A characterization of Ti-Beta zeolites synthesized under various conditions as well as an investigation of their photocatalytic properties for the reduction of CO_2 with H_2O at 323 K to produce CH_4 and CH_3OH were carried out. In situ XAFS spectra measurements indicated that a highly dispersed tetrahedral titanium oxide species was present in the zeolite framework and an increase in the coordination number of the titanium oxide species by the addition of H_2O and CO_2 molecules could be detected. The Ti-Beta zeolite having a hydrophilic property (Ti-Beta(OH)) exhibited a more dramatic increase in the coordination number than the Ti-Beta(F) zeolite which had a hydrophobic property. These results suggest that CO_2 and H_2O molecules can be adsorbed efficiently onto the highly dispersed tetrahedrally coordinated titanium oxide species. UV irradiation of these Ti-Beta zeolite catalysts in the presence of H_2O and CO_2 led to the formation of CH_4 and CH_3OH. Ti-Beta(OH) exhibited a higher reactivity than Ti-Beta(F), while the selectivity for the formation of CH_3OH on Ti-Beta(F) was higher than that for Ti-Beta(OH). These results indicated that the reactivity and selectivity of the zeolite catalyst can be determined by the hydrophilic and hydrophobic properties of the zeolites

Anpo, Masakazu

Davis, Mark E.

Ikeue, Keita

Takewaki, Takahiko

Yamashita, Hiromi

Caltech Authors - Main

catalysis
602 # 2001 International Union of Crystallography  Printed in Great Britain ± all rights reserved J. Synchrotron Rad. (2001). 8, 602±604
Characterization of Ti-Beta zeolites and
their reactivity for the photocatalytic
reduction of CO2 with H2O
Keita Ikeue,a Hiromi Yamashita,a Takahiko
Takewaki,b Mark E. Davis,b and Masakazu Anpo*a
a Department of Applied Chemistry, Graduate School of
Engineering, Osaka Prefecture University, 1-1 Gakuen-cho,
Sakai, Osaka 599-8531, Japan.
E-mail: anpo@ok.chem.osakafu-u.ac.jp
b Division  of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California,
91125, USA.
  
A characterization of Ti-Beta zeolites synthesized under various
conditions as well as an investigation of their photocatalytic
properties for the reduction of CO2 with H2O at 323 K to produce
CH4 and CH3OH were carried out. In situ XAFS spectra
measurements indicated that a highly dispersed tetrahedral
titanium oxide species was present in the zeolite framework and
an increase in the coordination number of the titanium oxide
species by the addition of H2O and CO2 molecules could be
detected.  The Ti-Beta zeolite having a hydrophilic property ( Ti-
Beta(OH) ) exhibited a more dramatic increase in the
coordination number than the Ti-Beta(F) zeolite which had a
hydrophobic property.  These results suggest that CO2 and H2O
molecules can be adsorbed efficiently onto the highly dispersed
tetrahedrally coordinated titanium oxide species.  UV irradiation
of these Ti-Beta zeolite catalysts in the presence of H2O and CO2
led to the formation of CH4 and CH3OH.  Ti-Beta(OH) exhibited
a higher reactivity than Ti-Beta(F), while the selectivity for the
formation of CH3OH on Ti-Beta(F) was higher than that for Ti-
Beta(OH). These results indicated that the reactivity and
selectivity of the zeolite catalyst can be determined by the
hydrophilic and hydrophobic properties of the zeolites.
Keywords:  in-situ XAFS, Photocatalyst, Ti-Beta, CO2
1.
 
Introduction
The photocatalytic reduction of CO2 with H2O is of interest not
only as a reaction system utilizing artificial photosynthesis but
also as a way to use carbon sources for the synthesis of
oxygenates and hydrocarbons such as CH4 and CH3OH.  We
have previously reported that UV-irradiation of highly dispersed
tetrahedrally coordinated titanium oxide photocatalysts led to the
highly selective formation of CH3OH in the photocatalytic
reduction of CO2 with H2O as compared to octahedrally
coordinated bulk TiO2 photocatalysts (Anpo et. al., 1992; Anpo
et. al., 1997; Ikeue et. al., 1999).  The incorporation of highly
dispersed tetrahedrally coordinated titanium oxide species into
zeolite cavities which are known to have unique properties made
it possible to achieve a more active and selective photocatalytic
system.  It was recently reported that Ti-containing zeolites,
especially the Ti-Beta zeolite synthesized using OH- ions and F-
ions as the counter anions of the structure directing agent,
exhibited hydrophilic and hydrophobic properties, respectively
(Blasco et. al, 1998).
In the present study, two different Ti-Beta zeolites having
hydrophilic and hydrophobic properties were synthesized and the
photocatalytic reduction of CO2 with H2O to produce CH4 and
CH3OH was carried out as the photocatalytic reaction. The effect
of these properties of zeolites and the local structure of the
titanium oxide species incorporated into the zeolite frameworks
on the reactivity and selectivity for their photocatalytic reactions
have been investigated.
2.
 
Experimental section
The Ti-Beta zeolites were synthesized under hydrothermal
conditions using two kinds of structure directing agents (SDA)
from material mixtures with the compositions expressed below:
Two different Ti-Beta will be denoted according to the kind of
counter anion of the SDA, i. e., Ti-Beta(OH) and Ti-Beta(F).
Ti-Beta(OH); 1 SiO2: 0.02 TiO2: 0.2 SDA(OH-): 30 H2O
Ti-Beta(F); 1 SiO2: 0.02 TiO2: 0.56 SDA (F-): 7 H2O
SDA(OH-) and SDA(F-) were N, N'-dibenzyl-4, 4’-trimethylene
bis(N-methylpiperidinium)dihydroxide and tetraethylammonium
fluoride (TEAF), respectively.  The products were washed with
distilled water and dried in air at 353 K. To remove the occluded
organic molecules, the samples were heated under a flow of dry
air at 823 K for 4 h.
The photocatalytic reduction of CO2 with H2O was carried out
with the catalysts (50 mg) in a quartz cell with a flat bottom (88
cm3) connected to a conventional vacuum system (10-4  Pa range).
Prior to the photoreactions and spectroscopic measurements, the
catalysts were heated in O2 at 725 K and evacuated at 475 K to
10-4 Pa. UV-irradiation of the catalysts in the presence of CO2
(36 µmol) and gaseous H2O (120 µmol) was carried out using a
75-W high pressure Hg lamp (λ > 250 nm) at 323 K. The
reaction products collected in the gas phase were analyzed by gas
chromatography.
The photoluminescence spectra of the catalysts were
measured at 298 K and 77 K using a SPEX FLUOLOG-3
spectrophotofluorometer. The UV-VIS adsorption spectra were
measured at 298 K by a Shimadzu UV-2200A
spectrophotometer.  The XAFS spectra (XANES and EXAFS) of
the catalysts were measured at the BL-7C facility of the Photon
Factory at the National Laboratory for High-Energy Physics,
Tsukuba.  The X-ray source was monochromatized using a
Si(111) monochromater.  Ti K-edge absorption spectra were
recorded in the fluorescence mode at 295 K.  Fourier
transformation was performed on k3-weighted EXAFS
oscillations in the range of 3-10 Å-1.  The zeolite samples,
obtained as thin self-supporting pellets (50 mg), were placed
inside a Pyrex sample cell.
3.
 
Results and discussion
In the diffuse reflectance UV-VIS absorption spectra, both the
Ti-Beta(OH) and Ti-Beta(F) catalysts exhibited an absorption
band  in the wavelength region of 200-260 nm, attributed to the
LMCT (ligand-to-metal charge transfer) band of the highly
dispersed tetrahedrally coordinated titanium oxide species
(Marchese et. al., 1998).
Figure 1 shows the Ti K-edge XAFS (XANES and EXAFS)
spectra of Ti-Beta(OH) and Ti-Beta(F).  In the tetrahedral
symmetry, one intense preedge peak corresponding to the
dipolar-allowed transition from the 1s to t2 molecular levels built
from the 3d and 4p metal orbital and from the neighboring orbital
could be observed (Babonneau et. al., 1988).  As shown in Fig. 1
(a-c), Ti-Beta(OH), Ti-Beta(F), and TS-1 all exhibited one
intense preedge peak.  The local structure of the titanium oxide
species of these zeolites were all found to be in tetrahedral
coordination.
Figure 1 also shows the FT-EXAFS spectra of the zeolite
catalysts. All of the catalysts exhibited a strong peak due to the
neighboring oxygen atoms (a Ti-O bond). These zeolites
J. Synchrotron Rad. (2001). 8, 602±604 Keita Ikeue et al. 603
catalysis
exhibited only a Ti-O peak indicating the presence of the isolated
titanium oxide species.  From the curve-fitting analysis of the
EXAFS spectra, it was found that the tetrahedrally coordinated
titanium oxide species consists of four Ti-O bonds having a bond
distance of 1.84 Å for Ti-Beta(OH) and 1.83 Å for Ti-Beta(F),
respectively.  These XAFS and UV-VIS absorption
investigations clearly indicate that the local structure of the
titanium oxide species of Ti-Beta(OH) and Ti-Beta(F) are highly
dispersed and exist in a tetrahedral coordination, and that a
difference in the coordination geometry of these species for Ti-
Beta(OH) and Ti-Beta(F) could scarcely be observed.
Figure1.
The Ti K-edge XANES (a, b) and FT-EXAFS (A, B) spectra of Ti-
Beta(OH) (a, A) and Ti-Beta(F) (b, B). Debye-Waller factors were fixed
at 0.003 Å2.  Coordination number within an error ± 0.4.  Ti-O bond
distance within an error ± 0.01 Å.  CN: coordination number, R: Ti-O
bond distance (Å)
From investigations on the adsorption isotherm of H2O at 298
K, it was found that the amount of adsorbed H2O molecules on
Ti-Beta(OH) was more than those for Ti-Beta(F). These results
indicated that Ti-Beta(OH) and Ti-Beta(F) have hydrophilic and
hydrophobic properties, respectively. It could, therefore, be
expected that the H2O molecules are easily accessible to the
titanium oxide species on Ti-Beta(OH).
Figure 2 shows the effect of the addition of H2O molecules on
the preedge peak intensity of the XANES spectra. In both Ti-
Beta zeolites, the addition of H2O molecules leads to a decrease
in the preedge peak intensity.  This decrease was found to be
longer for Ti-Beta(OH) than for Ti-Beta(F).  Moreover, on Ti-
Beta(OH), the position of  the preedge peak shifts to a high
energy region by the addition of excess amounts of H2O
molecules (4.6 mmol/g-cat).  The shift in the preedge peak
position was attributed to a modification in the local geometry of
the Ti atoms from the tetrahedral coordination to a 5 or 6-fold
coordination.  On the other hand, the large decrease in the
preedge peak intensity and the shift in the position of the preedge
peak could not be observed on Ti-Beta(F). These results indicate
that H2O molecules are easily accessible to the tetrahedrally
coordinated titanium oxide species of Ti-Beta(OH), in agreement
with the results obtained from the adsorption isotherms of the
H2O molecules.
UV-irradiation of Ti-Beta zeolite catalysts in the presence of
CO2 and H2O at 323 K led to the formation of CH4, CH3OH, and
trace amounts of CO, C2H4, and O2, their yields increasing with
the irradiation time.
  
Figure 3 shows the product distribution in
the reactions on the catalysts.  The Ti-Beta(OH) catalyst exhibits
a higher reactivity as compared to the Ti-Beta(F) catalyst.  On the
other hand, the selectivity for the formation of CH3OH on Ti-
Beta(F) is higher than that for Ti-Beta(OH).
Figure 2
The effect of the addition of H2O molecules upon the preedge peak
intensity of the XANES spectra.  (a) Ti-Beta(OH), (b) Ti-Beta(F).  The
amount of added H2O molecules; 0, 1.4, 3.0, 4.6 mmol/g-cat (from top to
bottom).
Figure 3
The yields of CH4 and CH3OH in the photocatalytic reduction of CO2
with H2O at 323 K on Ti-Beta(OH), TS-1, Ti-Beta(F), and TiO2 (P-25) as
the reference catalyst.
0 1 2 3 4 5 6
M
ag
nit
ud
e 
(a.
 u.
 )
No
rm
al
ize
d 
ab
so
rp
tio
n 
 (a
. u
.) 
4960 5000 5040
(a)
(b)
(A)
(B)
No
rm
al
ize
d 
ab
so
rp
tio
n 
(a.
 u.
)
4960  5000  5020  5040  4980 
(a)
(b)
Energy (eV)
4965 4970 4975
4965 4970 4975
0
2
4
6
P -25 Ti-Beta(F) TS-1 Ti-Beta(OH)
CH4
CH3OH
Photoc atalysts
Yi
el
ds
 
of
 
pr
od
uc
t (µ
m
ol
•
g-
Ti
-
1 •
h-1
)
Energy / eV                                   Distance / Å
   preedge
Ti-O       CN: 3.9
                        R: 1.84
CN: 3.9
R: 1.83
X
X X X
catalysis
604 Received 31 July 2000  Accepted 12 December 2000 J. Synchrotron Rad. (2001). 8, 602±604
Ti-Beta(OH) and Ti-Beta(F) both exhibited a
photoluminescence spectrum at around 480 nm by excitation at
260 nm at 77 K.  This spectrum was attributed to the radiative
decay process from the charge transfer excited state of the highly
dispersed tetrahedrally coordinated titanium oxide species to its
ground state. Ti-Beta(OH) exhibit a higher yield of
photoluminescence as compared to Ti-Beta(F).  The yield of the
photoluminescence is related to the number of charge transfer
excited complexes (Ti3+ - O-)*.  The higher reactivity of Ti-
Beta(OH) over Ti-Beta(F) was attributed to the higher
concentration of the charge transfer excited complexes for Ti-
Beta(OH).
For Ti-Beta(F) having a hydrophobic property, the relative
concentration of H2O is much lower than Ti-Beta(OH), resulting
in the low reactivity and the high selectivity for the formation of
CH3OH.  Thus, the chemical nature, i.e., the hydrophilic-
hydrophobic properties were found to be the deciding factor
controlling the reactivity and selectivity in the photocatalytic
reduction of CO2 with H2O on these zeolite catalysts.
4. Conclusions
The characterization of Ti-Beta(OH) and Ti-Beta(F) catalysts
by means of XAFS (XANES and FT-EXAFS) and
photoluminescence clearly indicated that both catalysts involve
highly dispersed tetrahedrally coordinated Ti-oxide species as the
major species within zeolite framework.  Both Ti-Beta(OH) and
Ti-Beta(F) catalysts were found to exhibit high photocatalytic
reactivity for the reduction of CO2 with H2O.  The Ti-Beta(OH)
catalyst having a hydrophilic property exhibited higher reactivity
than Ti-Beta(F) due to the higher concentration of the charge
transfer excited complexes (Ti3+ - O-)*.  On the other hand, the
highly selective formation of CH3OH was observed on the Ti-
Beta(F) catalyst having a hydrophobic property.  The change of
Ti coordination in the rehydration process indicated that the H2O
molecules are more easily accessible to the tetrahedrally
coordinated titanium oxide species on Ti-Beta(OH) than Ti-
Beta(F), attributed to the affinity for the H2O molecules for
adsorption. The hydrophilic-hydrophobic properties of the zeolite
were, therefore, found to be the controlling factor in the
reactivity and selectivity for the photocatalytic reduction of CO2
with H2O on these zeolite catalysts.
References
Anpo, M. & Chiba, K. (1992). J. Mol. Catal. 74, 207-212.
Anpo, M., Yamashita, H., Ichihashi, Y., Fujii, Y. & Honda, M. (1997). J
   Phys. Chem. B 101, 2632-2636.
Babonneau,  F., Doeuff, S.,  Leaustic, A., Sanchez,  C., Cartier, C. &
   Verdaguer, M. (1988). Inorg. Chem. 27, 3166-3172.
Blasco, T., Camblor, M. A., Corma, A., Esteve, P., Guil, J. M., Martínez,
   Perdigón-Melón,  J. A., Valencia, S. (1998).  J. Phys. Chem. B 102,
   75-88.
Ikeue, K., Yamashita, H. & Anpo, M. (1999). Chem. Lett. 1135-1136.
Marchese, L., Maschmeyer, T., Gianotti, E., Coluccia, S. & Thomas J. M.
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Characterization of Ti-Beta zeolites and their reactivity for the photocatalytic reduction of CO_2 with H_2O

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Characterization of Ti-Beta zeolites and their reactivity for the photocatalytic reduction of CO_2 with H_2O

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