DE19757496A1 - Photocatalysts comprising metal oxide and metal ions - Google Patents

Abstract

Photocatalysts that absorb light in the visible wavelength region (380-780 nm) comprise a homogeneous mixture of at least 60% Ti, Zn, Fe, Mn, Mo and/or W oxide(s) and less than 40% Pt, Rh, Mn, Cr, Ru, Ni, Pd, Fe, Co, Ir, Cu, Mo, Zr, Re, Ag and/or Au ions in oxide or halide form. Also claimed is a process for producing the above photocatalysts by polymerisation or polycondensation of hydrolysable soluble compounds of the above metals at pH 0 to 10 in a sol-gel process at pH value between 0-7, followed by mild drying and slow calcination.

Description

The invention relates to photocatalysts, consisting essentially of Metal oxides doped metal oxides with special photocatalytic activity, their production in a one-step sol-gel process and their utilization of light in the wavelength range of visible light.

The use of photocatalysts for the environmentally friendly treatment of wastewater is becoming increasingly important. Previous photocatalysts have been based on the development of titanium oxide materials (MR Hoffmann, ST Martin, W. Choi, DW Bahnemann, Chem. Rev. 1995, 95, 69). These semiconducting materials generate highly active oxidants on their surface by irradiation with UV light and are therefore able to oxidatively decompose organic air and water contents without an additional external energy source (AL Linsebigler, G.Lu, JT Yates Jr., Chem. Rev. 1995, 95, 735). Various studies report increased photoefficiency with decreasing particle size (1 nm to 20 nm), other studies report that the massive phase is more active than nanoparticles (AJ Hoffmann, ER Carraway, MR Hoffmann, Environ. Sci. Technol. 1994, 28, 776). An increase in the photocatalytic activity of TiO ₂ was observed by doping TiO ₂ particles with various metal ions (W. Choi, A. Termin, MR Hoffmann, Angew. Chem. 1994, 106, 1148), but these materials show no activity visible light.

Most of the previously known photocatalytic processes for drinking water treatment require ultraviolet light, since TiO ₂ only absorbs in this spectral region (a) Schiavello, M., Ed. Photocatalysis and environment; Kluwer Academic Publishers: Dordrecht, 1988. (b) Ollis, DF; Al-Ekabi, H., Eds. Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993. (c) Fox, MA; Dulay, MT Chem. Rev. 1993, 93, 341. (d) Legrini, O .; Oliveros, E .; Braun, AM Chem. Rev. 1993, 93, 671. (e) Lewis, LN Chem. Rev. 1993, 93, 2693. (f) Matthews, RW Pure Appl. Chem. 1992, 64, 1285. EP 0633 064 A1). The use of such photocatalysts requires only weak UV light and permits the purification of water and air (Environmental Photocatalysis for Purification of Air and Water, Part 1. [In: Res. Chem. Intermed., 1997; 23 (3), Vinodgopal , K .; Editor, VSP: Utrecht, Neth. 1997). The cleaning of air with weak UV light can also be accomplished by applying TiO ₂ particles to paper (H. Matsubara, M. Takada, S. Koyama, K. Hashimoto, A. Fujishima, Chem. Lett. 1995, 767).

The greatest disadvantage of the previous applications of photocatalysts is their binding to high-energy UV light, which is not available to a sufficient extent both with increasing cloudiness and indoors. The development of photocatalytically active materials with the possibility of utilizing visible light is therefore of great technical interest. The photosensitization of titanium dioxide by subsequent doping with transition metal oxides or by adsorptive or covalent linkage with dyestuffs plays an outstanding role. An example of the former approach are mixed titanium-iron oxide colloids, which allow the degradation of dichloroacetate with visible light. However, this catalyst is decomposed during the reaction, a process that can only be partially suppressed by adding hydrogen peroxide (Bahnemann, DW; Bockelmann, D .; Goslich, R .; Hilgendorff, M. In Aquatic and Surface Photochemistry; Helz, GR ; Zepp, RG; Crosby, DG, Eds .; Lewis Publishers: Boca Raton, 1994, pp349-367.). The decomposition of NO _x with visible light is also reported for TiO ₂ , SrTiO ₃ , WO _x and SiC doped with V, Cr, Mn, Fe, Co, Ni or Cu (JP 09192496 A2); however, it has not been established whether these reactions proceed according to a semiconductor mechanism and whether these materials are capable of generating a photocurrent in a photoelectrochemical cell. Doping of nanocrystalline TiO ₂ with divalent and trivalent transition metal ions and its use in a photoelectrochemical cell has also been reported (WO 91/16719). The most important example of the second procedure, the photosensitization by means of a dye, is a nanocrstalline TiO ₂ electrode loaded with a ruthenium (II) bipyridyl complex (DE 42 07 659 A1). It is questionable whether the organic ligands are photostable with longer exposure times.

We have now found that, surprisingly, highly porous sol-gel materials based on metal oxide, preferably titanium dioxide, also enable photocatalytic degradation reactions and generation of a photocurrent with visible light. This visible light photocatalytic activity is linked to the presence of second ions in the porous metal oxide matrix. The material differs from known materials primarily in that the second ions are not only present on the inner and outer surface, but represent an integral part of the entire material. It is also important that the doping with the photocatalytic active second ions is an integral part of the production and is not carried out subsequently. Investigations of the materials with high-resolution transmission electron microscopy and X-ray backscatter analysis show that the photocatalytically active ions are evenly distributed in the material. EXAFS investigations (Extended X-ray Adsorption Fine Structure) using the example of the photocatalytically active Pt-TiO ₂ catalyst made from Na ₂ PtCl ₆ × 6 H ₂ O and titanium isopropylate demonstrate the presence of atomically isolated PtCl ₄ units in titanium dioxide. ESCA investigations (electron spectroscopy for chemical analysis) on the same material confirm that Pt is only in ion form. High-resolution transmission electron microscopic examinations show that the titanium oxide is present as an amorphous material with nanometer-sized crystalline domains enclosed therein, and Pt can be found homogeneously distributed throughout the material. Studies with Ar adsorption isotherms show that these materials consistently over large surfaces (greater than 10, typically 30 to 1000 m ² / g) and thus over a high porosity (pore size <0.5 nm, typically 0.8 to 10 nm) feature. This property is significant because the large surface area ensures a particularly effective interaction of the catalyst material with the medium (air or water).

Suitable catalyst materials with a large surface are materials which essentially contain more than 60% a metal oxide or mixtures of metal oxides of titanium, zinc, iron, manganese, molybdenum or tungsten and less than 40% ions from the group Pt, Rh, Contain Mn, Cr, Ru, Ni, Pd, Fe, Co, Ir, Cu, Mo, Zr, Re, Ag, Au and absorb the light in the visible wavelength range. TiO ₂ is preferably used as the metal oxide, preferably doped with Pt or Rh.

The photocatalysts presented thus work with visible light, and are not subject to decomposition. The production is preferably carried out wet chemical according to a one-step sol-gel process. The procedure has the advantage that it immediately supplies materials with large surfaces and that both the pore size and the concentration of the active centers in allow wide limits to vary through the process.

Since phenolic compounds occur in many waters as impurities, the breakdown of 4-chlorophenol (Hoffmann, MR; Martin, ST; Choi, W .; Bahnemann, DW Chem. Rev. 1995, 95, 69.) is based on the new one The process is described using some typical examples. Solar air purification is documented by the breakdown of acetaldehyde and ethyl mercaptan. Fig. 1 shows the decrease in the 4-chlorophenol concentration when exposed to visible light (455 nm) in the presence of a conventional (Degussa P25) and a new hybrid catalyst.

These new photocatalysts enable the much more effective use of solar energy and visible light in closed rooms to purify air and water and to generate electricity. A great advantage here is the optical transparency of the metal oxide used as the base material, e.g. B. TiO ₂ , which allows a much greater depth of penetration of the incident light into the material and thus a more effective use of the existing active centers. Through the use of wallpapers, which have been mixed with this photocatalyst, an efficient cleaning of the room air is conceivable during the entire period of use, here is especially the cleaning of the air in the presence of smokers, the removal of allergens in rooms with sick or allergy sufferers Cleaning in heavily used rooms such as restaurants, open-plan offices, production plants, at meetings and lectures, in school classes, and many other useful applications possible. Since visible light is much less harmful to wallpapers than UV light, the use of the catalysts presented here should not only affect the effectiveness of cleaning by using visible light, but also the effectiveness and durability of such applications. Air purification indoors and outdoors by photocatalytic degradation of pollutants can also be carried out by coating such thin photocatalytically active layer on the interior walls of rooms, exterior walls of buildings including window and roof areas, walkway slabs and road surfaces. In such applications in particular, the simplicity of manufacturing these photocatalysts and their adhesion to all mineral surfaces should have an advantageous effect. The use of visible light also enables the use of visible light to be used much more effectively: Since water itself is not completely transparent to UV light but visible light, the covering of the catalysts with water is much less critical than when using UV light using conventional technology. The use of the materials as a solar cell to generate electricity from visible light opens up completely new uses for solar cells, since, in contrast to silicon, the sol-gel materials presented here as a thin film at low cost on many materials, such as. B. tiles, glass panels, building cladding, etc. can be applied.

Example 1a-u, catalyst preparation Production of the metal-containing TiO ₂ catalysts 1a-1t

In a beaker covered with parafilm (flushed with argon), 2.4 ml of titanium isopropoxide (8.1 mmol) mixed with 8 ml of dry ethanol. After 30  min. 20 μl of 8N HCl were added with stirring. After 2 minutes, 20 µl conc. HCl added dropwise. After a further 5 min, 60 ul conc. HCl admitted. Finally, after a waiting time of 10 min, 60 µl conc. HCl added to the mixture. With stirring, a further 2 ml of ethanol admitted. The yellowish, clear mixture was mixed with the metal salt solution (see below) and the mixture was stirred for a few hours (12 h). That loose covered sol stand for another 6 days without stirring and gelled During this time. For mild predrying, the gel was left without Cover for another 10 days in the hood, hardening and became brittle. The resulting glass was now with a Heating rate heated from 0.1 ° C / min to 65 ° C at this temperature Hold for 100 min and then heat up at the same rate Heated to 250 ° C. The material was kept at this temperature for 5 h and cooled then with a cooling rate of 0.5 ° C / min to room temperature from.

To prepare various metal-containing catalysts, the following amounts of metal salt were dissolved in 5 ml of ethanol (metal salt solutions):

• a) 0.5 wt% Pt / TiO ₂ : 9.5 mg Na ₂ PtCl ₆ × 6H ₂ O = 1.7 × 10 ^-5 mol
• b) 0.7 wt% Pt / TiO ₂ : 13.4 mg Na ₂ PtCl ₆ × 6H ₂ O = 2.38 × 10 ^-5 mol
• c) 1.1 wt% Pt / TiO ₂ : 21 mg Na ₂ PtCl ₆ × 6H ₂ O = 373 × 10 ^-5 mol
• d) 2.0 wt% Pt / TiO ₂ : 38.1 mg Na ₂ PtCl ₆ × 6H ₂ O = 6.78 × 10 ^-5 mol
• e) 3.0 wt% Pt / TiO ₂ : 57.2 mg Na ₂ PtCl ₆ × 6H ₂ O = 1.02 × 10 ^-4 mol
• f) 4 wt% Pt / TiO ₂ : 76.4 mg Na ₂ PtCl ₆ × 6H ₂ O = 1.36 × 10 ^-4 mol
• g) 5 wt% Pt / TiO ₂ : 96 mg Na ₂ PtCl ₆ × 6H ₂ O = 1.71 × 10 ^-4 mol
• h) 1 wt% Cr / TiO ₂ : 44.34 mg Cr (acac) ₃ = 1.27 10 ^-4 mol
• i) 1 wt% Mn / TiO ₂ : 32.2 mg Mn (Oac) ₃ = 1.2 10 ^-4 mol
• j) 1 wt% Fe / TiO ₂ : 45.2 mg Fe (NO ₃ ) ₃ × 9H ₂ O = 1.18 × 10 ^-4 mol
• k) 1 wt% Ru / TiO ₂ : 26 mg Ru (acac) ₂ = 6.52 × 10 ^-5 mol
• l) 1 wt% Co / TiO ₂ : 32.2 mg CoCl ₂ × 6H ₂ O = 1.35 × 10 ^-4 mol
• m) 1 wt% Rh / TiO ₂ : 13.4 mg RhCl ₃ = 6.41 × 10 ^-5 mol
• n) 1 wt% Ir / TiO ₂ : 11.46 mg IrCl ₄ = 3.43 × 10 ^-5 mol
• o) 1 wt% Ni / TiO ₂ : 26.7 mg NiCl ₂ × 6H ₂ O = 1.12 × 10 ^-4 mol
• p) 1 wt% Pd / TiO ₂ : 54.9 mg PdCl ₂ = 3.1 × 10 ^-4 mol
• q) 1 wt% Cu / TiO ₂ : 27.2 Cu (acac) ₂ = 1.04 × 10 ^-4 mol
• r) 1 wt% Ag / TiO ₂ : 10.4 mg AgNO ₃ = 6.12 × 10 ^-5 mol
• s) 1 wt% Au / TiO ₂ : 10.16 mg AuCl ₃ = 3.5 × 10 ^-5 mol
• t) 1 wt% Zn / TiO ₂ : 13.8 mg ZnCl ₂ = 1.01 × 10 ^-4 mol

Example 1u Production of the metal-containing TiO ₂ catalysts (supercritical drying)

In a beaker covered with parafilm (flushed with argon), 2.4 ml of titanium isopropoxide (8.1 mmol) were mixed with 8 ml of dry ethanol. After 30 min, 20 μl of 8N HCl were added with stirring. After 2 minutes, 20 ul conc. HCl added dropwise. After a further 5 min, 60 ul conc. HCl added. Finally, after a waiting time of 10 min, 60 µl conc. HCl added to the mixture. A further 2 ml of ethanol were added with stirring. The yellowish, clear mixture was mixed with the metal salt solution (see below) and the mixture was stirred for a few hours (12 h). The loosely covered sol now stood for another 8 days without stirring and gelled during this time. The gelled pieces were placed in a 250 ml autoclave and 5 bar of N _{2 were} added. The autoclave was slowly (1 ° C / min) heated to 270 ° C, the pressure rising to about 22 bar. After the end temperature had been reached, the autoclave was slowly depressurized at 0.1 bar / min. The heating was then switched off and, after reaching room temperature, the autoclave was opened and the finished catalyst was removed.

Example 2

The catalyst (0.5 g / l) was suspended in an ultrasonic bath for 15 min in 14 ml of a solution of 4-chlorophenol (2.5 × 10 ^-4 M). The mixture was then magnetically stirred for 20 min and then exposed to an Osram XBO 150 W xenon lamp for 3 h with further stirring; an edge filter with a wavelength of λ ≧ 400 nm and λ ≧ 455 nm ensured the elimination of UV radiation. The degradation of 4-chlorophenol was followed by high pressure liquid chromatography.

Table 1

Example 3

Like example 2, but an edge filter λ ≧ 335 nm was used.

Table 2

Example 4

As example 2, no. 3. After exposure for 6 h, TiO ₂ / Pt was filtered off, washed with H ₂ O and used again as a photocatalyst. The reaction rate was now 90% of the original.

Example 5

Like Example 3, but the suspension was on October 6 and 7, 1997, respectively from 11.30 a.m. to 4.30 p.m. in diffuse sunlight in a glass bulb (λ ≧ 320 nm) stirred (see Tab. 1). (October 6: clear sky; October 7: cloudy). The degradation was 100 or 69% (Tab. 1, Example 5 and 8).

Example 6

The measurement of the apparent quantum yield for the degradation of 4-chlorophenol at the different wavelengths (335, 366, 400, 436, 546 nm) was carried out with an electronically integrating actinometer. 3.0 ml of a suspension of TiO ₂ / Pt (0.25 g I ^-1 ), which contained 4-chlorophenol in a concentration of 2.5 × 10 ^-5 M, were treated in an ultrasound bath for 15 min and then transferred to the substance cuvette. An SiO ₂ suspension with the same light scattering properties as TiO ₂ / Pt was filled into the solvent cuvette and both cuvettes were stirred magnetically. All measurements were carried out at 25 ° C.

Table 3

Example 7

In a conventional photoelectrochemical cell (3-electrode structure), an indium tin oxide electrode coated with TiO ₂ / Pt (layer thickness approx. 3 µm) is exposed from the back with light of different wavelengths and the resulting current is measured (Tab. 4).

Table 4

Action spectrum of the Degussa-P25 photocurrent and TiO ₂ / Pt hybrid catalyst

Example 8

Construction of a photovoltaic cell for power generation.

An aqueous paste of the catalyst was used to produce the photoelectrode (1c) applied to indium tin oxide glass and dried at 140 ° C. for 30 min. The glass plate was placed in the rubber-sealed cell introduced, covered with the redox electrolyte and with the second Indium tin oxide plate tightly sealed. Exposure to visible light leads to the occurrence of photo tension.

Claims (16)

1. Photocatalysts, consisting essentially of at least 60% an oxide of titanium, zinc, iron, manganese, molybdenum or tungsten or from mixtures of such oxides, and less than 40% from one or several ions, selected from the group Pt, Rh, Mn, Cr, Ru, Ni, Pd, Fe, Co, Ir, Cu, Mo, Zr, Re, Ag, Au in the form of their oxides or halides, absorption in the wavelength range of visible light (380-780 nm) and an even distribution of all atoms through the whole Material. 2. Photocatalysts according to claim 1, characterized in that the material has a porosity greater than 5% and a total surface area greater than 10 m ² / g according to BET. 3. Photocatalysts according to claim 1-2 with TiO ₂ as the metal oxide. 4. Photocatalysts according to claims 1-3 with Pt or Rh ions. 5. Photocatalysts according to claims 1-4 with a total surface area between 30 and 1000 m ² / g according to BET. 6. Photocatalysts according to claims 1-5 with a pore diameter between 0.8 and 4 nm. 7. The method for producing photocatalysts according to claim 1 by catalyzed polymerization or polycondensation of hydrolyzable soluble Compounds of the elements according to claim 1, preferably pure Alkoxy, mixed alkoxy, alkoxyoxo, or acetylacetonate derivatives or Chlorides, nitrates, citrates or other carboxylates of the selected elements, at pH 0 to 10 in a sol-gel process at pH between 0-7, followed by mild drying and slow calcining.   8. The method of claim 7, wherein the drying and calcining done slowly under normal pressure. 9. The method of claim 7, wherein the drying and calcining under supercritical conditions. 10. The method according to claims 7-9, wherein the final calcining temperature in Range is from 120 to 800 ° C. 11. Use of photocatalysts according to claims 1-6 for photocatalytic degradation of pollutants in air or water by exposure of light. 12. Use according to claim 11, wherein the catalyst is supported such as glass, ceramics, paper, mortar, cement, wood, plastic or natural material is applied. 13. Use according to claim 11-12, wherein the pollutant degradation with light in Wavelength range of visible light is carried out. 14. Use according to claim 11-12, wherein the pollutant degradation with Sunlight is carried out. 15. Use according to claim 11-12, wherein the pollutant degradation with artificial light is performed. 16. Use of the photocatalysts according to claims 1-6 for the production of photocurrent based on the principle of a photoelectrochemical solar cell, the photocatalyst being used as a photoactive layer.