ARTICLE pubs.acs.org/IECR
Kinetic Study of Diesel Soot Combustion with Perovskite Catalysts Simelys Hernandez CSHR-Center for Space Human Robotics, Italian Institute of Technology, Corso Trento 21, 10129, Turin, Italy
Gian Andrea Blengini DISPEA—Department of Production Systems and Business Economics, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Turin, Italy CNR-IGAG: Institute of Environmental Geology and Geo-Engineering, Corso Duca degli Abruzzi 24, 10129 Turin, Italy
Nunzio Russo and Debora Fino* DISMIC—Department of Materials Science and Chemical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Turin, Italy ABSTRACT: The objective of this paper is to analyze the kinetics of soot combustion, promoted by several catalysts, through differential scanning calorimetry (DSC) and thermogravimetic analysis (TGA). All the catalysts are substoichiometric or alkalimetal substituted perovskites (La0.9CrO3, La0.8CrO3, La0.9Rb0.1CrO3, La0.9Na0.1CrO3, La0.9K0.1CrO3, La0.8Cr0.9Li0.1O3) on the basis of the LaCrO3 standard, which was found to be active for this reaction in previous studies conducted by our group. Different model-free iso-conversion methods with the DSC data were used to determine the activation energies. Instead, thermogravimetric data were employed to determine the oxygen reaction order.
1. INTRODUCTION The application of stringent emission standards for diesel engines has directed toward a reduction in both nitrogen oxides and soot particulates in exhaust gases. The health effects caused by soot emissions from diesel engine exhaust gases are already well-known.1 Diesel particulate matter (soot) mainly consists of agglomerated solid carbonaceous materials, ash, volatile organics, and NOx.2 A diesel particulate filter (DPF) is used to remove the carbonaceous particles; this process involves mechanical filtration and subsequent combustion of the soot particles in order to avoid a pressure drop due to filter plugging. Since these materials burn at high temperatures (>550 °C), but diesel exhaust gas temperatures lie between 200 °C to 400 °C, a suitable catalyst is required to promote soot combustion.3,4 Thus, it is very important to develop catalytic materials which are sufficiently active to ignite the soot at lower temperatures. Most studies have been performed on readily available commercial soot samples. The use of soot, such as carbon black5 (i.e., Printex- U by Degussa), as a substitute for real soot, can be considered acceptable for catalyst screening, since the real diesel soot features are heterogeneous.6 Many different soot combustion catalysts have been developed. It has been shown that several oxides act as soot combustion catalysts.711 However, ceria is commonly considered to exhibit considerable oxidation activity, high thermal stability, and poor chemical reactivity, compared with many other ceramic supports and diesel emission components.1214 Several perovskiteand spinel-type oxides have been reported to be effective for soot combustion, especially under close contact conditions.1517 r 2011 American Chemical Society
Platinum-based catalysts have exhibited high activity for NOassisted soot oxidation, but they are expensive.18,19 Recently, catalysts based on transition metals such as cobalt, manganese, and copper have shown remarkable soot oxidation activity, because of their strong oxidative properties.15,20 Some catalysts can oxidize soot particulates by catalyzing the formation of a mobile oxygen species, by providing redox sites for the oxidation or by dissociating O2 and transferring the resulting active adsorbed atomic oxygen (Oads) to the soot particle via a spillover mechanism.21 The objective of this paper is to analyze the kinetics of soot combustion promoted by several catalysts through thermal analyses (DSC and TGA). All the catalysts are substoichiometric or alkali-metal substituted perovskites (La0.9CrO3, La0.8CrO3, La0.9Rb0.1CrO3, La0.9Na0.1CrO3, La0.9K0.1CrO3, La0.8Cr0.9Li0.1O3), according to the LaCrO3 standard, which was determined to be active for this reaction in previous studies carried out by our group.16,17 Different model-free isoconversion methods have been used to analyze the DSC data in order to determine the activation energies. These models were compared to establish the most reliable ones and to have indications for future works. One of the general purposes of modeling thermally activated reactions is to obtain a complete description of the progress of a reaction that Special Issue: Russo Issue Received: September 20, 2011 Accepted: December 1, 2011 Revised: November 30, 2011 Published: December 01, 2011 7584
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Industrial & Engineering Chemistry Research would be valid for any thermal treatment (e.g., isothermal, temperature-programmed linear rise, or any other nonisothermal treatment) and the reliability of the model is essential for this reason.22 Finally, thermogravimetric data have been used to determine the oxygen reaction order.
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Table 1. Collection of Results of Catalyst Characterization Tests Concerning Catalytic Activity Peak Temperature (Tp) and BET Specific Surface Areaa
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. Several
catalysts were synthesized via a highly exothermic and self-sustaining reaction (the so-called “combustion synthesis” method): LaCrO3, La0.9CrO3, La0.8CrO3, La0.9Rb0.1CrO3, La0.9Na0.1CrO3, La0.9K0.1CrO3, La0.8Cr0.9Li0.1O3.16,17 X-ray diffraction (Philips Model PW1710 diffractometer equipped with a monochromator for Cu Kα radiation) was used on all the fresh catalysts, to establish whether the desired structure was actually achieved. The amorphous Printex U carbon by Degussa (average particle size: 25 nm; 0.32% ashes after calcinations; 92 m2/g) was used, instead of real diesel soot, in order to compare the oxidation activity of the catalysts and to assess their prevalent kinetic parameters. This material was preferred over its real counterpart in order to avoid any interfering effect due to the presence of adsorbed hydrocarbons, sulfates, or fly ashes, which are commonly encountered in real diesel soot. Furthermore, the commercial amorphous carbon employed is more difficult to burn than real diesel soot, which renders the achieved activity results conservative. There are no standard methods to test the soot oxidation activity of a catalyst, and the type of contact applied in each specific study is very important. When the soot and a catalytic powder are mixed with a spatula, the contact mode is loose, and the actual contact conditions that are encountered in a real particulate trap are simulated. When mixing is carried out in a ball mill, it is possible to achieve a close contact condition, which is useful to define intrinsic catalytic activity under optimal conditions. Some catalysts show high activity under tight contact and no significant activity under loose contact.23 The intrinsic catalytic activity is not the only requirement to obtain high carbon conversion rates: the mobility of the catalytic species is also an important requirement for applications in diesel exhaust treatments. It is clear that the performance of a catalyst mixed with soot in tight mode cannot reproduce real diesel engine tests. However, it is compulsory to perform kinetic and mechanistic studies in tight contact in order to establish good reproducibility of the experiments and to evaluate the intrinsic kinetic parameters. In order to complement the above characterization experiments, Table 1 lists the BrunauerEmmettTeller (BET) specific surface area of the various catalysts have been prepared and the temperature-programmed oxidation peak temperature (Tp) values that were obtained in previous investigations conducted by the group.17 Particularly, the latter data represent the temperatures at which the maximum CO2 concentration was registered at the outlet of the microreactors hosting a given initial catalystsoot mixture and flushed by air under increasing temperature conditions.17 The Tp value of a catalyst represents an index of its catalytic activity: the lower the Tp value, the higher the catalyst activity. 2.2. Activation Energy Theory. The modeling of thermally activated reactions is very difficult for many reactions because any
Tp (K)
BET (m2/g)
LaCrO3
768
17.53
La0.9CrO3
720
16.46
La0.8CrO3
714
13.54
La0.8Cr0.9Li0.1O3
681
12.96
La0.9Na0.1CrO3 La0.9Rb0.1CrO3
728 721
17.09 7.72
La0.9K0.1CrO3
727
17.62
noncatalytic combustion
923
catalyst
a
Data taken from ref 17.
reaction could occur with different mechanisms and intermediate stages, all of which could have different temperature dependencies. This is especially true for solidsolid reactions. To overcome this problem, most researchers adopt a few reasonable simplifying assumptions. Several publications have detect with research in which the transformation rate during a reaction is assumed to be the product of two functions, one of which is dependent only on the temperature (T), while the other is dependent only on the converted soot fraction (α).22 The temperature-dependent function is generally assumed to follow an Arrhenius-type dependency: dα Ea ¼ k∞exp f ðαÞ ð1Þ dt RT Thus, the function f(α) and the constants k∞ and Ea must be determined, to describe the progress of the reaction at each temperature and for each temperaturetime program. In general, the reaction function f(α) is unknown at the outset of the analysis. A range of standard functions, which represent particular idealized reaction models, have been proposed.2427 Under nonisothermal conditions, in which the reacting catalyst soot sample is heated at a constant rate, the explicit time dependence in eq 1 is eliminated through the minor transformation dα k∞ Ea ¼ exp f ðαÞ ð2Þ dT β RT where β is the heating rate (β = dT/dt). After some other steps, the previous equation changes to Z α dα Z T Ea dT ¼ k∞ exp gðαÞ ¼ 0 f ðαÞ RT β T0 k∞ Z T Ea ¼ exp dT ð3Þ β T0 RT and gðαÞ ¼
k∞ Ea Z ∞ expð xÞ k∞ Ea dx ¼ pðxÞ x x2 β R βR
ð4Þ
where x = Ea/(RT), and p(x) is generally called the temperature integral. Many different methods have been considered in this investigation to determine the activation energy. Their basic features are listed in Table 2. All these methods require the determination of the temperatures at which an equivalent stage of the reaction 7585
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Table 2. Activation Energy Analysis Methods method
type
year
p(x) approximation
Ozawa24
Type B
1970
pðxÞ =
expð Ax þ BÞ x5
25
Starink
Type B
1992
pðxÞ =
expð Ax þ BÞ x5
Flynn26
Type B
1983
pðxÞ =
Type B
1957
pðxÞ ¼
expð Ax þ BÞ x5 expð xÞ x2
Type C
1997
Kissinger27 22
Vyazovkin
dm 1 ¼ kðTÞPOn 2 dt S 3 mcat
ð5Þ
The integral value can be obtained for different conversions as follows: Int ¼
Z α 0
vðαÞ dα ¼
Z α 0
kðTÞPOn 2 dα
ð6Þ
and it is possible to obtain the following reaction order using different oxygen concentrations, as well as leading isothermal experiments: Pn IntA ¼ On 2 A IntB P O2 B n¼
lnðIntA =IntB Þ lnðPO2 A =PO2 B Þ
n
IðEa , Ti ðtα ÞÞ ΘðEa Þ ¼ ∑ni6¼ j ∑ IðE ¼ minimum a , Tj ðtα ÞÞ
no approximation
occurs for various heating rates (T(β)) (hence, the term isoconversion methods). The equivalent stage (also called the fixed or identical stage) can be defined as the stage at which a fixed amount of soot is transformed, or at which a fixed fraction of the total amount of soot is transformed. Isoconversion methods can be categorized as one of two main groups of methods. One set of methods provides an approximation of the so-called temperature integral and only requires data on Tα(β). The term p(x)isoconversion methods is generally adopted to describe the methods in this category and to classify them as Type B methods. Another set of methods does not use any mathematical approximation, but instead provides a determination of the reaction rate at an equivalent stage of the reaction for various heating rates. Therefore, this group of methods is known as rate-isoconversion methods, or Type A methods.22 No Type A method will be considered here. Another type of method, generally used to determine the activation energy, is called the “advanced model-free isoconversion method” or Type C methods: they do not use any approximation of the p(x) function, but propose more-complex and nonlinear expressions, whose solutions can only be obtained using computer algorithms. 2.3. Oxygen Reaction Order. The order of the reaction of oxygen is, by definition, expressed as an exponent of the partial pressure PO2 or concentration reaction gas. Ciambelli et al.28 proposed an expression for the catalytic conversion of soot that considers the mass of the catalyst and the surface between the soot and catalyst: vðαÞ ¼
solving equation Ea þ constant lnðβÞ ¼ 1:0518 RT α β Ea ln T 1:92 ¼ 1:0008 RT þ constant α α dðln βÞ Ea dð1=Tα Þ ¼ BðxÞ R Ea þ constant ln Tβp ¼ 1 RT p
ð7Þ
ð8Þ
2.4. Thermal Analysis Experiments. Differential scanning calorimetry (DSC) experiments have been performed on all the carboncatalyst mixtures to measure the heat released as an index of the evolution of the catalytic combustion. A tightcontact mixture of catalyst and carbon (2:1 mass basis) was
Table 3. Activation Energies of Perovskite Catalysts Activation Energy (kJ/mol) Ozawa24
Flynn26
LaCrO3
147.02
141.47
140.74
141.61
141.46
La0.9CrO3 La0.8CrO3
133.83 129.29
128.24 123.31
106.62 122.32
128.35 123.37
128.15 123.22
La0.8Cr0.9Li0.1O3 125.54
119.54
109.80
119.49
119.29
La0.9Na0.1CrO3
134.53
128.91
117.23
128.85
128.82
La0.9Rb0.1CrO3
148.89
143.82
129.37
143.74
143.64
La0.9K0.1CrO3
121.33
114.96
109.48
115.02
114.84
noncatalytic
159.00
catalyst
Kissinger27 Vyazovkin22 Starink25
combustion
placed in the sample crucible under an air flow of 100 mL/min, and an equal weight of alumina was used as a reference. The temperature was increased from 50 °C to 700 °C, with different heating rates (Φ = 5, 10, 20, 30, and 40 °C/min). Several plots, representing exothermal combustion peaks, were obtained. These patterns were processed to establish the temperature corresponding to the temperature peaks and the cumulative combustion (α) of 25, 50, and 75 wt % of the total initial carbon located in the sample holder. Thermogravimetric analysis (TGA) experiments were performed on the same carboncatalyst mixtures in order to measure the mass reduction as an index of the evolution of the catalytic combustion. The same catalyst carbon ratio (2:1 mass basis) and an air flow of 100 mL/min were used: three different oxygen concentrations (1%, 2.5%, and 5% O2 in helium) were adopted under experimental isothermal conditions.
3. RESULTS AND DISCUSSION The activation energy values determined with the models listed in Table 2, on the grounds of the gathered DSC data, are listed in Table 3. Surprisingly, the Type B Ozawa and Kissinger method, which are the two methods most commonly used for activation energy calculations, showed the maximum deviations from the values provided by the Vyazovkin method (Table 2), which can be considered the most accurate, because it is not dependent on temperature integral approximation. The Ozawa method, in particular, showed higher values than the most accurate method for each catalyst. Conversely, the Type B method proposed by Kissinger always gave lower Ea values than those obtained using the Vyazovkin method. The deviation cannot be traced back to the temperature integral because the approximation used for the Kissinger method leads to negligible errors in our Ea range. The deviation must be due to another reason: the maximum reaction 7586
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Figure 1. Tp/Ea values of the catalytic activity peak temperature (Tp) and activation energy (Ea) ratio for all the catalysts analyzed for the Flynn, Vyazovkin, and Starink models.
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Figure 3. Activation energy (Ea) versus carbon conversion (α) for La0.9CrO3.
Table 4. Reaction Order (n) Obtained by TGA Data catalyst
Figure 2. Ozawa plots for the determination of the activation energy of soot combustion over the LaCrO3 catalyst.
rate, corresponding to the thermal analysis peak, does not always correspond to the same conversion fraction for the different heating rates. The Starink method showed minor differences than the Vyazovkin method: this feature confirms, as mentioned in the literature,25 that the approximation proposed by Starink for the temperature integral is the real expression of p(x). Therefore, values with a similar accuracy can be obtained using a method with a lower computational complexity than the Vyazovkin method. This feature is shared by the Flynn method.26 Both the activation energy (Ea) and the peak temperature (Tp) of the temperature-programmed oxidation runs are indexes of catalytic activity and decrease as the activity increases. Hence, the Tp/Ea ratio was calculated for all the catalysts and for the three models that provided the best Ea predictions (Flynn, Vyazovkin, and Starink). Figure 1 shows the derived Tp/Ea values. The values show that the data for the analyzed class of catalysts congregate rather well around the average value of 5.65, which suggests that a rough estimation of the activation energy can be derived from a simple temperature-programmed oxidation run adopting this conversion factor. The only two catalysts that show appreciable deviations from the average Tp/Ea value of 5.65 are the La0.9Rb0.1CrO3 and La0.9K0.1CrO3 catalysts. In the former case, the deviations are likely to be due to the rather low specific surface area of the catalysts (Table 1), which may influence the contact conditions between the catalysts and the soot.
reaction order, n
LaCrO3 La0.9CrO3
0.70 0.74
La0.8CrO3
0.84
La0.8Cr0.9Li0.1O3
0.75
La0.9Na0.1CrO3
0.92
La0.9Rb0.1CrO3
0.85
La0.9K0.1CrO3
0.81
noncatalytic combustion
1.00
Moving on to the activation energy of each catalyst, it can be noticed that the values for each catalyst are lower than for the noncatalytic conversion. This reduction reaches 30% for the La0.9K0.1CrO3 catalysts. The average reduction is 19%, and this emphasizes the high activity of LaCrO3 perovskite-based catalysts. The high activity of these catalysts can be explained by their oxygen-pumping ability.16,17 Focusing attention on the different activation energy values, at different conversions (see Figure 2, including the model lines obtained via the Ozawa method), it is possible to notice that the Ea value decreases as the soot conversion increases for the La0.9CrO3 catalyst (see the plot in Figure 3). The same behavior has been observed for the other catalysts. Two different explanations can be given for this phenomenon: carbon conversion is a process that not only involves kinetics, but also both heat-transfer and mass-transfer phenomena. The obtained activation energy is, in fact, an apparent activation energy that combines these three factors. The mass transfer is less important at the beginning of the combustion, because of the excellent carboncatalyst contact. During the evolution of the reaction, the amount of carbon in close contact with the active sites decreases and new carbon particles reach the active sites. This explanation has been described clearly in the literature.28 Mass transfer is characterized by a much lower intrinsic activation energy (ca. 40 kJ/mol) than that of the kineticscontrolled catalytic combustion of soot. When the mass transfer becomes predominant in the conversion, the apparent activation energies gradually decrease. 7587
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Industrial & Engineering Chemistry Research Another explanation can be given concerning the fact that reactive free carbon (Cf) is generated during the early stages of catalytic combustion when a surface carbon atom is reacted away by oxygen. This reactive carbon is oxidized more easily (i.e., with ower activation energy) than the original surface carbon atoms.29 The soot particle can be compared to an apple attacked by a worm: once the skin is perforated, it becomes easier to eat inside the apple. The reaction order results obtained from TGA data are listed in Table 4. The variation in the reaction order, from that of the noncatalytic carbon combustion is clearly induced by the presence of the catalyst. The reaction order is lower in all the tested catalysts. This result can be explained considering the ability of these catalysts to chemisorb and release oxygen molecules according to a Langmuir-type mechanism. Considering that the reaction rate is proportional to the absorbed oxygen molecules, the reaction order to the oxygen becomes fractional. It is possible to assume that the lower the reaction order of the catalytic reaction, the stronger the capability of the catalysts to adsorb and release oxygen.
4. CONCLUSIONS A kinetic study of seven different catalysts has been carried out: the activation energies and oxygen reaction orders of LaCrO3, La0.9CrO3, La0.8CrO3, La0.9Rb0.1CrO3, La0.9Na0.1CrO3, La0.9K0.1CrO3, and La0.8Cr0.9Li0.1O3 perovskite catalysts have been calculated. Two different benefits could be obtained from this study: in-depth knowledge of the different isoconversion methods in order to determine the activation energy of the catalytic conversion of soot, and evidence on the increased activity of the A-site-substituted perovskite. Vyazovkin proposed the best fitting method, which can adequately be approximated by the Flynn and Starink models, while the Kissinger and Ozawa methods give the worst predictions. The computational complexity of the Vyazovkin method is not a problem for present-day computer technology. The activation energy values of the different catalysts showed a great influence of the alkali-metal substitutions, except for Rb. The oxygen reaction order values, obtained for the various catalysts from the TGA data, showed a reduction in the influence of oxygen in the catalytic conversion of soot. Finally, the reader should be aware of the fact that the derived intrinsic kinetic study cannot be used directly in numerical models for the design optimization of diesel particulate filters (DPFs). Several exhaust gas components (water, NOx, SOx, hydrocarbons, etc.), which were not considered here, may have a significant effect on catalyst activity and stability.30,31 This study can be considered a step forward with regard to selecting the proper model for the evaluation of the activation energy of this particular solidsolid reaction and the perovskite class of catalysts, as well as toward evaluating the activation energies of all the catalysts selected to be implemented in a complete kinetic expression to use for model purposes. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: +39-011.090.4710. Fax: +39-011-090.4699. E-mail: debora. fi
[email protected].
’ LEGEND α = fraction of burned carbon B = Ozawa constant
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β = heating rate [°C/min] Ea = activation energy [kJ/mol] k∞ = pre-exponential constant [Paβ s1] mcat = catalyst mass [g] n = reaction order s = Ozawa constant S = specific area of carbon [m2 g1] R = gas constant; R = 8.314 J mol1 K1 T = temperature [K] Tα = temperature corresponding to the consumption of a fixed fraction α of carbon [K] Tp = peak temperature-programmed oxidation runs [K] t = time [s] k = kinetic constant PO2 = oxygen partial pressure
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