IT201800007578A1 - PHOTOCATALYTIC MATERIAL INCLUDING A SILICEOUS SUBSTRATE SUPERFICALLY TREATED WITH TITANIUM DIOXIDE PARTICLES, AND ITS USE FOR THE REMOVAL OF POLLUTING PRODUCTS. - Google Patents

Description

PHOTOCATALYTIC MATERIAL INCLUDING A SILICEOUS SUBSTRATE SURFACE TREATED WITH TITANIUM DIOXIDE PARTICLES, AND ITS USE FOR THE REMOVAL OF POLLUTING PRODUCTS.

The present invention relates to a photocatalytic material comprising a siliceous substrate surface treated with particles of titanium dioxide, and relative use for the removal of polluting products, in particular from water or air.

Titanium dioxide is a product widely used in many industrial fields, thanks to its stability, high refractive index, non-toxicity and low costs. Furthermore, it is known that titanium dioxide has semiconductor properties and, when superficially activated with radiation of a suitable wavelength (generally UVA, UVB or UVC radiation), it generates, both superficially and by diffusion of radical cascades, a oxidizing potential capable of attacking and decomposing a wide range of pollutants, both organic and inorganic, which can be present in the air or in liquid media, such as water for civil or industrial use.

The realization of devices suitable for the removal of pollutants according to the aforementioned photocatalytic mechanism requires the availability of titanium dioxide in the form of particles with a high surface area and in a crystalline form which is active from the photocatalytic point of view (in particular the anatase form). Furthermore, these particles must be supported on a suitable material that allows the creation of filters or other means configured in such a way as to allow contact of the fluid to be purified with the photocatalyst particles and at the same time their illumination by means of a UV lamp in as homogeneous as possible.

A review of the use of titanium dioxide for the photocatalytic degradation of organic pollutants is reported in the article by Haoran Dong et al., Water Research, 79 (2015), 128-146, where various stabilization techniques of titanium particles are described. dioxide on substrates of different nature, for example glass, clays, steel, textile fibers, plastic films, activated carbon and zeolites. The authors note that, despite the various solutions proposed, a drawback that is observed following stabilization on such supports is a significant reduction in the photocatalytic activity of titanium dioxide compared to free particles, most likely due to a reduction in the active surface of the photocatalyst. and its activation by the incident radiation which is hindered by the mass of the support on which the titanium dioxide is deposited.

In the article by Sung-Bong Yang et al., J. Air & Waste Management Association, 65 (3): 365-373 (2015), a process for the preparation of titanium dioxide, functionalized with iron, on fibers of flexible glass using a sol-gel method. The process involves the preparation of an acid solution of tetra (n-butyl) titanate in water / ethanol, to which Fe (III) nitrate is added. A sheet of glass fibers is then immersed in the solution. The sheet is then dried, and the dipping and drying process repeated three times. The sheet thus treated is then subjected to calcination at 400 ° C, obtaining glass fibers coated with Fe-TiO2 in crystalline form.

Patent US 8,617,300 describes a filter medium having photocatalytic activity with a thickness of at least 2 mm, homogeneous and without holes visible to the naked eye, which comprises a felt of inorganic fibers coated with a layer of a photocatalyst. The latter can be chosen from TiO2, ZnO and CeO2, while the inorganic fibers can be glass fibers, ceramic material or pure silica (quartz). For the realization of the coating with the photocatalyst, the fibers are impregnated with a solution of an organic precursor of silica (in particular a tetraalkyl orthosilicate mixed with an alkoxysilane) and a dispersion of particles of the photocatalyst. The fibers thus impregnated are then subjected to calcination at a temperature up to 550 ° C, preferably 450 ° C. In this way, a silica-based ceramic layer is obtained on the fibers in which the photocatalyst particles are dispersed.

The Applicant has set itself the problem of obtaining a photocatalytic material in which particles of titanium dioxide are supported on a siliceous substrate, in such a way as to guarantee a high photocatalytic activity which remains substantially stable over time even without the use of binders, matrices or adhesives for titanium dioxide particles.

These and further objectives which will be better illustrated in the following have been achieved by the Applicant through the deposition of particles of titanium dioxide by means of heterocoagulation on the siliceous substrate. This allows you to avoid subsequent heat treatments, such as calcination processes, which would inevitably have the effect of significantly reducing the surface area and therefore the activity of the photocatalyst.

As is known, the phenomenon of heterocoagulation allows particles of different nature to be bound together, thanks to the electrostatic attraction that is generated when the different materials have opposite surface charges, or zeta potentials (ζ) of opposite sign. In this regard, see for example P. Kralchevsky, R. Miller, F. Ravera, “Colloid and Interface Chemistry for Nanotechnology”, CRC Press (2013), page 148.

The Applicant has surprisingly found that particles of titanium dioxide are able to bind stably to a siliceous support thanks to this phenomenon, which is further favored by the surface micro-roughness of the support itself, which has a larger size than the photocatalyst particles. Without wishing to bind to an interpretative theory, the Applicant believes that a partial amorphous nature of the titanium dioxide particles also allows to generate stable covalent bonds over time by condensing their surface hydroxyl functions with the free ones of the siliceous support. This leads to obtaining a particularly stable functionalization of the substrate surface by the photocatalyst particles. Furthermore, the Applicant has found that, unlike the processes of the known art, it is not necessary to subject the titanium dioxide to densification by calcination at a high temperature (generally at about 500 ° C), neither before nor after the deposition on the substrate. : this allows to obtain particularly high specific surface area values. These high values, combined with the fact that the photocatalyst particles are not trapped in a matrix, but fully exposed to the outside and well dispersed on the surface, help to increase the catalytic yield.

Therefore, according to a first aspect, the present invention relates to a photocatalytic material comprising a siliceous substrate having at least one surface on which particles of titanium dioxide are present, said particles being adhered to the surface by heterocoagulation and such particles having a degree of crystallinity of 60% to 90%, preferably from 70% to 85%, more preferably from 80% to 85%.

The degree of crystallinity can be determined by methods known in the art, for example by analysis of the X-ray diffraction spectrum, by calculating the ratio between the area of the peak (or peaks) corresponding to the crystalline phase and the area due at the bottom (background). The degree of crystallinity can be determined for example by the method described in the ISO 13778-3: 2008 standard.

Preferably, the titanium dioxide particles have a specific surface area from 150 m <2> / g to 400 m <2> / g, more preferably from 200 m <2> / g to 350 m <2> / g.

The specific surface area can be determined using methods known in the industry, for example through BET (ISO 9277: 2010), on free particles, before their adhesion to the substrate.

The Scherrer equation (Scherrer, P., Göttinger Nachrichten Gesell., Vol. 2, 1918, p 98) allows to estimate the size of the diffraction centers of the above titanium dioxide particles present on the siliceous substrate, starting from the analysis of the diffraction peaks. This value can also be related to the degree of regularity of the crystallites that form the particles. In the case of the present invention, in which this value is relatively low, it can also be related to the partial amorphous nature of the particles.

Preferably, the titanium dioxide particles present on the siliceous substrate have a diffraction center size of from 50 to 150 Å, more preferably from 50 to 80 Å.

As mentioned above, the size of the diffraction centers can be determined using the Scherrer equation, as known to the skilled in the art (Scherrer, P. 1918).

A particularity of the photocatalytic material of the present invention is also that of not creating a continuous covering film, but exhibiting a potentially discontinuous spatial distribution of the titanium dioxide particles on the siliceous substrate, such that the aforementioned surface of the substrate has exposed surface areas. The presence of aggregates is also found, as shown by the scanning electron microscope images of Figures 3 and 4a. Furthermore, the distribution of the photocatalyst particles, although discontinuous, is substantially homogeneous on the surface of the substrate, as shown in Figure 4b, which reports a point compositional analysis (obtained by means of an EDS probe) in which the titanium signal is observed in the spectrum of issue.

The discontinuous distribution described above gives the particles of titanium dioxide adhering to the substrate a particularly high surface area exposed (that is, available for the photocatalytic activity) of the particles, and in any case higher than that which would occur if the same quantity by weight of the same particles were distributed in the form of a continuous layer of uniform thickness on the surface of the substrate, as occurs for example in the case in which a precursor of the photocatalyst is subjected to calcination processes after deposition.

In the prior art, a high photocatalytic activity is usually associated with a high degree of crystallinity of the particles (see Yang et al. 2015), their uniformity of distribution and the absence of aggregation phenomena (Dong et al. 2015) , paragraph 3.3.).

It has now been surprisingly found that it is possible to obtain a high photocatalytic activity, comparable or even higher than that reported by the known art, by using titanium dioxide particles having a relatively low degree of crystallinity, especially when distributed discontinuously on the substrate surface. As already seen above, the low degree of crystallinity also allows the adhesion of the particles even in the absence of stabilizers, binders, matrices or adhesives for the titanium dioxide particles and without the need to subject the substrate to calcination.

Preferably, the titanium dioxide particles have a median diameter (d50) in aqueous suspension from 100 nm to 500 nm, more preferably from 200 nm to 400 nm. The median diameter can be determined by methods known in the art, for example by dynamic light scattering (DLS), according to ISO 22412 (2008). As is known, d50 is the median value, i.e. the diameter value below which 50% by weight of the particle population is found (see "A Guidebook to Particle Size Analysis" published by Horiba Instruments Inc. - 2016 , available at https://www.horiba.com/fileadmin/uploads/ Scientific / eMag / PSA / Guidebook / pdf / PSA_Guidebook.pdf).

Through transmission electron microscope (TEM) analysis, it can be seen that in reality the titanium dioxide particles are made up of aggregates of the above mentioned dimensions of crystallites having nanometric dimensions, generally from 5 nm to 80 nm, preferably from 10 nm to 50 nm, more preferably 20 nm to 40 nm.

As regards the crystalline form of titanium dioxide, this is mainly in anatase form, possibly in a mixture with other crystalline forms, in particular with brookite: in this case the weight ratio between anatase and brookite is preferably from 60/40 to 95 / 5, more preferably 70/30 to 90/10.

It has in fact been observed that a mixture of anatase and brookite increases the photoactivity of the particles compared to the same amount of the anatase form alone.

As regards the siliceous substrate, this can be preferably selected from: amorphous silica, crystalline silica, fumed silica, fused silica, quartz, silicates (for example phyllosilicates), preferably amorphous silica.

Preferably the siliceous substrate is in the form of fibers or granules, even more preferably in the form of fabrics or non-woven fabrics of silica fibers, in particular glass or quartz fibers. As far as glass is concerned, it is known that it is mainly formed by silica mixed with other oxides, for example Al2O3, CaO, MgO, Na2O3, B2O3, Fe2O3, TiO2, ZrO2, and others.

As mentioned above, the siliceous substrate must support the catalyst particles allowing the creation of filters or other means configured in such a way as to improve the contact of the fluid to be purified with the photocatalyst particles and at the same time allow their illumination by means of a lamp. UV.

In a preferred embodiment, the siliceous substrate is a glass fiber fabric having a grammage from 100 to 1000 g / m <2>, more preferably from 100 to 900 g / m <2>, even more preferably from 100 to 300 g / m <2>.

The weight according to the selected siliceous substrate is within the skills of the skilled in the art to obtain specific characteristics of contact with the fluid and lighting.

Preferably, the photocatalytic material comprises a glass fiber fabric having a grammage from 100 to 300 g / m <2>, preferably from 150 to 250 g / m <2>, more preferably about 200 g / m <2>, and the particles of titanium dioxide are present on the fabric in an amount, in percentage by weight with respect to the weight of the fabric as such, from 0.5 to 5%, preferably from 1 to 3%, more preferably about 2%.

According to another aspect, the present invention relates to a process for the production of a photocatalytic material as described above, which comprises:

preparing a suspension of titanium dioxide particles in a hydroalcoholic medium at a pH value from 3 to 7;

immersing a siliceous substrate in said suspension at a suspension temperature of from 0 to 80 ° C, preferably from 10 to 70 ° C, so as to obtain heterocoagulation of the titanium dioxide particles on the siliceous substrate;

wash and dry the siliceous substrate thus treated.

Preferably, the dipping step is carried out at least twice in succession, each dipping step being separated from the next by a drying step of the siliceous substrate. In this way it is possible to gradually increase the saturation of the titanium dioxide layer until the desired value is obtained.

Two immersions are usually sufficient to achieve complete saturation of the siliceous substrate.

Preferably, in the step of immersing a siliceous substrate in the suspension, the temperature is from 15 to 60 ° C.

As already mentioned above, heterocoagulation allows you to bind particles of different nature to each other, thanks to the electrostatic attraction that is generated when the different materials have opposite surface charges, or zeta potentials (ζ) of opposite sign.

It has been observed by the Applicant that when the suspension of particles of the process of the invention has a pH from 3 to 7, the zeta potential of the particles is positive. In this same pH range, the siliceous substrate has a negative zeta potential instead. The marked difference in zeta potential that is observed in the aforementioned pH range between the particles and the substrate is such as to favor, in the heterocoagulation phase, their mutual electrostatic attraction, and therefore the formation of particularly stable bonds between them.

The zeta potential can be measured according to methods known in the industry, such as according to the ISO 13099-2: 2012 standard, with the use of the DLS optical instrument ("Dynamic light scattering").

Without wishing to bind to any interpretative theory, it is believed that the partial amorphous nature of the particles, which involves the surface presence of a relatively high number of reactive -OH groups, determines, in the indicated pH range, a particularly high zeta potential value. . As already noted above, the presence of a relatively high number of surface hydroxyl functions, in turn, would entail the remarkable ability of the particles to adhere to the siliceous support through heterocoagulation, even in the absence of stabilizers, surfactants, or other additives, and at the same time determines the high reactivity of the titanium dioxide particles according to the invention.

Preferably, before immersing the siliceous substrate in the suspension, the latter is treated with a basic solution.

Following the treatment with the basic solution and before the immersion phase in the suspension of the siliceous substrate, the latter is subjected to washing to remove the residues of the basic solution.

The treatment with the basic solution, by increasing the number of available -OH groups, has the effect of increasing the absolute value of the (negative) zeta potential of the surface, and therefore allowing greater adhesion of the titanium dioxide particles on the surface of the substrate.

Preferably, this basic solution comprises a strong base, more preferably selected from potassium, sodium, calcium hydroxide, or mixtures thereof.

Preferably, this basic solution has a pH from 7.5 to 14, more preferably from 8 to 12.

Preferably, the hydroalcoholic medium in which the titanium dioxide particles are dispersed consists of a mixture of water with an alcohol, more preferably with isopropanol. Isopropanol is particularly preferred as it is completely miscible with water and nevertheless has a low dipolar moment which gives the mixture with water a relatively low dielectric constant, which favors the heterocoagulation process of titanium dioxide.

The titanium dioxide particles are present in the suspension in a concentration, in percentage by weight on the volume of the suspension, generally from 0.1 to 5%, preferably from 0.5 to 5%, more preferably from 1 to 3%.

Preferably, according to the process of the invention, the siliceous substrate is not subjected, after the immersion step, to a heat treatment step, in particular of densification. This guarantees a high specific surface area of the photocatalyst and therefore an improved catalytic yield of the same.

The production process of a photocatalytic material according to the present invention is therefore simplified compared to the conventional processes of the known art.

In accordance with the present invention, the suspension of titanium dioxide particles is preferably obtained by hydrolysis of a solution of an alcoholic titanium Ti (OR) 4, where R is a C1-C6 alkyl, linear or branched, in an aqueous medium, preferably the same aqueous medium used subsequently for the heterocoagulation process, so as to be able to directly use the titanium dioxide suspension obtained from hydrolysis, without having to subject the photocatalyst particles to isolation from the hydrolysis solvent and subsequent redispersion in the heterocoagulation solvent, which it can cause unwanted aggregation phenomena. Preferably, R is methyl, ethyl, n-propyl, i-propyl, n-butyl, ibutyl, t-butyl, n-pentyl, i-pentyl, n-hexyl, ihexyl. The hydrolysis reaction is preferably carried out at a temperature from 40 ° C to 100 ° C, more preferably from 70 ° C to 95 ° C.

Preferably, the hydrolysis reaction is carried out by dropping a solution of the titanium alcohol in a C1-C6 alcohol in water maintained under stirring at a temperature within the above defined ranges. At the end of the dripping, the titanium dioxide suspension is kept under stirring at a temperature within said intervals, for a time ranging from 0.5 hours to 5 hours, preferably from 1 hour to 3 hours.

This hydrolysis process is therefore carried out, in a simple way, without the aid of acid or basic catalysts, which can pollute the photocatalyst and decrease its activity, and leads to the obtainment of a highly photoactive titanium dioxide suspension. This suspension can be used as it is for the heterocoagulation step, without the need for any heat treatment.

It is important to note that the deposition of titanium dioxide particles on a siliceous support allows to obtain a particularly active photocatalytic material, thanks to the fact that the silica of the support acts as a filter by adsorbing the molecular species present in the environment so as to facilitate the action. demolition of the photocatalyst. Furthermore, being hygroscopic, amorphous silica acts as a reserve of water molecules which are useful for the formation of hydroxyl radicals (OH), which participate in the decomposition process, thus increasing the photocatalytic yield especially in the case of heterogeneous solid-gas photocatalysis. with low relative humidity values, i.e. particularly dry environments.

The relatively high surface area of the titanium dioxide particles helps to increase the adhesion of the particles to the substrate.

Brief description of the figures

Figure 1 represents an X-ray diffraction spectrum obtained from the titanium dioxide particles of Example 1, according to the invention (in black). The Figure also shows, for comparison purposes, the X-ray diffraction spectrum obtained from a very crystalline Titanium Dioxide consisting solely of anatase phase (Brenntag S.p.A., Germany) (in gray).

Figure 2 is a graph showing the dimensional distribution, expressed in nanometers (d. Nm), from DLS ("dynamic light scattering" analysis, determined according to ISO 22412: 2008) on the suspended particles of Example 1, according to the invention.

Figures 3a, 3b and 3c are scanning electron microscopy (SEM) images of glass fibers loaded with particles of titanium dioxide according to Example 1.

Figure 4a is a further scanning electron microscopy (SEM) image of the glass fibers loaded with titanium dioxide particles according to Example 1. Figure 4b is an elementary mapping image for the element Titanium obtained on the same detail of figure 4a by means of an EDS probe (“Energy Dispersive Spectrometry”, spectrometry for energy dispersion) which equips the electron microscope; the image demonstrates the homogeneous distribution of the catalyst on the support fibers.

Figure 5a shows the emission spectra for the lamp as is of Example 2 ("9 W lamp"), filtered by a layer of the fiber fabric according to Example 2 ("1 layer"), and by two layers of the fiber fabric ("2 layers"). Figure 5b shows a detail at the emission peak of the lamp, centered at a wavelength of 254 nm.

Figure 6 is a graph that summarizes the results of Example 3.

Figure 7a summarizes the results of Example 4 in terms of absolute VOC. Figure 7b summarizes the results of Example 4 in terms of percentage VOC (VOC: volatile organic compounds ("volatile organic compounds")).

The following embodiment examples are provided for illustrative purposes only of the present invention and should not be construed as limiting the scope of protection defined by the attached claims.

EXAMPLE 1

Preparation of a photocatalytic glass fiber fabric

A suspension of titanium dioxide particles was prepared according to a sol-gel synthesis method as follows.

In a 500 ml flask, 148 ml of double distilled water were heated up to 80 ° C. Separately, 16 ml of titanium isopropoxide were dissolved cold in 47 ml of 2-propanol, taking care to minimize the contact of the solution with ambient humidity.

The alcoholic phase was then slowly dripped (4 ml / min) into the water, kept under modest stirring at 80 ° C, in the absence of acidic or basic catalysts.

Once the dripping was finished, the suspension was kept under stirring and at temperature for 2 hours.

A stable white suspension was thus obtained containing about 2.5% by weight of titanium dioxide in hydro-alcoholic suspension.

The titanium dioxide particles had the following characteristics:

- Degree of crystallinity of 80% (Figure 1);

- Anatase / Brookite crystalline phase ratio equal to 80/20 (± 5) (Figure 1, the ratio is determined by the ratio between the areas of the peaks corresponding to the two crystalline phases according to techniques known in the sector, for example the RIR method (Chung , F.H. (1994) "Quantitative interpretation of XR diffraction patterns and mixtures. Matrix flushing method for quantitative multicomponent analysis" J.of Applied Cryst., Vol.

7, 519-525). The respective characteristic peaks are determined by comparison with the peaks of samples of known composition.

- From DLS analysis in suspension, performed according to the ISO 22412 (2008) protocol, the average diameter of the nanostructured sub-micrometric particles was equal to 250 nm; from TEM analysis, the crystallites had a diameter of 25-50 nm; from Scherrer equation, the size of the diffraction centers was equal to 57 Å (considering shape factor 0.9); Figure 2 shows the size distribution obtained from DLS analysis on suspended particles;

- Specific surface area of the particles equal to 240 m <2> / g, measured according to the BET method described in the ISO 9277: 2010 standard.

Separately, a 10 cm x 10 cm glass fiber fabric with a weight of 200 g / m <2> was prepared, which was immersed in the aforementioned suspension of titanium dioxide particles at room temperature by dip- coating.

The fabric thus treated was air dried and the immersion in the suspension was repeated.

At the end, the treated fabric was washed with water (to remove excess deposited material) and dried.

The photocatalytic material thus obtained had the surfaces partially covered with titanium dioxide particles. It was determined that the treated tissue had loaded 50 mg of titanium dioxide particles.

Figure 3 (3a, 3b, 3c) and Figure 4a show scanning electron microscopy images of glass fibers loaded with titanium dioxide particles. As can be seen, the particles are distributed on the surface, with the formation of some clearly visible aggregates.

Figure 4b shows an elementary analysis obtained using an EDS probe ("Energy Dispersive Spectrometry", spectrometry for energy dispersion) of the same glass fibers. Specifically, the elementary mapping of Titanium is shown, reconstructed through its characteristic signal in the emission spectrum. As commented above, this analysis shows that the distribution of the photocatalyst particles, although discontinuous, is substantially homogeneous on the surface of the substrate.

EXAMPLE 2

Absorption tests

The fiber fabric loaded with photocatalyst particles described in Example 1 was irradiated with a 9 W power UVC lamp (Philips 254 nm TUV T8). Since the amount of radiation absorbed by a said layer is a fraction of the total incident, this layer was coupled to a second layer, prepared in the same way, thus making the most of the radiant capacity of the lamp and at the same time increasing the catalytic exchange surface. . Figure 5 shows the emission spectra of the lamp as it is, filtered by 1 layer, and filtered by 2 layers (Figure 5a), and the detail of the maximum emission peak at 254 nm (Figure 5b).

EXAMPLE 3

Catalytic activity evaluation test

For this test, the provisions of the ISO 10678: 2010 standard were followed, under the following specific conditions.

A 10x10 cm piece of fiberglass fabric was loaded with about 50 mg of titanium dioxide particles according to the procedure described in Example 1.

The fabric thus loaded was then immersed in a container containing 500 ml of aqueous solution 5.3 * 10 <-6> M of methylene blue under mechanical stirring at room temperature. After 30 minutes of conditioning in the dark, the solution was irradiated with the same UVC lamp of Example 2, placed 30 cm away from the container for 120 minutes. The measurement of the absorbance value of the solution at 660 nm made it possible to measure the quantity of dye adsorbed in the 30 minutes of conditioning and degraded in the 120 minutes of irradiation.

The test was repeated under the same experimental conditions on the same weight of the same particles of Titanium dioxide in powder form, not deposited on a siliceous support.

Results:

Dye Adsorbed by supported catalyst vs unsupported powder = 3350% Dye degraded by supported catalyst vs unsupported powder = 400%

Figure 6 shows a graph summarizing the results.

EXAMPLE 4

Acetaldehyde abatement test; comparison with a system of the known art in static conditions

A laboratory test was carried out to evaluate the effectiveness of a prototype photocatalytic system in the abatement of a volatile organic species.

The prototype is composed of a metal structure supporting a lamp (UVC, 18W, OSRAM HNS-L UVC) inside which the catalytic material composed of two layers of glass fiber fabric with a weight of 200 g / m <2> having a surface area of 0.544 m <2> (6.8 m x 8 cm) loaded with TiO2 particles (for a total of about 4.5 g).

The test was performed by placing the prototype inside a glass chamber with a volume of 72000 cm <3>, a volatile contaminants (VOC) detector (ION SCIENCE mod. CORVUS) equipped with a photoionization detector was positioned in the chamber. (PID) capable of continuously monitoring the concentration of volatile species and other parameters relating to air quality. A quantity of 10 microliters of acetaldehyde in liquid form was then injected inside the chamber and the air inside the sealed chamber was monitored. After an initial peak of analyte concentration detected during the drop evaporation phase, it is expected to have a homogeneous distribution of the vapors in the test chamber (considered reached when a constant concentration value (75 ppmv) is recorded for 3 minutes); from this point (considered time 0) the prototype UV lamp was turned on and various parameters of the air inside the chamber were monitored for about 200 minutes of testing.

The test was repeated under the same conditions using a sanitizing device available on the market based on the same technology (UV activated titanium dioxide), equal to the prototype for lamp power and volume of air treated (from declared technical specifications, Activtek Induct 2000) . The results obtained are shown in the graph (in absolute value (Figure 7a) and percentage (Figure 7b) in which the "prototype" corresponds to the device containing the photocatalytic material of the invention, while "A" corresponds to the sanitizing device available on the market.

The VOC abatement value recorded after 120 minutes for the prototype is approximately 490% higher than that recorded for the sanitizing device “A”. It should be noted that in the two tests the relative humidity values were on average low (44% and 35% respectively). These are humidity levels in which the prior art devices have poor VOC abatement capacity. On the contrary, the prototype of the invention is able to give satisfactory results at these humidity values and even lower (for example at HR 29%).

EXAMPLE 6

ICP and gravimetric analysis

To confirm the stability of the catalyst particles on the surface of the support, a 10 x 10 cm square of glass fiber cloth loaded with titanium dioxide particles, as described in Example 1, was repeatedly immersed in 500 ml of water under vigorous shaking for periods of 2 hours, and left to dry (with the use of a hair dryer). After three soaking-drying cycles, the catalyst showed no instrumental weight changes compared to the start of the test, and ICP analyzes carried out on the washing water did not allow to detect releases of titanium in solution (Ti <LOD = 0.02 ppm) .

Claims (10)

CLAIMS 1. Photocatalytic material comprising a siliceous substrate having at least one surface on which particles of titanium dioxide are present, said particles being adhered to the surface by heterocoagulation and said particles having a degree of crystallinity from 60% to 90%, preferably from 70% to 85 %, more preferably 80% to 85%. 2. Photocatalytic material according to claim 1, wherein said titanium dioxide particles have a specific surface area from 150 m <2> / g to 400 m <2> / g, preferably from 200 m <2> / g to 350 <2 > m / d. 3. Photocatalytic material according to claim 1 or 2, wherein said particles of titanium dioxide present on said siliceous substrate have a diffraction center dimension from 50 to 150 Å, preferably from 50 to 80 Å. 4. Photocatalytic material according to any one of the preceding claims, wherein the titanium dioxide has a crystalline form mainly in anatase form, preferably in admixture with brookite in a weight ratio between anatase and brookite, preferably from 60/40 to 95/5, plus preferably from 70/30 to 90/10. 5. Photocatalytic material according to any one of the preceding claims, wherein said siliceous substrate is selected from: amorphous silica, crystalline silica, fumed silica, fused silica, quartz, silicates, preferably amorphous silica. 6. Photocatalytic material according to any one of the preceding claims, wherein said siliceous substrate is a glass fiber fabric having a basis weight from 100 to 1000 g / m <2>, preferably from 100 to 900 g / m <22>, plus preferably from 100 to 300 g / m <2>. 7. Process for the production of a photocatalytic material according to any one of the preceding claims, which comprises: preparing a suspension of titanium dioxide particles in a hydroalcoholic medium at a pH value from 3 to 7; immersing a siliceous substrate in said suspension at a suspension temperature of from 0 to 80 ° C, so as to obtain heterocoagulation of the titanium dioxide particles on the siliceous substrate; wash and dry the siliceous substrate thus treated. 8. Process according to claim 7, wherein in said step of dipping a siliceous substrate in said suspension is carried out at least twice in succession, each dipping step being separated from the next by a drying step of the siliceous substrate. 9. Process according to claim 7 or 8, wherein said particles of titanium dioxide are present in said suspension in a concentration, in percentage by weight on the volume of the suspension, from 0.1 to 5%, preferably from 0.5 to 5 %, more preferably 1 to 3%. Process according to any one of claims 7 to 9, wherein the siliceous substrate is not subjected, after the dipping step, to a heat treatment step.