介孔分子筛的合成

Materials Letters 168(2016)111–115

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Materials Letters

journal homepage:www.elsevier.com/locate/matlet

Co-templating synthesis of mesoporous hollow silica spheres and their application in catalytic oxidation with low Pt loading

Wenxiang Tang 1, Xiaofeng Wu, Yunfa Chen n

State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China

a r t i c l e i n f o

Article history:

Received 28August 2015Received in revised form 6December 2015

Accepted 27December 2015

Available online 12January 2016Keywords:

Mesoporous silica Hollow sphere Pt catalyst

VOCs oxidation

a b s t r a c t

Mesoporous hollow silica spheres (MHSS)with uniform size were successfully prepared by introducing monodispersed polystyrene (PS)spheres as hard template and selected surfactant as soft template in acid condition. With assistance of different surfactants, a series of mesoporous shells were created and the used surfactants have a signi ﬁcant effect on the structural properties of MHSS. The highest surface area was 690.3m 2g À1on the MHSS which was achieved by using P123as a soft template. The MHSS was further used for loading active Pt through the reaction between chloroplatinic acid or and sodium bor-ohydride. The as-prepared Pt/MHSScatalyst exhibited high activity for VOCs' deep oxidation.

1. Introduction

Mesoporous hollow silica spheres (MHSS)with low density and large surface area have attracted great interest in the past few years because of their wide range of potential applications in thermal insulation, lithium-ion batteries, controlled drug delivery, biomolecule separation, enzyme immobilization and catalysis etc [1–3]. Compared with conventional hollow spheres of solid shells, the hollow spheres with hierarchical porous structure will own high surface area, and penetrating pores in the shell that facilitate mass transfer and improve reaction ef ﬁciencies. Many chemical and physicochemical methods such as self-assembly techniques, sol –gel process and template-sacri ﬁcial techniques have been ap-plied to fabricate various mesoporous hollow spheres [1, 2, 4–7]. In this regard, template-sacri ﬁcial techniques, which can control the size of hollow microspheres by selecting the various size of tem-plates, are the most widely used to the fabrication of hollow structures. The sacri ﬁcial template cores including hard templates (e.g.,polymer latex, inorganic nanoparticles, and carbon) and soft templates (e.g.,vesicles, emulsion and gas bubbles) have been extensively employed [2–5, 8, 9]. Recently, polymer spheres in-cluding polystyrene and PMMA are highly dispersible and can be obtained easily in a large-scale, which are often used as templates for preparation of hollow structures. With assistance of hard templates, the nanoparticles are formed on the surface of cores in

Corresponding author.

E-mail addresses:[email protected](W.Tang), [email protected](Y.Chen). 1

Present address:Institute of Materials Science, University of Connecticut, 97N Eagleville Rd, Storrs, CT, 06269, United States. http://dx.doi.org/10.1016/j.matlet.2015.12.142

0167-577X/&2016Elsevier B.V. All rights

reserved.

highly alkaline media and the hollow structures can be obtained by removal of sacri ﬁcial templates [4, 8]. However, the formation of nanoparticles took place quickly under concentrated aqueous ammonia media and multilayer of nanoparticles was needed to obtain stable hollow spheres which would be dif ﬁcult to control. On the contrary, the formation of shells on the templates will be controllable in acid condition because of the slow hydrolysis of TEOS and the related process for synthesis of MHSS is rarely re-ported. Herein, we are proposing an acid approach to fabricate hollow spheres with mesoporous silica shell by using different surfactants and polystyrene spheres as co-templates. The role of surfactants on the structures was investigated. As the catalytic support with higher speci ﬁc area, the as-prepared MHSS was loaded with Pt nanoparticles which was further applied in the removal of volatile organic compounds such as benzene, n-hexane, and toluene.

2. Experimental 2.1. Synthesis

The PS spheres were prepared according to procedures re-ported previously [8]. Typically, 1.0g PS particles and an appro-priate amount of surfactants (P123,Brij58and CTAB) were dis-solved into 80ml H 2O and 20ml ethanol by supersonic oscillation. After the PS particles were dispersed, 8.6g HCl (36wt%)was ad-ded into the above solution. The mixture was stirred at 35°C for 2h and following 2.5g tetraethyl orthosilicate (TEOS)was added. Stirring was continued for another 24h and then the solution was

112W. Tang et al. /Materials Letters 168(2016)111–115

aged at 100°C for 48h. The solid product was washed, ﬁltered, and then dried at 50°C overnight. Finally, the material was treated in air at 550°C with a heating rate of 1°C/minand kept for 5h to remove all templates. Pt was loaded onto HMS via a reduction route with sodium chloride platinum as precursor and NaBH 4as reducing agent. The weight percent of Pt metal to the HMSS support was set at 0.5wt%.The Pt/HMSSsample was dried at 80°C and further tested for catalytic oxidation of VOCs.

2.2. Characterization and activity testing

The morphologies the as-prepared materials were performed by using scanning electron microscopy (SEM,JEOL JSM-6700F, Ja-pan) and the microstructures were recorded with transmission electron microscopy (TEM,JEOL JEM-2010). N 2adsorption –deso-rption measurements were operated on an automatic surface analyser (AS-1-CTCD, Quantachrome Cor., USA). Every sample was degassed at 200°C for 3h before measurement. The speci ﬁc sur-face areas and pore size distribution of all samples were

obtained

Fig. 1. SEM and TEM images of as-prepared MHSS by using different surfactants (P123(a,b and c), Brij 58(d,e and f), CTAB (g,h and i)).

W. Tang et al. /Materials Letters 168(2016)111–115113

by Brunauer-Emmett-Teller (BET)and Barrett-Joyner-Halenda (BJH)analyses. Catalytic activity testing was carried out according to procedures reported previously [10]and 1000ppm of valatile organic compounds (VOCs)from gas tank such as benzene, to-luene or n-hexane in air (BeijingHuayuan, China) were used.

Table 1

Surface areas, average pore sizes, and pore volumes of prepared MHSS. Sample

BET surface area (m2g À1) 690.3641.1564.6694.2

Pore size (nm)5.63.83.45.6

Pore volume (cm3g À1) 1.010.850.651.01

3. Results and discussion

Uniform PS spheres could be obtained through a dispersion polymerization technique which was carried out in an alcohol/water medium with a steric stabilizer, PVP K-30. The average size of PS spheres can be controlled easily by the dispersion medium and concentration of styrene [11]. As shown in Fig. S1, the ob-tained spheres are highly uniform and the size of the spheres is about 450nm. The as-prepared PS spheres can be highly dispersed in an alcohol/watermixed solution. With the assistance of acid (HCl),the hydrolysis of TEOS takes place and a PS@SiO2core –shell organized structure can be obtained through the combination surfactant self-assembly with spheres templates. After removal of templates via calcination at 550°C for 5h, a uniform sphere structure was obtained as shown in Fig. 1. It can be seen clearly that the spheres are nearly uniform and the average size the spheres is about 400–500nm. Some broken spheres can be seen clearly, indicating the spheres are hollow structures. The hollow spherical morphologies are further proved by TEM images as displayed Fig. 1b, e and h. The dimension of the cavity of this hollow structure is around 420–450nm, which is almost equal to that of PS spherical templates. Moreover, it is very clear that the shells are built up with the aggregated ultra ﬁne particles and the thicknesses are about 20–30nm. By using different surfactants as structure directing agent, a disordered worm-like structure are formed and the resultant surface roughness is different which can be seen from the HRTEM micrographs of HMSS.

Fig. 2shows the N 2adsorption –desorption isotherms and pore size distributions of the synthesized HMSS and the results are listed in Table 1. The type IV isotherm curve with well-developed H1type hysteresis loops are related to the capillary condensation in mesopores, showing the mesoporous structure of all MHSS samples. Correspondingly, the pore size distributions of the sam-ples show narrow pore distribution with a peak value of 5.6, 3.8, 3.4nm, respectively. By using the BET and BJH methods, the sur-face area and pore volume were obtained and the values were 690.3m 2g À1and 1.01cc g À1on P123-derived sample, 641.1m 2g À1and 0.85cc g À1on Brij58-derived sample, 564.6m 2g À1and 0.65cc g À1on CTAB-derived sample, respectively.

HPMSS-P123HPMSS-Brij 58HPMSS-CTAB Pt/HPMSS-P123

In order to investigate the application of MHSS, the P123-de-rived sample with highest surface area was selected as a support for loading Pt nanoparticles with a low loading of 0.5wt%which was further used for deep oxidation of VOCs. As displayed in Fig. 2, the N 2adsorption –desorption isotherm and pore size distribution were kept very well after loading Pt particles on P123-derived MHSS. For the XRD pattern shown in Fig. 3a, both MHSS and Pt-MHSS have a broad peak (2θ¼15–30°) about amorphous silica. Meanwhile, a small peak was found in Pt-MHSS which could be indexed to the (111)crystal phase of Pt (JCPDS001-1194). By checking the TEM images, some big Pt clusters were observed as displayed in Fig. 3b and c. However, some ultra ﬁne Pt particles (o 2nm) can be seen in the porous silica as shown in Fig. 3d. The porous structure of MHSS provides a good place for holding the Pt nanoparticles which would make the active Pt sites more func-tional because of their favorable dispersion. The catalytic activities of benzene, toluene and n-hexane oxidation were shown in Fig. 4. As expected, the T 10%, T 50%, T 90%values obtained over the Pt-MHSS catalyst were 130, 146and 150°C for benzene oxidation, 147, 164and 170°C for ethyl acetate oxidation, 155, 172and 190°C for n-hexane oxidation, respectively. All VOCs can be totally oxidized below 200°C over this catalyst while the carbon balance was above 99.5%.The superior catalytic activity can be ascribed to the high surface area and rich porous structure provided by the MHSS support. The reduction of Pt precursor can take place at the limited pore space which will prevent the growth of Pt and be good for the dispersion of Pt nanoparticles.

4. Conclusions

Mesoporous hollow silica spheres with uniform size, high surface area and pore volume have been successfully fabricated by using PS spheres and different surfactants as co-templates in acid condition. With the assistance of surfactants (P123,Brij58, CTAB) in acid condition, the porous silica shell was formed through the hydrolysis of TEOS on the PS spheres by simple one-step

method.

Fig. 2. The nitrogen adsorption –desorption isotherms (a)and pore size distributions (b)of the as-prepared MHSS.

114W. Tang et al. /Materials Letters 168(2016)111–115

Fig. 3. XRD pattern and TEM images of the Pt/MHSS

catalyst.

Acknowledgments

This research was supported by the 863Hi-tech Research and Development Program of China (Grantno. 2012AA062702) and the National Natural Science Foundation of China (No.51002154).

Appendix A. Supplementary material

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2015.12.142.

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