FR2939327A1 - Preparing nitrogen-doped titanium dioxide comprises hydrolysis of precursor of titanium dioxide with aqueous ammonia solution, recovering the product, cleaning the product with another aqueous ammonia solution, drying and calcining - Google Patents

Abstract

Preparing nitrogen-doped titanium dioxide (N-TiO 2) comprises: hydrolysis of a precursor of titanium dioxide with a first aqueous ammonia solution until it reaches substantially neutral pH; recovering the solid hydrolysis product; cleaning the product with a seconds aqueous ammonia solution; and drying and calcining the product.

Description

PROCESS FOR THE PREPARATION OF TITANIUM DIOXIDE HAVING PHOTOCATALYTIC ACTIVITY BY IRRADIATION WITH VISIBLE LIGHT

In its most general aspect, the present invention relates to a process for the preparation of a titanium dioxide having a photocatalytic activity by irradiation of light energy, in particular, but not exclusively, in the wavelength range of visible light.

In particular, the present invention relates to a process for the preparation of a titanium dioxide of the above type, wherein the titanium dioxide is doped with nitrogen. The use of titanium dioxide as anatase as a photocatalyst is widely known. In this connection, titanium dioxide can be used as such or more frequently, in compositions and products having photocatalytic properties, for decomposing organic substances, in particular environmental pollutants, in the presence of light and oxygen. From the moment titanium dioxide has a rather high bandwidth energy, it is able to absorb light radiation essentially in the ultraviolet range (i.e. with an equal wavelength at 400 nm or less). However, these light radiations are contained only in small amounts in sunlight reaching the Earth and this therefore limits the efficiency of the photocatalytic activity of titanium dioxide, and its possible applications.

To overcome these drawbacks, photocatalysts based on titanium dioxide suitably modified in its chemical structure have been studied and sometimes developed to obtain a photocatalytic response to irradiation with visible light. These photocatalysts are generally obtained by preparation processes in which the titanium dioxide is doped to a small extent, with appropriate doping elements, among which nitrogen, which must be substituted for the oxygen atoms in the chemical structure of the titanium dioxide, which therefore has a slightly lower oxygen content than the stoichiometric value.

A process for the preparation of a nitrogen-doped titanium dioxide photocatalyst (hereinafter also simply referred to as N-TiO 2) is described in the publication of T. Ihara et al Applied Catalysis B: Environmental 42 ( 2003) 403-409. This process provides for the hydrolysis of a precursor of titanium dioxide, more specifically of titanium (IV) sulphate [Ti (SO4) 2], by adding an aqueous ammonia solution to this precursor, up to reach a neutral pH. The hydrolysis product is washed with deionized water until a concentration of sulfate ions in the wash water of less than 0.5 mg / l is obtained, then dried and calcined. The N-TiO 2 material obtained with said preparation method has a photocatalytic response to irradiation with visible light and, under these conditions, it has been found to be capable of catalyzing the oxidation of acetone to carbon dioxide in phase gas. However, it is frequently observed that the N-TiO 2 obtained with said preparation process exhibits even large variations in the photocatalytic activity in the visible range, from one sample to another. Said method of preparation is therefore unsuitable for industrial application since the N-TiO 2 obtained from it is not very reproducible. The technical problem underlying the present invention is that of providing a process for the preparation of N-TiO 2 which is simple to use and which, at the same time, guarantees a suitable reproducibility of this photocatalyst, in particular with regard to relates to its photocatalytic activity, in order to overcome the aforementioned disadvantages of the art. This technical problem is solved by a process for the preparation of N-TiO 2, comprising the hydrolysis of a precursor of titanium dioxide with a first aqueous solution of ammonia until a substantially neutral pH is reached, the recovery of the product of solid hydrolysis, then its treatment by drying and calcination after at least one washing with a second aqueous solution of ammonia. It has surprisingly been found that the N-TiO 2 obtained with the method according to the invention has a high photocatalytic activity in the visible range, which remains substantially constant from one sample to another. The process according to the invention, which provides for the washing of the hydrolysis product of the titanium dioxide precursor with an aqueous solution of ammonia rather than water, thus makes it possible to prepare NTiO 2 having a high reproducibility and is advantageous for a industrial application. In the figures: FIG. 1 shows a diffractogram of a sample of N-TiO 2 according to the invention; FIG. 2 shows diffuse reflectance spectra of an N-TiO 2 sample according to the invention, compared with an undoped commercial TiO 2 sample; FIG. 3 shows diffuse reflectance spectra of samples of N-TiO 2 obtained by a method according to the known technique; FIG. 4 shows the reduction of NO on said samples of N-TiO 2 obtained by a method according to the known technique. FIG. 5 shows diffuse reflectance spectra of samples of N-TiO 2 obtained according to the method of the invention; FIG. 6 shows the reduction of NO on a sample of N-TiO 2 obtained according to the invention, and on a sample of commercial TiO 2; FIG. 7 shows the reduction of NO on a cement matrix containing 3% by weight of N-TiO 2 by irradiation with visible light; FIG. 8 shows diffuse reflectance spectra of N-TiO 2 obtained according to the invention, as such, and mixed (3 and 10% by weight) with a photocatalytically inert matrix of CaCO 3; FIG. 9 shows the reduction of NO on mixtures of N-TiO 2 (3 and 10% by weight) in CaCO 3 matrix, by irradiation with visible light, FIG. 10 shows the reduction of NO 2 on mixtures of N-TiO 2 (3 and 10%). by weight) in the CaCO3 matrix, by irradiation with visible light, - Figures 11 and 12 show the degradation of 2-propanol on a sample of N-TiO 2 obtained according to the invention by irradiation with visible light, Figures 13 and 14 show the degradation of 2-propanol on a mixture of 10% N-TiO 2 -CaCO 3 by irradiation with visible light. In the process according to the invention, the titanium dioxide precursor may be any hydrolysable titanium compound, usually a titanium salt. Preferably, the precursor is a sulphate-containing titanium salt selected from the group consisting of titanium sulphate (TiOSO4), titanium (IV) sulphate [(Ti (SO4) 2] and titanium (III) sulphate [Ti 2 (SO 4) 3] The hydrolysis of the titanium dioxide precursor is generally carried out from a solution of the precursor in water and preferably adding at a temperature between 20 ° C and 25 ° C, predefined amounts of an aqueous ammonia solution, until a pH of about 7 is reached. Preferably, the concentration of ammonia in the aqueous solution used for the hydrolysis of the precursor is between 20% and 28% by weight relative to the weight of the solution, and preferably is equal to 25% by weight relative to the weight of the solution Under the treatment conditions, the precursor is completely hydrolysed to form a solid hydrolysis product ( usually a titanium hydroxide) which precipitates in the medium In the process according to the invention, the solid hydrolysis product is separated from the liquid phase in a conventional manner, for example by decantation of the solid product and subsequent removal of the liquid phase or by filtration. According to the present invention, the hydrolysis product is washed at least once, preferably twice, with aqueous ammonia solution. The temperature of the wash solution is generally between 20 and 25 ° C. Preferably, the aqueous solution used for the washing has an ammonia concentration between 0.15 M and 0.30 M. According to a particularly preferred embodiment, the aqueous solution used for washing has an ammonia concentration. of about 0.23 M. Preferably, each wash is carried out using 400 ml of 0.23 M ammonia solution per gram of precipitate (corresponding to 1.56 g of NH3 / g of precipitate); using solutions having different ammonia molarities, a volume will be used such that it can reproduce the same ratio by weight ammonia / precipitate. At the end of washing (or washes), the solid product is recovered from the solution, then dried and calcined. Drying and calcination can be carried out conventionally per se, preferably at a temperature between 90 ° C and 100 ° C, in the case of drying and at a temperature between 400 ° C and 470 ° C for 2 hours in the case of calcination. The N-TiO 2 material obtained with the process according to the invention has a photocatalytic activity similar to that of the best known products, advantageously associated with a much higher level of reproducibility. In particular, N-TiO 2 as it is obtained according to the invention has a high photocatalytic activity in the reduction of atmospheric pollutants, such as NOx, by irradiation with visible light. In compositions or products containing N-TiO 2 obtained according to the invention, in particular compositions or cement products, the content of N-TiO 2 is preferably equal to at least 10% by weight. The photocatalytic activity of N-TiO 2, obtained according to the invention, in the reduction of NOx, is otherwise important by irradiation with UV light, both when N-TiO 2 is used as it is, and when it is contained in compositions or products, in particular of the cement-based type.

The N-TiO 2 obtained according to the invention also has a high photocatalytic activity by irradiation with visible light in the degradation of 2-propanol to CO2. Other features and advantages of the present invention will become apparent in the following examples given for information only and not limiting. 20 Experimental part

Example I

Preparation of N-TiO 2 according to the invention The precursor is a 5% by weight aqueous solution of titanium sulphate [Ti (SO4) 2], produced by Aldrich Japan. To 800 ml of water are added 16 ml of the titanium sulphate solution and 25% aqueous ammonia solution (Fluka product) is added dropwise under vigorous stirring to the resulting solution. until the pH 7 is reached. The suspension obtained is stirred mechanically for 5 h and then decanted for another 12 hours.

At this stage, the liquid phase is removed and 800 ml of distilled water containing 14 ml of 25% NH 3 are added to the solid hydrolysis product and the product is shaken vigorously for 2 hours. The suspension obtained is filtered and the solid is washed with 800 more ml of distilled water containing 14 ml of 25% NH 3. Finally, after having been dried in an oven at 100 ° C., the solid is calcined at 450 ° C. for 2 hours.

Example 2

Microstructural characterization The solid material prepared according to Example 1 was characterized by X-ray diffraction measurements (FIG. 1) with a Philips diffractometer, using CuKc radiation (X = 0.154 nm) and a scanning speed of 2 °. / min. X-ray diffraction was used to identify the crystalline phase and to evaluate the particle size. The size of the crystals was estimated using Scherrer's formula: c = KX / ((3cos6) where (ID is the size of the crystals, X is the wavelength of the X radiation used (0.154 nm) , K is a constant usually equal to 0.89 and 13 is the peak width at half height.Under the diffractogram analysis, the sample after calcination was found to be in 100% anatase form. did not find any variations in the XRD spectra, indicating that there was no appreciable formation of other phases, in addition to anatase, and the amount of doping nitrogen The estimate of the mean particle size according to the Scherrer formula gave a result of 26.5 nm as a result.

Example 3

Diffuse Reflectance Measurements The absorption of visible light by a sample of N-TiO 2 according to the invention, prepared according to Example 1, and that by a sample of undoped TiO 2 (commercial product PC-105 sold by Millenium Chemicals) were were measured by diffuse reflectance measurements and compared with each other.

In particular, the UV-visible spectra of said samples were recorded in the range 200-700 nm, using a Jasco V-570 diffuse reflectance spectrophotometer and using BaSO4 as a reference sample. As can be seen in Figure 2, the visible light absorption (X> 400 nm) by the product PC-105 is almost zero (reflectance around 100%), while for the sample of N-TiO2 according to the invention, there are spectral variations due to the presence of nitrogen in the reticle, which corresponds to the absorption of visible light. In particular, these spectral variations are characterized by a decrease in reflectance (increase in light absorption) in the 400-500 nm range.

Example 4 (comparative example on reproducibility of data)

N-TiO 2 obtained by a method known from the state of the art Two samples of N-TiO 2 (sample 1 and sample 2) were prepared by following, in the same way for the two samples, the procedure described by T. Ihara and al, in Applied Catalysis B .: Environmental 42 (2003) 403-409, thus carrying out precipitation by the Ti (SO4) 2 precursor and washing the precipitate with distilled water. Said samples were therefore subjected to diffuse reflectance measurements as indicated in Example 3 and to NO reduction measurements with irradiation at X> 450 nm. The results are shown in Figure 3 and Figure 4, respectively. Figure 3 shows a significant spectral variation between the two samples in the 400-500 nm wavelength range, with a decrease in reflectance (increase in visible light absorption) for the sample. 2. In addition, it is observed (FIG. 4) that the sample 1, by which the absorption at 450 nm linked to the presence of nitrogen is almost not visible, has a lower photocatalytic activity of elimination in% of NO compared to sample 2, demonstrating that washing with water gives samples with non-reproducible activity.

Example 5 (comparative example on reproducibility of the data) N-TiO 2 obtained according to the process of the invention Two samples of N-TiO 2 (sample 1 and sample 2) were prepared by following the procedure indicated in example 1. Said The samples were therefore subjected to diffuse reflectance measurements as indicated in Example 3 and the results are shown in FIG. 5. It may be noted that the spectra of the two samples are completely superimposable and are both characterized by a decrease in reflectance. (increase in light absorption) in the 400-500 nm range, already demonstrated above with reference to Example 3. In contrast to the data shown in Figure 4, it was observed in this case, high reproducibility for photocatalytic reactions of NO reduction. These results clearly demonstrate that the process according to the invention and, in particular, the washing with ammonia water, gives samples of N-TiO 2 having reproducible photocatalytic properties. Similar conclusions have been verified by starting with other precursors based on sulphate salts.

Example 6

Comparison between NO reduction with N-TiO 2 according to the invention and undoped commercial TiO 2. The following procedure was used to prepare a commercial TiO2 film (PC-105, Millenium Chemicals) or N-TiO 2 obtained according to the invention. 0.2 g of commercial TiO 2 anatase (or of N-TiO 2 obtained according to the invention) were dispersed in ethanol and deposited on a nylon inert support of 64 cm 2 of surface area. After evaporation of the solvent under vacuum, a semiconductor film is obtained in a highly dispersed form. NO reduction analyzes were performed with each of these films by irradiating with visible light (λ> 450 nm) and the results are shown in Fig. 6. For irradiation at λ> 450 nm, the commercial compounds are no longer active while the N-TiO2 prepared according to the invention is very active, even by radiation at wavelengths close to the maximum emission of the solar spectrum.

Example 7 Hotocatal Activity ti. on cement and matrix. hotochimi. CaCO3) NO NO reduction measures, Cement samples were prepared (surface area 64 cm 2), by mixing N-TiO 2 obtained according to the invention with a cement of type I 52.5 R (Italbianco, cement plant). Rezzato), in order to have a percentage by weight of the photocatalyst equal to 3%. These cement samples were tested for NOx reduction, irradiating with visible light (X> 420 nm). The results are shown in Figure 7. Some activity can be observed with respect to N-TiO 2 as it is. It has been assumed that the different behavior of N-TiO 2 on this cement matrix, as compared to N-TiO 2 as it is, should be attributed to a non-optimal concentration of this semiconductor in the cement matrix. In order to verify the validity of this hypothesis, the procedure was as described below. Since the photocatalytic response in the visible range is due to the presence of a band at X> 400 nm in N-TiO 2, as demonstrated by the reflectance spectrum shown in Figure 3, it is obvious that when the photocatalyst is "diluted" in the cement matrix, the absorption between 400 and 500 nm decreases accordingly. This seems to indicate that there is an optimal amount of TiO2 for the response in the visible range. Experiments were therefore carried out in which the N-TiO 2 catalyst was dispersed in a photochemically inert matrix of CaCO 3 and the UV-visible reflectance spectra of these mixtures were recorded (FIG. 8). In this figure, we see how, to have an appreciable absorption in the visible range (400-500 nm), it is necessary to prepare composite materials containing at least 10% of N-TiO2. The visible range response of these N-TiO 2 / CaCO 3 blends for NOx reduction is shown in Figures 9 and 10.5. In the case of NO, the photocatalytic activity of the 10% N-TiO 2 mixture by radiation with light at a wavelength λ> 450 nm, close to the maximum of the emission spectrum of sunlight, is good (Figure 9). Irradiation at lower wavelengths, but still in the visible range (X> 420 nm), of N-TiO2 / CaCO3 shows that the activity of the 10% sample is excellent (90% reduction in 60 minutes). The activity of the 3% sample is good (50% reduction in 60 minutes) and comparable to that of cement composite materials containing the same percentage (3%) of N-TiO2 (see Figure 7 and Figure 9) .

Example 8

Cement photocatalytic activity (CEN mortar): irradiation with visible UV light Photocatalytic activity (NOx reduction) was evaluated by irradiation with UV-visible light (with elimination of wavelengths below 310 nm) on cement samples (CEN mortar), containing respectively 1% and 3% by weight of N-TiO2. The results are shown in the tables below.

Mortar reduction test CEN Area S = 0.0061 m2 Luminous intensity L = 20 W / m2 Photocatalytic reduction of NO under UV-visible radiation TiO2 on cim. Conv. 30 '(%) Conv. 60%) F 30 m / h) AF 60 m / h) 1% 31 52 768.02 1287.82 3% 61 85 1613.40 2230.32 Photocatalytic reduction of NO2 under UV-visible radiation TiO2 on cim. Conv. 30 '(%) Conv. 60%) AF 30 m / h) AF 60 m / h) 1% 30 50 755.52 1237.85 3% 55 80 462.05 2112.7920 (AF: photocatalytic activity) It can be noted that composite materials of N TiO 2 in the cement matrix exhibit good activity for irradiation with UV-visible light, even for relatively low semiconductor quantities. Example 9

Photodegradation of 2-propanol with N-TiO 2 obtained according to the invention The photocatalytic activity of a sample of N-TiO 2 according to the invention, obtained as indicated in Example 1, was evaluated with regard to the degradation of 2-propanol. The rate of degradation of 2-propanol was evaluated in a reactor per Pyrex batch (diameter = 12.5 cm; Volume = 0.9 dm3) in a gas-solid state by irradiation with visible light. Thin layers of the dusts of said samples were prepared on a carrier, dispersing the obtained suspensions by mixing the powders of each sample with water on respective glass holders, which were then dried at 50 ° C for 30 minutes. minutes. Visible light experiments were performed using a 1500 W xenon lamp. A 1M KNO2 solution was used as a cut filter. The reactor was saturated with oxygen and then fixed amounts of 2-propanol were introduced. 500 l samples were taken at predetermined time intervals using a gas-tight syringe. The concentrations of 2-propanol and propanone were determined via a GC Shimadzu 17A gas chromatograph, equipped with an HP-1 column and FID developer, using He as gas transport. The amount of CO2 developed during photodegradation was monitored using an HP 6890 Gas Chromatograph, equipped with a Superlco CarboxenTM column. Figures 11 and 12 show the degradation of 2-propanol on the N-TiO 2 sample, depending on irradiation time with visible light. This photocatalyst showed high photocatalytic activity and optimal reproducibility. Indeed, for both tests, after 7 hours of irradiation, the disappearance of propanone was observed and simultaneously the concentration of CO2 reached the value corresponding to the total mineralization of 2-propanol. Figures 13 and 14 show the degradation of 2-propanol on a sample of 10% N-TiO 2 dispersed in CaCO 3 as a function of irradiation time with visible light. The sample shows a discrete photoactivity, also in the presence of the photochemically inert matrix (CaCO3). 2-Propanol disappears after 100 minutes of irradiation, while longer times are required for the complete disappearance of acetone, because the sample is diluted in the inert matrix.

Claims (7)

REVENDICATIONS1. Process for the preparation of N-TiO2 comprising hydrolyzing a titanium dioxide precursor with a first aqueous ammonia solution until a substantially neutral pH is reached, recovering the solid hydrolysis product and then treating it with drying and calcining after at least one wash with a second aqueous ammonia solution. 2. The method of claim 1, wherein said first aqueous ammonia solution has an ammonia concentration of between 20% and 28% by weight relative to the weight of the solution. 3. The process according to claim 1 or 2, wherein said second aqueous ammonia solution has an ammonia concentration between 0.15 M and 0.30 M. The process according to any one of the preceding claims, wherein said precursor is a titanium salt selected from the group consisting of titanyl sulfate (TiOSO4), titanium (IV) sulfate [Ti (SO4) 2] and titanium (III) sulfate [Ti2 (SO4) 3] The method of claim 4, wherein said precursor is titanium (IV) sulfate [Ti (SO4) 2]. 6. A process according to any one of the preceding claims, wherein said drying of the solid hydrolysis product is carried out at a temperature of between 90 ° C and 100 ° C. A process according to any one of the preceding claims, wherein said calcining of the solid hydrolysis product is carried out at a temperature between 400 ° C and 470 ° C for 2 hours.