Soot Oxidation Research Articles

Real-world aging and deactivation of catalytic diesel particulate filters in heavy-duty vehicles

Low-temperature soot oxidation in catalytic diesel particulate filters (DPF) is important for maintaining high efficiency of heavy-duty vehicles. This can be achieved by coating DPFs with an oxidation catalyst. In this work, catalytic DPFs have been collected from real-world operating heavy-duty vehicles for assessment of their catalytic activity and subsequent characterization. Testing of catalytic activity revealed the diminishing nitric oxide (NO) oxidation of the aged catalysts. The apparent reaction rates showed that the number of available catalytic sites decreased with mileage explaining the loss in activity. Characterization of the samples showed a decreasing surface area as well as an accumulation of metals and poisonous elements. An important finding from SEM-EDS analysis is the evident accumulation of phosphorus and sulfur in the washcoat in the absence of other ash-related elements, potentially explaining the decreased activity.

Advancements in cobalt‐based oxide catalysts for soot oxidation: Enhancing catalytic performance through modification and morphology control

AbstractThe widespread use of diesel engines results in significant environmental contamination due to emitted pollutants, particularly soot particles. These pollutants are detrimental to public health. At present, one of the most effective ways to remove soot particles is the catalytic diesel particulate filter after‐treatment technology, which requires the catalyst to have superior low temperature activity. Compared with cerium oxide which is widely used, cobalt oxide in transition metal oxides has been widely studied in recent years because of its high redox ability and easy to control morphology. This paper elaborates on the influence of modification techniques such as doping, loading, and solid solution on the catalytic performance of cobalt‐based catalysts in soot oxidation. Along the same lines, it further reviews the research progress on cobalt‐based oxide catalysts with specific dimensional structures and morphologies in soot oxidation. Finally, it provides an outlook on the challenges faced by the theoretical basis and applied research of cobalt‐based catalysts in soot oxidation.

Evaluation of soot emission and wall heat loss for the fuels with high aromatic contents using the coupled analysis of chemical luminescence images with heat flux

The stricter regulations on the sulfur content of marine fuel oil in the general sea area that came into effect in January 2020 have led to a prediction of the increase of light cycle oil with high aromatic contents. The higher aromatic content is expected to result in higher soot in the exhaust gas. During combustion, soot emits radiant heat because of blackbody radiation, some of which passes through the walls of the combustion chamber and is released outside. Therefore, the exhaust emission characteristics and wall heat loss are affected if soot formation during combustion differs depending on the aromatic content of the fuel oil. This study developed an analytical method that combines the temporal and spatial distributions of C2 and OH radicals (these species contribute to soot formation and oxidation) to evaluate the emission and wall heat loss characteristics of different aromatics in fuel oils via combustion and heat flux analysis. The method was applied to test fuels containing different aromatic contents. Two test fuels containing 60 vol% each of toluene (monocyclic) and 1-methylnaphthalene (bicyclic) were evaluated in a rapid compression machine with a 100 mm wall-to-wall distance combustion chamber. The results suggested that the differences in soot emissions were observed because of differences in aromatics, and no differences in the wall heat flux were observed. The distributions of C2 and OH radical luminescence intensities between the walls were examined, and the differences in soot production were attributed to differences in the C2 radical production upstream of the flame in the region of low air entrainment. The lack of difference in the heat flux between the test fuels can be attributed to the the differences in the C2 and OH luminescence intensities, which are related to the heat release rate and heat flux, being almost nonexistent near the walls.

Temperature-induced evolution of CuOx clusters in CuOx/TiO2 catalyst for boosting auto-exhaust oxidation

Developing non-noble metal oxides, replacing platinum group catalysts for auto-exhaust purification, where their usage amount exceeds 40 % of global demand, remains a challenge. Herein, we presented a combined approach on the synthesis and application of robust CuOx/TiO2 catalysts with ultra-fine CuOx clusters (ca. 1 nm). The strong interaction between CuOx and TiOx induces favorable interfacial charge accumulation, facilitating the extraction of the interfacial oxygen atoms in Cu2-x-O-Ti4-y chains and the surface lattice oxygen on CuOx clusters. Isotope 18O2 experiments and DFT calculations jointly confirmed that these oxygen atoms preferentially participate in the oxidation through the Mars-van Krevelen mechanism below 250 °C. The resulted oxygen vacancies facilitate the adsorption and activation of O2 molecules, which will participate the oxidation above 250 °C through Eley-Rideal mechanism. The increase in the temperature also drives the synergy of active oxygen species in the interfacial Cu2-x-O-Ti4-y bond chains and on CuOx clusters, cooperatively promoting the conversion of NO to NO2. As a result, the CuOx/TiO2 catalyst exhibits exceptional catalytic performance in soot oxidation, with a four-fold higher reaction rate (4.17 μmol g−1 min−1) than that of single-atom Cu₁-TiO2 catalyst. Such findings provide a novel strategy for the advancement of Cu-based cluster catalysts in environmental applications.

Correction to: The effect of supported metal Species on soot oxidation over PGM/CeO2-ZrO2

Correction to: The effect of supported metal Species on soot oxidation over PGM/CeO2-ZrO2

Plasma treatment on flame soot: view from nanostructure and reactivity of soot

In recent years, the use of plasma for pollutant emission control has an expected potential. In this study, soot particles from flames were sampled from an ethylene inverse diffusion flame, which were placed in a self-designed dielectric barrier discharge plasma device for reaction treatment. The nanostructure and oxidation reactivity of soot particles after plasma treatment were evaluated by transmission electron microscopy, Raman spectroscopy, X-ray diffraction analysis and X-ray photoelectron spectroscopy. With 16 kV, 2.5 kHz as an example, with the treatment of plasma, the fringe length decreases (1.3% and 9.6%), and the fringe tortuosity increases (1.1% and 8.7%) in both high-maturity and low-maturity soot particles, which means that the oxidation activity of soot is enhanced after plasma discharge. Raman spectra, X-ray diffraction analysis spectra and X-ray photoelectron spectroscopy spectra have confirmed this phenomenon, especially the oxidation activity of soot particles under the action of plasma at high maturity is more obvious. It is worth noting that the enhancement of soot oxidation activity after plasma discharge has certain limitations, that is, the oxidation activity will not continue to increase after it is enhanced to a certain intensity, which may be related to the selection of Ar as the atmosphere gas.

The effect of supported metal Species on soot oxidation over PGM/CeO2-ZrO2

Abstract Herein is studied the soot oxidation over platinum group metal oxide/CeO2–ZrO2 (PGM/CZ) catalysts. The ability to capture gas-phase oxygen was in sequence of Ru/CZ > Rh/CZ > Ir/CZ > Pt/CZ > Pd/CZ. A soot oxidation test by TG-DTA showed that Ru/CZ, Rh/CZ, and Ir/CZ are highly active catalysts. It was found that there is a good correlation between the ability to capture gas-phase oxygen and soot oxidation activity. TEM observation revealed that soot oxidation mainly occurs at the interface between soot and CZ surfaces. The Ea values and soot oxidation test using labelled oxygen suggest that highly active catalysts oxidize soot by CZ lattice oxygen. For Ir/CZ, soot oxidation at 270 °C occurred due to the reduction by soot. Ru/CZ and Rh/CZ captured gas-phase oxygen spontaneously below 250 °C, resulting in soot oxidation at 270 °C. H2-TPR results suggest that the reactivity of lattice oxygen in the CZ surface, improved by PGM, is also related to soot oxidation activity. This suggests that the ability to capture gas-phase oxygen and the reactivity of lattice oxygen in the CZ surface determine the soot oxidation activity, and that Ru, Rh, and Ir have the effect of enhancing these properties.

Soot and Flame Structures in Turbulent Partially Premixed Jet Flames of Pre-Evaporated Diesel Surrogates with Admixture of OMEn

In this study, the soot formation and oxidation processes in different turbulent, pre-evaporated and partially premixed diesel surrogate flames are experimentally investigated. For this purpose, a piloted jet flame surrounded by an air co-flow is used. Starting from a defined diesel surrogate mixture, different fuel blends with increasing blending ratios of poly(oxymethylene) dimethyl ether (OME) are studied. The Reynolds number, equivalence ratio, and vaporization temperature are kept constant to ensure the comparability of the different fuel mixtures. The effects of OME addition on flame structures, soot precursors, and soot are investigated, showing soot reduction when OME is added to the diesel surrogate. Using chemiluminescence images of C2 radicals (line of sight) and subsequent Abel-inversion, flame lengths and global flame structure are analyzed. The flame structure is visualized by means of planar laser-induced fluorescence (PLIF) of hydroxyl radicals (OH). The spatial distribution of soot precursors, such as polycyclic aromatic hydrocarbons (PAHs), is simultaneously measured by PLIF using the same excitation wavelength. In particular, aromatic compounds with several benzene rings (e.g., naphthalene or pyrene), which are known to be actively involved in soot formation and growth, have been visualized. Spatially distributed soot particles are detected by using laser-induced incandescence (LII), which allows us to study the onset of soot clouds and its structures qualitatively. Evident soot formation is observed in the pure diesel surrogate flame, whereas a significant soot reduction with changing PAH and soot structures can be identified with increasing OME addition.

Ce(III) formate-derived hierarchical cerium oxide particles with icosahedral symmetry-based architectures: Effect of third level of structural hierarchy in soot and propane oxidation

The article investigates the role of architecture of complex hierarchically structured ceria and gadolinium doped ceria particles in soot combustion (solid-solid reaction) and propane oxidation (gas-solid reaction). Synthesis of hierarchical ceria mesostructures showing icosahedral symmetry-based architecture has been presented for the first time. Carefully choosing synthetic conditions allows to obtain particles with intended morphology of dodecahedron, rhombic hexecontahedron, great stellated dodecahedron, fractal-like branched great stellated dodecahedron and great dodecahemicosacron polyhedra. Twinning-based multiple branching of Ce(III) formate crystals by {101} twin orientation relation was proposed as a driving mechanism for the formation of macroparticles architecture that has been confirmed by the construction of macroparticle models using the identified twinning law. In the catalytic studies, the third level of the material organizational structure referring to shape of hierarchical macroparticles was chosen as a tested variable, while other architecture-related properties were controlled to be alike. In result, the shape of hierarchical particles plays a vital role in the catalytic soot oxidation, and architecture modulation allows the catalytic properties of the material to be enhanced through increasing the geometrical fit between ceria and soot particles. The best sample lowers temperature of half oxidation by 143 °C as compared to non-hierarchically organized ceria nanocrystals. The shape of hierarchical particles does not play leading role in gas-solid propane oxidation, but introduction of Gd dopant diversifies catalytic activity. The architecture of the particles may serve as a modifier of the catalytic activity through grain boundary engineering of gadolinium doped ceria.

Catalytic performance of Cs-V-based non-noble soot oxidation catalyst used for DPF and its enhancement by cerium addition

Non-noble metal catalysts have great potential for controlling soot emissions due to their effective oxidation properties and low cost. This study investigated the characteristics of Cs-V-based non-noble metal soot catalyst and Ce doping enhancement effect through comprehensive characterization of physicochemical properties, catalytic activity evaluation, and density functional theory (DFT) theoretical calculations. The results demonstrate that Ce doping improves the degree of surface particle aggregation and the spatial distribution morphology of Cs-V-based catalyst, with a 36.7 % increase in the specific surface area. Additionally, Ce doping enhances the probability of oxygen adsorption and dissociation activation on the Cs-V-based catalyst, accelerating the soot-oxidation process, improving the soot-oxidation activity, with a decrease in the characteristic temperature T10 (temperature at 10 % conversion rate of soot) from 349.7 ℃ to 281.1 ℃, and decreases the activation energy for soot-oxidation to 5.8 kJ/mol. DFT calculations reveal that Ce doping strengthens the pulling effect of interatomic chemical bonds, facilitating the removal of oxygen atoms and dissociated reactive oxygen (Oasd), resulting in an increase in the adsorption energy of C4 molecules and a shorter bond length between C4 molecules and the catalytic surface, facilitating the desorption of post-oxidation soot products.

Insight into the mechanism of solution organic fractions on soot oxidation activity enhancement

The particulate matter and soluble organic fraction emitted by diesel engine are hazardous to environment and human health. Exploring the effect mechanism of soluble organic fraction on soot oxidation is beneficial for reducing the emissions. In this study, the effects of four different types of soluble organic fractions on the soot oxidation activity and physicochemical properties are investigated. The results show that the attachment of oxygen-containing soluble organic fractions enhances the soot oxidation and reduces the peak characteristic temperature. However, the low volatility soluble organic fractions without oxygen element inhibit soot oxidation. Additionally, the high volatility soluble organic fractions without oxygen element elicit limited effects on soot oxidation. the contents of aliphatic C-H functional groups, carbonyl CO functional groups, and carboxylic acid O-CO functional groups significantly increase after adding oxygen-containing soluble organic fractions, while the limited increase in functional groups is observed in soluble organic fractions without oxygen element. Solid soluble organic fractions adhere to soot particles in the form of small particles, leading a reduction in the initial particle size distribution, while liquid soluble organic fractions exhibit block and chain shapes around the soot particles, which makes the initial particle size distribution increasing. Moreover, the attachment of all soluble organic fractions disrupts the surface order of soot particle, leading to a decrease in soot graphitization. This study is beneficial for revealing the interaction mechanism between soot and soluble organic fractions.

Co3O4-modified 3DOM LaFe0.6Mg0.4O3 perovskite as robust catalysts for soot oxidation

In this study, 3DOM LaFe0.6Mg0.4O3 perovskite was prepared using a colloidal crystal template technique and subsequently decorated with Co3O4 for soot combustion. The results indicated that the LaFe0.6Mg0.4O3 perovskite with three-dimensional ordered macropores structure could facilitate the dispersion of Co3O4, boost the number of active sites for soot combustion, and significantly enhance the contact efficiency between the catalyst and soot. The interaction between Co3O4 and perovskite created additional oxygen vacancies and enhanced the redox ability of the catalysts. Meanwhile, the presence of Co3O4 facilitated the adsorption and activation of O2, which could accelerate the oxidation of soot particles. Notably, the 2.5 %Co/LaFe0.6Mg0.4O3 catalyst exhibited the highest performance, achieving a T50 of 407 °C for soot combustion in the presence of 500 ppm NO and 5 % O2. The catalyst also demonstrated excellent stability and water resistance, making it a promising candidate for practical applications.

High performance of PrMnO3 perovskite catalysts for low-temperature soot oxidation

In the field of heterogeneous catalysis for diesel vehicle exhaust, designing low-cost catalysts with high efficiency and low-temperature catalytic oxidation to reduce the ignition temperature of soot remains a significant challenge. In this study, a series of praseodymium manganese perovskites (PrMnO3) were synthesized using a solution combustion method that regulated the molar ratio of glycine to nitrate (φ). Notably, when φ = 1, the prepared catalyst (PMO-1) exhibited a multistage pore structure and large specific surface area, demonstrating excellent catalytic oxidation performance for soot (T10 = 280 °C, T50 = 377 °C, TOF=16.2 × 10-4 s−1, SCO₂ > 99 %) and NO (maximum NO conversion rate of 80 %). During soot combustion in a NO+O2 reaction atmosphere, the strong NO oxidation capacity of the catalyst provides sufficient NO2 as an oxidant for soot combustion, thereby further enhancing its oxidation performance (T50 = 360 °C). This study concludes that the excellent low-temperature catalytic oxidation performance of the PrMnO3 catalyst is attributed to the large specific surface area provided by its multistage pore structure and the extensive dispersion of active sites, which enhance the contact efficiency between soot particles and active sites. Additionally, based on multiple characterization results such as XPS, TOF calculations, and Soot-TPR, the PMO-1 catalyst exhibits a high content of lattice oxygen and efficient lattice oxygen migration. These properties further supply active oxygen for the NOx-assisted soot oxidation mechanism. This study offers a valuable, simple, and cost-effective strategy for designing perovskite catalysts and efficiently removing PM particles.

Modeling of Soot Emission in Turbulent Diffusion Flames Impinging on a Cold Surface

ABSTRACT A hydrocarbon flame impinging on a cold surface produces an enhanced soot emission compared to the identical free flame. Cooking with biomass fuels is one practical process where soot emissions due to flame impingement has important environmental and health consequences. This problem was examined in a simplified system in which a turbulent non-premixed ethylene flame impinged on a cold surface. Previous experiment on this configuration examined the influence of a number of parameters on soot emission. The present paper uses numerical modeling to investigate the mechanism of soot emission enhancement in this configuration. Variations in the height of the surface above the fuel jet inlet showed going through a maximum, replicating the data. Since soot behavior can be affected by both thermal quench and flow field disruption, we decoupled the processes by replacing the cold surface with an adiabatic surface. These results indicated that most of the emission enhancement was due to the flow field disruption and not due to thermal quench. The modeling suggests that the presence of the surface changes the scalar dissipation rate, which influences C2H2 concentration, a key species in the soot surface growth process. Specifically, the surface causes a spike in the scalar dissipation rate adjacent to the surface, which significantly influences C2H2 concentration and results in increased soot production. The surface also forces the flame to spread radially, allowing more O2 to penetrate the flame immediately, enhancing the soot oxidation process. The combination of the enhanced soot production and oxidation produces the observed behavior.

Role of morphology in improving the catalytic performance of ZnCo2O4 for soot oxidation

The extent of interaction between carbon and catalyst profoundly shapes soot oxidation results. The catalyst configuration notably influences the frequency of contact points in solid–solid interactions. This investigation studies the impact of three distinct ZnCo2O4 catalyst morphologies and their redox property on soot oxidation. The formation of the cubic phase of ZnCo2O4 via three distinct methods was revealed during XRD analysis. SEM analysis unveiled varying morphologies, including rod-shaped, rose petal-shaped, and bead-like structures. Notably, ZnCo2O4 exhibiting bead-like morphology demonstrated heightened levels of chemisorbed oxygen species which was observed during XPS analysis. The presence of Co2+ and Co3+ occupied at octahedral site ZnCo2O4 acted as the active sites for soot oxidation. With continuous redox property (Co 3+ → Co 2+) leading to the generation of active oxygen species and with an added advantage of surface morphology, the M2 sample (with bead-like morphology) exhibited superior soot oxidation activity, which is evident by its T50% value of 402 °C. This study underscores the essential role of catalyst morphology in influencing soot oxidation activity. Through a comprehensive array of structural, morphological, and catalytic analyses, this work sheds light on the correlation between catalyst architecture and enhanced soot oxidation performance.

Facile preparation of Fe-Beta zeolite-supported transition metal oxide catalysts and their catalytic performance for the simultaneous removal of NOx and soot

Diesel engine exhaust comprises nitrogen oxides (NOx) and soot particles, which cause serious air pollution. However, owing to the contradictory nature of NOx reduction and soot oxidation, a trade-off exists in the simultaneous removal of NOx and soot. Consequently, catalytic technology has become a hot research topic. This study prepared MOδ/Fe-Beta (M=Fe, Co, Ni, Mn, Cu) catalysts through incipient wetness impregnation using Fe-Beta as the support and explored the catalytic performance of the above catalysts. The results exhibited the good performance of the prepared catalysts. The introduction of Mn resulted in a lower peak temperature of soot combustion for the catalyst, and slightly decreased deNOx performance of Fe-Beta. The soot combustion temperature was as low as 422 °C, and the temperature window for 80% NO conversion was 164–423 °C. The interaction between MnOδ and zeolite can regulate the acid sites and produce sufficient active oxygen species for the catalyst. The catalytic activity of the MnOδ/Fe-Beta catalyst is due to its strong redox property, appropriate number of acid sites and sufficient number of active oxygen species. In addition, the catalyst had good stability and water and sulfur resistance, therefore it had great potential for future application in simultaneous removal of NOx and soot from diesel engine exhaust.

Influence of spray-to-spray interaction after wall impingement of spray flames on diesel combustion characteristics

The influence of spray-to-spray interaction after wall impingement of spray flames on the combustion characteristics in high pressure and high temperature ambient gas like in combustion chambers of diesel engines was examined with a constant volume vessel. Fuel was injected onto a flat wall from two nozzles to form two parallel, adjacent sprays in the vessel, causing the spray-to-spray interaction after the wall impingement. The combustion was analyzed with the rate of heat release calculated from the pressure transition in the vessel and the spray flame was visualized by high-speed video. The 310 nm UV light images of the chemiluminescence from OH radicals are recorded to demonstrate the reaction activity in the spray flame. The images of transmitted light throughout the constant volume vessel were recorded to visualize the soot formation and oxidation processes as well as to quantify the soot concentrations as the KL factors. The results showed that the rate of heat release from the main combustion decreases and the afterburning increases with the spray-to-spray interaction after the wall impingement of the spray flame. Combustion suppression with the spray-to-spray interaction occurred in all the conditions of the experiments here when changing the distance from the nozzle to the impinging wall between 25 and 40 mm and the fuel injection pressures between 100 and 200 MPa. Inside the spray-to-spray interaction zone, the chemiluminescence from OH radicals is weaker, supporting the inactive combustion due to difficulties of the air entrainment, and the lower transmitted light intensities with larger KL factors, indicating higher soot concentrations. The spray-to-spray interaction zone on the impingement wall advances toward the inside of the vessel between the sprays and it moves away from the wall, entraining the unutilized air and causing a relatively active combustion as well as rapid soot oxidation during the late afterburning stage.

Preparation and application of SiO2–Fe2O3 and SiO2–Fe2O3–Fe for soot oxidation: A step toward decarbonization by reducing soot particle emissions

The article presents a method for obtaining catalytic systems: SiO2–Fe2O3, SiO2–Fe2O3–Fe and verification of their catalytic properties in the oxidation process of technical soot N550. The process of immobilization of Fe3+ ions on microsilica-SiO2 was investigated in the batch system (equilibrium, kinetics, thermodynamics). The process was aimed at obtaining a system with a developed surface and using less iron while maintaining the same catalysis active surface. In the next stages, the SiO2–Fe3+ systems were modified to obtain SiO2–Fe2O3 and SiO2–Fe2O3–Fe materials, which exhibited catalytic properties. To obtain catalytic systems, the processes of Fe3+ ions sorption, iron oxide precipitation - Fe2O3 and Fe reduction using a plant extract were used. Catalytic systems were applied in the N550 technical soot oxidation process to reduce the conversion temperature and increase its efficiency. The soot oxidation process was carried out in a muffle furnace using variable process parameters, i.e. temperature (450, 475, 500, 525 and 550oC), time (1, 2 and 3h), type of catalytic system (SiO2–Fe2O3, SiO2–Fe2O3–Fe) and its % content relative to the constant mass of soot (0, 10, 20 and 30%). The greatest increase in the conversion efficiency of soot particles was obtained using the SiO2–Fe2O3 system with a content of 20% at a temperature of 550oC and for 3 h.

The effect of deposit morphology on soot oxidation in non-catalytic and catalytic processes

Investigating the oxidation behavior of soot particles with varying deposit morphologies is crucial for understanding the mechanisms behind uneven regeneration in diesel particulate filters and enhancing regeneration strategies. This study explores the impact of two distinct deposit morphologies on soot oxidation within non-catalytic and catalytic processes, with a particular focus on the associated changes in pore structure during oxidation. Findings indicate a minimal difference in oxidation activity between loose and compacted soot particles in non-catalytic oxidation, while a significant enhancement in oxidation activity is observed for compacted soot particles during catalytic oxidation. Specifically, compacted soot particles exhibit a pronounced reduction in total pore volume, particularly in macropores, and a shift in pore size distribution towards smaller diameters during non-catalytic oxidation. In the catalytic oxidation process, compacted soot particles predominantly form 2–10 nm mesopores, with limited formation of pores larger than 10 nm. Oxidation activity in the non-catalytic process positively correlates with pore volume, especially for 100–200 nm macropores. In contrast, during catalytic oxidation, oxidation activity inversely relates to pore volume, emphasizing the critical role of catalyst-soot contact in catalytic oxidation efficiency.

Soot evolution in ethylene combustion catalyzed by electric field: experimental and ReaxFF molecular dynamics studies

In this study, an electric field (E-field) was used to regulate the formation and oxidation of soot in combustion. In the procedure, an E-field is applied to the ethylene non-premixed flame, the flame is stretched horizontally and the flame height is reduced. By comparing the partially premixed flame to eliminate the effect of the E-field on the increase of combustion temperature caused by premixing, transmission electron microscope (TEM) detection revealed that the soot particles were more dispersed in the action of E-field, the “core-shell” structure of soot disappeared, and the markedly improved oxidation rate of soot in the flame. The evolution process of soot in ethylene combustion was studied by reactive molecular dynamics. The 9 ns simulation was performed at E-field strengths of 0, 0.001, and 0.1 V/Å by three successive modelings. The results showed that the E-field reduced the growth of the largest molecule in the system and increased the coagulation time of the initial soot particles. With increasing E-field strength, the “core-shell” structure of soot gradually broke. An increased hydrogen-carbon ratio and decreased number of six-membered rings are the internal factors for the more scattered structure of soot, which allows the soot to be more easily oxidized.