Catalytic Combustion of Soot over the Highly Active (La0.9K0.1CoO3

Sep 10, 2009 - (MRU company, Germany). Another group of NO oxidation experiments were also performed on the above fixed beds for studying the formatio...
0 downloads 0 Views 2MB Size
Download PDF
17114

J. Phys. Chem. C 2009, 113, 17114–17123

Catalytic Combustion of Soot over the Highly Active (La0.9K0.1CoO3)x/nmCeO2 Catalysts Jian Liu, Zhen Zhao,*,† Jie Lan,† Chunming Xu,† Aijun Duan,† Guiyua Jiang,† Xinping Wang,‡ and Hong He§ State Key Laboratory of HeaVy Oil Processing, China UniVersity of Petroleum, Beijing, 102249, People’s Republic of China, State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian, 116024, China, and Laboratory of Catalysis Chemistry and Nanoscience, College of EnVironmental and Energy Engineering, Beijing UniVersity of Technology, Beijing 100124, People’s Republic of China ReceiVed: June 16, 2009; ReVised Manuscript ReceiVed: August 21, 2009

The highly active (La0.9K0.1CoO3)x/nmCeO2 catalysts consisting of comparably sized La0.9K0.1CoO3 active component and CeO2 support nanoparticle were studied on the catalytic combustion of soot by temperatureprogrammed oxidation (TPO) technique. The properties of oxygen species and the redox performances of catalysts were investigated by the method of O2-TPD and H2-TPR. During TPO process, all supported oxide catalysts showed lower Tm than the corresponding nanometer CeO2 support or pure La0.9K0.1CoO3 perovskite. The best catalytic activity was obtained over (La0.9K0.1CoO3)20/nmCeO2 catalyst (Tm) 354 °C), and its catalytic activity for the combustion of soot is as good as supported Pt catalysts. During the catalytic combustion of soot, the surface R oxygen species should play an important role. The formation of surface oxygen-containing complexes (SOC) may be one of the key intermediate species according to the results of in situ DRIFT, and NO2 released by surface nitrate species remarkably promote the soot oxidation. 1. Introduction The awareness of the detrimental effects of automotive emissions to the environment has brought about a great deal of research works to the area of catalytic deep oxidation of the emitted harmful matters (e.g., CO, hydrocarbons, and soot particulates). The invention and application of three-way catalytic (TWC) technology, whose catalyst is composed of Pt and Rh noble metal particles dispersed on γ-Al2O3 surfaces, has successfully lowered the harmful gas emissions of gasolinepowered engines to acceptable levels. However, the TWC noble metal catalyst was not designed to efficiently operate in diesel engine exhaust for it functions at higher temperatures (650 °C) than the temperature range of the diesel exhaust (typically 180-400 °C), and specifically, the conditions with excessive oxygen were not suitable for TWC catalysts.1,2 In recent years, Oi-Uchisawa et al.3,4 and Hinot et al.5 studied the combustion of soot over supported noble metal Pt catalysts using NO/O2 mixtures as oxidizing agents. In the temperature range of the typical diesel exhaust, the use of NO/O2 as oxidizing agents promotes soot combustion over these materials through the operation of a NO2 intermediate in the reaction mechanism. The supported Pt catalysts exhibited a high level of catalytic activity to promote soot oxidation. This catalyst system is the best one so far reported for soot combustion under loose contact conditions. However, Pt is expensive and scarce. Thus, the research works were oriented to oxide materials which are capable of reducing the diesel emissions (much richer than the gasoline emissions is soot particulates and unburned large aromatics) in the low temperature region. Various different oxide materials have been tested as soot oxidation catalysts: single oxides like PbO, Co3O4, V2O5, MoO3, * Corresponding author. E-mail: [email protected]. † China University of Petroleum. ‡ Dalian University of Technology. § Beijing University of Technology.

CeO2, and CuO and some mixtures of them,6-9 and some complex oxides such as spinel-type oxides10 and perovkite-type oxides11-14 all showed soot oxidation activities. Perovskite oxides are a class of complex oxides with a specific structure, having the general formula ABO3, where lanthanide elements usually are at the A-site position and the first row transition metals are at the B-site. They are less expensive and thermally more stable than noble metals. Some compositions were reported to possess similar or higher activity than Pt-supported catalysts.15 However, in spite of their good catalytic activity toward soot oxidation, the use of perovskites is generally limited by the low surface area. The most common approach to increase the contact surface between the perovskite and the reactant is the deposition of them on supports. With this respect, up to date, deposition of perovskites on thermally stable oxides seems to be the apparent solution. Since Gallagher reported the preparation of supported LaMnO3,16 other attempts were made, changing the nature or the shape of the support.14,17-19 For supported perovskites, three factors should be considered: the nature of the support, loading amount of perovskite, and the preparation method chosen for active phase deposition. The appropriate support is very important for the preparation of supported perovskite mixed oxide catalysts. Unfortunately, the most common and largely accessible supports such as Al2O3 are reactive toward the elements contained in the perovskites, resulting in the formation of catalytically inert compounds such as spinels.20 This is mainly the case of Co-containing perovskites, since Co easily diffuses into alumina network yielding CoAl2O4. Ce is inert toward the components of the perovskite. Moreover, the presence of Ce might be helpful to enhance the catalytic activity of Co-based perovskite for soot combustion. The present work is to explore CeO2 nanometer materials as carriers for La0.9K0.1CoO3 perovskite and to investigate the influence of the support composition and the perovskite loading amounts on their catalytic performances for soot oxidation. The

10.1021/jp9056303 CCC: $40.75  2009 American Chemical Society Published on Web 09/10/2009

Catalytic Combustion of Soot catalytic activities for soot oxidation were greatly enhanced with the nature of the oxide carrier. High catalytic activities, even exceeding the one of bulk La0.9K0.1CoO3, were obtained by using nanometer CeO2 support. On the other hand, several problems are still unsolved for the research of the intrinsic nature of soot combustion, although the amount of work toward a better understanding is impressive. In particular, the reaction mechanism of the catalytic removal of soot over perovskite-type catalysts remains unclear. It is due to the deep black color of soot, which makes it very difficult to acquire some useful information about the catalytic oxidation of soot under real reaction conditions. Several researches already pointed out the quite complex nature of this reaction mechanism. For instance, Teraoka et al.21,22 applied the kinetic methods speculating on the reaction mechanism and considered that NO2 played an important role in the reaction process. Mul23 and Minogue24 et al. used KBr as a diluent in IR spectroscopic studies to obtain high quality in situ spectra of carbonaceous materials. However, the interactions between metal oxides and KBr cannot be excluded, especially when analyses are performed at high temperatures. When black perovskite-type oxides supported over nanometer CeO2 support, due to the light color of CeO2, it can be used as a diluent; thus, the relative high quality in situ spectra of soot oxidation over supported perovskite oxide catalysts can be obtained. According to the results of in situ DRIFT (diffuse reflection infrared Fourier transformed) spectra and other characterization results, the molecular-level understanding of soot combustion over (La0.9K0.1CoO3)x/nmCeO2 can be obtained in this study. On this basis, a new reaction mechanism is proposed, which can preferably explain the reaction process of the catalytic combustion of diesel soot over supported perovskite oxide catalysts. 2. Experimental Section 2.1. Catalyst Preparation. Nanometric CeO2 powder was prepared by the autocombustion method assisted by rotating evaporation, which was described in ref 25. Briefly, Ce(NO3)3 was dissolved in distilled water in appropriate amounts and mixed with citric acid 100% in excess. The resulting solution was heated by a rotating evaporator at 50 °C in water bath under P ) 0.02 MPa vacuum. After about 80 % of the water was evaporated, the mixing solution was heated at a flow air atmosphere by an electric furnace with 2 kW power and evaporated to dryness with vigorous stirring, followed by burning, giving off a great amount of gases, and finally, the precursor was calcined at 800 °C for 6 h in a static air. Nanometric CeO2-supported La0.9K0.1CoO3 perovskite catalysts were prepared by the method of ultrasonic-assisted incipient-wetness impregnation. The nitrates of La, K, and Co were used as starting materials with appropriate stoichiometry. The catalysts will be generically named as (La0.9K0.1CoO3)x/ nmCeO2, where x is the molar ratio of 100 Co/Ce (x ) 1,4,10,20,50,100). 2.2. Catalyst Characterization. The BET specific surface areas were measured with linear parts of the BET plot of the N2 isotherms, using a Micromeritics ASAP 2010 analyzer. The crystal structures of the fresh catalysts were determined by a powder X-ray diffractometer (XRD, Shimadzu 6000), using Cu KR (λ ) 1.54184 Å) radiation combined with nickel filter operating at 40 kV and 10 mA. The diffractometer data were recorded for 2θ values between 15° and 80° with a scanning rate of 4°/min. The patterns were compared with JCPDS reference data for phase identification. The morphology of the catalysts was observed by scanning electron microscope (SEM,

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17115 S-4800, Japan). The UV-vis experiments were performed on Hitachi U-4100 UV-vis spectrophotometer with the integration sphere diffuse reflectance attachment. The powder samples were loaded in a transparent quartz cell and were measured in the region of 200-800 nm at room temperature. The nanometric CeO2 support reflectance was used as the baseline for the corresponding catalyst measurement. FT-IR spectra were measured with a FTS-3000 spectrophotometer in the wavenumber ranging from 400 to 4000 cm-1. For the transmission IR experiments under ambient conditions, the measured wafer was prepared as KBr pellet with the weight ratio of sample to KBr 1/100. The resolution was set at 4 cm-1 during measurement. For in situ diffuse reflection infrared Fourier transformed (DRIFT) experiments, it did not need to use KBr as diluent. The spectra were recorded in the range of 400-4000 cm-1 after 256 scans at a resolutiom of 4 cm-1. The powder sample was loosely mixed and placed in a sample holder assembly in a Harrick praying mantis DRIFT cell. The gases were supplied by individual mass flow controllers with a total flow rate of 50 mL/min. Before reactant gases enter, the sample was pretreated with 5% O2 in helium at 300 °C for 30 min. The sample was then cooled to reaction temperature and equilibrated in a helium atmosphere. After the sample had cooled to the reaction temperature, a spectrum of the treated sample was taken as the background at that temperature. The in situ experiment was performed by the introduction of 5% O2/He or 2000 ppm NO + 5% O2 in helium. Meanwhile, the IR spectra were sequentially recorded at the different reaction temperatures of 100 °C, 200 °C, 300 °C, 350 °C, 400 °C, 450 °C, and 500 °C. Oxygen temperature-programmed desorption (O2-TPD) experiments were performed in a quartz reactor with a TCD as detector. In each analysis, 200 mg samples were pretreated with helium gas, and the temperature was increased from room temperature to 700 °C at a rising rate of 10 °C/min. The samples were oxidized with a 20% O2/He mixture at a total flow rate of 30 cm3/min at 700 °C for 30 min. Then they were cooled to room temperature in the oxidizing mixture and flushed with a stream of purified helium for 30 min. When the baseline of the chromatograph was stabilized, the desorption was carried out from room temperature to 900 °C at a rising rate of 10 °C/min in helium gas stream. NO adsorption on (La0.9K0.1CoO3)20/nmCeO2 supported oxide catalyst was carried out in the quartz flow reactor using 200 mg sample. The catalyst was pretreated in a gas flow of 10% O2 in helium at 500 °C for 1 h and then cooled to the experimental temperature. When the temperature had stabilized at 350 °C, 2000 ppm NO and 5% O2 in He were introduced at a rate of 200 mL/min for 20 min for NO adsorption. The NO concentration was monitored by a Vario Plus gas analyzer (MRU company, Germany). Another group of NO oxidation experiments were also performed on the above fixed beds for studying the formation of NO2. Reactant gas compositions were 5% O2 and 0.2% NO with He balanced gas. The reaction temperature ranged from 100 to 600 °C. The catalyst used was 200 mg and without soot during the reaction. The other reaction conditions were the same as ones following the TPO experiment. The NO2 concentration in the outlet gas was in situ detected by the mass spectrometer detector. Temperature-programmed reduction experiments under hydrogen atmosphere (H2-TPR) were performed in the apparatus used for O2-TPD. In this case, 200 mg samples were pretreated with helium gas, and the temperature was increased from room temperature to 700 °C at a rate of 10 °C/min and then was

17116

J. Phys. Chem. C, Vol. 113, No. 39, 2009

Liu et al.

TABLE 1: BET Surface Areas and Catalytic Performances of nmCeO2, La0.9K0.1CoO3, and (La0.9K0.1CoO3)x/nmCeO2 Catalysts for the Catalytic Combustion of Soot under Loose Contact Conditions between the Catalysts and Soot (x ) 1, 4, 10, 20, 50, 100) catalyst no catalyst nmCeO2 La0.90K0.10CoO3 (La0.9K0.1CoO3)1/nmCeO2 (La0.9K0.1CoO3)4/nmCeO2 (La0.9K0.1CoO3)10/nmCeO2 (La0.9K0.1CoO3)20/nmCeO2 (La0.9K0.1CoO3)50/nmCeO2 (La0.9K0.1CoO3)100/nmCeO2

BET surface area/m2/g Tm/°C SCO2m/% 48.8 11.1 39.4 35.3 29.1 26.6 22.8 19.7

594 465 398 399 390 383 354 380 363

55.0 90.4 98.3 98.1 98.4 98.9 98.8 98.2 98.4

cooled down to 100 °C. The reducing atmosphere was a 10% H2/He mixture introduced at a total flow rate of 30 cm3/min. The temperature was increased at a rising rate of 10 °C/min from 100 to 700 °C. The hydrogen consumption signal was monitored by a thermal conductivity detector (TCD) in an online gas chromatograph (GC). For eliminating the effect of the outlet reactor gas containing other components which mainly consisted of H2O and CO2 on the TCD during H2-TPR measurements, a filter was set that contained a 5A molecular sieve (60-80 meshes) for adsorption of H2O and CO2 before the reaction gases entered the TCD. 2.3. Activity Measurement. The catalytic activities of the prepared samples were evaluated with a TPO reaction on a fixed-bed tubular quartz system. The reaction temperature was controlled through a PID-regulation system based on the measurements of a K-type thermocouple and varied during each TPO run from 200 to 700 °C at a 2 °C/min rate. The soot used in this work was Printex-U which was supplied by Degussa as a model soot. Its primary particle size was 25 nm, and its specific surface area was 100 m2/g. The catalyst and soot were mixed with a spatula in order to reproduce the loose contact mode, which is the most representative model of diesel particles flowing through a catalytic filter.26 A 100 mg sample of the mixture (catalyst to soot, 10:1, w/w) was placed in the tubular quartz reactor (di ) 6 mm) in every test. Reactant gases containing 5% O2 and 0.2% NO balanced with He were passed through a mixture of the catalyst and soot at a flow rate of 50 mL/min. The outlet gas from the reactor passed through a 1 cm3 sampling loop of a six-point gas-sampling valve before it was injected into an online GC. The GC used both a thermal conductivity detector (TCD) and a flame ionization detector (FID) to analyze the gaseous mixture composition. The TCD was used to measure the concentration of O2, N2, and CO after separating these gases over a molecular sieve 5A column. The FID was employed to determine CO and CO2 concentrations after separating these gases over a Porapak N column and converting them to methane over a Ni catalyst at 380 °C. 3. Results 3.1. Specific Surface Area Measurement. Surface areas of the supports and catalysts are listed in Table 1. As shown in Table 1, all catalysts exhibit lower surface areas than the corresponding nanometer CeO2 supports. This drop in SSA increases with the perovskite loading amount, showing that the deposited material blocks the entrance of the pores. However, supported perovskites show clearly larger surface areas than bulk La0.9K0.1CoO3. It would be beneficial to enhance the catalytic activity of supported catalysts for the catalytic combustion of soot.

Figure 1. X-ray diffraction patterns of nmCeO2 and (La0.9K0.1CoO3)x/ nmCeO2 catalysts. (x ) 1, 4, 10, 20, 50, 100) (1) (La0.9K0.1CoO3)1/ nmCeO2, (2) (La0.9K0.1CoO3)4/nmCeO2, (3) (La0.9K0.1CoO3)10/nmCeO2, (4) (La0.9K0.1CoO3)20/nmCeO2, (5) (La0.9K0.1CoO3)50/nmCeO2, and (6) (La0.9K0.1CoO3)100/nmCeO2 .

3.2. X-ray Diffraction (XRD) Results. The crystal structures of supported (La0.9K0.1CoO3)x/nmCeO2 catalysts after calcination at 800 °C are revealed by XRD. The XRD patterns of the solid mixture are displayed in Figure 1. These patterns show strong signals of the supports, a very strong peak at 2θ ) 28.5° and other weak peaks indicating the characteristics of fluorite structure, which corresponds to a face-centered cubic (fcc) fluorite structure of CeO2 (JCPDS Card No. 34-0394). As shown in Figure 1, no perovskite crystal phases were detected when the loading amount of La0.9K0.1CoO3 x e 20. It indicated that La0.9K0.1CoO3 perovskite highly dispersed on the surface of nmCeO2 support. The weak and broad line centered around 2θ ) 40.8° and 41.5° that can be assigned to the planes (2 0 2) and (0 0 6) of rhombohedral La0.9K0.1CoO3 (JCPDS 48-0123). Stronger and better defined reflections of La0.9K0.1CoO3 appear when the loading amount of x g 50 together with another line accounting for perovskite phase formation, centered at 2θ ) 23.4. No other phases, such as La2O3 or Co3O4, that might suggest an incomplete perovskite formation, are detected. 3.3. Scanning Electron Microscope (SEM). Figure 2 shows the SEM photograph of the (La0.9K0.1CoO3)20/nmCeO2 sample. For comparison, the SEM photographs of nanometric CeO2 and La0.9K0.1CoO3 samples have also been shown in Figure 2.8,27 It can be seen that the nanometric CeO2 sample has an average particle size around 20 nm and is very well-dispersed, and the average particle size of La0.9K0.1CoO3 is about 150 nm; thus, the particle size of support CeO2 is smaller than that of La0.9K0.1CoO3 perovskite oxide. When perovskite oxide was supported on nanometric CeO2 support, as shown in Figure 2c, the active component La0.9K0.1CoO3 has been well-dispersed on CeO2 support, and its particle size has changed and is smaller. The boundary and phase interface between active component La0.9K0.1CoO3 and support CeO2 are very unclear. It might indicate that the nanocomposite (La0.9K0.1CoO3)x/nmCeO2 has been produced, and the conventional idea has been weakened for active component and support of catalysts. 3.4. Results of UV-vis Characterization. The UV-vis diffuse reflection spectroscopy can be applied to investigate the structures of supported oxide catalysts due to the ligand-to-metal charge transfer (LMCT) transitions of transitional metal ions in the 200-500 nm region and d-d transition bands in the 600-800 nm region due to d-d electron transferring. The diffuse reflection spectral features are sensitive to nanosized oxide

Catalytic Combustion of Soot

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17117

Figure 3. UV-vis spectra of (La0.9K0.1CoO3)x/nmCeO2 catalysts. (Nanometer CeO2 support was used as reference baseline. x ) 1, 4, 10, 20, 50, 100) (1) (La0.9K0.1CoO3)1/nmCeO2, (2) (La0.9K0.1CoO3)4/ nmCeO2, (3) (La0.9K0.1CoO3)10/nmCeO2, (4) (La0.9K0.1CoO3)20/nmCeO2, (5) (La0.9K0.1CoO3)50/nmCeO2, and (6) (La0.9K0.1CoO3)100/nmCeO2.

Figure 4. FT-IR spectra of nmCeO2 and (La0.9K0.1CoO3)x/nmCeO2 catalysts. (x ) 1, 4, 10, 20, 50, 100) (1) (La0.9K0.1CoO3)1/nmCeO2, (2) (La0.9K0.1CoO3)4/nmCeO2, (3) (La0.9K0.1CoO3)10/nmCeO2, (4) (La0.9K0.1CoO3)20/ nmCeO2, (5) (La0.9K0.1CoO3)50/nmCeO2, and (6) (La0.9K0.1CoO3)100/ nmCeO2, (7) nmCeO2 . Figure 2. SEM photographs of three catalysts: (a), nmCeO2; (b), La0.9K0.1CoO3; (c), (La0.9K0.1CoO3)20/nmCeO2.

particle. The UV-vis diffuse reflection spectra of nanometric CeO2-supported La0.9K0.1CoO3 pervoskite catalysts using nanometric CeO2 support as reference sample are shown in Figure 3. Both maximal characteristic bands at 720 and 470 nm are due to the charge transfer of cobalt.8,28 Herein, it is worth noting that these bands become more intense as the pervoskite loading amount of catalyst increases. For the pervoskite-loading sample, an intense absorption band centered at about 720 nm was observed, indicating some surface Co2+/Co3+ species existing in the samples. 3.5. Results of IR Characterization. Consistent with the previous XRD results, IR transmission spectroscopy gave some information about the surface compositions and structures of the catalysts. Figure 4 shows the IR spectra of the (La0.9K0.1CoO3)x/ nmCeO2 supported oxides, and for comparison, the spectrum of nmCeO2 is also included in Figure 4. The spectra of the (La0.9K0.1CoO3)x/nmCeO2 oxides of low perovskite loading show strong absorption bands below 600 cm-1 assigned to the Ce-O lattice vibration of cubic fluorite structure in CeO2.29 For BO6

octahedron in the perovskite phase, which has A-site cation in their clearance, there is a repeatable structure unit of ABO3 crystalline structure. There are six kinds of vibrations in their IR spectra, and the stretching vibration (v3) is IR inactive if three pairs of B-O bonds have the same length, that is, BO6 octahedron having high symmetry. On the contrary, the B-O stretching vibration v3 is IR active if the symmetry of BO6 is low. For high loading samples (m g 50), there are three vibration bands at 421 cm-1, 560 cm-1, and 602 cm-1, in the IR spectra of all (La0.9K0.1CoO3)x/nmCeO2 samples. The vibration band at 421 cm-1 belongs to the bending vibration of Co-O bonding in the BO6 octahedron, and the bands at 560 cm-1 and 602 cm-1 can be assigned to two kinds of Co-O bond stretching vibration in the BO6 octahedron.27 These results further prove that the oxides of (La0.9K0.1CoO3)x/nmCeO2 possessed ABO3 perovskitetype structures. The IR results are very consistent with the XRD analysis. However, it is noted that all of the three peaks at 421, 560, and 602 cm-1 are weak. It may be due to the fact that the nanocomposite (La0.9K0.1CoO3)x/nmCeO2 supported oxide has been produced, and the characteristic IR peaks of perovskite become weak. The in situ DRIFT spectra for soot combustion over the typical (La0.9K0.1CoO3)20/nmCeO2 supported catalyst under 5%

17118

J. Phys. Chem. C, Vol. 113, No. 39, 2009

Liu et al.

Figure 6. O2-TPD curves of (La0.9K0.1CoO3)x/nmCeO2 catalysts. (x ) 1, 4, 10, 20, 50,100) (2) (La0.9K0.1CoO3)4/nmCeO2, (3) (La0.9K0.1CoO3)10/ nmCeO2, (4) (La0.9K0.1CoO3)20/nmCeO2, (5) (La0.9K0.1CoO3)50/nmCeO2, and (6) (La0.9K0.1CoO3)100/nmCeO2.

Figure 5. The in situ DRIFT spectra of the catalytic combustion of soot NOx over (La0.9K0.1CoO3)20/nmCeO2 catalysts at different temperatures. (a) at O2/He atmosphere, (b) at (NO + O2)/He atmosphere.

O2/He atmosphere as a function of temperature are shown in Figure 5a. The DRIFT spectra contain two main absorption bands located at around 1608 cm-1 and 1258 cm-1. The absorption band of 1608 cm-1 is caused by aromatic stretching vibrations of the soot, which are enhanced by polar functional groups like quinone.23,30 The other absorption band can be assigned to ether-like oxygen complexes formed on the soot surface.23,31 Furthermore, the two absorption bands slowly disappeared when the reaction temperature reached 400 °C. Two possible reasons may explain this experimental phenomenon. One reason may be that the reaction has completed and soot has been completely oxidized; thus, these surface oxygencontaining complexes (SOC) disappeared. Another reason may be due to the low stability of the SOC at high temperatures; that is, it was easily changed to CO2. Another important absorption peak located at about 2354 cm-1 is due to the vibration of CO2 molecule. When reaction temperature was less than 300 °C, CO2 concentration in the reaction gas was low; thus, the intensity of IR peak was also low. With the increase of reaction temperature, CO2 concentration was also increased. When reaction temperature exceeded 400 °C, though the reaction has completed and soot has been completely oxidized, a lot of CO2 was still in the reaction gas instead of being replaced because of the low reaction gas flow velocity. Another possible reason was that some amounts of CO2 were still adsorbed in the surface of catalysts. Thus, the intensity of CO2 IR peak was still very high. Figure 5b shows the in situ DRIFT spectra for soot combustion over (La0.9K0.1CoO3)20/nmCeO2 supported catalyst under

(2000 ppm NO + 5% O2)/He atmosphere as a function of temperature. The absorption bands at 1608 cm-1 and 1258 cm-1 are still assigned to oxygen-containing complexes formed on the soot surface. The absorption peak of CO2 was located at about 2356 cm-1, and its IR peak intensity was larger than that for soot combustion under 5% O2/He atmosphere over the same (La0.9K0.1CoO3)20/nmCeO2 supported catalyst. It may be indicated that the CO2 concentration became high and the soot combustion was accelerated under (2000 ppm NO + 5% O2)/ He atmosphere. The other main absorption bands located at 949, 1010, 1180-1238, 1435, and 1540 cm-1 indicate the presence of the adsorbed species which can be assigned to various kinds of nitrites and nitrates. A strong absorption band at 1180-1238 cm-1 can be assigned to the bridging bidentate nitrite, and the two absorption peaks located at 949 and 1010 cm-1 are attributed to the symmetric stretching vibration of bidentate nitrates. The weak peak located at about 1435 cm-1 is ascribed to the symmetric stretching vibration of monodentate nitrates (νsymNO2), and a weak absorption peak at 1540 cm-1 is due to νNdO stretching vibration of chelating bidentate nitrates.32-37 It is noted that the IR absorption peak positions of NOx are basically the same at different temperatures. However, their relative adsorption amounts, that is, the IR peak intensities, are different. With the increase of reaction temperature, the IR peak intensities also increase, indicating that NOx adsorption is enhanced. It may influence on the catalytic combustion of soot. 3.6. Temperature-Programmed Desorption of Oxygen (O2-TPD). Typical O2-TPD traces are shown in Figure 6. For low loading amount samples, they desorbed very low amounts of oxygen, and the desorption peaks can hardly be observed as indicated in Figure 6. For all high loading samples of (La0.9K0.1CoO3)x/nmCeO2, the first smaller peaks of oxygen between 200 and 500 °C were observed. The most energetically labile oxygen desorbed at lower temperature can most likely be related to simple surface chemisorption. The oxygen species associated with these signals are generally denoted by R oxygen, and they are oxygen species adsorbed on the surface oxygen vacancies.38 But the second peak was observed only above 800 °C. The larger quantity desorbed at higher temperature may be due to the mobilization of bulk oxygen associated with the defective structure of the main (most abundant) phase of a given composition. This signal is generally denoted by β oxygen, and it is associated with the lattice oxygen or with oxygen species occupying the inner vacancies created by substitution of K for

Catalytic Combustion of Soot

Figure 7. H2-TPR curves of (La0.9K0.1CoO3)x/nmCeO2 catalysts. (m ) 1, 4, 10, 20, 50,100) (1) (La0.9K0.1CoO3)1/nmCeO2, (2) (La0.9K0.1CoO3)4/ nmCeO2, (3) (La0.9K0.1CoO3)10/nmCeO2, (4) (La0.9K0.1CoO3)20/nmCeO2, (5) (La0.9K0.1CoO3)50/nmCeO2, and (6) (La0.9K0.1CoO3)100/nmCeO2.

Figure 8. Outlet NO and NO2 concentration plots for NO adsorption on (La0.9K0.1CoO3)20/nmCeO2 at 350 °C.

La. In the case of (La0.9K0.1CoO3)20/nmCeO2, the relatively large quantity of oxygen desorbed at higher temperatures is due, in part, to partial removal of lattice oxygen of La0.9K0.1CoO3 perovskite. 3.7. Temperature-Programmed Reduction of Hydrogen (H2TPR). Figure 7 shows the results of H2-TPR for (La0.9K0.1CoO3)x/ nmCeO2 supported oxide catalysts with different loading amounts of perovskite. As it can be observed, there is no appreciable difference in the reduction temperatures of the different samples below 200 °C, whereas a decrease in the reduction peak temperature and an increase in the amount of hydrogen consumed should be noticed with the increase of perovskite loading amount until x ) 20. Then, the amount of hydrogen consumption is still constantly increased, but the reduction peak temperature is slowly shifted in high temperature direction. These results agree with those obtained by O2-TPD, which indicated the amount of reducible species increases as the perovskite loading amount increases, whereas the (La0.9K0.1CoO3)20/nmCeO2 catalyst has the strongest redox ability because of its lowest reduction peak temperature. 3.8. Results of NO Adsorption and Oxidation on (La0.9K0.1CoO3)20/nmCeO2 Catalyst. The adsorption profile of NO on (La0.9K0.1CoO3)20/nmCeO2 supported oxide catalyst is presented in Figure 8. The NO concentration rapidly decreases at the beginning because of the NO adsorption on the sample and rapidly gains the equilibrium, only three minutes, attributing to the NO adsorption saturation. However, the equilibrium concentration of NO adsorption does not obtain the initial

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17119

Figure 9. Outlet NO2 ion current curves for reaction gas going through soot, nmCeO2, and (La0.9K0.1CoO3)20/nmCeO2.

concentration because of NO transferring to NO2. It would be beneficial to promoting soot oxidation. Figure 9 exhibits the NO oxidation profile on (La0.9K0.1CoO3)20/ nmCeO2 supported oxide catalyst. For comparison, the NO2 concentration profiles of NO oxidation on nmCeO2 and the case without catalyst are also displayed on the figure.8 As shown in Figure 9, NO2 produced on (La0.9K0.1CoO3)20/nmCeO2 supported oxide catalyst is clearly detected by the means of mass spectroscopic measurement. Almost no NO2 was detected in the case of no catalyst when the combustion temperature was less than 600 °C, as shown in plot a. However, when nmCeO2 was present, the NO2 concentration remarkably increased, as shown in plot b. The NO2 concentration was largest on a typical catalyst (La0.9K0.1CoO3)20/nmCeO2, as shown in plot c. 3.9. Catalytic Activity. The combustion peak temperature (Tm) of soot combustion was used as a primary measure of catalytic activity. The selectivity to CO2 formation (SCO2) was defined as the CO2 outlet concentration (CCO2) divided by the sum of the CO2 and CO outlet concentrations; that is, SCO2 ) CCO2/ (CCO + CCO2). SCO2m was denoted as the SCO2 at the maximum temperature at which soot-burnt rate was the highest. Under the investigated conditions, CO2 was the main reaction product and the amount of CO was very little, indicating that the selectivities of (La0.9K0.1CoO3)x/nmCeO2 supported oxide catalysts were very high as shown in Table 1. Catalytic activities of the nmCeO2 supports and La0.9K0.1CoO3 perovskite toward the total oxidation of soot are shown in ref 8 and ref 27, respectively. Pure perovskite was more active than that of the nmCeO2 support. As shown in Table 1, the deposition of La0.9K0.1CoO3 on nmCeO2 brings about a lower Tm by comparing with pure support and bare perovskite. The presence of perovskite in the samples promotes the oxidation activity, and the optimal performance is obtained in a medium loading amount samples (x ) 20). In accordance, the Tm temperatures are further lowered by about 40 °C for (La0.9K0.1CoO3)20/ nmCeO2 supported oxide compared with corresponding La0.9K0.1CoO3 catalysts. Catalytic behavior is described by a “parabola shape” light-off curve for (La0.9K0.1CoO3)x/nmCeO2 supported oxide catalysts, as shown in Figure 10. Since the supported catalysts and bare support possessed comparable surface area, other intrinsic factors seem to be responsible for their different catalytic behaviors. 4. Discussion 4.1. Nanocomposite (La0.9K0.1CoO3)x/nmCeO2 Supported Catalysts for Soot Combustion. Supported oxide catalysts are a very important class of industrial catalysts that are closely

17120

J. Phys. Chem. C, Vol. 113, No. 39, 2009

Figure 10. Curves of CO2 concentration for soot combustion over (La0.9K0.1CoO3)x/nmCeO2 catalysts under loose contact conditions between the catalysts and the soot (x ) 1, 4, 10, 20, 50,100). (1) (La0.9K0.1CoO3)1/nmCeO2, (1) (La0.9K0.1CoO3)1/nmCeO2, (2) (La0.9K0.1CoO3)4/ nmCeO2, (3) (La0.9K0.1CoO3)10/nmCeO2, (4) (La0.9K0.1CoO3)20/nmCeO2, (5) (La0.9K0.1CoO3)50/nmCeO2, (6) (La0.9K0.1CoO3)100/nmCeO2.

related to many key technologies in chemical industries and in environmental protection. Often in this kind of catalyst system, discrete active component nanocrystals are dispersed on support particles that are one to several orders of magnitude larger than the active component nanoparticles. The most important function of a support is to provide proper texture/pore structure and high surface area to disperse and maintain the active component function. When the particle sizes of an oxide support are reduced to such an extent that they become comparable to that of the active component particles, the oxide may deviate dramatically from its function as a conventional catalyst support. Such catalyst with size-comparable nanocrystal active component and nanometer oxide support may be better called nanocomposite oxides rather than “supported-oxide” catalysts.39-41 This raises the possibility to tune the catalytic behavior of supported oxide catalysts by reducing the particle size of support to make nanocomposite catalysts. In contrast to conventional supported oxide, the stable (La0.9K0.1CoO3)x/nmCeO2 catalysts appear as nanocomposites of comparably sized perovskite active component and nanometer CeO2 support as shown by SEM photographs in Figure 2. Perovskite oxide supported on conventional oxide supports are not able to avoid sintering, and thus, they easily deactivate rapidly under the same conditions of the high temperature combustion reaction. Attempts in reducing the particle sizes of support oxides to approach active-component/support nanocomposites have also resulted in significant performance improvement of Au catalysts “supported” on TiO2 and ZrO2.40,41 In the present work, the nanocomposite (La0.9K0.1CoO3)x/nmCeO2 supported catalyst for the soot combustion reaction was tested. It was proved that the nanocomposite catalyst is a highly active catalyst for soot oxidation reaction. It is believed that the oxygen transfer ability was significantly enhanced by reducing the particle size of CeO2 or by the formation of nanocomposites. This is not a surprise because a much higher percentage of perovskite/oxide boundaries exist in the nanocomposite system. When sizes of the support particles become even smaller, the perovskite/support catalyst forms nanocomposites that could show distinctive catalytic performances. This may have an important implication on the design and preparation of advanced supported oxide catalysts for many key technologies. Since the percentage of (La0.9K0.1CoO3)x/ nmCeO2 boundary or perimeter is much higher in the nanocomposites than that in the conventional systems, it is not

Liu et al. surprising that nmCeO2 in the nanocomposite catalysts can have higher redox ability. The high percentage of oxide-support interface may also be responsible for the higher reactivity of soot combustion on the nanocomposite catalysts in the TPO experiments because some amounts of carbon might locate near the perimeter and can easily be attacked by activated oxygen on the perimeter. 4.2. Oxygen Species and Their Roles for the Catalytic Combustion of Soot. In oxidation reactions, the oxygen vacancies play an important role in oxidation reactions since they are responsible for the adsorption-desorption properties of the gas phase and they facilitate the diffusion of lattice oxygen from the bulk to the surface. Both cases would lead to the enhancement of the catalytic activity. In the elucidation of the predominant role of the oxygen vacancies in the catalytic activity, studies of O2-TPD and H2-TPR can be valuable techniques. In fact, the O2-TPD results shown in Figure 6 reveal very interesting information. A first observation indicates that the perovskite loading amount in the catalysts modifies the oxygen desorption curves, where new signals appear. According to the higher desorption signal intensity, adsorption properties of the catalyst toward oxygen would be enhanced when the perovskite loading amount is increased. By taking into account the temperature at which the different signals appear, the nature and reactivity of the different released oxygen species can be elucidated. The possible peak below 150 °C that is assigned to physisorbed oxygen species is not observed in all of the samples because of helium clearing. These species would hardly participate in the soot catalytic oxidation, since they are released at temperatures lower than those at which the reaction starts. More relevant to the catalytic activity can be the oxygen species releasing at about the temperature range from 230 to 500 °C, since the adsorbed oxygen species that can be desorbed under relatively low temperatures are able to participate in the oxidation reactions. These species are called R oxygen, and they are oxygen species such as O- or O2- adsorbed on surface oxygen vacancies.8,33 Thus, the intensity of this desorption signal can indicate the surface oxygen vacancy amount, and the amount of oxygen vacancies increases when the perovskite loading amount is increased. It is well-known that, in the case of all solids, the composition of the surface layer is usually different from the composition of the bulk. This is of great importance, since the catalytic properties of the solid are determined by the surface nature. Therefore, the presence of oxygen vacancies on the surface can be different from that in the bulk. In fact, an interesting characteristic of the O2-TPD curves is that the intensity of the R oxygen desorption signal increases when the perovskite loading amount increases. The last oxygen desorption signal (above 700 °C) is called β oxygen, and it is generally ascribed to lattice oxygen O2-.12,33 A higher intensity of the β oxygen signal indicates a higher mobility of lattice oxygen when the perovskite loading amount increases. Up to the moment, there is disagreement over whether these species derive from the inner oxygen vacancies in the bulk or if they are directly associated with the B-site cation reduction in the perovskite oxide framework. The reduction process with hydrogen undoubtedly involves lattice oxygen species of the perovskite oxide and causes the reduction of the B-site cation (cobalt). Thus, studies of H2-TPR (as shown in Figure 7) could shed light on the lattice oxygen reactivity. The most relevant features of the perovskite loading amount effect on the catalyst reducibility are the onset temperatures of the reduction profiles. These temperatures remarkably decrease when the perovskite loading amount increases, suggesting that the reduction is facilitated. The

Catalytic Combustion of Soot (La0.9K0.1CoO3)20/nmCeO2 catalyst has the lowest reduction peak temperature indicating its strongest redox ability. These results are in good agreement with the desorption signal assigned to β oxygen species in the O2-TPD curves, where the signal intensity increases linearly with the perovskite loading amount. However, it cannot be ensured that the β oxygen signal in O2-TPD corresponds to the same oxygen species involved in the reduction with hydrogen because it is not exactly certain that the β oxygen release is accomplished by the cobalt reduction. The β oxygen could be oxygen species resulting from inner oxygen vacancies in the bulk. The participation of β oxygen species in the oxidation reaction would be unexpected because of the release of these species at relatively high temperatures. However, it must be considered that the oxygen adsorption capacity of perovskite-type oxides changes under different surrounding atmospheres (O2 mixed with NO or another preadsorbed gas). In fact, because of the stronger reduction ability of H2 compared with that of He, during the reduction with hydrogen, the lattice oxygen activity starts at 400-550 °C (depending on the perovskite loading amount in the catalyst), whereas in the O2-TPD studies, they become active at temperatures higher than 700 °C. The lattice oxygen can become surface electrophilic oxygen as an intermediate in the oxygen transfer from the lattice to the gas phase during the process of solid dissociation or in the course of its reduction. Thus, the β oxygen species could play an important role under reaction conditions. However, this should not be taken as an indication that it is the nucleophilic lattice oxygen that is participating in the reaction.42,43 It is noted that both R oxygen and β oxygen may originate from active component La0.9K0.1CoO3 perovskite and nanometer CeO2 support due to its large oxygen storage capacity via the redox process Ce4+ T Ce3+. Some relevant hints about which oxygen species are participating in the reaction can be directly revealed by catalytic activity results. It has been found that the soot combustion temperature decreases when a small amount of perovskite is loaded on the catalysts, but the soot combustion temperature increases markedly for higher x. Both the desorption signal intensity of β oxygen species (lattice oxygen) and the reduction temperature in the H2-TPR profiles vary linearly with the perovskite loading amount. In contrast, the desorption signal intensity of R oxygen species (mainly that below 400 °C assigned to oxygen species adsorbed on the oxygen vacancies) follows the order shown for catalytic activity. The close parallel relationship between catalytic activity for soot combustion and the extent of oxygen adsorption indicates that adsorbed oxygen is the dominant oxygen species participating in this reaction. In general, oxygen species might have a very important affect on the catalytic combustion of soot. 4.3. Reaction Mechanism. Elucidation of the mechanism of catalytic oxidation reactions of carbonaceous materials has been the subject of many researches over the past decades.22-24 It was reported that noncatalytic oxidation yielded surface oxygen complexes (SOC), which played an essential role in the oxidation mechanism. These complexes were formed in the presence of different oxidizing agents, such as NO or O2. Researches involved in the catalytic oxidation of graphite, chars, and/or carbon black by metal oxides usually described a redox mechanism to explain experimental phenomena: in the first step, the catalyst was reduced by carbon and followed by reoxidation of the oxide by molecular oxygen. Apparently, the formation and subsequent decomposition of SOC has only been discussed in mechanistic studies of potassium- and calcium-catalyzed carbon oxidation.44 In all previous studies, because of the deep black color of soot, usually KBr was used as a diluent in IR

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17121 spectroscopic studies to obtain high quality in situ spectra of carbonaceous materials. However, the interactions between SOC, metal oxides, or both and KBr could not be excluded, especially when analyses were performed at high temperatures. When black perovskite-type oxides supported on nanometer CeO2 support, due to the light color of CeO2, and it can substitute KBr and be used as a diluent; thus, the relative high quality in situ IR spectra of soot oxidation over supported-perovskite catalysts were obtained. According to the results of in situ IR spectra and NO in situ adsorption and oxidation, the molecular-level understanding of soot combustion over (La0.9K0.1CoO3)x/nmCeO2 supported oxides can be obtained in this study. As shown in Figure 5, the two broad bands could be identified in the in situ DRIFT spectra, located at 1258 and 1608 cm-1. The 1258 cm-1 absorption band can be assigned to ether-like oxygen complexes formed on the soot surface. The other absorption peak at 1608 cm-1 is caused by aromatic stretching vibrations of the soot, which are enhanced by polar functional groups like quinine.28-30 The formation of these SOC was observed upon the catalytic oxidation of soot over (La0.9K0.1CoO3)x/ nmCeO2 supported oxide catalysts. Furthermore, as shown in Figure 5b, NO was adsorbed on the catalyst surface as various kinds of nitrites and nitrates. The nitrates can decompose and release NO2 to the gas phase or the NO is directly oxidized to NO2 in the gas phase. Then, NO2 may act as the oxidizing agent for soot combustion. The strong promotion role of NO2 for the soot oxidation has been demonstrated by many researches.3-5,8 For example, Oi-Uchisawa et al.3,4 reported that a platinum catalyst could oxidize NO to NO2, which subsequently oxidize soot to CO and CO2. So, NO2 is used as intermediate to facilitate an indirect contact between the platinum catalyst and soot. This mechanism subtly changed solid (soot)-solid (catalyst) contact into solid (soot)-gas (NO2)-solid (catalyst) contact. Thus, the platinum catalyst system obtained the best results so far reported for soot combustion under loose contact conditions. The high oxidation rate of soot is due to the strong oxidizing ability of NO2. A similar mechanism is found for soot combustion over (La0.9K0.1CoO3)x/nmCeO2 supported oxide catalysts. The formation of NO2 may be one of the important factors that (La0.9K0.1CoO3)x/nmCeO2 supported oxide catalysts are very highly active for the soot oxidation. The obtained DRIFT spectra (Figure 5) of adsorbed species are in a good agreement with the results described in the literature.32-37 The strong adsorption peaks only observed for the mixed oxides correlate well with high NOx storage capacity of the mixed oxides compared with that of the individual oxides. The identification of the adsorbed species proves that NOx is stored in the form of nitrite/nitrates on the catalyst surface, for which NO must be partially oxidized to NO2 in a preceding step. This also explains why NO2 can be released from the catalyst at higher temperatures. The NO adsorption experiment on (La0.9K0.1CoO3)x/nmCeO2 supported oxide catalyst (Figure 8) shows that the NO uptake drops rapidly, and almost full saturation is achieved within less than 3 min. It is suggestive of NO being stored only on the surface and not transferred to the bulk in a slow diffusion process. The storage of NOx on the surface and not in the bulk of the catalyst would be further beneficial to the production of NO2.45 In order to provide some insights into the NOx promotion function for the soot combustion, NO oxidation experiments were carried out. The NO2 concentration in the outlet gas mixture is plotted as a function of temperature in Figure 9. It is important to note that the NO2 level in the gas stream is very little at the entrance of the reactor, with NO being the main component of the NOx binary. NO

17122

J. Phys. Chem. C, Vol. 113, No. 39, 2009

Liu et al.

Figure 11. Scheme of the reaction mechanism for the catalytic combustion of soot over (La0.9K0.1CoO3)x/nmCeO2 supported-perovskite oxide catalysts.

can be oxidized to NO2 by O2 according to the reaction NO + 1/2 O2 f NO2. The predicted NO2 level considering the thermodynamic equilibrium of this reaction was described in literature.46 As shown in Figure 9, soot exhibited no activity for NO2 production, and the obtained NO2 level with soot is the same as that measured in the initial time of the reaction. In comparison, nmCeO2 was quite effective for NO oxidation to NO2 from 300 °C. The NO2 profile obtained with this catalyst increased with temperature until the thermodynamic equilibrium of the above equation was fulfilled, and then decreased at higher temperatures following thermodynamics. The highest NO2 concentration was reached at 450 °C. Figure 9 further shows that the NO2 concentration was about four times as much on (La0.9K0.1CoO3)20/nmCeO2 catalyst as that on nmCeO2 oxide. At the same time, the peak temperature of the production of NO2 on (La0.9K0.1CoO3)x/nmCeO2 supported oxide catalyst is about 360 °C, which is much lower than that on nmCeO2. In fact, the stronger oxidation ability of (La0.9K0.1CoO3)x/nmCeO2 supported oxides for NO oxidation is consistent with the results of H2-TPR as shown in Figure 7. Thus, (La0.9K0.1CoO3)x/ nmCeO2 supported oxide catalyst should have a higher excellent catalytic activity for soot combustion. As shown in Figure 8 and Figure 9, the direct evidence of NO2 produced is detected by the means of in situ NO adsorption and mass spectroscopic measurement. It was found that NO is oxidized to NO2 over the catalyst, which is stored on the catalyst as nitrite/nitrate. The nitrates can decompose and release NO2 to the gas phase or the NO is directly oxidized to NO2 in the gas phase. Then, NO2 acts as the oxidizing agent for soot combustion. The nitrate storage capacity of (La0.9K0.1CoO3)20/nmCeO2 is much higher than that of the bare nmCeO2 oxide resulting in a major contribution of the produced NO2 to the soot oxidation process. It explains the highly catalytic activity of (La0.9K0.1CoO3)20/ nmCeO2 on the soot combustion. In summary, three reasons can lead to the highly catalytic activities of (La0.9K0.1CoO3)x/nmCeO2 for soot combustion. First of all, the catalysis nature of soot combustion is a kind of deep oxidation process. The amounts of active oxygen species and redox property of the catalyst determines its intrinsic activity. Both nanometric CeO2 support and La0.9K0.1CoO3 perovskite active component possess strong redox ability, and the active oxygen species could be generated by (La0.9K0.1CoO3)x/nmCeO2 supported-perovskite oxides, which would be beneficial to enhancing the redox ability of the catalyst. Therefore, the intrinsic activities of (La0.9K0.1CoO3)x/nmCeO2 supported oxide catalysts are very good. Second, the contact between catalyst and soot is a necessary external condition which is significantly important for the catalyst to play the role of catalysis. The good contact between the catalyst and the soot can be obtained by synthesizing nanocomposite (La0.9K0.1CoO3)x/nmCeO2 catalysts. The nanocomposite (La0.9K0.1CoO3)x/nmCeO2 could provide more contact points between catalyst and soot particles and

promote the contact between them. It is because the surface particle sizes of the nanoparticle catalyst are small. Surface atoms of nanoparticle catalysts have extra and high surface energies and they are good at mobility. Third, similar to noble metal Pt catalysts, (La0.9K0.1CoO3)x/nmCeO2 supported oxide can efficiently catalyze the NO to NO2. The oxidizing ability of NO2 is much stronger than that of O2, and it can directly oxidize the soot particle to CO2 and ensure the good indirect contact performances between catalysts and soot particles. Thus, the oxidation temperature of soot becomes lower even under loose contact conditions. Therefore, the catalytic activities of (La0.9K0.1CoO3)x/nmCeO2 supported oxide catalysts prepared in this study are very high for soot combustion under loose contact conditions between the soot and the catalyst. The best catalytic activity was obtained over (La0.9K0.1CoO3)20/nmCeO2 supported oxide catalyst that Tm was 354 °C, and its catalytic activity for the combustion of soot particle is as good as supported Pt catalysts, which is the best catalyst system so far reported for soot combustion under loose contact conditions. On the basis of in situ characterization results and the above discussion, the reaction mechanism for soot combustion over (La0.9K0.1CoO3)x/nmCeO2 supported oxides can be proposed. There are three reaction pathways during soot combustion: (1) The active components of perovskite oxides adsorb and activate molecular oxygen, release reactive surface oxygen species, and then produce reactive surface oxygen complex intermediate SOC by transferring to the surface of soot and releasing out CO2 (or CO), followed by the soot being oxidized. (2) The nanometric CeO2 support activates molecular oxygen or releases lattice oxygen atoms, transfers reactive oxygen species to the surface of catalysts to attack soot surface, and then forms SOC and oxidizes soot. (3) NO adsorb over the catalyst surface by forming surface nitrate species and then releases NO2 to promote soot oxidation. The whole reaction process is described in Figure 11. 5. Conclusions (1) Nanocomposite (La0.9K0.1CoO3)x/nmCeO2 supported oxide catalysts are investigated on the catalytic combustion of soot. High catalytic activities, even exceeding the bulk La0.9K0.1CoO3, were obtained by depositing La0.9K0.1CoO3 perovskite oxide on nanometer CeO2 carrier. The best catalytic activity was obtained over (La0.9K0.1CoO3)20/nmCeO2 supported oxide catalyst (Tm ) 354 °C), and its catalytic activity for the combustion of soot particle is as good as that of supported Pt catalysts, which is the best catalyst system so far reported for soot combustion under loose contact conditions. (2) In soot oxidation reactions, the predominant roles of the oxygen species in the catalytic activity were observed in studies by the O2-TPD and H2-TPR methods. The oxygen vacancies play an important role in oxidation reactions since they are

Catalytic Combustion of Soot responsible for the adsorption-desorption properties of the gas phase and they facilitate the diffusion of lattice oxygen from the bulk to the surface. It would lead to the enhancement of the catalytic activity. Both surface R oxygen and lattice β oxygen may be originated from La0.9K0.1CoO3 perovskite and nanometer CeO2 support. During the catalytic combustion of soot, the surface R oxygen species should play a major role. (3) The reaction mechanisms for soot combustion over (La0.9K0.1CoO3)x/nmCeO2 supported oxides were studied by means of in situ DRIFT spectroscopy and other characterization techniques. The results demonstrate that the formation of surface oxygen-containing complexes (SOC) may be one of the key intermediate species, and NO is adsorbed over the catalyst surface by forming surface nitrate or nitrite species and then released as NO2 to promote soot oxidation. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20803093, 20833011, and 20525621), the Doctor select Foundation for the University of State Education Ministry (No. 200804251016), and the 863 program of China (No. 2006AA06Z346). References and Notes (1) Labhsetwar, N.; Biniwale, R. B.; Kumar, R.; Rayalu, S.; Devotta, S. Catal. SurVeys from Asia 2006, 10, 55–64. (2) Liu, J.; Zhao, Z.; Xu, C.; Duan, A.; Zhu, L.; Wang, X. Appl. Catal., B 2005, 61, 36–46. (3) Oi-Uchisawa, J.; Wang, S.; Nanba, T.; Ohi, A.; Obuchi, A. Appl. Catal., B 2003, 44, 207–215. (4) Oi-Uchisawa, J. A.; Obuchi, Wang, S.; Nanba, T.; Ohi, A. Appl. Catal., B 2003, 43, 117–129. (5) Hinot, K.; Burtscher, H.; Weber, A. P.; Kasper, G. Appl. Catal., B 2007, 71, 271–278. (6) Uner, D.; Demirkol, M. K.; Dernaika, B. Appl. Catal., B 2005, 61, 334–345. (7) Liu, J.; Zhao, Z.; Liang, P.; Xu, C.; Duan, A.; Jiang, G.; Lin, W.; Wachs, I. E. Catal. Lett. 2008, 120, 148–153. (8) Liu, J.; Zhao, Z.; Xu, C.; Duan, A.; Jiang, G.; Yang, Q. Appl. Catal., B 2008, 84, 185–195. (9) Palma, V.; Russo, P.; Matarazzo, G.; Ciambelli, P. Appl. Catal., B 2007, 70, 254–260. (10) Shangguan, W. F.; Teraoka, Y.; Kagawa, S. Appl. Catal., B 1997, 12, 237–247. (11) Teraoka, Y.; Nakano, K.; Kagawa, S. Appl. Catal., B 2001, 34, 73–78. (12) Liu, J.; Zhao, Z.; Xu, C.; Duan, A. Appl. Catal., B 2008, 78, 61– 72. (13) Hong, S. S.; Lee, G. D. Catal. Today 2000, 63, 397–404. (14) Biamino, S.; Fino, P.; Fino, D.; Russo, N.; Badini, C. Appl. Catal., B 2005, 61, 297–305. (15) Klvana, D.; Kirchnerova, J.; Chaouki, J.; Delval, J.; Yaıc¨i, W. Catal. Today 1999, 47, 115–121.

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17123 (16) Gallagher, P. K.; Johnson, D. W., Jr.; Schrey, F. Mater. Res. Bull. 1974, 9, 1345–1352. (17) Alifanti, M.; Florea, M.; Paˆrvulescu, V. I. Appl. Catal., B 2007, 70, 400–405. (18) Alifanti, M.; Florea, M.; Somacescu, S.; Paˆrvulescu, V. I. Appl. Catal., B 2005, 60, 33–39. (19) Alifanti, M.; Blangenois, N.; Florea, M.; Delmon, B. Appl. Catal., A 2005, 280, 255–265. (20) Peter, S. D.; Garbowski, E.; Perrichon, V.; Primet, M. Catal. Lett. 2000, 70, 27–33. (21) Shangguan, W. F.; Teraoka, Y.; Kagawa, S. Appl. Catal., B 1998, 16, 149–154. (22) Shangguan, W. F.; Teraoka, Y.; Kagawa, S. Appl. Catal., B 1996, 8, 217–227. (23) Mul, G.; Kapteijn, F.; Moulijn, J. A. Appl. Catal., B 1997, 12, 33– 47. (24) Minogue, N.; Riordan, E.; Sodeau, J. R. J. Phys. Chem. A 2003, 107, 4436–4444. (25) Liu, J.; Zhao, Z.; Xu, C.; Duan, A.; Wang, L.; Zhang, S. Catal. Commun. 2007, 8, 220–224. (26) Neeft, J. P. A.; Michiel, M.; Jacob, A. Appl. Catal., B 1996, 8, 57–78. (27) Wang, H.; Zhao, Z.; Liang, P.; Xu, C.; Duan, A.; Jiang, G.; Xu, J.; Liu, J. Catal. Lett. 2008, 124, 91–99. (28) Tang, C.; Kuo, C.; Kuo, M.; Wang, C.; Chien, S. Appl. Catal., A 2006, 309, 37–43. (29) Bumajdad, A.; Zaki, M. I.; Eastoe, J.; Pasupulety, L. Langmuir 2004, 20, 11223–11233. (30) Morterra, C.; Low, M. J. D. Spectrosc. Lett. 1982, 15, 689–697. (31) Mul, G.; Neeft, J. P. A.; Kapteijn, F.; Moulijn, J. A. Carbon 1998, 36, 1269–1276. (32) Fino, D.; Fino, P.; Saracco, G.; Specchia, V. Appl. Catal., B 2003, 43, 243–259. (33) Liu, J.; Zhao, Z.; Xu, C.; Duan, A.; Jiang, G. J. Phys. Chem. C 2008, 112, 5930–5941. (34) Yu, J. J.; Jiang, Z.; Zhu, L.; Hao, Z. P.; Xu, Z. P. J. Phys. Chem. B 2006, 110, 4291–4300. (35) Prinetto, F.; Ghiotti, G.; Nova, I.; Lietti, L.; Tronconi, E.; Forzatti, P. J. Phys. Chem. B 2001, 105, 12732–12745. (36) Sedlmair, C.; Seshan, K.; Jentys, A.; Lercher, J. A. J. Catal. 2003, 214, 308–316. (37) Nova, I.; Castoldi, L.; Lietti, L.; Tronconi, E.; Forzatti, P.; Prinetto, F.; Ghiotti, G. J. Catal. 2004, 222, 377–388. (38) Zhao, Z.; Yang, X.; Wu, Y. Appl. Catal., B 1996, 8, 281–297. (39) Zhang, Q.; Li, Y.; Xu, B. Catal. Today 2004, 98, 601–605. (40) Xu, B.; Wei, J.; Yu, Y.; Li, Y.; Li, J.; Zhu, Q. J. Phys. Chem. B 2003, 107, 5203–5208. (41) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301–309. (42) Merino, N. A.; Barbero, B. P.; Grange, P.; Cadus, L. E. J. Catal. 2005, 231, 232–244. (43) Kremenic˘, G.; Nieto, J. M. L.; Tasco´n, J. M. D. J. Chem. Soc., Faraday I 1985, 81, 939–950. (44) Cerfontain, M. B.; Moulijn, J. A. Fuel 1983, 62, 256–258. (45) Tikhomirov, K.; Kro´cher, O.; Elsener, M.; Wokaun, A. Appl. Catal., B 2006, 64, 72–78. (46) Atribak, I.; Such-Basa´n˜ez, I.; Bueno-Lo´pez, A.; Garcı´a, A. J. Catal. 2007, 250, 75–84.

JP9056303