The invention relates to colloidal suspensions of titanium dioxide and a corresponding method for obtaining them. The invention further relates to methods for eliminating polluting substances and for eliminating microorganisms near a surface.

As is known, the titanium dioxide possesses optimal photocatalytic properties with regard to a multiplicity of organic and inorganic pollutants. Owing to these properties, titanium dioxide-based colloidal suspensions have already been studied and implemented, being for example used in water treatment plants or in apparatuses for purifying air of polluting agents. Titanium dioxide has, in fact, great oxidating power when it is subjected to ultraviolet light radiation.

Titanium dioxide-based solutions are also known that can be activated when irradiated by radiation having a wavelength comprised in the range comprising visible light and ultraviolet light.

Furthermore, it is known that objects covered with titanium dioxide can absorb bad odours from the environment into which they are introduced and are self-cleaning. Nevertheless, the colloidal suspensions of known type are obtained by means of reactions of titanium compounds subjected to numerous chemical processes, that require the use of very complex systems, with consequently very high costs . Furthermore, the known titanium dioxide-based colloidal suspensions are rather instable, because the titanium dioxide tends to precipitate and separate from a liquid phase of the suspension, in particular when ionic substances such as inorganic or organic salts are added. The period of stability of the known colloidal suspensions is not greater than a year.

This entails problems of transporting and in particular storing the colloidal suspension, which cannot be stored for long periods .

An object of the invention is to improve titanium dioxide- based colloidal suspensions and the corresponding known production methods .

Another object of the invention is to provide a titanium dioxide-based colloidal suspension that is simple and relatively low-cost. Still another object is to obtain a titanium dioxide-based colloidal suspension that enables the problems of precipitation of the titanium dioxide during storage to be solved.

Another object of the invention is to provide a titanium dioxide-based colloidal suspension that is easy to apply and ready for use .

Still another object is to obtain a titanium dixoide-based colloidal suspension having good adhesive properties with respect to a support to which it is applied. A further object is to obtain a method for preparing a titanium dioxide-based colloidal suspension that does not require complex systems .

In a first aspect of the invention there is provided a colloidal suspension comprising titanium dioxide of about 0.5% w/v to approximately 5% w/v, a surface-active agent from approximately 0.1% w/v to approximately 5% w/v and water .

In a second aspect of the invention there is provided a paint comprising a colloidal suspension containing titanium dioxide from approximately 0.5% w/v to approximately 5% w/v, a surface-active agent from approximately 0.1% w/v to approximately 5% w/v and water.

Owing to the first and second aspect of the invention it is possible to obtain a colloidal suspension of titanium dioxide and a paint comprising this colloidal suspension in a simple manner and at relatively low costs .

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In a third aspect of the invention there is provided a method for eliminating polluting substances near a surface comprising preparing a colloidal suspension of titanium dioxide and depositing said colloidal suspension on said surface.

In a fourth aspect of the invention there is provided a method for eliminating microorganisms near a surface comprising preparing a colloidal suspension of titanium dioxide and depositing said colloidal suspension on said surface .

In an embodiment of the methods according to the third and fourth aspect, preparing the colloidal suspension comprises mixing a solid part comprising titanium dioxide with a liquid part comprising a surface-active agent and water. Owing to the above methods it is possible to prepare a titanium dixoide-based colloidal suspension in a rapid manner and in sufficient quantity to coat the surface that it is desired to treat without it being necessary to store the colloidal suspension, thus avoiding the risk that the colloidal suspension disintegrates. In fact, it is possible to mix the solid part and the liquid part only at the moment in which the colloidal suspension actually has to be used. In a fifth aspect of the invention there is provided the use of a colloidal suspension of titanium dioxide, comprising titanium dioxide from approximately 0.5% w/v to approximately 5% w/v, ^' a surface-active agent from approximately 0.5% w/v to approximately 5% w/v and water, for eliminating polluting agents. In a sixth aspect of the invention there is provided the use of a colloidal suspension of titanium dioxide, comprising titanium dioxide from approximately 0.5% w/v to approximately 5% w/v, a surface-active agent from approximately 0.1% w/v to approximately 5% w/v and water, for eliminating microorganisms . Owing to the fifth and sixth aspect of the invention, it is possible to eliminate effectively polluting substances

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present in the air near the surface, such as, for example, NO _x , and to eliminate bacteria, or fungi from crops or moulds from walls of buildings.

The invention can be better understood and implemented with reference to the attached figures that illustrate some embodiments thereof by way of non-limitative example, in which:

Table 1 is a table showing the number of various bacterial colonies that survive after treatment of colloidal suspensions with increasing concentrations of titanium dioxides;

Figures 1 to 3 are frontal views of bacterial cultures of Erwinia Amylovora treated with colloidal suspensions according to the concentrations of titanium dioxide shown in Table 1;

Figures 4 to 6 are frontal views of bacteria cultures of Staphylococcus treated with colloidal suspensions according to the concentrations of titanium dioxide shown in Table 1; Figure 7 is a schematic view of an apparatus for detecting the percentage of pollutants, in particular NO _x , in a mixture of air that has been in contact with a portion of asphalt treated with a colloidal suspension of titanium dioxide according to the invention; Figure 8 is a detail of the apparatus in Figure 7; Figures 9 and 10 are graphs illustrating the percentage of NO _x and of NO removed over time from a mixture of air in contact with a first sample of asphalt coated by a first quantity of colloidal suspension of titanium dioxide, both in illuminated conditions of the sample and in non- illuminated conditions;

Figures 11 and 12 are graphs illustrating the percentage of NO _x and of NO removed over time from a mixture of air in contact with a second sample of asphalt coated by a second quantity of colloidal suspension of titanium dioxide, both in illuminated conditions' of the sample and in non- illuminated conditions;

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Figures 13 and 14 are graphs illustrating the percentage of NO _x and of NO removed over time from a mixture of air in contact with a third reference sample of asphalt not coated by the colloidal suspension of titanium dioxide, both in illuminated conditions of the sample and in non-illuminated conditions .

The titanium dioxide is a semiconductor material with a crystalline structure, having a valence band separated from a conduction band by a given energy difference. Like most materials, when titanium dioxide is hit by electromagnetic radiation it absorbs energy from the radiation. When the absorbed energy is greater than or the same as the energy difference between the valence band and the conduction band, an electron is caused to pass from the valence band to the conduction band, generating an excess electronic charge (e ^" ) in the conduction band and an electron hole (h ⁺ ) in the valence band.

Solid-state titanium dioxide in crystalline form like anatase, rutile, or brookite. Anatase is the most active crystalline form from the photocatalytic point of view and has an energy difference between the valence band and the conduction band of 3.2 eV. As a result, if this material is irradiated with photons having energy greater than or the same as 3.2 eV, i.e. with an electromagnetic radiation with a wavelength the same as or less than 390 nra, an electron is caused to pass from the valence band to the conduction band-. This occurs in particular when the titanium dioxide is hit by ultraviolet (UV) radiation, for example emitted by an ultraviolet ray lamp, or by solar radiation. The electronic holes can oxidate most organic contaminants. Such electronic holes can, for example, react with a molecule of water (H ₂ O) generating a hydroxyl radical ( ^" OH) that is highly reactive. The excess electrons have very great reducing power and can react with the molecule of the oxygen to form the superoxide anion (O ₂ ^*" ) . The oxidation reaction of the water molecule is

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shown in the formula (i) and the reduction reaction of the oxygen is shown in the formula (ii) : TiO ₂ (h ⁺ ) + H ₂ O ^■ * TiO ₂ + ^' OH + H ⁺ ; (i)

TiO ₂ (e ^~ ) + O ₂ ^• * ^• TiO ₂ + O ₂ ^"" (ii) The hydroxyl radical ( ^" OH) is particularly active both for the oxidation of organic and inorganic substances, for example present in the air, both for deactivating microorganisms, that may, for example be harmful to cultivated plants and people. In particular, the organic compounds are oxidized to carbon dioxide (CO ₂ ) and water (H ₂ O) , the nitrogen compounds are oxidized to nitrate ions (NO ₃ ^" ) , the sulphur compounds are oxidized to sulphate ions (SO ₄ ^2" ) . The titanium dioxide furthermore has an anti—microbial, anti—bacterial and anti-mould action that is very effective. The titanium dioxide is furthermore able to decompose many gases or harmful substances such as thiols or mercaptans, formaldehyde, having an unpleasant smell. The decomposition of such gases or substances eliminates the bad smells associated therewith.

The titanium dioxide-based colloidal suspension according to the invention was developed to be distributed in the form of film on solid surfaces such as for example roads, external walls or internal walls of buildings and dwellings or plants or crops .

The titanium dixoide-based colloidal suspension disclosed below is suitable for being distributed by means of spray techniques or by means of techniques that provide for spreading a film of colloidal suspension on the surfaces to be treated at ambient temperature or at higher temperatures . In particular, the colloidal suspension remains substantially unaltered in a temperature range comprised between approximately 10 ⁰ C and approximately 120 ⁰ C. This temperature range enables the colloidal suspension to be distributed, for example onto the asphalt that has just been cast, having a temperature above ambient temperature.

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Furthermore, in the case of use with a spray technique, the heating due to pressure and friction of the colloidal suspension against walls of a nebulizer apparatus does not entail any alteration of the colloidal suspension. In irradiation conditions with sunlight and ultraviolet light

(UV) , the colloidal suspension distributed on the surface is able to eliminate very effectively polluting agents in the atmospheric air, such as, for example, nitrogen oxides (NO _x ) , sulphur oxides (SO _x ) , or volatile organic substances, such as benzene (C ₆ H ₆ ) .

Furthermore, the colloidal suspension is able to denature bacterial strains or fungi that are particularly harmful for agriculture . The colloidal suspension is obtained by mixing a solid part and a liquid part.

The solid part comprises titanium dioxide between an approximately 0.5% w/v and an approximately 5% w/v suspension. The titanium dioxide used comprising approximately 80% anatase and approximately 20% rutile, with a density from approximately 3.6 g/cm ³ to approximately 3.9 g/cm ³ and a surface area from approximately 52 m ² /g to approximately 56 m ² /g. The titanium dioxide furthermore has an average dimension of particles from approximately 25 nm to approximately 30 nm and a granulometric distribution that may vary in the range from approximately 5 nm to approximately 50 nm.

The titanium dioxide of the solid part may be of the type commercially known as Degussa P 25. The liquid part of the colloidal suspension comprises distilled water having electric conductibility not greater than 1.5 μS and pH comprised in the range between approximately pH 5 and approximately pH 7, and approximately 0.1% w/v to approximately 5% w/v of a surface-active agent. The surface-active agent, also known as a surfactant, is a glycol or a polyethylene glycol or a polyethylene glycol- phenyl-ether or a polyethylene-glycol-ether or a

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polyoxyethylene-stearyl-ether or a polyethylene-glycol- hexadecyl-ether or a polyethylene-glycol-octadecyl-ether or a polyethylene-glycol-dodecyl-ether . In particular, the surface-active agent can be chosen from a group of non-ionic surfactants comprising: Triton X-45, Triton X-IOO, Triton X-114, Triton X-165, Triton X-305, Triton X-405, Triton X-705-70, Triton CFlO, Brij 30, Brij 35 P, Brij 52, Brij 56, Brij 58 P, Brij 72, Brij 76, Brij 78 P, Brij 92V, Brij 96 V. The surfactants act as stabilisers of the suspension of nanoparticles of titanium dioxide and enable the colloidal suspension to be distributed evenly on any support. An example of the composition of the colloidal suspension comprises a liquid part obtained by dissolving 2 ml of surfactant, for example Triton X 100, in 1 1 of distilled water having electric conductibility that is not greater than 1.5 μS and pH comprised in the range between approximately pH 5 and approximately pH 7 , and a solid part comprising 30 g of TiO ₂ Degussa P 25, that is mixed with the liquid part. The fluid mixture that is thus prepared is stirred for a time comprised between approximately 2 and approximately 5 minutes so as to form a colloidal suspension that is ready for use. The colloidal suspension, which has a paintlike consistency, can, in fact, be distributed immediately by means of spraying onto the desired surface. Owing to the simple formulation of the colloidal suspension, it is possible to prepare the suspension in a short time and with facility. It is therefore not necessary to store the colloidal suspension before it is used. As the liquid part and the solid part do not undergo alterations of the physical state thereof or of the chemical composition thereof over time, it is possible to store and keep separate the solid part and the liquid part until it is desired to apply the final product. This brings a great advantage over prior-art

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colloidal suspensions in titanium dioxide, which tend to give rise to the formation of a precipitate over time. Triton X 100 is the surfactant that is particularly indicated if the colloidal suspension has to be used in agriculture. In fact, it is known that such a surfactant does not cause undesired effects in the human organism. In the building trade, the colloidal suspension can furthermore be used as a component of a paint, which thus acquires the anti-mould and anti-bacterial properties of the suspension.

In an embodiment, the liquid part of the colloidal suspension may contain ionic substances or other additives suitable for enhancing the antibacterial properties and/or accentuating the adhesion of the colloidal suspension to the surface to be treated.

For example, .the liquid part can be obtained by adding to the water and to the surfactant one or more of the substances indicated below:

- colloidal silicon (SiO ₂ ) having particles of dimensions comprised in the range from approximately 4 nm to approximately 30 nm and in a concentration comprised in the range from approximately 0.5% w/v to approximately 1.4% w/v, in particular 0.8% w/v;

- sodium hydroxide (NaOH) in a concentration comprised in the range from approximately 0.01% w/v to approximately

0.08% w/v, in particular 0.04% w/v;

-. lithium oxide (LiO ₂ ) in a concentration comprised in the range from approximately 0.1% w/v to approximately 0.16% w/v, in particular 0.12% w/v; - silver acetate (CH ₃ COOAg) in a concentration comprised in the range from approximately 0.001% w/v to approximately 0.008% w/v, in particular 0.004% w/v;

- sodium sulphite (Na ₂ SO ₃ ) in a concentration comprised in the range from approximately 0.01% w/v to approximately 0.04% w/v, in particular 0.012% w/v;

- sodium thiosulphate(Na ₂ S ₂ 0 ₃ ) in a concentration from

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approximately 0.01% w/v to approximately 0.06% w/v, in particular 0.025% w/v.

The colloidal silicon increases the degree of adhesion of the colloidal suspension; the silver acetate, the sodium sulphite and the sodium thiosulphate accentuate inactivation of the bacteria of the colloidal suspension.

Also in this embodiment, the liquid part obtained is stable over time, can therefore be stored separately from the solid part and be mixed with the latter before use . The embodiment disclosed above is particularly suitable for being used in a building environment so as to prevent the formation of fungi or to eliminate possible bacteria, for example on walls of buildings. In another embodiment, applying to the surface a first film formed by a solution, also said primer, arranged to improve the adhesion of a second film of colloidal solution of titanium dioxide distributed on the first film is provided. In a first version, the primer is a peroxytitanic acid solution. This solution is obtained by arranging 150 ml of titanium chloride (TiCl ₄ ) in 20% hydrochloric acid (HCl) in a beaker with a l l volume. To this solution 826 ml of ammonia hydroxide (NH ₄ OH) are added, diluted 1:9 with distilled water. The pH of the solution is neutral (pH =7) and the titanic acid (Ti(OH) ₄ ) is precipitated. This precipitate has the consistency of a gel that is white in colour. The precipitate is gathered on a filter, with G3 porosity, and washed with approximately from 750 ml to 1000 ml of distilled water until the chlorides are eliminated, as can be checked by treating the filtered liquid with silver nitrate (AgNO ₃ ) . The presence of chlorides in the filtered liquid is shown by the precipitation of the cheese-white silver chloride (AgCl) . The titanic acid (Ti(OH) ₄ ) is collected and suspended in 200 ml of distilled water with conductivity that is no greater than 1.5 μS and with pH comprised in the range between approximately pH 5 and approximately pH 7 and slowly, in a period of time between approximately 20 min and approximately

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30 min, 92 ml of 30% hydrogen peroxide (H ₂ O ₂ ) is added. The dissolution of the precipitate and the formation of a solution of a yellow colour are noted, containing peroxytitanic acid with a general formula [Ti ₂ (0) ₅ (OH) _x ] ^(x"2)~ , in which the value x may vary from 3 to 6 and so the coordination number of the hydroxyl group (OH) cannot be specified.

The solution is then heated for 1 hour at aproximately 70 ⁰ C in order to decompose the excess of hydrogen peroxide (H ₂ O ₂ ) . The spray distribution of the peroxytitanic acid solution onto a solid support, followed by the spray deposition of a colloidal suspension of titanium dioxide according to the embodiments disclosed above enables the degree of adhesion of the film of colloidal solution to the support to be increased and exhalts the photocatalytic properties thereof.

An application example provides for a solid surface, for example asphalt, cement or a ceramic support, being coated by means of spraying technique with a quantity varying from approximately 30 g/m ² to approximately 50 g/m ² of peroxytitanic acid solution at a temperature of between approximately 60 ⁰ C and approximately 80 ⁰ C, and subsequently with a quantity from approximately 50 g/m ² to approximately 110 g/m ² of colloidal solution according to any of the embodiments disclosed above. The peroxytitanic acid solution can also be sprayed substantially simultaneously to the colloidal suspension of titanium dioxide.

In a second version, the primer is a colloidal silicon solution. This solution contains water and colloidal silicon (Siθ2) having particles of dimensions comprised in the range from approximately 4 nm to approximately 30 nm and in a concentration comprised in the range from approximately 4% w/v to approximately 8% w/v, in particular 7% w/v. The colloidal silicon solution may furthermore comprise one or more of the following substances:

- Dodecyltrimethylammonium bromide, in a concentration

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from approximately 0.5% w/v to approximately 2% w/v, in particular 0.9% w/v;

- Dodecyltrimethylammonium chloride, in a concentration from approximately 0.5% w/v to approximately 2% w/v, in particular 0.9% w/v; ^■

- Hydroxyethyl methacrylate in a concentration comprised in the range from approximately 1% w/v to approximately 2% w/v, in particular 1.5% w/v;

- Sodium hydroxide, in a concentration from approximately 0.05% w/v to approximately 0.2% w/v, in particular 0.09% w/v.

Also the second version of primer enables the titanium dioxide to be anchored better to the support on which it is applied. Experiments conducted on various bacterial strains, according to experimental procedures disclosed below, have shown great antibacterial activity of the colloidal suspensions of titanium dioxide according to the invention. Tests have been conducted on the growth of microorganisms in the presence and absence of a colloidal suspension of titanium dioxide obtained by mixing a solid part comprising Degussa P 25, and a liquid part comprising distilled water, having electric conductivity that is not greater than 1.5 μS and pH comprised in the range between approximately pH 5 and approximately pH 7, and 2 ml of Triton X 100. The tests on the growth of the microorganisms were conducted on cultures sowed on plates and considering the action of three colloidal suspensions containing various concentrations of titanium dioxide, namely a first colloidal suspension containing approximately 0.025% w/v, a second colloidal suspension containing approximately 0.125% w/v and a third colloidal suspension containing approximately 0.250% w/v of titanium dioxide. The microorganisms analysed are Staphylococci, Escherichia CoIi, Pseudomonas, Erwinia Amylovora and Candida Albicans, and were cultivated in illuminated liquid terrains for 12

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hours at a temperature of 37 ⁰ C, except for Erwinia Amylovora, which has an optimal reproduction temperature from between approximately 26 ⁰ C to approximately 27 ⁰ C. The tests were conducted by applying to each culture the three colloidal suspensions having titanium dioxide in different concentrations and comparing the development of the microorganisms with a control plate containing only the bacterial culture . As the titanium dioxide is activated and becomes effective through excitation with visible UV light, the cultures were illuminated for an average period of 12 hours both with a lamp that simulates the solar spectrum and in effective solar radiation conditions for a time equal to 12 hours . In the laboratory experiments the light source was placed at a distance comprised between approximately 60 cm and 70 cm from the plates in which the microorganisms were cultivated, filtering the infrared radiation acting thereupon with water filters. At the end of growth in a liquid terrain, from each plate liquid constant quantities were taken that were sowed in an agar terrain to determine the number of live bacteria present there .

The sowed plates were placed at 37 ⁰ C for 24 hours and the bacterial colonies that had developed were then counted. The obtained results are summarised in Table 1.

From an examination of the results it is clear that the colloidal suspension has great antibacterial efficacy that varies according to the microrganism considered. With the first colloidal suspension having a concentration of titanium dioxide of 0.025% w/v, Escherichia CoIi mortality is total whereas concentrations of the of the second colloidal suspension, namely of 0.125% w/v, are necessary for eliminating Erwinia Amylovora, Staphylococcus, Pseudomonas and Candida. In Figures 1 to 3 images of the culture plates of Erwinia Amylovora are shown after treatment with resepctively the

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third, first and second colloidal suspension. In Figure 1, a plate is shown corresponding to the control plate in which a virtually unlimited number of colonies is evident, whilst in Figure 3 , which shows a plate corresponding to the treatement with the second colloidal suspension, no bacterial colony is present .

Similarly, Figures 4 to 6 show culture plates of Staphylococcus, in which the plate in Figure 4 is the control plate, i.e. has not been treated with the colloidal suspension, and has a virtually unlimited number of colonies, the plate in Figure 5 was treated with the first colloidal suspension and has a limited number of colonies, the plate in Figure 6 was treated with the second colloidal suspension and does not have any colony. In addition to the tests that show the efficacy of the colloidal suspensions according to the invention on the inactivity of microorganisms, measurements were conducted of the elimination of polluting substances, such as nitrogen oxides (NO _x ) , from air in contact with a reference surface of road asphalt .

The colloidal suspension of titan dioxide used for the tests comprised in the solid part 30 g of Degussa P 25 and in the liquid part 1 1 of distilled water, having electric conductivity not greater than 1.5 μS and pH comprised in the range between approximately pH 5 and approximately pH 7, and 2 ml of Triton X 100.

Two samples of asphalt were prepared, indicated as I and II, sample I of which was covered with 65 g/m ² of solution whereas sample II was covered with 115 g/m ² of solution. The respective surfaces of samples I and II thus have a different concentration of titanium dioxide on the respective surface. Furthermore, a further, non-treated, sample III used as a reference sample was provided. The measurements were conducted by using an experimental apparatus, shown in Figure 7 and comprising an inlet E for introducing into the apparatus a mixture of NO _x in air, a

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mixing chamber A having a volume of 25 1, a reaction chamber B, in which the titanium dioxide is activated, an analyser C for measuring the concentration of the nitrogen oxides (NO and NO ₂ ) in the mixture and a recirculation pump P. A main line 1 connects the mixing chamber A to the reaction chamber B and the latter to the recirculation pump P. On the main line 1 upstream and downstream of the reaction chamber B valves V are provided that enable air to be taken from the main line 1 to take it to the analyser C through respective conduits 2 and 3 , so as to be able to measure the concentration of nitrogen oxide both before and after treatment of the air in the reaction chamber B. Lastly, a further conduit 4 connects the recirculation pump P to the mixing chamber A. The main line 1, the conduits 2 and 3 and the further conduit 4 are all of a material that does not alter the concentration of the nitrogen oxides, in particular polytetrafluoroethylene . The reaction chamber B comprises a Pyrex chamber having a volume of 3 1, inside which the sample of asphalt G is arranged to be examined on a support D, which is also of Pyrex, as shown in Figure 8. The reaction chamber B is provided with a first portion Bl in which the sample of asphalt G is housed and a second portion B2 arranged for closing and/or opening the reaction chamber B. The first portion Bl and the second portion B2 are kept clamped to one another by a flange G that extends outside the two portions Bl, B2. A seal H of the 0-ring type sealingly closes the two portions Bl, B2 with respect to an external environment. To measure the elimination of the NO _x gases, the concentration of the nitrogen oxides in the mixture of NO _x and air is monitored in function of time, in recycling conditions of the mixture through the reaction chamber B containing the sample. The measurement of the concentration of the initial NO _x gases and the measurement of the NO _x gases at different

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irradiation times were conducted using an analytic methodology based on chemiluminescence, illustrated in standard UNI 10878.

Once air has been introduced into the inlet E, it reaches the mixing chamber A and continues on the main line 1. An initial sample of air through the conduit 2 enables an initial concentration of the NO _x gases to be measured, which is of the order of 0.6 parts per million (PPM) . The air then takes up the main line 1 and passes through the reaction chamber B with a flow that varies around 5 1/min, between approximately 4.5 1/min and approximately 5.5 1/min. At preset intervals of time an air flow is sampled through the conduit 3 of Figure 7. The sample F is illuminated with a lamp, for example of the Vitalux type produced by the company Osram and is kept at the temperature of approximately 27 ⁰ C + 2 ⁰ C, by means of external ventilation.

The illuminated surface of the sample in question is 80 cm ² ± 20 cm ² . The measurements of the concentration of nitrogen oxides are conducted both in the presence of and in the absence of light in order to be able to measure the contribution of an adsorption effect of the surface of the sample F and through the difference the effective photocatalytic contribution due to the TiO ₂ deposit.

The results of the photocatalytic action of samples I and II in eliminating the nitrogen oxides are shown in Figures 9 to 12, that illustrate the deterioration of the nitrogen oxides in the presence of and in the absence of light. The removal of the nitrogen oxides (NO _x ) in the presence of light occurs in extremely short times .

The sample I enables the nitrogen oxides to be removed completely in a time of the order of approximately 50 min. The sample II enables 100% of the nitrogen oxide to be removed in a time of the order of 30 min with a reduction of over 80% in 15 minutes.

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From the results of the experiments conducted on the reference sample III, shown in Figures 13 and 14, whilst the nitrogen oxide (NO) is not adsorbed by the asphalt, the nitrogen dioxide (NO ₂ ) , which mixed with NO causes the composition of the NO _x gases, is partially adsorbed also by the non-treated asphalt.

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