CZ2011430A3 - Process for preparing photocatalytically active material with foamy structure - Google Patents

Abstract

A method for preparing a photocatalytically active material, ammonium peroxotitanite in the form of a very low bulk weight foam of 1 to 10 g.dm.sup.-3. for titanium dioxide, which retains the morphological structure of the ammonium perititanate, such that the hydrated titanium dioxide resulting from dissolution precursor in water and then precipitating it with an alkaline precipitating agent, in aqueous suspension it is treated with hydrogen peroxide to form a yellow transparent gel which is further lyophilized at a temperature below 0 degC to -64 degC and the water vapor pressure corresponding to the respective temperature to remove all the aqueous phase to form an ammonium peroxotitanite foam, wherein the precursor content in the aqueous solution is 1 to 25% by weight. This results in a yellow very porous foam, the texture of which depends in particular on the concentration of titanium dioxide in the colloidal solution, with a bulk density of 1 to 10 g.dm.sup.-3.n. 450 to 1100 degC, while retaining the texture of the starting perititanium gel.

Description

Method of preparation of photocatalytically active material with foam structure

Field of technology

The present invention relates to a process for the preparation of ammonium peroxotitanate in the form of foam and titanium dioxide, which preserves the morphology of the starting peroxotitanate with a very low bulk density of 1 to 10 .mu.g.dm @ ³ , which are useful, for example, as sorbents, photocatalysts surfaces or catalysts.

State of the art

Titanium dioxide is an important material widely used as a white pigment, a component of various catalysts or photocatalysts, and in many other applications. Two of its crystallographic modifications, anatase and rutile, are particularly technically important. In addition to chemical purity and phase composition, the texture of the material is crucial for the practical use of both of these modifications. The form of titanium dioxide foam with low bulk density, which is characterized by high porosity and high gas phase permeability, high sorption capacity and consequently high photocatalytic activity, plays an important role. For these reasons, the synthesis of titanium dioxide in the form of foam has received considerable attention in the past. In particular, various modifications of sol-gel processes have been used in connection with the application of gel layers to the surface of various polymer templates and their subsequent annealing: e.g. Ren, J .; Du, H. J .; Li, H. Q., Porous titania monoliths prepared through sol-gel method using polystyrene foam as template. Abstr. Pap. Am. Chem. Soc. 2005, 229, 270-PMSE; Ren, J .; Du, Z. J .; Zhang, C .; Li, H. Q., Macroporous titania monolith prepared via sol-gel process with polymer foam as the template. Chin. J. Chem. 2006, 24, (7), 955-960; Saha, M. C .; Kabir, Μ. E.; Jeelani, S., Enhancement in thermal and mechanical properties of polyurethane foam infused with nanoparticles. Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 2008, 479, (1-2), 213-222; Gao, N .; Schumacker, A .; Hepel, M .; Yartym, J., Photocatalytic degradation of naphthol blue black dye with TiO2 nanoparticles immobilized on polyethylene foam sheet. Abstr. Pap. Am. Chem. Soc. 2000, 219, 226-ENVR. The starting polymeric foam is immersed in a solution of titanium (IV) isopropoxide in an alcohol, e.g. 2-propannol. It is used as a polymeric carrier

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polyacrylamide, polyurethane, polyethylene. Other procedures (Zhao, Y .; Zhang, X .; Zhai, J .; Jiang, L .; Liuy, ZY; Nishimoto, S .; Murakami, T .; Fujishima, A .; Zhu, DB, Ultrastable TiO2 foams derived macro- / meso-porous material and its photocatalytic activity Microporous Mesoporous Mat. 2008, 116, (1-3), 710-714; Cam, F .; Colin, A .; Achard, MF; Deleuze, H .; Sanchez Backov, R., Anatase and rutile TiO2 macrocellular foams: Air-liquid foaming sol-gel process towards controlling cell sizes, morphologies, and topologies Adv. Mater. 2005, 17, (1), 62) are established on the action of surfactants such as hexylamine, which acts on a colloidal solution of titanium dioxide nanoparticles while bubbling air to form a stable moist foam, which is further dried and annealed. Other methods use tetradecyltrimethylammonium bromide combined with pluronic P-123 copolymer (Arabatzis, IM; Falaras, P., Synthesis of porous nanocrystalline TiO2 foam. Nano Lett. 2003, 3, (2), 249-251; Cam, F .; Achard, MF, Babot, O .; Deleuze, H .; Reculusa, S .; Backov, R., Syntheses and characterization of highly mesoporous crystalline TiO2 macrocellular foams J. Mater. Chern. 2005, 15, (35-36), 3887 -3895), the processing process is similar. The foams thus prepared are pure titanium dioxide, but the synthesis processes are generally environmentally difficult, expensive and the resulting foams are relatively large particles, given the need to use organic substances in the synthesis process. Another type of foam is TiO2 layers deposited on porous ceramics or glasses, e.g., Boccaccini, A. R .; Rossetti, M .; Roether, J. A .; Zein, S. H. S .; Ferraris, M., Development of titania coatings on glass foams. Constr. Build. Mater. 2009, 23, (7), 2554-2558; Haugen, H .; Will, J .; Kohler, A .; Hopfner, U .; Aigner, J .; Wintermantel, E., Ceramic TiO2-foams: characterization of a potential scaffold. J. European Ceram. Soc. 2004, 24, (4), 661-668; Kim, H .; Lee, S .; Han, Y,; Park, J., Preparation of dip-coated TiO2 photocatalyst on ceramic foam pellets. J. Mater. Sci. 2005, 40, (19), 5295-5298; Ochuma, I. J .; Osibo, O. O .; Fishwick, R. P .; Pollington, S .; Wagland, A .; Wood, J .; Winterbottom, J. M., Three-phase photocatalysis using suspended titania and titania supported on a reticulated foam monolith for water purification. Catal. Today 2007, 128, (1-2), 100-107; Plesch, G .; Gorbar, M .; Vogt, U. F .; Jesenak, K .; Vargova, M., Reticulated macroporous ceramic foam supported TiO2 for photocatalytic applications. Mater. Lett. 2009, 63, (3-4), 461-463 Schimmoeller, B .; Schulz, H .; Pratsinis, S. E .; Bareiss, A .; Reitzmann, A .; Kraushaar-Czametzki, B., Ceramic foams directly-coated with fame-made V2O5 / TÍO2 for synthesis of phthalic anhydride. J. Catal. 2006, 243, (1), 82-92; Wang, H. N .; Yuan, P .; Zhou, L .; Guo, Y. N .; Zou, J .; Yu, A. M .; Lu, G. Q .; Yu, C. Z., Synthesis and characterization of TiO2-incorporated silica foams. J. Mater. Sci. 2009, 44, (24), 6484-6489; Wu, P. G .; Xie, R. C .; Imlay, K .; Shang, J. K., Monolithic Ceramic Foams for Ultrafast

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Photocatalytic Inactivation of Bacteria. J. Am. Ceram. Soc. 2009, 92, (8), 1648-1654. In this case, too, the sol-gel precursor is used, i.e. solutions of titanium (IV) isopropoxide in alcohol, e.g. 2-propannol, which is applied to the surface of the ceramic or glass foam by spraying or immersion, followed by drying and annealing.

The disadvantage of these processes is the high price of raw materials as well as the environmental problems associated with a considerable amount of liquid and gaseous wastes generated during synthesis. In most cases, the materials have a not entirely optimal texture, especially too thick cell walls, which results in degraded sorption or photocatalytic properties. Ceramic carrier materials also have a relatively low titanium dioxide content and the resulting low photoactivity.

The essence of the invention

The solution according to the invention relates to a process for the preparation of a photocatalytically active material, ammonium peroxotitanate in the form of a foam with a very low bulk density of 1 to 10 g.dm ^-3 and titanium dioxide, which preserves the morphological structure of ammonium peroxotitanate, such that hydrated titanium dioxide, formed by precipitation with an alkaline precipitating agent from an aqueous precursor solution, is treated in aqueous suspension with hydrogen peroxide to give a yellow transparent gel which is further lyophilized at less than 0 ° C but the lowest possible - 64 ° C and water vapor pressure corresponding to temperature until the removal of all the aqueous phase to form an ammonium persulfate foam, wherein the proportion of the precursor in the aqueous solution is 1 to 25% by weight.

The starting material for the synthesis is thus an aqueous solution of a precursor, a titanium salt (titanium salt - titanyl sulphate or titanium salt) soluble in water, preferably eg TiOSO 4, TiCl 4. The proportion of precursor, salt, in the aqueous solution corresponds to 1 to 25% by weight. precursor. Hydrated titanium dioxide T1O2 x nHiO is then precipitated from this aqueous precursor solution with a basic precipitating agent, e.g. aqueous ammonia, the precipitating agent being added until the pH in the aqueous phase reaches more than 7. The aqueous solution with the precipitate is then cooled. , to the solidification temperature of the aqueous solution or lower, preferably to the solidification temperature of the aqueous solution, 0 ° C, and cooled until crushed, at least until the first crystals formed, preferably until homogeneous crushed, and preferably further stirred for up to 90 minutes. The precipitate of hydrated titanium dioxide thus formed is filtered off and washed with water of a purity corresponding to the purity requirements of the final product until

• The disappearance of the content of the corresponding anions (SO4 ² , Cf) in the filtrate and the prepared precipitate of hydrated titanium dioxide is resuspended in water. An aqueous solution of hydrogen peroxide is then added to the washed suspension in an amount of 1: 1 or more in the titanium present. Excess hydrogen peroxide does not significantly affect the properties of the resulting gel or products resulting from its subsequent processing. The resulting yellow completely transparent gel, a false solution of ammonium peroxotitanate, is then lyophilized, i. is rapidly frozen and dewatered by vacuum sublimation at a temperature below the freezing point of water (temperature below 0 ° C to -64 ° C) and the corresponding saturated water vapor pressure, the material is thus dried until sublimation of all ice, to form a yellow very porous foam, the texture of which depends mainly on the concentration of titanium dioxide in the colloidal solution, with a bulk density of up to 10 g.dm ³ . This method of drying preserves the original structure of the false solution, which is formed by a network of very thin ammonium peroxotitanate membranes (see Fig. 1-2). Lyophilization allows complete removal of the aqueous phase by sublimation under vacuum without aggregation of the individual films and their solid adhesion. Thus, the lyophilized material consists of a network of thin films of ammonium persulphate, which can be advantageously annealed and thus converted to crystalline titanium dioxide in the form of anatase or, at annealing temperatures above 950 ° C, a mixture of anatase and rutile or pure rutile (see FIG. 3). The lyophilized material, yellow foam, is thus preferably annealed to a temperature in the range of 450 to 1100 ° C (e.g. in air or in an inert atmosphere, annealing is not limited by the composition of the atmosphere). The annealing product is a porous three-dimensional system of ultra-thin membranes, composed of nanocrystalline titanium dioxide anatase or at annealing temperatures above 950 ° C a mixture of anatase and rutile or even pure rutile at even higher temperatures with typical slit-shaped pores (see Fig. 4). Due to the layered structure of the peroxygenate foam, one of the possible growth directions of the anatase nanocrystals is blocked during annealing and the particles therefore remain very small even at annealing temperatures around 1000 ° C and the pores formed have a slit shape. Annealing therefore produces a very porous foam with a bulk density of 1 to 10 g.dm.

The prepared material, both the lyophilized material and the product of its annealing, is highly photocatalytically active.

The photoactivity of the materials thus prepared was determined by measuring the decomposition kinetics of aqueous 4-chlorophenol solutions according to the procedure described in our previous work Bakardjieva, S .; Subrt, J .; Stengl, V .; Dianez, MJ; Sayagues, MJ, Photoactivity of anatase-rutile TiO ₂ nanocrystalline mixtures obtained by heat treatment of homogeneously precipitated anatase. Appl. Catal. B-Environ. 2005, 58, (3-4), 193-202. The photoactivity of the annealed materials is significantly higher than that of the Degussa P-25 comparative standard sample (see Fig. 5). For annealing at 600 ° C, a rate constant of 1 order of 4-chlorophenol decomposition reaction of 0.011 min ^-1 was measured, for samples annealed at 800 ° C 0.022 min ^-1 and for samples annealed at 950 ° C 0.033 min ^-1 compared to the sample. Degussa P 25, where this constant was measured at 0.014 min ^-1 . Degussa P 25 is a commercial material used as a standard and is a mixture of anatase and rutile nanocrystals.

Overview of figures in the drawings

Giant. 1. shows a leaf image of a lyophilized ammonium persulfate gel. The image was taken using a transmission electron microscope.

Giant. 2. shows an image of a lyophilized ammonium persulfate gel. The image was taken on a scanning electron microscope.

Giant. 3 shows an image of a lyophilized ammonium persulfate gel after annealing at 950 ° C. The image was taken on a scanning electron microscope

In FIG. 4. the adsorption and desorption curve of titanium dioxide, prepared according to Example 7, annealing temperature 900 ° C are shown

Giant. 5. shows the photocatalyzed decomposition of an aqueous solution of 4-chlorophenol on samples prepared according to Example 9, annealing temperature 600 ° C (L600), 800 ° C (L800) and 950 ° C (L950) and the reference material DEGUSSA P-25.

Examples of embodiments of the invention

Example 1

4.80 g of titanyl sulphate (T1OSO4 x H2O) was dissolved with stirring at 35 ° C for 30 min in 150 ml of demineralized water, until complete dissolution and clarification of the solution. An aqueous solution of ammonia (25 to 39% NH 3) was then added dropwise to the solution with stirring, and the total reaction was added over a period of 5 min. 3.9 ml of 26% NH 3 are added. The pH, solution temperature and volume of ammonia added were monitored. Ammonia was added until the pH of the solution reached about 8. During precipitation, the temperature of the reaction mixture rose from 0 ° C to about 7 ° C. Subsequently, the precipitate, hydrated titanium dioxide gel, was filtered off and transferred to the beaker with demineralized water, mixed and filtered again. After the second filtration, the precipitate was washed 3 more times with demineralized water on a filter to remove residual sulfate and ammonium ions. Precipitate, hydrated

151 titanium dioxide, was transferred to a beaker and made up to 350 ml with demineralized water to form a suspension. After stirring, 26 ml of 30% hydrogen peroxide (H 2 O 2) was added to the suspension. The addition of H2O2 changed the pH of the suspension from pH 8 to pH 1.5 and the original white color of the suspension changed to yellow. The resulting clear, colloidal yellow solution, yellow gel, can be lyophilized immediately or allowed to stand for 10 to 90 minutes, or preferably stirred. In this case, the solution was stirred at room temperature 23 ° C for 90 minutes. The clear yellow colloidal solution, a yellow gel, was then dried by lyophilization at -64 ° C and 6 mtorr for 72 hours until complete removal of the sublimable aqueous portion. The resulting solid ammonium peroxotitanate in the form of foam is formed by a network of thin films, the bulk density of the product is 1 g / dm ³ , crystallographically it is amorphous. The resulting ammonium peroxynitrite foam decomposes organic substances in the same way as anatase under light (rate constant 0.025 rpm ⁾ , but this photoactivity is due to the presence of peroxide in the structure, after depletion of the peroxide this photoactivity is lost. The anatase formed by annealing the foam, or anatase-rutile or rutile mixtures, are stable and permanently photoactive.

Example 2

The material was prepared in the same way as in Example 1, except that the amount of hydrogen peroxide added was 4 times higher than in Example 1. The morphological properties and bulk density of the product are identical to the product prepared according to Example 1. decomposes organic substances in the same way as anatase (rate constant 0.023 min ^-1 ).

Example 3

The material was prepared in the same manner as in Example 1, except that an equimolar TiCl ₄ solution was used instead of an aqueous T1OSO4 solution. The morphological properties and the bulk density of the product are identical to the product prepared according to Example 1. In this case, the resulting ammonium peroxynitrate foam decomposes organic substances in the same way as anatase (rate constant 0.021 min ^-1 ).

Example 4

The material was prepared according to Example 1 or 3, but after the addition of aqueous ammonia was complete, the precipitate solution, aqueous precursor solution after ammonia precipitation, was placed in a box at -5 ° C for 90 minutes until shattering formed. temperature 0 ° C. The resulting ammonium persulfate prepared in this way is

1 * 1

consists of thinner leaves and does not contain any particles with an irregular shape. Even in this case, the resulting ammonium peroxynitrite foam decomposes organic matter in the same way as anatase under the influence of light (rate constant 0.035 min ^-1 ).

Example 5

The material was prepared according to Example 4, after which the precipitate solution was stirred for a further 30 minutes (precipitate maturation) and the pH was maintained at 8 during this time. This procedure further improved the properties and homogeneity of the colloidal ammonium persulfate solution. The morphological properties and bulk density of the product are identical to the product prepared according to Example 1. In this case, too, the resulting ammonium peroxynitrate foam decomposes organic substances in the same way as anatase under the action of light (rate constant 0.034 min ¹ ).

Example 6

The material was prepared according to the procedure described in Example 1 or 3, but the amount of titanyl sulfate added was 48 g (or equimolar amount of TiCl 4) and the concentration of the starting solution T1OSO4 χ H2O (T1Cl4 χ H2O) was 10 times higher. The amount of precipitating (neutralizing) agent, aqueous ammonia solution and hydrogen peroxide used was thus 10 times higher. The other reaction conditions were maintained. The resulting gel has a significantly higher viscosity, but it can still be processed by lyophilization. At concentrations higher than 25 wt. precursor it is no longer possible. The resulting foam is also formed by a network of thin films of ammonium persulfate, the bulk density of the product is higher, up to 10 g / dm ³ , crystallographically it is amorphous and mechanically stronger than materials formed at lower concentrations of titanium salts. Its photoactivity is analogous to the materials obtained according to Examples 1 to 4. The morphological properties and bulk density of the product are similar to those of the product prepared according to Example 1, but the leaves are thicker. Even in this case, the resulting ammonium peroxynitrate foam decomposes organic matter in the same way as anatase under the influence of light (rate constant 0.017 rpm ⁾ .

Example 7

The materials prepared according to Examples 1 to 6 were annealed at 900 ° C for 30 min. with the formation of crystalline modifications of anatase while maintaining the morphological characteristics of the starting gel, resp. foamy structure. The obtained material is characterized by high porosity with pores in the shape of slits (see Fig. 4). The heating rate was 5 ° C / min, after reaching those

temperature, the material was tempered for 30 min. and freely cooled in air. The material thus prepared is highly photoactive, its photoactivity expressed as the rate constant of the 1-order photocatalyzed decomposition of 4-chlorophenol was 0.033 min ^-1 . The photoactivity of all annealed materials prepared according to Examples 1-6 is similar and depends only on the annealing temperature, since the own photoactive primary nanoparticles of anatase, resp. anatase-rutile or rutile mixtures are practically identical in size and structure.

Example 8

The material prepared according to Example 3 was annealed at 950 ° C for 30 minutes to give crystalline modifications of anatase while maintaining the morphological characteristics of the starting gel, i.e. the foamy structure. The adsorption and desorption curve of the material thus obtained is shown in Fig. 4. The heating rate was 5 ° C / min, after reaching the temperature the material was tempered for 30 min. and freely cooled in air. The material thus prepared is highly photoactive, its photoactivity expressed as the rate constant of the 1-order photocatalyzed decomposition of 4-chlorophenol was 0.033 min ^-1 .

Example 9

The material prepared according to Example 1 was annealed at 600 ° C, 800 ° C, 950 ° C for 30 min. with the formation of crystalline modifications of anatase while maintaining the morphological characteristics of the starting gel, resp. foamy structure. The heating rate was 5 ° C / min, after reaching the temperature the material was tempered for 30 min. and freely cooled in air. The photoactivity of the samples thus prepared was determined by measuring the decomposition kinetics of aqueous 4-chlorophenol solutions. The results of the measurements are shown in Fig. 5, which shows that the photoactivity of the samples thus prepared is significantly higher than in the comparative sample of Degussa P-25. For annealing at 600 ° C, the rate constant of 1 order of 4-chlorophenol decomposition reaction is 0.011 min ^-1 , at 800 ° C 0.022 min ^-1 and at 950 ° C 0.033 min ^-1 compared to the Degussa P-25 sample, where this constant was measured at 0.014 min ¹ .

Industrial applicability

The products prepared according to the process of the invention can find industrial use, for example, as photocatalysts for cleaning or disinfecting water and air, sorbents, UV filters, etc.

Claims (6)

PATENT CLAIMS A process for the preparation of a photocatalytically active material with a foam structure, characterized in that the hydrated titanium dioxide formed by precipitation with an alkaline precipitating agent from an aqueous precursor solution is treated with hydrogen peroxide in aqueous suspension to give a yellow transparent gel which is further lyophilized at lower temperature. than 0 ° C but the lowest possible - 64 ° C and water vapor pressure corresponding to the appropriate temperature until the removal of all the aqueous phase to form an ammonium persulfate foam, the precursor being present in the aqueous solution being 1 to 25% by weight. Preparation process according to Claim 1, characterized in that the ammonium peroxotitanate foam is calcined at temperatures of up to 1100 ° C. Preparation process according to Claim 1 or 2, characterized in that T 1 OSO 4 is used as precursor. Preparation process according to Claim 1 or 2, characterized in that TiCl 4 is used as precursor. Preparation process according to any one of Claims 1 to 4, characterized in that the aqueous solution of the precursor is cooled in the aqueous solution after precipitation with a basic reagent, preferably stirred for up to 90 minutes after cooling. Preparation process according to any one of Claims 1 to 5, characterized in that an aqueous ammonia solution is used as the basic precipitating agent.