Photocatalytic reaction kinetics on TiO2 thin films under light-limited and mass transport-limited conditions

Article abstract of DOI:10.1524/zpch.1999.213.Part_1.033

This paper describes our recent studies on kinetics and mechanism of photocatalytic decomposition of reactions on TiO₂ thin films under weak UV illumination. Separate observation of reactions at oxidation and reduction sites of a photocatalyst indicated that the radical chain reactions occur at oxidation sites during the photodegradation of ethanol. The quantum yields (QY) for degradation of 2-propanol and acetaldehyde were determined by the normalized absorbed photon number (I_norm). High QY values were obtained for acetaldehyde in comparison to 2-propanol indicating the existence of radical chain reactions for the former reaction. Based on the kinetic studies of gas phase 2-propanol degradation under wide range of light intensities and initial concentrations, we have mapped various regions on a light intensity vs. initial reactant concentration plot, showing pure mass transport-limited conditions and pure light intensity-limited conditions. by Oldenbourg Wissenschaftsverlag, Muenchen.

Full text of DOI:10.1524/zpch.1999.213.Part_1.033

tur
Zeitschrift
©
Chemie, Bd. 213, S. 33-42
München
Physikalische
(1999)
Oldenbourg
by
Wissenschaftsverlag,
Reaction Kinetics
Photocatalytic
Thin
under
Films
on
Ti02
Light-Limited
and Mass
Conditions
Transport-Limited
Y.
K.
T. N.
K.
and
A.
Hashimoto2*
Ohko1,
Ikeda1,
Tokyo,
Rao',
By
Fujishima1
'
The
of
of
7-3-1
Department
University
Applied Chemistry,
Graduate School of
The
4-6-1
Hongo, Bunkyo-ku, Tokyo 113-8656,
Japan
Engineering,
2
of
Research Center for Advanced Science
and
University
Tokyo,
Technology,
Komaba, Meguro-ku, Tokyo 153-8904, Japan
December
(Received
8, 1998;
10,
1998)
August
accepted
I
Weak
recent
light I Quantum
yield I Kinetic
UV
Ti021
Photocatalyst
study
This
describes our
studies on kinetics and mechanism of
paper
decomposition
observation of reactions at oxidation and reduction sites of
photocatalytic
of reactions on
thin films under weak
illumination.
UV
Ti02
Separate
a
indicated that
photocatalyst
the radical chain reactions occur at oxidation sites
the
and
of
ethanol.
during
photodegradation
The
for
of
were deter-
obtained
quantum yields (QY)
degradation
2-propanol
acetaldehyde
mined
for
the normalized absorbed
number
values
were
by
photon
(I„„r,„).
QY
High
in
the existence of
to
radical chain
acetaldehyde
comparison
2-propanol indicating
reactions for the former reaction. Based on
kinetic
the
studies of
intensities and initial
phase
gas
2-propanol
under wide
of
concentrations, we have
degradation
range
light
various
mass
on
a
vs. initial
concentration
reactant
mapped
regions
light intensity
plot, showing
conditions and
conditions.
transport-limited
light
pure
pure
intensity-limited
Introduction
The
of
for environmental
is
of
use
as
a
Ti02
photocatalyst
cleanup
great
and
interest because of its
oxidation
chemical inertness. The
power
high
holes in
to
have
V
vs.
Ti02
(~3.0
photogenerated
oxidizing
strong
power
organic
it
oxidize
all
of toxic
SHE),
enabling
nearly
types
compounds.
Extensive studies
carried
for the
out
purpose
have been
[1—4].
of air and
water
used in
The
sources
were
Earlier,
purification
high intensity light
mainly
research
achieve
such
as
most
to
rates
[5, 6].
photocatalytic
high degradation
and
use
of weak UV
sources
fluo-
sunlight
light
black-light-type
author.
Corresponding
Unauthenticated
Download Date | 5/28/16 12:32 PM

and A.
.
T.
.
.
Y.
34
Hashimoto
after the
Ohko,
much
Ikeda.
Rao.
Fujishima
demonstration
has
rescent
of
9]
[7
gained
importance
lamps
slicks
The role
carrier
with
of oil and
on water
chemical
Ti02-coated
photodegradation
of
in
Heller's
microbubbles
[10].
photocata-
oxygen
glass
by
group
as
recombination
in the
it
is
by trapping
highly significant
lysis
prevents
reac-
radical chain
electrons
and
the
[11]
participates
photogenerated
reactions
results in efficient
The
tions which in
in the
turn
[12].
photodegradation
participate
in
for
the
rates
gas-phase
high mass-transport
oxygen
of gas-phase
organic compounds.
degradation
is
use
of the
interest in the field of
make
Our main
to
photocatalysis
in
fluorescent
that the
available in the solar radiation and
toxic
weak UV
ordinary
light
so
to
contaminants,
appli-
gas-phase organic
lamps
decompose
in
indoor
and
environments be-
cation of
more
comes
living
working
photocatalysis
a
of
level
For
on
UV
example,
typical
light intensity
practical.
1
of
illuminated with white fluo-
rooms
is obtained
the floors
of the
pW/cm2
in
environment takes
The
reactants
rescent
living
lamps.
transport
the
UV
anti-
natural convection. We have demonstrated
self-cleaning,
place by
illumination
and
effects under low
bacterial
intensity
using
deodorizing
tiles
and
materials such
as
[14]
[15, 16].
for the
[13],
glass
Ti02-containing
paper
exhibit
materials
found
to
Such
were
photocatalytic activity
high
in
an
indoor
are
of
air,
particularly
acetaldehyde,
odor-causing
degradation
gas
to
hu-
odors that
in
mans are
smoke
to
18].
[17,
compounds
volume
objectionable
Generally,
cigarette
due
are
in the order of 10
available from
which
parts
only
present
million
the UV
Hence,
should be sufficient
light
ordinary
by
(ppmv).
per
fluorescent
the
such
in
to
compounds
lighting
decompose
kinetics of
the
is also
of
under
It
to
Ti02.
photocata-
interesting
study
presence
reactions
in
UV
order
obtain
illumination
to
new
low-intensity
lytic
about TiO,
photocatalysis.
insight
In
mechanistic
summarize
work
the
and
this
recent
our
on
we
paper,
in
of alcohols
kinetic studies
films under
of ethanol
microelectrode
at
on
Ti02-thin
photocatalytic decomposition
illumination. To
UV
understand the mechanism
low-intensity
degradation
have used
we
which involves radical chain
which enable the
reduction sites of
reaction,
of reactions
solutions
separate
in
monitoring
technique
con-
oxidation and
dissolved
photocatalyst
aqueous
kinetic
have chosen
For
we
studies,
[19].
2-propa-
taining
oxygen
it
efficient
substrate because
which
to
nol
as
a
undergoes
photodegradation
pro-
a
further reactions
much slower
were
at
rate
duce
The
the values
volves
reactions,
very
[20].
acetone,
quantum
undergoes
with
values obtained for this reaction
compared
yield
of
which in-
obtained for the
radical chain reactions
photodegradation
acetaldehyde
the existence of radical chain
Due to
of
for
[21].
be
to
the basis
the
in
for
was
photodegradation
acetaldehyde
expected
QY
the values
to
on
2-propanol. Finally,
comparison
high
have various
of results obtained with
we
on a
regions
2-propanol,
mapped
initial
concentration
reactant
mass
vs.
showing
plot,
light intensity
pure
[22].
conditions
conditions and
intensity-limited
light
transport
pure
Unauthenticated
Download Date | 5/28/16 12:32 PM

Reaction Kinetics on TiO,
35
Thin Films
Photocatalytic
section
Experimental
of ethanol
The mechanism of the
was
photodecomposition
investigated by
microelectrode
The details of the microelectrode fabri-
using
technique.
cation
were
elsewhere
carbon
a
was
fiber
heating
inserted
[19].
Briefly,
reported
into thin
the tube
obtain
to
a
was
to
tube, and,
pulled
Pyrex glass
by
fiber
it
a
fiber. After
the
was
glass-coated
cutting
glass-coated
polished
For
a
carbon disk.
electrochemical
Pd-covered
these microelectrodes
studies,
generate
were
as
act
a
above
films which
model
a
Ti02
placed
partially
for metal modified
The concentrations of dissolved and
Ti02
02
particle.
determined
the microelectrode
under
were
H202
amperometrically
positioning
surface
by
Pd
were
site
site
illumination.
above
the
or
on
Ti02
photocatalyst
in
M
solution
of ethanol.
The measurements
carried out
0.1
an
containing
aqueous
in
kinetic
in
the
both
the absence and
K2S04
For the
and
presence
of
we
of
were
2-
analysis
photocatalytic degradation
gas-phase
have used films which
Ti02
commercial
acetaldehyde,
propanol
prepared
sol.
a
a
anatase
Ti02
method,
factor of the
spin-coating
by
using
aqueous
was
to
film
The
found
be —150. An
02 (20%)-N2
fill reaction
the
roughness
of
mixture
vessel. The
sured
relative
50%
used
to
to
a
was
adjusted
humidity
mixture
of
with
the
was
mea-
prepared
injecting
2-propanol
by
(approximately
gas
5
vol% in the
quantity
2-propanol-saturated
gas
into the reaction vessel. In
of
The
filters
case
02-N2
mixture)
dry
acetaldehyde,
gas
gas
commercial
vol% in
used for
Neutral
thin film
used
to
was
Ti02
were
the
(5
N2)
injection.
density
a
illuminated with
was
lamp.
Hg-Xe
ratio of
flux absorbed
film
The details
control the
The
the
Ti02
intensity.
photon
by
determined
the incident
flux
be —0.65
to
was
to
[20].
photon
materials
in this work
described
about
and methods used
were
previously
[20-22].
and discussion
Results
In
order
understand the mechanism of
radical
to
photocatalysis involving
the
we
of ethanol in
reactions
studied
have
chain
ous
aque-
that
photodecomposition
solution
the dissolved
and
microelectrode
It
be
can
[19].
using
technique
expected
in the
consumed
the reduction site
at
solution
gets
at
oxygen
of
takes
reduction
the oxidation site due
both Pd
to water
production
place
oxygen
oxidation. The
and
current at
(reduction)
Ti02
oxygen
sites
measured
to
in the
at
of
absence ethanol.
were
(oxidation)
amperometrically
cathodic
the microelectrode de-
The
current
(due
reduction)
oxygen
illumination.
under
creased above Pd and increased above
Ti02
+
+
hp —hH
e"
+
Ti02
(1)
(2)
'
H
4
+
4 h
2
H20
02
(above
Ti02)
Unauthenticated
Download Date | 5/28/16 12:32 PM

and
A.
.
Hashimoto
.
T.
.
Y.
36
Ikeda.
Rao.
Ohko.
Fujishima
a
UV irradiation
UV irradiation
-3001
-200
-100
<
CL
5
3
min
4
0
1
2
/
4
3
2
5
0
1
Time
Time / min
C
Pd
Above
(
Q
fiöj)
(.Above
O
UV irradiation
irradiation
UV
+150
50
<
CL +100
M00
c
—
ím
12
=5
+50
+50
Ü
0
12
Time min
3
4
0
3
/
Time min
/
Above
Above
TÌO2
PcP)
(
(
)
A)
to
of dissolved
concentration
vs.
detection of
-1000 mV
at
1.
Oj (proportional
changes
or , ,
and Pd
Separate
current,
Fig.
to anodic current,
which results from
SCE,
(oxidation)
presence
cathodic
IOOO1T1V vs.
(proportional
sites
+
(reduction)
SCE,
Ti02
B)
ethanol.
in the
of
reaction
the
photocatalytic
—
+
e
O;
(3)
(4)
(above Pd)
02
'
-»
H
+
HO,
of
02
In
cathodic
the
Ti02
(Fig.
current at
the decrease
results
These
ethanol
the
however,
(5
vol%),
presence
microelectrode decreased
Pd
both
and
Pd site
above
the
sites,
1
more
that
TiO, site than
above
above
a).
being
Unauthenticated
Download Date | 5/28/16 12:32 PM

Reaction
50
Thin
Films
Kinetics
on TiCK
37
Photocatalytic
1000
100
10
•
ppmv
ppmv
ppmv
ppmv
40
30
20
A
O
1
T3
I
ce
3
o
µ
O
I
liti-1-l—L
10 14
1
I
I,
ul_ -
10 13
10 10
1012
photons/
on
10'
absorbed
s"
quanta.cm
absorbed
2.
Fig.
Quantum
yield dependence
at various initial
photons
2-propanol
concentrations.
indicate
that the
consumed
was
even at
sites
reactions
following
due
Ti02
(oxidation)
oxygen
to
the
oxidation
of
molecular
oxidation
in
the
participation
oxygen
of ethanol
photooxidation
at
sites.
The
is initiated
the
by
reactions :
+
h'
+
OH
H+
H20—
(5)
(6)
—
OH
+
+
CH,CH2OH
CH,CTHOH
H20.
The
tion
the
and
formation
ethanol
of final
can
radicals
with
react
molecular
and
in
the forma-
lb shows
above Pd
the
reaction
oxygen
at
resulting
sites.
H202
products,
acetaldehyde
Ti02
Fig.
current at
anodic
the
microelectrode
due
oxidation
to
of
H202
anodic
current at
sites.
of
The
H202
peroxyl
of
Ti02
site
confirms
appearance
Ti02
as
of
radical
The termination
coupling
chain
molecular
expected.
a
occurs
radical
reaction,
by
producing
oxygen
[19].
in
does
for kinetic
which
the
not
the
Unlike
of
case
of
ethanol,
2-
photocatalytic
decomposition
involve
chain
reactions.
it acts
as
a
model
propanol
Hence,
it is
simple
photodecomposed
with
substrate
studies.
Furthermore,
be detected
efficiently
to
can
acetone,
sensi-
by
gas
chromatography
high
does
The
chain
mechanism
reactions
of
of
tivity.
not in-
generation
is
photodecomposition
2-propanol
volve
one
in
the
[20].
Only
photon
participates
of
molecule
one
of
acetone. Hence
the
to
apparent
yield
quantum
equal
the
ratio of the
number of
acetone molecules
to number of ab-
generated
sorbed
photons.
Unauthenticated
Download Date | 5/28/16 12:32 PM

Y.
38
.
T.
Ikeda,
.
.
Hashimoto and
lOOOppmv
A.
Ohko,
Rao.
Fujishima
50
40
30
20
10
•
100
10
1
ppmv
ppmv
ppmv
U
A
o
!§
E
3
I
a
L
O
O
' '
illuni
illuni
limili
111
IO"7
IO"6
10"5 0.0001 0.001
0.01
0.1
1
/s"1
I
3.
on the normalized absorbed
concentrations.
number
at
Fig.
Quantum yield
photon
(I„orm)
dependence
various initial
2-propanol
the
acetone
concentrations.
2
shows
of absorbed
versus
(I)
Fig.
semilog
apparent
plots
photons
were
values for
These
values in
each
obtained
for various
QY
generation.
plots
The
saturated
initial
increased
with
QY
plot
gradually
and
I
for the lowest initial
concentration
was
decreasing
except
condition
finally
reached
intensi-
at
Thus,
lower
—28% for the
observation in this
(1
ppmv).
light-limited
light
ties.
maximum concentration observed
The
was
2-
is
to
highest
Another
concentration
(1000
propanol
ppmv).
figure
curve
that when the
the lower
concentration
decreased, the
shifted
was
2-propanol
was not
direction.
that
The shift
with the
light intensity
systematic
not
value is
concentration.
concentration. This indicates
the
determined
function of
normalized
the
QY
by
ratio of the
to
the
light intensity
gas
3
variation of
shows the
This
as
a
Fig.
QY
photon
number
number is defined
the ratio
of number of
as
absorbed
(Inurm)-
(per
the number of adsorbed
to
molecules. The
on
second)
photons
2-propanol
obtained for different initial
concentrations of
fall
the
became
that
and the
the de-
plots
2-propanol
same
The value of
—28% for
increase
decreased
curve.
as
and
Inorm
QY
finally
below 10~4/s.
constant at
either
values
These
the
the
results indicate
Inorm
molecules diffuse
or
surface
on
Ti02
2-propanol
collision
of
these
determine
one can
of
probability
species
efficiency
reaction. From the
the lowest initial
a
value of 28%
values of
3,
Fig.
composition
QY
expect
at
even
1
for
lower
concentration,
Inorm.
ppmv
Based
this
on
have calculated intermolecular
we
distance of
concentration,
Unauthenticated
Download Date | 5/28/16 12:32 PM

Thin Films
39
Reaction Kinetics on Ti02
Photocatalytic
1000
•
2-proponal
Acetaldehyde
3100
o
u
10
3
3
11
'
.1_.1_ 111 il ill_ . !_1
jjul_
10"b
110"7
10"b
0.0001 0.001 0.01
0.1
1
I
Is1
4.
of
on
between
and acetal-
Fig.
Comparison
yield dependence
In,„.m
2-propanol
quantum
dehyde.
ca.
adsorbed
possible
of
11 from
isotherm
Hence the
[20].
adsorption
2-propanol
or
radicals
diffusion
of either
be
least
at
can
length
2-propanol
11
nm.
also
have
the
values for
with the
For
we
comparison,
plotted
QY
acetaldehyde
correspond-
plot
a function of
as
Inorm
4)
together
(Fig.
degradation,
was
The calculation of
for this reaction
acetaldehyde
to
(1000
QY
ing
2-propanol
ppmv).
the
of
curves
earlier
coincided
the
for both
and 2-
described
[21]. Interestingly,
wide
are
values. This observation indi-
over
a
I„orm
propanol
range
highly
nature.
radicals
reactive with
The
kinds of
many
values
that
cates
organic
for
molecules
the
reactions.
The
of their
QY
high
(>100%)
irrespective
chain
is due
the involvement of the radical
to
acetaldehyde degradation
in the
Ti02
of
is known
to
of
02
presence
surface. Either the
degradation
aldehydes
on
proceed
via
radical chain reactions
the
photogenerated
or
hole
atom
the
radical
with
reacts
a
abstracting
acetaldehyde,
hydrogen
radical which
radical. This
further with
radical
reacts
to form
reacts
to
02
a
CH3CO
produce
unstable
with another
radical.
CH3(CO)00'
peroxo
acetic acid and the
CH3CO
subsequently generating
acetaldehyde,
in
acetic
molecule of
0.5
CH,CO
generated
Over
one
all,
generating
only
photons participate
back
with
acid. The
is
radical is
from acetic acid
to
react
02.
recycled
Subsequently,
the
of
If these
at
1.5
C02
photons.
expense
are
values
exceed 100%.
to
radical reactions
are
involved,
QY
expected
evident from the data shown in
4.
This is
Fig.
Unauthenticated
Download Date | 5/28/16 12:32 PM

40
Hashimoto and
A.
T.
. .
Rao.
Y.
.
Ikeda.
Ohko,
Fujishima
15
14
13
12
11
10
10
iiinii|
iiniii| iiiini|
limili
iiini|
CU
0.1
ppmv
A
region
region
10
18
13
1Q
1017
10
1Q14
1015
1Q16
Absorbed
/quanta'crri^s"1
photons
absorbed
rate
initial
various
at
of reaction
on
5.
photons
2-propanol
Fig.
Dependence
concentrations.
on
of
studies
further
carried
We have
out
photodegradation
2-propanol
in
achieve
intensities
order to
of the
mass
at
transport-limited
higher
relatively
conditions.
shows
rates
5
degradation
2-propanol
Fig.
log-log plots
ab-
the
different
versus
number of
initial concentrations
at
(I)
(R)
photons
were
at
carried
obtain
unit time. These studies
out
mass
sorbed
higher
the
Ti02
per
range
relatively
by
intensity
illumination in order
of UV
to
transport-
concen-
for each
the
limited conditions
rate
[22].
degradation
Although,
with
at
lower
number of absorbed
tration increased
light
trans-
mass
increasing
photons
and
reached
mass
it
saturated
intensities,
completely
finally
which
gradually
The
I
at-
is
at
values
conditions.
transport-control
port-limited
concentrations. Based
initial
with the results
increased with
results,
tained,
on
increasing
2-propanol
obtained under
low-
these
extremely
together
inten-
lim-
have
UV
we
various
regions
on
a
illumination,
light
mapped
intensity
mass
vs.
initial
concentration
reactant
showing
plot,
transport
(Fig.
sity
pure
conditions
of
orders
is
reactant
It
ited conditions and
6).
light intensity-limited
pure
6
of
that the
covers
to note
magnitude
plot
of
interesting
of
concentration and
8
orders
magnitude
light intensity.
Conclusions
of
of alcohols
kinetics and mechanism
were
The
The
investigated.
degradation
and
oxidation
reduction sites
reactions
were
at
photocatalytic
occurring
the
dissolved
and
monitored
a
H202
using
separately by determining
oxygen
Unauthenticated
Download Date | 5/28/16 12:32 PM

41
Films
on
Reaction
Thin
TiO:
Kinetics
Photocatalytic
-hoie
106
105
7
cr
mass-transport
controlled
region
p^curve
for outdoor
target
air
purification
^102
=3
co
H1013B-
for
target
cz
0)
c
indoor air
purification
103
102
101
10-3
1
10-2
10-1
reactant concentration
/ppmv
mass trans-
of gas-phase
concentration
initial
intensity-limited
reactant
Plots of
vs.
6.
showing
Fig.
pure
intensity
light
pure
for
conditions
and
light
photodegradation
port-limited
organics.
of ethanol
that the
indicated
results
microelectrode. These
photodegradation
for
The
chain reactions.
radical
involves
of
of
photodegradation
yields
quantum
were
values
values
the
QY
high
values
the
obtained
and
absorbed
using
by
acetaldehyde
2-propanol
gas-phase
The
number
photon
normalized
(Inorm).
in
to
for
obtained
for
(~28%)
comparison
acetaldehyde
(>100%)
chain
involvement of radical
attributed
the
were
to
obtained
2-propanol
the
from
the results
Based
reaction.
on
in the former
obtained
reactions
wide
con-
reactant
able
have been
to
out
a
we
of
range
map
magnitude
2-propanol,
photodegradation
of
of
in
vs.
orders
6
of
conditions,
covering
experimental
terms
of
and
vs.
8
mass
of
From
of
orders
centration
intensity,
intensity
magnitude
light
concentration
the
control.
intensity
light
transport
plot,
the
be
obtain
reactant
can
is
which
concentration
to
value of
the maximum
intensity
necessary
light
a
of
rate
for
reaction
maximum
given target
estimated.
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