Transparent, stable titanium dioxide sols

[0001] The present invention refers in general to compositions that add a photocatalytic coating to a surface. More specifically, the invention refers to suns of titanium dioxide nanoparticles, which are useful for providing transparent photocatalytic coatings on a substrate that are decontaminating and / or self-replacing.

BACKGROUND OF THE INVENTION [0002] The photocatalytic properties of the titanium dioxide of the semiconductor material come from the promotion of electrons from the valence band to the conduction band under the Influence of ultraviolet (UV) rays and near UV. The pairs of reactive electron holes that are created migrate to the surface of the titanium dioxide particles, where the holes oxidize the adsorbed water to produce reactive hydroxyl radicals and the electrons reduce the adsorbed oxygen to produce superoxide radicals, which can degrade NO_X and volatile organic compounds (VOC) in the air. Given these properties, photocatalytic titanium dioxide has been used in coatings and similar alternatives to remove air pollutants. Such coatings can also have the advantage of being self-replacement, since dirt (grease, fungus, mold, algae, etc.) oxidizes on the surface. [0003] In many applications, it is desirable that the titanium dioxide coating be transparent to maintain the original appearance of the substrate (eg, ceramic tiles, floor block, shed, etc.) or its original transparency (for example, window glass, windshield

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cars, etc.) Colloidal titanium dioxide sols have proven to be a useful precursor material for forming such transparent coatings.

[0004] For example, US Pat. UU. No. 6,420,437 to Morí et al., Which is incorporated herein by reference, has a colloidal sol of neutral titanium dioxide that has high stability in the neutral range and is capable of forming a colorless transparent coating. , even when it dries at room temperature. The sun is produced by mixing an acid titanium dioxide sol comprising between 50 and 100 parts by weight of colloidal particles of titanium dioxide and between 5 and 50 parts by weight of a chelating agent for titanium ions with 1 to 50 parts by weight of an alkaline substance comprising at least one alkali metal compound and amine compounds, and optionally adjusting the pH value of the liquid mixture between 5 and 10, or adjusting the pH value of the liquid mixture between 6 and 10 Y, then, by applying a deionization treatment to the mixture to charge the titanium dioxide particles with negative electricity.

[0005] US Pat. UU. 6,627,336 in the name of Ohmori et al., The disclosure of which is incorporated herein by reference, describes an aqueous dispersion of titanium oxide particles, which is predominantly composed of brookite titanium oxide particles, comprising a chloride ion and a non-chloride ion base, preferably nitrate ion and / or phosphate ion. The aqueous titanium oxide dispersion is prepared by means of hydrolyzed titanium tetrachloride in the presence of at least one type of Bronsted acid. Thin films formed from the dispersion of aqueous titanium oxide are said to have good photocatalytic activity, transparency and adhesion to a base material.

882289 v1 [0006] US Pat. UU. 6,824,826 in the name of Amadelli et al., Whose disclosure is incorporated herein by reference, refers to the use of photocatalytic preparations of colloidal titanium dioxide, modified with the addition of a metal chosen from the l-VA groups, and the lanthanide and actinide series of the periodic table, to preserve the original appearance of cementitious, stone and marble products. The preparation of colloidal titanium dioxide by controlled hydrolysis of titanium isopropoxide with nitric acid is shown in Example 1 of that patent.

[0007] US Patent Publication UU. 2004/0241502 on behalf of Chung, Hoon et al., Whose disclosure is incorporated herein by reference, refers to a method for manufacturing a colloidal solution of neutral and transparent titanium dioxide, where titanium dioxide nanoparticles are Disperse and prepare a colloidal solution of titanium dioxide according to the method. In a method for preparing suns, a titanium compound and a stabilizer are added in alcohol, neutralized by adding a basic solution and then heated at a temperature above 75 ° C for more than 7 hours. The titanium compound may be, among others, tetraisopropanol titanium (titanium isopropoxide) and among the numerous stabilizers, glycolic acid is suitable. The colloidal neutral titanium dioxide solution prepared with the method is said to be stable and transparent.

[0008] Despite these advances in art, there is a considerable place for improvement, since each of the known suns has its disadvantages. Therefore, one of the objects of the invention is to provide soles comprising photocatalytic titanium dioxide that are transparent. One of the objectives of the invention is also to provide such transparent soles that

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be stable for a prolonged period of time. Another of the objects of the invention is to provide novel methods for preparing such transparent and stable soles that are rapidly implemented on a commercial scale. [0009] The preceding analysis is presented, only, to provide a better explanation of the nature of the problems that art encounters and should not be interpreted as an acknowledgment of the prior art nor should the citation of the references of the present should interpreted as an acknowledgment that said reference constitutes a “prior art” to the immediate application

SUMMARY OF THE INVENTION [0010] In accordance with the preceding and other objectives, it has been surprisingly discovered that titanium dioxide soles, which are stable and transparent, are formed by heat treatment of a suspension of amorphous titanium dioxide in presence of certain alpha-hydroxy acids. [0011] In one aspect of the invention, there is provided a method for preparing a transparent photocatalytic titanium dioxide sol comprising: (I) providing a solution comprising a titanium compound, for example, a titanium alkoxyl, oxychloride titanium, titanium sulfate or titanium acetllacetonate; (I) precipitating aqueous titanium dioxide from said titanium compound solution; (¡II) form an aqueous dispersion of the precipitated aqueous titanium dioxide; and (iv) mixing the aqueous dispersion in the presence of one or more aifa-hydroxycarboxylic acids, for example, lactic acid, tartaric acid, malic acid, citric acid and combinations thereof, at a temperature between approximately 70 ° C and 150 ° C for a period of time between three hours and 3 days. While it is required that one or more of those hydroxycarboxylic alpha882289 v1 acids be present during the heat treatment stage, it will be understood that they may be incorporated during one of the stages (i) to (iv). In a variant of the method of the invention, the heat treatment is best carried out without a prior neutralization step. In another variant, one or more of those alpha-hydroxycarboxylic acids will not include glycolic acid.

[0012] The resulting titanium dioxide sol is surprisingly stable and transparent over a wide range of pH values, for example, over the entire pH range of 2 to 12. The sun will typically comprise crystallites of titanium dioxide having a average particle size less than 10 nm or approximately 1 nm to 10 nm, most of which are anatase-shaped. In a variant, the crystallites have an average particle size between approximately 1 nm and 5 nm and / or at least 90% of the crystallites are in anatase form.

[0013] These and other aspects of the present invention will be better understood by reference to the following detailed description and the accompanying figures

BRIEF DESCRIPTION OF THE FIGURES [0014] Figure 1 compares the photocatalytic activity with NOx contaminants with the passage of time in four designated coatings Sample AD formed from the soles of Examples 2, 4, 5 and 8, respectively.

DETAILED DESCRIPTION [0015] All terms used are expected to have their common meaning, unless otherwise indicated. The term "sun" refers to a colloidal suspension of particles. The term "NO_X”Refers to the

882289 v1 species NO (nitrogen oxide) and NO₂ (nitrogen dioxide), either collectively or individually.

[0016] If reference is made to the "removal" of air pollutants, it is understood that it includes the complete or partial removal of air pollutants. To determine if the removal is "substantial", the methods provided in the examples can be used, where "substantial" means the reduction in the total concentration of a fixed amount of contaminants given by at least about 5%, preferably in at least about 10% and more preferably about 15%.

[0017] The method for preparing stable and transparent soles of colloidal photocatalytic titanium dioxide according to the invention generally comprises: (1) providing a solution comprising a titanium compound; (2) precipitate amorphous aqueous titanium dioxide from said solution; (3) form an aqueous dispersion of the precipitated titanium dioxide in water and mix with a peptizing agent at a temperature between approximately 70 ° C and 150 ° C for a period of time between three hours and 3 days, where the peptizing agent comprises alpha-hydroxycarboxylic acid; thus, a stable and transparent sun is provided comprising nanoparticles of anatase titanium dioxide having a particle size less than or equal to about 10 nanometers (nm) in diameter and preferably less than or equal to 5 nm in diameter .

[0018] The titanium compound may be any compound capable of forming a precipitate of titanium dioxide, which includes, without limitation, a titanium alkoxide, titanium oxychloride, titanyl sulfate, titanium acetylacetonate and the like. Suitable titanium alkoxides include, among others,

882289 v1 titanium ethoxide, titanium n-propoxide, titanium isopropoxide, titanium terbutoxide and titanium n-butoxide, to name a few. It is also envisioned that mixed alkoxides are suitable. Titanium isopropoxide is a titanium compound, currently preferred, according to the invention, in part due to its low cost and relative ease of hydrolysis.

[0019] The solution of the titanium compound may be an aqueous solution or may comprise a suitable organic solvent such as an alcohol, for example, ethane or isopropanol. Essentially, there is no limitation on the concentration of the titanium compound solution, even though it is concentrated, preferably properly, so that the precipitation kinetics is optimized. Precipitation can be affected by any suitable method, including without limitation, hydrolysis, pH adjustment or solvent displacement. The precipitation method used will be determined primarily by the selection of a titanium compound. For example, hydrolysis is the preferred precipitation method where the titanium compound is a titanium alkoxide or titanium acetylacetonate. As for titanium oxychlorides or titanium sulfates, which are soluble in water, the best way to perform precipitation is by adjusting pH (for example, raising the pH) or by adding a solvent in which the compound is , essentially, soluble, such as acetone or higher alcohols ("solvent displacement"). By "essentially insoluble," it is understood that the solubility of the titanium compound is sufficiently low in the solvent to allow the titanium dioxide to precipitate from the solution when it comes into contact with the second solvent. By "higher" alcohols are meant C alcohols₅ or higher, including, among others, pentanol, hexanol, heptanol, octanol, etc.

882289 v1 [0020] The amorphous aqueous titanium dioxide precipitate is collected, in general, by filtration and washed thoroughly with water stripped before redispersion. The washed and wet filtered cake is again dispersed in a volume of deionized water with vigorous agitation (for example, stirring with a deep vortex, agitation, etc.). The de-tonated water will comprise, but not always, a peptlzant agent in a solution, before the dispersion forms. Since the benefit of the peptlzante agent is realized, mainly, during the subsequent heat treatment step, it is not strictly necessary that the peptlzante agent be present in the aqueous solution before the precipitate is dispersed again. On the contrary, the peptlzante agent can also be added after the dispersion has formed or the titanium precursor can be added before precipitation is carried out. The amount of deionized water will preferably be such that the weight ratio of the original titanium compound (e.g., titanium sopropoxide) with the total weight of the dispersion is 1: 2 to about 1:10, more typically 1: 3 at about 1: 6, and, it is preferred that it be about 1: 4 to about 1: 5.

[0021] It would be ideal if the peptizing agent is an organic acid (for example, a carboxylic acid) that preferably has a first dissociation constant pK_to3.5 to 25 ° C. Preferred peptlzant agents according to one embodiment are alpha-hydroxycarboxylic acids. Suitable alpha-hydroxy carboxylic acids will typically comprise one, two or three groups of carboxylic acids, and include, without limitation, lactic acid, malic acid, tartaric acid and citric acid, to name a few. In some embodiments, the salts of the acids mentioned are also considered

882289 v1 are suitable. It is also considered that the salts of the mentioned acids are useful. In one embodiment, the solution will be free or essentially free of glycolic acid, since this alpha-hydroxy acid has a pK_toi of 3.83 at 25 ° C and, therefore, is not preferred in the practice of the invention. By "essentially free of", it is understood that the glycolic acid as a whole comprises a maximum of 5% of the total weight of the peptizing agent, preferably less than about 2.5% by weight and, even more preferably, less than 1% by weight, approximately. The amount of peptizing agent required will generally range from 0.1 to about 0.5 moles per mole of TiO precipitate.₂. Without intentions to follow a particular theory, it is believed that the peptizing agent prevents or inhibits flocculation and has a chelating effect on the crystallites that grow, during heat treatment, to limit the size of the crystallite and provide stability to the resulting sun.

[0022] Peptization is generally performed at a temperature between about 70 ° C and about 150 ° C (heat treatment) for a period of time from about 3 hours to about 3 days by means of a stirring process. It is not necessary to neutralize the solution before heat treatment. Therefore, in one embodiment, the dispersion comprising the peptizing agent is not subject to the neutralization step, such as the addition of a basic solution, before or after heat treatment. It has been found that it is useful to perform peptization in a sealed hydrothermal reactor due to the concomitant increase in pressure. It has been found that pump-type hydrothermal reactors, such as those manufactured by Parr Instruments, are suitable for use in the hydrothermal reaction. Pump reactors can be

882289 v1 place in a roller oven or similar to provide thermal conditions and achieve agitation.

[0023] In some embodiments, the resulting soles are stable and transparent with essentially any pH (acidic, neutral or basic) and, therefore, pH adjustment is not necessary. However, optionally adjusting the pH as desired is within the scope of the invention. The pH of the acid peptized soles can be adjusted with the addition of an organic or inorganic base, which includes, without limitation, terbutylamine, diethylamine, tetramethylammonium hydroxide, ammonium hydroxide and the like. The transparency of the suns can be observed visually or by UV-visible spectroscopy. The stability of the suns can be measured as a function of the change in transparency over time. A "stable" sun is one whose transparency does not change visibly during an observation period of one, two or preferably three months at room temperature. It is not necessary to include a stabilizing agent, such as those described in US Pat. UU. 2004/0241502, which is incorporated herein by reference, to provide stability to the suns according to the invention.

[0024] Particle size and crlstalinity (anatase / rutile) can be determined by transmission electron microscopy (TEM) or other suitable means. Titanium dioxide crystallites typically have a particle size between about 1 nm to about 20 nm, more especially, between about 1 nm to about 10 nm, and preferably between about 1 nm and about 5 nm. nm. In other embodiments, substantially all titanium dioxide crystallites

882289 v1 will have particle sizes smaller than or equal to 10 nm and, preferably, less than or equal to approximately 5 nm. By “substantially all, it is understood that the value D₉₀ It is less than or equal to the indicated size, depending on the weight. Titanium dioxide soles generally comprise a majority of (ie,> 50%) of anatase-shaped crystallites, with only a minority of rutile-shaped crystallites. In several embodiments, at least 60%, at least 70%, at least 80% or at least 90% of the crystallites are anatase-shaped. In other embodiments, the titanium dioxide soles will be substantially free of the rutile crystalline form, which means less than 5%, preferably less than 2.5% and more preferably less than 1% of titanium dioxide. It has a crystalline form of rutile. In another embodiment, the titanium dioxide is 100% anatase. [0025] The soles generally comprise between 0.5% and 20% by weight, approximately, of titanium dioxide, based on the total weight of the composition. More typically, the soles generally comprise between about 1% and 10% by weight of titanium dioxide, based on the total weight of the composition. The soles have excellent transparency and stability over a wide range of pH values, even in arid, neutral and basic conditions. In one embodiment, the suns will be stable and transparent in the whole of each pH range from 1 to <6; 6-8; and> 8 to 13. In addition, the pH of the suns can be adjusted from acid to neutral, and vice versa, without having a measurable impact on transparency or stability.

[0026] The suns of the present invention may optionally include additional ingredients, as long as the addition of such Ingredients does not have a measurable negative impact on either transparency or stability

882289 v1 of the sun. For example, soles are expected to include smaller amounts of bactericidal agents, organic solvents (eg, alcohols), film-forming auxiliaries, sequestering agents, pH adjusters, etc. In one embodiment, the soles are free of metal ions chosen from the l-VA group, and the lanthanide or actinide series of the periodic table, which means that no additional amount of metal ions is added to the soles or intermediate preparations that exceeds minute quantities, which are present in the form of impurities in the starting material of titanium or other reagents.

[0027] While the soles according to the invention are transparent, it has also been found that films formed from soles have the advantage that, when applied to a substrate, they are also transparent. Therefore, a method for forming a transparent and self-cleaning photocatalytic decontaminating film or coating on a substrate comprising applying to the substrate any of the soles according to the invention is included in the invention. The films are allowed to dry until they become a transparent coating that has good adhesion to the substrate. There is essentially no limit on the nature of the substrate. Cement, metal, glass, polymeric, wooden, ceramic, paper, textile and leather substrates are considered suitable.

[0028] Stable and transparent suns will have a special utility in any application where photocatalytic activity is desired. Due to the transparent nature of the suns, they are ideal for coating surfaces that are, in themselves, transparent (for example, glass) or to provide a coating that does not alter the appearance of the substrate

882289 underlying v1. Among the applications, stand out, without limitation, photocatalytic coatings to decontaminate the air on road surfaces, cobblestones and ceramic tiles, building exteriors, window panes, car windscreens and the like. The suns will also be useful in fabrics, furniture, works of art, etc., due to their self-cleaning properties to provide stainless and dirt-free products as well as UV protection.

Example 1 [0029] A clear titanium dioxide sol according to the invention is prepared as follows: 50 g of titanium isopropoxide (Alfa Aesar, 95%) diluted with 50 g of isopropanol, slowly added to 250 g of deionized water, stirring vigorously. After precipitation of titanium dioxide, stirring is maintained for approximately 10 more minutes. The precipitate was filtered and washed with 500 g of deionized water. The washed and wet filtered cake was dispersed again in deionized water to a total dispersion weight of 250 g. 6 g of lactic acid (Alfa Aesar, 85% in water) was added to the dispersion and mixed well. The dispersion was placed in pump-type hydrothermal reactors (125 ml capacity, Parr Instruments) aligned with Teflon cups and lids. The pumps were then placed in a roller oven with a rolling speed of 25 rpm and a fixed temperature of 80 ° C. The treatment was maintained for 24 hours. After cooling the sun to room temperature, its pH was 2.3. Terbutylamine (Alfa Aesar, 98%) was added to the sun to adjust the pH to 7.0. The final product of the sun was stable and completely transparent and the TEM test revealed that it contained £ 5 nm crystalline T1O2 nanoparticles. The measurement of space d in images

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High resolution TEM indicated that most of the crystallites were anatase (> 50%) with rutile as a secondary phase (> 50%).

Comparative Example 1 [0030] A sample was prepared following the same method as in Example 1, except 7.6 g of nitric acid (69 weight percent solution) was used as a peptizing agent instead of lactic acid. After peptization, the sun was stable, but it looked milky, not transparent.

Example 2 [0031] The same procedure as in Example 1 was used to prepare a titanium dioxide sol, according to the invention, except that 4.5 g of tartaric acid (Alfa Aesar, 99%) was used instead of acid lactic as a peptizing agent. The resulting sun was stable and transparent. The TEM test revealed that it contained TiO nanoparticles₂ at 5 nm. Measurement of the d space in high-resolution TEM images indicated that the majority of the crystallites were anatase (> 50%) with a secondary phase (> 50%) showing an unusually large d space ~ 0.6 nm.

Example 3 [0032] A procedure identical to Example 1 was used to prepare a transparent titanium dioxide sol according to the invention, except that 10.5 g of citric acid was used instead of lactic acid as a peptizing agent and The peptization treatment was maintained for three days. The resulting sun was stable and transparent. TEM examination revealed that it contained 5 nm of TiO nanoparticles₂. Measurement of the high resolution TEM space d revealed that the majority of the crystallites were anatase (> 50%)

882289 v1 with an unknown secondary phase (&lt;50%) that showed an unusually large d space of ~ 0.6 nm.

Example 4 [0033] The procedure identical to Example 1 was used to prepare a transparent titanium dioxide sol according to the invention, except that 4.0 g of lactic acid were used and the peptization was carried out at 120 ° C under hydrothermal conditions for 2 days. The acid sol was neutralized to pH 8.0 with tert-butylamine. The sun product was stable and transparent. TEM images show 5 nm of TiO nanoparticles₂ well crystallized Measurement of space d in high-resolution TEM images indicated that most of the crystallites were anatase (> 50%) with rutile as a secondary phase (<50%).

Example 5 [0034] The procedure identical to Example 1 was used to prepare a clear titanium dioxide sol according to the invention, except that 6.0 g of tartaric acid was used and the peptization was carried out at 120 ° C under hydrothermal conditions for 2 days. The acid sol was neutralized to pH 8.0 with tert-butylamine. The sun product was stable and transparent. TEM images show 5 nm of TiO nanoparticles₂ well crystallized Measurement of the d space in high-resolution TEM images revealed that most of the crystallites were anatase (> 50%) with an unknown secondary phase (<50%) indicating an unusually large d space of ~ 0.6 nm.

Example 6 [0035] A clear titanium dioxide sol according to the invention was prepared as follows. 50 g of

882289 v1 titanium isopropoxide with an isopropanolic solution of tartaric acid (12.5 g of tartaric acid in 100 g of isopropanol). To this mixture, 125 g of deionized water was added slowly under strong stirring. After hydrolysis, stirring was maintained for 15 minutes, after which the precipitate was separated by centrifugation and the upper liquid layer was decanted. The wet solid layer was redispersed in water to a total weight of 250 g. It was loaded into pump reactors and treated in a roller oven at 80 ° C for 3 days. The sol sample with a pH of about 2.0 as prepared was adjusted to pH 8.0 with tert-butlamine. It was stable and transparent.

Example 7 [0036] This example provides a larger scale preparation of the sun from Example 5. The sample was the same as that used in Example 5, except that instead of 125 ml of pumps, a two-hydrothermal reactor was used liters equipped with a heater, a Teflon coating and a magnetic stirrer for sample preparation. With this reactor, a sample of approximately 1.5 kg in each batch was obtained. The appearance, properties and photocatalytic performance were substantially equal to Example 5.

Example 8 [0037] In this example, titanium oxysulfate (TiOSCU) was used as a TiO precursor₂ and the precipitation of TiO₂ It was performed by adjusting the pH with an ammoniacal solution. 950 g of titanium oxysulfate solution in water (7.9% based on TiO analysis were added₂, Millennium Inorganic Chemicals) at 25 ml / min. To a 4 L beaker containing 950 g of deonized water. Simultaneously, the ammoniacal solution (29%) was added to the beaker at

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a rate that maintained the pH of the reaction mixture at approximately 8.0 throughout the entire precipitation process. The precipitate was stirred for another 30 minutes before filtering and washed with 5 L of deionized water. The wet washed precipitate was then redispersed with deionized water to a total dispersion weight of approximately 1,400 g. 45 g of tartaric acid were added to the dispersion with stirring and the dispersion was loaded into the 2 L hydrothermal reactor described in Example 7. Hydrothermally treated at 120 ° C for 2 days. A transparent T1O2 with similar appearance and properties to the sample described in Example 5 was obtained.

Example 9 [0038] To investigate the photocatalytic activity of the coatings prepared from soles according to the invention, the soles of Examples 2, 4, 5, and 8 were deposited as thin layers on concrete substrates (approximately 0, 3 mi of sun on an area of ​​18 cm²) to give samples A, B, C, and D, respectively. The activity against NOx pollutants under UV radiation (2 W / m²) was measured at various intervals over a period of approximately 4,000 hours. The methodology for determining NOx reduction was substantially as described in US Patent Publication 2007/0167551, the description of which is incorporated herein by reference. As indicated in Figure 1, each sample showed substantial photocatalytic activity (i.e., greater than about 15%), expressed as% NOx removal, throughout the period. It was interesting that samples A, B, and C, which were prepared by hydrolysis of titanium isopropoxide, exhibited a higher% NOx removal in

882289 v1 function of time, while sample D, which had been prepared by precipitate Induced by pH from titanium oxy sulfate (TIOSO₄), showed an extremely high Initial NOx removal% (for example, between approximately 50% and approximately 75%) up to approximately 1,500 hours with a gradual leveling to more than approximately 45% NOx removal.

[0039] All references that include patent applications and publications cited herein are hereby incorporated by reference in their entirety and for all purposes, to the same extent as if it had been indicated that each publication or patent or application for Individual patent should be incorporated specifically and individually by reference in its entirety for all purposes. Many modifications and variations of this invention can be made without departing from its spirit and scope, as will be obvious to those skilled in the art. The specific embodiments described herein are offered by way of example only, and the invention should be limited only by the terms of the appended claims, together with the full scope of equivalents to which these claims are entitled.

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