Process for producing a photocatalyst based on titanium dioxide

The present invention relates to a method for producing a titanium dioxide-based iron-containing photocatalyst that is also active in visible light, and its use.

The material of the photocatalyst is a semiconductor in which electron-hole pairs that generate highly reactive free radicals are generated under the influence of light on the surface of the material. Titanium dioxide is such a semiconductor. Titanium dioxide irradiates the impurities in air and water with UV light by reducing air oxygen to oxidize (mineralize) natural and artificial impurities into a final product that does not harm the environment. It is known that it can be removed by. Furthermore, the surface of titanium dioxide becomes superhydrophilic due to the adsorption of UV light. The anti-fog effect of thin titanium dioxide films on mirrors and windows is based on this.

A significant drawback of titanium dioxide is that it can only utilize the UV component of sunlight, which is 3-4% of the radiation, and has no or very weak catalytic activity in diffuse daylight. That's what it means.

Therefore, for a long time, attempts have been made to modify titanium dioxide so that the main components of sunlight (the visible spectrum region of 400 to about 700 nm) that act photochemically can also be used in order to cause the above-mentioned phenomenon. There is.

TiO₂One way to make the photocatalytic activity against daylight is Ti⁴⁺By reduction of TiO₂It is to create oxygen vacancies in the crystal lattice. Another strategy is to dope with a non-metal, such as nitrogen, carbon or sulfur. Similarly, it is also known to dope with a metal such as vanadium, chromium, platinum or the like. However, these developments require partially costly manufacturing techniques such as ion implantation or plasma treatment.

Furthermore, it is known that titanium dioxide particles containing an iron component have improved photocatalytic activity even in visible light. Various methods have been published for the production of iron-containing titanium dioxide photocatalysts.

EP0666107B1 discloses a titanium dioxide photocatalyst containing 5 ppm to 10% by weight of iron. This photocatalyst is produced by hydrolysis of an aqueous solution containing a titanium compound and an iron compound in a dissolved form.

EP1036593A1 discloses a photocatalyst doped with trivalent iron and equimolar amounts of pentavalent ions, which are hydrolyzed and subsequently dried and the individual components also dissolved in the solvent. Manufactured by sintering.

US2007 / 0193875A1 discloses a method for producing an iron ion-doped titanium dioxide photocatalyst, where each mixture of precursor compounds (eg, each chloride) is dialyzed to remove chlorine ions. The resulting product is subsequently dried and fired.

An object underlying the present invention is to provide a novel and economical method for producing a titanium dioxide-based photocatalyst that contains iron and is active in the visible spectrum region.

The problem is solved by a method for producing a titanium dioxide-based photocatalyst, which comprises the following steps: -Step to prepare an aqueous suspension containing titanium oxide hydrate nanoparticles Step of adding iron (III) ions Step of precipitating iron (III) hydrate -Step of separating the mixture containing titanium oxide hydrate and iron (III) hydrate A step of heat treating the mixture at a temperature of at least 100 ° C.

Further advantageous embodiments of the present invention are set forth in the subclaims.

The object of the present invention is a simple and economical method for producing a titanium dioxide-based iron-containing photocatalyst having remarkable activity in the visible spectrum region, and the photocatalyst in liquid, gas and on the surface. It is used for decomposing impurities and harmful substances in the photocatalyst, and for water separation.

All indications disclosed below, such as pH value, temperature, concentration in% by weight, etc., are to be construed as including all values within the range of their respective measurement accuracy known to those of skill in the art.

The first step of the method according to the invention involves the preparation of an aqueous suspension containing titanium oxide hydrate nanoparticles. In the following, the titanium oxide hydrate is also understood as metatitanium acid, titanium oxyhydrate, titanium hydrate, titanium hydroxide, amorphous titanium dioxide or titanium oxide, hydrous titanium dioxide or titanium hydroxide. .. Nanoparticles are understood to be particles that have a primary particle size of less than 100 nm.

The titanium oxide hydrate can be produced by a sulfuric acid method or a chloride method, for example, in the production of titanium dioxide, and is precipitated by hydrolysis of titanyl sulfate or titanyl chloride. The titanium oxide hydrate is preferably amorphous to microcrystalline. In particular, the primary particle size is less than 10 nm (calculated from electron microscopy). When the titanium oxide hydrate is derived from the sulfuric acid method, it is preferable that the titanium oxide hydrate be washed before the production of the suspension.

The suspension is SO₃The content is preferably 10% by mass at the maximum, preferably 4 to 7% by mass. The iron content of this suspension is preferably less than 100 ppm, especially less than 50 ppm, particularly preferably less than 20 ppm. In a further embodiment of the invention, the pH value of this suspension is up to about 7.

The second step of the method according to the invention involves the addition of trivalent iron ions to the suspension. This iron ion is added in the form of a water-soluble inorganic iron compound or a short-chain organic iron compound, for example, as a solution of iron (III) sulfate or iron oxalate.

In one embodiment of the invention, a water-soluble salt can be used with divalent iron, which salt (eg, iron (II) sulphate) is first dissolved in an oxidizing agent, eg, iron (II) sulfate. H₂O₂Is quantitatively oxidized to iron (III) sulfate, which is subsequently added to the suspension. The iron (II) sulfate is a so-called green salt in the elution (Aufschluss) of ilmenite ore (Eisentitanerz) that occurs during the sulfuric acid process for the production of titanium dioxide, such as iron (II) sulfate heptahydrate. It may be.

The amount of iron (III) ions added is TiO₂On the other hand, Fe is preferably 0.05 to 5% by mass, particularly 0.05 to 3% by mass, and particularly preferably 0.05 to 1% by mass.

The third step of the method according to the invention includes precipitation of iron (III) hydrate. Preferably, the pH value of the suspension is such that a suitable pH control substance (eg, alkali, eg, NaOH or acid, eg, H).₂SO₄) Is adjusted to 6-8, and iron (III) hydrate is quantitatively precipitated.

The temperature of the precipitate is preferably 20 to 80 ° C.

The fourth step of the method according to the invention involves the separation of a mixture of titanium oxide hydrate and iron (III) hydrate. This separation is preferably done by filtration. Appropriate methods, such as Vakuumplatten filtration or Vakuumplan filtration, are known to those of skill in the art. The filter cake is preferably washed. Ultimately, this filter cake preferably has one or more of the following characteristics: SO₃The content is up to 1.5% by weight, preferably less than 0.3% by weight. The sodium content is up to 0.1% by weight, preferably less than 0.05% by weight. The maximum conductivity is 5 mS / cm. The wet content is up to 70% by mass, preferably less than 65% by mass.

The fifth step of the method according to the invention includes heat treatment of a mixture containing titanium oxide hydrate and iron (III) hydrate at temperatures above 100 ° C. The temperature is preferably 100 ° C to 900 ° C, particularly preferably 100 ° C to 400 ° C. This heat treatment may be carried out in a conventional device such as a fluidized bed device (Wirbelschichtanlage), a spray dryer or a rotary furnace. When the heat treatment is performed at 100 ° C to about 150 ° C, it is preferably performed in a fluidized bed device or a spray dryer.

In one embodiment of the method, the heat treatment may be carried out in two steps, first drying at about 100 ° C.

The residual moisture content of the filter cake is preferably 3 to 10% by mass, particularly 5 to 7% by mass after drying.

The photocatalyst produced by the method according to the invention is preferably about 25-300 m.²It has a BET surface area of / g. It is known to those skilled in the art that the specific surface area of this product can be affected by the temperature of the heat treatment, and the specific surface area decreases with increasing temperature.

The photocatalyst produced by the method according to the invention is photoactive in both the UV and visible spectral regions and is outstandingly suitable for harmful substances in liquids and gases, as well as for self-cleaning surfaces in the indoor and outdoor ranges. Is. This photocatalyst may be further used in photovoltaic cells and photolysis.

<u style="single">Example</u> The present invention will be described with reference to the following examples; however, this is not understood to limit the invention.

Example 1 Titanium oxide hydrate from titanium dioxide production by the sulfuric acid method is kneaded with 90 L of water in a temperature-controlled jacket tank (40 ° C) and TiO.₂ It was made into a suspension having 300 g / L. In a separate tank, dissolve 1.33 kg of iron (II) sulfate heptahydrate (green salt) in 5.2 L of water with an iron (II) content of 17.8% by weight and stir for 30% H.₂O₂274 g of solution was added, thereby quantitatively oxidizing Fe (II) to Fe (III). 6.8 kg of a solution containing iron (III) was added to the titanium oxide hydrate suspension with stirring. Subsequently, the pH value of this suspension was adjusted to about 7 by the addition of NaOH.

The suspension is filtered through a vacuum plate and the filter cake is SO with warm water at 50 ° C-60 ° C.₃Washing was performed until the content was less than 1% by mass, the Na content was less than 0.02% by mass, and the conductivity in the filtrate was less than 5 mS / cm.

Subsequently, the filter cake was kneaded with water (solid content 25 to 27%) and heat-treated with a spray dryer at about 120 ° C.

The residual moisture content of this dried product was 5-7% by mass. This product has a Fe content of 1.0% by mass and SO.₃Content 0.11% by mass, and specific surface area (BET) 310m²Had / g.

Example 2 As in Example 1, however, with the following differences: 17 kg of solution containing iron (III) was added to the suspension. After drying with a spray dryer at 120 ° C, the product was heat treated at 510 ° C for 2 hours: the residual moisture content of this dried product was 5-7% by weight. This product has a Fe content of 2.5% by mass and SO.₃Content 0.10% by weight, and specific surface area (BET) 89m²Had / g.

Comparative example As in Example 1, however, the following differences were made. No iron-containing solution was added to the suspension. After drying with a spray dryer at 120 ° C, the product was heat treated at 300 ° C for 2 hours: the residual moisture content of this dried product was 5-7% by weight. This product has a Fe content of 0% by weight and SO.₃Content 0.35% by mass and specific surface area (BET) 98m²Had / g.

<u style="single">Test method</u> Chemical analysis Fe content, SO₃The content and Na content were measured by fluorescent X-ray.

Specific surface area by BET BET surface area was measured by the static volumetric principle using a Micromeritics Tristar 3000.

Photocatalytic activity The photocatalytic activity of the photocatalyst was measured by decomposition of isopropanol.

Measurements of isopropanol degradation are described in the publication of R. Nothhelfer-Richter et al. ("New methodology for the determination of the photocatalytic stability and activity of pigments and organic coatings", Congress Proceedings Nuernberg Congress, Vincentz Network, Hannover 2007). It was performed with the measuring device of. A 450 W XBO lamp was used for exposure.

The measuring device and measuring method were developed by JR Earle ("Titanium dioxide pigment photocatalysed degradation of PVC and plasticized PVD coatings", PhD-thesis, University of Swansea, Wales, 2002).

Prior to the actual measurement, the powdered sample was exposed for 4 days without a UV filter to adjust the conditions. Here, the absorbed organic compound is decomposed and CO₂And H₂It became O. Subsequently, the laboratory was filled with a new test gas (isopropanol) and waited for a new equilibrium state.

Subsequently, isopropanol was introduced into the measurement room using 5 μl of a glass injection tube. The sample was subjected to Dunkela adsorption. This adsorption was terminated when the isopropanol measurement changed by less than 2 ppm within 10 minutes.

Continued exposure. The following edge filters were used: 305 nm and 385 nm.

The change in isopropanol concentration was performed using an FTIR detector. Isopropanol decomposition (ppm / m) from a negative gradient (decomposition line) between 95% and 85% after 5% of isopropanol is decomposed²* min) was calculated.

Test results The measured photocatalytic activity of the photocatalysts according to the invention, Examples 1 and 2, and Comparative Examples are summarized in Table 1.

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The above results show that the photocatalytic activity of the photocatalyst according to the present invention in the visible region (edge filter 385 nm) is about 10% of the total activity (edge filter 305 nm). In comparison, the Fe-free photocatalyst according to the comparative example has only a small amount of catalytic activity in the spectral region larger than 385 nm (edge filter 385 nm), but it is still up to 50 ppm in the photocatalyst due to the cleaning process. It is especially derived from the possible presence of trace amounts of iron.