Synthesis and Characterization of High Surface Area

Tin Oxide Nanocatalyst for the Hydrogenation of Styrene

J. Chin. Chem. Soc., Vol. 57, No. 2, 2010

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Synthesis and Characterization of High Surface Area Tin Oxide Nanoparticles via the Sol-Gel Method as a Catalyst for the Hydrogenation of Styrene Rohana Adnan,* Nur Ariesma Razana, Ismail Abdul Rahman and Muhammad Akhyar Farrukh‡ School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia

A systematic study on the preparation of SnO2 nanoparticles using a simple sol-gel technique has been conducted by varying reaction parameters such as concentration of ammonia, ammonia feed rate and reaction temperature. The tin oxide obtained was characterized by using FTIR, BET, XRD and TEM. Particles size was obtained in the range of 4 to 5.6 nm and the surface area was found to be between 76 to 114 m2 g-1 depending on the reaction parameters. Meanwhile, the catalytic activity of SnO2 was first time investigated for the hydrogenation reaction of styrene using ethanol as the solvent at 70 °C and 1 atmospheric pressure. It is found that SnO2 acts as a good catalyst in this hydrogenation process. The product conversions in the presence of catalysts prepared at different conditions were between 37 to 72%. Keywords: Tin oxide nanoparticles; Sol-gel method; Catalysis; Hydrogenation; Styrene.

INTRODUCTION Tin oxide is an important material due to its properties such as high degree of transparency in the visible spectrum, strong physical and chemical interaction with adsorbed species, low operating temperature and strong thermal stability in air (up to 500 °C).1 It is an n-type semiconductor with a band gap of 3.6-3.8 eV.2 Tin oxide has been used as gas sensing material for gas mixture such as CO and O2.3 In addition to that, tin oxide is also widely used in optoelectronic devices,4 electrochemical properties5 and lithium batteries.6 Tin oxide received little attention in the catalysis 7 field compared to other metal oxides. However, tin oxide supported catalysts have been reported to be active for oxidative dehydrogenation of propane, 8 CO oxidation, 9 esterification reaction,10 reduction of NO/NO2 to N211 and hydrogenation reaction of nitrate.12 Tin oxide has been more commonly used as a catalyst for the oxidation of organic compounds.13 In most of its applications, high surface area metal oxides such as SnO2 is favorable and preferred due to high number of surface active groups. Generally, the high surface area SnO2 can be produced with various surfactants such as cetyltrimethylammonium bromide, CTAB, 14 dodecylamine,15 tetradecylamine1,16 and sodium dioctylsul-

fosuccinate, AOT.17 However, the difficulty to remove the surfactant from the sample, affects the properties of the final product. SnO2 can be synthesized using a variety of techniques such as sol-gel, hydrothermal method, precipitation, carbothermal reduction, polymeric precursor18 and thin film by chemical vapor deposition.19 In a sol-gel process the precursor solution is converted into an inorganic solid by a) dispersion of colloidal particles in a liquid (sol) and b) conversion of sol into rigid phase (gel) by hydrolysis and condensation reactions.20 The sol-gel method is well applicable for the synthesis of nanoparticles of oxides of different metals like Sn, Ti, V, Zr, Ta, Nb, Hf, In, Fe, Cr, Ni, Mn, Sm, W, Li, Al in aqueous,21,22 non-aqueous (organic)23,24 mediums with or without surfactants.25,26 Hydrogenation of styrene has been done by a number of catalysts composed of a) transition metals of rhodium,27 palladium, platinum,28 iron, 29 and nickel/sepiolite30 b) complexes of perfluoroalkylated pyridine–palladium,31 [H3Os4(CO)12]- 32 and c) nanomaterials of NaH,33 TiO2.34 There is no study concerning the catalytic properties of SnO2 alone in hydrogenation reaction has been reported in the literature. In this research, we had prepared tin oxide using sol-gel technique, a versatile process for making ce-

* Corresponding author. Tel: +6046533549; Fax: +6046574854; E-mail: [email protected] ‡ Permanent address: Department of Chemistry, GC University Lahore, Pakistan, E-mail: [email protected]

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ramic and glass materials. The sol-gel process allows better control on the properties of nanoparticles such as volatilization, contamination, phase separation, etc. The advantage of being able to control these properties lead to the formation of highly pure and homogeneous multi component compounds.35,36 This method involves hydrolysis and condensation processes and ammonia is commonly used to force in hydrolysis to form tin oxide nanoparticles. However, there were no reports by other researchers regarding the study of the effect of ammonia feed rate. Herein, we present a systematic study on the effect of ammonia concentration, ammonia feed rate and reaction temperature on the particles size, particles distribution and surface area of tin oxide nanoparticles using sol-gel method. The catalytic activity of tin oxide nanoparticles for the hydrogenation reaction of styrene to produce ethyl benzene was also investigated. EXPERIMENTAL Chemicals The chemicals used in this study were tin(IV) chloride tetrahydrate (SnCl4.5H2O, Fluka), ammonium hydroxide (NH4OH 25%, Merck), sodium hydroxide (NaOH, R & M Chemicals) and ultra pure water obtained by SQ-Ultra Pure Water Purification System, Germany. All reagents were used as received without any further purification. Synthesis of SnO2 nanoparticles Tin oxide nanoparticles were prepared by the sol-gel route. Generally, 2.94 g tin(IV) chloride was added to 50 mL ultra pure water in a round bottom flask and was stirred for 20 min. A certain amount of ammonia solution (25%) was added into the mixture under a controlled feed rate (0.01-0.1 mL min-1) and constant stirring. The pH values range between 1 to 10.2 and reaction temperatures were varied between 30 to 90 °C. After 2 h of stirring, the sol was aged at room temperature for 24 h. The resulting gel was then washed with ethanol until the pH of the solution became 7. After drying at 80 °C for 24 h in air, the obtained powder was ground using mortar and pestle and finally calcined at 400 °C for 2 h. Catalyst characterization The SnO2 samples were characterized by FTIR spectroscopy (Perkin-Elmer System 2000), BET surface area analysis (Micromeritics Instrument Corporation Model ASAP 2000), powder X-ray diffractometry (Siemens dif-

Adnan et al.

fractometer D5000, Kristalloflex), and SEM/EDX analysis (Edax Falcon System). The morphology of the samples was examined using Philips CM12, 80 kV transmission electron microscopy (TEM). Meanwhile the particle size was measured using analysis Docu Version 3.2 image processing software. The average particle size and statistical parameters were determined based on the measurement of more than 300 particles from TEM micrographs. Catalytic reaction Hydrogenation process was performed at 70 °C under 1 atmospheric pressure in a closed 50 mL three neck flask (glass reactor) equipped with magnetic stirrer in a thermostated water bath. The flask has a gas inlet and outlet and a silicon-rubber septum sample inlet. An amount of 0.01 g catalyst was fed into the flask containing 5 mL ethanol (solvent). The catalyst was activated by flowing hydrogen gas for 1 h at 70 °C under vigorous stirring. The reaction was started immediately after 0.5 mL styrene was injected. The mixture was stirred for 5 h and then filtered to remove the catalyst. The filtrate was analyzed by using GC (Hitachi, Japan, supelcowax capillary column, 30 m). The column temperature was programmed as follows; temperature: 40150 °C, time: 1-10 min, and heating rate: 5 °C min-1. RESULTS AND DISCUSSION Effect of ammonia concentration on the synthesis of tin oxide nanoparticles The study on the effect of ammonia concentration on tin oxide nanoparticles was carried out by varying the concentration of ammonia (from 1.07 to 10.67 M) while the feed rate of ammonia and reaction temperature were kept constant at 0.1 mL min -1 and 30 °C, respectively. Fig. 1 shows the FTIR spectra of SnO2 synthesized using different concentration of ammonia. The broad band around 3394-3409 cm-1 region is due to the stretching vibration of O-H bond. This band is due to the OH groups and the adsorbed water bound at the SnO 2 surface. 37 The peak at 1620-1630 cm-1 is attributed to the bending vibration of water molecules, trapped in the SnO2 sample.38 The peak at 521 cm-1 in the sample prepared using 1.07 M of ammonia (Fig. 1a), agrees with the stretching vibrations of the terminal Sn-OH,17,39 while a peak at 660-600 cm-1 region corresponds to the stretching modes of the Sn-O-Sn.39 As the concentration of ammonia is increased, the peak at 521



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Fig. 1. The FTIR spectra of SnO2 prepared different using ammonia concentration (a) 1.07 M, (b) 3.20 M, (c) 5.34 M, (d) 7.48 M and (e) 10.67 M.

cm-1 disappears while the peak at 669 cm-1 becomes stronger. According to Ibarguen et al., 18 at low pH value, the Sn-OH group is formed. At higher pH, the Sn-OH groups disappear and the band at 630 cm-1 which correspond to Sn-O-Sn becomes stronger17,18,39 indicating the following reaction has taken place. SnOH + SnOH ® Sn-O-Sn + H2O

(1)

Table 1 shows the mean particle sizes and surface area of SnO2 particles using different ammonia concentration. Consistently, the increase in particle sizes is followed by the decrease in surface area. As the concentration of ammonia varies from 1.07 to 10.67 M, which result in the increase of the pH from 1 to 10.2, the particle size increases marginally from 4.2 to 5.6 nm. Meanwhile, the surface area dropped from the highest, 114 to 76 m2 g-1. The value of tin oxide surface area obtained in this study is higher compared to the sample prepared using water-in-oil microemulsion technique in the presence of surfactant which were around 107 m2 g-1,40 86 m2 g-1,41 and 73 m2 g-1.17 The concentration of ammonia fed into the reaction mixture was found to have significant effect on SnO2 particle distribution as shown in TEM images (Fig. 2). This be-

Table 1. Particle sizes and BET surface area of SnO2 samples synthesized using different ammonia concentrations at 30 °C and 0.1 mL min-1 feed rate Sample

[NH3] (M)

pH

Particle size (nm)

Surface area (m2 g-1)

BD1 BD2 BD3 BD4 BD5

01.07 03.20 05.34 07.48 10.67

01.0 08.7 09.7 09.9 10.2

4.2 ± 0.9 4.6 ± 1.1 4.7 ± 1.3 5.4 ± 1.3 5.6 ± 1.8

114 111 089 088 076

havior is similar with the result reported by Park et al. on silica nanoparticles.42 At low ammonia concentration, 1.07 M, well-dispersed SnO2 nanoparticles with soft aggregation were obtained. The particles showed better distribution when the concentration of ammonia is increased up to 5.34 M. However, a further increased in the concentration of ammonia results in high particle agglomeration believed to be due to excessive generation of primary particles at super saturation state.43 Effect of ammonia feed rate on the synthesis of tin oxide nanoparticles Table 2 shows the effect of ammonia feed rate on par-

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Table 2. Particle sizes and BET surface area of tin oxide using different ammonia feed rate. Ammonia concentration: 3.20 M and reaction temperature 30 °C Sample BF1 BF2 BF3 BF4

NH3 feed rate (mL min-1)

Particles size (nm)


0.100 0.050 0.017 0.010

4.5 ± 1.7 4.4 ± 1.0 4.0 ± 0.9 4.7 ± 1.1

111 95 99 88

ticle size and surface area of the tin oxide nanoparticles prepared. Fig. 3 shows the TEM images of SnO2 nanoparticles obtained with feed rate from 0.1 to 0.01 mL min-1 at constant temperature of 30 °C and concentration of ammonia of 3.20 M. The results show that ammonia feed rate significantly effect the particle sizes. The rapid growth of the particle size is observed when the faster feed rate is used. This behavior can be explained by the higher rate of hydrolysis and condensation process44 thus affecting the growth of tin oxide particles. As a result, bigger particles are formed and the larger standard deviation is contributed by particles aggregation. Rahman et al.45 in their study on the prepara-

Fig. 2. TEM micrographs of SnO2 at different concentration of ammonia (a) 1.07 M, (b) 3.20 M, (c) 5.34 M, (d) 7.48 M and (e) 10.67 M. [Scale bar: 50 nm]

tion of silica also found that slower ammonia feed rate results in smaller, homogeneous and narrowly distributed particles size of silica. The authors claimed that this is due to the extended induction period, which generates primary particles at slower feed rate. In this study, slower ammonia feed into solution resulted in smaller and highly dispersed particles as shown in Table 2. However, too slow of ammonia feed rate, 0.001 mL min-1, causes the formation of bigger particle, 4.7 nm compared to only 4.0 nm. The increases of the particle size is believed to be due to the aggregation of the particle (Fig. 3d) whereby smaller and less stable particles were deposited on more stable particles to form larger particles.44 In general, the surface area of tin oxide decreases when slower NH3 feed rate is used. Effects of reaction temperature on the synthesis of tin oxide nanoparticles Temperature is known to be one of the most important parameter which influences particles size37 and their distributions. In this study, the experiment was carried out by varying the reaction temperature from 30 to 90 °C, while the concentration of ammonia and its feed rate were kept constant at 3.20 M and 0.1 mL min-1, respectively. Fig. 4 shows the XRD patterns of SnO2 prepared at different reaction temperatures. Diffraction patterns in all samples prepared agree well with that of SnO 2 (cassiterite) (JCPDS 21-1250). The grain size of the nanocrystals was estimated

Fig. 3. TEM micrographs of SnO2 at different ammonia feed rate (a) 0.1 mL min -1 , (b) 0.05 mL min -1 , (c) 0.017 mL min -1 and (d) 0.01 mL min-1. [Scale bar: 50 nm]



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Fig. 4. XRD patterns of SnO2 prepared at different reaction temperature (a) 30 °C (b) 50 °C (c) 70 °C and (d) 90 °C.

using the Scherrer equation based on the full width at half maximum of the 110 diffraction peak. The crystallite size of nanoparticles obtained from XRD spectra are in agreement with the results obtained from TEM analysis (Table 3). It is observed that the peaks become broader as the reaction temperature increases indicating smaller particles size. Zhang and Gao39 found that tin oxide samples prepared at 100 °C and calcined at 300, 400 and 500 °C also showed broader peaks in the XRD diffractogram when the calcination temperature is increased. However, the size of tin oxide particles prepared in this study is smaller than tin oxide prepared by the authors at 100 °C and calcined at the same temperature which was around 8.4 nm.39 The higher temperature favors a fast hydrolysis reaction and results in high supersaturation which also can be attributed to high nucleation rate.42 This will lead to the formation of a large number of small nuclei and eventually lead to the formation of small particles. Consistently, the surface area of the samples is larger when higher reaction temperature is used. Fig. 5 shows the TEM images of SnO2 synthesized at

Table 3. Particle sizes and surface area of SnO2 prepared at different reaction temperatures. Ammonia concentration: 3.20 M and ammonia feed rate: 0.1 mL min-1 Sample BT1 BT2 BT3 BT4

Particles size (nm)

Temperature (°C)

(TEM)

(XRD)


30 50 70 90

4.8 ± 1.3 4.5 ± 1.2 4.4 ± 1.2 4.1 ± 1.0

5.9 5.6 5.1 4.8

79 83 92 94

different reaction temperature. At low temperature (30 °C and 50 °C), the SnO2 nanoparticles obtained are uniformly spherical with some degree of agglomeration. As the reaction temperatures were increased, the aggregation of the particles becomes more dominant. This can be attributed to the higher solubility and collision between particles.46 Catalytic hydrogenation of styrene The hydrogenation reaction of styrene (vinyl benzene) to form ethyl benzene can be shown by following simple reaction;

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Fig. 6 shows the percentage of product conversion in the hydrogenation reaction of styrene using selected tin oxide samples at 70 °C and 1 atm. The tin oxide prepared at low ammonia concentration, 1.07 M gave the percentage of product conversion around 55%. When the concentration of ammonia in the preparation of tin oxide was increased to 3.20 M (pH 8.7), the percentage of conversion to ethyl benzene also increased to 58%. We believed the good catalytic activity of the catalysts was influenced by their high surface area which was around 111 to 114 m2 g-1. Further increase in the concentration of ammonia up to 5.34 M resulted in lower surface area as shown in Table

Fig. 5. TEM micrograph of SnO2 at different of reaction temperature (a) 30 °C (b) 50 °C (c) 70 °C and (d) 90 °C. [Scale bar: 50 nm]

Fig. 6. The product conversion of hydrogenation reaction of styrene using the tin oxide prepared using different of ammonia concentration.

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Fig. 7. The product conversion of hydrogenation reaction of styrene using the tin oxide prepared at different reaction temperature.

1. The sharp decreases of the tin oxide surface area also resulted in low activity as can be seen from the percentage of product conversion. However, the tin oxide prepared using higher ammonia concentration, 7.48 M, the percentage of product conversion increases back up to 50% although the surface area did not increase. The percentage of product conversion reached the highest value of 72% using the tin oxide prepared with 10.67 M of ammonia. The high activity of the catalyst prepared in high alkaline condition was probably influenced by its surface active groups, Sn-OH. The reaction temperature used during the preparation of tin oxide also influenced the catalytic activity of tin oxide in the hydrogenation reaction of styrene. At low reaction temperature, 30 °C, percentage of product conversion of 37% was obtained as shown in Fig. 7. The percentage of product conversion increases up to 54% as the temperature in tin oxide preparation was increased to 90 °C. As discussed earlier, higher preparation temperature produced higher surface area tin oxide particles. The catalytic behavior of the tin oxide samples is also consistent with their particles size where smaller particles size was obtained at higher reaction temperature (Table 3) and shows better product conversion. Catalytic activity of tin oxide nanoparticles towards conversion of styrene to ethyl benzene is more than the other nanocatalysts reported. Product conversion by TiO2, TiO2/Si, TiO2/CN was achieved to 10, 14, and 40% respectively.34 CONCLUSION Tin oxide nanoparticles were successfully synthe-


sized using a simple sol-gel method. It was found that the surface area decreases with increasing ammonia concentration due to particles agglomeration. It was also observed that the slower ammonia feed rate produced a significant effect on particles size and distribution. However, a very slow feed rate led to bigger particles. Meanwhile, higher reaction temperature results in smaller particle sizes and larger surface area. The catalytic activities of tin oxide nanoparticles prepared were investigated in the hydrogenation reaction of styrene. It was revealed that tin oxide prepared in high alkaline condition (pH 10.2) exhibited high catalytic activity with the percentage of conversion around 72%. Meanwhile, higher reaction temperature during the preparation of tin oxide also resulted in higher product conversion. ACKNOWLEDGMENTS The authors acknowledge the supports by MOSTI and USM through Escience Fund grant (305/PKIMIA/ 613309) and FRGS grant (203/PKIMIA/671083) as well as TWAS. Received January 14, 2010. REFERENCES 1. Zhu, J.; Tay, B. Y.; Ma, J. Mater. Lett. 2006, 60, 1003. 2. Bagheri-Mohagheghi, M. M.; Shahtahmasebi, N.; Alinejad, M. R.; Youssefi, A.; Shokooh-Saremi, M. Physica B. 2008, 403, 2431. 3. Fort, A.; Mugnaini, M.; Rocchi, S.; Serrano-Santos, M. B.; Vignoli, V.; Spinicci, R. Sens. Actuators B. 2007, 124, 245. 4. You, Z. Z.; Dong, J. Y. Microelectron. J. 2007, 38, 108. 5. Kobayashi, H.; Uebou, Y.; Ishida, T.; Tamura, S.; Mochizuki, S.; Mihara, T.; Tabuchi, M.; Kageyama, H.; Yamamoto, Y. J. Power Sources. 2001, 97, 229. 6. Kwon, C. W.; Campet, G.; Portier, J.; Poquet, A.; Fournès, L.; Labrugère, C.; Jousseaume, B.; Toupance, T.; Choy, J. H.; Subramanian, M. A. Inter. J. Inorg. Mater. 2001, 3, 211. 7. Pyke, D. R.; Reid, R. T.; Tilley, R. J. D. J. Chem. Soc. Faraday Trans. 1. 1980, 76, 1174. 8. Sadykov, V. A.; Pavlova, S. N.; Saputina, S. N.; Zolotarskii, I. A.; Pakhomov, N. A.; Moroz, E. M.; Kuzmin, V. A.; Kalinkin, A. V. Catal. Today. 2000, 61, 93. 9. Hagemeyer, A.; Hogan, Z.; Schlichter, M.; Smaka, B.; Streukens, G.; Turner, H.; Volpe, J. A.; Weinberg, H.; Yaccato, K. Appl. Catal. A. Gen. 2007, 317, 139. 10. Khder, A. E. R. S. Appl. Catal. A. Gen. 2008, 343, 109.


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