Synthesis and Characterization of Silica Nanoparticles Preparing by

Jul 10, 2014 - An economical and environmentally benign method is proposed for the preparation of silica nanoparticles by hydrolysis of silicon tetrac...
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Synthesis and Characterization of Silica Nanoparticles Preparing by Low-Temperature Vapor-Phase Hydrolysis of SiCl4 Feng Yan,† Jianguo Jiang,*,†,‡,§ Xuejing Chen,† Sicong Tian,† and Kaimin Li† †

School of Environment, Tsinghua University, Beijing 100084, China Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education, Beijing 100084, China § Collaborative Innovation Center for Regional Environmental Quality, Beijing 100084, China ‡

ABSTRACT: An economical and environmentally benign method is proposed for the preparation of silica nanoparticles by hydrolysis of silicon tetrachloride vapor with water vapor at a low temperature (∼150 °C). Analysis by X-ray fluorescence, X-ray diffraction, and scanning electron microscopy, and examination of nanoparticle size distribution, specific surface area, and pore diameter revealed that dry processing was more suitable than wet processing for collection of nanoparticles. The porous amorphous silica had high purity (99.89 wt % SiO2), large specific surface area (342.44 m2/g), and a size distribution (162.8 ± 41.0 nm, polydispersity index = 0.221), and was prepared at a reaction temperature of 150 °C and reaction time of 5 s. The thermal stability of the silica nanoparticles was verified by demonstrating that calcination at temperatures ≤ 600 °C could remove surface hydroxyl groups, achieving hydrophobic modification, while maintaining particle mesostructure. The reduced cost of the synthesis route presented here is a result of the low reaction temperature and the inexpensive materials used in the process, making this a promising method for wide use in various high-end applications.

1. INTRODUCTION Silica nanoparticles are advanced functional materials used widely as additives to rubbers,1 plastics, and paints2 because of their high specific surface area, dispersion, purity, and ease of use.3 Silica has recently attracted significant attention for its utility in a wide range of emerging applications, such as catalyst support,4,5 adsorption of CO2,6,7 and drug delivery in biomedical engineering.8,9 A large number of methods have been reported for the preparation of silica nanoparticles, these can be categorized as either wet- or vapor-phase processes. Wet-phase methods involve simultaneous hydrolysis and condensation, including sol−gel processing,10 chemical precipitation,11 microemulsion processing,12 hydrothermal techniques, and pressurized carbonation.13 A vapor-phase method, in which silicon tetrachloride (SiCl4) is hydrolyzed in a hydrogen−oxygen flame (1800 °C) to synthesize fumed silica, was developed in the 1960s.14 Tetraethoxysilane (TEOS), which is easier to handle than SiCl4, was also used in many studies as a precursor for silica particles through the vapor-phase route.15 Compared with the wet-phase method, the product obtained using vapor-phase techniques is of higher purity and has a more uniform particle-size distribution, larger surface area and a smoother nonporous surface.16 However, the development of alternative methods for preparation of high-quality silica nanoparticles by lowering energy consumption and production costs is of considerable interest, and lowering temperature is a good solution to save energy consumption and lower requirement for expensive equipment. A promising method involving direct hydrolysis between chloride vapor and water vapor at relatively low temperatures was recently proposed for the preparation of nanoparticles.17 SiCl4 is a byproduct of the polysilicon industry that can cause severe environmental pollution.18 Thus, preparation of silica © 2014 American Chemical Society

nanoparticles using this byproduct has the potential to be commercially advantageous while helping to ameliorate environmental pollution. Isobe et al.19 prepared porous silica from SiCl4 at 300 °C using an ultrasonic spray method, but the water was not in the vapor phase and this lead to a large range in particle size (1−3 μm). In Park’s study,20 SiCl4 vapor was hydrolyzed at 150 °C to allow for uniform particle growth and formation of oxychloride particles, which were then converted to nearly monodispersed silica spheres with a uniform size distribution through further hydrolysis at 1000 °C; however, the impurity of the chlorine content was determined to be 0.5 at. % and this method also required high temperatures. To our knowledge, a low-temperature vapor-phase hydrolysis method for synthesizing silica nanoparticles in a single step has not been reported previously, nor has a comprehensive examination of the key parameters of this process been performed. In the present work, silica nanoparticles were obtained by the hydrolysis of SiCl4 vapor with water vapor at a low temperature range from 125 to 300 °C. The advantages of this method for synthesizing silica nanoparticles include inexpensive starting materials, a simple operation process, and low energy consumption. Two recovery methods for nanoparticle collection were compared first using a series of characterization studies, and silica nanoparticles with a high specific surface area and uniform particle-size distribution were obtained at an optimal reaction temperature and reaction time. The properties of the silica nanoparticles after thermal treatment were then investigated. Received: Revised: Accepted: Published: 11884

April 29, 2014 June 13, 2014 July 10, 2014 July 10, 2014 dx.doi.org/10.1021/ie501759w | Ind. Eng. Chem. Res. 2014, 53, 11884−11890

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Figure 1. Schematic drawing of the experimental apparatus.

Table 1. Chemical Composition of Dry-SiO2 and Wet-SiO2

a

sample

SiO2 (%)

Na2O (%)

Cl (%)

CaO (%)

MgO (%)

SO3 (%)

Fe2O3 (%)

Cr2O3 (%)

dry-SiO2 wet-SiO2

99.8944 87.0046

n.d.a 7.7208

n.d. 4.7170

0.0109 0.1779

n.d. 0.1765

n.d. 0.1418

0.0645 0.0357

0.0303 0.0256

Element not detected in sample.

volume of ∼40 cm3 in total measured by water capacity; the main pipe was 3 cm in length and 4.5 cm in diameter, and the branch pipe was 5 cm in length and 0.8 cm in diameter. The elbow pipe was kept warm (∼120 °C) by heat tape, resulting in the silica nanoparticles agglomerating in the collector while being exposed to the NaOH solution. The product suspension was centrifuged, filtered, washed, and dried at 105 °C in a vacuum oven (Labcab DZF-6020, China). The reaction temperature, reaction time, and recovery method were different for each sample; samples were thus designated by their reaction conditions (e.g., SiO2-125 for 125 °C reaction temperature; SiO2-4s for 4-s reaction time; dry-SiO2 for the dry process). 2.2. Characterization. Chemical analysis of the particles was conducted using an X-ray fluorescence analyzer (XRF, Shimadzu XRF-1800, Japan). The crystal structure of particles was recorded by a high-resolution X-ray diffraction (XRD, Siemens D8 Advance, Germany) using Cu Kα radiation (λ = 0.154 18 nm) in the 2θ range of 10−70° (scanning rate of 6°/ min). The size of silica agglomerates and the particle-size distribution was examined using a nanoparticle size analyzer (Beckman coulter DelsaNano C, USA); 0.3 wt % nanoparticle was dispersed in water by ultrasonic waves for 20 min before the measurement. The morphology of silica nanoparticles was characterized by scanning electron microscopy (SEM, Hitachi S-4500, Japan). Nitrogen adsorption/desorption isotherms of nanoparticles at 77 K were collected on a gas adsorption analyzer (Micrometrics Instrument ASAP2020 HD88, USA); all samples were degassed in a vacuum at 90 °C for 1 h and at 160 °C for 2 h before measurement. Specific surface area (SSA) was calculated using the Brunauer−Emmett−Teller (BET) method, and the total pore volume was estimated from the amount of nitrogen adsorbed at a relative pressure (P/P0) of 0.99. The average pore diameter was derived from the desorption branch of the N2 isotherm using the Barrett−

2. METHODS 2.1. Particle Preparation. The experimental setup consisted of a precursor evaporator unit, a tubular reactor unit, and a particle collector unit (Figure 1). The evaporator (1 L) was made of quartz glass and was heated by a heating mantle (Taisite 98-1-C, China), and the rate of evaporation was controlled by heating power. Distilled water and SiCl4 (99.5%) were injected at rates of 1 and 0.33 mL/min, respectively, into individual evaporators; the injection speed was controlled by a syringe pump (Longer TJ-3A, China). Water vapor and SiCl4 vapor were then introduced into the same reactor by nitrogen carrier gas, which was preheated to 100 °C. A mass flow meter (Horiba Metron S49 32/MT, China) and a float flow meter (Yinhuan LZB-6, China) were used to control the nitrogen flow rate. The reactor was a quartz glass tube (Zhonghuan, SKG04123K, China) of 40 cm in length and 4 cm in diameter; the reactor tube was heat-resistant, corrosion-resistant, heated by electric power, and capable of maintaining a consistent temperature from 125 to 300 °C. The nitrogen flow rate was adjusted to 100−450 L/h to achieve a reaction time (means the residence time in the tube reactor) from 4 to 18 s. After the reaction, two recovery methods (a wet and a dry process) were used to collect silica nanoparticles and hydrochloric acid (HCl). In the dry process, the collector linked together after the tube reactor was cooled down, causing the silica nanoparticles to agglomerate and deposit on the collector wall. The HCl was blown by carrier gas (N2) into the tail gas absorbed equipment through the branch pipe of the collector, and then the HCl was absorbed by the sodium hydroxide (NaOH) solution (2 mol/L) while the N2 was discharged into the atmosphere. In the wet process, the mixture of silica nanoparticles and HCl was blown by N2 into the collector containing NaOH solution (2 mol/L). The tube reactor and the collector were linked by an elbow pipe with a 11885

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Joyner−Halenda (BJH) method.21 Fourier transform infrared (FT-IR) spectroscopy was performed using a FT-IR spectrometer (Nicolet 8700, America) with the KBr method.

silica.22 However, six new diffraction peaks appeared in the wetSiO2 at 2θ = 27.42°, 31.76°, 45.49°, 53.99°, 56.53°, and 66.27°, which were assigned to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), and (4 0 0) reflections of sodium chloride (NaCl) crystal, respectively (JCPDS 72-1668). This suggested that relatively larger NaCl crystal particles were formed in the wet-SiO2 sample, corresponding to the elemental contents of wet-SiO2 (Table 1). 3.1.2. Morphological Characteristics. The wet-SiO2 was highly agglomerated, whereas the dry-SiO2 was better dispersed and not so highly agglomerated (Figure 3). The primary silica nanoparticles were dehydrated during the process of agglomeration, leading to the secondary silica nanoparticles (agglomerates) observed by SEM; this process was irreversible. These secondary nanoparticles contained mesopores which were intergranular gaps between the primary silica particles.19 There was some distribution in particle size for both samples (Figure 4), especially for the wet-SiO2; the average nanoparticle sizes of dry-SiO2 and wet-SiO2 were measured as 162.8 and 360.2 nm, respectively. The polydispersity index (PDI), an important parameter for evaluating particle-size distribution, was 0.221 for dry-SiO2, indicating a fairly uniform particle size. However, the PDI for wet-SiO2 was 0.404 due to interference of the solvent and solute during agglomeration in aqueous phase. 3.1.3. Porous Structure. Both dry-SiO2 and wet-SiO2 possessed type IV N2-adsorption/desorption isotherms, which is a typical characteristic of mesoporous materials as classified by the International Union of Pure and Applied Chemistry (IUPAC) (Figure 5). The adsorption isotherms could be divided into the following three stages: low-pressure stage (P/ P0 = 0.0−0.2), medium-pressure stage (P/P0 = 0.2−0.8), and high-pressure stage (P/P0 = 0.8−1.0). During the low-pressure stage, the isotherms increased due to monolayer or multilayer adsorption on the inner surface, which is a characteristic of microporous materials.21 Adsorption continued to increase steadily during the medium-pressure stage in connection with the size distribution of pore diameter. The isotherms exhibited a sharp increase in adsorption during the high-pressure stage, which was caused by intergranular gaps between primary silica particles that adsorbed N2.19 Although the adsorption isotherms were similar, there was a clear hysteresis for the isotherm of dry-SiO2 at P/P0 < 0.1, indicating the presence of intrapore diffusion. This wide range of adsorption hysteresis was observed because diffusion in the micropores was too slow to attain equilibrium adsorption within the span of each adsorption run and because a higher dose of nitrogen accelerated the diffusion.23 Moreover, the desorption and

3. RESULTS AND DISCUSSION 3.1. Comparison of the Two Recovery Methods. Silica nanoparticles were synthesized at 150 °C with a 5-s reaction time. Several characteristic indicators of the nanoparticles were used to compare the effects of dry and wet processing on the properties of the silica products. 3.1.1. Chemical Composition. Chemical composition, except for H2O content, was determined by X-ray fluorescence (Table 1). The SiO2 content of dry-SiO2 was ≥99.8 wt %, which was considerably pure (based on DIN EN ISO 3262-19: 2000); impurities such as Fe and Ni might have originated from the reactant (SiCl4) because the content of these elements was almost equal in the dry- and wet-processed SiO2. The SiO2 content of wet-SiO2 was 87 wt %, which was not a satisfactory result. The primary impurities were Na2O and Cl (12.5 wt % combined); the remaining impurities may have originated from the NaOH solution. Because the wet-SiO2 was washed sufficiently prior to measurement, we presume that the Na2O and Cl were trapped inside the silica nanoparticles and formed a copolymer during the agglomeration process. The XRD patterns of dry-SiO2 and wet-SiO2 revealed that recovery method had a significant influence on the crystal structure of the nanoparticles (Figure 2). The wide-angle XRD

Figure 2. X-ray diffraction patterns of (a) dry-SiO2 and (b) wet-SiO2.

patterns showed a broad diffraction peak at 2θ ≈ 22.46° in the dry-SiO2 only, which could be attributed to the amorphous

Figure 3. Scanning electron micrographs of (a) dry-SiO2 and (b) wet-SiO2. 11886

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Figure 4. Particle-size-number distribution curves of (a) dry-SiO2 and (b) wet-SiO2.

adsorption isotherms were not consistent over a wide range of relative pressure (P/P0 = 0.2−1.0 and P/P0 = 0.7−1.0 for drySiO2 and wet-SiO2, respectively). The hysteresis loop type was of IUPAC H3, caused by capillary condensation of nitrogen in mesopores.24 Because mesopores consist mainly of intergranular gaps between primary silica particles, the smaller the average size of primary silica particles, the narrower the average pore size. The pore diameters were 9.08 and 9.78 nm for dry-SiO2 and wetSiO2, respectively, indicating a similar primary silica particle size. However, the surface area of dry-SiO2 was 342.44 m2/g and 3.87-fold greater than that of wet-SiO2, showing that interference of the solvent and solute accelerated the agglomeration process of primary silica particles in the wet process. In addition, there were few micropores in the wet-SiO2 (micropore surface area and volume = 0.11 m2/g and 0.0001 cm3/g, respectively). In summary, the dry process was suitable for the collection of silica nanoparticles, as indicated by the pure chemical

Figure 5. Nitrogen adsorption and desorption isotherms of dry-SiO2 and wet-SiO2.

Table 2. Main Parameters Affecting the Properties of Silica Nanoparticles sample

reaction temperature (°C)

reaction time (s)

BET surface area (m2/g)

pore volume (cm3/g)

average pore diameter (nm)

SiO2-125-7s SiO2-150-7s SiO2-175-7s SiO2-200-7s SiO2-225-7s SiO2-250-7s SiO2-275-7s SiO2-300-7s SiO2-150-4s SiO2-150-5s SiO2-150-10s SiO2-150-14s SiO2-150-18s

125 150 175 200 225 250 275 300 150 150 150 150 150

7 7 7 7 7 7 7 7 4 5 10 14 18

293.70 302.03 265.20 258.10 250.77 235.15 237.50 203.50 312.53 342.44 270.58 281.40 263.21

0.55 0.51 0.44 0.39 0.43 0.43 0.36 0.33 0.51 0.64 0.57 0.52 0.64

7.40 6.70 6.57 6.03 6.93 7.23 6.09 6.37 6.54 9.08 8.40 7.42 9.69

11887

average particle size (nm)

polydispersity index

± ± ± ± ± ± ± ± ± ± ± ± ±

0.237 0.239 0.269 0.208 0.175 0.252 0.246 0.232 0.219 0.221 0.236 0.276 0.278

195.4 170.3 148.7 124.6 152.4 170.4 180.7 199.4 141.6 162.8 182.9 302.3 330.2

36.9 42.9 38.2 29.9 37.5 39.6 35.8 49.9 32.5 41.0 44.2 73.4 59.4

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different temperatures (Figure 6). The absorption bands at 472, 809, and 1105 cm−1 corresponded to the rocking vibration,

composition, smaller secondary nanoparticle size, more uniform particle-size distribution, considerably larger surface area, and greater volume of micropores compared to the wet process. 3.2. Optimization of the Main Parameters. Silica nanoparticles were collected by dry processing, which was demonstrated to be more suitable than wet processing. We investigated the effect of reaction temperature and reaction time on the properties (BET surface area, pore volume, average pore diameter, and average particle size of the agglomerates) of silica nanoparticles; the experimental conditions and results are presented in Table 2. 3.2.1. Effect of Reaction Temperature. Reaction temperature is the key parameter related to the production cost of fumed silica because the temperature of the reaction zone can be as high as 1800 °C during the oxyhydrogen flame method.25 In this study, fumed silica was prepared at a maximum temperature of 300 °C to reduce energy consumption; the temperature should not be lower than 100 °C, the boiling point of water. To determine the effects of reaction temperature, silica nanoparticles were prepared at 125, 150, 175, 200, 225, 250, 275, and 300 °C at a reaction time of 7 s. The BET surface area of all samples was >200 m2/g and reached a maximum of 302.03 m2/g at 150 °C, which is important for ensuring highquality particles within certain temperature limits during the actual production process. The BET surface area decreased with increasing temperature because the primary nanoparticles agglomerated (a process of dehydroxylation) more easily during the longer cooling process. Water vapor may liquefy at 125 °C, leading to a decrease in BET surface area. The particle size of all samples ranged from 100 to 200 nm and reached a minimum of 124.6 nm at 200 °C; all samples had a relatively narrow particle size distribution (Table 2). 3 Considering the expense and quality, a temperature of ∼150 °C was determined to be suitable for the production of silica nanoparticles, similar to the conclusions of Park et al.20 3.2.2. Effect of Reaction Time. To investigate the effect of reaction time, the time was set as 4, 5, 7, 10, 14, and 18 s, while the reaction temperature was maintained at 150 °C. The BET surface area of all samples was >260 m2/g and reached a maximum of 342.44 m2/g at a reaction time of 5 s, which was superior to that reported by Isobe et al.19 and Luo et al.,3 and to the fumed silica (e.g., N20 and T30) produced by Wacker Chemie (Germany). Also, the BET surface area decreased with increasing reaction time, possibly because of the greater opportunity for agglomeration, which could also explain the increase in particle size with increasing reaction time. However, the mixed vapors could not make good contact during a reaction time of 4 s; thus, a portion of the reaction may occur after the reaction zone, leading to decreased surface area. 3.3. Study of the Effect of Calcination Temperature. Water vapor is adsorbed on the surface of silica nanoparticles during their synthesis; thus, the ability of this hydrophilic product to combine with organic groups is poor and its dispersion and wetting in the organic phase are difficult to achieve. To broaden the potential range of application, it is necessary to take measure for hydrophobic modifications and enhance the compatibility of silica nanoparticles with organic groups.26 To investigate the effects of calcination on the properties of the product, silica nanoparticles were calcined at 200, 400, 600, 800, and 1000 °C for 2 h; the SiO2-150-5s was used as a representative sample. 3.3.1. Changes in Hydroxyl Content. The FT-IR spectra were used to characterize uncalcined and calcined silica at

Figure 6. FT-IR spectra of silica nanoparticles calcined at (a) 1000, (b) 800, (c) 600, (d) 400, (e) 200 °C, and (f) uncalcined silica nanoparticles.

symmetric stretching vibration, and asymmetric stretching vibration, respectively, of the Si−O−Si bond according to silica peaks reported in the literature.27 All samples showed almost equal transmittance for the Si−O−Si bond, indicating that this bond was stable after calcination. However, the adsorption band at 963 cm−1, corresponding to the bending vibration of the Si−OH bond, decreased as the calcination temperature increased and disappeared completely at temperatures >600 °C. And the absorption bands at 1637 and 3441 cm−1, which were respectively assigned to the bending and stretching vibrations of the H−OH bond, decreased as calcination temperature increased. In addition, for each water molecule formed, the presence of two silanols on the silica surface was needed, so the isolated silanols was difficult to be removed.28 Thus, the weak absorption band at 3740 cm−1, which was assigned to the stretching vibration of isolated silanols or terminal silanols,29 was almost same for all samples. The FT-IR spectra indicated that most of the surface hydroxyl groups and absorbed water could be removed by calcination, which created a hydrophobic surface on the silica nanoparticles, while part of them remained in the inner pores of the nanoparticles. 3.3.2. Changes in Chemical Structure. All calcined samples exhibited an intense and broad diffraction peak at 2θ = 21°− 24°, which was attributed to the amorphous silica (Figure 7). However, the broad peaks tended to sharpen as the calcination temperature increased, corresponding to a slight decrease in the full width at half-maximum (fwhm) of each peak, listed in Figure 7. This suggested that the samples were gradually converted to a crystalline state. Some studies have also indicated that amorphous silica will be converted entirely to crystalline form (e.g., quartz) at temperatures >1200 °C.30 This conclusion is substantiated by the decrease in BET surface area with increasing calcination temperature (Table 3). Careful comparison of the patterns revealed that all peaks shifted slightly, from 23.14° to lower angles (21.34°), as the calcination temperature increased from room temperature to 1000 °C. According to Bragg’s Law, 2d × sin θ = n × λ, this shift 11888

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Figure 7. X-ray diffraction spectrogram of silica nanoparticles calcined at (a) 1000, (b) 800, (c) 600, (d) 400, (e) 200 °C, and (f) uncalcined silica nanoparticles.

Figure 8. Nitrogen adsorption and desorption isotherms of uncalcined SiO2 and of SiO2 calcined at 600 and 1000 °C.

Table 3. Structural Parameters of SiO2 Samples from Nitrogen-Adsorption Studies

remained, which explained the sharp increase in adsorption at P/P0 = 0.9−1.0. In general, the porous nature and large surface area of SiO2 samples can be well maintained when the calcination temperature dose not exceed 600 °C; at higher temperatures, the mesostructure will be destroyed and the physical quality of the samples will deteriorate.

calcination temperature (°C)

BET surface area (m2/g)

micropore surface area (m2/g)

pore volume (cm3/g)

micropore volume (10−2 cm3/g)

average pore diameter (nm)

uncalcined 200 400 600 800 1000

342.44 339.28 287.88 201.78 117.08 38.49

101.33 99.75 87.76 17.19 0.42 0.35

0.64 0.62 0.55 0.51 0.41 0.22

5.39 5.16 4.66 0.88 0.03 0.02

9.08 9.12 9.18 9.71 11.66 20.27

4. CONCLUSIONS Silica nanoparticles were prepared by the hydrolysis of SiCl4 vapor with water vapor at a low temperature. This economical and environmentally benign method for preparation of silica nanoparticles has been demonstrated to be entirely feasible; the method is highly attractive because of the absence of a hydrogen−oxygen flame, the use of inexpensive materials, and the lack of requirement for expensive equipment. The chemical composition, morphology, average particle size, and porous structure of the nanoparticles were affected by the two typical recovery methods, and the dry process proved to be more satisfactory than the wet process. Porous silica nanoparticles with a uniform size distribution (162.8 ± 41.0 nm, PDI = 0.221) and high surface area (342.44 m2/g) were prepared under relatively better conditions (150 °C reaction temperature and 5-s reaction time). Both the surface area and hydroxylgroup content decreased with increasing calcination temperature to 1000 °C. The silica nanoparticles could be hydrophobically modified by calcination at a maximum temperature of 600 °C, to remove the surface hydroxyl groups and absorbed water while maintaining the mesostructure. The relatively highpurity silica that results from this process is suitable for use in various high-end applications such as catalyst support and pharmaceuticals.

suggested that the frameworks of the nanoparticles became slightly enlarged during the calcination process.22 3.3.3. Changes in Porous Structure. The surface area and pore volume of silica nanoparticles decreased markedly as the calcination temperature increased (Table 3), which indicated that sintering processes occurred due to particle agglomeration and dehydroxylation at higher temperatures.31 Pore diameter did not increase substantially until the calcination temperature exceeded 600 °C, showing that the intergranular gaps and mesostructure were not destroyed by calcination at 600 °C. However, the surface area and volume of micropores decreased rapidly from 200 to 800 °C, indicating the disappearance of micropores. We deduce from these results that the sintering processes began with the disappearance of micropores. Sintering was more dominant at temperature from 600 to 1000 °C, as confirmed by the XRD results showing increased order in atomic arrangement at higher temperatures. When the calcination temperature exceeded 600 °C, few micropores were present and the mesostructure was destroyed, which explained the sharp reduction in surface area and pore volume. Similar trends in decreasing surface areas of nanoparticles with increasing calcination temperature have been reported elsewhere.32,33 Three typical adsorption/desorption isotherms are displayed in Figure 8 for uncalcined SiO2 and for SiO2 calcined at 600 and 1000 °C. Hysteresis in the isotherm for uncalcined SiO2 at P/P0 < 0.1 disappeared when the SiO2 was calcined at 1000 °C, which was attributed to the disappearance of micropores. The adsorption isotherm of SiO2 calcined at 1000 °C increased slowly at P/P0 = 0.0−0.9, showing that only mesopores

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AUTHOR INFORMATION

Corresponding Author

*Prof. Dr. Jiang Jianguo. Tel.: +8610-62783548. Fax: +861062783548. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Hi-Tech Research and Development Program (863) of China for financial support (Grant No. 2012AA06A116). 11889

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dx.doi.org/10.1021/ie501759w | Ind. Eng. Chem. Res. 2014, 53, 11884−11890