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IntroductionIntroductionPhotocatalysis - Need of the day Photocatalysis - Need of the day
PhotocatalysisPhotocatalysis
Photocatalyst
Starch + O2
Organic compound
CO2
H2O
CO2 + H2O
Organic Compound
+ H2O + O2
Chlorophyll
What is Photocatalysis?What is Photocatalysis?Photocatalysis is a technique used to degrade toxic species Photocatalysis is a technique used to degrade toxic species into more environmentally friendly formsinto more environmentally friendly forms
Absorption of light with energy ≥ band gap energyAbsorption of light with energy ≥ band gap energy
Positively charged hole in the valence bandPositively charged hole in the valence band
Negatively charged electrons in the conduction band Negatively charged electrons in the conduction band
Potential for oxidation/reduction reactionsPotential for oxidation/reduction reactions
In the year 1972, Fujishima and Honda discovered the photocatalytic splitting of water.
the usage of semiconductor photocatalysis for environmental protection.
various environmental process such as deodorization, water purification, air purification, sterilization and soil proof.
degradation and mineralization of variety of toxic substances
Different types of semiconductors were used as photocatalyst
PhotocatalysisPhotocatalysis
Mechanism of PhotocatalysisMechanism of Photocatalysis
3.12 eV (380 nm)
Mechanism of Photocatalysis using Mechanism of Photocatalysis using PhotocatalystPhotocatalyst
Conduction Band
Valence Band
e-
h+ Recom
bination
O2
O2•–
TiOH
TiOH·
UV Light< 380nm
Eg =3.2 eV
10-15s
10-9s
10-3s
10-8s
Inactivation by generation of reactive species (H2O2, hydroxyl radical)
e-
+
hν
MechanismMechanism
O2
e-
+
e-
+
O2 O2
O2- O2
- O2-
H2O H2O H2O
e- e- e-
OH OH OH
Conduction Band (CB)
Valence Band (VB)
e−cb
h+vb
Excite e-Recombination
Photon (hν)
Photon (hν)
Redox potential of hRedox potential of h++
The redox potential for photogenerated h+ is +2.53 V vs. the SHE
After reaction with water, these h+ can produce •OH
Both h+ and •OH are more positive compare to ozone
Mechanism of Photocatalysis using Mechanism of Photocatalysis using PhotocatalystPhotocatalyst
Photocatalysts and their band gap energy
Semiconductor Valence band
Conductance band
Band gap(eV)
Band gap wavelength
(nm)
SnO2 +4.1 +0.3 3.8 318
ZnO +3.0 -0.2 3.2 390
ZnS +1.4 -2.3 3.7 336
CdS +2.1 -0.4 2.5 497
CdSe +1.6 -0.1 1.7 730
TiO2 +3.1 -0.1 3.0 380
GaP +1.3 -1.0 2.3 540
Requirements of good photocatalyst
Criteria for material selection of photocatalystCriteria for material selection of photocatalyst
active sites in the catalyst
mobility and life time of electrons and holes
the extent of charge separation
recombination rate of electrons and holes.
high activity and selectivity towards a particular decomposition reaction
high stability
it stays active over many catalytic cycles
High surface area .
Some commonly Used DYES
Methylene Blue (MB)
4-chlorophenol
Rhodamine B (Rh B)
Methyl Orange (MO)
Photocatalyst
+ + + +
electron/hole
transfer reactionsCO2 + H2O+simple molecules
sunlight
- - - -
Organic Dyes + H2O
e
h
+
Proposed degradation pathway for methylene blue (MB) during photocatalysis
Proposed degradation pathway for rhodamine B (Rh B) during photocatalysis
Proposed degradation pathway for methyl orange (MO) during photocatalysis
Mechanism of decomposition of 4-chlorophenol during photocatalysis
Mechanism of decomposition of 4-chlorophenol during photocatalysis
Photocatalysis Set Up : Solar Stimulator
Photo reactor employed under visible light irradiation: (1) xenon lamp; (2) air outlet and reactant inlet; (3) glass cooling jacket with water; (4) interlayer; (5) outer vessel for reaction mixture; (6) aeration board; (7) discharge outlet; (8) gas flow meter; (9) buffer bottle and (10) air compressor.
Photodegradation of Rhodamine Photodegradation of Rhodamine B over Ilmenite AgSbOB over Ilmenite AgSbO33 Ilmenite Ilmenite
underunder (a) UV light (b) Visible (a) UV light (b) Visible
lightlight
0 50 100 150 200 250 300 350 4000.0
0.2
0.4
0.6
0.8
1.0
C /
C0
Irradiation time (min.)
Repeated cycles of photodecomposition of MB in presence of AgSbO3 Ilmenite
PhotocatalyticPhotocatalytic Applications Applications
Self-Cleaning EffectSelf-Cleaning Effect
References
1.M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann, Chemical Reviews 1995, 95, 69-96.2.A. L. Linsebigler, G. Lu, J. T. Yates, Jr. Chem. Rev. 1995, 95, 735-758.3.J. Singh, S. Uma, J. Phys. Chem. C 2009, 113, 12483–12488.
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