Preparation of LaSrCoO4 mixed oxides and their catalytic properties in the oxidation of CO and C3H8

Preparation of LaSrCoO4 mixed oxides and their catalytic properties in the oxidation of CO and C3H8

Catalysis Communications 6 (2005) 13–17 www.elsevier.com/locate/catcom Preparation of LaSrCoO4 mixed oxides and their catalytic properties in the oxi...

199KB Sizes 1 Downloads 75 Views

Catalysis Communications 6 (2005) 13–17 www.elsevier.com/locate/catcom

Preparation of LaSrCoO4 mixed oxides and their catalytic properties in the oxidation of CO and C3H8 Xiaomao Yang

a,b

, Laitao Luo

a,*

, Hua Zhong

a

a

b

Institute of Applied Chemistry, Nanchang University, Nanchang, Jiangxi, 330047, PR China Institute of Materials, Jiangxi University of Finance and Economics, Nanchang, Jiangxi, 330013, PR China Received 22 February 2004; accepted 8 October 2004 Available online 24 November 2004

Abstract The perovskite-like LaSrCoO4 mixed oxides were prepared by the gelatin, polyglycol gel and polyacrylamide gel methods and were used successfully for CO and C3H8 oxidation. These samples were investigated by using the XRD, TEM, BET and TPD methods. The effects of preparation methods on structure and performance of LaSrCoO4 were studied. The catalytic activity of LaSrCoO4 prepared by polyacrylamide gel method is the best among all samples, and this is explained in terms of its more oxygen vacancies and mobile lattice oxygen, smaller particle size and larger BET surface areas. Ó 2004 Elsevier B.V. All rights reserved. Keywords: LaSrCoO4; Perovskite-like; Preparation method; Oxidation

1. Introduction The precious metal catalysts are widely studied and adopted for automotive exhaust gas purification in order to eliminate the contaminations of CO, low HC (hydrocarbon) and NOx (nitrogen oxides) [1–5], whereas, because of their limited supply and high cost, the replacement of precious metal catalysts by low cost active catalysts is highly desirable. The perovskite-like A2BO4 mixed oxides of K2NiF4 structure show high catalytic activity in the reactions involved in the motor vehicles exhaust gas after-treatment, and are recently studied as new materials for their low cost, high catalytic activity and high thermal stability [6,7]. In this A2BO4 structure, the redox property of the mixed oxides can be controlled and improved by changing the valence of B-site ion, which is caused by substitution at A-site ion with ions of different valence [8–12]. Presently, there

*

Corresponding author. Tel.: +86 791 3891294; fax: +86 791 3891364. E-mail address: [email protected] (L. Luo). 1566-7367/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2004.10.003

are some studies on LnSrCuO4 and LnSrNiO4 [11–14], but few on LnSrCoO4, especially, the effects of preparation methods on structure and performances of LnSrCoO4. So it is very significant for us to study the relationship among preparation methods, structure and performance of LnSrCoO4 in order to choose better catalytic materials. In this paper, the perovskite-like LaSrCoO4 mixed oxides of K2NiF4 structure were prepared by different methods and characterized by XRD, TEM, BET and TPD; their catalytic activities were determined in the oxidization of CO and C3H8; in addition, the effects of preparation methods on structure and performance of LaSrCoO4 were studied for the first time. 2. Experimental 2.1. Catalyst preparation 2.1.1. Polyglycol gel method Briefly, lanthanum, strontium, and cobalt nitrates in a desired molar ratio were dissolved in a citric solution

14

X. Yang et al. / Catalysis Communications 6 (2005) 13–17

at 80 °C with constant stirring. The polyglycol 20,000 was added in when the solution was evaporated to 40 mL. The stirring was continued until a viscous gel was formed. The resulting gel was evaporated to dryness, and the obtained precursor was calcined at 650 °C for 6 h, followed by pelletization and calcination once more at 950 °C in air for 12 h. The synthesized pellet was pulverized to ca. 40–80 mesh size. 2.1.2. Gelatin method The gelatin was added in when the solution was evaporated to 40 mL, the other processes of gelatin method are the same as those of polyglycol gel method. 2.1.3. Polyacrylamide gel method Briefly, lanthanum, strontium, and cobalt nitrates in a desired molar ratio were dissolved in a citric solution. The pH of the solution was adjusted to 6–7 by addition of concentrated ammonia. Acrylamide and N,N 0 -methylenebisacrylamide (reticulating agent) were added, and the resulting solution was heated at 90–95 °C. Polymerization of the organic monomer was initiated by adding azo-bis-isobutyronitrile (AIBN) dissolved in ethanol, and N,N,N 0 ,N 0 -tetramethylethylenediamide (TEMED) which acts as a radical transfer agent. Polymerization occurs generally within a few minutes. The resulting gel was evaporated to dryness, and the obtained precursor was calcined at 650 °C for 6 h, followed by pelletization and calcination once more at different

temperatures in air for 12 h. The synthesized pellet was pulverized to ca. 40–80 mesh size. 2.2. Characterisation Powder X-ray diffractometer (type D/max-3B made in Japan) over the range 20° 6 2h 6 80°, at room temperature, operating at 40 kV and 10 mA, using Cu Ka radiation combined with the nickel filter. The changes of the specific surface areas of catalysts were determined by gas chromatography. Methanol was used as adsorbate, and the molecular diameter of ˚ at the adsorption temmethanol at critical state is 4.4 A perature which was maintained at 25 ± 0.1 °C. The nitrogen pressure was controlled to let the P/Ps = 0.200. The examination of the catalyst by TEM was done by means of a Hitachi 600 with an acceleration voltage of 75 kV. Temperature-programmed desorption of O2 was carried out in the in-house apparatus over 0.5 g catalyst. The samples were first heated from room temperature to 850 °C at a rate of 10 °C/min and kept at 850 °C for 1 h, then cooled to room temperature in O2, finally heated at a rate of 10 °C/min in He for recording the O2-TPD spectra. 2.3. Catalytic activity measurements The CO + O2 and C3H8 + O2 reactions were carried out in a flow reactor by feeding a gas mixture of CO

Fig. 1. XRD patterns of the LaSrCoO4 catalysts (different methods) from (a) to (e): gelatin method (950 °C), polyglycol gel method (950 °C), and polyacrylamide gel method (950, 850 and 750 °C) T:T phase mixed oxide, T*:T* phase mixed oxide, L:La2O3 and C:CoO.

X. Yang et al. / Catalysis Communications 6 (2005) 13–17

15

(3.5 vol%), C3H8 (1.0 vol%), O2 (7.0 vol%), and N2 (balanced) over 0.2 g catalyst (W/F = 1.2 g s/cm3). The gas composition was analyzed before and after the reaction by an online gas chromatography with thermal conductor detector (TCD), connected with a computer integrator system and using TDX-01 column for the oxidative products of CO and Porapak Q column for the oxidative products of C3H8, respectively. The temperatures of the columns and TCD are 60 and 100 °C, respectively.

3. Results and discussion 3.1. Solid characteristics of catalysts The perovskite-like A2BO4 mixed oxides belong to K2NiF4-type structure, Ganguli [15] proposed that the K2NiF4-type structure needs two conditions: structure factor (1.7 6 t 6 2.4) and balance of electrovalence. LaSrCoO4 mixed oxide satisfies the two conditions. XRD patterns of these LaSrCoO4 mixed oxides prepared by different methods are presented in Fig. 1. The results of phase analysis obtained by XRD clearly show that all samples are tetragonal K2NiF4-type mixed oxides, although LaSrCoO4 prepared by the gelatin method contains minor contributions of La2O3 and CoO. In general, tetragonal K2NiF4-type A2BO4 mixed oxides exist as three kind of phases (T, T* and T 0 ) [16], B ions are coordinated with six, five and four oxygen atoms (BO6, BO5 and BO4) in T, T* and T 0 phases, respectively, as shown in Fig. 2. T* and T 0 phases result from abundant oxygen vacancies. LaSrCoO4 mixed oxides prepared by different methods belong to T phase, but T* phase partly appears in LaSrCoO4 prepared by polyacrylamide gel method, and the lower the calcination temperature is, the more the T* phase appears, but LaSrCoO4 mixed oxides of K2NiF4-type structure did not form when the calcination temperature is below 700 °C. The TEM micrographs show that the particle sizes of LaSrCoO4 catalysts varied from 30 to 140 nm in Fig. 3. The particle size of LaSrCoO4 catalyst prepared by gelatin method is the largest, whereas the particle size of LaSrCoO4 catalyst prepared by polyacrylamide gel

Fig. 2. BOx structure.

Fig. 3. TEM micrographs of the LaSrCoO4 catalysts (different methods) from (a) to (e): gelatin method (950 °C), polyglycol gel method (950 °C), and polyacrylamide gel method (950, 850 and 750 °C).

method is the smallest, and the lower the calcination temperature is, the smaller the particle size is. At the order of LaSrCoO4 prepared by gelatin method (950 °C), Polyglycol gel method (950 °C) and polyacrylamide gel method (calcination temperature 950, 850 and 750 °C), their BET surface areas are 2.34, 3.02, 6.57, 9.23, 12.45 m2 g1, respectively. The results of the BET surface areas of these samples are in accordance with the results of the TEM. 3.2. TPD studies The O2-TPD curves were obtained over LaSrCoO4 prepared by different methods, and there are three O2desorption peaks (a, a 0 and b), respectively, as shown in Fig. 4. The a peak (at 200 °C) could be attributed 0 to the ordinary chemisorbed oxygen ðO 2 Þ. The a peak

Fig. 4. O2-TPD profiles over LaSrCoO4 catalysts (different methods) top to bottom: polyacrylamide gel method (750, 850 and 950 °C), polyglycol gel method (950 °C), and gelatin method (950 °C).

16

X. Yang et al. / Catalysis Communications 6 (2005) 13–17

Fig. 5. Schemes of oxygen transformation in the CoOx structure (x = 5, 6)

(at 385 °C) corresponds to the desorption of the oxygen chemically adsorbed on oxygen vacancies [9], namely, the oxygen is released by reduction of Co3+ according to the following reaction: 2Co3þ þ O2 ! 2Co2þ þ Vo þ 1=2O2 The b peak (500–800 °C) might be attributed to the lattice oxygen associated with the redox steps of Co ions as shown in Fig. 5. Comparing different preparation methods, we find that at the order of LaSrCoO4 prepared by gelatin method (950 °C), polyglycol gel method (950 °C) and polyacrylamide gel method (950 °C), their a peak areas are approximately equal, indicating that different preparation methods have no effects on ordinary chemisorbed oxygen over catalysts, whereas the a 0 and b peak areas increase gradually, implying that oxygen vacancies and mobile lattice oxygen also increase [10]. Comparing different calcination temperatures, we find that at the order of LaSrCoO4 prepared by the polyacrylamide gel method (950, 850 and 750 °C), their a peak areas are approximately equal, indicating that different calcination temperatures also have no effects on ordinary chemisorbed oxygen over catalysts, whereas the a 0 and b peak areas increase gradually with the decrease of calcination temperatures, implying that oxygen vacancies increase, lattice oxygen is more easily mobile [7] and mobile lattice oxygen also increase.

s:OI; d:OII;

: Oxygen vacancy.

3.3. Catalytic activities of LaSrCoO4 mixed oxides The results of oxidation of CO and C3H8 over LaSrCoO4 catalysts are summarized in Table 1 by means of the 50% and 100% conversion temperatures (T50, T100). By comparing T50, T100, we find that compared to LaSrBO4 (B = Fe, Ni, Cu, Cr, Mn) [13,17,18], the LaSrCoO4 catalysts possess higher catalytic oxidative activities, and at the mentioned order, the catalytic activities of LaSrCoO4 for oxidation of CO and C3H8 improve gradually. More details can be furnished here. The oxidation mechanism over LnSrCoO4, which is similar to that over LnSrNiO4 [13], CO and C3H8 are adsorbed on Co3+, and then react with the lattice oxygen, whereas the oxygen chemically adsorbed on oxygen vacancy is transformed into the lattice oxygen to replenish the consumed lattice oxygen. The oxygen transformation in the CoO6 octahedral is proposed in Fig. 5. We can therefore deduce that the oxygen vacancies are the one of the key of the chemical reaction, on one hand, they continuously transform the chemisorbed oxygen into the lattice oxygen to replenish the consumed lattice oxygen, on the other hand, they make the lattice oxygen more mobile, which are advantageous to chemical reaction. In LaSrCoO4, when the three valent ion La3+ at A site is substituted by a lower valent ion Sr2+, according to the principle of electroneutrality [19], the reduced po-

Table 1 Catalytic activities at T50 (°C) and T100 (°C) of LaSrCoO4 (prepared by different methods) for CO and C3H8 oxidation Samples of different preparation methods (°C)

CO oxidation

C3H8 oxidation

T50 (°C)

T100 (°C)

T50 (°C)

T100 (°C)

Gelatin method (950) Polyglycol gel method (950) Polyacrylamide gel method (950) Polyacrylamide gel method (850) Polyacrylamide gel method (750)

250 210 200 180 170

300 250 230 210 200

400 350 320 300 280

460 400 370 340 320

X. Yang et al. / Catalysis Communications 6 (2005) 13–17

sitive charge could be balanced either by the formation of higher oxidation state ion at B site, i.e. Co2+ ! Co3+, or by the formation of oxygen vacancy (Vo). These processes could be explained by the following defect equations: 3þ 0  LaSrCo2þ 1x Cox O4 ! La2 CoO4 þ SrLa þ xCoCo

LaSrCo2þ O4k ðVoÞk ! La2 CoO4 þ Sr0La þ kVo ðx ¼ 2kÞ where Sr0La is Sr2+ which has substituted La3+, CoCo is Co3+ which has substituted Co2+. In fact, the two cases often exist at the same time, so LaSrCoO4 has many oxygen vacancies. In addition, in macromolecule gel method, a great deal of macromolecule polymer is added in mixed solution of citric acid and nitrates. The macromolecule polymer will consume a great deal of the lattice oxygen and chemisorbed oxygen which cannot be fully and timely replenishd by environmental oxygen, so the samples have more oxygen vacancies after being calcined. Comparing the different preparation methods and different calcination temperatures, we find that at the order of LaSrCoO4 prepared by gelatin method (950 °C), polyglycol gel method (950 °C) and polyacrylamide gel method (calcination temperature 950, 850 and 750 °C), the oxygen vacancies increase gradually as proved by the results of XRD and O2-TPD. For gelatin method, the gelatin formed a gel while being added to the solution of citric acid and nitrates. This impedes a uniform dispersion of metal ion complexes in solution. Consequently, the particles are not uniformly dispersed and, partly, mixed phases are formed. The resulting higher particle sizes lead to lower BET surface areas, which are lowest for samples prepared by the gelatin method. For polyglycol gel method, polyglycol is dissolved first before the gel forms, and the metal ion complexes are better separated than they are in case of the gelatin method. But, nevertheless, the solubility of the polyglycol is limited. For polyacrylamide gel method, this method is intermediate between the Ôamorphous citrateÕ and a sol–gel process. The metal cations are complexed by citric acid, and yield a stable aqueous solution, which is gelled by radical polymerization of organic monomers that evenly disperse in solution. The metal ions complex are evenly and perfectly fixed in the gel, so particle size of the sample is the smallest, and its BET surface areas is the largest among these catalysts. High temperature calcination can partly cause agglomeration of particles, so a decrease of the calcination temperature leads to smaller particle sizes, simultaneously, the BET surface areas increases, this is favorable for the chemical reaction.

17

4. Conclusion The perovskite-like LaSrCoO4 mixed oxides of K2NiF4 structure have been prepared by different methods. All samples are tetragonal K2NiF4-type mixed oxides, at the same time, T* phase partly appears in LaSrCoO4 prepared by polyacrylamide gel method, and the lower the calcination temperature is, the more the T* phase appears. These catalysts possess high catalytic activities towards the oxidation of CO and C3H8, and the catalytic activity of LaSrCoO4 prepared by polyacrylamide gel method (calcination temperature 750 °C) is the best among these samples in oxidation reaction. This is explained in terms of their structure: oxygen vacancies, mobile lattice oxygen, particle sizes and BET surface areas.

Acknowledgement This work described above was fully supported by a grant from the Nanchang University.

References [1] Z. Hu, F.M. Allen, C.Z. Wan, R.M. Heck, J.J. Steger, R.E. Lakis, C.E. Lyman, J. Catal. 174 (1998) 13. [2] Z. Hu, C.Z. Wan, Y.K. Liu, J. Dettling, J.J. Steger, Catal.Today 30 (1996) 83. [3] D.H. Kim, S.I. Woo, J. Noh, O.B. Yang, Appl. Catal. A 207 (2001) 69. [4] T. Kobayashi, T. Yamada, K. Kayano, Appl. Catal. B 30 (2001) 287. [5] H. He, H.X. Dai, L.H. Ng, K.W. Wong, C.T. Au, J. Catal. 206 (2002) 1. [6] N. Guilhaume, S.D. Peter, M. Primet, Appl. Catal. B 10 (1996) 325. [7] S.D. Peter, E. Garbowski, N. Guilhaume, V. Perrichon, M. Primet, Catal. Lett. 54 (1998) 79. [8] A.K. Ladavos, P.J. Pomonis, Appl. Catal. A 165 (1997) 73. [9] Z. Zhao, X. Yang, Y. Wu, Appl. Catal. B 8 (1996) 281. [10] L.Z. Gao, C.T. Au, Catal. Lett. 65 (2000) 91. [11] Z. Zhao, X. Yang, Y. Wu, Sci. China Ser. B 40 (1997) 464. [12] Y. Wu, Z. Zhao, Y. Liu, X. Yang, J. Mol. Catal. A 155 (2000) 89. [13] H. Lou, H. Zhen, J. Yang, Z. Yao, S. Yu, S. Du, F. Ma, Chem. J. Chinese Uni. 16 (1995) 107. [14] Z. Zhao, X. Yang, Y. Liu, Y. Wu, J. Chinese Rare Earth Soc. 16 (1998) 325. [15] D. Ganguli, J. Solid State Chem. 30 (1979) 353. [16] Y. Tokura, H. Takagi, S. Uchida, Nature 337 (1989) 345. [17] H. Zhen, S. Du, G. Lv, H. Lou, F. Ma, J. Wang, Acta Chim. Sinica 51 (1993) 373. [18] Z. Ma, Y. Qin, X. Qi, Z. Liang, F. He, Acta Phys.-Chim. Sin. 14 (1998) 453. [19] Y. Wu, T. Yu, B.S. Dou, C.X. Wang, J. Catal. 120 (1989) 88.