Relationship between the local structures of titanium oxide photocatalysts and their reactivities in the decomposition of NO

Article abstract of DOI:10.1021/jp012080e

Photocatalysts involving titanium oxides were prepared by an ion-exchange and impregnation method in which zeolites with different Si/Al ratios were used as supports. Those photocatalysts with low Si/Al ratios exhibited high and unique photocatalytic reactivity for the direct decomposition of NO into N₂, O₂, and N₂O at 275 K. In sim photoluminescence, diffuse reflectance absorption, and XAFS investigations indicated that the titanium oxide species are highly dispersed within the zeolite and exist in tetrahedral coordination. The charge transfer excited state of these highly dispersed titanium oxide species plays a significant role in the decomposition of NO with a high selectivity for the formation of N₂ and O₂, while the catalysts involving the aggregated octahedrally coordinated titanium oxide species show a high selectivity to produce N₂O, being similar to the reactions observed on the powdered bulk TiO₂ photocatalysts.

Full text of DOI:10.1021/jp012080e

J. Phys. Chem. B 2001, 105, 8395-8398
8395
Relationship between the Local Structures of Titanium Oxide Photocatalysts and Their
Reactivities in the Decomposition of NO
Jinlong Zhang,† Yun Hu, Masaya Matsuoka, Hiromi Yamashita, Madoka Minagawa,
,‡
†
†
†
†
§
,†
Hisao Hidaka, and Masakazu Anpo*
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture UniVersity, Gakuen-cho,
Sakai, Osaka 599-8531, Japan, and Frontier Research Center for the Global EnVironment Protection,
Meisei UniVersity, 2-1-1 Hodokubo, Hino, Tokyo 191, Japan
ReceiVed: May 31, 2001
Photocatalysts involving titanium oxides were prepared by an ion-exchange and impregnation method in
which zeolites with different Si/Al ratios were used as supports. Those photocatalysts with low Si/Al ratios
exhibited high and unique photocatalytic reactivity for the direct decomposition of NO into N , O , and N O
2 2 2
at 275 K. In situ photoluminescence, diffuse reflectance absorption, and XAFS investigations indicated that
the titanium oxide species are highly dispersed within the zeolite and exist in tetrahedral coordination. The
charge transfer excited state of these highly dispersed titanium oxide species plays a significant role in the
decomposition of NO with a high selectivity for the formation of N
aggregated octahedrally coordinated titanium oxide species show a high selectivity to produce N
similar to the reactions observed on the powdered bulk TiO photocatalysts.
2
and O
2
, while the catalysts involving the
2
O, being
2
Introduction
relationship between the photocatalytic properties of titanium
oxides prepared within zeolites and the role of their local
structures.
In recent years, much attention has been focused on how to
reduce NO from the air, and research into the removal of NOx
is one of the most important and urgent fields of study.
1
-5
Experimental Section
Photocatalysis is a clean and attractive, low-temperature,
nonenergy-intensive approach, significantly, for chemical waste
remediation. Titanium oxide catalyst is one of the most
investigated photocatalytic systems and has been found to be
capable of decomposing a wide variety of organic and inorganic
pollutants and toxic materials, in both liquid and gas-phase
Ex.Ti/Y(x) and ex.Ti/ZSM-5 catalysts were prepared by an
ion exchange method with an aqueous titanium ammonium
oxalate solution using Y-zeolite samples (Si/Al ) 5.6, 13.9 and
6
3
90) and ZSM-5 supplied by the TOSOH corporation. Fur-
thermore, the imp.Ti/ZSM-5 catalyst was prepared by impreg-
nating ZSM-5 with an aqueous titanium ammonium oxalate.6
The actual compositions of the photocatalysts were determined
by atomic absorption. Samples were dissolved in an HF solution
and measured on a Perkin-Elmer system with a sensitivity of
6
-8
systems.
However, their reactivity and selectivity are not
enough for large scale applications. On the other hand, highly
dispersed titanium oxides prepared in zeolites or in silicalite
cavities have also been found to exhibit unique and high
photocatalytic reactivity in the reduction of CO2 with H2O to
produce CH3OH and CH4, as well as the direct decomposition
of NO into N2 and O2.6 These photocatalytic reactions are very
important with regard to reducing the greenhouse effect and
controlling global air pollution as well as the development of
highly active environmentally friendly and safe catalytic
technologies using solar energy4
1
.7 mg/mL.
The final catalysts were denoted as ex.Ti/Y(x), ex.Ti/ZSM-
, and imp.Ti/ZSM-5. TiO2 powered catalysts (JRC-TIO-4:
5
,9
anatase 92%, rutile 8%) were supplied as a standard reference
catalyst by the Catalysis Society of Japan. The photocatalytic
reactions of NO molecules were carried out with the catalysts
(
100 mg) in a quartz cell with a flat bottom (60 mL) connected
,6,9
-6
to a conventional vacuum system (10 Torr range). Prior to
photoreactions and spectroscopic measurements, the catalysts
were degassed at 725 K for 2 h, heated in O2 at the same
To address the removal of nitrogen oxides (NOx, x ) 1, 2)
from exhaust gases, semiconductors have been applied as
photocatalysts which react under UV or visible light.4
,6,9,10
The
-6
temperature for 2 h, and finally evacuated at 475 K to 10
design of molecular and/or cluster size titanium oxide photo-
catalysts within zeolite cavities and frameworks is of special
interest because zeolites offer unique nanoscale pore reaction
fields, an unusual internal surface topology, and ion-exchange
capacities. The aim of the present work is to examine the
Torr. UV irradiation of the catalysts in the presence of NO (7.8
µmol) was carried out using a 75 W high-pressure Hg lamp (λ
> 280 nm) at 275 K. The reaction products were analyzed by
gas chromatography. The photoluminescence spectra of the
catalysts were measured at 77 K using a Shimadzu RF-5000
spectrophotofluorometer. The UV absorption spectra were
recorded with a Shimadzu UV-2200A spectrometer at 295 K.
The XAFS spectra were measured at the BL-7C facility of the
Photon Factory at the National Laboratory for High-Energy
Physics, Tsukuba. The Ti K-edge absorption spectra were
recorded in the transmission mode or fluorescence mode at 295
*
To whom correspondence should be addressed. E-mail: anpo@
ok.chem.osakafu-u.ac.jp.
†
Department of Applied Chemistry.
Permanent address: Institute of Fine Chemicals, East China University
‡
of Science and Technology, Shanghai 200237, P. R. China.
§
Frontier Research Center for the Global Environment Protection.
1
0.1021/jp012080e CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/09/2001

8396 J. Phys. Chem. B, Vol. 105, No. 35, 2001
Zhang et al.
Figure 1. Diffuse reflectance UV-vis absorption spectra of ex.Ti/Y.
a) ex.Ti/Y(5.6), (b) ex.Ti/Y(13.9), and (c) ex.Ti/Y(390).
(
Figure 3. Effect of the addition of NO upon the photoluminescence
spectrum of ex.Ti/Y (5.6) catalyst. (a) 0 Torr, (b) 0.1 Torr, (c) 3.2 Torr,
(d) 6 Torr, and (e) 8 Torr.
ligands having different second coordination spheres are quite
arbitrary. Therefore, the above equation cannot be quantitatively
used to account for the shift in the LMCT transitions of the Ti
atoms upon changes in coordination and ligand environments.
As can be seen in Figure 1, the transition from sample ex.Ti/
Y(5.6) to ex.Ti/Y(13.9) and ex.Ti/Y(390) causes only a slight
red shift of the LMCT transitions, indicating that the coordina-
tion geometry changes as well as any changes in the ligand
due to the relatively minor contribution of the titanium to the
shift in the LMCT transitions. From Figure 2, the significant
red shift of the LMCT transitions and the decrease in the edge
energy with the changing TiO2 loading methods from ex.Ti/
ZSM-5 to imp.Ti/ZSM-5 and P-25 can be observed. The
decrease in the edge energy of the LMCT transitions of the Ti
atoms may be associated with the increase in the number of
the nearest Ti atoms, which suggests the polymerization of the
surface Ti atoms with the impregnation method. These results
clearly suggest that the dispersion of the Ti oxide species on
the catalyst prepared by an ion-exchange is higher than that of
the catalysts prepared by impregnation methods. According to
Figure 2. Diffuse reflectance UV-vis absorption spectra of the various
Ti/ZSM-5 catalysts. (a) ex. Ti/ZSM-5(23.8), (b) imp. Ti/ZSM-5(23.8),
and (c) P-25 as reference.
K. The normalized spectra were obtained by a procedure
described in previous literature.11
Results and Discussion
Figure 1 shows the diffuse reflectance UV spectra of the
various photocatalysts. They were prepared by an ion-exchange
method, and the Y-zeolite were used as supports, the Si/Al ratios
of which were 5.6, 13.9, and 390, respectively. From Figure 1,
it was observed that the UV absorption of the samples shifted
to longer wavelengths with the increase in the ratio of Si/Al.
This meant that the titanium oxide species became aggregated
with the increase. As can be seen in Figure 2, the UV absorption
spectrum of ex. Ti/ZSM-5 shifts to shorter wavelengths, and
that of P-25 locates at the longest wavelength. The diffuse
reflectance UV-vis spectra are usually considered to provide
information on the coordination geometry of the Ti cation (the
first coordination sphere) and the change in the ligands (the
1
4
several studies, the absorption shoulder at 260-270 nm can
be attributed to the presence of site-isolated Ti atoms in penta-
or octahedral coordination. The ex.Ti/Y-zeolite sample exhibited
a photoluminescence spectrum at around 450-600 nm by
excitation at around 280 nm at 77 K.
Figure 3 shows the typical photoluminescence observed on
ex.Ti/Y-zeolite by excitation with UV light. The observed
photoluminescence and absorption bands are in good agreement
with those previously observed with the highly dispersed
tetrahedrally coordinated titanium oxides prepared in silica
1
2
second coordination sphere) under various conditions. Jor-
gensen13 proposed that the LMCT transitions between the ligand
(
X ) H-O-, Si-O-, Ti-O-, etc.) and the empty d orbital
1
5,16
4
+
o
matrices,
where the absorption of UV light at around 280
of Ti can be estimated from the optical electronegativities ø
of ligand X and Ti by the following equation:
4+
nm brings about an electron transfer from the lattice oxygen
2-
4+
(
O1 ) to the titanium ion (Ti ) to form a charge transfer excited
λ ) (1000/3)[øopt(X) - øopt(Ti)]-1
state. We can, therefore, conclude that the observed photolu-
minescence spectrum is attributed to the radiative decay process
from the charge transfer excited state formed in this way to the
ground state of the highly dispersed titanium oxide species
having a tetrahedral coordination, as shown in the following
scheme:
The increase in the coordination number of Ti from 4-fold
to 6-fold increases the øopt(Ti) value from 1.85 to 2.0. Therefore,
the LMCT band for Ti in the octahedral sites is located at a
longer wavelength relative to Ti in the tetrahedral sites. The Ti
atoms in either the anatase or rutile phase are located at the
octahedral sites, and these results indicate that a single value
of optical electronegativity may not properly represent the Ti
atoms in the different coordination and ligand environments.
Moreover, the electronegativity values used for oxygenated
hν
4+
2-
3+
-
(
Ti -O ) { } (Ti -O )*
hν′
The addition of NO onto the ex.Ti/Y-zeolite catalyst led to an

Titanium Oxide Photocatalysts
J. Phys. Chem. B, Vol. 105, No. 35, 2001 8397
TABLE 1: Comparisons of the Yields of N and N O and
2
2
Their Selectivities in the Direct Photocatalytic Decomposition
of NO at 275 K on the Various Types of the Ti/Zeolite
Catalysts
yields µmol/g
of TiO
2
h
selectivity (%)
Ti content
catalysts
(wt % as TiO
2
)
N
2
N
2
O
total
N
2
2
N O
ex. Ti/Y(5.6)
ex. Ti/Y(13.9)
ex. Ti/Y(390)
ex. Ti/ZSM-5
imp. Ti//ZSM-5
P-25
1.0
1.0
1.0
1.1
1.0
23
19
17
14
8
6
29
26
25
16
19
11
79
73
68
88
42
27
21
27
32
12
58
73
7
8
2
11
8
3
neighboring oxygen atoms (Ti-O). The ex.Ti/Y(5.6) exhibits
only a Ti-O peak, indicating the presence of an isolated
titanium oxide species, while the ex.Ti/Y(390) exhibits an
intense peak at around 2.7 Å. This peak can be assigned to the
aggregation of the titanium oxide species in the catalyst. From
these results, it can be seen that ex.Ti/Y(5.6) contains isolated
four-coordinate titanium ions. On the other hand, in the ex.Ti/
Y(390), the presence of the aggregated octahedral titanium oxide
species was clearly suggested.
Figure 4. (a-c) XANES and (A-C) FT-EXAFS spectra of (a, A) ex.
Ti/Y(390), (b, B) ex. Ti/Y(13.9), and (c, C) ex. Ti/Y(5.6).
efficient quenching of the photoluminescence spectrum of the
catalyst, as shown in Figure 3. The intensity of the spectrum
depended on the amount of NO added. The lifetime of the charge
transfer excited state was also found to be shortened by the
addition of NO. These results indicate not only that the
tetrahedrally coordinated titanium oxide species locate at
positions accessible to the added NO but also that the added
NO easily interacts with the charge transfer excited state of the
titanium oxide species.
The DRS results alone are insufficient to determine the
coordination of Ti atoms because of the mixed contribution to
the LMCT transitions from both coordination states and ligands.
Combining the in situ DRS and XANES spectroscopies was
found to provide the most direct and reliable structural informa-
tion on the surface titanium oxide species.
The chemical structural information on the titanium oxide
species within the zeolite is strongly dependent on the amount
of Ti loading on the support. The edge region in the X-
absorption spectra provides much information on the environ-
ment geometry and the electronic structure of the absorbing
atom. Figure 4 shows the XANES spectra of three different
samples used in this present work. The lack of inversion
symmetry in the tetrahedral environment results in a single
intense preedge peak. This peak is assigned to the allowed 1s
f t2 transition for tetrahedral symmetry.17 The intensities of
the preedge peaks were in the following order: ex.Ti/Y-zeolite-
UV irradiation of the ex.Ti/Y-zeolite(5.6), ex.Ti/Y-zeolite-
(13.9), ex.Ti/Y-zeolite(390), ex.Ti/ZSM-5, and imp.Ti/ZSM-5
photocatalysts in the presence of NO was found to lead to the
evolution of N2, O2, and N2O in the gas phase at 275 K with
different yields and different product selectivities. The yields
of these photoformed N2, O2, and N2O increased linearly to the
UV irradiation time and the reaction immediately stopped when
irradiation was ceased. The formation of these reaction products
was not detected under dark conditions nor in UV irradiation
of the zeolites without titanium.
The photocatalytic reactivities of various titanium oxide
catalysts for the direct decomposition of NO into N2 and O2
are shown in Table 1. Table 1 also shows the yields of the
photoformed N2 (O2) and N2O (efficiency) and their selectivity
to produce N2 in decomposition of NO. From Table 1, it is clear
that the efficiency and selectivity for the formation of N2
strongly depend on the type of catalyst. It was observed that
the photocatalytic reactivity and selectivity for the formation
of N are much higher in low ratios than for high ratios of Si/
2
Al.
Of special interest is the comparison of the photocatalytic
reactivities of ex.Ti/ZSM-5 catalysts with that of imp.Ti/ZSM-5
in Table 1. The photocatalytic performance of the titanium oxide
appears to be completely modified by its incorporation into the
ZSM-5 zeolite, which is also associated with changes in the
molecular structure and coordination environment. The unique
catalytic performance of the highly dispersed titanium photo-
catalyst, ex.Ti/ZSM-5, in comparison with the aggregated
titanium oxide photocatalyst shows higher photocatalytic reac-
tivity and selectivity for the formation of N and O . It is thus
(5.6) > ex.Ti/Y-zeolite(13.9) > ex.Ti/Y-zeolite(390). The
preedge features of the samples show differences with the
increasing ratio of Si/Al in the catalysts, with the peak intensity
in ex.Ti/Y-zeolite(5.6) being significantly higher, suggesting that
the average coordination number of the Ti atoms is lower in
samples with a lower ratio of Si/Al. As can be seen in Figure
2
2
clear that the selectivity for the formation of N is much higher
2
when prepared by an ion-exchange method than by an impreg-
nation method. Moreover, the catalytic activity and selectivity
4a for the ex.Ti/Y-zeolite(390), the single preedge peak is rather
weak, indicating that the catalyst consists of a mixture of
tetrahedrally and octahedrally coordinated oxide species. Previ-
for the formation of N are associated with changes in the
coordination geometry and the degree of the polymerization of
the surface titanium oxide species. From these results, it was
found that the photocatalytic reactivity and selectivity for the
2
1
8
ous literature has shown that the preedge peak becomes less
intense and wider, and shifts to a higher energy value when
octahedral Ti is present in the form of an extraframework anatase
or in systems with significant disorder in the Ti-O distances
and Ti-O-Si angles.
formation of N decrease with an increase in the aggregated
2
titanium oxides in the photocatalysts. Polymerization of the
titanium oxide species when the titanium oxide are loaded by
an impregnation method leads to a decrease in the fraction of
the Ti-O-Si bonds and, therefore, significantly decreases the
photocatalytic reactivity of the catalyst. Thus, in the case of
Figure 4 also shows the FT-EXAFS spectra of the catalysts.
All of the catalysts exhibit a strong peak at around 1.6 Å
(without phase shift correction) which can be assigned to the

8398 J. Phys. Chem. B, Vol. 105, No. 35, 2001
Zhang et al.
the highly dispersed tetrahedrally coordinated TiO4 species,
under UV irradiation, two NO molecules can be activated on a
photoexcited TiO4 species, resulting in the formation of N2 and
Acknowledgment. The present work has been supported in
part by the Ministry of Education of Japan, Grant-in-Aid for
JSPS Fellows (Grant 98096).
3
O2. Meanwhile, in the case of the aggregated octahedrally
coordinated titanium oxide particles, the photoformed electrons
and holes react with NO molecules at different sites, respec-
tively, and this situation results in the formation of NO2 and/or
N2O but not N2 and O2, since N and O formed in the primary
processes of the decomposition of NO immediately react with
another NO molecules to form NO2 and N2O. Thus, the
photocatalytic reaction mechanism on the highly dispersed
isolated tetrahedrally coordinated TiO4 species is completely
different from the mechanism operating on titanium oxide
semiconductor particles.2
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(
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