Characterization of the local structures of Ti-MCM-41 and their photocatalytic reactivity for the decomposition of NO into N2 and O2

Article abstract of DOI:10.1021/jp058240u

Ti-MCM-41 mesoporous molecular sieves were prepared at ambient temperature and were characterized by X-ray absorption near-edge structure and extended X-ray absorption fine structure, UV - vis, Fourier transform infrared spectroscopy, and photoluminescenc

Full text of DOI:10.1021/jp058240u

1680
J. Phys. Chem. B 2006, 110, 1680-1685
Characterization of the Local Structures of Ti-MCM-41 and Their Photocatalytic Reactivity
for the Decomposition of NO into N2 and O2
†
‡
§
†
‡
Yun Hu, Gianmario Martra, Jinlong Zhang, Shinya Higashimoto, Salvatore Coluccia, and
Masakazu Anpo*,†
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture UniVersity,
1
-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan, Dipartimento di Chimica IFM and NIS Center of Excellence,
UniVersit a` di Torino, Via P. Giuria 7, 10125 Torino, Italy, and Institute of Fine Chemicals, East China
UniVersity of Science and Technology, Shanghai 200237, P.R. China
ReceiVed: August 26, 2005; In Final Form: NoVember 16, 2005
Ti-MCM-41 mesoporous molecular sieves were prepared at ambient temperature and were characterized by
X-ray absorption near-edge structure and extended X-ray absorption fine structure, UV-vis, Fourier transform
infrared spectroscopy, and photoluminescence spectroscopic analyses. It was found that an increase in the Ti
content caused the structure of the Ti-oxides in Ti-MCM-41 to change from an isolated tetrahedral coordination
4+
to adjacent Ti-oxide species with Ti of tetrahedral coordination. The photocatalytic reactivity of these catalysts
for the decomposition of NO into N and O was found to strongly depend on the local structure of the
2
2
Ti-oxide species including their coordination and distribution, i.e., the charge transfer excited state of the
highly dispersed isolated tetrahedrally coordinated Ti-oxides act as the active sites for the photocatalytic
decomposition of NO into N and O .
2 2
1
. Introduction
are only a few reports on the photocatalytic reactivity of these
materials.
1
9-21
Nitric oxide is an especially harmful atmospheric pollutant
In the present work, characterizations of a series of Ti-MCM-
1 involving different loadings of Ti-oxides and their photo-
and the main cause of acid rain and photochemical smog, which
is emitted largely from the reaction of N2 with O2 in high-
temperature combustion processes. In recent times, the removal
of nitrogen oxides (NOx: NO, N2O, NO2), i.e., the direct
decomposition of NOx into N2 and O2, has been a great challenge
for many researchers.1
4
catalytic reactivities for the direct decomposition of NO into
N2 and O2 have been investigated by applying various molecular
spectroscopic techniques such as photoluminescence, UV-Vis,
X-ray absorbtion fine structure (XAFS) (X-ray absorbtion near
edge structure [XANES] and Fourier transform extended X-ray
absorbtion fine structure [FT-EXAFS]), and Fourier transform
infrared spectroscopy (FT-IR). Special attention has been
focused on understanding the relationship between the local
-6
Photocatalysis is an attractive, clean, safe, low-temperature
reaction and a nonenergy-intensive approach, especially for for
chemical waste remediation. As one of the most popular
photoactive catalysts, titanium dioxide has attracted extensive
interest due to its various properties such as its nontoxicity, high
stability, low cost, and high reactivity for the complete oxidation
of a wide variety of organic pollutants and toxic materials into
4
+
structures of the Ti centers in MCM-41-type materials and
their photochemical properties. Moreover, the prepared samples
were compared with Ti-HMS mesoporous molecular sieves,
which have a thicker pore wall, and also with the TS-1
microporous material to clarify the effect of the support on the
photocatalytic reactivity for NO decomposition.
7
-10
CO2 and H2O.
When the powdered TiO2 is applied as a
photocatalyst for the decomposition of NO in the absence of
O2, it was reported that the formation of N2O and NO2 is the
major reaction, and the formation of N2 and O2 scarcely
2. Experimental Section
proceeds.1
1,12
On the other hand, highly dispersed titanium
oxides prepared in zeolite cavities have been found to exhibit
a high and characteristic photocatalytic reactivity for the direct
2
.1. Catalysts Preparation. MCM-41 and Ti-MCM-41(X)
(Ti content as wt %: X ) 0.15, 0.60, 0.85, 2.00, 5.00) were
synthesized by following method reported in previous literature
using tetraethyl orthosilicate (TEOS) and tetraisopropyl ortho-
titanate (TPOT) as the starting materials and cetyltrimethyl-
decomposition of NO into N2 and O2 as well as the reduction
of CO2 with H2O into CH4 and CH3OH.1
2-16
And recently, Ti-
containing mesoporous materials such as MCM-41, MCM-48,
22
ammonium bromide as a structure directing agent. The molar
and HMS have attracted interest as active catalysts for the partial
composition of the reaction mixture was 1.0 Si/(0.01-0.10) Ti/
oxidation of alkenes.1
7,18
Such systems exhibit the advantage
0
.20 [C16H33N(CH3)3] Br/160 H2O. A high Ti/Si gel ratio was
of having a high dispersion of Ti-oxides due to their high internal
surface area and nanoscale pore reaction fields. However, there
used for this preparation due to the high solubility of Ti under
strongly acidic conditions. The reaction mixture was stirred for
5
days at room temperature and then the solid product obtained
*
Corresponding author phone, +81-72-254-9282; fax, +81-72-254-9910,
was filtered, washed, dried, and calcined at 823 K for 6 h with
airflow.
Ti-HMS (0.60 wt % as Ti) was prepared from a material
mixture having the following composition: 1.0 TEOS/0.0076
e-mail, anpo@chem.osakafu-u.ac.jp.
†
Osaka Prefecture University.
Universit a` di Torino.
East China University of Science and Technology.
‡
§
1
0.1021/jp058240u CCC: $33.50 © 2006 American Chemical Society
Published on Web 01/07/2006

Characterization of Local Structures of Ti-MCM-41
J. Phys. Chem. B, Vol. 110, No. 4, 2006 1681
TABLE 1: The Basic Physicochemical Properties of
Ti-containing Samples
BET surface
area/m g
sample (x)a
MCM-41
Ti-MCM-41(100)
Ti-MCM-41(25)
Ti-MCM-41(14)
Ti-MCM-41(10)
Ti-HMS(130)
TS-1(130)
wt % (solid)
a
/Å
2
-1
o
0
37.82
38.90
41.43
42.05
44.12
58.01
1027
997
915
879
780
759
457
0.15
0.60
0.85
2.00
0.60
0.60
a
x ) Si/Ti mol ratio in the starting solution.
TPOT/0.2 dodecylamine (DDA)/9.0 EtOH/160 H2O.22 TS-1
0.60 wt % as Ti) was prepared by a hydrothermal synthesis
(
2
3
method with a gel mixture of the following ratio: 1.0 TEOS/
0
3
.0076 TPOT/0.33 tetrapropylammonium hydroxide (TPAOH)/
0 H2O.
The Ti content in each sample synthesized was determined
using atomic absorption method. Prior to spectroscopic mea-
surements and photocatalytic reactions, the catalysts were
degassed at 723 K for 1 h and calcined in O2 (>20 Torr) at
7
23 K for 2 h, then degassed at 473 K for 2 h.
.2. Catalyst Characterizations. The powder X-ray diffrac-
2
tion (XRD) patterns of all the samples were recorded on a
Shimadzu XD-D1 using Cu KR radiation (λ ) 1.5406 Å).
Diffuse reflectance UV-vis spectroscopic measurements were
carried out on a Shimadzu UV-vis recording spectrophotom-
eter, UV-2200A. The photoluminescence and lifetimes were
measured at 295 K using a Spex 1943D3 spectrofluorophotom-
eter and an apparatus for lifetime measurements, respectively.
The XAFS (XANES and EXAFS) spectra were obtained at
the BL-7C facility of the Photon Factory at the National
Laboratory for High-Energy Physics, Tsukuba, Japan. The
storage ring energy was set at 2.5 GeV and positron current
was 260-370 mA. The Ti K-edge absorption spectra were
recorded in the fluorescence mode at 295 K. The EXAFS data
were examined by an EXAFS analysis program, Rigaku
Figure 1. XANES and FT-EXAFS spectra of Ti-MCM-41 with various
Ti contents: (A, a) 0.60; (B, b) 2.0; (C, c) 5.0 wt %.
TABLE 2: Curve-fitting Results of FT-EXAFS Data for
Ti-MCM-41 and Reference Samples
2
2 c
sample
shell
C. N.a
R (Å)b
σ (Å )
Ti-MCM-41(0.6)
Ti-MCM-41(2.0)
Ti-O
Ti-O
Ti-O
Ti-O
Ti-O
3.8
4.1
5.8
4
1.80
1.81
1.93
1.76
1.96
0.004
0.004
0.004
0.0009
0.0009
Ti-MCM-41(5.0)
i
Ti(OPr )
P-25
4
6
a
Coordination number. b Bond distance. c Debye-Waller factor.
ions into the framework structure of MCM-41 since the Ti-O
bond distance is longer than the Si-O bond distance, and the
thickening of the pore wall due to a transition metal promoted
3
EXAFS. Fourier transform was performed on k -weighted
-
1
EXAFS oscillations in the region of 3-10 Å to obtain the
radial structure function.
22
a cross-linking of the amorphous silica walls. A similar
observation was made of the titanium-incorporated ZSM-5 and
The FT-IR spectra were recorded with a resolution of 4 cm-
at 295 K using a JASCO FT/IR-660 spectrometer. The powdered
samples were compressed into thin disk-shaped pellets and
mounted in a purpose-made quartz cell attached to a vacuum
line similar to that used for photoluminescence in situ analyses.
1
17,22,24
HMS frameworks.
The XANES spectra of the Ti-oxide catalyst at the Ti K-edge
show several well-defined pre-edge peaks which are related to
the local structure surrounding the Ti atom. Also, the relative
intensity of the pre-edge peak provides useful information on
the coordination number surrounding the Ti atom. Figure 1
shows the XANES spectra of the Ti-MCM-41 catalysts having
various Ti-contents. Ti-MCM-41 (0.6 wt %) and Ti-MCM-41
2
.3. Photocatalytic Reactions. The photocatalytic decom-
positions of NO were performed under a closed system.
Typically, 80 mg of the sample was loaded in a quartz cell with
a flat bottom (30 mL), which was connected to a vacuum system
(2.0 wt %) exhibit an intense single pre-edge peak which can
-5
-1
(
10 Torr range). After pretreatment, NO (180 µmol_Eg-cat. )
be assigned to the so-called 1s-3d transition, consistent with
was introduced into the quartz cell. UV irradiation of the sample
was carried out using a high-pressure mercury lamp (100 W)
through a UV cut filter (λ > 240 nm) at 295 K, and a water
bath was employed to keep the flat bottom at a constant tem-
perature. The products were analyzed by gas chromatography.
12,25
the tetrahedral coordination for the Ti atoms.
These two
samples show a pre-edge peak of the same intensity and position,
indicating that they have the same titanium coordination. On
the other hand, Ti-MCM-41(5.0 wt %) shows three characteristic
weak pre-edge peaks due to the presence of the crystalline
anatase TiO2 with an octahedral coordination.
Figure 1 also shows the FT-EXAFS spectra of the catalysts
while Table 2 shows the results obtained by the curve fitting
analysis of the EXAFS spectra. All of the catalysts exhibit a
strong peak at around 1.6 Å (without phase shift correction)
that can be assigned to the neighboring oxygen atoms (Ti-O).
The Ti-MCM-41 (0.6 wt % and 2.0 wt %) catalysts exhibited
only a Ti-O peak while Ti-MCM-41 (5.0 wt %) exhibited an
additional strong peak at around 2.7 Å due to the
3
. Results and Discussion
The powder XRD patterns showed that the Ti-MCM-41
catalysts have a typical hexagonal mesoporous MCM-41
structure. An increase in the d spacing or (a0 values) was
observed with an increase in Ti loading for Ti-MCM-41 and
the results obtained are shown in Table 1. The increase in the
unit cell parameter compared to a siliceous analogue, MCM-
41, can be taken as an indication of the incorporation of titanium

1682 J. Phys. Chem. B, Vol. 110, No. 4, 2006
Hu et al.
Figure 3. FT-IR spectra of NH
3
molecules adsorbed on MCM-41 (a)
and Ti-MCM-41 (b, 0.15; c, 0.60; d, 0.85; e: 2.0 Ti wt %) observed
after admission of 10 Torr NH
for 1 min.
3
and subsequent outgassing at 295 K
upon outgassing of the samples at 295 K, exhibiting a spectrum
similar to that of the bare MCM-41. The complexes formed
with NH3 adsorption on MCM-41 were also formed on
Ti-MCM-41 (Figure 2B). Besides these bands, irreversible
absorptions at 3402, 3298, and 1608 cm assigned, respectively,
to the asymmetric stretching, symmetric stretching, and asym-
metric bending modes of NH3 adsorbed on the Ti(IV) Lewis
Figure 2. FT-IR spectra of NH
3
molecules adsorbed on: (A) MCM-
was decreased
41 and (B) Ti-MCM-41 (2.0 wt %). The pressure of NH
3
-1
from 10 Torr to 0 Torr by outgassing at 295 K for 1 min. Inset: (a')
Original IR spectra in the high-frequency region of the sample in a
vacuum and (b') in the presence of 10 Torr NH .
3
acid sites (Scheme 1B) were observed. The bands at around
SCHEME 1
-1
1
725, 1472, 1660 cm can be assigned to the bending modes
+
of NH4 formed by the protonation of the ammonia molecules
by the acidic surface hydroxyl groups. The first two bands were
observed to shift to 1708 and 1453 cm , respectively, after
outgassing of the sample at 295 K. The band at 1553 cm can
be attributed to the Ti-NH2 or Si-NH2 bending mode formed
by the irreversible reaction of NH3 with the Si-O-Ti bridges
or distorted surface Si-O-Si bridges formed due to the
-
1
-
1
2
7
incorporation of Ti into the Si-O-Si networks. Bands for
the NH3 absorbed on Ti-MCM-41 were not observed on TiO2.
Figure 3 shows the IR spectra resulting from the adsorption
of 10 Torr NH3 on MCM-41 and Ti-MCM-41 (0.15-2.0 wt
%), followed by outgassing of the system at 295 K for 1 min.
The increase in the Ti content led to an increase in the intensity
of this band, indicating that, up to a Ti content of 2.0 wt %, the
amount of tetrahedrally coordinated Ti(IV) sites exposed at the
surface walls of the channels of the Ti-MCM-41 materials
increased with the Ti content.
Nevertheless, as can be seen in Figure 4, the diffuse
reflectance UV-Vis spectra exhibited different degrees of
dependence on the Ti-loading. In fact, besides a progressive
increase in intensity, the increase in the Ti content resulted in
a progressive shift of the absorption maximum from 205 to 208
nm [Ti-MCM-41(0.15) and Ti-MCM-41(0.60)] (Figure 4a, b)
to 215 nm [Ti-MCM-41(2.0)] (Figure 4d), while a shoulder at
ca. 240 nm became evident in the spectrum of Ti-MCM-41(0.85)
(Figure 4c), showing a larger contribution in the spectrum of
aggregation of the Ti oxide species. The results of the curve-
fitting analysis confirmed that even for a Ti content as high as
2.0 wt %, the Ti(IV) centers are located in tetrahedral coordina-
tion with four oxygen atoms in the Ti-MCM-41 catalysts.
Ammonia adsorption was used both to monitor the acidity
of the surface hydroxyl groups and to elucidate the local
environment of the Ti(IV) centers. Figure 2 shows the FT-IR
spectra for ammonia adsorption on MCM-41 and Ti-MCM-41
at room temperature. At high NH3 pressure, a band at 1635
-1
cm appeared on the siliceous MCM-41 due to the asymmetric
bending mode of the NH3 molecules adsorbed on the Si-OH
-
1
groups, while bands at 3407 and 3323 cm appeared due to
the asymmetric and symmetric stretching vibrations of the
adsorbed NH3, respectively (Scheme 1A).2 Such adsorptions
were almost completely reversible at 295 K since all of the IR
features associated with the complex completely disappeared
6,27

Characterization of Local Structures of Ti-MCM-41
J. Phys. Chem. B, Vol. 110, No. 4, 2006 1683
Figure 4. Diffuse reflectance UV-vis spectra of Ti-MCM-41 with
different Ti content: (a) 0.15; (b) 0.60; (c) 0.85; and (d) 2.0 wt %.
Figure 6. The photoluminescence spectra of Ti-MCM-41 with different
Ti content: (a) 0.15; (b) 0.60; (c) 0.85; (d) 2.0 wt %, measured at 295
K.
SCHEME 2
loadings. Ti-HMS and TS-1 shows a similar UV absorption to
Ti-MCM-41 centered at around 210 nm, which indicates that
the Ti-oxides exist mainly in isolated tetrahedral coordination,
as seen in Figure 5. For Ti-HMS and TS-1, the absorption at
around 250 nm is slightly more intensive in yield than that of
Ti-MCM-41, and this may be due to the lower dispersibility of
the isolated tetrahedral Ti-oxides for Ti-HMS and TS-1.
As shown in Figure 6, Ti-MCM-41 exhibits a typical
photoluminescence spectrum at around 400-600 nm upon
excitation of its charge-transfer band at around 230-260 nm at
Figure 5. Diffuse reflectance UV-vis spectra of (a) Ti-MCM-41; (b)
TS-1; and (c) Ti-HMS (0.6 wt %).
2
95 K, coinciding well with those previously observed for the
the Ti-MCM-41(2.0) sample by reflecting a larger extent of the
Ti-oxide species observed at ca. 250 nm. All the components
shown in the absorption ranges are associated with the ligand-
to-metal charge transfer processes of the tetrahedrally coordi-
isolated tetrahedrally coordinated Ti-oxides species highly
32,33
dispersed in silica matrixes.
The photoluminescence spectra
can be attributed to the reverse radiative decay process from
the charge transfer excited triplet to the ground state of the
isolated Ti-oxides with a tetrahedral coordination. The intensity
of the photoluminescence increased with an increase in the Ti
content up to 0.60 wt %, and then sharply decreased with a
higher Ti content, as can be seen in Figure 6. Furthermore, it
was found that an increase in the Ti content from 0.60 wt % to
0.85 wt % led to a decrease in the phosphorescence lifetime
from 0.1 to 0.025 ms. On the basis of these data, when the
Ti-MCM-41 materials are photoexcited by UV light, only the
isolated tetrahedrally coordinated Ti-oxide species (Scheme 2,
A), which are more abundant for Ti contents up to 0.60 wt %,
may produce the excited-state long enough to allow the
appearance of photoluminescence as a radiative decay to the
2
-
nated Ti-oxide species involving an electron transfer from O
to Ti4 to form its charge-transfer excited state, (Ti -O )*.
+
3+
-
28,29
The absorption bands at 200-210 nm and 220-230 nm are
attributed to the isolated tetrahedrally coordinated Ti-oxide
species with different framework sites [Ti(OSi)4 and Ti(OH)-
(
OSi)3], while the band at ca. 250 nm is assigned to the
18,30,31
tetrahedral titanium dimers or small oligomers.
It should
be noted that the band of UV absorption depends not only on
the structure but also on the dispersibility of the Ti-oxides.
Taking the XAFS results into consideration, it is assumed that
the band at 240-250 nm is associated with a decrease in the
dispersibility of the isolated tetrahedral Ti-oxides at higher Ti

1684 J. Phys. Chem. B, Vol. 110, No. 4, 2006
Hu et al.
Figure 8. Relationship among the yields of N
intensities of the IR band of δ asymNH
b) photoluminescence spectra of Ti-MCM-41 with various Ti contents.
2
2
and N O, and the
3
adsorbed on (a) Ti(IV) and
(
Figure 7. Effect of the addition of NO on the photoluminescence
spectra of Ti-MCM-41 (0.60 wt %) measured at 295K. Amount of
added NO (in Torr): (a) 0; (b) 0.1; (c) 1.0; (d) 6.0; (e) 20; (f) degassed
after (e).
ground state. However, a decrease in the photoluminescence
intensity and lifetime for Ti loadings above 0.6 wt % was
observed, probably due to the nonradiative deactivation of the
photoluminescence arising from an efficient energy transfer to
a nearby Ti when the dispersibility of isolated tetrahedral Ti
oxide species decrease at higher Ti content. These results suggest
that the Ti oxide species exist in different microenvironments
from that of highly dispersed isolated tetrahedral coordination.
By considering these results, the other structure of the Ti oxide
species, different from the isolated tetrahedral species for Ti-
MCM-41 which has a high Ti content, can be proposed as
Structure B (Scheme 2, B), i.e., the tetrahedral Ti oxide species
are adjacent to other Ti-oxide species.
Figure 7 shows that the addition of NO molecules onto the
Ti-MCM-41 catalyst leads to an efficient quenching of the
photoluminescence as well as a shortening of its lifetime, their
extents depending on the amount of added gas, indicating that
the added NO molecules interact with the Ti-oxide species in
its charge transfer excited triplet state. After degassing of NO
at 295 K for 30 min, the photoluminescence recovered to the
original intensity level, suggesting that the NO dynamically or
collisionally interact with the excited Ti-oxides species.
Figure 8 shows the relationship between the intensity of the
δasymNH3 band adsorbed on the Ti(IV) centers and the yield of
the photoluminescence spectra of Ti-MCM-41 with varying Ti-
contents. The intensity of the δasymNH3 band (Figure 8a)
monitors the total amount of the tetrahedral Ti-oxide species
exposed at the surface walls of the Ti-MCM-41 channels, and
the yield of the photoluminescence spectrum (Figure 8b)
monitors the amount of the isolated tetrahedrally coordinated
Ti-oxide species. The yield of the photoluminescence increased
with an increase in the Ti content up to 0.6 wt % as well as an
increase in the intensity of the NH3 IR band; however, it
decreased with a further increase in Ti content even though the
amount of the tetrahedral Ti-oxide species increased. From these
results, the Ti-MCM-41 catalyst is proposed to include only an
isolated tetrahedral Ti-oxide species with low Ti content while,
2 2
Figure 9. The yields of N and N O formation in the photocatalytic
decomposition of NO on Ti-MCM-41, Ti-HMS, and TS-1 (0.6 wt %)
under UV irradiation at 295 K.
with higher Ti content, the isolated and also adjacent oxides
with a tetrahedral Ti-oxide species are included.
UV irradiation of Ti-MCM-41 in the presence of NO was
observed to lead to the formation of N , O , and N O at 295 K,
2
2
2
their yields increasing in proportion to the irradiation time. No
products could be detected either under dark conditions or in
UV light irradiation of the siliceous MCM-41. These results
clearly indicate that the reaction proceeds photocatalytically on
Ti-MCM-41. Figure 8 also shows the effect of the Ti content
on the photocatalytic reactivity of the decomposition of NO into
N2 and O2. The yield and selectivity for the formation of N2 in
the reaction is seen to be the highest for Ti-MCM-41 (0.60 Ti
wt %), and an increase in Ti content led to a decrease in the
reactivity, showing a good correspondence with the intensities
of the photoluminescence spectra due to the isolated tetrahedrally
coordinated Ti-oxide species. The amount of the isolated
tetrahedrally coordinated Ti-oxide species and not the total
amount of the tetrahedral Ti-oxide species exposed at the surface
walls of the Ti-MCM-41 channels were, thus, seen to play an
important role in the reaction. These results indicate that only
the highly dispersed isolated tetrahedrally coordinated Ti-oxide

Characterization of Local Structures of Ti-MCM-41
J. Phys. Chem. B, Vol. 110, No. 4, 2006 1685
species acted as the active sites in the photocatalytic decomposi-
tion of NO into N2 and O2.
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(
The photocatalytic decomposition of NO into N2 and O2
proceeded on both Ti-mesoporous and microporous silicate
catalysts, as shown in Figure 9. It is clear that the photocatalytic
reaction rate and selectivity for the formation of N2 strongly
depended on the type of catalyst. Ti-MCM-41 exhibited much
higher activity and selectivity for the formation of N2 than Ti-
HMS and TS-1. It was also found that the yields of N2 have a
good relationship with the intensities of the photoluminescence
spectra due to the isolated tetrahedrally coordinated Ti-oxide
species. Considering the higher surface area of Ti-MCM-41,
the higher activity and selectivity for the formation of N2
observed for this catalyst may be a contribution of the high
dispersion state of the Ti-oxide species.
1
(
(
7) Photocatalytic Purification and Treatment of Water and Air; Ollis,
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(
(
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(
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4. Conclusions
(
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In situ characterizations of Ti-MCM-41 prepared at ambient
(
temperature by combined UV-vis, XAFS, photoluminescence,
and FT-IR spectroscopies were seen to be very important in
gaining an understanding of the local structures of catalysts with
different Ti contents. The local structures of these photocatalysts
were found to be very sensitive to the differences in the
molecular environment of the reaction field. For catalysts with
a Ti content of up to 0.60 wt %, the isolated tetrahedrally
coordinated Ti-oxide species existed as the major species, while
in catalysts with higher Ti content, the adjacent Ti-oxide species
with Ti in tetrahedral coordination were seen to be over-
whelming in Ti-MCM-41. Their photocatalytic reactivity for
the decomposition of NO into N2 and O2 was found to strongly
depend on the local structure of the Ti-oxide species, i.e., the
coordination as well as the dispersibility of the Ti-oxides. The
yield and selectivity for the formation of N2 corresponded well
with the yield of the photoluminescence from the charge-transfer
excited state of the isolated tetrahedrally coordinated Ti-oxides
species, indicating that only these species were involved in the
photocatalytic reactions. Ti-MCM-41 showed higher photocata-
lytic reactivity than Ti-HMS and TS-1 for the decomposition
of NO.
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Acknowledgment. This work has been supported by a
Grant-in-Aid for Scientific Research on Priority Area (#417)
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) of Japan, and M.A. expresses his thanks
for their support. Y.H. acknowledges and thanks the Japan
Society for the Promotion of Science (JSPS) for their kind
financial support.
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