CN101524642A - Hydrothermal surface fluorination method for preparing high photocatalytic activity mesoporous titanium dioxide powder - Google Patents

Abstract

A hydrothermal surface fluorination method for preparing mesoporous titanium dioxide powder with high photocatalytic activity. The method is to put an aqueous solution of ammonium bifluoride into a 200 ml polytetrafluoroethylene lining, and then add an ethanol solution of tetrabutyl titanate dropwise to the lining, and the tetrabutyl titanate is hydrolyzed to produce precipitation. Then put the lining into another 200ml stainless steel reaction kettle, seal the reaction kettle, heat the reaction kettle to 120-200°C, and keep it for 5-24 hours. After the reaction, the reaction kettle is cooled to room temperature and discarded. Remove the supernatant, wash the obtained precipitate with distilled water and ethanol in sequence, then vacuum-dry and grind into fine powder to obtain the surface fluorinated mesoporous titanium dioxide photocatalyst. The obtained photocatalyst has high activity. The method is simple, low temperature and environment friendly.

Description

A hydrothermal surface fluorination method for preparing mesoporous titanium dioxide powder with high photocatalytic activity

technical field

The invention relates to a method for improving the photocatalytic activity of titanium dioxide by hydrothermal surface fluorination treatment.

technical background

Since the discovery of titanium dioxide photocatalytic water splitting by Fujishima and Honda in 1972, titanium dioxide and other semiconductor photocatalytic materials have attracted close attention due to their wide potential applications in water and air purification as well as solar energy. Among various oxide and non-oxide semiconductor photocatalytic materials, titanium dioxide is the most suitable photocatalytic material due to its biological and chemical inertness, strong oxidation ability, low cost, and long-lasting stability against light and chemical corrosion. From a practical and commercial perspective, the photocatalytic activity of titania must be further improved. In recent years, many studies have found that fluorine doping can improve the crystallinity and photocatalytic activity of anatase phase titanium dioxide, and surface fluorination has become a new method for surface modification of titanium dioxide. Surface fluorination can be achieved simply by an ion-exchange reaction between hydroxyl groups on the titanium dioxide surface and fluoride ions.

Surface fluorinated TiO 2 (F-TiO ₂ ) showed higher photocatalytic activity than non-fluorinated TiO 2 during the photocatalytic oxidation of Acid Orange 7 and phenol in aqueous solution. The degree of fluorination of titanium dioxide depends largely on the pH value, with the highest degree of fluorination (approximately 99%) being at pH 3-4. Studies have shown that the fluorinated surface is conducive to the generation of hydroxyl radicals (non-surface hydroxyl groups), and hydroxyl radicals help to improve the oxidation capacity. Since the surface fluoride itself does not react with the valence band holes, the enhanced photocatalytic oxidation ability of the F- _TiO2 suspension can be attributed to the increase of hydroxyl radicals (reaction 2), while the free hydroxyl radicals on the surface of unfluorinated _TiO2 groups tend to adsorb more on the surface (reaction 3).

These reactions indicate that the photocatalytic reaction in the F- _TiO2 suspension can proceed inside the solution away from the _TiO2 surface. The current method for surface fluorination of titania is achieved by an ion exchange reaction between hydroxyl groups on the titania surface and fluorine ions (reaction 3), and surface fluorinated titania prepared by this method usually exhibit low photocatalytic activity.

This patent proposes a method for hydrothermal surface fluorination treatment to improve the photocatalytic activity of titanium dioxide, that is, by hydrothermally treating the precipitate of tetrabutyl titanate (TBOT) in a mixed solution of ammonium bifluoride, ethanol and water, one-step preparation of Surface fluorinated mesoporous titania powder with high photocatalytic activity.

Contents of the invention

The purpose of the present invention is to propose a hydrothermal surface fluorination method for preparing mesoporous titanium dioxide powder with high photocatalytic activity according to the current research status at home and abroad, considering the shortcomings of the usual method for preparing high-activity nano-titanium dioxide photocatalyst. The method is simple, low temperature, water is a solvent, and is environmentally friendly. The method can prepare a highly active mesoporous titanium dioxide photocatalyst with a grain size of 10-12nm, a mesopore diameter of 5.5-6.5nm and a specific surface area of 120-125m ² /g.

The technical scheme that realizes the object of the present invention is:

A hydrothermal surface fluorination method for preparing nano-titanium dioxide powder with high photocatalytic activity, which is characterized in that catalyst preparation and surface fluorination treatment are completed in one step, and the method steps are as follows:

1. Configure ammonium bifluoride aqueous solution and tetrabutyl titanate ethanol solution. The solution configuration is: 0.01-1 gram of ammonium bifluoride is added to 150 ml of water; 12 ml of butyl titanate is added to 10-40 ml of ethanol;

2. Add the ethanol solution of butyl titanate dropwise into the ammonium bifluoride aqueous solution under magnetic stirring;

3. Transfer the mixed solution to a 200 ml hydrothermal kettle so that 80% of the volume of the hydrothermal kettle can be filled, cover the hydrothermal kettle tightly, and conduct a hydrothermal reaction at 100-200°C for 5-24 hours;

4. Collect the resulting white solid precipitate, wash it with water and ethanol in sequence, and then dry it in a vacuum oven at 30-100°C for 1-3 hours to obtain nano-titanium dioxide powder with high photocatalytic activity.

The preferred preparation conditions are: the aqueous solution of ammonium bifluoride is prepared by adding 0.29-0.57 grams of ammonium bifluoride to 120 milliliters of water;

The tetrabutyl titanate ethanol solution is configured by adding 5-10 ml of tetrabutyl titanate to 30 ml of ethanol, preferably by adding 6-7 ml of tetrabutyl titanate to 30 ml of ethanol;

The hydrothermal reaction temperature is 140-160°C;

The described hydrothermal reaction time is 8-12 hours;

Step 4 Dry in a vacuum oven at 80°C for 2 hours.

The photocatalytic activity of the as-prepared highly active surface fluorinated TiO2 was characterized by the photocatalytic degradation of acetone in air. The experimental procedure is as follows: The test of the photocatalytic degradation of acetone of the _TiO2 sample was carried out in a 15-liter airtight rectangular container, and the initial concentration of acetone was 275 ± 25 ppm. The photocatalyst samples were prepared by uniformly coating the _TiO2 suspension onto three petri dishes with a diameter of 7 cm, drying the petri dishes at 80 °C, and then cooling to room temperature for use. The mass of each test sample was kept at 0.3 g. During the experiment, the petri dish was put into the reactor, and then acetone was injected into the reactor with a micro-injector. The reactor was directly connected to a desiccator containing calcium chloride in order to control the initial humidity inside the reactor. Allow acetone vapor to reach adsorption-desorption equilibrium with the catalyst prior to UV light exposure. The intensity of ultraviolet light irradiated on the surface of the sample was measured with an ultraviolet photometer (UV-A type, manufactured by Beijing Normal University Optoelectronic Instrument Factory). The intensity was 2.5mW/cm ² , and the peak wavelength of ultraviolet light was 365nm. The concentrations of acetone, carbon dioxide and water vapor in the reactor were detected and analyzed online using a photoacoustic IR multigas monitor (INNOVA air techinstruments model 1312). The photocatalytic activity of the _TiO2 samples was quantitatively characterized by comparing the respective apparent reaction rate constants. The photocatalytic oxidation reaction of acetone is a quasi-first-order reaction, and its kinetic equation can be expressed as: ln(C ₀ /C)=kt, k is the apparent rate constant, C ₀ and C are the initial and reaction process of acetone, respectively concentration in . In addition, the application of the prepared product in degrading dye pollutants in water is also verified by the test of fading rhodamine B aqueous solution. As the photocatalytic reaction progressed, the concentration change of Rhodamine B was detected by measuring the absorbance of its aqueous solution. The absorbance was measured by a Shimadzu UV-2550 ultraviolet-visible spectrophotometer.

The characterization method of the microstructure of the highly active surface fluorinated titanium dioxide photocatalyst is: the X-ray diffractometer (HZG41/B-PC type) obtained on the Cu target as the X-ray source and the scan rate of 0.05 degrees/second. - Ray Diffraction (XRD) patterns to determine crystal phase and grain size. The specific surface area of the powder sample was tested on a nitrogen adsorption instrument modeled as Micromeritics ASAP 2020 (USA) by nitrogen adsorption. All samples were degassed at 100°C for 2 hours prior to testing. The Brunauer-Emmett-Teller (BET) surface area (S _BET ) of the samples was calculated by the multi-point BET method using the adsorption data with relative pressure (P/P ₀ ) in the range of 0.05-0.3. The pore size distribution was determined from the desorption isotherm using the Barret-Joyner-Halender (BJH) method and assuming that the pores are cylindrical. The pore volume and average pore diameter were determined at a nitrogen adsorption volume at a relative pressure (P/P ₀ ) of 0.994. The size and shape of the grains of mesoporous titanium dioxide powder were observed by transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM). The samples required for TEM observation were first prepared by dispersing _TiO2 powder in absolute ethanol under ultrasonic conditions, and then adding the dispersion liquid onto the carbon film-copper composite mesh.

The hydrothermal surface fluorination method of the present invention is also suitable for the preparation of other high active materials.

The present invention will be further described below in conjunction with the accompanying drawings and examples.

Description of drawings

Fig.1 XRD patterns of _TiO2 samples prepared under different _RF conditions

Fig.2 Nitrogen adsorption-desorption isotherms of _TiO2 samples at R _F =0 and R _F =0.5

Fig.3 Pore size distribution curves of _TiO2 samples when R _F =0 and R _F =0.5

Figure 4 TEM of _TiO2 sample with R _F = 0.5

Fig.5 HRTEM of _TiO2 sample with R _F = 0.5

Fig. 6 Comparison of rate constants of _TiO2 samples with different _RF

where R _F is the atomic ratio of fluorine and titanium.

Detailed ways

Example 1

To prepare nanocrystalline _TiO2 powder, tetrabutyl titanate was used as titanium source. The detailed experimental process is as follows: First, take 120 ml of distilled water and place it in a 200 ml polytetrafluoroethylene liner, add 0.29-0.57 g of ammonium bifluoride to it and keep stirring to completely dissolve it. Then 6.8 milliliters of tetrabutyl titanate was dissolved in 30 milliliters of ethanol and mixed thoroughly. At room temperature, this solution was added dropwise to the aqueous solution of ammonium bifluoride with constant stirring. Tetrabutyl titanate will be hydrolyzed when it meets water, resulting in precipitation. After sealing, the autoclave was heated to 150°C and maintained at this temperature for 10 hours. After the reactor was cooled to room temperature, the white product was collected and washed with deionized water and ethanol in sequence, and then the powder was dried in a drying oven at 80° C. for 2 hours. Finally, the dried product was ground into a fine powder with an agate mortar to obtain the desired surface fluorinated TiO ₂ powder sample.

The phase structure of the prepared samples was characterized by XRD. The XRD patterns and related physical properties of _TiO2 samples prepared at different temperatures are shown in Fig. 1 and Table 1. It can be seen from Fig. 1 that the titanium dioxide (R _F =0) prepared in pure water is mainly anatase phase and contains a small amount of brookite phase. The single peak at 2θ=30.7° corresponds to the diffraction peak of the (121) plane of the titanium dioxide brookite phase. There is no brookite phase in TiO ₂ prepared by adding NH ₄ HF ₂ , because NH ₄ HF ₂ inhibits the formation of brookite. Further observation found that with the increase of _RF , the XRD peak of the anatase phase gradually strengthened, and the width of the diffraction peak gradually narrowed, indicating the growth of TiO ₂ grains and the increase of crystallinity. The crystallinity of _TiO2 nanoparticles was calculated from the relative intensity of the (101) plane diffraction peak of anatase. Table 1 lists the average grain size and relative crystallinity of anatase phase of _TiO2 samples under different _RF . It can be seen from the table that the average grain size and the relative crystallinity of the anatase phase increase with the increase of R _F .

Figure 2 shows the nitrogen adsorption-desorption isotherms of titanium dioxide samples when _RF = 0 and _RF = 0.5. The nitrogen adsorption-desorption isotherm of all titanium dioxide is type IV, and there is a hysteresis loop between the relative pressure of 0.5-0.8. The loop type is H2 type, usually an ink bottle hole (small mouth and large cavity).

Figure 3 shows the corresponding pore size distribution curves of the prepared samples when R _F =0 and R _F =0.5. The pore size distribution curve obtained from the nitrogen desorption isotherm by the BJH (Barrett-Joyner-Halenda) method is narrow (3.0-8.0 nm), and the peak pore diameter is about 5.4 nm. These mesopores come from the agglomeration of primary particles. The narrow pore distribution indicates that the _TiO2 particles prepared by this method have a uniform pore size distribution.

Figure 4 presents the TEM of the _TiO2 samples prepared at _RF = 0.5. It can be seen from the figure that the TiO ₂ nanocrystals are in an agglomerated state and contain a disordered mesoporous structure. The average size of the primary particles estimated from the TEM photos is about 11±2nm, which is in good agreement with the grain size (11.2nm) calculated from the XRD pattern using the Scherrer equation (as shown in Table 1).

Figure 5 shows the HRTEM photographs of TiO ₂ samples prepared at _RF = 0.5. Clear lattice fringes can be seen from the figure, indicating that the sample is highly crystalline. The spacing of the lattice fringes is 0.35nm, which coincides with the spacing of the crystal planes of the anatase phase TiO ₂ (101).

Figure 6 presents the effect of _RF on the apparent reaction rate constant of the as-prepared _TiO2 powders under UV light irradiation and the comparison with the photocatalytic activity of P25. The unfluorinated _TiO2 prepared in pure water has better photocatalytic activity because of its larger specific surface area and smaller grain size. While all fluorinated _TiO2 has higher activity than unfluorinated and P25. With the increase of _RF , the crystallization of fluorinated _TiO2 is enhanced, and its photocatalytic activity is significantly improved. When R _F =0.5, k reaches the maximum value, which is 17.35×10 ^-3 min ^-1 . The photocatalytic reaction rate constant of P25, which is recognized as having good photocatalytic activity, is 5.64×10 ^-3 min ^-1 . When R _F =0.5, the photocatalytic activity of fluorinated _TiO2 is 3.01 times that of P25, because it has larger specific surface area, smaller grain size and mesoporous structure than P25. In general, the specific surface area and grain size of P25 are 50m ² g ^-1 and 30nm, respectively. With the further increase of _RF , the value of k decreases obviously.

Example 2:

In order to test the effect of hydrothermal time on the photocatalytic activity of the sample, the hydrothermal temperature was fixed at 150°C. Except for the different hydrothermal time, other reaction conditions such as: tetrabutyl titanate, absolute ethanol, ammonium bifluoride and the amount of water, etc. All are identical with embodiment 1. The results showed that the sample prepared at 0.5–1 h had essentially no photocatalytic activity due to its amorphous structure. With the prolongation of hydrothermal time, the photocatalytic activity of the samples gradually increased. When the hydrothermal time was increased to 8 h, the photocatalytic activity of the sample increased significantly and was higher than that of P25, which may be due to the former having larger specific surface area, higher pore volume and smaller grain size. The photocatalytic activity of the sample prepared at 10 h was the highest. Although this sample has a smaller specific surface area than the 5 h prepared sample, it has larger grain size and higher crystallinity. Generally, the higher the crystallinity, that is to say, the fewer surface and bulk defects, leading to a decrease in the recombination probability of photogenerated electrons and holes and an increase in photocatalytic activity. Continue to extend the hydrothermal time to 15-24 hours, the photocatalytic activity begins to decrease, which may be due to the sharp decrease of the specific surface area and pore volume due to the increase of the grain size. According to the above results, it can be deduced that _TiO2 particles with higher crystallinity and larger pore volume are more conducive to the application of photocatalysis. In this embodiment, the hydrothermal time of 5-24 hours can improve the photocatalytic activity, and the optimum hydrothermal time is 8-12 hours.

Example 3:

In order to test the effect of hydrothermal temperature on the photocatalytic activity of the sample, in addition to the different hydrothermal temperature, other reaction conditions such as: hydrothermal time (10 hours), tetrabutyl titanate, absolute ethanol, ammonium bifluoride and the amount of water, etc. All are identical with embodiment 1. The results show that when the hydrothermal temperature is lower than 100 °C, the as-prepared samples are basically amorphous and have low photocatalytic activity. With the increase of temperature, the photocatalytic activity of titanium dioxide also gradually increased, because higher temperature is more conducive to the growth of crystal grains. When the temperature rises to 140-160 °C, the photocatalytic activity of the prepared titanium dioxide reaches the highest value. Continuing to increase the temperature to 200 °C, the activity of the sample began to decrease, which was caused by a sharp decrease in the specific surface area. Therefore, the optimum hydrothermal temperature for preparing highly active titanium dioxide by this method is 140-160°C.

Example 4:

In order to test the effect of the amount of tetrabutyl titanate on the photocatalytic activity of the sample, in addition to the different amount of tetrabutyl titanate, other reaction conditions such as: reaction temperature (150 ° C), reaction time (10 hours), the amount of water (120 milliliters) etc. are all identical with embodiment 1. The results show that when the amount of tetrabutyl titanate is in the range of 5-10 ml, the prepared titanium dioxide samples all have high photocatalytic activity. In experiments, it was found that the optimum amount of butyl titanate was 6-7 milliliters.

Table 1 Effect of different R _F on the physical properties of _TiO2 samples

R _F phase composition Specific Surface(m ² /g) Pore volume (cm ³ /g) Average pore size (nm) Porosity(%) Grain size (nm) 0 A, B 196.6 0.28 5.7 50.9 8.1(1.00) 0.25 A 126.6 0.19 5.8 41.3 10.9(1.35) 0.5 A 122.0 0.19 6.1 41.3 11.2(1.38 1 A 120.9 0.18 6.2 40.0 11.5(1.42)

In Table 1, A represents the anatase phase, and B represents the brookite phase; the BET surface area is calculated by the linear part (P/P ₀ =0.05-0.3) of the adsorption isotherm; the pore volume (total pore volume), from P/ The _N adsorption volume of P ₀ = 0.994 was obtained; the average pore size was estimated by the desorption isotherm and the Barrett-Joyner-Halenda (BJH) method; the porosity was estimated by the pore volume; the average grain size of _TiO was obtained by XRD with Scherrer Equation calculation; relative crystallinity: the relative intensity of the diffraction peaks of the (101) crystal plane of the anatase phase (indicated in parentheses).

Claims (6)

1, a kind of hydrothermal surface fluorination method for preparing the highlight catalytic active nano titania powder is characterized in that method step is followed successively by:

1st, the configuration ammonium acid fluoride aqueous solution and butyl titanate ethanolic solution, solution allocation is: 0.01-1 gram ammonium acid fluoride adds 150 ml waters; 12 milliliters of butyl titanates add in 1040 milliliters of ethanol;

2nd, the ethanolic solution of the butyl titanate that is disposed dropwise joins in the ammonium acid fluoride aqueous solution under magnetic agitation;

3rd, this mixed solution is transferred in 200 milliliters of water heating kettles, 80% volume of water heating kettle is filled, cover water heating kettle completely, at 100-200 ℃ of hydro-thermal reaction 5-24 hour;

4th, the white solid of gained precipitation is collected water and ethanol washing successively, then in vacuum drying chamber in 30-100 ℃ dry 1-3 hour down, promptly make the highlight catalytic active nano titania powder.

2, the hydrothermal surface fluorination method of preparation highlight catalytic active nano titania powder as claimed in claim 1 is characterized in that: the described ammonium acid fluoride aqueous solution adds the configuration of 120 ml waters by 0.29-0.57 gram ammonium acid fluoride.

3, the hydrothermal surface fluorination method of preparation highlight catalytic active nano titania powder as claimed in claim 1 is characterized in that: described butyl titanate ethanolic solution adds 30 milliliters of ethanol configurations by butyl titanate 5-10 milliliter.

4, as the hydrothermal surface fluorination method of described each preparation highlight catalytic active nano titania powder of claim 1-3, it is characterized in that: described hydrothermal temperature is 140-160 ℃.

5, as the hydrothermal surface fluorination method of described each preparation highlight catalytic active nano titania powder of claim 1-3, it is characterized in that: the described hydro-thermal reaction time is 8-12 hour.

6, as the hydrothermal surface fluorination method of described each preparation highlight catalytic active nano titania powder of claim 1-3, it is characterized in that: the 4th step in vacuum drying chamber in 80 ℃ of dryings 2 hours.

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