- Email: [email protected]
Experimental evidence for the sintering of primary soot particles
Journal of Aerosol Science 105 (2017) 1–9
Contents lists available at ScienceDirect
Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci
Experimental evidence for the sintering of primary soot particles a,b,⁎
a
a,b
a
Kiminori Ono , Kazuki Dewa , Yoshiya Matsukawa , Yasuhiro Saito , Yohsuke Matsushitaa, Hideyuki Aokia, Koki Eraa,c, Takayuki Aokic, Togo Yamaguchic
MARK
a
Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Miyagi , Aoba-ku, Sendai 9808579, Japan JSPS, Japan c ASAHI CARBON CO., LTD., 2 Kamomejima-cho, Higashi-ku, Niigata 950-0883, Japan b
AR TI CLE I NF O
AB S T R A CT
Keywords: Soot Sintering Surface growth Aggregate Tandem differential mobility analysis Aerosol
Sintering behavior and the contribution of soot produced in the absence of oxygen are experimentally investigated. A tandem differential mobility analyzer technique equipped with thermophoretic sampling was used to reveal the change in the mobility diameter of soot aggregates, which is proportional to the surface area. The mobility diameter decreased with an increase in reheating temperature. The decrease in mobility diameter clearly shows a decrease in the aggregate surface area. SEM images also clearly showed the decrease in the surface area and the simplification after heat treatment. These are experimental evidence that sintering of carbonaceous particle occurs in the absence of oxygen. Acetylene, which is considered to promote surface growth, is also added to size-selected soot particles before heat treatment to investigate the effect of surface growth on simplification of aggregate structure. The addition of acetylene, which is relevant to the surface growth via the hydrogen-abstraction/carbon-addition (HACA) mechanism, did not affect the mean mobility diameter at low temperature around 1200 K. These results showed that simplification of soot aggregates would not be affected by surface growth caused via the HACA mechanism.
1. Introduction Soot is a nanoparticle produced via the formation of polycyclic aromatic hydrocarbons (PAHs), and ultrafine soot particles affects health since they can penetrate the respiratory system more deeply than larger particles (D'Anna, 2009). Although soot is required to be reduced, carbon black, soot produced industrially, has been widely used as an important material in automobile tires, battery electrodes, and pigments in toners for laser printers. When soot is used as a material, the control over morphological features such as primary particle diameter and aggregate structure is important. Nevertheless, the techniques used to control soot morphology are based on trial and error, and the fundamental aspects of these processes are not well understood (Xiong & Pratsinis, 1991). Therefore, to reduce soot formation and control the morphology of carbon black, the formation mechanism of soot needs to be comprehensively understood. Although several soot formation mechanisms have been proposed (D'Anna, 2009; Frenklach, 2002a; Richter & Howard, 2000; Wang, 2011), there is considerable agreement on the general features of the processes involved; these processes are summarized as follows. PAHs are produced by pyrolysis of hydrocarbons, which are mainly produced by the incomplete combustion of fossil fuels.
⁎ Corresponding author at: Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Miyagi , Aoba-ku, Sendai 980-8579, Japan. Fax: +81 22 795 6165. E-mail address: [email protected] (K. Ono).
http://dx.doi.org/10.1016/j.jaerosci.2016.11.013 Received 9 April 2016; Received in revised form 18 October 2016; Accepted 25 November 2016 Available online 27 November 2016 0021-8502/ © 2016 Elsevier Ltd. All rights reserved.
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
The molecular precursors of soot particles are thought to be heavy PAHs with molecular weights of 500–1000 amu (Richter & Howard, 2000). The hydrogen-abstraction/carbon-addition (HACA) mechanism captures the essence of the thermodynamic and kinetic requirements for the formation of PAHs(Frenklach, 2002a). During the mass growth process, these particles collide to produce larger spherical particles, which then aggregate into the final carbon black clusters. The particles are converted into amorphous carbon and a progressively more graphitic material in the furnace. Our recent experimental studies revealed the morphological changes in carbon black that occur with changes in temperature, residence time, and feedstock composition (Ono et al., 2012, 2013). In addition, a detailed kinetic analysis of these experimental conditions indicated that the nuclei concentration and nucleation rate strongly affect the morphology of carbon black (Ono et al., 2014a, 2014b). Our experimental results showed that the aggregate structure becomes simple one with less surface asperity with increasing residence time (Ono et al., 2012). Surface growth has proposed as the main factor responsible for this simplification and increasing particle sphericity (Balthasar & Frenklach, 2005; Morgan et al., 2007). Morgan, Patterson, and Kraft (2008) claimed that sintering, which is important for inorganic materials, is unlikely to be relevant to soot formation. Some characteristic sintering time models for inorganic materials such as Cu (Chepkasov, Gafner, & Gafner, 2016), TiO2 or SiO2 (Goudeli, Eggersdorfer, & Pratsinis, 2015; Hao, Zhao, Xu, & Zheng, 2015) have been developed continuously, some sintering model for soot have been proposed (Chen, Totton, Akroyd, Mosbach, & Kraft, 2014; Chen et al., 2013; Shishido et al., 2007). In contrast to inorganic materials, no reports of experimental evidence for the sintering of soot primary particles are found in the literature, although recent models of soot formation have included sintering. In addition, the contribution of surface growth and sintering to the simplification of soot aggregates is still not well understood, either experimentally or theoretically. Several studies on the experimental investigation of the sintering behavior of inorganic and metal nanoparticles have been reported previously. Tandem differential mobility analyzer (TDMA) is a powerful tool that has been used to investigate the sintering behavior and rate of silver (Shimada, Seto, & Okuyama, 1994), titania (Nakaso et al., 2001; Seto, Hirota, Fujimoto, Shimada, & Okuyama, 1997; Seto, Shimada, & Okuyama, 1995), silica (Seto et al., 1997), and gold (Nakaso, Shimada, Okuyama, & Deppert, 2002) nanoparticles. TDMA has also been employed to reveal the oxidation behavior of soot (Higgins, Jung, Kittelson, Roberts, & Zachariah, 2001; Ma, Zangmeister, & Zachariah, 2013). Ma et al. recently introduced a new tandem ion-mobility method (differential mobility analyzer-aerosol particle mass (DMA-APM) analyzer) to study the size-resolved oxidation kinetics of freshly generated flame soot (Ma et al., 2013). Herein, sintering behavior and the contribution of soot produced in the absence of oxygen are experimentally investigated. Accordingly, we introduce size-selected soot particles produced by the pyrolysis of hydrocarbons in the first electric furnace in the absence of oxygen using DMA on the basis of a measurement of its equivalent mobility diameter to the second furnace, allowing the investigation of primary particle sintering from the differences in mobility diameter. In addition, acetylene, which is considered to play an important role in surface growth, is introduced to the gas including monodisperse soot aggregates. 2. Experimental The experimental setup (Fig. 1) shows a laminar flow reactor equipped with an electric furnace, a probe sampling system equipped with the first DMA, a second electric furnace, a scanning mobility particle sizer (SMPS) consisting of the second DMA, and an ultrafine condensation particle counter (UCPC). Soot was generated by the thermal pyrolysis of ethylene or benzene in an alumina tube (ϕ=11 mm and length=640 mm) that is heated using an electric furnace from 1500 to 1800 K. Liquid benzene was introduced via a syringe pump into the nitrogen flow (TAIYO NIPPON SANSO, 99.9999%) and subsequently into the reaction tube through mass flow controllers at a flow rate of 3.0 NL/min. The benzene concentration was 0.14 vol% to prevent the formation of coke in the tube. The ethylene concentration was 1.0 vol% and the flow rate was 2.0 NL/min. The condition is summarized in Table 1. The gas, including soot particles, was drawn into a sampling probe that was horizontally connected to the end of the reactor. The
Fig. 1. Schematic diagram of the experimental apparatus used for sintering by the TDMA method.
2
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Table 1 Experimental condition of sample preparation. Sample
Feedstock
Temperature [K]
Concentration [vol%]
Benzene-1500 K Benzene-1700 K Ethylene-1600 K Ethylene-1800 K
Benzene Benzene Ethylene Ethylene
1500 1700 1600 1800
0.14 0.14 1.0 1.0
dilution probe used during these experiments was based on the design of Zhao et al. (Zhao et al., 2003; Zhao, Yang, Li, Johnston, & Wang, 2005; Zhao, Yang, Wang, Johnston, & Wang, 2003). The probe was made of a 1/4-inch (OD) stainless steel tube that was horizontally connected to a 1/2-inch (OD) stainless steel tube. The sample gas entered the probe through a 4.0-mm-diameter orifice and was immediately diluted in the probe by nitrogen flowing at 30–39 NL/min. The dilution ratio was typically set to be greater than 104 to prevent the coagulation of products in the probe (Zhao, Yang, Wang, Johnston & Wang, 2003). Sampling from centerline of the reactor allows the radial distribution of temperature and the residence time to be neglected. The polydisperse soot particles were first sent through a neutralizer to establish a known equilibrium charge distribution and then passed to the first DMA for size selection. The sample and sheath flows through the DMA were 1.5 and 15 L/min, respectively. The mobility diameters used in the sintering studies were 50, 100, 140, and 180 nm. The mobility diameter distribution was previously examined using the SMPS system, and it was found to be monodisperse. The size-selected soot particles were sent to an alumina tube (ϕ=11 mm and length = 640 mm) heated and 1000−1700 K with a nominal residence time of approximately 200 ms at an aerosol flow rate of 1.5 L/min at room temperature. The reheated soot particles were sent to the SMPS system for measuring the mean mobility diameter of sintered soot. A TSI model 3938 SMPS was used to analyze the size distributions of nascent soot particles in the flow reactor. The SMPS system comprises a diffusion charger (Kr-85 Bipolar), a long-DMA (TSI 3081 A), and an ultrafine condensation particle counter (UCPC, TSI 3776). TDMA experiments were conducted to measure the change in particle size resulting from the sintering of soot. In a TDMA setup, the first DMA was used for particle size selection, and the second DMA was used to measure the size change of the monodisperse particles resulting from a physical or chemical transformation. The size-selected soot particles and reheated sample were collected on the grid by thermophoretic sampling, mostly by direct sampling from the flame or the reactor using the probe (Abid et al., 2008; Manzello et al., 2007; Schenk et al., 2015). Aggregates classified by the DMA or reheated samples were collected on a collodion-coated grid (Rogak, Flagan, & Nguyen, 1993) mounted on a thermophoretic sampler positioned between DMA and UCPC. The sampler comprised a flexible heater, a nozzle to direct the hot aerosol over the microscope grid, and a copper rod to keep the grid cool. The collected samples were imaged using a Hitachi HighTechnologies S5500 scanning electron microscope (SEM) operated at 3.0 kV. The samples were collected for approximately 30 min because they were further diluted in DMA. 3. Results and discussion The change in mobility diameter with heat treatment provides the sintering behavior. Mobility diameter is pointed out to be proportional to the surface area of an aggregate at some level when the specific conditions are met, such as the fractal dimension of aggregates and the mobility diameter ranges (Cho, Hogan, & Biswas, 2007; Ku & Kulkarni, 2012); therefore, the change in mobility diameter indicates a change in the structure of an aggregate if the mass of the aggregate is not changed by surface growth or oxidation. Fig. 2 shows the change in the mobility diameter of reheated soot that was originally formed by ethylene pyrolysis at 1800 K. In all cases, mobility diameter decreased with an increase in reheating temperature. Considering the inert atmosphere, the decrease in mobility diameter clearly shows a decrease in the aggregate surface area. The mobility diameter began to decrease at 1200 K, and the steep decrease showed from 1500 to 1700 K. This indicates that sintering would be promoted above 1500 K. Fig. 3 shows SEM and scanning transmission electron microscope (STEM) images of soot aggregates before and after heat treatment. The
Fig. 2. Change in the mean mobility diameter of reheated soot aggregates produced by ethylene pyrolysis at 1800 K.
3
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Fig. 3. SEM and STEM images: (a) SEM image of soot aggregate produced by ethylene pyrolysis at 1800 K with a mobility diameter of 180 nm before heat treatment; (b) SEM image of soot aggregate with a mobility diameter of 180 nm after heat treatment at 1500 K; (c) SEM image of soot aggregate with a mobility diameter of 180 nm after heat treatment at 1700 K; (d) SEM image of soot aggregate produced by ethylene pyrolysis at 1800 K with a mobility diameter of 100 nm before heat treatment; and (e) STEM image of soot aggregate with a mobility diameter of 100 nm after heat treatment at 1700 K.
soot aggregate that had a mobility diameter of 180 nm before heat treatment exhibited an aggregate with primary particle diameter of approximately 20 nm [Fig. 3(a)]. In contrast, the soot aggregates heat treated at 1500 and 1700 K [Fig. 3(b) and (c), respectively] exhibited spherical aggregate with primary particle diameters of 20–50 nm. The soot aggregate with a mobility diameter of 100 nm before heat treatment [Fig. 3(d)], which consisted of a few dozen primary particles, also changed to spheroidal particles consisting of a few primary particles [Fig. 3(e)]. This study indicates that the sintering of carbon nanoparticles occurs regardless of the inert atmosphere. Soot properties such as crystallite size differ with the temperature profile or feedstocks (Ono et al., 2012; Santamaria, Yang, Eddings, & Mondragon, 2010). Fig. 4 shows the change in the mobility diameter of reheated soot that was originally formed by ethylene pyrolysis at 1600 K. The decrease in the mobility diameter was larger than that of ethylene pyrolysis at 1700 K as shown in Fig. 2. The steep decrease from 1200 to 1500 K would be due to the presence of aliphatic structure and low crystallinity of PAHs because the pyrolysis temperature was lower than that of the soot shown in Fig. 2. To elucidate the effect of feedstock on the
Fig. 4. Change in the mean mobility diameter of reheated soot aggregates produced using ethylene pyrolysis at 1600 K.
4
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Fig. 5. Change in the mean mobility diameter of reheated soot aggregates produced using benzene pyrolysis at 1700 K.
Fig. 6. Change in the mean mobility diameter of reheated soot aggregates produced using benzene pyrolysis at 1500 K.
morphological change with heat treatment, benzene was used as a feedstock. Figs. 5 and 6 show the change in the mobility diameter of reheated soot that was originally formed by benzene pyrolysis at 1700 K and 1500 K. Although the mobility diameter decreased with an increase in reheating temperature as is the case with ethylene pyrolysis, the decrease in the mobility diameter was smaller than that of ethylene pyrolysis. To explain effects of non-reheated soot properties on sintering behavior, the change in the mean mobility diameter of the sample heat treated at 1700 K was examined. The mobility diameter of the non-heated sample was 180 nm. As shown in Table 2, the mean primary particle diameter of non-heat-treated samples with mobility diameters of 180 nm increased in the order of ethylene-1800 K > benzene-1700 K > ethylene-1600 K > benzene-1500 K. Fig. 7 shows the primary particle diameter size distribution of non-heat treated aggregates. The distribution is increased at low temperatures regardless of feedstock. As shown in Table 3, the mean mobility diameter decreases in the order of benzene-1500 K > benzene-1700 K > ethylene-1800 K > ethylene-1600 K, indicating that sintering rate increases in that order. Ostwald ripening, which is the phenomenon by which small particles dissolve into larger particles, will also occur. Considering that the sintering rate was low for benzene-1500 K, which had a large primary particle distribution, the effect of Ostwald ripening would be small for the sintering of soot particles. The characteristic sintering time, τ, is expressed as follows:
τ =Ad pm exp(E / Rg T ),
(1)
where A is a constant, dp is the primary particle diameter, E is the activation energy for self-diffusion, Rg is the gas constant, and T is the temperature. Therefore, the sintering rate should increase with a decrease in primary particle diameter. However, considering that the sintering rate was the highest in the case of ethylene-1600 K, which had a relatively large primary particle diameter, the effect of primary particle diameter on the sintering of soot particles would be small. The most plausible parameters that affect the sintering of soot are the compositions and crystallinities of primary particle surfaces and interiors. Fig. 8 shows the results for the nonheat treated sample analyzed using gas chromatography mass spectrometry (GCMS) equipped with a pyrolyzer. The analytical Table 2 Mean primary particle diameter of non-heat-treated samples with mobility diameters of 180 nm with standard deviation. Sample
Mean primary particle diameter [nm]
Benzene-1500 K Benzene-1700 K Ethylene-1600 K Ethylene-1800 K
30.6 ± 1.6 26.1 ± 0.5 28.6 ± 1.1 16.9 ± 0.4
5
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Fig. 7. Size distribution of aggregate primary particle diameter for a mobility diameter of 180 nm (non-heat treated): (a) ethylene-1800 K, (b) ethylene-1600 K, (c) benzene-1700 K, and (d) benzene-1500 K.
conditions and chemical composition are summarized in Tables S-1 and S-2 in Supporting information. The chemical composition of surface soot deposits produced using ethylene pyrolysis showed aliphatic hydrocarbons with relatively high molecular weights such as decane and undecane. In contrast, the soot surface deposits produced by benzene pyrolysis did not produce an aliphatic peak; instead, their chemical composition was dominated by high-molecular-weight PAHs. These analytical results strongly suggest that the sintering rate of soot is affected by the aliphatic hydrocarbons and low-molecular-weight PAHs on the soot surface deposits, which lead to a decrease in the melting point of the soot particles. The micro-Fourier transform infrared spectroscopy results along with TEM and atomic force microscopy images suggested that a large amount of aliphatic compounds was present in nascent soot formed in the ethylene–argon–oxygen flame; in addition, the nascent soot was liquid-like and far from carbonized (Barone, D’Alessio, & D’Anna, 2003; Cain, Camacho, Phares, Wang, & Laskin, 2011; Cain, Gassman, Wang, & Laskin, 2010; Dobbins, 2007; Dobbins, Fletcher, & Lu, 1995). These previous results support the findings and theory presented in this study. Surface growth includes chemical growth and physisorption of PAHs (Morgan et al., 2007). The chemical growth was often modeled as the HACA mechanism (Frenklach, 2002b), and the condensation of PAHs is often modeled as the condensation of pyrene (Appel, Bockhorn, & Frenklach, 2000; Morgan et al., 2007). Acetylene is the key role for the surface growth for the addition of carbon source to the armchair edge of PAHs (Frenklach, 1996). To examine the effect of surface growth on the morphological change of soot, acetylene was added to size-selected soot particles before heat treatment on the surface growth of soot particles. Table 4 shows the mean mobility diameters of the samples with mobility diameters of 140 nm reheated at 1000 and 1200 K with and without 1.0 vol% acetylene. The mean mobility diameters decreased from the size-selected diameter of 140 nm after reheating. The p value given by Welch's t-test showed the mean diameter was almost equivalent at 1000 K. We can close discussion at 1000 K from this statistical analysis. On the other hand, we cannot judge the statistical difference of the mean mobility diameter at 1200 K was not enough or not enough at the p=0.05 level, and the mean mobility diameter at 1200 K slightly increased. As we cannot conclude that there is no difference at 1200 K is statistically, we discuss this change for confirmation. This slight increase was probably due to surface growth or the condensation of nascent soot because the temperature of soot production was reached. The surface growth must
Table 3 Mean mobility diameter after heat treatment at 1700 K with standard deviation. Sample
Mean mobility diameter [nm]
Benzene-1500 K Benzene-1700 K Ethylene-1600 K Ethylene-1800 K
145.7 ± 3.9 138.4 ± 3.1 123.8 ± 1.6 135.4 ± 7.0
6
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Fig. 8. Total ion chromatogram showing the chemical composition of soot surface deposits (non-heat treated) obtained using GC–MS equipped with a pyrolyzer: (a) soot produced by ethylene pyrolysis at 1800 K; and (b) soot produced by benzene pyrolysis at 1700 K (1 carbon dioxide, 2 benzene, 3 toluene, 4 decane, 5 undecane, 6 naphthalene, 7 biphenyl, 8 biphenylene or acenaphthylene, 9 phenanthrene, 10 anthracene, 11 2-phenylnaphthalene, 12 fluoranthene, 13 benzene,1,1′-(1,3butadiyne-1,4-diyl)bis-, 14 pyrene, 15 triphenylene or benzo[c]phenanthrene or benzo[a]anthracene, 16 cyclopenta[cd]pyrene).
entail mass growth, if the aggregate structure became spheroidal in shape because of the surface growth (Balthasar & Frenklach, 2005; Morgan et al., 2007), the mobility diameter would decrease further due to the addition of acetylene. Morgan et al. (2008) insisted that the amount of surface growth required to build the two newly joined particles into one larger, approximately spherical particle would be reduced. If surface growth drives aggregate structure into spherical particle, the mobility diameter would decrease by the addition of acetylene to size-selected soot particles. However, the mobility diameter slightly increased at 1200 K. It suggests that surface growth would not affect the simplification of soot aggregates. Instead, sintering is important role for the morphological change of soot. Recent theoretical investigation suggested that sintering is the dominant mechanism for particle rounding compared to surface reaction because the simple surface growth model overpredicted the surface area (Chen et al., 2013). The modeling study strongly supported our experimental results.
4. Conclusion In this paper, we introduced size-selected soot particles using DMA, which are produced by pyrolysis of hydrocarbons in the absence of oxygen, to second furnace to elucidate sintering behavior and the contribution of soot. A tandem differential mobility analyzer technique equipped with thermophoretic sampling provided the evidence that the mobility diameter, which is proportional to the surface area, decreased in an inert atmosphere by reheating process. The addition of acetylene, which is considered to promote surface growth via the HACA mechanism, to size-selected soot particles before heat treatment would not affect the morphological change of the structure by surface growth via the HACA mechanism. Although the previous study (Morgan et al., 2008) claimed that sintering is unlikely to be relevant to soot formation due to the carbonaceous properties, this experimental study clearly showed that sintering of carbonaceous particles occurs in an inert atmosphere. While surface growth has proposed as the main factor responsible for this simplification and increasing particle sphericity, this experimental study showed that simplification of soot aggregate would be affected by sintering rather than surface growth. These new findings will shed light on the modeling study the growth of soot aggregate structure, and would be enhanced by the kinetic study. Table 4 Mean mobility diameter of the sample with a mobility diameter of 140 nm reheated at 1000 and 1200 K with and without 1.0 vol% acetylene with standard deviation and p Value by Welch's t-test.
Reheated at 1000 K Reheated at 1200 K
Without acetylene
With acetylene (1.0 vol%)
p Value
124.3 ± 3.0 nm 126. 7 ± 1.4 nm
123.3 ± 2.5 nm 129.8 ± 3.4 nm
0.96 0.33
7
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jaerosci.2016.11.013. References Abid, A. D., Heinz, N., Tolmachoff, E. D., Phares, D. J., Campbell, C. S., & Wang, H. (2008). On evolution of particle size distribution functions of incipient soot in premixed ethylene–oxygen–argon flames. Combustion and Flame, 154(4), 775–788. http://dx.doi.org/10.1016/j.combustflame.2008.06.009. Appel, J., Bockhorn, H., & Frenklach, M. (2000). Kinetic modeling of soot formation with detailed chemistry and physics: laminar premixed flames of C2 hydrocarbons. Combustion and Flame, 121(1–2), 122–136. http://dx.doi.org/10.1016/S0010-2180(99)00135-2. Balthasar, M., & Frenklach, M. (2005). Detailed kinetic modeling of soot aggregate formation in laminar premixed flames. Combustion and Flame, 140(1–2), 130–145. http://dx.doi.org/10.1016/j.combustflame.2004.11.004. Barone, A. C., D’Alessio, A., & D’Anna, A. (2003). Morphological characterization of the early process of soot formation by atomic force microscopy. Combustion and Flame, 132(1–2), 181–187. http://dx.doi.org/10.1016/S0010-2180(02)00434-0. Cain, J. P., Camacho, J., Phares, D. J., Wang, H., & Laskin, A. (2011). Evidence of aliphatics in nascent soot particles in premixed ethylene flames. Proceedings of the Combustion Institute, 33(1), 533–540. http://dx.doi.org/10.1016/j.proci.2010.06.164. Cain, J. P., Gassman, P. L., Wang, H., & Laskin, A. (2010). Micro-FTIR study of soot chemical composition-evidence of aliphatic hydrocarbons on nascent soot surfaces. Physical Chemistry Chemical Physics, 12(20), 5206–5218. http://dx.doi.org/10.1039/B924344E. Chen, D., Totton, T. S., Akroyd, J., Mosbach, S., & Kraft, M. (2014). Phase change of polycyclic aromatic hydrocarbon clusters by mass addition. Carbon, 77(0), 25–35. http://dx.doi.org/10.1016/j.carbon.2014.04.089. Chen, D., Zainuddin, Z., Yapp, E., Akroyd, J., Mosbach, S., & Kraft, M. (2013). A fully coupled simulation of PAH and soot growth with a population balance model. Proceedings of the Combustion Institute, 34(1), 1827–1835. http://dx.doi.org/10.1016/j.proci.2012.06.089. Chepkasov, I. V., Gafner, Y. Y., & Gafner, S. L. (2016). Changing of the shape and structure of Cu nanoclusters generated from a gas phase: MD simulations. Journal of Aerosol Science, 91, 33–42. http://dx.doi.org/10.1016/j.jaerosci.2015.09.004. Cho, K., Hogan, C. J., & Biswas, P. (2007). Study of the mobility, surface area, and sintering behavior of agglomerates in the transition regime by tandem differential mobility analysis. Journal of Nanoparticle Research, 9(6), 1003–1012. http://dx.doi.org/10.1007/s11051-007-9243-5. D'Anna, A. (2009). Combustion-formed nanoparticles. Proceedings of the Combustion Institute, 32(1), 593–613. http://dx.doi.org/10.1016/j.proci.2008.09.005. Dobbins, R. A. (2007). Hydrocarbon nanoparticles formed in flames and diesel engines. Aerosol Science and Technology, 41(5), 485–496. http://dx.doi.org/10.1080/02786820701225820. Dobbins, R. A., Fletcher, R. A., & Lu, W. (1995). Laser microprobe analysis of soot precursor particles and carbonaceous soot. Combustion and Flame, 100(1–2), 301–309. http://dx.doi.org/10.1016/0010-2180(94)00047-V. Frenklach, M. (1996). On surface growth mechanism of soot particles. Symposium (International) on Combustion, 26(2), 2285–2293. http://dx.doi.org/10.1016/S0082-0784(96)80056-7. Frenklach, M. (2002). Method of moments with interpolative closure. Chemical Engineering Science, 57(12), 2229–2239. Frenklach, M. (2002). Reaction mechanism of soot formation in flames. [10.1039/B110045A]. Physical Chemistry Chemical Physics, 4(11), 2028–2037. http://dx.doi.org/10.1039/B110045A. Goudeli, E., Eggersdorfer, M. L., & Pratsinis, S. E. (2015). Aggregate characteristics accounting for the evolving fractal-like structure during coagulation and sintering. Journal of Aerosol Science, 89, 58–68. http://dx.doi.org/10.1016/j.jaerosci.2015.06.008. Hao, X., Zhao, H., Xu, Z., & Zheng, C. (2015). Identification of the compensation effect in the characteristic sintering time model for population balances. Journal of Aerosol Science, 82, 1–12. http://dx.doi.org/10.1016/j.jaerosci.2014.12.005. Higgins, K. J., Jung, H., Kittelson, D. B., Roberts, J. T., & Zachariah, M. R. (2001). Size-selected nanoparticle chemistry: kinetics of soot oxidation. The Journal of Physical Chemistry A, 106(1), 96–103. http://dx.doi.org/10.1021/jp004466f. Ku, B. K., & Kulkarni, P. (2012). Comparison of diffusion charging and mobility-based methods for measurement of aerosol agglomerate surface area. Journal of Aerosol Science, 47, 100–110. http://dx.doi.org/10.1016/j.jaerosci.2012.01.002. Ma, X., Zangmeister, C. D., & Zachariah, M. R. (2013). Soot oxidation kinetics: a comparison study of two tandem ion-mobility methods. The Journal of Physical Chemistry C, 117(20), 10723–10729. http://dx.doi.org/10.1021/jp400477v. Manzello, S. L., Lenhert, D. B., Yozgatligil, A., Donovan, M. T., Mulholland, G. W., Zachariah, M. R., & Tsang, W. (2007). Soot particle size distributions in a well-stirred reactor/plug flow reactor. Proceedings of the Combustion Institute, 31(1), 675–683. Morgan, N., Kraft, M., Balthasar, M., Wong, D., Frenklach, M., & Mitchell, P. (2007). Numerical simulations of soot aggregation in premixed laminar flames. Proceedings of the Combustion Institute, 31(1), 693–700. http://dx.doi.org/10.1016/j.proci.2006.08.021. Morgan, N. M., Patterson, R. I. A., & Kraft, M. (2008). Modes of neck growth in nanoparticle aggregates. Combustion and Flame, 152(1–2), 272–275. Nakaso, K., Fujimoto, T., Seto, T., Shimada, M., Okuyama, K., & Lunden, M. M. (2001). Size distribution change of titania nano-particle agglomerates generated by gas phase reaction, agglomeration, and sintering. Aerosol Science and Technology, 35(5), 929–947. http://dx.doi.org/10.1080/02786820126857. Nakaso, K., Shimada, M., Okuyama, K., & Deppert, K. (2002). Evaluation of the change in the morphology of gold nanoparticles during sintering. Journal of Aerosol Science, 33(7), 1061–1074. http://dx.doi.org/10.1016/S0021-8502(02)00058-7. Ono, K., Watanabe, A., Dewa, K., Matsukawa, Y., Saito, Y., Matsushita, Y., & Yamaguchi, T. (2014). Detailed kinetic analysis of the effect of benzene–acetylene composition on the configuration of carbon nanoparticles. Chemical Engineering Journal, 250(0), 66–75. http://dx.doi.org/10.1016/j.cej.2014.03.091. Ono, K., Watanabe, A., Dewa, K., Matsukawa, Y., Saito, Y., Matsushita, Y., & Yamaguchi, T. (2014). Detailed kinetic analysis of the impact of nucleation behavior and particle size distribution on the configurations of carbon black. Journal of Nanoparticle Research, 16(7), 1–11. http://dx.doi.org/10.1007/s11051-014-2519-7. Ono, K., Yanaka, M., Saito, Y., Aoki, H., Fukuda, O., Aoki, T., & Yamaguchi, T. (2013). Effect of benzene–acetylene compositions on carbon black configurations produced by benzene pyrolysis. Chemical Engineering Journal, 215–216(15), 128–135. http://dx.doi.org/10.1016/j.cej.2012.10.085. Ono, K., Yanaka, M., Tanaka, S., Saito, Y., Aoki, H., Fukuda, O., & Yamaguchi, T. (2012). Influence of furnace temperature and residence time on configurations of carbon black. Chemical Engineering Journal, 200–202(0), 541–548. http://dx.doi.org/10.1016/j.cej.2012.06.061. Richter, H., & Howard, J. B. (2000). Formation of polycyclic aromatic hydrocarbons and their growth to soot – a review of chemical reaction pathways. Progress in Energy and Combustion Science, 26(4-6), 565–608. http://dx.doi.org/10.1016/S0082-0784(00)80679-7. Rogak, S. N., Flagan, R. C., & Nguyen, H. V. (1993). The mobility and structure of aerosol agglomerates. Aerosol Science and Technology, 18(1), 25–47. http://dx.doi.org/10.1080/02786829308959582. Santamaria, A., Yang, N., Eddings, E., & Mondragon, F. (2010). Chemical and morphological characterization of soot and soot precursors generated in an inverse diffusion flame with aromatic and aliphatic fuels. Combustion and Flame, 157(1), 33–42. http://dx.doi.org/10.1016/j.combustflame.2009.09.016. Schenk, M., Lieb, S., Vieker, H., Beyer, A., Gölzhäuser, A., Wang, H., & Kohse-Höinghaus, K. (2015). Morphology of nascent soot in ethylene flames. Proceedings of the Combustion Institute, 35(2), 1879–1886. http://dx.doi.org/10.1016/j.proci.2014.05.009. Seto, T., Hirota, A., Fujimoto, T., Shimada, M., & Okuyama, K. (1997). Sintering of polydisperse nanometer-sized agglomerates. Aerosol Science and Technology, 27(3), 422–438. http://dx.doi.org/10.1080/02786829708965482. Seto, T., Shimada, M., & Okuyama, K. (1995). Evaluation of sintering of nanometer-sized titania using aerosol method. Aerosol Science and Technology, 23(2), 183–200. http://dx.doi.org/10.1080/02786829508965303. Shimada, M., Seto, T., & Okuyama, K. (1994). Size change of very fine silver agglomerates by sintering in a heated flow. Journal of Chemical Engineering of Japan, 27(6), 795–802. http://dx.doi.org/10.1252/jcej.27.795.
8
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Shishido, F., Hashiguchi, H., Matsushita, Y., Morozumi, Y., Aoki, H., & Miura, T. (2007). An investigation of primary particle growth and aggregate formation of soot using a numerical model considering the sintering of primary particles. Kagaku Kogaku Ronbunshu, 33(4), 306–314. Wang, H. (2011). Formation of nascent soot and other condensed-phase materials in flames. Proceedings of the Combustion Institute, 33(1), 41–67. http://dx.doi.org/10.1016/j.proci.2010.09.009. Xiong, Y., & Pratsinis, S. E. (1991). Gas phase production of particles in reactive turbulent flows. Journal of Aerosol Science, 22(5), 637–655. http://dx.doi.org/10.1016/0021-8502(91)90017-C. Zhao, B., Yang, Z., Johnston, M. V., Wang, H., Wexler, A. S., Balthasar, M., & Kraft, M. (2003). Measurement and numerical simulation of soot particle size distribution functions in a laminar premixed ethylene–oxygen–argon flame. Combustion and Flame, 133(1–2), 173–188. http://dx.doi.org/10.1016/s0010-2180(02)00574-6. Zhao, B., Yang, Z., Li, Z., Johnston, M. V., & Wang, H. (2005). Particle size distribution function of incipient soot in laminar premixed ethylene flames: effect of flame temperature. Proceedings of the Combustion Institute, 30(1), 1441–1448. http://dx.doi.org/10.1016/j.proci.2004.08.104. Zhao, B., Yang, Z., Wang, J., Johnston, M. V., & Wang, H. (2003). Analysis of soot nanoparticles in a laminar premixed ethylene flame by scanning mobility particle sizer. Aerosol Science and Technology, 37(8), 611–620. http://dx.doi.org/10.1080/02786820300908.
9
Contents lists available at ScienceDirect
Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci
Experimental evidence for the sintering of primary soot particles a,b,⁎
a
a,b
a
Kiminori Ono , Kazuki Dewa , Yoshiya Matsukawa , Yasuhiro Saito , Yohsuke Matsushitaa, Hideyuki Aokia, Koki Eraa,c, Takayuki Aokic, Togo Yamaguchic
MARK
a
Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Miyagi , Aoba-ku, Sendai 9808579, Japan JSPS, Japan c ASAHI CARBON CO., LTD., 2 Kamomejima-cho, Higashi-ku, Niigata 950-0883, Japan b
AR TI CLE I NF O
AB S T R A CT
Keywords: Soot Sintering Surface growth Aggregate Tandem differential mobility analysis Aerosol
Sintering behavior and the contribution of soot produced in the absence of oxygen are experimentally investigated. A tandem differential mobility analyzer technique equipped with thermophoretic sampling was used to reveal the change in the mobility diameter of soot aggregates, which is proportional to the surface area. The mobility diameter decreased with an increase in reheating temperature. The decrease in mobility diameter clearly shows a decrease in the aggregate surface area. SEM images also clearly showed the decrease in the surface area and the simplification after heat treatment. These are experimental evidence that sintering of carbonaceous particle occurs in the absence of oxygen. Acetylene, which is considered to promote surface growth, is also added to size-selected soot particles before heat treatment to investigate the effect of surface growth on simplification of aggregate structure. The addition of acetylene, which is relevant to the surface growth via the hydrogen-abstraction/carbon-addition (HACA) mechanism, did not affect the mean mobility diameter at low temperature around 1200 K. These results showed that simplification of soot aggregates would not be affected by surface growth caused via the HACA mechanism.
1. Introduction Soot is a nanoparticle produced via the formation of polycyclic aromatic hydrocarbons (PAHs), and ultrafine soot particles affects health since they can penetrate the respiratory system more deeply than larger particles (D'Anna, 2009). Although soot is required to be reduced, carbon black, soot produced industrially, has been widely used as an important material in automobile tires, battery electrodes, and pigments in toners for laser printers. When soot is used as a material, the control over morphological features such as primary particle diameter and aggregate structure is important. Nevertheless, the techniques used to control soot morphology are based on trial and error, and the fundamental aspects of these processes are not well understood (Xiong & Pratsinis, 1991). Therefore, to reduce soot formation and control the morphology of carbon black, the formation mechanism of soot needs to be comprehensively understood. Although several soot formation mechanisms have been proposed (D'Anna, 2009; Frenklach, 2002a; Richter & Howard, 2000; Wang, 2011), there is considerable agreement on the general features of the processes involved; these processes are summarized as follows. PAHs are produced by pyrolysis of hydrocarbons, which are mainly produced by the incomplete combustion of fossil fuels.
⁎ Corresponding author at: Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Miyagi , Aoba-ku, Sendai 980-8579, Japan. Fax: +81 22 795 6165. E-mail address: [email protected] (K. Ono).
http://dx.doi.org/10.1016/j.jaerosci.2016.11.013 Received 9 April 2016; Received in revised form 18 October 2016; Accepted 25 November 2016 Available online 27 November 2016 0021-8502/ © 2016 Elsevier Ltd. All rights reserved.
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
The molecular precursors of soot particles are thought to be heavy PAHs with molecular weights of 500–1000 amu (Richter & Howard, 2000). The hydrogen-abstraction/carbon-addition (HACA) mechanism captures the essence of the thermodynamic and kinetic requirements for the formation of PAHs(Frenklach, 2002a). During the mass growth process, these particles collide to produce larger spherical particles, which then aggregate into the final carbon black clusters. The particles are converted into amorphous carbon and a progressively more graphitic material in the furnace. Our recent experimental studies revealed the morphological changes in carbon black that occur with changes in temperature, residence time, and feedstock composition (Ono et al., 2012, 2013). In addition, a detailed kinetic analysis of these experimental conditions indicated that the nuclei concentration and nucleation rate strongly affect the morphology of carbon black (Ono et al., 2014a, 2014b). Our experimental results showed that the aggregate structure becomes simple one with less surface asperity with increasing residence time (Ono et al., 2012). Surface growth has proposed as the main factor responsible for this simplification and increasing particle sphericity (Balthasar & Frenklach, 2005; Morgan et al., 2007). Morgan, Patterson, and Kraft (2008) claimed that sintering, which is important for inorganic materials, is unlikely to be relevant to soot formation. Some characteristic sintering time models for inorganic materials such as Cu (Chepkasov, Gafner, & Gafner, 2016), TiO2 or SiO2 (Goudeli, Eggersdorfer, & Pratsinis, 2015; Hao, Zhao, Xu, & Zheng, 2015) have been developed continuously, some sintering model for soot have been proposed (Chen, Totton, Akroyd, Mosbach, & Kraft, 2014; Chen et al., 2013; Shishido et al., 2007). In contrast to inorganic materials, no reports of experimental evidence for the sintering of soot primary particles are found in the literature, although recent models of soot formation have included sintering. In addition, the contribution of surface growth and sintering to the simplification of soot aggregates is still not well understood, either experimentally or theoretically. Several studies on the experimental investigation of the sintering behavior of inorganic and metal nanoparticles have been reported previously. Tandem differential mobility analyzer (TDMA) is a powerful tool that has been used to investigate the sintering behavior and rate of silver (Shimada, Seto, & Okuyama, 1994), titania (Nakaso et al., 2001; Seto, Hirota, Fujimoto, Shimada, & Okuyama, 1997; Seto, Shimada, & Okuyama, 1995), silica (Seto et al., 1997), and gold (Nakaso, Shimada, Okuyama, & Deppert, 2002) nanoparticles. TDMA has also been employed to reveal the oxidation behavior of soot (Higgins, Jung, Kittelson, Roberts, & Zachariah, 2001; Ma, Zangmeister, & Zachariah, 2013). Ma et al. recently introduced a new tandem ion-mobility method (differential mobility analyzer-aerosol particle mass (DMA-APM) analyzer) to study the size-resolved oxidation kinetics of freshly generated flame soot (Ma et al., 2013). Herein, sintering behavior and the contribution of soot produced in the absence of oxygen are experimentally investigated. Accordingly, we introduce size-selected soot particles produced by the pyrolysis of hydrocarbons in the first electric furnace in the absence of oxygen using DMA on the basis of a measurement of its equivalent mobility diameter to the second furnace, allowing the investigation of primary particle sintering from the differences in mobility diameter. In addition, acetylene, which is considered to play an important role in surface growth, is introduced to the gas including monodisperse soot aggregates. 2. Experimental The experimental setup (Fig. 1) shows a laminar flow reactor equipped with an electric furnace, a probe sampling system equipped with the first DMA, a second electric furnace, a scanning mobility particle sizer (SMPS) consisting of the second DMA, and an ultrafine condensation particle counter (UCPC). Soot was generated by the thermal pyrolysis of ethylene or benzene in an alumina tube (ϕ=11 mm and length=640 mm) that is heated using an electric furnace from 1500 to 1800 K. Liquid benzene was introduced via a syringe pump into the nitrogen flow (TAIYO NIPPON SANSO, 99.9999%) and subsequently into the reaction tube through mass flow controllers at a flow rate of 3.0 NL/min. The benzene concentration was 0.14 vol% to prevent the formation of coke in the tube. The ethylene concentration was 1.0 vol% and the flow rate was 2.0 NL/min. The condition is summarized in Table 1. The gas, including soot particles, was drawn into a sampling probe that was horizontally connected to the end of the reactor. The
Fig. 1. Schematic diagram of the experimental apparatus used for sintering by the TDMA method.
2
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Table 1 Experimental condition of sample preparation. Sample
Feedstock
Temperature [K]
Concentration [vol%]
Benzene-1500 K Benzene-1700 K Ethylene-1600 K Ethylene-1800 K
Benzene Benzene Ethylene Ethylene
1500 1700 1600 1800
0.14 0.14 1.0 1.0
dilution probe used during these experiments was based on the design of Zhao et al. (Zhao et al., 2003; Zhao, Yang, Li, Johnston, & Wang, 2005; Zhao, Yang, Wang, Johnston, & Wang, 2003). The probe was made of a 1/4-inch (OD) stainless steel tube that was horizontally connected to a 1/2-inch (OD) stainless steel tube. The sample gas entered the probe through a 4.0-mm-diameter orifice and was immediately diluted in the probe by nitrogen flowing at 30–39 NL/min. The dilution ratio was typically set to be greater than 104 to prevent the coagulation of products in the probe (Zhao, Yang, Wang, Johnston & Wang, 2003). Sampling from centerline of the reactor allows the radial distribution of temperature and the residence time to be neglected. The polydisperse soot particles were first sent through a neutralizer to establish a known equilibrium charge distribution and then passed to the first DMA for size selection. The sample and sheath flows through the DMA were 1.5 and 15 L/min, respectively. The mobility diameters used in the sintering studies were 50, 100, 140, and 180 nm. The mobility diameter distribution was previously examined using the SMPS system, and it was found to be monodisperse. The size-selected soot particles were sent to an alumina tube (ϕ=11 mm and length = 640 mm) heated and 1000−1700 K with a nominal residence time of approximately 200 ms at an aerosol flow rate of 1.5 L/min at room temperature. The reheated soot particles were sent to the SMPS system for measuring the mean mobility diameter of sintered soot. A TSI model 3938 SMPS was used to analyze the size distributions of nascent soot particles in the flow reactor. The SMPS system comprises a diffusion charger (Kr-85 Bipolar), a long-DMA (TSI 3081 A), and an ultrafine condensation particle counter (UCPC, TSI 3776). TDMA experiments were conducted to measure the change in particle size resulting from the sintering of soot. In a TDMA setup, the first DMA was used for particle size selection, and the second DMA was used to measure the size change of the monodisperse particles resulting from a physical or chemical transformation. The size-selected soot particles and reheated sample were collected on the grid by thermophoretic sampling, mostly by direct sampling from the flame or the reactor using the probe (Abid et al., 2008; Manzello et al., 2007; Schenk et al., 2015). Aggregates classified by the DMA or reheated samples were collected on a collodion-coated grid (Rogak, Flagan, & Nguyen, 1993) mounted on a thermophoretic sampler positioned between DMA and UCPC. The sampler comprised a flexible heater, a nozzle to direct the hot aerosol over the microscope grid, and a copper rod to keep the grid cool. The collected samples were imaged using a Hitachi HighTechnologies S5500 scanning electron microscope (SEM) operated at 3.0 kV. The samples were collected for approximately 30 min because they were further diluted in DMA. 3. Results and discussion The change in mobility diameter with heat treatment provides the sintering behavior. Mobility diameter is pointed out to be proportional to the surface area of an aggregate at some level when the specific conditions are met, such as the fractal dimension of aggregates and the mobility diameter ranges (Cho, Hogan, & Biswas, 2007; Ku & Kulkarni, 2012); therefore, the change in mobility diameter indicates a change in the structure of an aggregate if the mass of the aggregate is not changed by surface growth or oxidation. Fig. 2 shows the change in the mobility diameter of reheated soot that was originally formed by ethylene pyrolysis at 1800 K. In all cases, mobility diameter decreased with an increase in reheating temperature. Considering the inert atmosphere, the decrease in mobility diameter clearly shows a decrease in the aggregate surface area. The mobility diameter began to decrease at 1200 K, and the steep decrease showed from 1500 to 1700 K. This indicates that sintering would be promoted above 1500 K. Fig. 3 shows SEM and scanning transmission electron microscope (STEM) images of soot aggregates before and after heat treatment. The
Fig. 2. Change in the mean mobility diameter of reheated soot aggregates produced by ethylene pyrolysis at 1800 K.
3
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Fig. 3. SEM and STEM images: (a) SEM image of soot aggregate produced by ethylene pyrolysis at 1800 K with a mobility diameter of 180 nm before heat treatment; (b) SEM image of soot aggregate with a mobility diameter of 180 nm after heat treatment at 1500 K; (c) SEM image of soot aggregate with a mobility diameter of 180 nm after heat treatment at 1700 K; (d) SEM image of soot aggregate produced by ethylene pyrolysis at 1800 K with a mobility diameter of 100 nm before heat treatment; and (e) STEM image of soot aggregate with a mobility diameter of 100 nm after heat treatment at 1700 K.
soot aggregate that had a mobility diameter of 180 nm before heat treatment exhibited an aggregate with primary particle diameter of approximately 20 nm [Fig. 3(a)]. In contrast, the soot aggregates heat treated at 1500 and 1700 K [Fig. 3(b) and (c), respectively] exhibited spherical aggregate with primary particle diameters of 20–50 nm. The soot aggregate with a mobility diameter of 100 nm before heat treatment [Fig. 3(d)], which consisted of a few dozen primary particles, also changed to spheroidal particles consisting of a few primary particles [Fig. 3(e)]. This study indicates that the sintering of carbon nanoparticles occurs regardless of the inert atmosphere. Soot properties such as crystallite size differ with the temperature profile or feedstocks (Ono et al., 2012; Santamaria, Yang, Eddings, & Mondragon, 2010). Fig. 4 shows the change in the mobility diameter of reheated soot that was originally formed by ethylene pyrolysis at 1600 K. The decrease in the mobility diameter was larger than that of ethylene pyrolysis at 1700 K as shown in Fig. 2. The steep decrease from 1200 to 1500 K would be due to the presence of aliphatic structure and low crystallinity of PAHs because the pyrolysis temperature was lower than that of the soot shown in Fig. 2. To elucidate the effect of feedstock on the
Fig. 4. Change in the mean mobility diameter of reheated soot aggregates produced using ethylene pyrolysis at 1600 K.
4
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Fig. 5. Change in the mean mobility diameter of reheated soot aggregates produced using benzene pyrolysis at 1700 K.
Fig. 6. Change in the mean mobility diameter of reheated soot aggregates produced using benzene pyrolysis at 1500 K.
morphological change with heat treatment, benzene was used as a feedstock. Figs. 5 and 6 show the change in the mobility diameter of reheated soot that was originally formed by benzene pyrolysis at 1700 K and 1500 K. Although the mobility diameter decreased with an increase in reheating temperature as is the case with ethylene pyrolysis, the decrease in the mobility diameter was smaller than that of ethylene pyrolysis. To explain effects of non-reheated soot properties on sintering behavior, the change in the mean mobility diameter of the sample heat treated at 1700 K was examined. The mobility diameter of the non-heated sample was 180 nm. As shown in Table 2, the mean primary particle diameter of non-heat-treated samples with mobility diameters of 180 nm increased in the order of ethylene-1800 K > benzene-1700 K > ethylene-1600 K > benzene-1500 K. Fig. 7 shows the primary particle diameter size distribution of non-heat treated aggregates. The distribution is increased at low temperatures regardless of feedstock. As shown in Table 3, the mean mobility diameter decreases in the order of benzene-1500 K > benzene-1700 K > ethylene-1800 K > ethylene-1600 K, indicating that sintering rate increases in that order. Ostwald ripening, which is the phenomenon by which small particles dissolve into larger particles, will also occur. Considering that the sintering rate was low for benzene-1500 K, which had a large primary particle distribution, the effect of Ostwald ripening would be small for the sintering of soot particles. The characteristic sintering time, τ, is expressed as follows:
τ =Ad pm exp(E / Rg T ),
(1)
where A is a constant, dp is the primary particle diameter, E is the activation energy for self-diffusion, Rg is the gas constant, and T is the temperature. Therefore, the sintering rate should increase with a decrease in primary particle diameter. However, considering that the sintering rate was the highest in the case of ethylene-1600 K, which had a relatively large primary particle diameter, the effect of primary particle diameter on the sintering of soot particles would be small. The most plausible parameters that affect the sintering of soot are the compositions and crystallinities of primary particle surfaces and interiors. Fig. 8 shows the results for the nonheat treated sample analyzed using gas chromatography mass spectrometry (GCMS) equipped with a pyrolyzer. The analytical Table 2 Mean primary particle diameter of non-heat-treated samples with mobility diameters of 180 nm with standard deviation. Sample
Mean primary particle diameter [nm]
Benzene-1500 K Benzene-1700 K Ethylene-1600 K Ethylene-1800 K
30.6 ± 1.6 26.1 ± 0.5 28.6 ± 1.1 16.9 ± 0.4
5
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Fig. 7. Size distribution of aggregate primary particle diameter for a mobility diameter of 180 nm (non-heat treated): (a) ethylene-1800 K, (b) ethylene-1600 K, (c) benzene-1700 K, and (d) benzene-1500 K.
conditions and chemical composition are summarized in Tables S-1 and S-2 in Supporting information. The chemical composition of surface soot deposits produced using ethylene pyrolysis showed aliphatic hydrocarbons with relatively high molecular weights such as decane and undecane. In contrast, the soot surface deposits produced by benzene pyrolysis did not produce an aliphatic peak; instead, their chemical composition was dominated by high-molecular-weight PAHs. These analytical results strongly suggest that the sintering rate of soot is affected by the aliphatic hydrocarbons and low-molecular-weight PAHs on the soot surface deposits, which lead to a decrease in the melting point of the soot particles. The micro-Fourier transform infrared spectroscopy results along with TEM and atomic force microscopy images suggested that a large amount of aliphatic compounds was present in nascent soot formed in the ethylene–argon–oxygen flame; in addition, the nascent soot was liquid-like and far from carbonized (Barone, D’Alessio, & D’Anna, 2003; Cain, Camacho, Phares, Wang, & Laskin, 2011; Cain, Gassman, Wang, & Laskin, 2010; Dobbins, 2007; Dobbins, Fletcher, & Lu, 1995). These previous results support the findings and theory presented in this study. Surface growth includes chemical growth and physisorption of PAHs (Morgan et al., 2007). The chemical growth was often modeled as the HACA mechanism (Frenklach, 2002b), and the condensation of PAHs is often modeled as the condensation of pyrene (Appel, Bockhorn, & Frenklach, 2000; Morgan et al., 2007). Acetylene is the key role for the surface growth for the addition of carbon source to the armchair edge of PAHs (Frenklach, 1996). To examine the effect of surface growth on the morphological change of soot, acetylene was added to size-selected soot particles before heat treatment on the surface growth of soot particles. Table 4 shows the mean mobility diameters of the samples with mobility diameters of 140 nm reheated at 1000 and 1200 K with and without 1.0 vol% acetylene. The mean mobility diameters decreased from the size-selected diameter of 140 nm after reheating. The p value given by Welch's t-test showed the mean diameter was almost equivalent at 1000 K. We can close discussion at 1000 K from this statistical analysis. On the other hand, we cannot judge the statistical difference of the mean mobility diameter at 1200 K was not enough or not enough at the p=0.05 level, and the mean mobility diameter at 1200 K slightly increased. As we cannot conclude that there is no difference at 1200 K is statistically, we discuss this change for confirmation. This slight increase was probably due to surface growth or the condensation of nascent soot because the temperature of soot production was reached. The surface growth must
Table 3 Mean mobility diameter after heat treatment at 1700 K with standard deviation. Sample
Mean mobility diameter [nm]
Benzene-1500 K Benzene-1700 K Ethylene-1600 K Ethylene-1800 K
145.7 ± 3.9 138.4 ± 3.1 123.8 ± 1.6 135.4 ± 7.0
6
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Fig. 8. Total ion chromatogram showing the chemical composition of soot surface deposits (non-heat treated) obtained using GC–MS equipped with a pyrolyzer: (a) soot produced by ethylene pyrolysis at 1800 K; and (b) soot produced by benzene pyrolysis at 1700 K (1 carbon dioxide, 2 benzene, 3 toluene, 4 decane, 5 undecane, 6 naphthalene, 7 biphenyl, 8 biphenylene or acenaphthylene, 9 phenanthrene, 10 anthracene, 11 2-phenylnaphthalene, 12 fluoranthene, 13 benzene,1,1′-(1,3butadiyne-1,4-diyl)bis-, 14 pyrene, 15 triphenylene or benzo[c]phenanthrene or benzo[a]anthracene, 16 cyclopenta[cd]pyrene).
entail mass growth, if the aggregate structure became spheroidal in shape because of the surface growth (Balthasar & Frenklach, 2005; Morgan et al., 2007), the mobility diameter would decrease further due to the addition of acetylene. Morgan et al. (2008) insisted that the amount of surface growth required to build the two newly joined particles into one larger, approximately spherical particle would be reduced. If surface growth drives aggregate structure into spherical particle, the mobility diameter would decrease by the addition of acetylene to size-selected soot particles. However, the mobility diameter slightly increased at 1200 K. It suggests that surface growth would not affect the simplification of soot aggregates. Instead, sintering is important role for the morphological change of soot. Recent theoretical investigation suggested that sintering is the dominant mechanism for particle rounding compared to surface reaction because the simple surface growth model overpredicted the surface area (Chen et al., 2013). The modeling study strongly supported our experimental results.
4. Conclusion In this paper, we introduced size-selected soot particles using DMA, which are produced by pyrolysis of hydrocarbons in the absence of oxygen, to second furnace to elucidate sintering behavior and the contribution of soot. A tandem differential mobility analyzer technique equipped with thermophoretic sampling provided the evidence that the mobility diameter, which is proportional to the surface area, decreased in an inert atmosphere by reheating process. The addition of acetylene, which is considered to promote surface growth via the HACA mechanism, to size-selected soot particles before heat treatment would not affect the morphological change of the structure by surface growth via the HACA mechanism. Although the previous study (Morgan et al., 2008) claimed that sintering is unlikely to be relevant to soot formation due to the carbonaceous properties, this experimental study clearly showed that sintering of carbonaceous particles occurs in an inert atmosphere. While surface growth has proposed as the main factor responsible for this simplification and increasing particle sphericity, this experimental study showed that simplification of soot aggregate would be affected by sintering rather than surface growth. These new findings will shed light on the modeling study the growth of soot aggregate structure, and would be enhanced by the kinetic study. Table 4 Mean mobility diameter of the sample with a mobility diameter of 140 nm reheated at 1000 and 1200 K with and without 1.0 vol% acetylene with standard deviation and p Value by Welch's t-test.
Reheated at 1000 K Reheated at 1200 K
Without acetylene
With acetylene (1.0 vol%)
p Value
124.3 ± 3.0 nm 126. 7 ± 1.4 nm
123.3 ± 2.5 nm 129.8 ± 3.4 nm
0.96 0.33
7
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jaerosci.2016.11.013. References Abid, A. D., Heinz, N., Tolmachoff, E. D., Phares, D. J., Campbell, C. S., & Wang, H. (2008). On evolution of particle size distribution functions of incipient soot in premixed ethylene–oxygen–argon flames. Combustion and Flame, 154(4), 775–788. http://dx.doi.org/10.1016/j.combustflame.2008.06.009. Appel, J., Bockhorn, H., & Frenklach, M. (2000). Kinetic modeling of soot formation with detailed chemistry and physics: laminar premixed flames of C2 hydrocarbons. Combustion and Flame, 121(1–2), 122–136. http://dx.doi.org/10.1016/S0010-2180(99)00135-2. Balthasar, M., & Frenklach, M. (2005). Detailed kinetic modeling of soot aggregate formation in laminar premixed flames. Combustion and Flame, 140(1–2), 130–145. http://dx.doi.org/10.1016/j.combustflame.2004.11.004. Barone, A. C., D’Alessio, A., & D’Anna, A. (2003). Morphological characterization of the early process of soot formation by atomic force microscopy. Combustion and Flame, 132(1–2), 181–187. http://dx.doi.org/10.1016/S0010-2180(02)00434-0. Cain, J. P., Camacho, J., Phares, D. J., Wang, H., & Laskin, A. (2011). Evidence of aliphatics in nascent soot particles in premixed ethylene flames. Proceedings of the Combustion Institute, 33(1), 533–540. http://dx.doi.org/10.1016/j.proci.2010.06.164. Cain, J. P., Gassman, P. L., Wang, H., & Laskin, A. (2010). Micro-FTIR study of soot chemical composition-evidence of aliphatic hydrocarbons on nascent soot surfaces. Physical Chemistry Chemical Physics, 12(20), 5206–5218. http://dx.doi.org/10.1039/B924344E. Chen, D., Totton, T. S., Akroyd, J., Mosbach, S., & Kraft, M. (2014). Phase change of polycyclic aromatic hydrocarbon clusters by mass addition. Carbon, 77(0), 25–35. http://dx.doi.org/10.1016/j.carbon.2014.04.089. Chen, D., Zainuddin, Z., Yapp, E., Akroyd, J., Mosbach, S., & Kraft, M. (2013). A fully coupled simulation of PAH and soot growth with a population balance model. Proceedings of the Combustion Institute, 34(1), 1827–1835. http://dx.doi.org/10.1016/j.proci.2012.06.089. Chepkasov, I. V., Gafner, Y. Y., & Gafner, S. L. (2016). Changing of the shape and structure of Cu nanoclusters generated from a gas phase: MD simulations. Journal of Aerosol Science, 91, 33–42. http://dx.doi.org/10.1016/j.jaerosci.2015.09.004. Cho, K., Hogan, C. J., & Biswas, P. (2007). Study of the mobility, surface area, and sintering behavior of agglomerates in the transition regime by tandem differential mobility analysis. Journal of Nanoparticle Research, 9(6), 1003–1012. http://dx.doi.org/10.1007/s11051-007-9243-5. D'Anna, A. (2009). Combustion-formed nanoparticles. Proceedings of the Combustion Institute, 32(1), 593–613. http://dx.doi.org/10.1016/j.proci.2008.09.005. Dobbins, R. A. (2007). Hydrocarbon nanoparticles formed in flames and diesel engines. Aerosol Science and Technology, 41(5), 485–496. http://dx.doi.org/10.1080/02786820701225820. Dobbins, R. A., Fletcher, R. A., & Lu, W. (1995). Laser microprobe analysis of soot precursor particles and carbonaceous soot. Combustion and Flame, 100(1–2), 301–309. http://dx.doi.org/10.1016/0010-2180(94)00047-V. Frenklach, M. (1996). On surface growth mechanism of soot particles. Symposium (International) on Combustion, 26(2), 2285–2293. http://dx.doi.org/10.1016/S0082-0784(96)80056-7. Frenklach, M. (2002). Method of moments with interpolative closure. Chemical Engineering Science, 57(12), 2229–2239. Frenklach, M. (2002). Reaction mechanism of soot formation in flames. [10.1039/B110045A]. Physical Chemistry Chemical Physics, 4(11), 2028–2037. http://dx.doi.org/10.1039/B110045A. Goudeli, E., Eggersdorfer, M. L., & Pratsinis, S. E. (2015). Aggregate characteristics accounting for the evolving fractal-like structure during coagulation and sintering. Journal of Aerosol Science, 89, 58–68. http://dx.doi.org/10.1016/j.jaerosci.2015.06.008. Hao, X., Zhao, H., Xu, Z., & Zheng, C. (2015). Identification of the compensation effect in the characteristic sintering time model for population balances. Journal of Aerosol Science, 82, 1–12. http://dx.doi.org/10.1016/j.jaerosci.2014.12.005. Higgins, K. J., Jung, H., Kittelson, D. B., Roberts, J. T., & Zachariah, M. R. (2001). Size-selected nanoparticle chemistry: kinetics of soot oxidation. The Journal of Physical Chemistry A, 106(1), 96–103. http://dx.doi.org/10.1021/jp004466f. Ku, B. K., & Kulkarni, P. (2012). Comparison of diffusion charging and mobility-based methods for measurement of aerosol agglomerate surface area. Journal of Aerosol Science, 47, 100–110. http://dx.doi.org/10.1016/j.jaerosci.2012.01.002. Ma, X., Zangmeister, C. D., & Zachariah, M. R. (2013). Soot oxidation kinetics: a comparison study of two tandem ion-mobility methods. The Journal of Physical Chemistry C, 117(20), 10723–10729. http://dx.doi.org/10.1021/jp400477v. Manzello, S. L., Lenhert, D. B., Yozgatligil, A., Donovan, M. T., Mulholland, G. W., Zachariah, M. R., & Tsang, W. (2007). Soot particle size distributions in a well-stirred reactor/plug flow reactor. Proceedings of the Combustion Institute, 31(1), 675–683. Morgan, N., Kraft, M., Balthasar, M., Wong, D., Frenklach, M., & Mitchell, P. (2007). Numerical simulations of soot aggregation in premixed laminar flames. Proceedings of the Combustion Institute, 31(1), 693–700. http://dx.doi.org/10.1016/j.proci.2006.08.021. Morgan, N. M., Patterson, R. I. A., & Kraft, M. (2008). Modes of neck growth in nanoparticle aggregates. Combustion and Flame, 152(1–2), 272–275. Nakaso, K., Fujimoto, T., Seto, T., Shimada, M., Okuyama, K., & Lunden, M. M. (2001). Size distribution change of titania nano-particle agglomerates generated by gas phase reaction, agglomeration, and sintering. Aerosol Science and Technology, 35(5), 929–947. http://dx.doi.org/10.1080/02786820126857. Nakaso, K., Shimada, M., Okuyama, K., & Deppert, K. (2002). Evaluation of the change in the morphology of gold nanoparticles during sintering. Journal of Aerosol Science, 33(7), 1061–1074. http://dx.doi.org/10.1016/S0021-8502(02)00058-7. Ono, K., Watanabe, A., Dewa, K., Matsukawa, Y., Saito, Y., Matsushita, Y., & Yamaguchi, T. (2014). Detailed kinetic analysis of the effect of benzene–acetylene composition on the configuration of carbon nanoparticles. Chemical Engineering Journal, 250(0), 66–75. http://dx.doi.org/10.1016/j.cej.2014.03.091. Ono, K., Watanabe, A., Dewa, K., Matsukawa, Y., Saito, Y., Matsushita, Y., & Yamaguchi, T. (2014). Detailed kinetic analysis of the impact of nucleation behavior and particle size distribution on the configurations of carbon black. Journal of Nanoparticle Research, 16(7), 1–11. http://dx.doi.org/10.1007/s11051-014-2519-7. Ono, K., Yanaka, M., Saito, Y., Aoki, H., Fukuda, O., Aoki, T., & Yamaguchi, T. (2013). Effect of benzene–acetylene compositions on carbon black configurations produced by benzene pyrolysis. Chemical Engineering Journal, 215–216(15), 128–135. http://dx.doi.org/10.1016/j.cej.2012.10.085. Ono, K., Yanaka, M., Tanaka, S., Saito, Y., Aoki, H., Fukuda, O., & Yamaguchi, T. (2012). Influence of furnace temperature and residence time on configurations of carbon black. Chemical Engineering Journal, 200–202(0), 541–548. http://dx.doi.org/10.1016/j.cej.2012.06.061. Richter, H., & Howard, J. B. (2000). Formation of polycyclic aromatic hydrocarbons and their growth to soot – a review of chemical reaction pathways. Progress in Energy and Combustion Science, 26(4-6), 565–608. http://dx.doi.org/10.1016/S0082-0784(00)80679-7. Rogak, S. N., Flagan, R. C., & Nguyen, H. V. (1993). The mobility and structure of aerosol agglomerates. Aerosol Science and Technology, 18(1), 25–47. http://dx.doi.org/10.1080/02786829308959582. Santamaria, A., Yang, N., Eddings, E., & Mondragon, F. (2010). Chemical and morphological characterization of soot and soot precursors generated in an inverse diffusion flame with aromatic and aliphatic fuels. Combustion and Flame, 157(1), 33–42. http://dx.doi.org/10.1016/j.combustflame.2009.09.016. Schenk, M., Lieb, S., Vieker, H., Beyer, A., Gölzhäuser, A., Wang, H., & Kohse-Höinghaus, K. (2015). Morphology of nascent soot in ethylene flames. Proceedings of the Combustion Institute, 35(2), 1879–1886. http://dx.doi.org/10.1016/j.proci.2014.05.009. Seto, T., Hirota, A., Fujimoto, T., Shimada, M., & Okuyama, K. (1997). Sintering of polydisperse nanometer-sized agglomerates. Aerosol Science and Technology, 27(3), 422–438. http://dx.doi.org/10.1080/02786829708965482. Seto, T., Shimada, M., & Okuyama, K. (1995). Evaluation of sintering of nanometer-sized titania using aerosol method. Aerosol Science and Technology, 23(2), 183–200. http://dx.doi.org/10.1080/02786829508965303. Shimada, M., Seto, T., & Okuyama, K. (1994). Size change of very fine silver agglomerates by sintering in a heated flow. Journal of Chemical Engineering of Japan, 27(6), 795–802. http://dx.doi.org/10.1252/jcej.27.795.
8
Journal of Aerosol Science 105 (2017) 1–9
K. Ono et al.
Shishido, F., Hashiguchi, H., Matsushita, Y., Morozumi, Y., Aoki, H., & Miura, T. (2007). An investigation of primary particle growth and aggregate formation of soot using a numerical model considering the sintering of primary particles. Kagaku Kogaku Ronbunshu, 33(4), 306–314. Wang, H. (2011). Formation of nascent soot and other condensed-phase materials in flames. Proceedings of the Combustion Institute, 33(1), 41–67. http://dx.doi.org/10.1016/j.proci.2010.09.009. Xiong, Y., & Pratsinis, S. E. (1991). Gas phase production of particles in reactive turbulent flows. Journal of Aerosol Science, 22(5), 637–655. http://dx.doi.org/10.1016/0021-8502(91)90017-C. Zhao, B., Yang, Z., Johnston, M. V., Wang, H., Wexler, A. S., Balthasar, M., & Kraft, M. (2003). Measurement and numerical simulation of soot particle size distribution functions in a laminar premixed ethylene–oxygen–argon flame. Combustion and Flame, 133(1–2), 173–188. http://dx.doi.org/10.1016/s0010-2180(02)00574-6. Zhao, B., Yang, Z., Li, Z., Johnston, M. V., & Wang, H. (2005). Particle size distribution function of incipient soot in laminar premixed ethylene flames: effect of flame temperature. Proceedings of the Combustion Institute, 30(1), 1441–1448. http://dx.doi.org/10.1016/j.proci.2004.08.104. Zhao, B., Yang, Z., Wang, J., Johnston, M. V., & Wang, H. (2003). Analysis of soot nanoparticles in a laminar premixed ethylene flame by scanning mobility particle sizer. Aerosol Science and Technology, 37(8), 611–620. http://dx.doi.org/10.1080/02786820300908.
9