Lanthanum Based High Surface Area Perovskite-Type Oxide and Application in CO and Propane Combustion
Total Page：16
File Type：pdf, Size：1020Kb
EQ05-AF:MODELO-AF 7/21/09 11:46 AM Página 31 www.scielo.br/eq www.ecletica.iq.unesp.br Volume 34, número 1, 2009 Lanthanum based high surface area perovskite-type oxide and application in CO and propane combustion. P. R. N. Silva 1, A. B. Soares 1,2 1 Universidade Estadual do Norte Fluminense – CCT/LCQUI, Av. Alberto Lamego 2000 – 28013602 Campos dos Goytacazes, RJ 2 Centro Federal de Educação Tecnológica do Espírito Santo, 29040-780 Vitória – ES *[email protected] Abstract: The perovskite-type oxides using transition metals present a promising potential as catalysts in total oxidation reaction. The present work investigates the effect of synthesis by oxidant co-precip - itation on the catalytic activity of perovskite-type oxides LaBO 3 (B= Co, Ni, Mn) in total oxidation of propane and CO. The perovskite-type oxides were characterized by means of X-ray diffraction, nitro - gen adsorption (BET method), thermo gravimetric and differential thermal analysis (ATG-DTA) and X-ray photoelectron spectroscopy (XPS). Through a method involving the oxidant co-precipitation it’s possible to obtain catalysts with different BET surface areas, of 33-44 m 2/g, according the salts of metal used. The characterization results proved that catalysts have a perovskite phase as well as lan - thanum oxide, except LaMnO 3, that presents a cationic vacancies and generation for known oxygen excess. The results of catalytic test showed that all oxides have a specific catalytic activity for total oxi - dation of CO and propane even though the temperatures for total conversion change for each transition metal and substance to be oxidized. Keywords: perovskites; oxidation; propane. Introduction the advantages of lower costs and the potential market in energy generation systems in domestic The removal of CO, unburned hydrocar - and small scale industrial applications. For this bons (HC), and NO from automotive exhaust reason perovskite type compounds have received requires catalytic devices in which these pollu - wide attention, which have been incorporated tants are eliminated. Catalytic combustion offers into the design of the novel combustors (2,3). one of the most efficient means for controlling A perovskite-type oxide has an ABO 3 type atmospheric pollution (1). Although noble met - crystal structure wherein cations with a large als, such as palladium, platinum, and Rhodium ionic radius have twelve coordination to oxygen are well known with higher activity (per site than atoms and occupy A-sites, and occupy B sites. A metal-oxide catalysts, they present several disad - and O form a cubic closest packing, and B is con - vantages, such as higher volatility, high cost, and tained in the octahedral voids in the packing. If poor availability. Compared with noble metals, the ionic radii are r A, r B and r O, to form a per - base metal catalysts present a lower but still suf - ovskite crystal structure, the tolerance factor (t) = ficient activity as oxidation catalysts, and have (r A+ r O)/ (r B + r O) must lie within the range 0.8 < Ecl. Quím., São Paulo, 34(1): 31-38, 2009 31 EQ05-AF:MODELO-AF 7/21/09 11:46 AM Página 32 t < 1.0, and r A > 0.090 nm, r B > 0.051 nm. A great pared using 20 mmol of aqueous solution (100 many elements can form ideal or modified per - mL) of metal’s salts (Co(NO 3)2.6H 2O to LaCoO 3, ovskites depending on the tolerance factor. The MnCl 2.4H 2O to LaMnO 3 and Ni(NO 3)2.6 H 2O to ideal perovskites structure appears only in a few LaNiO 3) mixed with 100 mL of aqueous solution cases for tolerance factors very close to 1 and at of lanthanum nitrate (20 mmol). The mixture was right temperatures In others conditions different added drop wise to a rapidly stirred (nitrogen distortions of the perovskites structure will stream) solution of sodium hydroxide (7.2g 0.18 appear. Deviations from the ideal structure with mol) in sodium hypochlorite (3.5 M. 30ml). Use orthorhombic, rhombohedral, tetragonal, mono - of inert atmosphere minimizes carbonate contam - clinic and triclinic symmetry are known, although ination in the final product. The resultant gel was the latter three are scarce and poorly character - washed repeatedly with deionized water (3x200 ized [4]. Nonstoichiometry in perovskites can ml), then washed with acetone (3x 50ml) and arise from cation deficiency (in the A or B site), dried under vacuum. anion deficiency or anion excess. Much attention The powder was dried at 60 ºC for 12h, has particularly been paid to lanthanum – transi - after that it was calcinated at 600ºC for 2 hours. tion metal based perovskites oxides, which were The catalysts were characterized by X-ray introduced into catalysis some 30 years ago (5,6). powder diffraction (XRD) on a 65- Rich Seifert but they are available for perovskites in which the & Co diffractometer by using CuK a radiation for metal in position A and/or B has been partly sub - determination of the final structure. Patterns were stituted with another metal, thus establishing a recorded between a 2 q of 20º and 65º with step large group of active oxides described by the gen - size of 0.05º with 30kV and 40mA. eral formula A 1-x A´ xB1-y B´ yO3±d , where d stands The specific surface areas were obtained for excess oxygen resulting from non-stoichiom - by nitrogen adsorption at 77 K, evaluated using etry of these species [7]. the BET equation on an automatic Quantachrome Lanthanum-based perovskites containing Aparatus, Autosorb – 1 C, in the 0, 05-0,095 rel - transition metal in B-site, (LaBO 3, B = Co, Fe, Ni ative pressure range. The powders were degassed or Mn), show catalytic activity close to the noble at 300ºC for 1h before the measurements. metal, presenting low cost and high thermal stabil - The thermal decomposition with the tem - ity [8]. The perovskites type oxides are of interest perature of the powder before heating at 60°C for for catalytic oxidation and reduction reactions asso - 12h and passed by a sieve of 200 mesh was per - ciated with automotive exhaust emission control formed in N 2 by using a TGA-DTA of Schimadzu -1 [9]. They have been tested in oxidation of CO and and TA Instruments, under N 2 flow (50 m l min ), hydrocarbons, NOx reduction and hydrogenation and 10 0C min -1 from room temperature to 600ºC. and hydrogenolysis reactions [7]. The X-ray photoelectron spectra (XPS) In this work, the effect of oxidant - copre - were obtained with a VGESCALAB MKII, cipitation synthesis on the catalytic activity of employing Mg K a radiation (1253.6 eV) under a -8 perovskite-type oxides LaBO 3 (B= Co, Ni, Mn) is high vacuum of 3x10 Torr. The binding energies investigated, using as a model the total oxidation were calibrated by using a C1s peak (284. 6 eV) of propane and CO. as a background. Temperature programmed reduction (TPR) experiments were performed in a system Experimental with a thermal conductivity detector. The reduc - ing gas was a mixture of 3% H 2/N 2 (1.8 l/h) and A number of methods have been used in heating 10°C/min was employed. the synthesis of perovskites; the choice of a par - The catalysts were tested in combustion of ticular one depends mostly on the expected use propane and CO. The catalytic measurements for these oxides. The LaCoO 3, LaMnO 3 and were realized in a fixed-bed reactor, working in LaNiO 3 perovskites oxides were prepared by oxi - atmospheric pressure. For characterization of the dant – coprecipitation. The catalysts were pre - catalytic activity, it was used 10mg of powder. 32 Ecl. Quím., São Paulo, 34(1): 31-38, 2009 EQ05-AF:MODELO-AF 7/21/09 11:46 AM Página 33 Before the reaction, the powder is pretreated for peak the range, centered at 318 and 440 0C for 0 1.5h at 400ºC with synthetic air: O 2 (20 ±0.5)% LaCoO 3, 340 and 420 C for LaMnO 3, 318 and 0 and N 2 (80 ±0.5)%, 1.8 l/h. The feed gas compo - 470 C for LaNiO 3.This different behavior prob - sition was CO (0.97%), C 3H8 (0.507%), O 2 ably due to the different decomposition mecha - (2.02%) and N 2 (balance), 1.8 l/h, on-line contin - nism of hydroxide gels. In LaNiO 3 catalyst, the uous analysis of reactants and products was per - carbonate quantity is bigger, when compared to formed using a gas chromatograph CG 9001 another catalyst, due to the catalyst has been Finnigan, provided with a thermal conductivity stayed more time during the filtration process (72 detector. For CO and CO 2 analyses were utilized h), once fine grain was formed, difficulting the an independent ND-IR detector. filtration process. Results and discussion The method employed was oxidant-copre - cipitacion, developed by Barnard et al [10] to LaCoO 3 catalyst synthesis. The catalyst preparation in basic condi - tions, due to the presence the sodium hydroxide and sodium hypochlorite as oxidant agent, yields probably LaBOH (B= Co, Mn, Ni). This was reported by Vydiasagar et al [11] as the hydroxi- gel to be dehydrated. The advantage in this method is that the dehydratation of the gel occurs at low tempera - tures (60°C), due to the initial dehydratation in the presence of acetone. Controlled dehydratation of hydroxi-gel, using water and a miscible solvent, presents pos - itive results in obtain solids with high specific surface areas. The displacement of water by ace - tone allows a reduction of superficial tension and leads a smallest capillary force; producing weak agglomeration and keeping opened the particle structure.