Energy Fuels 2010, 24, 3719–3726 Published on Web 02/10/2010
: DOI:10.1021/ef901279w
Soot Combustion over Lanthanum Cobaltites and Related Oxides for Diesel Exhaust Treatment† Runduo Zhang,‡,§ Na Luo,‡ Biaohua Chen,*,‡ and Serge Kaliaguine§ ‡ State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, People’s Republic of China, and §Department of Chemical Engineering, Laval University, Ste Foy, G1V 0A6 Qu ebec, Canada
Received November 1, 2009. Revised Manuscript Received January 22, 2010
This paper reports a comparative study of a series of La1-x(M)xCoO3 (M = Ce and Sr) perovskites and the related M0 Ox (M0 = La, Co, Ce, and Sr) sample oxides. These catalysts were characterized by N2 adsorption, X-ray diffraction (XRD), temperature-programmed desorption (TPD) of oxygen, temperature-programmed reduction (TPR) by hydrogen, and temperature-programmed combustion (TPC) of soot. In the presence of perovskite-type catalysts, the soot oxidation was promoted, showing a shift of CO2 peaks toward lower temperature compared to the non-catalytic process. In addition, the catalytic activity of these perovskites was found to follow the trend La0.8Ce0.2CoO3 > La0.9Ce0.1CoO3 > La0.9Sr0.1CoO3 > La0.8Sr0.2CoO3 ≈ LaCoO3. A mechanism was proposed with an attack of soot by surface oxygen species, which migrate from the perovskite surface. The generation of CO (or carbonyl groups) as an intermediate and the possible formation of carbonate species over the most basic compounds are also discussed.
regeneration. This device is then designated as the “catalytic filter”.4 Although in the past efforts to seek soot combustion catalysts involved precious metals,5,6 zeolites,7,8 molten salts,9,10 simple metal oxides,11,12 and mixed oxides,13-19 much attention has been recently paid to perovskite-type oxides. These studies focused on the following major compositions: La(0.8,0.9)(Li, Na, K, Rb)0.1Cr(0.9,1)O3,13,14 Sr1-x(Li, K, Cs)xTiO3,15 La1-xKxMnO3,16,17 La1-x(K, Li, Sr)x(Co, Mn, Fe)1-yCuyO3,18 and La1-x(Sr, Ba, Cs)x(Co, Mn, Fe)O3.19 The usual ABO3 perovskite has a super structure with a ReO3-type framework built up by incorporation of A cations into BO6 octahedra. The great diversity, excellent redox properties, high thermal and mechanical strength, and good oxidation ability of perovskites make them ideal materials for soot catalytic combustion.
1. Introduction Diesel engines are widely used for heavy-duty transportation (such as in trucks, buses, passenger vans, trains, and ships) as well as machinery for excavation, exploitation, and construction, because of their desirable power and fuel economy performances as well as lower emissions of CO and hydrocarbons (HCs). The latter is owing to a more complete fuel combustion at excess oxygen in the compression-ignition chamber in comparison to gasoline engines.1 Particulate matters (PMs), mainly composed of primary carbonaceous particles (soot) and a soluble organic fraction (SOF) condensed on these particulates, are however also produced as specific pollutants from the diesel engine. These particulates were classified as suspected carcinogen and mutagen hazards for human health.2 As a result, the regulations on PM emissions have become more and more stringent. For instance, emission standards for new heavy-duty vehicles were tightened from 0.10 g kW-1 h-1 in 2000 (Euro III) to 0.02 g kW-1 h-1 in 2005 (Euro IV).3 PMs can also lead to a rapid activity loss of post-treatment catalysts for the elimination of nitrogen oxides (NOx), which are other serious pollutants in the diesel exhaust gases. The PMs control is thus thought as being necessary for the practical application of diesel engines. This is commonly realized using a diesel particulate filter (DPF) to capture these carbonaceous particulates. The filter may also be coated with a catalytic layer active in soot combustion during in situ
(5) Chien, C.; Huang, T. Ind. Eng. Chem. Res. 1995, 34, 1952. (6) Stein, H. J. Appl. Catal., B 1996, 10, 69. (7) Fritz, A.; Pitchon, V. Appl. Catal., B 1997, 13, 1. (8) Xiao, S.; Ma, K.; Tang, X.; Shaw, H.; Pfeffer, R.; Stevens, J. G. Appl. Catal., B 2001, 32, 107. (9) Milt, V. G.; Querini, C. A.; Miro, E. E.; Ulla, M. A. J. Catal. 2003, 220, 424. (10) Badini, C.; Saracco, G.; Serra, V.; Specchia, V. Appl. Catal. 1998, 18, 137. (11) Neeft, J. P. A.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 1996, 8, 57. (12) Mul, G.; Zhu, W.; Kapteijn, F.; Moulijn, J. A. Appl. Catal., B 1998, 17, 205. (13) Russo, N.; Fino, D.; Saracco, G.; Specchia, V. J. Catal. 2005, 229, 459. (14) Fino, D.; Russo, N.; Canda, E.; Saracco, G.; Specchia, V. Catal. Today 2006, 114, 31. (15) Bialobok, B.; Trawczy nski, J.; Rzadki, T.; Mista, W.; Zawadzki, M. Catal. Today 2007, 119, 278. (16) Peng, X.; Lin, H.; Shangguan, W.; Huang, Z. Catal. Commun. 2007, 8, 157. (17) Teraoka, Y.; Kanada, K.; Kagawa, S. Appl. Catal., B 2001, 34, 73. (18) Teraoka, Y.; Nakano, K.; Shangguan, W.; Kagawa, S. Catal. Today 1996, 27, 107. (19) Hong, S.; Lee, G. Catal. Today 2000, 63, 397.
† This paper has been designated for the Asia Pacific Conference on Sustainable Energy and Environmental Technologies (APCSEET) special section. *To whom correspondence should be addressed. Telephone: 86-1064429057. E-mail:
[email protected]. (1) van Setten, B. A. A. L.; Makkee, M.; Moulijn, J. A. Catal. Rev. 2001, 43, 489. (2) United States Environmental Protection Agency (EPA). Review of EPA’s Health Assessment Document for Diesel Exhaust, EPA 600/890/057E, 2000. (3) Dieselnet Diesel Emissions Online. http://www.dieselnet.com. (4) Walker, A. P. Top. Catal. 2004, 28, 165.
r 2010 American Chemical Society
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Table 1. Physicochemical Properties of Prepared Oxide Samples after Calcination at 500 °C for 5 h sample
specific surface area (m2/g)
LaCoO3 La0.9Ce0.1CoO3 La0.8Ce0.2CoO3 La0.9Sr0.1CoO3 La0.8Sr0.2CoO3 Co3O4 La2O3 CeO2 SrO
29 34 32 34 19 2 4 7 5
crystal structure
impurity phase identification
rhombohedral rhombohedral rhombohedral rhombohedral rhombohedral cubic hexagonal cubic tetragonal
Co3O4 CeO2 SrO La2O2CO3 SrCO3
Previous work dealt with the development of such catalysts and the mechanism of diesel particulate combustion. Fino et al. suggested that soot combustion catalyzed by Cr-, Mn-, and Fe-based perovskites with A-site substitution by alkali metals underwent a suprafacial process and weakly chemisorbed oxygen upon vacancies played a crucial role for this reaction.13,20 The importance of “triple contact point”, where perovskite catalyst, soot, and gaseous reactants meet together, was stated by Teraoka et al.18 Badini et al. expressed the view that the mobility of catalyst components because of the formation of a vapor or liquid phase was beneficial to particulate conversions.10 Therefore, it seems that the agreement on key issues related to soot combustion was far not achieved. Lanthanum cobaltite was tested as a catalyst for the combustion of carbon black, owing to its reported abundance of surface anion vacancies and good activity in several complete oxidation reactions.21,22 The effect of various gases (hydrocarbons, CO, H2, NOx, SO2, and H2O vapor) present in the diesel exhaust on soot combustion using LaCoO3 as a catalytic material was investigated by us.23 As a continuation of this work, the redox properties and catalytic performance of LaCoO3 were further modified through A-site substitution by Ce4þ or Sr2þ in the present study. Reactive grinding was used for the preparation of La1-x(Sr, Ce)xCoO3 (x = 0, 0.1, and 0.2) to ensure relatively high surface areas and nanoscale crystallites.22,24 These perovskites as well as the related simple metal oxides were characterized by N2 adsorption, X-ray diffraction (XRD), temperature-programmed desorption (TPD) of O2, temperature-programmed reduction (TPR) by H2, and temperature-programmed combustion (TPC) of soot, aiming at correlating the physicochemical properties of these catalysts with their TPC performance.
R-O2 (μmol/g)
β-O2 (μmol/g)
Tmax TPC (°C)
133.2 (597 °C) 160.5 (581 °C) 198.4 (576 °C) 140.8 (590 °C) 145.6 (586 °C) 5.7 (700 °C) 7.5 (>700 °C)
485 454 435 475 486 455 722 505 >700
oxide catalysts, namely, CeO2, SrO, and Co3O4, were obtained by calcining the commercial products at 500 °C for 5 h. 2.2. Physical Properties. Specific surface areas of the materials calcined at 500 °C were determined from nitrogen adsorption equilibrium isotherms at -196 °C. These were measured via an automated gas sorption system (NOVA2000, Quantachrome) operating in continuous mode using a multipoint BrunauerEmmett-Teller (BET) method. Samples of about 200 mg were degassed at 300 °C under vacuum until complete removal of humidity (about 6 h) prior to adsorption/desorption experiments. The specific surface area was determined from the linear part of the BET curve (P/P0 = 0.01-0.10). The crystal structure was established by XRD using a diffractometer (Siemens D 5000) and Cu KR radiation (λ = 1.5406 A˚). Patterns were recorded with step scans from 20° to 80° in 2θ angle and 2.4 s for each 0.05° step. The identification of the crystal phases was performed using the JCPDS data bank. 2.3. O2-TPD and H2-TPR. O2-TPD and H2-TPR were performed using a temperature-programmed characterization system (RXM-100, ASDI), equipped with a UTI 100 quadrupole mass spectrometer (MS) and a thermal conductivity detector (TCD). The samples (50 mg) were pretreated under 10% O2/He at 20 cm3/min total flow rate [standard temperature and pressure (STP)] for 1 h at 500 °C, subsequently, cooled to room temperature under the same atmosphere, and finally, purged with 20 cm3/min of helium for 40 min to remove the physisorbed O2 before temperature-programmed experiments. For the O2TPD, 20 mL/min He passed through the reactor heated at 10 °C/ min up to 800 °C and the desorbed O2 was monitored by mass spectrometry (mass number of 32). For the H2-TPR, 5% H2/Ar at a total flow rate of 20 cm3/min was flowing over the sample heated at 5 °C/min up to 900 °C and H2 consumption was recorded online using a TCD. The gas responses obtained by MS and TCD were calibrated using standard mixtures. 2.4. TPC of Soot. The evaluation of catalytic activity for soot combustion was performed in a tubular fixed-bed quartz reactor under atmospheric pressure with 50 mg of mixture (at a catalyst/ carbon weight ratio of 95:5) loaded between two quartz wool plugs. This mixture was obtained by fully mixing catalyst and carbon powders in an agate mortar, to obtain a “tight” contact.11 This procedure allows for a much higher reproducibility for activity screening, although this type of contact is much more intimate than what is actually achieved in a catalytic soot trap. Amorphous carbon black (Printex XE2, Degussa) was used for the sample preparation. This type of carbonaceous particulate (∼70 nm in diameter; 200 m2/g in BET surface area) was chosen because its properties are close to those of the real diesel soot.11 The system was heated externally via a tubular furnace, regulated with a temperature programmer (Omega CN3240) to raise the reactor temperature from ambient temperature to 800 °C at a heating rate of 5 °C/min. The gas feeds were controlled using mass flow meters to yield an inlet mixture of 0, 1, and 10% O2/He, at a total flow rate of 60 mL/min, which resulted in a gas hourly space velocity of about 100 000 h-1 (STP). The effluent gases (CO2 and CO) were analyzed using a FTIR gas analyzer (FTLA 2000, ABB, Inc.). The carbonaceous material balance was always achieved with an error below 5%, assuming that the soot consists of only carbon.
2. Experimental Section 2.1. Catalyst Preparation. The perovskite-type mixed oxides were prepared using reactive grinding, as described in our previous reports.22,24 Powders of La2O3, CeO2, SrO (Alfa), and Co3O4 (Baker and Adamson) were mixed in the desired proportions and then introduced into a vial with three tempered steel balls. The high-energy grinding was performed via a SPEX 800 shaker mill normally at an agitation speed of 1100 rpm for approximately 20 h to ensure a good crystallization of the perovskite phase. The La2O3 powder was calcined at 600 °C for 24 h to transform any La(OH)3 to La2O3 before reactive grinding or before its use as a simple oxide catalyst. The other simple metal (20) Fino, D.; Russo, N.; Saracco, G.; Specchia, V. J. Catal. 2003, 217, 367. (21) Szabo, V.; Bassir, M.; Van Neste, A.; Kaliaguine, S. Appl. Catal., B 2002, 37, 175. (22) Kaliaguine, S.; Van Neste, A. U.S. Patent 6,017,504, 2000. (23) Zhang, R.; Yang, W.; Xue, J.; Chen, B. Catal. Lett. 2009, 132, 10. (24) Kaliaguine, S.; Van Neste, A.; Szabo, V.; Gallot, J. E.; Bassir, M.; Muzychuk, R. Appl. Catal., A 2001, 209, 345.
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3.2. Redox Properties. The ascription of oxygen species of perovskite-type oxides desorbed during O2-TPD is welldocumented in the literature. These are commonly classified as R- and β-oxygen.24,26-28 R-O2 is desorbed as a broad plateau-like peak, appearing below ca. 700 °C.24 It is formed via a rapid adsorption of molecular oxygen upon anion vacancies (noted here as 0).27
Au et al.28 recently reported a further and much slower dissociation of this adsorbed molecular oxygen, yielding a negatively charged atomic oxygen (O-). This process can be formulated as
Figure 1. XRD profiles of La1-x(Sr, Ce)xCoO3 perovskites.
The whole process might be considered reversible
3. Results and Discussion 3.1. Physical Properties. The BET surface areas of the prepared samples calcined at 500 °C determined by N2 adsorption are listed in Table 1. Most lanthanum cobaltites exhibited specific surface areas around 30 m2/g (except 19 m2/g for La0.8Sr0.2CoO3). The simple metal oxides including Co3O4, La2O3, CeO2, and SrO showed relatively lower surface areas, with values below 8 m2/g. XRD patterns (Table 1 and Figure 1) indicated that the perovskite main phase was well-crystallized with rhombohedral symmetry (JCPDS 48-0123 and 50-0298) for LaCoO3. The other minor phase of Co3O4 (JCPDS 42-1467) was also detected in addition to the major ABO3 perovskite phase. With 10% A-site substitution of La3þ by Sr2þ or Ce4þ cations, no diffraction lines belonging to SrO (JCPDS 271304) or CeO2 (JCPDS 34-0394) were clearly discerned from Figure 1. However, the peaks corresponding to SrO and CeO2 were visible in the XRD patterns of La0.8Sr0.2CoO3 and La0.8Ce0.2CoO3, respectively. In addition, cubic symmetry for Co3O4 (JCPDS 42-1467) and CeO2 (JCPDS 34-0394), hexagonal symmetry for La2O3 (JCPDS 83-1350), and tetragonal symmetry for SrO (JCPDS 27-1304), together with small fractions of impurity phases of La2O2CO3 (JCPDS 370804) in La2O3 and SrCO3 (JCPDS 74-1491) in SrO, were also evidenced by XRD. The ideal perovskite-type structure, which happens only in a few cases for tolerance factors very close to one and at high temperatures, is cubic, with a space group Pm3m-O1h.25 Diverse distortions of the perovskite structure would take place in other conditions. LaCoO3 contains both paramagnetic (t42ge2g configuration) and diamagnetic (t62ge0g configuration) Co3þ ions. A reductive non-stoichiometry appears for lanthanum cobaltite together with an increasing fraction of trivalent cobalt ions with t42ge2g configuration and formation of the rhombohedral structure after high-temperature treatment.26 Because some deviations from the ideal cubic structure were found for all lanthanum cobaltites prepared in this work, the rhombohedral symmetry perhaps leads to the most significant oxygen non-stoichiometry (oxygen vacancies) because of a much more severe distortion in the crystal structure.
The reverse first step corresponds to the desorption of R1-O2, whereas the overall reversed process is the desorption of R2O2 at higher temperatures. β-O2 desorption is considered as the liberation of lattice oxygen usually represented by a sharp peak above 700 °C simultaneously with the generation of surface oxygen vacancies and reduced metal cations.27
Then, the bulk oxygen ions must diffuse toward the surface for continued desorption
with simultaneous diffusion of the anion vacancy toward the bulk. The oxygen species formed over different catalysts after a pretreatment under flowing 10% O2/He at 500 °C were studied via O2-TPD experiments, as illustrated in panels a and b of Figure 2 (the corresponding TPD profile for Co3O4 was shrinked into 1/50 of its original value in Figure 2b). Their amounts released from the various samples were calculated after a Lorentzian deconvolution of the O2 desorption curve, as reported in Table 1. A total of 133.2 μmol/g of R-oxygens was formed over parent LaCoO3. When the A-site ion is (27) Zhang, R.; Villanueva, A.; Alamdari, H.; Kaliaguine, S. J. Catal. 2006, 237, 368. (28) Au, C. T.; Chen, K. D.; Dai, H. X.; Liu, Y. W.; Luo, J. Z.; Ng, C. F. J. Catal. 1998, 179, 300.
(25) Pe na, M. A.; Fierro, J. L. G. Chem. Rev. 2001, 101, 1981. (26) Seiyama, T. Catal. Rev.;Sci. Eng. 1992, 34, 281.
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Figure 2. O2-TPD profiles of prepared catalysts: (a) La1-x(Sr, Ce)xCoO3 and (b) (La, Sr, Ce, and Co)Ox.
partially substituted with an ion of different oxidation state, a charge compensation is required to achieve electroneutrality. From the results in Table 1, an enhancement of R-oxygen desorption was observed upon substitution of trivalent La3þ ions by divalent Sr2þ. The resulting positive charge deficiency is compensated for by oxygen vacancies. Thus, the enhanced R-O2 reflects the fact that Sr2þ has been partially incorporated into the lattice A site even though a small fraction of the SrO phase appeared in the XRD pattern of La0.8Sr0.2CoO3 (Figure 1). In line with the electroneutrality principle, the A-site substitution of La3þ by higher valence Ce4þ should depress the R-O2 formation over the surface. Contrarily, the R-oxygen over lanthanum cobaltites became quite significant with the introduction of Ce4þ ions (160.5 μmol/ g for La0.9Ce0.1CoO3 and 198.4 μmol/g for La0.8Ce0.2CoO3). Fino and Specchia29 found that a mechanical mixture of perovskite with ceria did not result in any significant enhancement of desorbed oxygen. Conversely, the deposition of fine CeO2 over a perovskite was realized using a cosynthesis technique, which led to a significant increase of the oxygen-transfer potential of the catalyst. They believed that oxygen chemisorbed over CeO2 could migrate to its neighbor perovskite via a subsequent surface diffusion, which further enhances the concentration of oxygen species over active perovskite. The appearance of CeO2 crystallites is evidenced by the XRD pattern of La0.8Ce0.2CoO3 (Figure 1). A synergetic role of small CeO2 particles beside perovskite substrate may thus be responsible for the intense R-O2 of Cesubstituted lanthanum cobaltites (Table 1). The surface R-type oxygen species are believed to play a crucial role in promoting the catalytic activity of perovskites in HCs and CO oxidations via a suprafacial catalytic process.21,26,30 Therefore, their amounts desorbed can be regarded as an indicator for the oxidative ability of materials. A trend in oxidation capability of La0.8Ce0.2CoO3 >La0.9Ce0.1CoO3 > La0.8Sr0.2CoO3, La0.9Sr0.1CoO3 > LaCoO3 may thus be deduced from the O2-TPD data in Table 1. In comparison to perovskite-type oxides, only minor R-oxygen (below 6 μmol/g) was formed over sample oxides (Co3O4,
La2O3, CeO2, and SrO), implying their scarcity in surface anion vacancies (see Figure 2b). The β-type oxygen is considered to be more specifically associated with the nature of the B-site cation and responsible for the catalytic activity in natural gas combustion.31,32 Hydrogen abstraction from the methane molecule by the reaction with monatomic intrafacial oxygen (O2-) is the first step for an Elay-Rideal mechanism.31 The desorbing temperature and amount of lattice oxygen listed in Table 1 reflect the reducibility of these solid materials. LaCoO3 (112.4 μmol/g) and Co3O4 (348.6 μmol/g) showed a rather higher mobility of their lattice oxygens bonded to cobalt ions. The introduction of Ce4þ cations into lanthanum cobaltite could also facilitate the lattice oxygen desorption (281.8 μmol/g for La0.8Ce0.2CoO3), although the β-O2 depletion from bulk CeO2 is difficult (3.7 μmol/g). The reducibility of oxides was thereafter investigated by H2-TPR experiments, as illustrated in panels a and b of Figure 3. For simple oxides (Figure 3a), the complete reduction of Co3O4 was realized at rather low temperature (354 °C). This high reducibility feature is in accordance with its significant β-O2 desorption (Table 1). In contrast, the reduction of bulk CeO2 is quite difficult, appearing as a H2 consumption peak at 812 °C. The slight reduction peak at 482 °C for CeO2 was possibly related to a minor population of fine CeO2 particles. Because La2O3 cannot be reduced under the investigated TPR conditions, the trace H2 consumption occurring at 589 °C was speculated to correspond to the reduction of some surface impurities, such as La2O2CO3, whose phase is visible in the XRD pattern of La2O3 (Table 1). The reduction occurring at 741 °C over SrO was also attributed to some carbonate species, because of the simultaneous CO2 desorption detected by MS (not shown) and the appearance of the SrCO3 impurity phase evidenced by XRD (Table 1). The H2-TPR profile of LaCoO3 presents a peak at 311 °C assigned to Co3þ f Co2þ and a peak at 578 °C with shoulders (31) Saracco, G.; Geobaldo, F.; Baldi, G. Appl. Catal., B 1999, 20, 277. (32) Szabo, V.; Bassir, M.; Van Neste, A.; Kaliaguine, S. Appl. Catal., B 2003, 43, 81.
(29) Fino, D.; Specchina, V. Chem. Eng. Sci. 2004, 59, 4825. (30) Viswanathan, B. Catal. Rev.;Sci. Eng. 1992, 34, 337.
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Figure 4. (a) CO2 and (b) CO profiles during TPC of soot over (La, Sr, Ce, and Co)Ox sample metal oxides.
Figure 3. H2-TPR profiles of prepared catalysts: (a) (La, Sr, Ce, and Co)Ox and LaCoO3 and (b) La1-x(Sr, Ce)xCoO3.
was also observed. Conversely, the catalytic combustion of carbonaceous particulates over SrO seems to be difficult, appearing as a light CO2 trace with an onset at about 700 °C. The formation of small amounts of CO over metal oxides was also found in the temperature range of 350-680 °C (Figure 4b), with fractional conversions toward this byproduct of 0.3% for Co3O4, 0.9% for CeO2, 2.2% for La2O3, and 10.3% for SrO. It was noticed that the appearance of CO correlated well with the CO2 peaks in the case of CeO2 (at 505 °C) and La2O3 (at 515 °C). In contrast, the formation of CO2 over Co3O4 was found at 454 °C, which is much lower than the temperature for CO generation and corresponds to a shoulder in the CO2 trace. The opposite occurred over SrO, showing a CO peak at 447 °C followed by a CO2 peak at >700 °C. In the early 1960s, Amariglio and Duval found that only metals that could “oscillate” between two oxidation states were able to catalyze the oxidation of graphite.33 There are both magnetic Co2þ ions and diamagnetic low-spin Co3þ ions located in the respective tetrahedral (Td) and octahedral (Oh) coordination sites present in the Co3O4 spinel.34,35 The
at 454 and 720 °C assigned to Co2þ f Co0 (Figure 3a). Furthermore, the sharp peak appearing at 352 °C is accordingly attributed to the Co3O4 reduction according to the present H2TPR result for the related simple oxide. The effect of La3þ-site substitution by Ce4þ or Sr2þ on the reducibility of LaCoO3 was also studied by means of H2TPR, as depicted in Figure 3b. It seems that only 20% Ce4þ ion substitution can effectively facilitate the initial reduction of lanthanum cobaltite. Interestingly, the Co2þ f Co0 reduction peak became flat with the introduction of cerium. The special characteristic of ceria addressed as “oxygen storage capacity” possibly makes the related reduction process conducted more smoothly. On the contrary, Sr2þ substitution slightly decreased the reducibility of LaCoO3. This coincides with the difficulty of SrO reduction (Figure 3b). 3.3. Soot Combustion over Simple Oxides and the Related LaCoO3 Perovskites. The ability of simple oxides, including La2O3, Co3O4, CeO2, and SrO, to facilitate the soot combustion was investigated, with their TPC results being shown in panels a (toward CO2) and b (toward CO) of Figure 4. Co3O4 displayed the best performance, showing a CO2 peak at a temperature as low as 454 °C. A moderate activity for the oxidation of carbon black into CO2 was achieved in the case of CeO2 (at 505 °C) and La2O3 (at 515 °C). Interestingly, another CO2 peak at high temperature (722 °C) for La2O3
(33) Amariglio, H.; Duval, X. Carbon 1966, 4, 323. (34) van Elp, J.; Wieland, J. L.; Eskes, H.; Kuiper, P.; Sawatzky, G. A.; de Groot, F. M.; Turner, T. S. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 44, 6090. (35) Kim, K. J.; Park, Y. R. Solid State Commun. 2003, 127, 25.
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Figure 5. CO2 profiles during TPC of soot over the La2O3-Co3O4 mixture, LaCoO3 perovskite, and in the absence of catalyst.
Figure 6. CO2 profiles during TPC of soot over La1-x(Sr, Ce)CoO3 perovskites.
excellent redox properties of Co3O4 because of an easy “oscillation” of Co2þ T Co3þ, associated with the depletion of β-O2 as already evidenced by O2-TPD (Figure 2b and Table 1) and H2-TPR (Figure 3a), likely determine its good performance for soot catalytic combustion. Similarly, unsupported Co3O4 powder having a satisfactory activity for catalytic CO oxidation (at T > 150 °C)36 and methane combustion (at T > 300 °C)37 was also reported in the literature. The minor reduction peak at 482 °C in the TPR plot of ceria was related to its fine particulates, while the reduction of bulk CeO2 is much harder, appearing as a TPR peak at 812 °C. The lower redox ability of CeO2 leads to its TPC CO2 peak shifting to a higher temperature (505 °C) with respect to Co3O4 (Figure 4a). Furthermore, La2O3 and SrO are hardly reduced by hydrogen over the entire H2-TPR temperature range investigated (700 °C for SrO take place under such high temperatures, these CO2 formations are likely involving a decomposition of surface carbonate species. The basic properties of La2O3 and SrO (especially SrO, which belongs to the base-earth metal groups) make the carbonate formation over their surfaces easier. This may explain that two CO2 peaks are observed over La2O3 (Figure 4a), with the carbonate formed by the reaction with CO2 at 515 °C being decomposed at 722 °C. The above conclusion is further confirmed by the experiment reported in Figure 5. A mixture of La2O3 and Co3O4 with a La/Co molar ratio equal to 1 was prepared, and its catalytic activity for soot combustion was tested and compared to the result of non-catalytic combustion and LaCoO3catalyzed combustion. In the absence of catalytic material, carbon black is oxidized directly by gas-phase oxygen at 645 °C. This process was proposed to involve the ketone, semiquinone, and carbonyl groups with the assistance of the
absorbed oxygen atom over carbonaceous particulates.39 A significant CO production was found at 645 °C (∼8.5% of total soot conversion) during the non-catalytic combustion, while the CO formation is marginal in the presence of LaCoO3 and the La2O3-Co3O4 mixture (not shown). These results allow several conclusions to be made: Both LaCoO3 and the La2O3-Co3O4 oxidation catalysts allow for soot combustion at relatively lower temperatures than the non-catalytic process. Both catalysts depress the emission of CO by fast catalytic combustion of any CO produced. Considering that the CO2 peak at 705 °C is observed at a temperature where all carbon would be consumed even by the non-catalytic process, it can only correspond to the decomposition of lanthanum carbonate. The decomposition of lanthanum carbonate at a similar temperature range (525-725 °C) was indeed previously reported.11 Because no high-temperature CO2 peak is observed when TPC is performed in the presence of the perovskite, it seems that the formation of carbonates is not significant in these conditions. 3.4. Effect of A-Site Substitution on Soot Combustion over Lanthanum Cobaltites. Ferri and Forni demonstrated that substitution at the A site in families of La1-xAx(Co, Fe, Ni)O3 oxides has a strong influence on methane combustion.40 Because LaCoO3 showed good performance as a catalyst for soot combustion, the effect of partial substitution of A-site La3þ ions by Ce4þ or Sr2þ ions was examined (Figure 6). All lanthanum cobaltites showed high selectivity (essentially 100%) to CO2 in accordance with their good oxidative properties (panels a and b of Figure 2 and Table 1). The substitution of trivalent La3þ ions by lower valent Sr2þ ions should result in an enhanced R-O2 adsorption. The slight shift of the CO2 trace of La0.9Sr0.1CoO3 to lower temperatures compared to that of the parent LaCoO3 is likely owing to this reason. As the Sr2þ substitution degree reach 20%, the temperature of the maximum of CO2 peak for La0.8Sr0.2CoO3 and unsubstituted LaCoO3 remain close. The presence of SrO in addition to the main La0.8Sr0.2CoO3
(36) Yao, Y.-F. Y. J. Catal. 1974, 33, 108. (37) Zavyalova, U.; Scholz, P.; Ondruschka, B. Appl. Catal., A 2007, 323, 226. (38) Iwamoto, M.; Yoda, Y.; Yamazoe, N.; Seiyama, T. J. Phys. Chem. 1978, 82, 2564.
(39) Chen, S. G.; Yang, R. T.; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem. Res. 1993, 32, 2835. (40) Ferri, D.; Forni, L. Appl. Catal. 1998, 16, 119.
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Energy Fuels 2010, 24, 3719–3726
: DOI:10.1021/ef901279w
Zhang et al.
3.6. Mechanism of Catalytic Soot Oxidation. Several mechanisms for soot combustion were documented in the literature. The first mechanism, although not firmly proven, is designated as the “electron-transfer” mechanism.42 It states that the alternation of π-electron distribution in the coke makes the carbon substrate more readily oxidizable. Thereafter, a “redox” mechanism was proposed by Amariglio and Duval.33 They suggested that only metals with an “oscillation” between two oxidation states can catalyze the graphite oxidation. Moreover, some catalysts were believed to dissociate molecular oxygen and subsequently transfer it to the soot particles via a “spillover” mechanism.43 Furthermore, a more complex “triple contact point” mechanism18 suggested that the simultaneous removal of soot and O2 is taking place where the solid catalyst, the solid soot, and the gaseous O2 meet together. In such a case, the specific surface area seems to be of less importance than in the usual solid-gas reaction. Fino et al. recently pointed out that the suprafacial oxygen of perovskites is crucial for catalytic combustion of soot.13,14,20,29 Voorhoeve et al. first classified the catalytic oxidation reactions over perovskites into suprafacial and interfacial processes that respectively occur at low and high temperatures.44 If the attention is focused on the temperature range of 400-500 °C, where most perovskites displayed their best soot combustion activities, “suprafacial” oxidation processes should be accepted. This is realized through transferring the R-O2 species (O2- and O-) over perovskites to carbonaceous particulates to accelerate the oxidation reaction. The issue of CO appearing in the gas phase in some of our experiments is somewhat confusing, because CO could be either a direct product of soot combustion or a secondary product obtained by the Boudouard reaction
Figure 7. CO2 profiles during TPC of soot over LaCoO3 under various O2 atmospheres or pretreatment.
phase was established by XRD (Figure 1). This suggests that the 20% substitution was not completed. The introduction of Ce4þ into LaCoO3 always results in a significant enhancement in soot oxidation activity. CO2 peaks with a maximum at 435 °C for La0.8Ce0.2CoO3 and 454 °C for La0.9Ce0.1CoO3 are respectively observed. This coincides with the intense R-O2 desorption peak during the O2-TPD tests, which is proportional to the ceria addition. As discussed above (see section 3.2), the high density of R-O2 over cerium-substituted samples is believed to be related to the presence of CeO2 clusters on the surface of these perovskites. Reactive grinding facilitates the short distance interaction between CeO2 and perovskite. This coincides with the fact that more grain boundaries of perovskite-type mixed oxides can be generated through high-energy-ball milling.41 In this case, the role of fine CeO2 clusters is to supply and subtract oxygen to and from the perovskite active sites through “oxygen storage”, resulting in a total improvement of the catalytic performance for soot combustion. 3.5. Effect of Gaseous Oxygen on Soot Combustion over LaCoO3. In the case of LaCoO3, the role of gas-phase and adsorbed surface oxygen on TPC behavior was further investigated, as shown in Figure 7. Some soot oxidation is indeed observed in the absence of gaseous oxygen. The shape of the CO2 profile became, however, flat, showing two maxima at 525 and 685 °C. The complete combustion of soot was never accomplished, even at temperatures up to 800 °C. At T > 700 °C, minor CO formation was also observed. These results reflect a relatively low reaction rate in the absence of gaseous oxygen. This process is believed to involve soot oxidation by adsorbed oxygen species (O2- and O-) present on the Co-based perovskite at temperatures
