ITMI20091764A1 - PROCEDURE FOR THE PREPARATION OF DOPED TITANIUM DIOXIDE WITH CARBON - Google Patents

Description

DESCRIPTION

FIELD OF INVENTION

The present invention relates to the field of photocatalysts and methods for regulating and improving their abatement capacity of pollutants present in the atmosphere.

TECHNIQUE NOTE

Titanium dioxide, in its crystalline form of anatase, is a well-known photocatalytic agent. In the presence of light it catalyzes the oxidation of various contaminants present in the atmosphere, in particular aromatic hydrocarbons, favoring their abatement process (see, for example, Int. RILEM Seminar on Photocatalysis, Florence, 8-9 Oct. 2007, Photocatalytic and Surface Abatement of Organic Hydrocarbons by Anatase).

A characteristic limit to the photocatalytic action of titanium dioxide is that it uses only the ultraviolet component of sunlight (about 4% of the radiation) and therefore is photocatalytically not very active, especially in environments where sunlight is reduced. .

To solve this problem, an attempt was made to modify titanium dioxide by doping it with other elements, making it able to use the most consistent part of sunlight, ie the spectrum of visible light, between 400 and 700 nm. For this purpose, titanium dioxide has been doped with metal ions such as lanthanum and iron, or with nitrogen (eg EP1 178011 and EP 1254863). The benefits achieved are, however, modest. Another possibility is to dop the titanium dioxide with carbon; however the related doping methods (see US 2005/0226761, Kronos Inc.) are complex and expensive; in particular, they require the titanium dioxide to be intimately mixed with carbon-containing compounds, e.g. sugars; the mixture is then subjected to expensive heat treatments (generally between 250 and 400 ° C) in an oxidizing atmosphere: these treatments involve a significant loss of carbonaceous material in the form of CO2 and / or CO; the treatment also involves a sintering of the photocatalyst, with a strong reduction of its specific surface and, consequently, of the photocatalytic activity; at the end of the treatment the product must be subjected to grinding in order to be used. Any mixing with active carbons with a high surface area has proved insufficient to obtain significantly active products.

Therefore, the need for carbon doping procedures that are simple, reproducible, effective and inexpensive is still not satisfied. There is also the need to obtain carbon-doped titanium dioxide with a high specific surface and a strong and stable photocatalytic activity over time.

SUMMARY

The object of the present invention is a process for the preparation of titanium dioxide doped with carbon, comprising rirradiating the titanium dioxide at a wavelength between 300 and 400 nm, said titanium dioxide being exposed to a gaseous flow comprising an inert gas and an organic compound. It has been observed that the titanium dioxide thus treated acquires a high and stable carbon content; moreover, advantageously with respect to the known technique, there is no decrease in the specific surface, but the same remains substantially unchanged: it is therefore possible, starting from a titanium dioxide with a desired specific surface value, to obtain a reproducible product doped with carbon having the same specific surface value. This avoids having to resort to complex processes of mixing in the liquid phase, evaporation, drying, high temperature calcination, grinding, which, in addition to being ineffective, complicate the methodology and make it much more expensive. The overall cost-effectiveness of the process is also increased by the fact of avoiding two typical phases of known doping systems, namely the preliminary mixing of titanium dioxide and organic compound, and the subsequent step of high heat treatment. In particular, in the present invention the irradiation occurs at a typically low temperature, eg. ambient temperature, and the energy consumption linked to irradiation is decidedly lower than that necessary for the heat treatments described in the known art.

DESCRIPTION OF THE FIGURES

Figure 1: TPO graph (oxidation at programmed temperature) related to doped titanium dioxide according to the invention.

DETAILED DESCRIPTION

The term â € œtitanium dioxide doped with carbonâ € identifies a titanium dioxide containing carbon: it can be present in the elementary state and / or in the form of organic substance. The carbon title (doping title) is expressed as a percentage by weight of elemental carbon with respect to the weight of the doped titanium dioxide: it can be measured using known methods such as oxidation at programmed temperatures, as shown in the experimental part. The present process is particularly (though not exclusively) suitable for obtaining a doping titer between 0.03% and 5%, preferably between 0.3 and 3%, more preferably between 1 and 1.6%.

The titanium dioxide used as starting reagent can be any commercial titanium dioxide, present at least in part in the form of anatase; It is normally used in powder form; conveniently, it has a specific BET surface value corresponding to that desired in the doped final product: this value, depending on the needs, can be chosen in the range between 50 and 450 m <2> / g, preferably between 300 and 350 m <2 > / g, e.g. 330 m <2> / g.

The present process has proved particularly useful in obtaining carbon-doped titanium dioxide having a specific BET surface between 200 and 400 m <2> / g, preferably between 255 and 400 m <2> / g.

The organic compound contained in the gaseous flow (here also defined as carbonaceous compound) can be chosen from those that can be easily vaporized, such as to be conveniently transported by a gaseous flow; for the rest there is no particular limit regarding the chemical structure of this compound: they can be used eg. hydrocarbons or their derivatives optionally functionalized with groups such as alkyl, hydroxy, formyl, acetyl, carboxy, alkoxycarbonyl, aryloxycarbonyl, amino, alkylamino, thio, alkylthio, etc .: examples of preferred products are toluene, benzene, xylene, naphthalene, their derivatives and their blends; a particularly preferred example is ethylbenzene.

The gaseous carrier (carrier gas) used to transport the above compounds is an inert gas, for example nitrogen, helium, argon, etc., or their mixtures, possibly mixed with further gases; for example, it is possible, for convenience, to use air: however the presence of reactive gases (oxygen or others) as components of the carrier gas is in no way indispensable, as this process does not require oxidation of the organic compound; in a specific embodiment of the invention, the carrier gas consists exclusively of one or more inert gases.

The speed of the gaseous flow can be suitably chosen according to the quantity of titanium dioxide to be treated, eg. for quantities of the order of 100-200 mg, flows preferably between 5 and 30 cm <3> / min are used; evidently the applied fluxes and the concentrations of organic compounds can be increased or decreased, having to treat respectively higher or lower quantities of titanium dioxide. For example, in the case of processes on an industrial scale, the concentrations of organic compound can be between 500 and 10000 ppm.

The flow of the carrier gas can be ensured by known systems (pumps, containers under pressure, etc.), suitably controlled and possibly corrected by known systems. In particular, the doping plant may include analyzers capable of evaluating the quantity of carbonaceous compound present in the carrier gas before and / or after contact with titanium dioxide. The difference between the two concentrations, in particular the variation over time of this value, is an indication of the progress of the doping procedure: a time-varying differential indicates that the procedure is in progress; a stable, non-zero differential indicates that there is no doping in progress.

The mode of contact between the gas and the titanium dioxide is not decisive in itself and can be suitably varied with reactor solutions well known to those skilled in the art.

An important aspect of the present process is the irradiation of titanium dioxide, which must occur simultaneously with the flow of carbonaceous compound thereon. The irradiation was important for obtaining an adequate doping of the titanium dioxide, obtaining a consistent and stable doping titer. The irradiation is carried out in a specific band of ultraviolet light, the one between the wavelengths of 300 and 400 nm. For this purpose, lamps of adequate power are used, placed at an adequate distance from titanium dioxide, eg. between 5 and 25 cm or even immersed in the same. The intensity of irradiation on titanium dioxide is preferably between 10 and 1000 W / m <2>.

The treatment temperature, ie that of the reaction environment and of the titanium dioxide, is not decisive; it can be eg. below 50 ° C, conveniently including the ambient temperature. Useful temperature ranges are e.g. 10-50 ° C, or 20-40 ° C, etc. The reaction temperature can be controlled by thermally controlling the reactor within which the contact between titanium dioxide and carrier gas occurs; the fluxed gaseous mixture is used in a temperature range such that the temperature in the reactor is maintained in the desired range.

The procedure is carried out in an adequate amount of time, eg. between 100 and 400 minutes, until the desired doping title is reached. The doped titanium dioxide according to the invention has a higher photocatalytic activity than that conferred by the C-doped titanium dioxide according to the known technique. This property, exploited for the preparation of cementitious products and products with prolonged photocatalytic activity, is the subject of a co-pending application in the name of the Applicant.

The invention is illustrated here without limitation by means of the following examples.

EXPERIMENTAL PART

Example 1

Preparation of doped titanium dioxide

Operating conditions:

Titanium dioxide: anatase, PC-500 (Millenium) 150 mg (average diameter 0.2-0.3 mm / 50- 70 mesh). Gas composition: oxygen-helium 3: 1 Ethylbenzene concentration: 1000 ppm

Flow rate: 16 cm <3> / min

Irradiation wavelength: 315-400 nm.

Irradiation intensity: 20-21 W / m <2>

Reactor temperature: 45 ° C.

The reactor consists of a U-shaped sample holder (height approx. 15 cm; average internal diameter 2 mm). At a distance of about 15 cm there is a 125 W Hg vapor UV lamp (mod. GN 125, Helios Interquartz) which radiates it from the front.

Next to the sample are positioned a UV probe to measure the intensity of irradiation (W / m <2>) and a thermocouple to measure the temperature. The reactor is equipped with a bypass to analyze the gaseous mixture before and after the sample, recording the relative concentrations of ethylbenzene. The gaseous mixture is analyzed by gas chromatographic analysis (PORAPAK Q column).

At the beginning, the reactor is placed in bypass: the saturator is opened and the reaction mixture is sent (1000 ppm EB O2 He). When the system has stabilized (EB values constant), the reactor is switched on, sending the mixture on the irradiated sample. At the exit of the reactor the presence of the hydrocarbon is not detected, which indicates that the doping is in progress. After a certain time, the outgoing hydrocarbon becomes measurable again, growing until it reaches a constant value; this indicates that the doping process is over.

Example 2

Assessment of the degree of doping.

The oxidation analyzes at programmed temperature are carried out to quantify the presence of carbon in the sample treated in example 1. The procedure includes heating the sample under current of an oxidizing mixture (5% O2 / He) and continuously analyzing the quantity of oxygen consumed. A positive band is thus recorded corresponding to the oxidation of the different oxidizable components present. The area subtended by the band, corresponding to the oxygen consumed, is suitably calibrated with a known sample.

The system is equipped with a flow regulator connected to a 5% O2 / He oxidant mixture cylinder. The reactor consists of a quartz U-shaped sample holder inserted in an oven connected to a temperature programmer (Eurotherm 808). The temperature of the sample is monitored by means of a thermocouple inserted in the sample itself. After the sample holder there is a trap filled with soda lime and anhydrous (anhydrous magnesium sulphate) which allows to block CO2 and H2O formed during the reaction. The outgoing gas is sent to a heat conductivity detector interfaced with a computer.

Operating conditions:

Sample quantity: 50 mg (average diameter 0.2-0. 3 mm / 50- 70 mesh)

Flow rate: 40cm <3> / min

Heating speed: 10 ° C / min up to 800 ° C

The oxidation test carried out on the product of example 1 highlighted the presence of carbon in a quantity equal to 1.3%.

Example 3

Characterization of the product

The specific BET surface of titanium dioxide was determined by nitrogen adsorption, before and after the doping procedure carried out in example 1. The value of the two measurements was the same, equal to 330 m <2> / g. The doping method used did not therefore lead to any reduction of the specific surface of the photocatalyst.

In parallel, the effect produced on the specific surface by the heat treatment described in the preparation method referred to in US 2005/0226761 was verified. The specific surface before and after this heat treatment was respectively 330 m <2> / g and 160 m <2> / g. The method described in US 2005/0226761 therefore resulted in a reduction of the specific surface of the photocatalyst equal to 170 m <2> / g.

Example 4

Evaluation of the photocatalytic activity

The system is equipped with two flow regulators connected respectively to a cylinder with 1000 ppb NO / air and to an air cylinder. In this way, by means of adequate dilution, it is possible to send to the NOx analyzer a mixture with a known concentration of NO / air (about 100 ppb NO / air, obtained by diluting the initial mixture 1/10). The reactor part of the plant consists of a U-shaped sample holder (height approx. 15 cm; internal diameter 2 mm). About 15 cm away there is a visible lamp (low consumption, 14 W) which radiates it from the front. Next to the sample are positioned a probe in the visible (400-1050 nm) to measure the intensity of irradiation (W / m <2>) and a thermocouple to measure the temperature.

The reactor is equipped with a bypass to analyze the gaseous mixture before and after returning it to the sample, recording the relative concentrations of NO. The reactor is kept covered in order not to let light reach the sample before the start of the reaction.

After the analyzer preheating phase and before starting the measurement, the entire line and the sample are cleaned in a flow of chromatographic air (at least 1000 ml / min). The reaction mixture is then sent to the bypass. When the system has stabilized, the NO / air mixture is sent to the sample. Once the read NO value (initial NO) has been stabilized, the visible lamp is turned on, the reactor is discovered and the sample is irradiated. There is a rapid decrease in the amount of

NO, which reaches a minimum in a few minutes (NO. The% NO conversion value is calculated on the basis of the initial NO value and the NO value according to the formula:

conversion% = [(initial NO- minimum NO) / initial NO] X 100

Operating conditions:

Sample quantity: 100 mg (50-70 mesh).

Flow rate: 1000 ml / min total

Irradiation intensity: 7W / m <2>

Temperature: 23-25 ° C.

The doped product obtained according to example 1, subjected to the photocatalytic activity test described above, showed a NO conversion of 88%.

In similar conditions, the reference product obtained according to example 1 described in US 2005/0226761 was tested: in this case the NO conversion was only 10%.

In parallel, a further product was prepared using the same methodologies and components of example 1, with the only difference that the mixture (O2 He) was replaced by nitrogen. This product, tested under the same operating conditions, showed a 91% NO conversion. This result shows that the presence of oxygen in the carrier gas does not contribute to obtaining the doped product according to the invention.

Claims (14)

CLAIMS 1. Process for the preparation of carbon-doped titanium dioxide, comprising irradiating titanium dioxide at a wavelength between 300 and 400 nm, said titanium dioxide being exposed to a gaseous flow comprising an inert gas and a organic compound. 2. Process according to claim 1, wherein the irradiation intensity is between 10 and 1000 W / m <2>. 3. Process according to claims 1-2, wherein the organic compound is selected from hydrocarbons or their derivatives optionally functionalized with one or more groups selected from alkyl, hydroxy, formyl, carboxy, alkoxycarbonyl, aryloxycarbonyl, amino, alkylamino, thio, alkylthio. 4. Process according to claims 1-3, wherein the organic compound is selected from among toluene, benzene, xylene, naphthalene, their derivatives and their mixtures. 5. Process according to claims 1-4, wherein the organic compound is ethylbenzene. 6. Process according to claims 1-5, wherein the gaseous stream includes the organic compound at a concentration comprised between 500 and 10000 ppm. Method according to claims 1-6, wherein the BET specific surface value of the titanium dioxide before and after the process remains substantially unchanged. 8. Process according to claim 7, wherein the substantially unchanged value of specific surface BET is comprised between 50 and 450 m <2> / g. 9. Process according to claim 7, wherein the substantially unchanged value of specific surface BET is comprised between 300 and 350 m <2> / g. 10. Process according to claims 1-9, wherein the obtained titanium dioxide has a carbon content comprised between 0.03 and 5% by weight. 11. Process according to claims 1-9, wherein the obtained titanium dioxide has a carbon content comprised between 0.3% and 3% by weight. 12. Process according to claims 1-9, wherein the obtained titanium dioxide has a carbon content comprised between 1% and 1.6% by weight. 13. Carbon doped titanium dioxide, obtained according to the process described in claims 1-12. 14. Titanium dioxide according to claim 13, characterized by the oxidation pattern at programmed temperature of Figure 1.