Environmental Engineering Reference
In-Depth Information
Hydrolysis
:
M(OR)
4
+ H
2
O → HO-M(OR)
3
+ ROH → M(OH)
4
+ 4ROH
Condensation
:
(OR)
3
M-OH + HO-M(OR)
3
→ (RO)
3
M-O-M(OR)
3
+ H
2
O
(OR)
3
M-OH + RO-M(OR)
3
→ (OR)
3
M-O-M(OR)
3
+ ROH
Metal alkoxides are represented by (MOR)
z
, where M may be Si, Ti, Zr, or Al, and R is an
alkyl group. The sol-gel process involves an inorganic precursor that undergoes hydroly-
sis and condensation reactions.
The sol-gel methods give amorphous TiO
2
particles that become crystalline on heating.
Alternatively, another solution with titanium isopropoxide (iPr) as precursor was prepared
by mixing titanium (IV) in tetraisopropoxide, ethanol, and acetic acid in a molar ratio of
1:100:0.05. It was hydrolyzed by glacial acetic acid. It was found that use of titanium (IV) iPr
yielded a better immobilized coating. The reactions involved are as follows:
Ti(O-iPr)
4
+ H
2
O → Ti(OH)
4
+ 4PrOH
Ti(OH)
4
→ TiO
2
+ 2H
2
O
Ti(O-iPr)
4
+ 4EtOH → Ti(OEt)
4
+ 4PrOH
Ti(O-iPr)
4
or Ti(OEt)
4
+ H
2
O → Ti(OH)
4
+ 4Pr
4
OH or EtOH
Ti(OH)
4
→ TiO
2
+ 2H
2
O
(EtOH = ethanol)
After coating one exposure of water to UV radiation, it was found that the required time
for killing
E. coli
and disinfecting water was reduced by 20% as a result of photocatalysis
due to the release of OH
∙
free radicals. The following reactions occur [18]:
Excitation
:
+
−
TiOh he fs (femtosecond)
vb
2
+→+
ν
cb
Adsorption
:
4
−
2
−
4
−
Ti
++→+
OHOOH i
−
OH
L
2
L
Ti
4
+ H
2
O → Ti
4
- H
2
O
Site + R
1
→ R
1,ads
4
4
OH
+→∫
Ti
Ti
OH
