Energy Fuels 2010, 24, 3778–3783 Published on Web 01/14/2010
: DOI:10.1021/ef901366v
Comparative Study on Physicochemical Properties and Combustion Behaviors of Diesel Particulates and Model Soot† Jian Liu, Zhen Zhao,* Chunming Xu, Aijun Duan, and Guiyuan Jiang State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Received November 14, 2009. Revised Manuscript Received December 29, 2009
The physicochemical properties and combustion characteristics of model soot (i.e., Printex U) and diesel particulates with different origins were compared. Their elemental composition and structures were determined using several kinds of techniques including element analysis, infrared spectroscopy (IR), X-ray fluorescence spectroscopy (XRF), and X-ray photoelectron spectroscopy (XPS), and their different reactivities were measured by a number of temperature-programmed oxidation (TPO) experiments. It was found that the Brunauer-Emmett-Teller (BET) surface area of the particulates increased with the increase of the amount of adsorbed hydrocarbons. The main element components of model carbon were basically consistent with those of the practical diesel particulates, including C, H, O, etc. However, their element contents and surface properties were very different. The major ingredient of Printex U was carbon, and its content reached 92%; however, the carbon content of diesel particulates was about 65-74%, and the contents of O, H, and N in the diesel particulates were about 3-10 times as much as those of Printex U. TPO results showed that the diesel particulates could be more easily removed than Printex U by the combustion method, and the combustion temperatures for both Printex U and diesel particulates could be lowered when catalysts were present. Printex U is very well-suited as a kind of model soot for the study on the combustion performances of the soot fraction of diesel particulates.
considerably because of variation in the condition of the engine, ambient conditions, and temperature change.4 Thus, the model soot can be a good candidate and is often used in laboratory research. However, the physicochemical properties and combustion characteristics between diesel particulates and the model soot are obviously different.5-7 Some pioneering works have illustrated that different engine load conditions could produce different diesel particulates in morphology and nanostructure.6-8 The objective of this study was to determine the properties and oxidation characteristics for diesel particulates collected from a diesel engine as well as for model soot (Printex U). The model soot was compared to the diesel particulates to validate its choice. After an eventual positive validation, it can be used as the model soot for the research about the catalytic combustion of soot and diesel particulates.
1. Introduction Diesel engine emissions are a source of environmental pollution and have been causing a threat to the environment and people’s health. Diesel particulates and NOx emissions are the main problems because very small particles carrying various suspectedly mutagenic polynuclear aromatic hydrocarbons (PAHs) can penetrate deeply into the lungs, while NOx compounds contribute to both acid rain and photochemical smog. The standard for the diesel particulates emission control has been increasingly strict over the last 2 decades. The combination of traps and oxidation catalysts is one of the most plausible approaches to eliminate soot particulates.1-3 The specific composition of the diesel particulates determines its reactivity in the catalytic oxidation reaction. Therefore, the composition is one of the starting points and considering options to increase the oxidation reactivity of diesel particulates. Diesel particulates, which are often erroneously referred to as diesel “soot”, are composed of a number of different constituents. The relative amounts of these constituents of diesel particulates vary with the diesel engine and the conditions at which the engine is operated, such as temperatures, type of fuel and lubricating oil, and engine loading. In addition, the composition of diesel particulates from a certain engine under defined operating conditions may also vary
2. Experimental Section 2.1. Collection of Diesel Particulates. Yezerets et al.7-9 did excellent work about the collection and characterization of diesel particulates. In this work, DPI is diesel particulates that were collected from a JX493ZQ light-duty diesel engine and (4) Neeft, J. P. A.; Makkee, M.; Moulijn, J. A. Fuel Process. Technol. 1996, 47, 1–69. (5) Su, D. S.; Jentoft, R. E.; M€ uller, J. O.; Rothe, D.; Jacob, E.; M€ Simpson, C. D.; Tomovic, Z.; ullen, K.; Messerer, A.; P€ oschl, U.; Niessner, R.; Schl€ ogl, R. Catal. Today 2004, 90, 127–132. (6) Bonnefoy, F.; Gilot, P.; Stanmore, B. R.; Prado, G. Carbon 1994, 32, 1333–1340. (7) Yezerets, A.; Currier, N. W.; Kim, D. H.; Eadler, H. A.; Epling, W. S.; Peden, C. H. F. Appl. Catal., B 2005, 61, 120. (8) Wal, R. L. V.; Yezerets, A.; Currier, N. W.; Kim, D. H.; Wang, C. M. Carbon 2007, 45, 70. (9) Yezerets, A.; Currier, N. W.; Eadler, H. A.; Suresh, A.; Madden, P. F.; Branigin, M. A. Catal. Today 2003, 88, 17.
† This paper has been designated for the Asia Pacific Conference on Sustainable Energy and Environmental Technologies (APCSEET) special section. *To whom correspondence should be addressed. E-mail: zhenzhao@ cup.edu.cn. (1) Liu, J.; Zhao, Z.; Xu, C.; Duan, A.; Jiang, G.; Yang, Q. Appl. Catal., B 2008, 84, 185–195. (2) Liu, J.; Zhao, Z.; Xu, C.; Duan, A. Appl. Catal., B 2008, 78, 61–72. (3) Liu, J.; Zhao, Z.; Xu, C.; Duan, A.; Jiang, G. J. Phys. Chem. C 2008, 112, 5930–5941.
r 2010 American Chemical Society
3778
pubs.acs.org/EF
Energy Fuels 2010, 24, 3778–3783
: DOI:10.1021/ef901366v
Liu et al.
DPII is diesel particulates that were collected from an ISBE18030 heavy-duty diesel engine. The low-sulfur fuel (50 ppm) from the Petrochina company was consumed by these two engines. The full exhaust gas stream was led through the filter. A bypass with pressure control was installed to avoid exhaust gas pressures from becoming too high, because the combustion efficiency of the engine would be decreased at high exhaust gas back pressures. Particulates were collected up to a back pressure of 25 kPa. After collection, particulates were prudently scraped off the filter, avoiding small pieces of filter from coming off. The particulates were stored in closed vessels. Because hydrocarbon desorption might continue and the reaction (e.g., with air) could take place, no samples older than a week were used. No difference in hydrocarbon content was found when fresh particulates were compared to particulates that had been stored for about a week in a closed vessel. The model soot (Printex U) was supplied by Degussa AG. 2.2. Characterization of Samples. Elemental composition analysis of soot particulates was performed on FlashEA 1112, made in the U.S. Samples were dried at 373 K prior to analysis. The Brunauer-Emmett-Teller (BET) specific surface areas were measured with linear parts of the BET plot of the N2 isotherms, using a Micromeritics ASAP 2010 analyzer. Before analyses, samples were heated for 10 h at 425 K under vacuum. The morphology of the catalysts was observed by a scanning electron microscope (SEM, S-4800, Japan). A transmission electron microscope (TEM) was carried out on Philips JRM 2010. Collected diesel particulates were suspended in ethanol using an ultrasonic bath kept for 30 min. A small droplet of the dispersion was deposited on a small copper frit with a carbon film on top. Fourier transform infrared (FTIR) spectra were measured with a FTS-3000 spectrophotometer in the wavenumber ranging from 400 to 4000 cm-1. For the transparent IR experiments under ambient conditions, the measured wafer was prepared as the KBr pellet, with the weight ratio of the sample/KBr of 1:200. X-ray fluorescence spectrometry (XRF) was performed on a Philips PW 1400 spectrometer. X-ray photon electron spectroscopy (XPS) measurements were recorded using a Perkin-Elmer PHI 5600 ci spectrometer with a standard Al KR source (1486.6 eV), working at 350 W. The working pressure was less than 1 10-8 Pa. The spectrometer was calibrated by assuming the binding energy (BE) of the Au 4f7/2 line at 84.0 eV with respect to the Femi level. The standard deviation in the BE values of the XPS line is 0.10 eV. 2.3. Measurement for the Combustion of Model Soot and Diesel Particulates. The combustion behavior of the prepared samples was evaluated with a temperature-programmed oxidation (TPO) reaction on a fixed-bed tubular quartz system. The reaction temperature was controlled through a proportionalintegral-derivative (PID) regulation system based on the measurements of a K-type thermocouple and varied during each TPO run from 200 to 700 °C at a 2 °C/min rate. The catalyst and soot were carefully mixed; 100 mg of the mixture (10:1 catalyst/ soot, w/w) was placed in the tubular quartz reactor (di = 6 mm) in every test; and the mixture of 10 mg of soot and 100 mg of quartz sand was placed in a reactor for the reaction without catalysts. Reactant gases containing 5% O2 and 0.2% NO balanced with He were passed through a mixture of the catalyst and soot at a flow rate of 50 mL/min. The outlet gas from the reactor passed through a 1 cm3 sampling loop of a six-point gassampling valve before it was injected into an online gas chromatograph (GC). The GC used a flame ionization detector (FID) to analyze the gaseous mixture composition and determine CO and CO2 concentrations after separating these gases over a Porapak N column and converting them to methane over a Ni catalyst at 380 °C.
Table 1. Results for EA of Printex U or Diesel Particulates element content
Printex U (%)
DPI (%)
DPII (%)
92.0 0.7 3.5 0.1 0.2
65.6 4.1 21.8 2.8 0.9 0.24 3.86 0.7
73.64 1.7 14.2 1.1 0.7 0.41 9.05 0.2
C H O N S Si Fe othera a
3.5
Calculation values.
particulates and Printex U: elemental analysis (EA), XPS, and XRF. Elemental compositions of diesel particulates and Printex U are depicted in Table 1. These results revealed that the major constituents of diesel particulates were carbon, oxygen, and hydrogen. The concentrations of nitrogen and sulfur were lower. Sulfur was found by the above techniques, which showed good agreement. The sulfur content was too low to be detected by XPS. XPS is a surface technique, and thus, it is applicable to the surface rather than the bulk of diesel particulates. XRF is quite a sensitive technique and is good for quantitative analysis of concentrations. Relative concentrations by XRF include the surface and bulk phase of the samples. 3.2. Results of XPS Characterization. Diesel particulates and Printex U samples were investigated by the XPS technique. As shown in Figure 1, the two weak XPS peaks are located at 284.9 and 532.8 eV for real diesel particulates (DPI and DPII), which can be assigned to the C1s line and O1s line, respectively.10,11 However, for the Printex U sample, only one XPS peak located at 284.9 eV can be observed. It indicates that very little adsorbed oxygen existed on the model carbon Printex U. In addition, the other elements were not detected because of their low surface concentration. Furthermore, the intensity of XPS peaks indicates the element concentration of the sample surface. Figure 1 exhibits that C1s line peak intensity gradually deceased from Printex U to DPII and DPI. It indicates that the carbon content follows the order Printex U > DPII > DPI. In addition, O1s line peak intensity of DPI is higher than that of DPII, indicating that the oxygen content is larger in DPI than that in DPII. 3.3. Surface Area and Amount of Adsorbed Hydrocarbons. BET surface areas of diesel particulates and Printex U are shown in Table 2. The BET surface area decreased from 398.2 to 217.8 m2/g for diesel particulates originating from a light-load engine to a heavy-load engine. In addition, the lower BET surface area of Printex U was 100 m2/g. The hydrocarbon fraction of diesel particulates is known to consist of paraffinic, monoaromatic, polyaromatic, and oxygenated hydrocarbons. These groups of hydrocarbons are composed of a large number of individual compounds with a range of molecular masses (typically, monoaromatics from C8 to C15 and paraffin from C14 to C26).12,13 Because these compounds consist of a large amount of pore structures, the surface area of the particulates would be increased. (10) Kirchner, U.; Vogta, R.; Natzeck, C.; Goschnick, J. J. Aerosol Sci. 2003, 34, 1323–1346. (11) Collura, S.; Chaoui, N.; Azambre, B.; Finqueneisel, G.; Heintz, O.; Krzton, A.; Koch, A.; Weber, J. V. Carbon 2005, 43, 605–613. (12) Barbella, R.; Bertoli, C.; Ciajolo, A.; D’anna, A. Combust. Sci. Technol. 1988, 59, 183–198. (13) Ikegami, M.; Li, X.; Yoshihara, Y.; Inagaki, H. SAE Tech. Pap. 890465, 1989.
3. Results 3.1. Elemental Composition. Several techniques were employed to assess the elemental composition of diesel 3779
Energy Fuels 2010, 24, 3778–3783
: DOI:10.1021/ef901366v
Liu et al.
Figure 1. Survey XP spectra of several diesel particulates and Printex U: (a) Printex U, (b) DPI, and (c) DPII. Table 2. BET Surfaces and the Combustion Temperatures and sCO2m of Model Soot and Diesel Particulates over V2O5
samples Printex U DPI DPII
BET surface area (m2/g)
T10 (°C)
T50 (°C)
T90 (°C)
Tm (°C)
sCO2m (%)
100
316
390
479
391
87
398.2 217.8
301 303
369 372
429 431
376 378
88 88
Figure 2. TEM micrographs of diesel particulates and Printex U: (a) Printex U, (b) DPI, and (c) DPII.
3.4. TEM and SEM Results. Micrographs of diesel particulates were obtained with TEM and SEM, and typical examples are shown in Figures 2 and 3. From these images, it can be inferred that particulates have a very loose structure, which is the result of the coagulation process. The average sizes of the elemental particles were determined from a large number of elemental particles on several
It is very clear that the adsorbed hydrocarbons result in a large increase in the BET surface area for DPI (from a lightload diesel engine) compared to DPII (from a heavy-load diesel engine) and Printex U. 3780
Energy Fuels 2010, 24, 3778–3783
: DOI:10.1021/ef901366v
Liu et al.
Figure 3. SEM micrographs of diesel particulates and Printex U: (a and b) Printex U, (c and d) DPI, and (e and f) DPII.
located at 570, 680, and 977 cm-1 are assigned to the -C-Cand -CdC- vibrations of the carbon framework. In the IR spectra of real diesel particulates, the absorption peaks at around 2928 and 3048 cm-1 are corresponding to the C-H vibration of the aromatic cycle, the peak at around 764 cm-1 is due to the vibration of substituted aromatic carbon, and the absorption peaks at around 1600 and 1724 cm-1 are assigned to the -CdO- vibration.10,16 There are scarcely any vibration peaks of C-H, -CdO-, and aromatic carbon in the IR spectra of Printex U. It indicated that the structure of Printex U mainly consisted of the carbon framework and did not contain adsorbed hydrocarbon and oxygen-containing species. On the contrary, besides the -C-C- and -CdCvibrations of the carbon framework, the aromatic hydrocarbon and oxygen-containing species existed in the diesel
micrographs. The average size of the elemental particles was found to be 20-25 nm. These results agree well with the literature,14,15 where diameters of elemental particles are reported in the range from 10 to 40 nm. The size of the elemental particles of DPII is smaller than that of DPI, indicating that the primary particle size becomes small with the increase of the diesel engine load. Furthermore, as shown in Figure 3, some particles with a snowflake shape were formed on the surface of diesel particulates. It is because some hydrocarbons that were not completely combusted in the diesel engine were adsorbed on the diesel particulates. 3.5. Results of IR Characterization. FTIR spectroscopy gave some information about the surface compositions and structures of the catalysts. Figure 4 shows the IR spectra of diesel particulates and Printex U. Several absorption peaks
(16) Kirchner, U.; Scheer, V.; Vogt, R. J. Phys. Chem. C 2000, 104, 8908–8915.
(14) Medalia, A. I.; Rivin, D. Carbon 1982, 20, 481–492. (15) Stevenson, R. Carbon 1982, 20, 359–365.
3781
Energy Fuels 2010, 24, 3778–3783
: DOI:10.1021/ef901366v
Liu et al.
Figure 4. FTIR spectra of diesel particulates and Printex U.
Figure 6. CO2 concentration profiles for Printex U, DPI, and DPII combustion on the V2O5 catalyst.
combustion curve of Printex U is a “parabola shape”.17 It is due to the fact that a large amount of hydrocarbon was adsorbed on the diesel particulates and they were easy to ignite. The results in Table 2 and Figure 6 also reveal that the V2O5 catalyst can improve the activity and selectivity of soot combustion. The amount of CO is greatly decreased. Thus, the catalytic combustion is one of the efficient ways to improve soot oxidation. 4. Discussion The elemental composition of diesel particulates indicates that their major constituents are carbon, hydrogen, and oxygen and a small amount of nitrogen and sulfur. The oxygen amount was found to be rather high. This can be understood well by the fact that XPS is a technique that yields information on the surface layer of the diesel particulates. The surface of diesel particulates contains the highest oxygen concentrations in the form of partly oxygenated absorbed hydrocarbons, sulfate, and water and surface oxygen complexes on the carbon core of the particulates, as phenol, carbonyl, and carboxyl groups. The hydrocarbon content of diesel particulates is higher than that of Printex U of the model soot. A difference is also found between the two diesel particulate samples. Hydrocarbons have a higher hydrogen/ carbon molar ratio, which fluctuates between 1 and 2 (depending upon the amount of double or triple bonds and the number of aromatic rings), compared to the graphite-like carbon, with a hydrogen/carbon molar ratio of much less than 1. Apart from carbon, hydrogen, nitrogen, and oxygen, also sulfur, iron, and silicon have been found to be the major inorganic components of diesel particulates. The diesel particulates are formed from diesel fuel components. Sulfur originates mainly from the diesel fuel, and the diesel fuel sulfur content determines the sulfur content in the diesel particulates. The iron and silicon data are varied. Iron is thought to originate from diesel fuel additives or wearing and oxidation of the engine. For the high iron concentrations of 3.86 and 9.05 wt % in diesel particulates DPI and DPII, apparently, the amount of wear greatly varies between different diesel engines. Iron and silicon particles that originate from wearing are probably much larger compared to the other elements that
Figure 5. CO and CO2 concentration profiles for DPI and DPII combustion (without catalyst).
particulates. Furthermore, the intensity of IR vibration peaks of C-H, -CdO-, and aromatic carbon in DPI is larger than that in DPII, indicating that hydrocarbon and oxygen-containing species were more adsorbed in the diesel particulates emitted from a light diesel vehicle than in the diesel particulates emitted from a heavy diesel vehicle. This result is in agreement with the results of EA, BET surface measurement, and SEM observation. 3.6. Combustion Behavior of Model Soot and Diesel Particulates. The combustion curves of diesel particulates and Printex U are shown as a function of the temperature at a heating rate of 2 °C/min in Figure 5. The two curves shown in Figure 5 denote the CO and CO2 data. CO2 is the main product, and there is still much CO produced. The ratio of CO/CO2 is higher at 400 °C than that at 550 °C, which is attributed to the fact that the reactivity of CO oxidation to CO2 is enhanced with the increase of the TPO temperature. In addition, CO is partially oxidized to CO2. The combustion temperature range is very high under the condition without catalyst. However, as shown in Table 2 and Figure 6, the combustion of diesel particulates and Printex U over the V2O5 catalyst brings about a lower combustion temperature range. The presence of the catalyst in the samples promotes the oxidation activity. In accordance, the Tm temperatures are lowered by about 213, 196, and 198 °C for Printex U, DPI, and DPII, respectively, compared to the case of without the catalyst. The combustion behavior of diesel particulates is described by two “parabola shape” light-off curves, and the
(17) Liu, J.; Zhao, Z.; Lan, J.; Xu, C.; Duan, A.; Jiang, G.; Wamg, X.; He, H. J. Phys. Chem. C 2009, 113, 17114–17123.
3782
Energy Fuels 2010, 24, 3778–3783
: DOI:10.1021/ef901366v
Liu et al.
surface groups and a lower surface area, more like graphite. Printex U has a small amount of adsorbed hydrocarbons and contains a trace amount of inorganic compounds. Its hydrogen content is less than that of diesel particulates. The oxidation temperature of Printex U was very similar to that of the carbon nucleus of diesel particulates. In general, it can be concluded that Printex U is a suitable model soot for the study on the combustion performances of the soot fraction of diesel particulates. 5. Conclusions Diesel particulates and a model soot Printex U were characterized by several kinds of analysis techniques, and their combustion performances were also evaluated. The main conclusions are summarized as follows. Diesel particulates contain elements such as C, H, N, O, S, Fe, Si, and other elements, and the high Fe and Si concentrations are probably due to engine wear. The amount of carbon, hydrogen, nitrogen, and sulfur in diesel particulates emitted from a light-load diesel engine are 65.6, 4.1, 21.8, 2.8, and 0.9 wt %, respectively. Printex U has higher carbon (92 wt %) and lower hydrogen (0.7 wt %) contents. In Printex U, the trace elements other than C, H, N, O, and S could not be detected. Both diesel particulates and Printex U consist of carbon spheres. The carbon spheres have sizes of 20-25 nm, which are coagulated to form larger agglomerates with sizes ranging from 50 nm to a micrometer scale. Diesel particulates contain considerable amounts of adsorbed compounds in the form of hydrocarbons, sulfate, and water. The amounts of adsorbed hydrocarbons and water on Printex U are much less than those on diesel particulates. The amount of adsorbed hydrocarbons in the diesel particulates varies with the engine load. The BET surface area of the particulates is proportional to the amount of adsorbed hydrocarbons. Printex U has much less adsorbed hydrocarbons and a lower surface area compared to diesel particulates. The oxidation reactivities of diesel particulates depend upon the engine load at which they were formed. Diesel particulates that were produced at low engine loads appeared to be the most reactive. It is suggested that this higher reactivity is due to a higher content of hydrocarbons and a higher active surface area. Printex U is similar to the carbon nucleus of diesel particulates and very well-suited as a kind of model soot for the study on the combustion performances of the soot fraction of diesel particulates.
Figure 7. Scheme for the composition of diesel particulates assessed by several kinds of analysis techniques.
originate from fuel oil and are, therefore, better distributed in the diesel particulates. As a result, the scatter in iron and silicon concentrations between different measurements can be expected to be larger than fluctuations in the concentrations of other elements. For real diesel particulates emitted from the diesel engine, iron and silicon formed can be ignored during the wear sampling. The overall scheme of diesel particulates is given in Figure 7. A carbon framework that contains typically a small amount of inorganic material is surrounded by adsorbed species: adsorbed hydrocarbons, sulfate, etc. The surface contains a significant amount of oxygen, in the form of sulfate, water, oxygenated hydrocarbons, as well as surface groups on the carbon nucleus itself. The particulates that are collected at low engine loads are more reactive than the particulates obtained at higher engine loads. This higher reactivity can be explained by the higher hydrocarbon content because hydrocarbons are easier to oxidize than soot.18 In the literature, no direct relation between soot or diesel particulate oxidation reactivity and surface area was reported. It was suggested that the oxidation reactivity of carbonaceous materials is proportional to the “active surface area”, which is only a part of the total surface area. This active surface area is composed of all kinds of surface groups containing adsorptive oxygen. Oxidation reactivity can be influenced by this amount of surface groups. As temperatures in a diesel combustion chamber increase with increasing loads, soot particles formed at high loads are thought to have lower surface areas and less surface groups than diesel particulates formed at low loads. Thus, DPI exhibited higher reactivity than DPII. According to the previous results, it is also known that diesel particulates have high surface areas. However, Printex U has a structure with less
Acknowledgment. This work was supported by the National Natural Science Foundation of China (20803093, 20833011, and 20525621), the Doctor Select Foundation for the University of State Education Ministry (200804251016), the Beijing Outstanding Ph.D. Thesis Foundation (YB 20091141401), and 863 Program (2009AA06Z3488052).
(18) Messerer, A.; Niessner, R.; P€ oschl, U. Carbon 2006, 44, 307–324.
3783
