photocatalysis.ppt

University of SalernoHeterogeneous photocatalysisDepartment of Chemical and Food Engineering

Department of Chemical and Food Engineering

Eng. Roberto S. Mazzei

CatalysisA chemical reaction can be catalyzed so that the energy of the transition state is lower than without a catalyst, and the rate can be increased. Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst.

Catalytic reactions have a lower rate-limiting free energy of activation than the corresponding uncatalyzed reaction, resulting in higher reaction rate at the same temperature. However, the mechanistic explanation of catalysis is complex.


Chemical reactions activationThermal activation: the reaction rate is increased by increasing the temperature (Arrhenius law)

Catalytic activation: The reaction rate is increased through the action of a catalytic substance (temperature lower than uncatalyzed reactions)

Photochemical activation: Some chemical reactions are activated by electromagnetic radiation (light)

Radiochemical activation: a, b, g and x rays, due to their high energy, could activate chemical reactions also at very low temperature

Radiochemical activation: a, b, g and x rays, due to their high energy, could activate chemical reactions also at very low temperature

Electrochemical processes: Electrical current flow can provide the energy required for the activation of chemical reactions


Types of catalysisHomogeneous catalysts: If the catalyst and reactants or their solution form a common physical phase, then the reaction is called homogeneously catalyzed. Metal salts of organic acids, organometallic complexes, and carbonyls of Co, Fe, and Rh are typical homogeneous catalysts.

Examples of homogeneously catalyzed reactions are oxidation of toluene to benzoic acid in the presence of Co and Mn benzoates and hydroformylation of olefins to give the corresponding aldehydes. This reaction is catalyzed by carbonyls of Co or Rh.

Heterogeneous catalysts: Heterogeneous catalysis involves systems in which catalyst and reactants form separate physical phases.Typical heterogeneous catalysts are inorganic solids such as metals, oxides, sulfides, and metal salts, but they may also be organic materials such as organic hydroperoxides, ion exchangers, and enzymes.

An example of heterogeneously catalyzed reaction is ammonia synthesis from the elements over promoted iron catalysts in the gas phase.


Different forms of heterogeneous catalystsCatalysts can be used in different forms:

Powder: mainly used in slurry reactors for the treatment of wastewater

Pellets: mainly used in fixed-bed reactors for the treatment of gaseous streams

Structured (monoliths): used in processes to minimize the pressure drop- pollutants abatement in power plants (SCR for NOx abatement, HC and NOx removal in engine exhaust after treatment device)


Advanced oxidation processesAOPs (advanced oxidation processes) to be taken in consideration for wastewater treatmentCatalysis Pulsed plasma Electrochemistry Super-critical water oxidation Fenton UltrasoundsUV UV/H2O2Ionizing Radiation MicrowavesO3 Wet-air Oxidation

Photochemical treatment and in particular


Heterogeneous catalysis VS PhotocatalysisClassical heterogeneous catalytic process can be divided into five independent stepsTransfer of the reactants in the fluid phase to the surface.Adsorption of at least one of the reactants.Reaction in the adsorbed phase.Desorption of the product(s).Removal of the products from the interface region.The only difference with conventional catalysis is the way of the catalyst activation . Thermal activation is replaced by a photonic activationLight wave lenght: Light frequency: = c / Ep = h *c/ =h


SemiconductorsDifference in the energy arrangement between an isolated atom and the atom in a solidBand gap, also called energy gap, is the energy range in a solid where no electron existBand gap of some semiconductorsBand gap determines if a substance is an insulator, semiconductor, or conductor.Energy level diagram380 nm440 nm


Department of Chemical and Food EngineeringHeterogeneous Photocatalysise- + p+ Energyh > EbgA- and D+ are highly reactive species which oxidize or reduce the compounds adsorbed on the catalyst surface .Ebg=Band gap Energy



Semiconductors in photocatalysisThe most used semiconductor in photocatalytic processes is TiO2TiO2 crystallographic forms: Anatase (important in photocatalysis). Rutile Brookite


Reactions in photocatalytic process with TiO2


Main factors affecting kinetics of a photocatalytic process Catalyst weight.Light wavelength.Initial concentration of reactants.Reaction temperature.Photonic flux.


Influence of photocatalyst weight The reaction rate of a photocatalytic reaction increases with catalyst weight. Above a certain value of m the reaction rate becomes independent of the mass.


Influence of photocatalyst weight The reaction rate of a photocatalytic reaction increases with catalyst weight. Above a certain value of m the reaction rate becomes independent of the mass.All particles of photocatalyst are fully illuminatedPart of the photosensitive area is masked


Influence of light wavelength The trend of reaction rate as a function of wavelength follows the absorption spectrum of the catalyst, with a threshold that corresponds to the energy of Band-Gap.

It 'worth checking that there is no absorption by the reagent, so as to ensure that all the photons hitting the photocatalyst, activating it.


Influence of initial reactant concentration Generally, the kinetics follows the Langmuir-Hinshelwood mechanism.

The reaction rate r varies proportionally with the coverage degree according to:

KC > 1Order zero kinetics


Influence of reaction temperature Initially, the reaction rate increases with temperature (Arrhenius-type dependency).There is also a temperature range within the reaction rate is almost constant or increases slightly as there are other limiting factors such as the photon flux.Further increasing the temperature, the reaction rate decreases markedly, as in this state the desorption of reagents is favored decreasing the reaction rate.


Effect of photonic flux The reaction rate is proportional to the photon flux .Above a threshold value, the reaction rate becomes proportional to 1/2: the rate of formation of electron-hole pairs is higher than the rate of photocatalytic reaction, and this promotes the phenomenon of recombination.


Cyclohexane total oxidation: thermodynamic evaluationC6H12+9O2 6CO2+6H2O

Grafico1

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100

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100

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conversione

T, C

C6H12 conversion, %

Gaseq

Reactant C6H12Reactant N2Reactant O2Product TT, CC6H12N2O2CO2COH2OX C6H12, %resa CO2

0.179212982507920.10.600.6100100

0.179213487507920.10.600.6100100

0.1792139812507920.10.600.6100100

0.1792144817507920.10.600.6100100

0.1792149822507920.10.600.6100100

0.1792154827507920.10.600.6100100

0.1792159832507920.10.600.6100100

0.1792164837507920.10.600.6100100

0.1792169842507920.10.600.6100100

0.1792174847507920.10.600.6100100

0.1792179852507920.10.600.6100100

0.1792184857507920.10.600.6100100

0.1792189862507920.10.600.610099.9999999997

0.1792194867507920.10.600.610099.9999999979

0.1792199872507920.10.59999999990.00000000010.610099.9999999873

0.17921104877507920.10000000020.59999999960.00000000040.610099.9999999355

0.17921109882507920.10000000080.59999999830.00000000170.610099.999999718

0.17921114887507920.10000000330.59999999350.00000000650.610099.9999989163

0.17921119892507920.10000001120.59999997770.00000002230.610099.999996282

0.17921124897507920.10000003460.59999993070.00000006930.610099.9999884539

0.179211298102507920.10000009850.5999998030.0000001970.610099.9999671683

0.179211348107507920.1000002590.5999994820.0000005180.610099.999913673

0.179211398112507920.1000006350.599998730.000001270.610099.9997883348

0.179211448117507920.10000146240.59999707520.00000292480.610099.9995125273

0.179211498122507920.10000318350.59999363310.00000636690.610099.9989388479

0.179211500122707920.10000328050.5999934390.0000065610.610099.9989065061

Gaseq

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0

0

0

0

0

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conversione

T, C

C6H12 conversion, %


Photocatalysis applicationsEnvironmentalWastewater treatmentAir purificationDisinfectionSelf cleaning materials

Novel synthesesSelective synthesis of organic chemicals Hydrogen production by direct pthotocatalytic water splitting


Photocatalysis and EnvironmentWastewater treatment.

Abatement of pollutants in gas streams (VOC, NOx, sulfur compounds and chlorinated hydrocarbons)

Disinfection


Reactions of photocatalysis for water purification: advantages The photocatalytic reactions are non-specific, able to degrade a wide range of organic compounds (hydrocarbons, halogenated solvents, pesticides and toxic organic compounds).

The process is able to completely mineralize organic substances

The process is suitable for degradation of xenobiotic toxic compounds


Commercial applications of photocatalysis (1/4)Photo-Cat water treatment systems (Purifics) is able to treat wastewaters having high turbidity, high levels of dissolved solids and high concentrations of metals.


Commercial applications of photocatalysis: depuration of aquariums (2/4)


Commercial applications of photocatalysis: (3/4)Depuration of aquariums


Commercial applications of photocatalysis: (4/4)Depuration of aquariums


Examples of photoreactors for the treatment of wastewater: double layer sheet solar photoreactor The reactor consists of a series of channels through which circulates waterThe photoreactor is made of Plexiglas (polymethyl meta acrylate) which is very transparent to UV radiation.


Examples of photoreactors: photoreactor for the degradation of the CN- ionA catalytic photoreactor is currently used for the degradation of the CN- ion (Plataforma Solar de Almera, Spain)

Solar photoreactor and scheme of the pilot plant : A) sampling valves, B) thermocouples, C) reservoirs containing reagents, D) pumps, PFP) plug flow photoreactor



Examples of photoreactors: photoreactor for the cyanide degradation A photocatalytic reactor that uses UV lamps is currently the heart of a pilot plant used to degrade cyanide, contained inside an in industrial waste at Puertollano, Spain.Schematic and photograph of the pilot reactor tank containing reagents


Photocatalysis for disinfection Its possible to consider photocatalytic disinfection processes using suspended TiO2 E. Coli destruction by irradiated TiO2


Department of Chemical and Food EngineeringBenzene [X. Fu, W. A. Zeltner, M. A. Anderson ;(1995). N. N. Lichtin, M. Sadeghi;(1998)]Trichloroethane [L. A. Dibble, G. B. Raupp; (1988). D. M. Blake, W. A. Jacoby and M. R. Nimlos; (1992)]Acetone [Jun Lin, Jimmy C. Yu; (1998)]Methanol [C. S. Turchi, R. Rabago; (1995)]Acetaldehyde [I. Sopyan, M. Watanabe, S. Murasawa, K. Hashimoto, A. Fujishima; (1996). E. Obuchi, T. Sakamoto and K. Nakano; (1999)]Toluene [ O. dHennezel, P. Pichat, D. F. Ollis; (1998)]Cyclohexane [ H. Einaga, S. Futamura, T. Ibusuki; (2002)]Photocatalytic treatment of gaseous streamsPHOTOCATALYST: TiO2 anatase


Photocatalytic oxidation systems: advantages Room temperatureHigh efficiency for substance degradation also when using low-power radiation sources.Complete oxidation of hydrocarbons, with the formation of CO2 and H2O, without undesired products (CO and NOx)Use of cheap and non-toxic catalysts (TiO2).


Main odorous compounds and their originVolatile organic acidsAmmonia and aminesAromatic and other ring structuresSulfur compounds (H2S, mercaptans, etc.). Peculiar activities able to origin disturbing smells:

Wastewater treatment plantsComposting plantsTanneriesPetrochemical planesFood industries


Typical stinking compounds degraded by means of catalytic processes


Photocatalytic Purification Systems

A typical application of photocatalytic materials is for air purification using UV lampsAir purifiers are designed in different sizes, ranging from household (100 m3/h) to ventilation systems for tunnels (1,500,000 m3/h).Many of these systems work in combination with filters or electrostatic precipitators which remove some of the dangerous gases and airborne particles


Photocatalytic materials Cementitious materials, containing titanium dioxide (within 1 - 5%) and irradiated with sunlight, have a high efficiency in the oxidation of the organic substances which settle on them, keeping their colour unchanged in the timeAntibacterial materials (glass and ceramics containing TiO2, for hospitals information from literature confirms the possibility to destroy bacteria and viruses thanks to the photocatalytic activity of TiO2)


Photocatalytic materials Self-cleaning materials (cement, glass) coated with TiO2 filmsPhoto-induced super-hydrophilicity


NOx decomposition promoted by eco-coatingsA photocatalytic application, still under study but already tested in laboratory and in situ, deals with the possibility to reduce the nitrogen oxides (NOx), currently produced by the exhaust of cars and vehicles, by using cementitious materials (paints, floorings or self-locking blocks)UV light promotes the activation of the TiO2, contained in the materials, and the subsequent degradation of pollutants such as NO and NO2, which are first adsorbed on the particles and then converted into nitric acid (HNO3).Rain washes away the nitric acid as harmless nitrate ions, which may be used to fertilize the soil, or the acid can be neutralized by the alkaline calcium carbonate contained in the materials.


Self-cleaning coatings: applications(1/3)


Dives in Misericordia (Roma)Self-cleaning coatings: applications(2/3)


Facade and structural detail of the of the City of Music and Fine Arts in Chambery (France)Self-cleaning coatings: applications(3/3)


Department of Chemical and Food EngineeringPhotocatalysis for green chemistryAdvantagesPossibility to operate at ambient temperature and pressureCatalysts relatively inexpensive (mainly TiO2 based) Photocatalysis can lead to more sustainable processes by :Increasing process selectivity to the required products Different chemistry Decreasing energy consumption of the process Heterogeneous photocatalysis is a promising technology for a wide range of applications, such as organic synthesis, photo-destruction of volatile organic compounds and purification of waste water



Department of Chemical and Food EngineeringHydrocarbons selective Photo-oxidation Reactions

ReactantsCataystsProductsReferencesOlephins(Prophilene)TiO2, MoO3, WO3, CdSepoxides, alcohols, aldehydes, and ketonesWard, M. D.; Brazdill, J. F., Jr.; Grasselli, R. K. U.S. Patent 4,571,290, 1986TolueneTiO2BenzaldehydeGonzalez, M. A.; Howell S. G. and Sikdar, S. K. J. Catal. 1999, 183, 159.Fujihira, M.; Satoh, J.; Osa, T. Nature 1981, 293, 206NaphthaleneTiO22-formylcinnamaldehydes, naphthoquinoneSoana, F.; Sturini, M.; Cermenati L.; Albini, A. J. Chem. Soc.,Perkins Trans. 2 2000, 649.

Linear olephinsTiO2epoxidesOhno, T.; Nakabeya, K.; Mutsumura, M. J. Catal. 1998, 176, 76.CumeneTiO2acetophenoneErmolenko, L. P.; Giannotti, C. J. Chem. Soc., Perkins Trans. 2 1996, 1205.

ButadienesTiO2acetaldehydeOShea, K. E.; Jannach, S. H.; Garcia, I. J. Photochem. Photobiol., A 1999, 122, 127.

PropeneTiO2 SnO2, WO3, Sb2O4Acetaldehyde, acetone, acroelinPichat, P.; Herrmann, J. M.; Didier, J.; Mozzanega, M. N. J.Phys. Chem. 1979, 83, 3122.

CyclohexaneV2O5/ZrO2 cyclohexanol, cyclohexanoneK. Teramura, T. Tanaka, T. Yamamoto, T. Funabiki, J. Mol. Catal. 165 (2001) 299CyclohexaneMoO3/TiO2Benzene, cyclohexeneP. Ciambelli, D. Sannino, V. Palma, V.Vaiano , Catal. Today 99 (2005) 143EthylbenzeneMoO3/g-Al2O3StyreneP. Ciambelli, D. Sannino, V. Palma, V. Vaiano, Italian patent ApplicationSAA2008000012



Aroma green synthesisAldehydes are widely applicated as aromas to enrich the flavor of different types of products.If we consider the case of vanillin, its extensive utilization (more than 12 thousand tons per year) makes it the most important aroma. Currently, more than 99% of vanillin is produced by chemical synthesis and is sold at a much lower price (typically less than 1%) than the natural vanilla extract, which requires very long, complex and expensive procedures of cultivation of the plant, induced production of the beans, curing of the pods and extraction of the aromas. In spite of the high price, the demand of the natural product remains noteworthy. On the other hand, the processes of chemical synthesis are not environmentally safe and the "bouquet" of the obtained "artificial" vanillin is usually considered to be of lower quality due to the lack of many trace components which substantially contribute to the flavor of the "natural" product. In the last decades many researchers investigated also the possibility of producing vanillin through a "green" biotechnological route. But, till now, only one commercial biotech product is present on the market. In fact, even if future improvements are hoped, the proposed biotechnology methods show some limitations such as relatively high costs, low yields, long production times, difficult purification and necessity of selected strains of microorganisms.


Photocatalytic synthesis of vanillin

ferulic acid was chosen since it can be of natural origin,

AdvantagesA substantial improvement of the yield and of the selectivity; A satisfactory purification of the product, that is in some case does not need any further separation. Note also that the product stream is in any case free from the photocatalytic powders, which are preferably used instead of the photocatalytic films, due to their higher photocatalytic activity, but are usually problematic to separate without the integrated process; Low energy demand for the separation step; Green process (mild conditions, that is low harmless temperatures and atmospheric pressure, reagent in aqueous solution without any chemical additive, possibility of using natural precursors, possibility of a complementary utilization of the solar radiation to satisfy the energy requirements); modularity (both the capacity of the photocatalytic and of the membrane separation apparatuses can be varied by adding or removing modules); possibility of choice between continuous or discontinuous (batch) processing; very simple control so that qualified technicians are not required and the actions of operators are minimized.


Main reactions


The membrane PEBAX2533 (polyether block amide by Arkema) It has been observed that the life of the membrane is significantly shortened when it is exposed to the UVA radiation in presence of the photocatalyst, so the pervaporation membrane has been kept separated from the reactor.Photocatalytic system





Experimental Apparatus

Experimental set up apparatus: (1) rotameter; (2) mass flow controllers; (3) MFC control unit; (4) hydrocarbons saturator; (5) manometer; (6) photoreactor; (7) thermocouple; (8) GC-MS;(9) CO-CO2 analyzer


Cohesive tendenciesThe gas opens channels PHYSICAL MIXTURES WITH - AL2O3 OR SILICA WITH TITANIA BASED CATALYSTS WERE REALIZEDParticles expand uniformly and gas bubbles form-Al2O3, SiO2TiO2Fluidized bed photoreactor


Department of Chemical and Food EngineeringIllumination systemsEXTERNAL HEATINGHg LAMPS Light intensity: 30 mW/cm2INTERNAL HEATINGUV LEDs Light Intensity: 10-122 mW /cm2TraditionalMicroscale



Department of Chemical and Food EngineeringEffect of UV sourceCyclohexane conversion (X) and benzene production (P)Ligth intensity: 30mW/cm2

Catalyst : 10MoPCAl, 28gTemperature: 120C Flow rate: 830 Ncc/min Carrier: HeCyclohexane : 0.1% Oxygen/Cyclohexane ratio: 1.5Water/Cyclohexane : 1.6These results indicate that UVA-LEDs allow better performances in terms of catalyst illumination avoiding photons dispersion


Grafico2

000000

1.922.455466.6844.91092

2.852.850551218.349749.174875.7011

3.996253.3671616.326.1119613.055986.73432

4.935824.7110824.732.8944416.447229.42216

4.985.5078941.238.4270219.2135111.01578

5.845.7172449.8482411.43448

5.876.2255.860.816830.408412.44

5.876.9462758.11369.434.713.89254

5.877.2658.11381.2536840.6268414.52

5.97.6558.41103.5349651.7674815.3

5.98.1758.41113.856.916.34

5.838.6657.717128.7206264.3603117.32

5.869.5157.123179.689.819.02

5.89.8457.42206108.719.68

5.7710.0757.12322211421.2

5.8110.1648156.727227114.621.4

5.8416910.2743957.3226114.621.7

5.8310.4211256.727225115.221.8

5.8210.4657.123220116.422

5.8710.5458.113220116.821.7

5.8810.5558.212220118.1313821.7

X (UV LAMPS)

X (two modules UVA- LEDs)

P (UV LAMPS)

P (two modules UVA-LEDs)

P (four modules UVA-LEDs)

X (four modules UVA-LEDs)

Irradiation time, min

Cyclohexane conversion, %

Benzene, ppm

Grafico1

0

6.6

12

16.3

24.7

41.2

49.8

55.8

58.113

58.113

58.41

58.41

57.717

57.123

57.42

57.123

56.727

57.3

56.727

57.123

58.113

58.212

ppm benzene

Foglio1

illumination time, mincyclohexane conversion, %10MoPC100Al (4 lampade UV)ppm benzenelampadeledconvppm benzene40 LEDillumination time, minconvconversione cicloesanoppm benzeneppm benzene80 LED

0.01400000.0140000

1.61.926.62.4554641.64.910929.82184816

3.0142.85122.850559.174873.0145.701111.402218.3497436.69948

4.033.9962516.33.3671613.055984.036.7343213.4686426.1119652.22392

5.0474.9358224.74.7110816.447225.0479.4221618.8443232.8944465.78888

6.0134.9841.25.5078919.213516.01311.0157822.0315638.4270276.85404

75.8449.85.7172424711.4344822.868964896

8.0465.8755.86.2230.40848.04612.4424.8860.8168121.6336

9.0125.8758.1136.9462734.79.01213.8925427.7850869.4138.8

10.0295.8758.1137.2640.6268410.02914.5229.0481.25368162.50736

11.55.958.417.6551.7674811.515.330.6103.53496207.06992

12.55.958.418.1756.912.516.3432.68113.8227.6

155.8357.7178.6664.360311517.3234.64128.72062257.44124

18.15.8657.1239.5189.818.119.0238.04179.6359.2

21.55.857.429.84108.721.519.6839.36206412

31.75.7757.12310.0711431.721.242.4222444

40.0735.8156.72710.16481114.640.07321.442.8227454

49.635.8416957.310.27439114.649.6321.743.4226452

59.5435.8356.72710.42112115.259.54321.843.6225450

69.5075.8257.12310.46116.469.5072244220440

79.5215.8758.11310.54116.879.52121.743.4220440

89.5365.8858.21210.55118.1313889.53621.743.4220440

Foglio2

Foglio3


Homogeneous vs. Heterogeneous catalysisHomogeneous Fenton processAdvantagesRoom pressure and temperature Cheap and non-toxic catalysts High process efficiencyDisadvantagesHandling of chemical reactantsWorking pH range (pH=2-4)High cost of chemical reagentsCatalyst recoveryFormation of sludge Heterogeneous Fenton processpH working range (2-5)Separation and catalyst recovery Potentially, lower catalytic activity compared to homogeneous catalytic process


Photo-Fenton apparatus which does not require a separation system of the catalyst EFFECTIVE IN:Removal of acetic acid, ethanol, methanolRemoval of MTBEWine wastewater treatment monoliteP. Ciambelli, D. Sannino, M. Ricciardi, L. Isupova; (2007).


Homogeneous Photo-Fenton vs. Heterogenous Photo-Fenton omogeneaThe most promising catalyst are LaFeO3 e LaMnO3Catalysts were supplied by Boreskov Insitute of CatalysisAcetic acid solution Initial TOC : 500 ppmCatalyst: monilite V tot.: 100 mlH2O2/CH3COOH= 4 (stoichiometric ratio)C0H2O2= 0.083 mol/lC0Fe3+= 0.0296 M (homogeneous)P=1 atmT=25CQair=250 Ncc/minpH=3.9


Heterogeneous photo-Fenton process: mechanism


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photocatalysis.ppt