Catalytic substrate surface

Abstract

There is disclosed a catalytic substrate surface where in the substrate surface comprises particles in the size range 2-500 nm, said particles comprising TiO.sub.2, wherein at least 50 wt % of the Ti in the particles is leachable in 37% HCl. A method for its manufacture is also provided. One advantage is that the higher fractions of Ti leachable in 37% HCl give a better coating with better adhesion, in particular on objects comprising protonatable acidic oxides, such as SiO.sub.2. The higher fraction of Ti leachable in 37% HCl gives better transparency and dispersion stability. The resulting surface exhibits both improved activity and improved adhesion of the particles. There is no color change and/or speckles on the substrate, allowing anti-pollution surfaces to be produced also on decorative surfaces. The TiO.sub.2-comprising layer is thin and easily applied in the production line.

Claims

1. A substrate comprising a surface, said surface comprising particles having an average size in the range 5-50 nm, said particles comprising TiO.sub.2, wherein at least 55 wt % of the total amount of Ti in the particles is leachable in 37% HCl, wherein the leachability is determined by treating TiO.sub.2 particles in 37% HCl at 55 C. for an extended period of several hours, until a plateau value for the concentration of leached material in the HCl is reached, wherein said substrate comprises protonatable acidic oxides, wherein at least a part of the particles comprise at least one core and a surrounding layer, and wherein the at least one core comprises at least 95 wt % anatase.

2. The substrate according to claim 1, wherein said substrate comprises at least one material selected from the group consisting of cement, stone, silicate, and aluminasilicate.

3. The substrate according to claim 1, wherein each of said particles comprise at least one crystalline part and a layer, which layer at least partially surrounds the at least one crystalline part, wherein 90 wt % or more of the total amount of Ti leachable in 37% HCl is in the layer.

4. The substrate according to claim 1, wherein the substrate is porous and wherein there is a porous substructure below the surface and wherein at least a part of the particles have penetrated at least 200 m into the porous substructure.

5. The substrate according to claim 4, wherein at least 65 wt % of catalytically active TiO.sub.2 is in the porous substructure.

6. The substrate according to claim 1, wherein the substrate exhibits a removal of nitric oxide as measured according to ISO 22197-1:2007 of at least 60 wt %.

7. The substrate according to claim 1, wherein the surface is free of binder.

8. The substrate according to claim 1, wherein at least 60 wt % of the total amount of Ti in the particles is leachable in 37% HCl.

9. The substrate according to claim 1, wherein at least 65 wt % of the total amount of Ti in the particles is leachable in 37% HCl.

10. The substrate according to claim 1, wherein at least 70 wt % of the total amount of Ti in the particles is leachable in 37% HCl.

11. A method of manufacturing a substrate with surface particles according to claim 1, said method comprising the steps of: a) preparing a dispersion comprising particles having an average size in the range 5-50 nm, said particles comprising TiO.sub.2, wherein at least 55 wt % of the total amount of Ti in the particles is leachable in 37% HCl, wherein the leachability is determined by treating TiO.sub.2 particles in 37% HCl at 55 C. for an extended period of several hours, until a plateau value for the concentration of leached material in the HCl is reached, wherein at least a part of the particles comprises at least one core and a surrounding layer, and wherein the at least one core comprises at least 95 wt % anatase; b) contacting the dispersion with a substrate comprising protonatable acidic oxides, and c) drying the substrate.

12. The method according to claim 11, wherein the dispersion comprises at least one selected from the group consisting of monoethanolamine, diethanolamine, and triethanol amine.

13. The method according to claim 11, wherein the amount of TiO.sub.2 is from 0.5 to 200 g/m.sup.2.

14. The method according to claim 11, wherein the amount of TiO.sub.2 is lower than 30 g/m.sup.2.

15. The method according to claim 11, wherein a precursor dispersion is contacted with the substrate prior to contacting the dispersion with the substrate.

16. The method according to claim 11, wherein the pH of the dispersion comprising particles is adjusted to a value of 8-11.

17. The method according to claim 11, wherein the substrate is dried by heating to a temperature not exceeding 250 C.

18. The method according to claim 11, wherein the dispersion is aqueous and has the ability to form a gel with increased viscosity upon drying.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is now described, by way of example, with reference to the accompanying drawings, in which:

(2) FIG. 1a shows a scanning electron micrograph of a cross section through a fractured piece of a sample according to the invention, with concentrations of TiO.sub.2 given as gray levels in FIG. 1b using energy dispersive x-ray imaging on the same sample area as FIG. 1a. High concentrations of TiO.sub.2 derived from the precursor dispersion were seen at least 300 micrometer into the paving stone porous substructure.

DETAILED DESCRIPTION

(3) Before the invention is disclosed and described in detail, it is to be understood that this invention is not limited to particular compounds, configurations, method steps, substrates, and materials disclosed herein as such compounds, configurations, method steps, substrates, and materials may vary somewhat. It is also to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention is limited only by the appended claims and equivalents thereof.

(4) It must be noted that, as used in this specification and the appended claims, the singular forms a, an and the include plural referents unless the context clearly dictates otherwise.

(5) If nothing else is defined, any terms and scientific terminology used herein are intended to have the meanings commonly understood by those of skill in the art to which this invention pertains.

(6) A dispersion as used herein refers to a system in which particles are dispersed in a continuous phase of a different composition. The term dispersion includes but is not limited to suspensions, colloids, and sols.

(7) The present invention refers to a functionalized surface structure produced by applying TiO.sub.2 nanoparticles to a substrate.

(8) The adhesion of the particles is improved compared to prior art. Without wishing to be bound by any specific scientific theory the inventors believe that the improved adhesion is due to a combination of at least two causes. Firstly, due to good distribution of nanoparticles on the surface and in the pores. Secondly, due to chemical binding between the TiO.sub.2 and SiO.sub.2 without a binder. The improved chemical binding between the TiO.sub.2 and SiO.sub.2 without a binder is attributed to the high fraction of Ti leachable in HCl. The adhesion is in general improved between TiO.sub.2 and protonatable acidic oxides due to the high fraction of Ti leachable in HCl.

(9) The inventors believe that the catalytic action of the particles on the substrate surface can be described as a two-step process, in particular for particles comprising at least one core and a surrounding layer. The compound to be reacted, for instance NO is in a first step adsorbed to the surrounding layer. Subsequently the compound to be reacted is transported to the core(s). The majority of the molecules react at the core(s). A good catalytic particle should thus have a surrounding layer where the adsorption of the compound to be reacted is adsorbed in a good way and a core(s) where the catalyzed reaction can take place.

(10) A beneficial distribution of nanoparticles is in one embodiment obtained by preparing a pH-matched dispersion with very good nanoparticle dispersability. The pH matching to the substrate is to prevent aggregation during the application and subsequent curing. A pH value which is suitable with respect to the substrate is selected so that the particles do not aggregate or aggregate only to a minor extent. In one embodiment the pH of the dispersion comprising particles is adjusted to a value within pH 8-11. 8-11 is suitable for cement. For old cement a lower pH is suitable, such as 8-9, whereas for newer cement a higher pH is suitable such as 9-10. In one embodiment the pH of the dispersion comprising particles is adjusted to a value of 9 or higher. In an alternative embodiment where for instance a fabric is to be coated the pH is adjusted to a neutral value around pH 7.

(11) The invention give well distributed small particles that can be absorbed faster and deeper into the pores of the substrate. As shown in example 6, particles can penetrate several hundreds of micrometers into the material and in some embodiments of the invention; the majority of the active material exists in the substructure of the substrate. A large fraction of active material in the substructure gives better wear properties.

(12) In one embodiment of the invention, the dispersion is transparent or semi-transparent.

(13) As shown in example 1, the adhesion of the TiO.sub.2 to the substrate is proven. The adhesion increases significantly when curing over several days.

(14) Since the good adhesion is not based on the use of a binder, adhesion is not reduced due to the originally present organic materials being broken down by the photocatalysis.

(15) The active surface available for pollution breakdown depends on the specific surface of the substrate, and optimizing the substrate porosity and specific surface gives further increase in performance.

(16) The particle dispersion can be used for many different purposes. One purpose is for preparing a surface with adsorbed particles where the catalytic properties of the particles are utilized. The inventors believe that the catalytic action of the particles can be described as a two-step process, in particular for particles comprising at least one core and an at least partially surrounding layer. The compound to be reacted is in a first step adsorbed to the surrounding layer. Subsequently the compound to be reacted is transported to the core(s). The majority of the molecules to be reacted react at the core(s). A good catalytic particle should thus have a surrounding layer where the adsorption of the compound to be reacted is adsorbed in a good way and a core(s) at which the catalyzed reaction can take place. A high (i.e 50 wt % or more) fraction of Ti leachable in 37% HCl give improved adsorbtion of NO.sub.x and other gases. Further it has turned out that the at least partially surrounding layer with a high content of Ti leachable in 37% HCl is porous facilitating transport of gases. The layer with a high content of Ti leachable in 37% HCl is further rich in hydroxyl groups.

(17) According to the invention, the nanoparticles comprise a significant fraction of Ti leachable in HCl 37% (50 wt % or more). The leachability is determined by treating TiO.sub.2 particles in 37% HCl at 55 C. for an extended period of several hours, such as at least 5 hours or more such as 17 hours. The treatment time should be sufficiently long to reach a plateau value for the concentration of leached material in the HCl. The skilled person can perform a routine experiment as already described in this application and verify a sufficiently long treatment time to reach equilibrium. The particles should be leached until equilibrium. A skilled person realizes that in theory equilibrium may not be reached even after many hours, but in practice a value of 1 wt % within the true equilibrium value is considered to be equilibrium in a practical experiment. The leached Ti in solution is measured and the fraction of leached Ti can be calculated. All fractions are calculated by weight.

(18) As shown in example 4, the fraction of amorphous material can be reduced by thermal treatment. Since the particles can show a high leachable fraction while XRD analysis shows 100 wt % anatase, this leachable fraction is concluded to consist of one of more amorphous forms of titanium oxides (TiO.sub.2 or substochiometric TiO(.sub.2-x)). It is conceived that the particles comprise titanic acid with the general formula [TiO.sub.x(OH).sub.4-2x].sub.n. Zeolitic water is present in the particles.

(19) The high fraction of Ti leachable in 37% HCl at 55 C. is obtained by carefully controlling the growth of nanoparticles. It is possible to start with a solution/dispersion of titanic acid which is heated to initiate growth of nanoparticles. When the nanoparticles are formed amorphous particles start to form which to a large extent are leachable in 37% HCl at 55 C. When the material which is leachable in 37% HCl has formed the crystalline material starts to grow. The crystalline material is not or only to a very minor extent leachable in 37% HCl. Thus it is possible to abort the heating early in order to obtain particles with a high fraction of leachable material.

(20) The high amorphous content nanoparticles (more than 50 wt % of the Ti leachable in 37% HCl) are observed to give better transparency and dispersion stability in the precursor dispersion. Powders with high amorphous content in the nanoparticles are also observed have better redispersability than highly crystalline powders. A model of the particles as at least one crystalline core surrounded by layer(s) of amorphous material can explain the advantages in crosslinking, dispersion stability and powder redispersability since all of these are connected to surface mechanisms and not bulk composition.

(21) As shown in example 1, the precursor dispersion contains no organic or inorganic binders, nor is it necessary to apply a binder on the substrate before applying the precursor dispersion.

(22) The preparation of the precursor dispersion is not observed to have any significant effect on the basic nanoparticle properties, only on additives on the nanoparticle surface. The precursor dispersion can be applied to the substrate in a single step by spraying or by conventional coating methods such as roll-coating, doctor blading, dip-coating etc. As shown in example 5, the precursor dispersion can be applied during the production of cementitous products such as bricks and paving stones. The precursor dispersion can be applied to the surface before, after or during the curing of the cementitous material. Combinations of application before, during and after curing are also encompassed.

(23) The amount of TiO.sub.2 used in the product can be varied in the order of 0.5-200 g per square meter. Higher numbers give higher absolute efficiency, while lower numbers give higher efficiency per amount of TiO.sub.2 added.

(24) The thickness of the TiO.sub.2-containing layer is mainly determined by viscosity of the precursor dispersion and the pores of the substrate, since the nanoparticles enter the material via physical absorption into the pores. As shown in example 6 and FIG. 1, high concentrations of TiO.sub.2 can be found in pores at least 300 micrometers into the substructure of the substrate. Preparing precursor dispersions with different viscosity controls the penetration and thus the thickness of the functionalized layer, allowing the surface to be optimized for cost-effectiveness versus wear of the substrate.

(25) As shown in example 3, the pollution breakdown performance of the present product is significantly better than the best ISO-test results published for comparable products, such as the investigation by Hsken et al (2007).

(26) In a first aspect there is provided a substrate comprising a surface, said surface comprising particles in the size range 2-500 nm, said particles comprising TiO.sub.2, wherein at least 50 wt % of the Ti in the particles is leachable in 37% HCl.

(27) It is understood that the beneficial effects of a high fraction of Ti leachable in 37% HCl start already at 50 wt %, but that the beneficial effects are improved at 55 wt % and more pronounced at 60 wt %. At even higher fractions of leachable material the advantages can be further pronounced. However one drawback of a very high fraction of leachable material (such as 70-80 wt % Ti) is that the yield during the manufacturing process is impaired. Thus in one embodiment at least 50 wt % of the Ti in the particles is leachable in 37% HCl. In another embodiment at least 55 wt % of the Ti in the particles is leachable in 37% HCl. In yet another embodiment at least 60 wt % of the Ti in the particles is leachable in 37% HCl. In a further embodiment at least 65 wt % of the Ti in the particles is leachable in 37% HCl. In another embodiment at least 70 wt % of the Ti in the particles is leachable in 37% HCl.

(28) In one embodiment at least of part of said particles comprise at least one core and an at least partially surrounding layer, wherein more than 90 wt % of the total amount of Ti leachable in 37% HCl is in the surrounding layer. In an alternative embodiment more than 95 wt % of the total amount of Ti leachable in 37% HCl is in the surrounding layer. The embodiments with at least one core and a surrounding layer is often several cores embedded in a surrounding layer.

(29) In one embodiment the substrate comprises protonatable acidic oxides. In one embodiment the substrate comprises at least one selected from the group consisting of cementitous, stone, silicate, and aluminasilicate. In one embodiment the substrate comprises SiO.sub.2.

(30) It is conceived that the particles comprise at least one, and often a number of parts which are crystalline, and which are surrounded by material which to a large extent is amorphous. In one embodiment each of said particles comprise at least one crystalline part and a layer, which layer at least partially surrounds the at least one crystalline part, wherein 90 wt % or more of the total amount of Ti leachable in 37% HCl is in the layer. In an alternative embodiment more than 95 wt % of the total amount of Ti leachable in 37% HCl is in the layer. In these embodiments the crystalline parts or core(s) comprises Ti which is not leachable to any significant extent. The leachable Ti is essentially in the layer at least partially surrounding the crystalline parts.

(31) In one embodiment the substrate is porous and there is a porous substructure below the surface and wherein at least a part of the particles have penetrated at least 200 m into the porous substructure. In one embodiment at least 65 wt % of the catalytically active TiO.sub.2 is in the porous substructure. In one embodiment the substrate is porous and there is a porous substructure below the surface and wherein at least a part of the particles have penetrated at least 300 m into the porous substructure.

(32) In one embodiment the average size of the particles is in the range 5-50 nm. For a spherical particle the size is defined as the diameter. For a non-spherical particle the size is the largest size in any direction. Average size of the particles is intended to mean the average of all particles. In an alternative embodiment the average size of the particles is in the range 15-30 nm.

(33) In one embodiment at least a part of the particles comprise at least one core and a surrounding layer, and wherein the at least one core comprises at least 95 wt % anatase. In an alternative embodiment the core comprises at least 50 wt % anatase.

(34) In one embodiment the removal of nitric oxide as measured according to ISO 22197-1:2007 is at least 60 wt %. In an alternative embodiment the removal of nitric oxide as measured according to ISO 22197-1:2007 is at least 40 wt %. removal of nitric oxide as measured according to ISO 22197-1:2007 is at least 50 wt %. removal of nitric oxide as measured according to ISO 22197-1:2007 is at least 70 wt %.

(35) In a second aspect there is provided a method of manufacturing a coated substrate, said method comprising the steps of: c) preparing a dispersion comprising particles in the size range 2-500 nm, said particles comprising TiO.sub.2, wherein at least 50 wt % of the Ti in the particles is leachable in 37% HCl, d) contacting the dispersion with the substrate to be coated.

(36) In one embodiment the dispersion to be contacted with the substrate comprises at least one selected from the group consisting of monoethanolamine, diethanolamine, and triethanolamine.

(37) In one embodiment the amount of TiO.sub.2 is from 0.5 to 200 g/m.sup.2. In an alternative embodiment the amount of TiO.sub.2 is lower than 30 g/m.sup.2. In another alternative embodiment the amount of TiO.sub.2 is lower than 15 g/m.sup.2. In another alternative embodiment the amount of TiO.sub.2 is lower than 50 g/m.sup.2. The amounts include TiO.sub.2 in any pores of the substrate. TiO.sub.2 in a porous substructure, if present is included in these amounts. The porous substructure is about the outer 300 m of an ordinary cementitous substrate.

(38) In one embodiment a precursor dispersion is contacted with the substrate to be coated prior to contacting the dispersion with the substrate to be coated.

(39) In one embodiment the substrate is dried by heating to a temperature not exceeding 250 C. In one embodiment the substrate is dried by heating to a temperature not exceeding 200 C. If the temperature is increased to a value of about 300-400 C. or more the advantageous high fraction (50 wt % or more) of Ti leachable in 37% HCl is lost.

(40) In one embodiment the aqueous dispersion has the ability to form a gel with increased viscosity upon drying and also at other conditions. The viscosity is increased compared to the aqueous dispersion before gelation. The fact that at least 50 wt % of the Ti in the particles is leachable in 37% HCl gives that the particles form a gel like phase upon standing, drying and/or heating. The gelation is reversible and the gel can quickly and easily be turned into a liquid with dispersed particles by shaking. The gelation has the advantage that the liquid can be shaken and applied to the substrate as a liquid where the liquid penetrates the pores of the material and wets the surfaces including the surfaces of the pores. When the liquid has entered the pores and starts to dry, the liquid forms a gel and gelates in the pores, subsequently it dries. The gelation during the drying after penetration of the pores gives a better particle coating.

(41) Other features and uses of the invention and their associated advantages will be evident to a person skilled in the art upon reading the description and the examples.

(42) It is to be understood that this invention is not limited to the particular embodiments shown here. The embodiments are provided for illustrative purposes and are not intended to limit the scope of the invention since the scope of the present invention is limited only by the appended claims and equivalents thereof.

EXAMPLES

(43) All percentages are calculated by weight if not clearly stated otherwise.

(44) Producing an Acidic Titania

(45) 27.1 kg of TiOCl.sub.2 was diluted with 13.8 kg deionized water and left overnight. The diluted TiOCl.sub.2 solution was neutralized with 77.2 kg of aqueous NaOH and 16.2 kg of (Na).sub.2CO.sub.3 in a stirred reactor to yield a final solution pH below 5. To this was added a solution of 63.7 kg TiOCl.sub.2 and 1.9 kg surfactant yielding a transparent solution with a temperature of 40.4 C. To this clear solution, 1.9 kg of acetylacetone was added followed by heating for 35 minutes until the temperature was 81.9 C. The temperature was held constant for a further 75 minutes, followed by cooling from 81.7 C. to 50.9 C. The resulting 198.8 kg nanotitania dispersion yielded 196.6 kg of reaction product. The product was filtered through a nanofiltration unit combining washing and concentration steps to obtain a final pH of 1.1 and approximately 20% weight concentration of TiO.sub.2. These particles were utilized for the following examples unless otherwise is clearly stated.

Example 1

Preparation of Functionalized Surface from High Stability, pH-matched TiO2 Dispersion

(46) An aqueous titanium dioxide dispersion was prepared comprising 15 wt % titania as 3 wt % triethanolamine and 3 wt % monoethanolamine. The pH was further adjusted to pH=10 by addition of potassium hydroxide. The dispersion was roll coated or sprayed onto grey and red brick surfaces and dried in ambient conditions for several days. After drying, the brick surface appeared unchanged with no visible change in color and no speckles. Under UV-vis lamp, the functionalized surfaces displayed high degradation rates for Rhodamine-B on the surface.

Example 2

Investigation of Adhesion

(47) Two brick samples were coated with a titanium dioxide composition comprising 15 wt % titania as nanoparticles, 3 wt % triethanolamine and 3 wt % monoethanolamine. The pH was further adjusted to pH=10 by addition of potassium hydroxide. After overnight drying at room temperature, the first sample was washed with water by placing it upside-down in a vigorously stirred water bath overnight. The presence of TiO.sub.2 nanoparticles were clearly detected in the water upon addition of 1:1 ratio of 37% hydrochloric acid.

(48) The second sample was further cured at ambient conditions for 5 days before undergoing the same washing cycle. In the case of the second sample, no TiO.sub.2 was detected in the water after the test. The results clearly demonstrate that several days curing embedded the titania nanoparticles into the brick without being washed away.

Example 3a

Investigation of NO Removal Rate

(49) Five samples of coated surfaces were sent for analysis of air purification performance according to ISO 22197-1:2007, using NO as the model pollutant. Procedures and equipment used were identical to that described in Hsken et al (2007) and Yu and Brouwers (2009). All samples were tested at 10.01 ppm NO inlet concentration, 50% relative humidity, 19.80.1 C., 3.0 l/min flow rate and 10.0 W/m.sup.2 irradiance. Samples VI-I/II and VII-I/II were roll-coated samples according to the present invention, whereas sample IV was produced by mixing precursor dispersion into the cement mix before making the substrate.

(50) TABLE-US-00001 Results show very high NO and total NOx removal rates for the samples according to the present invention. Sample NOcon(wt %) NO2, gen(wt %) NOx, con(wt %) IV 1.7 0.0 1.7 VI-I 78.6 28.2 50.4 VI-II 63.0 29.5 33.5 VII-I 74.0 25.5 48.5 VII-II 75.3 19.0 56.3

Example 3b

Investigation of NO Removal Rate

(51) Protoype samples for photocatalytic paving stones were sent for analysis of air purification performance according to ISO 22197-1:2007, using NO as the model pollutant. Procedures and equipment used were identical to that described in Hsken et al (2007) and Yu and Brouwers (2009). All samples were tested at 10.01 ppm NO inlet concentration, 50% relative humidity, 19.80.1 C., 3.0 l/min flow rate and 10.0 W/m.sup.2 irradiance. All samples contained Joma nanoparticles and were spray-coated and cured according to the present invention. Note that the amount of material sprayed varied, so that the 5% samples contained more than 5 times the TiO.sub.2 of the1% dispersion sample.

(52) TABLE-US-00002 The results show record high numbers for the largest amount if TiO.sub.2 used, but better performance per gram TiO.sub.2 for the 5% samples. Using too low amounts of TiO.sub.2 gives very poor results, which is explained by essentially all the nanoparticles being sucked into the deepest pores where performance is poor. The effect of adding an IR lamp is inconclusive. Sample NOcon(%) NOx, con(%) 1% dispersion 4.3 4.0 5% with IR 76.3 35.0 5% without IR 75.0 39.5 15% dispersion 87.6 63.1

Example 4

Investigation of Amorphous Content of Nanoparticles

(53) Various samples containing TiO.sub.2 nanoparticles were dissolved in concentrated HCl in an amount of ca 10 g HCl 37% per gram of TiO2-containing product, enough to leach any amorphous TiO.sub.2 present in the product.

(54) Two different Joma acidic TiO.sub.2 dispersions were treated with nano filtration, cationic and anionic exchangers until a pH=6.2 was reached. Excess amounts of 37% HCl was added to the samples. The vials were shaken at 55 C. for 17 hours.

(55) Two samples of the second Joma dispersion, after treatment to pH=6.2, were dried at 60 C. and sintered at different temperatures. The sintered samples were added to excess amounts of 37% HCl. The vial was shaken at 55 C. for 17 hours.

(56) A third Joma dispersion, after treatment to pH=6.2, was dried at 100 C. and sintered at 200 C. The samples were added to excess amounts of 37% HCl. The vials were shaken at 55 C. for 18 hours and the solid fraction weighed after washing and air drying for 96 hours followed by heating at 50 C. for 1 hour.

(57) Two different Cristal Millennium product samples (dispersions) were mixed with excess amounts of 37% HCl. The vials were shaken at 55 C. for 6-40 hours.

(58) Evonik Degussa P25 powder was added to excess amounts of 37% HCl. The vial was shaken at 55 C. for 24 hours.

(59) TABLE-US-00003 In all the cases the resulting almost clear samples were analysed for Ti content Unleach- Initial Leaching Clear solution ed/initial Sample TiOwt % time Ti[aq]wt % TiO2 Joma sample A 0.68 17 h 0.30 0.25 Joma sample B 1.02 17 h 0.44 0.27 Joma sample 2.44 17 h 0.72 0.71 B sintered at 300 C. for 90 min Joma sample 2.19 17 h 0.24 0.89 B sintered at 500 C. for 30 min Joma sample C 7.75 18 h 0.54 Joma sample 7.75 18 h 0.64 C sintered at 200 C. for 18 h Cristal 4.09 6 h 0.386 0.84 Millennium s5300A Cristal 2.95 24 h 0.616 0.64 Millennium s5300A Cristal 40 h 0.637 0.63 Millennium s5300A Cristal 4.45 18 h 0.69 0.73 Milennium PC500 Degussa P25 17.5 24 h 0.27 0.974 Joma sample A 0.68 17 h 0.30 0.25 Joma sample B 1.02 17 h 0.44 0.27 Joma sample 2.44 17 h 0.72 0.71 B sintered at 300 C. for 90 min Joma sample 2.19 17 h 0.24 0.89 B sintered at 500 C. for 30 min Cristal 4.09 6 h 0.386 0.84 Millennium s5300A Cristal 2.95 24 h 0.616 0.64 Millennium s5300A Cristal 40 h 0.637 0.63 Millennium s5300A Cristal 4.45 18 h 0.69 0.73 Milennium PC500 Degussa P25 17.5 24 h 0.27 0.974

(60) The results support the following three claims: First, that leaching times of 17 hrs are sufficiently long to obtain reliable data for comparison. Second, that the amount of crystalline TiO.sub.2 depends on the method of producing TiO.sub.2 and can be controlled by thermal treatment: Evonik Degussa process is known to have a calcination step that ensures highly crystalline particles, while Cristal and Joma use wet chemical processes. Thirdly, that the Joma TiO.sub.2 nanoparticles show a significant difference in structure from the other tested TiO.sub.2 nanoparticles.

Example 5

Investigation of Industrial Applicability

(61) A spray unit and an IR heating unit were added to a commercial production line for paving stones. The precursor dispersion was applied to the stones via the spraying unit. Production tests were run with and without IR-treating the stones after applying the precursor dispersion. It was found that the added steps to create the functionalized surface did not affect the production rate, scalability or safety of the production line. The time needed to coat and IR treat a paving stone was a few seconds.

Example 6

Investigation of Pore Penetration

(62) A paving stone sample according to the invention was fractured with a hammer and the cross section analysed by SEM. As seen in FIG. 1, the TiO.sub.2 dispersion was observed to have infiltrated, wet and set in small and large pores at least 300 micrometer below the surface. The wetting was evidenced by thin 1-2 micrometer films on the inner pores. Analysis of the coverage indicated around 2.75 g/m.sup.2 on the surface and around 8 g/m.sup.2 in the substructure.