Article pubs.acs.org/EF
Multiscale Characterization of Some Commercial Carbon Blacks and Diesel Engine Soot Giovanni Ferraro,† Emiliano Fratini,*,† Riccardo Rausa,‡ Paolo Fiaschi,‡ and Piero Baglioni*,† †
Department of Chemistry “Ugo Schiff” and CSGI, University of Florence, via della Lastruccia 3-Sesto Fiorentino, Florence, Italy Eni S.p.A. Research & Technological Innovation Department, S. Donato Research Center, Via Maritano 26, 2009, San Donato Milanese, Italy
‡
ABSTRACT: This paper reports on a detailed physical−chemical characterization of different carbonaceous particulates (commercial carbon blacks and diesel engine-derived soot) by using a combination of several techniques such as scanning electron microscopy (SEM), absorption isotherms, infrared spectroscopy, and small angle X-ray scattering (SAXS), to provide information on the chemical composition and structure at the micro- and nanoscale level. SEM micrographs indicate an almost spherical primary unit for all investigated samples with a diameter ranging from 5 to 100 nm. On the other hand, SAXS allows extracting the fractal dimension of both the primary units and the aggregates. All these results, taken together, can be used as a reference point to define an appropriate and rather cheap commercial surrogate for engine-derived soot and, alternatively, to choose the better carbon black according to the final industrial application.
1. INTRODUCTION Over the past decades, carbon black (CB) has attracted increasing interest because of its optical and electronic properties and for its relatively low production costs. About 65% of the world’s produced carbon black is used in the manufacturing of tires and other vehicle products; the remaining is used in the production process of a large variety of materials,1 such as printing inks, paints, and plastics, among others. The wide use of carbon black in different fields comes from its ability to confer new properties to nanocomposites. For example, carbonaceous particles can be used to improve the mechanical properties and the lifetime of rubber (reinforcing filler and antioxidant, respectively),2 to make a conductive material starting from an insulating one (conductive filler),3 and as pigment for inks and varnishes.4 Nowadays, CB for industrial use is mainly produced following two main procedures5 usually referred as f urnace black and thermal black. The furnace black process is an incomplete combustion of heavy aromatic oils in a reactor where the feedstock is vaporized and subsequently pyrolyzed to obtain carbonaceous particles. The thermal black process uses methane or heavy aromatic oils as starting feedstock. These substances are injected into a furnace where, in the absence of air, they are decomposed, thus producing carbon black and hydrogen. Sometimes, carbon black is also produced with a different process (i.e., channel black), as is the case with one of the carbon blacks reported in this work (Degussa S170). This process involves the combustion of natural gas in small glowing flames in contact with water-cooled iron rails (so-called “channels”) from which the carbonaceous particulate is recovered at the end of the combustion step. The obtained particulate results in spherical primary particles with diameter in the 10−70 nm range,6 which can coalesce to form aggregates. The characteristics of the carbonaceous particulate produced during the combustion of diesel fuel (soot) have been extensively studied in the past because of the importance of © 2016 American Chemical Society
this byproduct in the whole engine lifecycle. When the primary soot unit is formed and transported into the oil sump, a series of subsequent oxidizations and reactions with the oil and the other combustion products takes place. As a result, soot particles grow in size and aggregate, giving rise to a series of complications (i.e., oil gelation, increased viscosity, wear, etc.) strictly connected to engine failure and lower fuel economy.7 The structure of both the primary units and aggregates present in CB particulates is a key parameter for understanding the macroscopic properties of these nanomaterials and related nanocomposites. The internal structure of primary particles has been revealed by transmission electron microscopy (TEM), and a graphiticlike structure with parallel stacked layers of polyaromatic planes8,9 has been highlighted. Both the particulates show the same internal morphology, the only difference being in the dimension of crystal domains which can change depending on the production procedure.10 This feature is strictly connected to both the strong optical absorption11 and the good conductive behavior12 of these materials. Moreover, also other structural parameters, such as the chemical nature, the surface roughness, and the dimension of primary particles have been reported.12 Together, these characteristics play a key role in practical applications; for example, an increase of carbon black roughness improves the binding of elastomer in rubber materials, thus increasing mechanical and conductive properties of the final composites.13 Soot importance, as mentioned before, is related to the problems this substance generates in the diesel engine. To counteract these complications, modern lubricant formulations contain dispersant additives, which are especially developed to reduce soot impact by stabilizing the primary particles against Received: July 15, 2016 Revised: September 12, 2016 Published: October 10, 2016 9859
DOI: 10.1021/acs.energyfuels.6b01740 Energy Fuels 2016, 30, 9859−9866
Article
Energy & Fuels
of the test. The performances of the oil under investigation are evaluated by comparing the viscosity increase when at least 6% of soot is obtained with that of some reference oils. Soot samples were obtained at the end of the test (EOT) by draining the used oil from the engine and then recovering the solid particulate using a procedure modified from Clague et al.14 Briefly, a sample of oil was first diluted with hexane, and then the mixture was centrifuged (15 500g for 40 min.). The solid after separation was further purified in a Soxhlet apparatus using dichloromethane (48 h) and toluene (48 h) as solvents. Finally, the recovered soot was dried from toluene at 200 °C for 2 h. 2.2. Elemental Analysis. Carbon, hydrogen, nitrogen, and oxygen were determined by a routine flash combustion technique with a Perkin−Elmer CHNS/O Analyzer model 2400 Series II. All samples (about 10−20 mg) were dried at 80 °C just before the analysis was performed. 2.3. Small-Angle X-ray Scattering. SAXS measurements were carried out using a HECUS S3-Micro (Kratky camera) equipped with two position-sensitive detectors (PSD-50M) containing 1024 channels of 54 μm width. Cu Kα radiation of wavelength 1.542 Å was provided by a GeniX X-ray generator (Xenocs, Grenoble) working with a microfocus sealed-tube operating at a power of 12 W. The sample− detector distance was 281 mm. The volume between the sample and the detector was kept under vacuum during the measurements to minimize scattering from the air. The Kratky camera was calibrated using silver behenate, which is known to have a well-defined lamellar structure (d = 58.38 Å).21 Scattering curves were monitored in a qrange from 0.01 to 0.55 Å−1. Samples were placed into demountable cells; a transparent film of Nalophan was used as window. All measurements were done at 25 °C. The temperature was controlled by a Peltier element, with an accuracy of ±0.1 °C. All the scattering curves were corrected for the empty cell contribution considering the relative transmission factors. Desmearing of the SAXS curves was not necessary because of the sophisticated point microfocusing system. 2.4. Field Emission Scanning Electron Microscopy. FE-SEM was conducted with a ΣIGMA high-resolution scanning electron microscope (Carl Zeiss) based on the GEMINI column which features a high-brightness Schottky field emission source, beam booster, and inlens secondary electron detector. Measurements were conducted on uncoated samples with an acceleration potential of 2 kV and at a working distance of about 3 mm. 2.5. Surface Area. Surface area (SA) was determined by N2 adsorption isotherms (BET) using a Coulter SA 3100 analyzer (Beckman Coulter). Experiments were performed on 0.15 g of carbon black or soot preliminary degassed for 2 h at 130 °C. 2.6. Fourier Transform Infrared Spectroscopy. Infrared spectroscopy was performed with a Nexus 870 FT-IR spectrometer from Thermo Nicolet. Data were collected in attenuated total reflectance (ATR) mode. The spectra were obtained at room temperature, and 128 scans were gathered to get an acceptable signal-to-noise ratio. The optical resolution was 8 cm−1, and the spectral range was from 400 to 4000 cm−1. 2.7. Differential Thermogravimetric Analysis. Thermogravimetric analysis was carried out with a SDT Q600 Instrument (TA) that combines two thermal analysis measurements (DSC-TGA) in a single instrument. The instrument works in the range from room temperature to 1500 °C, and the balance sensitivity is 0.1 μg with respect to the weight change in the sample. Measurements were performed in a nitrogen atmosphere with a flow rate of 100 mL/min. The samples were analyzed in alumina open pans.
aggregation, thus reducing all the adverse factors that lead to the loss of the original lubricant oil performance. The evaluation of dispersant properties of these additives is normally carried out by performing sophisticated, as well as specific, engine tests (e.g., DV4 in the case of light-duty diesel engines). However, these tests are quite expensive and some preliminary evaluations are necessary at a lab-scale level to rank new additive candidates and reduce the final screening costs. Therefore, the study and development of new dispersants necessarily require either recovery of the real soot or the acquisition of some suitable soot substitute to be utilized for lab dispersion tests. The production, extraction, and purification of real soot from exhaust oils are complicated, expensive, and time-consuming tasks; therefore, a reproducible and easily available substrate, able to mimic soot characteristics and behavior, is necessary to evaluate the performance of new additives. The use of carbon black provides a simple way to utilize an easily accessible particulate with very reproducible composition and controlled characteristics (e.g., size distribution of primary particles). Consequently, it is important to assess how carbon black properties and its characteristics match those of the actual soot. In the past, both raw and modified CB have been studied, and their characteristics have been evaluated via spectroscopic, morphological, and tribological techniques.14−16 Various empirical methods based on CB particulates used to evaluate the dispersing properties of engine lubricants as well as methods to study other phenomenology related to soot (e.g., wear) are reported in the literature.17−20 Nevertheless, because of the low availability and high variability of actual soot, the assessment of the possibilities offered by carbon blacks as a soot surrogate is fundamental. Particularly, it is essential to verify that commercial carbon blacks and actual soot (obtained from a standard engine test) possess similar morphology and composition. In this study, we report on the physicochemical characterization of a series of commercially available carbon blacks and one soot extracted from an exhaust oil produced in a standard (light-duty) engine test (Peugeot DV4) by means of scanning electron microscopy (SEM), nitrogen adsorption isotherm (BET), and small-angle X-ray scattering (SAXS). The chemical composition and main functional groups present on the two classes of carbonaceous particles were determined by Fourier infrared spectroscopy (FT-IR) and elemental analysis.
2. MATERIALS AND METHODS 2.1. Materials. The carbon blacks investigated in this study are commercial materials produced by Cabot Corporation (Corax N110, Corax N115, Corax N234, Corax N326, Corax N539, Corax N550, Corax N351, and Vulcan XC72R) and Degussa Italia (S170). They have been used as received. All the listed carbon blacks are produced by the furnace black process except for S170, which is obtained through the channel black approach. Soot was obtained as a soot-in-oil sample produced in a standard, CEC L-93-04, light-duty, diesel, Peugeot DV4 test. The objective of this engine test is to evaluate the effect of combustion soot on engine oil viscosity increase and piston cleanliness. This procedure simulates high-speed highway service in a diesel-powered passenger car. The test is run for a total duration of 120 h (+10 h run-in phase), alternating medium-high load/speed to fast idling. Kinematic viscosity at 100 °C, soot content, and iron content in the used oil are evaluated at 24 h intervals during the procedure. The final oil drain is used in conjunction with the intermediate samples to interpolate the absolute viscosity increase at 6% soot. This approach has the value of exactly reproducing the behavior of the lubricant under the defined conditions
3. RESULTS AND DISCUSSION 3.1. Composition and Thermal Stability. Elemental composition of carbon blacks and soot is reported in Table 1 and Figure 1. The major constituents are carbon, oxygen, and hydrogen. Moreover, small amounts of nitrogen (0.2 Å−1), the scattering is dominated by the internal atomic arrangement of the carbonaceous particle.30 In the intermediate q-range (i.e., 0.05 Å−1< q < 0.2 Å−1), 3 < P < 4 corresponds to the scattering of the surface of the primary unit; finally, at low q (
