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The purpose of the present study is to better understand the reaction kinetics of diesel exhaust soot during oxidation process. A thermogravimetric analyzer was used to oxidize real diesel exhaust soot generated from a Euro VI diesel engine under non-isothermal conditions. The Friedman-Reich-Levi method and the Sestak-Berggren model were used to determine the oxidation kinetics. Raman spectroscopy and high-resolution transmission electron microscopy were employed to follow the changes of the soot structure during oxidation. The activation energy gradually increased with increasing conversion level during soot oxidation. The oxidation process of diesel exhaust soot could be described as three-step kinetics, and the calculated conversions fitted the experimental results very well. The kinetic predictions of diesel soot oxidation that were obtained using the proposed kinetic models were more accurate and precise than those with the common first-order model. The structural order increased as oxidation progressed, which was responsible for the increased activation energy. The structural ordering was principally caused by the preferential oxidation of the disordered fraction in the diesel soot, especially for the amorphous carbon, which was oxidized in the initial stage of the oxidation reaction., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

This paper focuses on the correlation between the oxidative reactivity and electrical properties of soot particles. Soot particles were produced from an n -heptane inverse diffusion flame and were sampled at different heights above the burner (HAB) using a thermophoretic and probe sampling techniques. The electrical properties were characterized by the electrical conductivity and work function, which were evaluated by an atomic force microscopy. The soot reactivity was assessed in terms of activation energy (E a) and characteristic oxidation temperatures, including peak temperature (T P) and burnout temperature (T b), using thermogravimetric analysis. As the soot particles gradually age, they have various electrical conductivity distributions. The average electrical conductivity presents an increase by three orders of magnitude when increasing the HAB from 10 to 60 mm. The work function of bulk soot particles shows an increase with the soot maturation. The soot reactivity gradually reduces during the soot maturation process because the values of E a , T P , and T b show increases with the increase in HAB. For the soot particles, the E a and T p have strongly positive correlations with the work function and with the logarithm of the electrical conductivity. Thus, electrical properties can serve as an indicator of soot reactivity. [Display omitted] • The electrical properties and reactivity of soot particles were assessed. • The electrical conductivity and work function increase as soot particles mature. • Soot reactivity decreases with the increase in logarithm of electrical conductivity. • Soot reactivity decreases with the increase in work function. • Soot electrical properties can serve as an indicator of soot reactivity. [ABSTRACT FROM AUTHOR]

Atomic force microscopy (AFM) was used to characterize the attractive force, adhesive force and adhesion energy between an AFM probe tip and nanometric soot particle generated by a premixed methane/oxygen flame. Different attractive force distributions were found when increasing the height above burner (HAB), with forces ranging from 1.1–3.5 nN. As the HAB was increased, the average attractive force initially increased, briefly decreased, and then underwent a gradual increase, with a maximum of 2.54 nN observed at HAB = 25 mm. The mean adhesive force was 6.5–7.5 times greater than the mean attractive force at the same HAB, and values were in the range of 13.5–24.5 nN. The adhesion energy was in the range of 2.0–5.6 × 10 −17 J. The variations observed in the average adhesion energy with increasing HAB were different from those of the average adhesion force, implying that the stretched length of soot particles is an important factor affecting the average adhesion energy. The Hamaker constants of the soot particles generated at different HABs were determined from AFM force-separation curves. The average Hamaker constant exhibited a clear correlation with the graphitization degree of soot particles as obtained from Raman spectroscopy. [ABSTRACT FROM AUTHOR]

We investigate effects of post injection strategies on regular exhaust emissions and soot particulate matters on in-cylinder emission control. A trade-off relation indicates that CO and THC emissions increase as post injection timing delays, but NOx emission significantly decreases when crank angle intervals increase because of cylinder volume expansion and heat transfer. In PM emission, total particle concentration and particle size distributions indicate that, at early post injection timing, SOF emission is relatively low but soot emission is relatively large caused by high temperature and local lean oxygen conditions. As crank angle intervals increasing, soot emission decreases quickly and becomes insensitive to the post injection timing. When SOF emission has an obvious increasing trend, there is a lowest level for the PM emission at intermediate post injection timing. Moreover, variations for total particle concentration are similar with soot emission at different post injection strategies. Particle size distribution result illustrates that PS is insensitive to post injection timing at different post injection strategies. [ABSTRACT FROM AUTHOR]

This work compared the surface functional group (SFG) types and concentrations and sp 3 /sp 2 hybridization ratios of in-cylinder soot samples generated by a heavy-duty diesel engine when employing n -heptane and a toluene/ n -heptane mixture (20% toluene by volume) as the fuels. In-cylinder soot samples were obtained from a total cylinder sampling system, and the SFGs and sp 3 /sp 2 hybridization ratios were analyzed using Fourier transform infrared and X-ray photoelectron spectroscopy. Despite the differences in fuel formulation, both n -heptane and n -heptane/toluene soot exhibited similar trends in terms of changes in the SFGs concentrations and sp 3 /sp 2 hybridization ratios during the combustion process. However, the addition of toluene to the n -heptane was found to increase the concentrations of all SFGs as well as the sp 3 /sp 2 hybridization ratio. The C OH and C O group concentrations exhibited a bimodal distribution for both the n -heptane and n -heptane/toluene soot throughout the combustion process, with the concentrations peaking in the premixed and diffusion combustion phases, respectively. In contrast, the relative amounts of aliphatic C H groups decreased in the premixed combustion phase, increased in the early diffusion combustion phase, and then decreased in the subsequent combustion phase. The sp 3 /sp 2 hybridization ratios obtained from both fuel soot were observed to initially decrease, then to increase before a decrease during the combustion process. There was a definite correlation between the sp 3 /sp 2 hybridization ratio and the relative concentration of aliphatic C H groups. [ABSTRACT FROM AUTHOR]

La 1− x Ce x Mn 1− y Co y O 3 catalysts were prepared by the “glucose method”. The structures and physico-chemical properties for these catalysts were characterized using X-ray diffraction (XRD), nitrogen adsorption, scanning electron microscopy (SEM), Fourier transform infrared spectra (FT-IR), H 2 -temperature-programmed reduction (H 2 -TPR) and O 2 -tempreature-programmed desorption (O 2 -TPD). Results showed that cerium substitution at the A-site in LaMnO 3 produced a CeO 2 phase. The cobalt can be introduced into the B-site in La 0.8 Ce 0.2 MnO 3 at any substitution ratio because of the similar ionic radii between cobalt and manganese. The catalytic activity for soot combustion in air was evaluated using a TG/DTA analyzer. Cerium substitution at A-site enhances the catalytic activity, while cobalt substitution at B-site inhibits the catalytic activity. The activation energy for soot combustion was calculated using the Horowitz method. The activation energy for non-catalytic soot combustion was 164.1 kJ mol − 1 . The addition of catalysts decreased the activation energy by about 26–63 kJ mol − 1 . Among the applied catalysts, Ce20Mn exhibited the lowest activation energy (101.1 kJ mol − 1 ). [ABSTRACT FROM AUTHOR]

Abstract: Zirconium doped Cu/ZSM-5 catalysts were prepared and characterized in this investigation. Catalytic activity during soot combustion was determined in both O2/He and NO/O2/He atmospheres by temperature-programmed oxidation. The use of zirconium reduces the temperature of maximum soot oxidation rate by 229°C in O2/He atmosphere and 270°C in NO/O2/He atmosphere. The promoting effect of zirconium is discussed in terms of surface dispersion, enrichment of active components, and creation of oxygen vacancies where molecular oxygen or NO x is adsorbed forming basic surface oxygen species active for soot oxidation. The NO2 formed at the copper–zirconium interface sites leads to the ignition temperature being significantly decreased to 93°C, which is inside the exhaust temperature range of diesel engines. To understand the combustion reaction kinetics, the activation energy and reaction order of soot combustion were evaluated. According to the Redhead method, the activation energy for non-catalyzed reaction is 164kJ/mol under the O2/He atmosphere. For the Cu/ZSM-5 and Cu–Zr/ZSM-5, the activation energies under the O2/He atmosphere (134–151kJ/mol) are slightly higher than those under the NO/O2/He atmosphere (128–135kJ/mol). The Freeman–Carroll method is suitable to describe the soot combustion in the NO/O2/He atmosphere, with the activation energies for the catalysts in the range of 97–112kJ/mol and the average value of reaction order equal to 1.36. [Copyright &y& Elsevier]

The oxidation rates of diesel soot from the combustion chamber of a running diesel engine were calculated based on the particle size distributions at different crank angles. The primary particle diameter and nanostructure of soot were obtained by means of high-resolution transmission electron microscopy (HRTEM). The soot characteristics were also investigated by oxidative thermogravimetry and Raman scattering spectrometry. The results showed the soot nanostructures were dependent on engine operation conditions and combustion phases. The oxidation rates were found to differ by nearly fourfold from that calculated by the Nagle/Strickland-Constable (NSC) model for the soots studied here. The varied oxidation rates were interpreted in terms of differences in nanostructure between the soots. The experimental results were used to modify the NSC model and the fringe length of in-cylinder diesel soot was chosen to describe the influence of graphitisation on the oxidation of soot. The modified NSC model lessened the deviation between measurements and predictions. [ABSTRACT FROM AUTHOR]

Abstract: The nanostructure, fractal dimension and size of in-cylinder soot during diesel combustion process have been investigated for a heavy-duty direct injection diesel engine, using a total cylinder sampling system followed by high-resolution transmission electron microscopy and Raman scattering spectrometry. Different structural organizations of in-cylinder soot are found depending upon the combustion phase. It is revealed that both the fringe tortuosity and separation distance decrease as combustion proceeds, while the mean fringe length increases distinctly from 1.00 to 2.13nm, indicating the soot evolution toward a more graphitic structure during the combustion process. The fractal dimensions of aggregates are in a range of 1.20–1.74 at various crank angles under the applied engine operating conditions. As temperature and pressure increase, the fractal dimension decreases significantly to a minimum at the early diffusion combustion stage. The soot particles become more compact again as the fractal dimension increases during the subsequent combustion period. Primary particle sizes start small, go through a maximum in the early diffusion combustion phase and decline again as combustion proceeds. [Copyright &y& Elsevier]

Abstract: The diesel soot was extracted from the chamber of a running diesel engine using a total cylinder sampling system. A structural description and size distribution of the soot spherules were performed by means of high-resolution transmission electron microscopy (HRTEM). HRTEM image analysis was carried out to achieve semi-quantitative information about the soot organization. Different nanostructures were found in dependence upon the engine test conditions. A better organization of the graphitic layers occurred for the in-cylinder soot as the expansion stroke developed. The oxygen concentrations at sampling angles and the cylinder pressure were measured. The oxidation rates of the in-cylinder soot during the late combustion phase were calculated according to the differences of spherules size distributions between two consecutive sampling angles. The soot reactivity in dependence on the nanostructure was shown after deducting the effect of oxygen concentration and temperature. The increase of resistance toward oxidation was associated to the increase of the structural order for the in-cylinder soot. The results are a useful database for testing the structure–property relationship between the nanostructure and the reactivity of the in-cylinder diesel soot. [ABSTRACT FROM AUTHOR]

• The physical properties of CFT and ULSD soots are studied at various engine modes. • CFT produces the soot with smaller fractal dimensions and primary particle sizes. • CFT soot has more ordered nanostructure and lower reactivity than the ULSD soot. • The fringe length and tortuosity show definite correlations with soot reactivity. This study investigated the physical properties of the exhaust soot particles from a light-duty diesel engine fueled with an ultra-low sulfur diesel fuel (ULSD) and Fischer–Tropsch diesel fuel synthesized from coal (CFT). The morphology and nanostructure of the obtained soot were characterized using a high-resolution transmission electron microscopy, while the oxidation reactivity of soot was evaluated by the thermogravimetric analysis (TGA). Despite the different fuel properties, both the ULSD and CFT soot exhibits similar changes in fractal dimension, primary particle size and nanostructure parameters (including fringe length, separation distance and tortuosity) when varying the engine speed or load. However, the CFT soot has smaller fractal dimensions and primary particle sizes and a more ordered nanostructure as compared to the ULSD soot. The TGA results demonstrate that the CFT soot has the higher apparent activation energy, indicating that it is less reactive than the ULSD soot. The length and tortuosity of the fringes within the soot structure are found to have definite correlations with soot reactivity. [ABSTRACT FROM AUTHOR]

The present study investigated the effects of adding 20 vol.% biodiesel to petroleum diesel (to produce a mixture termed B20) on the physical properties and reactivity of the resulting exhaust soot particles. Tests were performed at different engine loads of a constant speed, and the soot particles from the combustion of B20 and petroleum diesel fuel (DF) were collected from the engine exhaust stream. Transmission electron microscopy and Raman spectroscopy were employed for the analysis of soot morphology and nanostructure. The thermogravimetric analysis was used to determine the oxidative reactivity of the soot. For both the DF and B20 soot, increased engine loads result in soot aggregates with more compact morphology and primary soot particles with larger size and more organized structure. Compared to the DF soot, the B20 aggregates have a slightly more compact morphology and smaller primary particle size. No appreciable differences are observed in nanostructure between the DF and B20 soot. The thermogravimetric analysis demonstrates that the B20 soot is associated with lower peak temperature, burnout temperature and apparent activation energy, suggesting that it is more reactive than the DF soot. [ABSTRACT FROM AUTHOR]

Experiments were performed on a CH 4 lean premixed flame to better understand the effect of the oxidation-induced fragmentation of primary particles on soot oxidation reactivity. An aerosol generator was used to homogeneously disperse diesel soot into the flame. Because minimal soot was formed in this flame itself, the information about diesel soot fragmentation was readily obtained. The thermophoretic and probe sampling techniques were used to obtain soot particles at various heights above the burner. The particle size distribution, oxidation reactivity, nanostructure and carbon chemical state of each sample were characterized. The soot reactivity was evaluated in terms of activation energy. The results indicate a higher extent of aggregate fragmentation occurs at moderate soot burnout percentages and that the internal structure in soot is not destroyed during aggregate fragmentation. At higher soot burnout percentages, the internal burning produces more primary particle fragmentation, so that the soot particles exhibit an increase in fringe tortuosity and decreases in fringe length and sp2/sp3 hybridization ratio. These variations in physicochemical properties increase the oxidation reactivity of the soot particles. [ABSTRACT FROM AUTHOR]