The invention relates to a titanium dioxide-based photocatalyst containing carbon that is photoactive in the visible range, also referred to as vlp-TiO2 below.
The invention further relates to a method for producing a titanium dioxide containing carbon (vlp-TiO2) that is effective as a photocatalyst when irradiated with visible light.
Photocatalytic materials are semiconductors in which, when exposed to light, electron-hole pairs develop, which generate highly reactive free radicals on the material surface. Titanium dioxide is a semiconductor of this kind. It is known that titanium dioxide is capable of removing natural and artificial contaminants in air and water by irradiation with UV light, in that the atmospheric oxygen is reduced and the contaminants oxidized (mineralized) into environmentally friendly end products. In addition, the surface of titanium dioxide becomes superhydrophilic as a result of absorbing UV light. This is the basis of the anti-fogging effect of thin titanium dioxide films on mirrors and windows.
One serious disadvantage of titanium dioxide is the fact that it can only utilize the UV component of sunlight, i.e. only 3 to 4% of the radiation, and displays either only very weak catalytic activity, or none at all, in diffuse daylight.
Consequently, attempts have for some time been made to modify titanium dioxide in such a way that it can also utilize the main component of photochemically active sunlight—the visible spectral range between roughly 400 nm and roughly 700 nm—to produce the above-mentioned phenomena.
One way of making TiO2 photocatalytically active in daylight is to dope it with metal ions, such as V, Pt, Cr or Fe. Another possibility is to create oxygen vacancies in the TiO2 crystal lattice by reduction of Ti4+. Both developments require complex production techniques, such as ion implantation or plasma treatment.
Numerous patents describe nitrogen-modified titanium dioxide that is photocatalytically active when irradiated in the visible range (e.g. EP 1 178 011 A1, EP 1 254 863 A1).
It is furthermore known that modification with carbon increases the photocatalytic activity of titanium dioxide when irradiated with visible light. For example, JP 11333304 A describes a titanium dioxide whose surface at least partly displays a precipitate of graphite, amorphous carbon, diamond-like carbon or hydrocarbons. EP 0 997 191 A1 reports on a titanium dioxide with titanium carbide applied to its surface by means of vapor-phase deposition.
Photocatalytic materials in which titanium dioxide contains, inter alia, nitrogen, sulfur, carbon or other elements as anions, are disclosed in EP 1 205 244 A1 and EP 1 205 245 A1, for example. The anions are said to be located either at oxygen sites, at interstitial sites or at the grain boundaries of a polycrystalline titanium oxide particle. No information is given as regards the characterization of the material and about catalytic or physical properties.
The production of titanium dioxide containing 1.0 to 1.7% by weight carbon from titanium alcoholates by hydrolysis with hydrochloric acid and subsequent heating to 350° C. is also known (C. Lettmann et al., Applied Catalysis B 32 (2001) 215). In this case, the carbon originates from the ligand of the titanium compound.
According to a further publication, it has been found that hydrolysis of titanium tetrachloride with tetrabutylammonium hydroxide, followed by calcination for one hour at 400° C., yields a titanium dioxide preparation containing 0.42% by weight carbon (S. Sakthivel & H. Kisch, Angew. Chem. Int. Ed. 42 (2003) 4908). In this case, the carbon originates from the precipitant and is presumably dispersed relatively uniformly in the volume (volume doping).
The disadvantage of the known photocatalytic materials is that the methods for producing them are not suitable for industrial-scale production. Either the methods cannot be realized on an industrial scale for technical reasons, or they would then no longer be economical. In addition, most of the products obtained display insufficient photocatalytic activity in the degradation of pollutants in visible light in the range of λ>400 nm, and an only slight light-induced increase in hydrophilicity.
Moreover, the products have so far only been optimized in respect of their photocatalytic properties. The color and brightness, i.e. the optical properties, have been disregarded to date. However, use of a very bright vlp-TiO2 with little inherent color and high photocatalytic activity has advantages in all applications that tolerate only little or no inherent color of the vlp-TiO2, such as applications in coatings, specifically in paints and plasters.
The object of the invention is to provide a daylight-active, highly effective photocatalyst on the basis of a carbon-modified titanium dioxide, and to specify an economical method for producing it.
According to the invention, the object is solved by a titanium dioxide containing carbon which, compared to pure titanium dioxide, displays significant light absorption in the range of λ≧400 nm (vlp-TiO2) and whose electron spin resonance (ESR) spectrum measured at a temperature of 5 K displays only one significant signal in the g value range from 1.97 to 2.05.
The object is further solved by a production method in which a titanium compound having a specific surface area of at least 50 m2/g according to BET (Brunauer-Emmett-Teller) is mixed intimately with a carbon containing compound, and the mixture is treated thermally at a temperature of up to 400° C.
Further advantageous developments of the invention are indicated in the sub-claims.
The vlp-TiO2 according to the invention displays greater photocatalytic activity than the types described in the prior art. The measure of photocatalytic activity (hereinafter referred to as “photoactivity”) is the degradation of 4-chlorophenol by a defined quantity of vlp-TiO2 during 120-minute irradiation with light having a wavelength λ of ≧455 nm. The measuring method is described in detail below. Under the specified measuring conditions, the photoactivity of the vlp-TiO2 according to the invention is in the region of at least 20%, preferably in the region of at least 40%, particularly in the region of at least 50%.
The carbon content is in the range from 0.05 to 4% by weight, referred to TiO2, preferably 0.05 to 2.0% by weight, and particularly preferably 0.3 to 1.5% by weight. The best results are obtained with carbon contents of 0.4 to 0.8% by weight.
The titanium dioxide particles contain carbon in a surface layer only, and are thus referred to as “carbon-modified” below - as opposed to the volume-doped titanium dioxide produced according to Sakthivel and Kisch (2003). The carbon or carbon compounds of the vlp-TiO2 according to the invention are presumably primarily covalently bonded to the TiO2 surface via oxygen, and alkaline-leachable.
The photocatalyst can additionally contain nitrogen.
In contrast to unmodified TiO2, the vlp-TiO2 according to the invention absorbs visible light with a wavelength λ≧400 nm. In this context, compared to the value at 400 nm, the Kubelka-Munk function F(R∞), which is proportional to the absorbancy, is roughly 50% at 500 nm and roughly 20% at 600 nm.
The electron spin resonance (ESR) spectrum of the vlp-TiO2 according to the invention, measured at a temperature of 5 K, is characterized by a strong signal at a g value of 2.002 to 2.004, particularly 2.003. No further signals occur in the g value range from 1.97 to 2.05. The intensity of the signal at a g value of roughly 2.003 is increased by irradiation with light having a wavelength of λ≧380 nm (UV-free 100 W halogen lamp, KG5 cold-light filter), compared to the measurement in darkness.
The X-ray photoelectron spectrum (XPS) of the vlp-TiO2 according to the invention is characterized by the occurrence of a strong absorption band at a bond energy of 285.6 eV, referred to the O1s band at 530 eV.
A further characteristic is that, in contrast to the photocatalyst according to Sakthivel & Kisch (2003), the vlp-TiO2 does not display carbonate bands, either in the X-ray photoelectron spectrum (XPS), or in the infrared spectrum.
When irradiated with visible light, the vlp-TiO2 displays a water contact angle of roughly 8°, whereas unmodified TiO2 displays a contact angle of roughly 21°.
The new photocatalyst enables pollutant degradation not only using artificial visible light, but also with diffuse, indoor daylight. It can be used to degrade contaminants and pollutants in liquids or gases, particularly in water and air.
The photocatalyst can advantageously be applied to various substrates, such as glass (plain and metallized), wood, fibers, ceramics, concrete, building materials, SiO2, metals, paper and plastics, in the form of a thin layer. In combination with its simple production, this thus opens up potential applications in a wide variety of sectors, e.g. in the construction, ceramics and automotive industries for self-cleaning surfaces, or in environmental engineering (air-conditioning equipment, devices for air purification and air sterilization, and in the purification of water, especially drinking water) as well as for antibacterial and antiviral purposes. The photocatalyst can be used in coatings for interior and exterior purposes, such as paints, plasters and glazes for application to masonry, plaster surfaces, paint coats, wallpapers and wood, metal, glass or ceramic surfaces, or to components, such as composite heat insulation systems and curtain-wall facade elements, as well as in road surfacings and in plastics, plastic sheeting, fibers and paper. The photocatalyst can moreover be used in the production of prefabricated concrete elements, concrete paving stones, roof tiles, ceramics, decorative tiles, wallpapers, fabrics, panels and cladding elements for ceilings and walls in indoor and outdoor areas.
The light-induced increase in the hydrophilicity of the TiO2 surface gives rise to additional applications, such as non-fogging mirrors and windows in the sanitary sector or in the automotive and construction industries.
The photocatalyst is moreover suitable for use in photovoltaic cells and for photolysis.
The vlp-TiO2 according to the invention is described in more detail below with reference to FIGS. 1 to 11.
FIG. 1 shows the Kubelka-Munk function F(R∞) (arbitrary units), which is proportional to the relative absorbancy, for unmodified TiO2 and for C-modified TiO2 (vlp-TiO2) as a function of the wavelength, and indicates that, in contrast to unmodified titanium dioxide, the vlp-TiO2 absorbs in the visible spectral range. F(R∞) is roughly 50% at 500 nm and roughly 20% at 600 nm compared to the value at 400 nm.
FIG. 2 shows the electron spin resonance (ESR) spectra of the vlp-TiO2 according to the invention (spectrum A) and of the TiO2 produced according to Sakthivel & Kisch (spectrum B), recorded in darkness at a temperature of 5 K. Spectrum A displays only one significant signal at a g value of 2.003. In addition to the principal signal at a g value of roughly 2.003, spectrum B displays three further signals in the g value range from 1.97 to 2.05.
FIG. 3 contains the X-ray photoelectron spectra (XPS) of the vlp-TiO2 according to the invention (spectrum A) and of the previously known TiO2 according to Sakthivel & Kisch, precipitated from titanium tetrachloride with tetrabutylammonium hydroxide (spectrum B). The spectrum of the vlp-TiO2 displays a pronounced C1s signal at a bond energy of 285.6 eV, referred to the O1s absorption band at 530 eV, this indicating elementary carbon. In contrast, spectrum B displays C1s signals for elementary carbon at a bond energy of 284.5 eV, as well as additional bands at 289.4 eV and 294.8 eV, these indicating carbonate. Corresponding IR spectra likewise display typical carbonate bands at 1738, 1096 and 798 cm−1.
FIG. 4 illustrates the photocatalytic activity of vlp-TiO2 compared to unmodified TiO2 in the degradation of 4-chlorophenol (in the form of a 2.5×10−4 molar aqueous solution) by means of artificial visible light (λ≧455 nm). The diagram presents the decrease in the total content of organic carbon (TOCt) in the solution relative to the starting value (TOC0). With the vlp-TiO2, complete degradation is achieved after 3 hours.
FIG. 5 illustrates the photocatalytic activity of vlp-TiO2 compared to unmodified TiO2 in the degradation of 4-chlorophenol (in the form of a 2.5×10−4 molar aqueous solution) by means of diffuse, indoor daylight. The diagram presents the decrease in the total content of organic carbon (TOCt) in the solution relative to the starting value (TOC0). Even in low-intensity, diffuse daylight (7 to 10 W/m2 in the range from 400 to 1200 nm), the vlp-TiO2 leads to 80% degradation within six hours.
Even when illuminated by very-low-intensity diffuse daylight (1.6 to <1 W/m2), the vlp-TiO2 still displays significant photoactivity, unlike commercially available TiO2 photocatalysts (Degussa P25, Kemira UV-Titan, Sachtleben Hombikat, Tayca MT-100SA). The degradation rate of 2.5×10−4 molar 4-chlorophenol solution was measured as described above.
a) Light intensity: 1.6 W/m2; duration: 12 h