Patent Application
20100087310
This application claims priority to German patent application Serial Nos. DE 10 2008 050 133.6 filed Oct. 2, 2008 and DE 10 2009 017 409.5 filed Apr. 8, 2009.
The invention relates to a heptazine modified photocatalyst based on titanium dioxide that is photoactive in the visible range. The new photocatalyst permits pollutant degradation not only with artificial visible light but also with the diffuse daylight in rooms. The invention furthermore relates to a method for manufacturing a heptazine modified titanium dioxide that is effective as a photocatalyst when irradiated with visible light.
Photocatalysts are substances that form highly reactive oxygen radicals on their surface by absorbing light. These radicals can oxidise (mineralise) pollutants in air and water to form inorganic end products. In the case of titanium dioxide, however, this requires UV light, which accounts for only roughly 3% of sunlight. There are consequently many attempts to modify titanium dioxide in such a way that it can also utilise the main component of photochemically active sunlight, corresponding to a wavelength range from roughly 400 nm to roughly 700 nm.
Modification of this kind can essentially be accomplished in three ways. First, by doping with transition elements, such as platinum, iron, chromium and niobium. Second, by doping with main-group elements, such as nitrogen (e.g. EP 1 178 011 A1, EP 1 254 863 A1) and carbon (e.g. JP 11333304, EP 1 205 244 A1, EP 0 997 191 A1, DE 10 2004 027 549 A1). Third, by sensitisation with dyes.
The latter method is primarily used for generating electricity in photoelectrochemical cells, and there are only few reports regarding the use of such systems for oxidative elimination of pollutants in air and water. The reason for this is that most dyes are not photostable in the presence of titanium dioxide and air, likewise being degraded themselves after only brief exposure. These photocatalysts are customarily obtained by preparing a suspension of titanium dioxide in a dye solution. This results in physisorption of the dye on the surface of the solid. A characteristic example is the TiO2/metal phthalocyanine system (metal: Fe, Cu), as reported in patent application CN 2005-10111249. These systems thus appear hardly suitable for use in technical applications.
WO 02/38272 A1 discloses the manufacture of UV photoactive transparent TiO2 films prepared from TiO2 precursor compounds via a sol-gel process. For improvement of the UV photoactivity and for chemical stabilisation of the films on the carrier the TiO2 is doped by mixing an s-triazine-derivate, urea or dicyanamide to the liquid TiO2 precursor compound. The doping imparts stability against treatment with alkalies and a higher photocatalytic activity in the ultraviolet spectral region to the photocatalytic TiO2 film.
Y. Nosaka et al. (“Nitrogen-doped titanium dioxide photocatalysts for visible response prepared by using organic compounds”, Science and Technology of Advanced Materials 6 (2005), 143-148) disclose N-doped visible light photoactive TiO2 photocatalysts prepared by calcining powderous TiO2 together with guanidine carbonate, guanidine hydrochloride and urea at 350 to 550° C.
Kisch et al. (“A low-bandgap, nitrogen modified titania visible light photocatalyst”, J. of Physical Chemistry C 211 (2007) 11445-11449) report on visible light photoactive titanium dioxide which has been produced by calcining a mixture of titanium hydroxide and urea at 400° C.
The present invention is an innovative heptazine modified photocatalyst and a method for manufacturing a photochemically and thermally stable titanium dioxide photocatalyst in which a metal-free sensitiser is bonded to the surface of the semiconductor in covalent fashion. According to the invention, a titanium compound is mixed with at least one heptazine derivate or oligo-heptazine derivate or with at least one precursor of a heptazine derivate or an oligo-heptazine derivate and subjected to thermal treatment at temperatures of about 300 to about 500° C., preferably at about 400° C. The TiO2 photocatalyst is also referred to as TiO2—(N═C)x below. As used herein, heptazine derivate includes oligo-heptazine derivate. In this context, the expression “(N═C)x” symbolises oligonuclear azine compounds, where x is a positive integer. When necessary, the precursor compound of the (oligo-)heptazine derivate is also added to this acronym, e.g. melamine (TiO2—(N═C)x/melamine). The TiO2—(N═C)x photocatalyst obtained in this way is characterised in that it degrades pollutants with visible light (λ>400 nm).
a-9c illustrate structures for heptazine and heptazine derivates.
The TiO2—(N═C)x according to the invention possesses greater photocatalytic activity than the types described in the prior art. This activity is measured on the basis of the degradation of formic acid by a defined quantity of TiO2—(N═C)x during 120-minute irradiation with light having a wavelength ≧455 nm. The nitrogen content is 0.70% to 2.50% by weight, referred to titanium dioxide, preferably 0.70% to 2.20% by weight, and particularly preferably 0.60% to 1.90% by weight. The carbon content is in the range from 0.10% to 2.00% by weight, referred to TiO2, preferably 0.30% to 1.50% by weight, and particularly preferably 0.50% to 1.20% by weight. The hydrogen content is 0.50% to 2.00% by weight, referred to TiO2, preferably 0.50% to 1.50% by weight, and particularly preferably 0.80% to 1.20% by weight.
In contrast to unmodified TiO2, the TiO2—(N═C)x according to the invention absorbs visible light with a wavelength of λ≧400 nm (
The X-ray photoelectron spectrum (XPS) of TiO2—(N═C)x is characterised preferably by the occurrence of an absorption band at a bonding energy of about 400.0 eV, referred to the O1s band at 530 eV (
The TiO2—(N═C)x according to the invention preferably displays a quasi Fermi potential of −0.45 to −0.52 V (rel. to NHE) at pH 7 (
A surface layer of the titanium dioxide particles contains a heterocyclic aromatic compound of the heptazine derivate or oligo-heptazine derivate type, which is bonded to the titanium dioxide probably in covalent fashion via Ti—N bonds (
The new photocatalyst permits pollutant degradation not only with artificial visible light, but also with the diffuse daylight in rooms. It can be used to degrade contaminants and pollutants in liquids or gases, particularly in water and air.
The photocatalyst can advantageously be applied as a thin layer to various substrates, such as glass, wood, fibres, ceramics, concrete, building materials, SiO2, metals, paper and plastics. Together with simple manufacture, this opens up application options in a variety of sectors, such as for self-cleaning surfaces in the construction, ceramics and automotive industry, or in environmental engineering (air-conditioning equipment, equipment for air purification and air sterilisation, and in water purification, particularly potable water, e.g. for antibacterial and antiviral purposes).
The photocatalyst can be used in coatings for indoor and outdoor purposes, such as paints, plasters, varnishes and glazes for application to masonry, plaster surfaces, coatings, wallpapers, and wood, metal, glass or ceramic surfaces, or on components, such as composite heat insulation systems and curtain-type façcade elements, as well as in road surfacings and in plastics, plastic films, fibres and paper. The photocatalyst can moreover be used in the production of prefabricated concrete elements, concrete paving stones, roof tiles, ceramics, floor and wall tiles, wallpapers, fabrics, panels and cladding elements for ceilings and walls in indoor and outdoor areas.
Since TiO2—(N═C)x is stable in air at up to 400° C., it can be used in extrusion systems in the plastics industry. It is moreover suitable for use in photovoltaic cells and for water splitting.
The TiO2—(N═C)x according to the invention is described in more detail below in reference to
a)Cyanuric acid
nEF*
a)By elemental analysis
b)Cyanuric acid
The method according to the invention consists in a titanium compound including titanium dioxide with a specific surface of at least 30 m2/g (according to BET) being mixed, preferably intimately mixed, with at least one heptazine derivate or oligo-heptazine derivate or with at least one heptazine derivate precursor or oligo-heptazine derivate precursor, referred to as an N,C compound below, and subsequently subjected to thermal treatment at about 300° C. to about 500° C., preferably at about 400° C.
The titanium compound is titanium oxide. In the following, as used herein, titanium oxide is understood to include titanium dioxide. It can be used in the form of a fine powder or a suspension. The titanium oxide may be of crystalline or semi-crystalline structure. The titanium oxide displays a specific surface of at least 30 m2/g according to BET.
The N,C compound can be of an organic or inorganic nature and must contain carbon and nitrogen. Compounds containing functional groups, such as OH, CN, SCN, CO, CHO, COOH, NHx and SO3H, have proven to be particularly suitable. Typical examples include cyanamides, thiocyanates like ammonium thiocyanate, melamine, cyanuric acid and other (N,C)xH precursors for heptazine derivates or oligo-heptazine derivates, as well as melem and melon, as shown in
The N,C compound can be used in the form of a solid, or a solution, or a suspension.
The titanium compound is mixed with the N,C compound in the production process. This can be done by dissolving the N,C compound in the suspension of the titanium dioxide or by mixing the suspension of the N,C compound with the suspension of the titanium compound. Intensive mixing of the N,C compound with a previously dried, powdery titanium dioxide is also possible. In the finished mixture of original titanium dioxide and N,C compound, the quantity of N,C compound referred to TiO2 is 1% to 40% by weight. If the finished mixture is present in the form of a suspension, it can be dried by familiar methods to obtain a powdery solid before further processing.
The finished mixture is subjected to thermal treatment at temperatures of about 300 to about 500° C., preferably at about 400° C. in the presence of air or oxygen/air mixtures. This leads to the formation of heptazine derivates and/or oligo-heptazine derivates, such as melem and melon, which are bonded to the titanium dioxide surface probably via covalent Ti—N bonds (see
The thermal treatment is preferably performed in such a way that the product (TiO2—(N═C)x) obtained has a nitrogen/carbon ratio of 1.30 to 1.85, preferably 1.40 to 1.70, particularly preferably 1.50 to 1.65. A colour change from white to yellowish occurs in the course of thermal treatment. The end product is preferably characterised by the fact that heptazine derivates, such as cyameluric acid, can be extracted with sodium hydroxide solution (see
The invention is described in more detail on the basis of the following examples, this not being intended to restrict the scope of the invention.
A mixture of 1 g of commercially available titanium dioxide (Sachtleben Hombikat UV 100) with twice the quantity of melamine is ground in an agate mortar and thermally treated in an open, rotating glass flask at 400° C. for 1 hour. After cooling to room temperature, the product is washed six times, using 40 ml double-distilled water each time, and then dried at 80° C. for 1 hour.
Same procedure as in Example 1, the difference being that cyanuric acid is used as the N,C compound.
Same procedure as in Example 1, the difference being that cyanuric acid in an ammonia atmosphere is used as the N,C compound.
Same procedure as in Example 1, the difference being that a mixture of melem and melon is used as the N,C compound. The melem/melon mixture is prepared by tempering 5 g melamine in an open Schlenk tube at 450° C. for 5 hours.
As a modification of Examples 1 to 5, thermal treatment is performed in a continuously operated rotary kiln.
Extraction of cyameluric acid: 0.8 g TiO2—(N═C)x/melamine are refluxed overnight in 80 ml 0.01 mol l−1 NaOH, and the supernatant solution is subsequently evaporated into a beige powder that is identified as cyameluric acid.
To coat a metal foil, a powder manufactured according to Examples 1 to 6 is suspended in a liquid, such as methanol or ethanol, using an ultrasonic bath, and the resultant suspension is applied to the foil as thinly as possible by means of a spray bottle. After subsequent drying at temperatures of up to 400° C., the procedure can be repeated until the required film thickness is reached. Other substrates can be used instead of the metal foil, e.g. paper, wood and plastic.
As a reference example commercially available unmodified titanium dioxide (Sachtleben Hombikat UV 100) was used.
a) Determination of the photoactivity (pollutant degradation) 20 ml of the powder suspension (1 g l−1) in 10−3 mol l−1 formic acid are treated in the ultrasonic bath for 15 minutes before the start of exposure. Subsequent exposure to determine the photoactivity is performed with an Osram XBO 150 W xenon short-arc lamp installed in a focusing lamp housing (AMKO, Model A1020, focal length 30 cm). The reactions are carried out in a water-cooled, 20 ml round cell with an inside diameter of 30 mm and a layer thickness of 20 mm. The reaction suspension can be stirred with a laterally mounted stirrer motor and stirring magnets. The cell is fixed at the focus of the lamp. The light is focused in such a way that only the reaction chamber of the cell is irradiated. All components are rigidly mounted on an optical bench. To eliminate UV light, a cut-off filter (Messrs. Schott) transmitting at λ≧455 nm is installed in the beam path. To prevent potential heating of the reaction chamber as a result of exposure, an IR filter is additionally fitted in the beam path. This filter is a water-filled cylinder (diameter 6 cm, length 10 cm). Samples taken are pressed through a micropore filter, and the formic acid is determined by means of ion chromatography. In no instance could oxalate be detected (Dionex DX120; column: Ion Pac 14, conductance detector; eluent: NaHCO3/NaCO3=0.001/0.0035 mol l−1); all activity data refer to the degradation after 3-hour exposure. Initial rates were calculated from the formic acid concentrations determined after one hour. The term photoactivity is used below to denote the percentage degradation measured after 3 hours.
b) Determination of the specific surface area according to BET (Brunauer-Emmett-Teller). The BET surface is measured according to the static volumetric principle, using a Tristar 3000 from Messrs. Micromeritics.
c) XPS measurements
The bonding energies were measured using a Phi 5600 ESCA spectrometer (pass energy of 23.50 eV; Al standard; 300.0 W; 45.0°).
d) Measurement of the reflectance spectra (Kubelka-Munk function) The reflectance spectra of the powders were measured using a Shimadzu UV-2401 PC UV/V is spectrometer equipped with an Ulbricht sphere. The white standard used was barium sulphate, with which the powders were ground in a mortar before measurement. The Kubelka-Munk function is proportional to the absorbance.
| Number | Date | Country | Kind |
|---|---|---|---|
| DE 102008050133.6 | Oct 2008 | DE | national |
| DE 102009017409.5 | Apr 2009 | DE | national |
