Gas-phase photo-oxidations of organic compounds over different forms of zirconia

Article abstract of DOI:10.1016/j.molcata.2004.11.029

Nanosized zirconia powders with different phase structure and specific surface area were prepared by various methods and were used as catalysts for gas-phase photocatalytic oxidations of methanol and hexane. The phase structure of the samples was analyzed by X-ray diffraction and Raman spectroscopy. The photocatalytic activity of the amorphous zirconia synthesized in this experiment is much higher than that of the crystalline samples. The activity of hexane oxidation on ZrO₂-0-110 reaches 25.6%, which is even higher than that of Degussa P25 (17.3%). Thermal gravimetric studies show that the high activity is due to the abundant surface hydroxyl groups on the catalysts. Sulfation of zirconia samples induces a decrease in the amount of surface hydroxyl groups and has a negative influence on the activity of the zirconia photocatalysts. The results of a 100 h test show that the activity of the synthesized amorphous zirconia catalyst is very stable.

Full text of DOI:10.1016/j.molcata.2004.11.029

Journal of Molecular Catalysis A: Chemical 229 (2005) 233–239
Gas-phase photo-oxidations of organic compounds
over different forms of zirconia
∗
Chunying Wu, Xiping Zhao, Yingjie Ren, Yinghong Yue , Weiming Hua,
Yong Cao, Yi Tang, Zi Gao
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,
Fudan University, Shanghai 200433, PR China
Received 9 November 2004; received in revised form 29 November 2004; accepted 29 November 2004
Abstract
Nanosized zirconia powders with different phase structure and specific surface area were prepared by various methods and were used
as catalysts for gas-phase photocatalytic oxidations of methanol and hexane. The phase structure of the samples was analyzed by X-ray
diffraction and Raman spectroscopy. The photocatalytic activity of the amorphous zirconia synthesized in this experiment is much higher than
that of the crystalline samples. The activity of hexane oxidation on ZrO
2
-0-110 reaches 25.6%, which is even higher than that of Degussa P25
(17.3%). Thermal gravimetric studies show that the high activity is due to the abundant surface hydroxyl groups on the catalysts. Sulfation
of zirconia samples induces a decrease in the amount of surface hydroxyl groups and has a negative influence on the activity of the zirconia
photocatalysts. The results of a 100 h test show that the activity of the synthesized amorphous zirconia catalyst is very stable.
©
2004 Elsevier B.V. All rights reserved.
Keywords: Nanosized zirconia; Photocatalytic oxidation; Amorphous; Crystalline; Surface hydroxyl group; Sulfation
1
. Introduction
most frequent and accepted value is 5.0 eV, with a conduc-
tion band potential of −1.0 V versus NHE at pH 0 [3,4].
Previous literature [2–7] reported the application of zirconia
photocatalyst for several reactions, including H2 evolution
from water, oxidation of 2-propanol to acetone, oxidation
of propene and ethane, photodegradation of 4-chlorophenol,
4-nitrophenol and 1,4-pentanediol, oxidation of CO, photore-
duction of aqueous carbonate, oxygen isotopic exchange, and
reduction of CO2 with H2.
In the past decade, the significance of semiconductor pho-
tocatalysis as a potential environmental control technology
has aroused tremendous interest in semiconductor photo-
chemistry and photophysics [1]. Many oxide and sulfide
semiconductors such as TiO2, WO3, ZnO, SrTiO3, ZnS and
CdS are found to exhibit photocatalytic activity for a wide
range of chemical reactions. Among the metal oxides, ZrO2
has been considered recently as a photocatalyst in photo-
chemical heterogeneous reactions due to its relatively wide
Eg value and the high negative value of the conduction band
potential [2–4]. The reported values of the energy of the band
gap energy of this oxide range between 3.25 and 5.1 eV, de-
pending on the preparation technique of the sample, and the
The activity of the photocatalyst is influenced by its crys-
tallinity as well as other factors such as surface area, crystal
size, synthesis method, band gap, porosity, crystal phase, and
surface acidity. On the basis of extensive studies on the pho-
tocatalysis of TiO2, high crystallinity reduces defective sites
−
+
of the catalyst, decreases the probability of e –h recombi-
nation, and hence shows high activity. However, Kominami
et al. [8] found that amorphous Nb2O samples prepared by
5
∗
the solvothermal method exhibited much higher activity than
that of the crystalline Nb2O5 samples for the photocatalytic
Corresponding author. Tel.: +86 21 65642409; fax: +86 21 65641740.
E-mail address: yhyue@fudan.edu.cn (Y. Yue).
1
381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.11.029

2
34
C. Wu et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 233–239
dehydrogenation of methanol and the photocatalytic miner-
alization of acetic acid in aqueous solutions.
formed on a Rigaku Thermoflex System with a temperature
◦
−1
ramp rate of 10 C min in flowing N2 and the amount of
sample used was 30 mg. Raman spectra was obtained using a
confocal microprobe Raman system (Lab Ram Infinity). The
excitation wavelength was 514.5 nm from an internal argon
ion laser with a power of 8 mW. UV–vis diffuse reflectance
spectra were obtained with a JASCO-V-550 UV-Vis spec-
trometer.
In this work, nanosized zirconia samples were prepared
by different methods and characterized by XRD, TG/DTA,
UV–vis spectra and N2 adsorption method. The gas-phase
photocatalytic oxidations of methanol and hexane on the zir-
conia catalysts were studied and correlated to the phase struc-
ture, specific surface area and the surface property of the cata-
lysts. The influence of sulfation on the photocatalytic activity
of the catalysts was investigated as well.
2.3. Photocatalytic oxidations of hexane and methanol
The photocatalytic oxidation of hexane was carried out
2
. Experimental
in a continuous fix-bed flow microreactor under atmospheric
pressure. The 0.3 g (20–40 mesh) catalyst was pretreated in
flowing N2 for 0.5 h. Hexane (1700 ppm) and balance air with
2
.1. Catalyst preparation
−
1
a space velocity (GHSV) of 4000 h were passed through
the catalyst overnight to reach adsorption equilibrium, and
then the catalyst was illuminated by two 8 W UV-lamps with
the optimal wavelength of 365 nm. The main oxidation prod-
ucts were CO2 and H2O and only a small amount of ac-
etaldehyde was produced during reaction. Unreacted hexane
was analyzed by a gas chromatograph equipped with a flame
ionization detector (FID). The photocatalytic activity of the
catalysts reached steady state after about 6 h and all the ac-
tivity data referred in this work were obtained at steady state.
The photocatalytic oxidation of methanol (1100 ppm) was
carried out in the same fix-bed flow reactor under similar
reaction conditions.
The zirconia precursors were synthesized by a precipita-
−
1
tion method [9]. The 6.0 mol l aqueous NH3 solution was
added dropwise to 0.4 mol l ZrOCl2 solution under stir-
ring until pH 10.0, and then aged at 0 C for 24 h. The ob-
tained white precipitate was filtered, washed thoroughly with
−
1
◦
◦
deionized water, dried at 110 C for 24 h, ground into a fine
powder and labeled as ZrO2-0-110. To obtain the amorphous
and tetragonal ZrO2 catalysts, the zirconia precursor was cal-
cined at different temperatures for 3 h. These catalysts were
labeled as ZrO2-0-110-t, where t stood for the calcination
temperature.
The monoclinic ZrO2 catalyst was prepared by the hydrol-
ysis of Zr-alkoxide [10]. In a typical synthesis, 0.0223 mol
of zirconium propoxide (Zr(OC3H7)4) was first dissolved in
5
3
0 ml of 1-propanol and the solution was slowly poured into
00 ml distilled water with vigorous stirring at room temper-
3. Results
ature. The mixed solution was allowed to continue stirring
for 1 h. The precipitate obtained was filtered, washed with
3
.1. Characterization of nanosized zirconia
◦
distilled water and dried at 110 C for 24 h, and then was
In order to obtain the zirconia catalysts with different
◦
calcined in air at 650 C for 3 h. The obtained product was
forms, the prepared zirconia precursor was calcined at var-
ious temperatures, their XRD patterns are shown in Fig. 1.
The crystalline phase of these calcined catalysts are differ-
ent. All of the catalysts are amorphous after calcination at
labeled as ZrO2-110-650.
Sulfated zirconia catalysts were prepared by immersing
−
1
uncalcined zirconia precursors (ZrO2-0-110) in 1 mol l
◦
H2SO4 for 45 min, followed by filtering, drying at 110 C
and calcining at 650 C for 3 h. The samples before and af-
ter calcination were labeled as ZrO2-0-110/SO4 -110 and
◦
2−
ZrO2-0-110/SO42 -650, respectively.
−
2
.2. Characterization
The crystalline phases of the zirconia catalysts were char-
acterized by X-ray powder diffraction (XRD). A Siemens D8
Advanced X-ray diffractometer with a Cu K␣ radiation, a
◦
−1
◦
scan speed of 8 min and a scan range of 20–70 was used.
The surface areas of the catalysts were measured by nitrogen
adsorption at 77 K using a Micromeritics ASAP 2000 instru-
ment and calculated by BET method. The morphologies of
representative zirconia catalysts were examined by transmis-
sion electron microscopy (TEM) using a JEOL-2011 instru-
ment. Thermogravimetric analysis (TG/DTA/DTG) was per-
Fig. 1. XRD patterns of zirconia samples. From bottom to top: ZrO2-0-110,
ZrO -0-110-200, ZrO -0-110-300, ZrO -0-110-450 and ZrO -110-650.
2
2
2
2

C. Wu et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 233–239
235
Table 1
Textural properties and activities of various photocatalysts
Sample
Surface area Conversion of Conversion of
2
−1
)
hexane (%)a
methanol (%)a
(m g
ZrO2-0-110
289
248
181
121
34
25.6
22.1
2.9
3.7
4.5
0
31.6
27.2
10.2
15.4
12.3
9.7
ZrO2-0-110-200
ZrO2-0-110-300
ZrO2-0-110-450
ZrO2-110-650
ZrO2-0-110/SO4 -110 168
ZrO2-0-110/SO4 -650
2
−
−
2
82
48
0
17.3
5.6
40.3
P25
a
7
h after illuminated by UV-lamps with the optimal wavelength of
365 nm.
ticles can be observed and all are adhere to each other, form-
ing hard agglomerates. The ZrO2-0-110-450 sample consists
of irregular spherical grains and the grain sizes are in the
Fig. 2. Raman spectra of zirconia samples. From bottom to top: ZrO2-0-110,
ZrO2-0-110-450, ZrO2-110-650; (᭹) tetragonal; (ꢀ) monoclinic.
◦
range of 15–30 nm. The ZrO -110-650 sample has the similar
morphology, but its gain sizes are about 30–50 nm, relatively
larger compared with ZrO2-0-110-450.
a temperature below 300 C. When the temperature reaches
4
2
◦ ◦ ◦ ◦ ◦
50 C, the peaks at 29.8 , 34.2 , 49.6 and 59.5 appears,
corresponding to the diffractions of (1 0 1), (0 0 2), (1 1 2)
and (2 1 1) of tetragonal ZrO2, respectively, indicating zir-
conia catalyst with pure tetragonal phase is obtained. These
results are in good agreement with previous literature [11,12],
which reported that the phase transformation of zirconia from
amorphous to tetragonal form takes place at about 400 C. A
mixture of monoclinic and tetragonal zirconia catalyst, pre-
dominantly in monoclinic phase was obtained by the hydrol-
UV–vis diffuse reflectance spectra of representative sam-
ples, ZrO2-0-110, ZrO2-0-110-450 and ZrO2-110-650 are
shown in Fig. 4. For all the samples, a non-negligible ab-
sorption in the near-UV range of 290–390 nm is observed,
which agrees with previous reports [3,4]. Crystallization of
amorphous samples on calcinations induces a red shift of the
absorption edge [8]. The spectra of three samples are similar
in the absorption peak shape and strength, indicating their
similar photoabsorbed properties despite of their different
phase structure.
◦
◦
◦
◦
◦
ysis of Zr(OC3H7)4. The peaks at 24.0 , 28.2 , 31.5 , 41.0 ,
5.0 and 55.8 are the diffraction of (0 1 1), (1 1 1), (1 1 1),
2 1 1), (1 1 2) and (1 3 0) of monoclinic ZrO2, respectively.
◦ ◦
¯
4
(
¯
Raman spectroscopy was applied in the present investiga-
tionbecauseofitssensitivityindeterminingthelocalorder, so
as to compensate for XRD shortcomings dealing with short-
range structures. Fig. 2 shows the Raman spectra of three rep-
resentative zirconia samples. According to previous Raman
studies, ZrO2 in tetragonal form has six Raman active modes
3.2. Photocatalytic activity of various zirconia samples
Using gas-phase photo-oxidation of hexane and methanol
as probe reactions, the photocatalytic activity of various
forms of zirconia was tested under the illumination of two
UV-lampswiththeoptimalwavelengthof365 nm. Thesteady
state conversions of hexane andmethanol are given in Table 1,
together with Degussa P25 for comparison. The activities
of crystalline zirconia catalysts, either tetragonal or mono-
clinic, are quite low, while the amorphous catalysts prepared
−
1
and is identified by peaks at 148 and 263 cm , whereas
monoclinic ZrO2 has 18 Raman active modes with diagnos-
−
1
tic twin-peaks at 170–180 and 180–190 cm [13–15]. In
the Raman spectrum of ZrO2-0-110-450, the peaks at 150,
−
1
2
64, 321, 464 and 640 cm are observed, characteristic of
◦
tetragonal phases of zirconia. For ZrO2-110-650, multiple
Raman peaks assignable to monoclinic zirconia are found.
Only several weak bands are present in the Raman spectrum
of ZrO2-0-110, which could be assigned to the bending and
stretching of zirconium–oxygen bond, indicating its amor-
phous structure. These results agree quite well with the XRD
measurements.
The specific surface areas of the prepared catalysts were
measured by N2 adsorption methods and listed in Table 1.
The surface area of the catalysts decreased gradually with
the increased calcination temperature. Moreover, the surface
area of monoclinic catalyst is much lower, only one-third of
that of tetragonal one.
by calcining at 100 and 200 C are very active. Their activi-
ties of hexane oxidation reach 25.6 and 22.1%, respectively,
which are even higher than that of Degussa P25 (17.3%).
◦
The zirconia catalyst prepared by calcination at 300 C has
a very low activity, though it retains amorphous structure
and comparable surface area. To explain such an unexpected
result, thermal gravimetric analysis of ZrO2-0-110, ZrO2-0-
110-200, ZrO2-0-110-300 and ZrO2-0-110-450 was carried
out in flowing N2. The DTG/DTA profiles of the samples are
shown in Fig. 5. Two endothermic peaks are observed on the
DTG profiles of ZrO2-0-110 and ZrO2-0-110-200, which are
associated with the desorption of physically adsorbed water
and the dehydroxylation of surface hydroxyls on ZrO2. But
◦
TEM micrographs of some of the representative catalysts
are shown in Fig. 3. In the ZrO2-0-110 samples, no clear par-
only peaks around 88 C are observed on the DTG profiles of
ZrO2-0-110-300 and ZrO2-0-110-450, indicating that there

2
36
C. Wu et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 233–239
Fig. 4. UV–vis diffuse reflectance spectra of various zirconia samples: (a)
2
−
ZrO2-0-110/SO4 -650; (b) ZrO2-0-110-450; (c) ZrO2-0-110; (d) ZrO2-
10-650.
1
are little surface hydroxyls left on these catalysts after calci-
nations at relative high temperature. The existence of surface
hydroxyls is probably the cause for the high photocatalytic
activities of ZrO2-0-110 and ZrO2-0-110-200.
In the UV–vis diffuse reflectance spectra, ZrO2-0-110
and ZrO2-0-110-450 show a maximum absorption around
250 nm. Since the photocatalytic performance of the cata-
lyst is related to the wavelength of illumination light [16,17],
Fig. 3. TEM micrographs of the samples: (a) ZrO2-0-110; (b) ZrO2-0-110-
50; (c) ZrO2-110-650.
4
Fig. 5. Thermograms of various zirconia samples: (a) ZrO2-0-110; (b) ZrO2-
0-110-200; (c) ZrO2-0-110-300; (d) ZrO2-0-110-450.

C. Wu et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 233–239
237
Table 2
Activities of various zirconia samples illuminated by UV-lamps with differ-
ent optimal wavelengths
Sample
Conversion of hexane (%)
I
II
ZrO2-0-110
ZrO2-0-110-450
ZrO2-110-650
25.6
3.7
4.5
29.4
9.3
9.7
I: 7 h after illuminated by UV-lamps with the optimal wavelength of 365 nm;
II: 7 h after illuminated by UV-lamps with the optimal wavelength of
2
53.7 nm.
UV-lamps with the optimal wavelength of 253.7 nm matching
the band gap energy of these samples was used. The activities
of various samples under such lamps are shown in Table 2.
Obviously, the photocatalytic activities of these samples are
improved under illumination light with suitable wavelength.
Among the three catalysts tested, amorphous zirconia cal-
◦
cined at 110 C exhibits highest activity, just as above.
3
.3. Effect of sulfation on the photoactivity of ZrO2
Sulfated zirconia catalysts were prepared by immersing
−
1
the ZrO2-0-110 sample in 1 mol l H2SO4 and then drying
◦
at 110 C. The XRD patterns of the sulfated catalysts before
and after calcined at 650 C are shown in Fig. 6. The structure
◦
of the sulfated catalyst before calcination retains amorphous.
◦
After calcination at 650 C, the catalyst shows a mixture of
2
−
-
Fig. 7. Thermograms of sulfated zirconia samples: (a) ZrO2-0-110/SO4
monoclinic and tetragonal structure, in which tetragonal is
the main phase. The surface area of the sulfated catalysts
measured by N2 adsorption is listed in Table 1. The ZrO2-
2
−
650; (b) ZrO2-0-110; (c) ZrO2-0-110/SO4 -110.
methanol over ZrO -0-110/SO42 -110 are much lower than
that of ZrO2-0-110, and the activities decrease further after
calcination at 650 C due to the decreasing of the surface
area. Sulfation renders a negative effect on the photocatalytic
activity of zirconia catalysts.
Thermal gravimetric analysis was again employed to ex-
plain this negative effect. Fig. 7 depicts the DTG/DTA pro-
files of sulfated catalysts. Only one endothermic weight loss
−
-110/SO42 -110 sample has much smaller specific surface
−
0
2
area after sulfation, and moreover, the surface area is reduced
further to 82 m g after calcination at 650 C.
The photocatalytic activities of the sulfated catalysts on
the photo-oxidation of hexane and methanol were tested and
the results are listed in Table 1 as well. The activities reduced
dramatically after sulfation, the conversion of hexane and
◦
2
−1
◦
◦
peak centered at 80 C was observed on both sulfated cata-
lysts, corresponding to the desorption of physically adsorbed
water. In the meantime, an extra exothermic peak around
◦
7
00 C is observed, corresponding to the release of SO3. No
evident peak of the dehydroxylation of surface hydroxyls is
observed for both of the sulfated samples, which probably
account for their low photocatalytic activity. The details of
this phenomenon deserve further studies.
UV–vis diffuse reflectance spectrum of ZrO2-0-
1
10/SO42 -650 is shown in Fig. 4. As compared with
−
zirconia with amorphous or pure tetragonal phase, ZrO2-0-
1
10/SO42 -650 calcined at a higher temperature induces
−
a red shift of the absorption edge probably due to grain
growth. However, the absorption spectrum is similar in peak
shape and strength as non-sulfated sample, which indicates
that sulfation has no significant effect on the photoabsorbed
properties of the sample.
Fig. 6. XRD patterns of sulfated zirconia samples. From bottom to top:
ZrO2-0-110/SO4 -110, ZrO2-0-110/SO4 -650; (᭹) tetragonal; (ꢀ) mon-
oclinic.
2
−
2−

2
38
C. Wu et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 233–239
Fig. 8. Activity of hexane oxidation over ZrO2-0-110 as a function of re-
action time. Reactant: 1700 ppm hexane in air; catalyst: 0.3 g ZrO2-0-110;
GHSV: 4000 h ; (᭹) UV-lamps with wavelength of 253.7 nm; (ꢀ) UV-
lamps with wavelength of 365 nm.
Fig. 10. Long-term test results of ZrO -0-110 illuminated by UV-lamps
2
with the optimal wavelength of 365 nm. Reactant: 1700 ppm hexane in air;
−1
−1
catalyst: 0.3 g; GHSV: 4000 h
.
ity and stability, which predict them to be a potential practical
photocatalyst.
3
.4. Stability of the catalysts
The conversion of hexane and methanol over ZrO2-0-110
4
. Discussion
catalysts as a function of reaction time was studied and de-
picted in Figs. 8 and 9. For all the reactions, the conver-
sion drops gradually in the initial reaction period and reaches
steady state after about 7 h, which is just the same as that over
titania catalysts [18]. According to previous studies [19], the
adsorption of less reactive partial oxidation intermediates on
the catalyst surface may be responsible for the initial deacti-
vation.
The photocatalytic oxidation reaction of hexane has been
run continuously on ZrO2-0-110 catalyst for 100 h, which
showed a maximum catalytic activity in the reaction, and the
results are shown in Fig. 10. It can be seen that the catalyst
was rather stable after the initial deactivation. The steady-
state conversion of hexane during the reaction is about 23%,
higher that of standard photocatalyst Degussa P25. The long-
term test results show that amorphous zirconia samples pre-
pared in this experiment possess both rather high photoactiv-
The valence band electrons of ZrO2 can be excited to the
conduction band under illumination of ultraviolet light, and
highly reactive electron–hole pairs are produced. The elec-
trons and holes after migration to the catalyst surface are ca-
pable of reducing and oxidizing surface-adsorbed substrates
in the gas-phase photo-oxidation. Furthermore, the positive
hole can also react with surface hydroxyl to form OH radical,
whereas the electron is trapped by surface-adsorbed oxygen
to form superoxide:
−
+
ZrO2 + hν ꢁ e + h
e− + O2 → O2−
h+ + S → Sox+
+
−
h + OH → OH
−
+
where e is an electron and h is a positive hole, and S and
Sox are the adsorbed organic substrate and its oxidized prod-
+
uct, respectively. The OH radical and superoxide are both ef-
fective oxygenation agents, which are able to degrade a great
variety of organic compounds [1,18].
Since the photocatalytic oxidation reactions via interfa-
cial charge transfer between the surface-adsorbed species,
reduction in the surface recombination of electron–hole pair
lead to a high efficiency of the photocatalytic process. Some
researchers [8,20] claim that high crystallinity, decreasing
defects acting as the electron–hole recombination center, im-
prove photocatalytic activity of TiO2 catalysts. In contrast
to the general results for TiO2, amorphous but not crys-
talline ZrO2 catalysts show the higher activity. This high
photo-activity may be due to the abundance of hydroxyl
groups present on the catalyst surface, which can prevent
Fig. 9. Activity of methanol oxidation over ZrO2-0-110 as a function of
reaction time. Reactant: 1100 ppm methanol in air; catalyst: 0.3 g ZrO2-0-
−1
1
10; GHSV: 4000 h ; UV-lamps with wavelength of 365 nm.

C. Wu et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 233–239
239
the electron–hole recombination by trapping hole and gen-
Acknowledgements
erating OH radicals acting as powerful oxidants. This has
been proved by the thermal gravimetric experiments shown
This work was supported by the Chinese Major State Ba-
sic Research Development Program (grant no. 2000077507),
the National Natural Science Foundation of China (grant no.
20303004) and the Shanghai Major Basic Research Program
(grant no. 03DJ14004).
in Fig. 5. Similar results were observed for the Nb2O pho-
5
tocatalyst by Kominami et al. [8].
It was reported that sulfation of anatase titania enhances
its activity for photocatalytic oxidation of CH3Br, C H and
6
6
C2H4. The improved activity for the sulfated titania cata-
lysts is not only due to a greater surface area but also the
increased surface acidity [18]. It is well known that sulfa-
tion of amorphous ZrO2 will create strong acid sites on the
surface, which have high catalytic activity for the conversion
of small hydrocarbon at low temperature [21]. But sulfation
does not improve the photo-catalytic activity of ZrO2 cat-
alysts in the gas-phase oxidation of hexane or methanol as
expected. On the contrary, the activities reduced dramatically
after sulfation. This negative effect can also be explained by
the decreasing of the hydroxyl group on the surface, which
may be caused by the reaction between the hydroxyl group
and the sulfate group. The result indicates that the hydroxyl
groups not the acid sites play an important role in the ZrO2
photo-catalytic reactions.
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[
[
face area were prepared and characterized. The effect of the
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◦
[
[
[
systematically. The amorphous ZrO2 prepared below 200 C
232.
in this experiment shows much higher activity than that of
the crystalline samples for the gas-phase photocatalytic oxi-
dations of hexane and methanol, presumably due to the abun-
dance of the hydroxyl groups on the catalyst surface. Sulfa-
tion of the ZrO2 results in a decrease in the amount of surface
hydroxyl groups of the catalysts, leading to a negative effect
on the photocatalytic activity of the catalysts. The results of
a long-term test of 100 h show that the activity of the amor-
phous zirconia photocatalysts is very stable.
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