Photocatalytic dechlorination of air free aqueous carbon tetrachloride solutions in TiO2 layer systems. A chain reaction mechanism

Article abstract of DOI:10.1524/zpch.1999.213.Part_1.093

Quantum yields of chloride ions exceeding unity are observed upon illumination of TiO₂ layers in aqueous solutions containing CCl₄ and methanol. The layers were produced by spin coating of TiO₂ (prepared by hydrolysis of the propoxide) from concentrated colloid solutions. Absorbed light intensities ranged from 7 × 10^-11 to 3 × 10^-8 ein cm^-2 s^-1. No effect of alcohol concentration on the yield is observed in the range 0.01-12 M. A negligible yield of chloride ion is obtained in the absence of methanol. Pre-equilibration of the layer with the CCl₄ solution proved necessary for maximization of the yields of photo-dechlorination. The rate of build up of the chloride ions increases nearly linearly with the square root of the light intensity at the lowest intensities which were employed. At the lowest intensity, the quantum yield φ (corrected for dark reaction) increases from ≈1 in the pH range 2-6 to φ > 6 at pH 12.2. There is no observable effect of CCl₄ concentration above 1 mM. The high quantum yields and the roughly square root dependence on light intensity (at very low intensities) indicate that a chain reaction takes place. by Oldenbourg Wissenschaftsverlag, Muenchen.

Full text of DOI:10.1524/zpch.1999.213.Part_1.093

Zeitschrift für Physikalische Chemie, Bd. 213, S. 93-98 (1999)
by Oldenbourg
München
©
Wissenschaftsverlag,
Dechlorination
Carbon
Photocatalytic
of Air Free
Aqueous
Tetrachloride Solutions in
Ti02 Layer Systems.
A
Chain Reaction Mechanism
By Johannes
Stark and
Rabani*
Joseph
The Hebrew University of Jerusalem, Department of Physical
Chemistry
and the Farkas Center, Jerusalem 91904, Israel
(
Received August 3, 1998; accepted December 10, 1998)
Quantum yields of chloride ions exceeding unity are observed upon illumination of Ti02
in
solutions
CC14 and methanol. The
were
layers
aqueous
containing
layers
produced by
of TiO, (prepared
of the
from concentrated colloid
to 3X10" ein ern es '. No
spin coating
by hydrolysis
propoxide)
"
solutions. Absorbed
intensities ranged from 7X10
light
effect of alcohol concentration on the yield is observed in the
0.01 —12 M.
A
range
of chloride ion is obtained in the absence of methanol.
negligible yield
Pre-equilibration
of the layer with the CC14 solution
for maximization of the yields of
proved necessary
The rate of build
of the chloride ions increases nearly linearly
up
photo-dechlorination.
with the root of the
at the lowest intensities which were employed.
square
At the lowest intensity, the quantum yield (corrected for dark reaction) increases from
2—6 to
light intensity
at pH 12.2. There is no observable effect of CC14
root dependence on light intensity
~
1
in the pH
>
6
range
concentration above
1
mM.
The high quantum yields and the roughly
square
chain reaction takes place.
(
at very low intensities) indicate that
a
Introduction
Interest in the detailed mechanism of illuminated TiO, reactions arises from
the intensive use of TiO, in
reactions and
aqueous
photocatalytic organic
from waste waters [1 19].
its
The
for detoxification of
application
pollutants
in the reaction mechanism is the
of electrons and holes
reaction 2) and holes (reaction
of
primary step
photo-generation
pairs
to reaction 1.
of the electrons
according
3) occurs within less than 30
ps [20-25].
Trapping
(
Ti02
hVB
+
eCB
(1)
(2)
>
6cb
,
*
Corresponding author.
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94
Johannes Stark and Joseph Rabani
hvB
h¿
(3)
In the absence of
recombination of electrons and holes takes
scavengers,
place (reactions 4—6).
+
hvB
h^B
h,:
Ti02
(4)
(5)
(6)
eCB
+
Ti02
etr
+
Ti02
eIr
hole, h¿, has been assumed to be
a
surface bound
OH'
The
trapped positive
oxidation of surface water or OH" ions (reaction 7a
radical, produced by
or 7b
respectively).
+
'
(7a)
7b)
focused on photocatalytic
hVB+Ti,v-0 2-Ti,vOH2^ {Ti,v-0 2-Ti,v}-OH' (OH¡ds)
—
OH'lUs
(
hVB+Tilv-0"2-TilvOH-
Considerable amount of work has been
recently
removal of chlorinated methane and other hydrocarbons, both in gas phase
and in solution [29 39]. Thermal catalysis of organic halogens in
as well as liquid [30] phase has also been reported. The present
[
26—28]
gas [40]
concerns mechanistic studies on dechlorination of CC14 induced
manuscript
by photolysis
of Ti02
in the absence of oxygen.
layers
Experimental
Formaldehyde
was
its reaction with
analyzed by
measured
dinitrophenylhydrazinc
DNPH). Chloride was
a
chloride sensitive electrode (ORION
(
by
617BN), connected to an
ORION A720 monitor. TiO, colloidal solution
of titanium isopropoxide (Aldrich) according to
9
was
prepared by hydrolysis
the
out
[41]. Preparation of thin layers on ITO was carried
reported procedure
successive
TiO,
with optical absorption
layers
by
spin coatings, yielding
0
.3 at 355 nm and thickness (1 ±0.15) µ .
Illumination
The excitation
Pyrex glass (2
source was
a
75
W
Xe
The
light
was filtered
by
light
lamp.
mm thick, cutting off below 300 nm) and an ORIEL 59800
cut-off filter (transmitting below 400 nm). The light intensity was adjusted
neutral
filters.
the IR irradiation was
by appropriate
density
Heating by
cm). The illumination area was 1.2 cm2.
(9.5
minimized
0
a
water filter
by
.2
as within 20 minutes prior to photolysis the solution
netic stirrer. Fe3+ oxalate was used for actinometry [42].
M
methanol was used as an OH' scavenger. During photolysis, as well
was stirred
a
by mag-
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Dechlorination of Air Free
Aqueous Carbon
Tetrachloride Solutions
95
Photocatalytic
Table 1. Rates and quantum yields of HCOH and Cl formation at pFf 12.2. 3.2 cm3
solutions. Results corrected for dark formation.
çi(Cl-)
^(HCOH)1
CrxiO10
mois"1
HCHOX10'"
mois"'
IX108
ein cm"2
s
3
.8
0.008
1.5
1.1
0.31
0.36
0.19
0.12
0
2.2
(blank)
0
0
0
0
.06
.185
.34
.04
.6
0.007
0.017
0.016
0.02
0.05
0.17
12.2
12.2
7.2
1.6
6.1
4.7
0.67
0.21
0.212
0.038
0.007
1
6.7
'
Initial yields (in the range where [HCHO] increases linearly with illumination
time).
2
3
10"2
H2P04/HP04 buffer, pH 7.
Results and discussion
Illumination of the Ti02
in the
of air free solution
methanol induces formation of chlo-
layer
presence
containing
5
X
10~3
M
CC14 and 0.2
M
typically
ride ions and
anol] (>0.2M) and [CC14] (7.5
building up depend
and to lesser extent
The
are
on [meth-
independent
formaldehyde.
quantum yields
X
10"4—5
X
10"3 M), although the rates of
on
as shown in Table 1. Chloride
also in the dark.
light intensity
ions,
Although
are
formaldehyde
produced
the mechanism of the dark reactions has not been studied, the results of
Table have been corrected for the dark reaction
subtraction, the
tude of which is shown in the first line of Table 1.
in the rate of chloride build is observed at
1
by
magni-
remarkable increase
9: at 2X10"9 ein cm"2
A
>
up
pH
s"1, the rate of chloride ion formation (mol s"') increases from
1
X
10"10 at
pH7Thteo
spective
3X10"10
rate of
8.8), 7.5 XlO"10
formation is
12.2) and 2.2X10"9
(pH
13).
(pH
(pH
much lower than the re-
formaldehyde
always
rate of chloride. Both rates show deviations from linear
depen-
on I"2,
approaching constant values
at
relatively high intensities.
dency
Mechanism
We
Reaction
that chloride ions are
a
chain reaction mechanism.
suggest
produced by
removes adsorbed OH' radicals,
and leaves electrons
This reduction initiates
(
8)
OH¡ds
available for the slow reduction of CC14 (reaction
9
).
a
chain
reaction (11) and terminated
Reac-
process, propagated by
by (12).
tions (13) and
additional chloride ions. Reaction (11) accounts
(
14) produce
for the
large
difference between chloride and
because
qualita-
formaldehyde yields,
this
reaction does not involve methanol. This reaction
propagation
also the
strong pH effect on
Cl" build
The "saturation"
up.
tively explains
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96
Johannes Stark and Joseph Rabani
of Cl" formation rate at elevated intensities is attributed to
the slow rate of
reaction (9),
with build
of electrons which compete for the Ti02
resulting
up
holes. This
is expected to affect the yield of HCHO too. The
competition
linear increase of the rate of formaldehyde build up with
I"2 at low
nearly
intensities
cess. If this is so, the
that formaldehyde is also produced by
a
chain
pro-
may suggest
difference between the yields
of chloride and
large
would indicate that the two chain processes take place in
reaction (10) and (15) account for the propagation of form-
concentrations of chloride ions,
formaldehyde
parallel, e.g.
aldehyde formation, producing equivalent
and chloroform. Such chain process cannot be a major path
formaldehyde
for Cl formation since the experiments show that ç01ICHo
^
<Pc\ However,
between reaction (4), (5) and (7) may also account for the
competition
observation in view of the relatively high steady state concentration
electrons.
of the
CH,OH' H20
CH20 +H20
cci;+cr
+
k=
1
X109 M"1 s"1
[43]
[44]
(8)
OH¡ds+CH,OH
Oad:+CH3OH
k
7X108M"' s"1
Initiation
(9)
^
TiO: CCI4
Doubling electron yield
CH2OH+Ti02
(10)
ecB+CCl,OH
C,C1„
Propagation
Termination
(11)
cci;+tío2+oh¡^
2
cci;
(12)
(13)
(14)
(15)
+
H20+Cr
Thermal
hydrolysis
CCLOH+OH
CCl20+3 OH
CCl20
HCOï+2 C1+H20
Thermal
hydrolysis
+
CH,OH
CHCI3
+
CH2OH'
cci;
Acknowledgment
This work was supported by the Joint Israel —German
Research
W. Bahnemann and Prof. Robert
Project
MOS/BMBF. We wish to thank Dr. D.
Schiller for most useful discussions.
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