Transcript MS PowerPoint - Catalysis Eprints database

Photocatalysis by modified Titania
V. Jeyalakshmi
30-07-2011
Photocatalysis is defined as the combination of Photochemistry and catalysis. Which
implies that the light and a catalyst are essential to enhance the rates of
thermodynamically favored but kinetically slow photophysical and photochemical
Transformations.
Photocatalysis - simultaneous oxidation and reduction
Photocatalyst
Metals
No band gap, Only reduction or oxidation Depends on the band position
Insulators
High band gap, High energy requirement.
Semiconductor
•The redox potential of the photo-generated VB hole should be sufficiently positive for the
hole to act as an acceptor;
• The redox potential of the photo-generated CB electron should be sufficiently negative for
the electron to act as a donor;
• Photocatayst should be economically available and be environmentally inert;
• Photo-catalyst should be stable (Photo-stable) in a wide pH range and in a variety of
electrolytes
A. Millis and S. L. Hunte J. Photochem. Photobiol. A: Chem 180 (1997) 1
Titanium oxide has been widely used as catalyst for UV irradiation and was
considered the best choice among several other oxides.
The advantages of titania photocatalysts: strong resistance to chemical and
photocorrosion, low operational temperature, low cost, non-toxic significantly low
energy consumption, making it a perfect candidate for photocatalytic process .
Even though titania is the widely used semiconductor, it has some disadvantage like
low surface area, fast recombination and wavelength maximum lies in UV region.
Visible-Light Active Photocatalyst
TiO2 – efficient photocatalyst under UV light
Yet, need visible-light active photocatalyst for practical purpose.
Modification of Titania can enhance photocatalytic activity:
•Inhibiting recombination by increasing the charge separation and therefore increase
the efficiency of the photocatalytic process;
•Increasing the wavelength response range (i.e. excitation of wide band gap
semiconductors by visible light);
•Changing the selectivity or yield of a particular product.
Modifications
•
Titania can be modified by the following methods:
•
Dye sensitization
•
Surface modification of the semiconductor to improve the stability
•
Multi layer systems (coupled semiconductors)
•
Doping of wide band gap semiconductors like TiO2 by nitrogen, carbon
Sulphur
•
New semiconductors with metal 3d valence band instead of Oxide 2p contribution.
•
All these attempts can be understood in terms of some kind sensitization and hence
the route of charge transfer has been extended and hence the efficiency could not be
increased considerably.
and
Modification of TiO2
Metal doping
Coupled Semiconductors
Surface sensitization
Surface sensitization
Schematic diagram for the visible light photocatalytic mechanisms of surfacesensitized TiO2.
The photosensitizers readily desorb and decompose, which limit their wider application.
Miao Zhang et al, Angew. Chem. Int. Ed. 2008, 47, 9730 –9733
Coupled Semiconductors
The conduction and valence band potentials of the coupled semiconductor should be
more negative and less positive, respectively, than those of titania.
The hole produced in the CdS valence band from the excitation process remains in the CdS
particle while the electron transfers to the conduction band of the Ti02 particle. The electron
transfer from CdS to Ti02 increases the charge separation and efficiency of the photocatalytic
process.
Sclafani, A.; Mozzanega, M.-N.; Pichat, P. J. Photochem. Photobiol. A: Chem. 1991, 59, 81.
Doping:
Non metals: carbon, Nitrogen, sulphur, phosphorus, boron, etc.
Metals
: Various metals especially transition metals are doped.
Non – metals:
Mechanism of S-doped TiO2 for the visible light activity photocatalyst
S.X. Liu, X.Y. Chen, J. Hazard. Mater. 152, 48–55 (2008)
N- type semiconductor
Schematic illustration of nitrogen doping into oxygen site
K. HASHIMOTO et al. Jpn. J. Appl. Phys., Vol. 44, No. 12 (2005)
Nitrogen doping
States generated by substitutional (a) and interstitial (b) N-doping in
titania.
Photocatalytic activity of non- metal doped Titania
Decomposition rate of rhodamine B for 1 h on undoped TiO2 and TiO2
doped with different N/Ti ratios under visible light irradiation.
Ye Cong et al., J. Phys. Chem. C, Vol. 111, No. 19, 2007, 6976-6982
MECHANISM OF RECOMBINATION REDUCTION BY METAL DOPING
Conduction Band
- - e- e- e- e- e- e - e- e- e- e- ee-(M) <-- M+ee
Eg
Electron/hole pair
recombination
Electron/hole pair
generation
Valence Band
h+ h+ h+ h+ h+ h+ h+ h+ h+ h+
Metallic promoter attracts electrons from TiO2 conduction band and slows
recombination reaction.
(below the conduction band)
(above the valence band)
Doping metal:
• must be similar ionic radii as that of Ti4+ ,
• should exhibit two or more oxidation states,
• Mn+ /M(n+1) energy levels should lies closer to Ti3+ /Ti4+ ,
• Higher electronegativity than Titanium,
• Should posses incompletely filled electronic configuration,
• Should possess both e- /h+ trapping capacity,
• Should posses both e-/h+ detrapping capacity.
ionic radii of various metals
Energy levels of impurity ions in rutile proposed by Mizushima et al and Triggs
Comparison between Fe3+ and Cr3+
Fe3+ is more active,
the Fe2+ /Fe3+ energy level lies close to Ti3+ /Ti4+ level,
Fe2+ is relatively unstable due to the loss of exchange energy on going from d5 (half-filled
high spin) to d6 and tends to return to Fe3+ (d5),
Fe3+ possess both e-/h+ trapping capacity,
the trapped electron in Fe2+ can be easily transferred to a neighboring surficial Ti4+ which
then leads to interfacial electron transfer.
Photocatalytic activity of metal loaded Titania
Effect of Pt-metal content in Pt/TO-NP catalysts on CH4 yield for
photocatalytic reduction of CO2 after 7 h UV irradiation at 323 K, H2O/CO2 = 0.02
Q.-H. Zhang et al. / Catalysis Today 148 (2009) 335–340
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