrical stretching of −CH2− and antisymmetrical stretching of −CH3 in alkyl moieties, further suggesting the substituted-aromatic characteristics of ASR volatile. The band between 2300 and 5443


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ACS Sustainable Chemistry & Engineering 2370 cm−1 is related to OCO asymmetrical stretch of CO2 gas due the presence of oxygen containing functional groups such as alkoxy and carbonyl groups. As shown in Figure 3a, the CO2 gas continues to evolve as temperature increases and it becomes significant after 1300 °C. Another small IR band (1300 cm−1) starts to appear at around 650 °C and continues onward; this band is related to the methane gas which is postulated to evolve due to the carbonization of the stable methylated-polycyclic aromatic hydrocarbons (PAHs) through aryl−aryl bridging mechanism. The FTIR spectrum of ASR/ TiO2 shows similar bands to those of ASR. However, a few distinguishing features can be noticed; as can be observed in Figure 3b, a continuous evolution of aromatics can be noticed between 400 and 1300 °C, which indicates that TiO2 also acts as a catalyst to break the stable PAHs into smaller and less stable aromatic hydrocarbons at higher conversion levels, which also justifies the low carbon content in the remaining residues (i.e., TiN ceramic) reported in a previous work.29 One of the most interesting observations is that the CO2 gas does not seem to be evolved during the thermal degradation of ASR/TiO2 as its FTIR spectrum does not show any peaks related to CO2. This phenomenon is attributed to the evolution of CO2 and its simultaneous catalytic reduction by TiO2. TiO2 is known for its catalytic performance, and it has been widely used as a catalyst for many applications. The CO2 is suggested to transform into several products such as CO, C, CH4, and CH3OH according to eqs 4−9. TiO2 is an intrinsic semiconductor that has similar characteristics of n-type semiconductors;34 when it receives external energy in the form of heat or light, electrons (e−) are excited and expelled from the valence band to the conduction band of TiO2, creating free-moving electrons and vacancies (i.e., holes, h+) in the structure of TiO2. The extracted electrons react with CO2 to produce O−C•−O− intermediate, that subsequently reacts with the intermediate H•+, which is a common intermediate of polymers thermal degradation, to form CO gas. The CO gas can also undergo further catalytic reduction to form carbons according to eq 7. Other subsequent reactions are postulated to occur and additional products such as methane and methanol can be formed as given in eqs 8 and 9. Heat

TiO2 ⎯⎯⎯→ e−(TiO2 ) + h+(TiO2 )

(4)

CO2 + e− → O−C·−O−

(5)

·

−

·+

O−C −O + H → CO + OH

·−

Figure 4. IR gas analysis of (a) ASR and (b) ASR/TiO2 mixture at isothermal conditions.

formation of reasonable concentration of CO2 and its simultaneous evolution with CO suggest that the Boudouard reaction is less likely to be the dominant cause of CO formation. As shown in Figure 4b, the ASR/TiO2 results in CO that continues to evolve for 35 min due to the reduction of TiO2 whereas low concentrations of CH4 and almost negligible concentrations of CO2 were evolved. The low evolution of CO2 is consistent with the results indicated from the FTIR gas analysis at nonisothermal conditions, and further indicates the catalytic reduction of CO2 by TiO2.

(7)

C + 4H·+ → CH4

(8)

C + 3H·+ + OH·− → CH3OH

(9)

(10)

2M′O2 + C → M′2 O3 + CO

(11)

M′2 O3 + 5C → 2M′C + 3CO

(12)

M′2 O3 + N2 + 3C → 2M′N + 3CO

(13)

M′O2 + 2CH4 → M′C + CO2 + 4H 2

(14)

M′2 O3 + 5CH4 → 2M′C + 3CO + 10H 2

(15)

M′2 O3 + N2 + CH4 → 2M′N + CO + 2H 2O

(16)

35,36

where M′ is a metal. The evolution of CO2 during thermal processes is not favorable as it provides oxidizing conditions for the formation of high-level toxins such as dioxins and furans, especially when the combusted material contains halogenated-substances. It is worth mentioning that the CO gas can be also harmful when it is generated at low yield and released to the atmosphere. However, CO gas has a high market value and it is important for a wide range of industrial applications.37 In our process, the high yield of CO can make its trapping for commercial purposes a possible and profitable approach. The oils resulting from the isothermal treatment of ASR and ASR/TiO2 at 1550 °C were trapped in different types of solvents; acetone, DCM, THF, and n-hexane. The collected oils exhibit several different colors; pale yellow, orange, red, and black as shown in Figure 5. The ASR was generally observed to yield higher content of oil compared to ASR/TiO2. The low yield of oils for ASR/TiO2 is due to the consumption of these oils by CO to produce carbons as will be explained later on. To observe the chemical variations of the collected oils, their GC−

(6)

CO + H·+ + e− → C + OH·−

M′O2 + 2C → M′C + CO2

To confirm the CO2 catalytic reduction, ASR and ASR/TiO2 were heat-treated again at isothermal conditions at 1550 °C and the evolution of three noncondensable gases (CO, CO2, and CH4) was studied. As shown in Figure 4a, ASR yields high concentration of CO due to the presence of trace levels of oxide minerals (ash; e.g., TiO2 and SiO2) that undergo specific reactions according to eqs 10−16). The low concentration of CH4 is due to its consumption by the reduction of oxide minerals to form CO. The presence of oxygen-containing group in ASR plastics can also contribute to the formation of CO and CO2. Although CO2 at high temperature tends to get reduced by carbon to form CO according to Boudouard reaction, the 5444


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Figure 5. General appearance of collected oils from ASR and ASR/TiO2 mixture at isothermal conditions. Oil#1, n-hexane from ASR/TiO2; Oil#2, acetone from ASR/TiO2; Oil#3, DCM from ASR/TiO2; Oil#4, THF from ASR/TiO2; Oil#5, n-Hexane from ASR; Oil#6, acetone from ASR; Oil#7, DCM from ASR; Oil#8, THF from ASR.

(Supporting Information). The calculated percentages suggest that styrene is the dominant hydrocarbon in the trapped oils of both ASR and ASR/TiO2. Toluene and naphthalene are present in most trapping solvents of ASR with almost similar percentages in the range between 10 and 15%. Acenaphthylene and phenanthrene percentages are also similar in the trapping solvents of both ASR and ASR/TiO2 in the range between 3 and 7%. However, it can be noticed that the ASR/TiO2 tends to result in comparatively more naphthalene and indene, and less toluene. The presence of persistent organic pollutants (POPs) such as dioxins and furans are not evident in all samples. ASR is postulated to not result in such compounds as its content of halogenated compounds is very low, and the procedure under which the oils were collected does not provide oxidizing gases (e.g., O2 and CO2) that could lead, when combined with halogenated compounds, to the formation of POPs.38,39 After the isothermal treatment of ASR/TiO2, significant quantities of black powder (∼60 to 110 mg per run) were found residing in the cold zone of the furnace (i.e., mouth of the furnace). The diffraction pattern of this powder, Figure 7a, shows a typical pattern of pure graphitic carbon; a large peak at around (2θ) 24.8° related to the diffraction from the 002 plane of graphite and other two smaller peaks at 44.2° and 81.6° of 101 and 112 planes of graphite, respectively. The peaks centered at 1358 cm−1 (D peak) and 1584 cm−1 (G peak) in Figure 7a are in accordance with the two distinct Raman modes of graphitic structure, which confirms the formation of graphitic carbon. The N2 adsorption/desorption isotherm, Figure 7b, shows a slow increase in the adsorption volume in the first half of the convex curve, and a subsequent increase after a relative pressure (P/P°) of 0.7. This type of hysteresis loops (H1 type) is typically associated with uniformly arranged spherical particles. The calculated value of BET surface area of these particles was relatively high around 84.1 m2 g−1; such value indicates that the carbon spheres have a very small particle size. As displayed in Figure 7c, the Barrett−Joyner−Halenda (BJH) pore size distribution shows a dominant pore diameter of approximately 5 nm. The FE-SEM image, Figure 7d, reveals nanoparticles with around 70 to 130 nm particle size, which is in correspondence with the N2 physisorption analysis. The TEM image, Figure 7e, shows regular-shaped spherical nanoparticles of similar size to that observed by FE-SEM and even smaller around 40 nm size.

MS chromatograms were obtained, Figure 6a. Although oils show different colors, their contents are qualitatively similar.

Figure 6. (a) GC−MS chromatograms of oils collected from ASR and ASR/TiO2 mixture at isothermal conditions. It also shows some of the hydrocarbons that were identified. More hydrocarbons and their fractions are given in Table S1 (Supporting Information); (b) percentages of dominant hydrocarbons in the collected oils.

The dominant PAHs in both ASR and ASR/TiO2 are styrene, naphthalene, toluene, phenanthrene, indene, and acenaphthylene. The percentages of these hydrocarbons in the different solvents were calculated from the GC−MS chromatograms, and the results are presented in Figure 6b. All other detected hydrocarbons and their percentages are given in Table S1 5445


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Figure 7. Structural characterization of the materials found in the cold zone of the furnace. (a) XRD and Raman patterns showing typical features of graphitic carbon, (b) N2 adsorption/desorption isotherm, (c) BJH pore size distribution, (d) FE-SEM, (e) low magnification TEM, and (f) high magnification TEM images of the resulting carbon. Inset of panel f shows the SAED pattern of the resulting carbon nanoparticles.

Figure 8. Suggested mechanism for the formation of OLC-NPs from the gas phase during the catalytic conversion of ASR by TiO2; through a simultaneous formation of thermally stable molecules and their carbonization on the surface of carbon nuclei.

the cold zone of the furnace near the outlet (the point of least temperature and turbulence). The carbonization of gas phase precursors to OLC requires several factors to occur: first, presence of small particulates to act as nuclei; second, oils (i.e., PAHs) should turn into molecules of a relatively higher thermal stability; third, heat for further cyclization and aromatization on the surface of the nuclei. The suggested mechanism for the formation of OLCNPS from the gas phase is schematically presented in Figure 8. Carbon nuclei are suggested to form first from the catalytic reduction of carbon monoxide according to eq 7. Because

The carbon structure or interlayer spacing orientation was further studied by TEM. The TEM image at high magnification, Figure 7f, shows a concentric orientation for the interlayer spacing of graphite (i.e., onion-like structure), which indicates that these particles are composed of fullerene-like carbon spheres stacking concentrically. Because the OLC-NPs were found in the cold zone of the furnace during the treatment of ASR/TiO2, it is suggested that they form from a gas phase intermediate (particularly PAHs). The relatively lower yield of oil for ASR/TiO2 also supports this conclusion. The resulting OLC-NPs are carried away with the gas flow and precipitate in 5446



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PAHs (e.g., styrene) and CO are dominant in the gas phase, the formation of stable molecules is postulated to be a product of reaction between them. These two components with the presence of a catalyst tend to form aromatic polyketones.40 Aromatic polyketones form matrices of relatively more stable molecules that undergo simultaneous polymerization and carbonization on the surface of the carbon nuclei, eventually resulting in OLC-NPs. The formation mechanism of OLC-NPs is far more complex and involves many reactants. However, in Figure 8, styrene and its derivative 3-phenyl-2-pentanone were taken as examples of PAHs and aromatic polyketones, respectively.

CONCLUSION Based on the calculated thermal degradation kinetics, it was confirmed that the TiO2 interacts chemically and physically with ASR at elevated temperatures; the addition of TiO2 to ASR catalyzes its degradation and facilitates the heat-transfer within the solid residues while it decomposes. At temperature above 980 °C, residual carbons of ASR with TiO2 undergo carbothermic reduction reactions resulting in significant yield of CO. The TiO2 was also found to react with the gas phase resulting from the decomposition of ASR; it limits the formation of CO2 gas, which can be harmful during the thermal processes as it oxidizes the halogenated compounds and forms dioxins and furans. Several PAHs were dominant in both ASR and ASR/TiO2 such as styrene, naphthalene, toluene, phenanthrene, indene, and acenaphthylene. Although the TiO2 addition did not change the quality of oils, it was observed that the TiO2 results in lower quantities of oils, and alternatively, results in considerable quantities of OLC-NPs (40 to 130 nm particle size and ∼60 to 110 mg per run) that deposit at a specific point inside the furnace. The formation mechanism of OLC-NPs from the gas phase was suggested to be through several simultaneous stages of polymerization, nucleation, and carbonization. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00774. Table listing all hydrocarbons identified by GC/MS analysis and their contents in the oil samples collected from the pyrolysis of ASR and ASR/TiO2 mixture (PDF)

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REFERENCES

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Research Article

AUTHOR INFORMATION

Corresponding Author

*Tel: +61 (2) 9385 4433. E-mail: mohanad.mayas@yahoo. com, [email protected] (Mohannad Mayyas). ORCID

Mohannad Mayyas: 0000-0003-1852-6687 Samane Maroufi: 0000-0001-5553-8519 Zhao Liu: 0000-0002-6697-855X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported under Australian Research Council’s Industrial Transformation Research Hub funding scheme (project IH130200025). 5447


Research Article


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