Transparent, stable titanium dioxide sols

Stable transparent colloidal solutions of titanium dioxide

Transparent, stable titanium dioxide solutions

Full description

Background of the invention:

The present invention relates generally to compositions for embedding a photocatalytic coating on a surface. More specifically, the invention relates to colloidal solutions of nano-sized titanium dioxide particles that are useful for providing photocatalytic transparent coatings on a substrate that are de-polluting and/or self-cleaning.

The photocatalytic properties of the semiconductor material titanium dioxide result from the promotion of electrons from the valence band to the conduction band under the influence of ultraviolet (UV) and near-UV radiation. The reactive electron-holepairs that are formed travel to the surface of titanium dioxide particles, where the holes oxidize adsorbed water to produce reactive hydroxyl radicals and the electrons reduce adsorbed oxygen to produce superoxide radicals. It can analyze both NOx and volatile organic compounds (VOCs) in the air. Due to these properties, titanium dioxide photocatalytic has been used in coatings and similar processes to remove pollutants from the air. Such coatings may also have the advantage of being self-cleaning as soil is also oxidized (grease, mildew, mold, algae, etc.) on the surface.

In many applications, it is desirable for the titanium dioxide coating to be transparent in order to maintain the original appearance or transparency of the substrate (e.g., ceramic tile, paving block, siding, etc.) (For example, window glass, car windshield, etc.). Colloidal solutions of titanium dioxide have proven to be a useful precursor material for forming such transparent coatings.

For example, U.S. Patent No. 6,420,437 in the name of Mori et al., which is incorporated herein by reference, discloses a neutral colloidal solution of titanium dioxide that is said to have high stability in the neutral range and which is capable of forming a transparent, colorless shell. Color even when dry at room temperature. The colloidal solution is produced by mixing an acid colloidal solution of titanium dioxide comprising 50 to 100 parts by weight colloidal particles of titanium dioxide and 5 to 50 parts by weight of an emulsifying agent for titanium ions with 1 to 50 parts by weight of an alkaline including on at least one of the alkali metal compounds and the amine compounds, optionally by adjusting the pH of the liquid mixture to 5 to 10, or adjusting the pH of the liquid to 6 to 10, and then using a treatment Deionization treatment with the mixture to charge colloidal particles of titanium dioxide with negative electricity.

U.S. Patent No. 6,627,336 in the name of Ohmori et al. explains, and the disclosure thereof is incorporated by reference, an aqueous dispersion of titanium dioxide particles, preferably composed predominantly of titanium oxide particles, comprising a chloride ion, and a Brunsted base other than the chloride ion, preferably nitrate ion and/or phosphate ion. An aqueous dispersion of titanium oxide is prepared by hydrolyzing titanium tetrachloride in the presence of at least one type of Bransted acid. Thin films composed of aqueous titanium oxide dispersions are said to exhibit photo-catalytic activity, transparency, and adhesion to a substrate.

US Patent No. 6,824,826 relates to Amadelli et al., the disclosure of which is hereby incorporated by reference, using photocatalytic preparations of colloidal titanium dioxide optionally pasted with a metal selected from groups 1-5a, the lanthanide series and actinide of the periodic table, to preserve the original appearance For cementitious products, stone, and marble.

Colloidal preparation of titanium dioxide by standardized hydrolysis of titanium isopropoxide with nitric acid is provided in Example 1 of this patent.

US Patent Publication No. 0241502/2004 relates to Chimg; Hoon et al., and their disclosure is hereby incorporated by reference, by a method for manufacturing a neutral and transparent colloidal solution of titanium dioxide in which nano-sized titanium dioxide particles are dispersed, and preparing a colloidal solution of titanium dioxide by the method. In one method for preparing colloidal solutions, a titanium compound and a stabilizer are added to alcohol, neutralized by adding a basic solution, and then heated at a temperature above 75°C for more than 7 hours. Titanium compound can be, among other materials:

tetraisopropanol titanium (titanium isopropoxide) Among the many stabilizers, glycolic acid is said to be suitable. The colloidal solution of neutral titanium dioxide prepared by the method is said to be stable and transparent.

Despite advances in the field, there is much room for improvement as not all known colloidal solutions are without their drawbacks. Therefore, the object of the invention is to provide colloidal solutions containing photocatalytic titanium dioxide, which are transparent. It is also an object of the invention to provide such transparent colloidal solutions of titanium dioxide that are stable over an extended period of time. It is also an object of the invention to provide new methods for preparing such stable and transparent colloidal solutions which are easily accomplished on a commercial scale.

The preceding explanation is presented individually to provide a better understanding of the nature of the problems confronting the art and should not be construed in any way as a submission that the prior art is, nor should the listing of any reference in this patent be construed as a submission that such a reference constitutes “prior art” for the application. Present.

General description of the invention:

In accordance with the above and other objectives, it has been unexpectedly discovered that colloidal solutions of titanium dioxide, which are stable and transparent, are formed by thermal treatment of a suspension of amorphous titanium dioxide in the presence of certain alpha-hydroxy acids.

In one aspect of the invention, a method is provided for preparing a stable and transparent colloidal solution of photocatalytic titanium dioxide, comprising: (1) providing a solution comprising a compound containing titanium, for example, titanium alkoxide, titanium oxychloride, titanyl sulfate, or titanyl acetylacetonate; (ii) precipitating aqueous titanium dioxide from said solution of a titanium-containing compound; (iii) formation of an aqueous dispersion of aqueous titanium dioxide; and (4) mixing the aqueous dispersion in the presence of one or more alpha-hydroxy carboxylic acids, e.g., lactic acid, tartaric acid, malic acid, citric acid, and combinations thereof, at Temperature between about 70°C and about 150°C for a period of time between about three hours and about 3 days. While one or more alpha-hydroxy carboxylic acids need to be present during the thermal treatment step, it will be understood that they can be introduced during any of steps (1) through (4). In one variation of the creative method, heat treatment is advantageously carried out without a prior neutralization step. In another variant, the one or more alpha-hydroxy carboxylic acids will not include a glycolic acid.

The resulting colloidal solution of titanium dioxide is surprisingly stable and transparent over a wide range of pH values, for example, throughout the pH range from 2 to 12.

A colloidal solution will typically contain titanium dioxide crystallites with an average particle size of less than about 10 nm, or between about 1 nm and about 10 nm, with most of the crystallites being in the anatase form. In one variant, the crystallites have an average particle size between about 1 nm and about 5 nm and/or at least 90% of the crystallites are in the anatase form. These and other features of the present invention will be better understood by reference to the following detailed explanation and accompanying figures.

Brief explanation of the drawings:

Figure (1) compares the photocatalytic activity against pollutants NOx over time for four coatings called 2-D composed of colloidal solutions for examples Nos. 2, 4, 5, and 8, respectively.

Detailed description:

All expressions in this patent are intended to have their ordinary meaning unless otherwise provided. The term "colloidal solution" refers to a colloidal suspension of particles. The term "NOx" refers to the type of NO (nitrogen oxide) and NO2 (nitrogen dioxide) either collectively or individually.

When reference is made to “removal” of pollutants from the air, it will be understood to include the complete or partial removal of pollutants from the air. Whether the removal is “substantial” may be determined by methods given in the examples, where the expression “substantial” removal refers to a reduction in the final concentration of a fixed amount of a known contaminant by about 5%, preferably at least 10%, and more preferably about 15%. % at least.

The method for preparing stable transparent sols of colloidal photocatalytic titanium dioxide according to the invention generally includes: (1) Providing a solution of a titanium-containing compound; (ii) precipitating amorphous hydrous titanium dioxide from solution; (3) dispersing the precipitated titanium dioxide in water and mixing with a peptizing agent at a temperature between about 70°C and about 150°C for a period of time between about three hours and 3 days; Wherein the colloid conversion agent includes alpha-hydroxy carboxylic acids; This provides a stable, transparent colloidal solution containing nano-sized particles of:

anatase titanium dioxide with a particle size of less than or equal to about 10 nanometers in diameter, preferably less than or equal to about 5 nanometers in diameter.

A titanium-containing compound may be any compound capable of forming a precipitate of titanium dioxide, including, without limitation, titanium alkoxide, titanium oxychloride, titanyl sulfate, titanyl acetylacetonate, and the like. Suitable titanium alkoxides include, without limitation, titanium ethoxide, titanium n-propoxide, titanium isopropoxide, titanium tert-butoxide, and titanium n-butoxide, to name a few. Mixed alkoxides are also expected to be suitable.

Titanium isopropoxide is a currently preferred titanium-containing compound according to the invention partly because of its low cost and relative ease of hydrolysis.

The solution of the titanium-containing compound may be an aqueous solution or may include a suitable organic solvent such as alcohol, for example, ethanol or iso-propanol. There are no necessary restrictions on the solution concentration of the titanium-containing compound, although it is preferable to concentrate it appropriately so that the deposition kinetics are improved. Precipitation may be effected by any appropriate method, including, without limitation, hydrolysis, pH adjustment, or solvent-shifting. The precipitation method used will be largely determined by the choice of titanium-containing compound. For example, hydrolysis is the preferred precipitation method where the titanium-containing compound is titanium alkoxide or titanium acetylacetonate. For titanium oxychloride or titanyl sulfates, which are water soluble, precipitation is best accomplished by adjusting the pH (e.g. raising the pH) or by adding a solvent in which the compound is not necessarily insoluble. Such as acetone or higher alcohols (“solvent shifting”). By “necessarily insoluble” we mean that the solubility of the titanium-containing compound is low enough in the solvent to allow titanium dioxide to precipitate from the solution upon contact with the second solvent. By "higher" alcohols we mean alcohols C5 or greater, including without limitation, pentanol, hexanol, heptanol, octanol, etc.

The aqueous precipitate of amorphous titanium dioxide is typically collected by filtration and careful washing with de-ionized water before redispersion.

The washed wet filter cake is then redispersed in a volume of deionized water with vigorous agitation (eg with deep vortex shaking, etc.). De-ionized water will usually, although not always, include a peptizing agent in solution before the dispersion is formed, as the benefit of the colloidal agent is largely realized during the subsequent thermal treatment step. It is not strictly necessary that the colloid conversion agent be present in the aqueous solution before the precipitate can be re-dispersed. Ideally, the conversion agent may also be added to the colloid after the dispersion has been formed, or it may be added to the titanium precursor before precipitation is performed. The amount of deionized water used will preferably be such that the weight ratio of the parent titanium-containing compound (eg, titanium isopropoxide) to the total weight of dispersion is from about 1:2 to about 1:10, more typically , about 1:3 to about 1:6, and preferably about 1:4 to about 1:5.

The peptizing agent is typically an organic acid (e.g., carboxylic acid) which will preferably have a solubility constant pKa1 ≤ 3.5 at 25°C.

The preferred colloidal conversion agents according to the embodiments are alpha-hydroxy carboxylic acids. Suitable alpha-hydroxy carboxylic acids will typically include one, two, or three carboxylic acid groups, and include, without limitation, lactic acid, malic acid, tartaric acid, and citric acid, to name a few. In some embodiments, the preceding salts of acids are also expected to be suitable. Combinations of the above acids are also expected to be beneficial. In one embodiment, the solution will be free or necessarily free of glycolic acid, such alpha-hydroxy acid having a pKa of 3.83 at 25°C and is not thus favored in practice of the invention. By “necessarily free of” we mean that the glycolic acid comprises in total no more than 5% of the total weight of the colloidal transfer agent, preferably less than about 2.5% by weight, and, more preferably, less than about 0.1 to about 0.5 mol. per mole of TiO2 precipitate. Without wishing to be bound by any particular theory, it is believed that the colloidal transfer agent prevents or inhibits flocculation and exerts an emulsifying effect on growing crystallites during thermal treatment to reduce crystallite size and to provide stability to the resulting colloidal solution.

Colloidal conversion is typically carried out at a temperature of about 70°C to about 150°C (heat treatment) for a period of time from about 3 hours to about 3 days under agitation. It is not necessary to neutralize the solution before thermal treatment. Therefore, in one embodiment, the dispersion comprising the peptizing agent is not subjected to a neutralization step, such as by adding a basic solution, before or during heat treatment. It has been found advantageous to perform colloidal peptization in a hydrothermal reactor because of the increase in inherent pressure. It has been found that vessel type hydrothermal reactors are suitable for use in hydrothermal reaction. Vessel reactors may be placed in a roller oven or similar to provide thermal conditions and to achieve stirring.

In some embodiments, the resulting colloidal solutions are necessarily stable and transparent at any pH (acidic, neutral, or basic) and no pH adjustment is necessary. However, within the scope of the invention it is possible to optionally adjust the pH as desired.

The pH of acid peptized colloidal solutions may be adjusted by the addition of an organic or inorganic base, including without limitation, tert-butylamine, diethylamine, tetramethylammonium hydroxide, ammonium hydroxide, and the like. The transparency of colloidal solutions may be observed either visually or by UV-visible spectroscopy. The stability of colloidal solutions may be measured as a function of change in transparency with time. A “stable” colloidal solution is a colloidal solution that does not change visibly in transparency over an observation period of one, two, or preferably three months at room temperature. It is not necessary to include a stabilizing agent, such as that described in US Patent Publication No. 0241502/2004, incorporated herein by reference, to stabilize colloidal solutions according to the invention.

Particle size and crystallinity (anatase/rutile) may be determined by transmission electron microscopy (TEM) or other suitable means. Titanium dioxide crystallites will typically have an average particle size between about 1 and about 20 nm, more typically, between about 1 nm and about 10 nm, and preferably between about 1 and about 5 nm. By “substantially all” we mean that the D90 value is less than or equal to the indicated volume on a weight basis. Colloidal solutions of titanium dioxide will typically contain the majority (i.e., >50%) of the crystallites as anatase, with a trace amount as rutile. In various embodiments, at least 60%, at least 70%, at least 80%, or at least 90% of the crystals are in the anatase crystal form. In other embodiments, colloidal solutions of titanium dioxide will be substantially devoid of a crystalline form of the rutile, by which we mean that less than 5%, preferably less than 2.5%, and more preferably less than 1% of the titanium dioxide have a crystalline form crystalline for rutile. In another embodiment, the titanium dioxide is 100% in the form of anatase.

Colloidal solutions will typically contain from about 0.5 to about 20 wt% titanium dioxide based on the total weight of the composition. More typically, colloidal solutions will contain from about 1% to about 10% by weight titanium dioxide, based on the total weight of the composition. Colloidal solutions exhibit excellent transparency and stability over a wide range of pH values, including acidic, neutral, and basic conditions. In one embodiment the colloidal solutions will be stable and transparent over the entire range of pH values ​​from pH 1 to < 6; 6-8; and > 8 to 13. Furthermore, the pH adjustment of colloidal solutions may be adjusted from acidic to neutral to basic, and vice versa, without a measurable effect on transparency and stability. Colloidal solutions according to the invention may typically include additional components provided that the addition of such components does not have a measurable adverse effect on either the transparency or stability of the colloidal solution. For example, it is believed that colloidal solutions may include trace amounts of bactericidal agents, organic solvents (e.g., alcohols), film-forming aids, sequestering agents, and pH-adjusting agents pH adjusters, etc. In one embodiment, the colloidal solutions will be free of metal ions selected from group 1-5a, the lanthanide series or the actinide series of the periodic table, meaning that no additional amounts of such metal ions are added to the colloidal solutions or Intermediate preparations are any trace amounts that are present as impurities in the titanium starting material or other reactants.

While colloidal solutions according to the invention are transparent, it has also been found usefully that films formed when placed on a substrate are also transparent. Included in the invention is a method for forming a self-cleaning transparent photocatalytic decontamination film or coating on a substrate including applying the substrate to any of the colloidal solutions according to the invention. The films are allowed to dry to a transparent film that has good adhesion to the base material. There is no basis depending on the nature of the base material. Cement, metal, glass, polymer, wood, ceramic, paper, textile, and leather are all believed to be suitable foundation materials.

Stable, transparent colloidal solutions will be of particular benefit in any application where photocatalytic activity is desired. Because of the transparent nature of colloidal solutions, they are ideally suited to coating surfaces that are themselves transparent (i.e., glass) or to provide a coating that does not alter the appearance of the underlying substrate. Notable applications include, but are not limited to, photocatalytic coatings for removing air pollution on road surfaces, pavers and ceramic tiles, building exteriors, window glass, automobile windshields, and the like. Colloidal solutions will also find benefit on fabrics, furniture, artwork, etc. due to their self-cleaning properties to provide stainless, soil-less and also UV protection.

Example No. (1)

A transparent colloidal solution of titanium dioxide was prepared according to the invention as follows.

(50 g) titanium isopropoxide (Alfa Aesar, 95%) diluted with (50 g) iso-propanol was slowly added to (250 g) de-ionized water under vigorous agitation. The precipitate was then filtered and the precipitate was washed with (500 grams) deionized water.

The wet filter cake was re-dispersed in deionized water to a total dispersion weight (250 g). (6 grams) lactic acid (Alfa Aesar, 85% in water) was added to the dispersion and mixed well. The dispersion was discharged into bowl-type hydrothermal reactors (125 ml, Parr Instruments) lined with cups and Teflon lids. The vessels were then placed in a roller oven at a rolling speed of 25 rpm and the temperature was set at 80°C. Then maintain the treatment for 24 hours. After the colloidal solution was cooled to room temperature, its pH was 2.3. tert-butylamine (Alfa Aesar, 98%) was added to the colloidal solution to adjust the pH to 7. The final colloidal solution product was completely stable and transparent and TEM examination revealed that it contained crystalline TiO2 nanoparticles ≤ 5 nm, most of which were Crystallites anatase (> 50%) with rutile as secondary phase (< 50%).

Comparative Example No. (1):

A sample was prepared using the same method as Example No. (1), except that (7.6 grams) nitric acid (69% solution by weight) was used as a colloidal peptizing agent instead of lactic acid. After peptization, the colloidal solution appeared stable but with an opaque milky appearance.

Example No. (2):

A procedure identical to Example 1 was used to prepare a transparent colloidal solution of titanium dioxide according to the invention, except that (4.5 g, tartaric acid (Alfa Aesar, 99%)) was used instead of lactic acid as the colloidal conversion agent. The product was stable and transparent. TEM examination showed that it contained TiO2 nanoparticles ≤ 5 nm. D-space measurement by high-resolution TEM revealed that the majority of crystallites were anatase (>50%) with an unknown secondary phase (<50%) unusually showing a d-space of ~0.6 nm.

Example No. (3):

A procedure identical to Example No. (1) was used to prepare a transparent colloidal solution of titanium dioxide according to the invention, except that (10.5 grams, citric acid) was used instead of the lactic load as the colloidal conversion agent and the colloidal peptization treatment was maintained for 3 days. The resulting colloidal solution was stable and transparent. TEM examination showed that it contained TiO2 nanoparticles ≤ 5 nm.

D-space measurement by high-resolution TEM revealed that the majority of crystals were anatase (>50%) with an unknown secondary phase (<50%) unusually showing a d-space of ~0.6 nm.

Example No. (4):

A procedure identical to Example No. (1) was used to prepare a transparent colloidal solution of titanium dioxide according to the invention, except that (4 grams) lactic acid was used and the colloidal conversion was carried out at 120°C under hydrothermal conditions for two days. The acidic sol was neutralized to pH 8 with tert-butylamine. The sol product was stable and transparent. TEM images show well-crystallized 5 nm TiO2 nanoparticles.

D-space measurement on high-resolution TEM images showed that most of the crystallites were anatase (>50%) with rutile as the secondary phase (<50%).

Example No. (5):

A procedure identical to Example No. (1) was used to prepare a transparent colloidal solution of titanium dioxide according to the invention, except that (6 grams) tartaric acid was used and colloidal peptization was carried out at 120°C under hydrothermal conditions for two days. The acidic colloidal solution was neutralized to pH 8 by tert-butylamine. The sol product was stable and transparent. TEM images show well-crystallized 5 nm TiO2 nanoparticles. Measurement of the d-space on high-resolution TEM images showed that most of the crystals were anatase (>50%) with an unknown secondary phase (<50%) showing an unusually large d-space of ~0.6 nm.

Example No. (6):

A transparent colloidal solution of titanium dioxide was prepared according to the invention as follows. (50 grams) titanium isopropoxide was mixed with an iso-propanol solution of tartaric acid (12.5 grams tartaric acid in 100 grams iso-propanol). To this mixture, 125 grams of deionized water was slowly added under vigorous stirring. After hydrolysis, stirring was maintained for 15 minutes, after which the precipitate was separated by centrifugation and the upper liquid layer was separated by decantation. The wet solid layer was re-dispersed in water to a total weight of (250 grams). They were packed into vessel reactors and processed in a roller oven at 80°C for 3 days. The colloidal solution sample was adjusted to a pH of approximately 2 and prepared to pH 8 by tert-butylamine. It was stable and transparent.

Example No. (7):

This example provides a large-scale preparation of the colloidal sol of Example 5. The sample was the same as the sample used in Example 5, except that instead of 125 ml bombs, a 2 liter hydrothermal reactor equipped with a heater, a Teflon liner, and a magnet drive stirrer was used to prepare the sample. With this reactor, a sample of about 1.5 kg was obtained in each batch. Appearance, properties and photocatalytic performance were largely the same as Example 5.

Example No. (8):

In this example, titanium oxysulfate (TiOSO4) was used as the precursor material for TiO2 and precipitation of TiO2 was induced by pH adjustment with an ammonia solution. A (950 g) solution of titanium oxysulfate in water (7.9% TiO2 based analysis, Millennium Inorganic Chemicals) was added at 25 ml/min to a large pharmacy beaker containing (950 g) de-ionized water. At the same time, the ammonia solution (29%) was added to the beaker at a rate that maintained the pH of the reaction mixture at approximately 8 throughout the precipitation process. The precipitate was stirred for another 30 minutes before being filtered and washed with 5 L of deionized water. The washed wet precipitate was then re-dispersed with de-ionized water to a total weight of dispersion of approximately 1400 grams. (45 grams) of tartaric acid were added to the dispersion with stirring, and the dispersion was filled in the 2-liter hydrothermal reactor described in Example No. (7). It was hydrothermal treated at 120°C for two days. A transparent TiO2 colloidal solution was obtained with appearance and properties like those described in Example No. (5).

Example No. (9):

To examine the photocatalytic activity of coatings prepared from colloidal solutions according to the invention, the colloidal solutions of Examples (2), (4), (5), and (8) were deposited in the form of thin layers on concrete base materials. substrates (about 0.3 ml over an area of ​​18 cm2, giving samples A, B, C, and D, respectively. Activity against pollutants measured NOx was measured under UV radiation (2 W/m2) at different intervals over a period of about 4000 hours. The method for determining NOx reductase was essentially as described in US Patent Publication No. 2007/0167551, the disclosure of which is incorporated herein by reference. As shown in Figure 1, each of the samples showed substantial photocatalytic activity (i.e. greater than about 5%) expressed as % NOx removal, over the entire time period. Interestingly, titanium oxysulfate (TiOSO4)-catalyzed samples A, B, and C showed very high initial NOx% removal (i.e., between about 50% and about 75%) up to about 1500 h with a higher gradual leveling. Of about 45% NOx removed.

All references, including patent applications and publications mentioned in this patent, are hereby incorporated by reference in their entire contents for all purposes to the same extent as if they had been specified individually, and each individual publication, patent or patent application is hereby incorporated by reference in their entire contents for all purposes. Many modifications and changes can be made to this invention without departing from its spirit and scope, as will be obvious to those skilled in the art. The specific embodiments described in this patent are provided by example only, and the invention shall be identified only by expressions for the appended claims, with the full range of equivalents being interpreted for such claims.