Photocatalytic degradation of gaseous toluene over hollow "spindle-like" α-Fe2O3 loaded with Ag

Article abstract of DOI:10.1016/j.materresbull.2012.02.039

In this work, hollow "spindle-like" α-Fe₂O ₃ nanoparticles were synthesized by a hydrothermal route. The Ag/α-Fe₂O₃ catalyst was prepared based on the spindle-shaped α-Fe₂O₃ with CTAB as th

Full text of DOI:10.1016/j.materresbull.2012.02.039

Materials Research Bulletin 47 (2012) 1459–1466
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Materials Research Bulletin
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Photocatalytic degradation of gaseous toluene over hollow ‘‘spindle-like’’
loaded with Ag
a
-Fe O
2
3
a,b
a
a,d,
a
c
d
d,
Hong Li , Qidong Zhao , Xinyong Li *, Yong Shi , Zhengru Zhu , Moses Tade , Shaomin Liu **
a
State Key Laboratory of Fine Chemical, Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian
University of Technology, Dalian 116024, China
b
Department of Basic, Dalian Naval Academy, Dalian 116018, China
c
Research Center of Hydrology and Engineering, Academy of City and Environment, Liaoning Normal University, Dalian 116029, China
d
Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
2 3
In this work, hollow ‘‘spindle-like’’ a-Fe O nanoparticles were synthesized by a hydrothermal route.
Received 20 October 2011
Received in revised form 12 February 2012
Accepted 23 February 2012
Available online 3 March 2012
2 3 2 3
The Ag/a-Fe O catalyst was prepared based on the spindle-shaped a-Fe O with CTAB as the surfactant,
which showed excellent photoelectric property and photocatalytic activity. The structural properties of
these samples were systematically investigated by X-ray powder diffraction, scanning electronic
microscopy, transmission electronic microscopy, energy-dispersive X-ray spectra, and UV–Vis diffuse
reflectance spectroscopy techniques. The photo-induced charge separation in the samples was
Keywords:
demonstrated by surface photovoltage measurement. The photocatalytic performances of the Ag/
Fe and -Fe samples were comparatively studied in the degradation of toluene under xenon lamp
irradiation by in situ FTIR spectroscopy. Benzaldehyde and benzoic acid species could be observed on the
-Fe surface rather than Ag/ -Fe surface. The results indicate that the Ag/ -Fe sample
exhibited higher photocatalytic efficiency.
a-
A. Composites
2
O
3
a
2 3
O
B. Chemical synthesis
C. Electron microscopy
C. Infrared spectroscopy
D. Catalytic properties
a
2
O
3
a
2
O
3
a
2 3
O
ß 2012 Elsevier Ltd. All rights reserved.
1
. Introduction
efficiently and possess proper band-gaps and advanced micro-
scopic structure is still indispensable. As an important n-type
Volatile aromatic compounds are particularly important con-
semiconductor, hematite (
chemical sensor, and magnetic applications due to its low cost,
non-toxicity and high resistance to corrosion. -Fe has also
been selected as the support material for noble metal [11–14]. The
notable advantages of supported noble metal catalysts are
relatively high activity, mild process conditions, easy separation,
and better handling properties. The choice of an efficient support
could significantly improve the activity, selectivity, recycling, and
reproducibility of Ag catalyst systems.
2 3
a-Fe O ) is widely used for catalyst,
stituents and comprise up to 44% of the volatile hydrocarbon
mixture in the urban atmosphere [1]. Toluene is a major volatile
aromatic pollutant. Various studies have tested the potential use of
gas-phase photocatalytic oxidation of toluene for air detoxification
2–6]. The emphasis on photocatalytic oxidation of toluene has
shifted toward the application of photocatalysis for air treatment
7]. Recently, increasing interest has grown in the photocatalytic
oxidation of toluene by using visible-light active photocatalysts.
Florea et al. [8] and Zhu et al. [9] reported the photooxidation of
toluene over ferrite-type catalysts. Obee [10] reported the
investigation of photooxidation of toluene and formaldehyde
using another type of reactor, titania on glass-plate reactor and
compared the results of two reactor designs.
a
2 3
O
[
[
However, to date,
tures have not been sufficiently explored. Many synthesis methods
have been developed for the preparation of hematite ( -Fe
2 3
a-Fe O materials with various nanostruc-
a
2 3
O )
including sol–gel, hydrolysis of iron salt, solvothermal, and
hydrothermal synthesis [15–18]. The catalytic properties depend
on the morphology, particle size, and surface area, which are very
sensitive to the preparation method used in their synthesis. The
hydrothermal method is most favorable for its simplicity and good
control of grain size in synthesizing magnetic oxides with
controlled nanostructures.
From the viewpoint of solar energy utilization, the exploration
of semiconductor photocatalysts that could utilize visible light
*
Corresponding author at: State Key Laboratory of Fine Chemical, Key Laboratory
of Industrial Ecology and Environmental Engineering (MOE), School of Environ-
mental Science and Technology, Dalian University of Technology, Dalian 116024,
China. Tel.: +86 411 8470 7733; fax: +86 411 8470 7733.
2 3
In this study, hollow ‘‘spindle-like’’ a-Fe O was fabricated by a
facile hydrothermal method and then was employed as the
support of Ag nanoparticles. After systematic characterization of
** Corresponding author. Tel.: +61 08 9266 9056; fax: +61 08 9266 2681.
2 3 2 3
the bulk and surface structures of hollow a-Fe O and Ag/a-Fe O
E-mail addresses: xyli@dlut.edu.cn (X. Li), shaomin.liu@curtin.edu.au (S. Liu).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2012.02.039

1
460
H. Li et al. / Materials Research Bulletin 47 (2012) 1459–1466
catalytic materials, their performances in photocatalytic degrada-
tion of gaseous toluene and the detailed reaction mechanism were
then comparatively studied by in situ FTIR spectroscopy. Positive
illuminated by an XQ-500W xenon lamp. The distance between the
lamp and sample was about 15 cm. The light intensity at the sample
holder was about 40.5 mW cm . Two pellets were prepared in
À2
effect of Ag species on the photocatalytic properties of Ag/
a
-Fe
2
O
3
parallel.
composite material was expected.
Thereactioncellwaspurgedbydryairfor1 h.After1 h,thefluxof
dry air was set at 20 mL/min. Spectra of the clean catalyst surface
were collected after this process and utilized as the background.
2
. Experimental
Subsequently, toluene species was fed at the flow rate of 2
mL/h for
2
.1. Preparation of catalysts
ca. 30 min using a syringe pump to a mixing tee where it was
vaporized and mixed with the dry air. The reactant mixture then
flowed through the reaction cell and allowed to equilibrate at room
temperature (293 K). We determined whether the reactant concen-
trationwasstabilizedbycollectingitsinfraredspectrumevery5 min
until a stable peak line on the infrared spectrum was obtained. Once
the reactant concentration was stabilized, the inlet and outlet ports
were shut off and the lamps were turned on. The infrared spectra
were continuously collected during the reaction. The infrared
All the materials were analytical grade regents and used
without further purification.
Hollow ‘‘spindle-like’’ -Fe
mal route. FeCl O (18 mmol) was added into distilled water
Á6H
60 mL) to form a clear solution. Then 4.32 g urea (CH O) and
mL ethylene glycol, a protective agent were added into the
a
2 3
O was obtained via a hydrother-
3
2
(
9
4 2
N
solution. Subsequently under magnetic stirring for 30 min at room
temperature, the resultant solution was put into a Teflon-lined
stainless steel autoclave of 100 mL in capacity, which was sealed
and maintained at 180 8C for 12 h. After cooling to room
temperature naturally, the obtained precipitate was collected by
filtration, and then washed with absolute ethanol and distilled
water in sequence for several times. The final product was dried in
a vacuum box at 80 8C for 12 h.
À1
spectra were collected with a resolution of 1 cm and 20 scans in
À1
the region of 4000–1000 cm
.
The reactant concentration was measured by gas chromatog-
raphy (Aligent 7890A, USA). During the reaction process, samples
of approximately 1
mL at the outlet every 0.5 h, were collected and
the change in concentration of toluene was analyzed by a gas
chromatography equipped with FID (HP-5 capillary column
0
.3335 g of as-prepared hollow spindle-shaped
a
-Fe
2
O
3
was
(30 m  320
m
m  0.25 m)) and TCD (Porapak Q). When the peak
added into distilled water (20 mL) to form a solution under
ultrasonic stirring for 10 min at room temperature. 0.45 g CTAB
was added into the above solution, then after magnetic stirring for
intensity of toluene did not obviously change, the xenon lamp was
turned off and the cell was purified with the nitrogen.
3
0 min and ultrasonic stirring for 30 min, the solution was added
3. Results and discussion
into 25% NH O (3 mL) and 0.105 g AgNO under magnetic
3
ÁH
2
3
stirring for 20 min at room temperature. Then the precipitation
was collected by centrifugation, and washed with absolute ethanol
and distilled water in sequence for several times. After evapora-
tion, the catalyst was then dried at 100 8C for 12 h and calcined at
3.1. XRD analysis
The X-ray diffraction patterns of
samples are shown in Fig. 1. Seven major characteristic peaks can
be indexed as the rhombohedral structure (JCPDS 84-0311) [20],
2 3 2 3
a-Fe O and Ag/a-Fe O
5
50 8C in air for 2 h followed by slow cooling under air atmosphere.
which indicates the
2 3
a-Fe O phase with high purity and
2.2. Characterizations
crystallinity (Fig. 1). No peaks from other phases are found,
suggesting that the as-synthesized sample is pure highly. The
The phase structures of the
a-Fe O
2 3
based samples prepared
characteristic peaks at 2u of 24.188, 33.158, 35.758, 40.938, 49.438,
were determined by X-ray diffraction (XRD, RIGAKU, Dmax22000)
with Cu K radiation ( = 0.15418 nm) over the 2 range of 20–808.
The morphology of the samples was investigated by scanning
electronic microscopy (SEM) with a JSM-6700 LV electron micro-
54.028, 57.568, 62.518, 64.058, 71.948 and 75.488 are corresponding
to (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (0 1 8), (2 1 4),
(3 0 0), (0 1 1 0) and (2 2 0) crystallographic nucleation planes of
hematite phase, respectively. In addition, the peak of (1 0 4) planes
a
l
u
scope operating at 5.0 kV and transmission electron microscope
2 3
at 2u = 33.158 indicated a fine preferential growth of the a-Fe O
sample in the (1 0 4) direction. Two peaks of (1 1 1) and (2 0 0)
planes corresponding to Ag species (JCPD No. 00-004-0783) could
2
(TEM, FEITecnaiG 20).Thecompositionswereexaminedbyenergy-
dispersive X-ray spectroscopy (EDX) in the SEM. The amount of Ag
dopant was determined by inductive coupling IPS-AEC (Optima
2
000 DV, Perkin Elmer, USA). Light absorption property was
examined using a UV–Vis diffuse reflectance spectrophotometer
JASCO, UV-550). The separation and transfer behaviors of the
(
photogenerated charge carriers in the samples were investigated
using a lock-in-based surface photovoltage (SPV) measurement
system, which consists of a monochromator (model Omni-
and a lock-in amplifier (model SR830-DSP) with an optical chopper
model SR540) running at a frequency of 20 Hz. All of the SPV
l 3005)
(
measurements were performed at room temperature. The XPS data
were recorded using an ESCALAB250 electron spectrometer using
+
achromatic Al K
a
radiation (1486.6 eV) with Ar sputtering to
remove the surface layer of the sample.
2.3. Photocatalytic performance of the
2 3 2 3
a-Fe O and Ag/a-Fe O
sample by GC and in situ FTIR
In situ FT-IR spectra were collected with a Fourier Transform
Infrared Spectrophotometer (BRUKER VERTEX 70 Optics) and a self-
made in situIRquartz photoreactioncell[19]. The photocatalyst was
Fig. 1. The X-ray diffraction patterns of
2 3 2 3
a-Fe O and Ag/a-Fe O samples.

H. Li et al. / Materials Research Bulletin 47 (2012) 1459–1466
1461
be observed in the X-ray diffraction patterns (Fig. 1). From ICP
analysis, 0.632 wt% Ag was loaded into the -Fe sample.
As calculated from the XRD line broadening of the (1 0 4) peak
using the Scherrer equation D = 0.89 cos , the average sizes of
the -Fe and Ag/ -Fe samples are ca. 27 nm and 21 nm,
respectively. The crystallite size of Ag/ -Fe sample seems to
decrease with Ag loading. Previously, the crystallite size of
mesoporous TiO was also found to become smaller with Ag
doping [21]. Herein, the decreased crystallite size of
a
-Fe
2
O
3
with
a
2
O
3
Ag loading was probably due to the ultrasonic treatment, which
destroyed some of the -Fe particles into smaller ones.
a
2 3
O
l
/
b
u
a
2
O
3
a
2
O
3
3.2. SEM and TEM analysis
a
2 3
O
The SEM and TEM images in Fig. 2a–d show the morphology of
2
the
a-Fe O
2 3
and Ag/a-Fe O
2 3
samples, respectively. The two samples
Fig. 2. The SEM images of the
a-Fe
2
O
3
(a) and Ag/a-Fe
2
O
3
samples (b), the TEM images of the
a-Fe
2
O
3
(c) and Ag/a-Fe
2
O
3
samples (d), and EDS spectrum of the Ag/
a-Fe
2
O
3
sample (e).

1
462
H. Li et al. / Materials Research Bulletin 47 (2012) 1459–1466
1
/2
Fig. 3. The UV–Vis absorption spectra and (
Fe and Ag/ -Fe samples.
ah
n
)
vs. hn curve (inset) of the a-
2
O
3
a
2 3
O
Fig. 5. The photocatalytic oxidation of toluene over the commercial
2 3
a-Fe O ,
fabricated -Fe , and Ag/ -Fe samples after the reaction proceeding for 6 h
a
2
O
3
a
2 3
O
under xenon lamp irradiation conditions.
are completely composed of hollow ‘‘spindle-like’’ particles. It could
also be seen that spindle-shaped samples have the dimension size of
micrometer. Statisticalanalysisofdimensionaldistributions(shown
inFig. 2cand d)basedonTEM indicatesthatthelengthandthewidth
3.4. SPV analysis
The SPV technique can provide a rapid, nondestructive
monitor of the surface properties of semiconductors [23]. Fig.
are about 1.5
mm and 0.5 mm, respectively.
Fig. 2d shows the TEM micrograph of Ag/
shown by the image, some dots exist on the surface of the sample
distinctly. Hence, we can distinguish the Ag particles from the
a
-Fe
2
O
3
samples. As
4 shows the SPV spectra of the
It could be observed by systematic comparison of the change of
the SPV spectra that the Ag/ -Fe sample exhibits a more
distinguished SPV response, and its performance is evidently
higher than the -Fe sample. Hence, the higher photocata-
lytic activity might be expected for Ag/
the Ag species adsorbed onto the surface of
2 3 2 3
a-Fe O and Ag/a-Fe O samples.
a
2 3
O
spindle-like support. The EDX patterns of the synthesized Ag/
a-
Fe sample were taken to screen the composition of the metal.
2
O
3
a
2 3
O
Metallic silver was successfully identified by referring to the
corresponding EDX patterns of the samples (Fig. 2e).
a
-Fe
2
O
a
3
sample. Probably,
-Fe play a key
2 3
O
role in trapping the photo-generated electrons, which is due to
3
.3. DRS analysis
that the Fermi level of Ag (ca. À4.3 eV) [24] is much lower than
the conduction band edge of the
electronic energy.
2 3
a-Fe O nanocrystals in
The UV–Vis absorption spectra of the
2 3 2 3
a-Fe O and Ag/a-Fe O
samples are given in Fig. 3. A more broad and strong absorption
with the absorption edge at about 674 nm was found for Ag/
Fe sample, which absorbed more visible light than -Fe . The
inset in Fig. 3 shows a plot of ( against the energy of
absorbed light, from which the direct allowed band gaps can be
estimated. Using this method, the estimated band gap of the Ag/
Fe sample is found to be 1.7 eV. Compared with the reported
band gap values of -Fe (1.95 eV) and bulk -Fe (2.2 eV)
22], the absorption edge of the -Fe sample is red-shifted.
Therefore, a better absorption capability of visible light is obtained
with the Ag/ -Fe sample.
a
-
2 3 2 3
3.5. The photocatalytic properties of the a-Fe O and Ag/a-Fe O
samples for degradation of gaseous toluene by GC analysis and in situ
study
2
O
3
a
2
O
3
1
/2
ahn)
a
-
Fig. 5 shows the photocatalytic oxidation of toluene over the as-
2
O
3
2 3 2 3
prepared a-Fe O and Ag/a-Fe O samples after the reaction
a
2
O
3
a
2
O
3
proceeding for 6 h under xenon lamp irradiation conditions. The
initial concentration of 100 ppm for toluene was used for reaction.
The degradation percentage of toluene reaches up to 88%, whereas
[
a
2 3
O
a
2
O
3
the degradation percentage of toluene over commercial
fresh -Fe , and without any catalyst reaches to 56%, 77% and
7%, respectively (see Fig. 5). It is obviously seen that the
photocatalytic activity of the Ag/ -Fe sample is more active
than that of the -Fe sample.
We ascribe the enhanced photocatalytic activity of Ag/
2 3
a-Fe O ,
a
2 3
O
3
a
2 3
O
a
2 3
O
a
2 3
-Fe O
sample to a synergistic effect between loaded Ag species and the
hollow supports. Firstly, the loaded silver content could not change
the band gap of
band in the visible-light range instead. Hence, we believe that the
loaded Ag on the surface of the -Fe might be excited by proper
2 3
a-Fe O , but induce a more intensive absorption
a
2 3
O
incident photons, producing excitons via the plasmon effect [25].
There is certain possibility that the excitons dissociate. Some of the
yielded charges could transfer across the interface between Ag
nanoparticle and the
formation of active species at the surface. In addition, the excited
electrons in the CB of -Fe would be trapped by Ag species
adsorbed onto the surface of -Fe , and the Ag could enhance
2 3
a-Fe O support, contributing to the
a
2 3
O
a
2 3
O
the separation of the photo-generated electrons from the holes,
which would subsequently help form active radicals to degrade the
Fig. 4. The SPV spectra of the prepared
2 3 2 3
a-Fe O and Ag/a-Fe O samples.

H. Li et al. / Materials Research Bulletin 47 (2012) 1459–1466
1463
Fig. 6. The infrared absorbance spectra of toluene over the
2 3
a-Fe O sample under xenon lamp irradiation for 6 h.
adsorbed pollutants. The above discussion explains the reason
a
-Fe
2 3 2 3
O and Ag/a-Fe O samples surface by water incorporation
why Ag/
sample.
2 3 2 3
a-Fe O catalyst is much more active than the a-Fe O
(Figs. 6b and 7b).
For Fig. 6, some bands related to the adsorbed water and
hydroxyl groups [27] in the range of 1300–1800 cm . The band in
À1
À1
3.6. In situ FTIR study of photodegradation mechanism of toluene
the region of 1260–1410 cm
could be assigned to be the
corresponding aliphatic C–H bending vibration. After 2 h of
illumination, the formation of several small bands in the 1900–
In situ infrared reaction study provides real-time monitoring of
transient events which are occurring on the catalyst during the
reaction. A set of infrared absorbance spectra obtains during the
À1
À1
À1
À1
À1
1100 cm
range (1559 cm
,
1511 cm
,
1489 cm
and
1338 cm ). Similar bands have been reported for benzaldehyde
photocatalytic oxidation of toluene over
samples are shown in Figs. 6 and 7, respectively. The spectra were
collected using the -Fe and Ag/ -Fe samples as the
a
-Fe
2
O
3
and Ag/
a
-Fe
2
O
3
adsorption on nanostructured TiO
2
and Degussa P25 [27–29]. As
the irradiation time was increased, new bands appeared at
À1 À1
and 1338 cm . These results indicate that weakly
a
O
2 3
a
2
O
3
1489 cm
background, respectively. Prior to xenon lamp illumination (t = 0),
adsorbed benzaldehyde was formed as the reaction progressed
[30]. Benzaldehyde molecules resulting from the photo-oxidation
of toluene weakly interact with the catalyst surface so that they
can be released to the gas phase. By contrast, hydroxyl groups on
the spectrum displays the characteristic toluene bands at
À1
À1
À1
À1
À1
À1
À1
À1
À1
2
1
987 cm
,
2969 cm
,
2927 cm
,
2901 cm
,
2901 cm ,
À1
539 cm , 1451 cm , 1407 cm , 1394 cm
and 1382 cm
[
26], respectively. Upon irradiation, the intensity of these peaks
2 3
the surface of a-Fe O are able to react with photo-produced
began to decrease slowly. The signal corresponding to CO
2
benzaldehyde, which is then retained on the catalyst surface.
Subsequently it can be converted into other products, like benzoic
À1
À1
(2362 cm and 2340 cm ) increased as the reaction progressed
Figs. 6c and 7c). The photocatalytic oxidation pathway of toluene
À1
À1
(
acid (1489 cm and 1338 cm ) [31], which adsorbed on the
a-
over commercial
shown) (Fig. 6).
The FTIR spectra display a marked increase of the broad
absorption in the 3800–2800 cm range, indicating the regener-
a
-Fe
2
O
3
catalyst looks pretty similar (data not
Fe surface, leading to the progressive deactivation of the
2 3
O
catalyst in the gas-solid system. These hydroxyl groups play a key
role in the photocatalytic process and then this dehydroxylation
should be responsible for the deactivation observed in the reaction
carried out in the absence of water vapor.
À1
ation of interacting hydroxyl groups and adsorbed molecules on
the sample surface, a small increase of the isolated hydroxyl
Insights on the behaviors of the surface species of the
2 3
a-Fe O
À1
bands, mainly the one at 3690 cm
is observed. The results
and Ag/ -Fe samples during the photo-oxidation process were
a
2 3
O
indicated that some adsorbed surface species molecules are
released from the adsorption sites but probably remain on the
obtained by analyzing the high frequency region of the spectrum
of the catalyst, where the absorptions due to the stretches of

1
464
H. Li et al. / Materials Research Bulletin 47 (2012) 1459–1466
2 3
Fig. 7. The infrared absorbance spectra of toluene over the Ag/a-Fe O sample under xenon lamp irradiation for 6 h.
hydroxyl groups and adsorbed water molecules are observed. The
2 3
toluene occurred on the Ag/a-Fe O surface under the experimen-
spectrum of the catalyst exhibits an asymmetric peak at
tal conditions used. These results show that toluene was
mineralized into carbon dioxide and water as the major species.
À1
3
672 cm , due to the stretching mode of free hydroxyl groups.
À1
The broad band at 3700–3500 cm (Fig. 6), resulting from the
overlap of the bands is ascribed to the stretching modes of
hydrogen bonded –OH groups and water molecules coordinated
However, only a partial oxidation was achieved on the
surface, due to benzaldehyde and benzoic acid species adsorbed on
the -Fe surface, leading to the progressive deactivation of the
2 3
a-Fe O
a
2 3
O
3
+
to surface Fe cations [30,31,6].
catalyst in the gas–solid system.
For Fig. 7, no bands assigned to be immediate species appeared
during the reaction. This indicates that complete degradation of
2 3
3.7. Study on the stability of the catalyst Ag/a-Fe O
We carried out the experiments on the stability of the catalyst
Ag/ -Fe for toluene photodegradation. Every repetition exper-
a
2 3
O
iment lasted up to 6 h under the same in situ FT-IR test condition.
The photodegradation ratio of the substrate was measured as a
function of time for the Ag/a-Fe O catalyst. The results are shown
2 3
in Fig. 8. It is found that the photodegradation ratio of toluene
reaches to 76% after the fifth repetition, which is slightly lower
than that (88%) with the fresh Ag/
a
2 3
-Fe O catalyst. The results
show that the Ag/
reusability.
2 3
a-Fe O catalyst has good stability and
In order to get a better understanding of the stability of the
obtained composite Ag/Fe
2
O
3
catalyst, additional XPS analysis of
2 3
the fresh and used Ag/Fe O samples was performed and the
results are shown in Fig. 9. Fig. 9b shows the high-resolution XPS
spectrum of the Ag in the fresh Ag/Fe . The spectrum provides
the binding energies of the Ag 3d3/2 and Ag 3d5/2 peaks as 374.6 and
68.7 eV, respectively, which reveals that the Ag species exists
2 3
O
3
predominantly in the metallic form [32]. Meanwhile, the 6.0 eV
gap between the two states is also characteristics of metallic Ag.
2 3
Fig. 8. Cyclic photodegradation of toluene by the Ag/a-Fe O sample.

H. Li et al. / Materials Research Bulletin 47 (2012) 1459–1466
1465
Fig. 9. XPS spectra of the reused Ag/Fe
2
O
3
(after the fifth operation of photodegradation) and the fresh Ag/Fe
2 3 2 3
O sample. (a) The whole XPS spectrum of the fresh Ag/Fe O
sample, (b) the XPS spectrum for the Ag in the fresh Ag/Fe
2
O
3
, and (c) the XPS spectrum for the Ag in the used Ag/Fe O .
2 3
The high resolution XPS spectrum of the Ag of the used Ag/Fe
2
O
3
is
Acknowledgments
shown in Fig. 9c, according to which two components of Ag
2
O
0
(367.4 eV) and Ag (368.2 eV) can be found in the used sample [33].
This work was supported financially by the National Nature
Science Foundation of China (NSFC-RGC 21061160495), the
National High Technology Research and Development Program
of China (863 Program) (No. 2010AA064902), the Major State Basic
Research Development Program of China (973 Program) (No.
2011CB936002), the Excellent Talents Program of Liaoning
Provincial University (LR2010090) and the Key Laboratory of
Industrial Ecology and Environmental Engineering, China Ministry
of Education.
2
Thus, the Ag particles and Ag O coexist on the surface of the sample
after the cyclic photodegradation, the small Ag particles on the Ag/
-Fe surface are partially oxidized into Ag O. Moreover, the
Ag O prevents further oxidation of the Ag particles on the Ag/
Fe
responsible for the fine stability of the composite Ag/
catalyst. The used catalyst sample of Ag/ -Fe still has a certain
stability and favorable activity because there still exist Ag species
on the surface of -Fe after more reaction cycles.
a
2
O
3
2
2
a-
0
2
O
3
surface. Hence, highly dispersed Ag species would be
-Fe
a
2 3
O
a
2 3
O
0
a
2 3
O
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