MICROREACTOR FOR PHOTOCATALYTIC REACTIONS

Abstract

The present invention relates to a device for the photocatalytic reduction of a substance with a structured reaction plate and/or a structured housing, wherein the reaction plate has, at least in some regions, a surface which contains a material with negative electron affinity and which can be electronically excited with radiation having a wavelength of ≥180 nm.

Claims

1-25. (canceled)

26. A device for photocatalytic reduction of a substance comprising at least one reactor housing with a reaction plate disposed therein, the reaction plate and/or the reactor housing having a structuring and being insulated electrically from each other, the reactor housing consisting, at least in regions, of a material which is transparent for radiation of a wavelength of ≥180 nm, and the reaction plate having, at least in regions, a surface which comprises a material with negative electron affinity and which can be excited electronically with radiation of a wavelength of ≥180 nm.

27. The device according to claim 26, wherein the structuring of the reaction plate and/or of the reactor housing are/is suitable for the transport of a liquid thin film with a film thickness of max. 120 μm.

28. The device according to claim 26, wherein the structuring comprises at least one channel-like depression.

29. The device according to claim 28, wherein the at least one channel-like depression extends along a preferential flow direction of the liquid thin film.

30. The device according to claim 26, wherein the material with negative electron affinity is selected from the group consisting of doped or pure diamond, boron nitride, silicon carbide, gallium nitride, gallium arsenide, and mixtures thereof.

31. The device according to claim 26, wherein the material with negative electron affinity is bonded covalently to at least one photosensitizer.

32. The device according to claim 31, wherein the at least one photosensitizer is a compound with an absorption wavelength of ≥180 nm.

33. The device according to claim 31, wherein the at least one photosensitizer is an organic or organometallic colorant.

34. The device according to claim 33, wherein the organic or organometallic colorant is selected from the group consisting of rylene diimide derivatives, squaraines, porphyrines, phthalocyanines, xanthene colourant derivatives, metal complexes of the porphyrines and phthalocyanines, noble metal complexes, non-noble metal complexes, and mixtures thereof.

35. The device according to claim 26, wherein the reaction plate consists entirely of a material with negative electron affinity or consists of a substrate with a surface coating, comprising a material with negative electron affinity.

36. The device according to claim 35, wherein the substrate is a material selected from the group consisting of metal, non-oxide ceramic, plastic material, and mixtures thereof.

37. The device according to claim 26, wherein the device comprises in addition at least one direct radiation source and/or a reflector and/or a mirror.

38. The device according to claim 37, wherein the direct radiation source is selected from the group consisting of a laser, a light-emitting diode, a gas discharge lamp, and the sun.

39. The device according to claim 26, wherein the reactor housing has at least one inlet and at least one outlet for the supply of educts and discharge of products.

40. The device according to claim 39, wherein a first inlet is a liquid inlet.

41. The device according to claim 39, wherein a second inlet is a gas inlet.

42. The device according to claim 41, wherein the reactor housing, apart from the at least one inlet and at least one outlet, is fluid- and/or gas-impermeable.

43. The device according claim 26, wherein the reaction plate is connected to an external voltage source.

44. The device according to claim 26, wherein the reactor housing has a cooling circulation, independent of the reaction plate, for cooling the reaction plate.

45. A device for photocatalytic reduction of a substance, comprising a plurality of reactor housings with a reaction plate disposed therein.

46. A continuous method for photocatalytic reduction of a substance in a device comprising a reactor housing with reaction plate disposed therein according to claim 26, in which a) a liquid and the substance to be reduced are introduced into the device such that a liquid thin film is formed on the surface of the reaction plate, through which the substance to be reduced diffuses, b) the reaction plate is irradiated with light of a wavelength of ≥180 nm so that electrons are excited and emitted from the surface of the material with negative electron affinity, and c) the electrons reduce the substance to be reduced which diffuses through the liquid thin film.

47. The method according to claim 46, wherein the liquid thin film has a film thickness of 5 to 150 μm.

48. The method according to claim 46, wherein electrons of a photosensitizer being excited, which photosensitizer is connected to the material with negative electron affinity.

49. The method according to claim 46, wherein the substance to be reduced is a gas, liquid, or solid.

Description

EXAMPLE 1 FOR THE PRODUCTION OF A REACTION PLATE

[0067] By way of example, the process starts in this example with titanium as material for the substrate of the reaction plate. The substrate cut to size is processed with the help of spark erosion in order to provide it on both sides with channel-like depressions extending parallel to each other. Normally, the width of the channel-like depressions is 600 μm and the depth thereof 200 μm. Other dimensions are however likewise possible, e.g. 1,200 μm width and 400 μm depth or 300 μm width and 100 μm depth.

[0068] The number and the length of the channel-like depressions are essentially dependent upon the dimensioning of the device (e.g. the falling film microreactor), in which the finished reaction plate is intended to be inserted finally. It is assumed here, by way of example, that 32 channel-like depressions per side of the substrate are produced, which have respectively a length of 79.4 mm. The total channel volume is 609.8 μl in this case.

[0069] The substrate produced in this way is cleaned, subsequently electropolished and etched in a bath of a solution comprising HCl and sulphuric acid at increased temperature. Subsequently, the substrate prepared in this way is coated in order to provide a reaction plate in the sense of the present invention. The growth of the boron-doped diamond film is effected according to methods known from the literature (T. Grögler, E. Zeiler, M. Dannenfeld, S. Rosiwal, R. Singer, Diamond & Related Materials, 1997, 6, 1658-1667; T. Grögler, E. Zeiler, A. Hörner, S. Rosiwal, R. Zeiler, Surf Coat. Tech., 1998, 98, 1097-1091; E. Zeiler, T. Grögler, G. Heinrich, S. Rosiwal, R. Singer, Titanium '99: Science and Technology, Proceedings of the Ninth World Conference on Titanium, 2000, 1, 884-891.) and takes place in a CVD furnace under hydrogen gas-, methane gas- and trimethylborane flow. As a result, a homogeneous diamond layer of approx. 4 μm thickness is produced on the microstructured substrate.

[0070] The thus prepared reaction plate is vapour-coated at suitable points outside the structuring with a thin gold layer in order to provide electrical contact points.

EXAMPLE 2 METHOD FOR THE FUNCTIONALISATION/COATING WITH PHOTOSENSITISER

[0071] The functionalisation of the diamond surface is implemented wet-chemically and is based on the introduction of a linker unit with an azide group for subsequent coupling to an alkyne function (Click Chemistry).

[0072] In the first step, the diamond-coated reaction plate is made completely wet at 80° C. with an aqueous solution of isopentylnitrite and the linker molecule, e.g. 4-azidoaniline. The in situ diazotisation of the linker molecule leads, at increased temperature via splitting of molecular nitrogen, to the carbon-carbon bond formation to the diamond surface and hence to a functionalisation of the diamond surface with azide groups. The plate is thereupon cleaned and rinsed multiple times with water and acetone.

[0073] In the second step, the azide-functionalised reaction plate is made completely wet with an aqueous dimethylformamide solution which comprises an alkyne-functionalised photosensitiser, e.g. iron(II)-(4′ethinyl-2,2′:6′,2-terpyridine)(2,2′:6′,2-terpyridine), and also copper sulphate and sodium ascorbate. In this coupling step, the azide unit of the surface bonds with the alkyne unit of the photosensitiser forming a stable covalent-bonded triazole linker unit. The plate is cleaned and rinsed also after this step multiple times with water and acetone.

EXAMPLE 3 FOR A METHOD USING THE REACTION PLATE

[0074] By way of example, the reduction here of carbon dioxide (CO.sub.2) in an aqueous liquid thin film is illustrated. A reaction plate (both sides respectively 32 channel-like depressions; 600 μm deep; 200 μm wide; 64 mm long), coated with a boron-doped diamond film which is functionalised with a bis(terpyridine)iron(II) complex (λ.sub.max=580 nm), was incorporated in the described falling film microreactor.

[0075] By means of an HPLC pump, water is pumped into the reactor with a flow rate of 1 ml/min in order to wet the channel-like depressions with a continuous water thin film. By distributing the total water flow to all 64 channel-like depressions, a water thin film with a thickness of on average 50 μm with a dwell time of approx. 3 seconds is formed in each channel-like depression.

[0076] The CO.sub.2 is introduced into the device as gas and guided in counterflow to the liquid film. The gas flow rate is 20 ml/min and the gas flow is distributed uniformly on both sides of the reaction plate.

[0077] The system pressure is adjusted to 4 bar by a retaining pressure valve. A voltage is applied between the reaction plate in the falling film microreactor and a platinum net which is immersed in a product vessel. The electrical potential between the platinum net in the product vessel and the reaction plate in the falling film microreactor is maintained below 2 V. Under these conditions, an LED array which is used as radiation source is switched on and the reduction process starts.

[0078] The gas-liquid reaction mixture is collected in the product vessel and both the gas phase and the liquid phase are analysed by gas chromatography with mass detector. In the gas phase, there are CO.sub.2, CO, methane and ethane. In the liquid phase, formic acid, formaldehyde and methanol are contained.

EXAMPLE 4 FOR THE SELECTION OF THE LIQUID

[0079] For a microchannel dimension of 600 μm width and 200 μm depth, in the case of methanol (density: 0.79 g/ml at 20° C.; dynamic viscosity: 0.544 mPa s at 20° C.), a theoretical film thickness of 45 μm and at a flow rate of 0.5 ml/min is formed. This corresponds to a dwell time of approx. 8 seconds in a 79 mm long channel.

[0080] A fourfold flow rate of 2 ml/min leads to a film thickness of 70 μm and a dwell time of 1.1 seconds.

[0081] The change to water (density: 1 g/ml at 20° C.; dynamic viscosity: 1 mPa s at 20° C.) as solvent leads under the same technological conditions to a theoretical film thickness of 51 μm and 3.1 seconds dwell time (at 0.5 ml/min) or 80 μm film thickness and 1.2 seconds dwell time (at 2 ml/min).

[0082] The change to an even more viscous solvent, e.g. propylene carbonate (density: 1.21 g/ml at 20° C.; dynamic viscosity: 2.8 mPa s at 20° C.) leads correspondingly to a theoretical film thickness of 67 μm and a dwell time of 4.1 seconds (at 0.5 ml/min) or 106 μm and 1.6 seconds dwell time (at 2 ml/min).

[0083] In FIG. 1, the front- and rear-side of a reaction plate is shown. The reaction plate 1 is manufactured either completely from a diamond material (or another material with negative electron affinity) or composed of a substrate and a coating. The coating is applied on the front-side of the reaction plate and comprises at least one material with negative electron affinity. The structuring 13 is likewise present only on the front-side and consists of a large number of parallel, straight-line channel-like depressions. The channel-like depressions are situated on a surface which corresponds to at least 50% of the total surface of the reaction plate.

[0084] In FIG. 2, the side (left) orientated towards the reaction plate 1 and the side (right) orientated away from the reaction plate 1 of a first half 14 of the reactor housing is illustrated. The side of the base plate orientated towards the reaction plate 1 has a sink 15 for inserting the reaction plate 1. Furthermore, the illustration shows, on both sides of the reactor housing part, a window 16 made of a colourless material which is transparent for light of the wavelength ≥180 nm. Also the inlet for the heat exchanger liquid 17 and the outlet for the coolant liquid 18 can be seen in both illustrations.

[0085] FIG. 3 shows the side (left) orientated away from the reaction plate 1 and the side (right) orientated towards the reaction plate of a second half 19 of the reactor housing. The side orientated towards the reaction plate 1 has a window 16 made of a colourless material which is transparent for light of the wavelength ≥180 nm. Furthermore, the inlets and/or outlets (21, 22, 23, 24) and the corresponding distributor structures (25, 26, 27, 28) are shown. The liquid can be conducted through the liquid inlet 21 into the device. The liquid inlet 21 opens into the first distributor structure 25 which is configured as a slot and discharges the liquid uniformly over the entire width of the reaction plate 1. The first distributor structure 25 ensures that the reaction plate 1 is made completely wet. A gaseous substance to be reduced can be supplied to the device through the gas inlet 23. Here, a second distributor structure 27 is provided. The liquid discharge is ensured through the liquid outlet 22. This connects to a third distributor structure 26 to which a reverse function is assigned, namely collecting the liquid which is distributed over the entire width of the reaction plate 1. The gas discharge can be effected through the fourth distributor structure 28 and the gas outlet 24. Gas outlet 24 and gas inlet 23 can however also be exchanged in their function so that liquid and gas are guided in parallel flow within the device.

[0086] FIG. 4 shows an exploded illustration of a device according to the invention with a first half 14 of the reactor housing, a reaction plate 1 and a second half 19 of the reactor housing (left) and finished assembled device (right).

[0087] In FIG. 5, the device according to the invention is illustrated as module. A plurality of reactor housings with reaction plates contained therein are coupled together here.

[0088] FIG. 6 shows an embodiment of the device according to the invention in which a reactor housing 30 is irradiated with natural sunlight and a parabolic mirror 29 is installed in the light path of the sun behind the reactor housing. In this way, radiation can impinge on the device from two opposite directions.

[0089] FIG. 7 shows the front-side of a reaction plate 1a which has a modified structuring 13a. The modified structuring 13a consists of a plurality of parallel channels which have a sub-structuring 31. In the enlargement 32 of one of these modified, channel-like depressions, it can be detected clearly that the sub-structuring 31, which is configured here as herringbone pattern, has been made very filigree. Whilst the width of the channel-like depressions is approx. 1 mm, the width of the cavities which are produced by the sub-structuring is at less than 0.5 mm.

[0090] FIG. 8 shows the front-side of a reaction plate 1b. The reaction plate 1b thereby has channel-like depressions 13b which intersect at regular spacings. In the entire impression, a structuring with diamond-shaped raised portions is hence produced.

[0091] FIG. 9 shows schematically the electronic excitation states and the transfers of an electron in the method according to the invention. By absorption of a photon of energy hv1, an electron is excited from the highest occupied molecule orbital (HOMO Sens) into the lowest unoccupied molecule orbital of the photosensitiser (LUMO Sens). A further transfer of the electron into a level of the conduction band of the boron-doped diamond material (CB BDD) is triggered by the absorption of a second photon with the energy hv2. From this level, thermalisation to the band edge (CBM BDD) is effected, at the level of which then the emission of the electron from the crystal lattice onto the surface of the reaction plate 13 takes place. After the substance to be reduced has been reduced by absorption of an electron (reaction RX), the reaction products are collected in a product vessel 40. Also the counterelectrode 41 which typically consists of a platinum net, is immersed in the product vessel 40. The electron deficit in the HOMO of the sensitiser, which is produced by excitation and transfer of an electron into the LUMO thereof, is filled again by an electron from the valence band of the boron-doped diamond (VB BDD). The electron deficit (h.sup.+) produced consequently in the valence band of the boron-doped diamond material is filled again via the applied current source at low voltage.