- Email: [email protected]
Porous acicular mullite ceramics fabricated with in situ formed soot oxidation catalyst obtained from waste MoSi2
Ceramics International 43 (2017) 9815–9822
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Porous acicular mullite ceramics fabricated with in situ formed soot oxidation catalyst obtained from waste MoSi2 ⁎
Dusan Bucevaca, , Jelena Maletaskicb, Mia Omerasevicb, Branko Matovicb, Chang-An Wangc, a b c
MARK ⁎
Department of Mechanical and Materials Engineering, Queen's University, Kingston, Ontario, Canada K7L 3N6 Vinca Institute of Nuclear Sciences, University of Belgrade, Belgrade 11000, Serbia State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
A R T I C L E I N F O
A BS T RAC T
Keywords: A: sintering B: porosity C: thermal properties D: acicular mullite E: engine components
Porous acicular mullite (3Al2O3·2SiO2) ceramics containing Cu3Mo2O9 as a soot oxidation catalyst was fabricated by a novel approach using commercial powders of Al2O3 and CuO, and powder obtained by controlled oxidation of ground waste MoSi2. The obtained material consisted of elongated mullite grains which are known to be effective in carbon soot removal from diesel engine exhaust. The presence of in situ created Cu3Mo2O9 was found to catalyze the carbon burnout which is an extremely important feature when it comes to filter regeneration, i.e., the captured soot removal. The carbon burnout temperature in the sample containing 12 wt% CuO was by 90 °C lower than that in the sample without CuO. Effect of sintering temperature as well as the effect of amount of CuO additive on mullite properties were studied. It was found that the increase in amount of CuO in samples sintered at 1300 °C decreased porosity and increased compressive strength of the porous mullite ceramics. The addition of 12 wt% CuO increased the strength of the porous mullite ceramics up to 70 MPa, whereas the porosity was reduced from 62% in the mullite without CuO to 44% in the mullite ceramics containing 12 wt% CuO. Although affected by the amount of CuO, the microstructure still consisted of elongated mullite grains.
1. Introduction
passive regeneration may fail during low engine loads that do not generate enough heat for sufficiently long time to oxidise the soot [10]. When a certain level of carbon soot particles is reached in the filter, the active regeneration is required to oxidise the soot particles to carbon dioxide with the help of oxygen at an exhaust gas temperature of 600– 650 °C [11]. Two approaches can be used to accomplish the task of a carbon soot combustion. One is to heat the exhaust gas and/or filter to the certain ignition temperature, which is normally done by injection of an additional amount of fuel [12]. The other approach is to lower the ignition temperature with the aid of oxidation catalyst which can be either coating [13] or fuel-born [14]. Both approaches increase the engine operation/maintenance cost either through an increased fuel consumption, use of expensive noble catalyst or vehicle's onboard dosing system. In the case of catalytic coated DPF, the both approaches are combined [15]. The most frequently used commercial ceramics DPFs are made of SiC [16] and cordierite coated with a mixture of Al2O3 and CeO2 which serves as a carrier layer for platinum catalyst [17]. However, acicular mullite was found to be a particularly convenient material for production of honeycomb filters [18] owing
The reduction of particulate emissions from diesel engines has been a great challenge over the past three decades [1–3]. Carbon soot particles that are inherent in diesel emissions due to an incomplete fuel combustion [4] can be effectively removed from exhaust gas by ceramic diesel particular filters (DPFs) [5]. DPF normally comprises of a honeycomb structure with many narrow and parallel channels which are blocked at alternate ends. The exhaust gas is thus forced to flow through the walls between channels and the carbon particles are deposited as a soot cake on the walls. Since the continuous deposition of the soot will make a filter resistant to the exhaust flow through it and therefore interfere with an efficient engine operation, it is necessary to regenerate the filtration properties of the filter by burning off the collected soot on a regular basis [6,7]. During so called passive regeneration, the trapped soot is converted into carbon dioxide by continuous reaction with nitrogen oxide (NO2) which is much stronger particular oxidizer than oxygen [8] and normally produced by upstream oxidation (Pt-based) catalyst [9]. Although the oxidation temperature is relatively low ( < 250 °C), the
⁎
Corresponding authors. E-mail addresses: [email protected] (D. Bucevac), [email protected] (C.-A. Wang).
http://dx.doi.org/10.1016/j.ceramint.2017.04.161 Received 20 December 2016; Received in revised form 18 March 2017; Accepted 27 April 2017 Available online 29 April 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 43 (2017) 9815–9822
D. Bucevac et al.
sintered samples were machined to make upper and lower planes parallel. The representative samples containing different amount of CuO were crushed in mortar and pestle and mixed with carbon (graphite) in order to determine the catalytic effect of CuO on carbon oxidation. Graphite was used as a substitute for diesel soot. The obtained mixtures with 10 wt% of graphite were pressed into cylindrical samples and heated to temperature of 575 °C in air in 25 °C increments. After 10 min hold at each temperature the weight of samples was measured in order to determine the weight loss and therefore the effect of CuO on carbon burnout. Please note that although CuO will be converted into Cu3Mo2O9 during sintering, the sintered samples will be designated according to the CuO amount in starting mixtures.
to its good thermo-mechanical properties [19,20] and microstructure consisting of elongated and interlocked prism-like grains [21,22]. Although sufficiently strong and rigid, this material contains large, open, pores which create remarkably low back pressure which is extremely important for an efficient engine operation. At the same time the long, elongated grains projecting from the filter wall surface offer a high surface areas for catalyst coatings and collection of soot particles. Similar to SiC and cordierite, mullite possesses good thermal shock resistance, high melting point, sufficient chemical resistance and good adhesion compatibility with catalyst coatings. Pyzik et al. have developed acicular mullite starting from kaolinite clay as a source of alumina and silica [23]. They have found that acicular mullite grans can be formed if mullite precursor is heated in a laboratory quartz reactor filled with SiF4. The obtained mullite showed better resistance to ash components than commercially available SiC and cordierite filter material. The present paper describes a simple, low-cost, method for fabrication of acicular mullite material with in situ formed catalyst which lowers the temperature of carbon oxidation by 90 °C. It will be shown that the acicular, prism-like, mullite grains can be obtained without the use of gaseous components or expensive equipment necessary to handle pressurised gasses. The method is based on a complete reuse of waste MoSi2 which can be converted into mixture of amorphous SiO2 and MoO3 by simple heating at 500 °C in air [24]. It appears that the use of fine amorphous SiO2 promotes formation of acicular mullite grains through reaction with commercial Al2O3 [25]. At the same time MoO3 reacts with CuO additive to form copper molybdate (Cu3Mo2O9) known to catalyze carbon oxidation (combustion) [26]. The excess of MoO3 evaporates introducing considerable porosity. As Eq. 1 shows, the final product is porous mullite-Cu3Mo2O9 composite which can catalyze the reaction of carbon soot oxidation.
2.2. Characterization The thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses of the starting mixture without CuO and the mixture containing 4 wt% CuO were performed in air (flow rate: 200 cm3/min; heating rate: 20 °C/min) from room temperature up to 1400 °C on a SDT Q600 instrument (TA Instruments). The same parameters were used to analyze the powder compacts made of ground (powdered) sintered samples with 10 wt% carbon. The phase composition of sintered samples was determined by X-ray diffraction (XRD) analysis on a Siemens D500 difractometer using Cu Ka radiation with a Ni filter. Bulk density and open porosity were determined by the Archimedes’ method using xylene. The specific surface area and average pore size were determined by the mercury intrusion measurements on a Porosimeter 2000 (Fisons Instruments) in the pressure range from ambient pressure up to 2000 bar. The calculation was done for a
cylindrical pore geometry. The thermal conductivity of sintered samples was determined using a hot-disk thermal analyzer (TPS2200, Hot disk AB Co., Sweden) at 300 °C. The coefficients of thermal expansion were measured by the means of dilatometer (Bahr, DIL801) in temperature range from 200 to 1000 °C. Compressive strength measurement was carried out on Instron M 1185 testing machine. Microstructure was examined by Scanning electron microscopy (SEM) using a JEOL-JSM-5800LV microscope. The composition of sintered samples was also analysed by Energy dispersive X-ray spectroscopy (EDS).
Bearing in mind that the annual production of MoSi2 heating elements is more than 500 t/year and knowing that no relevant report on MoSi2 recycling was published, it is evident that large amount of waste MoSi2 is available for this method. In this way, the waste MoSi2 can be completely reused without the use of any complicated process or poisonous chemical. 2. Material and methods 2.1. Preparation of samples
3. Results and discussion
The waste MoSi2 heating element (Super Kanthal 1700 °C, Bulten– Kanthal AB, Sweden) was crushed and pulverized in vibratory mill for 1 h. The obtained powder was calcined at 500 °C for 24 h in order to convert MoSi2 into a blend of MoO3 and amorphous SiO2. Small amount of residual MoSi2 of ~ 7.5 wt% was present despite the quite long calcination. The detail procedure as well as analysis of the obtained powder were described in our previous study [25]. This blend along with Al2O3 powder (Alcoa A-16 SG, Bauxite, USA) and CuO powder were homogenised in a plastic jar using Al2O3 balls as milling media. The amount of Al2O3 was adjusted to Al2O3/SiO2 ratio of 72/ 28 wt% whereas the CuO amount in the mixtures was varied from 4 to 12 wt%. These starting mixtures were dried, sieved and pressed into 8 mm cylindrical compacts under a pressure of 60 MPa. The samples were sintered at temperature ranging from 1300 to 1500 °C for 4 h in air with a heating rate of 5 °C/min. Four-hour long hold at 750 °C was employed to ensure a complete evaporation of excess MoO3. The
3.1. TG-DSC analysis of starting mixtures The change of weight and thermal effects during the heating of starting mixture without and with 4 wt% CuO were recorded by TGADSC analyzer and presented in Fig. 1. The small weight increase observed on TG curves (Fig. 1a) at approximately 500 °C is due to the oxidation of residual MoSi2 present in the starting mixtures. As can be seen, the weight loss at temperature bellow 800 °C is somewhat larger for the powder containing CuO. It is likely that the additional weight loss comes from decomposition of Cu(OH)2 which takes place bellow 200 °C. Kaur et al. have reported that CuO present in SnO2:CuO films converts to Cu(OH)2 on exposure to moisture [27]. It is believed that this effect is even more pronounced when CuO is in a powder form. The following weight decrease in temperature range 800–1300 °C is mainly 9816
Ceramics International 43 (2017) 9815–9822
D. Bucevac et al.
100
Weight (wt%)
95 90
0 wt% CuO 85
4 wt% CuO
80 75
0
200
400
600
800
1000 1200 1400
Temperature (oC)
(a) 25
Fig. 2. XRD patterns of samples containing different amount of CuO sintered at 1300 °C for 4 h. The inserted diagram is XRD pattern of the sample containing 12 wt% CuO sintered at 1500 °C for 4 h.
Heat flow (W/g)
20
phases are not formed in the presence of MoO3 as it reacts with CuO to form Cu3Mo2O9 which was detected by the XRD analysis. Thus, it is believed that the presence of Cu3Mo2O9 prevents direct contact and therefore inhibits the reaction between Al2O3 and SiO2 particles leading to somewhat increased temperature of mullite formation. However, mullite can be obtained at lower temperature such as 1300 °C with the aid of prolonged heat treatment.
15 10 5
exo up
0 -5
(b)
0 wt% CuO 4 wt% CuO
0
200
400
600
3.2. XRD analysis and weight loss
800 1000 1200 1400
Temperature (oC)
Fig. 2. shows XRD patterns of the samples containing different amount of CuO sintered at 1300 °C for 4 h. The sintering temperature was chosen based on our previous study which showed that 1300 °C is the lowest temperature at which porous mullite samples with sufficient mechanical integrity could be obtained. As can be seen in Fig. 2, mullite and small amount of residual Al2O3 were found in all samples regardless of the CuO amount. On the other side, the amount of Cu3Mo2O9 detected in samples containing CuO, continuously increases with the fraction of CuO added indicating the presence of excess MoO3 in the starting mixtures available for reaction with CuO. It is quite interesting that only one crystalline phase containing Cu and Mo was detected despite the fact that the amount of CuO, and therefore, MoO3/ CuO ratio varies. MoO3-CuO phase diagram constructed by Machej and Ziolkowski predicts formation of two phases; CuMoO4 in CuO lean region ( < 60 mol% CuO) and Cu3Mo2O9 in CuO reach region ( > 50 mol%) [32]. For instance, the molar MoO3/CuO ratio in samples with 8 and 12 wt% CuO is 60/40 and 49/51 meaning that CuMoO4 should be detected in both samples as the molar fraction of CuO is less than 60 mol%. The absence of CuMoO4 in samples sintered at 1300 °C (Fig. 2) is believed to be partly the result of evaporation of MoO3 prior to the reaction with CuO. The evaporation of MoO3 increases the fraction of CuO and favors formation of Cu3Mo2O9. This evaporation also causes a weight loss which is strongly affected by the amount of CuO. As Fig. 3 reveals, the highest weight loss at 1300 °C was measured in the sample without CuO as the result of complete evaporation of MoO3. As the amount of CuO increases the weight loss decreases due to the reaction of MoO3 with CuO and formation of Cu3Mo2O9, and possible CuMoO4, both of which have higher decomposition temperature than MoO3 itself. However, it is evident that a certain amount of MoO3 evaporates even from sample with the highest content of CuO. It is worth mentioning that the evaporation of MoO3 is not considered an environmental issue as MoO3 deposits as soon as it gets out of the hot furnace zone making long interlocked needles. Another reason for the absence of CuMoO4 is its thermal decomposition. Although both Cu3Mo2O9 and CuMoO4 decompose above 800 °C the temperature of CuMoO4 decomposition is always lower than
Fig. 1. (a) TG curves and (b) DSC curves of starting mixtures without CuO (MoO3-SiO2Al2O3) and with 4 wt% CuO.
the result of the evaporation of MoO3 which was initialized by melting at temperature slightly lower than 800 °C [28]. It is important to note that the evaporation of MoO3 from powder containing CuO is less intensive than that from the powder without CuO making TG curve less steep. This can be explained by the fact that the certain amount of MoO3 reacts with CuO to form Mo/Cu containing compounds such as CuMoO4 and mainly Cu3Mo2O9 which will be confirmed later by XRD analysis. Bearing in mind that Cu3Mo2O9 slowly decomposes at temperature above 850 °C [29], it is expected that MoO3 which is constituent of Cu3Mo2O9 does not evaporate at 800 °C which is typical for MoO3 compound present in the sample without CuO. As Fig. 1a shows, the weight of sample with CuO continuously decreases as the temperature increases from 800 to 1300 °C. The above mentioned processes of evaporation and decomposition, which are endothermic in nature, were also confirmed by DSC analysis (Fig. 1b) which shows the broad endothermic peaks in temperature range from 700 to 1250 °C. These endothermic processes in the sample containing CuO consume more energy and terminate at the somewhat higher temperature than those in the sample without CuO. This finding is believed to be the result of evaporation of Cu3Mo2O9 and probably CuMoO4 which takes place at higher temperatures. As shown by DSC curves (Fig. 1b), the first significant exothermic peak for the powder without CuO occurs at approximately 1250 °C and should be attributed to the mullite formation. However, the first peak for powder containing CuO is shifted to the higher temperature indicating that mullite forms at ~1350 °C. This is contradictory to the results of several studies showing that the presence of CuO normally promotes the formation of mullite and therefore lowers the formation temperature [30,31]. Although the various mechanisms were proposed, the accelerated mullite formation is generally ascribed to the formation of intermediate quartz-type and spinel-type phases reach in copper. Evidently these 9817
Ceramics International 43 (2017) 9815–9822
26
Table 1 Specific surface area, average pore size, thermal conductivity and coefficient of thermal expansions of samples with different CuO content sintered at 1300 °C for 4 h.
Weight loss (wt%)
D. Bucevac et al.
24 22
Specific surface area (m2/g)
Average pore size (µm)
Thermal conductivity at 300 °C (W/m K)
Coefficient of thermal expansion (x 10−6/°C)
0 4 8 12
0.982 0.841 0.820 0.707
3.082 2.762 1.961 1.442
0.98 1.03 0.97 0.98
4.82 4.51 4.47 4.96
20
0 wt% CuO 4 8 12
18 16 14 1250
1300
1350
1400
1450
1500
(1500 °C) is the lowest despite the fact that the weight loss of this sample is the highest (Fig. 3). This implies that the porosity is also controlled by the ability of CuO to promote sintering either by dissolution of small amount of Cu into mullite matrix or by formation of Mo-containing liquid phase. Although the liquid phase will eventually evaporate it is believed that its presence promotes densification. Seemingly, this effect is more pronounced at higher temperature. When it comes to the effect of sintering temperature on porosity of the samples containing different amount of CuO it can be concluded that the porosity of all compositions increases in temperature range from 1300 to 1450 °C due to decomposition of Cu3Mo2O9. Apparently the decomposition of Cu3Mo2O9 is almost complete after four-hour long sintering at 1450 °C. Further increase in temperature to 1500 °C promotes sintering by increasing diffusion coefficients which leads to the small reduction in porosity of all compositions, including samples without CuO, as depicted in Fig. 4. Similarly to the open porosity, the specific surface area of samples was also affected by the amount of CuO, though the effect was much less pronounced. The results presented in Table 1 point out that the surface area of samples sintered at 1300 °C decreased from 0.982 m2/g in sample without CuO to 0.707 m2/g in sample with 12 wt% CuO whereas the average pore size decreased from 3.08 µm to 1.44 µm, respectively. Knowing that the primary soot particles, which are smaller than 50 nm in diameter [33], form agglomerates with diameter normally larger than 1 µm it can be concluded that the obtained material is expected to be efficient in carbon soot removal. The other two properties which are very important for application of mullite as DPF material are thermal conductivity and coefficient of thermal expansion. As Table 1 shows, the change of these properties due to the presence of CuO can be considered as negligible. It is worth mentioning that the coefficient of thermal expansion of mullite is just slightly higher than that of SiC which is used for production of commercially available DPFs (~ 4×10−6/°C).
Sintering temperature (oC) Fig. 3. Effect of sintering temperature on weight loss of samples containing different amount of CuO sintered for 4 h.
that of Cu3Mo2O9 by 10–35 °C [32] causing faster decomposition of CuMoO4 which was not detected in samples sintered at 1300 °C. As Fig. 3 shows, the weight loss due to the decomposition of Cu3Mo2O9 is even more pronounced at higher temperature. The weight loss at 1500 °C was the highest in sample containing the highest amount of CuO (12 wt%). The inserted XRD pattern in Fig. 2 reveals that the amount of Cu3Mo2O9 considerably decreases after sintering at 1500 °C. It is also worth mentioning the presence of small amount of CuAl2O4 in the samples containing 8 and 12 wt% CuO. Similarly to Cu3Mo2O9, CuAl2O4 disappears after sintering at 1500 °C. The evaporation of MoO3 (primarily), CuMoO4 and Cu3Mo2O9 during sintering is expected to affect the porosity of sintered samples which will be discussed in the following section. 3.3. Porosity and thermal properties The effect of sintering temperature on the open porosity of samples containing different amount of CuO is presented in Fig. 4 showing that the all porosity values fall in the range from 44% to 62% which is actually the porosity measured in commercially available DPFs. Similarly to the weight loss (Fig. 3), the porosity of samples sintered at low temperature (1300 °C) decreases with the amount of CuO indicating that the porosity of these samples is mainly the result of evaporation of MoO3 which was the most pronounced in sample without CuO. This way MoO3, i.e., waste MoSi2, was also employed as a pore former. It is interesting to note that the porosity of sample containing 12 wt% CuO sintered at the higher sintering temperatures
Open porosity (%)
CuO content (wt%)
65
3.4. Compressive strength and expansion
60
Knowing that the compressive strength of samples is strongly affected by the porosity, an attempt to find the relationship between strength and porosity will be made in the following section. The effect of sintering temperature on compressive strength of samples with different CuO content is presented in Fig. 5. As the figure shows, the strength of all compositions decreases with increasing sintering temperature reaching a minimum value in samples sintered at 1450 °C. The decrease in strength is the result of the increase in porosity which was discussed and presented in Fig. 4. The samples sintered at 1450 °C possess the maximum porosity and the minimum compressive strength at the same time. Further increase in sintering temperature slightly improves strength which is, again, the result of the small decrease in porosity (Fig. 4). This inversely proportional relation between strength and porosity is well documented and therefore expected. This relationship can be used to design porous material with specific values of porosity and compressive strength which are required for particular application. According to Fig. 4 and Fig. 5, the boundary
55 50
0 wt% CuO 4 8 12
45 40 1250
1300
1350
1400
1450
1500
Temperature (oC) Fig. 4. Effect of sintering temperature on open porosity of samples containing different amount of CuO sintered for 4 h.
9818
Ceramics International 43 (2017) 9815–9822
Compressive strength (MPa)
D. Bucevac et al.
0 wt% CuO 4 8 12
70 60 50
addition of CuO reduces the amount of MoO3 that evaporates, promotes sintering and therefore shrinkage at high temperature which decreases the overall expansion measured after sintering. As Fig. 6 shows, the overall expansion of samples containing 12 wt% CuO is zero. In this case the expansion that occurs at ~ 500 °C is completely compensated by the shrinkage that takes place at sintering temperature. The zero expansion can considerably facilitate the process of compaction of ceramic powder and therefore fabrication of ceramic components. In the case of zero expansion, the dimensions of the die used for powder pressing are the same like the dimensions of final (sintered) sample. There is no need for estimation of shrinkage/ expansion which must be taken into account while selecting the die.
40 30 20 10 0 1250
1300
1350
1400
1450
3.5. Microstructure
1500
Sintering temperature (oC)
Fig. 7 shows SEM micrographs of the samples with different amount of CuO sintered at 1300 °C. This temperature was chosen as it was the lowest temperature at which mullite was formed. Fig. 7a reveals that the microstructure of sample without CuO consists of interpenetrating, elongated mullite grains with bimodal grain size distribution. The large grains are normally in the form of rectangular prisms with a rectangular base and sharp, well defined, edges. The grain diameter varies from 1 to 5 µm whereas the length might be up to 30 µm. These relatively thick grains are interlocked with very thin and elongated mullite grains, whiskers, with diameter less than 0.5 µm. Besides the elongated mullite grains, Fig. 7a also shows the presence of small spherical grains which are identified as residual alumina. Unlike the pure mullite sample, the sample with 4 wt% CuO does not contain thin mullite whiskers. As Fig. 7b indicates there are much bigger mullite grains which were formed by coalescence of several elongated mullite grains. Although the grains are still elongated, the prism-like grain morphology is not as pronounced as in sample without CuO. According to Fig. 7c and d the coalescence of mullite grains becomes more pronounced with an increase in the amount of CuO. The coalescence might be ascribed to the presence of liquid phase which contains Cu and solidifies during cooling from sintering temperature as white phase (Fig. 7b-d). The elemental composition of both white and grey phase was determined by EDS analysis and presented in Fig. 8. As expected, constitutive elements of mullite, i.e., Al, Si and O were found in the grey phase without any trace of Mo or Cu (Fig. 8a). On the other side, Fig. 8b reveals that the main elements in the white phase are Mo and Cu. This finding, along with XRD analysis (Fig. 2) confirms that the white phase is Cu3Mo2O9. Moreover, relatively large amount of Mo in white phase rules out the possibility that CuAl2O4, also detected by XRD analysis, is colored white. Further analysis of location of CuAl2O4 was not conducted as the amount of this phase was small. The fractions of white areas (Cu3Mo2O9) in the figures showing samples with 4 wt%, 8 wt% and 12 wt% CuO were roughly estimated to be 3.7%, 7.3% and 10.5%, respectively. The exact amount of Cu3Mo2O9 was not crucial in this study as the amount of CuO in starting mixtures was the parameter that had been optimized.
Fig. 5. The effect of sintering temperature on compressive strength of samples containing different amount of CuO sintered for 4 h.
values for strength and porosity are measured in sample without CuO sintered at 1450 °C and sample with 12 wt% CuO sintered at 1300 °C. Namely, the sample without CuO has the highest porosity of 63% and the lowest compressive strength of only 4 MPa, whereas the sample containing 12 wt% CuO has the lowest porosity of 44% and the highest compressive strength of 70 MPa. Evidently, the variation of sintering temperature and the amount of CuO allows fabrication of filter material with specific values of porosity and compressive strength. Besides porosity and strength, the variation of amount of CuO can be also used to control shrinkage of ceramic compacts after sintering. Since the samples without CuO actually expand after sintering, the term expansion will be used instead of shrinkage. Fig. 6 shows the effect of sintering temperature on expansion of samples containing different amount of CuO. The expansion of sintered samples is affected by the amount of CuO rather than sintering temperature. The small changes in porosity of the samples containing certain amount of CuO with temperature are not sufficient to cause any considerable change of the diameter of sintered samples. On the other side, the amount of CuO strongly affects the expansion. As Fig. 6 reveals, samples without CuO expands by ~ 6% after sintering. This expansion comes from the oxidation of residual MoSi2 present in the starting powder and consequent volume increase due to the formation of MoO3 and SiO2. The oxidation, and therefore expansion, takes place at ~ 500 °C and it is inevitably followed by a certain shrinkage at high temperature such as 1300 °C. The shrinkage that takes place at sintering temperature is strongly affected by the amount of CuO. The
7
Expansion (%)
6 5
0 wt% CuO 4% 8% 12%
4 3 2
3.6. Effect of Cu3Mo2O9 on carbon burnout It was expected that the amount of CuO and therefore the amount of Cu3Mo2O9 would affect the rate of carbon oxidation. Thus, the sintered samples with different amount of CuO were crushed and mixed with carbon to make powder mixtures containing 10 wt% of carbon which were subsequently compacted and heated to temperature of 575 °C in 25 °C increments. Fig. 9 shows the effect of temperature on weight loss of the powder compacts containing mullite with different amount of CuO and 10 wt% of carbon. The loss of mullite-Cu3Mo2O9 material itself was not detected even after prolonged heat treatment pointing out a good thermal stability of mullite as well as Cu3Mo2O9. Therefore, the weight loss is solely a result of carbon burnout (oxidation) which is presented on the right vertical axis as a percentage
1 0 -1 1250
1300
1350
1400
1450
1500
Sintering temperature (oC) Fig. 6. The effect of sintering temperature on expansion of samples containing different amount of CuO sintered for 4 h.
9819
Ceramics International 43 (2017) 9815–9822
D. Bucevac et al.
(a)
(c)
10 µm
10 µm
(d)
(b)
10 µm
10 µm
Fig. 7. SEM (back scatter mode) micrographs of fracture surface of samples containing different amount of CuO sintered at 1300 °C for 4 h: (a) 0 wt%, (b) 4 wt%, (c) 8 wt% and (d) 12 wt% CuO. The white phase is Cu3Mo2O9.
of total carbon amount. As the figure evidences, the weight loss of all samples increases with temperature. It is important to note that the weight loss of samples containing CuO considerably increases when the temperature was raised to 575 °C. Evidently, at this temperature, the presence of Cu3Mo2O9 (CuO) accelerates the oxidation of carbon. The weight loss continuously increases with the amount of CuO reaching the highest value of 7.1 wt% in sample containing 12 wt% CuO, which means that 71% of the added carbon was removed by oxidation. Now, the temperature was kept at 575 °C and heating time was extended up to ~ 4 h. Fig. 10 shows that after 20 min at 575 °C, the weight loss of sample with 12 wt% CuO is 9.4 wt%. That means that almost all carbon (94%) was removed from the powder compact. At the same time the weight loss of sample without CuO is only 2.8 wt%. Moreover, the complete removal of carbon (10%) from mullite without CuO takes more than 4 h which is considerably longer than the time required for samples containing CuO. These results clearly indicate that the presence Cu3Mo2O9 in mullite matrix catalyzes the reaction of carbon oxidation. The TGA analysis was employed to quantify the effect of Cu3Mo2O9 (wt% CuO) on carbon oxidation by determining the reduction in temperature at which certain weight loss was achieved. As Fig. 11
shows, the weight loss of 5% in the sample without CuO was achieved at 710 °C whereas the same loss in sample containing 12 wt% CuO was achieved at 620 °C. Knowing that samples contain 10 wt% of carbon, it is evident that the temperature at which 50% of carbon was oxidized was by 90 °C lower for sample containing 12 wt% CuO. Based on these results it can be concluded that the presence of Cu3Mo2O9 in porous mullite lowers the carbon oxidation temperature. Therefore mullite Cu3Mo2O9 composite is a kind of promising material for fabrication of DPFs as it lowers the temperature of so called active filter regeneration, i.e., carbon soot oxidation. 4. Conclusions Porous acicular mullite ceramics containing in situ created Cu3Mo2O9 as a soot oxidation catalyst was fabricated using the commercial powders of Al2O3 and CuO, and the powder obtained by controlled oxidation of waste MoSi2. Samples containing 0−12 wt% CuO were sintered in temperature range between 1300 and 1500 °C. Both the amount of CuO and sintering temperature influenced the porosity and compressive strength of the samples. Sintering temperature of 1300 °C was found to be optimal as it was the lowest 9820
Weight loss (wt%)
Intensity (a.u.)
Al
O
Si Au 0
10
100
8
80
6 4 2
2
4
6
8
10
0 wt% CuO 4 8 12 0
40
80
Energy(keV)
(b)
240
100
0
Sample weight (wt%)
Intensity(a.u.)
Cu
Cu
Si
Cu
2
4
6
8
10
Energy (keV) Fig. 8. EDS spectra of the sample containing 12 wt% CuO sintered at 1300 °C for 4 h taken at (a) grey phase and (b) white phase.
80 70
0 wt% CuO 4 8 12
6
4
60 50 40 30 20
2
10 475
500
525
550
575
98
710 oC
620 oC 96 94 92
0 wt% CuO
90 88 200
20
12 wt% CuO 400
600
800
0 10 20 30 40 50 60 70 80 90 100 1000
Temperature (oC)
Carbon burnout (wt%)
8
Weight loss (wt%)
200
Fig. 10. Effect of time on weight loss of powder compacts containing 10 wt% carbon at 575 °C. The carbon was mixed with powders obtained by grinding samples with different amount of CuO sintered at 1300 °C for 4 h. The right vertical axis shows the percentage of carbon burnt during heating.
Al
0 450
160
40
Time (min)
Mo
O
120
60
Carbon burnout (wt%)
(a)
Carbon burnout (wt%)
Ceramics International 43 (2017) 9815–9822
D. Bucevac et al.
Fig. 11. TGA curves of powders containing 10 wt% carbon. The carbon was mixed with powders obtained by grinding samples with 0 wt% and 12 wt% CuO sintered at 1300 °C for 4 h. The right vertical axis shows the percentage of carbon burnt during heating.
was almost negligible. The obtained material consisted of elongated mullite grains with dispersed gains of in situ created Cu3Mo2O9 which catalyzes the carbon oxidation (burnout). The addition of 12 wt% CuO decreases the carbon oxidation temperature by 90 °C when compared to pure mullite. This feature along with porous elongated microstructure makes mullite-Cu3Mo2O9 composite a promising material for DPF fabrication. In addition, the described novel approach is environmentally friendly as the waste MoSi2 is completely reused to fabricate mullite material as well as Cu3Mo2O9 catalyst.
0
Temperature (oC)
Acknowledgement
Fig. 9. Effect of temperature on weight loss of powder compacts containing 10 wt% carbon. The carbon was mixed with powders obtained by grinding samples with different amount of CuO sintered at 1300 °C for 4 h. The hold at each temperature was 10 min. The right vertical axis shows the percentage of carbon burnt during heating.
This work was supported by the Ministry of education, science and technological development of Serbia (project number: 45012). References
temperature at which mullite - Cu3Mo2O9 composite with high porosity and relatively high strength was obtained. At this temperature, the addition of CuO decreased porosity from 62% in pure mullite to 44% in sample containing 12 wt% CuO. At the same time the compressive strength increased from 19 MPa in pure mullite to 70 MPa in sample containing 12 wt% CuO. On the other side, the change of thermal conductivity and coefficient of thermal expansion with the CuO content
[1] M. Fiebig, A. Wiartalla, B. Holderbaum, S. Kiesow, Particulate emissions from diesel engines: correlation between engine technology and emissions, J. Occup. Med. Toxicol. 9 (2014) 2–18. [2] U. Mathis, M. Mohr, R. Kaegi, A. Bertola, K. Boulouchos, Influence of diesel engine combustion parameters on primary soot particle diameter, Environ. Sci. Technol. 39 (2005) 1887–1892. [3] H. Burtscher, Physical characterization of particulate emissions from diesel
9821
Ceramics International 43 (2017) 9815–9822
D. Bucevac et al.
[19] H. Schneider, J. Schreuer, B. Hildmann, Structure and properties of mullite-A review, J. Eur. Ceram. Soc. 28 (2008) 329–344. [20] E.C. Hammel, O.L.R. Ighodaro, O.I. Okoli, Processing and properties of advanced porous ceramics: an application based review, Ceram. Int. 40 (2014) 15351–15370. [21] C.H.H. Hsiung, A.J. Pyzik, F. De Carlo, X. Xiao, S.R. Stock, K.T. Faber, Microstructure and mechanical properties of acicular mullite, J. Eur. Ceram. Soc. 33 (2013) 503–513. [22] R.R. Monteiro, A.C.S. Sabioni, Preparation of mullite whiskers derived from topaz doped with rare earth oxides for applications in composite materials, Ceram. Int. 42 (2014) 49–55. [23] A.J. Pyzik, C.S. Todd, C. Han, Formation mechanism and microstructure development in acicular mullite ceramics fabricated by controlled decomposition of fluorotopaz, J. Eur. Ceram. Soc. 28 (2008) 383–391. [24] S. Lohfeld, M. Schütze, Oxidation behaviour of particle reinforced MoSi2 composites at temperatures up to 1700°C part I: literature review, Mater. Corros. 56 (2005) 93–97. [25] D. Bučevac, A. Dapčević, V. Maksimović, Porous acicular mullite obtained by controlled oxidation of waste molybdenum disilicide, Mater. Res. Bull. 50 (2014) 155–160. [26] G. Mul, J.P.A. Neeft, F. Kapteijn, M. Makkee, J.A. Moulijn, Soot oxidation catalyzed by a Cu/K/Mo/Cl catalyst: evaluation of the chemistry and performance of the catalyst, Appl. Catal. B, Environ. 6 (1995) 339–352. [27] M. Kaur, D.K. Aswal, J.V. Yakhmi, Chemiresistors Gas Sensors: materials, Mechanisms and Fabrication, in: D. Aswal, S. Gupta (Eds.), Sci. Technol. Chemiresistors Gas Sensors, Nova Science Publishers, Inc, New York, 2007, pp. 33–94. [28] J. Bygden, D. Sichen, S. Seetharaman, A thermodynamic study of molybdenumoxygen system, Metall. Mater. Trans. B. 25B (1994) 885–891. [29] L. Kihlborg, R. Norrestam, B. Olivecrona, The crystal structure of Cu3Mo2O9, Acta Crystallogr. Sect. B Struct. Crystallogr. Cryst. Chem. 27 (1971) 2066–2070. [30] T. Martisius, R. Giraitis, Influence of copper oxide on mullite formation from kaolinite, J. Mater. Chem. 13 (2003) 121–124. [31] E.R. Segnit, T. Geleb, Metastable quartz-type structures formed from kaolinite by solid state reaction, Am. Mineral. 57 (1972) 1505–1514. [32] T. Machej, J. Ziołkowski, Phase diagram of the CuOMoO3 system, Mater. Chem. 4 (1979) 113–121. [33] S. Yoon, D.C. Quiros, H.A. Dwyer, J.F. Collins, M. Burnitzki, D. Chernich, J.D. Herner, Characteristics of particle number and mass emissions during heavyduty diesel truck parked active DPF regeneration in an ambient air dilution tunnel, Atmos. Environ. 122 (2015) 58–64.
engines: a review, J. Aerosol Sci. 36 (2005) 896–932. [4] M. Tsukahara, T. Murayama, Y. Yoshimoto, Influence of fuel properties on the combustion in diesel engine driven by the emulsified fuels, Bull. JSME 25 (1982) 612–619. [5] M.V. Twigg, Progress and future challenges in controlling automotive exhaust gas emissions, Appl. Catal. B Environ. 70 (2007) 2–15. [6] V. Sibanda, R.W. Greenwood, J.P.K. Seville, Particle separation from gases using cross-flow filtration, Powder Technol. 118 (2001) 193–202. [7] F. Millo, M. Andreata, M. Rafigh, D. Mercuri, C. Pozzi, Impact on vehicle fuel economy of the soot loading on diesel particulate filters made of different substrate materials, Energy 86 (2015) 19–30. [8] S. Bai, J. Tang, G. Wang, G. Li, Soot loading estimation model and passive regeneration characteristics of DPF system for heavy-duty engine, Appl. Therm. Eng. 100 (2016) 1292–1298. [9] M. Shrivastava, A. Nguyen, Z. Zheng, H.W. Wu, H.S. Jung, Kinetics of Soot Oxidation by NO2, Environ. Sci. Technol. 44 (2010) 4796–4801. [10] V. Di, G. Landi, L. Lisi, A. Saliva, A. Di, Applied catalysis B: environmental the sootcatalyst contact on the regeneration performance, "Appl. Catal. B, Environ. 197 (2016) 1–9. [11] D. Fino, Diesel emission control: catalytic filters for particulate removal, Sci. Technol. Adv. Mater. 8 (2007) 93–100. [12] C. Beatrice, S. Di Iorio, C. Guido, P. Napolitano, Detailed characterization of particulate emissions of an automotive catalyzed DPF using actual regeneration strategies, Exp. Therm. Fluid Sci. 39 (2012) 45–53. [13] V.R. Pérez, A. Bueno-López, Catalytic regeneration of Diesel Particulate Filters: comparison of Pt and CePr active phases, Chem. Eng. J. 279 (2015) 79–85. [14] Z. Stępień, L. Ziemiański, G. Zak, M. Wojtasik, Ł. Jęczmionek, Z. Burnus, The evaluation of fuel borne catalyst (FBC's) for DPF regeneration, Fuel 161 (2015) 278–286. [15] D. Fino, S. Bensaid, M. Piumetti, N. Russo, A review on the catalytic combustion of soot in Diesel particulate filters for automotive applications: from powder catalysts to structured reactors, Appl. Catal. A Gen. 509 (2016) 75–96. [16] Y.Y. Choi, J.G. Kim, K.M. Lee, H.C. Sohn, D.J. Choi, A study of surface-coated SiC whiskers on carbon fiber substrates and their properties for diesel particulate filter applications, Ceram. Int. 37 (2011) 1307–1312. [17] B. Guan, R. Zhan, H. Lin, Z. Huang, Review of the state-of-the-art of exhaust particulate filter technology in internal combustion engines, J. Environ. Manag. 154 (2015) 225–258. [18] A.J. Pyzik, C.G. Li, New design of a ceramic filter for diesel emission control applications, Int. J. Appl. Ceram. Technol. 2 (2005) 440–451.
9822
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Porous acicular mullite ceramics fabricated with in situ formed soot oxidation catalyst obtained from waste MoSi2 ⁎
Dusan Bucevaca, , Jelena Maletaskicb, Mia Omerasevicb, Branko Matovicb, Chang-An Wangc, a b c
MARK ⁎
Department of Mechanical and Materials Engineering, Queen's University, Kingston, Ontario, Canada K7L 3N6 Vinca Institute of Nuclear Sciences, University of Belgrade, Belgrade 11000, Serbia State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
A R T I C L E I N F O
A BS T RAC T
Keywords: A: sintering B: porosity C: thermal properties D: acicular mullite E: engine components
Porous acicular mullite (3Al2O3·2SiO2) ceramics containing Cu3Mo2O9 as a soot oxidation catalyst was fabricated by a novel approach using commercial powders of Al2O3 and CuO, and powder obtained by controlled oxidation of ground waste MoSi2. The obtained material consisted of elongated mullite grains which are known to be effective in carbon soot removal from diesel engine exhaust. The presence of in situ created Cu3Mo2O9 was found to catalyze the carbon burnout which is an extremely important feature when it comes to filter regeneration, i.e., the captured soot removal. The carbon burnout temperature in the sample containing 12 wt% CuO was by 90 °C lower than that in the sample without CuO. Effect of sintering temperature as well as the effect of amount of CuO additive on mullite properties were studied. It was found that the increase in amount of CuO in samples sintered at 1300 °C decreased porosity and increased compressive strength of the porous mullite ceramics. The addition of 12 wt% CuO increased the strength of the porous mullite ceramics up to 70 MPa, whereas the porosity was reduced from 62% in the mullite without CuO to 44% in the mullite ceramics containing 12 wt% CuO. Although affected by the amount of CuO, the microstructure still consisted of elongated mullite grains.
1. Introduction
passive regeneration may fail during low engine loads that do not generate enough heat for sufficiently long time to oxidise the soot [10]. When a certain level of carbon soot particles is reached in the filter, the active regeneration is required to oxidise the soot particles to carbon dioxide with the help of oxygen at an exhaust gas temperature of 600– 650 °C [11]. Two approaches can be used to accomplish the task of a carbon soot combustion. One is to heat the exhaust gas and/or filter to the certain ignition temperature, which is normally done by injection of an additional amount of fuel [12]. The other approach is to lower the ignition temperature with the aid of oxidation catalyst which can be either coating [13] or fuel-born [14]. Both approaches increase the engine operation/maintenance cost either through an increased fuel consumption, use of expensive noble catalyst or vehicle's onboard dosing system. In the case of catalytic coated DPF, the both approaches are combined [15]. The most frequently used commercial ceramics DPFs are made of SiC [16] and cordierite coated with a mixture of Al2O3 and CeO2 which serves as a carrier layer for platinum catalyst [17]. However, acicular mullite was found to be a particularly convenient material for production of honeycomb filters [18] owing
The reduction of particulate emissions from diesel engines has been a great challenge over the past three decades [1–3]. Carbon soot particles that are inherent in diesel emissions due to an incomplete fuel combustion [4] can be effectively removed from exhaust gas by ceramic diesel particular filters (DPFs) [5]. DPF normally comprises of a honeycomb structure with many narrow and parallel channels which are blocked at alternate ends. The exhaust gas is thus forced to flow through the walls between channels and the carbon particles are deposited as a soot cake on the walls. Since the continuous deposition of the soot will make a filter resistant to the exhaust flow through it and therefore interfere with an efficient engine operation, it is necessary to regenerate the filtration properties of the filter by burning off the collected soot on a regular basis [6,7]. During so called passive regeneration, the trapped soot is converted into carbon dioxide by continuous reaction with nitrogen oxide (NO2) which is much stronger particular oxidizer than oxygen [8] and normally produced by upstream oxidation (Pt-based) catalyst [9]. Although the oxidation temperature is relatively low ( < 250 °C), the
⁎
Corresponding authors. E-mail addresses: [email protected] (D. Bucevac), [email protected] (C.-A. Wang).
http://dx.doi.org/10.1016/j.ceramint.2017.04.161 Received 20 December 2016; Received in revised form 18 March 2017; Accepted 27 April 2017 Available online 29 April 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 43 (2017) 9815–9822
D. Bucevac et al.
sintered samples were machined to make upper and lower planes parallel. The representative samples containing different amount of CuO were crushed in mortar and pestle and mixed with carbon (graphite) in order to determine the catalytic effect of CuO on carbon oxidation. Graphite was used as a substitute for diesel soot. The obtained mixtures with 10 wt% of graphite were pressed into cylindrical samples and heated to temperature of 575 °C in air in 25 °C increments. After 10 min hold at each temperature the weight of samples was measured in order to determine the weight loss and therefore the effect of CuO on carbon burnout. Please note that although CuO will be converted into Cu3Mo2O9 during sintering, the sintered samples will be designated according to the CuO amount in starting mixtures.
to its good thermo-mechanical properties [19,20] and microstructure consisting of elongated and interlocked prism-like grains [21,22]. Although sufficiently strong and rigid, this material contains large, open, pores which create remarkably low back pressure which is extremely important for an efficient engine operation. At the same time the long, elongated grains projecting from the filter wall surface offer a high surface areas for catalyst coatings and collection of soot particles. Similar to SiC and cordierite, mullite possesses good thermal shock resistance, high melting point, sufficient chemical resistance and good adhesion compatibility with catalyst coatings. Pyzik et al. have developed acicular mullite starting from kaolinite clay as a source of alumina and silica [23]. They have found that acicular mullite grans can be formed if mullite precursor is heated in a laboratory quartz reactor filled with SiF4. The obtained mullite showed better resistance to ash components than commercially available SiC and cordierite filter material. The present paper describes a simple, low-cost, method for fabrication of acicular mullite material with in situ formed catalyst which lowers the temperature of carbon oxidation by 90 °C. It will be shown that the acicular, prism-like, mullite grains can be obtained without the use of gaseous components or expensive equipment necessary to handle pressurised gasses. The method is based on a complete reuse of waste MoSi2 which can be converted into mixture of amorphous SiO2 and MoO3 by simple heating at 500 °C in air [24]. It appears that the use of fine amorphous SiO2 promotes formation of acicular mullite grains through reaction with commercial Al2O3 [25]. At the same time MoO3 reacts with CuO additive to form copper molybdate (Cu3Mo2O9) known to catalyze carbon oxidation (combustion) [26]. The excess of MoO3 evaporates introducing considerable porosity. As Eq. 1 shows, the final product is porous mullite-Cu3Mo2O9 composite which can catalyze the reaction of carbon soot oxidation.
2.2. Characterization The thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses of the starting mixture without CuO and the mixture containing 4 wt% CuO were performed in air (flow rate: 200 cm3/min; heating rate: 20 °C/min) from room temperature up to 1400 °C on a SDT Q600 instrument (TA Instruments). The same parameters were used to analyze the powder compacts made of ground (powdered) sintered samples with 10 wt% carbon. The phase composition of sintered samples was determined by X-ray diffraction (XRD) analysis on a Siemens D500 difractometer using Cu Ka radiation with a Ni filter. Bulk density and open porosity were determined by the Archimedes’ method using xylene. The specific surface area and average pore size were determined by the mercury intrusion measurements on a Porosimeter 2000 (Fisons Instruments) in the pressure range from ambient pressure up to 2000 bar. The calculation was done for a
cylindrical pore geometry. The thermal conductivity of sintered samples was determined using a hot-disk thermal analyzer (TPS2200, Hot disk AB Co., Sweden) at 300 °C. The coefficients of thermal expansion were measured by the means of dilatometer (Bahr, DIL801) in temperature range from 200 to 1000 °C. Compressive strength measurement was carried out on Instron M 1185 testing machine. Microstructure was examined by Scanning electron microscopy (SEM) using a JEOL-JSM-5800LV microscope. The composition of sintered samples was also analysed by Energy dispersive X-ray spectroscopy (EDS).
Bearing in mind that the annual production of MoSi2 heating elements is more than 500 t/year and knowing that no relevant report on MoSi2 recycling was published, it is evident that large amount of waste MoSi2 is available for this method. In this way, the waste MoSi2 can be completely reused without the use of any complicated process or poisonous chemical. 2. Material and methods 2.1. Preparation of samples
3. Results and discussion
The waste MoSi2 heating element (Super Kanthal 1700 °C, Bulten– Kanthal AB, Sweden) was crushed and pulverized in vibratory mill for 1 h. The obtained powder was calcined at 500 °C for 24 h in order to convert MoSi2 into a blend of MoO3 and amorphous SiO2. Small amount of residual MoSi2 of ~ 7.5 wt% was present despite the quite long calcination. The detail procedure as well as analysis of the obtained powder were described in our previous study [25]. This blend along with Al2O3 powder (Alcoa A-16 SG, Bauxite, USA) and CuO powder were homogenised in a plastic jar using Al2O3 balls as milling media. The amount of Al2O3 was adjusted to Al2O3/SiO2 ratio of 72/ 28 wt% whereas the CuO amount in the mixtures was varied from 4 to 12 wt%. These starting mixtures were dried, sieved and pressed into 8 mm cylindrical compacts under a pressure of 60 MPa. The samples were sintered at temperature ranging from 1300 to 1500 °C for 4 h in air with a heating rate of 5 °C/min. Four-hour long hold at 750 °C was employed to ensure a complete evaporation of excess MoO3. The
3.1. TG-DSC analysis of starting mixtures The change of weight and thermal effects during the heating of starting mixture without and with 4 wt% CuO were recorded by TGADSC analyzer and presented in Fig. 1. The small weight increase observed on TG curves (Fig. 1a) at approximately 500 °C is due to the oxidation of residual MoSi2 present in the starting mixtures. As can be seen, the weight loss at temperature bellow 800 °C is somewhat larger for the powder containing CuO. It is likely that the additional weight loss comes from decomposition of Cu(OH)2 which takes place bellow 200 °C. Kaur et al. have reported that CuO present in SnO2:CuO films converts to Cu(OH)2 on exposure to moisture [27]. It is believed that this effect is even more pronounced when CuO is in a powder form. The following weight decrease in temperature range 800–1300 °C is mainly 9816
Ceramics International 43 (2017) 9815–9822
D. Bucevac et al.
100
Weight (wt%)
95 90
0 wt% CuO 85
4 wt% CuO
80 75
0
200
400
600
800
1000 1200 1400
Temperature (oC)
(a) 25
Fig. 2. XRD patterns of samples containing different amount of CuO sintered at 1300 °C for 4 h. The inserted diagram is XRD pattern of the sample containing 12 wt% CuO sintered at 1500 °C for 4 h.
Heat flow (W/g)
20
phases are not formed in the presence of MoO3 as it reacts with CuO to form Cu3Mo2O9 which was detected by the XRD analysis. Thus, it is believed that the presence of Cu3Mo2O9 prevents direct contact and therefore inhibits the reaction between Al2O3 and SiO2 particles leading to somewhat increased temperature of mullite formation. However, mullite can be obtained at lower temperature such as 1300 °C with the aid of prolonged heat treatment.
15 10 5
exo up
0 -5
(b)
0 wt% CuO 4 wt% CuO
0
200
400
600
3.2. XRD analysis and weight loss
800 1000 1200 1400
Temperature (oC)
Fig. 2. shows XRD patterns of the samples containing different amount of CuO sintered at 1300 °C for 4 h. The sintering temperature was chosen based on our previous study which showed that 1300 °C is the lowest temperature at which porous mullite samples with sufficient mechanical integrity could be obtained. As can be seen in Fig. 2, mullite and small amount of residual Al2O3 were found in all samples regardless of the CuO amount. On the other side, the amount of Cu3Mo2O9 detected in samples containing CuO, continuously increases with the fraction of CuO added indicating the presence of excess MoO3 in the starting mixtures available for reaction with CuO. It is quite interesting that only one crystalline phase containing Cu and Mo was detected despite the fact that the amount of CuO, and therefore, MoO3/ CuO ratio varies. MoO3-CuO phase diagram constructed by Machej and Ziolkowski predicts formation of two phases; CuMoO4 in CuO lean region ( < 60 mol% CuO) and Cu3Mo2O9 in CuO reach region ( > 50 mol%) [32]. For instance, the molar MoO3/CuO ratio in samples with 8 and 12 wt% CuO is 60/40 and 49/51 meaning that CuMoO4 should be detected in both samples as the molar fraction of CuO is less than 60 mol%. The absence of CuMoO4 in samples sintered at 1300 °C (Fig. 2) is believed to be partly the result of evaporation of MoO3 prior to the reaction with CuO. The evaporation of MoO3 increases the fraction of CuO and favors formation of Cu3Mo2O9. This evaporation also causes a weight loss which is strongly affected by the amount of CuO. As Fig. 3 reveals, the highest weight loss at 1300 °C was measured in the sample without CuO as the result of complete evaporation of MoO3. As the amount of CuO increases the weight loss decreases due to the reaction of MoO3 with CuO and formation of Cu3Mo2O9, and possible CuMoO4, both of which have higher decomposition temperature than MoO3 itself. However, it is evident that a certain amount of MoO3 evaporates even from sample with the highest content of CuO. It is worth mentioning that the evaporation of MoO3 is not considered an environmental issue as MoO3 deposits as soon as it gets out of the hot furnace zone making long interlocked needles. Another reason for the absence of CuMoO4 is its thermal decomposition. Although both Cu3Mo2O9 and CuMoO4 decompose above 800 °C the temperature of CuMoO4 decomposition is always lower than
Fig. 1. (a) TG curves and (b) DSC curves of starting mixtures without CuO (MoO3-SiO2Al2O3) and with 4 wt% CuO.
the result of the evaporation of MoO3 which was initialized by melting at temperature slightly lower than 800 °C [28]. It is important to note that the evaporation of MoO3 from powder containing CuO is less intensive than that from the powder without CuO making TG curve less steep. This can be explained by the fact that the certain amount of MoO3 reacts with CuO to form Mo/Cu containing compounds such as CuMoO4 and mainly Cu3Mo2O9 which will be confirmed later by XRD analysis. Bearing in mind that Cu3Mo2O9 slowly decomposes at temperature above 850 °C [29], it is expected that MoO3 which is constituent of Cu3Mo2O9 does not evaporate at 800 °C which is typical for MoO3 compound present in the sample without CuO. As Fig. 1a shows, the weight of sample with CuO continuously decreases as the temperature increases from 800 to 1300 °C. The above mentioned processes of evaporation and decomposition, which are endothermic in nature, were also confirmed by DSC analysis (Fig. 1b) which shows the broad endothermic peaks in temperature range from 700 to 1250 °C. These endothermic processes in the sample containing CuO consume more energy and terminate at the somewhat higher temperature than those in the sample without CuO. This finding is believed to be the result of evaporation of Cu3Mo2O9 and probably CuMoO4 which takes place at higher temperatures. As shown by DSC curves (Fig. 1b), the first significant exothermic peak for the powder without CuO occurs at approximately 1250 °C and should be attributed to the mullite formation. However, the first peak for powder containing CuO is shifted to the higher temperature indicating that mullite forms at ~1350 °C. This is contradictory to the results of several studies showing that the presence of CuO normally promotes the formation of mullite and therefore lowers the formation temperature [30,31]. Although the various mechanisms were proposed, the accelerated mullite formation is generally ascribed to the formation of intermediate quartz-type and spinel-type phases reach in copper. Evidently these 9817
Ceramics International 43 (2017) 9815–9822
26
Table 1 Specific surface area, average pore size, thermal conductivity and coefficient of thermal expansions of samples with different CuO content sintered at 1300 °C for 4 h.
Weight loss (wt%)
D. Bucevac et al.
24 22
Specific surface area (m2/g)
Average pore size (µm)
Thermal conductivity at 300 °C (W/m K)
Coefficient of thermal expansion (x 10−6/°C)
0 4 8 12
0.982 0.841 0.820 0.707
3.082 2.762 1.961 1.442
0.98 1.03 0.97 0.98
4.82 4.51 4.47 4.96
20
0 wt% CuO 4 8 12
18 16 14 1250
1300
1350
1400
1450
1500
(1500 °C) is the lowest despite the fact that the weight loss of this sample is the highest (Fig. 3). This implies that the porosity is also controlled by the ability of CuO to promote sintering either by dissolution of small amount of Cu into mullite matrix or by formation of Mo-containing liquid phase. Although the liquid phase will eventually evaporate it is believed that its presence promotes densification. Seemingly, this effect is more pronounced at higher temperature. When it comes to the effect of sintering temperature on porosity of the samples containing different amount of CuO it can be concluded that the porosity of all compositions increases in temperature range from 1300 to 1450 °C due to decomposition of Cu3Mo2O9. Apparently the decomposition of Cu3Mo2O9 is almost complete after four-hour long sintering at 1450 °C. Further increase in temperature to 1500 °C promotes sintering by increasing diffusion coefficients which leads to the small reduction in porosity of all compositions, including samples without CuO, as depicted in Fig. 4. Similarly to the open porosity, the specific surface area of samples was also affected by the amount of CuO, though the effect was much less pronounced. The results presented in Table 1 point out that the surface area of samples sintered at 1300 °C decreased from 0.982 m2/g in sample without CuO to 0.707 m2/g in sample with 12 wt% CuO whereas the average pore size decreased from 3.08 µm to 1.44 µm, respectively. Knowing that the primary soot particles, which are smaller than 50 nm in diameter [33], form agglomerates with diameter normally larger than 1 µm it can be concluded that the obtained material is expected to be efficient in carbon soot removal. The other two properties which are very important for application of mullite as DPF material are thermal conductivity and coefficient of thermal expansion. As Table 1 shows, the change of these properties due to the presence of CuO can be considered as negligible. It is worth mentioning that the coefficient of thermal expansion of mullite is just slightly higher than that of SiC which is used for production of commercially available DPFs (~ 4×10−6/°C).
Sintering temperature (oC) Fig. 3. Effect of sintering temperature on weight loss of samples containing different amount of CuO sintered for 4 h.
that of Cu3Mo2O9 by 10–35 °C [32] causing faster decomposition of CuMoO4 which was not detected in samples sintered at 1300 °C. As Fig. 3 shows, the weight loss due to the decomposition of Cu3Mo2O9 is even more pronounced at higher temperature. The weight loss at 1500 °C was the highest in sample containing the highest amount of CuO (12 wt%). The inserted XRD pattern in Fig. 2 reveals that the amount of Cu3Mo2O9 considerably decreases after sintering at 1500 °C. It is also worth mentioning the presence of small amount of CuAl2O4 in the samples containing 8 and 12 wt% CuO. Similarly to Cu3Mo2O9, CuAl2O4 disappears after sintering at 1500 °C. The evaporation of MoO3 (primarily), CuMoO4 and Cu3Mo2O9 during sintering is expected to affect the porosity of sintered samples which will be discussed in the following section. 3.3. Porosity and thermal properties The effect of sintering temperature on the open porosity of samples containing different amount of CuO is presented in Fig. 4 showing that the all porosity values fall in the range from 44% to 62% which is actually the porosity measured in commercially available DPFs. Similarly to the weight loss (Fig. 3), the porosity of samples sintered at low temperature (1300 °C) decreases with the amount of CuO indicating that the porosity of these samples is mainly the result of evaporation of MoO3 which was the most pronounced in sample without CuO. This way MoO3, i.e., waste MoSi2, was also employed as a pore former. It is interesting to note that the porosity of sample containing 12 wt% CuO sintered at the higher sintering temperatures
Open porosity (%)
CuO content (wt%)
65
3.4. Compressive strength and expansion
60
Knowing that the compressive strength of samples is strongly affected by the porosity, an attempt to find the relationship between strength and porosity will be made in the following section. The effect of sintering temperature on compressive strength of samples with different CuO content is presented in Fig. 5. As the figure shows, the strength of all compositions decreases with increasing sintering temperature reaching a minimum value in samples sintered at 1450 °C. The decrease in strength is the result of the increase in porosity which was discussed and presented in Fig. 4. The samples sintered at 1450 °C possess the maximum porosity and the minimum compressive strength at the same time. Further increase in sintering temperature slightly improves strength which is, again, the result of the small decrease in porosity (Fig. 4). This inversely proportional relation between strength and porosity is well documented and therefore expected. This relationship can be used to design porous material with specific values of porosity and compressive strength which are required for particular application. According to Fig. 4 and Fig. 5, the boundary
55 50
0 wt% CuO 4 8 12
45 40 1250
1300
1350
1400
1450
1500
Temperature (oC) Fig. 4. Effect of sintering temperature on open porosity of samples containing different amount of CuO sintered for 4 h.
9818
Ceramics International 43 (2017) 9815–9822
Compressive strength (MPa)
D. Bucevac et al.
0 wt% CuO 4 8 12
70 60 50
addition of CuO reduces the amount of MoO3 that evaporates, promotes sintering and therefore shrinkage at high temperature which decreases the overall expansion measured after sintering. As Fig. 6 shows, the overall expansion of samples containing 12 wt% CuO is zero. In this case the expansion that occurs at ~ 500 °C is completely compensated by the shrinkage that takes place at sintering temperature. The zero expansion can considerably facilitate the process of compaction of ceramic powder and therefore fabrication of ceramic components. In the case of zero expansion, the dimensions of the die used for powder pressing are the same like the dimensions of final (sintered) sample. There is no need for estimation of shrinkage/ expansion which must be taken into account while selecting the die.
40 30 20 10 0 1250
1300
1350
1400
1450
3.5. Microstructure
1500
Sintering temperature (oC)
Fig. 7 shows SEM micrographs of the samples with different amount of CuO sintered at 1300 °C. This temperature was chosen as it was the lowest temperature at which mullite was formed. Fig. 7a reveals that the microstructure of sample without CuO consists of interpenetrating, elongated mullite grains with bimodal grain size distribution. The large grains are normally in the form of rectangular prisms with a rectangular base and sharp, well defined, edges. The grain diameter varies from 1 to 5 µm whereas the length might be up to 30 µm. These relatively thick grains are interlocked with very thin and elongated mullite grains, whiskers, with diameter less than 0.5 µm. Besides the elongated mullite grains, Fig. 7a also shows the presence of small spherical grains which are identified as residual alumina. Unlike the pure mullite sample, the sample with 4 wt% CuO does not contain thin mullite whiskers. As Fig. 7b indicates there are much bigger mullite grains which were formed by coalescence of several elongated mullite grains. Although the grains are still elongated, the prism-like grain morphology is not as pronounced as in sample without CuO. According to Fig. 7c and d the coalescence of mullite grains becomes more pronounced with an increase in the amount of CuO. The coalescence might be ascribed to the presence of liquid phase which contains Cu and solidifies during cooling from sintering temperature as white phase (Fig. 7b-d). The elemental composition of both white and grey phase was determined by EDS analysis and presented in Fig. 8. As expected, constitutive elements of mullite, i.e., Al, Si and O were found in the grey phase without any trace of Mo or Cu (Fig. 8a). On the other side, Fig. 8b reveals that the main elements in the white phase are Mo and Cu. This finding, along with XRD analysis (Fig. 2) confirms that the white phase is Cu3Mo2O9. Moreover, relatively large amount of Mo in white phase rules out the possibility that CuAl2O4, also detected by XRD analysis, is colored white. Further analysis of location of CuAl2O4 was not conducted as the amount of this phase was small. The fractions of white areas (Cu3Mo2O9) in the figures showing samples with 4 wt%, 8 wt% and 12 wt% CuO were roughly estimated to be 3.7%, 7.3% and 10.5%, respectively. The exact amount of Cu3Mo2O9 was not crucial in this study as the amount of CuO in starting mixtures was the parameter that had been optimized.
Fig. 5. The effect of sintering temperature on compressive strength of samples containing different amount of CuO sintered for 4 h.
values for strength and porosity are measured in sample without CuO sintered at 1450 °C and sample with 12 wt% CuO sintered at 1300 °C. Namely, the sample without CuO has the highest porosity of 63% and the lowest compressive strength of only 4 MPa, whereas the sample containing 12 wt% CuO has the lowest porosity of 44% and the highest compressive strength of 70 MPa. Evidently, the variation of sintering temperature and the amount of CuO allows fabrication of filter material with specific values of porosity and compressive strength. Besides porosity and strength, the variation of amount of CuO can be also used to control shrinkage of ceramic compacts after sintering. Since the samples without CuO actually expand after sintering, the term expansion will be used instead of shrinkage. Fig. 6 shows the effect of sintering temperature on expansion of samples containing different amount of CuO. The expansion of sintered samples is affected by the amount of CuO rather than sintering temperature. The small changes in porosity of the samples containing certain amount of CuO with temperature are not sufficient to cause any considerable change of the diameter of sintered samples. On the other side, the amount of CuO strongly affects the expansion. As Fig. 6 reveals, samples without CuO expands by ~ 6% after sintering. This expansion comes from the oxidation of residual MoSi2 present in the starting powder and consequent volume increase due to the formation of MoO3 and SiO2. The oxidation, and therefore expansion, takes place at ~ 500 °C and it is inevitably followed by a certain shrinkage at high temperature such as 1300 °C. The shrinkage that takes place at sintering temperature is strongly affected by the amount of CuO. The
7
Expansion (%)
6 5
0 wt% CuO 4% 8% 12%
4 3 2
3.6. Effect of Cu3Mo2O9 on carbon burnout It was expected that the amount of CuO and therefore the amount of Cu3Mo2O9 would affect the rate of carbon oxidation. Thus, the sintered samples with different amount of CuO were crushed and mixed with carbon to make powder mixtures containing 10 wt% of carbon which were subsequently compacted and heated to temperature of 575 °C in 25 °C increments. Fig. 9 shows the effect of temperature on weight loss of the powder compacts containing mullite with different amount of CuO and 10 wt% of carbon. The loss of mullite-Cu3Mo2O9 material itself was not detected even after prolonged heat treatment pointing out a good thermal stability of mullite as well as Cu3Mo2O9. Therefore, the weight loss is solely a result of carbon burnout (oxidation) which is presented on the right vertical axis as a percentage
1 0 -1 1250
1300
1350
1400
1450
1500
Sintering temperature (oC) Fig. 6. The effect of sintering temperature on expansion of samples containing different amount of CuO sintered for 4 h.
9819
Ceramics International 43 (2017) 9815–9822
D. Bucevac et al.
(a)
(c)
10 µm
10 µm
(d)
(b)
10 µm
10 µm
Fig. 7. SEM (back scatter mode) micrographs of fracture surface of samples containing different amount of CuO sintered at 1300 °C for 4 h: (a) 0 wt%, (b) 4 wt%, (c) 8 wt% and (d) 12 wt% CuO. The white phase is Cu3Mo2O9.
of total carbon amount. As the figure evidences, the weight loss of all samples increases with temperature. It is important to note that the weight loss of samples containing CuO considerably increases when the temperature was raised to 575 °C. Evidently, at this temperature, the presence of Cu3Mo2O9 (CuO) accelerates the oxidation of carbon. The weight loss continuously increases with the amount of CuO reaching the highest value of 7.1 wt% in sample containing 12 wt% CuO, which means that 71% of the added carbon was removed by oxidation. Now, the temperature was kept at 575 °C and heating time was extended up to ~ 4 h. Fig. 10 shows that after 20 min at 575 °C, the weight loss of sample with 12 wt% CuO is 9.4 wt%. That means that almost all carbon (94%) was removed from the powder compact. At the same time the weight loss of sample without CuO is only 2.8 wt%. Moreover, the complete removal of carbon (10%) from mullite without CuO takes more than 4 h which is considerably longer than the time required for samples containing CuO. These results clearly indicate that the presence Cu3Mo2O9 in mullite matrix catalyzes the reaction of carbon oxidation. The TGA analysis was employed to quantify the effect of Cu3Mo2O9 (wt% CuO) on carbon oxidation by determining the reduction in temperature at which certain weight loss was achieved. As Fig. 11
shows, the weight loss of 5% in the sample without CuO was achieved at 710 °C whereas the same loss in sample containing 12 wt% CuO was achieved at 620 °C. Knowing that samples contain 10 wt% of carbon, it is evident that the temperature at which 50% of carbon was oxidized was by 90 °C lower for sample containing 12 wt% CuO. Based on these results it can be concluded that the presence of Cu3Mo2O9 in porous mullite lowers the carbon oxidation temperature. Therefore mullite Cu3Mo2O9 composite is a kind of promising material for fabrication of DPFs as it lowers the temperature of so called active filter regeneration, i.e., carbon soot oxidation. 4. Conclusions Porous acicular mullite ceramics containing in situ created Cu3Mo2O9 as a soot oxidation catalyst was fabricated using the commercial powders of Al2O3 and CuO, and the powder obtained by controlled oxidation of waste MoSi2. Samples containing 0−12 wt% CuO were sintered in temperature range between 1300 and 1500 °C. Both the amount of CuO and sintering temperature influenced the porosity and compressive strength of the samples. Sintering temperature of 1300 °C was found to be optimal as it was the lowest 9820
Weight loss (wt%)
Intensity (a.u.)
Al
O
Si Au 0
10
100
8
80
6 4 2
2
4
6
8
10
0 wt% CuO 4 8 12 0
40
80
Energy(keV)
(b)
240
100
0
Sample weight (wt%)
Intensity(a.u.)
Cu
Cu
Si
Cu
2
4
6
8
10
Energy (keV) Fig. 8. EDS spectra of the sample containing 12 wt% CuO sintered at 1300 °C for 4 h taken at (a) grey phase and (b) white phase.
80 70
0 wt% CuO 4 8 12
6
4
60 50 40 30 20
2
10 475
500
525
550
575
98
710 oC
620 oC 96 94 92
0 wt% CuO
90 88 200
20
12 wt% CuO 400
600
800
0 10 20 30 40 50 60 70 80 90 100 1000
Temperature (oC)
Carbon burnout (wt%)
8
Weight loss (wt%)
200
Fig. 10. Effect of time on weight loss of powder compacts containing 10 wt% carbon at 575 °C. The carbon was mixed with powders obtained by grinding samples with different amount of CuO sintered at 1300 °C for 4 h. The right vertical axis shows the percentage of carbon burnt during heating.
Al
0 450
160
40
Time (min)
Mo
O
120
60
Carbon burnout (wt%)
(a)
Carbon burnout (wt%)
Ceramics International 43 (2017) 9815–9822
D. Bucevac et al.
Fig. 11. TGA curves of powders containing 10 wt% carbon. The carbon was mixed with powders obtained by grinding samples with 0 wt% and 12 wt% CuO sintered at 1300 °C for 4 h. The right vertical axis shows the percentage of carbon burnt during heating.
was almost negligible. The obtained material consisted of elongated mullite grains with dispersed gains of in situ created Cu3Mo2O9 which catalyzes the carbon oxidation (burnout). The addition of 12 wt% CuO decreases the carbon oxidation temperature by 90 °C when compared to pure mullite. This feature along with porous elongated microstructure makes mullite-Cu3Mo2O9 composite a promising material for DPF fabrication. In addition, the described novel approach is environmentally friendly as the waste MoSi2 is completely reused to fabricate mullite material as well as Cu3Mo2O9 catalyst.
0
Temperature (oC)
Acknowledgement
Fig. 9. Effect of temperature on weight loss of powder compacts containing 10 wt% carbon. The carbon was mixed with powders obtained by grinding samples with different amount of CuO sintered at 1300 °C for 4 h. The hold at each temperature was 10 min. The right vertical axis shows the percentage of carbon burnt during heating.
This work was supported by the Ministry of education, science and technological development of Serbia (project number: 45012). References
temperature at which mullite - Cu3Mo2O9 composite with high porosity and relatively high strength was obtained. At this temperature, the addition of CuO decreased porosity from 62% in pure mullite to 44% in sample containing 12 wt% CuO. At the same time the compressive strength increased from 19 MPa in pure mullite to 70 MPa in sample containing 12 wt% CuO. On the other side, the change of thermal conductivity and coefficient of thermal expansion with the CuO content
[1] M. Fiebig, A. Wiartalla, B. Holderbaum, S. Kiesow, Particulate emissions from diesel engines: correlation between engine technology and emissions, J. Occup. Med. Toxicol. 9 (2014) 2–18. [2] U. Mathis, M. Mohr, R. Kaegi, A. Bertola, K. Boulouchos, Influence of diesel engine combustion parameters on primary soot particle diameter, Environ. Sci. Technol. 39 (2005) 1887–1892. [3] H. Burtscher, Physical characterization of particulate emissions from diesel
9821
Ceramics International 43 (2017) 9815–9822
D. Bucevac et al.
[19] H. Schneider, J. Schreuer, B. Hildmann, Structure and properties of mullite-A review, J. Eur. Ceram. Soc. 28 (2008) 329–344. [20] E.C. Hammel, O.L.R. Ighodaro, O.I. Okoli, Processing and properties of advanced porous ceramics: an application based review, Ceram. Int. 40 (2014) 15351–15370. [21] C.H.H. Hsiung, A.J. Pyzik, F. De Carlo, X. Xiao, S.R. Stock, K.T. Faber, Microstructure and mechanical properties of acicular mullite, J. Eur. Ceram. Soc. 33 (2013) 503–513. [22] R.R. Monteiro, A.C.S. Sabioni, Preparation of mullite whiskers derived from topaz doped with rare earth oxides for applications in composite materials, Ceram. Int. 42 (2014) 49–55. [23] A.J. Pyzik, C.S. Todd, C. Han, Formation mechanism and microstructure development in acicular mullite ceramics fabricated by controlled decomposition of fluorotopaz, J. Eur. Ceram. Soc. 28 (2008) 383–391. [24] S. Lohfeld, M. Schütze, Oxidation behaviour of particle reinforced MoSi2 composites at temperatures up to 1700°C part I: literature review, Mater. Corros. 56 (2005) 93–97. [25] D. Bučevac, A. Dapčević, V. Maksimović, Porous acicular mullite obtained by controlled oxidation of waste molybdenum disilicide, Mater. Res. Bull. 50 (2014) 155–160. [26] G. Mul, J.P.A. Neeft, F. Kapteijn, M. Makkee, J.A. Moulijn, Soot oxidation catalyzed by a Cu/K/Mo/Cl catalyst: evaluation of the chemistry and performance of the catalyst, Appl. Catal. B, Environ. 6 (1995) 339–352. [27] M. Kaur, D.K. Aswal, J.V. Yakhmi, Chemiresistors Gas Sensors: materials, Mechanisms and Fabrication, in: D. Aswal, S. Gupta (Eds.), Sci. Technol. Chemiresistors Gas Sensors, Nova Science Publishers, Inc, New York, 2007, pp. 33–94. [28] J. Bygden, D. Sichen, S. Seetharaman, A thermodynamic study of molybdenumoxygen system, Metall. Mater. Trans. B. 25B (1994) 885–891. [29] L. Kihlborg, R. Norrestam, B. Olivecrona, The crystal structure of Cu3Mo2O9, Acta Crystallogr. Sect. B Struct. Crystallogr. Cryst. Chem. 27 (1971) 2066–2070. [30] T. Martisius, R. Giraitis, Influence of copper oxide on mullite formation from kaolinite, J. Mater. Chem. 13 (2003) 121–124. [31] E.R. Segnit, T. Geleb, Metastable quartz-type structures formed from kaolinite by solid state reaction, Am. Mineral. 57 (1972) 1505–1514. [32] T. Machej, J. Ziołkowski, Phase diagram of the CuOMoO3 system, Mater. Chem. 4 (1979) 113–121. [33] S. Yoon, D.C. Quiros, H.A. Dwyer, J.F. Collins, M. Burnitzki, D. Chernich, J.D. Herner, Characteristics of particle number and mass emissions during heavyduty diesel truck parked active DPF regeneration in an ambient air dilution tunnel, Atmos. Environ. 122 (2015) 58–64.
engines: a review, J. Aerosol Sci. 36 (2005) 896–932. [4] M. Tsukahara, T. Murayama, Y. Yoshimoto, Influence of fuel properties on the combustion in diesel engine driven by the emulsified fuels, Bull. JSME 25 (1982) 612–619. [5] M.V. Twigg, Progress and future challenges in controlling automotive exhaust gas emissions, Appl. Catal. B Environ. 70 (2007) 2–15. [6] V. Sibanda, R.W. Greenwood, J.P.K. Seville, Particle separation from gases using cross-flow filtration, Powder Technol. 118 (2001) 193–202. [7] F. Millo, M. Andreata, M. Rafigh, D. Mercuri, C. Pozzi, Impact on vehicle fuel economy of the soot loading on diesel particulate filters made of different substrate materials, Energy 86 (2015) 19–30. [8] S. Bai, J. Tang, G. Wang, G. Li, Soot loading estimation model and passive regeneration characteristics of DPF system for heavy-duty engine, Appl. Therm. Eng. 100 (2016) 1292–1298. [9] M. Shrivastava, A. Nguyen, Z. Zheng, H.W. Wu, H.S. Jung, Kinetics of Soot Oxidation by NO2, Environ. Sci. Technol. 44 (2010) 4796–4801. [10] V. Di, G. Landi, L. Lisi, A. Saliva, A. Di, Applied catalysis B: environmental the sootcatalyst contact on the regeneration performance, "Appl. Catal. B, Environ. 197 (2016) 1–9. [11] D. Fino, Diesel emission control: catalytic filters for particulate removal, Sci. Technol. Adv. Mater. 8 (2007) 93–100. [12] C. Beatrice, S. Di Iorio, C. Guido, P. Napolitano, Detailed characterization of particulate emissions of an automotive catalyzed DPF using actual regeneration strategies, Exp. Therm. Fluid Sci. 39 (2012) 45–53. [13] V.R. Pérez, A. Bueno-López, Catalytic regeneration of Diesel Particulate Filters: comparison of Pt and CePr active phases, Chem. Eng. J. 279 (2015) 79–85. [14] Z. Stępień, L. Ziemiański, G. Zak, M. Wojtasik, Ł. Jęczmionek, Z. Burnus, The evaluation of fuel borne catalyst (FBC's) for DPF regeneration, Fuel 161 (2015) 278–286. [15] D. Fino, S. Bensaid, M. Piumetti, N. Russo, A review on the catalytic combustion of soot in Diesel particulate filters for automotive applications: from powder catalysts to structured reactors, Appl. Catal. A Gen. 509 (2016) 75–96. [16] Y.Y. Choi, J.G. Kim, K.M. Lee, H.C. Sohn, D.J. Choi, A study of surface-coated SiC whiskers on carbon fiber substrates and their properties for diesel particulate filter applications, Ceram. Int. 37 (2011) 1307–1312. [17] B. Guan, R. Zhan, H. Lin, Z. Huang, Review of the state-of-the-art of exhaust particulate filter technology in internal combustion engines, J. Environ. Manag. 154 (2015) 225–258. [18] A.J. Pyzik, C.G. Li, New design of a ceramic filter for diesel emission control applications, Int. J. Appl. Ceram. Technol. 2 (2005) 440–451.
9822