Titanium dioxide photocatalyst containing carbon and method for its production   

Abstract: The invetion relates to a titanium dioxide-based photocatalyst containing carbon that is highly photoactive in visible light (vlp-TiO2) and to a method of manufacture. The vlp-TiO2is manufactured by mixing a fine grained titanium compount (BET≧50 m2/g) with an organic carbon compound and subsequent thermal treatment at temperatures up to 350° C. The carbon content amounts to 0.05 to 4% by weight, preferably 0.4 to 0.8% by weight. The product is characterized by an ESR spectrum which displays only one significant signal in the g value range from 1.97 to 2.05 at g about 2.003. The inventive photocatalyst can be used for to degrade contaminants and pollutants in liquids and gases.

The invention relates to a titanium dioxide-based photocatalyst containing carbon that is photoactive in the visible range, also referred to as vlp-TiO2 below.

The invention further relates to a method for producing a titanium dioxide containing carbon (vlp-TiO2) that is effective as a photocatalyst when irradiated with visible light.

Photocatalytic materials are semiconductors in which, when exposed to light, electron-hole pairs develop, which generate highly reactive free radicals on the material surface. Titanium dioxide is a semiconductor of this kind. It is known that titanium dioxide is capable of removing natural and artificial contaminants in air and water by irradiation with UV light, in that the atmospheric oxygen is reduced and the contaminants oxidized (mineralized) into environmentally friendly end products. In addition, the surface of titanium dioxide becomes superhydrophilic as a result of absorbing UV light. This is the basis of the anti-fogging effect of thin titanium dioxide films on mirrors and windows.

One serious disadvantage of titanium dioxide is the fact that it can only utilize the UV component of sunlight, i.e. only 3 to 4% of the radiation, and displays either only very weak catalytic activity, or none at all, in diffuse daylight.

Consequently, attempts have for some time been made to modify titanium dioxide in such a way that it can also utilize the main component of photochemically active sunlight—the visible spectral range between roughly 400 nm and roughly 700 nm—to produce the above-mentioned phenomena.

One way of making TiO2 photocatalytically active in daylight is to dope it with metal ions, such as V, Pt, Cr or Fe. Another possibility is to create oxygen vacancies in the TiO2 crystal lattice by reduction of Ti4+. Both developments require complex production techniques, such as ion implantation or plasma treatment.

Numerous patents describe nitrogen-modified titanium dioxide that is photocatalytically active when irradiated in the visible range (e.g. EP 1 178 011 A1, EP 1 254 863 A1).

It is furthermore known that modification with carbon increases the photocatalytic activity of titanium dioxide when irradiated with visible light. For example, JP 11333304 A describes a titanium dioxide whose surface at least partly displays a precipitate of graphite, amorphous carbon, diamond-like carbon or hydrocarbons. EP 0 997 191 A1 reports on a titanium dioxide with titanium carbide applied to its surface by means of vapor-phase deposition.

Photocatalytic materials in which titanium dioxide contains, inter alia, nitrogen, sulfur, carbon or other elements as anions, are disclosed in EP 1 205 244 A1 and EP 1 205 245 A1, for example. The anions are said to be located either at oxygen sites, at interstitial sites or at the grain boundaries of a polycrystalline titanium oxide particle. No information is given as regards the characterization of the material and about catalytic or physical properties.

The production of titanium dioxide containing 1.0 to 1.7% by weight carbon from titanium alcoholates by hydrolysis with hydrochloric acid and subsequent heating to 350° C. is also known (C. Lettmann et al., Applied Catalysis B 32 (2001) 215). In this case, the carbon originates from the ligand of the titanium compound.

According to a further publication, it has been found that hydrolysis of titanium tetrachloride with tetrabutylammonium hydroxide, followed by calcination for one hour at 400° C., yields a titanium dioxide preparation containing 0.42% by weight carbon (S. Sakthivel & H. Kisch, Angew. Chem. Int. Ed. 42 (2003) 4908). In this case, the carbon originates from the precipitant and is presumably dispersed relatively uniformly in the volume (volume doping).

The disadvantage of the known photocatalytic materials is that the methods for producing them are not suitable for industrial-scale production. Either the methods cannot be realized on an industrial scale for technical reasons, or they would then no longer be economical. In addition, most of the products obtained display insufficient photocatalytic activity in the degradation of pollutants in visible light in the range of λ>400 nm, and an only slight light-induced increase in hydrophilicity.

Moreover, the products have so far only been optimized in respect of their photocatalytic properties. The color and brightness, i.e. the optical properties, have been disregarded to date. However, use of a very bright vlp-TiO2 with little inherent color and high photocatalytic activity has advantages in all applications that tolerate only little or no inherent color of the vlp-TiO2, such as applications in coatings, specifically in paints and plasters.

The object of the invention is to provide a daylight-active, highly effective photocatalyst on the basis of a carbon-modified titanium dioxide, and to specify an economical method for producing it.

According to the invention, the object is solved by a titanium dioxide containing carbon which, compared to pure titanium dioxide, displays significant light absorption in the range of λ≧400 nm (vlp-TiO2) and whose electron spin resonance (ESR) spectrum measured at a temperature of 5 K displays only one significant signal in the g value range from 1.97 to 2.05.

The object is further solved by a production method in which a titanium compound having a specific surface area of at least 50 m2/g according to BET (Brunauer-Emmett-Teller) is mixed intimately with a carbon containing compound, and the mixture is treated thermally at a temperature of up to 400° C.

Further advantageous developments of the invention are indicated in the sub-claims.

The vlp-TiO2 according to the invention displays greater photocatalytic activity than the types described in the prior art. The measure of photocatalytic activity (hereinafter referred to as “photoactivity”) is the degradation of 4-chlorophenol by a defined quantity of vlp-TiO2 during 120-minute irradiation with light having a wavelength λ of ≧455 nm. The measuring method is described in detail below. Under the specified measuring conditions, the photoactivity of the vlp-TiO2 according to the invention is in the region of at least 20%, preferably in the region of at least 40%, particularly in the region of at least 50%.

The carbon content is in the range from 0.05 to 4% by weight, referred to TiO2, preferably 0.05 to 2.0% by weight, and particularly preferably 0.3 to 1.5% by weight. The best results are obtained with carbon contents of 0.4 to 0.8% by weight.

The titanium dioxide particles contain carbon in a surface layer only, and are thus referred to as “carbon-modified” below - as opposed to the volume-doped titanium dioxide produced according to Sakthivel and Kisch (2003). The carbon or carbon compounds of the vlp-TiO2 according to the invention are presumably primarily covalently bonded to the TiO2 surface via oxygen, and alkaline-leachable.

The photocatalyst can additionally contain nitrogen.

In contrast to unmodified TiO2, the vlp-TiO2 according to the invention absorbs visible light with a wavelength λ≧400 nm. In this context, compared to the value at 400 nm, the Kubelka-Munk function F(R∞), which is proportional to the absorbancy, is roughly 50% at 500 nm and roughly 20% at 600 nm.

The electron spin resonance (ESR) spectrum of the vlp-TiO2 according to the invention, measured at a temperature of 5 K, is characterized by a strong signal at a g value of 2.002 to 2.004, particularly 2.003. No further signals occur in the g value range from 1.97 to 2.05. The intensity of the signal at a g value of roughly 2.003 is increased by irradiation with light having a wavelength of λ≧380 nm (UV-free 100 W halogen lamp, KG5 cold-light filter), compared to the measurement in darkness.

The X-ray photoelectron spectrum (XPS) of the vlp-TiO2 according to the invention is characterized by the occurrence of a strong absorption band at a bond energy of 285.6 eV, referred to the O1s band at 530 eV.

A further characteristic is that, in contrast to the photocatalyst according to Sakthivel & Kisch (2003), the vlp-TiO2 does not display carbonate bands, either in the X-ray photoelectron spectrum (XPS), or in the infrared spectrum.

When irradiated with visible light, the vlp-TiO2 displays a water contact angle of roughly 8°, whereas unmodified TiO2 displays a contact angle of roughly 21°.

The new photocatalyst enables pollutant degradation not only using artificial visible light, but also with diffuse, indoor daylight. It can be used to degrade contaminants and pollutants in liquids or gases, particularly in water and air.

The photocatalyst can advantageously be applied to various substrates, such as glass (plain and metallized), wood, fibers, ceramics, concrete, building materials, SiO2, metals, paper and plastics, in the form of a thin layer. In combination with its simple production, this thus opens up potential applications in a wide variety of sectors, e.g. in the construction, ceramics and automotive industries for self-cleaning surfaces, or in environmental engineering (air-conditioning equipment, devices for air purification and air sterilization, and in the purification of water, especially drinking water) as well as for antibacterial and antiviral purposes. The photocatalyst can be used in coatings for interior and exterior purposes, such as paints, plasters and glazes for application to masonry, plaster surfaces, paint coats, wallpapers and wood, metal, glass or ceramic surfaces, or to components, such as composite heat insulation systems and curtain-wall facade elements, as well as in road surfacings and in plastics, plastic sheeting, fibers and paper. The photocatalyst can moreover be used in the production of prefabricated concrete elements, concrete paving stones, roof tiles, ceramics, decorative tiles, wallpapers, fabrics, panels and cladding elements for ceilings and walls in indoor and outdoor areas.

The light-induced increase in the hydrophilicity of the TiO2 surface gives rise to additional applications, such as non-fogging mirrors and windows in the sanitary sector or in the automotive and construction industries.

The photocatalyst is moreover suitable for use in photovoltaic cells and for photolysis.

The vlp-TiO2 according to the invention is described in more detail below with reference to FIGS. 1 to 11.

FIG. 1 shows the Kubelka-Munk function F(R∞) (arbitrary units), which is proportional to the relative absorbancy, for unmodified TiO2 and for C-modified TiO2 (vlp-TiO2) as a function of the wavelength, and indicates that, in contrast to unmodified titanium dioxide, the vlp-TiO2 absorbs in the visible spectral range. F(R∞) is roughly 50% at 500 nm and roughly 20% at 600 nm compared to the value at 400 nm.

FIG. 2 shows the electron spin resonance (ESR) spectra of the vlp-TiO2 according to the invention (spectrum A) and of the TiO2 produced according to Sakthivel & Kisch (spectrum B), recorded in darkness at a temperature of 5 K. Spectrum A displays only one significant signal at a g value of 2.003. In addition to the principal signal at a g value of roughly 2.003, spectrum B displays three further signals in the g value range from 1.97 to 2.05.

FIG. 3 contains the X-ray photoelectron spectra (XPS) of the vlp-TiO2 according to the invention (spectrum A) and of the previously known TiO2 according to Sakthivel & Kisch, precipitated from titanium tetrachloride with tetrabutylammonium hydroxide (spectrum B). The spectrum of the vlp-TiO2 displays a pronounced C1s signal at a bond energy of 285.6 eV, referred to the O1s absorption band at 530 eV, this indicating elementary carbon. In contrast, spectrum B displays C1s signals for elementary carbon at a bond energy of 284.5 eV, as well as additional bands at 289.4 eV and 294.8 eV, these indicating carbonate. Corresponding IR spectra likewise display typical carbonate bands at 1738, 1096 and 798 cm−1.

FIG. 4 illustrates the photocatalytic activity of vlp-TiO2 compared to unmodified TiO2 in the degradation of 4-chlorophenol (in the form of a 2.5×10−4 molar aqueous solution) by means of artificial visible light (λ≧455 nm). The diagram presents the decrease in the total content of organic carbon (TOCt) in the solution relative to the starting value (TOC0). With the vlp-TiO2, complete degradation is achieved after 3 hours.

FIG. 5 illustrates the photocatalytic activity of vlp-TiO2 compared to unmodified TiO2 in the degradation of 4-chlorophenol (in the form of a 2.5×10−4 molar aqueous solution) by means of diffuse, indoor daylight. The diagram presents the decrease in the total content of organic carbon (TOCt) in the solution relative to the starting value (TOC0). Even in low-intensity, diffuse daylight (7 to 10 W/m2 in the range from 400 to 1200 nm), the vlp-TiO2 leads to 80% degradation within six hours.

Even when illuminated by very-low-intensity diffuse daylight (1.6 to <1 W/m2), the vlp-TiO2 still displays significant photoactivity, unlike commercially available TiO2 photocatalysts (Degussa P25, Kemira UV-Titan, Sachtleben Hombikat, Tayca MT-100SA). The degradation rate of 2.5×10−4 molar 4-chlorophenol solution was measured as described above. a) Light intensity: 1.6 W/m2; duration: 12 h

Catalyst BET surface Degradation rate vlp-TiO2 170 m2/g  16% P25 50 m2/g 4% UV-Titan 20 m2/g 5% Hombikat 240 m2/g  9% MT-100SA 50 m2/g 5% b) Light intensity: <1 W/m2; duration: 24 h

Catalyst BET surface Degradation rate vlp-TiO2 170 m2/g 18% Hombikat 240 m2/g 3%

FIG. 6 illustrates the photocatalytic activity of vlp-TiO2 compared to unmodified TiO2 in the degradation of benzole (5% by volume), acetaldehyde (2% by volume) and carbon monoxide (5% by volume) by diffuse, indoor daylight. The reaction vessel used is a 1-liter, round-bottom flask, fitted with a filter-paper disk (d=15 cm) coated with 12 mg titanium dioxide. The diagram presents the decrease in the total content of organic carbon (TOCt) in the atmosphere relative to the starting value (TOC0). The curves show the degradation of benzole, acetaldehyde and carbon monoxide by the vlp-TiO2 according to the invention, as well as the degradation of acetaldehyde by unmodified titanium dioxide.

FIG. 7 shows an X-ray powder diffractogram of the vlp-TiO2, which displays only anatase reflections. The crystallite size calculated by the Scherrer method is in the region of 10 nm.

FIG. 8 shows a photograph of vlp-TiO2, produced by means of high-resolution transmission electron microscopy (HTEM), with the lattice lines of the crystallites. The crystallite size can be estimated as having an order of magnitude of 10 nm.

FIG. 9 shows a carbon depth profile of the vlp-TiO2, presented as the C/Ti ratio. It was determined by means of ion bombardment (Ar+) and ESCA analysis. The indicated bombardment time of 5×103 seconds corresponds to a depth of roughly 5 nm.

The method according to the invention is based on a titanium compound, which is present in the form of an amorphous, semicrystalline or crystalline titanium oxide or hydrous titanium oxide and/or a titanium hydrate and/or titanium oxyhydrate, and is referred to below as the parent titanium compound.

The parent titanium compound can, for example, be produced during the manufacture of titanium dioxide, either by the sulfate process or by the chloride process. Titanium hydrate, titanium oxyhydrate or metatitanic acid is, for example, precipitated during hydrolysis of titanyl sulfate or titanyl chloride.

The parent titanium compound can be present in the form of a fine-grained solid, or in a suspension of particles with a corresponding solids content of at least 15% by weight, in which context the specific surface area of the solid is at least 50 m2/g according to BET, preferably roughly 150 to 350 m2/g according to BET, particularly 150 to 250 m2/g according to BET. For economical reasons, titanium hydrate from the sulfate process is preferred as the parent titanium compound for industrial implementation of the method according to the invention. This titanium hydrate is advantageously freed of adhering sulfuric acid by prior neutralization and washing, such that the sulfate content of the solid after drying is <1% by weight, calculated as SO3.

Organic carbon containing compounds useful for the invention have a decomposition temperature of 400° C. at most, better <350° C., preferably <300° C. Materials containing carbon, such as wood, carbon black or activated carbon, and especially hydrocarbons with at least one functional group, have proven to be suitable. The functional group can be: OH; CHO; COOH; NHx; SHx; COOR in which R is an alkyl or aryl residue. Suitable are—e e.g. succinic acid, glycerol or ethylene glycol. Sugars or other carbohydrates can also be used, as can organoammonium hydroxides, especially tetraalkylammonium. Mixtures of the compounds mentioned are also suitable. Preferably, water-soluble polyalcohols with a carbon/oxygen ratio of roughly 0.7 to 1.5, preferably of roughly 1, are used, particularly pentaerythritol. The carbon compound can be used in the form of a solid, or as a solution, or as a suspension.

The organic carbon compound should have the greatest possible affinity for the surface of the parent titanium compound, in order to be able to enter into an intimate bond with the latter.

The parent titanium compound is mixed intimately with the organic carbon compound in such a way that the surface of the parent titanium compound is coated with the carbon compound. In this context, the organic carbon compound can be present on the surface of the parent titanium compound in physisorbed or chemisorbed form. The surface of the parent titanium compound can be coated by dissolution of the carbon compound in the suspension of the parent titanium compound, or by mixing of the suspension of the carbon compound with the suspension of the parent titanium compound. Intensive mixing of the carbon compound with a previously dried, powdery parent titanium compound is likewise possible. If titanium hydrate is used, the carbon compound can alternatively also already be admixed to the solution to be hydrolyzed during production of the titanium hydrate.

In the finished mixture of parent titanium compound and carbon compound, the quantity of carbon compound, referred to the parent titanium compound (as solid), is 1 to 40% by weight.

If the finished mixture is present in the form of a suspension, it can be dried into a powdery solid prior to further processing. Known methods, such as spray drying or fluidized-bed drying, are suitable for this purpose.

The finished and, where appropriate, predried mixture is treated thermally at temperatures of max. 400° C. The thermal treatment is performed in an oxidizing atmosphere preferably in air or in a mixture of oxygen and air. This process results in decomposition of the organic carbon compound on the surface of the parent titanium compound, and in the release of H2O, CO2 and CO. Although thermal treatment can be performed in batch mode, e.g. in a commercially available laboratory furnace, a continuous process in which a specific temperature profile can be maintained is preferred for economical reasons. The continuous methods open to consideration include all methods with which a corresponding temperature profile and the necessary residence time can be realized. Indirectly and directly heated rotary kilns are particularly suitable units. Continuously operated fluidized-bed reactors, fluidized-bed driers and heated ploughshare mixers can also be used. The three last-named units can also be operated in batch mode.

Thermal treatment is preferably performed in such a way that a product (vlp-TiO2) is obtained which has a carbon content of 0.05 to 4.0% by weight, preferably 0.05 to 2.0% by weight, particularly preferably 0.3 to 1.5% by weight, and especially 0.4 to 0.8% by weight. A change of color, from white to brown and finally to beige, takes place in the course of the thermal treatment. The end product is characterized by a beige to slightly yellowish-brownish color. It is characterized by the fact that the carbon can be detected in amorphous and polycrystalline regions of the surface layer, as well as on the surface itself. The product is photoactive in visible light.

Following thermal treatment, the product is disagglomerated by known methods, e.g. in a pin mill, jet mill or opposed jet mill. In the case of powdery, predried mixtures, thermal treatment mostly leads to agglomerate-free products not requiring further milling. The fineness of grind to be achieved depends on the particle size of the parent titanium compound. The fineness of grind or specific surface area of the product is only marginally lower, but in the same order of magnitude as that of the educt. The targeted fineness of grind of the photocatalyst depends on the field of application of the photocatalyst. It is usually in the same range as for TiO2 pigments, although it can also be lower or higher. The specific surface area according to BET is in the region of 100 to 250 m2/g, preferably 130 to 200 m2/g, particularly 130 to 170 m2/g.

The invention is described more precisely on the basis of the following examples, although this is not intended to restrict the scope of the invention.

An aqueous titanium oxyhydrate paste (35% by weight solids), produced by the sulfate process, is diluted with sufficient distilled water at room temperature to obtain a stirrable suspension. The solids content is in the region of 20 to 25%. NaOH solution (36% by weight) is added until a pH value between 6.0 and 7.0 is obtained. The suspension is subsequently filtered and washed with distilled water until the SO3 content measured on the dried residue is below 1% by weight.

The titanium oxyhydrate neutralized and washed in this way is subsequently again diluted into a stirrable suspension (25% solids) with distilled water, and 12% by weight succinic acid, referred to the solid, are added. The succinic acid is added to the suspension in solid form, and the suspension stirred until the succinic acid has dissolved completely. The suspension is heated to roughly 60° C. to improve the solubility of the succinic acid. While stirring, the suspension prepared in this way is dried under a surface evaporator (IR radiator) to such an extent that a pasty mass is obtained from the suspension. The pasty mass is subsequently dried in a laboratory drying oven at 150° C. until the solids content is >98%.

300 g of the dried titanium oxyhydrate/succinic acid mixture are finely crushed (e.g. by grinding in a mortar and sieving), and the resultant powder is placed in a laboratory furnace at 290° C. in a quartz dish with cover. At intervals of 1 to 2 hours, the quartz dish is removed and the powder mixed again. After 13 to 15 hours in the laboratory furnace, the color of the powder has changed from initially yellowish to gray-black and finally to yellowish-brown. The thermal treatment to obtain vlp-TiO2 is complete when the carbon content has decreased from the initial 5 to 5.5% by weight to roughly 0.65 to 0.80% by weight.

The photocatalyst is subsequently disagglomerated and analyzed to determine its carbon content, optical properties, BET surface and photoactivity.

Procedure similar to that in Example 1, except that 12% by weight pentaerythritol are added to the titanium oxyhydrate suspension as the solid.

Procedure similar to that in Example 2, except that 5% by weight pentaerythritol are added to the titanium oxyhydrate suspension as the solid.

A titanium oxyhydrate/pentaerythritol suspension containing 5% by weight pentaerythritol is prepared as described in Example 1. As a modification of Example 1, the thermal treatment of the suspension obtained in this way is performed in a continuously operated rotary kiln, as follows:

The rotary kiln is operated in counter-current mode and heated directly by means of a gas burner. The naked flame of the gas burner is protected by a fire tube, thus preventing direct contact with the product (vlp-TiO2). The heated kiln length is 7 m, and the inside diameter 0.3 m. The suspension is finely sprayed in at the kiln inlet. The suspension is added at a feed rate of 40 kg/h. Chains installed in the kiln inlet ensure good turbulence, and thus rapid drying and subsequent comminution of the dried material. The time required to pass through the continuously operated rotary kiln is 1 hour. The kiln temperature in the area of the outlet is regulated to 260° C. by means of the gas volume at the burner. The vlp-TiO2 obtained at the outlet of the kiln has the form of a fine powder with a yellowish-brown color. The vlp-TiO2 is subsequently disagglomerated in a laboratory mixer (Braun, MX 2050) and analyzed to determine its carbon content, optical properties, BET surface and photoactivity.

Procedure similar to that in Example 4, except that the kiln temperature in the area of the outlet is regulated to 280° C. by means of the gas volume at the burner.

A titanium oxyhydrate/pentaerythritol suspension containing 5% by weight pentaerythritol is prepared as described in Example 1. As a modification of Example 1, the suspension is predried in an electrically heated furnace to obtain a powdery solid with a residual moisture content of 22%. The thermal treatment of the predried, powdery feed material is performed in a continuously operated, indirectly heated rotary kiln, as follows:

The rotary kiln is operated in co-current mode and heated electrically in three zones. The total heated kiln length is 2,700 mm, and the inside diameter 390 mm. The powdery solid is fed to the kiln inlet via a metering screw. Chains installed over the full length of the rotary kiln ensure homogeneous distribution in the kiln and prevent caking on the kiln wall. The feed rate is 25 kg solid per hour. The time required to pass through the continuously operated rotary kiln is 0.5 hours. The kiln temperature is regulated electrically in three heating zones. The temperature of each of the three heating zones can be controlled individually. The vlp-TiO2 obtained at the outlet of the kiln has the form of a beige-colored, fine powder. The vlp-TiO2 is subsequently disagglomerated in a laboratory mixer (Braun, MX 2050) and analyzed to determine its carbon content, optical properties, BET surface and photoactivity.

A TiO2 pigment (anatase) with a BET surface of roughly 10 m2/g (commercial product Kronos 1000) is mixed with 12% pentaerythritol and thermally treated as in Example 2.

Thermal Photoactivity

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