Adsorption-driven photocatalytic activity of mesoporous titanium dioxide

Article abstract of DOI:10.1021/ja053265s

Titanium dioxide with a mesoporous structure, when photoactivated in water, demonstrates an unprecedented photocatalytic activity, driven strongly by an adsorption degree of molecules onto the catalyst surface, which promotes a preferential conversion of a well-adsorbed molecule. This catalyzes a selective transformation of a well-adsorbed molecule into a less-adsorbed molecule, so-labeled "stick-and-leave" transformation, which promotes a direct hydroxylation of benzene to phenol, one of the most difficult synthetic reactions, with very high selectivity (> 80%) and using water as a source of oxidant. Copyright

Full text of DOI:10.1021/ja053265s

Published on Web 08/27/2005
Adsorption-Driven Photocatalytic Activity of Mesoporous Titanium Dioxide
Yasuhiro Shiraishi,* Naoya Saito, and Takayuki Hirai
Research Center for Solar Energy Chemistry, and DiVision of Chemical Engineering, Graduate School of
Engineering Science, Osaka UniVersity, Toyonaka 560-8531, Japan
Received May 18, 2005; E-mail: shiraish@cheng.es.osaka-u.ac.jp
Development of highly selective methods for photochemical
organic synthesis, driven by a heterogeneous catalyst, is one of the
2
content of the catalyst. In contrast, mTiO (65, 61, 57, 37)
demonstrates peculiar relationships (Figure 1B): (i) conversions
of substrates with low D value (<0.05) are nearly zero; and (ii)
conversions of substrates with higher D value are obviously higher.
None of the properties of catalysts [band gap energy (Ebg), particle
size, and surface hydrophobicity (monolayer adsorption capacity
1
biggest challenges in chemistry. Much investigation has been made
2
based on systems using a semiconductor, titanium dioxide (TiO
In most cases, photocatalytic reactions on TiO
2
).
2
proceed via the
following steps: (i) generation of electron-positive hole pairs by
an absorption of supra-band gap photons; (ii) production of hydroxyl
radicals (•OH) via reaction of the hole with surface -OH groups
2
of H O, qH2O)] or substrates [oxidation potential, HOMO level, and
hydrophobicity (log P)], which usually affect the catalytic activity
1
0
or adsorbed H
OH. However, oxidation by •OH is nonselective, resulting in
insufficient product selectivity. Recently, two innovative methods
were proposed for improvement of catalytic selectivity on TiO
i) creation of molecular recognition site (â-cyclodextrin) for
2
O molecules; and (iii) oxidation of substrates by
of nTiO
2
,
2
can explain the unusual activity of mTiO (Table 1;
7
•
Table S1 ), leading to the degree of substrate adsorption as the
strong factor.
2
:
On mTiO
(Figure 1B), as is also observed on nTiO
because the photoformed electron and positive hole recombine
2
(0), conversion for all tested substrates is nearly zero
(
2
(0) (Figure 1A). This is
3
preferential decomposition of targeted substrate; and (ii) creation
easily on the amorphous surface.1 AggTiO
0a
(36), which was
6c
of nonsemiconducting micropores, leading to selective decomposi-
2
tion of large molecules on an external semiconducting TiO
However, a photocatalytic system based on TiO , enabling selective
transformation” of molecules and production of “fine chemicals”,
2
surface.4
prepared by aggregation method without calcination and which
7
2
contains high surface area but almost no mesopores (Figure S2 ),
“
does not show such adsorption-dependent profile (Figure 1A).
had not been developed.
Our system employs TiO
Calcination of aggTiO
2
leads to a formation of mesopores due to
2
2
with a mesoporous structure (mTiO ).
sintering of the particles, along with an increase in anatase content
So far, various mTiO
2
with different pore size and surface area
[mTiO
adsorption-dependent profile (Figure 1B), as do mTiO
These findings strongly suggest that the unusual activity of mTiO
is triggered by a combination of anatase phase and mesopores on
the catalyst. It is noted that mTiO (57), prepared by aggregation
method, has more disordered mesostructure than mTiO
2
(57)]6 (Figure S2 ). The use of mTiO
c
7
2
(57) shows an
have been synthesized by means of (i) surfactant-templating
2
(65, 61, 37).
5
method and (ii) aggregation method consisting of an aggregation
2
of nanosized TiO
A few reports revealed that mTiO
nonporous TiO (nTiO ), catalyzes a rapid photodecomposition of
some substrates, such as 2,4,6-trichlorophenol, n-pentane,
particles followed by sintering of the particles.6
2
2
, as compared to a conventional
2
6
2
2
2
(65, 61, 37)
5c
5f,6a,e
7
and
acetone.6 These enhanced activities are, however, inspired simply
by the high surface area of mTiO ; structural advantage of the
mesoporous system is left unexploited. Herein, we report an
unprecedented photocatalytic activity of mTiO driVen by an
(Figure S1 ), indicating that a well-ordered porous system is not
necessary for the catalytic activity.
b,f
2
ESR spin-trap technique with R-phenyl-N-tert-butylnitrone (PBN)
was used to clarify the catalytic mechanism on mTiO
2
involving
containing anatase phase, when
suspended in aqueous PBN solution, gives PBN-•OH spin adduct
signals, as also for nTiO
(100) (Figure 2A).11 ESR silence with
nTiO (0) and mTiO (0) (Figure 2A) indicates that •OH is formed
mainly on the anatase phase. Considering the large internal
surface area of mTiO , most of anatase phase should exist within
the mesopores; meaning, •OH production on mTiO must occur
mainly on the internal surface. Meanwhile, substrate adsorption onto
mTiO should take place mainly within the pores of high surface
7
2
•OH. UV irradiation to mTiO
2
adsorption degree of molecules onto the catalyst surface, which
promotes a preferential conversion of well-adsorbed molecules. We
highlight a successful application of this property to selective
transformation of a well-adsorbed molecule into a less-adsorbed
molecule, labeled as “stick-and-leave” transformation, which
enables a transformation of benzene into phenol, one of the most
difficult synthetic reactions, with very high selectivity (>80%).
2
2
2
10a
2
2
We used five kinds of mTiO
2
(x) [x ) anatase content (%)] with
2
pores of >3 nm diameter (Table 1), prepared by the above two
methods, and nTiO (x) catalysts for comparison. XRD analysis
2
area. Therefore, the concentrations of substrates with higher D value
inside the pore are expected to be higher than those with lower D
value; the substrates with higher value may be attacked effectively
by the •OH inside the pore.
7
confirmed that all catalysts consisted of anatase and amorphous
7
phases (Figure S1). Catalytic conversions were carried out by
photoirradiation (λ > 320 nm) to a buffered aqueous solution (pH
PBN is scarcely adsorbed on all of the catalysts (D ) 0). On the
basis of the above assumption, the PBN concentration within the
pore is expected to be lower than that in bulk solution. Figure 2B
shows the PBN-•OH intensity, obtained by double integration of
the lowest magnetic field signal of the PBN-•OH (Figure 2A).
7
; 10 mL) containing respective substrate (20 µmol) and catalyst
7
(10 mg). The degree of substrate adsorption onto the catalyst was
8
defined as the distribution ratio, D, based on the adsorption test.
Figure 1 shows a relationship between D and conversion of 15
The intensity on nTiO
content, as expected. However, the intensity on mTiO
mTiO (61) is obviously lower than that on nTiO (58), although
anatase content is higher. As reported, •OH is trapped on the TiO
2
increases linearly with an increase in anatase
9
2
(65) and
2
2
12
2
12820
9
J. AM. CHEM. SOC. 2005, 127, 12820-12822
10.1021/ja053265s CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Properties of Catalysts
anatase
(%)a
A
BET
A
ext
D
p
E
bg
particle size
m)f
q
H2O
catalyst
(m2g-1)b
(m2g-1)c
(nm)d
(eV)e
(
µ
(molecules nm-2)g
synthesis
mTiO2(65)
mTiO2(61)
mTiO2(57)
mTiO2(37)
mTiO2(0)
nTiO2(100)
nTiO2(58)
nTiO2(0)
65
61
57
37
0
>99
58
0
171
308
164
161
135
50
81
127
297
25
33
29
17
38
48
76
112
44
4.6
3.2
3.0
3.4
6.8
3.01
3.06
3.03
3.08
2.89
3.08
3.08
3.10
3.01
2.3
2.2
2.1
1.8
1.9
7.0
5.6
1.9
3.3
8.0
4.8
8.1
6.7
8.4
6.6
7.7
10.4
12.9
h
h
i
h
h
j
j
aggTiO2(36)
36
k
a
10a
b
BET surface area. c External surface area determined by the t-plot volumetric measurements with N2.
Measured according to literature procedure.
d
e
f
Determined by the DH method. Measured by a plot of the Kubelka-Munk function versus the energy of light absorbed. Determined by a laser scattering
technique. Monolayer adsorption capacity of H2O, as determined by dividing the amount of H2O adsorbed on the catalyst by ABET. By surfactant templating
method.
g
h
5
e,7
i
6c,7
j
By calcination of nTiO2(0).7 By aggregation method without calcination.
k
6c,7
By aggregation method.
Figure 2. (A) ESR spectra of PBN-•OH spin adduct obtained by
photoirradiation to a catalyst-suspended aqueous PBN solution. (B) The
PBN-•OH intensity obtained by double integration of the lowest magnetic
field signal of the adduct, where the intensity obtained with nTiO2(100) is
set as 1. (C) Photoconversion (0.5 h) of phenol (1) on respective catalyst
as a function of anatase content of the catalyst. The catalysts are: (a) mTiO2-
(
(
65), (b) mTiO2(61), (c) mTiO2(57), (d) mTiO2(37), (e) mTiO2(0), (f) nTiO2-
100), (g) nTiO2(58), (h) nTiO2(0), and (i) aggTiO2(36).
solution, thus leading to higher PBN-•OH intensity (Figure 2B).
When conversion of 1, which is scarcely adsorbed on all of the
catalysts (D ) 0) as also for PBN, is plotted against anatase content
of the catalyst (Figure 2C), the obtained profile is similar to that
of the PBN-•OH intensity (Figure 2B). This fact explains the full
Figure 1. Relationship between the distribution ratio, D, and substrate
conversion (0.5 h) on (A) nTiO2(x) (green open circle, x ) 100; red open
circle, 58; black open circle, 0) and aggTiO2(36) (blue open circle) and
2
picture of the adsorption-driven activity of mTiO (Figure 1B): a
(B) mTiO2(x) (blue open circle, x ) 65; green open circle, 61; solid green
well-adsorbed molecule (high D) diffuses well inside the pores and
reacts easily with •OH, whereas a less-adsorbed molecule (low D)
scarcely enters the pores and cannot react with •OH. As shown in
circle, 57; red open circle, 37; black open circle, 0). Substrates used are
denoted as numbers in the figure: 1, phenol; 2, 2,4,6-trichlorophenol; 3,
chlorohydroquinone; 4, 2,4-dichlorophenol; 5, 3-chlorophenol; 6, 4-chlo-
rophenol; 7, 2-chlorophenol; 8, benzyl alcohol; 9, 2,4-dichlorophenoxyacetic
acid; 10, p-cresol; 11, phenoxyacetic acid; 12, 1,2,4-trihydroxybenzene; 13,
Figure 1A, on aggTiO
and conversions are lower than those on mTiO
aggTiO (36) shows rather lower PBN-•OH intensity and conver-
sion of 1 than does mTiO (37) (Figure 2). This may be because
substrates cannot diffuse smoothly inside the narrow micropores
on aggTiO , indicating that the presence of “mesopores”, which
2
(36), no adsorption dependence is observed,
2
(37). In addition,
1
,3,5-trihydroxybenzene; 14, 4-chlorophenoxyacetic acid; 15, 2,6-bis-
(
hydroxymethyl)-p-cresol. Detailed data for D and conversion of substrates
2
7
on mTiO2 are shown in Table S1.
2
surface and is converted to an inactive surface -OH group in a
2
near-diffusion-controlled rate. Calculation based on a simple
cylindrical model (see discussion in the Supporting Information)
allow a smooth diffusion of molecules, is necessary for the
adsorption-driven activity.
7
13
reveals that diffusion distance of •OH formed inside the mesopore
The unique activity of mTiO
transformation of several molecules (Table 2). Photoirradiation of
2
is applicable to a selective
on mTiO
2
is 1.3-2.4 nm, which is smaller than the pore diameter
(Table 1), suggesting that •OH formed inside the pore is
of mTiO
2
2
11 on nTiO gives rise to 1 as an initial product (runs 1, 2) via
deactivated rapidly and, hence, scarcely diffuses out of the pores.
These findings clearly support the above assumption: on mTiO
most of PBN exists out of the pores and cannot react sufficiently
with •OH formed inside the pore. In contrast, on nTiO , PBN reacts
easily with •OH formed on the external surface facing the bulk
•OH-induced ether bond cleavage,14 but the selectivity is not so
high (<35%) because sequential oxidation of 1 by •OH occurs on
2
,
15
nTiO
twice the selectivity (72%). This is because 1 is scarcely adsorbed
on mTiO (D ) 0) and, hence, suppresses the sequential oxidation
2 2
. However, on mTiO (run 3), 1 is obtained with more than
2
2
J. AM. CHEM. SOC.
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C O M M U N I C A T I O N S
Table 2. Photocatalytic Transformation of Various Substratesa
a
Reaction conditions were: reactants, 20 µmol; photoirradiation time, 2 h; catalyst, 10 mg; buffered (pH 7) aqueous solution, 10 mL; temperature, 313
K. b Photoirradiation time, 6 h. Reactant, 0.5 mmol (which is not fully dissolved in the aqueous solution).
c
of 1. For chloro-substituted phenoxyacetic acids (14 and 9) on nTiO
runs 4, 5 and 7, 8), the initially formed chlorophenols (6 and 4)
are converted rapidly via substitution of -Cl by -OH to form
2
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2
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2
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2
, the initially
(
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7
proposed so far (<21% selectivity) (Table S2 ). As well-known,
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selectivity of 1 than the system proposed here, but they require
(7) See Supporting Information.
(
8) D ) (C
0
- C
e
)/C
e
, where C
e
0
denotes the initial concentration of substrate
in solution and C denotes the equilibrium concentration of the substrate
in the solution after stirring with catalyst (313 K; 0.5 h).
(
9) Almost no reaction occurs in the presence of D-mannitol, a typical •OH
scavenger (100-fold molar excess based on the substrate).
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7
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(
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and environmentally friendly way.
13) Catalytic activity of mTiO
area; mTiO (65) clearly shows higher substrate conversions than mTiO
(37) (Figure 1B). This is because the former contains a higher quantity
of anatase phase and hence produces a higher quantity of •OH. mTiO
61) shows higher substrate conversions than mTiO (65) (Figure 1B),
2
depends on its anatase content and surface
2
2
-
2
-
Acknowledgment. This work is supported by the Grant-in-Aids
(
2
despite the lower anatase content. This is because the former, having twice
larger surface area than the latter (Table 1), has larger anatase surface
area.
for Scientific Research (No. 15360430) and on Priority Areas
“
Fundamental Science and Technology of Photofunctional Inter-
faces (417)” (No. 17029037) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan (MEXT).
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(
4
Supporting Information Available: Materials and Methods,
Discussion, Tables S1-S4, and Figures S1 and S2. This material is
available free of charge via Internet http://pubs.acs.org.
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