Study on Hydrothermal Stability and Catalytic Activity of Nanosphere

Mar 12, 2008 - The RMS had a three-dimensionally interconnected and disordered wormhole-like mesostructure, and its pore size was easily controlled by...
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5136

J. Phys. Chem. C 2008, 112, 5136-5140

Study on Hydrothermal Stability and Catalytic Activity of Nanosphere-Walled Mesoporous Silica Dong-Wook Lee, Chang-Yeol Yu, and Kew-Ho Lee* National Research Laboratory for Functional Membranes, EnVironment and Energy Research Center, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yuseong, Daejeon 305-606, Republic of Korea ReceiVed: December 24, 2007; In Final Form: February 4, 2008

The Rh-embedded mesoporous silica (RMS) with a silica-nanosphere pore wall was successfully synthesized using silica nanospheres as a framework and citric acid as a nonsurfactant template. The RMS had a threedimensionally interconnected and disordered wormhole-like mesostructure, and its pore size was easily controlled by simply changing the citric acid concentration or carrying out the aging treatment. In particular, we have first reported high hydrothermal stability and catalytic activity of the nanosphere-walled mesoporous silica in an ethanol steam reforming reactor at high temperature. As a result, the RMS showed 1.8% and 1.3% reduction of BET surface area and total pore volume after the ethanol steam reforming test for 5 h in the temperature range of 400 °C to 600 °C, whereas Rh-impregnated SBA-15 (RSBA15) showed 31.3% and 14.0% reduction of those. It is well-known that mesoporous materials have not been widely used as catalysts or catalyst supports in industry yet because of their poor hydrothermal stability. Accordingly, we have improved the hydrothermal stability of mesoporous silica through using silica nanospheres as a framework and have confirmed that the high hydrothermal stability of the RMS was attributed to the thermodynamically stable sphere-shaped pore wall, large intra-micropore volume of the silica-nanosphere wall, the thicker pore wall, and the highly branched structure of colloidal silica synthesized under base-catalyzed conditions. Moreover, compared to the RSBA15, the RMS gave a higher catalytic activity for the ethanol steam reforming because of its larger pore size and three-dimensionally interconnected pore structure, which lead to improvement in accessibility of reactants to the catalytic active sites.

1. Introduction Since surfactant-organized mesoporous silica was first prepared in 1992,1,2 much research has been reported on the synthesis of various mesoporous materials using supramolecular assembly of surfactant molecules.3-11 The mesoporous materials are generally synthesized with ionic11-13 or neutral14-17 surfactants as a structure-directing agent, and pore size of those materials is adjusted by changing the chain length of the surfactant molecules or hydrothermal treatment conditions or using swelling agents. Recently, since Wei et al.18,19 reported a novel preparation method of mesoporous silica via a nonsurfactant-templating route, which is versatile, low-cost, and nontoxic, several research groups synthesized nonsurfactanttemplated mesoporous silica using polymeric silica as a silica source and hydroxy-carboxylic acid compounds as a nonsurfactant template.20-26 Differently from the surfactant-templating route, using the nonsurfactant-templating route, pore size was readily controlled in the range from 2 to 10 nm by simply changing the concentration of nonsurfactant templates. More recently, we reported that pore diameter of mesoporous silica can be easily tailored up to 15 nm with high pore properties using the silica nanosphere-citric acid nanocomposite27 and successfully synthesized various nanosphere-walled mesoporous materials (NWM), which consist of a nanosphere framework and three-dimensionally interconnected wormhole-like pore structure.28-31 In the case of mesoporous titania-silica, the pore size was controlled up to 25 nm by only changing the concentration of citric acid.28 * To whom correspondence [email protected].

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In this study, we have synthesized Rh-embedded mesoporous silica (RMS) using silica nanospheres as a framework and citric acid as a nonsurfactant template. The pore size was easily controlled by changing the citric acid concentration or conducting aging treatment. In addition, we have first reported high hydrothermal stability and catalytic activity of the nanospherewalled mesoporous silica in a steam reforming reactor at high temperature. It is well-known that mesoporous materials have not been widely used as catalysts or catalyst supports in industry yet because of their poor hydrothermal stability under the critical conditions of industrial applications, where the materials are often exposed to water steam at high temperature.32 In order to improve their hydrothermal stability, several methods have been suggested, which includes pore wall-thickening,33,34 silylation of surface silanol groups,35,36 stabilization by addition of inorganic salts,37,38 and addition of aluminum.39,40 However, in the case of the wall-thickening approach, synthetic strategies for systematic control of the wall thickness have not been found. The silylation method is undesirable for hydrophilic applications because of the hydrophobicity of the pore walls. The stabilization by the salt effect is a time-consuming technique.38 In contrast, we have easily improved the hydrothermal stability of mesoporous silica through using silica nanospheres as a framework. In addition, while few publications have reported the hydrothermal stability of mesoporous silica during a true catalytic reaction test, we have reported their hydrothermal stability in a fixed-bed reactor for an ethanol steam reforming at high temperature.

10.1021/jp712051p CCC: $40.75 © 2008 American Chemical Society Published on Web 03/12/2008

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2. Experimental Section 2.1. Synthesis of Colloidal Silica Sol. Transparent colloidal silica sol was synthesized from base-catalyzed condition via hydrolysis of tetraethyl orthosilicate (TEOS) and condensation reaction. A TEOS:NH3:H2O:EtOH molar ratio was 1:0.086:53.6: 40.7. Prior to addition of a NH3/H2O mixture, a TEOS/ethanol mixture was stirred vigorously at 50 °C. The addition of the NH3/H2O mixture was carried out dropwise, followed by refluxing the final mixture for 3 h, resulting in a transparent colloidal silica sol including silica nanospheres with about 6-7 nm in diameter.27,29 2.2. Synthesis of Rh-Embedded Mesoporous Silica. Citric acid (CA) and 0.017 g of RhCl2 were added into 10 mL of 0.5 M aqueous HCl solution, and the resulting solution was vigorously stirred for 12 h at room temperature. Subsequently, the homogeneous solution was added into 50 mL of the transparent colloidal silica sol including 0.014 mol of Si, followed by vigorous stirring for 3 min at room temperature, resulting in a silica nanosphere-CA nanocomposite sol. After drying for 12 h in a vacuum oven at 70 °C and 9.5 Pa, and calcination for 2 h at 500 °C in air (heating rate: 1 °C/min), Rh(1wt.%)-embedded mesoporous silica was successfully synthesized. The Rh-embedded mesoporous silica is denoted RMSx, where x is a CA/Si molar ratio in the silica nanosphere-CA nanocomposite. To expand pore size of the RMS, we aged the silica nanosphere-CA nanocomposite sol for 20 h at 100 °C and then conducted drying of the aged nanocomposite for 12 h in a vacuum oven at 70 °C and 9.5 Pa and calcination for 2 h at 500 °C in air (heating rate: 1 °C/min). The Rh-embedded mesoporous silica synthesized with aging treatment is designated as RMS-x-ag, where x is a CA/Si molar ratio in the silica nanosphere-CA nanocomposite. 2.3. Synthesis of Rh-Impregnated SBA-15. For comparison, SBA-15-type mesoporous silica was also prepared via the synthetic method reported by Schmidt-Winkel et al.41 In a typical synthesis, 2.0 g of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer (EO20PO70EO20, Pluronic P123, Mav ) 5800, BASF) was dissolved in 75 mL of 1.6 M aqueous HCl solution at room temperature. A 4.4 g amount of TEOS was added into the HCl solution. After aging of the solution for 24 h at 35 °C, the cloudy mixture was aged for 24 h at 100 °C. After the white precipitate was filtered, washed, dried, and calcined for 8 h at 550 °C, SBA-15-type mesoporous silica was prepared. The Rh(1wt.%)/SBA-15, designated as RSBA15, was prepared by impregnating the SBA15 with RhCl2 precursor dissolved in a 0.5 M aqueous HCl solution by means of the incipient wetness method. Drying and calcination were carried out for 12 h at 70 °C and 9.5 Pa, and for 2 h at 500 °C in air (heating rate: 1 °C/min), respectively. 2.4. Catalytic Activity Test. In order to estimate the catalytic activity of the RMS, ethanol steam reforming tests were carried out in the temperature range of 400 to 600 °C for 5 h by using 1 g of RMS-0.46-ag, RMS-0.74-ag, RMS-1.49-ag and RSBA15 as a catalyst (180-250 µm powder). The ethanol steam reforming test was conducted for 1 h at each reaction temperature of 400 °C, 450 °C, 500 °C, 550 °C, and 600 °C. The catalysts were reduced in a hydrogen flow (70 mL/min) at 200 °C for 30 min and then purged with Ar (purity, 99.9999%). The liquid feed flow rate of a H2O/EtOH mixture with a molar ratio of 5 was 0.03 mL/min. The H2O/EtOH mixture was evaporated in a preheating line and diluted by Ar carrier gas with 70 mL/min of a flow rate. The concentrations of products and reactants were analyzed using a gas chromatograph

Figure 1. (a) Nitrogen sorption isotherms and (b) pore size distributions of the RMS-x.

equipped with a packed porapak T column and 5 Å molecular sieves column. 2.5. Characterization. TEM analysis was carried out on an FE-TEM, JEM-2100F Jeol operated at 200 kV. The X-ray diffraction (XRD) patterns were collected on a Rigaku D/MAX2200V instrument (operated at 1.6 kW) at a scanning rate of 1°/min in the 2θ range of 0.6-8°. Nitrogen adsorption/ desorption isotherms were taken by a Micromeritics ASAP 2020 instrument. Pore size distribution curves were obtained from the desorption branch by using the Barrett-Joyner-Halenda (BJH) method. Micropore volume and surface area were calculated from a t-plot. To evaluate the hydrothermal stability of the RMS, the pore properties of the RMS and RSBA15 were investigated after the ethanol steam reforming test for 5 h in the temperature range of 400 °C to 600 °C. 3. Results and Discussion Figure 1 presents nitrogen sorption isotherms and pore size distributions of the RMS-x with different concentrations of citric acid. The RMS gave a typical type IV isotherm with a H2 hysteresis loop as defined by IUPAC.42 It is clearly shown that the pore size distribution and sharp inflection of the hysteresis loop shifted toward higher pore size and higher P/Po value with an increase in the CA/Si molar ratio. As shown in Table 1, the average pore size of the RMSs increased from 6.8 to 8.9 nm with an increase in the CA/Si molar ratio and the pore volume from 0.9 cm3/g to 1.24 cm3/g. The BET surface area (SA) of the RMS was relatively constant in the range of 538 m2/g to 577 m2/g. In addition, the presence of intramicropore volume and surface area was confirmed by the t-plot of nitrogen sorption

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TABLE 1: Pore Properties of the RMSs Synthesized under Various Conditions sample

SABET a (m2/g)

SAmi b (m2/g)

SAme c (m2/g)

Vtot d (cm3/g)

Vmi e (cm3/g)

Df (nm)

RMS-0.46 RMS-0.74 RMS-1.49 RMS-0.46-ag RMS-0.74-ag RMS-1.49-ag

546 538 577 488 512 507

119 117 119 119 118 119

427 421 458 369 394 388

0.90 1.10 1.24 1.08 1.42 1.59

0.050 0.049 0.049 0.050 0.049 0.050

6.8 8.3 8.9 8.5 10.1 12.0

a BET surface area. b Micropore surface area calculated from a t-plot. c Mesopore surface area calculated from SABET - SAmi. d Total pore volume taken from the volume of N2 adsorbed at P/Po ) 0.995. e Micropore volume calculated from a t-plot. f BJH desorption average pore diameter.

Figure 3. The TEM image and XRD pattern (inset) of the RMS-0.46ag.

Figure 2. (a) Nitrogen sorption isotherms and (b) pore size distributions of the RMS-x-ag.

tests. The pore size of the RMS can be also controlled through aging treatment of the silica nanosphere-CA nanocomposite sol. Figure 2 and Table 1 exhibit the various pore properties of the RMS-x-ag after aging treatment for 24 h at 100 °C. The aging treatment resulted in an increase in pore size of the RMS from 6.8-8.9 to 8.5-12.0 nm. Their total pore volume increased from 0.9-1.24 cm3/g to 1.08-1.59 cm3/g. Figure 3 shows the TEM image and XRD pattern of the RMS-0.46-ag. It can be seen that the RMS has a three-dimensionally interconnected and disordered wormhole-like pore structure, and the pore wall of the RMS is composed of silica nanospheres with 6-7 nm in particle diameter. The RMS exhibits an intense reflection at a low 2θ corresponding to a pore-pore correlation distance of 12.1 nm with a broad shoulder in the 2θ range of 1.6-2.8°. The pattern is typical of disordered wormhole-like pore structures and is similar to that of MSU-type mesoporous silica. In order to estimate the catalytic activity of the RMS, ethanol steam reforming tests were carried out with the RMS-x-ag for 5 h in the temperature range of 400 °C to 600 °C. For comparison, the Rh-impregnated SBA-15 (RSBA15) was also

Figure 4. Ethanol steam reforming results of the RMS-x-ag and RSBA15 in the temperature range of 400 °C to 600 °C.

synthesized with the nonionic surfactant P123. Figure 4 presents the hydrogen yield from the ethanol steam reforming with the RMS-x-ag and RSBA15. The hydrogen yield of the RMS-1.49ag was much higher than the RSBA15 in the temperature range of 400 °C to 600 °C. The RMS-0.46-ag and RMS-0.74-ag gave higher catalytic activity compared to the RSBA15 above 500 °C. In spite of relatively lower surface area (Tables 1 and 2), the RMS showed higher catalytic activity compared to the RSBA15. Moreover, the hydrogen yield of the RMS- x-ag increased with an increase in the x (a CA/Si molar ratio). As shown in Figure 2 and Table 1, the pore size and pore volume

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TABLE 2: Variation in Pore Properties of the RSBA15 and RMS-1.49-ag after the Ethanol Steam Reforming Test for 5 h in the Temperature Range of 400 °C to 600 °C sample RSBA15 before reaction RSBA15 after reaction RMS-1.49-ag before reaction RMS-1.49-ag after reaction

SABET (m2/g)

SAmi (m2/g)

SAme (m2/g)

Vtot (cm3/g)

Vmi (cm3/g)

761

209

552

0.93

0.088

523

90

433

0.80

0.035

507

119

388

1.59

0.050

498

111

387

1.57

0.037

TABLE 3: Reduction Rates of Pore Properties after the Ethanol Steam Reforming Test for 5 h in the Temperature Range of 400 °C to 600 °C sample RSBA15 after reaction RMS-1.49-ag after reaction

SAme Vtot Vmi SABET SAmi reductiona reduction reduction reduction reduction (%) (%) (%) (%) (%) 31.3

56.9

21.6

14.0

60.2

1.8

6.7

0.3

1.3

26.0

Figure 5. Nitrogen sorption isotherms of the RMS-1.49-ag and RSBA15 before and after the ethanol steam reforming test for 5 h in the temperature range of 400 °C to 600 °C.

a The reduction rates were calculated from (SA before reaction SA after reaction)/SA before reaction × 100.

of the RMS increased with increasing the citric acid concentration. That is, the RMS with larger pore size and pore volume gave better catalytic activity for the ethanol steam reforming. Therefore, it was revealed that the catalytic activity of the RMS can be readily controlled through tuning the pore size and pore volume of the RMS, which can be tailored by simply changing the concentration of citric acid. In addition, it was confirmed that the three-dimensionally interconnected and disordered mesostructure could be favorable for the ethanol steam reforming compared to two-dimensional hexagonal and ordered pore structure. In other words, the higher catalytic activity of the RMS was attributed to the larger pore size and pore volume and the three-dimensionally interconnected mesostructure. This is because the well-connected mesostructure and large pore size and volume allow the reactant molecules to more readily access the active sites.43 For evaluation of the hydrothermal stability of the RMS, the various pore properties of the RMS and RSBA15 were investigated after the ethanol steam reforming test for 5 h in the temperature range of 400 °C to 600 °C. Figure 5 and Table 2 exhibit the variation in the nitrogen sorption isotherm of the RMS-1.49-ag and RSBA15 after the ethanol steam reforming. The nitrogen sorption isotherm of the RSBA15 before the ethanol steam reforming test is a type IV curve with a type H1 hysteresis loop (Figure 5), and its BET surface area and total pore volume were 761 m2/g and 0.93 cm3/g, respectively. After the ethanol steam reforming, the adsorbed nitrogen volume of isotherm significantly decreased. The BET surface area of the RSBA15 decreased considerably from 761 m2/g to 523 m2/g, and the pore volume from 0.93 cm3/g to 0.8 cm3/g. In contrast, the RMS showed a type IV nitrogen sorption isotherm with a type H2 hysteresis loop (Figure 5). After the ethanol steam reforming, little change in the isotherm of the RMS was observed, and its BET surface area and pore volume were almost constant. Table 3 presents reduction rates of pore properties after the ethanol steam reforming. As a result, the RMS showed 1.8% and 1.3% reduction of BET surface area and total pore volume after the ethanol steam reforming test for 5 h in the temperature range of 400 °C to 600 °C, whereas the RSBA15 showed 31.3% and 14.0% reduction of those. The reduction rates of micropore

Figure 6. Hydrogen yield from ethanol steam reforming as a function of reaction time at 500 °C.

and mesopore surface area of the RSBA15 calculated from the t-plot were 56.7% and 21.6%, respectively. In contrast, those of the RMS were 6.7% and 0.3%. While both micropore and mesopore surface areas of the RSBA15 considerably decreased, the RMS showed a slight decrease in those. Figure 6 shows the hydrogen yield from ethanol steam reforming as a function of reaction time at 500 °C. Compared to RMS-1.49-ag, RSBA15 showed more severe deactivation till 5 h of reaction time. From the results shown in Figure 5 and Tables 2 and 3, it is concluded that the RMS has higher hydrothermal stability compared to the RSBA15. Differently from the various mesoporous silica synthesized with surfactant or nonsurfactant templates, we employed the transparent colloidal silica sol including a silica nanosphere of 6-7 nm in particle diameter as a framework of the mesoporous silica. The silica-nanosphere pore wall is a key factor to improve the hydrothermal stability of mesoporous silica in our study. In the case of the polymeric silica sol synthesized under acidcatalyzed condition, the rate of hydrolysis is faster compared to the rate of condensation. After monomers are depleted, condensation between completely hydrolyzed species occurs by cluster-cluster aggregation, leading to weakly branched structures of primary silica particles with low fractal dimension. On the contrary, for colloidal silica sol prepared under basecatalyzed conditions, dissolution and redistribution reactions provide a continual source of monomers which condense preferentially with clusters rather than each other. Therefore,

5140 J. Phys. Chem. C, Vol. 112, No. 13, 2008 growth occurs primarily by monomer-cluster aggregation, resulting in highly branched primary particles with high fractal dimension.44 It is considered that the highly branched structure of the colloidal silica leads to rigidity of the silica framework, making it possible to maintain the high surface area and pore volume during the steam reforming reaction at high temperature. Moreover, the sphere shape with minimized surface energy is the thermodynamically stable form of amorphous particles.45 Meanwhile, as shown in Table 2, for both samples of the RMS and RSBA15, the reduction rate of the micropore surface area was higher than that of mesopore surface area. According to the publication reported by Zhang et al.,32 the mesoporous materials with a larger micropore volume and thicker pore wall have better hydrothermal stability in the presence of steam at high temperature because of recombination of Si-O-Si on the amorphous silica pore wall. That is, in the case of the mesoporous silica with a thinner wall, the recombination occurring primarily around the micropores in the wall would cause direct destruction of the pore wall and mesostructure. However, for the mesoporous silica with a thicker wall, the recombination does not severely destroy the mesostructure. Therefore, it is concluded that the large intramicropore volume and thick pore wall of the RMS also contributed to the high hydrothermal stability. In summary, the hydrothermal stability of mesoporous silica can be easily improved through using silica nanospheres as a framework, and the high hydrothermal stability of the RMS was attributed to a thermodynamically stable sphereshaped pore wall, large intramicropore volume of silica-nanosphere wall, thicker pore wall, and highly branched structure of colloidal silica synthesized under base-catalyzed conditions. 4. Conclusions We have successfully synthesized the RMS with a silicananosphere pore wall via a nonsurfactant-templating route. Its pore size was readily tuned by changing the citric acid concentration or conducting the aging treatment. For confirmation of hydrothermal stability and catalytic activity of the RMS, ethanol steam reforming tests were carried out in the temperature range of 400 °C to 600 °C. As a result, the RMS showed higher catalytic activity compared to the RSBA15. The RMS with a larger pore size and pore volume gave better catalytic activity for the ethanol steam reforming. The higher catalytic activity of the RMS was attributed to the larger pore size and pore volume and the three-dimensionally interconnected mesostructure. This is because the well-connected mesostructure and large pore size and volume allow the reactant molecules to more readily access the active sites. In addition, the RMS has high hydrothermal stability compared to the RSBA15. The RMS showed 1.8% and 1.3% reduction of BET surface area and total pore volume after the ethanol steam reforming test, whereas the RSBA15 showed 31.3% and 14.0% reduction of those. It is considered that the high hydrothermal stability of the RMS is attributed to a thermodynamically stable sphere-shaped pore wall, large intramicropore volume of silica-nanosphere wall, thicker pore wall, and highly branched structure of colloidal silica synthesized under base-catalyzed conditions. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.;

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