WO2014008953A1 - Photocatalytically active material for purifying air - Google Patents

Description

 Photocatalytically active material for air purification Description

The invention relates to a method for reducing particulate matter using a photocatalyst and a photocatalyst, which is active even in the absence of UV light

Photocatalysts are excited in a photoassisted catalytic reaction. This is used, for example, to generate oxygen radicals on the surface of photocatalysts, which in turn can then oxidize pollutants and thus make them harmless. A well-known photocatalytically active material is titanium dioxide (Ti0 ₂ ). To excite titanium dioxide UV light is necessary.

An object of the present invention was to utilize photocatalytic effects for new applications and also to provide new photocatalysts which are photocatalytically active even in the absence of UV light.

This object is achieved by a method for reducing particulate matter, which is characterized in that particulate matter is separated from a gaseous medium by means of a photocatalytic material.

In particular, the invention relates to a method for fine dust reduction, which is characterized in that fine dust is separated from a gaseous medium by means of a photocatalytic material in daylight.

In particular, the invention relates to a method for reducing particulate matter, which is characterized in that particulate matter from a gaseous medium is separated by means of a photocatalytic material without UV light.

According to the invention, it has been found that by using the photo-assisted catalytic reaction of photocatalytic materials, an electrostatic attraction of fine dusts takes place, which can thereby be separated from gaseous media, in particular air. In this case, the photocatalytic material is caused by the action of light of a suitable wavelength in an excited state catalyst. It is to maintain the reaction, a continued exposure to light required.

Photocatalytic material is also referred to herein as a photocatalytically active material.

The photocatalytic materials which can be used according to the invention preferably comprise a photocatalyst and optionally further materials such as, for example, binders. The photocatalyst, which may also be referred to as a photocatalytically active substance, preferably has semiconductor properties. In a photocatalytically active semiconductor, electrons can be put into an energetically higher state by excitation by light photons. If the energy of the incoming photon is sufficiently large, then an electron can change from an energetic state, for example from an energetic ground state, to the energetic state of the conduction band at room temperature. At the original part of the electron (negative charge carrier), a defect electron or hole is created, ie a positive charge carrier. At this point, an adjacent electron can change. In this process, a defect electron arises again, etc. As a result, the positive and negative charge carriers are distributed and can diffuse to the surface of the material. There arise now special charge distributions. These charge distributions cause an electrostatic attraction, through which fine dust is separated from air can be. This effect was first established by the inventors of the present application. In particular, according to the invention, fine dust particles can accumulate on the photocatalytically active surfaces through the existing charge distributions.

Insofar as the photocatalyst is a material which is also active when exposed to visible light, ie in particular also in the absence of UV light, the charge distributions are also formed without irradiation of UV light. Such materials can thus also be used for reducing particulate matter in the interior, for example in interior interiors or vehicle interiors, in which no photo-assisted catalytic reaction occurs when conventional photocatalysts such as titanium dioxide are used because of the absence of UV radiation

The inventive method can be carried out in daylight. In particular, it is carried out in daylight without additional UV light. In a preferred embodiment, the method is attenuated in daylight with a weakened portion of UV light, more preferably at least 20%, more preferably at least 30%, even more preferably at least 50%, and most preferably at least 80%, most preferably at least 90% Proportion of light with wavelengths of <350 nm, preferably <380 nm and in particular <390 nm performed. An example of such a preferred embodiment is indoor implementation in which daylight first has to pass through glass, for example window glass, which has only a partial UV transmittance, in a preferred embodiment the process is in daylight without UV light components performed. This, for example, when daylight passes through thicker glass and thus the UV light component is completely separated. However, the method according to the invention can also be carried out under artificial light, in particular with artificial light having a wavelength of 380 nm to 800 nm, in particular from 400 to 700 nm. The method according to the invention can also be carried out in environments in which no additional UV light is irradiated especially in environments where no UV light is present. Thus, the method according to the invention can be carried out in particular even in the absence of light having a wavelength of <400 nm, in particular <380 nm, even more preferably <350 nm.

According to the invention, the photocatalyst preferably comprises at least one of Sn, Zn, Bi, Ga, Ge, In, Ta, Ti, V, W, Sb or ΤΊ, in particular at least one of Sn, Zn, Ta, Bi, In, V, W, Sb, Ge, Ga, TI or / and titanium, and even more preferably Sn or / and Zn.

In a preferred embodiment, the photocatalyst Sn comprises. In another preferred embodiment, the photocatalyst comprises Zn. In yet another preferred embodiment, the photocatalyst comprises Sn and Zn.

In the photocatalyst, the elements are preferably in the form of compounds having semiconductor properties, for example as oxides. Most preferably, the photocatalyst comprises tin oxide (SnO ₂ ) or / and zinc oxide (ZnO).

An inventively used as a photocatalyst material is Sn0 _second Sn0 ₂ has an energy band gap of 3.5 to 3.7 eV corresponding to a wavelength range of 354 to 335 nm.

Charged SnO * has an energy band gap of about 2.9 eV corresponding to a wavelength of 428 nm. Thus, such Sn0 ₂ be excited by blue light in the electromagnetic spectrum of visible light.

A further preferred material used as the photocatalyst is ZnO. 5 ZnO has an energy band gap of about 3.37 eV corresponding to a wavelength of 368 nm.

In a particularly preferred embodiment, the photocatalyst used is a mixture of SnO ₂ and ZnO, in particular in a weight ratio of from 1:10 to 10: 1, more preferably from 1: 3 to 3: 1.

In order to further increase the usability of the photocatalysts in the process according to the invention, the band gaps of the active particles used are preferably reduced

5

 According to the invention, it is therefore preferable to use a photocatalyst which is doped with one or more elements, in particular selected from Co, C, N, P, S or H. Preferably, a photocatalyst is used which is doped with Co, C and / or N, most preferably with Co. When doping, foreign atoms are incorporated into the molecular structure of the photocatalyst. The doping reduces the band gap and thus the activation energy.

According to the invention, ZnO or / and SnO ₂ doped with Co 5, in particular Co doped ZnO, is particularly preferably used as the photocatalyst. The weight ratio Zn: Co is preferably from 8: 1 to 15: 1, in particular from 9: 1 to 11: 1 and even more preferably from 9.5: 1 to 10.5: 1. It has been found according to the invention that Co is incorporated into the crystal structure of ZnO. In particular, doped ZnO photocatalysts with particle sizes in a range from about 50 nm to 1.5 μm, in particular from about 100 nm to 1.2 μm, could be produced. Co-loaded ZnO has energy band gaps at about 1.8 eV and 2.7 eV, corresponding to a wavelength of 690 nm and 460, respectively nm. Due to the shift of the necessary activation energy in the visible wavelength range, these materials can be used as a photocatalyst even when irradiated with light without any UV component.

Doped photocatalysts can be obtained, for example, by wet-chemical or thermal processes.

In a further preferred embodiment, the photocatalyst is provided with one or more elements, in particular selected from Pb, Au, Ag, Pt, Al, Cu, Sb, Mo or Cd, preferably selected from Au, Pt, Ag, Sb, Fe, Al, Cd, Cu or Pb loaded. Most preferred is a loading of Au or / and Pt. Particular preference is given to loading with nanoparticles, in particular with nanoparticles, which have an average particle size of <10 nm, in particular <5 nm. In a particularly preferred embodiment, loading takes place with Au and / or R nanoparticles which have an average particle size -S 10 nm, in particular s 5 nm. During loading, smaller particles are brought into stable contact with larger particles by surface effects. Also, a loading reduces the band gap and thus the activation energy of the active substance. In a further preferred embodiment, different particles and / or different particle sizes are mixed for the photocatalyst. For example, the mixture of ZnO and SnO _{2 is} preferred. Furthermore, identical or different materials in different particle sizes are preferably used, for example in mixtures of particle sizes <50 nm, in particle sizes of 50 to <150 nm and in particle sizes of 150 to 300 nm. It has been found that the particle size also has an influence on the band gap , In particular, small particles are preferred. On the other hand offer larger particles more potential attachment surface for fine dust. Thus, according to the invention, preference is given to using a mixture of different particle sizes

In a particularly preferred embodiment, the photocatalytic material used according to the invention comprises

 (i) a photocatalyst as well

 (ii) a binder.

The photocatalyst is preferably used in the form of small particles. Preferably, the photocatalytic material has a particle size of έ 1000 nm, more preferably 500 500 nm, more preferably s 300 nm, most preferably 150 150 nm, and most preferably 50 50 nm. By particle size herein is meant, unless otherwise stated, the average particle diameter.

Silanes have proven to be particularly suitable as binders. Particularly suitable are tetra-alkoxysilanes and in particular tetra-ethyl-ortho-silicates (TEOS). The silanes preferably have a SiCVAnteil of 5 to 70 mol%, preferably from 10 to 50 mol% and in particular from 15 to 30 mol%.

As solvents, for example for applying the photocatalytic material to a substrate, preference is given to using alcohols, in particular primary, secondary and tertiary alcohols, and also water and mixtures thereof. Particular preference is given to using primary alcohols, in particular methanol, ethanol or propanol.

The compositions intended for applying the photocatalytic material to a substrate furthermore preferably comprise surfactants. Such surfactants prevent agglomeration of the active particles, so that the composition can be applied by spraying, for example. The process according to the invention is preferably carried out with light having a wavelength of 400 nm to 800 nm.

In a particularly preferred embodiment, the method according to the invention for fine dust reduction is carried out in an open system. An open system is characterized in particular by the fact that the normal air circulation is sufficient. The photocatalytically active coatings come out with little to no UV light. Other requirements that must be met by an open system are health safety of the coating and abrasion resistance.

In particular, no external energy must be supplied to an open system and there is no forced guidance of air flows, as is the case with closed systems, for example air conditioning systems. In closed systems, the photocatalytically active surfaces are inside a device. The pollutant-laden atmosphere, such as gases or bioaerosols are passed by forced air flow to these surfaces. The placement of the surfaces inside the device in closed systems has the advantage that they can be irradiated with strong, harmful UV light. However, such irradiation is not required according to the invention. In summary, it can be stated that, in contrast to conventional closed systems, the open coating systems preferred according to the invention do not require forced air guidance and no irradiation with wavelengths and / or intensities of electromagnetic radiation harmful to human health.

By virtue of the effect determined according to the invention, fine dusts can be removed from gaseous media, in particular from air. In particulate matter, these are in particular particles that are in the air in Floating state, so in particular particles with a mean particle diameter of 50 pounds μιτι.

Preferably, the fine dust particles having a size of -S 40 μπι on. However, according to the invention, it is also possible to separate fine dusts with a particle size of <350 nm and ultrafine dusts with a particle size -S 100 nm. Particularly with fine dusts with a mean particle diameter of £ 100 nm excellent results could be obtained. In the context of fine dust, however, pollen grains were also understood according to the invention which could also be successfully separated from air. Pollen grains usually have an average particle diameter of 10 pm to 30 pm.

Preferably, the photocatalytic material is applied to a substrate for the inventive method, which is then brought into contact with the gaseous medium. Suitable substrates are, for example, glass, plastic, textile structures, masonry, concrete, metal, ceramic, wood or / and composite materials. Particular preference is given to using glass as the substrate. By coating glass panes on the inside of a building or vehicle with a photocatalytic material, which is also active without UV light, fine dust can be separated continuously and without further measures. The photocatalytically active coatings are preferably used indoors, such as private living spaces, public buildings, hospitals, schools, etc. or in vehicles such as buses, passenger compartments of cars and trucks, watercraft or aircraft.

The application of the photocatalytic material to the substrate can be carried out by conventional methods, for example by spraying, rolling, painting, spraying, dip coating or vapor deposition. The present invention further includes a photocatalyst which is active without UV light. The photocatalyst has one or more elements selected from Sn, Zn, Ta, Bi, In, V, W, Sb, Ge, Ga, Tl or / and Ti, in particular Sn and / or Zn. Particularly preferred is a photocatalyst comprising SnO ₂ , ZnO or mixtures thereof. Particularly preferred is a photocatalyst doped with one or more elements selected from Co, C, N, P, S or H. Particularly preferred is a photocatalyst doped with Co, C and / or N, most preferably Co.

In a particularly preferred embodiment, the photocatalyst according to the invention comprises Co-doped ZnO. The weight ratio of Zn: Co is preferably 8: 1 to 12: 1, in particular 9: 1 to 11: 1.

Instead of or in addition to the doping elements, the photocatalyst according to the invention may be loaded with one or more elements, in particular selected from Au, Ag, Pt, Al, Cu, Sb, Mo or Cd. Most preferred is a loading of Au or / and Pt.

Particularly preferred is a photocatalytic material comprising such a photocatalyst and a binder, wherein the binder is selected from silanes. The binder preferably comprises tetra-alkoxysilanes and in particular tetra-ethyl-ortho-silicates (TEOS). The S1O2 content of the silane is preferably 5 to 70 mol%, preferably 10 to 50 mol%, and more preferably 15 to 30 mol%. In a particularly favorable embodiment of the method according to the invention for fine dust reduction of air, fine dust is separated from air by means of a photocatalytic material applied to glass panes without UV light. For this embodiment, the photocatalyst preferably has particle sizes of less than 300 nm in order to be transparent to the human eye. The invention will be further elucidated by the attached figures and the following examples.

Figure 1 shows XRD analyzes of SnQr samples.

Figure 2 shows XRD analyzes of ZnO samples.

FIG. 3 shows an XRD measurement of Co-doped ZnO. The incorporation of Co into the crystal structure of ZnO is clearly visible.

FIG. 4 shows a scanning electron micrograph of Co-doped ZnO particles.

Examples

example 1

Co-doped ZnO

ZnO was doped with Co by two different methods, namely wet-chemical and thermal. The weight fraction Zn: Co was in each case 10: 1. It was determined by means of energy-dispersive X-ray analysis (EDX) that the mass ratio of the elements in the doped photocatalyst corresponded to the mass ratio of the educts, ie a ratio Zn: Co = 10: 1. By UV / Vis NIR analysis, it was found that Co-doped ZnO has at least two active band gaps at about 1.8 eV (690 nm) and 2.7 eV (460 nm). The particle sizes of the photocatalyst obtained are in a range of about 100 nm to 1.2 μπι (mean particle diameter), as determined by scanning electron microscopy (SEM) (see Figure 4). On the basis of cathodoluminescence measurements it could be confirmed that almost all ZnO was converted. The two ZnO-doped photocatalytic materials (once wet-chemically and once thennically doped) were applied as coatings to substrates. Fine dust reduction measurements resulted in a reduction of fine dusts after 30 minutes to 8% and 6.35%, respectively. In the case of control measurements without the photocatalytically active material according to the invention, the corresponding final value was significantly higher, namely at 15.84%. This shows that with the photocatalytic active coatings according to the invention air purification processes can be significantly accelerated.

Claims

claims Method for reducing particulate matter, characterized, that particulate matter is separated from a gaseous medium by means of a photocatalytic material. Method for reducing particulate matter, characterized, that fine dust is separated from a gaseous medium by means of a photocatalytic material in daylight. Method according to claim 1 or 2, characterized, that fine dust is separated from a gaseous medium by means of a photocatalytic material without UV light. Method according to one of the preceding claims, characterized, in that the photocatalytic material comprises a photocatalyst and a binder Method according to one of the preceding claims, characterized, in that the photocatalyst comprises at least one of Sn, Zn, Bi, Ga, Ge, In, Ta, Ti, V, W, Sb or Tl. Method according to one of the preceding claims, characterized, the photocatalyst comprises Sn or / and Zn. 7. The method according to any one of the preceding claims,  characterized,  in that the photocatalytic material comprises silanes as binders. 5 8. The method according to any one of the preceding claims,  characterized,  in that the photocatalyst is doped with one or more elements, selected from Co, C, N, P, S or H. 9. Method according to one of the preceding claims,  characterized,  the photocatalyst is loaded with one or more elements selected from Pb, Au, Ag, PL Al, Cu, Sb, Mo, Fe or Cd. 10. The method according to any one of the preceding claims,  characterized,  the photocatalytic material is applied to a substrate, in particular to glass, metal, building materials and / or ceramics. 30 1. Method according to one of the preceding claims,  characterized,  that the method for fine dust reduction is carried out in an open system. 12. Method according to one of the preceding claims,  characterized,  that it is carried out in the absence of light with a wavelength <350 nm. o 13. The method according to any one of the preceding claims,  characterized, that the particulate matter is particles having a mean particle diameter of £ 50 pm, in particular £ 100 nm. Photocatalyst, which is active without UV light,  characterized,  in that the photocatalyst comprises one or more elements selected from Sn, Zn, Ta, Bi, In, V, W, Sb, Ge, Ga, Tl or / and Ti. 15. Photocatalyst according to claim 14,  characterized,  that it comprises tin oxide and / or zinc oxide.  10  16. Photocatalyst according to claim 14 or 15,  characterized,  the photocatalyst is doped with one or more elements selected from Co, C, N, P, S or H, or the i5 photocatalyst with one or more elements selected from  Pb, Au, Ag, PL AI, Cu, Sb, Mo, Fe or Cd. A photocatalytic material comprising a photocatalyst, in particular a photocatalyst according to any one of claims 14 to 16, and a binder,  characterized,  that the binder is a silane. Method for reducing particulate matter of air,  characterized,  that fine dust is separated from air by means of a photocatalytic material applied to glass panes without UV light. 19. The method according to claim 18, 0 characterized  the photocatalyst has particle sizes of <300 nm.