The invention is directed to a method of performing a

photooxidation and photoreduction reaction in a liquid phase and to an apparatus for performing said method.

The increasing demand for global energy has drawn much attention to the field of energy development. It is predicted that the annual amount of energy requirement will double in the next fifty years. At present, the main energy output comes from hydrocarbon fuels, and only 20 % comes from other energy sources such as tidal power, nuclear energy, biomass, photovoltaics, etc. Hydrocarbon fuels have many advantages over the other types of fuels, including easy storage and transportation, availability and a high volumetric energy density. The total amount of global hydrocarbon fuel available is limited and the large amounts of carbon dioxide emitted from burning hydrocarbon fuels are a significant drawback against their application. Moreover, safety and health issues behind the storage of hydrocarbon fuels are often ignored (viz. oil spills). Taking into account all these factors, the development of new sources of renewable and clear energy to replace fossil fuels has become one of the most important research topics.

Sunlight is the most abundant renewable energy source in the world. It has long been recognised that the sunlight falling on the earth's surface is more than adequate to supply all the energy that human activity requires. In addition, solar energy is clean, non-monopolised, and

environmentally friendly. The challenge is to collect and convert this dilute and intermittent energy to forms that are convenient and economical or to use solar photons in place of those from lamps. Solar technologies can roughly be divided into two main groups: thermochemical processes and photochemical processes. In thermochemical processes the solar radiation is converted into thermal energy that causes a chemical reaction. Such as chemical reaction is produced by thermal energy obtained from the sun for the general purpose of substituting fossil fuels. In photochemical processes solar photons are utilised by reactants and/or catalysts to cause a reaction. This path leads to a chemical reaction produced by the energy of the sun's photons, for the general purpose of carrying out new processes. The invention is concerned with photochemical conversion of solar energy.

Because they are very technologically and environmentally attractive, solar chemical processes have seen spectacular development in recent years.

In seeking applications in which the use of solar photons is possible and economically feasible, it is interesting to learn from nature. Natural photochemical phenomena have contributed to the evolution of life as we know it, and permit its continued existence on earth.

Photosynthesis is perhaps the most important of the many interesting photochemical processes known in biology. Not only was the evolution of the earth's atmosphere dependent on it, but also animal life derives energy from the sun via photosynthesis by eating plants. In natural photosynthesis, plants oxidise water in the PSII reaction centre leading to the formation of oxygen and production of reducing equivalents. These are further used in the Calvin cycle in the reduction of carbon dioxide. Following a similar mechanism, an artificial system can be designed not only to oxidise the water in a light driven process to produce oxygen, but also to drive the reduction of protons to yield hydrogen. Hydrogen has the highest energy density of fuels by weight, and the only combustion product of a hydrogen driven system is water. Due to these advantages, hydrogen generation by a sunlight driven water splitting process has been widely investigated.

Besides the concern of the limited amount of hydrocarbon fuels reserve, the impact of burning fossil fuels, and thereby causing carbon dioxide emission, is huge. While discussions have begun on means to reduce carbon dioxide emissions, it is apparent tat the atmospheric carbon dioxide concentration will continue to increase due to hydrocarbon consumption. An advanced solution for reducing carbon dioxide emissions is recycling carbon dioxide and converting it into a high-energy fuel. However, carbon dioxide is the thermodynamic final product of combustion, thus it is very stable and carbon dioxide conversion requires huge energy input. Carbon dioxide can be converted e.g. by thermochemical methods, electrochemical methods, and photochemical methods. Unlike photoreduction of carbon dioxide, thermal or electrochemical process requires very high temperatures or a strong external voltage bias to provide energy to drive the reaction, which lowers the efficiency of the device and limits its deployment. Photocatalysis can reduce carbon dioxide to produce hydrocarbons in a similar way to the Calvin cycle in natural photosynthesis. This is also another approach to convert and store solar energy.

The main hurdles in developing artificial photosynthesis devices include scalability, long term stability, price, and solar-to-fuel efficiency. Various research groups have focussed on optimising different aspects of such a device (such as the photocatalyst or electron/hole transport).

However, there has been less effort in designing a scalable device wherein the different aspects are integrated into one simple device. For example, US-A-2012/0 222 951, US-A-2009/0 013 593 and US-A-2009/0 016 948 each describe separate photovoltaic cells that are connected to an electrolysis device.

An objective of the invention is to provide a device suitable for solar-to-fuel applications having a simple design which allows it to be produced on large scales.

A further objective of the invention is to provide a device that is cost efficient, stable on the long term and low in production costs.

Yet a further objective of the invention is to provide a device which may be used for carrying out coupled photooxidation/photoreduction reactions. Yet a further objective of the invention is to provide a method of performing a coupled photooxidation and photoreduction reaction which does not require a costly proton selective membrane.

Yet a further objective of the invention is to provide a method allowing cost-efficient conversion of solar energy into fuel.

The inventors found that one or more of these objectives can, at least in part, be met by the device and method of the invention wherein photooxidation and photoreduction are physically separated and reaction products of the photooxidation and photoreduction reactions are removed in degassers and/or debubblers.

Accordingly, in a first aspect the invention is directed to an apparatus for performing a coupled photooxidation and photoreduction reaction, the apparatus comprising at least a first channel providing a first liquid flow path for a liquid to be treated and a second channel arranged adjacent the first channel providing a second liquid flow path downstream the first flow path for the liquid to be treated, wherein the first and second channels each comprise a light receiving surface provided with respective first and second photocatalysts arranged for initiating a photoreaction inside the respective channel to produce a photoreaction product, wherein downstream the first and second channels, first and second separators, respectively, are arranged for removing the photoreaction product from the liquid passing said respective separator, wherein the first and second channels are conductively interconnected such that the liquid passing along the first liquid flow path is in electrical contact with the liquid passing along the second flow path.

In a second aspect the invention is directed to a method of performing a coupled photooxidation and photoreduction reaction, said method comprising the steps of

i) feeding a first container with a liquid to be treated, said first container comprising a first photocatalyst and optionally a first light absorber, ii) exposing said first photocatalyst and/or optional first light absorber to light thereby initiating a first photoreaction in said first container and producing a first photoreaction product,

iii) removing at least part of said first photoreaction product by passing the liquid from said first container through a first separator,

iv) feeding the liquid of which at least part of said first photoreaction

product has been removed to a second container, said second container comprising a second photocatalyst and optionally a second light absorber,

v) exposing said second photocatalyst and/or optional second light absorber to light thereby initiating a second photoreaction and producing a second photoreaction product,

vi) removing at least part of said second photoreaction product by passing the liquid from said second container through a second separator, wherein one of said first and second photoreactions is photooxidation and the other is photoreduction, and wherein the liquid in said first container and the liquid in said second container are conductively connected.

The method of the invention for performing a coupled

photooxidation and photoreduction reaction is preferably performed in the apparatus of the invention. Likewise, the apparatus of the invention is preferably for carrying out the method of the invention.

In accordance with the method of the invention a first container is fed with a liquid to be treated. The first container may suitably be in the form of a channel. In case the first container is used for performing a photooxidation reaction, then the first container can be considered an oxidation channel. In case the first container is used for performing a photoreduction reaction, then the first container can be considered a reduction channel.

The liquid to be treated is preferably an aqueous liquid, such as water or an aqueous solution. The first container comprises a first photocatalyst. This first catalyst can be either a photooxidation catalyst or a photoreduction catalyst. The term "photocatalyst" as used in this application is meant to refer to a catalyst that is able to produce, upon absorption of light, a chemical transformations of reactants. The excited state of the photocatalyst repeatedly interacts with the reactants forming reaction intermediates and regenerates itself after each cycle of such interactions. The first

photocatalyst may be optionally be sensitised by a first light absorber present in the first container. The first photocatalyst and optional first light absorber may be provided as a thin layer on the wall of the first container. The first photocatalyst may suitably be present as a layer of nanop articles, which results in an increase of the surface area.

Photocatalysts are well-known in the art. They can be either inorganic or organic. Suitably photocatalysts include, for instance, metal oxides (such as T1O2, Fe203, ^O, ZnO, MgO and Sn02), CdS, and silicon. Also metal oxides doped with other elements such as carbon, nitrogen, fluorine, boron, platinum and/or gold may be employed. Some more sophisticated photocatalysts include Pt/WO ₃ , Ru02/TaON, B1VO4, B12M0O6, W0 ₃ ,Pt/SrTi0 ₃ :Cr,Ta, Pt/TaON, and Pt/SrTi0 ₃ :Rh. Of these photocatalysts Pt/WO ₃ , RuO ₂ /TaON, B1VO4, Bi ₂ MoO ₆ , and WO ₃ have been reported as useful oxygen evolution photocatalysts in photocatalytic water splitting, while Pt/SrTiO ₃ :Cr,Ta, Pt/TaON, and Pt/SrTiO ₃ :Rh have been reported as useful hydrogen evolution photocatalysts in photocatalytic water splitting. More specifically, the following combinations of oxygen evolution

photocatalysts and hydrogen evolution catalysts are considered appropriate for photocatalytic water splitting: WO ₃ as O2 photocatalyst with Pt/TaON as H2 photocatalyst, B1VO4 as O2 photocatalyst with Pt/SrTiO ₃ :Rh as ¾ photocatalyst, B12M0O6 as O2 photocatalyst with Pt/SrTiO ₃ :Rh as ¾ photocatalyst, and WO ₃ as O2 photocatalyst with Pt/SrTiO ₃ :Rh as ¾ photocatalyst. It is also possible to apply mixtures of different types of oxygen evolution photocatalysts and/or mixtures of different types of hydrogen evolution photocatalysts, for example to extend the usable range of the solar spectrum. Further, suitable catalysts can have a formula selected from A _x B _y C _z Ot or A _x ByC _z DqOt. In these formulae A, B, C, and D are independently selected from Si, Al, Fe, Ca, Na, K, Mg, Ti, P, Mn, F, Ba, C, Sr, S, Zr, W and V. Indexes x, y, z, q, and t are independently any positive number, typically less than 100.

Light absorbers are also well-known in the art. The first light absorber can suitably be selected from inorganic stable semiconductors (in particular narrow band-gap semiconductors), organic dyes, polymers, etc. Some suitably examples of semiconductor light absorbers include silicon, copper indium selenides, cadmium telluride and sihcon-hydrogen alloy. Also nanoparticles layers can be used, such as nanoparticles of titanium dioxide (T1O2), zinc oxide (ZnO), tin dioxide (Sn02), nickel oxide (NiO) and the like. Organic dyes such as are known and used in the solar cell arts will be apparent to those of skill in the art. Some well-known examples include azo dyes, methine dye, fullerene derivatives, quinones, coumarin, eosine, rhodamine, merocyanine, or the like; metal complex dyes such as porphyrin, phthalocyanine, or the like; or ruthenium complex dyes; and natural dyes such as those derived from plants, or the like. Some well-known examples of polymers include conjugated polymers, such as polyacetylene,

polythiophene, polyaniline, polypyrrole, polyfluorene, polyphenylene, and poly(phenylene vinylene). The light absorber can be an organic solar cell and/or a polymer photovoltaic cell.

The first photocatalyst and/or first light absorber are exposed to light, preferably sunlight or part of the solar spectrum. Preferably, the first container has at least one transparent section (or wall) through which the light can reach the first photocatalyst and/or first light absorber. Hence, in a preferred embodiment the first container is at least partly transparent, in particular for sunlight. The term "transparent" as used in this application is meant to refer to a material which transmit more than 50 % of incident light, preferably more 70 %, more preferably more than 80 %, such as more than 90 % or more than 95 %. Preferably, the first container is transparent at one or more wavelengths selected from 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm and 1000 nm. More preferably, the first container is transparent at two or more, or three or more wavelengths selected from 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm and 1000 nm.

Exposing the first photocatalyst to light will cause a first photoreaction to initiate, which photoreaction is selected from

photooxidation and photoreduction. For example, the first photoreaction in the first container can be photooxidation which means that the

photocatalyst oxidises chemicals in the liquid. In case of photocatalytic water splitting, oxygen and electrons will be generated in the oxidation channel by oxidation of water. The first photocatalyst and/or the first light absorber are preferably both in direct contact with the liquid in the first container.

The liquid that exits the first container and in which the first photoreaction has taken place is passed through a first separator in order to remove at least part of a product of the first photoreaction. In case the first photoreaction product is gaseous (such as oxygen), then suitably a degasser, a debubbler or a membrane contactor can be employed. If the product is dissolved in solution then first photoreaction product may be removed by membrane distillation. Preferably, the first separator is selected from a degasser, debubbler and a membrane contactor. More preferably, the first contactor is a degasser or debubbler.

After removal of at least part of the first photoreaction product, the liquid is fed to a second container. The second container may suitably be in the form of a channel. In case the second container is used for performing a photooxidation reaction, then the second container can be considered an oxidation channel. In case the second container is used for performing a photoreduction reaction, then the second container can be considered a reduction channel.

The second container comprises a second photocatalyst. This second catalyst can be either a photooxidation catalyst or a photoreduction catalyst. The second photocatalyst may be optionally be sensitised by a second light absorber present in the second container. The second

photocatalyst and optional second light absorber may be provided as a thin layer on the wall of the second container. The second photocatalyst may suitably be present as a layer of nanoparticles, which results in an increase of the surface area.

Photocatalysts are well-known in the art. Suitable examples for the second photocatalysts include those mentioned hereinabove for the first photocatalyst. Preferably, the first photocatalyst and the second

photocatalysts are not identical.

Light absorbers are also well-known in the art. Suitable examples for the second light absorber include those mentioned hereinabove for the first light absorber. Preferably the first light absorber and the second light absorber are not identical.

The second photocatalyst and/or second light absorber are exposed to light, preferably sunlight or part of the solar spectrum. Preferably, the second container has at least one transparent section (or wall) through which the light can reach the second photocatalyst and/or second light absorber. Hence, in a preferred embodiment the second container is at least partly transparent, in particular for sunlight.

Exposing the second photocatalyst to light will cause a second photoreaction to initiate, which photoreaction is selected from

photooxidation and photoreduction. For example, the second photoreaction in the second container can be photoreduction which means that the photocatalyst reduces chemicals in the liquid. In case of photocatalytic water splitting, hydrogen will be generated in the reduction channel by reduction of protons. The second photocatalyst and/or the second light absorber are preferably both in direct contact with the liquid in the second container.

The liquid that exits the second container and in which the second photoreaction has taken place is passed through a second separator in order to remove at least part of a product of the second photoreaction. In case the second photoreaction product is gaseous (such as hydrogen), then suitably a degasser, a debubbler or a membrane contactor can be employed. If the product is dissolved in solution then second photoreaction product may be removed by membrane distillation. Preferably, the second separator is selected from a degasser, debubbler and a membrane contactor. More preferably, the second contactor is a degasser or debubbler.

Preferably the first photoreaction product and/or the second photoreaction product is gaseous.

In accordance with the method of the invention the liquid in the first container and the liquid in the second container are conductively connected. This means that the liquid in the first container and the liquid in the second container are in electrical contact, i.e. electrons can freely travel from one container to the other and vice versa. Electrical conductivity may be provided, e.g. by a transparent conductive layer provided on the wall of the containers which electrically connects the liquid in the first container with the liquid in the second container. Various transparent conductive oxides are known in the art, including fluorine-doped tin oxide (FTO), indium tin oxide (ITO, also referred to as tin-doped indium oxide), indium zinc oxide (IZO), indium zinc tin oxide (IZTO), aluminium zinc oxide (AZO), indium tin oxide-silver-indium tin oxide (ITO-Ag-ITO), indium zinc oxide-silver-indium zinc oxide (ITO-Ag-IZO), indium zinc tin

oxide-silver-indium zinc tin oxide (IZTO-Ag-IZTO), and aluminium zinc oxide-silver-aluminium zinc oxide (AZO-Ag-AZO) and the like. Preferred transparent conductive oxides are fluorine-doped tin oxide and indium tin oxide. Thin transparent metal layers can also be employed. In an

embodiment, the transparent conductive layer is in the form of a

nanop article layer or a nanostructured layer. This advantageously increases the surface area. In case nanop articles are used, then the layer as such must still be electrically conductive. An example of such an embodiment is a layer of nanocrystalline T1O2 such as commonly known from Gratzel-type dye sensitised solar cells.

In an embodiment the first container is not in the form of a single container, but comprises a plurality of containers, each of which is fed with liquid to be treated, and each of which comprises first photocatalyst and optionally first light absorber. In such a case, it is preferred that the first photocatalyst and/or first light absorber in each of the first containers is exposed to light. Likewise, the second container can comprise a plurality of containers, each of which is fed with liquid from the first separator, and each of which comprises second photocatalyst and optionally second light absorber. In such a case, it is preferred that the second photocatalyst and second light absorber in each of the second containers is exposed to light.

In a further embodiment, the liquid exiting the second separator and of which at least part of the second photoreaction product has been removed is used as new feed for a method according to the invention. This embodiment may be particularly useful, when the method of the invention is used for a purifying application (such as for water purification).

The method of the invention can be used for photocatalytic water splitting. An example of using the method of the invention for photocatalytic water splitting is as follows. When the method of the invention is used for photocatalytic water splitting, the liquid that is fed into the first container comprises water (preferably is water). In case the first container is the oxidation channel, then photooxidation of water takes place which

photoreaction generates oxygen gas and electrons. Since the liquids in the first container and the second container are conductively connected, electrons generated in the first container (oxidation channel) can freely travel to the second container (reduction channel). Oxygen generated in the first container is removed in the first separator while the remaining liquid, including protons (H ⁺ ions) are fed into the second container. In this case, photoreduction takes place in the second container, which photoreaction consumes protons and electrons to generate hydrogen gas. The electrons stem from the photooxidation reaction in the first container. Hydrogen generated in the second container is removed in the second separator.

Optionally, the remaining liquid can be recycled into the first container for renewed water splitting.

The method of the invention can further be used for

photoreduction of carbon dioxide. An example of using the method of the invention for photoreduction of carbon dioxide is as follows. When the method of the invention is used for photoreduction of carbon dioxide, the liquid that is fed into the first container comprises water and carbon dioxide (preferably is a solution of carbon dioxide in water). In case the first container is the oxidation channel, then photooxidation of water takes place which photoreaction generates oxygen gas and electrons. Since the liquids in the first container and the second container are conductively connected, electrons generated in the first container (oxidation channel) can freely travel to the second container (reduction channel). Oxygen generated in the first container is removed in the first separator while the remaining liquid, including protons (H ⁺ ions) and carbon dioxide are fed into the second container. In this case, photoreduction of carbon dioxide takes place in the second container, which photoreaction consumes carbon dioxide, protons and electrons to generate methanol. Depending on the initial content of the liquid also other alcohols and/or hydrocarbons can be generated. The electrons stem from the photooxidation reaction in the first container.

Methanol generated in the second container is removed in the second separator. Optionally, the remaining liquid can be recycled into the first container.

The aforementioned and other features and advantages of the invention will be more fully understood from the following detailed description of certain embodiments of the invention, taken together with the accompanying drawings, which are meant to illustrate and not to limit the invention.

Brief description of the drawings

Figure 1 shows a schematic top view of a first embodiment of the apparatus according to the invention;

Figure 2 shows a schematic cross sectional view A-A of the apparatus of Figure 1;

Figure 3 shows a schematic cross sectional view of a second embodiment of the apparatus according to the invention; and

Figure 4 shows a schematic perspective view of a third embodiment of the invention.

It is noted that identical or corresponding elements in the different drawings are indicated with identical or corresponding reference numerals.

Detailed description of the apparatus

In Figure 1, an embodiment of an apparatus 1 for performing a coupled photooxidation and photoreduction reaction according to an aspect of the invention is shown. The apparatus 1 comprises a first channel 2 providing a first liquid flow path 3 for a liquid to be treated, for instance an aqueous liquid. The apparatus 1 further comprises a second channel 4 that is arranged adjacent the first channel 2. The second channel 4 provides a second liquid flow path 5 that is arranged downstream the first flow path 3. The apparatus 1 comprises a pump unit 20 configured for pumping the liquid to be treated through the respective channels 2, 4. Each channel 2, 4 comprises a light receiving surface 6 (see Figures 2-4) provided with a respective first and second photocatalyst that is arranged for initiating a photoreaction inside the respective channel 2, 4 to produce a photoreaction product. The first channel 2 is conductively interconnected with the second channel 4 such that the liquid to be treated that is passing along the first liquid flow path 3 is in electrical contact with the liquid to be treated that is passing along the second flow path 5. Via said electrical contact, electrons can pass from the first channel 2 to the second channel 4.

Between the first and second channels 2, 4, at least seen in the flow direction of the liquid, a first separator 7 is provided. The separator 7 may be one of a degasser, debubbler and a membrane contactor dependent on the kind of reaction that is performed with the apparatus 1. In the shown embodiment, the first channel 2 is an oxidation channel and the second channel 4 is an reduction channel. In the oxidation channel 2 oxygen is produced and in the reduction channel 4 hydrogen or hydrocarbons are produced. Between the first and second channel 2, 4 a degasser 7 is provided for removing the photoreaction product from the liquid passing said degasser 7, in the shown embodiment thus for removing the oxygen.

Downstream of the second channel 4, a second separator 8 is provided. The second separator 8 may also be one of a degasser, debubbler and a

membrane contactor. The second separator 8 is arranged to remove, from the liquid that has passed along the second liquid flow path 5, the produced hydrogen or hydrocarbons.

To provide the electrical contact between the liquid passing the first liquid flow path 3 and the liquid passing the second liquid flow path 5, at least part of at least an inner surface 9 of the channels 2, 4 is provided with a layer 10 of conductive material (see Figures 2-4). The first and second photocatalysts may be provided in a layer 11, 12 on the respective light receiving surfaces 6 inside the respective channels 2, 4. In the shown embodiments, the apparatus 1 further comprises a layer of light absorber 13 adjacent the photocatalyst layers 11, 12. However, such layer of light absorber 13 is optionally. The channels 2 may comprise substantially elongate hollow tubes at least arranged in pairs, i.e. at least one pair comprising an oxidation channel 2 and a reduction channel 4 provided adjacent said oxidation channel 2 (see Figure 1). The apparatus 1 may have different constructions for instance the channels 2, 4 may be manufactured by different methods and may have different configurations and be of different materials.

In operation with the method of the invention, the first channel 1 is fed with liquid to be treated. First photocatalyst on the light receiving surface of first channel 1 is exposed to light and as a result initiates a first photoreaction, optionally sensitised by a first light absorber. The first photoreaction provides at least a first photoreaction product. Pump 20 ensures that the liquid flows through first channel 1 towards first separator 7. Upon passing first separator 7, at least part of the first photoreaction product is removed from the liquid. The remaining liquid is then fed to the second channel 2. Second photocatalyst on the light receiving surface of second channel 2 is exposed to light and as a result initiates a second photoreaction, optionally sensitised by a second light absorber. The second photoreaction provides at least a second photoreaction product. Pump 20 ensures that the liquid flows through second channel 2 towards second separator 8. Upon passing second separator 8, at least part of the second photoreaction product is removed from the liquid. Either the first

photoreaction is photooxidation and the second reaction is photoreduction, or the first photoreaction is photoreduction and the second photoreaction is photooxidation. Since the first channel 1 and the second channel 2 are conductively connected electrons generated in the oxidation channel as a result of the photooxidation reaction can freely travel to the reduction channel where they can be consumed in the photoreduction reaction. In Figures 2-4 three different embodiments of the apparatus 1 are shown and described.

In Figure 2, a cross sectional view A- A of the channels 2, 4 of the apparatus 1 of Figure 1 for performing the coupled photooxidation and photoreduction reaction is given. The elongate hollow tubes forming channels 2, 4 are formed by means of a substantially sheet shaped base part 14 and a cover part 15 connected to the base part 14. In the shown

embodiment, the base part 14 comprises a glass or plastic substrate and the cover part 15 comprises an amount of substantially concave elongate cover elements 16, for instance glass or plastic tubes that have been cut in half along the central axis of said tubes. Alternatively, the concave elongate cover elements may be part of a single cover part wherein the elongate cover elements may be formed by recessed provided in the cover part.

The conductive layer 10, the photocatalyst layer 11, 12 and the hght absorber layer 13 may be provided on the upper surface 14a of the base part 14, at least on the surface of the base part 15 facing the cover part 15. The conductive layer 10 extends, at least partly, along the upper surface 14a of the base part 14 such that part of said layer 10 is in conductive

connection with the liquid inside the first channel 2 and at the same time in conductive connection with the liquid inside the second channel 4. The cover part 15 may be transparent or at least translucent to allow incident light passing through said cover part 15 to enable the coupled photooxidation and photoreduction reaction.

In Figure 3 a further embodiment of the apparatus 1 according to the invention has been shown. This embodiment differs from the

embodiment as shown in Figures 1 and 2, in that the base part 14 comprises channel forming recesses 17. The respective photocatalyst layers 11, 12 and the optional light absorber layer 13 may be provided on the bottom surface 14b of the respective recesses 17 in the base part 14. The conductive layer 13 extends at least from a recess wall 14c of the first channel 2 along an intermediate base part surface 14d to a recess wall 14e of the second channel 4. In this embodiment, the cover part 15 may comprise a

substantially flat substrate, which substrate may be of a transparent or at least translucent material. In a different embodiment, the base part 14 may additionally or alternatively be of a transparent or translucent material. In such embodiment, the respective layers 10, 11, 12, 13 may be provided on the substrate that forms the cover part 15 instead of on the base part 14 and as such the cover part 15 does not need to be substantially transparent.

In Figure 4 a third embodiment of the apparatus 1 according to the invention is shown. In this embodiment, the channels 2, 4 comprise substantially elongate hollow, for instance cylindrical, tubes 2, 4. The tubes 2, 4 may be of a substantially transparent or at least translucent material. The tubes may be coated with a substantially thin conductive layer 10. The layer 10 extends along at least part of the tube inner surface 9. To be able to provide the liquid to be treated in the first channel 2 in electrical contact with the liquid to be treated in the second channel 4, the conductive layer 10 also extends partly along a tube end surface 2a, 4a and along at least part of the tube outer surface 2b, 4b at least at the location thereof at which said tube 2, 4 is connected to the adjacent tube 4, 2. On the channel inner surface 9, at least on the light receiving surface 6 on the inner surface 9, the photocatalyst layer 11, 12 and/or the light absorber layer 13 are provided. In the shown embodiment, the respective layers 11, 12, 13 extend along the entire channel inner surface. In use, the liquid passes along the first liquid flow path 3 that extends through the first tube 2 and subsequently along the second flow path 5 that extends through the second tube 4, after passing the first separator (not shown in Figure 4). The apparatus 1 according to any of the shown embodiments has a relatively simple construction that may enable easy manufacturing and scaling thereof. The amount of channels shown in the embodiments of figures 1-4 is merely illustrative and may be more than shown in these figures. In an embodiment, the amount of channels is an even amount.

Although illustrative embodiments of the present invention have been described above, in part with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Reference

throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment in the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment.

Furthermore, it is noted that particular features, structures or

characteristics of one or more embodiments may be combines in any suitable manner to form new, not explicitly described embodiments.