Photocatalytsis_ significance and Applications.pptx

PHOTOCATALYSIS

1. What is Photocatalysis? The word photocatalysis is a composite word which is composed of two parts, “ photo ” and “ catalysis ”. Photo means Light Catalysis is the process where a substance participates in modifying the rate of a chemical reaction without being altered or consumed. This substance is known as the catalyst which increases the rate of a reaction by reducing the activation energy. Therefore Photocatalysis is a reaction which uses light to activate a substance which modifies the rate of a chemical reaction without being involved itself. And the photocatalyst is the substance which can modify the rate of chemical reaction using light irradiation.

2. How photocatalyst works? It involves the reaction of catalyst (nanoparticles) with chemical compounds in the presence of light. The mechanism of this reaction is that when nanoparticles of specific compounds are subjected to UV light, the electrons in the outermost shell (valence electrons) are excited resulting in the formation of electron hole pairs, i.e. negative electrons and positive holes.

3. Photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the Sun , into chemical energy that can be later released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules , such as sugars , which are synthesized from carbon dioxide and water .

4. Photosynthesis and photocatalysis Chlorophyll of plants is a typical natural photocatalyst. The difference between chlorophyll photocatalyst to man-made nanophotocatalyst e.g TiO2 is, usually chlorophyll captures sunlight to turn water and carbon dioxide into oxygen and glucose, but on the contrary photocatalyst creates strong oxidation agent and electronic holes to breakdown the organic matter to carbon dioxide and water in the presence of photocatalyst, light and water

5. Applications 1. Conversion of water to hydrogen gas by photocatalytic water splitting. . Use of titanium dioxide in self-cleaning glass Free radicals generated from TiO 2 oxidize organic matter. . Disinfection of water by supported titanium dioxide photocatalysts, a form of solar water disinfection. . Self-sterilizing photocatalytic coatings (for application to food contact surfaces and in other environments where microbial pathogens spread by indirect contact)

6. Photocatalytic Degradation Photocatalytic effect: UV + SC → SC (h + + e − ) Here SC stands for Semiconductor Photocatalyst Oxidative reactions due to Photocatalytic effect h + + H 2 O → H + + •OH 2 h + + 2 H 2 O → 2 H + + H 2 O 2 H 2 O 2 → HO• + •OH The reductive reaction due to photocatalytic effect: 2 e − + O 2 → •O − •O 2 − + HO•2 + H + → H 2 O 2 + O 2 HOOH → HO• + •OH Photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole pairs, which generate free radicals (e.g. hydroxyl radicals: OH) able to undergo secondary reactions leading to degradation of pollutants

7. Artificial Photosynthesis/Artificial Leaf The world’s first practical ‘artificial leaf’ was invented by MIT researchers last year: it is a solar cell composed of a thin sheet of semiconducting silicon coated with different catalysts on each side. It is placed into a water container and exposed to sunlight to generate bubbles of hydrogen on one side, and oxygen on the other.

8. The efficiency of the classical photocatalysts is often lowered owing to some drawbacks, such as i)- very fast recombination of the photogenerated electron-hole pairs which results as waste heat, ii)- the possibility of backward or secondary reactions with the formation of undesirable by- products and iii)- low absorption in the visible region which determines their inability to use solar light. Approaches for Photocatalytic Efficiency Enhancement

9. In recent years, therefore, the de v elop m ent of photocatalysts seeking ability to overco m e t h ese problems has become one of the ho t are a s in t h e photocatalytic research. In order to enhan c e the photocatalytic activity of semiconductors under visible light, several techniques are used. Some of these techniques which are most are: i)- Nanoscale Size and Morphology of Photocatalyst Material ii)-Noble Metal Loading or Decoration of Photocatalyst iii)- Coupling or Composition of two Semiconductors

10. Th e c o n v e r si o n o f s o lar ene r gy t o h y d r o g en b y m eans of photocatalysis is one of the most interesting ways to achieve clean and renewable energy systems. However if this process is assisted by photocatalysts suspended directly in water instead of using a photovoltaic and electrolytic system the reaction is in just one step, and can therefore be more efficient Nanowires of silicon and titanium oxide show promise for the efficient production of hydrogen through solar water splitting.

11. Photocatalytic Degradation