WO2012098346A1

WO2012098346A1 - A portable apparatus for measuring the efficiency of a photocatalyst

Info

Publication number: WO2012098346A1
Application number: PCT/GB2011/052532
Authority: WO
Inventors: Stephen Lynch; Patrick Nickels; Ahmed A. AL-GHAMDI; Abdullah OBAID
Original assignee: Bio Nano Consulting; King Abdulaziz University
Priority date: 2011-01-17
Filing date: 2011-12-20
Publication date: 2012-07-26
Also published as: GB201312481D0; GB201100709D0; GB2503813B; GB2503813A

Abstract

The present invention relates to a portable apparatus and methods for measuring the efficiency of a photocatalyst. The invention is also concerned with the use of the apparatus for measuring the efficiency of a photocatalyst and/or to determine the purity of a sample of water. The invention is further concerned with water purification systems comprising the apparatus according to the present invention.

Description

A PORTABLE APPARATUS FOR MEASURING THE EFFICIENCY OF A

PHOTOCATALYST

Field of the Invention

Background of the Invention

Water pollution is becoming a global problem and there is an increasing demand to develop rapid and reliable methods for measuring water quality. This problem is partly due to the presence of compounds including natural organic matter and synthetic organic micro-contaminants, such as hydrocarbons, polychlorinated biphenyls and pesticides, which are not removed by conventional treatment processes.

Although several advance treatment processes are being adopted, these are often expensive to run and are subsequently increasing the cost of water. There is therefore a need to provide a cheaper and more energy efficient method of breaking down large toxic molecules present in water into smaller non-toxic molecules.

One of the most promising techniques which has recently been developed to tackle this issue has involved the use of semiconductor photocatalysts which promote the degradation of harmful compounds present in water. The investigation into and development of efficient photocatalysts has thus become a worldwide challenge. A key step in the development process of semiconductor photocatalysts is to understand how well the photocatalyst behaves under different environmental conditions. The efficacy of a photocatalyst is usually evaluated by monitoring the degradation rate of a specific compound in aqueous solution, under controlled conditions such as concentration, irradiance, pH and volume. One compound that the Applicant has identified as being desirable to monitor is the relatively benign chemical, methyl orange. Methyl orange is beneficial to use because it shows a strong colour and conventional spectrometers can be used to assess the concentration by absorption spectroscopy. However conventional bench-top spectrometers, such as UV-Vis or FTIR spectrometers, have several disadvantages. These instruments are often bulky and very expensive. Moreover they contain complicated and expensive optics and moving parts, which make them necessarily delicate instruments and quite unsuitable for fieldwork. In addition, other existing analysis methods such as high pressure (performance) liquid phase chromatography and liquid chromatography mass spectroscopy are not compatible with real-time monitoring.

In recent years, a number of attempts have been made to develop methods and devices for measuring the quality of a sample of water. One such example disclosed in US 2010/0007888, is directed to a device to measure the amount of light transmission through a test liquid sample. The device disclosed therein uses a multiple path length design to efficiently and accurately measure and compute the ultraviolet transmittance/ultraviolet absorbance of a test water source. As such, the device is able to provide a measurement of UV-active natural organic matter which is present in water. However such a device is not optimised for in-line use. The device is also not able to provide recording of time traces and therefore does not allow the efficiency of a photocatalyst to be evaluated. In addition, the device relies upon the use of a rotation mechanism to bring about the multiple path length design and such moving parts are impractical for fieldwork.

A further device disclosed in US 2010/0045979 measures the efficiency of a photocatalyst by monitoring the degradation rate of methylene blue. However this device has no ability to connect to a circulating system.

Therefore, the present invention seeks to provide a compact, robust and cheap solution aimed at directly addressing these problems. The device and method of the present invention allows the absolute concentration of a test compound in solution to be monitored directly in real time. This information can then be used to calculate the rate constants of a particular photocatalytic reaction and allow the efficiency of the new photocatalyst to be evaluated.

Furthermore, there is currently no accepted industrial standard for comparing the efficiency of new photocatalysts developed in different labs or different companies. Therefore the present invention could be used as a means for standardisation such that different companies are able to benchmark their new photocatalysts against a competitor. Summary of the Invention

According to a first aspect of the present invention, there is provided a portable apparatus for measuring the efficiency of a photocatalyst comprising:

a) a sample cell capable of transmitting light emitted from a light source and comprising a means for allowing a continuous flow of sample fluid through the sample cell,

b) a reference cell capable of transmitting light emitted from a light source and comprising a means for allowing a continuous flow of reference fluid through the reference cell,

c) a light emitter which is configured to emit light of a particular wavelength or range of wavelengths, and

d) a detector configured to detect the amount of light transmitted through the sample and reference cells,

wherein said sample fluid comprises a compound capable of being degraded by a photocatalyst and absorbing light of a particular wavelength or range of wavelengths emitted by the light emitter and wherein the amount of transmitted light detected by the detector allows the efficiency of a photocatalyst to be determined.

In a preferred embodiment, the amount of transmitted light detected by the detector is an indicator for the concentration of the compound in the fluid and as such allows the efficiency of a photocatalyst to be determined by monitoring the decay of the concentration over time.

One of the major advantages of the present invention is that the apparatus is portable, making it suitable for fieldwork. The term "portable apparatus" as used herein refers to any device which is compact and light enough to be carried by one person. In this respect, the apparatus of the present invention should preferably have a weight of less than 5 kg, preferably less than 3 kg, and even more preferably less than 1 kg. The size of the device should be less than 1 m (width) x 1 m (length) x 1 m (height), preferably less than 0.5 m (width) x 0.5 m (length) x 0.5 m (height) and even more preferably less than 0.3 m (width) x 0.3 m (length) x 0.3 m (height). In a particularly preferred embodiment, the apparatus should also have no moving parts. In this respect, all of the essential components of the apparatus should be fixed in place, so that the user does not need to assemble the apparatus. This does not however refer to other components of the apparatus which may be present in preferred embodiments. Such embodiments include for example, a reaction chamber for carrying out the photocatalysis reaction, which is not an essential component of the portable apparatus.

According to the present invention, the sample cell and the reference cell each has at least one set of opposed side walls which is capable of transmitting light emitted from a light source. In a preferred embodiment, the set of opposed walls is made out of a substantially transparent material. As used herein, the term "transparent" refers to any material which is capable of transmitting light of a wavelength between 200 and 12000 nm, preferably 200 and 2000 nm without substantially affecting the properties of the light being emitted. In a preferred embodiment, the reference and sample cells each have at least one set of opposed side walls made out of quartz.

In an alternative embodiment according to the present invention, each cell may have one set of opposed walls made out of quartz and another set of opposed walls made out of zinc selenide. In this case, the opposed walls made out of quartz will be capable of transmitting light of a wavelength between 200 and 2000 nm, whilst the opposed walls made out of zinc selenide will be capable of transmitting light of a wavelength between 600 and 12000 nm.

In a particularly preferred embodiment, the sample and reference cells are each capable of holding a volume of fluid between 1 to 50 ml, preferably 1 to 30 ml, even more preferably 1 to 10 ml at any given time.

The sample cell and the reference cell also each has a means for allowing a continuous flow of fluid through the cell. This may be in the form of an inlet and an outlet means. Such an embodiment may comprise an inlet means at one end of the cell and an outlet means at another end of the cell. In this embodiment, the sample/reference fluid enters the cell through an inlet means, then out through the outlet means back into the source of the sample/reference fluid. This provides a continuous flow of fluid flowing through the sample/reference cell.

The sample fluid according to the present invention comprises a compound capable of being degraded by a photocatalyst. The term "photocatalyst" as used herein refers to a compound wherein the valence band electrons, upon exposure to photons of a given energy, are promoted into the conduction band with the simultaneous generation of corresponding holes in the valence band. As such, a compound capable of being degraded by a photocatalyst can be any compound which is capable of being at least partially degraded, directly or indirectly, by photocatalytically driven reaction(s). The compound capable of being degraded by a photocatalyst must also be able to absorb light of a particular wavelength or range of wavelengths emitted by the light emitter. In a preferred embodiment, the compound is capable of absorbing light between 200 and 2000 nm. In a particularly preferred embodiment, the compound is methyl orange or methylene blue, most preferably methyl orange.

In order to ensure that only the compound capable of being degraded by the photocatalyst is absorbing the light from the light emitter, the sample/reference fluid should comprise a solvent which is substantially transparent, preferably at least in the region of the spectrum having the characteristic absorption peaks of the compound capable of being degraded by a photocatalyst. In a preferred embodiment, the solvent is water.

The light emitter according to the present invention may be any means capable of emitting a light of a particular wavelength or range of wavelengths. In a preferred embodiment, the light emitter is capable of emitting light between 200 and 2000 nm. In a particularly preferred embodiment, the light emitter is an illuminating LED. Term "illuminating LED" as used herein, refers to an LED light source that may be in a form of an LED package, or any other form providing LED-emitted light. Some examples of LED packages have one or multiple number of light-emitting diodes on a base. Such multiple diodes may emit light with the same wavelength which produces a common-colour light. Alternatively, multiple diodes may emit light of different wavelengths and thus different colours may be blended to achieve a desired colour of light. For example, when the compound to be detected is methyl orange, the light emitter is configured to emit light at a wavelength of 465 nm. The central emission of 465 nm corresponds to the strongest peak in the visible absorption spectrum of methyl orange. In yet another preferred embodiment, when the compound to be detected is methylene blue, the light emitter is configured to emit light at a wavelength of 650 nm.

In a further preferred embodiment, when the compound to be detected is a mixture of compounds, a plurality of light emitters can be provided which are each tailored to emit at the absorbance of one of said plurality of compounds. Thus, for example, where two compounds are to be detected, two light emitters may be provided, one tuned to emit at approximately the absorption peak of the first compound, and a second emitter tuned to emit at approximately the absorption peak of the second compound. In such a configuration, the light emitters would not be switched on simultaneously, but each spectrally resolved measurement would be synchronised to the switching on of the relevant light emitter. In one embodiment according to the present invention, there may be one light emitter present to emit light to both the reference and sample cells. In another embodiment, each of the sample and reference cells may have a separate light emitter.

The apparatus according to the present invention also requires a detector configured to detect the amount of light transmitted through the sample and reference cells. In a preferred embodiment, the detector is a photodiode. A photodiode as used herein refers to a photodetector capable of converting light into either current or voltage, depending upon the mode of operation. In a preferred embodiment, the spectral range of the photodiode is chosen to overlap with the spectral range of the illuminating LED. Therefore if the light emitter is configured to emit light at a wavelength of 465 nm, the photodiode is also configured to detect light at a wavelength of 465 nm. In such a case, a silicon photodiode may be used which has a spectral response between about 400 nm and 950 nm, which contains the peak emission of 465 nm. In one embodiment according to the present invention, there may be one detector present to detect the transmitted light from both the reference and sample cells. In another embodiment, each of the sample and reference cells may have a separate detector.

The amount of light absorbed by the compound in the fluid in the sample cell is directly related to the concentration of the compound in the streaming fluid through the Beer-Lambert law. The essential components of the apparatus of the present invention therefore allow the concentration to be monitored by detecting how much of the emitted light from the light emitter reaches the detector after it has been transmitted through the sample fluid.

In a preferred embodiment, the present invention uses the reference cell as a standard so that the absorbance, a value directly proportional to the concentration of the compound in the sample cell, can be recorded. Assuming the light intensity measured by the reference cell emitter is 100%, because there is no absorption from the compound, the intensity measured at the sample cell detector will be less than this value because the compound will absorb a fraction of the emitted light. The absolute transmission (fraction of light transmitted) is the amount of transmitted light through the sample, divided by the amount of transmitted light through the reference. In the preferred embodiment, the fraction of transmitted light is calculated by measuring the quotient of the voltage produced by the sample detector electronics and the voltage produced by the reference detector electronics. The absorbance is then calculated by taking the base 10 logarithm of the reciprocal of the fraction of the transmitted light. According to the Beer-Lambert law, the absorbance is directly proportional to the concentration with a multiplicative scaling factor. This multiplicative scaling factor includes the geometrical, electrical, and material conditions, and is a constant for the apparatus. The present invention therefore allows the absolute concentration of the compound in the sample cell to be measured.

In a preferred embodiment the present invention incorporates a mechanism to calibrate the sample cell and the reference cell. This can be a dimmer or other electrical or electronic means to adjust and control the light intensity from the light emitter so that the light intensity transmitted through the reference and the sample cell is the same if both chambers contain the same concentration of the compound which in the preferred case is zero.

The present invention may also comprise an integrated electronic system to convert the amount of light measured by the detectors into a signal proportional to the concentration of the compound in the sample fluid. Such an electronic circuit would first amplify the signal from the reference cell detector and the sample cell detector and then process the signals to produce an output proportional to the concentration of the compound in the sample cell.

The output signal of the electronic circuit will be monitored over time. This allows the change in concentration of the compound to be recorded and photoreaction rate to be calculated. This allows the efficiency of the photocatalyst to be evaluated.

The user is able to monitor the output signal using standard data acquisition tools such as a digital multimeter, an analogue-to-digital (A D) card or a digital oscilloscope. An alternative implementation of the invention may have integrated digitizing electronics with an USB output so that the device can be plugged directly into, and operated directly from a computer such as laptop or desktop computer.

This could then provide a means for calculating and displaying instantaneous reaction rate as well as a time-averaged value.

In order to ensure that accurate measurements of the efficiency of the photocatalyst are obtained, it is preferred that the apparatus further comprises a light-tight outer casing. This ensures that the light emitter is the only source of transmitted light, and it also ensures that the detector only detects the transmitted light from the light emitter.

In yet another preferred embodiment, the apparatus may further comprise two pumps, wherein each pump is configured to continuously circulate the sample and reference fluids through the sample and reference cells respectively. Examples of such pumps may include centrifugal or diaphragm pumps providing adequate pump rates for water between 50 to 5000 ml/min. Such features would particularly be useful in systems where analysis of still water is required.

As previously mentioned, the apparatus only provides a means of measuring the efficiency of the photocatalyst, and therefore the actual photocatalysis reaction takes place elsewhere. However in another embodiment, the apparatus may further comprise a reaction chamber for carrying out the photocatalysis reaction.

In yet another embodiment, the apparatus may be integrated into existing water purification systems. The apparatus according to the present invention could then be adapted as an inline quality-testing tool that could raise an alarm should the water quality fall outside a control sample. Therefore water purification systems comprising the apparatus of the present invention is also included within the scope of the claims.

In another embodiment according to the present invention, there is also provided an apparatus further comprising one or more light emitters, each configured to emit light at a wavelength or range of wavelengths which is different from any other light emitters. In addition, such an embodiment may further comprise one or more detectors, each configured to detect the wavelength or range of wavelengths of light emitted by a particular light emitter. In a preferred embodiment, the apparatus could also comprise a sequential timing scheme which would allow switching between each of a multiple series of light emitters, and so that the detector signals for each light emitter may be recorded separately. The invention could thus be extended to monitor more than one compound at the same time. However this will require each of the compounds being monitored to display a different peak in the visible absorption spectrum.

The present invention is also directed to a method of measuring the efficiency of a photocatalyst, comprising the steps of:

a) providing a continuous flow of a reference fluid and a sample fluid; and b) measuring the degradation rate of a compound capable of being degraded by a photocatalyst using the apparatus as defined herein.

In addition, the use of the apparatus to determine the efficiency of a photocatalyst and/or the purity of water also falls within the scope of the present invention. Brief Description of the Drawings

Figure 1 is a schematic diagram showing how the illuminating LED and the interrogating photodiode fit together with the liquid flow cell.

Figure 2 is a schematic diagram showing the two flow cells together with the functionality of the integrated electronics.

Figure 3 is a schematic of the closed water circuit for evaluation of photocatalytic reactions. The reactor is driven by UV-LEDs illuminating a substrate coated with photocatalytic material on the base of the reactor vessel. A micro-pump provides constant mixing and mass flow over the catalyst and serves the in-stream sensor unit to monitor the concentration of pollutants in real time by detecting light absorption.

Figure 4(a) is a schematic showing the reactor, which consists of a glass vessel equipped with in- and outlet, with the photocatalyst fixed on its base. UV-LEDs are fixed into the cover of the reactor. Figure 4(b) is an emission spectrum of the illuminating LEDs showing a peak emission wavelength centered on 375 nm.

Figure 5(a) is a photograph of the UV-LED pattern in the reactor cover, Figure 5(b) is a photograph of the resulting illuminated area on the photocatalyst coated wafer, Figure 5(c) is a simulated irradiant power distribution, based upon geometric arrangement assuming Gaussian distribution of the power for each LED on the photocatalytic disc, and Figure 5(d) is a measured irradiant power distribution reaching up to 2.1 mW/cm²

Figure 6 shows two investigated chamber geometries in (a) and (b). The design and results of two-dimensional CFD simulations are presented. For illustration purposes we have included arrows indicating the speed and flow direction in the vector diagrams. The first design in a) was implemented in the reactor. Figure 6(c) shows mixing of methyl orange in the reactor vessel monitored by the in-stream sensor. Each step represents adding one drop (ca. 0.05 mL) of a 100 ppm MO solution to 150 mL of clear water.

Figure 7(a) shows the absorption measurement is performed in special flow-through cells, which have fittings for the tubing in one direction and two windows on each side on one of the orthogonal axes. On the windows attached are holders for the light source (LED) and the photo-detector to measure the light absorption in the liquid. Figure 7(b) is a photograph of the cell. Figure 8 is an absorption spectrum showing the absorption values for methyl orange and the matching emission of the blue LED, which is used in the concentration sensor. The UV-LED spectrum is plotted as a reference.

Figure 9 is a flow diagram of the read out and signal processing electronics. The signal from the photodiode (/) is amplified and passed through comparator electronics to produce an output directly proportional to the concentration. The other input to the differential amplifier is the calibrated reference intensity (/₀).

Figure 10 is a powder X-ray diffraction patterns of (a) the pristine P25 Ti0₂ and (b) P25 Ti0₂ coated onto the glass wafer.

Figure 1 1 is a representative SEM images of the coated wafer surface before (a) and after (b) photocatalytic reaction.

Figure 12(a) is a degradation measurement of methyl orange solution in the reactor, showing an exponential decay of concentration (C) with decay rate (k). Figure 12(b) is the negative logarithm of the concentration divided by the initial concentration (C₀) and the observed decay rate (/ ).

Figure 13(a) refers to several cleaning cycles of freshly added polluted water (methyl orange) which demonstrates the possibility of continuous operation. Figure 13(b) is a UV-Vis of the prepared solution before and after cleaning shows the complete removal. In the contaminated water methyl orange is expressing the typical peak around 465 nm.

Figure 14 shows the degradation rates of methyl orange depending on the initial concentration of methyl orange used. The graph shows derived values (black squares) from measurements and a fit (dotted line) using the Langmuir-Hinshelwood kinetic rate model.

Figure 15(a) shows measurements at constant initial concentrations (0.35 ppm) varying the UV irradiance in the center of the light field illuminating the fixed photocatalyst. Figure 15(b) is a plot showing the obtained decay rates against irradiance. The line is a guide, demonstrating the linear increase of the rate with increasing the irradiance.

Figure 16 shows rate constants were derived from measurements made at constant initial concentrations, varying the UV irradiance in the center of the light field illuminating the fixed photocatalyst on two different wafers (a) and (b). The first wafer in a shows two series where the position of the wafer is changed leading to large fluctuations in the rate. The second wafer in b shows two series at lower and higher irradiant power. The lines in both graphs are guides to the eye showing trends in the change of rates with increasing UV power. Both wafers show a region of linear increase at lower power but the rate reaches saturation at higher power, the threshold being between around 1000 μνν/cm².

Figure 17(a) shows measurements at different volumes in the reactor vessel. Figure 17(b) is a graph to show the observed decay rate against volume showing an exponential slowdown of the rate as the volume of polluted water increases.

Detailed Description of the Invention

General

The term "comprising" encompasses "including" as well as "consisting" e.g. a composition "comprising" X may consist exclusively of X or may include something additional e.g. X + Y.

The term "about" or "approximately" in relation to a numerical value x means, for example, x+5%. Where necessary, the word "about" or "approximately" may be omitted from the definition of the invention.

The word "substantially" does not exclude "completely". Where necessary, the word "substantially" may be omitted from the definition of the invention.

Examples of the Present Invention

The following examples of the present invention are merely exemplary and should not be viewed as limiting the scope of the invention.

Example 1

A specific example of the invention would consist of two liquid cells with front and back quartz observation windows. At one of the windows, there is a light-tight tube containing an illuminating LED with spectral properties matching the peak visible absorption of methyl orange, along with some collimation optics. At the other window, there is a light-tight tube containing a photodiode with spectral range chosen to match the illuminating LED and some focusing optics. The fraction of illuminating light not absorbed by the methyl orange solution registers on the photodiode in the form of an electrical signal. A similar reference signal is produced by the photodiode on the other reference cell. The photocurrent generated by the light intensity incident on each photodiode is taken into a transimpedance amplifier. This converts the photocurrent to a voltage which is proportional to the size of the photocurrent. The transmittance is the quotient of the sample voltage and the reference voltage. The quotient is performed by subtracting the voltage outputs from two logarithmic amplifiers (using the logarithmic law log(A/B) = log(A) -log(B) ). Without converting back to the linear quotient A/B by an exponential amplifier the logarithmic quotient log(A B) is given to the output. This value is directly proportional to the absorbance and to the concentration through the Beer-Lambert law. This signal is then digitized and recorded over time allowing the decay rate of the methyl orange to be calculated.

Figure 1 shows how the illuminating LED and the interrogating photodiode fit together with the liquid flow cell.

Figure 2 is a schematic diagram showing the two flow cells together with the functionality of the integrated electronics. Each cell has an illuminating LED and a photodiode. The signals from each photodiode are amplified and passed through the comparator electronics to produce the transmission signal.

Example 2

2. Experimental Section

2.1 Overview

Figure 3 shows a schematic of the closed water circuit system containing a reactor where the photocatalytic degradation of chemicals or organic pollutants is carried out. The water flow is driven through a centrifugal micro-pump to guarantee constant mixing in the reactor vessel. The heart of the monitoring system can be seen on the left side in Figure 3. A liquid cell is placed in the flow circuit; here a light (LED source) passes through the water/pollutant stream so that a measurement of the light absorption can be made. The signal from a photodiode is then processed by analogue electronic circuitry, and the resulting signal corresponds to the concentration of the absorbing chemical, which, in this instance, is methyl orange.

2.2 Chemicals and Photocatalyst Preparation

Methyl orange (MO) sourced from Sigma-Aldrich was dissolved in deionized (Dl) water in typical concentrations ranging from 100 to 10 ppm. Drops of the MO solution were added into the reactor containing Dl water. We monitored the real- time photodiode signal during this process and observed that a homogenous mixture was produced on a timescale of seconds. This timescale was negligible when compared to the rate-constant of any of the reactions we studied.

The photocatalyst used for these experiments was Evonik Ti0₂ Aeroxide P25, which we will subsequently refer to in this paper as P25. P25 consists of a mixture of 20% rutile and 80% anatase Ti0₂. The responsible photocatalytic mechanism that makes P25 one of the most photocatalytic active material on the market (Kinetic Study of Photocatalytic Degradation of Carbamazepine, Clofibric Acid, lomeprol and lopromide Assisted by Different Ti0₂ Materials-Determination of Intermediates and Reaction Pathways, Doll, T.E., Frimmel, F.H., Water Res., 2004, 38, 955-964.), is under constant debate; some believe the rutile Ti0₂ acts as an antenna, which due to a smaller bandgap absorbs a larger range of wavelengths, whilst others claim that at the interface between the two materials charge separation and prolongation of lifetimes enhance the photocatalyst's activity (Recombination Pathways in the Degussa P25 Formulation of Ti0₂: Surface versus Lattice Mechanisms., Hurum, D.C., Gray, K.A.; Rajh, T., Thurnauer, M.C., J. P ys. C em. B, 2005, 109, 977-980; The Solid-Solid Interface: Explaining the High and Unique Photocatalytic Reactivity of Ti0₂-based Nanocomposite Materials., Li, G.H., Gray, K.A., Chem. Phys., 2007, 339, 173-187).

For coating, we adapted a spin casting method where Ti0₂ nanoparticle suspensions were formed by mixing Ti0₂ (400 mg) with ethanol (4 mL) and Triton X- 100 surfactant (250 μί) (ZnO Nanowire/Ti0₂ Nanoparticle Photoanodes Prepared by the Ultrasonic Irradiation Assisted Dip-Coating Method., Gan, X., Li X., Gao X., Zhuge, F., Yu, W., Thin Solid Films, 2010, 518, 4809-4812). Thin films were fabricated by spin casting the Ti0₂ suspension onto three inch glass wafers. For several cycles, 0.5 mL of suspension was drop cast onto the substrate surface and then spun at 300 rpm for 20 seconds. The wafer was rapidly heated to 450 °C for 10 minutes. The function of this processing step was to remove any traces of the organic surfactant used for spin coating. We have chosen a temperature of 450 °C because this is well below the annealing temperature required for microstructural transformation of the film, as discussed by Zhang et al. (Importance of the Relationship Between Surface Phases and Photocatalytic Activity of Ti0₂, J. Zhang, Q. Xu, Z. Feng, M. Li, C. Li, Angew. Chem. Int. Ed., 2008, 47, 1766 -1769).

2.3 Characterization

Powder X-ray diffraction (XRD) experiments on the Ti0₂ powder and coated Ti0₂ samples were performed at room temperature using a Philips PW 3040 DY640 diffractometer equipped with a graphite monochromator using Cu K-a radiation (λ = 0.1541 nm). The samples were scanned over a 2Θ range of 10 ⁰ - 80 ⁰ in steps of 0.02 °. To verify the surface coverage and morphology of the Ti0₂ on the glass wafers both before and after reaction, field-emission scanning electron microscopy (SEM, Carl Zeiss XB 1540) at 5 kV acceleration voltage was employed. The thicknesses of the films were measured using a Dektak profilometer.

2.4 Reactor Assembly

The water circuit was assembled by connecting a small batch reactor, made from a glass with in- and outlets, in series with a small centrifugal pump and the liquid cell. Figure 4 shows a schematic of the reactor vessel on the left panel. The glass reactor vessel has a height of 70 mm and has an inner diameter of 84 mm. The reactor cover holds 15 UV-LEDs and the coated wafer is fixed to the reactor base. The distance of the UV-LEDs to the photocatalyst is 65 mm. The in- and outlets are 4 mm diameter glass tubes situated 10 mm from the base of the reactor. The reactor was filled with the test liquid in volumes ranging from 100 mL to 250 mL. In most experiments 100 mL or 150 mL was used, which give water depths of approximately 20 mm or 30 mm, respectively.

For the UV light source, we have used Ultra Bright Deep Violet LED370E UV-LEDs sourced from Thorlabs. The emission spectrum is indicated on the right panel in Figure 4 with a main emission peak at 375 nm and a line width of approximately 10 nm. Thus, the emitted light lies well in the absorption spectrum of P25. Each UV- LED has a half viewing angle of 19° and a forward optical power of 2 mW at the drive current of 20 mA.

The arrangement of the 15 UV-LEDS is shown in Figure 5a with a slight prolongation along one axis. The real light field was photographed and is shown in Figure 5b. The ideal light field generated, at a distance of 65 mm, (the position of the photocatalyst surface) gives an almost circular illumination, as shown in the simulation in Figure 5c. The real illuminated area deviates due to non-ideal soldering of the UV-LEDs on the lid plate and possible inhomogeneous molding of the light emitting semiconductor chips. The intensity distribution was measured with a Newport 918D-UV-OD3 detector and power meter (results are shown in Figure 5d) at a step distance of 1 cm. The maximum irradiance is 2.1 W/m², with the peak center shifted slightly to the right of the ideal position. The integrated power of the measured irradiated field from the measurement is 31.2 mW, which has to be corrected by a factor of 4/π due to the circular aperture of the detector and the square type measurement matrix and amounts to 24.5 mW of total irradiant power. The intensity of the light can be changed by a potentiometer set in series to the UV- LEDs. For measurements of the light intensity, we have plotted the irradiance observed in the center of the light field and assumed a linear relationship with total power. The total area of the coated wafer is 45.6 cm². All 15 UV-LEDs irradiate approximately three quarters of the coated surface.

The reactor vessel design was chosen to ensure both efficient mixing of the MO solution and at a steady but controlled mass flow rate over the photocatalytic surface. In Figures 6a and b we show two-dimensional computational fluid dynamics (CFD) simulations and subsequent distribution of flow rates indicated by velocities for two different designs respectively. Both designs have a central circular chamber, the design in Figure 6a has opposing in- and outlets, whereas the design in Figure 6b has a linear arrangement for the inlet and outlet. CFD simulations were performed with EasyCFD in the steady state regime with turbulent flow, isothermal and non-buoyant settings. A fast converging steady state solution depending on the grid size was confirmed. The in and out mass flow rate was set to 0.5 L/min similar to the real pump rate. The first design (Figure 6a) shows a slow flow in the middle of the reactor and faster flow at the edge and also has turbulence due to the direction change at the outlet from the water stream coming from the inlet. It, therefore, gives better mixing properties as opposed to the design suggested in Figure 6b, where most of the liquid flows directly in a linear stream from the inlet to the outlet. In addition, the design in Figure 6a guarantees a constant flow rate and mass exchange in the center of the photocatalytic wafer, the area that receives the highest photon flux and is expected to have the highest photocatalytic activity, while at the same time providing a fast mixing and an instant sensor reading of the real concentration. Figure 6c shows the concentration of MO, measured by the sensor upon drop wise addition of approximately 0.05 mL of 100 ppm MO solution into the reactor containing 150 mL of water. The concentration increases in a stepwise manner and demonstrates fast and homogeneous mixing in three to five seconds.

2.5 In-stream Sensor Unit

The sensor system consists of an aluminium milled liquid flow cell with front and back quartz observation windows (see Figure 7). Quartz has been chosen for this application because it is transparent in the range 200-2500 nm. Other window materials could be used to access alternative spectral bands. At one of the windows there is a light-tight tube containing an illuminating LED with spectral properties matching the visible absorption of methyl orange, along with collimation optics. The emission spectrum of the LED (Hyper blue LED LB3333 from OS RAM Opto Semiconductor GmbH) is taken from the datasheet and presented in Figure 8. When compared to the measured absorption of MO in Figure 8a, both spectra have the same maximum wavelength at 465 nm. At the other window of the liquid cell, is a light-tight tube containing a photodiode (visible light photodiode BPW21 from OSRAM Opto Semiconductor GmbH), here the spectral range is chosen to match the illuminating LED.

The fraction of illuminating light not absorbed by the MO solution registers on the photodiode in the form of an electrical signal / (Figure 9). A calibrated reference signal representing the intensity l₀ of a total absence of MO is produced either by the photodiode on a similar reference cell containing pure water or alternatively by an adjustable constant voltage source. Both signals are then passed first through a trans-impedance amplifier and second a logarithmic amplifier. In the last stage, a differential amplifier compares the amplified logarithmic signals. In this way, the output signal produced is proportional to the quotient of the sample / and reference signal /₀:

f

log ¹ = log i - log % (1 ) which, in turn, is proportional to the concentration C of the monitored chemical, according to the Beer Lambert law:

where / and l₀ are the intensities of the transmitted and incident light, a is the absorption coefficient, / the path length and C the concentration. The resultant logarithmic quotient is therefore directly proportional to the concentration of the monitored chemical.

3. Results & Discussion

Before considering the properties of the catalytic reactor/sensor system, some basic material characterization of the Ti0₂ film was performed. This aim of this exercise was to establish whether the coating process itself affected the catalytic properties of the Ti0₂. Factors such as the microcrystalline structure, and the film uniformity, had been studied in previous investigations (Environmental Applications of Semiconductor Photocatalysis., Hoffmann, M.R., Martin, ST., Choi, W.Y., Bahnemann, D.W., Chem. Rev., 1995, 95, 69-96; Ti0₂ Photocatalysis: A Historical Overview and Future Prospects., Hashimoto, K., Irie, H., Fujishima, A., Jpn. J. Appl. Phys., 2005, 44, 8269-8285; Heterogeneous Photocatalytic Degradation of Organic Contaminants Over Titanium Dioxide: A Review of Fundamentals, Progress and Problems., Gaya, U.I., Abdullah, A.H., J. Photoch. Photobio. G, 2008, 9(1), 1 -12; Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications., Chen, X., Mao, S.S., Chem. Rev., 2007, 107, 2891 -2959; The Solid- Solid Interface: Explaining the High and Unique Photocatalytic Reactivity of Ti0₂- based Nanocomposite Materials., Li, G.H., Gray, K.A., Chem. Phys., 2007, 339, 173-187). XRD results before and after confirm that no major phase transitions have occurred after coating. Some broadening of the peaks was observed (Figure 10) but this was attributed to the decreased sample volume in the film, compared to the powdered state. Analysis of the SEM images (Figure 1 1 ) shows that the coating method resulted in a uniform coverage with an average thickness of about 40 nm (measured by Dektak). This measurement was repeated after several catalytic reactions had been performed. While the later SEM images did show some minor changes to the surface morphology, showing some additional agglomeration of the particles, the average thickness of the film remained more or less constant at 40 nm. From this, we concluded that the films had remained relatively stable during the catalytic reactions, and consequently leaching of the Ti0₂ nanoparticles into the water was negligible.

The batch reactor, micro-pump and liquid cell were connected by flexible 3 mm diameter tubing and loaded with Dl water. The sensor system was calibrated to zero output before mixing the MO solution into the reactor. To initiate the photocatalytic reaction, the Ti0₂ coated glass wafer was fixed on the bottom and was illuminated by the UV-LEDs.

Figure 12 depicts typical results from an experiment measuring the degradation of MO, where the initial concentration of MO in the solution is 0.6 ppm and after approximately six to eight hours the orange solution becomes colorless. Control experiments using a wafer prepared without Ti0₂ and experiments with no UV illumination confirmed that the decolorization is due to the photocatalytic reaction. Figure 12a shows the measured data from the output of the sensor unit - an almost perfect exponential decay. Hence we are assuming first order kinetics, where the concentration C at time f is described by

C = C^exp(-kt) (3) with initial concentration C₀, and observed decay rate k. The rate k is determined by the slope of a linear fit to - ln(C/C₀) over f (See Figure 10b). From the data, we observe a decay rate / of 0.5 hr^"1. Taking into account the amount of water (150 ml_ in this instance) and the initial concentration of 0.52 ppm, we can estimate the cleaning capacity to be in the range of 0.0036 μηηοΙ L^"1 hr^"1. This rate depends on the geometry of the reactor, which includes the ratio of photocatalytic surface area and water volume. To achieve an improved cleaning rate this ratio has to be optimized through the reactor design.

For continuous operation it is essential to demonstrate that the reactor is stable and can be operated for many cycles. Figure 13a shows four consecutive runs, where in each run the concentration was set to 0.35 ppm in the reactor vessel at a filling level of 100 ml_. As can be seen in the plot, the rate gives similar results for each run. This demonstrates, therefore, that the system is stable and the efficacy of the photocatalyst is conserved.

In Figure 13b the UV-Vis spectra of the contaminated model water (MO solution) and the clean water after the photocatalytic reaction is shown. The water containing MO exhibits the typical peak around 465 nm. After the reaction, the peak disappears, demonstrating complete removal. Baiocchi et al. have shown that in the photocatalytic process the MO molecule is decomposed into smaller molecules (Characterization of Methyl Orange and its Photocatalytic Degradation Products by HPLC/UV-VIS Diode Array and Atmospheric Pressure Ionization Quadrupole Ion Trap Mass Spectrometry., Baiocchi, C, Brussino, M.C., Pramauro, E., Prevot, A.B., Palmisano, L, Marci, G., Int. J. Mass Spectrom., 2002, 214, 247-256). The reaction proceeds through a number of steps including demethylation, hydroxyl attack on the phenyl ring and eventually cleavage of the azo bond. The final end products are sulfate, water and carbon dioxide (Room Temperature Oxidation of Methyl Orange and Methanol Over Pt-HCa₂Nb₃Oi₀ and Pt-W0₃ Catalysts Without Light., Dvininov, E., Joshi, U.A., Darwent, J.R., Claridge, J.B., Xu, Z., Rosseinsky, M.J., Chem. Comm., 2011 , 47, 881 -883).

In order to understand the influence of essential parameters and to demonstrate the capability of our setup, a series of experiments were performed with variations in the initial MO concentration, the irradiance and the liquid volume.

Figure 14 presents observed decay rates from measurements at varied initial concentrations. All measurements were performed with a filling volume of 150 mL. It can be seen that there is a steep increase in decay rate with increasing initial concentration, which plateaus at higher initial concentrations. A reaction model often used to explain this kinetic behavior is the Langmuir-Hinshelwood (L-H) model (Kinetic Disguises in Heterogeneous Photocatalysis., Ollis, D.F., Top. Catal., 2005, 35, 217-223) where the adsorption constant K_ads describes the rate of ad- and desorption of the chemical under investigation on the surface and the constant k_LH describes other influences such as the light intensity. In the model it follows that the degradation rate η depends on the initial concentration C₀ in the form of: ^{L H} l÷ A\-/ - iC- ] (4)

This model was fitted to our data (see Figure 14 dotted line) resulting in values _LH = 0.32 μηηοΙ L^"1 hr^"1 and K_ads = 0.45 μη"ΐοΓ¹Ι_. These values give an indication of the photocatalytic efficiency of our coated surface.

The L-H model has been criticized as an oversimplification (Dogmas and Misconceptions in Heterogeneous Photocatalysis. Some Enlightened reflections., Emeline, A.V., Ryabchuk, V.K., Serpone, N., J. Phys. Chem. B, 2005, 109, 18515- 18521 ) due to the very complex nature of photocatalytic processes involving a series of steps from light absorption, transfer of excited states to the surface and production of active oxygen species before a reduction/oxidation of a given molecule can take place (Kinetics of Liquid Phase Photocatalyzed Reactions: An illuminating approach., Ollis, D.F., J. Phys. Chem. B, 2005, 109, 2439-2444). An important parameter is the incident light intensity, which will influence the charge carrier dynamics in the semiconductor and can affect both constants (Kinetics of Liquid Phase Semiconductor Photoassisted Reactions: Supporting Observations for a Pseudo-Steady-State Model., Mills, A., Wang, J.S., Ollis, D.F., J. Phys. Chem. B, 2006, 110, 14386-14390). To see the dependence of the irradiant power of the UV in our system, the light intensity was varied at constant initial concentration (0.35 ppm) and constant filling levels of 100 mL. In Figure 15, measurements and derived rate constants for light intensities from 50 μ\Λ/ to 200 μ\Λ/ per cm² are plotted. The rates follow an almost linear increase as indicated by the dotted line in Figure 15b. The small deviation of the point measured at 100 μνν/cm² can be attributed to measurement errors.

To test the linear increase of the rate with light intensities further, measurements at higher power intensities up to a maximum of 2000 μνν/cm² were performed. In Figures 16a and b measurements on two different wafers are shown. We found that the coating in our process results in inhomogeneous thickness and together with the non-uniform light field (as seen in Figure 5) the system is sensitive to the exact position of the wafer resulting in large fluctuations in the decay rates. Depending on the position of the wafer relative to the illuminating UV-LEDs the rates can double as can be seen in Figure 16a. An overall trend in all the measurements is the linear increase in the irradiance range below 1000 μW cm² and a saturation effect that appears at higher UV power. We have also tested a reduced set of five UV-LEDs and found that a similar effect occurs. There appears to be a transition where the rate and its dependence on the light intensity saturate at a similar position around 1000 μνν/cm². It was identified that the reaction rate follows a linear relationship when the process is dominated by the chemical reaction. If, however, the light flux reaches a threshold the internal processes in the semiconductor can become dominant and recombination of charges controls the reaction. Similar behavior on light intensities was found by Stefanov et al. and Wang et al. (Novel Integrated Reactor for Evaluation of Activity of Supported Photocatalytic Thin Films: Case of Methylene Blue Degradation on Ti0₂ and Nickel Modified Ti0₂ Under UV and Visible Light., Stefanov, B.I., Kaneva, N.V., Puma, G.L., Dushkin, CD., Colloid Surface A., 2011 , 382, 219-225; Dimethyl Sulfide Photocatalytic Degradation in a Light-Emitting- Diode Continuous Reactor: Kinetic and Mechanistic Study., Wang, Z., Liu, J., Dai, Y., Dong, W., Zhang, S., Chen, J., Ind. Eng. Chem. Res., 2011 , 50, 7977-7984).

In a third series of experiments we tested the dependence of decay rate on water volume in the reactor (see Figure 17). As expected, there was a clear reduction in degradation rate when the volume and, therefore the total number of molecules which have to be degraded, increases. The measured rates follow an exponential curve, as can be seen in Figure 17b. This can be reasonably explained by changes in the mass flow rate over the photocatalytic surface, which is kept constant.

Although energy efficient, due to UV-LEDs, the reactor design would need to be enhanced in order to reach performances of a suspension based system (Study on UVLED/Ti0₂ Process for Degradation of Rhodamine B Dye, T.S. Natarajan, M. Thomas, K. Natarajan, H.C. Bajaj, R.J. Tayade, Chem. Eng. J., 169, 2011 , 126- 134). To enhance the reactor's cleaning capacity the geometric arrangement of light source, liquid and catalyst or periodic illumination has to be optimized (Photocatalytic Degradation of Reactive Red 22 in Aqueous Solution by UV-LED Radiation., Wang, W.Y., Ku, Y., Water Res., 2006, 40, 2249-2258). Another possibility is to increase the ratio of coated surface to water volume, introducing coated light guides (Photocatalytic Reactor Based on UV-LED/T1O2 Coated Quartz Tube for Degradation of Dyes, Kalithasan Natarajan, Thillai Sivakumar Natarajan, H.C. Bajaj, Rajesh J. Tayade., Chem. Eng. J., 178, 2011 , 40-49).

4. Conclusions

We have assembled a cheap, robust and small closed circulating water system and have integrated a sensor unit that can measure the concentration of chemicals in a water stream. We have also developed a photocatalytic test reactor and demonstrated its function by measuring the degradation of methyl orange. The sensor system allows us to monitor the degradation of the concentration in real-time and also records degradation curves. From the data, we can calculate the first- order rate constant, which is a measure of the efficiency of the reaction. Our system provides the possibility to investigate a range of important parameters which can affect the reaction rate. We have demonstrated its ability by showing its stability in operation and by investigating the dependence of the reaction rate on initial concentration, light intensity and liquid volume to catalyst surface.

Whilst our set up is designed specifically to study photocatalytic degradation of methyl orange, in principle, it could be used to monitor any liquid-phase chemical or biochemical reaction in real time. Some alternative reactions that our system may be able to be adapted and optimized to study include: monitoring fermentation reactions to detect changes in turbidity, detecting changes in metabolic product concentrations, and assessing the effect of antibiotics on bio organisms.

Recently, efforts have been made to introduce standards (e.g. BSI:ISO 10678:2010) to enable comparison of the efficiency of new photocatalysts developed in different labs or companies. Our cheap and simple setup could potentially be incorporated into standard procedures which would allow different laboratories and companies to benchmark their new photocatalysts against a competitor.

Because our system is very cheap it would be easy to scale-up by purchasing additional units. For example, several tens of our invention could be operated in parallel for the price of one UV-Vis spectrometer.

In a real water purification plant, the water is assessed throughout the treatment process as part of quality control. Our sensor could be easily adapted as an in-line quality-testing tool that could raise an alarm should the water quality fall outside sample limits.

Claims

1 . A portable apparatus for measuring the efficiency of a photocatalyst comprising:

(a) a sample cell capable of transmitting light emitted from a light source and comprising a means for allowing a continuous flow of sample fluid through the sample cell;

(b) a reference cell capable of transmitting light emitted from a light source and comprising a means for allowing a continuous flow of reference fluid through the reference cell;

(c) a light emitter which is configured to emit light of a particular wavelength or range of wavelengths; and

(d) a detector configured to detect the amount of light transmitted through the sample and reference cells;

wherein said sample fluid comprises a compound capable of being degraded by a photocatalyst and absorbing light of a particular wavelength or range of wavelengths emitted by the light emitter; and

wherein the amount of transmitted light detected by the detector allows the efficiency of a photocatalyst to be determined.

2. An apparatus according to claim 1 , wherein said detector is a photodiode.

3. An apparatus according to claim 1 or claim 2, wherein the sample cell and the reference cell consist of a set of opposed side walls made out of quartz.

4. An apparatus according to any preceding claim, wherein the light emitter is an illuminating LED.

5. An apparatus according to any preceding claim, wherein said compound capable of being degraded by a photocatalyst, absorbs light of any wavelength between 200 to 12000 nm.

6. An apparatus according to claim 5, the range of wavelength is between 200 and 2000 nm.

7. An apparatus according to claim 6, wherein said compound is methyl orange and the light emitter is configured to emit light at a wavelength of about 465 nm.

8. An apparatus according to claim 7, wherein said compound is methylene blue and the light emitter is configured to emit light at a wavelength of about 650 nm.

9. An apparatus according to any preceding claim, wherein the sample fluid and the reference fluid comprise a transparent solvent.

10. An apparatus according to claim 9, wherein the transparent solvent is water.

1 1 . An apparatus according to any preceding claim, further comprising a light- tight outer casing.

12. An apparatus according to any preceding claim, wherein there is a separate light emitter for the sample and reference cells.

13. An apparatus according to any preceding claim, wherein there is a separate detector for the sample and reference cells.

14. An apparatus according to any preceding claim, wherein the absorption of emitted light from the light emitter which is subsequently detected by the detector in the reference cell and the sample cell is compared to determine the concentration of said component.

15. An apparatus according to any preceding claim, wherein the efficiency of the photocatalyst is determined by monitoring the degradation rate of said compound, by recording the concentration change over time.

16. An apparatus according to any preceding claim, further comprising a separate inlet and outlet means on the sample and reference cells.

17. An apparatus according to any preceding claim, further comprising an analogue electronic circuit which compares the transmitted light signal of the sample and reference cells.

18. An apparatus according to any preceding claim, wherein there are no moving parts.

19. An apparatus according to any preceding claim, wherein said apparatus allows real-time monitoring of the degradation of said compound.

20. An apparatus according to any preceding claim, wherein each of said sample and reference cells can hold a volume of fluid between 1 to 50 ml.

21 . An apparatus according to claim 20, wherein each of said sample and reference cells can hold a volume of fluid between 1 to 10 ml.

22. An apparatus according to any preceding claim, wherein said apparatus can be integrated into existing water purification systems.

23. An apparatus according to any preceding claim, further comprising an amplifier to amplify the signal received from the detector.

24. An apparatus according to any preceding claim, further comprising a comparator to produce a transmission signal.

25. An apparatus according to any preceding claim, further comprising an integrated electronics system, to convert the amount of light detected by the detector into a signal which is proportional to the absolute transmission in the sample fluid.

26. An apparatus according to any preceding claim, further comprising a reaction chamber for carrying out the photocatalysis reaction.

27. An apparatus according to any preceding claim, further comprising one or more light emitters, each configured to emit light at a wavelength or range of wavelengths which is different from any other light emitters.

28. An apparatus according to claim 27, further comprising one or more detectors, each configured to detect the wavelength or range of wavelengths of light emitted by a particular light emitter.

29. An apparatus according to any preceding claim further comprising two pumps.

30. A water purification system comprising an apparatus according to any preceding claim.

31 . A method of measuring the efficiency of a photocatalyst, comprising the steps of:

a) providing a continuous flow of a reference fluid and a sample fluid; and b) measuring the degradation rate of a compound capable of being degraded by a photocatalyst using the apparatus according to any of claims 1 to 29.

32. Use of the apparatus according to any of claims 1 to 29 to determine the efficiency of a photocatalyst and/or the purity of water.