Environ. Sci. Technol. 2005, 39, 1319-1323
Nanoporous Silica-Supported Nanometric Palladium: Synthesis, Characterization, and Catalytic Deep Oxidation of Benzene JIN-JUN LI, XIU-YAN XU, ZHENG JIANG, ZHENG-PING HAO,* AND CHUN HU Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, People’s Republic of China
In this present study, nanoporous silica SBA-15 supported palladium catalysts are prepared through two different methods. The catalysts are employed for catalytic deep oxidation reaction of benzene at a high gas hourly space velocity of 100 000 h-1. It is found that the traditional aqueous impregnation method has some difficulties and disadvantages in obtaining highly dispersed palladium active phases. Whereas, when a grafting procedure is employed, palladium tends to be highly dispersed as nanoparticles due to the confinement of the nanosized pore channels of the support materials. The catalysts prepared via the grafting procedure catalyze the benzene oxidation far more effectively than those prepared via aqueous impregnation method, and complete conversion of benzene can be achieved below 190 °C over the most active catalyst. The nanoporous silica-supported palladium catalysts are promising materials for the control of some types of VOCs such as benzene.
Introduction Volatile organic compounds (VOCs) are a large class of air pollutants that can give rise to many environmental problems due to their toxic or photochemical properties. A wide range of industrial and commercial processes are responsible for the discharge of VOCs. Various end-of-pipe techniques have been developed to reduce VOCs emissions. Among them, catalytic deep oxidation that converts VOCs into carbon dioxide and water has been recognized as one of the most promising methods for the control of VOCs (1, 2). Catalytic deep oxidation is an energy-saving technique, for it generally operate at a low temperature less than 500 °C. In contrast, conventional thermal incineration always requires operation temperatures as high as over 1000 °C. The activity of the catalysts employed is an important factor determining the effectiveness of the catalytic deep oxidation technique. To prepare catalysts with high activity, porous materials with large surface areas are often used as support to disperse the active metals in nanoscale, forming large amounts of active sites. Besides, the pore size of the support materials should be considered when designing the catalysts. The materials with too small pore size take the risk of blocking the porosity after the loading of active metals, thus it is difficult for the VOC molecules to reach the inner active sites. In 1992, the synthesis of a new type of nanoporous * Corresponding author telephone: +86-10-62849194; fax: +8610-62923564; e-mail:
[email protected]. 10.1021/es0491174 CCC: $30.25 Published on Web 12/31/2004
2005 American Chemical Society
silica molecular sieve (M41S) was reported by researchers of the Mobil Corporation (3, 4), who used micelles constituted by surfactant molecules in solution as the templates to direct the assembly of silica gel. Such materials have desirable properties in the field of catalysis and pollution control (5) (e.g., high specific surface areas, controllable pore sizes, and uniform mesopores). Since then, a great deal of work has been followed to the synthesis of various types of mesostructured materials. Zhao et al. (6, 7) synthesized another type of mesoporous silica (SBA-15) under acidic conditions, using a triblock copolymer surfactant as the template. Such materials attracted wide attention for they have even larger pore diameter and better thermal stability. While purely siliceous materials are catalytically inactive, various methods to introduce active atoms into the silica-based matrix, including direct synthesis (8-10), wet impregnation (1113), and post-synthesis grafting (14-17) have been attempted in the past decade. The validity of the direct synthesis method is limited in the synthesis of metal-containing SBA-15 due to the acidic synthetic condition, under which the metal tends to exist as free ions and would not bond to the silica wall. The wet impregnation method is the traditional and generally used way to prepare supported catalysts. However, this method has some difficulties in preventing the active phases from agglomerating outside the pores of the supports. Postsynthesis grafting procedures involve modifying the silica host with some types of organosiloxane containing organic functional groups, such as thiol (14), amine (15), EDTA (16), etc. In most cases, the organic moieties are planted onto the silica wall by refluxing the dry toluene or hexane containing both pre-synthesized silica and organosiloxane. Then the resultant materials are dispersed in solutions containing metal ions, and the metal ions are adsorbed by the functional groups. Upon calcination, the metals are transformed into oxides located within the inner pore channels of the silica host. This is an attractive way to obtain well-defined metallic nanoparticles, for the silica host could confine the size and the shape of the guest materials in nanoscales. Furthermore, the size of the nanoparticles could be controlled by tuning the pore diameters of the host. In the present study, we prepared SBA-15-supported palladium catalysts via an aqueous impregnation method. Meanwhile, we also prepared silica-supported nanometric palladium catalysts via post-synthesis route. In this way, amino groups are used to adsorb palladium ions, and the organic amino groups are anchored at the silica wall in one step through co-condensation of the silica source and organosiloxane when preparing SBA-15. The prepared catalysts are characterized by TG/DSC analysis, FTIR, nitrogen adsoption/desorption, and TEM techniques; their activity for deep oxidation of benzene (a typical VOC) is tested in a continuous-flow fixed-bed reactor.
Experimental Section Catalyst Preparation. The nonionic triblock copolymer surfactant Pluronic P123 (EO20PO70EO20) and organosiloxane 3-aminopropyltrimethoxysilane (APTMS) were from SigmaAldrich. Tetraethyl orthosilicate (TEOS) was used as the silica source, and concentrated HCl was used as the acid source. In a typical synthesis of purely siliceous SBA-15, 5 g of P123 was dissolved in 155 g of water, and 31.5 g of concentrated HCl was added, followed by the addition of 10.5 g of TEOS as silica source to the above solution while stirring. After being stirred for 2 h, the mixture was aged at 35 °C for 22 h, and then it was transferred into an autoclave and aged at 100 °C for 48 h. The solid product was filtered, VOL. 39, NO. 5, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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dried at 100 °C for 24 h, and then washed in 250 mL of ethanol containing a little HCl. After drying, the material was calcined in a muffle furnace at 550 °C for 3 h to decompose the surfactant P123 completely, thus forming the siliceous SBA15. Various amounts of palladium were loaded onto the siliceous SBA-15 via the aqueous impregnation method with palladium chloride as precursor. The impregnated solids were calcined at 500 °C for 3 h and then reduced in hydrogen at 400 °C for 1 h. The samples labeled as PS(w), the value of w in the parentheses is the weight percent of palladium metal to the SBA-15 support. The preparation of the amine-functionalized silica sample was similar to that of siliceous SBA-15. The difference was that both 9.4 g of TEOS and 0.9 g of APTMS were added to the acidic P123 solution at the same time. After the mixture has been aged at 35 °C for 22 h and at 100 °C for 48 h, the solid product was filtered, and the surfactant in the wet cake was removed by repeated extraction with ethanol containing a little HCl instead of calcination. The resultant aminefunctionalized silica was labeled as SAF. To load palladium onto SAF, 0.5 g of this material was dispersed in 50 mL of palladium chloride solution of various concentrations, thus the palladium ions can be chelated to the amino groups, contact kept for 24 h, followed by filtration and drying. The solids were calcined at 500 °C for 3 h and then reduced in hydrogen at 400 °C for 1 h. The obtained catalysts were labeled as PSA(w), the value of w in the parentheses is the weight percent of palladium metal in the solution to the SAF support. Catalyst Characterizations. Thermal analysis (TG/DSC) was conducted on a Setaram Labsys-16. In each experiment, about 15 mg of a sample was used, and the temperature was raised from 20 to 900 °C with a heating rate of 10 °C/min, under an air flow of 30 mL/min. Infrared analysis of the samples was recorded on Bruker Tensor27 using DRIFT technique, scanned from 4000 to 650 cm-1. Nitrogen adsorption/desorption isotherms were obtained at liquid nitrogen temperature, using NOVA1200 gas sorption analyzer. Prior to the experiments, all the samples were degassed under vacuum at 300 °C for more than 5 h except for the sample SAF, which was degassed under vacuum at room temperature for 36 h in order to prevent the organic moiety from decomposition. The specific surface areas (SBET) were determined from the linear part of the BET plot (P/P0 ) 0.05-0.25). The mesopore size distributions (PSD) were derived from the desorption branch of the N2 isotherms using the Barrett-Joyner-Halenda (BJH) method (18). The total pore volumes were estimated from the amount of nitrogen adsorbed at a relative pressure (P/P0) of ca. 0.99. TEM images were taken on a JEOL JEM-200CX electron microscope at an accelerating voltage of 200 kV. All samples were crushed, dispersed in ethanol, and deposited on a microgrid prior to observation. Catalytic Activity Evaluation. Catalytic activity tests were performed in a continuous-flow fixed-bed reactor of 6 mm i.d. that was placed in a tubular electrical furnace equipped with a temperature programmer. In each test run, 0.05 g of the catalyst diluted with appropriate amount of inert quartz beads (40-60 mesh) was placed at the center of the reactor, above which a thermocouple was located to monitor the reaction temperatures. To create the stream containing benzene, one stream of pure air was passed through a boatshaped saturator in an ice-bath to make a mixed gas containing benzene of high concentration and then further diluted with another stream of pure air before reaching the catalyst bed. And the total flow rate was set to be 320 mL/ min with the concentration of ca. 1050 ppm by adjusting the two flow rates. The gas hourly space velocity (GHSV) in the tests was kept ca. 100 000 h-1. An on-line gas chromatograph equipped with FID detector was used to analyze the concentration of benzene in the inlet and outlet gas. Another 1320
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FIGURE 1. Thermal analysis of the amine-functionalized silica. on-line gas chromatograph was used to analyze the concentration of carbon monoxide in the outlet gas. Before each test, the temperature of the catalytic bed was raised to 120 °C under the feed stream and stabilized at that temperature until the concentration of benzene became constant, generally no benzene oxidation being observed at this temperature, then the temperature of the catalyst bed was raised with a heating rate of 5 °C/min, and the temperature was kept constant for 2 min at each datum point for equilibrium prior to analyzing the benzene concentration of the outlet gas. Catalyst stability tests for both PS and PSA samples were carried out under the same conditions as the activity evaluation runs. In each run, the catalyst bed was first heated to a temperature at which complete conversion was achieved, then decreased to a certain temperature, and maintained constant at that temperature for about 60 h.
Results and Discussion Thermal Analysis of the Amine-Functionalized Silica. Figure 1 displays the TG/DSC profile of the as-synthesized aminefunctionalized silica SAF. Three apparent weight loss steps can be seen in the TG curve. The first one below 150 °C is attributed to the desorption of adsorbed water or ethanol, which is also reflected by the endothermic effect centered at 96 °C in the DSC profile. The second step between 200 and 300 °C can be attributed to the decomposition of the polymeric template, which is accompanied by the intense exothermic effect (19). The release of water formed from the condensation of silanol groups in the silica framework could also contribute to a smaller extent to this weight loss step (19). It can be deduced that some surfactant molecules still remain in the pore despite the fact that the material has been washed by ethanol repeatedly. Probably the strong interaction between the polymer and the amine-containing organic moieties that are chemically bonded to the silica wall make it difficult to remove the surfactant completely. The third weight loss step occurs between 550 and 600 °C, which is reasonably assigned to the decomposition of amino groups. About 10% of the weight is lost in this step. FIIR Study of the Materials. Figure 2 shows the DRIFT spectra of the silica and the supported palladium catalysts. It is well-known that the adsorption band at 3745 cm-1 for both siliceous SBA-15 and PS(5%) sample is attributed to the stretching vibrations of isolated silanol groups, which is accompanied by a broad band from 3700 to 2500 cm-1 assigned to stretching frequencies of hydrogen-bonded silanols, often called silanol nests (20). In the organic aminemodified sample SAF, the band at 3745 cm-1 disappeared. Meanwhile, the absorption associated with silanol nests is quite broadened. It has been reported that the silylation of silica would develop a band at 2975 cm-1 attributed to the C-H stretching vibration of methyl groups (21). Therefore, the broadening of the band could be a combined result of both silanol nests and organic moiety. Furthermore, it could be expected that the organosiloxane APTMS would assemble
FIGURE 2. DRIFT spectra of the materials. in priority around the micelles of P123 due to the strong van der Waals force between the two phases during the hydrothermal synthesis procedure. Thus the organic moieties would tend to locate on the mesopore surface, reducing the amount of the exposed terminal silanols. In addition, a band around 1509 cm-1 appeared only in the SAF sample that could be attributed to C-H bending vibration of organics (22), also indicating the presence of the organic moiety. Upon calcination, the band at 3745 cm-1 appears again in the PSA(5%) sample, and the hydrogen-bonded silanols absorption is extensively intensified as compared to the siliceous SBA-15, indicating that the decomposition of the organic moiety leads to a large amount of uncondensed Si-O units exposed, forming lots of Si-OH groups. In all the spectrums, the band at ca. 1869 and 1634 cm-1 is caused by SiO2 overtone and deformational vibrations of adsorbed water molecules (22, 23), respectively. The band at 815 cm-1 is assigned to symmetric Si-O-Si stretching (2224), and the band at around 1177 cm-1 could be attributed to the asymmetric stretching of the same units that become less intense in the functionalized sample SAF (24). The interpretation for the band at ca. 970 cm-1 is still a matter of debate. Some researchers have taken this band as evidence for the isomorphous substitution of Si atoms by heteroatoms (22). However, a similar band has also been found to be present in purely siliceous materials, which has been attributed to another absorption band of silanol groups in addition to the band at 3745 cm-1. Some authors attributed this band to the Si-O stretching vibrations of Si-Oδ--Rδ+ (22), as RdH in the purely siliceous materials. In our work, this band in SAF and PSA(5%) sample is more intense than that in SBA-15, thus we propose that Cδ+ and Pdδ+ could act as counterions for Si-Oδ- defect sites. Surface and Textural Properties. Figure 3 shows the nitrogen adsorption/desorption isotherms and the derived pore size distributions of siliceous SBA-15, SAF, and PSA(5%). All the isotherms with an irreversible hysteresis loop are of type IV as defined by IUPAC (25). The N2 isotherms of purely siliceous SBA-15 agrees well with that reported in the literature, which feature a hysteresis loop of H1 type and a sharp step in the P/P0 range from 0.6 to 0.8 (6, 7). The inflection point of the step corresponds to a diameter in the mesopore range, and the sharpness of the step indicates the uniformity of the pore size distribution. The loading of palladium on SBA-15 had little influence on its textural properties (Table 1), since nearly identical isotherms were obtained for both SBA-15 and palladium-loaded sample PS(5%) (not shown). Both amine-functionalized silica SAF and palladiumincorporated PSA(5%) feature a hysteresis loop of H2 type. In the case of SAF sample, the step occurs in a relative pressure range similar to that of purely siliceous SBA-15, while the amount of nitrogen adsorbed is higher than that of the latter,
FIGURE 3. Nitrogen adsorption/desorption isotherms (a) and pore size distributions (b) of the materials.
TABLE 1. Textural Properties of the Materials sample
BET surface areas (m2/g)
pore vol (cm3/g)
mean pore diameter (nm)
SBA-15 PS(5%) SAF PSA(5%)
642 626 609 770
1.08 1.05 1.17 0.60
6.7 6.7 7.7 3.1
which is indicative of a larger pore volume of the former (Table 1). In fact, the mean pore diameter of the former is also a little larger (Table 1). In the case of PSA(5%), the step occurs in a relatively lower P/P0 range, indicating that the material possessed a relatively narrower pore size though it is still in the mesopore range (Figure 3b). Also a decrease of the total pore volume is observed (Table 1). Apparently, the addition of organosiloxane during the synthesis brings some change to the textural properties of the obtained sample. On one hand, the organosiloxane APTMS could possibly be dissolved in the formed micelle of P123 surfactant and induce an expansion of the diameter of the micelle template, thus resulting in an increased pore diameter of the templated functionalized silica SAF. On the other hand, the surfactant removed by extraction instead of high-temperature calcination during the synthesis of SAF, there would be ample uncondensed silanol groups (Si-OH) on the surface of SAF. Furthermore, some organic moieties would be embedded in the silica wall of the pore, leading to a less condensed structure of the pore wall. Upon calcination, the organic moieties decomposed and a further condensation of silanol groups happened, causing the contraction and the collapse of the pore. As a result, the PSA have a decreased pore diameter (Table 1). Notably, the PSA samples have considerably increased specific surface areas in comparison with SAF sample (Table 1), probably the decomposition of the organic moieties let some pore surface exposed. Microscopic Images. The TEM images of PS(5%) and PSA(5%) samples are shown in Figure 4. Siliceous SBA-15 support has ordered pore structure characterized by parallel channels and uniform pore diameters. The pore channels are rather long in length up to more than 250 nm. In contrast, VOL. 39, NO. 5, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Stability tests for benzene combustion with time-onstream over PS(5%) and PSA(9%) catalysts (4) PS(5%) at 240 °C; (0) PSA(9%) at 185 °C. FIGURE 4. TEM images of PS(5%) (a) and PSA(5%) (b).
FIGURE 5. Conversion curves of benzene catalytic deep oxidation over mesoporous silica-supported palladium catalysts. the length of the pore channel of the PSA sample is obviously short, and most of them are less than 20 nm. In addition, the pore channels of the PSA sample tend to be randomly arrayed. It could be proposed that the decomposition of the organic moieties during calcinations caused the rupture of the silica wall to a certain extent, leading to such properties of the PSA sample. Unfortunately, there are palladium particles existing outside the pore channels of both samples. In particular, crystallites of palladium with extremely large diameter could be observed when taking the microscopic images of PS(5%) sample (not shown), indicating that severe sintering of the active phase occurred. Activity of the Catalysts. The ignition curves of benzene catalytic deep oxidation over PS and PSA series of catalysts are displayed in Figure 5. The conversion values in the plot are calculated based on the concentration of benzene in the inlet and outlet gas. Among the PS series of catalysts prepared via aqueous impregnation method, the sample with a 5% Pd loading seems to be the most active, upon which the complete conversion of benzene was reached at ca. 270 °C. Some decrease could be observed when the loading increased to 7%, as is indicated by the fact that the conversion curve is slightly shifted toward higher temperatures. A phenomenon has been observed during the impregnation procedure that the aqueous solution of palladium chloride had some difficulties in spreading over the surface of SBA-15 possibly due to the hydrophobic property of the silica surface. Thus, we proposed that the palladium species would aggregate on the external surface when too many palladium ions were in the solution, resulting in the poor dispersion of the active phase. Whereas, in the case of PSA series of catalysts prepared via a grafting procedure, the activity is monotonically improved with increasing the concentration of palladium chloride in the solution. But when the value of w exceeds 5%, the increase of the activity appears to be less remarkable, which could be reasonably explained in terms of a gradual saturation of the amine groups by the palladium ions. The surplus metallic ions remained non-incorporated in the solution when the concentration of the precursor was too high. 1322
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Apparently, the activity of the catalysts prepared via a grafting procedure, which has steep conversion curves, are remarkably superior to those prepared via an impregnation method. As for PSA(9%) catalyst, the conversion of benzene becomes appreciable at less than 140 °C, approaching a conversion of 50% at ca. 173 °C, and quickly achieving complete conversion at less than 190 °C. Considering the high space velocity of 100 000 h-1 during the activity tests, it seems to be a rather good result of catalytic performance. Whereas, as for PS(5%) catalyst, 50% and 99% of benzene conversion were reached at ca. 205 and ca. 270 °C, respectively, which are even higher than that of PSA(1%). It could be proposed that the amino groups dispersed on the SAF host surface chelated the palladium ions in stoichiometry. Hence, the palladium ions would tend to be anchored homogeneously at the functionalized silica surface, and most palladium nanoparticles formed upon calcinations and reduction would be well-dispersed on the internal surface of the mesoporous support, and the crystallites would not grow up due to the confinement of the nanosized pore channels. Meanwhile, the relatively short pore channels of PSA series of catalysts could favor the diffusion of the reactant molecules to reach the active nanoparticles inside the pore. In contrast, extremely long pore channels of PS series of catalysts would make a long way for the reactants to reach the inner active sites. Furthermore, the impregnated palladium tends to agglomerate on the hydrophobic surface of SBA-15, thus the pore channels could be jammed to some extent, making the palladium in the intermediate position less accessible. In conclusion, the traditional aqueous impregnation method has some difficulties and disadvantages in preparing highly dispersed palladium catalysts supported on SBA-15, though the support material possessed high surface areas. Whereas, a grafting procedure seems promising in designing nano-dispersed catalysts on these mesoporous materials, and these catalysts should be effective and promising materials for the elimination of some benzene-like VOCs. It should be noted that carbon monoxide often appears in the oxidation of hydrocarbon over some catalysts. However, we detected no sign of carbon monoxide at any temperature over both PS and PSA catalysts in our work. Stability Tests of the Catalysts. The stability of the catalysts are crucial in application because the water produced during the reaction could possibly cause the change of the catalysts surface, decreasing the catalytic activity. Figure 6 shows the stability tests of PS(5%) and PSA(9%) catalysts. The operation temperature is 240 °C for PS(5%) and 185 °C for PSA(9%), respectively. Though some decrease of the conversion value happened within the first 5 h for both catalysts, no noticeable further decrease was observed in the following 55 h, indicating that the catalytic performance of both catalysts are well-sustained.
Acknowledgments Financial support from the National Basic Research Program of China (2004CB719500), the key project of Knowledge
Innovation of Chinese Academy of Sciences (KZCX3-SW430), and the project of China Natural Science Foundation (20322201) is gratefully acknowledged.
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Received for review June 11, 2004. Revised manuscript received October 28, 2004. Accepted November 1, 2004. ES0491174
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