One-pot synthesis and characterization of high surface area perovskite-type BaTiO3 with mesoporous texture

465

Studies in Surface Science and Catalysis 156 M. Jaroniec and A. Sayari (Editors) 9 2005 ElsevierB.V. All rights reserved

One-pot synthesis and characterization of high surface area perovsldte-type BaTiO3 with mesoporous texture Bo Hou a'b, Zhijie Li a'b, Yao Xu a, Dong

Wu a

and Yuhan Sun a*

aState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China. E-mail: [email protected] bGraduate School of the Chinese Academy of Sciences, Beijing, 100039, China.

High surface area perovskite-type BaTiO3 with mesoporous texture were prepared via hydrothermal and solvothermal method and then characterized by TEM, XRD and BET. The results revealed that samples synthesized by the different methods had high surface area of 52 and 61 m2.g-1, respectively. The as-synthesized sample via hydrothermal method has better thermal stability. The catalytic performance of the samples was investigated by selective oxidation and dehydrogenation of benzyl alcohol, which indicated that the high surface area of BaTiO3 could enhance the total conversion and the selectivity ofbenzylaldehyde.

1. INTRODUCTION Barium titanate, an important perovskite oxide, has been extensively investigated in the past decades due to its application in the manufacture of multilayer ceramic capacitors, thermistors, fuel cell electrodes, gas detection sensors, electrooptic components, and so on [1-4]. Recently, the perovskite oxides were reported to be used as catalysts or catalyst supports for complete oxidation of hydrocarbons and exhaust gases [5, 6], CO2 reforming of CH4 [7-10], spatial separation of photochemical oxidation [ 11], etc. Also, barium titanate used as catalyst or catalyst supports for these reactions was reported [7, 11-12]. In these studies, barium titanates were indirectly used to decompose gaseous pollutants [ 12] or used as catalyst supports for CO2 reforming of CH4 [7-8]. T. Hayakawa et al. [7] studied Ni/perovskite catalysts for CO2 reforming of CH4 and found that the CaTiO3 and BaTiO3 perovskite materials could afford alkaline earth metals in the catalyst, which may result in a high resistance against coke formation. The perovskites BaTiO3 was also reported to be used as the low-cost substitute for expensive metal three-way catalyst, which has potential application in reducing gaseous pollutant emissions. T. Kawasaki et al. [ 12] investigated the influence of the physical properties of BaTiO3 pellets on the efficiency of NOx decomposition in BaTiO3 packed-bed reactors. The efficient NO removal without using catalysts and/or additional reductants in BaTiO3 packed-bed reactors could be achieved by adjusting the preparation method of BaTiO3 powder and the manufacture of BaTiO3 pellets. The surface areas and thermal properties of BaTiO3 powder are very important to attain efficient BaTiO3 pellets or BaTiO3-type catalyst. R. Sumathi et al. [13] studied the selective oxidation and

466

dehydrogenation of benzyl alcohol on ABB'O3 (A=Ba, B=Pb, Ce, Ti and B=Bi, Cu, Sb)-type perovskite oxide and found that the reducibility of the perovskite oxides depended on the nature of B site cation and extent of substitution at B sites. As used as catalyst, the surface areas of materials are the most important physical performance but there are few reports about the preparation of high surface area perovskite-type BaTiO3 and its catalysis capabilities. Apart from solid-state synthesis [14-16] and coprecipitation method [ 17, 18], which are industrialized processes to attain micrometer barium titanate powder, there are wet chemical methods to prepare barium titanate, such as sol-gel method [19], solvothermal method [20-22], sol-precipitation process [23], combustion synthesis method [24], low temperature direct synthesis method [25] et al. Generally, the wet chemical method could attain nanometer BaTiO3 powder with average particle size <_ 100 nm, but the surface areas of these materials are mostly in 10-25 m2.g1 range which is a range acceptable for applications in catalysis, but higher areas are desirable. M. Penarroya Mentruit et al. [26] reported that BaTiO3 having surface area of 45 m2.g-lwere prepared by sol-gel method using titanium ethoxide as precursor. In addition, Z. Novak et al. [27] and K.M. Hung et al. [28] also attained nanometer BaTiO3 having higher surface area of 147 m2.g! and 60 m2.g"1 via sol-gel method, respectively, but these methods used costly reagent and required especial post-treatment or high temperature calcination. The high surface area barium titanate can also be attained by other wet chemical methods, in which the solvothermal method is the most potential method in industry. Comparing with the sol-gel method and coprecipitation method, the solvothermal and hydrothermal methods have many advantages including low cost, simple operation, and, in particular, avoiding high temperature calcination. H.J. Chen et al. [29] prepared nanometer BaTiO3 with surface area of 53 m 2 .g- 1 and average particle size 40 nm by hydrothermal method using newly-prepared Ti(OH)4 as precursor. In the present work, we reported one-pot hydrothermal and solvothermal preparation of high surface area perovskite-type BaTiO3 with mesoporous texture. Moreover, the influence of BaTiO3 surface area on the catalytic performance was investigated by selective catalytic oxidation and dehydrogenation of benzyl alcohol.

2. MATERIALS AND METHODS

2.1. Preparation of BaTiO3 Sample 1 was prepared by following procedure: at ice bath, 6 ml of triethanolamine was added into 80 ml of distilled water, and stirred for 10 min to form a solution A. 2 ml of titanium tetrachloride (TiCI4) was dropped slowly into the solution A and stirred to attain a clear solution B. Excess NaOH (5 g) was added to the solution B to form a white suspension and keep stirring for 10 min. The suspension was placed into a 100 ml autoclave and hydrothermally treated at 418 K for 48 h. After the reaction completed, cool down to room temperature, and 5.33 g BaCI2"2H20 was added in the white suspension. The autoclave was sealed and treated at 393 K for 12 h. After reaction completed, the gained precipitation was filtrated and washed with distilled water, then dried in oven at 383 K for 12 h. Sample 2 was prepared by non-aqueous solvothermal method. At room temperature, 1 ml of titanium tetrachloride (TIC14) was dropped slowly into 40 ml of absolute ethanol to form a clear yellowish solution A. 3.44 g barium hydroxide (Ba(OH)2-8H20) ( Ba/Ti mol ratio is 1.2) was dissolved into 40 ml of ethylene glycol monomethyl ether (HO(CH2)OCH3) to form solution B. The solution A was added dropwise into the solution B and then a white stiff suspension appeared. Excess NaOH, 5 g, was added the above-mentioned suspension. Then,

467 the white suspension was placed into a 100 ml autoclave, sealed and treated at 473 K for 48 h. After reaction completed, the gained precipitation was filtrated and washed with distilled water, then dried in oven at 383 K for 12 h. Sample 3, comparing with the above as-prepared samples, was also prepared by routine hydrothermal method without pre-treatment of Ti-processor. At ice bath, 2 ml of titanium tetrachloride (TiCI4) was dropped slowly into 80 ml of distilled water to form a clear solution, and then 5 g NaOH and 5.33 g BaCI2"2H20 were added in the solution, and stirred for 10 min to attain a white suspension. The white suspension was placed into a 100 ml autoclave, sealed and treated at 393 K for 12 h. After reaction completed, the gained precipitation was filtrated and washed with distilled water, then dried in oven at 383 K for 12 h.

2.2. Characterization of BaTiO3 powder and catalysts Their crystal structures were investigated at room temperature using a powder X-ray diffractometer (XRD) (Philips X' PERT-Pro-MPD, Cu-Ka, 40 kV, 20 mA) with a 0.02 ~ step size at the range of 20~ ~ The morphologies of the samples were examined using transmission electron microscopy (Hitachi-600-2, Japan) and the average particle sizes were calculated from about 200 particles. N2 adsorption/desorption isotherms of the samples were measured at 77 K on a Micromeritics Tristar 3000 sorptometer. The surface areas were calculated by BET method, and the pore diameter distributions were determined from the adsorption branch of the isotherms using BJH method. 2.3. Catalysis test The as-synthesized samples were pressed under 40 MPa for 30 min, then crushed and sifted. The particles were collected in the range of 20-40 mesh to be used as catalyst. The catalytic performance was characterized by the reaction of selective oxidation and dehydrogenation of benzyl alcohol. Gas phase catalytic oxidation of benzyl alcohol was carried out in a fixed bed continuous flow reactor made of stainless steel with an i.d. of 10 mm. The temperature of the catalyst bed was measured with the help of a thermocouple placed at the centre of the catalyst bed. The benzyl alcohol was fed into the reactor by means of an infusion pump. The catalyst was aged at 723 K for 4 h before the reaction. The reaction temperature was 673 K and the carrier gas was air. The flow rate of carrier gas is 20 ml.min ~ and contact time (W/F; W = weight of the catalyst, F = the flow rates of benzyl alcohol) was kept constant at 1.25 g.h.ml 1. Products were collected in an ice-cold trap after attaining steady state conditions. The quantitative analyses of products were identified by Gas-Chromatography (GC-920, SICT).

3. RESULTS AND DISCUSSION Fig. 1 displays the XRD pattems of the samples prepared in our experiment. The XRD patterns of samples reveal a simple cubic perovskite due to the absence of splitting for (002)/(200)(20~45 ~ peak, which implies the powders are stabilized in the cubic phase at room temperature [25]. There are no other impurities existing except for barium carbonate, which is labeled with asterisk mark in Fig.1. Comparing with Sample 1 and 3, Sample 2 has lower diffraction intensity and broader diffraction peak, which indicate lower crystallization and smaller crystal size [20].

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, BaCO 3 sample 3 200

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Figure 1. XRD patterns of the samples Fig.2 shows the TEM images of the samples. Sample 2 and 3 have uniform particle shape; Sample 1 has an irregular particle shape. The average particle size of Sample 3 is 70 nm, which is the smallest among the three samples. Sample 1 and 2 before and atter calcinations at 823 K for 2 h were characterized by nitrogen adsorption at 77 K in order to measure their surface area and to determine their porosity. The results of N2 adsorption/desorption and the pore size distributions are showed in Fig. 3 and Fig. 4. The pore size distribution of each sample was analyzed from the adsorption branch of corresponding isotherm using BJH method. It is seen that both samples have a hysteresis loop at a high relative pressure, suggesting that Sample 1 and 2 prepared in our experiment are basically mesoporous materials [26]. Fig. 3 (lett) displays the N2 adsorption/desorption isotherms of Sample 1 before and aider calcined at 823 K, both of the isotherms are very similar, indicating a well thermal stabilization mesoporous structure of sample 1 even aider calcinated at high temperature, which is important to be used as catalyst support prepared by high temperature impregnation method. N2 adsorption/desorption isotherms and pore size distribution plot of Sample 2 change greatly and the surface area decrease from 61 m2.g~ to 28 m2.g~ atter calcination, indicating that the structure stability of Sample 2 is weaker than that of sample 1. In addition, the most frequent pore radius increase and the pore volume decrease atter both samples are calcined, but the change trend of

Figure 2. TEM images of the samples

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Figure 3. N2 adsorption/desorption isotherms (left) and pore size distribution (right) of Sample 1 before (solid) and after (dot) calcined at 823 K for 2 h Sample 2 is more obvious. Except of micropores and mesopores, there are some obvious macropores in Sample 1 (see Fig. 3, right) resulting from accumulation between particles resulting from its irregular particle shape. Comparing with the two samples, the Sample 3 has surface area of 15 m2.g1 and the particle size calculated from surface area is 68 nm, which is consistent with that from TEM, this suggests there are no pores in Sample 3 [27]. The catalytic performance was investigated by benzyl alcohol oxidation in the presence of oxygen. The major products obtained in the partial oxidation of benzyl alcohol on all the perovskite oxides in the absence of oxygen are benzaldehyde and toluene. However, when the reaction is carried out in the presence of oxygen, the small amounts of benzoic acid and benzyl benzoate are also obtained [13]. The carrier gas in our experiment is air, so the product contains the above-mentioned compounds. Conversion rate of benzyl alcohol oxidation on different samples is shown in Fig. 5. Initially, the conversion on Sample 3 is higher than that of Sample 1 and 2; but, when the reaction runs longer than 3 h, the result is opposite. At the same time, in most reaction time, the conversion rate of sample 1 is higher than that of Sample 2. Benzaldehyde selectivity of benzyl alcohol oxidation on different samples is shown in Fig. 6. During almost the whole reaction course, the benzaldehyde selectivities on sample 1 and 2 are both better than that of Sample 3. It is observed that the conversion and selectivity passes

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Table 1. Catalytic performance for benzyl alcohol oxidation on different samples

NO Sample 1 Sample 2 Sample 3

Rpa

Vpb

(nm)

(cm3.g "l)

area

0.16 0.14 -

(m2'g-~) 52 61 15

10.4 10.2 -

Surface Conversion (mole %) 69.9 56.5 61.6

Selectivity (mole %) Benzal -dehyde 91.6 91.2 83.1

Toluene 1.6 2.3 5.8

Benzoic Other acid impurity 0.1 6.7 0.6 5.9 11.1

a: most frequent pore radius, b: pore volume

through a maximum at the range of 3-4 h and subsequently decrease with the processing of reaction. This is may be due to coke formation in the course of reaction. Catalytic partial oxidation of alcohols is a useful method to prepare aldehydes and ketones [ 13]. So, although Sample 3 has high conversion initially, its benzaldehyde selectivity is low, indicating that the high surface BaTiO3 can greatly enhance the benzaldehyde selectivity in whole reaction course and the conversion in the upper reaction. The highest benzaldehyde selectivity of Sample 1 appears about at 4 h. It should be pointed out that the change trend of conversion rate of sample 2 is relatively steady. The catalytic activities and the values obtained from the Brunauer-Emmett-Teller surface area, pore volume and the most frequent pore radius of different samples are summarised in Table 1. The catalytic performance data for benzyl alcohol oxidation on different samples are attained from the top conversion rate of each sample. It can be seen that Sample 1, whose conversion rate and benzaldehyde selectivity are the highest, is more active than other two samples. Although the surface area of Sample 2 is higher than that of Sample 1, its catalysis activity is lower than Sample 1, because the calcinations stability of Sample 2 is weaker. It can be pointed out that both sample 1 and 2 have a considerably high surface area and enough large pore to satisfy gas phase catalytic reaction. In contrast to Sample 3, Sample I and 2 having high surface area exhibit better catalysis selectivity.

471 4. CONCLUSIONS The high surface area BaTiO3 with mesoporous texture were synthesized via one-pot hydrothermal and solvothermal methods, their surface area are 52 and 61 m2.gl respectively. From the results of BET measured before and after calcination at 823 K for 2h, the samples have mesoporous texture and the hydrothermal BaTiO3 has higher structure stability than that from solvothermal method. Furthermore, the high surface area BaTiO3 with mesoporous texture may be used as catalyst supports prepared by high temperature impregnation method. It can be concluded from benzyl alcohol catalytic oxidation in the presence of oxygen that high surface area BaTiO3 can enhance the benzaldehyde selectivity in the course of the whole reaction and the conversion rate in upper reaction. The catalytic mechanism of high surface area BaTiO3 needs further investigation. ACKNOWLEDGEMENT The financial support from the National Key Nature Science Foundation (No. 20133040) was gratefully acknowledged. REFERENCES

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