Patent Description:
Photocatalysis is a promising technology for pollution abatement in aqueous and gaseous environment, especially degradation of volatile organic compounds (VOC). The photocatalytic process is based on light-induced generation of electron-hole pairs that initiate oxidation and reduction reactions of pollutants on the catalyst surface. Typically, photocatalytic reactors for air purification are based on the immobilization of the catalyst onto a surface. An ideal photoreactor for air purification should have (<NUM>) an appropriate light source irradiating the catalyst surface, (<NUM>) the catalyst coating should have a large surface area to ensure availability of many active sites, (<NUM>) high mass transfer and (<NUM>) low pressure drop. The most widely used gas phase photocatalytic reactors are the flat plate reactor and the annular reactor. The flat plate reactor is one of the most straightforward geometries and consists of a coated flat substrate that is illuminated from the top by UV lamps. The airflow runs parallel to the flat plate. Advantages of this type of reactor are simplicity and small pressure drop. A big disadvantage is that only a small reaction area can be achieved. The annular type reactor is built up from two concentric cylinders resulting in an annular gap. The catalyst is coated on the interior wall(s) of the cylinder(s) and the light source is usually positioned longitudinally in the center of the reactor. Also for this geometry the small reaction area remains an important disadvantage. More complex reactor types have also been developed and include for example a monolith reactor. This reactor contains a certain number of channels and provides a large catalyst surface area and a low pressure drop. The major drawback of this type is the top irradiation that is insufficient, and results in a limited reaction rate.

<CIT> discloses a material that is able to selectively inactivate a specific biologically harmful substance through photocatalysis. Moreover, it describes in one embodiment a photocatalytic treatment unit comprising-on an outer peripheral region of a light source-a plurality of columns connected together tail-to-head in a series via infusion tubes. Each column is formed in a long thin hollow cylindrical shape from an optically transparent material that transmits ultraviolet radiation relatively well. The columns are detachably installed with the axial directions thereof aligned in the vertical direction.

<CIT> discloses a microfluidic reactor including at least one reaction cell for a photocatalytic reaction. The at least one reaction cell includes at least one channel having an inlet port, an outlet port and a surface transparent to actinic radiation. A photoactive catalyst is disposed within the at least one channel.

<CIT> discloses a purifier for purifying a fluid by eliminating contaminants from the fluid. The purifier includes a fluid passage, through which the fluid flows, formed by an ultraviolet ray transmitting material. A plurality of photocatalytic pipes are arranged in the fluid passage. Each of the photocatalytic pipes has an inner surface and an outer surface on which a thin film of a photocatalyst is applied. The photocatalytic thin film is excited by ultraviolet rays irradiated from a source located near the fluid passage, thereby oxidizing and decomposing the contaminants and purifying the fluid.

Asapu et al. ) disclosed silver-polymer core-shell nanoparticles prepared using the layer-by-layer (LbL) technique. The metallic silver core is encapsulated with an ultra-thin protective shell that prevents oxidation and clustering without compromising the plasmonic properties. The particles are used to prepare a plasmonic Ag-TiO<NUM> photocatalyst.

It is an object of embodiments of the present invention to provide good systems and methods for photocatalytic gas phase pollutant degradation.

It is an advantage of embodiments of the present invention that it provides a homogeneous light distribution over the entire reactor.

It is an advantage of embodiments of the present invention that an intense contact between pollutants and the catalyst surface is allowed.

It is an advantage of embodiments of the present invention that a large reaction area is provided in the reactor.

It is an advantage of embodiments of the present invention that pollutants in high concentrations can be degraded efficiently.

It is an advantage of embodiments of the present invention that it provides high pollutant degradation efficiency.

It is an advantage of embodiments of the present invention that a very compact reactor can be obtained.

It is an advantage of embodiments of the present invention that removal of volatile hazardous particles from air can be obtained by photocatalytic destruction.

It is an advantage of embodiments of the present invention that a compact design is provided that allows making use of natural light e.g. sun light and/or make use of a planar, e.g. flat planar radiation source.

It is an advantage of embodiments of the present invention that the configurations provided allow to use direct sunlight and therefore allow to use a sustainable energy source.

The reactor may be a glass reactor or a reactor made of any other UV-VIS transparent material. Advantageously, the reactor deteriorates not by the reaction caused.

The photocatalytic material may be any of TiO<NUM>, ZnO, WO<NUM> or composite materials containing a combination of these oxides.

The coating on the reactor furthermore may comprise nanostructures comprising metals such that the nanostructures display surface plasmon resonance effects.

The nanostructures may comprise metals are nanostructures comprising any of Ag, Au, Cu, Pd, Pt or a combination thereof.

The nanostructures may be covered by a polymeric shell comprising at least one layer of polymer.

The polymeric shell may comprise at least one layer of polyelectrolytes.

The layer may comprise at least poly(allylamine hydrochloride) (PAH) and/or PAA.

The device may comprise Ag nanostructures and TiO<NUM> photocatalytic material.

The device furthermore may comprise a mirroring surface positioned at one side of the reactor tube for reflecting radiation towards the reactor tube. The present invention also relates to a photocatalytic reactor system comprising a device for providing photocatalytic reaction as described above.

The system may comprise a plurality of devices for providing photocatalytic reaction as described above, the devices for providing photocatalytic reaction being arranged parallel to each other.

The reactor tubes of the different devices for providing photocatalytic reaction may be interconnected with each other so as to form a single reactor tube.

The system furthermore may comprise a substantially planar radiation source for irradiation the reactor tube. In between the devices for providing photocatalytic reaction planar radiation sources may be positioned.

Use of a system as described above for photocatalytic gas phase pollutant degradation based on sunlight.

Use of a system as described above for photocatalytic gas phase pollutant degradation The system furthermore comprises a source of UV light adapted for illuminating the substrate. It is an advantage of embodiments of the present invention that an energy efficient photocatalytic reactor is obtained which can work at room temperature.

It is an advantage of embodiments of the present invention that efficient capture of light from the source can be obtained.

The surface for providing photocatalytic reaction further may comprise Au or Ag or Cu or Pt or Pd particles or their combinations, for allowing photocatalysis using the whole light spectrum.

The present invention also relates to a method of obtaining a catalytic surface, the method comprising the steps of.

In some embodiments, the photocatalyst may be any of TiO<NUM>, ZnO, WO<NUM> or composite materials containing a combination of these oxides and/or the nanostructures may comprise metals are nanostructures comprising any of Ag, Au, Cu, Pd, Pt or a combination thereof.

The latter may include a deposition wherein a mixture of a photocatalyst, e.g. TiO<NUM>, and said nanostructures, e.g. Ag comprising nanostructures, are deposited in a single coating or wherein at least a layer comprising the catalyst, e.g. TiO<NUM>, and at least a layer comprising said nanostructures, e.g. Ag comprising nanostructures, is deposited. A multi-stack of alternating layers also may be deposited.

The application of said nanostructures, e.g. Ag comprising nanoparticles, e.g. gold/silver modified nanoparticles, result in a modified glass spiral reactor offering an attractive solution for photocatalytic air purification with high degradation efficiencies at short gas residence times compared to conventional reactor geometries of the same dimensions and catalyst loading.

Depositing nanostructures and a photocatalyst may comprise.

It is an advantage of embodiments of the present invention that a stable nanostructure coating is obtained, avoiding oxidation and/or clustering of the nanostructures.

Providing a photocatalyst coating, e.g. a TiO2 coating, may comprise providing a liquid suspension comprising the photocatalyst, e.g. TiO2, on the surface of the substrate and subsequently evaporating the liquid. Alternatively for example also sol-gel methods may be applied.

Providing a photocatalyst, e.g. TiO2 coating, may comprise providing a coating such that the light transmission through a single coated substrate surface is at least <NUM>%, at most <NUM>%, for example about <NUM>%.

It is an advantage of embodiments of the present invention that the substrate can be kept substantially transparent.

Obtaining nanostructures may comprise obtaining the nanostructures, e.g. Ag comprises nanostructures, with at least one layer of polyelectrolytes. It is an advantage of embodiments of the present invention that this polymer layer protects the nanostructures, e.g. silver comprising nanostructures, from oxidation and clustering. The polyelectrolytes may be polycations or polyanions. The polymers may comprise poly(acrylic acid) (PAA), poly(allylamine hydrochloride) (PAH), or a combination thereof. Another example is PSS. It is an advantage of embodiments of the present invention that these ionic polymers are inexpensive.

It is an advantage of embodiments of the present invention that this polymer layer is sufficiently thin so enhanced electromagnetic fields induced by surface plasmon resonance effects of the metal nanostructures protrude beyond the polymer shell. The shell thickness can be varied from one polymer layer and should not exceed a total thickness of <NUM>.

Coating the nanostructures may comprise coating the nanostructures with four polymer layers. The number of layers may be selected such that the optical properties of the capped silver nanoparticles are such that the plasmon resonance effect matches very well the peak intensity wavelength of the lamp used.

Obtaining coated nanostructures comprises providing layer-by-layer (LbL) deposition of polymers on the nanostructures. An advantage of the layer by layer technique is that sub-nanometer control over the layer thickness can be achieved.

The nanostructures comprising Ag may be round nanostructures comprising Ag. It is an advantage of embodiments of the present invention that no functionalization is necessary. It is a further advantage that layer-by-layer deposition can be obtained from an aqueous solution, with no need to provide buffer solution.

The present invention also relates to a substrate transparent to light comprising a surface suitable for providing photocatalytic reaction, the surface comprising nanostructures, e.g. Ag comprising nanostructures, and a photocatalyst, e.g. a TiO<NUM>, as a catalyst for photocatalytic gas phase pollutant degradation, the nanostructures being nanostructures comprising metals such that the nanostructures display surface plasmon resonance effects.

The photocatalyst may be any of TiO<NUM>, ZnO, WO<NUM> or composite materials containing a combination of these oxides and/or the nanostructures comprising metals may be nanostructures comprising any of Ag, Au, Cu, Pd, Pt or a combination thereof.

The substrate may be at least partially coated with the photocatalyst, e.g. TiO2, and may further comprise at least one layer of nanostructures, e.g. Ag comprising nanostructures, the nanostructures being covered by a polymeric shell comprising at least one layer of polymer. It is an advantage of embodiments of the present invention that long term stable nanostructures can be obtained. It is a further advantage that plasmon resonance of nanostructures, e.g. Ag nanostructures, is close to the absorption wavelength of a photocatalyst, e.g. TiO2, allowing coupling and locally increasing the electromagnetic field upon impinging UV radiation. This enhanced field in turn advantageously allows for a sharp increase in the number of charge carriers (electrons and holes) that are formed in the substrate, thereby improving photocatalytic destruction of volatile compounds.

The polymeric shell may comprise at least one layer of polyelectrolytes. It is an advantage of embodiments of the present invention that the polymers may not block the increased electromagnetic field associated by surface plasmon resonance on the silver comprising nanostructures.

The layer may comprise at least poly(allylamine hydrochloride) (PAH) and/or polyacrylic acid (PAA). It is an advantage of embodiments of the present invention that inexpensive nanostructures can be obtained.

The present invention also relates to a photocatalytic reactor for gas phase pollutant degradation, the reactor comprising a UV/Vis transparent spiral tube, coated on the inside with photocatalytic material, such as TiO2. The gas phase photocatalytic reactor may be based on a glass spiral surrounding the light source. The TiO2 material may be an easy TiO<NUM> coating, starting from commercially available TiO<NUM> and without the need of any further heat treatments. The advantages of this spiral reactor are (<NUM>) a very homogeneous light distribution over the entire reactor, (<NUM>) an intense contact between pollutants and the catalyst surface and (<NUM>) a large reaction area, and (<NUM>) the fact that it is leading to high pollutant degradation efficiency, (<NUM>) all in a very compact design.

The reactor may furthermore comprise long-term stable metal nanoparticles, protected by an ultra-thin polymer shell. The shell may be applied via a layer-by-layer (LbL) method. The metal nanoparticles typically may be Ag comprising particles. The silver comprising particles may be silver/gold alloy nanoparticles. When irradiated with light of the correct wavelength, noble metal nanoparticles display surface plasmon resonance (SPR), i.e. the coherent oscillation of the negative electron cloud against the restoring force of the positive nucleus. The resonant wavelength and SPR intensity depend on the nature of the metal, the surrounding environment, the size and the shape of the metallic nanostructures. Application of these particles results in shifting the activity window of a wide bandgap semiconductor like TiO<NUM> to the visible range of the spectrum. According to embodiments of the present invention this shift offers an innovative solution to the poor solar light response of TiO<NUM>, but furthermore the quantum efficiency under visible light is also improved by working with silver/gold alloy nanoparticles thus covering the entire range of the solar spectrum. Convenient use is made of the strong electric near-field enhancement which SPR entails. The build-up of these intense local electric fields allows an efficient concentration of the incident photon energy in small volumes near the nanostructures. Since the rate of electron-hole pair formation is proportional to the intensity of the electric field, a drastic increase in charge carrier formation occurs.

In order for this plasmonic "lens effect" to work, an energy match between the bandgap energy of the semiconductor, the spectral output of the light source and the energy associated with the SPR was selected. For TiO<NUM> under UVA irradiation, this criterion is met especially for silver nanoparticles, with an SPR band around <NUM>. Furthermore, the nanostructures are provided with a shell in order to overcome stability issues. The protective shell consists of polyelectrolytes which are wrapped around the nanoparticles, e.g. based on the layer-by-layer protocol.

In a first aspect, the present invention relates to a device for providing photocatalytic reaction for gas phase pollutant degradation, the reactor comprising a UV/vis transparent reactor tube for sending a gas therethrough, the UV/vis transparent reactor tube being coated on the inside with photocatalytic material as a catalyst for photocatalytic gas phase pollutant degradation, the reactor tube extending in different directions so as to form a substantially two dimensional pattern. The reactor tube has a planar two-dimensional spiral shape.

In one aspect, the present invention relates to as substrate transparent to light. The substrate comprises a surface suitable for providing photocatalytic reaction. The surface is therefore coated, the coating comprising nanostructures, e.g. Ag comprising nanostructures, and a photocatalyst, e.g. a TiO<NUM>, as a catalyst for photocatalytic gas phase pollutant degradation, the nanostructures being nanostructures comprising metals such that the nanostructures display surface plasmon resonance effects.

According to some embodiments of the above aspects, the photocatalyst may be any of TiO<NUM>, ZnO, WO<NUM> or composite materials containing a combination of these oxides and/or the nanostructures comprising metals may be nanostructures comprising any of Ag, Au, Cu, Pd, Pt or a combination thereof.

In some embodiments, the substrate is at least partially coated with the photocatalyst, e.g. TiO2, and further coated with said nanostructures, e.g. Ag comprising nanostructures, e.g. a layer of Ag comprising nanostructures, the nanostructures being covered by a polymeric shell comprising at least one layer of polymer.

In a second aspect, the present invention relates to a method for obtaining a catalytic surface, the method comprising the steps of.

In some embodiments, depositing nanostructures, e.g. Ag comprising nanostructures, and a photocatalyst, e.g. TiO2, comprises.

Further features and advantages of the first and second aspect will further be illustrated with reference to experimental results obtained and described below, embodiments not being limited thereto.

By way of illustration, embodiments of the present invention not being limited thereby, the results of an experimental study of photocatalytic degradation for gas phase pollutants using a spiral photocatalytic reactor and/or a metal nanoparticle including photocatalytic material are discussed below. The latter illustrates features and advantages of at least some embodiments of the present invention.

First the experimental setup and design will be discussed. The gas phase reactor for the present study was constructed using a spiral tube, transparent for both visible and UV light, in which a lamp was placed longitudinally in the center as schematically depicted in <FIG>. The reactor internal diameter and length can be varied over a large range. <FIG> depicts an example of a glass spiral reactor with an internal diameter of <NUM> and a total length of <NUM>, that is hand-blown into a spiral of <NUM> length and <NUM> width as outer dimensions.

To benchmark the results of the spiral reactor, an annular reactor of the same outer dimensions as the spiral reactor was also constructed. This annular reactor is schematically depicted in <FIG> and consists of two concentric cylinders, one outer cylinder of <NUM> and an inner cylinder of <NUM>, both <NUM> long. Both ends were sealed with butyl rubbers sandwiched between a dedicated closing mechanism to ensure airtightness (<FIG>). The same lamp as for the spiral reactor is positioned longitudinally in the center.

In the present study, silver nanoparticles were used for obtaining improved efficiency. The synthesis of silver nanoparticles was based on the procedure described by <NPL>). A <NUM> aqueous solution of sodium citrate (<NUM>) and tannic acid (<NUM>) was heated under vigorous stirring until boiling commenced. At this point, <NUM> of a <NUM> aqueous silver nitrate (AgNO<NUM>) solution was added and left boiling for <NUM>. Immediately after the addition of AgNO<NUM>, the solution became bright yellow. To remove the excess of tannic acid and citrate, the batch was centrifuged at <NUM>,<NUM> for <NUM> and the resulting nanoparticles were redispersed in <NUM> Milli-Q-water (ρ = <NUM> MΩ). Tannic acid is used to control the size of the Ag seeds. With this technique, nanoparticles can be synthesized in a large range of diameters.

Colloidal AuXAg(<NUM>-x) spherical nanoparticles can be synthesized using a modified Turkevich procedure. In this method, appropriate amounts of HAuCl<NUM>·<NUM><NUM>O and AgNO<NUM> precursor solutions are diluted to avoid precipitation of AgCl, mixed and brought to boil after which citrate is added as both the reducing and stabilizing agent. By altering the amount of gold ions, i.e. the amount of HAuCl<NUM>. <NUM><NUM>O, and silver ions, i.e. AgNO<NUM>, the ratio of gold versus silver can be varied. Fabrication of more intricate, 'cornered' nanostructures (cubes, stars, prisms,. ) is more challenging. The advantage of this type of nanostructures is that plasmonic hot spots are the strongest at spikes or corners. Fabrication of silver and Au@Ag nanocubes has been described by <NPL>. Nanostars are another interesting category of 'pointy' nanostructures. <NPL> on the synthesis of gold nanostars and silver-coated gold nanostars. There is also evidence for the synthesis of bare silver nanostars, fabricated in a two-stage process. It is possible to modify these synthesis strategies by mixing gold and silver precursors to obtain alloyed nanostructures, similar to what was done for modifying the Turkevich procedure for spherical gold nanoparticles.

To avoid oxidation and aggregation of metal nanoparticles, a core-shell structure of polymers based on the LBL method was applied. The silver core-shell particles were constructed using stock solutions of polyelectrolytes polyallylamine hydrochloride (PAH, Mwt <NUM> KDa, Sigma-Aldrich) and polyacrylic acid (PAA, MW <NUM> KDa, Sigma-Aldrich) prepared in Mili-Q-water. Prior to use, the polymer solutions were sonicated for <NUM>. For the deposition of the first polyelectrolyte layer, i.e. PAH, <NUM> of silver colloidal solution was added drop-wise to <NUM> of <NUM>/L PAH in a glass vial under vigorous stirring, which continued for <NUM> under dark conditions at room temperature. The resulting PAH capped silver nanoparticles were centrifuged (<NUM>,<NUM>) for <NUM> in <NUM> Eppendorf tubes in order to remove the excess polyelectrolyte. The particles were redispersed in Milli-Q water as a washing step, centrifuged once more and finally the obtained colloids were again redispersed in <NUM> of Milli-Q-water. The deposition of the second layer was achieved using <NUM>/L PAA using a similar approach as for the first layer, only the centrifugation speed was lowered (<NUM>,<NUM>) to avoid the formation of hard pellets. The procedure was repeated until silver nanoparticles were obtained with four polymer layers. A FEI Tecnai Transmission Electron Microscope (TEM) operated at <NUM> kV was used to visualize the core-shell structure of nanoparticles. The same procedure can be followed for the synthesis of other types of metal nanostructures such as gold or silver/gold alloys including different shapes such as cubes, triangles, stars,.

The photocatalytic degradation of air pollutants is studied under ambient conditions. Acetaldehyde is studied as a model compound. Firstly, the TiO2 coating was introduced in the spiral photoreactor via an easy and affordable coating protocol. An important design feature is the optimization of the catalyst loading in conjunction with the resulting light transparency of the coated surface. Rather than performing thickness measurements, the optimization method was purely based on light transmission. In order to establish the most ideal coating conditions, different glass slides were coated with three different suspensions of TiO2 in ethanol (<NUM>, <NUM> and <NUM> wt%) by pouring <NUM> of the suspension over the glass slide in one smooth motion, dried at <NUM> for <NUM> and then the transmission of UV light passing through the glass slides was measured using a calibrated spectroradiometer (Avantes Avaspec). The coating procedure was repeated to introduce higher catalyst loadings. Using this approach the goal was to obtain a coating condition that results in <NUM>% light transmission which is considered optimal since both in the spiral and annular reactors, emitted photons are assumed to cross two coating layers before exiting the geometry. The results of the 'coating-transmission calibration' in <FIG> show that one coating cycle using a <NUM> wt% suspension resulted in approximately <NUM>% transmission, as required.

For coating the spiral reactor, <NUM> of TiO<NUM> P-<NUM> (Evonik) was added to <NUM> of ethanol to obtain a <NUM> wt% suspension and stirred ultrasonically for <NUM>. The suspension was poured in one smooth motion through the spiral reactor to obtain a thin semi-transparent white layer of TiO<NUM>. It was verified that the appearance of the coating was indeed similar to what was obtained by coating a planar glass slide with the same <NUM> wt% suspension. To remove the solvent, a continuous air flow was sent through the reactor overnight in the presence of UV light (leading to photocatalytic degradation of adsorbed ethanol).

The procedure described above resulted in a catalyst coverage of <NUM> ± <NUM> cm-<NUM> (corresponding to a total catalyst loading of <NUM> ± <NUM> in the entire reactor). The latter is determined by constructing a calibration curve that relates UV LED light transmission to well-known surface coverages of TiO<NUM> on glass, obtained by (repeated) drop casting of known amounts of a very well dispersed <NUM> wt% TiO<NUM> suspension in ethanol on cleaned cover slips (<FIG>).

Deposition of polymer-capped silver nanoparticles was achieved by simply pouring the as-obtained colloidal solution as obtained with the technique described above through the coated spiral reactor once. The reactor was dried overnight by flushing with air. Quantization of the total metal loading on TiO<NUM> was, unfortunately, not feasible under these conditions.

The reactor (as shown in <FIG>) was mounted on a fully automated photocatalytic gas test setup. In all tests, acetaldehyde was selected as target pollutant (Messer, <NUM>% in N<NUM>). The total flow rate was altered between <NUM> and <NUM> min-<NUM> to investigate the effect on the degradation efficiency. Depending on the experiment, the inlet concentration of acetaldehyde was set at <NUM> ppmv, <NUM> ppmv or <NUM> ppmv, as will be specified below. The degradation of acetaldehyde was determined as the steady state concentration of CO<NUM> produced under UV illumination, over twice the acetaldehyde inlet concentration (as two moles of CO<NUM> are produced per mole of acetaldehyde).

For the validation of the spiral photocatalytic reactor, the efficiency was compared with an annular reactor of the same outer dimensions and identical catalyst loading. Two different cases were considered for this annular reactor. In the first case, only the inner wall of the outer cylinder was coated with <NUM> of a <NUM> wt% suspension of TiO2 in ethanol to result in the same total amount of TiO2 as present in the spiral reactor (<NUM>). In the second case, both the inner wall of the outer cylinder, as well as the outer wall of the inner cylinder were coated. In that case both cylinders were coated with a <NUM> wt% suspension of TiO2 in <NUM> ethanol that again resulted in the same total amount of TiO<NUM> as in the spiral reactor (<NUM>).

Following results were obtained :
At first, our hypothesis of achieving an optimal catalyst coating by calibrating the resulting light transmission was verified. The idea was to aim for <NUM>% transmission, since the light roughly has to cross two coated surfaces. A single coating step with a <NUM> wt% suspension of TiO<NUM> P25 in ethanol leads to this transmission target of <NUM>%. This hypothesis was proven experimentally by coating the spiral reactor with increasing amounts of TiO<NUM> and measuring the resulting photocatalytic degradation of <NUM> ppmv acetaldehyde at a total flow rate of <NUM> min-<NUM>. The results in <FIG> clearly illustrate that the degradation efficiency increases when increasing the suspension concentration up to <NUM> wt%, which can be explained by more effective use of incident photons by the increasing amount of catalyst present in the reactor. After reaching a maximum for the <NUM> wt% suspension, the degradation efficiency again drops for the <NUM> wt% suspension, which is attributed to an excessive amount of coating that blocks (scatters) part of the incident photon flux and prevent full photo-activation of the inner spiral surface in contact with the pollutants.

The photocatalytic activity of the two different reactor geometries (spiral versus annular) were compared towards the photocatalytic degradation of acetaldehyde. In these experiments, the concentration of acetaldehyde was kept constant at <NUM> ppmv while the total volumetric flow rate was altered between <NUM> min-<NUM> and <NUM> min-<NUM>, which are the extrema of our setup. Since our aim was to compare reactor geometries of the same outer dimensions, both the spiral and the annular reactors were <NUM> long and <NUM> wide. Since the annular type reactor is composed of two cylinders, the most evident way to compare it with the spiral reactor is to coat both cylinders. The outer cylinder only coated from the inside and the inner cylinder only coated on the outside. This means, however, that although the amount of catalyst was kept the same, i.e. <NUM>, the total available surface was significantly larger for the annular reactor than for the spiral reactor (<NUM><NUM> and <NUM><NUM> respectively). Alternatively, only the outer cylinder of the annular reactor was coated from the inside. This way, a total amount of <NUM> TiO<NUM> was coated on a surface of <NUM><NUM> which is very close to the total available surface of the spiral reactor. All geometrical parameters are summarized in Table <NUM>. As can be derived from the results in <FIG>, the spiral reactor outperforms both the single coated cylinder and the double coated cylinder over the entire set of reaction conditions. Although the outer dimensions of the reactor are identical, the internal volumes are not. For the annular reactor, the volume is <NUM><NUM> whereas the reactor volume of the spiral is only <NUM><NUM>. This means that the residence time of acetaldehyde in the spiral reactor is about <NUM> times shorter than for the annular reactor. In other words, the same amount of acetaldehyde is flowing through the spiral reactor <NUM> times as fast as through the annular reactor and still the degradation efficiency is significantly higher, especially at higher flow rates (i.e. shorter residence time). It can thus be concluded that the spiral reactor provides a very compact solution for the effective and fast degradation of VOC in a short period as compared to conventional photocatalytic reactor types. This outstanding performance is attributed to (<NUM>) homogeneous illumination over the entire reactor length, (<NUM>) with a coated layer optimized towards light transmission, and (<NUM>) intense (yet short) contact between gaseous reagents and the irradiated surface over the entire spiral length due to the narrow tube diameter.

Finally, silver nanoparticles, stabilized by four polymer layers (Ag/(PAH/PAA)) were added to the TiO<NUM> coating in the spiral reactor. <FIG> shows a TEM image in which a thin, uniform polymer shell is clearly visible. This image shows the degree of nanometer-level control on the thickness of the polymer shell that is offered by the LbL method.

The degradation of acetaldehyde was again used as a test to evaluate the difference between bare TiO<NUM> and silver-modified TiO<NUM>. In these experiments a total flow rate of <NUM> min-<NUM> was used with an acetaldehyde concentration of <NUM> ppmv. A drastic increase in the inlet concentration was required since for the <NUM> ppmv inlet concentration in the experiments above <NUM>% acetaldehyde degradation was already achieved over the entire flow regime. The results show that under these conditions the TiO<NUM>-coated spiral results in <NUM>% acetaldehyde degradation. Upon addition of LbL-stabilized silver nanoparticles, an activity increase of <NUM>% is obtained and complete removal of acetaldehyde is achieved. This is attributed to the enhanced electric near-field accompanied by SPR on these protected silver nanoparticles under UV illumination. The core-shell silver nanoparticles show a plasmon band around <NUM> which overlaps with the TiO<NUM> P25 absorption spectrum and therefore an increased number of charge carriers is generated that result in a higher photocatalytic activity. The degradation behavior is shown in <FIG>.

The addition of gold or silver/gold alloy nanostructures would not only enable to harvest UV photons, but photons from the entire UV/Vis region of the solar spectrum, as we recently showcased by means of a 'rainbow' broadband plasmonic catalyst. For the effective harvesting of solar photons a slight variation on the reactor geometry shown earlier may be used by translating a 3D spiral (helix) to a planar 2D spiral that can be oriented directly towards the sun in <FIG>. Another example of a planar 2D reactor tube is illustrated in <FIG>. The reactor used in experiments for evaluating the 2D reactor tube is configured as shown in <FIG>. It consists of <NUM> tubes, each having a length of <NUM> and being connected with short turns to form a <NUM> long reaction tube.

By way of illustration, efficiency of the 2D planar configuration is illustrated below, referring to some experimental results. The experiment as performed for the 3D spiral reactor was repeated for the 2D planar configuration. <NUM> ppmv acetaldehyde was fed to the reactor at a total flow rate of <NUM> min-<NUM>. The degradation efficiency of this 2D spiral reactor (ca. <NUM>%) was lower than the corresponding 3D spiral (<NUM>%) but the intensity of the UV lamps has to be taken into account. In the case of the 3D spiral, <NUM> mW/cm<NUM> UV lamps were used, whereas in the case of the 2D spiral one was only able to use UV lamps of <NUM> mW/cm<NUM>. Upon addition of core-shell stabilized silver nanoparticles, an activity increase of <NUM>% was achieved leading to a degradation efficiency of <NUM>%.

In addition, formal quantum efficiencies (FQE) were calculated using the following equation: <MAT>.

The results are listed in the table below. The FQE for the bare TiO2 is very similar for the 2D spiral and the 3D spiral. For the silver modified catalyst, the FQE is clearly increased especially in the case of the 3D spiral reactor. According to Sopyan et al. <NUM><NUM> holes are needed for a complete photocatalytic degradation of acetaldehyde following the equation below:.

CH<NUM>CHO + <NUM><NUM>O + <NUM>+ → 2CO<NUM> + <NUM>+.

Since already <NUM> excitation events are needed for a complete degradation, the maximum achievable quantum efficiency can be only <NUM>%. One achieves an apparent efficiency that is one order of magnitude higher compared to literature reported values.

Formal quantum efficiencies for the degradation of acetaldehyde.

The above results illustrate the advantageous effects of a 2D planar configuration.

Claim 1:
A device for providing photocatalytic reaction for gas phase pollutant degradation, the reactor comprising a UV/vis transparent reactor tube for sending a gas therethrough, the UV/vis transparent reactor tube being coated on the inside with photocatalytic material as a catalyst for photocatalytic gas phase pollutant degradation, the reactor tube has a planar two-dimensional spiral shape.