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La1xKxCoO3 and LaCo1yFeyO3 Perovskite Oxides: Preparation, Characterization, and Catalytic Performance in the Simultaneous Removal of NOx and Diesel Soot Feng Bin, Chonglin Song,* Gang Lv, Jinou Song, Cairong Gong, and Qifei Huang State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China ABSTRACT: Perovskite-type La1xKxCoO3 and LaCo1yFeyO3 catalysts were prepared and characterized by nitrogen sorption, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. Catalytic activity for the simultaneous removal of NOx and soot was investigated using temperatureprogrammed reactions. For the La1xKxCoO3 series, the introduction of K ions into the A-site caused the enhancement of Co valence state, which was beneficial to improving the catalytic activity. Excess K ions produced a Co3O4 phase adhering to the perovskite crystals, but the rhombohedral perovskite structure was well-maintained. In contrast, the B-site could be substituted by Fe ions with the doping ratio changing from null to 0.5, and no secondary phases were detected. With increasing K substitution, NOx conversion in the La1xKxCoO3 series showed a declining trend after an initial ascent. The Co3O4 particles produced at high K content were responsible for this falling catalytic activity. For the LaCo1yFeyO3 series, catalytic performances showed a monotonously decreasing trend as a function of Fe substitution. Among all of the perovskite oxides tested in this study, the La0.6K0.4CoO3 sample exhibited the highest catalytic activity for the simultaneous removal of NOx and soot.
1. INTRODUCTION In recent years, diesel engines have been extensively employed in automotive systems and suffer from high levels of exhaust NOx and carbon soot. The reduction of such pollutants to regulated levels cannot be accomplished solely by engine modification, and great efforts have been devoted to the development of catalytic after-treatment processes.1 In widespread use for the reduction of soot emission is the wall-flow filter, where soot is trapped and burned, owing to the presence of a catalyst deposited on the filter. NOx is eliminated with another converter, either by reaction with suitable reducing agents or by direct decomposition. These catalytic technologies have been successfully applied worldwide. However, the varying measures adopted in diesel engine treatment will inevitably lead to an increase in fabrication cost and exhaust backpressure.2,3 Ever since Yoshida et al.4 brought forward the NOxO2soot reaction mechanism in the early 1970s, perovskite structured metal oxides have been thought capable of promoting the simultaneous conversion of both NOx and soot into N2 and CO2, respectively, in a single catalytic trap.58 A perovskite-type oxide has an ABO3 crystal structure, in which large ionic radius cations are 12 coordinate to oxygen atoms and occupy A sites, and smaller ionic radius cations are 6 coordinate and occupy B sites. In general, the A site is usually occupied by a lanthanide ion, usually La, and the B site occupied by a transition metal ion of the d-transition series.9 For an unsubstituted perovskite, the catalytic activity is predominantly attributable to the metal ion at the B site. The A site metal has a strong effect on stability, and provides the possibility to improve catalyst performance through synergetic interactions with metals at the B site.10 Partial substitution at the A and/or B site with alternative A0 and B0 metal ions results in the composition A1xA0 xB1yB0 yO3. This substitution can induce structural modifications related to the generation of r 2011 American Chemical Society
crystal lattice defects and oxygen vacancies, thus allowing the tailoring of thermal stability and catalytic performance.1113 Considerable attention has recently been given to the factors affecting the catalytic performance of NOx and soot reduction, such as catalyst surface area, catalyst redox properties, and metal ion substitutions at A and/or B sites in perovskite oxides. Libby et al.14 and Voorhoeve et al.15 reported that perovskite catalysts with Co or Mn at the B site possessed high catalytic activity in purifying automotive exhaust. This result was supported by Hong et al.,16 whose study indicated that for La0.6Cs0.4BO3 (B = Co, Mn, Fe) catalysts, the ignition temperature of particulate carbon slightly decreased according to the order: Co > Mn > Fe, while almost the same NOx conversions were obtained. Shimokawa et al.17 presented evidence that for La0.8K0.2MnO3, surface area was an important factor controlling catalytic performance for diesel particulate combustion. The improvement of contact points between the catalyst and particulate matter was found to be critical for a high performance catalyst. Teraoka et al.13 showed that the activity and N2 selectivity of the La1xKxMnO3 series depended significantly on K content, and oxides with intermediate K contents (x = 0.20.25) exhibited the highest activity and selectivity. Peng et al.18,19 synthesized and evaluated the porous La1xKxMn1yCuyO3 oxides and found that the highest NO conversion efficiency to N2 was 91%, and the lowest soot ignition temperature was 260 °C for La0.8K0.2Mn0.95Cu0.05O3. These studies showed that a significant reduction in NOx and soot could be achieved with perovskite catalysts. Among all the Received: January 27, 2011 Accepted: April 20, 2011 Revised: April 15, 2011 Published: April 20, 2011 6660
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catalysts studied, LaCoO3 oxide was of particular interest because cobalt possessed excellent catalytic activity10,20 and the ability to form well-ordered perovskite crystallites even under highly substituted conditions.21 In this study, series of La1xKxCoO3 and LaCo1yFeyO3 perovskite catalysts were synthesized and characterized by nitrogen sorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The challenge was to find a catalyst which could decrease the combustion temperature of carbon soot and enhance NOx conversion by investigating the influence of K and Fe content on catalytic activity. Our results will provide a helpful estimation for the design of a commercial perovskite catalyst for diesel exhaust purification.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Series of La1xKxCoO3 and LaCo1yFeyO3 perovskite oxides were synthesized according to the citric acid method as described previously.22,23 Briefly, nitrate salts in desired stoichiometric ratio were dissolved in deionized water. Citric acid monohydrate was then added in 100 wt % excess to ensure metal ion complexation. The resulting solutions were evaporated to dryness at 90 °C with vigorous stirring until the spongy materials were formed. The obtained precursors were calcined at a suitable temperature for 3 h. Catalyst chemical composition was tested by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) to verify that experimental values were in agreement with theoretical values. 2.2. Catalyst Characterization. The BrunauerEmmett Teller (BET) specific surface area of the perovskite catalysts was derived from the corresponding nitrogen adsorption isotherm obtained at 196 °C, from a Quantachrome NOVA-2000 analyzer. Samples were degassed at 300 °C prior to adsorption measurements. Crystalline phases were measured by powder XRD using a Rigaku D/MAC/max 2500v/pc instrument with Cu KR radiation (40 kV, 200 mA, λ = 1.5418 Å). The lattice parameters of samples were analyzed by Jade 5.5 software. The diffractometer data were acquired with a step scan of 0.02° for 2θ values between 10° and 80°. The morphology and phase purity of samples were confirmed by SEM (Philips XL-30) and TEM (JEM-2010HR) coupled with an energy-dispersive X-ray spectrometer (EDX, Dxford-1NCA). Prior to TEM imaging, powders was sonicated in ethanol for 30 min and, then, deposited and dried on copper grids. XPS was performed on a PHI-1600 ESCA spectrometer with an Mg KR (hv = 1253.6 eV, 1 eV = 1.603 1019 J) X-ray source. The binding energies were calibrated using the C1s peak of contaminant carbon (BE = 284.6 eV) as an internal standard. 2.3. Activity Testing. Catalytic activity tests were undertaken with a temperature-programmed reaction (TPR) operated at atmospheric pressure. The surrogate for diesel soot in this experiment was a model carbon particle (Printex-U, Degussa AG). The properties of this model soot were similar to those of real particulate diesel soot.24 The average particle size, specific surface area, and ignition temperature were 25 nm, 100 m2 g1, and >300 °C, respectively. The catalyst (100 mg) and model carbon (10 mg) were mixed with a spatula and, then, were placed in the center of a cylindrical quartz tube reactor (10 mm internal diameter). To ensure the same degree of contact between air and carbon, inert SiO2 (100 mg) was added to the model carbon (10 mg) for the noncatalyzed combustion experiments. The feed
Figure 1. XRD patterns showing the effect of calcination temperature on La0.8K0.2CoO3 samples.
gas, containing 800 ppm NO þ 10 vol % of O2, was employed with He as the balance gas. The gas flow rate was set to 200 mL min1. During each TPR run, the reaction temperature range was 25700 °C with a heating rate of 2 °C min1. The outlet concentrations of NOx, CO, and CO2 were recorded by an online infrared gas analyzer.
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. XRD analysis was used to examine the catalyst phase segregation, and temperature windows where the perovskite structure was formed.25 Typical XRD patterns of La0.8K0.2CoO3 sample prepared by heating the corresponding precursors at different temperatures are illustrated in Figure 1. The La0.8K0.2CoO3 sample calcined at 450 °C did not form the perovskite structure, perhaps because impurities in the precursor were not completely eliminated by the pyrolysis process, but a few La2CoO4 crystal phases could be detected (PDF 34-1081). Well-crystallized perovskite phases was detected after calcination at 750 °C, and diffraction peaks were well resolved (2θ = 23°, 33°, 41°, 59°, 69°, PDF 48-0123). The loss of main diffractions at higher calcination temperature (1150 °C) confirmed the perovskite structure destruction, accompanied with the occurrence of La2O3 (PDF 05-0602) and Co3O4 (PDF 42-1467) phases. Thus calcination temperature for all of the precursors was fixed at 750 °C. Figure 2 shows typical XRD patterns of La1xKxCoO3 and LaCo1yFeyO3 catalysts calcined at 750 °C, with different K or Fe loadings. All La1xKxCoO3 catalysts predominantly presented the rhombohedral distortion of the ideal perovskite octahedral, characterized by the split of the principal reflection in the range 2θ = 3233°. K substitution led to a drastic decrease in the intensity of principal diffractions, and a slight shift toward smaller 2θ angles corresponds to lattice expansion. Intense peaks attributed to the LaCoO3 perovskite structure were identified without trace of any other compound when x increased from 0 to 0.2. A Co3O4 spinel appeared in low quantities (2θ = 19°, 31°, 36°, 44°, 55°, PDF 42-1467) at x = 0.3. With further increase in K substitution ratio, the sharpening and narrowing of Co3O4 diffractions indicated a gradual increase in the Co3O4 crystallite size. This suggested that the single perovskite structure was well retained up to a substitution level of x = 0.3. A site substitution for perovskite oxides often plays an important role in catalytic 6661
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Table 1. Crystal struCtures and BET Specific Surface Area Data for La1xKxCoO3 and LaCo1yFeyO3 Perovskite-type Oxide Catalysts BET specific surface area (m2/g)
lattice parameter (Å)
catalyst samples
fresh
used
a=b
c
LaCoO3
11.4
10.5
5.4479
13.1044
La0.9K0.1CoO3
12.9
11.4
5.4500
13.1136
La0.8K0.2CoO3
14.6
12.8
5.6184
14.6464
La0.7K0.3CoO3
17.1
15.2
5.6118
14.6787
La0.6K0.4CoO3 La0.5K0.5CoO3
18.5 18.4
18.1 17.3
6.0129 6.0213
13.8898 13.9088
LaCo0.9Fe0.1O3
10.5
9.7
5.4478
13.1226
LaCo0.8Fe0.2O3
17.4
16.6
5.4555
13.1741
LaCo0.7Fe0.3O3
12.2
10.1
5.4568
13.2311
LaCo0.6Fe0.4O3
16.2
14.3
5.4729
13.2023
LaCo0.5Fe0.5O3
13.7
11.8
5.4594
13.4858
Figure 3. SEM images for catalysts calcined at 750 °C with different K or Fe contents: (a) La0.9K0.1CoO3; (b) La0.6K0.4CoO3; (c) LaCo0.9Fe0.1O3; (d) LaCo0.7Fe0.3O3.
Figure 2. Typical XRD patterns of (a) La1xKxCoO3 and (b) LaCo1yFeyO3 catalysts, calcined at 750 °C with different K or Fe contents.
performance. To enhance catalytic activity, La substitution by Kþ ions was expected to maintain a pure well-defined perovskite structure rather than a phase mixture. The La3þ and Kþ ionic radii are 0.106 and 0.133 nm, respectively.26 Because the ionic diameter of Kþ was greater than that of La3þ, substitution with Kþ inevitably induced structural distortion, leading to a separate
Co3O4 phase. This distortion was also confirmed by the variation in unit cell parameters. As shown in Table 1, pure LaCoO3 has a rhombohedral structure with hexagonal cell parameters a = b = 5.4479 Å and c = 13.1044 Å. “a” and “b” increased with K doping, while the “c” parameter reached a maximum of 14.6787 Å at x = 0.3. Due to Fe3þ and Co3þ having similar ionic radii,26 LaCo1yFeyO3 oxides exhibited only a single perovskite-type structure regardless of Fe substitution level, as shown in Figure 2. The most intense diffraction for LaCoO3 in the range 2θ = 3233° was a doublet, and this pattern was indexed using a rhombohedral unit cell. According to the cell parameters listed in Table 1, it was obvious that the rhombohedral structure was conserved over the range of Fe substitution studied. The BET specific surface areas (SSAs) of the La1xKxCoO3 and LaCo1yFeyO3 catalysts, measured before and after catalytic testing, are also shown in Table 1. All samples had relatively low SSAs due to the high calcination temperature. SSAs of fresh 6662
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Figure 4. TEM images for catalysts calcined at 750 °C with different K or Fe contents: (a) La0.9K0.1CoO3; (b) La0.6K0.4CoO3; (c) LaCo0.9Fe0.1O3; (d) LaCo0.7Fe0.3O3. EDX quantitative analysis of the corresponding marked areas.
La1xKxCoO3 catalysts increased monotonously from 11.4 to 18.4 m2 g1, with increasing K substitution. Those of LaCo1yFeyO3 varied over the range of 10.517.4 m2 g1 with Fe substitution and exhibited a maximum of 17.4 m2/g at y = 0.2. After catalytic testing, SSAs of all catalysts were found to have decreased slightly. SEM images (Figure 3) show that La1xKxCoO3 and LaCo1yFeyO3 mainly consisted of spherical primary particles with a grain radius in the range of 150200 nm. These primary particles had condensed to form agglomerates. All samples were studied in detail by transmission electron microscopy. From Figure 4a, it could be seen that the spheres were fine and homogeneous for low K content. However, a few of impurities appeared at higher K content. This inhomogeneity was subsequently investigated by quantitative EDX measurements. As shown in Figure 4b, the large dark regions, which consisted of La, K, Co, O, and Cu (Cu peaks arose from the supporting
copper grid), were ascribed to perovskite particles. This indicated that the perovskite-type structure was maintained. Some gray small spots adhering to the perovskite crystals had a composition of solely Co and O rich phases and were identified as cobalt oxides. Their appearance was not surprising and was in good agreement with data estimated by the XRD analysis. Figure 4c and d clearly show that typical LaCo1yFeyO3 samples were well crystallized. 3.2. XPS Analysis. The chemical states and surface proportions of elements for the catalysts were characterized by XPS. The XPS survey scan (Figure 5) showed that carbon, lanthanum, cobalt, oxygen, potassium (only for La1xKxCoO3) and iron (only for LaCo1yFeyO3) were present on the catalyst surfaces. The binding energies of La 3d5/2, K 2p, Co 2p3/2, O 1s, and Fe 2p3/2 core levels obtained from spectral deconvolution are summarized in Table 2. The La 3d5/2 doublet contained components at 837.2 and 833.6 eV and indicated that lanthanum ions 6663
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were present in the trivalent form regardless of K substitution at A sites or Fe substitution at B sites. All La1xKxCoO3 samples showed binding energies of 292.4 eV for the K 2p level, which was characteristic of Kþ ions located at exchange positions. Figure 6a illustrates high resolution spectra of the Co 2p XPS region for LaCoO3. The peak at 779.4 eV arose from Co 2p3/2, and the doublet with maxima at 794.5 and 796.5 eV was due to Co 2p1/2. The distorted Co 2p3/2 peak confirmed the presence of at least two cobalt valence states. The peak binding energy for Co 2p3/2 was similar to that for Co3þ in other reported studies.2729 It is generally accepted that Co3þ is located at the B-site of LaCoO3 frameworks. The small difference of binding energy and band broadening of the Co 2p3/2 signal rendered the position of this band alone unreliable for unambiguously detecting Co2þ.
Figure 5. XPS survey scan spectra of LaCoO3, La0.7K0.3CoO3, and LaCo0.8Fe0.2O3.
Despite this, a satellite peak at around 787.5 ( 0.5 eV confirmed that Co2þ was present.30 With increasing K content, slight chemical shifts of the Co 2p3/2 peak to higher binding energies were probably attributable to the slight enhancement of valence state.31 Curve-fitting procedures were then applied to the O 1s region, as shown in Figure 6b. The peak at 528.3 eV corresponded to regular lattice oxygen (OL, metal oxygen bond), while that at 531.7 eV was assigned to adsorbed oxygen species (OA), such as O, O2, and O22. For La1xKxCoO3 samples, the ratio of adsorbed oxygen area to lattice oxygen area (OA/OL) increased monotonously with K content, as shown in Table 2. It was probable that oxygen vacancies had been created with K-substitution, and similar results were reported by other researchers.32 For LaCo1yFeyO3, the Co 2p3/2 signal was similar to that of LaCoO3, with a valence of þ3 state from a simple consideration of solid-state chemistry. The binding energy of Fe 2p3/2 was 710.2 ( 0.2 eV for all samples, which suggested that Fe ions existed mainly in the trivalent valence state. As the Co Auger peak partially overlapped it, the Fe 2p3/2 peak was difficult to deconvolute into different oxidation states, and hence, Fe4þ could not be distinguished from the Fe 2p XPS signal. The Fe/Co ratios agreed largely with the stoichiometric values at all levels of Fe content. Neither distinct shifts of Co 2p3/2 signal nor obvious changes in OA/OL ratio were apparent for LaCo1yFeyO3 series. 3.3. Simultaneous Catalytic Removal of NOx and Soot. Catalytic activities of La1xKxCoO3 and LaCo1yFeyO3 for the simultaneous removal of NOx and soot were evaluated in terms of Tig (the ignition temperature of soot oxidation, defined by extrapolating the steeply ascending portion of the CO2 concentration), Tm (the temperature at maximum CO2 concentration), XNO (the maximum NO conversion), and SCO2
Table 2. XPS Binding Energies of Photoelectron Peaks in La1xKxCoO3 and LaCo1yFeyO3 Samples binding energies of main peaks (eV) samples
Fe 2p3/2
Co 2p3/2
K 2p
LaCoO3
833.6 837.2
0
779.4
528.3 531.7
0.78
La0.9K0.1CoO3
834.0
292.3
779.3
528.6
0.92
837.3 La0.8K0.2CoO3
834.1 834.0
Fe/Co (atomic %)
531.9 292.6
779.5
837.3 La0.7K0.3CoO3
O 1s
OA/OL
La 3d5/2
528.7
1.25
531.9 292.4
779.5
837.6
528.9
1.57
531.3
La0.6K0.4CoO3
834.2 837.5
292.4
779.7
528.6 531.6
1.89
La0.5K0.5CoO3
834.2
292.5
779.8
528.6
2.29
837.3 LaCo0.9Fe0.1O3
833.6
531.5 710.4
779.3
837.2 LaCo0.8Fe0.2O3
834.1
710.2
779.2
837.1 834.2 837.4
710.1
779.3
LaCo0.6Fe0.4O3
834.2
710.4
779.1
837.5 833.4
0.1
0.73
0.31
0.72
528.6 531.7
0.39
0.84
528.3
0.74
1.04
0.96
0.75
528.4 531.4
LaCo0.7Fe0.3O3
LaCo0.5Fe0.5O3
528.3 531.6
531.6 710.3
779.2
837.6
528.4 531.7
6664
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Figure 6. High resolution XPS spectra of LaCoO3: (a) Co 2p and (b) O 1s spectral regions.
(the selectivity of CO2, defined as the maximum of CO2 outlet concentration divided by the sum of CO and CO2 outlet concentrations). Figures 7a and b show that the formation of CO and CO2, and reduction of NOx for noncatalyzed and catalyzed reaction conditions took place simultaneously when using a feed gas of 800 ppm NO þ 10 vol % O2. As a consequence of soot consumption at higher temperature, a sharp CO2 decrease and NOx concentration then rapidly rising back to the original level were observed. CO was only formed in a tiny amount when LaCoO3 was present, while high concentrations of up to 0.27 vol % CO were detected without a catalyst present. The overall molar ratio of CO2/CO was approximately 4.2 and 9.1 in the absence and presence of the catalyst, respectively. A small portion of NO2 was formed from the fed NO and O2 according to the NO þ 1/2O2 f NO2 equilibrium.33 The NO2 concentration decreased to null during soot combustion. This indicated that the oxidation reactivity of NO2 toward soot is much higher than that of NO, and thus NO2 oxidizes the soot more rapidly to produce N2 and CO2.3436 Catalytic performances of the materials for the simultaneous removal of NOx and soot are given in Table 3. Considerable differences were observed between noncatalyzed and catalyzed reactions for the reduction of NOx and soot. The TPR results showed that Tig, Tm, and XNO for noncatalyzed soot oxidation were 480 and 565 °C and 10.9%, respectively. All perovskite catalysts with more than 90% CO2 selectivity achieved higher
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Figure 7. Outlet concentrations of CO, CO2, NO, and NO2 (a) in the noncatalyzed NOxsootO2 reaction and (b) in the NOxsootO2 reaction using LaCoO3 as the catalyst. The gas feed was flow rate = 200 mL/min, NO = 800 ppm þ O2 = 10 vol % with He to balance.
Table 3. Catalytic Activities Tig (°C)
Tm (°C)
XNO (%)
SCO2 (%)
LaCoO3
316
406
31.7
90.1
La0.9K0.1CoO3
304
401
35.1
93.3
La0.8K0.2CoO3
292
397
38.9
95.7
La0.7K0.3CoO3
286
389
41.0
94.4
La0.6K0.4CoO3
283
382
41.6
96.2
La0.5K0.5CoO3
288
391
40.2
94.6
LaCo0.9Fe0.1O3 LaCo0.8Fe0.2O3
309 315
409 405
32.8 33.5
93.7 94.2
LaCo0.7Fe0.3O3
323
417
33.1
94.6
LaCo0.6Fe0.4O3
320
412
32.4
95.9
LaCo0.5Fe0.5O3
317
416
32.5
95.1
Co3O4
308
414
31.6
92.7
no catalyst
480
565
10.9
80.8
samples
catalytic activities than noncatalyzed SiO2. Tig and Tm of those decreased to 283323 and 382417 °C, respectively, and XNO increased by over 30%. It was also found that the activity of the La1xKxCoO3 series was enhanced with increasing K loading, but this increase diminished with K loading greater than 0.4. Tig, Tm, and SCO2 changed from 316 and 406 °C and 31.7% at x = 0 to 6665
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Industrial & Engineering Chemistry Research 283 and 382 °C and 41.6% at x = 0.4, respectively. The presence of Co3O4 crystallites at x g 0.4 was linked to the decrease in catalytic activity because pure Co3O4 (SSA = 17.6 m2/g) had a lower catalytic activity than each of the La1xKxCoO3 series at the similar SSA level. The catalytic performance of the LaCo1yFeyO3 series showed a monotonously decreasing trend as a function of Fe substitution. XPS results indicated that for La1xKxCoO3 samples, the Co ions mainly existed with a valence of þ3. When K ions entered the perovskite structure, charge compensation took place through the production of lattice oxygen vacancies and enhancement of Co valence state to achieve electrical neutrality. The defect reaction can be described by the Kr€ogerVink notation: 1 2CoCo • þ OO x S 2CoCo x þ V O •• þ O2 ð v Þ 2 where the subscripts Co and O denote their respective crystallographic sites and the superscripts • and x denote net positive and net null charges, respectively. VO represents a vacancy at the oxygen site. Both variable valence cobalt ions and adjacent oxygen vacancies were active centers for the simultaneous reduction of NOx and carbon soot. NO and adsorbed molecular oxygen enabled the reaction with carbon soot, accompanied by the valence transition of Co ions.37 No redox reaction could take place at the A site, because both La and K have only one stable valence state, i.e. þ3 and þ1, respectively. No secondary phase was detected upon varying Fe content at the B site, due to the similarity in ionic radii between Fe and Co. However, the presence of Fe in the perovskite structure decreased the catalytic activity. Co3þ ions have six electrons in the unfilled 3d shell. The transfer of d electrons from high-spin Co3þ (t2g4eg2) to adjacent low-spin CoIII (t2g6eg0), results in the formation of equal numbers of n- and p-type carriers.38 Fe3þ (3d5) contains five spins aligned in a half-filled 3d shell (t2g3eg2) high-spin configuration. Their low-lying level suggests that Fe3þ ions have extra stability. Thus, substitution of Fe for Co may have partially caused the decrease of catalytic activity. The decrease was probably also attributable to the strong interaction between Co and Fe. At low Fe-doping ratios, the Co3þCoIII couple played an important role in catalytic performance.39 With increasing Fe content, the Fe4þFe3þ couple gradually prevailed over the Co3þCoIII couple and consequently inhibited the simultaneous removal of NOx and soot.
4. CONCLUSION A series of perovskite-type La1xKxCoO3 and LaCo1yFeyO3 catalysts have been synthesized, characterized, and evaluated for their catalytic performance in the simultaneous removal of NOx and soot. The citric acid method was used for the preparation of the perovskite catalysts calcined at 750 °C and made it possible to form the perovskite structure. All samples had relatively low specific surface areas due to the high calcination temperature. After catalytic testing, surface areas decreased slightly. For the La1xKxCoO3 series, K ions doped into A sites caused a partial Co valence state enhancement. The rhombohedral perovskite structure could be well-maintained with K loadings increased to x = 0.6, but Co3O4 particles produced at high K content (x > 0.4) had a decreased catalytic performance. Variable valence of cobalt ions and adjacent oxygen vacancies are active centers for the soot catalytic activity. For LaCo1yFeyO3 catalysts, B site substitution by Fe ions was achieved at any substitution degree due to
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similarities in ionic radii of Fe and Co. Catalytic performances of LaCo1yFeyO3 catalysts showed a monotonously decreasing trend with Fe content. In this study, the most promising catalyst for the simultaneous removal of NOx and diesel soot was the partially substituted perovskite La0.6K0.4CoO3, with which the temperature of soot combustion dropped to 283 °C and NOx conversion reached 41.6%.
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