Soot Nanostructure Research Articles

Effects of a Catalyst on the Nanostructure and Reactivity of Soot under an Oxygen Atmosphere

The promotional effects of soot oxidation with an additional catalyst were studied via varying the oxidation temperature (650 and 1000 °C) at the same temperature ramp. The performance with or without the catalyst was evaluated through temperature-programmed oxidation in thermogravimetric analysis (TGA) mode. The soot structure, morphology, and surface properties were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The soot with the catalyst showed a less ordered crystalline structure under the low temperature period (50–650 °C). Under an increased temperature, the difference in crystal lattice between soot A and soot B increased. These results illustrate the soot oxidation process with catalysts in an O2 atmosphere.

The core–shell internal nanostructure of soot – A criterion to model soot maturity

A novel model called Surface Shell Formation (SSF) is developed which predicts soot maturity based on the equilibrium nanostructure of poly aromatic hydrocarbons (PAHs) inside soot primary particles. The characteristic isotropic core-graphitic shell internal nanostructure of soot primary particles is used to distinguish nascent from mature soot primary particles. A new sectional soot model is developed to track particle Hydrogen to Carbon (H/C) ratio and the growth in the molecular weight of PAHs inside soot primary particles. An independent Arrhenius term describes particle dehydrogenation/carbonization. It is shown that soot maturity depends on both particle size and H/C ratio. Graphitic shell formation in mature soot particles is related to the surface PAHs that change configuration from edge on surface for nascent soot to face on surface for mature soot particles. The new model is validated against experimental data for laminar premixed, partially premixed and diffusion flames. The SSF model addresses the two major limitations of the current soot modeling approaches. First, it predicts H/C ratio of soot particles by considering soot carbonization. Second, it distinguishes nascent soot from mature soot primary particles based on the internal nanostructure of soot primary particles and the presence of the graphitic shell.

Soot oxidation-induced fragmentation: Part 1: The relationship between soot nanostructure and oxidation-induced fragmentation

This paper tests the proposed mechanisms in the literature [1] where the fragmentation was suggested to be as either the result of aggregate break up due to preferential burning in the bridge sites or the result of primary particle fragmentation due to internal burning. In order to evaluate aggregate break up, high-resolution transmission electron microscopy (HR-TEM) images were analyzed to compare the soot nanostructure of the bridge sites with the particle sites. Results demonstrated that the bridge sites were formed by less-ordered nanostructure indicating a higher oxidation potential than particle sites. In addition, the feasibility of internal burning in the single particles was tested through an analysis of intraparticle diffusion. In this case, the actual oxygen diffusion pathway as well as effective spacing between the crystalline structures was estimated by the image analysis technique. By using the obtained data in the effectiveness factor calculation, it was found that the possibility of internal burning by O2 molecules increased for particles less than 10 nm as compared to larger particles. In addition, increasing temperature decreased the potential for internal burning.

Impact of dual-fuel combustion with n-butanol or hydrous ethanol on the oxidation reactivity and nanostructure of diesel particulate matter

This work evaluated the oxidation reactivity, nanostructure and morphological characteristics of the particulate matter (PM) produced by dual-fuel combustion with n-butanol or hydrous ethanol in an automotive diesel engine operating under two different engine loads (equivalence ratios) at the same engine speed (2410min−1). 10% and 15% of ultra-low sulfur diesel (ULSD) fuel on an energy basis were replaced by alcohol fumigation. Thermogravimetric analysis (TGA) showed that independently of engine load, the soot produced by alcohol fumigation was more reactive to oxidation and exhibited higher active surface area (ASA) than ULSD, following the order hydrous ethanol>n-butanol>ULSD. The same trend was obtained for the volatile organic fraction (VOF) of the particulate matter (PM). All PM samples exhibited higher soot reactivity, ASA and VOF at low load in comparison with high load. FTIR spectra showed significant differences on the content of OH group (3100–3600cm−1 wavelength) and aliphatic groups (2850–2950cm−1 wavelength) among all PM samples. Finally, it was found that soot nanostructure, determined with XRD and Raman spectroscopy, as well as PM morphological characteristics (primary particulate diameter and fractal dimension), determined through digital image analysis of TEM micrographs, were not modified significantly by alcohol fumigation neither by engine load. From this experimental setting and engine configuration, it can be concluded that independently of engine load, the intake fumigation of n-butanol or hydrous ethanol (ethanol to a greater extent) increases soot oxidation reactivity, ASA and VOF without affecting soot nanostructure, or PM morphology in comparison to ULSD fuel.

Effects of 2,5-dimethylfuran addition to diesel on soot nanostructures and reactivity

With the improvements in technologies to produce 2,5-dimethyfuran (DMF) from biomass such as cellulose, the potential of this biofuel to completely or partially replace fossil fuels in order to reduce our dependence on them and to curb pollutant emissions has been explored in some recent studies. In this work, the effect of the addition of DMF (up to 15% by volume) to diesel on the sooting propensity and on the oxidative reactivity and nanostructures of soot particles is studied using a diffusion flame. With increasing amount of DMF in the DMF/diesel blends, soot emission was found to reduce, while the oxidation reactivity of soot particles increased. The activation energies of soot oxidation from blended fuels were significantly lower than that of diesel soot. The increase in soot reactivity is linked with the changes in physical and chemical properties of soot nanostructures. Several experimental techniques such as high resolution transmission electron microscopy, electron energy loss spectroscopy, thermo-gravimetric analysis, elemental analysis, Fourier transform infrared spectroscopy, and X-ray diffraction have been used to identify the changes in structures, functional groups such as oxygenates and aliphatics, π and σ bonding, O/C and H/C ratios, and crystallite parameters of soot particles. With increasing DMF concentration in diesel, the sizes of primary particles and the polycyclic aromatic hydrocarbons (PAHs) comprising soot decreased, while the aliphatic character (sp3 hybridization) and the amount of oxygenated functional groups in it increased. The observations from soot characterization are used to explain the high susceptibility of soot particles from blended fuels toward oxidation.

The effect of oxidation pressure on the equilibrium nanostructure of soot particles

The notion of equilibrium soot nanostructure was introduced by Hurt et al., who argued that the peculiar turbostratic carbon structure is an equilibrium arrangement of lamellar, graphene-like molecules. It was proposed that the typical, onion-like internal structure of primary soot particles can be satisfactorily described by thermodynamic principles. There are two main objectives of this paper. First, the effects of oxidation pressures above atmospheric pressure on soot nanostructure are investigated experimentally. The analyzed soot was generated in premixed flames of liquid fuels: n-dodecane, m-xylene and n-butanol and further oxidized in a thermogravimetric analyzer under atmospheric, 10atm and 40atm pressures. Nanostructure is described by utilizing high-resolution transmission electron microscopy and recently developed image analysis techniques. Second, empirical observations are compared against behavior that is semi-quantitatively predicted by the thermodynamic model. The utilization of the novel analysis technique made direct comparison between observed and computed properties possible. Reasonable consistency was found between experimental and computational results. The results suggest that the known thermodynamic model can be used to predict equilibrium structure even when soot is oxidized under pressurized conditions. Since diesel and jet engines operate at elevated pressures, the conclusions drawn in this paper may find their use in predicting soot nanostructure in the limiting case of equilibrium conditions.

Effect of the flame environment on soot nanostructure inferred by Raman spectroscopy at different excitation wavelengths

Soot structural details were elicited through a detailed analysis of the main first-order Raman peaks measured at different excitation wavelengths (457, 514 and 633nm) on soot produced in premixed fuel-rich flames of different hydrocarbon fuels (namely, methane, ethylene and benzene) burning in similar temperature conditions. Information on the distribution of the aromatic clusters size and on the configuration of the sp2 bonds was obtained. In particular the analysis of the main spectral parameters, namely the G and D band position and widths as well as the I(D)/I(G) ratio, elucidates that the sp2-bonded phase featuring soot particles resulted to be mainly organized in clusters of nanometric size with some presence of sp2 olefinic chains. The Raman spectral analysis allowed to follow even small soot structural differences deriving from the effect of fuel molecule identity and soot aging. In the framework of soot formation and growth process, graphitization, intended as increase of in-plane layer length (as derived from the I(D)/I(G) ratio), was found to generally occur to a very low extent as soot is formed. On the other hand, the widening of the main Raman bands was observed and indicated as signature of the disorder increase likely due to the coagulation of PAH of diverse size and shape, possibly interconnected by odd-numbered and/or sp3 bonds causing the warping and puckering during the aromatic layers growth.

Impact of rail pressure and biodiesel fueling on the particulate morphology and soot nanostructures from a common-rail turbocharged direct injection diesel engine

An investigation of the impact of rail pressure and biodiesel fueling on exhaust particulate agglomerate morphology and primary particle (soot) nanostructure was conducted with a common-rail turbocharged direct injection diesel engine. The engine was operated at steady state on a dynamometer running at moderate speed with both low (30%) and medium–high (60%) fixed loads, and exhaust particulate was sampled for analysis. The fuels used were ultra-low sulfur diesel and its 20% v/v blends with soybean methyl ester biodiesel. Fuel injection occurred in a single event around top dead center at three different injection pressures. Exhaust particulate samples were characterized with transmission electronic microscopy imaging, scanning mobility particle sizing, thermogravimetric analysis, Raman spectroscopy, and X-ray diffraction analysis. Particulate morphology and oxidative reactivity were found to vary significantly with both rail pressure and biodiesel blend level. Higher biodiesel content led to an increase in the primary particle size and oxidative reactivity but had no impact on nanoscale disorder in the as-received samples. For particulates generated with higher injection pressures, the initial oxidative reactivity increased, but there was no detectable correlation with primary particle size or nanoscale disorder.

Effect of oxygenated fuels on physicochemical and toxicological characteristics of diesel particulate emissions.

A systematic study was conducted to make a comparative evaluation of the effects of blending five different oxygenates (diglyme (DGM), palm oil methyl ester (PME), dimethyl carbonate (DMC), diethyl adipate (DEA), and butanol (Bu)) with ultralow sulfur diesel (ULSD) at 2% and 4% oxygen levels on physicochemical and toxicological characteristics of particulate emissions from a nonroad diesel engine. All blended fuels led to an overall decrease in the particulate mass concentration and elemental carbon (EC) emissions, which was strongly associated with the oxygen content in fuels and the specific type of fuels used. In general, the proportion of particulate-bound organic carbon (OC) and water-soluble organic carbon (WSOC) increased while using oxygenated fuel blends. Compared to ULSD, all fuel blends showed different emission factors of particle-phase PAHs and n-alkanes, slight alterations in soot nanostructure, lower soot ignition temperature, and lower activation energy. The total counts of particles (≤ 560 nm diameter) emitted decreased gradually for ULSD blended with DMC, DEA, and Bu, while they increased significantly for other fuel blends. The in vitro toxicity of particulates significantly increased with ULSD blended with DMC and DEA, while it decreased when ULSD was blended with PME, DGM, and Bu.

Oxidative Reactivity of Particles Emitted from a Diesel Engine Operating at Light Load with EGR

The impact of exhaust gas recirculation (including three levels: 0, 10%, and 30%) on engine combustion characteristics, gaseous emissions, and particulate properties (i.e., oxidative reactivity, carbonaceous compositions, size distribution, and nanostructure) was studied on a common rail diesel engine operating at low engine load. The results showed that the lack of oxygen with EGR prolongated ignition delay and the premixed portion of combustion started to rise significantly. Higher EC (accumulation mode) with larger particle size could be observed with increasing EGR from 0 to 30%, which is attributed to the promotion of soot formation with less available oxygen and the inhibition of soot oxidation with low in-cylinder temperature with increasing EGR. The soot nanostructure observation showed that soot changed from smooth surface under 0 EGR to rugose surface under 10% EGR. Moreover, the amorphous core turned larger with increasing EGR. With increasing EGR to 30%, the amorphous core appeared to include ...

Effect of a homogeneous combustion catalyst on the nanostructure and oxidative properties of soot from biodiesel combustion in a compression ignition engine

The effect of a ferrous picrate based homogeneous combustion catalyst on the properties of soot from the combustion of a biodiesel in a compression ignition engine was systematically studied. Soot samples from the combustion of a reference petroleum diesel (RD) and the biodiesel (BD), with and without the catalyst treatment, were collected when the engine was maintained under a constant condition of 2800rpm speed and 5.5Nm load, and subsequently analysed using transmission electron microscopy (TEM) for their nanostructures and thermogravimetric analyser (TGA) for the oxidative reactivity. The results showed that compared to the soot from BD, the sizes of both primary soot and aggregates from the catalyst treated biodiesel were consistently smaller; while the nanostructure and fractal dimension remained similar, indicating that there were no dramatic changes in the mechanisms of the soot nucleation and agglomeration. Furthermore, soot from the catalyst treated biodiesel possessed higher oxidative reactivity as indicated by the lower ignition temperature and faster oxidation rate than those of soot from BD. In addition, the primary soot and aggregates from BD were smaller and the soot had a higher order in its nanostructure with greater oxidative reactivity, in comparison to those from RD. A mechanism of the working of the catalyst was proposed in that the catalyst promotes the biodiesel combustion process leaving fewer soot precursors and also accelerates the oxidation of soot particles, resulting in smaller sizes of the primary soot and soot aggregates and the reduced overall soot emissions.

Effects of Diesel Oxidation Catalyst on Nanostructure and Reactivity of Diesel Soot

In order to investigate the nanostructure changes of diesel soot during the oxidation process, two different types of diesel soot were collected, and their nanostructures were studied on the basis of thermogravimetric analysis and high-resolution transmission electron microscopy analysis. This work shows that the nanostructure alone does not dictate the reactivity of diesel soot, but rather, the oxidation mechanism has a strong effect on the oxidative reactivity. Soot emitted directly from the engine is oxidized under the surface burning mode, which makes the soot retain the typical core–shell structure. However, the diesel oxidation catalyst (DOC) has an influence on the oxidation mechanism of diesel soot as well as the evolution of nanostructure during the oxidation process. Soot sampled after DOC mainly undergoes an internal burning oxidation process that makes the oxidation more rapid, leading to a hollow capsule-like structure during the early stage of oxidation. However, soot becomes less reactive d...

Soot oxidation kinetics under pressurized conditions

The oxidation kinetics, under different pressures, of soot samples obtained from different liquid fuels and two standards (a commercial black carbon sample and a reference diesel soot) was studied. Soot samples were generated in a flat-flame, premixed burner under heavily-sooting conditions and captured on a water-cooled stabilization plate located above the burner surface. The collected soot was oxidized using a high-pressure thermogravimetric analyzer (HTGA). TGA operation was optimized to reduce mass transfer effects by adjusting the oxidizer flow rate and initial sample mass. Further corrections for mass transfer were accomplished by computing the effectiveness factors for intraparticle, interparticle, and external mass transfer. Two pressures were evaluated (1 and 10atm) and O2 concentration was varied between 10 and 21%.There was not a significant difference in the activation energies of the soot samples for either pure components or as a mixture, with exception of soot from oxygen-containing fuels, and activation energies for both standards were within the range reported by others.Pressure did not change the activation energies when mass transfer corrections were applied.Soot nanostructure for the soot collected from the flat flame “nascent”, and partially oxidized at 1, 10 and 40atm was studied by High-Resolution Transmission Electron Microscopy (HRTEM). The lattice fringe length and fringe tortuosity were estimated to correlate the nanostructure and the soot reactivity, and the nascent soot nanostructure was similar across the five fuels. n-butanol/n-dodecane soot showed a significant change in nanostructure with increasing pressure; however, there was no apparent change in the nanostructure (length or tortuosity) for the other fuels. Corroborating the HRTEM data, surface carbon oxidation studies by X-ray photoelectron spectroscopy (XPS) revealed that the sp2/sp3 content for the oxygenated fuel blend was six times lower than the other fuels leading to lower activation energy and external surface lamellae break-up.

Growth and oxidation of graphitic crystallites in soot particles within a laminar diffusion flame

The growth and oxidation behaviors of graphitic crystallites in soot particles within a laminar co-flow propane diffusion flame were investigated experimentally. Soot was sampled along the flame axis and collected on a quartz–fiber filter. The soot nanostructure, i.e., the graphitic crystallite size and the amorphous carbon content in the soot particles, was characterized by Raman spectroscopy. The spatial distributions of the fuel, PAHs, soot and OH measured by laser diagnostic techniques are presented to examine the effect of the flame structure on the soot nanostructure. The Raman spectra of the soot show that both the graphitic crystallite size and the amorphous carbon content in the soot particles increase during the soot growth process. In addition, the crystallite size and the amorphous carbon content simultaneously decrease during the soot oxidation process by OH. HRTEM images of soot particles support that these findings obtained from the Raman spectroscopy are reasonable. The influences of the soot formation and oxidation history within the flame on the soot nanostructure are validated.

Does Hydrophilicity of Carbon Particles Improve Their Ice Nucleation Ability?

Carbonaceous particles account for 10% of the particulate matter in the atmosphere. Atmospheric oxidation and aging of soot modulates its ice nucleation ability. It has been suggested that an increase in the ice nucleation ability of aged soot results from an increase in the hydrophilicity of the surfaces upon oxidation. Oxidation, however, also impacts the nanostructure of soot, making it difficult to assess the separate effects of soot nanostructure and hydrophilicity in experiments. Here we use molecular dynamics simulations to investigate the effect of changes in hydrophilicity of model graphitic surfaces on the freezing temperature of ice. Our results indicate that the hydrophilicity of the surface is not in general a good predictor of ice nucleation ability. We find a correlation between the ability of a surface to promote nucleation of ice and the layering of liquid water at the surface. The results of this work suggest that ordering of liquid water in contact with the surface plays an important role in the heterogeneous ice nucleation mechanism.

Heterogeneous Nucleation of Ice on Carbon Surfaces

Atmospheric aerosols can promote the heterogeneous nucleation of ice, impacting the radiative properties of clouds and Earth's climate. The experimental investigation of heterogeneous freezing of water droplets by carbonaceous particles reveals widespread ice freezing temperatures. It is not known which structural and chemical characteristics of soot account for the variability in ice nucleation efficiency. Here we use molecular dynamics simulations to investigate the nucleation of ice from liquid water in contact with graphitic surfaces. We find that atomically flat carbon surfaces promote heterogeneous nucleation of ice, while molecularly rough surfaces with the same hydrophobicity do not. Graphitic surfaces and other surfaces that promote ice nucleation induce layering in the interfacial water, suggesting that the order imposed by the surface on liquid water may play an important role in the heterogeneous nucleation mechanism. We investigate a large set of graphitic surfaces of various dimensions and radii of curvature and find that variations in nanostructures alone could account for the spread in the freezing temperatures of ice on soot in experiments. We conclude that a characterization of the nanostructure of soot is needed to predict its ice nucleation efficiency.

Sooting behaviors of n-butanol and n-dodecane blends

This work focuses on understanding the formation and oxidation of soot when adding n-butanol, an oxygenated fuel, to n-dodecane. A two-stage burner was used to characterize the oxidation of soot from different n-butanol blends, 10%, 30%, and 60mol% in n-dodecane. The two-stage burner isolates the soot oxidation process from the formation process. Soot is formed in a first-stage premixed burner under fuel-rich conditions, while in a second stage, the soot is oxidized under slightly fuel-rich conditions. A scanning mobility particle sizer (SMPS) was used to measure the soot particle size distributions in the flame at different heights during oxidation. Results showed a decrease in particle mass concentration (g/cm3) as the fraction of n-butanol increased, which indicates the capability of n-butanol to reduce soot particle number and mass. On the other hand, the results demonstrated that the increasing n-butanol reduces the difference between initial mass of soot particles entering and the final mass of soot particles leaving the second burner. This result implies that increasing the n-butanol concentration decreases the rate of soot oxidation. Two different fuel quality indicators are used to quantify our observations. The first one, “sooting tendency”, is calculated to show how the amount of soot formed in the flame is affected by using different n-butanol percentages. The second one, “sooting stability”, is defined for quantifying the stability of soot particles against oxidation. The results demonstrated that by increasing the n-butanol percentage, soot formation was suppressed. However, sooting stability increased with higher concentrations of n-butanol. The soot nanostructure was quantified by high-resolution electron microscopy and digital image processing. Image analysis revealed layer arrangement is in correlation with sooting stability. The results of interlayer spacing showed a decrease by increasing n-butanol at the same sampling height.

Insights into the combustion chemistry within a gas-turbine driven auxiliary power unit as a function of fuel type and power level using soot nanostructure as a tracer

Particulate emissions were collected from an Auxiliary power unit (APU) directly upon TEM grids for particle characterization by HRTEM. Carbonaceous emissions from two fuels, a coal-based Fischer–Tropsch and standard JP-8 were compared, each at three power levels. Differences in soot nanostructure, specifically fullerenic content reveal changes in the combustion chemistry with engine power level, as do differences in aggregate size between the two fuels. As inferred from the soot nanostructure, comparison between fuels demonstrates the impact of fuel structure upon soot formation chemistry.

Impact of engine operating modes and combustion phasing on the reactivity of diesel soot

The present work focuses on the impacts of engine operating conditions and combustion phasing on diesel soot properties. The study was carried out in a 2.5L direct injection common-rail turbodiesel engine using an ultra low sulfur diesel fuel (BP15). The study has two objectives. The first objective is to investigate the reactivity difference of the soot generated at four engine modes, spanning conditions of most interest in the engine operating map. The results show that the impact of engine speed at constant torque is more pronounced than the impact of engine torque (equivalence ratio) at constant engine speed. The effect of the engine torque at constant engine speed, especially at higher speed, is not observable in this experiment. The second objective is to investigate the reactivity and nanostructure of soot generated at different combustion phasing by advancing and retarding the fuel start of injection (SOI) timing. Reaction kinetics obtained from thermogravimetric analysis show that the rate constant of the sample for retarding SOI timing (retarded 2 CAD) is 2.3 times that for advancing SOI timing (advanced 2 CAD). In summary, the results presented here provide unique insights into the methodology that should be used when investigating soot samples generated by different fuels: matching operating modes and combustion phasing for different fuels is suggested while collecting soot samples for characterization.

Impact of fuel formulation on the nanostructure and reactivity of diesel soot

The work focuses on the impact of fuel on soot reactivity and nanostructure. A 2.5L, 4-cylinder, turbocharged, common rail, direct injection light-duty diesel engine was used in generating soot samples. Three test fuels were used: an ultra low sulfur diesel fuel (BP15), a pure soybean methyl-ester (B100), and a synthetic Fischer-Tropsch fuel (FT) produced in a gas-to-liquid process. A test condition of 2400rpm and 64Nm (low load), with single and split injection strategies, was chosen for studying the impacts of fuel formulation on the characteristics of diesel soot. The start of injection (SOI) and fuel rail pressures were adjusted such that the three test fuels have similar combustion phasing, thereby facilitating comparisons between soots from the different fuels. The collected soot samples were characterized using thermogravimetric analyzer (TGA), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high resolution transmission electron microscopy (HRTEM). According to TGA, B100 soot exhibits the fastest oxidation on a mass basis followed by BP15 and FT derived soots in order of apparent rate constant. XPS analysis indicates no relation between the surface oxygen content and the soot reactivity. The basal plane diameter obtained from XRD is inversely related to the apparent rate constants for soot oxidation. Quantitative analysis of HRTEM images also suggests soot nanostructure disorder correlates with a faster oxidation rate. XRD and HRTEM analysis results are compared, and they both show excellent correlation of soot nanostructure and reactivity.