Tntroduction to Road map for Photocatalysis .pptx

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1. Introduction and the Road Map for Photocatalysis By Dr.S.VIJAYAKUMAR (Formerly Senior Scientist, R&D, SPIC Ltd.) Dean-Projects, KNCET

2. PHOTOCATALYSIS Photocatalysis is a rapidly developing field of research with a high potential for a wide range of industrial applications, which include mineralization of organic pollutants, disinfection of water and air, production of renewable fuels, and organic syntheses. The word “ photocatalysis ” is of Greek origin and composes of two parts: the prefix “photo” ( phos : light) and the word “catalysis” ( katalyo : brake apart, decompose).

3. Photocatalysis , the term can be generally used to describe a process in which light is used to activate a substance, the photocatalyst , which modifies the rate of a reaction without being involved itself in the chemical transformation. The main difference between a conventional thermal catalyst and a photocatalyst is that the former is activated by heat whereas the latter is activated by photons of appropriate energy.

4. Catalysis 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. A 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.

5. Chemical reactions - Types of Activation Thermal 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)

6. Chemical reactions - Types of Activation 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

7. Types of catalysis Homogeneous 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. 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.

8. Photocatalytic reactions May occur homogeneously or heterogeneously, but heterogeneous photocatalysis is by far more intensively studied in recent years because of its potential use in a variety of environmental and energy-related applications as well as in organic syntheses. Heterogeneous Photocatalysis : The reaction scheme implies the previous formation of an interface between a solid photocatalyst (metal or semiconductor) and a fluid containing the reactants and products of the reaction.

9. Light Light is considered wave-like in nature as it consists of oscillating electric (E) and magnetic (M) fields. These fields are at right angles to each other, and travel at a constant velocity in a given medium. In a vacuum, this velocity is 3  10 8 ms -1 .

10. Wavelength and Frequency The energy associated with electromagnetic radiation can be defined as follows: Frequency is related to wavelength by : E Energy (J) h Planck’s constant (6.62  10 -34 Js) Frequency (s -1 ) c Speed of light (3  10 8 ms -1 )  Wavelength (m)

11. Heterogeneous Photocatalysis Mainly used in cases where a light-absorbing semiconductor photocatalyst is utilized, which is in contact with either a liquid or a gas phase. Semiconductors are particularly useful as Photocatalysts because of a favorable combination of electronic structure, light absorption properties, charge transport characteristics and excited-state lifetimes. Semiconductor, is nonconductive in its undoped ground state because an energy gap exists between the top of the filled valence band and the bottom of the vacant conduction band.

12. Heterogeneous Photocatalysis Semiconductor photocatalysis , excitation of an electron from the valence band to the conduction band is accomplished by absorption of a photon of energy equal to or higher than the band gap energy of the semiconductor. This light induced generation of an electron-hole pair is a prerequisite step in all semiconductor mediated photocatalytic processes. Photo generated species tend to recombine and dissipate energy as heat of photons, because the kinetic barrier for the electron-hole recombination process is low.

13. Energy arrangement - isolated atom & atom in a solid Band gap , also called energy gap , is the energy range in a solid where no electron exist

14. Energy arrangement - isolated atom & atom in a solid Band gap determines if a substance is an insulator, semiconductor, or conductor.

15. Semiconductors in Photocatalysis The most used semiconductor in photocatalytic processes is TiO 2 TiO 2 crystallographic forms: • Anatase (important in photocatalysis). • Rutile • Brookite

16. Reactions in photocatalytic process with TiO 2

17. Photocatalysis Applications Environmental Wastewater treatment Air purification Disinfection Self cleaning materials Novel syntheses Selective synthesis of organic chemicals Hydrogen production by direct pthotocatalytic water splitting

18. Photocatalysis and Environment Wastewater treatment . Abatement of pollutants in gas streams (VOC, NO x , sulfur compounds and chlorinated hydrocarbons ) Disinfection

19. 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

20. Commercial applications of photocatalysis Photo-Cat water treatment systems is able to treat waste waters having high turbidity, high levels of dissolved solids and high concentrations of metals.

21. Water Purification Photocatalyst coupled with UV lights can oxidize organic pollutants into nontoxic materials, such as CO2 and water and can disinfect certain bacteria . This technology is very effective at removing further hazardous organic compounds (TOCs) and at killing a variety of bacteria and some viruses in the secondary wastewater treatment.

22. Degradation of Cyanide A photocatalytic reactor that uses UV lamps is currently the heart of a pilot plant used to degrade cyanide. Degradation of Dyes using TiO2 Degrading Methylene Blue  

23. Photocatalytic Air Purification Systems A typical application of photocatalytic materials is for air purification using UV lamps Air purifiers are designed in different sizes, ranging from household (100 m 3 /h) to ventilation systems for tunnels (1,500,000 m 3 /h). Many of these systems work in combination with filters or electrostatic precipitators which remove some of the dangerous gases and airborne particles

24. Photocatalytic Air Purification Systems

25. NOx decomposition promoted by eco-coatings A 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 TiO 2 , contained in the materials, and the subsequent degradation of pollutants such as NO and NO 2 , which are first adsorbed on the particles and then converted into nitric acid (HNO 3 ).

26. NOx decomposition promoted by eco-coatings 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.

27. 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 time Antibacterial materials (glass and ceramics containing TiO 2 , for hospitals – information from literature confirms the possibility to destroy bacteria and viruses)

28. Photocatalytic Materials

29. Hydrocarbons selective Photo-oxidation Reactions

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