Patent Application: US-201615203563-A

Abstract:
a novel photocatalytic oxidation system that combines long lifetime , high - power light emitting diodes with efficient , visible light - activated photocatalysts for the destruction of volatile organic compounds and other pathogens in air and water flow systems under ambient conditions of temperature and pressure is described . the technology uses the combination of visible photocatalysts with robust visible leds , uniform side emission fiber optics , and efficient catalyst surface illumination technologies to create a photocatalytic oxidation unit for air and water purification . this combined approach leads to numerous performance benefits including high voc conversion efficiency , compact reactor volume , low pressure drop , and the elimination of conventional ultraviolet mercury lamp logistics and hazards .

Description:
oxidation reactions that produce free radicals are generally followed by a sequence of additional chemical reactions between the radical oxidants and other reactants ( both organic and inorganic ) until thermodynamically stable oxidation products are formed . the ability of an oxidant to initiate chemical reactions is measured in terms of its oxidation potential . the most powerful oxidants are fluorine , hydroxyl radicals ( ho □), and ozone with oxidation potentials of 2 . 85 , 2 . 70 , and 2 . 07 electron volts , respectively ( carey , j . h ., 1992 ; techcommentary , 1996 ). the end products of complete oxidation ( called mineralization ) of organic compounds are carbon dioxide ( co2 ) and water ( h2o ). there are several approaches to generating hydroxyl radicals including electrical discharge , photolysis , photocatalysis , sonolysis , electrochemical oxidation , fenton and photo - fenton chemistry , and ozone . the technique of photocatalytic oxidation involves the surface illumination of a semiconductor in which a photon with energy e hv equal to or greater than the semiconductor band gap e g excites an electron from the valence band ( vb ) to the conduction band ( cb ). when the semiconductor is photoexcited an electron ( e cb − ) is promoted to the cb and a hole ( h vb + ) is left behind in the vb . a variety of reaction dynamics can follow in which the energy carriers ( e cb − , h vb + ) can undergo recombination with heat generation , the electron can return to the ground state and radiate light or heat , or migrate to the surface of the semiconductor and undergo oxidation and reduction ( redox ) reactions with electron donor and acceptor molecules adsorbed on the surface . in the presence of o 2 , e cb − are trapped by oxygen which results in the formation of superoxide radical anion , o 2 − , which slows the rate of electron - hole recombination . as the holes h vb + migrate to the surface they can react with adsorbed h 2 o to form the hydroxyl radical , ho .. although both superoxide and hydroxyl radicals are chemically active , the hydroxyl radical is generally thought to be the primary oxidizing species in the photocatalytic oxidation of volatile organic compounds . the basic mechanism that describes the photocatalytic oxidative process of a volatile organic compound ( voc ) into co2 and h2o are presented below : cat + hv → cat ( e cb − + h vb + ) ( 1 ) titanium dioxide ( tio2 ) is the most widespread photocatalyst for voc removal in air and aqueous systems . it is inert , stable , inexpensive , and poses no harm to the environment or humans . in short , tio2 is an ideal photocatalyst except for two properties — its action spectrum does not extend into the visible portion of the spectrum , and its relatively low voc oxidation activity . the band gap in tio2 is 3 . 2 ev and therefore uv photons with wavelengths & lt ; 388 nm are required to activate the catalyst . the efficiency of tio2 photocatalysts are known to be limited by the high recombination rate of the electron - hole pairs ( ecb −, hvb +) during the semiconductor excitation process . various studies have shown that doping tio2 with nanoparticles such as the noble metals silver , gold and platinum ( okumura , k . et al ., 1997 , hou x . g ., et al ., 2009 , shiraishi , y ., et al ., 2012 ), or non - metals like nitrogen or carbon ( asahi , r . et al ., 2001 ; zou , x . et al ., 2012 ) can substantially reduce the recombination rate , and thereby increase the activity of tio2 by increasing the lifetime of the ecb − and hvb + pairs . doping the tio2 catalyst also has the added benefit of reducing the energy band gap to allow visible - light activation . visible light activation is desirable as it allows access to energy efficient light emitting diodes ( led ), and in some cases solar light . leds offer high power , high brightness ( up to 500 mw in a single led ), high reliability (& gt ; 50 , 000 hours ), high efficiency ( 10 - 20 % electrical - optical ), wide spectral source illumination in the uv - visible portions of the light spectrum , and low cost (& lt ;$ 5 / diode ). most industrial photocatalytic oxidation approaches that use tio2 use mercury vapor lamps as a light source . however , illumination sources other than mercury lamps are desired since lamp lifetime is relatively short with an operational lifetime of & lt ; 10 , 000 hours , and mercury presents a potential health hazard if the lamp is fractured . to shift the catalyst response from uv to visible wavelengths , the electronic structure of tio2 is typically modified by introducing metal and non - metal materials into the catalyst structure . there are a variety of proposed mechanisms that describe the origin of red - shifting to visible light absorption upon doping . these include band gap narrowing , formation of localized states above the vb and below the cb , generation of color - centers , and sensitization ( lu , g . q ., et al ., 2009 ). some of the more effective visible catalysts include nanoparticle noble metals , and nonmetal n and c doped tio2 systems . tio2 thin films can be immobilized on various solid substrates such as borosilicate glass , fused silica , ceramic tile , and plastics . fused silica has superior optical transparency and lends itself to a variety of readily available photocatalyst deposition techniques such as sol - gel , thermal spray , sputtering . for example , silver ( ag ) nanoparticle — tio2 films can be relatively easily prepared using a sol - gel method with ti ( obu ) 4 , titanium butoxide ( hou x . g ., et al ., 2009 ; garcia - serrano , j ., et al . 2009 ,). here an aqueous solution of ti ( obu ) 4 , ethanol , agno3 at ph4 produces a stable ag +/ tio2 sol . after drying and grinding a nano - particle aggregate is formed . this is then dissolved into solution to dip coat the photocatalyst substrate . fused silica is the preferred material if catalyst preparation and operating conditions require high temperatures , and uva ( 400 - 320 nm ) and uvb ( 320 - 290 nm ) wavelengths . organic polymer - based fiber optics exhibit a smaller yong &# 39 ; s modulus , and thus are more resilient and less prone to fracturing and breakage than glass or fused silica . an organic polymer - based substrate is also less expensive than fused silica or glass , and thus are amendable to use in industrial environments where robust operation and reduced replacement costs are desired . plastic fiber optics have larger core diameters ( up to 1 mm ) and operate multi - moded which allows higher transmission ( for an equivalent area ), higher numerical aperture , and more reflections inside the core . however , wavelengths less than 380 nm are absorbed or scattered by the impurities in the organic substrate , or absorbed by the substrate itself to lower its transmission . in addition , the temperature limit for the most polymer substrates is near 70 ° c . the rate of photocatalytic disappearance of voc species ci can be estimated by the following simple langmuir - hinshelwood relationship ( turchi , c . s ., et al . 1995 ): where i n o is the power dependence of the light intensity ( einstein / cm 2 - s ), n is unity for low intensity , n is ½ at high intensity , a sv is the catalyst surface area to reactor volume ( geometric surface area ), cm − 1 , φ i is the quantum efficiency for species i , c i is the concentration of species i , molec ./ cm − 3 , and k i is the binding constant of compound i , cm 3 / molec . inspection of this equation leads to a better understanding of effective device design . for example , a simple but necessary condition is that the photocatalyst is only activated if photons reach and illuminate the catalyst surface . partial illumination or intensity shadowing is known to be significant in externally illuminated substrates such as monoliths , micro - beads , zeolites . in these structures there can be large regions devoid of photon illumination . therefore , any reactor should be designed so that the gas flow does not exclusively flow in these dark regions , and that the reaction rate or residence time of the analyte in the reactor bed is to be of sufficient duration to allow the contaminant stream to be treated . the reaction rate is surface area dependent , the larger the surface area the higher the rate of conversion . the higher the surface area to reactor volume ratio , the smaller the generator footprint will be . further , a high a s enhances the surface contact time between contaminant and catalyst so that analytes and intermediates are afforded time to react and be oxidized . it has been long recognized that external light penetration presents a significant challenge for a variety of photocatalyst substrate configurations including monoliths , packed beds , microbeads , concentric tubes , rings , slurries , etc . the concept of using a fiber optic to transmit , illuminate and support photocatalysts was first proposed by marinangeli and ollis , ( 1977 ). later , hofstadler and bauer et al , ( 1994 ), and peill and hoffmann ( 1995 ), refined the method of driving a photocatalyst coated fiber optic reactor . however , these reactors were basically constructed of end - emitting , data transmission fibers which are designed to light guide down a single mode 10 □ m fiber core with total internal reflection for long distances . however , for illumination of a catalyst coated on the outer wall of a fiber a different approach is warranted . in principle , the uniform illumination of a catalyst coated fiber can be obtained with a side emission fiber optic over short distances . in this regard , the side - emitting fiber optic is well - suited to generating a uniform and a highly illuminated photocatalyst surface area . there are several commercial off the shelf polymer - based side emitting fibers that are readily available and can be used in this approach . an array of side emitting fiber optics can be arranged in a close - packed geometry resulting in a high reactor surface area to reactor volume ratio , asv . a reactor with a high surface area to volume substrate ratio is desirable since more reaction surface area is available for reactant conversion in a smaller reactor volume or package size . this means more fibers of shorter length can be used with less light attenuation and higher catalytic activity generated . for example , the surface to volume ratio asv of a fiber optic is approximated by equation 6 where df is the fiber diameter , and f is the area packing fraction of the fiber optics : for a simple square lattice arrangement for the fiber optic packing , f is given as : where a and b are the separation distance between fiber . consider a fiber diameter of 0 . 075 cm and a fiber separation a = b = 0 . 2 cm , the packing fraction is lowered to about 0 . 11 which gives a a sv ratio of 5 . 9 cm − 1 . this is a very good value , and with further packing refinement a uniformly illuminated fiber with geometric surface area of over 20 cm − 1 can be obtained . we note that the maximum triangular packing fraction is π / 2 √ 3 compared to π / 4 for a square packing arrangement . thus , we anticipate using a triangular lattice packing fraction would result in a substantially higher a sv . in comparison to 400 cell per square inch high performance monolith , the geometric surface area is about 22 cm − 1 . however , as previously noted , monoliths and nearly all others substrates suffer from illumination shadowing effects . the fiber density , defined as nf = 1 / ab , is 25 / cm2 for this example . using this value , the number of fibers needed to create a 5 . 9 cm - 1 geometric surface area in a 5 cm diameter round duct is nf (□ d2 / 4 ), or about 490 fibers . considering a commercial side emitting fiber strand typically consists of about 170 single fibers , thus 3 strands will produce the desired amount . a small pressure drop across the reactor is desired to reduce power requirements for gas flow through the reactor . even with the potential for high packing density , the pressure drop is still small . for example , taking the case for a pressure drop with a 2 - meter - long reactor with a surface area to volume ratio of 6 cm - 1 , f = 0 . 2 , a hydraulic diameter of dh = 4 ( 1 − f )/ asv = 0 . 005 m , and a velocity of 9 m / s . the calculated pressure drop for air based on the darcy - weibach equation , □ p = 2λ ( l / dh ) ( ρ v2 / 2 ), where the darcy - weibach friction factor □ is taken to be 0 . 035 , is less than 0 . 1 atm . table 1 summarizes the fiber based reactor surface area to volume for various geometries and the fiber number density to achieve the asv value . note that the actual photocatalyst surface area is much larger since the nanoparticle catalyst film on the fiber optic is highly porous and well dispersed . fig1 is a schematic block diagram of the photocatalytic system showing the general operating features of the invention . the system 0 shows a light generating module 1 attached to a fiber optic coupler 2 that gathers light from light generating chamber 1 and positions an array of side emitting fiber optics for light injection into the fiber array . the fiber optic array is fed into a reaction chamber coupler 3 which gathers the fiber optic strands and feeds them into a distributor plate 4 . the distributor plate 4 spreads the fiber optics out into an even arrangement consistent with maintaining a high surface to volume ratio and low pressure drop inside reactor chamber 5 . the distributor plate also generates an even gas flow field across the fiber optic array to avoid uneven reaction zones . the fiber optics are then gathered together and terminated into a reaction chamber terminator 6 . a reactant inlet port 7 delivers a stream of reactants to the reaction chamber 5 , and upon the combined action from the light , side emitting fiber optics , and photocatalyst , the oxidized reaction products exit from the reaction chamber at exhaust port 8 . a second light generating module 9 attached inside to reaction chamber 5 cross illuminates the fiber optic array for additional illumination of the photocatalyst . fig2 is a block diagram of the fiber optic distributor plate used in this demonstration to arrange the fiber optics in a geometrically desired orientation inside the reaction chamber . the system 20 shows a distributor plate 15 with a plurality of holes with a diameter large enough to concurrently pass the fiber optic strand and gas flow . a typical outside diameter for a bundle of side emitting fiber optics is 0 . 7 - 0 . 8 mm . to accommodate the fiber optic diameter and gas flow through the hole , a nominal hole diameter of 2 mm was made and placed in a staggered or triangular pattern arrangement 16 . the outer diameter of the plate is made to fit into the inside diameter of the reaction chamber . when the fiber strands from the reaction chamber couple 3 are run through the distributor plate 17 , the fibers spread out according to the defined geometric hole pattern . the fiber strands 18 extend from the holes for a length of 15 - 20 cm until they are threaded into a second distributor plate 19 . this process is repeated until the desired reactor length is achieved . in this manner , the fiber array is held to a defined geometric arrangement . keeping the fiber strands to an arrangement described here is beneficial in two regards . first , it keeps the fibers from bunching together to one side of the chamber , which results in the deleterious effect of flow bypassing or channeling around the photocatalyst . second , since the fiber geometry is reasonably well defined it allows the determination of the surface area to volume ratio . knowing this value is valuable in design and scale - up of larger systems . to illustrate the fiber distributor plate process , fig3 is a photograph of a segment of a fiber optic array placed inside of three distributor plates that spreads the fibers out into a distinct geometrical pattern free from entanglement and bunching . to demonstrate photocatalytic action of volatile organic chemicals using this mode and photochemistry , a test stand was constructed with the essential system elements . the process flow diagram of this apparatus is shown in fig4 . the device consists of three main subsystems which are ( 1 ) the voc gas delivery , ( 2 ) the photocatalytic reactor bed , and ( 3 ) the gas detection subsystems . the process starts in the voc gas delivery subsystem by splitting dry , carbon free air carrier gas into two streams . one stream passes into a gas sparger filled with water where it is humidified , while the second stream flows to a voc generator where the contaminant becomes entrained in the sample gas flow . in these experiments ethanol was chosen as a representative voc . the ethanol concentration admitted to the reactor chamber was ranged between 20 and 30 ppmv . the flow rate of the humidified carrier gas and voc streams are regulated and measured with flow control orifices and flow meters . both streams then recombine , and are passed into a static gas mixer where the streams are thoroughly mixed prior to being sent to the photocatalytic reactor bed . the stream exits the mixer and passes through a relative humidity ( rh ) and temperature monitor , which records the rh and temperature in real time . before the gas flows into the reactor , a gas chromatography sample port is placed in the stream to withdrawal samples for measuring the initial concentration of the voc . the humidified voc gas stream enters the photocatalytic reactor through a gas port upstream of the fiber array and distributor plate . the sample stream flows down through the reactor and exits out through a second distributor plate and gas port . concurrently , a 475 nm led illuminator propagates a beam of light down through the fiber array and which is then back reflected through the fiber array for a double pass of led light . a second set of 475 nm led emitters line the reaction chamber walls for additional light excitation of the photocatalyst surfaces . the reactor flow is then sent to a second gas chromatography sample port which measures the reacted voc outlet concentration . the reactor gas flows are then sent to flow meter and is exhausted . various nanoparticle photocatalysts were prepared including ag / tio2 , au / tio2 , ag — au / tio2 , and pt / tio2 using sol gel and impregnation methods , and characterized with uv - vis absorbance , x - ray diffraction , tem and eds methods . the collected uv - vis spectra of all prepared catalysts indicated a significant absorption feature resonant with the 475 □□ nm led emission band . the x - ray diffraction spectra confirmed crystalline morphology , and the tem and eds data indicated good dispersion with particle sizes in the 8 - 20 nm range . screening tests were performed on the noble metal photocatalysts and indicated that a 2 wt . % pt / tio2 formulation proved the most active towards ethanol . the 2 wt . % pt / tio2 catalyst was prepared using the method described by shiraishi , y ., et al ., 2012 . this was performed by taking 6 g of tio2 ( anatase ) and adding to 117 ml of deionized water with vigorous stirring . three ml of 8 wt . % h2ptcl6 was added drop - wise to the tio2 / h2o to the solution . the mixture was vigorously stirred while evaporated to dryness at 80 ° c . and then calcined in air at 400 ° c . the powder was then reduced with h at the same temperature . the heating rate was 2 degrees / min and held at 400 ° c . for 2 hours . a photograph of a portion of the photocatalyst coated fiber optic array is presented in fig5 . several tests were performed to demonstrate device performance using the apparatus described above in a once through , continuous mode operation of the reactor . in one test , the ethanol removal in a humidified air stream was tested with the pt / tio2 catalyst . the total flow rate that entered the reactor was 1 . 1 slpm . the temperature and relative humidity over the test period averaged about 35 ° c . and 54 %, respectively . after initial adsorption of ethanol onto the catalyst and other reactor surface , a steady ethanol concentration at the inlet and outlet stream ports was obtained . the concentration of the ethanol as measured by gc was determined to be 20 ppmv . at this point the led fiber illuminator and side mounted leds were powered on as shown in fig6 . once the light was activated the ethanol began to drop precipitously to 3 ppm in about 45 minutes . a second oxidation cycle was examined as the light sources were turn off . here the ethanol concentration climbed back to near 21 ppm . once it stabilized the lamps were activated again and the ethanol was oxidized back to about 2 ppmv . the ethanol conversion percentage , defined as : the first cycle gave about 85 % conversion while the second showed 90 %, for an average of 88 % ethanol removal over the two oxidation cycles . additional tests were performed over a range of flow conditions and these are summarized in table 2 . average values are reported for two cycle times , that is when the lamps are cycle twice on and twice off . the tests indicate a high ethanol removal efficiency in a relatively short reaction time . additional tests indicated that the reaction was temperature dependent . a plot of the ethanol conversion efficiency with internal reactor temperature is presented in fig7 , and shows that an optimum temperature is about 45 ° c . above this level the reaction levels off and additional heating has a small effect on the ethanol conversion . the entire disclosures of all documents cited throughout this application are incorporated herein by reference . asahi , r ., morikawa , t ., ohwaki , t ., aoki , k ., tage , y ., 2001 . “ visible - 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