Photocatalytic oxidation of acetaldehyde with oxygen on TiO2/ZSM-5 photocatalysts: Effect of hydrophobicity of zeolites

Article abstract of DOI:10.1016/j.jcat.2006.12.010

TiO₂ photocatalysts hybridized with ZSM-5 were prepared by the impregnation of (NH₄)₂[TiO(C₂O₄)₂] aqueous solution. This is one of the most ideal green processes in establishing TiO₂ photocatalysts because of an organic solvent-free preparation method. The highly hydrophobic H-ZSM-5 with low Al₂O₃ content was effective as an adsorbent for acetaldehyde molecules. These results suggest the close relationship between the hydrophobicity of the zeolite and the high photocatalytic reactivity of TiO₂ prepared on zeolites compared with unloaded TiO₂ catalysts. However, acetaldehyde molecules were strongly trapped on the Na⁺ sites of ZSM-5 and their efficient diffusion onto the TiO₂ sites was inhibited, resulting in low photocatalytic reactivity. The photocatalytic oxidation of acetaldehyde over TiO₂/ZSM-5 was improved in the coexistence of H₂O vapor. The role of the H₂O molecules was estimated to be the efficient formation of OH radicals and the prevention of strong adsorption of acetaldehyde on the TiO₂ sites.

Full text of DOI:10.1016/j.jcat.2006.12.010

Journal of Catalysis 246 (2007) 235–240
www.elsevier.com/locate/jcat
Photocatalytic oxidation of acetaldehyde with oxygen on TiO /ZSM-5
2
photocatalysts: Effect of hydrophobicity of zeolites
∗
Masato Takeuchi, Takashi Kimura, Manabu Hidaka, Diana Rakhmawaty, Masakazu Anpo
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan
Received 11 October 2006; revised 3 December 2006; accepted 8 December 2006
Available online 12 January 2007
Abstract
TiO photocatalysts hybridized with ZSM-5 were prepared by the impregnation of (NH ) [TiO(C O ) ] aqueous solution. This is one of the
2
4 2
2 4 2
most ideal green processes in establishing TiO photocatalysts because of an organic solvent-free preparation method. The highly hydrophobic
2
H-ZSM-5 with low Al O content was effective as an adsorbent for acetaldehyde molecules. These results suggest the close relationship between
2
3
the hydrophobicity of the zeolite and the high photocatalytic reactivity of TiO prepared on zeolites compared with unloaded TiO catalysts.
2
2
+
However, acetaldehyde molecules were strongly trapped on the Na sites of ZSM-5 and their efficient diffusion onto the TiO sites was inhibited,
2
resulting in low photocatalytic reactivity. The photocatalytic oxidation of acetaldehyde over TiO /ZSM-5 was improved in the coexistence of
2
H O vapor. The role of the H O molecules was estimated to be the efficient formation of OH radicals and the prevention of strong adsorption of
2
2
acetaldehyde on the TiO sites.
2
©
2006 Elsevier Inc. All rights reserved.
1
. Introduction
spaces and then decompose them into CO2 and H2O by photo-
catalytic oxidation. For this purpose, TiO2 fine particles with
large surface areas prepared by a solvothermal method have
been reported to show efficient photocatalytic reactivity for the
oxidation of acetic acid [7,8]. The hybridization of adsorbents
with large surface areas, such as zeolites [9–11] and active car-
In recent years, volatile organic compounds (VOCs), includ-
ing formaldehyde, acetaldehyde, toluene, and the like, emitted
from building and interior materials in newly built houses have
caused debilitating damage to our health in the so-called “sick
house syndrome.” This has become one of the most serious
environmental issues. Because TiO2 photocatalysts have the po-
tential to completely decompose various organic compounds
into harmless CO and H O under UV light irradiation due to
its high oxidative power, they have been widely studied for the
purification of air, water, and soil polluted with organic com-
pounds [1–3]. Removal systems incorporating TiO photocat-
alysts have been practically applied to purify water containing
dilute amounts of harmful and toxic compounds [4–6], and be-
cause the diffusion of the reactants toward the surface is limited
in aqueous solutions, adsorbents with large surface areas are not
always needed to condense the reactants on the near surface.
However, purifying polluted air requires hybrid photocatalysts
that can efficiently adsorb organic compounds diffused in wide
bon [5,6,12,13], with TiO fine particles also has been reported
2
to show efficient photocatalytic reactivity, especially for gas-
phase reactions.
2
2
ZSM-5 zeolites, with highly ordered micropores, surface
acidity, and ion-exchange capacities, are one of the most widely
applied inorganic materials as catalyst supports, adsorbents, and
molecular-sized spaces for various chemical or photochemi-
cal reactions. Highly siliceous ZSM-5 zeolites with low Al2O3
content are known to have highly hydrophobic surfaces [14].
H2O vapor has been reported to play an important role in
the photocatalytic degradation of formic acid [15,16], toluene
2
[
17–19], and trichloroethylene (TCE) [20] over TiO2 photo-
catalysts. In many cases, the improved photocatalytic reactivity
from the addition of H2O vapor could be explained by suppres-
sion of the strong adsorption of the reactants and/or interme-
diate species. From this standpoint, detailed investigations of
the relationship between the adsorption properties of zeolites as
*
Corresponding author.
E-mail address: anpo@chem.osakafu-u.ac.jp (M. Anpo).
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.12.010

236
M. Takeuchi et al. / Journal of Catalysis 246 (2007) 235–240
adsorbents and the photocatalytic reactivity of TiO2 photocat-
alysts hybridized with zeolites can lead to the development of
new photofunctional materials for environmental remediation.
In this work, the adsorption states of H2O and acetalde-
hyde molecules on H and Na type ZSM-5 zeolites of
different SiO2/Al2O3 ratios were investigated by adsorption
isotherm and temperature-programmed desorption (TPD) mea-
surements. The TiO2 photocatalysts hybridized with ZSM-5
zeolites were prepared by a simple impregnation method
from an aqueous solution of titanium(IV) ammonium oxalate
The zeolite samples (30 mg) in a quartz cell were pretreated
in the same way as done for the adsorption isotherm measure-
ments. Sufficient amounts of H O or CH CHO vapor were then
2
3
adsorbed on these zeolite samples for 30 min. Before TPD
measurements, the H O or CH CHO molecules in gas phase
+
+
2
3
were degassed at room temperature for 1 h. TPD profiles were
recorded by linear heating of the samples from 323 to 1023 K
at a constant rate of 5 K/min.
The photocatalytic reactivities of the prepared TiO /ZSM-5
2
catalysts were evaluated by the decomposition of acetaldehyde
((NH4)2[TiO(C2O4)2]). The photocatalytic reactivity of the
in the presence of O under UV light irradiation as a gas-phase
2
prepared TiO2/ZSM-5 samples was evaluated by the complete
oxidation of acetaldehyde with O2, as was the effect of H2O
vapor addition on photocatalytic reactivity.
reaction. TiO /ZSM-5 catalysts (50 mg) were placed onto a flat
2
3
bottom quartz cell (volume, ca. 33 cm ). The volume of the
3
reaction area, including the cell volume, was ca. 100 cm . Pre-
treatment of the catalysts was carried out in the same way as
done before the TPD and adsorption isotherm measurements.
A gas mixture of CH3CHO (0.27 kPa), O2 (1.07 kPa), and
2
. Experimental
Various ZSM-5 zeolite samples were supplied by Tosoh
2
H O (0–1.33 kPa) was then introduced into the reaction cell.
Co. Ltd. The ZSM-5 zeolite with a SiO2/Al2O3 ratio of 1880
was supplied as the H -type zeolite and NH4-ZSM-5 zeo-
The amount of acetaldehyde introduced into the reaction cell
+
was calculated as ca. 7 µmol (ca. 1500 ppm). After an adsorp-
tion equilibrium was reached, UV light was irradiated at 275 K
by a 100-W high-pressure Hg lamp (Toshiba, SHL-100UVQ-2)
through a cutoff filter (Toshiba Glass, UV-27, λ > 270 nm). To
avoid the heating effect from the Hg lamp, the photocatalysts in
the quartz cell were cooled in ice water during the photoreac-
lites with SiO2/Al2O3 ratios of 220 and 68, respectively, were
calcined in air at 723 K for 3 h to obtain H-ZSM-5(220)
and H-ZSM-5(68), respectively. Na-ZSM-5(23.8) was ion-
exchanged by a NH4NO3 solution to obtain NH4-ZSM-5(23.8)
and then calcined in air at 723 K for 3 h, resulting in the for-
mation of H-ZSM-5(23.8). These latter three samples were
confirmed to not desorb NH3 gas by TPD measurements.
TiO2 photocatalysts hybridized with ZSM-5 zeolites were
prepared by a simple impregnation method from a precursor so-
lution of titanium(IV) ammonium oxalate (NH4)2[TiO(C2O4)2]
tion. The amounts of CO produced and acetaldehyde decom-
2
posed were analyzed by thermal conductivity and flame ioniza-
tion detectors in a gas chromatograph (Shimadzu, GC-14A).
3
. Results and discussion
(
Kishida Chemicals Co.). After calcination at 723 K, the ZSM-5
zeolites were immersed in (NH4)2[TiO(C2O4)2] aqueous solu-
tion for 2 h, after which the water solvent was vaporized by
a rotary evaporator. The white powder samples thus obtained
were dried at 383 K for 12 h and then calcined at 773 K for 3 h.
The TiO2 content was adjusted to 10 wt%.
The adsorption isotherms of the H2O and CH3CHO mole-
cules on various ZSM-5 zeolites were measured at room
temperature. Before these measurements, the zeolite samples
The adsorption isotherms of the H2O molecules on H and
Na-ZSM-5 zeolites with SiO2/Al2O3 ratios of 1880, 220, 68,
and 23.8 are shown in Fig. 1. It is well known that the sur-
faces of the siliceous zeolites (e.g., silicalite, dealuminated
mordenite, USY-zeolites) show higher hydrophobic properties
than Al O -containing zeolites. H-ZSM-5(1880), with a low
2
3
Al2O3 content, exhibited the highest hydrophobicity among the
ZSM-5 zeolites with different SiO2/Al2O3 ratios. As the Al2O3
content increased to ca. 4 wt%, corresponding to a SiO2/Al2O3
ratio of 23.8, the amount of H2O adsorbed increased threefold
more than the highly siliceous H-ZSM-5(1880). These results
(
30 mg) were placed in a Pyrex glass cell, degassed at 723 K for
h, treated in sufficient amounts of O2 (ca. 6.7 kPa) at the same
temperature for 2 h, and then degassed at 373 K for 2 h up to a
2
−5
1
0
kPa range. To avoid temperature changes during adsorp-
tion isotherm measurements, the zeolite samples in the Pyrex
glass cell were immersed in a water bath at ca. 300 K. Given
amounts of H2O or CH3CHO vapor were admitted into the cell,
step by step, and the pressure changes were then recorded once
the adsorption equilibrium state was reached. The vapor pres-
sures of H2O and CH3CHO at 300 K were 3.56 and 129.3 kPa,
respectively.
For the TPD profiles of the H2O and CH3CHO molecules
+
adsorbed on these ZSM-5 zeolites, the ion current for H2O
(
1
the ratio of molecular weight [m] to charge careers [z]; m/z =
+
8) and CH3CHO (m/z = 44), which are ionized by high
voltages of 1200 V, were monitored by a gas desorption an-
alyzer (ANELVA, M-QA100TS) equipped with a quadrupole
mass analyzer in a high-vacuum chamber of 10 kPa range.
Fig. 1. Adsorption isotherms of H O molecules on (a–d) H-ZSM-5 and (e)
2
Na-ZSM-5 zeolites. SiO /Al O ratios: (a) 1880, (b) 220, (c) 68, (d) 23.8, and
2
2 3
−
9
(
e) 23.8. (Saturated vapor pressure of H O at 300 K: 3.56 kPa.)
2

M. Takeuchi et al. / Journal of Catalysis 246 (2007) 235–240
237
clearly indicate that the Al3 sites (i.e., Brönsted acid sites)
play a major role as the adsorption sites of H2O molecules. In
fact, FTIR investigations have revealed that the H2O molecules
adsorb on the Brönsted acid sites of H-ZSM-5 or SAPO-34 to
+
molecules at around 823–973 K for H-ZSM-5 zeolites with
low Al O content could not be observed for the samples af-
2
3
ter outgassing at 873 K, suggesting that the desorption of H O
2
at higher than 823 K may be attributed to the condensation of
the neighboring silanol groups on the zeolite surfaces. Further-
+
+
form oxonium ions (H3O or H2O ) [21–24] or H2O mole-
cules directly interacting with the H moieties of the Brönsted
5
+
more, the desorption of H O from Na-ZSM-5(23.8) broadened
2
acid sites by hydrogen bonding [25]. The strength of the inter-
actions between the H2O molecules and oxide surfaces can be
described by the bonding characteristics of the oxide surfaces.
The Si–O linkage is a covalent bond, whereas the Al–O link-
age has a combination of 40% covalent and 60% ionic bond
characters. Because H2O molecules are strongly polarized due
to the high electronegativity of the O atom, they strongly in-
teract with the cationic surfaces rather than with neutral or
anionic surfaces. Furthermore, because the addition of Al2O3
within the zeolite frameworks gives rise to Brönsted acid sites,
the surface of zeolites with high Al2O3 content have stronger
adsorption sites for H2O molecules compared with siliceous
zeolites. This interpretation explains why the amount of H2O
molecules adsorbed strongly depends on the SiO2/Al2O3 ra-
tios of the zeolites. Investigations of Na-ZSM-5(23.8) and H-
ZSM-5(23.8) showed that the adsorption of H2O molecules on
Na-ZSM-5(23.8) sharply increases even at low relative pres-
sure compared with H-ZSM-5(23.8). However, the saturated
amounts of H2O adsorbed on these ZSM-5 zeolites were almost
the same due to their similar surface areas. These results clearly
at higher temperatures compared with H-ZSM-5(23.8), sug-
gesting the stronger interaction between the H O molecules and
2
Na-ZSM-5(23.8) compared with H-ZSM-5(23.8). These results
clearly indicate that the desorption at 323–523 K and 523–
+
+
673 K (as a shoulder) can be attributed to the H and Na zeo-
lite sites, respectively. H O molecules strongly interact with the
2
cationic sites rather than with the hydroxyl groups on the oxide
surfaces. Three kinds of adsorbed H O species are summarized
2
in Table 1, according to the strength of the interactions between
the H O molecules and surface sites: (A) the H O molecules
2
2
interacting with the silanol groups are easily removed by evacu-
ation at room temperature. On the other hand, two kinds of H O
2
+
3+
species are adsorbed on the H sites neighboring the Al of
the zeolite surface, as proposed by FTIR investigations; (B-1)
the H O molecules interact only by hydrogen bonding [25];
2
+
(B-2) the oxonium ions, as with H O [21–24], desorb at 323–
3
523 K from the H-ZSM-5 with high Al O content; and (C)
2
3
+
the H O molecules interacting with the Na cation sites of Na-
2
ZSM-5 by electrostatic interaction desorb at 523–673 K. The
results of these adsorption and desorption measurements clearly
indicate that the hydrophilic/hydrophobic properties of ZSM-5
zeolites can be explained by the cationic characteristics of the
surface sites.
The adsorption properties of acetaldehyde molecules on
ZSM-5 zeolites were also investigated by adsorption isotherm
and TPD measurements. Adsorption isotherm analysis showed
the saturated amount of acetaldehyde on the ZSM-5 zeolites to
remain almost the same at ca. 2.0–2.5 mmol/g, despite their dif-
+
suggest that the Na cation sites on zeolite surfaces work as
strong adsorption sites for the H2O molecules.
TPD profiles of the H2O molecules adsorbed on H and Na-
ZSM-5 with different SiO2/Al2O3 ratios are shown in Fig. 2.
Desorption of the H2O molecules can be seen at 373–573 K for
H-ZSM-5 zeolites with low SiO2/Al2O3 ratios of 23.8 and 68.
However, very small amounts of H2O desorbed from H-ZSM-5
with low Al2O3 content (SiO2/Al2O3 = 220 and 1880). More-
over, as the SiO2/Al2O3 ratio decreased, the desorption temper-
ature shifted slightly toward higher-temperature regions. Gener-
ally, H2O molecules weakly adsorb on surface hydroxyl groups
by the hydrogen bonds and strongly adsorb on the cationic sites
of oxide surfaces by electrostatic interactions. Here the H2O
molecules interacted with the surface silanol groups of siliceous
H-ZSM-5 with low Al2O3 content. In contrast, the desorption
peak at around 373–573 K observed for the Al-containing H-
ZSM-5 could be assigned to the H2O molecules adsorbed on
ferent SiO /Al O ratios. These results indicate that the amount
2
2
3
of saturated adsorbed acetaldehyde depends on the surface ar-
2
eas of the ZSM-5 zeolites (ca. 300–350 m /g) but not on the
surface chemical properties, which originate from changes in
the SiO2/Al2O3 ratios. However, as shown in Fig. 3, the results
of TPD measurements revealed that the interaction of acetalde-
hyde molecules with the ZSM-5 surface depends on the dif-
+
the H sites of the zeolite surfaces. The desorption of the H2O
Fig. 3. TPD profiles of acetaldehyde molecules adsorbed on (a–d) H-ZSM-5
Fig. 2. TPD profiles of H O molecules adsorbed on (a–d) H-ZSM-5 and (e)
2
and (e) Na-ZSM-5 zeolites. SiO /Al O ratios: (a) 1880, (b) 220, (c) 68, (d, e)
2 2 3
Na-ZSM-5 zeolites. SiO /Al O ratios: (a) 1880, (b) 220, (c) 68, (d, e) 23.8.
2
2
3
23.8.

238
M. Takeuchi et al. / Journal of Catalysis 246 (2007) 235–240
Table 1
Interaction of H O molecules with various sites on the ZSM-5 zeolite surface
2
Species
(A)
(B)
(C)
H/ZSM-5 with low Al O content
H/ZSM-5 with high Al O content
Na/ZSM-5
523–673 K
2
3
2 3
Desorption temperature
Interaction
Room temperature
323–523 K
Hydrogen bonds
Weak
B-l; Hydrogen bonds or B-2; Static electricity (oxonium ion)
Medium
Static electricity
Strong
ferences in their SiO2/Al2O3 ratios. The desorption spectrum
of the molecule corresponding to the mass number (m/z = 44)
showed the same tendency of the spectrum for the mass num-
bers of 15, 29, and 43 which are assigned to CH , CHO, and
3
CH3CO, respectively, as fragments of the CH3CHO molecule.
From these results, the desorbed species of m/z = 44 can be
assigned to acetaldehyde and not to the CO2 molecules. The
desorption of acetaldehyde could be observed at 373–473 K
for the siliceous H-ZSM-5(1880) zeolite, showing that the ac-
etaldehyde molecules are well condensed on the surface hy-
droxyl groups in highly hydrophobic zeolite cavities. However,
a significant desorption of acetaldehyde could not be observed
Fig. 4. Diffuse reflectance UV–vis absorption spectra of (a–d) TiO prepared
2
on H-ZSM-5 zeolites and (e) TiO (Degussa, P-25) as reference. SiO /Al O
2
2
2
3
for the H-ZSM-5 (220 and 68) containing Al O , suggest-
ratios: (a) 1880, (b) 220, (c) 68, (d) 23.8.
2
3
ing that the Brönsted acid sites of the zeolite surfaces do not
work as strong adsorption sites for acetaldehyde. Moreover,
Na-ZSM-5(23.8) showed additional specific desorption profiles
for acetaldehyde molecules at 473–523 K. Onaka et al. have
reported that formaldehyde or acrolein molecules interacting
+
with the Na sites in NaY or NaX zeolite cavities are stored
in a stable form for long periods, and these formaldehyde or
+
acrolein molecules highly stabilized on the Na sites catalyze
the carbonyl-ene reaction of olefins [26] and a Friedel-Crafts
alkylation [27] with high selectivity, respectively. Acetaldehyde
molecules desorbed at 473–523 K were also estimated to be
+
highly stabilized on the Na sites of the Na-ZSM-5(23.8) zeo-
Fig. 5. TPD profiles of H O molecules adsorbed on TiO /H-ZSM-5 catalysts
(10 wt% as TiO ). SiO /Al O ratios: (a) 1880, (b) 220, (c) 68, (d) 23.8.
2 2 2 3
2
2
lite. In our work, H-ZSM-5(23.8) showed desorption profiles
not only at around 373 K, but also at ca. 573 K. H-ZSM-
+
5
(220 and 68) were supplied as H -type zeolites; however,
2 2
to the TiO particles; however, as the TiO content increased
up to 20 wt%, clear diffraction peaks attributed to the anatase
+
because H-ZSM-5(23.8) was prepared from H ion-exchange
of Na-ZSM-5(23.8), some Na ions still remained within H-
+
structure of TiO could be seen (data not shown). These results
2
ZSM-5(23.8). These results suggest that the desorption peak at
around 573 K may be attributed to the acetaldehyde molecules
highly stabilized on the Na ion sites.
clearly suggest much smaller TiO primary crystalline particles
of ca. 3–4 nm than the detection limit of XRD measurements.
However, because the diameter of the ZSM-5 zeolite cavity is
2
+
The diffuse reflectance UV–vis absorption spectra of TiO2
prepared on various ZSM-5 zeolites and of a commercial TiO2
about 0.55 nm, the major parts of the TiO particles are consid-
ered to exist on the outer surfaces of the ZSM-5 zeolites. SEM
2
(
Degussa, P-25) as reference are shown in Fig. 4. The ab-
2
images of the TiO /ZSM-5 samples in fact showed that the ag-
sorption edges of the TiO2/ZSM-5 samples were found to
shift slightly toward shorter wavelength regions compared with
P-25. The bandgap energies of the TiO2 prepared on ZSM-5 ze-
olites were estimated to be 3.3–3.4 eV based on the absorption
edges of ca. 365–375 nm. The TiO2/ZSM-5 catalysts (10 wt%
as TiO2) did not show any significant XRD peaks attributed
gregated TiO secondary particles of several 10 nm spread on
the outer surfaces of the zeolite crystals of ca. 3–4 µm (data not
shown).
2
The TPD profiles of the H O molecules adsorbed on the
2
TiO /ZSM-5 zeolite samples are shown in Fig. 5. Compared
2
with the TPD profiles of the ZSM-5 zeolites shown in Fig. 2,

M. Takeuchi et al. / Journal of Catalysis 246 (2007) 235–240
239
Fig. 6. Adsorption isotherms of CH CHO molecules on (a) dehydrated H-
3
ZSM-5, (b) hydrated H-ZSM-5(1880), and (c) TiO /H-ZSM-5(1880). (Satu-
rated vapor pressure of CH CHO at 300 K: 129.3 kPa.)
3
2
Fig. 7. Photocatalytic oxidation of acetaldehyde in the absence of H O vapor
2
over the TiO prepared on H- and Na-ZSM-5 zeolites under UV light irradia-
2
tion.
the desorption of H2O can be seen at around 373–423 K, espe-
cially for the TiO2 prepared on H-ZSM-5(1880 and 220) with
low Al2O3 content. For the TiO2 on H-ZSM-5(68, 23.8) with
high Al2O3 content, the main desorption peaks can be seen at
around 423 K, although the desorption profiles are broader than
those of the original H-ZSM-5(68, 23.8) before impregnation
with TiO2 particles. From these results, the order of interaction
between the H2O molecules and adsorption sites of the TiO2/H-
ZSM-5 zeolites can be estimated as follows: Brönsted acid sites
neighboring Al3 > TiO2 photocatalytic sites > surface silanol
+
groups of the zeolite framework.
The adsorption isotherms of the CH3CHO molecules on
the dehydrated, hydrated H-ZSM-5(1880), and TiO2/H-ZSM-
5
(1880) zeolites are shown in Fig. 6. The hydrated surfaces
Fig. 8. Effect of the addition of H O vapor on the photocatalytic oxidation of
2
were obtained through the preadsorption of H2O vapor and then
outgassing at room temperature. However, as shown in Fig. 2,
because the major part of physisorbed H2O molecules on the
hydrophobic H-ZSM-5(1880) were degassed at room tempera-
ture, the amount of CH3CHO adsorbed on the H-ZSM-5(1880)
was hardly affected by the preadsorption of H2O vapor. In
contrast, the amount of CH3CHO adsorbed on the hydrophilic
H-ZSM-5(23.8) was decreased by the preadsorption of H2O va-
por (data not shown). Furthermore, the adsorption of CH3CHO
molecules on the TiO2/H-ZSM-5(1880) increased by ca. 10–
acetaldehyde over TiO prepared on H- and Na-ZSM-5 zeolites under UV light.
2
and then efficiently provide them onto the TiO2 photocatalytic
sites, high photocatalytic reactivity could be obtained. On the
+
other hand, the H sites on H-ZSM-5(220 and 68) did not
work as efficient adsorption sites for the acetaldehyde mole-
cules, resulting in lower photocatalytic reactivity. Moreover,
+
the acetaldehyde molecules highly stabilized on the Na sites
of the Na-ZSM-5 zeolite could not efficiently spill over to the
TiO2 photocatalytic sites, resulting in no photocatalytic reac-
tivity. Although no experimental evidences for the formation
of Na2TiO3 phases as a low photocatalytic active site could be
1
5% compared with the H-ZSM-5(1880). This result indicates
that the CH3CHO molecules easily interact with TiO2 sites
rather than siliceous H-ZSM-5 surfaces. It also suggests that
once the CH3CHO molecules adsorbed on TiO2 photocatalytic
sites are decomposed into CO2 and H2O by UV irradiation,
the CH3CHO molecules condensed in zeolite cavities are ef-
ficiently supplied onto the TiO2 sites.
The photocatalytic reactivity of the TiO2/ZSM-5 samples for
the oxidation of acetaldehyde with O2 in the absence of H2O
under UV light irradiation (λ > 270 nm) is shown in Fig. 7. The
reactivity was found to depend on the SiO2/Al2O3 ratio of the
ZSM-5 zeolites. TiO2 prepared on H-ZSM-5(1880) showed the
highest photocatalytic reactivity among these hybrid photocata-
lysts. As the Al2O3 content increased, the reactivity decreased,
whereas the TiO2 prepared on Na-ZSM-5(23.8) showed no pho-
tocatalytic reactivity. Because the silanol groups in the H-ZSM-
+
obtained, the Na sites migrated into the TiO2 particles might
work as a recombination center to suppress the photocatalytic
reactivity.
The effects of H2O vapor on the photocatalytic oxidation re-
action of acetaldehyde with O2 on these TiO2/ZSM-5 samples
are shown Fig. 8. H2O vapor effectively promoted the photo-
catalytic oxidation of acetaldehyde with O2 on the TiO2 photo-
catalysts prepared on H-ZSM-5. The role of H2O molecules in
the photocatalytic oxidation of acetaldehyde is considered to be
−
the source supplying the OH species that are converted to ac-
tive OH radicals by their reaction with the photoformed holes.
4+
Moreover, because H O molecules strongly adsorb on the Ti
2
sites of TiO surfaces, they may inhibit the strong adsorption
2
of acetaldehyde as a reactant or acetic acid molecules as inter-
5
(1880) zeolite cavities can condense acetaldehyde molecules
mediate species on the TiO2 photocatalytic sites, followed by

240
M. Takeuchi et al. / Journal of Catalysis 246 (2007) 235–240
a decrease in photocatalytic reactivity. Further detailed spec-
troscopic investigations into the detection of the intermediate
species to clarify the effect of the H2O vapor on the photocat-
alytic reaction mechanism will be undertaken.
[2] M. Anpo, H. Yamashita, in: M. Schiavello (Ed.), Heterogeneous Catalysis,
Wiley, London, 1997.
[
3] A. Fujishima, K. Hashimoto, T. Watanabe (Eds.), TiO Photocatalysis
2
Fundamentals and Applications, BKC, Tokyo, 1999.
4] H. Yamashita, S. Kawasaki, Y. Ichihashi, M. Harada, M. Takeuchi, M. An-
po, G. Stewart, M.A. Fox, C. Louis, M. Che, J. Phys. Chem. B 102 (1998)
5870.
[
4
. Conclusion
[5] M. Harada, M. Honda, H. Yamashita, M. Anpo, Res. Chem. Intermed. 25
(
1999) 757.
TiO2 photocatalysts were prepared on ZSM-5 zeolites by
[
6] H. Yamashita, M. Harada, A. Tanii, M. Honda, M. Takeuchi, Y. Ichihashi,
M. Anpo, N. Iwamoto, N. Itoh, T. Hirao, Catal. Today 63 (1) (2000) 63.
[7] H. Kominami, J. Kato, Y. Takada, Y. Doushi, B. Ohtania, S. Nishimoto,
M. Inoue, T. Inui, Y. Kera, Catal. Lett. 46 (1997) 235.
8] H. Kominami, J. Kato, S. Murakamia, Y. Ishii, M. Kohno, K. Yabutani,
T. Yamamoto, Y. Kera, M. Inoue, T. Inui, B. Ohtani, Catal. Today 84
the impregnation of (NH4)2[TiO(C2O4)2] aqueous solution as
a green process. TiO2 prepared on siliceous H-ZSM-5 showed
higher photocatalytic reactivity compared with the commercial
TiO2 photocatalyst, because the zeolite worked as an effective
adsorbent to condense acetaldehyde molecules. The photocat-
alytic reactivity of TiO2/ZSM-5 was largely dependent on the
interaction of the reactants with the adsorption sites of the ze-
olite. The acetaldehyde molecules interacting with the silanol
groups of the hydrophobic H-ZSM-5(1880) can efficiently spill
over onto the TiO2 sites, producing high photocatalytic reac-
tivity. In contrast, the H-ZSM-5(220 and 68) with high Al2O3
content do not work as suitable adsorbents for acetaldehyde,
resulting in decreased photocatalytic reactivity. Because the ac-
[
(2003) 181.
[
9] N. Takeda, T. Torimoto, S. Sampath, S. Kuwabata, H. Yoneyama, J. Phys.
Chem. 99 (1995) 9986.
10] V. Durgakumari, M. Subrahmanyam, K.V. Subba Rao, A. Ratnamala,
M. Noorjahan, K. Tanaka, Appl. Catal. A Gen. 234 (2002) 155.
11] M. Anpo (Ed.), Photofunctional Zeolites, NOVA, New York, 2000.
12] T. Ibusuki, K. Takeuchi, J. Mol. Catal. 88 (1994) 93.
[
[
[
[
13] H. Uchida, S. Itoh, H. Yoneyama, Chem. Lett. (1993) 1995.
[14] N.Y. Chen, J. Phys. Chem. 80 (1976) 60.
15] L. Liao, W. Wu, C. Chen, J. Lin, J. Phys. Chem. B 105 (2001) 7678.
[16] D.S. Muggli, M.J. Backes, J. Catal. 209 (2002) 105.
17] R. Mendez-Roman, N. Cardona-Martinez, Catal. Today 40 (1998) 353.
18] G. Martra, S. Coluccia, L. Marchese, V. Augugliaro, V. Loddo, L. Palmi-
sano, M. Schiavello, Catal. Today 53 (1999) 695.
[
+
etaldehyde molecules strongly trapped on the Na sites of the
[
[
zeolite could not hardly transfer onto the TiO2 sites, TiO2/Na-
ZSM-5(23.8) showed no reactivity. Moreover, the oxidation of
acetaldehyde over the TiO2/ZSM-5 photocatalysts was effec-
tively enhanced by the addition of H2O vapor. The competitive
adsorption of H2O against acetaldehyde on the TiO2 photo-
catalytic sites may depress the highly stabilized acetaldehyde
molecules as reactants and/or acetic acid as the intermediate
species, resulting in enhanced photocatalytic reactivity from the
addition of H2O vapor.
[19] L. Cao, Z. Gao, S.L. Suib, T.N. Obee, S.O. Hay, J.D. Freihaut, J. Catal. 196
2000) 253.
(
[
[
20] J. Fan, J.T. Yates Jr., J. Am. Chem. Soc. 118 (1996) 4686.
21] A. Jentys, G. Warecka, M. Derewinski, J.A. Lercher, J. Phys. Chem. 93
(1989) 4837.
[
[
[
[
22] L. Marchese, J. Chen, P.A. Wright, J.M. Thomas, J. Phys. Chem. 97 (1993)
8109.
23] R. Buzzoni, S. Bordiga, G. Ricchinardi, G. Spoto, A. Zecchina, J. Phys.
Chem. 99 (1995) 11,937.
24] J.N. Kondo, M. Iizuka, K. Domen, F. Wakabayashi, Langmuir 13 (1997)
References
7
47.
25] A.G. Pelmenschikov, R.A. van Santen, J. Phys. Chem. 97 (1993) 10,678.
[
1] D.F. Ollis, H. Al-Ekabi (Eds.), Photocatalytic Purification and Treatment
of Water and Air, Elsevier, Amsterdam, 1993.
[26] T. Okachi, M. Onaka, J. Am. Chem. Soc. 126 (2004) 2306.
[27] S. Imachi, M. Onaka, Chem. Lett. 34 (5) (2005) 708.