Photocatalytic Reductive Defluorination of Fluorinated Compounds in Aqueous Alcohol Suspensions of a Metal-loaded Titanium(IV) Oxide

Article abstract of DOI:10.1002/cctc.202000299

Selective elimination of the fluorine element in organic fluorinated compounds under mild conditions is desired in order to resolve the environment and energy resource issues. Selective reductive defluorination of fluorine-containing organic compounds in 2-propanol-water suspensions of metal-loaded titanium(IV) oxide (TiO₂) photocatalysts is described in this paper. The effects of different types of metal co-catalysts, adsorption behavior of fluorobenzene (FB), stoichiometry, redox balance, scope, chemoselectivity and reaction temperature were examined. Photocatalytic defluorination of FB occurred over platinum-loaded TiO₂ at room temperature, and benzene and fluoride ion were produced with a high stoichiometry without degradation of the benzene structure, while hydrogen (H₂) production occurred as a competitive electron-consuming reaction. A slight elevation in the reaction temperature greatly accelerated defluorination and suppressed H₂ evolution, resulting in FB defluorination free from H₂ production at 333 K. Kinetic parameters of FB defluorination and H₂ evolution were determined, and the results obtained are explained on the basis of these parameters.

Full text of DOI:10.1002/cctc.202000299

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Photocatalytic reductive defluorination of fluorinated compounds
in aqueous alcohol suspensions of a metal-loaded titanium(IV)
oxide
Authors: Makoto Fukui, Atsuhiro Tanaka, and Hiroshi Kominami
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01/2020

10.1002/cctc.202000299
ChemCatChem
FULL PAPER
Photocatalytic reductive defluorination of fluorinated compounds
in aqueous alcohol suspensions of a metal-loaded titanium(IV)
oxide
Makoto Fukui,[a] Atsuhiro Tanaka[b, c] and Hiroshi Kominami*[b]
Selective elimination of the fluorine element in organic fluorinated compounds under mild conditions is desired in order to resolve the
environment and energy resource issues. Selective reductive defluorination of fluorine-containing organic compounds in 2-propanol-
water suspensions of metal-loaded titanium(IV) oxide (TiO2) photocatalysts is described in this paper. The effects of different types of
metal co-catalysts, adsorption behavior of fluorobenzene (FB), stoichiometry, redox balance, scope, chemoselectivity and reaction
temperature were examined. Photocatalytic defluorination of FB occurred over platinum-loaded TiO2 at room temperature, and benzene
and fluoride ion were produced with a high stoichiometry without degradation of the benzene structure, while hydrogen (H2) production
occurred as a competitive electron-consuming reaction. A slight elevation in the reaction temperature greatly accelerated defluorination
and suppressed H2 evolution, resulting in FB defluorination free from H2 production at 333 K. Kinetic parameters of FB defluorination
and H2 evolution were determined, and the results obtained are explained on the basis of these parameters.
high temperature. Recently, it has been reported that various
catalysts work for defluorination under mild conditions (ambient
pressure of H2 at room temperature);[9-12] however, not only the C-
F bond but also the aromatic rings are excessively hydrogenated
to aliphatics because of the high activity. Therefore, H2-free
Introduction
Organic fluorinated compounds, which are halogenated
compounds, are harmful to both the human body and the
environment, although they are used as a variety of chemical
conditions are preferable from the viewpoint of green
substances such as surfactants, coating agents, agrochemicals
chemistry.[13] A catalytic method for selective defluorination of
and polymer materials.[1] It is very difficult to remove fluorine
organic fluorinated compounds under milder conditions (e.g., H2-
atoms from organic fluorinated compounds because the carbon-
free and low temperature conditions) is therefore highly desired.
fluorine (C-F) bond is the strongest bond among carbon halogen
When TiO2 is photoirradiated by UV light, excited electrons
bonds.[2] Organic fluorinated compounds are persistent in the
and positive holes are formed in the conduction band (CB) and
environment due to the high stability of the C-F bond.[3,4]
the valence band (VB) in TiO2, and they induce reduction and
Incineration treatment is used for detoxifying organic fluorinated
oxidation, respectively. TiO2 powder can be easily recovered from
compounds; however, a large amount of energy is required to
a reaction mixture and is not toxic for humans or the environment,
cleave the strong C-F bond. In addition, a large amount of carbon
thus satisfying the policy of green chemistry.[13] A photocatalytic
dioxide (CO2) is released into the atmosphere. Therefore, a new
method is one of the possible methods for defluorination of
strategy that enables simultaneous elimination of fluorine atoms
organic fluorinated compounds under milder conditions because
and recovery of the organic parts (structure) of fluorinated
of its following advantages: 1) light is used as the driving force, 2)
compounds is required from both the viewpoints of environmental
the reaction can be controlled by light and 3) the reaction occurs
conservation and carbon recycling. Defluorination of organic
at room temperature and under ambient pressure. Many
fluorinated compounds over various metal catalysts has been
researchers have studied photocatalytic detoxifying pollutants by
reported.[5-8] However, these catalyst materials require harsh
using the strong oxidation power of positive holes and active
reaction conditions such as high pressure of hydrogen (H2) and
oxygen species.[14-17] There are various photocatalytic materials
that decompose fluorine-containing organic compounds under
[a]
M. Fukui
aerobic conditions;[18-24] however, the valuable organic
frameworks are simultaneously lost. Therefore, selective
elimination of fluorine atoms from fluorinated organic compounds
under milder conditions is highly desired for recycling of organic
compounds and detoxification.
Molecular and Material Engineering
Interdisciplinary Graduate School of Science and Engineering
Kindai University, 3-4-1, Kowakae, Higashiosaka, Osaka 577-8502
(Japan)
[b]
Dr. A. Tanaka, Dr. H. Komianmi
Department of Applied Chemistry
Faculty of Science and Engineering
Kindai University, 3-4-1, Kowakae, Higashiosaka, Osaka 577-8502
(Japan)
E-mail: hiro@apch.kindai.ac.jp
Dr. A. Tanaka
We have investigated reductive conversions of organic
compounds over a titanium(IV) oxide (TiO2) photocatalyst in
alcohols under irradiation of UV light. Various organic compounds
27]
such as nitrobenzenes[25] and benzaldehydes[26,
were
[c]
selectively hydrogenated in alcohol suspensions of a pristine TiO2
photocatalyst. Since the solvent alcohols also act as scavengers
for positive holes in the VB of TiO2, target organic compounds are
selectively reduced by photogenerated electrons in the CB of TiO2.
In other words, the alcohols can be regarded as hydrogen donors
(electron and proton donors) for the target compounds to be
Precursory Research for Embryonic Science and Technology
(PRESTO)
Japan Science and Technology Agency (JST)
4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
1
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10.1002/cctc.202000299
ChemCatChem
FULL PAPER
reduced. The introduction of a small metal co-catalyst to the
surface of TiO2 makes difficult conversions possible. For example,
selective hydrogenation of alkynes to alkenes and hydrogenation
of furan to tetrahydrofuran occurred over copper-loaded TiO2 (Cu-
TiO2)[28] and palladium-loaded TiO2 (Pd-TiO2),[29] respectively. In
photocatalytic conversions with the assistance of a metal co-
catalyst, active hydrogen species are formed on the surface of
metal particles by photogenerated electrons and these species
induce hydrogenation of organic compounds. We also found
reductive dechlorination of chlorobenzene, a kind of halogenated
organic compound, to benzene in a 2-propanol or aqueous 2-
propanol suspension of a Pd-TiO2 photocatalyst.[30-32] Benzene
and chloride ions were obtained stoichiometrically without
decomposition of benzene. To our knowledge, photocatalytic
selective defluorination of fluorinated compounds having the
stable C-F bond under mild conditions has not yet been reported.
In this study, we investigated photocatalytic defluorination of
fluorinated organic compounds in aqueous alcohol suspensions
of metal-loaded TiO2. We report here 1) the effects of different
kinds of metal co-catalysts and TiO2 on defluorination, 2)
expandability of the photocatalytic defluorination and 3) the
results of a kinetic study (effect of reaction temperature) on
defluorination and the side reaction (hydrogen evolution).
100
80
60
40
20
0
Pd
Au
Rh
Pt
Ag
Cu
0 0.1 0.2 0.3 0.4 0.5
Hydrogen overvoltage / V
Figure 2. Correlation between amount of H2 evolved in photocatalytic
defluorination of FB and hydrogen overvoltage of metal electrodes. Reaction
conditions are shown in Figure 1.
this system means that simple dehydrogenation of 2-propanol
occurred (CH3CH(OH)CH3 → H2 + CH3COCH3). When Rh-loaded
TiO2 was used, a small amount of benzene (BZ) was produced
together with a large amount of H2. We found that a Pt co-catalyst
promoted defluorination of FB to BZ and that H2 evolution was
greatly suppressed. These results indicate that suitable metal co-
catalysts for defluorination (Pt) and dechlorination (Pd)[30-32] were
different each other. We will discuss the reason later in this
section. We have reported that photocatalytic H2 evolution over
TiO2 was different depending on the type of metal co-catalyst
loaded on TiO2 and that the rate of H2 production decreased with
an increase in the hydrogen overvoltage of the metal
electrode.[33,34] The correlation between the hydrogen overvoltage
of the co-catalyst and H2 evolution in the present system is shown
in Figure 2, indicating that H2 evolutions in the case of Pt and Rh
were greatly different from those expected from the correlation of
the other co-catalysts (Au, Ag, Cu and Pd). This means that two
reduction reactions, i.e., FB defluorination and H2 evolution,
compete over Pt-TiO2 and Rh-TiO2 photocatalysts and that some
of photogenerated electrons were used for the former reaction. To
make the selectivity clear, a new indicator of the efficiency in
utilization of photogenerated electrons in the CB of TiO2, i.e.,
electron selectivity for the production of BZ (ES(BZ)), was
calculated by using Equation (1) and the values of ES(BZ) are
also shown in Figure 1.
Results and Discussion
Effects of metal co-catalysts on defluorination of
fluorobenzene (FB) over a TiO2 photocatalyst
The effects of metal co-catalysts on TiO2-photocatalyzed
defluorination of FB in 50 vol% 2-propanol-water solutions are
shown in Figure 1. No reaction occurred over co-catalyst-free
TiO2, indicating that electrons in the conduction band of TiO2 have
no ability to drive defluorination. When Au, Ag, Cu and Pd were
used as co-catalysts, H2 was produced. This means that these
metals did not show any catalysis for FB under the present
conditions and worked as co-catalysts for two-electron reduction
of protons (2H+ + 2e- → H2), although the potential of the CB
bottom (0 V vs. SHE) of rutile-type TiO2 (MT-150A) is very close
to the potential of H2 evolution (0 V vs. SHE). Evolution of H2 in
n(BZ)
0.4
0.2
0
(
)
ES BZ =
.
(1)
(
)
n BZ +n(H )
2
Of course, the values of ES(BZ) were zero in the case of Au, Ag,
Cu and Pd over which no defluorination occurred. The value of
ES(BZ) for Pt-TiO2 was calculated to be 0.42, which was much
higher than that for Rh-TiO2 (0.08). These results indicate that Pt
was the only effective co-catalyst for photocatalytic defluorination
of FB to BZ, indicating that Pt interacts with FB and provides a
new route with low activation energy for defluorination of FB.
Peköz et al. reported that Pt can readily adsorb a fluorinated
compound due to a strong interaction between Pt and the fluorine
element based on density functional theory calculation.[35] In order
to evaluate the interaction of FB with a metal co-catalyst and TiO2,
an experiment on FB adsorption in the dark was carried out and
the results are shown in Figure 3. The amounts of FB adsorbed
on Au-, Ag-, Cu- and Pd-TiO2 were almost the same as the
amount of FB adsorbed on TiO2, indicating that FB hardly
accumulates on these metal co-catalysts. In contrast to these
30
25
20
15
10
5
120
100
80
60
40
20
0
0
None Au
Cu Pd
Metal co-catalyst
Pt
Rh
Ag
Figure 1. Effects of different metal co-catalysts on the yields of BZ and H2
produced in photocatalytic reduction of FB (50 µmol) in 50 vol% 2-propanol-
water suspensions of 1.5 wt% M-TiO2 (MT-150A) for 1 h under irradiation of UV
light from a high-pressure mercury lamp at 293 K.
2
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10.1002/cctc.202000299
ChemCatChem
FULL PAPER
5
4
3
2
1
0
Table 1. Effects of different kinds of TiO2 on defluorination of FB (50 µmol) in
50 vol% 2-propanol-water suspensions of 1.5 wt%Pt-TiO2 under irradiation of
UV light from a high-pressure mercury lamp for 1 h at 293 K.
SBET
/
UV
light
BZ /
μmol
H2 /
μmol
ES
(BZ)
Entry
TiO2
Phase
m2 g-1
270
313
290
311
290
59
1
2
JRC-TIO-7
JRC-TIO-8
JRC-TIO-9
JRC-TIO-10
JRC-TIO-12
JRC-TIO-13
JRC-TIO-4
JRC-TIO-11
JRC-TIO-3
JRC-TIO-6
MT-150A
MT-150A
-
On
On
14
17
457
522
492
512
478
323
173
189
24
0.03
0.03
0.03
0.05
0.03
0.02
0.03
0.05
0.39
0.43
0.42
-
None Au Ag Cu Pd
Metal co-catalyst
Pt
Rh
3
On
16
Figure 3. Effects of metal co-catalysts on amounts of FB adsorbed on 1.5
wt%M-TiO2 (MT-150A) (200 mg) in 50 vol% 2-propanol-water solutions in the
dark for 20 h at 298 K.
A
4
On
15
5
On
15
results, the amounts of FB adsorbed on Rh-TiO2 and Pt-TiO2
were larger than the amount of FB adsorbed on metal-free TiO2,
indicating that FB strongly adsorbed on Rh and Pt particles
loaded on TiO2. Pt showed the largest amount of adsorption
among the metals examined, and the strong interaction with FB
reported by Peköz et al.[35] was experimentally confirmed in this
study. Therefore, the relatively high values of ES(BZ) over Pt-
TiO2 are explained by the large amount of FB adsorbed on the
Pt co-catalyst due to the strong interaction between them. The
adequate co-catalysts for defluorination (Pt) and dechlorination
(Pd) can be explained by the strength of the interaction with
target compounds.
The morphology of the Pt-TiO2 photocatalyst was observed
by TEM, and TEM images are shown in Figure 4. No deposits
were observed in the image of bare TiO2 (Figure 4a), whereas
fine Pt particles were observed in the image of Pt-TiO2 (Figure 4b).
Figure 4c shows the size distribution of Pt particles on Pt-TiO2. Pt
particles were highly dispersed on the surface of TiO2 and the
average diameter of the particles was determined to be 4.3 nm
from the size distribution.
6
On
6
7
54
On
6
A, (R)
8
97
On
10
9
On
15
10
11
12
13
14
100
93
93
-
On
23
30
R
On
25
35
(Dark)
On
n. d.
n. d.
n. d.
n. d.
n. d.
n. d.
-
-
-
-
-
(Dark)
-
The photocatalytic activity for defluorination of FB to BZ depended
on the crystalline phase of TiO2. In the case of anatase-type TiO2
modified with a Pt co-catalyst, H2 evolution from 2-propanol was
dominant and a small amount of BZ was formed, resulting in much
lower values of ES(BZ). Rutile-type TiO2 having a Pt co-catalyst
showed a higher value of ES(BZ) due to the greatly suppressed
H2 evolution. These results indicate that the use of rutile-type TiO2
is important for effective defluorination of FB. One of the major
differences between anatase-type TiO2 and rutile-type TiO2 is the
position of the CB, and the notable differences in values of ES(BZ)
can be reasonably explained by the potentials in the CB of TiO2 (-
0.3 V vs. SHE for anatase and 0 V vs. SHE for rutile). Since the
potential of the CB in anatase-type TiO2 is negative to the
potential for H2 evolution (0 V vs. SHE), H2 generation easily
occurred with the aid of a Pt co-catalyst. The CB potential in rutile-
type TiO2 is very close to the potential of H2 evolution, resulting in
a smaller amount of H2. In addition, Entries 12-14 show the effects
of the presence of UV light and Pt-TiO2 on the defluorination of
FB to BZ. No BZ was formed under the conditions in the absence
of UV light and/or Pt-TiO2, indicating that UV light irradiation and
a Pt-TiO2 photocatalyst were indispensable for the defluorination
of FB to BZ and that the defluorination of FB occurred through a
photocatalytic process.
50
(c)
40
30
20
10
0
0 1 2 3 4 5 6 7
8
Particle size of Pt / nm
Figure 4. TEM images of (a) TiO2 (MT-150A) and (b) 1.5 wt%Pt-TiO2, and
distribution of Pt particles in 1.5 wt%Pt-TiO2.
Effects of reaction conditions on defluorination of FB to BZ
over a Pt-TiO2 photocatalyst
Photocatalytic defluorination of FB in 50 vol% 2-propanol-
water suspensions of 1.5 wt%Pt-TiO2
The effects of different kinds of TiO2 on the yields of BZ and H2
produced in 50 vol% 2-propanol-water suspensions of 1.5 wt%
Pt-TiO2 under irradiation of UV light from a high-pressure mercury
lamp for 1 h were examined. The results are summarized in Table
1. There are differences in the crystalline phase and specific
surface area (SBET) among the commercial TiO2 samples.
Time courses of amounts of the reactant (FB), reduction products
(BZ, F- and H2) and oxidation product (acetone) during the
photocatalytic defluorination of FB in a 50 vol% 2-propanol-water
suspension of 1.5 wt% Pt-TiO2 were observed and the results are
shown in Figure 5. After 2-h photoirradiation time, FB was not
3
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10.1002/cctc.202000299
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FULL PAPER
detected in the liquid phase and 47.5 µmol of BZ corresponding
to 99% yield was obtained. The value of the turnover number
(TON) for production of BZ (47.5 µmol) based on the amount of
Pt (1.5 wt%, 3.8 µmol) was calculated to be 12.3, which means
that Pt worked as a catalytic material in this reaction. Material
balance (MB) calculated from the amounts of FB and BZ by using
Equation (2) was almost unity during the reaction, indicating that
no other intermediate(s) and by-product(s) were formed.
If BZ, acetone and HF in Equation (4) are regarded as valuable
compounds, the atom efficiency of this reaction calculated by
Equation (5) is 100%, which means that photocatalytic
defluorination matches the concept of green chemistry policy.[13]
Atom efficiency / % = molecular weight of desired products (BZ, acetone and HF) ×100.
(5)
molecular weight of all reactant (FB and 2-propanol)
It is important to confirm how efficiently incident photons are used
for a target reaction. In the present conditions, since a high-
pressure mercury lamp having several powerful emission lights
was used as a light source, we cannot estimate how much of the
incident photons are effectively used for the photocatalytic
defluorination of FB to BZ over 1.5 wt% Pt-TiO2. In order to
evaluate the utilization efficiency of incident photons, we chose a
n(FB) + n(BZ)
MB =
.
(2)
(
)
FB
n
0
Ion chromatograph analysis of the liquid phase revealed that
approximately equal amounts of F- (46.5 µmol) were produced
after 2 h, indicating that FB was converted to BZ and HF with a
high stoichiometry. From these results, it can be concluded that a
Pt-TiO2 photocatalyst makes it possible to detoxify and recycle
organic fluorinated compounds.
In the TiO2-photocatalyzed reaction process, one pair of
excited electrons and positive holes is formed and they work as a
reductant and an oxidant, respectively. In this photocatalytic
system in a 50 vol% 2-propanol-water suspension, oxidation of 2-
propanol occurred, while two reductions (FB defluorination and H2
production) competitively took place. Since CO2 was not observed
in the gas phase, decomposition of 2-propanol and acetone to
CO2 did not occur under the present conditions. In order to reveal
the total stoichiometry of redox reactions, we calculated the redox
balance (RB) by using Equation (3) and the values of RB are also
shown in Figure 5.
multi-wavelength
irradiation
monochromator
(MM-3PK,
Bunkoukeiki, Tokyo) that irradiates monochromatic light with a
specific energy. The light intensity was measured with a
spectroradiometer (USR-45D, Ushio, Tokyo). The apparent
quantum efficiency (AQE) at each centered wavelength of light
was calculated from the ratio of the amount of BZ and the amount
of incident photons by using Equation (6).
2 ×rete of BZ formation
AQE / % =
×100. (6)
rate of incident photons
Figure 6 shows the action spectrum in the defluorination of FB in
a 50 vol% 2-propanol-water suspension of 1.5 wt% Pt-TiO2.
Wavelength-dependency of AQE was similar to the
photoabsorption of TiO2, indicating that defluorination of FB in a
50 vol% 2-propanol-water suspension was induced by the band
gap excitation of TiO2 and that modified Pt particles worked only
as a co-catalyst site for the hydrogenation of FB.
n(BZ)×2 + n(H )×2
2
RB =
.
(3)
(
)
n acetone ×2
The values of RB were almost unity during the reaction. These
results (MB, RB and analysis of F- and CO2) show that
defluorination of FB over Pt-TiO2 utilizing 2-propanol as a hole
scavenger can be described as Equation (4) and that only two
redox reactions occur.
12
8
1
Pt-TiO
2
F
0.5
OH
O
+
+
+
HF
(4)
4
0.4
TiO
2
0.2
0
0
0
300 400 500 600
Wavelength / nm
2
1
0
Figure 6. Action spectrum in defluorination of FB (50 μmol) to BZ in a 50 vol%
2-propanol-water suspension of 1.5 wt%Pt-TiO2 (MT-150A, left axis) and
absorption spectrum of TiO2 (MT-150A, right axis).
300
BZ
50
40
30
20
10
0
FB
F-
Applicable scope of photocatalytic defluorination of
fluorinated compounds
200
100
0
Acetone
In order to demonstrate the applicable scope of photocatalytic
hydrogenation of fluorinated organic compounds over Pt-TiO2, we
examined the photocatalytic defluorination of various fluorinated
organic compounds under irradiation of UV light from a high-
pressure mercury lamp at 293 K. The results are shown in Table
2. In a recycling test for the photocatalytic defluorination of FB, BZ
was obtained with high levels of conversion and selectivity
(Entries 1-3), indicating that Pt-TiO2 powder exhibited excellent
recyclability without loss of activity. Several types of fluorinated
H
2
0
1
2
3
Irradiation time / h
Figure 5. Time courses of amounts of FB, BZ, F-, H2, acetone, MB, RB and
ES(BZ) in 50 vol% 2-propanol-water suspensions of 1.5 wt% Pt-TiO2 (MT-150A)
under irradiation of UV light from a high-pressure mercury lamp at 293 K.
4
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10.1002/cctc.202000299
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FULL PAPER
50
40
30
20
10
0
1
Table 2. Applicability of photocatalytic defluorination of various fluorine
compounds in 50 vol% 2-propanol-water suspensions of 1.5wt% Pt-TiO2 (MT-
150A) under irradiation of UV light from a high-pressure mercury lamp at 293 K.
0.8
0.6
0.4
0.2
0
Time
/ h
Conv.[a]
/ %
Sel.[b]
/ %
Entry
Substrate
Product
F
F
F
1
2
2
>99
>99
99
98
2[c]
293 308 323 333
Temperature / K
3[d]
2
2
3
2
3
98
99
99
99
99
98
99
97
97
98
Figure 7. Effect of reaction temperature on photocatalytic defluorination of FB
to BZ in a 50 vol% 2-propanol-water suspension of 1.5 wt%Pt-TiO2 under
irradiation of UV light from a UV-LED for 0.5 h.
F
4
elevated temperatures using a UV-LED as a light source. Yields
of BZ and H2 for 0.5 h are shown in Figure 7. At all temperatures,
defluorination of FB occurred free from other side reactions such
as ring hydrogenation and decomposition to CO2. With a slight
elevation in the reaction temperature, the BZ yield was greatly
increased. On the other hand, the competitive reaction, i.e., H2
production, was drastically suppressed and, finally, no H2 gas was
detected at 333 K. These results indicate that the reaction
temperature strongly affects the utilization of photogenerated
electrons. As shown in Figure 7, ES(BZ) increased with an
increase in the reaction temperature and the value of ES(BZ)
reached unity at 333 K, indicating that photogenerated electrons
were used only for FB defluorination at that temperature.
HO
F
HO
5
O
F
O
6
H2
N
F
H2N
7
F
8
3
99
98
F
9
4
5
99
99
97
97
F
10
To elucidate the electron selectivity for the reduction
products (BZ and H2) in this defluorination system, the effect of
reaction temperature on photocatalytic H2 evolution in the
absence of FB was examined under the same conditions. The
yield of H2 gradually increased with elevation in the reaction
temperature as shown in Figure 8. In order to obtain kinetic
parameters of FB defluorination and simple H2 evolution, the
results were analyzed on the basis of an Arrhenius plot as
expressed in Equation (7),
[a] Conversion of substrate; Conv. = decrement of substrate / initial amount of
substrate ×100. [b] Selectivity of product from substrate; Sel. = amount of
product / decrement of substrate ×100. [c] Second use. [d] Third use.
compounds having other functional groups including methyl,
hydroxy, methoxy and amino groups were examined. 4-
Fluorotoluene (Entry 4), 4-fluorophenol (Entry 5), 4-fluoroanisole
(Entry 6) and 4-fluoroaniline (Entry 7) were defluorinated to
toluene, phenol, anisole and aniline, respectively, with high
selectivity. Additionally, fluorinated organic compounds other than
E
a
ln k = - RT +ln A
(7),
fluorobenzenes,
i.e.,
1-fluoronaphthalene
(Entry
8),
fluorocyclohexane (Entry 9) and 1-fluorooctane (Entry 10), were
also defluorinated with high selectivity. These results show that
the combination of a Pt-TiO2 photocatalyst and 50 vol% 2-
propanol-water medium is a strong tool for defluorination of
various types of fluorinated organic compounds.
where k, Ea, R and A mean a rate constant, activation energy, gas
constant and frequency factor, respectively. Arrhenius plots
(logarithm of k vs reciprocal of T) for two reactions are shown in
Figure 9. Linear correlations between them were obtained and the
Effects of reaction temperature on defluorination of FB
50
40
30
20
10
In the present conditions, Pt metals loaded on TiO2 work as active
sites for the hydrogenation of FB, i.e., protons derived from 2-
propanol are reduced by exited electrons on Pt metals to form
active hydrogen species and these species are used for
defluorination of FB. The last process on Pt particles occurs
(thermo)catalytically and is different from the photocatalytic
process. During our studies on photocatalytic conversions of
organic compounds over M-TiO2, we found that a slight elevation
in the reaction temperature accelerates the reaction, which is
explained by the great increase in the reaction rate of the last
(thermo)catalytic process on metal particles.[36-38] In order to
hasten the photocatalytic defluorination, photocatalytic
defluorination of FB was carried out in a water bath kept at slightly
0
293 308 323 333
Temperature / K
Figure 8. Effect of reaction temperature on photocatalytic H2 evolution from a
50 vol% 2-propanol-water suspension of 1.5 wt%Pt-TiO2 under irradiation of UV
light from a UV-LED for 0.5 h.
5
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10.1002/cctc.202000299
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values of Ea and A for FB defluorination were calculated to be 24
kJ mol-1 and 4.4×105 µmol h-1, respectively, whereas the values
of Ea and A for H2 evolution were calculated to be 11 kJ mol-1 and
3.2×103 µmol h-1, respectively. The value of Ea for H2 evolution
(11 kJ mol-1) over Pt-TiO2 in this system was almost the same as
Ea (ca. 10 kJ mol-1) for H2 evolution over several photocatalysts.[39-
43] The much larger value of Ea for FB defluorination (24 kJ mol-1)
than that for H2 evolution (11 kJ mol-1) means that the rate-
determining step in this system is the last catalytic process (FB
defluorination) caused by active hydrogen species generated on
the surface of Pt metals. The improvement in ES shown in Figure
7 can be explained by the values of Ea and A for the two reactions.
Since the value of Ea for H2 evolution is much smaller than that for
FB defluorination, H2 was predominantly produced, resulting in a
smaller value of ES(BZ) at low temperatures. On the other hand,
the value of A for FB defluorination that is ca. 100-times larger
than that for H2 evolution contributes to the large increase in the
reaction rate and larger value of ES(BZ) at a higher temperature.
product selectivity and electron selectivity, providing a new
strategy for an efficient photocatalytic reaction.
Experimental Section
Preparation of metal-loaded TiO2 by photodeposition method
By using the photodeposition method, various metals were loaded on TiO2
(MT-150A, Tayca, Osaka, Japan) as co-catalysts. The metal sources were
HAuCl4, AgNO3, CuCl2, PdCl2, H2PtCl6 and RhCl3. Other commercial TiO2
samples registered at the Catalysis Society of Japan as Japan Reference
Catalysts (JRC-TIO series) were also used to obtain information on the
effects of different types of TiO2. TiO2 powder was suspended in a
methanol-water (9:1, 10 cm3) solution containing a metal source in a test
tube. The test tube was sealed with a rubber septum under argon (Ar). A
400 W high-pressure mercury arc (Eiko-sha, Osaka, Japan) was used as
the source of UV light to reduce the metal source to a metallic state on the
surface of TiO2. Photodeposition was performed for 2 h at 293 K in a water
bath. The metal-loaded TiO2 was washed with distilled water and then
dried for 1 h under vacuum.
5
4
3
5
4
3
(a)
(b)
lnk = -2.9/T+13
lnk = -1.3/T+8.1
Characterization
A high-resolution transmission electron microscope (TEM) (JEM-2100F,
JEOL, Tokyo, Japan) was used at 200 kV to observe the morphology of
samples. The microscope belongs to the Joint Research Center of Kindai
University. A UV-vis spectrometer (UV-2600, Shimadzu, Kyoto, Japan)
3
3.2
3.4
3
3.2
3.4
having
a diffuse reflectance measurement unit (ISR-2600PLUS,
103 / (T / K)
103 / (T / K)
Shimadzu) was used to obtain diffuse reflectance spectra of the samples,
and barium sulfate was used as a reference.
Figure 9. Arrhenius plots (logarithm of k vs reciprocal of T) for (a) photocatalytic
defluorination of FB to BZ and (b) H2 evolution.
Defluorination of fluorobenzene over an M-TiO2 photocatalyst in 50
vol% aqueous 2-propanol solution
Fifty mg of M-TiO2 was suspended in 5 cm3 of a 50 vol% aqueous 2-
propanol solution containing fluorobenzene (FB, 50 μmol) in a test tube.
After sealing the tube with a rubber septum under argon (Ar), the
suspension in the test tube was photoirradiated with the same high-
pressure mercury arc at 293 K. Products in the gas phase were analyzed
by a gas chromatograph (GC-8A, Shimadzu) and an MS-5A column.
Unreacted FB and product(s) in the liquid phase were analyzed by an FID-
type gas chromatograph (GC-2025, Shimadzu) and a DB-1 column, in
which toluene (5 mm3) as an internal standard substance was added to
the reaction solution (2 cm3). Ion chromatography (IC) (PU-2800 Plus,
Jasco, Tokyo) with an IC NI-424 column (Shodex, Tokyo, Japan) was used
to analyze a fluoride ion (F-) in the liquid phase.
Conclusions
Photocatalytic defluorination of FB in a 2-propanol suspension of
TiO2 having metal co-catalysts was examined. In the case of a Pt
co-catalyst, BZ and F- were formed with a high stoichiometry, and
the photocatalytic performance was attributed to the strong
interaction between FB and Pt. In addition, the Pt-TiO2
photocatalyst showed durability in several runs without loss of
activity and a wide applicability for defluorination of various
fluorinated compounds. Active hydrogen species, which were
formed through reduction of protons by photogenerated electrons
on the Pt co-catalyst, were competitively used for FB
defluorination and H2 evolution. The latter reaction decreased the
electron selectivity for BZ production. To accelerate FB
defluorination, several parameters were investigated and it was
found that the reaction temperature of the photocatalytic reaction
greatly affected the rates of two reactions. With elevation of the
reaction temperature, the rate of FB defluorination increased,
while the rate of H2 production decreased. Due to the effects on
the two reactions, the electron selectivity for BZ production was
greatly improved and, finally, the value reached unity at 333 K.
These results can be explained by the kinetic parameters of the
two reactions, i.e., larger values of Ea (24 kJ mol-1) and A (4.4×105
µmol h-1) for FB defluorination vs smaller values of Ea (11 kJ mol-
1) and A (3.2×103 µmol h-1) for H2 evolution. Most studies on
photocatalytic reactions have focused on new photocatalytic
materials. The present study revealed that reaction temperature
of the photocatalytic reaction greatly controls the reaction rate,
Adsorption of FB on metal-loaded TiO2 in 50 vol% aqueous 2-
propanol solution
M-TiO2 (200 mg) was suspended in a 50 vol% aqueous 2-propanol
solution (5 cm3) containing FB (50 μmol) in a test tube at 298 K. After the
test tube had been sealed with a rubber septum, the test tube was
magnetically stirred in a water bath for 20 h in the dark. The amount of FB
in the solution was determined in the same way as that for the
photocatalytic reaction after the particles had been removed by filtration.
Acknowledgements
This work was partially supported by Grant-in-Aid for Scientific
Research (No. 17H03462 and 17H04967) from the Japan Society
for the Promotion of Science (JSPS). M. F. is grateful to JSPS for
a Research Fellowship for young scientists (No. 18J12706). A. T.
6
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[35] R. Peköz, K. Johnston, D. Donadio, J. Phys. Chem. C 2014, 118, 6235-
6241.
is grateful for financial support from the Faculty of Science and
Engineering, Kindai University.
[36] H. Kominami, M. Shiba, A. Hashimoto, S. Imai, K. Nakanishi, A. Tanaka,
K. Hashimoto, K. Imamura, Phys. Chem. Chem. Phys. 2018, 20, 19321-
19325.
Keywords: Photocatalyst • TiO2 • Metal co-catalyst •
Defluorination • Fluorinated compounds
[37] A. Kinoshita, K. Nakanishi, A. Tanaka, K. Hashimoto, H. Kominami,
ChemPhotoChem 2019, 3, 559-567.
[38] M. Fukui, Y. Omori, S. Kitagawa, A. Tanaka, K. Hashimoto, H. Kominami,
J. Catal. 2019, 374, 36-42.
[1]
E. Kissa, Fluorinated Surfactants and Repellants, Marcel Dekker, New
York, 2001.
[39] G. R. Bamwenda, S. Tsubota, T. Kobayashi, M. Haruta, J. Photochem.
Photobio. A: Chem. 1994, 77, 59-67.
[2]
[3]
[4]
D. Huang, L. Yin, J. Niu, Environ. Sci. Technol. 2016, 50, 5857-5863.
J. P. Giesy, K. Kannan, Environ. Sci. Technol. 2002, 36, 146A-152A.
B. D. Key, R. D. Howell, C. S. Criddle, Environ. Sci. Technol. 1997, 31,
2445-2454.
[40] F. H. Hussein, R. Rudham, J. Chem. Soc. Faraday Trans. 1 1984, 80,
2817-2825.
[41] T. Hisatomi, K. Maeda, K. Takanabe, J. Kubota, K. Domen, J. Phys.
Chem. C 2009, 113, 21458-21466.
[5]
[6]
[7]
[8]
[9]
B. L. Edelbach, W. D. Jones, J. Am. Chem. Soc. 1997, 119, 7734-7742.
M. Aizenberg, D. Milstein, J. Am. Chem. Soc. 1995, 117, 8674-8675.
H. Yang, H. Gao, R. J. Angelici, Organometallics 1999, 18, 2285-2287.
R. J. Young, V. V. Grushin, Organometallics 1999, 18, 2942-296.
R. Baumgartner, K. McNeil, Environ. Sci. Technol. 2012, 46, 10199-
10205.
[42] T. Hisatomi, K. Miyazaki, K. Takanabe, K. Maeda, J. Kubota, Y. Sakata,
K. Domen, Chem. Phys. Lett. 2010, 486, 144-146.
[43] K. Shimura, K. Maeda, H. Yoshida, J. Phys. Chem. C 2011, 115, 9041-
9047.
[10] R. Baumgartner, G. K. Stieger,. K. McNeil, Environ. Sci. Technol. 2013,
47, 6545-6553.
[11] T. Braun, D. Noveski, M. Ahijado, F. Wehmeier, Dalton Trans. 2007,
3820-3825.
[12] H. Nakai, K. Jeong, T. Matsumoto, S. Ogo, Organometallics 2014, 33,
4349-4352.
[13] P. T. Anastas, J. C. Warner, Green Chemistry: Theory and Practice,
Oxford University Press, 1998.
[14] M.A. Fox and M.T. Dulay, Chem. Rev. 1993, 93, 341-357.
[15] M.R. Hoffman, S.T. Martin, W. Choi and D.W. Bahnemann, Chem. Rev.
1995, 95, 69-96.
[16] G. Palmisano, E. García-López, G. Marcì, V. Loddo, S. Yurdakal, V.
Augugliaro and L. Palmisano, Chem. Commun. 2010, 46, 7074-7089.
[17] W. Ji, T. Shen, J. Kong, Z. Rui, Y. Tong, Ind. Eng. Chem. Res. 2018, 57,
12766-12773.
[18] H. Hori, E. Hayakawa, H, Einaga, S. Kutsuna, K. Koike, T. Ibusuki, H.
Kiatagawa, R. Arakawa, Environ. Sci. Technol. 2004, 38, 6118-6124.
[19] H. Hori, Y. Nagaoka, M. Murayama, S. Kutsuna, Environ. Sci. Technol.
2008, 42, 7438-7443.
[20] E. Shang, Y. Li, J. Niu, S. Li, G. Zhang, Z. Wang, Chemosphere, 2018,
211, 34-43.
[21] B. Abada, T. E. G. Alivio, Y. Shao, T. E. O’Loughlin, C. Klemashevich, S.
Banerjee, A. Jayaraman, K.-H. Chu, Environmental Pollution 2018, 243,
637-644.
[22] J. Liu, D. J. V. Hoomissen, T. Liu, A. Maizel, X. Huo, S. R. Fernández, C.
Ren, X. Xiao, Y. Fang, C. E. Schaefer, C. P. Higgins, S. Vyas, T. J.
Strathmann, Environ. Sci. Technol. Lett. 2018, 5, 289-294.
[23] Z. Song, X. Dong, J. Fang, C. Xiong, N. Wang, X. Tang, Journal of
Hazardous Materials 2019, 377, 371-380.
[24] Y. Wu, Y. Li, C. Fang, C. Li, ChemCatChem 2019, 11, 2297-2303.
[25] K. Imamura, T. Yoshikawa, K. Hashimoto, H. Kominami, Appl. Catal. B:
Environ. 2013, 134-135, 193-197.
[26] M. Fukui, A. Tanaka, K. Hashimoto, H. Kominami, Chem. Lett. 2016, 45,
985-987.
[27] M. Fukui, H. Kouda, A. Tanaka, K. Hashimoto, H. Kominami,
ChemistrySelect 2017, 2, 2293-2299.
[28] H. Kominami, M. Higa, T. Nojima, T. Ito, K. Nakanishi, K. Hashimoto, K.
Imamura, ChemCatChem 2016, 8, 2019-2022.
[29] K. Nakanishi, A. Tanaka, K. Hashimoto, H. Kominami, Phys. Chem.
Chem. Phys. 2017, 19, 20206-20212.
[30] K. Fuku, K. Hashimoto, H. Kominami, Chem. Commun. 2010, 46, 5118-
5120.
[31] K. Fuku, K. Hashimoto, H. Kominami, Catal. Sci. Technol. 2011, 1, 586-
592.
[32] H. Kominami, T. Nishi, K. Fuku, K. Hashimoto, RSC Adv. 2013, 3, 6058-
6064.
[33] H. Kominami, H. Nishimune, Y. Ohta, Y. Arakawa, T. Inaba, Appl. Catal.
B: Environ. 2012, 112, 297-302.
[34] K. Nakanishi, R. Yagi, K. Imamura, A. Tanaka, K. Hashimoto, H.
Kominami, Catal. Sci. Technol. 2018, 8, 139-146.
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Entry for the Table of Contents
Organic fluorinated compounds were reductively converted to corresponding organic structures and fluoride ions with high stoichiometry
in aqueous 2-propanol suspensions of a metal-loaded TiO2 photocatalyst. A slight elevation of reaction temperature greatly improved
reaction rate and electron selectivity for defluorination. Catalytic defluorination process over Pt was the rate-determining step.
8
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