WO2010021454A2 - Method of preparing carbon-doped titanium dioxide nanoparticles by decompositing carbon dioxide using thermal plasma - Google Patents

Method of preparing carbon-doped titanium dioxide nanoparticles by decompositing carbon dioxide using thermal plasma Download PDF

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WO2010021454A2
WO2010021454A2 PCT/KR2009/003105 KR2009003105W WO2010021454A2 WO 2010021454 A2 WO2010021454 A2 WO 2010021454A2 KR 2009003105 W KR2009003105 W KR 2009003105W WO 2010021454 A2 WO2010021454 A2 WO 2010021454A2
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carbon
titanium dioxide
doped titanium
thermal plasma
doped
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WO2010021454A3 (en
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Dong-Wha Park
Jung Wan Park
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Inha-Industry Partnership Institute
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    • C01G23/00—Compounds of titanium
    • C01G23/04—Oxides; Hydroxides
    • C01G23/047—Titanium dioxide
    • C01G23/07—Producing by vapour phase processes, e.g. halide oxidation
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    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
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    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/34—Irradiation by, or application of, electric, magnetic or wave energy, e.g. ultrasonic waves ; Ionic sputtering; Flame or plasma spraying; Particle radiation
    • B01J37/341—Irradiation by, or application of, electric, magnetic or wave energy, e.g. ultrasonic waves ; Ionic sputtering; Flame or plasma spraying; Particle radiation making use of electric or magnetic fields, wave energy or particle radiation
    • B01J37/342—Irradiation by, or application of, electric, magnetic or wave energy, e.g. ultrasonic waves ; Ionic sputtering; Flame or plasma spraying; Particle radiation making use of electric or magnetic fields, wave energy or particle radiation of electric, magnetic or electromagnetic fields, e.g. for magnetic separation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/34—Irradiation by, or application of, electric, magnetic or wave energy, e.g. ultrasonic waves ; Ionic sputtering; Flame or plasma spraying; Particle radiation
    • B01J37/349—Irradiation by, or application of, electric, magnetic or wave energy, e.g. ultrasonic waves ; Ionic sputtering; Flame or plasma spraying; Particle radiation making use of flames, plasmas or lasers
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    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
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    • C01P2004/00—Particle morphology
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    • C01P2004/03—Particle morphology depicted by an image obtained by SEM
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    • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer

Definitions

  • the present invention relates to a method of preparing carbon-doped titanium dioxide nanoparticles by decomposing carbon dioxide using thermal plasma.
  • CO 2 carbon dioxide
  • Increased generation of carbon dioxide (CO 2 ) is regarded as the main cause of global warming. Because CO 2 has a great influence on the increase in atmospheric temperature, climate change, ecosystems and so on, the treatment of CO 2 is considered very significant in terms of reducing global warming.
  • TiO 2 titanium dioxide
  • TiO 2 has been receiving attention as an environmentally friendly material because it is chemically and photochemically stable and has high oxidizing power and is very harmless.
  • TiO 2 is typically utilized as a photocatalyst for removing an environmental pollutant, a material for a pigment, a plastic additive or a multicoating agent of glasses, and has two crystalline phases including an anatase phase and a rutile phase.
  • anatase TiO 2 having high photoactivity, is employed as a photocatalyst in systems for photodegradation of acetone, phenol and trichloroethylene or oxidation of nitrogen oxide such as nitrogen monoxide or nitrogen dioxide and the conversion of solar energy.
  • rutile TiO 2 having superior scattering effects for blocking UV light, is widely used as a material for white pigment. Furthermore, compared to other materials, rutile TiO 2 has a higher dielectric constant and refractive index and superior oil adsorption and coloring capacity, and is chemically stable against strong acids or strong bases and thus is applied to optical coatings, beam splitters, non-reflective coating films, etc. In addition, TiO 2 has wide chemical stability and non-stoichiometric phase region and thus exhibits various electrical properties depending on oxygen partial pressure. So, TiO 2 is also studied as a component of a humidity sensor and a high-temperature oxygen sensor and the application field thereof is gradually widened.
  • the chloride process which was industrialized by Du Pont, USA in 1956, is performed in a manner such that reaction occurs at high temperature of at least 1,000 °C using as a starting material titanium tetrachloride (TiCl 4 ) which is vigorously hydrolyzed through reaction with moisture in the air.
  • TiCl 4 titanium tetrachloride
  • the resultant TiO 2 particles are fine but are rough, and additional protector equipment should be used due to corrosive gases (e.g. HCl, Cl 2 ) occurring during the reaction, undesirably increasing the production costs.
  • the sulfate process which was industrialized by Titan Co., Norway in 1916, is performed in a manner such that amorphous hydroxide obtained through hydrolysis using titanium sulfate (TiSO 4 ) as a starting material is calcined again and milled, thus obtained TiO 2 powder.
  • TiSO 4 titanium sulfate
  • Russian Patent No. SU-1398321 discloses a method of preparing TiO 2 including adding an appropriate amount of titaniferous seed to a TiCl 4 solution, hydrolyzing it through heating, thus precipitating hydrated TiO 2 , which is then subjected to post treatment such as high-temperature heat treatment, thereby preparing TiO 2 .
  • This method has a relatively simple preparation process, but needs additional high-temperature heat treatment at 600 ⁇ 650 °C to obtain anatase TiO 2 or higher-temperature heat treatment to obtain rutile TiO 2 .
  • Japanese Patent Application No. Hei. 9-124320 discloses a method of preparing TiO 2 , including adding one selected from among acetate, carbonate, oxalate and citrate containing an alkali metal or alkali earth metal to a solution of TiCl 4 dissolved in an alcohol such as butanol along with water, thus preparing a gel, which is then subjected to high-temperature heat treatment, thereby preparing TiO 2 .
  • This method produces ultra-fine TiO 2 having good properties but is problematic in that third additives such as expensive organic acids should be used, and high-temperature heat treatment for removing the added organic acids should be performed again after the preparation of the gel.
  • An object of the present invention is to provide a method of preparing C-doped anatase TiO 2 nanoparticles using thermal plasma.
  • Another object of the present invention is to provide C-doped anatase TiO 2 nanoparticles prepared through the above method.
  • the present invention provides a method of preparing C-doped TiO 2 nanoparticles using thermal plasma, including injecting CO 2 and vaporized TiCl 4 , serving as reactant gases, into a reaction tube of a thermal plasma jet system, decomposing the injected CO 2 by a thermal plasma jet generated using argon gas, so that it reacts with the TiCl 4 transferred by a carrier gas, thus producing C-doped TiO 2 , and cooling the C-doped TiO 2 , thereby preparing C-doped TiO 2 nanoparticles.
  • the present invention provides C-doped anatase TiO 2 nanoparticles prepared through the above method.
  • the method according to the present invention can decompose CO 2 serving as a reactant gas, thus aiding the reduction of global warming. Also, because thermal plasma having features of high temperature and high activity is used, there is no need to consider post-treatment procedures nor to be concerned about the production of secondary pollutants. Furthermore, the method according to the present invention can decompose CO 2 within a short time, and thus shows decomposition efficiency superior to other methods.
  • the C-doped TiO 2 synthesized through the method according to the present invention can exhibit superior photocatalytic activity in the UV/visible range compared to TiO 2 synthesized through reaction with only oxygen. Also, the C-doped TiO 2 has a high specific surface area thanks to a particle size of 20 ⁇ 50 nm, and as well is in an anatase phase, and thus can be usefully employed as a photocatalyst.
  • FIG. 1 is a perspective view showing a system for preparing C-doped TiO 2 according to an embodiment of the present invention
  • FIG. 2 is a graph showing the decomposition rate of CO 2 depending on the flow rate of CO 2 using thermal plasma according to the embodiment of the present invention
  • FIG. 3 is a scanning electron microscope (SEM) image of C-doped TiO 2 nanoparticles synthesized according to the embodiment of the present invention
  • FIG. 5 is of X-ray photoelectron spectroscopy (XPS) graphs of the C-doped TiO 2 nanoparticles synthesized according to the embodiment of the present invention (at the flow rate of CO 2 of 2 L/min);
  • XPS X-ray photoelectron spectroscopy
  • FIG. 6 is of XPS graphs of the C-doped TiO 2 nanoparticles synthesized according to the embodiment of the present invention (at the flow rate of CO 2 of 4 L/min);
  • FIG. 7 is of transmission electron microscope (TEM) images of the C-doped TiO 2 nanoparticles synthesized according to the embodiment of the present invention and pure TiO 2 ((a): C-doped TiO 2 , (b): pure TiO 2 , (c): high performance TEM image around the lattices of the particles of (a));
  • TEM transmission electron microscope
  • FIG. 8 is a graph showing the light absorbance of the C-doped TiO 2 nanoparticles synthesized according to the embodiment of the present invention depending on the flow rate of CO 2 ;
  • FIG. 9 is a graph showing the decomposition of methylene blue over time using the C-doped TiO 2 nanoparticles synthesized according to the embodiment of the present invention.
  • the present invention provides a method of preparing C-doped TiO 2 nanoparticles using thermal plasma, including injecting CO 2 and vaporized TiCl 4 , serving as reactant gases, into a reaction tube of a thermal plasma jet system (step 1), decomposing the injected CO 2 by a thermal plasma jet generated using argon gas, so that it reacts with the TiCl 4 transferred by a carrier gas, thus producing C-doped TiO 2 (step 2), and cooling the produced C-doped TiO 2 , thus preparing C-doped TiO 2 nanoparticles (step 3).
  • the thermal plasma jet system includes a plasma torch 1 for supplying a heat source for decomposing CO 2 , a reaction tube 2 into which CO 2 and TiCl 4 are injected and in which decomposition of CO 2 and reaction thereof with TiCl 4 occur, a cooling tube 3, an exhaust portion 4 for discharging waste gas generated after the reaction, and a power supplier 5 for supplying power to the torch 1.
  • the thermal plasma jet system is operated using direct-current power, a voltage of 40 V and a current of 150 ⁇ 220 A.
  • the torch 1 includes a tungsten cathode bar and an anode nozzle, such that argon gas is allowed to flow between the anode nozzle and the cathode bar, thus generating a plasma jet. Furthermore, to protect the torch 1 from heat, both electrodes are cooled with water.
  • the reaction tube 2 is provided in the form of a window-equipped stainless double tube. The gas discharged via the exhaust portion 4 is passed through a scrubber 41 and then a gas chromatograph 42, thus determining the decomposition rate of CO 2 .
  • step 1 CO 2 and TiCl 4 are injected into the reaction tube of the thermal plasma jet system.
  • CO 2 which is the main cause of global warming has a major affect on the increase in atmospheric temperature, climate change, destruction of ecosystems and so on
  • the CO 2 decomposition may aid the reduction of global warming, and oxygen which results from the decomposition is utilized as a material adapted for synthesis of TiO 2 .
  • a mass flow controller (MFC) is mounted to the inlet of CO 2 so as to check the decomposition rate of CO 2 depending on the flow rate of CO 2 .
  • TiCl 4 functions to prevent the rebinding of oxygen and carbon in the course of dissociation/rebinding of CO 2 , and also plays a role as a reactant upon synthesis of TiO 2 .
  • TiCl 4 having a low boiling point, is easily vaporized even at room temperature, and is thus favorably injected using argon as the carrier gas.
  • a thermostat may be maintained at about 30 ⁇ 35 °C.
  • the CO 2 and TiCl 4 may be injected sequentially or simultaneously.
  • the CO 2 and TiCl 4 may be simultaneously injected.
  • the CO 2 and TiCl 4 may be injected in a direction parallel to the direction where the plasma jet is generated. This is because TiCl 4 is effectively decomposed in the high-temperature plasma region so as to efficiently achieve binding with an oxygen atom.
  • the CO 2 and TiCl 4 are injected at a position spaced apart from the plasma jet nozzle by a distance of not more than 2 mm so as to efficiently achieve decomposition and reaction. If the injection position falls outside 2 mm, decomposition and reaction by the plasma do not occur as desired.
  • a heat source used in this step is the thermal plasma obtained by the use of the direct-current thermal plasma jet system.
  • the thermal plasma is an ionized gas composed of electrons, ions, atoms and molecules generated in the plasma torch using direct-current arc discharge or high-frequency induction discharge.
  • the thermal plasma which is provided in the form of a high-speed jet flame having ultra high temperature ranging from thousands to tens of thousands of K and high thermal capacity is a fourth material state having physically and chemically properties quite different from solid, liquid and gas states.
  • the thermal plasma generating gas may include for example argon gas, air and nitrogen gas. Particularly useful is argon gas. Because argon is a Group 8 element of the periodic table, it easily emits electrons even in the presence of relatively low energy. Also, argon is inert and thus barely affects a chemical reaction, and thereby is the most widely used in the generation of thermal plasma.
  • the arc is formed by electrical energy between the cathode and anode of the power supplier of the thermal plasma jet system, and ultra high temperature plasma of about 10,000 K is produced by argon gas used as the thermal plasma generating gas. The ultra high temperature generated by such thermal plasma is much higher than the temperature generated by thermal treatment or combustion methods. Furthermore, the argon gas does not react with the other reactant gases and thus does not form by-products.
  • the CO 2 and TiCl 4 injected in step 1 are decomposed by the thermal plasma generated in the thermal plasma jet system, so that CO 2 is ionized into oxygen and carbon ions and TiCl 4 is ionized into chlorine and titanium ions.
  • the flow rate of CO 2 may be controlled to 1 ⁇ 3 L/min.
  • the oxygen and titanium ions bind to each other, thus forming the C-doped TiO 2 .
  • step 3 the produced TiO 2 is cooled, thereby preparing the TiO 2 nanoparticles.
  • the oxygen element resulting from the CO 2 decomposition in step 2 binds to titanium and a small amount of carbon ionized by the thermal plasma, and the C-doped TiO 2 nanoparticles are formed through cooling in the cooling tube.
  • the cooling process has an influence on crystallization of the C-doped TiO 2 to thus produce the nano-sized particles.
  • the particle size of TiO 2 is increased to nano size or more.
  • the synthesized C-doped TiO 2 may be rapidly cooled with cooling water.
  • the temperature of cooling water is maintained at 15 ⁇ 25 °C, so that the temperature of vaporized C-doped TiO 2 is drastically lowered.
  • the present invention provides C-doped TiO 2 prepared through the above method.
  • the C-doped TiO 2 prepared through the above method is observed to be nanoparticles having a particle size of 20 ⁇ 50 nm using an SEM (FIG. 3).
  • SEM SEM
  • FIG. 4 As a result of X-ray diffraction analysis, almost all of the C-doped TiO 2 can be seen to be anatase (FIG. 4).
  • XPS FIGS. 5 and 6) and TEM (FIG. 7) it can be seen that the TiO 2 is doped with C.
  • the C-doped TiO 2 according to the present invention can be seen to have superior photocatalytic activity in the UV/visible range to that of TiO 2 synthesized through reaction with only oxygen (FIG. 8).
  • the decomposition time is about 6 ⁇ 10 min, and thereby the C-doped TiO 2 according to the present invention can be seen to show shorter decomposition time and faster decomposition rate than those of TiO 2 synthesized through reaction with only oxygen (decomposition time: about 13 min) (FIG. 9).
  • the C-doped TiO 2 prepared through the method of the present invention is anatase and thus exhibits superior photocatalytic activity, and also is in the form of nano-sized particles and thus has a large surface area, resulting in increased photoactivity.
  • the C-doped TiO 2 can be imparted with novel functions.
  • the C-doped TiO 2 may be used as a conductive or sensor material using electrical properties, and may be used as a light absorbent, an optical filter, a photocatalyst, optical fiber or an IR sensor using optical properties.
  • C-doped TiO 2 was synthesized.
  • reactant gas inlets were mounted to a reaction tube of the thermal plasma jet system so as to be spaced apart from a plasma jet nozzle by a distance of 2 mm, and CO 2 and TiCl 4 serving as reactant gases were injected in a direction parallel to the direction of the generated plasma jet.
  • the flow rate of CO 2 was controlled using an MFC.
  • Ar gas was used as a carrier gas of vaporized TiCl 4 .
  • the flow rate of the carrier gas was set to 2 L/min.
  • the temperature of a thermostat was maintained at 30 °C.
  • the direct-current thermal plasma jet system was operated with direct-current power, a voltage of 40 V, and a current of 150 A.
  • Ar gas was used as the thermal plasma generating gas.
  • the Ar gas was allowed to flow between an anode nozzle and a cathode bar of the torch in the reaction tube, thus generating the plasma jet.
  • the CO 2 and TiCl 4 were decomposed and then transferred into the cooling tube, after which cooling water was supplied around the cooling tube to promote rapid cooling, thereby synthesizing C-doped TiO 2 nanoparticles.
  • the generated gas was passed through a scrubber and then a gas chromatograph, thus determining the decomposition rate of the CO 2 .
  • the synthesized C-doped TiO 2 nanoparticles were observed using an SEM. The results are shown in FIG. 3.
  • the produced particles were seen to be spherical and uniformly distributed and have a particle size of 20 ⁇ 50 nm.
  • FIG. 4 shows TiO 2 powder synthesized using only O 2
  • (b) to (d) show C-doped TiO 2 powder synthesized by injecting CO 2 at the flow rates of 0.3, 0.7 and 2 L/min.
  • the C-doped TiO 2 nanoparticles synthesized at the flow rates of CO 2 of 2 and 4 L/min were subjected to XPS.
  • the XPS was performed in a manner such that specific X-rays were applied and kinetic energy of emitted photoelectrons was measured, thus determining atomic binding energy. Thereby, internal C bonding of the synthesized TiO 2 could be confirmed.
  • FIG. 5 shows the C-doped TiO 2 synthesized at the flow rate of CO 2 of 2 L/min
  • FIG. 6 shows the C-doped TiO 2 synthesized at the flow rate of CO 2 of 4 L/min.
  • FIG. 7 shows the C-doped TiO 2 , (b) shows the pure TiO 2 , and (c) shows a high-performance TEM image around the lattices of the particles of (a).
  • the specific surface area of the C-doped TiO 2 nanoparticles according to the present invention was measured through a BET method, and determined to be 78 ⁇ 81 m2/g.
  • the C-doped TiO 2 nanoparticles according to the present invention had a high specific surface area, thereby exhibiting high catalytic activity.
  • the TiO 2 nanoparticles were synthesized at the flow rates of CO 2 of 1 ⁇ 4 L/min, after which the discharged gas was subjected to gas chromatography, thus determining the decomposition rate of CO 2 .
  • the light absorbance versus wavelength of the C-doped TiO 2 nanoparticles synthesized at the flow rates of CO 2 of 0.3, 0.7, 2 and 3 L/min and pure TiO 2 synthesized using only O 2 was measured using a UV-visible spectrophotometer. The results are shown in FIG. 8.
  • the absorption peak was shifted to the longer wavelength and the light absorbance was increased, compared to the pure TiO 2 synthesized using only O 2 .
  • the C-doped TiO 2 according to the present invention can exhibit photocatalytic activity in the UV/visible range superior to that of the TiO 2 synthesized through reaction with only O 2 , and thus can be usefully employed as a photocatalyst.
  • the C-doped TiO 2 nanoparticles according to the present invention can manifest superior photocatalytic activity to that of TiO 2 synthesized through reaction with only O 2 and used as a conventional photocatalyst, and thereby can be usefully employed as a photocatalyst in lieu of pure TiO 2 .

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Abstract

Disclosed is a method of preparing carbon-doped titanium dioxide nanoparticles by de¬ composing carbon dioxide using thermal plasma, including injecting carbon dioxide and titanium tetrachloride into a reaction tube of a thermal plasma jet system, decomposing the injected carbon dioxide by a thermal plasma jet generated using argon, so that it reacts with titanium tetrachloride transferred by a carrier gas, thus producing carbon-doped titanium dioxide, and cooling the carbon-doped titanium dioxide, thus preparing carbon-doped titanium dioxide nanoparticles. This method decomposes carbon dioxide within a short time thus reducing global warming and showing high decomposition efficiency, and does not need post-treatment thanks to the use of thermal plasma having high temperature and high activity, without concern about the production of secondary pollutants. The carbon-doped anatase titanium dioxide having a size of 20-50 nm exhibits superior photocatalytic activity in the UV/visible range, with a high specific surface area, and thus is efficiently employed as a photocatalyst.

Description

METHOD OF PREPARING CARBON-DOPED TITANIUM DIOXIDE NANOPARTICLES BY DECOMPOSITING CARBON DIOXIDE USING THERMAL PLASMA
The present invention relates to a method of preparing carbon-doped titanium dioxide nanoparticles by decomposing carbon dioxide using thermal plasma.
Increased generation of carbon dioxide (CO2) is regarded as the main cause of global warming. Because CO2 has a great influence on the increase in atmospheric temperature, climate change, ecosystems and so on, the treatment of CO2 is considered very significant in terms of reducing global warming.
On the other hand, titanium dioxide (TiO2) has been receiving attention as an environmentally friendly material because it is chemically and photochemically stable and has high oxidizing power and is very harmless. Also, TiO2 is typically utilized as a photocatalyst for removing an environmental pollutant, a material for a pigment, a plastic additive or a multicoating agent of glasses, and has two crystalline phases including an anatase phase and a rutile phase.
As such, anatase TiO2, having high photoactivity, is employed as a photocatalyst in systems for photodegradation of acetone, phenol and trichloroethylene or oxidation of nitrogen oxide such as nitrogen monoxide or nitrogen dioxide and the conversion of solar energy.
Also, rutile TiO2, having superior scattering effects for blocking UV light, is widely used as a material for white pigment. Furthermore, compared to other materials, rutile TiO2 has a higher dielectric constant and refractive index and superior oil adsorption and coloring capacity, and is chemically stable against strong acids or strong bases and thus is applied to optical coatings, beam splitters, non-reflective coating films, etc. In addition, TiO2 has wide chemical stability and non-stoichiometric phase region and thus exhibits various electrical properties depending on oxygen partial pressure. So, TiO2 is also studied as a component of a humidity sensor and a high-temperature oxygen sensor and the application field thereof is gradually widened.
General methods of preparing TiO2 which have been reported to date include a chloride process which is a gas method and a sulfate process which is a liquid method.
The chloride process, which was industrialized by Du Pont, USA in 1956, is performed in a manner such that reaction occurs at high temperature of at least 1,000 ℃ using as a starting material titanium tetrachloride (TiCl4) which is vigorously hydrolyzed through reaction with moisture in the air. As such, the resultant TiO2 particles are fine but are rough, and additional protector equipment should be used due to corrosive gases (e.g. HCl, Cl2) occurring during the reaction, undesirably increasing the production costs.
The sulfate process, which was industrialized by Titan Co., Norway in 1916, is performed in a manner such that amorphous hydroxide obtained through hydrolysis using titanium sulfate (TiSO4) as a starting material is calcined again and milled, thus obtained TiO2 powder. In this procedure, many impurities may be introduced, undesirably remarkably deteriorating the quality of a final product.
Recently, as a novel liquid method, Russian Patent No. SU-1398321 discloses a method of preparing TiO2 including adding an appropriate amount of titaniferous seed to a TiCl4 solution, hydrolyzing it through heating, thus precipitating hydrated TiO2, which is then subjected to post treatment such as high-temperature heat treatment, thereby preparing TiO2. This method has a relatively simple preparation process, but needs additional high-temperature heat treatment at 600~650 ℃ to obtain anatase TiO2 or higher-temperature heat treatment to obtain rutile TiO2.
Also, Japanese Patent Application No. Hei. 9-124320 discloses a method of preparing TiO2, including adding one selected from among acetate, carbonate, oxalate and citrate containing an alkali metal or alkali earth metal to a solution of TiCl4 dissolved in an alcohol such as butanol along with water, thus preparing a gel, which is then subjected to high-temperature heat treatment, thereby preparing TiO2. This method produces ultra-fine TiO2 having good properties but is problematic in that third additives such as expensive organic acids should be used, and high-temperature heat treatment for removing the added organic acids should be performed again after the preparation of the gel.
Thorough research into controlling properties of TiO2 powder including shape of the particles, a particle size and a particle size distribution using a sol-gel method, a hydrothermal synthesis method and so on in addition to the above methods is being conducted. To prepare spherical TiO2 powder having a uniform size, a method of using metal alkoxide is mainly utilized. Such a sol-gel method forms fine spherical powder having a uniform size of 1.0 ㎛ or smaller, but alkoxide itself used as the starting material is vigorously hydrolyzed in the air and thus reaction conditions should be strictly controlled. Also, because alkoxide is expensive, the above method has not yet been industrialized but is being studied merely on a laboratory scale. The hydrothermal synthesis method makes the state of the resultant powder good, but is disadvantageous because a continuous process is impossible attributable to the use of an autoclave under high-temperature and high-pressure conditions.
Therefore, as results of a study into the effective treatment of CO2 which is the main cause of global warming and the solution of problems encountered in the course of conventionally preparing TiO2 powder, the present inventors have discovered that carbon(C)-doped anatase TiO2 nanoparticles may be prepared by decomposing CO2 using thermal plasma, thus completing the present invention.
An object of the present invention is to provide a method of preparing C-doped anatase TiO2 nanoparticles using thermal plasma.
Another object of the present invention is to provide C-doped anatase TiO2 nanoparticles prepared through the above method.
In order to accomplish the above objects, the present invention provides a method of preparing C-doped TiO2 nanoparticles using thermal plasma, including injecting CO2 and vaporized TiCl4, serving as reactant gases, into a reaction tube of a thermal plasma jet system, decomposing the injected CO2 by a thermal plasma jet generated using argon gas, so that it reacts with the TiCl4 transferred by a carrier gas, thus producing C-doped TiO2, and cooling the C-doped TiO2, thereby preparing C-doped TiO2 nanoparticles.
In addition, the present invention provides C-doped anatase TiO2 nanoparticles prepared through the above method.
The method according to the present invention can decompose CO2 serving as a reactant gas, thus aiding the reduction of global warming. Also, because thermal plasma having features of high temperature and high activity is used, there is no need to consider post-treatment procedures nor to be concerned about the production of secondary pollutants. Furthermore, the method according to the present invention can decompose CO2 within a short time, and thus shows decomposition efficiency superior to other methods. In addition, the C-doped TiO2 synthesized through the method according to the present invention can exhibit superior photocatalytic activity in the UV/visible range compared to TiO2 synthesized through reaction with only oxygen. Also, the C-doped TiO2 has a high specific surface area thanks to a particle size of 20~50 nm, and as well is in an anatase phase, and thus can be usefully employed as a photocatalyst.
FIG. 1 is a perspective view showing a system for preparing C-doped TiO2 according to an embodiment of the present invention;
FIG. 2 is a graph showing the decomposition rate of CO2 depending on the flow rate of CO2 using thermal plasma according to the embodiment of the present invention;
FIG. 3 is a scanning electron microscope (SEM) image of C-doped TiO2 nanoparticles synthesized according to the embodiment of the present invention;
FIG. 4 is an X-ray diffraction graph of the C-doped TiO2 nanoparticles synthesized according to the embodiment of the present invention;
FIG. 5 is of X-ray photoelectron spectroscopy (XPS) graphs of the C-doped TiO2 nanoparticles synthesized according to the embodiment of the present invention (at the flow rate of CO2 of 2 L/min);
FIG. 6 is of XPS graphs of the C-doped TiO2 nanoparticles synthesized according to the embodiment of the present invention (at the flow rate of CO2 of 4 L/min);
FIG. 7 is of transmission electron microscope (TEM) images of the C-doped TiO2 nanoparticles synthesized according to the embodiment of the present invention and pure TiO2 ((a): C-doped TiO2, (b): pure TiO2, (c): high performance TEM image around the lattices of the particles of (a));
FIG. 8 is a graph showing the light absorbance of the C-doped TiO2 nanoparticles synthesized according to the embodiment of the present invention depending on the flow rate of CO2; and
FIG. 9 is a graph showing the decomposition of methylene blue over time using the C-doped TiO2 nanoparticles synthesized according to the embodiment of the present invention.
Hereinafter, a detailed description will be given of the present invention.
The present invention provides a method of preparing C-doped TiO2 nanoparticles using thermal plasma, including injecting CO2 and vaporized TiCl4, serving as reactant gases, into a reaction tube of a thermal plasma jet system (step 1), decomposing the injected CO2 by a thermal plasma jet generated using argon gas, so that it reacts with the TiCl4 transferred by a carrier gas, thus producing C-doped TiO2 (step 2), and cooling the produced C-doped TiO2, thus preparing C-doped TiO2 nanoparticles (step 3).
According to an embodiment of the present invention, as shown in FIG. 1, the thermal plasma jet system includes a plasma torch 1 for supplying a heat source for decomposing CO2, a reaction tube 2 into which CO2 and TiCl4 are injected and in which decomposition of CO2 and reaction thereof with TiCl4 occur, a cooling tube 3, an exhaust portion 4 for discharging waste gas generated after the reaction, and a power supplier 5 for supplying power to the torch 1. The thermal plasma jet system is operated using direct-current power, a voltage of 40 V and a current of 150~220 A. The torch 1 includes a tungsten cathode bar and an anode nozzle, such that argon gas is allowed to flow between the anode nozzle and the cathode bar, thus generating a plasma jet. Furthermore, to protect the torch 1 from heat, both electrodes are cooled with water. The reaction tube 2 is provided in the form of a window-equipped stainless double tube. The gas discharged via the exhaust portion 4 is passed through a scrubber 41 and then a gas chromatograph 42, thus determining the decomposition rate of CO2.
In step 1, CO2 and TiCl4 are injected into the reaction tube of the thermal plasma jet system.
Because CO2 which is the main cause of global warming has a major affect on the increase in atmospheric temperature, climate change, destruction of ecosystems and so on, the CO2 decomposition may aid the reduction of global warming, and oxygen which results from the decomposition is utilized as a material adapted for synthesis of TiO2. As such, a mass flow controller (MFC) is mounted to the inlet of CO2 so as to check the decomposition rate of CO2 depending on the flow rate of CO2.
Also, TiCl4 functions to prevent the rebinding of oxygen and carbon in the course of dissociation/rebinding of CO2, and also plays a role as a reactant upon synthesis of TiO2. As such, TiCl4, having a low boiling point, is easily vaporized even at room temperature, and is thus favorably injected using argon as the carrier gas. In order to inject a predetermined amount of TiCl4, a thermostat may be maintained at about 30~35 ℃.
In the method according to the present invention, the CO2 and TiCl4 may be injected sequentially or simultaneously. In particular, in order to increase the decomposition rate of CO2, the CO2 and TiCl4 may be simultaneously injected.
In the method according to the present invention, the CO2 and TiCl4 may be injected in a direction parallel to the direction where the plasma jet is generated. This is because TiCl4 is effectively decomposed in the high-temperature plasma region so as to efficiently achieve binding with an oxygen atom.
In the method according to the present invention, the CO2 and TiCl4 are injected at a position spaced apart from the plasma jet nozzle by a distance of not more than 2 mm so as to efficiently achieve decomposition and reaction. If the injection position falls outside 2 mm, decomposition and reaction by the plasma do not occur as desired.
Next, in step 2, the injected CO2 is decomposed by the thermal plasma jet generated using argon gas, so that it reacts with the TiCl4 transferred by the carrier gas, thus producing the C-doped TiO2.
A heat source used in this step is the thermal plasma obtained by the use of the direct-current thermal plasma jet system. The thermal plasma is an ionized gas composed of electrons, ions, atoms and molecules generated in the plasma torch using direct-current arc discharge or high-frequency induction discharge. The thermal plasma which is provided in the form of a high-speed jet flame having ultra high temperature ranging from thousands to tens of thousands of K and high thermal capacity is a fourth material state having physically and chemically properties quite different from solid, liquid and gas states.
In the thermal plasma jet system of the present invention, the thermal plasma generating gas may include for example argon gas, air and nitrogen gas. Particularly useful is argon gas. Because argon is a Group 8 element of the periodic table, it easily emits electrons even in the presence of relatively low energy. Also, argon is inert and thus barely affects a chemical reaction, and thereby is the most widely used in the generation of thermal plasma. The arc is formed by electrical energy between the cathode and anode of the power supplier of the thermal plasma jet system, and ultra high temperature plasma of about 10,000 K is produced by argon gas used as the thermal plasma generating gas. The ultra high temperature generated by such thermal plasma is much higher than the temperature generated by thermal treatment or combustion methods. Furthermore, the argon gas does not react with the other reactant gases and thus does not form by-products.
In this step, the CO2 and TiCl4 injected in step 1 are decomposed by the thermal plasma generated in the thermal plasma jet system, so that CO2 is ionized into oxygen and carbon ions and TiCl4 is ionized into chlorine and titanium ions. As such, in order to increase the decomposition rate of the CO2 which is decomposed by the thermal plasma, the flow rate of CO2 may be controlled to 1~3 L/min.
The oxygen and titanium ions bind to each other, thus forming the C-doped TiO2.
Next, in step 3, the produced TiO2 is cooled, thereby preparing the TiO2 nanoparticles.
The oxygen element resulting from the CO2 decomposition in step 2 binds to titanium and a small amount of carbon ionized by the thermal plasma, and the C-doped TiO2 nanoparticles are formed through cooling in the cooling tube.
The cooling process has an influence on crystallization of the C-doped TiO2 to thus produce the nano-sized particles. Specifically, in the case where the synthesized C-doped TiO2 is slowly cooled, the particle size of TiO2 is increased to nano size or more. Thus, in the present invention, the synthesized C-doped TiO2 may be rapidly cooled with cooling water. The temperature of cooling water is maintained at 15~25 ℃, so that the temperature of vaporized C-doped TiO2 is drastically lowered.
In addition, the present invention provides C-doped TiO2 prepared through the above method.
The C-doped TiO2 prepared through the above method is observed to be nanoparticles having a particle size of 20~50 nm using an SEM (FIG. 3). As a result of X-ray diffraction analysis, almost all of the C-doped TiO2 can be seen to be anatase (FIG. 4). Furthermore, through XPS (FIGS. 5 and 6) and TEM (FIG. 7) it can be seen that the TiO2 is doped with C.
Moreover, in the measurement of light absorbance versus wavelength, the C-doped TiO2 according to the present invention can be seen to have superior photocatalytic activity in the UV/visible range to that of TiO2 synthesized through reaction with only oxygen (FIG. 8). In the measurement of decomposition of methylene blue for evaluating activity of the C-doped TiO2 as the photocatalyst, the decomposition time is about 6~10 min, and thereby the C-doped TiO2 according to the present invention can be seen to show shorter decomposition time and faster decomposition rate than those of TiO2 synthesized through reaction with only oxygen (decomposition time: about 13 min) (FIG. 9).
Hence, the C-doped TiO2 prepared through the method of the present invention is anatase and thus exhibits superior photocatalytic activity, and also is in the form of nano-sized particles and thus has a large surface area, resulting in increased photoactivity. Furthermore, the C-doped TiO2 can be imparted with novel functions. For example, the C-doped TiO2 may be used as a conductive or sensor material using electrical properties, and may be used as a light absorbent, an optical filter, a photocatalyst, optical fiber or an IR sensor using optical properties.
A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed as limiting the present invention.
<EXAMPLE> Synthesis of C-doped TiO2 Nanoparticles using Thermal Plasma
Using the direct-current thermal plasma jet system of FIG. 1, C-doped TiO2 was synthesized.
Specifically, reactant gas inlets were mounted to a reaction tube of the thermal plasma jet system so as to be spaced apart from a plasma jet nozzle by a distance of 2 mm, and CO2 and TiCl4 serving as reactant gases were injected in a direction parallel to the direction of the generated plasma jet. The flow rate of CO2 was controlled using an MFC. To set the amount of the injected TiCl4, Ar gas was used as a carrier gas of vaporized TiCl4. The flow rate of the carrier gas was set to 2 L/min. To set the temperature of TiCl4, the temperature of a thermostat was maintained at 30 ℃. As such, the direct-current thermal plasma jet system was operated with direct-current power, a voltage of 40 V, and a current of 150 A. As the thermal plasma generating gas, Ar gas was used.
The Ar gas was allowed to flow between an anode nozzle and a cathode bar of the torch in the reaction tube, thus generating the plasma jet. Using the plasma, the CO2 and TiCl4 were decomposed and then transferred into the cooling tube, after which cooling water was supplied around the cooling tube to promote rapid cooling, thereby synthesizing C-doped TiO2 nanoparticles. After the reaction, the generated gas was passed through a scrubber and then a gas chromatograph, thus determining the decomposition rate of the CO2.
<Analysis>
(1) SEM Observation
The synthesized C-doped TiO2 nanoparticles were observed using an SEM. The results are shown in FIG. 3.
As is apparent from FIG. 3, the produced particles were seen to be spherical and uniformly distributed and have a particle size of 20~50 nm.
(2) X-ray Diffraction Analysis
The C-doped TiO2 nanoparticles synthesized at the flow rates of CO2 of 0.3, 0.7 and 2 L/min and pure TiO2 were subjected to X-ray diffraction analysis. The results are shown in FIG. 4.
In FIG. 4, (a) shows TiO2 powder synthesized using only O2, and (b) to (d) show C-doped TiO2 powder synthesized by injecting CO2 at the flow rates of 0.3, 0.7 and 2 L/min.
As shown in FIG. 4, compared to the pure TiO2 powder synthesized using only O2, the C-doped TiO2 powder had higher anatase peaks. As the flow rate of CO2 was increased, the molar fraction of anatase in the synthesized TiO2 was increased.
(3) X-ray Photoelectron Spectroscopy (XPS)
The C-doped TiO2 nanoparticles synthesized at the flow rates of CO2 of 2 and 4 L/min were subjected to XPS. The XPS was performed in a manner such that specific X-rays were applied and kinetic energy of emitted photoelectrons was measured, thus determining atomic binding energy. Thereby, internal C bonding of the synthesized TiO2 could be confirmed.
The analysis results are shown in FIGS. 5 and 6.
FIG. 5 shows the C-doped TiO2 synthesized at the flow rate of CO2 of 2 L/min, and FIG. 6 shows the C-doped TiO2 synthesized at the flow rate of CO2 of 4 L/min.
As shown in FIGS. 5 and 6, in the C 1s XPS region of (a), three peaks having binding energy of 284.4, 286.3 and 288.6 eV were observed. As such, the peaks of 286.3 eV and 288.6 eV indicate C-O bonding. In the Ti 2P region of (b), Ti 2p 3/2 peak (458.4 eV) had an energy higher than that of the pure TiO2 peak (457.3 eV). Such peak shift is based on distortion of the lattices by introducing not O but C into the lattices of TiO2. Furthermore, the peak (529.8 eV) in the O 1s region of (c) shows Ti-O bonding. From this, the TiO2 prepared through the method according to the present invention was seen to be doped with C.
(4) TEM Observation
The C-doped TiO2 nanoparticles according to the present invention and pure TiO2 were observed using a TEM. The results are shown in FIG. 7.
In FIG. 7, (a) shows the C-doped TiO2, (b) shows the pure TiO2, and (c) shows a high-performance TEM image around the lattices of the particles of (a).
As shown in FIG. 7, there were no changes in the shape and size of the particles depending on the types of reactant gas. However, in (a), partial C doping could be seen. Specifically, as seen in (c), while the direction of lattices was changed due to the C doping, defects in the lattices were seen to have occurred.
(5) Measurement of Specific Surface Area
The specific surface area of the C-doped TiO2 nanoparticles according to the present invention was measured through a BET method, and determined to be 78~81 ㎡/g. Thus, the C-doped TiO2 nanoparticles according to the present invention had a high specific surface area, thereby exhibiting high catalytic activity.
<EXPERIMENTAL EXAMPLE 1> Measurement of Decomposition Rate of CO2 depending on Flow Rate of CO2
The TiO2 nanoparticles were synthesized at the flow rates of CO2 of 1~4 L/min, after which the discharged gas was subjected to gas chromatography, thus determining the decomposition rate of CO2.
Furthermore, when only CO2 was injected as the reactant gas and when CO2 and TiCl4 were injected together as the reactant gases, the decomposition rate of CO2 was measured.
The measurement results are shown in FIG. 2 and Table 1 below.
Table 1
Flow Rate of CO2 (L/min) Decomposition Rate of CO2 (%)
CO2 CO2 + TiCl 4
1 90 95
2 81 87
3 76 84
4 67 72
As shown in FIG. 2 and Table 1, as the flow rate of CO2 was increased, the decomposition rate thereof was reduced. Compared to when only CO2 was injected, when CO2 and TiCl4 were injected together, the decomposition rate of CO2 was increased by about 5.5~9%.
<EXPERIMENTAL EXAMPLE 2> Measurement of Photocatalytic Properties of C-Doped TiO2 Nanoparticles
To evaluate the photocatalytic properties of the C-doped TiO2 nanoparticles according to the present invention, the following experiment was performed.
The light absorbance versus wavelength of the C-doped TiO2 nanoparticles synthesized at the flow rates of CO2 of 0.3, 0.7, 2 and 3 L/min and pure TiO2 synthesized using only O2 was measured using a UV-visible spectrophotometer. The results are shown in FIG. 8.
As shown in FIG. 8, in the case of the C-doped TiO2 according to the present invention, the absorption peak was shifted to the longer wavelength and the light absorbance was increased, compared to the pure TiO2 synthesized using only O2. Thus, the C-doped TiO2 according to the present invention can exhibit photocatalytic activity in the UV/visible range superior to that of the TiO2 synthesized through reaction with only O2, and thus can be usefully employed as a photocatalyst.
<EXPERIMENTAL EXAMPLE 3> Decomposition of Methylene Blue of C-Doped TiO2 Nanoparticles
To evaluate the photocatalytic properties of the C-doped TiO2 nanoparticles prepared in the present invention, the following experiment was performed.
The C-doped TiO2 nanoparticles synthesized at the flow rates of CO2 of 0.7 and 3 L/min, and pure TiO2 synthesized using only O2 were separately added to a 6 ppm diluted methylene blue solution, and the decomposition of methylene blue over time was then measured in the UV range. The results are shown in FIG. 9.
As shown in FIG. 9, in the case of pure TiO2 synthesized using only O2, about 13 min was required to decompose methylene blue. However, in the case of C-doped TiO2 nanoparticles according to the present invention, methylene blue was decomposed within 6~10 min, and the decomposition rate was seen to be faster. Thus, the C-doped TiO2 nanoparticles according to the present invention can manifest superior photocatalytic activity to that of TiO2 synthesized through reaction with only O2 and used as a conventional photocatalyst, and thereby can be usefully employed as a photocatalyst in lieu of pure TiO2.

Claims (9)

  1. A method of preparing carbon-doped titanium dioxide nanoparticles using thermal plasma, comprising:
    injecting carbon dioxide and vaporized titanium tetrachloride, serving as reactant gases, into a reaction tube of a thermal plasma jet system;
    decomposing the injected carbon dioxide by a thermal plasma jet generated using argon gas, so that it reacts with the titanium tetrachloride transferred by a carrier gas, thus producing carbon-doped titanium dioxide; and
    cooling the carbon-doped titanium dioxide, thus preparing carbon-doped titanium dioxide nanoparticles.
  2. The method according to claim 1, wherein the carbon dioxide and the titanium tetrachloride are injected at a position spaced apart from a plasma jet nozzle by a distance of not more than 2 mm in a direction parallel to a direction where the plasma jet is generated.
  3. The method according to claim 1, wherein argon gas is used as the carrier gas and a temperature of a thermostat is maintained at 30~35 ℃, in order to uniformly maintain an injection amount of the titanium tetrachloride.
  4. The method according to claim 1, wherein the cooling the carbon-doped titanium dioxide is performed through a water cooling process comprising rapid cooling using cooling water at 15~25 ℃.
  5. The method according to claim 1, wherein a flow rate of the carbon dioxide is controlled to a range of 1~3 L/min, in order to increase a decomposition rate of the carbon dioxide which is decomposed by the thermal plasma.
  6. Carbon-doped titanium dioxide nanoparticles prepared through the method of claim 1.
  7. The carbon-doped titanium dioxide nanoparticles according to claim 6, wherein the carbon-doped titanium dioxide nanoparticles have a particle size of 20~50 nm.
  8. The carbon-doped titanium dioxide nanoparticles according to claim 6, wherein the carbon-doped titanium dioxide nanoparticles are in an anatase phase.
  9. The carbon-doped titanium dioxide nanoparticles according to claim 6, wherein the carbon-doped titanium dioxide nanoparticles have a high specific surface area of 78~81 ㎡/g.
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