Spark Discharge Soot Research Articles

Spark discharge-generated soot: Varying nanostructure and reactivity against oxidation with molecular oxygen by synthesis conditions

Spark discharge generators enable a fast, reliable, continuous, and highly reproducible production of nanoparticle aerosols along with a widely tunable size distribution. Graphite spark discharge generators have been particularly applied for studies of instrument calibration, filter testing, soot reactivity studies, and in toxicological risk evaluation of soot. Until now, it has been consensus that soot produced in spark discharge generators is highly reactive because of its high surface area and its amorphous nanostructure. In this paper, we will demonstrate that the properties of soot produced in spark discharge generators strongly depend on the used carrier gas (i.e. argon, nitrogen). Soot produced in argon (spark discharge argon soot, SDS-Ar) as carrier gas is composed of a major fraction with high reactivity exhibiting a temperature at maximum oxidation rate Tmax between 796 K and 803 K. Soot produced in nitrogen (spark discharge nitrogen soot, SDS-N2) has a similar size distribution as SDS-Ar, whereas the main fraction is less reactive (963 K < Tmax < 983 K). This can be derived from temperature programmed oxidation (TPO) and thermogravimetric analysis (TGA) and also correlates well with a considerable higher BET surface area of SDS-Ar (679 m2 g−1) compared with SDS-N2 (426 m2 g−1). C/H/N Elemental Analysis, High-Resolution Transmission Electron Microscopy (HRTEM), Raman- and FTIR-spectroscopy complement the measurements, all leading to the observation that SDS-N2 has a more graphitic nanostructure than the highly amorphous nanostructure of SDS-Ar (Raman spectra SDS-Ar: I(D, disordered bands)/I(G, graphite band) = 0.76, SDS-N2: I(D)/I(G) = 1.0 or elemental analysis SDS-Ar: H/C = 0.25, SDS-N2: H/C = 0.10). These findings imply that studies applying spark discharge soot have to be critically viewed with respect to the used carrier gas. Spark-over field strength, Paschen's law, the energy release within the arc, decaying plasma or cooling rates, as well as dissociation/ionization processes are discussed to explain the observations above. Finally, we want to emphasize that it is possible to vary the reactivity towards oxidation and nanostructure of spark discharge soot simply by applying different mixtures of the carrier gas.

Technical Note: The single particle soot photometer fails to reliably detect PALAS soot nanoparticles

Abstract. The single particle soot photometer (SP2) uses laser-induced incandescence (LII) for the measurement of atmospheric black carbon (BC) particles. The BC mass concentration is obtained by combining quantitative detection of BC mass in single particles with a counting efficiency of 100% above its lower detection limit. It is commonly accepted that a particle must contain at least several tenths of a femtogram BC in order to be detected by the SP2. Here we show the result that most BC particles from a PALAS spark discharge soot generator remain undetected by the SP2, even if their BC mass, as independently determined with an aerosol particle mass analyser (APM), is clearly above the typical lower detection limit of the SP2. Comparison of counting efficiency and effective density data of PALAS soot with flame generated soot (combustion aerosol standard burner, CAST), fullerene soot and carbon black particles (Cabot Regal 400R) reveals that particle morphology can affect the SP2's lower detection limit. PALAS soot particles are fractal-like agglomerates of very small primary particles with a low fractal dimension, resulting in a very low effective density. Such loosely packed particles behave like "the sum of individual primary particles" in the SP2's laser. Accordingly, most PALAS soot particles remain undetected as the SP2's laser intensity is insufficient to heat the primary particles to their vaporisation temperature because of their small size (Dpp ≈ 5–10 nm). Previous knowledge from pulsed laser-induced incandescence indicated that particle morphology might have an effect on the SP2's lower detection limit, however, an increase of the lower detection limit by a factor of ∼5–10, as reported here for PALAS soot, was not expected. In conclusion, the SP2's lower detection limit at a certain laser power depends primarily on the total BC mass per particle for compact particles with sufficiently high effective density. By contrast, the BC mass per primary particle mainly determines whether fractal-like particles with low fractal dimension and very small primary particles are detectable, while their total BC mass has only a minor influence. This effect shifts the lower detection limit to much higher BC mass, or makes them completely undetectable. Consequently, care has to be taken when using the SP2 in applications dealing with loosely packed particles that have very small primary particles as building blocks.

Conductivity for Soot Sensing: Possibilities and Limitations

In this study we summarize the possibilities and limitations of a conductometric measurement principle for soot sensing. The electrical conductivity of different carbon blacks (FW 200, lamp black 101, Printex 30, Printex U, Printex XE2, special black 4, and special black 6), spark discharge soot (GfG), and graphite powder was measured by a van der Pauw arrangement. Additionally the influence of inorganic admixtures on the conductivity of carbonaceous materials was proven to follow the percolation theory. Structural and oxidation characteristics obtained with Raman microspectroscopy and temperature programmed oxidation, respectively, were correlated with the electrical conductivity data. Moreover, a thermophoretic precipitator has been applied to deposit soot particles from the exhaust stream between interdigital electrodes. This combines a controlled and size independent particle collection method with the conductivity measurement principle. A test vehicle was equipped with the AVL Micro Soot Sensor (photoacoustic soot sensor) to prove the conductometric sensor principle with an independent and reliable technique. Our results demonstrate promising potential of the conductometric sensor for on-board particle diagnostic. Furthermore this sensor can be applied as a simple, rapid, and cheap analytical tool for characterization of soot structure.

Development of a Spark Discharge PM Sensor for Measurement of Engine-Out Soot Emissions

There is interest in measuring the engine-out particulate emissions from diesel engines in real time in order to optimize the engine control strategies such as exhaust gas recirculation (EGR) and monitor the loading of the particulate filter. This paper presents experimental results obtained using a measurement technique that produced a signal proportional to the peak voltage of a spark discharge that was exposed to the exhaust gas. The sensor was tested on a turbocharged diesel engine with exhaust soot levels from &lt;0.1 FSN to &gt;3.5 FSN, and compared with reference measurements of filter smoke number (FSN) from an AVL 415S smokemeter. The sensor signal showed a high correlation with the reference FSN measurements. Conversion of the FSN values to mass concentration values (using published techniques for the reference instrument) indicated that the output of the spark discharge soot sensor was nearly linear with mass concentration over a substantial portion of the measuring range. The sensor showed a response time of less than 2 s to step changes in FSN levels.

Multiwavelength Raman Microspectroscopy for Rapid Prediction of Soot Oxidation Reactivity

Multiwavelength Raman microspectroscopy (MWRM) analysis for characterization of soot structure and reactivity was developed. This new method is based on the dispersive character of carbon D mode in Raman spectra (i.e., red shift and increase in intensity at higher excitation wavelength, λ(0)). The approach was proven by investigating various diesel soot samples and related carbonaceous materials at different λ(0) (785, 633, 532, and 514 nm). In order to compare the behavior of the D mode for various samples and to derive a single parameter characterizing the soot structure, the difference of integrals for pairs of spectra collected at different λ(0) was calculated. MWRM analysis revealed substantial differences in the structural ordering which decreases from graphite, over Printex XE2 and various diesel soot samples, to spark discharge soot. To obtain the relation between structure and reactivity of soot, MWRM analysis was combined with temperature-programmed oxidation (TPO). TPO allowed us to characterize the oxidation behavior of soot in terms of the maximum emission (CO + CO(2)) temperature and reactivity index. The latter was calculated by introducing the reactivity limits: spark discharge soot containing a large amount of disorder represents the upper limit, whereas the lower limit is given by graphite powder with high structural order. The comparison of MWRM (viz., the observed Raman difference integrals) and TPO data revealed a linear correlation between soot structure and oxidation reactivity. Thus, we demonstrated for the first time the potential of MWRM for a robust and rapid prediction of diesel soot reactivity based on the structure-reactivity correlation.

Nitro-PAH formation studied by interacting artificially PAH-coated soot aerosol with NO 2 in the temperature range of 295–523 K

Diesel particulate matter poses a threat to human health, and in particular nitrated polycyclic aromatic hydrocarbons (NPAHs) found within and on the surface of these particles. Although diesel particulate filters (DPFs) have been designed and implemented to reduce these and other harmful diesel emissions, the particle loaded filters may act as a reaction chamber for the enhanced production of NPAHs from the nitration of PAHs with NO 2. Focus is on the investigation of the heterogeneous reactions that occur on soot particles by exposing laboratory produced pyrene- or benzo(a)pyrene-coated spark discharge soot particles to varying concentrations of NO 2 and temperatures while following the formation of products over time. The sole nitration product that was observed throughout the experiments with pyrene-coated soot was 1-nitropyrene (1-NPYR), which increased linearly with reaction time for all NO 2 concentrations chosen (0.11, 1.0, 2.0, 4.0 ppm, m m −1). Resulting 1-NPYR formation rate increased exponentially with [NO 2]. Throughout the 3-h experiments less than 10% of pyrene has been converted to 1-NPYR and the partial reaction order with regard to [NO 2] was estimated to 1.52. Benzo(a)pyrene (BaP) was more reactive than pyrene. After 3 h reaction time almost 80% of the BaP has been converted to 6-NBaP. Highest 1-NPYR concentrations on particles were detected at 373 K, and at higher temperatures a considerable decrease in particulate 1-NPYR was observed. A similar trend was observed in a DPF simulation system (PM-Kat ®-like) with BaP-coated soot. In this case, highest 6-NBaP concentration on particles was detected at 423 K. Backed by corroborating results from separate gas/solid-phase partition experiments with 1-NPYR and 6-NBaP, it is likely that the newly formed 1-NPYR and 6-NBaP became transferred from particle to gas phase at higher temperatures. Results from this study confirm the presence of 1-NPYR and 6-NBaP in particulate and gas phase under conditions encountered in DPFs, especially when operated at low temperature situations of the aftertreatment system.

Raman Microspectroscopic Analysis of Changes in the Chemical Structure and Reactivity of Soot in a Diesel Exhaust Aftertreatment Model System

Raman microspectroscopy has been applied to follow structural changes in spark discharge (GfG) soot and light-duty diesel vehicle (LDV) soot upon oxidation and gasification by nitrogen oxides and oxygen in a diesel exhaust aftertreatment model system at 523 and 573 K. Raman spectra have been recorded before and during the oxidation process, and spectral parameters have been determined by curve fitting with five bands (G, D1-D4). For GfG soot, a steep initial decrease of the relative intensity of the D3 band suggested rapid preferential oxidation of a highly reactive amorphous carbon fraction, while a less steep but also substantial decrease of band widths (in particular, the D1 band) indicated a slower overall increase of chemical homogeneity and structural order in the partially oxidized material. The spectroscopic changes are in agreementwith a strong decrease of chemical reactivity at increasing mass conversion of GfG soot. In contrast, the spectral parameters and reactivity of partially oxidized LDV soot remained largely unchanged throughout the oxidation process. Overall, the spectroscopic and kinetic findings suggest that Raman spectroscopic parameters provide information about the relative abundance and structural order of graphitelike and amorphous carbon and can be used as proxies for the chemical reactivity of soot undergoing oxidation and gasification. Thus, Raman spectroscopy promisesto become an efficient tool forfurther investigation and optimization of diesel exhaust aftertreatment in continuously regenerating traps and particle filters.

Bulk and surface structural investigations of diesel engine soot and carbon black

The microstructure and electronic structure of environmentally relevant carbons such as Euro IV heavy duty diesel engine soot, soot from a black smoking diesel engine, spark discharge soot as model aerosol, commercial furnace soot and lamp black are investigated by transmission electron microscopy, electron energy-loss spectroscopy and X-ray photoelectron spectroscopy. The materials exhibit differences in the predominant bonding, which influences microstructure as well as surface functionalization. These chemical and physical properties depend on the formation history of the investigated carbonaceous materials. In this work, a correlation of the microstructure of the samples to the predominant bonding and incorporation of oxygen into the carbons is obtained. It is shown that a high amount of defects and the deviation of the carbons from a perfect graphitic structure results in a increased incorporation of oxygen and hydrogen. A correlation between the length and curvature of graphene layers with the bonding state of carbon atoms and incorporation of oxygen and hydrogen is established.

The effect of the contact between platinum and soot particles on the catalytic oxidation of soot deposits on a diesel particle filter

Experiments were performed with two model soot aerosols brought into different forms of contact with Pt aerosol particles, to investigate the effectiveness of this contact in lowering the catalytic soot oxidation temperature. The contact was either generated between individual particles in the aerosol state (Pt-doped soot to simulate a fuel borne catalyst), or by sequential or simultaneous deposition of separately generated soot and Pt aerosols onto a sintered metal filter. (Formation of a soot cake on previously deposited Pt aerosol would simulate a catalyst coated diesel particle filter.) The catalytic activity was determined in all cases from temperature ramped oxidation in air of the filtered particles, and defined as the 50% conversion temperature.It was found that Pt-doped soot and simultaneously filtered aerosols were both equally effective in reducing the oxidation temperature by up to 140–250°C for the spark discharge soot (with 3–47wt% Pt concentration in the soot cake), and by up to 140°C for the pyrolysis soot (3wt% Pt). Conversely, the deposition of a thin soot layer of 5–10μm thickness onto Pt, or vice versa, produced only a slight temperature reduction on the order of about 13–42°C. These results suggest that the distance between soot and Pt particles plays a key role in promoting an effective oxidation on the filter, which is consistent with the role of Pt particles as local generators of activated oxygen.

Comprehensive kinetic characterization of the oxidation and gasification of model and real diesel soot by nitrogen oxides and oxygen under engine exhaust conditions: Measurement, Langmuir–Hinshelwood, and Arrhenius parameters

The reaction kinetics of the oxidation and gasification of four types of model and real diesel soot (light and heavy duty vehicle engine soot, graphite spark discharge soot, hexabenzocoronene) by nitrogen oxides and oxygen have been characterized for a wide range of conditions relevant for modern diesel engine exhaust and continuously regenerating particle trapping or filter systems (0–20% O 2, 0–800 ppm NO 2, 0–250 ppm NO, 0–8% H 2O, 303–773 K, space velocities 1.3 × 10 4–5 × 10 5 h −1). Soot oxidation and NO 2 adsorption experiments have been performed in a model catalytic system with temperature controlled flat bed reactors, novel aerosol particle deposition structures, and sensitive multicomponent gas analysis by FTIR spectroscopy. The experimental results have been analyzed and parameterized by means of a simple carbon mass-based pseudo-first-order rate equation, a shrinking core model, oxidant-specific rate coefficients, Langmuir–Hinshelwood formalisms (maximum rate coefficients and effective adsorption equilibrium constants), and Arrhenius equations (effective activation energies and pre-exponential factors), which allow to describe the rate of reaction as a function of carbon mass conversion, oxidant concentrations, and temperature. At temperatures up to 723 K the reaction was driven primarily by NO 2 and enhanced by O 2 and H 2O. Within the technically relevant concentration range the reaction rates were nearly independent of O 2 and H 2O variations, while the NO 2 concentration dependence followed a Langmuir–Hinshelwood mechanism (saturation above ∼200 ppm). Reaction stoichiometry (NO 2 consumption, CO and CO 2 formation) and rate coefficients indicate that the reactions of NO 2 and O 2 with soot proceed in parallel and are additive without significant non-linear interferences. The reactivity of the investigated diesel soot and model substances was positively correlated with their oxygen mass fraction and negatively correlated with their carbon mass fraction.

Morphology-controlled reactivity of carbonaceous materials towards oxidation

Carbon samples are being investigated with thermogravimetry, infrared spectroscopy and transmission electron microscopy. We focus on a spark discharge soot, soot from a heavy-duty diesel engine, soot from a diesel engine in black smoking conditions and a furnace carbon black from Degussa. The aim of this study is to correlate reactivity towards oxygen, functional groups and nanostructure. It is found that the amount of defects as well as the functionalisation plays an important role in the onset of combustion in the thermogravimetric experiments. Clear differences in reactivity towards oxidation are observed.

Raman microspectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information

Experimental conditions and mathematical fitting procedures for the collection and analysis of Raman spectra of soot and related carbonaceous materials have been investigated and optimised with a Raman microscope system operated at three different laser excitation wavelengths (514, 633, and 780 nm). Several band combinations for spectral analysis have been tested, and a combination of four Lorentzian-shaped bands (G, D1, D2, D4) at about 1580, 1350, 1620, and 1200 cm −1, respectively, with a Gaussian-shaped band (D3) at ∼1500 cm −1 was best suited for the first-order spectra. The second-order spectra were best fitted with Lorentzian-shaped bands at about 2450, 2700, 2900, and 3100 cm −1. Spectral parameters (band positions, full widths at half maximum, and intensity ratios) are reported for several types of industrial carbon black (Degussa Printex, Cabot Monarch), diesel soot (particulate matter from modern heavy duty vehicle and passenger car engine exhaust, NIST SRM1650), spark-discharge soot (Palas GfG100), and graphite. Several parameters, in particular the width of the D1 band at ∼1350 cm −1, provide structural information and allow to discriminate the sample materials, but the characterisation and distinction of different types of soot is limited by the experimental reproducibility of the spectra and the statistical uncertainties of curve fitting. The results are discussed and compared with X-ray diffraction measurements and earlier Raman spectroscopic studies of comparable materials, where different measurement and fitting procedures had been applied.

Microstructure and oxidation behaviour of Euro IV diesel engine soot: a comparative study with synthetic model soot substances

In this study, the microstructure and oxidation behaviour of soot from the raw exhaust of a Euro IV test heavy duty (HD) diesel engine are investigated and compared to that of spark-discharge soot and hexabenzocoronene (HBC, C42H18). We find a microstructure-controlled reactivity towards oxidation of all three samples in 5% O2 in N2. The spark-discharge soot with its fine primary particles and fullerenoid structure has an onset temperature of 423K for combustion, while the HBC with its well-ordered crystallites has a high onset temperature of 773K. Due to an improved combustion process in the Euro IV HD diesel engine, the emitted soot consists of more fullerenoid- or onion-like particles agglomerated in a chain-like secondary structure. The onset temperature of the Euro IV HD engine soot combustion is 573K. Oxidation of the three samples produces only CO2 and H2O. The different H2O production profiles can be assigned to the functionalised surface of the samples and depend on the soot structure and preparation route.

Water adsorption and energetic properties of spark discharge soot: Specific features of hydrophilicity

Water adsorption was studied for spark discharge soot which is often used as a surrogate for atmospheric soot. Analysis by adsorption gravimetry and calorimetry showed that spark discharge soot exhibits specific features of hydrophilicity. Analysis of the isotherms and heats of water adsorption revealed the peculiarities of the mechanism of surface wetting related to significant changes in the soot microstructure. The first water adsorption/desorption cycle led to irreversible water adsorption and a decrease of the surface area available for water adsorption. Repeated water adsorption/desorption cycles demonstrated swelling phenomena related to the increase of the nitrogen surface area due to interparticle microporosity. The maximum effect of the transformation was observed under exposure to saturated water vapor following the microstructure stabilization. With decreasing temperature the amount of adsorbed water also decreased. Once emitted into the atmosphere the spark discharge soot particles will exhibit irreversible changes in surface area, porosity and hygroscopicity as a result of numerous humidity fluctuations until approaching a stable state of their microstructure due to long exposure to saturated water vapor.

Interaction of Ozone and Water Vapor with Spark Discharge Soot Aerosol Particles Coated with Benzo[a]pyrene: O3 and H2O Adsorption, Benzo[a]pyrene Degradation, and Atmospheric Implications

The interaction of ozone and water vapor with spark discharge soot particles coated with the five-ring polycyclic aromatic hydrocarbon benzo[a]pyrene (BaP) has been investigated in aerosol flow tube experiments at ambient temperature and pressure (296 K, 1 atm). The investigated range of ozone volume mixing ratio (VMR) and relative humidity (RH) was 0−1 ppm and 0−25%, respectively. The observed gas-phase ozone losses and pseudo-first-order BaP decay rate coefficients exhibited Langmuir-type dependencies on gas-phase ozone concentration and were reduced in the presence of water vapor, which indicates rapid, reversible and competitive adsorption of O3 and H2O on the particles followed by a slower surface reaction between adsorbed O3 and BaP. At low ozone VMR and RH, the half-life of surface BaP molecules was found to be shorter than previously reported (∼ 5 min at 30 ppb O3 under dry conditions). At higher RH and for multilayer BaP surface coverage, however, a strong increase of BaP half-life was observed a...