Ind. Eng. Chem. Res. 2008, 47, 8175–8183
8175
Strontium-Doped Lanthanum Cobaltite and Manganite: Highly Active Catalysts for Toluene Complete Oxidation Jiguang Deng,† Lei Zhang,† Hongxing Dai,*,† Hong He,† and C. T. Au*,‡ Laboratory of Catalysis Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of EnVironmental and Energy Engineering, Beijing UniVersity of Technology, Beijing 100124, China, and Department of Chemistry, Centre for Surface Analysis and Research, Hong Kong Baptist UniVersity, Kowloon Tong, Hong Kong, China
La1-xSrxMO3-δ (M ) Co, Mn; x ) 0, 0.4) catalysts have been fabricated via a strategy of citric acid complexing and hydrothermal treatment. The oxidation of toluene was used as a probe reaction for the evaluation of catalytic performance. The materials were characterized by a number of techniques. We observed that the catalytic activity (evaluated by the temperature required for 80% conversion of toluene, T80%) increased in the sequence of LaMnO3.10 (T80% ) 295 °C) < LaCoO2.89 (T80% ) 246 °C) < La0.6Sr0.4MnO3.03 (T80% ) 233 °C) < La0.6Sr0.4CoO2.76 (T80% ) 219 °C) at toluene/O2 molar ratio ) 1/400 and space velocity ) 20 000 h-1. Moreover, CO2 and H2O were the only products for toluene oxidation over the catalysts. It is concluded that factors such as enriched structural defects (oxygen vacancies) and good Mn4+/Mn3+ or Co3+/Co2+ redox ability are responsible for the excellent catalytic performance of the La1-xSrxMO3-δ nanomaterials. 1. Introduction Volatile organic compounds (VOCs) are pollutants detrimental to human health. From an economical viewpoint, control of VOCs emission by means of catalytic combustion is better done at low temperatures.1-6 The efficiency of such a technology is determined by the activity and stability of the catalyst. Various kinds of substances such as supported noble metals and base metal oxides have been investigated for the catalytic combustion of VOCs. Despite that the former are highly active at relatively low temperatures, their application is limited due to the high price of precious metals and problems related to sintering and volatility. Among the base metal oxides, perovskite-type oxides (ABO3) are promising for the complete oxidation of hydrocarbons and oxygenates.7-9 It has been shown that the La1-xSrxMO3-δ (M ) Co, Mn; x ) 0-0.4) perovskites with the La at the A-sites partially substituted by Sr exhibited wonderful catalytic activity.10-16 The good performance has been related to factors such as high surface area, richness of lattice defects, good redox ability of the M ions, and the presence of oxygen adspecies.17,18 Toluene is commonly adopted as a representative of VOCs because it is one that is hard to eliminate. Numerous supported single or mixed metal oxides19 as well as Pt, Pd, or Au20-22 catalysts have been applied for its complete oxidation. Over a Mn0.67-Cu0.33 mixed oxide catalyst, complete oxidation of toluene (toluene/O2 molar ratio ) 1/26) took place at ca. 220 °C.19 At a toluene/O2 molar ratio of 7/100, the total oxidation of toluene occurred above 400 °C over Au/Fe2O320 and at 360 °C over Au/CeO2.21 As compared to Pd/WO3 and Pd/MgO, Pd loaded on Al2O3, SiO2, SnO2, ZrO2, and Nb2O5 were found to be catalytically more active, among which the Pd/ZrO2 catalyst exhibited the highest activity but toluene conversion (toluene/ O2 molar ratio ) 1/21) was below 95% even at a temperature as high as 527 °C.22 Recently, several research groups conducted * To whom correspondence should be addressed. Tel.: +86-10-67396588. Fax: +86-10-6739-1983. E-mail:
[email protected] (H.D.). Tel.: +852-3411-7067. Fax: +852-3411-7348. E-mail:
[email protected] (C.T.A.). † Beijing University of Technology. ‡ Hong Kong Baptist University.
investigations on the combustion of various VOCs over Co- or Mn-based perovskite-type oxide catalysts9,12,23,24 and ascribed the enhancement of catalytic performance to the large surface area and high defect concentration. The Co-based perovksite catalysts are observed as active as the Mn-based perovskite ones.10-12 The partial substitution of La by Sr could significantly improve the catalytic performance of LaMO3-δ (M ) Co, Mn); for example, the temperature required for complete reactant conversion decreases by 30-105 °C.12,15,16,24 Over a La0.8Sr0.2MnO3-δ (δ < 0) catalyst, toluene conversion could reach 99% at 235 °C.24 Huang et al.16 reported that toluene conversion could be above 99% at 235 °C over La0.8Sr0.2CoO3, whereas 340 °C was needed for 99% toluene conversion over LaCoO3. Barnard et al. reported that the sequence of the reaction rate over the bulk perovskites was parallel to that of the BET surface area of these catalysts, especially in the complete oxidation of hydrocarbons and oxygenates.25 There are a number of methods for the preparation of perovskites, most of which adopt a hightemperature calcination process necessary for perovskite phase formation, and thus the solid materials generated are low in surface area. Although loading ABO3 onto porous Al2O3 or SiO2 could enhance its surface area by forming highly dispersed domains,26-29 the supports were found reactive toward the Aand B-site elements, giving rise to formation of catalytically inert structures (e.g., spinels).30 Thus, the preparation of highsurface-area perovskite-type oxides is highly desirable. It is generally accepted that the citric acid complexing method is an effective way to generate materials of high surface area. In the past years, we examined perovskite-type and perovskite-like oxide as well as their halo-oxide catalysts for the removal of carbon monoxide and automotive exhaust,31-33 and the oxidative dehydrogenation of ethane;34 we found that most of them performed well. Recently, we have prepared several series of perovskite-type oxides with high surface areas using a novel strategy, a combination of citric acid complexing and hydrothermal processing. The so-resulted materials showed excellent catalytic activity in the total oxidation of ethylacetate.35 In this Article, we report the preparation, characterization, and catalytic properties of the La1-xSrxMO3-δ (M ) Co, Mn; x ) 0, 0.4) nanomaterials for toluene complete oxidation.
10.1021/ie800585x CCC: $40.75 2008 American Chemical Society Published on Web 10/08/2008
8176 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
2. Experimental Section 2.1. Catalyst Preparation. The La1-xSrxMO3-δ (M ) Co, Mn; x ) 0, 0.4) perovskite-type oxides were prepared using a modified strategy of citric acid complexing coupled with hydrothermal treatment,35 in which the amount of citric acid added, the hydrothermal temperature, and the subsequent treatment conditions were modified. In a typical preparation procedure, lanthanum nitrate (0.01 mol) and cobalt (or manganese) nitrate (0.01 mol) for LaCoO3-δ (or LaMnO3-δ), lanthanum nitrate (0.006 mol), strontium nitrate (0.004 mol), and cobalt (or manganese) nitrate (0.01 mol) for La0.6Sr0.4CoO3-δ (or La0.6Sr0.4MnO3-δ), and a certain amount of citric acid (metal/ citric acid molar ratio ) 2/1) were mixed in deionized (DI) water under constant stirring for complete complexing of the metal ions. Next, a certain amount of ammonia (28 wt %) solution was added dropwise. With the neutralization of the unreacted citric acid, the solution was adjusted to a pH value of ca. 9.5 at which a sol was formed. The sol was transferred to a 50 mL Teflon-lined stainless steel autoclave with a packed volume of 40 mL, and the autoclave was placed in an oven (for hydrothermal treatment) at 180 °C for 20 h. Without filtering and washing, the obtained precursors were dried at 120 °C for 5 h. The dried materials were well ground and calcined in air at a ramp of 1 °C/min in a muffle furnace to 400 °C and kept at this temperature for 2 h and then heated to 650 °C where they remained at this temperature for 2 h. We also calcined the La0.6Sr0.4CoO3-δ catalyst at 900 °C for 4 h for the purpose of comparing their catalytic performance with the sample derived at 650 °C. Finally, the obtained materials (black in color) were in turn ground, pressed, crushed, and sieved to a size range of 40-60 mesh. 2.2. Catalyst Characterization. Powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a Bruker/ AXS D8 Advance X-ray diffractometer operated at 40 kV and 200 mA using Cu KR radiation and Ni filter. Diffraction peaks of crystal phases were referred to those of 1998 ICDD PDF Database for phase identification. The surface areas of the catalysts were measured via N2 adsorption at -196 °C on an ASAP 2020 apparatus. The surface area measurement error of each sample is less than 1%. X-ray photoelectron spectroscopy (XPS, VG CLAM 4 MCD analyzer) was used to determine the O 1s, Co 2p, Mn 2p, La 3d, and Sr 3d binding energies (BE) of surface oxygen, cobalt, manganese, lanthanum, and strontium species, respectively, with Mg KR (hν ) 1253.6 eV) as the excitation source. The C 1s signal at 284.6 eV was taken as a reference for BE calibration. High-resolution scanning electron microscopy (HRSEM) images of the samples were recorded using JEOL JSM 6500F equipment operated at a 30 kV accelerating voltage. Oxygen temperature-programmed desorption (O2-TPD) was recorded on a mass spectrometer (Hiden HPR20). For O2-TPD studies, the sample (0.1-0.2 g) was placed in the middle of a quartz microreactor (internal diameter (i.d.) ) 8 mm). The outlet gases were analyzed online by means of mass spectrometry. The heating rate was 10 °C/min, and the temperature range was 30-950 °C. Before each run, the sample was calcined at 650 °C in an O2 flow of 20 mL/min for 1 h, followed by cooling in oxygen to room temperature (RT) and purging with helium (first 80 mL/min for 2 h and then 20 mL/min for 2 h). The heliumpurging is to guarantee a thorough removal of gas-phase oxygen in the system. The amount of O2 desorbed from the catalyst was quantified by calibrating the peak area against that of a standard O2 pulse (50.0 µL). Hydrogen temperature-programmed reduction (H2-TPR) was carried out in a quartz fixed-bed
Table 1. Physical Properties of the La1-xSrxMO3-δ Catalysts
catalyst
crystal phase
LaCoO3-δ La0.6Sr0.4CoO3-δ LaMnO3-δ La0.6Sr0.4MnO3-δ
perovskite perovskite perovskite perovskite
Co3+/Co2+ Mn4+/Mn3+ (mol %/ (mol %/ mol %) mol %) 3.57 11.3 0.25 0.86
δ
surface area (m2/g)
0.11 0.24 -0.10 -0.03
29 15 27 19
microreactor (i.d. ) 6 mm) in the RT-900 °C range. The sample (0.1-0.2 g) was treated in an oxygen flow (50 mL/ min) at 500 °C for 1 h and then cooled to RT in the same atmosphere. After being purged in a N2 flow (50 mL/min) for 30 min, the treated sample was reduced in a 5% H2/Ar (balance) flow of 50 mL/min at a ramping rate of 10 °C/min. The effluent was monitored with a thermal conductivity detector (TCD). The thermal conductivity response was calibrated against the reduction of a known CuO powder sample (Aldrich, 99.995%). 2.3. Co and Mn Oxidation State Titration. The content of Mn4+ was determined by digesting the sample in an excess amount (known) of a standard solution of Mohr’s salt (Fe(NH4)2(SO4)2 · 6H2O) acidified with 1 mol/L H2SO4, and then the excess Fe2+ was determined by means of back-titration with 0.0167 mol/L K2Cr2O7 in 3 mol/L HCl.23,36 Before titration, a small amount of H2SO4-H3PO4 solution was added, and 0.5% sodium diphenylamine sulfonate solution was used as indicator. The Co3+ concentration of sample was determined by the iodometric titration method.36 The experimental error was estimated to be (0.10% for Mn and (0.50% for Co analysis. 2.4. Catalytic Activity Measurements. A continuous flow fixed-bed quartz microreactor (i.d. ) 8 mm) was employed for the determination of catalyst activity at atmospheric pressure for the complete oxidation of toluene. To minimize the effect of hot spots, quartz sands with weight equal to that (0.1-0.2 g) of the catalyst sample was used to dilute the perovskite particles (40-60 mesh) by means of thorough mixing. The total flow rate of the reactant mixture (consisting of 1000 ppm toluene + O2 + N2 (balance)) was 33.3 mL/min, giving a toluene/O2 molar ratio of 1/400 and a space velocity of 20 000 h-1. The 1000 ppm toluene was generated by passing a N2 flow through a bottle containing pure toluene (AR grade) chilled in an ice-water isothermal bath. For the changes of space velocity and toluene/oxygen molar ratio, we altered the amount of catalyst and mass flow of oxygen, respectively. Reactants and products were analyzed online by a gas chromatograph (GC14, Shimadzu) equipped with a flame ionization detector (FID) and a TCD, using a 1/8 in Chromosorb 101 column (3 m in length) for VOCs and a 1/8 in Carboxen 1000 column (3 m in length) for permanent gas separation. The balance of carbon throughout the investigation was estimated to be 99.5%. 3. Results 3.1. Crystal Structures, Oxygen Defects, Surface Areas, and Morphologies of Catalysts. The crystal structure, composition, oxygen nonstoichiometry, and surface area of the La1-xSrxMO3-δ catalysts are summarized in Table 1, and their XRD patterns are shown in Figure 1. By referring to the standard data of LaCoO3 (no. 75-0279) and LaMnO3.15 (no. 75-0440) in the ICDD PDF Database, one can see that the position and relative intensities of XRD peaks are consistent with those of the respective standard samples. The peaks can clearly be indexed as indicated in Figure 1b and d. The results disclose that the La1-xSrxMO3-δ (M ) Co, Mn; x ) 0, 0.4) catalysts mainly crystallize into the perovskite structure, although the presence of single oxides (La2O3) in low amount cannot be completely excluded. On the basis of the
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8177
Figure 1. XRD patterns of (a) LaCoO2.89, (b) La0.6Sr0.4CoO2.76, (c) LaMnO3.10, and (d) La0.6Sr0.4MnO3.03.
Figure 3. (A) O 1s and (B) Co 2p3/2 XPS spectra of (a) LaCoO2.89 and (b) La0.6Sr0.4CoO2.76; (C) O 1s and (D) Mn 2p3/2 XPS spectra of (c) LaMnO3.10 and (d) La0.6Sr0.4MnO3.03.
Figure 2. HRSEM images of (a) LaCoO2.89, (b) La0.6Sr0.4CoO2.76, (c) LaMnO3.10, and (d) La0.6Sr0.4MnO3.03.
assumption of electroneutrality and the content of cobalt or manganese ions of different oxidation states, we estimated the amount of oxygen nonstoichiometry (δ): 0.11 for LaCoO3-δ, 0.24 for La0.6Sr0.4CoO3-δ, -0.10 for LaMnO3-δ, and -0.03 for La0.6Sr0.4MnO3-δ. Obviously, there are oxygen vacancies (δ > 0) in La1-xSrxCoO3-δ,10 and overstoichiometric oxygen (δ < 0) in La1-xSrxMnO3-δ.37,38 It can also be seen from Table 1 that the surface areas of the four perovskite catalysts are relatively high, ranging from 15 to 29 m2/g. The doping of Sr induces a significant drop in surface area. Shown in Figure 2 are the representative HRSEM images of the La1-xSrxMO3-δ catalysts. The particles of LaCoO2.89 (Figure 2a) and La0.6Sr0.4CoO2.76 (Figure 2b) are relatively uniform in size and show a sphere-like morphology; the typical diameter is in the range of 30-140 nm. The LaMnO3.10 sample (Figure 2c) displays a sphere-like morphology with the diameter of 60-120 nm. With the doping of Sr2+, the so-obtained La0.6Sr0.4MnO3.03 particles (Figure 2d) aggregate significantly. 3.2. Surface Oxygen Species, Co and Mn Oxidation States, and Reducibility of Catalysts. XPS is a powerful tool to investigate the surface properties of solid materials. The
binding energies of La3d5/2 and Sr3d5/2 were 835.8 and 134.3 eV, respectively. It means that the La and Sr existed in tri- and divalency, respectively. Figure 3 shows the O 1s, Co 2p3/2, and Mn 2p3/2 spectra of La1-xSrxMO3-δ (M ) Co, Mn; x ) 0, 0.4). The asymmetrical XPS peaks were deconvoluted by the curvefitting approach. There are three O 1s signals at BE ) 528.9-529.0, 531.1, and 532.8-533.1 eV. The peak at BE ) 528.9-529.0 eV could be assigned to surface lattice oxygen (O2-latt), whereas that at BE ) 531.1 eV was assigned to adsorbed oxygen (O-, O2-, or O22-) species. The component at BE ) 532.8-533.1 eV is commonly ascribed to adsorbed H2O and/or surface carbonate.39-41 For the Co-based catalysts (Figure 3A), after Sr2+ doping, the intensity of the signal at BE ) 531.1 eV increases, whereas that at BE ) 529.0 eV decreases, indicating enhancement in the amount of oxygen adspecies after the partial substitution of Sr2+ for La3+. It is confirmed by the surface Oads/O2-latt ratio (Table 2) that increases from 0.77 to 1.05 after the doping of Sr2+ to the LaCoO3-δ lattice. On the other hand, the doping of Sr2+ to the LaMnO3-δ lattice causes the relative intensity of the signal at BE ) 531.1 eV to decrease (Figure 3C(d)), indicating a drop in the amount of oxygen adspecies. The significant decrease in the surface Oads/O2-latt ratio from 0.83 to 0.48 (Table 2) also supports such a deduction. Obviously, among the four catalysts investigated in the present study, the La0.6Sr0.4CoO2.76 one exhibits the biggest surface Oads/O2-latt ratio (1.05), an indication of the highest oxygen vacancy density. From Figure 3B, one can observe a strong Co 2p3/2 asymmetric signal at BE ) 780.4 eV and a weak satellite peak at BE ) 789.2 eV (characteristic of Co2+).42 The former asymmetric peak could be resolved into two components attributable to Co2+ and Co3+, respectively.42 The results suggest that there were Co2+ and Co3+ in the Cobased catalysts. It can also be observed that there is an increase in Co 2p3/2 signal intensity (Figure 3B(b)) after Sr2+ doping, indicative of a rise in Co3+ concentration. This is in agreement with the titration results of Co ions (Table 1). In the Mn 2p3/2 spectra (Figure 3D), the Mn 2p3/2 signal at BE ) 642.2 eV could be resolved into two components at BE ) 641.7 and 642.8 eV, the former due to Mn3+ ions, whereas the latter is due to Mn4+
8178 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 2. XPS-Derived Surface Molar Compositions of the La1-xSrxMO3-δ Catalysts catalyst
La/Co or La/Mn
LaCoO2.89 La0.6Sr0.4CoO2.76 LaMnO3.10 La0.6Sr0.4MnO3.03
0.35 (1.0)a 0.22 (0.6) 0.83 (1.0) 0.87 (0.6)
a
Sr/Co or Sr/Mn
Co/(La+Sr+Co) or Mn/(La+Sr+Mn)
La/Sr
0.28 (0.4)
0.80 (1.5)
0.55(0.4)
1.59 (1.5)
0.74 (0.5) 0.67 (0.5) 0.55(0.5) 0.41 (0.5)
Oads/O2-latt
Co3+/Co2+ or Mn4+/Mn3+
0.77 1.05 0.83 0.48
1.57 2.40 0.34 1.14
The data in parentheses are estimated according to the nominal bulk compositions.
Figure 4. O2-TPD profiles of (a) LaCoO2.89, (b) La0.6Sr0.4CoO2.76, (c) LaMnO3.10, and (d) La0.6Sr0.4MnO3.03.
ions.43,44 Similar to the case of the Co-based catalysts, the incorporation of Sr2+ to the LaMnO3-δ lattice brings about a rise in Mn4+ concentration (Figure 3D(d)). Although the A-site rare earth element can be readily enriched on the ABO3 surface, the enriching of the B-site element in ABO3 was also reported by some researchers. For example, Fierro’s group pointed out that there was an enrichment of Mn on the surface of La1-xSrxMnO3-δ.45 According to the surface compositions calculated on the basis of the XPS results (Table 2), one can find that all of the ratios of the surface La/Co, La/Mn, Sr/Co, Sr/Mn, and La/Sr deviate from the nominal bulk compositions. Furthermore, on the basis of the ratio of surface M/(La+Sr+M) (M ) Co or Mn), one can detect that there is Co enrichment on the surfaces of the Co-based catalysts. For the Mn-based catalysts, however, the surface Mn composition is similar to that of the bulk for LaMnO3.10, whereas there is La and Sr enrichment on the La0.6Sr0.4MnO3.03 surface. Shown in Figure 4 are the O2-TPD profiles of La1-xSrxMO3-δ. One can observe that for the LaCoO2.89 sample (Figure 4a) there is one broad peak at 510 °C, and another peak at 750 °C (with a shoulder at 710 °C), corresponding to an O2 desorption amount of 16.1 and 31.8 µmol/gcat, respectively. After Sr2+ doping to the LaCoO3-δ lattice, there are desorption peaks at 290, 510, and 807 °C (Figure 4b), and the amount of O2 desorption is 75.3, 18.4, and 206.4 µmol/gcat, respectively. For the LaMnO3.10 sample, one small peak at 540 °C appears with an O2 desorption amount of 13.7 µmol/gcat, and there is a big peak starting at 750 °C (Figure 4c). As for the La0.6Sr0.4MnO3.03 sample, there is one small desorption peak at 270 °C, one rather broad peak (onset temperature ) ca. 450 °C) centered at 600 °C, and another peak at 867 °C (Figure 4d), and the O2 desorption amount is 8.4 µmol/gcat for the former and 130.3 µmol/gcat for the latter two. The desorption below 400 °C could be attributed to the desorption of oxygen adspecies located at surface va-
Figure 5. (A) TPR profiles and (B) initial hydrogen consumption rate as a function of the reciprocal of temperature of (a) LaCoO2.89, (b) La0.6Sr0.4CoO2.76, (c) LaMnO3.10, and (d) La0.6Sr0.4MnO3.03.
cancies, that in the range of 400-700 °C to the desorption of surface O2-latt (resulting in partial reduction of Co3+ or Mn4+), and that above 700 °C to the reduction of Co2+ or Mn3+ as well as Mn4+.46 Figure 5A shows the H2-TPR profiles of La1-xSrxMO3-δ. It can be observed that there are two main peaks at 416 and 720 °C (with a shoulder at 620 °C) over LaCoO2.89 (Figure 5A(a)), and the corresponding H2 consumption is 1.61 and 4.01 mmol/ gcat. With the partial replacement of La3+ by Sr2+, the La0.6Sr0.4CoO2.76 sample becomes more reducible, and there are three peaks at 380 °C (with a shoulder at 340 °C), 708 °C (with a shoulder at 624 °C), and 873 °C (Figure 5A(b)), corresponding to a H2 consumption of 1.89 mmol/gcat for the first peak and 4.47 mmol/gcat for the latter two peaks. Over LaMnO3.10, there are two partially overlapped peaks in the 300-500 °C range (Figure 5A(c)) corresponding to a total H2 consumption of 1.02
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8179 Table 3. H2 Consumption of the La1-xSrxMO3-δ Catalysts in H2-TPR Experiments low temperature (500 °C)
H2 total consumption
catalyst
H2 consumption (mmol/gcat)
associated reaction
H2 consumption (mmol/gcat)
associated reaction
mmol/gcat
mol H2/molMb
LaCoO2.89 La0.6Sr0.4CoO2.76 LaMnO3.10 La0.6Sr0.4MnO3.03
1.61 (1.59)a 1.89 (2.04) 1.02 (0.83) 1.26 (1.04)
Co3+ + e- f Co2+ Co3+ + e- f Co2+ Mn4+ + e- f Mn3+ Mn4+ + e- f Mn3+
4.01 4.47 1.19 1.94
Co2+ + 2e- f Co0 Co2+ + 2e- f Co0 Mn3+ + e- f Mn2+ Mn3+ + e- f Mn2+
5.62 (5.66) 6.36 (6.48) 2.21 (2.48) 3.20 (3.30)
1.38 (1.39) 1.43 (1.46) 0.54 (0.60) 0.71 (0.73)
a The data in parentheses were calculated according to the Co3+/Co2+ or Mn4+/Mn3+ molar ratio in Table 1 and its associated reaction. denote the molar amount of H2 consumption per molar M (M ) Co or Mn).
mmol/gcat, and a big peak starting at 750 °C. With the inclusion of Sr2+ in the LaMnO3-δ lattice, however, there are two sets of partially overlapped peaks, one at 298 and 430 °C (with a weak shoulder at 185 °C), and the other at 772 and 842 °C (Figure 5A(d)). The former corresponds to a total H2 consumption of 1.26 mmol/gcat, whereas the latter corresponds to a total H2 consumption of 1.94 mmol/gcat. The H2-TPR results indicate that the total H2 consumption is 5.62, 6.36, 2.21, and 3.20 mmol/ gcat over LaCoO2.89, La0.6Sr0.4CoO2.76, LaMnO3.10, and La0.6Sr0.4MnO3.03, respectively. The values are only slightly lower than the corresponding theoretical values (5.66, 6.48, 2.48, and 3.30 mmol/gcat in Table 3) calculated according to the bulk Co3+/Co2+ or Mn4+/Mn3+ molar ratios (Table 1). The molar ratio of consumed H2 to Co is estimated to be 1.38 and 1.43 for LaCoO2.89 and La0.6Sr0.4CoO2.76, respectively. Because the two values are between 1.0 and 1.5, we are sure that there is the copresence of Co2+ and Co3+ in the Co-based samples, as evidenced by the results of chemical analysis (Table 1) and XPS investigation (Figure 3B). Similarly, from the molar ratios of consumed H2 to Mn of LaMnO3.10 (0.54) and La0.6Sr0.4MnO3.03 (0.71), which are in the range of 0.5-1.0, we deduce that there is the copresence of Mn3+ and Mn4+ in the Mn-based samples, also in good consistency with the results of chemical analysis (Table 1) and XPS investigation (Figure 3D). There is controversy on the reduction mechanism of LaCoO3 in the literature. Nakamura et al. considered that the perovskite reduction includes four stages: (i) oxide formation of B-site cation; (ii) occurrence of spinel structure; (iii) reduction of B-site cation; and (iv) formation of La2O3 and metallic Co0.47 However, there are researchers who proposed that the reduction of Co3+ in LaCoO3 occurs through two successive reduction steps: (i) reduction of LaCoO3 to an intermediate oxygen-deficient compound associated with low-temperature TPR bands; and (ii) reduction process above 550 °C leading to the generation of La2O3 and metallic Co0.12,48-50 Our results of the H2 consumption of Co-based catalysts (Table 3) support the second viewpoint. Therefore, the reduction profiles of La1-xSrxCoO3-δ could be reasonably divided into two stages: (i) that below 500 °C ascribable to the reduction of Co3+ (to Co2+) and oxygen adspecies; and (ii) that above 500 °C attributable to the reduction of the Co2+. According to Ponce et al.45 and Lisi et al.,51 as well as the data shown in Table 3, the reduction of Mn-based perovskites could be divided into two regions: (i) in the low-temperature region the reduction of Mn4+ to Mn3+ occurs; and (ii) in the hightemperature region the reduction of Mn3+ to Mn2+ occurs. For comparison purpose, we calculated the initial H2 consumption rate of the low-temperature reduction band in the range of 300-340 °C for the Co-based catalysts and that in the range of 230-295 °C for the Mn-based catalysts, within which the H2 consumption was 8.0%, 19.5%, 4.6%, and 17.5% for LaCoO2.89, La0.6Sr0.4CoO2.76, LaMnO3.10, and La0.6Sr0.4MnO3.03, respectively (Figure 5B). Apparently, in the considered temperature ranges, there is an increase in initial H2 consumption rate after Sr2+ doping. The initial H2 consumption rate decreases according to
b
The data
Figure 6. (A) Toluene conversion and (B) specific rate versus reaction temperature over (a) LaCoO2.89, (b) La0.6Sr0.4CoO2.76, (c) LaMnO3.10, (d) La0.6Sr0.4MnO3.03, and (e) 900 °C-calcined La0.6Sr0.4CoO3-δ under the conditions of toluene concentration ) 1000 ppm, toluene/O2 molar ratio ) 1/400, and space velocity ) 20 000 h-1.
the order of La0.6Sr0.4CoO2.76 > LaCoO2.89 for the Co-based catalysts and of La0.6Sr0.4MnO3.03 > LaMnO3.10 for the Mnbased catalysts, coinciding with the sequence of catalytic performance shown in section 3.3. 3.3. Catalytic Performance for Toluene Oxidation. There are many kinds of VOCs (such as aromatics, aldehydes, alcohols, carboxylic acids, ketones, ethers, and esters as well as light alkanes) that can induce pollution to our environment. Aromatics are believed to be relatively hard to eliminate. We selected toluene as an aromatics representative to examine the catalytic performance of the as-fabricated perovskites, and the results are shown in Figure 6. One can see from Figure 6A that under the conditions of toluene concentration ) 1000 ppm, toluene/O2 molar ratio ) 1/400, and space velocity ) 20 000 h-1, toluene conversion increased monotonously with increasing temperature, and above 200 °C, there is a sharp increase in toluene conversion. Among the four perovskite catalysts, La0.6Sr0.4CoO2.76 shows the best performance. Apparently, catalytic activity decreases according to the order of La0.6-
8180 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
Sr0.4CoO2.76 > La0.6Sr0.4MnO3.03 > LaCoO2.89 > LaMnO3.10. In the combustion of hydrocarbons, it is common to adopt T20% (the temperature required for 20% conversion of toluene) and T80% for the evaluation of catalytic activity. In the present study, the T20% and T80% are 205 and 219 °C over La0.6Sr0.4CoO2.76, 220 and 233 °C over La0.6Sr0.4MnO3.03, 220 and 246 °C over LaCoO2.89, and 245 and 295 °C over LaMnO3.10, respectively. These results reveal that the cobaltite catalyst outperforms the manganite counterpart notably. For comparison purpose, we measured the catalytic activity (Figure 6A(e)) of a La0.6Sr0.4CoO3-δ sample (surface area ) 6.9 m2/g) that was calcined at 900 °C for 4 h. Because the surface area of the catalyst prepared at 900 °C is much less than that of the catalyst obtained at 650 °C, it is understandable that the catalytic performance of the former is inferior to that of the latter. It is worth mentioning that CO2 and H2O are the only products and the estimated carbon balance is around 99.5%, indicating clearly that toluene can be completely oxidized over the perovskite materials. To assess the effect of specific surface area on catalytic activity of the materials, we determined the specific rate by normalizing reaction rate with surface area. Shown in Figure 6B is the specific rate as a function of reaction temperature. Apparently, the change trend of specific rate with the rise in temperature is in agreement with that of toluene conversion with the rise in temperature, and on equal temperature basis, the La0.6Sr0.4CoO2.76 catalyst exhibits the highest specific rate. From the above results, one can conclude that the surface area exerts a certain influence on the catalytic performance of the as-fabricated perovskite catalysts. The effects of space velocity and toluene/oxygen molar ratio on the catalytic activity of La0.6Sr0.4CoO2.76 were also examined, and the results are shown in Figure 7. As expected, there is a general trend of activity decrease at elevated space velocities (Figure 7A). Nonetheless, there is no significant difference in activity between 5000 and 20 000 h-1; further increase to 40 000 h-1 would result in a marked drop in activity. As for the influence of toluene/O2 molar ratio, a drop from 1/100 and 1/200 would result in lowering of T80% from 357 to 318 °C. When the toluene/O2 molar ratio is 1/400, the T80% is 219 °C (Figure 7B). It is clear that the enrichment of O2 in the reactant feed has a positive effect on complete toluene oxidation. One should note that at a toluene/O2 molar ratio of 1/100, the amount of oxygen is stoichiometrically much more than that required for complete oxidation of toluene. The result suggests that the oxygen adspecies plays an important role in the total oxidation of toluene. In other words, the oxygen nonstoichiometry related to structural defects is a critical factor in determining the catalytic performance of the perovskite oxides. To examine the catalytic stability, we carried out the on-stream reaction experiment over La0.6Sr0.4CoO2.76, and the results are shown in Figure 8. One can observe that there is no significant drop in catalytic activity within 40 h of on-stream reaction. It means that no deactivation took place. 4. Discussion 4.1. Structural Defects and Catalytic Performance. LaMO3-δ (M ) Co, Mn) and their Sr2+-doped counterparts have been generally accepted to be catalytically highly active in the oxidative abatement of pollutants.46 The doping of Sr2+ can alter the structural defects of the parent perovskite oxide and lead to changes in the Mn+1/Mn+ molar ratio. In addition to the increase in Co3+ and Mn4+ contents, Sr2+-substitution induces rise in oxygen vacancy density in the case of LaCoO3-δ (δ > 0), whereas in the case of LaMnO3-δ (δ < 0), it gives rise to
Figure 7. Catalytic performance of La0.6Sr0.4CoO2.76 as a function of (A) space velocity at toluene/O2 molar ratio ) 1/400 and (B) toluene/O2 molar ratio at space velocity ) 20 000 h-1.
Figure 8. Catalytic activity as a function of on-stream reaction time over La0.6Sr0.4CoO2.76 for toluene oxidation under the conditions of toluene concentration ) 1000 ppm, toluene/O2 molar ratio ) 1/400, and space velocity ) 20 000 h-1.
reduction in excess nonstoichiometric oxygen. Such effects are substantiated by the data of Tables 1 and 2. The La1-xSrxMO3-δ (M ) Co, Mn; x ) 0, 0.4) catalysts investigated in the present work mainly show perovskite-type structures (Figure 1) and exhibit sphere-like surface morphology (Figure 2). The specific surface areas of La-xSrxMO3-δ fabricated using the strategy of coupling citric acid complexing and hydrothermal processing are relatively high (15-29 m2/g), indicating the effectiveness of the approach in generating perovskite-type oxide nanoparticles of high specific surface area. With Sr2+ doping, the uniform nanoparticles aggregated to larger entities, resulting in a significant drop in specific surface area. Furthermore, there is Co enrichment on the surfaces of La-xSrxCoO3-δ as well as La
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and Sr enrichment on the surface of La0.6Sr0.4MnO3.03 (Table 2). It is apparent that the surface and bulk compositions of La-xSrxMO3-δ are different. As illustrated in the XPS investigation (Figure 3A and C), in addition to the molecularly adsorbed water and carbonate species, there was presence of oxygen adspecies on the surfaces of La-xSrxMO3-δ. Because the BEs of O-, O2-, and O22adspecies are very close, one cannot differentiate them only on the basis of O 1s BE values. It has been proven that the oxygen species at oxygen vacancies of ABO3 are dissociatively adsorbed oxygen,10,52 that is, O-. Therefore, we consider that the oxygen with O 1s BE ) 531.1 eV on the Co-based catalysts are mainly O- adspecies. Similar to O22-, O- is generally regarded as strongly electrophilic, and they attack an organic molecule where the electron density is the highest.53,54 Thus, if there were electrophilic oxygen on the surface of a catalyst, total oxidation of hydrocarbons would be efficient and the degradation of carbon skeletons facile. This could partly explain why in our study we did not detect products other than CO2 and H2O. From the O 1s spectra and Table 2, one can realize that the doping of Sr results in an increase of the surface Oads/O2-latt ratio on the cobaltite surface but a decrease of the surface Oads/O2-latt ratio on the manganite surface. In other words, the ratio of surface oxygen/bulk oxygen follows the order of La0.6Sr0.4CoO2.76 > LaCoO2.89 and LaMnO3.10 > La0.6Sr0.4MnO3.03 (Table 2), coinciding with the results of O2-TPD investigation. As revealed in Figure 4a and b, the R oxygen desorption amount over La0.6Sr0.4CoO2.76 (93.7 µmol/gcat) is significantly more than that over LaCoO2.89 (16.1 µmol/gcat), indicating that the oxygen vacancy density of the former is higher than that of the latter. For the Mn-based catalysts (Figure 4c and d), LaMnO3.10 (13.7 µmol/gcat) is more than La0.6Sr0.4MnO3.03 (8.4 µmol/gcat) in the amount of desorbable surface oxygen adspecies, indicating that the amount of excess oxygen in the former is more than that in the latter, as confirmed by the δ values estimated according to the titration results (Table 1). We have now La-xSrxCoO3-δ that possess oxygen vacancies and Co3+/Co2+, and La-xSrxMnO3-δ that contain excess oxygen (i.e., overstoichiometric oxygen) and Mn4+/Mn3+ redox couple. That overstoichiometric oxygen could influence the oxidation of hydrocarbons is known.8,55 Nevertheless, we would like to point out that the Mn4+/Mn3+ redox also plays an essential role in the catalytic action as illustrated later in section 4.2. As compared to organic compounds, O2 can be activated relatively readily at structural defects (such as anionic vacancies) of perovskite catalysts.54 At high oxygen partial pressure and long contact time (i.e., low space velocity), the amount of oxygen species at the oxygen vacancies would be enhanced. The outcome would be better performance of the oxygendeficient perovskite catalyst for complete toluene oxidation. This is confirmed by the results obtained at various toluene/O2 molar ratios (Figure 7B). Because the decrease in gas-phase oxygen concentration undermines the process of surface oxygen species replenishment, it is understandable that in an O2-rich atmosphere, the complete oxidation of VOCs over the ABO3 catalysts with oxygen excess or deficiency could proceed at a relatively lower temperature.23 In toluene oxidation over the fabricated catalysts, only CO2 and H2O were detected, and there was no generation of partially oxidized products. Under the adopted reaction condition, the Sr2+-doped catalysts outperform the undoped ones, and the Sr2+-doped cobaltite outperforms the Sr2+-doped manganite (Figure 6). Among the four catalysts, La0.6Sr0.4CoO2.76 shows the best catalytic performance for toluene complete oxidation. As shown in Figure 8, there is no
significant drop in catalytic activity within 40 h over La0.6Sr0.4CoO2.76. Such a stable catalytic behavior is associated with the factors that the ABO3 catalyst exhibits good thermal stability16 and no carbon deposition occurred on the catalyst surfaces (toluene was totally oxidized to CO2 and H2O, and the estimated carbon balance was 99.5%) at relatively low temperatures (
