LaZnxFe1-xO3钙钛矿型纳米催化剂的制备、表征及其甲苯燃烧性能

2010 Chinese Journal of Catalysis Vol. 31 No. 7 Article ID: 0253-9837(2010)07-0747-04 DOI: 10.1016/S1872-2067(09)60083-8 Article: 747750 Received date: 12 January 2010. *Corresponding author. Tel: +98-411-3393163; Fax: +98-411-3340191; E-mail: s_ali_hosseini English edition available online at ScienceDirect ( Synthesis, Characterization, and Performance of LaZnxFe1-xO3 Perovskite Nanocatalysts for Toluene Combustion Seyed Ali HOSSEINI1,*, Mohammad Taghi SADEGHI2, Abdolali ALEMI2, Aligholi NIAEI1, Dariush SALARI1, Leila KAFI-AHMADI2 1Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran 2Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran Abstract: Nanostructured LaFeO3 and substituted LaZnxFe1-xO3 (x = 0.01, 0.05, 0.1, 0.2, and 0.3) perovskites were synthesized by the sol-gel auto-combustion method and used in the catalytic combustion of toluene. Their structures and surface properties were investigated by X-ray diffraction, Fourier transmission infrared spectroscopy, BET surface area, and scanning electron microscopy. Characterization data revealed that the total insertion of zinc into LaFeO3 takes place when x 0.1. However, ZnO segregation occurs to some extent, especially at x 0.1. The performance of these perovskites was evaluated by toluene combustion. The catalytic activity of the catalysts increased substantially with an increase in zinc substitution. These results can be attributed to the cooperative effect between LaZnxFe1-xO3 and the zinc oxide phases. The relative concentration of these phases determines their oxygen activation ability and reactivity. Key words: sol-gel auto-combustion; perovskite; toluene; catalytic oxidation; LaZnxFe1-xO3 CLC number: O643 Document code: A Volatile organic compounds (VOCs) may undergo pho- tochemical reactions with nitrogen oxides in the presence of sunlight yielding even more hazardous compounds such as tropospheric ozone or organic peroxides (so-called photo- chemical smog) 1. Automobiles and industrial processes are mainly responsible for VOC emissions. VOCs such as hydrocarbons, alcohols, ketones, and aldehydes 2,3 are readily oxidized. When the recovery of these compounds is not desired, they are usually destroyed via deep oxidation. Catalytic combustion is regarded as an effective and eco- nomic way for air pollution control 4. Improved catalytic combustion performance would thus lead to considerable energy saving. The catalysts currently used for environ- mental purposes are either noble metals or metal oxides. Although noble metals are the most active and effective catalysts for VOC decomposition, they lack a high sintering rate, are volatile and are expensive 5. Under these circum- stances oxides appear to be a tempting solution, especially with regards to price and easier preparation. Perovskites are mixed oxides with a specific cubic struc- ture and can be described by the general formula ABO3. Perovskites have previously been investigated for VOC combustion. Chang et al. 6 found that La0.8Sr0.2CoO3 was highly effective for the deep oxidation of toluene and methyl ethyl ketone. Lintz et al. 7 studied the combustion of several VOCs over LaMnO3. Arai et al. 8 found a very high activity for the deep oxidation of methane with Sr-doped LaMnO3. At conversion levels below 80%, Sr-doped LaMnO3 was more active than Pt/Al2O3. High oxidation activities for La0.75Sr0.25MnO3+x and La0.8Sr0.2MnO3 have also been reported by McCarty et al. 9. It has been asserted that a correlation exists between VOC oxidation rate and the bond dissociation energy of the weakest CH bond 10,11. The main challenge in developing perovskite catalysts is to determine their structures while preserving sufficiently high surface areas. The preparation of a perovskite oxide involves a solid-state reaction of its precursor oxide to form the characteristic ABO3 structure. This requires significant exposure of the precursor oxide to high temperatures lead- ing to a low specific surface area for the catalyst. To cir- cumvent this limitation, a number of alternative preparation methods have been evaluated in an attempt to lower the firing temperature for perovskite synthesis. Fe and Zn are transition metals that have been used as catalysts of VOC because of their role in the improvement of catalyst activity 12,13. The objectives of this work are to synthesize LaZnxFe1-xO3 (x = 0, 0.01, 0.05, 0.1, 0.2, 0.3) perovskites in the form of nanoparticles by the sol-gel combustion method and also to evaluate their volatile organic compound com- bustion activities. The perovskites were characterized using 748 催 化 学 报 Chin. J. Catal., 2010, 31: 747750 X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area measurements, and Fourier trans- mission infrared spectroscopy (FT-IR). Toluene was chosen as a VOC model compound because of its application as an industrial solvent. Toluene is considered to be one of the main VOC compounds. 1 Experimental 1 Experimental 1.1 Synthesis of perovskites Analytical grade Fe(NO3)39H2O, La(NO3)36H2O, Zn(NO3)26H2O, and C6H8O7H2O were used as raw materi- als for the stoichiometric preparation. Specific amounts of Fe(NO3)39H2O, La(NO3)36H2O, and Zn(NO3)26H2O were dissolved in a citric acid solution as the sol to obtain substi- tuted LaZnxFe1-xO3 (x = 0, 0.01, 0.05, 0.1, 0.2, 0.3). The molar ratio of citric acid to the total metal nitrates in the solution mixture was kept at 1. The solutions were stirred vigorously and evaporated by heating at 80 C for 2 h and then at 100 C while the slurry gradually burned and turned into a brown powder. The brown powders obtained were calcined at 600 C for 1 h and subsequently at 800 C for 3 h. 1.2 Characterization of the perovskites The phases of the synthesized catalysts were character- ized by XRD using a Siemens D500 X-ray powder diffrac- tometer equipped with a position-sensitive detector allowing all angles between 4 and 70 to be read simultaneously at a scan rate of 2/min. Monochromatic Cu K was used as the radiation source. The mean crystal sizes were estimated using the Scherrer equation 14, D = k/cos, where k = 0.89, = 0.154 18 nm (Cu), is the half peak width of the X-ray reflection, and is the diffraction angle. The FT-IR transmission spectra from 400 cm1 to 4 000 cm1 were re- corded for all the samples using a FT-IR Nexus 670 instru- ment by the KBr pellet technique. SEM images were ob- tained using a JEOL JEM-100 CXII microscope. 1.3 Catalytic activity The activity of the catalysts was evaluated by toluene combustion in a U-shape quartz tube placed in an electric furnace working under atmospheric pressure. In each run, 0.2 g of catalyst was used for the catalytic test. The VOC concentration in the feed gas was 0.2 mol%, the gas hourly space velocity (GHSV) was 15 000 h1 and the reaction temperature ranged from 150 C to 400 C. The reactants and products were analyzed using a gas chromatograph (Shimadzu 2010) equipped with a flame ionization detector. The conversion was obtained by calculating the reactant concentration (in the inlet and the outlet of the reactor) at different temperatures. 2 Results and discussion 2 Results and discussion 2.1 Characterization of perovskites The XRD patterns of the perovskites are shown in Fig. 1. Figure 1(1) shows a pure perovskite-type oxide of LaFeO3 with an orthorhombic structure. The diffraction data are in good agreement with LaFeO3 (JCPDS 37-1493) 15. The XRD patterns of LaZnxFe1-xO3 are shown in Fig. 1(25). There is no distinguishable difference between the XRD patterns of LaFeO3 and LaZnxFeO3 (x 0.1). Some segrega- tion occurs for x 0.1 indicating a ZnO single phase (2 = 34.43 and 36.24). Furthermore, using the Scherrer equa- tion, the average particle sizes of the perovskites were de- termined to be 3050 nm. 10203040506070 (1) (3) (5) (2) (4) (6) Intensity 2/( o ) Fig. 1. XRD patterns of the LaZnxFe1-xO3 nanocatalysts. (1) x = 0 (LaFeO3); (2) x = 0.01; (3) x = 0.05; (4) x = 0.1; (5) x = 0.2; (6) x = 0.3. The FT-IR spectra of LaZnxFe1-xO3 calcined at 800 C for 3 h are shown in Fig. 2. Strong absorption bands are present at 547.6 and 446.9 cm1 indicating the formation of lantha- num orthoferrite. The band at 547.6 cm1 is attributed to the FeO stretching vibration (1 mode) and the band at 446.9 cm1 corresponds to the OFeO deformation vibration (2 mode) 16. All these spectra are similar in shape. The in- tensity of the band at 446.9 cm1 decreased with an increase in the substitution of Fe by Zn. This band seems to consist of two overlapped bands especially for x 0.2, which is due to the appearance of ZnO. The weak (very broad) band be- tween 3 600 and 3 200 cm1 is characteristic of the rapid absorption of moisture by KBr 17. Furthermore, the mor- phology and particle size of the perovskites were examined using SEM. SEM images of LaFeO3 and LaZn0.01Fe0.99O3 (as examples of the LaZnxFe1-xO3 perovskites) are shown in Fig. Seyed Ali HOSSEINI et al.: Synthesis, Characterization, and Performance of LaZnxFe1-xO3 Perovskite 749 3. These images show a uniform grain size distribution, a fine powder size, and homogenous nanostructures for the material obtained by the sol-gel auto-combustion method. A similar morphology and nanostructures were observed for the other perovskites and these are not shown. The mean particle size is within the nanoscale ( 100 nm). This value is in accordance with the results obtained by the measure- ment of X-ray peak broadening. The specific BET surface areas of these samples are summarized in Table 1. The largest BET surface area (30.1 m2/g) was obtained for the LaFeO3 perovskite. The partial substitution of iron by zinc results in a progressive decrease in the BET surface area and the lowest value (13.7 m2/g) was obtained for LaZn0.3Fe0.7O3. (a) (b) (c) (d) 10 m 10 m 1 m 1 m Fig. 3. SEM images of LaFeO3 (a,b) and LaZn0.01Fe0.99O3 (c,d). 2.2 Catalytic activity of perovskites The oxidation activity of the six perovskites under study was evaluated using the toluene oxidation reaction. A con- trol test without catalysts (thermal oxidation) was performed under the same conditions. The maximum conversion of toluene (35%) was obtained at 400 C. In the control test, toluene was not oxidized below 150 C. The activities of LaFeO3 and LaZnxFe1-xO3 for the oxidation of toluene are shown in Fig. 4. The catalysts apparently lowered the acti- vation energy as the oxidation of toluene required a lower initial burning temperature. We observed that compared to LaFeO3, the LaZnxFe1-xO3 catalysts were more active in the oxidation of toluene. 350030002500200015001000500 x = 0.3 x = 0.2 x = 0.1 x = 0.05 x = 0.01 Transmittance Wavenumber (cm1) x = 0 Fig. 2. FT-IR spectra of the LaZnxFe1-xO3 nanocatalysts calcined at 800 C in air for 3 h. Table 1 Specific BET surface area for the LaZnxFe1-xO3 perovskites Perovskite ABET/(m2/g) LaFeO3 30.1 LaZn0.01Fe0.99O3 29.8 LaZn0.05Fe0.95O3 26.4 LaZn0.1Fe0.9O3 22.6 LaZn0.2Fe0.8O3 15.2 LaZn0.3Fe0.7O3 13.7 750 催 化 学 报 Chin. J. Catal., 2010, 31: 747750 The activity of LaZnxFe1-xO3 increased with an increase in zinc insertion. The ignition temperature T50% defined as the temperature required for a 50% conversion of toluene over the catalysts is shown in Table 2. It is clear that T50% de- creased with an increase in zinc insertion. A similar trend is also observed for T96%. Taking the reaction temperature and conversion into account, it is apparent that the catalytic ac- tivity of LaZnxFe1-xO3 is higher than that of LaFeO3 under the same reaction conditions used for the oxidation of tolu- ene. Substitution modifies the surface structure of the cata- lysts by greatly increasing the oxygen valances in the sur- face regions, which results in higher catalytic activity. It has been reported that the catalytic activity of perovskite type ABxB1-xO3 is related to the metal-oxygen bond and the free energy of reduction of the cations at B and B sites 14,18,19. No relationship exists between the BET surface area and catalyst activity. Table 2 Reaction temperatures for 50% and 96% toluene conversion Catalyst T50%/oC T96%/oC LaFeO3 268 400 LaZn0.01Fe0.99O3 235 363 LaZn0.05 Fe0.95 O3 233 356 LaZn0.1Fe0.9O3 230 335 LaZn0.2Fe0.8O3 226 327 LaZn0.3Fe0.7O3 222 323 3 Conclusions 3 Conclusions Nanostructured LaFeO3 and LaZnxFe1-xO3 (x = 0.01, 0.05, 0.1, 0.2 and 0.3) perovskites were successfully synthesized using the sol-gel combustion method. Some segregation of single oxides was found for substitutions where x 0.1. A significant increase in catalytic activity with zinc substitu- tion was found. 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