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Catalytic diesel particulate filter
Applied Catalysis B: Environmental 49 (2004) 127–133
Catalytic diesel particulate filter Evaluation of parameters for laboratory studies Y. Nguyen Huu Nhon, H. Mohamed Magan, C. Petit∗ LMSPC, Laboratoire des Matériaux, Surfaces et Procédés pour la Catalyse, UMR 7515 CNRS-ECPM, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France Received 8 August 2003; received in revised form 27 November 2003; accepted 2 December 2003
Abstract The definition of experimental methods to be employed in the lab-scale for catalytic activity measurements for soot oxidation with either powder or coated catalysts is necessary before industrial applications. Characterisations of carbon black, a model soot of constant composition, have proved that mechanical grinding weakly affects its structure. The nature of carbon black can be modified by oxidation with O2 , H2 O or H2 O2 and confirmed by transmission electron microscopy (TEM). The temperature for half conversion of carbon (607–593 ◦ C after treatment) has revealed also the chemical stability of the structure. Molybdenum oxide is used as catalyst for the oxidation of carbon black. The reaction is greatly affected by the quality of the mixture between the two solid materials. For the preparation method by grinding, the effects of the amount of MoO3 and carbon black have been studied. At a high ratio of catalyst and carbon black (95/5), carbon black is well surrounded by MoO3 and the temperature for half conversion strongly decreases (from 607 to 459 ◦ C). This sample preparation condition can be adopted for the classification of catalysts. © 2003 Elsevier B.V. All rights reserved. Keywords: Carbon black; Combustion; Catalyst; Chemical structure; Reactivity
1. Introduction Removal of soot from diesel exhaust gas is a challenging topic. One of the best ways of preventing soot emissions is to place a particulate trap in a diesel engine exhaust pipe [1]. The filter durability is closely entailed by the successful control of periodic regeneration by combustion of the deposited particulate with post-injected fuel addition. Frequent regenerations prevent undesired back-pressure build-up which depends on the particulate matter accumulated in the filter. Any catalyst placed over the trap should possess high thermo-chemical stability and intrinsic oxidative properties to ignite reliably the soot as early as possible. Large varieties of catalytic materials for diesel soot abatement are being developed and evaluated [2–9]. The best system should permit the continuous regeneration of the filter with the aid of an oxidation catalyst. The performances of catalytic traps are affected by the intrinsic catalyst activity and the soot–catalyst contact ef-
∗
Corresponding author. Tel.: +33-390-242769; fax: +33-390-242768. E-mail address: [email protected] (C. Petit).
0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2003.12.005
ficiency [10–13], therefore, we need to define tests in the lab-scale for the catalytic evaluation. The nature of the contact between the carbon and the catalyst phases depends on two important parameters: the relative concentration of the solids and the mixing method used. Research of experimental tests in order to obtain optimal oxidative properties of powder catalyst and of catalyst under realistic conditions, i.e. when it is used as a coating on a monolith is necessary. For the latter case, a new equipment was designed by Nguyen Huu Nhon et al. [14] to test the carbon material deposited on a filter coated with a soot oxidation catalyst. As the composition of diesel particulate depends on several characteristics (engine load, speed, temperature) it is difficult to collect soot samples with constant properties. Catalytic oxidative reactions of carbonaceous materials (carbon black, char and graphite) have been extensively studied [15–19]. The present work aims at a better understanding of catalytic oxidation of carbon black in order to suggest one procedure for the classification of catalysts. The effect of pre-treatments (mechanical grinding, previous oxidation with water or hydrogen peroxide) on the particles size
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distribution, morphology and structural properties of carbon black is described. The nature of the contact between solid carbon particles and MoO3 catalyst, which is recognised as being among the best single-oxide catalyst to promote carbon black oxidation by O2 [20,21] via a Mars and Van Krevelen type mechanism [22], is studied in different cases.
2. Experimental 2.1. Materials 2.1.1. Carbon black samples A carbon black from Cabot Company is used for investigations. Before test, carbon black samples are ground in an agate mill by using various milling times: 0, 5, 10, 15, 40 and 60 min. Preliminary oxidations with water or hydrogen peroxide (30 vol.%) are realized by introducing the initial carbon black in the hot solution during 24 or 1 h, respectively. Then, the samples are dried at 120 ◦ C overnight. Partial oxidations under dilute air of carbon black at 500 ◦ C (630 ◦ C) to 48% (80%) burnout are also realized. Surface areas have been determined using an SA3100 Coulter sorptometer at 77 K. The bulk apparent density was measured by weighing a small known volume filled with carbon black samples [23]. The apparent densities of powder materials are highly dependent on the way the particles are packed together. The method used in this case is to place the samples in a biological capsule with no tapping. During tapping, smaller particles could scatter between larger particles and the apparent volume decreases. 2.1.2. Catalyst preparation The catalyst mainly presented in the paper is a commercial molybdenum trioxide MoO3 from Strem company (99.998%). The MoO3 phase was confirmed by X-ray diffraction (XRD). A special preparation of mixture of carbon black and molybdenum trioxide was obtained by the impregnation of a MoO2 (acac)2 salt, followed by a treatment into helium at 500 ◦ C with a temperature increase of 3 ◦ C min−1 . 2.2. Techniques of characterisation 2.2.1. Elemental analysis The total amounts of carbon, hydrogen, oxygen, nitrogen and sulphur were measured by elemental analysis in the CNRS Centre in Vernaison. Ash content was determined after heating carbonaceous samples at 900 ◦ C in a muffle furnace. 2.2.2. Physical characterisations Size distribution and structural information of amorphous carbon black aggregates were determined as a function of aggregation time using a Malvern Mastersizer/E (a 2 mW
He–Ne laser) which ascertains the size by analysis of frontal scattered light. This instrument operates on the principle of the Fraunhofer diffraction theory. The particles can be considered spherical. The size distribution is based on volume. Carbon black samples were introduced in the stirring cell which is connected to the measurement cell. A pump ensured the suspension (in water) circulation. The samples are carried out under variable stirring conditions. Transmission electron microscopy (TEM) analysis were performed on a Topcon EM002B apparatus to characterise firstly, the primary particle and aggregate sizes and secondly, bulk and surface microstructures of carbon black. A small amount of each sample was dispersed in toluene by ultrasonic agitation. The suspension was deposited on a copper grid coated with a holed carbon film. 2.2.3. Thermal gravimetric analysis (TGA) Thermal gravimetric analysis (TGA) was carried out with approximately 10 mg of sample using a Setaram 92–1200 device. The gas flow consisted of a mixture of 10 ml min−1 of helium and 15 ml min−1 of air. The activity of the catalysts can be expressed with reference to the temperature: T10 is the temperature at which the carbon starts to burn; T50 is the temperature at which 50% of the carbon is burnt in a linear temperature program. These temperatures are compared with the corresponding temperature found for a sample without catalyst. 2.2.4. Experimental set-up (RECP) A major disadvantage of TG experiments was that the test set-up does not represent the situation of wall-flow particulate filters. In reality, a soot bed is deposited on the catalyst material which is coated as a layer on the wall flow filter. Thus, the determination of the catalyst performance under realistic conditions requires a special experimental equipment. A new reactor called RECP was designed for this work [14]. The sample is implemented in a diesel particulate filter made of cordierite. It is square-shaped (25 mm×25 mm) and contains two successive porous walls of the DPF (2 mm). It can be covered by a carbon black deposit. The gas mixture (10% O2 , H2 O or H2 O2 addition, N2 balance) is set with mass-flow controllers. The gas flow during all experiments was 40 l h−1 . An electrical furnace is used to heat the reactor with a heating rate of 6 ◦ C min−1 . FTIR gas analysers for CO and CO2 are fitted to the outlet of the reactor to monitor the progress of oxidation. The oxidation rate reached for a given temperature is used as a measure to evaluate the activity of catalysts for the combustion.
3. Results and discussion 3.1. Studies of carbon black and ground samples The first step is to control if industrial carbon black can be used successfully to model the soot combustive behaviour.
Y. Nguyen Huu Nhon et al. / Applied Catalysis B: Environmental 49 (2004) 127–133 Table 1 Elemental composition of carbon black and diesel particulate Elemental composition (wt.%)
Carbon black Diesel soota
C
H
O
N
S
Ash
95.3 90.4
0.7 4.40
2.1 2.77
<0.3 0.24
1.0 0.79
∼0 n/a
n/a: not available. a From [24].
Table 1 lists the elemental composition of carbon black and diesel particulates collected by means of filter paper from the exhaust gases (cooled to 50 ◦ C in a dilution tunnel) of a small diesel engine [24]. Carbon is the major element in both materials. The carbon black contains less adsorbed water moisture, adsorbed hydrocarbons and sulphates than the diesel particulates. No ash has been found for carbon black sample. Carbon black is hydrophobic and can easily form large agglomerates of particles in water. Sedimentation occurs in the stirring cell and shows the difficulty of breaking up the agglomerates into individual particles. This fact complicates the analysis obtained with the Mastersizer. A negative correlation was observed between the mean size
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of the particles and the agitation rate. A bimodal distribution (25 and 600 m) was obtained with an agitation rate of 2000 rpm. Decreasing the pump rate led to a rise in the population of particles with higher mean size, indicating coagulation–flocculation of the carbon black. A dispersing agent (Triton C34 H62 O11 ) permits the stabilization of the suspension in water due to the modification of the carbon black surface. The particle sizes range (from 0.04 to 200 m) is lower than that observed without surfactant. Four mean particle sizes were observed at 0.1, 1.5, 12 and 120 m. A large part of the particles is very small (0.1 m) and constitutes the first aggregate of carbon black particles. The physical appearance of the carbon black is similar to diesel particulates. Therefore, it has been concluded that carbon black is a suitable model soot. Carbon black ground for 1 h showed dispersed agglomerates or aggregates of carbon black in the 1–10 m range, smaller than before milling. The results indicate a slight increase in the “compactness” of carbon black aggregates during the milling process. Smaller particles strongly tended to form agglomerates, thus, re-agglomeration of particles was observed after 5 min. Fig. 1 shows micrographs obtained by TEM and high resolution transmission electron microscopy (HRTEM) of the
Fig. 1. TEM micrographs of carbon black (a and b) and carbon black ground for 1 h (c and d). Magnification of 23,000× (a, c) and 390,000× (b and d).
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Table 2 Bulk density and BET surface area of carbon black samples Sample
Bulk density (kg m−3 )
BET surface area (m2 g−1 )
Carbon black (CB)
332
111
230 235 247 257 251
109 111 115 114 115
CB ground 5 10 15 40 1
(min) (min) (min) (min) (h)
morphology of carbon black before (Fig. 1a and b) and after a mechanical grinding (Fig. 1c and d). The carbon black samples consist of small primary particles with a nearly spherical shape and an average diameter of 32 nm which agglomerate forming a chain-like structure. The internal structure of carbon black sample has visible ordered graphitic regions. The substructures seen in the HRTEM pictures are bent and nearly concentrically arranged. The grinding does not modify the global structure, only an increase of compactness has been observed. It leads to a decrease of the dimensions of the bent graphitic subunits and to a slightly lower state of order (Fig. 1d). Bulk density and surface area values obtained for initial carbon black and samples modified by mechanical effects are presented in Table 2. The density of uncompacted bed of carbon black is 332 kg m−3 . The bulk density strongly decreases when the carbon black is ground for a short time (5 min) which indicates that it takes more space than the initial carbon black. This fact can be explained by a break-up of the agglomerate and the formation of a random arrangement. It is confirmed by the Mastersizer measurement showing the decrease (0.04–200 to 1–10 m) of the particle size during milling. Mechanical compaction by grinding of carbon bed increases the density from 230 to 251 kg m−3 . The surface area of carbon black samples is in the range 100–115 m2 g−1 . In the thermobalance experiments, the carbon black combustion starts at about 500 ◦ C (T10 = 552 ◦ C) and finishes at around 650 ◦ C, resulting in a T50 of 607 ◦ C (Fig. 3). When the carbon black was ground for 1 h, T10 and T50 are lowered by 20–30 ◦ C (532 and 593 ◦ C, respectively). These results indicate that the carbon black combustion is weakly affected by mechanical treatments and can be used for catalytic studies without important modifications. 3.2. Studies of carbon black pre-treated with O2 , H2 O or H2 O2 Some experiments have been performed with the RECP equipment in a linear temperature program with oxygen, water or hydrogen peroxide as reactants for the combustion of carbon black samples. The specific rates of carbon consumption are represented versus temperature in Fig. 4. The presence of H2 O or H2 O2 in the O2 /N2 gas feed has
an important influence on the rate during the oxidation process. Indeed, a significant increase of oxidation rate reached for a given temperature is observed by introducing water (relative increase of 107% at 500 ◦ C) or hydrogen peroxide (relative increase of 178% at 500 ◦ C) showing the activation of the carbon black surface during the combustion with O2 . Hydrogen peroxide, when dispersed in the gas flow, can interact with the carbon black at locations of sample surface/gas flow contact and cause high local oxidation rate. Combustion then accelerated and fell quickly, as observed in Fig. 4. In order to clarify the changes in structure of the carbon particles during the oxidation process, pre-treated samples with O2 , H2 O or H2 O2 were studied. Comparative results of the morphology evolution, bulk density, surface area and thermal gravimetric analysis of these preliminary oxidized samples are presented in this section. Granulometric investigations with the addition of surfactant show that pre-treatment of carbon black with water or hydrogen peroxide does not modify the particle size distribution. No flocculation occurs and the mean size of particles measured for carbon black samples pre-treated with H2 O2 is lower than with H2 O. The chemical modifications of carbon black generated in H2 O (micrographs not shown) and in H2 O2 (Fig. 2a and b) were observed by TEM and HRTEM. The products show a structure which is different compared to the initial carbon black particles. The morphology can be described as a very fluffy chain-like structure. With H2 O and H2 O2 treatments, the decrease of the average particle sizes (from 32 to 18 and 15 nm, respectively) indicates an oxidation from the surface of the particles. The internal structure of the carbon black materials generated in H2 O2 (Fig. 2b) indicates the presence of open shells surrounding of well-defined arrangement of carbon particles and bent substructure. The observations prove that the oxidation is an inward phenomenon. Carbon black samples at 30% (Fig. 2c and d) and 80% (Fig. 2e and f) carbon conversion under O2 are shown at low and high magnification. Modifications of the structure of graphitic carbon during oxidation are also noticed. The mean particle size decreases (from 30 to 22 nm) and a more porous structure is observed. The particles are oxidised inside and outside. Thus, a new arrangement of agglomerates is obtained. After chemical treatments by hot water or hydrogen peroxide the bulk density decreases from 332 to 271 and 297 kg m−3 . The surface area is almost the same as the initial product whereas an increase in surface area of 500% (from 111 to 609 m2 g−1 ) is observed for a carbon black at 48% burnoff. Ishiguro et al. [25] found similar observations. T10 and T50 obtained for a carbon black sample pre-treated with H2 O2 were 503 and 606 ◦ C and are close to initial carbon black data. Although oxygen can have an easier access through the pores of a carbon black sample at 80% carbon conversion, the combustion temperatures are similar to those of an untreated sample (Figs. 3 and 4).
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Fig. 2. TEM micrographs of carbon black pre-treated with H2 O2 (a and b) and carbon black at 48% (c and d) and 80% (e and f) burnoff under 10% O2 /N2 (e and f). Magnification of 23,000× (a, c and d) and 390,000× (b, d and f).
These results indicate that the carbon black combustion is not affected by oxidative or chemical treatments. The difference between the two combustion tests (TG analysis and RECP equipment) means that the additions in the feed gas modify the nature of the surface of carbon black temporarily. Unfortunately, the significant increase of reactivity is not observed for pre-treated carbon black with the same reactants. Modification of particles morphology has no influence on the combustion curves. The energy bond of C–C is high
and quite similar at the beginning an after a pre-treatment by O2 , H2 O or H2 O2 . 3.3. Studies of parameters on the catalysed oxidation of carbon black with catalyst In order to analyse the effects of mixing type, the carbon black and the catalyst were mixed in two different ways. In the literature, various types of physical contact
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Fig. 3. TG curves obtained on heating (at 6 ◦ C min−1 in a mixture of 10 ml min−1 of helium and of 15 ml min−1 of air) of standard carbon ); carbon black ground for 1 h (—); carbon black pre-treated black ( with H2 O2 (䊊) and carbon black at 48% ( ) and 80% ( ) burnoff.
have been reported. Following the concept introduced by Neeft et al. [26], when this contact is poor (“loose contact”), i.e. when the catalyst is mixed with the soot by shaking in a bottle, the catalytic activity of a metal oxide catalyst is much lower than when the catalyst is mixed with the soot in a ball mill (“tight contact”). Fig. 5 shows the difference in the oxidation activity presented for MoO3 catalyst differently mixed with the carbon black (shaken in a bottle during a few minutes and ground for 1 h by using a mechanical agate mill). A catalyst-to-carbon black ratio of 50/50 (on weight basis) is used. As expected, a higher T50 was found for a sample prepared by shaking (541 ◦ C) than that prepared by grinding (495 ◦ C). The temperature of carbon conversion decreases in the presence of an oxidation catalyst and depends on the nature of the contact. The catalytic performance of MoO3 is correlated with the oxidation activity and the mobility of MoO3 on the carbon black during the oxidation process as described by Liu et al. [21]. The results show the same effect of combustion for the quality of the carbon black/MoO3 contact and for the soot mixed with other catalysts presented by Neeft et al. [26].
Fig. 5. TG curves obtained on heating (at 6 ◦ C min−1 in a mixture of 10 ml min−1 of helium and 15 ml min−1 of air) of standard carbon black ) and mixtures of MoO3 and carbon black in high contact with a ( catalyst-to-carbon black ratio (on weight basis) of 50/50 (), 75/25 (䊏) and 95/5 (䊊). TG curve obtained for a 50/50 mixture in poor contact (䉱) is also reported.
Experiments with different catalyst-to-carbon black ratios (95/5, 75/25 and 50/50 on weight basis) showed significant variations in T10 and T50 , as observed in Fig. 5. The method used to prepare carbon black/catalyst mixtures is the milling of the two solid materials during 1 h. T10 and T50 diminished from 552 and 607 ◦ C without catalyst to 434 and 474 ◦ C when the catalyst is mixed with the carbon black in a ratio of 75/25. Whereas with a ratio of 95/5, the catalyst-to-carbon black ratio enhancement effect further decreased T50 by more than 150 ◦ C (T50 = 459 ◦ C). It should be noted that an impregnation with a molybdenum acetyl-acetonate salt on carbon black in the same ratio (95/5) permits also to lower T50 (decrease of about 20 ◦ C compared to the mixture by grinding MoO3 and carbon black). The increase of the contact quality decreases the temperature of oxidation until a constant value is reached. This temperature corresponds to the oxidation activity of MoO3 when the contact is considered as ideal. This value obtained for a ratio of 95/5 represents an optimal data for the determination of oxidative properties of the catalyst.
4. Conclusion
Fig. 4. Oxidation rates as a function of temperature measured in RECP ). Ten percent H2 O equipment of carbon black under 10% O2 /N2 ( ( ) and 10% H2 O2 (×) have been added to the gas feed.
Characterisations of a usable carbon black as a model of dry soot have been presented in this work. We tried to correlate the chemical properties (particles size distribution, bulk density, surface area and morphology) of pre-treated carbon black mixtures with their reactivity. Addition of H2 O or H2 O2 during the combustion process improves greatly the oxidation rates whereas mechanical treatments by grinding and preliminary chemical treatments by H2 O2 have a low effect on the reaction of combustion. Addition of MoO3 presents a catalytic effect on the temperature of combustion. The role of the nature of the contact (“loose” or “tight” referenced to [24,26]) depends on the manner of preparation and can inhibit largely the oxidative properties
Y. Nguyen Huu Nhon et al. / Applied Catalysis B: Environmental 49 (2004) 127–133
of the catalyst. The knowledge of these parameters is as important as the oxidative catalyst ability. The results are pointing out the way to the screening of candidate active catalysts. Acknowledgements Prof. Dr. J.-F. Brilhac (Laboratoire de Gestion des Risques et Environnement, Mulhouse), Dr. M. Guyon and Dr. N. Moral (Renault S.A., Lardy) are greatly acknowledged for helpful discussions. References [1] G. Boretto, M. Debenedetti, Adv. Propul. Emission Technol. (2001) 127. [2] A.F. Ahlström, C.U.I. Odenbrand, Appl. Catal. 60 (1990) 143. [3] G. Mul, J.P.A. Neeft, F. Kapteijn, M. Makkee, J.A. Moulijn, Appl. Catal. B: Environ. 6 (1995) 339. [4] J.P.A. Neeft, M. Makkee, J.A. Moulijn, Appl. Catal. B: Environ. 8 (1996) 57. [5] C. Badini, G. Saracco, V. Serra, V. Specchia, Appl. Catal. B: Environ. 18 (1998) 137. [6] C.A. Querini, M.A. Ulla, F. Requejo, J. Sorai, U.A. Sedron, E.E. Miro, Appl. Catal. B: Environ. 15 (1998) 5. [7] J. Jelles, B.A.A.L. van Setten, M. Makkee, J.A. Moulijn, Appl. Catal. B: Environ. 21 (1999) 35. [8] G. Neri, G. Rizzo, S. Galvagno, M.G. Musolino, A. Donato, R. Pietropaolo, Therm. Acta 381 (2002) 165.
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[9] A. Rougier, S. Soiron, I. Haihal, L. Aymard, B. Taouk, J.-M. Tarascon, Powder Technol. 128 (2002) 139. [10] P. Ciambelli, V. Palma, P. Russo, S. Vaccaro, CAPOC 4 2 (1997) 313. [11] B.A.A.L. Van Setten, J. Bremmer, S.J. Jelles, M. Makkee, J.A. Moulijn, Catal. Today 53 (1999) 613. [12] G. Saracco, N. Russo, M. Ambrogio, C. Badini, V. Specchia, Catal. Today 60 (2000) 33. [13] B.A.A.L. Van Setten, J.M. Schouten, M. Makkee, J.A. Moulijn, Appl. Catal. B: Environ. 28 (2000) 253. [14] Y. Nguyen Huu Nhon, J.-F. Brilhac, C. Petit, Chem. Eng. Technol., in press. [15] A. Pacault, Les Carbones, Groupe Français d’ Etude des Carbone, Ed Masson (1965), Part J.B. Donnet, Les noirs de carbone, 690. [16] F. Bonnefoy, P. Gilot, B.R. Stanmore, G. Prado, Carbon 32 (7) (1994) 133. [17] P. Ciambelli, M. d’Amore, V. Palma, S. Vaccaro, Combust. Flame 99 (1994) 413. [18] A.D.H. Clague, J.B. Donnet, T. Kwang, J.C.M. Peng, Carbon 37 (1999) 1553. [19] D. Fino, N. Russo, G. Saracco, V. Specchia, J. Catal. 217 (2003) 367. [20] M.A. Hasan, M.I. Zaki, K. Kumari, L. Pasupulety, Therm. Acta 320 (1998) 23. [21] S. Liu, A. Obuchi, J. Oi-Uchisawa, T. Nanba, S. Kushiyama, Appl. Catal. B: Environ. 30 (2001) 259. [22] G. Mul, F. Kapteijn, C. Doornkamp, J.A. Moulijn, J. Catal. 179 (1998) 258. [23] N. El Mejdoub, B. Calominus, G. Trouvé, P. Gilot, L. Delfosse, Carbon 33 (11) (1999) 1593. [24] J.P.A. Neeft, O.P. van Pruisen, M. Makkee, J.A. Moulijn, CAPOC III, Stud. Surf. Sci. Catal. 96 (1995) 549. [25] T. Ishiguro, N. Suzuki, Y. Fujitani, H. Morimoto, Combust. Flame 85 (1991) 1. [26] J.P.A. Neeft, M. Makkee, J.A. Moulijn, Chem. Eng. J. 64 (1996) 295.
Catalytic diesel particulate filter Evaluation of parameters for laboratory studies Y. Nguyen Huu Nhon, H. Mohamed Magan, C. Petit∗ LMSPC, Laboratoire des Matériaux, Surfaces et Procédés pour la Catalyse, UMR 7515 CNRS-ECPM, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France Received 8 August 2003; received in revised form 27 November 2003; accepted 2 December 2003
Abstract The definition of experimental methods to be employed in the lab-scale for catalytic activity measurements for soot oxidation with either powder or coated catalysts is necessary before industrial applications. Characterisations of carbon black, a model soot of constant composition, have proved that mechanical grinding weakly affects its structure. The nature of carbon black can be modified by oxidation with O2 , H2 O or H2 O2 and confirmed by transmission electron microscopy (TEM). The temperature for half conversion of carbon (607–593 ◦ C after treatment) has revealed also the chemical stability of the structure. Molybdenum oxide is used as catalyst for the oxidation of carbon black. The reaction is greatly affected by the quality of the mixture between the two solid materials. For the preparation method by grinding, the effects of the amount of MoO3 and carbon black have been studied. At a high ratio of catalyst and carbon black (95/5), carbon black is well surrounded by MoO3 and the temperature for half conversion strongly decreases (from 607 to 459 ◦ C). This sample preparation condition can be adopted for the classification of catalysts. © 2003 Elsevier B.V. All rights reserved. Keywords: Carbon black; Combustion; Catalyst; Chemical structure; Reactivity
1. Introduction Removal of soot from diesel exhaust gas is a challenging topic. One of the best ways of preventing soot emissions is to place a particulate trap in a diesel engine exhaust pipe [1]. The filter durability is closely entailed by the successful control of periodic regeneration by combustion of the deposited particulate with post-injected fuel addition. Frequent regenerations prevent undesired back-pressure build-up which depends on the particulate matter accumulated in the filter. Any catalyst placed over the trap should possess high thermo-chemical stability and intrinsic oxidative properties to ignite reliably the soot as early as possible. Large varieties of catalytic materials for diesel soot abatement are being developed and evaluated [2–9]. The best system should permit the continuous regeneration of the filter with the aid of an oxidation catalyst. The performances of catalytic traps are affected by the intrinsic catalyst activity and the soot–catalyst contact ef-
∗
Corresponding author. Tel.: +33-390-242769; fax: +33-390-242768. E-mail address: [email protected] (C. Petit).
0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2003.12.005
ficiency [10–13], therefore, we need to define tests in the lab-scale for the catalytic evaluation. The nature of the contact between the carbon and the catalyst phases depends on two important parameters: the relative concentration of the solids and the mixing method used. Research of experimental tests in order to obtain optimal oxidative properties of powder catalyst and of catalyst under realistic conditions, i.e. when it is used as a coating on a monolith is necessary. For the latter case, a new equipment was designed by Nguyen Huu Nhon et al. [14] to test the carbon material deposited on a filter coated with a soot oxidation catalyst. As the composition of diesel particulate depends on several characteristics (engine load, speed, temperature) it is difficult to collect soot samples with constant properties. Catalytic oxidative reactions of carbonaceous materials (carbon black, char and graphite) have been extensively studied [15–19]. The present work aims at a better understanding of catalytic oxidation of carbon black in order to suggest one procedure for the classification of catalysts. The effect of pre-treatments (mechanical grinding, previous oxidation with water or hydrogen peroxide) on the particles size
128
Y. Nguyen Huu Nhon et al. / Applied Catalysis B: Environmental 49 (2004) 127–133
distribution, morphology and structural properties of carbon black is described. The nature of the contact between solid carbon particles and MoO3 catalyst, which is recognised as being among the best single-oxide catalyst to promote carbon black oxidation by O2 [20,21] via a Mars and Van Krevelen type mechanism [22], is studied in different cases.
2. Experimental 2.1. Materials 2.1.1. Carbon black samples A carbon black from Cabot Company is used for investigations. Before test, carbon black samples are ground in an agate mill by using various milling times: 0, 5, 10, 15, 40 and 60 min. Preliminary oxidations with water or hydrogen peroxide (30 vol.%) are realized by introducing the initial carbon black in the hot solution during 24 or 1 h, respectively. Then, the samples are dried at 120 ◦ C overnight. Partial oxidations under dilute air of carbon black at 500 ◦ C (630 ◦ C) to 48% (80%) burnout are also realized. Surface areas have been determined using an SA3100 Coulter sorptometer at 77 K. The bulk apparent density was measured by weighing a small known volume filled with carbon black samples [23]. The apparent densities of powder materials are highly dependent on the way the particles are packed together. The method used in this case is to place the samples in a biological capsule with no tapping. During tapping, smaller particles could scatter between larger particles and the apparent volume decreases. 2.1.2. Catalyst preparation The catalyst mainly presented in the paper is a commercial molybdenum trioxide MoO3 from Strem company (99.998%). The MoO3 phase was confirmed by X-ray diffraction (XRD). A special preparation of mixture of carbon black and molybdenum trioxide was obtained by the impregnation of a MoO2 (acac)2 salt, followed by a treatment into helium at 500 ◦ C with a temperature increase of 3 ◦ C min−1 . 2.2. Techniques of characterisation 2.2.1. Elemental analysis The total amounts of carbon, hydrogen, oxygen, nitrogen and sulphur were measured by elemental analysis in the CNRS Centre in Vernaison. Ash content was determined after heating carbonaceous samples at 900 ◦ C in a muffle furnace. 2.2.2. Physical characterisations Size distribution and structural information of amorphous carbon black aggregates were determined as a function of aggregation time using a Malvern Mastersizer/E (a 2 mW
He–Ne laser) which ascertains the size by analysis of frontal scattered light. This instrument operates on the principle of the Fraunhofer diffraction theory. The particles can be considered spherical. The size distribution is based on volume. Carbon black samples were introduced in the stirring cell which is connected to the measurement cell. A pump ensured the suspension (in water) circulation. The samples are carried out under variable stirring conditions. Transmission electron microscopy (TEM) analysis were performed on a Topcon EM002B apparatus to characterise firstly, the primary particle and aggregate sizes and secondly, bulk and surface microstructures of carbon black. A small amount of each sample was dispersed in toluene by ultrasonic agitation. The suspension was deposited on a copper grid coated with a holed carbon film. 2.2.3. Thermal gravimetric analysis (TGA) Thermal gravimetric analysis (TGA) was carried out with approximately 10 mg of sample using a Setaram 92–1200 device. The gas flow consisted of a mixture of 10 ml min−1 of helium and 15 ml min−1 of air. The activity of the catalysts can be expressed with reference to the temperature: T10 is the temperature at which the carbon starts to burn; T50 is the temperature at which 50% of the carbon is burnt in a linear temperature program. These temperatures are compared with the corresponding temperature found for a sample without catalyst. 2.2.4. Experimental set-up (RECP) A major disadvantage of TG experiments was that the test set-up does not represent the situation of wall-flow particulate filters. In reality, a soot bed is deposited on the catalyst material which is coated as a layer on the wall flow filter. Thus, the determination of the catalyst performance under realistic conditions requires a special experimental equipment. A new reactor called RECP was designed for this work [14]. The sample is implemented in a diesel particulate filter made of cordierite. It is square-shaped (25 mm×25 mm) and contains two successive porous walls of the DPF (2 mm). It can be covered by a carbon black deposit. The gas mixture (10% O2 , H2 O or H2 O2 addition, N2 balance) is set with mass-flow controllers. The gas flow during all experiments was 40 l h−1 . An electrical furnace is used to heat the reactor with a heating rate of 6 ◦ C min−1 . FTIR gas analysers for CO and CO2 are fitted to the outlet of the reactor to monitor the progress of oxidation. The oxidation rate reached for a given temperature is used as a measure to evaluate the activity of catalysts for the combustion.
3. Results and discussion 3.1. Studies of carbon black and ground samples The first step is to control if industrial carbon black can be used successfully to model the soot combustive behaviour.
Y. Nguyen Huu Nhon et al. / Applied Catalysis B: Environmental 49 (2004) 127–133 Table 1 Elemental composition of carbon black and diesel particulate Elemental composition (wt.%)
Carbon black Diesel soota
C
H
O
N
S
Ash
95.3 90.4
0.7 4.40
2.1 2.77
<0.3 0.24
1.0 0.79
∼0 n/a
n/a: not available. a From [24].
Table 1 lists the elemental composition of carbon black and diesel particulates collected by means of filter paper from the exhaust gases (cooled to 50 ◦ C in a dilution tunnel) of a small diesel engine [24]. Carbon is the major element in both materials. The carbon black contains less adsorbed water moisture, adsorbed hydrocarbons and sulphates than the diesel particulates. No ash has been found for carbon black sample. Carbon black is hydrophobic and can easily form large agglomerates of particles in water. Sedimentation occurs in the stirring cell and shows the difficulty of breaking up the agglomerates into individual particles. This fact complicates the analysis obtained with the Mastersizer. A negative correlation was observed between the mean size
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of the particles and the agitation rate. A bimodal distribution (25 and 600 m) was obtained with an agitation rate of 2000 rpm. Decreasing the pump rate led to a rise in the population of particles with higher mean size, indicating coagulation–flocculation of the carbon black. A dispersing agent (Triton C34 H62 O11 ) permits the stabilization of the suspension in water due to the modification of the carbon black surface. The particle sizes range (from 0.04 to 200 m) is lower than that observed without surfactant. Four mean particle sizes were observed at 0.1, 1.5, 12 and 120 m. A large part of the particles is very small (0.1 m) and constitutes the first aggregate of carbon black particles. The physical appearance of the carbon black is similar to diesel particulates. Therefore, it has been concluded that carbon black is a suitable model soot. Carbon black ground for 1 h showed dispersed agglomerates or aggregates of carbon black in the 1–10 m range, smaller than before milling. The results indicate a slight increase in the “compactness” of carbon black aggregates during the milling process. Smaller particles strongly tended to form agglomerates, thus, re-agglomeration of particles was observed after 5 min. Fig. 1 shows micrographs obtained by TEM and high resolution transmission electron microscopy (HRTEM) of the
Fig. 1. TEM micrographs of carbon black (a and b) and carbon black ground for 1 h (c and d). Magnification of 23,000× (a, c) and 390,000× (b and d).
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Table 2 Bulk density and BET surface area of carbon black samples Sample
Bulk density (kg m−3 )
BET surface area (m2 g−1 )
Carbon black (CB)
332
111
230 235 247 257 251
109 111 115 114 115
CB ground 5 10 15 40 1
(min) (min) (min) (min) (h)
morphology of carbon black before (Fig. 1a and b) and after a mechanical grinding (Fig. 1c and d). The carbon black samples consist of small primary particles with a nearly spherical shape and an average diameter of 32 nm which agglomerate forming a chain-like structure. The internal structure of carbon black sample has visible ordered graphitic regions. The substructures seen in the HRTEM pictures are bent and nearly concentrically arranged. The grinding does not modify the global structure, only an increase of compactness has been observed. It leads to a decrease of the dimensions of the bent graphitic subunits and to a slightly lower state of order (Fig. 1d). Bulk density and surface area values obtained for initial carbon black and samples modified by mechanical effects are presented in Table 2. The density of uncompacted bed of carbon black is 332 kg m−3 . The bulk density strongly decreases when the carbon black is ground for a short time (5 min) which indicates that it takes more space than the initial carbon black. This fact can be explained by a break-up of the agglomerate and the formation of a random arrangement. It is confirmed by the Mastersizer measurement showing the decrease (0.04–200 to 1–10 m) of the particle size during milling. Mechanical compaction by grinding of carbon bed increases the density from 230 to 251 kg m−3 . The surface area of carbon black samples is in the range 100–115 m2 g−1 . In the thermobalance experiments, the carbon black combustion starts at about 500 ◦ C (T10 = 552 ◦ C) and finishes at around 650 ◦ C, resulting in a T50 of 607 ◦ C (Fig. 3). When the carbon black was ground for 1 h, T10 and T50 are lowered by 20–30 ◦ C (532 and 593 ◦ C, respectively). These results indicate that the carbon black combustion is weakly affected by mechanical treatments and can be used for catalytic studies without important modifications. 3.2. Studies of carbon black pre-treated with O2 , H2 O or H2 O2 Some experiments have been performed with the RECP equipment in a linear temperature program with oxygen, water or hydrogen peroxide as reactants for the combustion of carbon black samples. The specific rates of carbon consumption are represented versus temperature in Fig. 4. The presence of H2 O or H2 O2 in the O2 /N2 gas feed has
an important influence on the rate during the oxidation process. Indeed, a significant increase of oxidation rate reached for a given temperature is observed by introducing water (relative increase of 107% at 500 ◦ C) or hydrogen peroxide (relative increase of 178% at 500 ◦ C) showing the activation of the carbon black surface during the combustion with O2 . Hydrogen peroxide, when dispersed in the gas flow, can interact with the carbon black at locations of sample surface/gas flow contact and cause high local oxidation rate. Combustion then accelerated and fell quickly, as observed in Fig. 4. In order to clarify the changes in structure of the carbon particles during the oxidation process, pre-treated samples with O2 , H2 O or H2 O2 were studied. Comparative results of the morphology evolution, bulk density, surface area and thermal gravimetric analysis of these preliminary oxidized samples are presented in this section. Granulometric investigations with the addition of surfactant show that pre-treatment of carbon black with water or hydrogen peroxide does not modify the particle size distribution. No flocculation occurs and the mean size of particles measured for carbon black samples pre-treated with H2 O2 is lower than with H2 O. The chemical modifications of carbon black generated in H2 O (micrographs not shown) and in H2 O2 (Fig. 2a and b) were observed by TEM and HRTEM. The products show a structure which is different compared to the initial carbon black particles. The morphology can be described as a very fluffy chain-like structure. With H2 O and H2 O2 treatments, the decrease of the average particle sizes (from 32 to 18 and 15 nm, respectively) indicates an oxidation from the surface of the particles. The internal structure of the carbon black materials generated in H2 O2 (Fig. 2b) indicates the presence of open shells surrounding of well-defined arrangement of carbon particles and bent substructure. The observations prove that the oxidation is an inward phenomenon. Carbon black samples at 30% (Fig. 2c and d) and 80% (Fig. 2e and f) carbon conversion under O2 are shown at low and high magnification. Modifications of the structure of graphitic carbon during oxidation are also noticed. The mean particle size decreases (from 30 to 22 nm) and a more porous structure is observed. The particles are oxidised inside and outside. Thus, a new arrangement of agglomerates is obtained. After chemical treatments by hot water or hydrogen peroxide the bulk density decreases from 332 to 271 and 297 kg m−3 . The surface area is almost the same as the initial product whereas an increase in surface area of 500% (from 111 to 609 m2 g−1 ) is observed for a carbon black at 48% burnoff. Ishiguro et al. [25] found similar observations. T10 and T50 obtained for a carbon black sample pre-treated with H2 O2 were 503 and 606 ◦ C and are close to initial carbon black data. Although oxygen can have an easier access through the pores of a carbon black sample at 80% carbon conversion, the combustion temperatures are similar to those of an untreated sample (Figs. 3 and 4).
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Fig. 2. TEM micrographs of carbon black pre-treated with H2 O2 (a and b) and carbon black at 48% (c and d) and 80% (e and f) burnoff under 10% O2 /N2 (e and f). Magnification of 23,000× (a, c and d) and 390,000× (b, d and f).
These results indicate that the carbon black combustion is not affected by oxidative or chemical treatments. The difference between the two combustion tests (TG analysis and RECP equipment) means that the additions in the feed gas modify the nature of the surface of carbon black temporarily. Unfortunately, the significant increase of reactivity is not observed for pre-treated carbon black with the same reactants. Modification of particles morphology has no influence on the combustion curves. The energy bond of C–C is high
and quite similar at the beginning an after a pre-treatment by O2 , H2 O or H2 O2 . 3.3. Studies of parameters on the catalysed oxidation of carbon black with catalyst In order to analyse the effects of mixing type, the carbon black and the catalyst were mixed in two different ways. In the literature, various types of physical contact
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Fig. 3. TG curves obtained on heating (at 6 ◦ C min−1 in a mixture of 10 ml min−1 of helium and of 15 ml min−1 of air) of standard carbon ); carbon black ground for 1 h (—); carbon black pre-treated black ( with H2 O2 (䊊) and carbon black at 48% ( ) and 80% ( ) burnoff.
have been reported. Following the concept introduced by Neeft et al. [26], when this contact is poor (“loose contact”), i.e. when the catalyst is mixed with the soot by shaking in a bottle, the catalytic activity of a metal oxide catalyst is much lower than when the catalyst is mixed with the soot in a ball mill (“tight contact”). Fig. 5 shows the difference in the oxidation activity presented for MoO3 catalyst differently mixed with the carbon black (shaken in a bottle during a few minutes and ground for 1 h by using a mechanical agate mill). A catalyst-to-carbon black ratio of 50/50 (on weight basis) is used. As expected, a higher T50 was found for a sample prepared by shaking (541 ◦ C) than that prepared by grinding (495 ◦ C). The temperature of carbon conversion decreases in the presence of an oxidation catalyst and depends on the nature of the contact. The catalytic performance of MoO3 is correlated with the oxidation activity and the mobility of MoO3 on the carbon black during the oxidation process as described by Liu et al. [21]. The results show the same effect of combustion for the quality of the carbon black/MoO3 contact and for the soot mixed with other catalysts presented by Neeft et al. [26].
Fig. 5. TG curves obtained on heating (at 6 ◦ C min−1 in a mixture of 10 ml min−1 of helium and 15 ml min−1 of air) of standard carbon black ) and mixtures of MoO3 and carbon black in high contact with a ( catalyst-to-carbon black ratio (on weight basis) of 50/50 (), 75/25 (䊏) and 95/5 (䊊). TG curve obtained for a 50/50 mixture in poor contact (䉱) is also reported.
Experiments with different catalyst-to-carbon black ratios (95/5, 75/25 and 50/50 on weight basis) showed significant variations in T10 and T50 , as observed in Fig. 5. The method used to prepare carbon black/catalyst mixtures is the milling of the two solid materials during 1 h. T10 and T50 diminished from 552 and 607 ◦ C without catalyst to 434 and 474 ◦ C when the catalyst is mixed with the carbon black in a ratio of 75/25. Whereas with a ratio of 95/5, the catalyst-to-carbon black ratio enhancement effect further decreased T50 by more than 150 ◦ C (T50 = 459 ◦ C). It should be noted that an impregnation with a molybdenum acetyl-acetonate salt on carbon black in the same ratio (95/5) permits also to lower T50 (decrease of about 20 ◦ C compared to the mixture by grinding MoO3 and carbon black). The increase of the contact quality decreases the temperature of oxidation until a constant value is reached. This temperature corresponds to the oxidation activity of MoO3 when the contact is considered as ideal. This value obtained for a ratio of 95/5 represents an optimal data for the determination of oxidative properties of the catalyst.
4. Conclusion
Fig. 4. Oxidation rates as a function of temperature measured in RECP ). Ten percent H2 O equipment of carbon black under 10% O2 /N2 ( ( ) and 10% H2 O2 (×) have been added to the gas feed.
Characterisations of a usable carbon black as a model of dry soot have been presented in this work. We tried to correlate the chemical properties (particles size distribution, bulk density, surface area and morphology) of pre-treated carbon black mixtures with their reactivity. Addition of H2 O or H2 O2 during the combustion process improves greatly the oxidation rates whereas mechanical treatments by grinding and preliminary chemical treatments by H2 O2 have a low effect on the reaction of combustion. Addition of MoO3 presents a catalytic effect on the temperature of combustion. The role of the nature of the contact (“loose” or “tight” referenced to [24,26]) depends on the manner of preparation and can inhibit largely the oxidative properties
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