КИНЕТИКА И КАТАЛИЗ, 2011, том 52, № 5, с. 732-737
УДК 542.924.5:546.215:547.741
SYNTHESIS OF HOLLOW POLYPYRROLE—PLATINUM COMPLEX SPHERES AND THEIR SUCCESSFUL APPLICATION AS A CATALYST FOR DECOMPOSITION OF HYDROGEN PEROXIDE1 © 2011 г. Xia Lixin1, *, Liu Guangye1, Liu Jingfu2, Sun Mengtao3' *
1College of Chemistry, Liaoning University, Shenyang, China 2State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences,
Chinese Academy of Sciences, Beijing, China 3Beijing National Laboratory for Condensed Matter Physics and Laboratory of Soft Matter Physics, Institute of Physics,
Chinese Academy of Sciences, Beijing, China *E-mail: lixinxia@lnu.edu.cn, mtsun@aphy.iphy.ac.cn Received 12.04.2010
Hollow pyrrole—platinumpoly (PPy—Pt) complex spheres effectively catalyze the decomposition of hydrogen peroxide to form oxygen. The method used to synthesize the catalysts employs chemical polymerization of pyrrole with potassium hexachloroplatinate(IV) as oxidant and ZSM-5 molecular sieve as a hard template removable by dissolution. ZSM-5 molecular sieve used as template has the following merits: it can be easily removed, it is inexpensive and owing to micrometer size it shows a minimal aggregation. In addition, the use of the zeolite avoids the need for non-volatile surfactants which may be adsorbed onto the synthesized PPy—Pt complex spheres and interfere with their possible applications in catalysis. The new micron-sized hollow PPy—Pt complex spheres are produced simply and cost-effectively, and they can be expected to play an important role in wastewater treatment technologies. The synthetic method may represent a novel route to prepare hollow conductive polymer spheres doped by various metals for specific applications.
In recent decades, hollow spheres with nanometer to micrometer dimensions have been of great interest in many areas of science and technology. The wide variety of application includes the production of capsules suitable for controlled-release of various substances, catalysts, fillers, biomedical instrumentation and coatings. The use of hollow spheres in engineering, and the paint industry is also described. It is in particular their specific structure, surface permeability, large specific area and low density which have rendered them of such interest [1—3]. Polypyrrole (PPy) hollow spheres, particularly, have received a great deal of attention because they exhibit good environmental stability along with high and variable electric conductivity [4, 5]. Various methods have been developed to prepare hollow spheres including the template method in which polystyrene latex spheres [4], silica latex particles [5, 6], metals [7] and metal halides [8] are employed as templates removable by dissolution. The template method showed its feasibility, reproducibility, predictability and uniformity.
Compared to other template materials like polystyrene latex spheres [4, 9—12], silica latex spheres [5, 6, 13], metal [7] and metal halide [8], a template based on ZSM-5 molecular sieve has some advantages. They include quick removal, low cost and minimal aggregation owing to its micrometer size. In addition, the use
1 Статья публикуется на английском языке в авторском ва-
рианте.
of non-volatile surfactants can be avoided, which may be adsorbed onto the synthesized pyrrole—platinumpoly (PPy—Pt) complex spheres and interfere with their possible applications in catalysis. Very recently, we developed a new method for preparation of silver core—shell materials and hollow materials by using 5A and ZSM-5 molecular sieves as templates [14, 15]. To the best of our knowledge, there have been few reports on preparation of hollow conductive PPy spheres based on surface modified molecular sieves.
Catalytic decomposition of H2O2 is of great importance due to its vast applicability in water treatment technologies [16]. A variety of organic pollutants in wastewaters and soils can be successfully oxidized and degraded by hydrogen peroxide promoted by various catalysts such as pure metals (e.g. Ag, Cu, Fe, Mn, Ni, and Pt) and their oxides supported on silica, alumina and zeolites [16, 17]. Higher rates of decomposition of H2O2 are usually desired to increase the decomposition rates of the target pollutants [18]. The efficiency of supported catalysts has been found to be high in comparison to unsupported catalysts because in the unsupported state aggregation of catalyst reduces its efficiency [19].
In the present work, we report a facile method for synthesis of micron-sized hollow PPy—Pt complex spheres via chemical polymerization of pyrrole using potassium hexachloroplatinate(IV) as oxidant and ZSM-5 molecular sieve as a removable solid template.
The catalytic activity of the hollow PPy-Pt complex spheres was investigated in the decomposition of hydrogen peroxide.
EXPERIMENTAL
Materials
Pyrrole monomer was purchased from Sinopharm Chemical Reagent Co., Ltd., dried with potassium hydroxide (2—3 g per 10 ml of pyrrole), then filtered and distilled at reduced pressure. Potassium hexachloro-platinate(IV) was purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. (3-Chloropropyl)triethoxysilane (CPTE) was obtained from Aldrich, and potassium hydroxide (>82%) was acquired from Reagent Chemical Co., Ltd., Tian-jin, China. ZSM-5 molecular sieve, supplied by the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, was heated for 4 h at 600°C in a muffle. Hydrofluoric acid (40%) was supplied by Tianjin Institute of Chemicals, China. Hydrogen peroxide (30%) and sodium hydroxide (>96%) were purchased from Beihua Fine Chemicals Co., Ltd., Beijing, China. High purity water with resistance >18.0 Mfi cm was used in all preparations.
Synthesis and catalysis
ZSM-5 molecular sieve (1 g) was dispersed in a freshly prepared aqueous solution of CPTE (15 ml, 0.3 mol/l). To this mixture 2-propanol (80 ml) and ammonia solution (1 ml, 28.0-30.0 wt %) were added successively. The mixture was reacted for 3 h at 45°C under continuous stirring [20]. The deposits were collected and washed with ethanol and water several times to give CPTE-modified ZSM-5 particles.
The CPTE-modified ZSM-5 (0.2 g) was dispersed in ethanol (15 ml) by sonication for 10 min, then pyrrole monomer (0.4 ml, 5.6 mmol) was added to the dispersion. After sonication for 20 min, an aqueous solution (15 ml) containing K2PtCl6 (0.07 g, 0.14 mmol) was added to the suspension, and the mixture was stirred for 12 h to yield uniform ZSM-5/PPy-Pt core/shell particles. The solid particles were separated from the reaction medium by centrifuging at 3000 rpm. After thorough washing with water and ethanol, the powdered product was dried at room temperature under vacuum for 4 h. The molecular sieve cores were dissolved by subsequent treatment with dilute hydrofluoric acid to obtain hollow PPy-Pt complex spheres. Specifically, the PPy-Pt-coated ZSM-5 molecular sieve (30 mg) was initially added to aqueous HF solution (50 ml, 7 wt %), and mixed by hand using a shaking motion at a frequency of10 min-1. After storing for 1 h, the hollow PPy-Pt complex spheres were separated from the reaction medium by centrifuging at 3000 rpm. After thorough washing with water and ethanol, the product was dried at room temperature in vacuum for 4 h.
Decomposition of hydrogen peroxide was performed using a glass apparatus which was a modification of the apparatus described by Eyring and Daniels [21]. The apparatus was tested for gas leakage, then aqueous H2O2 solution (10 ml, 20 wt %) was poured carefully into an Erlenmeyer flask (25 ml) placed in the thermostat. After equilibration at 25°C, the hollow PPy-Pt complex spheres catalyst (3 mg) and NaOH (10 ml, 2 mol/l) were rapidly added successively to the flask, which was then closed with a stopper. The temperature of the reaction mixture was kept constant to within ±0.02°C, and the reaction mixture was continuously stirred at a constant rate to prevent the liberated oxygen from forming a supersaturated solution. Before reaching the gas burette the oxygen transferred through the glass wool was saturated with concentrated sulfuric acid to remove some impurities. The rate of decomposition of hydrogen peroxide was evaluated by measuring the volume of oxygen produced as a function of time. The experimental data reported herein are the average values recorded in five runs.
Characterization
Fourier transform infrared (FT-IR) spectra of the samples in KBr pellets were obtained with a NICOLET 5700 FT-IR spectrometer.
The Pt content in the hollow PPy-Pt complex spheres was measured using inductively coupled plasma mass spectrometry (ICP-MS, Nippon Jarrel Ash, ICAP575II) to analyze samples that had been dissolved in nitric acid.
Field emission scanning electron microscope (FESEM) images of ZSM-5/PPy-Pt core/shell particles and hollow PPy-Pt complex spheres were registered using a SUPRA35 FESEM instrument operated at 20.00 kV.
RESULTS AND DISCUSSION
The surface of ZSM-5 molecular sieve is hydro-philic and that of pyrrole is hydrophobic. Consequently, pyrrole is difficult to absorb on the surface of ZSM-5 and then polymerize to form a PPy shell. For that reason, prior to depositing pyrrole CPTE with bifunc-tional groups was assembled on the surface of the molecular sieve by forming Si-O-Si bonds under base-catalyzed hydrolysis. Due to the presence of the C-Cl groups of CPTE, PPy could be readily deposited on the CPTE-modified surface via hydrogen bonding interaction. A coating-like structure composed of PPy was then formed by chemical polymerization of the absorbed pyrrole in the presence of K2PtCl6 to cover the modified ZSM-5 core. The molecular sieve cores were dissolved by subsequent treatment with dilute hydrofluoric acid to obtain hollow PPy-Pt complex spheres. An outline of the synthesis is shown in Fig. 1.
Fig. 2 shows FT-IR spectra of the ZSM-5 molecular sieve, CPTE-modified ZSM-5,
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