Water splitting on semiconductor catalysts under visible light irradiation

Water splitting on semiconductor catalysts under visible light irradiation

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  Water Splitting onSemiconductor Catalysts under Visible Light Irradiation 2
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  Outlines  Current EnergySystem  Need for Hydrogen  Different Methods to Produce Hydrogen  Basic Principle of Photocatalytic water Splitting  Properties of Photocatalyst Used  Limitations 3
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  Current Energy System Current energy system is based upon fossil fuels consumption.  Fossil fuels such as coal, natural gas and oil are consumed to meet the energy demand round the globe.  Fossil fuels reserves are finite and depleting rapidly because of increase in population and improvement in living standard.  Other energy resources are wind energy, nuclear energy and solar energy.  Limitations of cost and efficiency are associated with all of them are responsible for their limited use. 4
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  Need For Hydrogen Water and solar light are renewable energy resources which on splitting produce hydrogen and oxygen  Hydrogen is highest energy yield per unit mass among the various fuels: 122kj/g that of fossil fuel is 40kj/g  Hydrogen is environment friendly and has zero negative impact on environment  Major outcome of hydrogen consumption is water which does not harm environment and also contains hydrogen 5
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  Different methods toproduce hydrogen 6  Hydrogen can be produced by steam methane reforming (SMR) by treating methane with steam but the process produce CO2 which is not desirable  Coal and biomass gasification can also be alternatives to produce hydrogen with low production of CO2  Electrolytic process is also used to convert water into hydrogen and oxygen by using electricity.  Electrolytic process is limited by cost of electricity and by its production method  Efficient way to produce hydrogen is by water splitting using photocatalyst
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  Basic Principle ofPhotocatalytic water splitting.  A photocatalyst is a substance that absorbs photon of light having energy equal or greater than its band gap to excite its electrons from valance band(VB) to conduction band(CB).  This excitation produce positive charge (hole) in valance band that of negative charge due to electron in conduction as charge carriers.  Photogenerated charges are then transferred to surface of photocatalyst over which water is adsorbed  The holes are then used to oxidize the water to produce oxygen and the electrons are used to reduce the hydrogen ions to hydrogen molecule  Hydrogen is then captured as a gas and can bee used for beneficial purposes. 7
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  Overall mechanism ofhydrogen production 8
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  Stepwise water splittingreaction H2O + h+  2H+ + 1 2 O2 2H+ + 2e-  H2 Overall reaction is H2O  H2 + 1 2 O2  The energy gap between the oxidation potential of oxygen and reduction potential of hydrogen in water is 1.23 eV.  Therefore, the minimum band gap is 1.23 eV for a suitable water splitting 9
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  Photocatalytic methods forhydrogen production 1. Particulate photocatalytic system 2. Photo electrochemical cell  Here photocatalyst is immersed as tiny particles in water based electrolyte  When the system interacts with solar light than absorption of light occur followed by the charge carrier generation.  Charges are moved to surface of photocatalyst to start the surface catalyzed chemical reaction with water  Holes (h+) are oxidizing the water and electrons reduce the hydrogen ions to hydrogen gas. Overall reaction is 2H2O 𝑃ℎ𝑜𝑡𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡+𝑠𝑢𝑛𝑙𝑖𝑔ℎ𝑡 2H2 + O2 10 Particulate photocatalytic system
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  Particulate photocatalyst foroverall water splitting 11
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  Photo electrochemical cell Here two electrodes are dipped in aqueous electrolyte  Out of two electrodes one is a semiconductor photocatalyst  When the photocatalyst irradiation takes place than it absorbs the radiations and produced the photogenerated charges  These charges react with water to oxidize and reduce the water for oxygen and hydrogen production  This process is advantage over particulate photocatalytic process because this process allows the separation of produced gases in order to prevent reverse reaction. 12
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  Systematic diagram ofPhoto electrochemical cell 13
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  Processes limitations Possible recombinationreactions between photogenerated charge carriers Due to recombination reactions, efficiency of photocatalysts to split the water is still limited by less than 10% Corrosion of photocatalyst also degrade the catalyst and efficiency becomes low 14
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  Thank You 15