An antimonate pyrochlore (H1.23Sr0.45SbO3.48) for photocatalytic oxidation of benzene: effective oxygen usage and excellent activity

Article abstract of DOI:10.1039/c6ta08652g

The sample H_1.23Sr_0.45SbO_3.48 crystallized in a pyrochlore structure is applied to photocatalytic oxidation of benzene in the gaseous phase. The pyrochlore structure is considered as the key factor for the effective oxygen adsorption, which makes the sample exhibit excellent photocatalytic activity.

Full text of DOI:10.1039/c6ta08652g

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An antimonate pyrochlore (H Sr0.45SbO3.48) for
1
.23
photocatalytic oxidation of benzene: effective
oxygen usage and excellent activity†
Cite this: DOI: 10.1039/c6ta08652g
Received 6th October 2016
Accepted 5th December 2016
Jing Chen, Yu Shao and Danzhen Li*
DOI: 10.1039/c6ta08652g
www.rsc.org/MaterialsA
The sample H1.23Sr0.45SbO3.48 crystallized in a pyrochlore structure is the oxidation of benzene is also ambiguous, which hampers the
applied to photocatalytic oxidation of benzene in the gaseous phase. development of an appropriate photocatalyst.
The pyrochlore structure is considered as the key factor for the
To develop an efficient photocatalyst for degradation of
effective oxygen adsorption, which makes the sample exhibit excellent benzene, efficient carrier capture is indispensable. Generally,
photocatalytic activity.
the photogenerated electrons and holes are captured by the
surface-adsorbed O and surface-adsorbed hydroxyl groups,
2
respectively. The oxidation reactions are reported to occur by
Benzene is one of the most fundamental organic materials.
Many important chemical compounds, such as phenol, toluene,
aniline, biphenyl, naphthalene, anthracene and dioxin, are
derived from benzene by replacing one or more of its hydrogen
atoms with another functional group. However, benzene and
most of its derivatives are poisonous. Thus, the cleavage of the
aromatic ring into low-toxic or non-toxic molecules is an
either indirect oxidation via the surface-bound hydroxyl radicals
3
and directly via the valence-band holes. However, in the
consideration of the kinetic factor, the capture of photo-
generated electrons by the surface-adsorbed O is relatively slow
2
4
(
about 10 ms) and acts as a rate determining reaction. This is
very important but oen neglected in the previous studies listed
in Table S1.† Thus, it is greatly desired to develop new photo-
catalysts to promote the adsorption of O2 for increasing the
quantum efficiency of PCO.
important issue. Photocatalytic oxidation (PCO) using TiO has
2
attracted considerable interest as a promising technology for
1
deep oxidation of benzene at room temperature. However, the
In this paper, we are intrigued by the pyrochlore compound.
The pyrochlore structure has Fd-3m symmetry. The general
2
low quantum yield of TiO due to fast recombination of pho-
togenerated electrons and holes greatly hampers its extensive
application for the removal of benzene. Several approaches have
been explored for the improvement of the PCO quantum yield,
0
composition of a typical pyrochlore oxide is A B O O (A B O ).
2
2
6
2 2 7
The crystallographic structure consists of the B
framework built by corner-sharing BO
2
O
6
open
0
6
2
octahedra and the A O
such as developing new photocatalysts as alternatives to TiO2
chains lled in the interstitial sites. The interactions between
and the combination of H O, H or thermal energy into pho-
0
2
2
the B O framework and A O chains are weak, leading to the
2 6 2
2
tocatalysis for obtaining the synergy effect. Great progress has
been made in this eld, such as ZnSn(OH) , b-Ga and
vanadate/TiO composites (Table S1†). Such studies pointed out
0
5
absence of A or O and the formation of defect pyrochlore. In
terms of electronic structure, a previous calculation has pointed
out that the 5s and 5p orbitals of the M cation (M ¼ Sn or Sb)
form covalent interactions with the O 2p orbitals. The LUMO
consists of Sb 5s-O 2p interaction, while the HOMO is made up
6
2 3
O
2
that both of hydroxyl radical (cOH) and super oxide radical
ꢀ
(O
2
c ) are the main active species during the benzene oxidation.
However, the mechanism of this reaction is very similar even
over different photocatalysts. The issue of how to choose an
efficient photocatalyst for degradation of benzene is seldom
discussed in these studies. The key inuencing factor during
6
of nonbonding O 2p states, when the A cation is Ca, Sr and Ba).
The band gap of this kind of antimonate mainly depends on the
0
cation at the B site, and the orbitals of A and O contribute
0
2
nothing to the HOMO and LUMO, but the defect in the A O
7
chains will drive the migration of O in the B O framework. The
2
6
unique crystalline and electronic structure makes pyrochlore
sustain a range of functions including as an oxygen electro-
State Key Laboratory of Photocatalysis on Energy and Environment, Research Institute
of Photocatalysis, Fuzhou University, Fuzhou, 350002, PR China. E-mail: dzli@fzu. catalyst, in lithium–O
batteries, and as an oxygen storage
material. Besides, pyrochlore can act as a proton conductor by
2
edu.cn; Fax: +86-591- 83779256; Tel: +86-591- 83779256
8
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ta08652g
9
2 6
allowing protons to diffuse along the O in the B O framework.
1
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˚
This proton conduction mechanism in the pyrochlore-structured parameter of a ¼ 10.37 A. In this structure, Sb (mixed valence
compound leads to the highly hydroxylated surface of pyrochlore, and B site cation, Fig. S1 and Table S2†) and O atoms, con-
4
8f
which is benecial for increasing the adsorption of oxygen on the sisting of the SbO octahedron, are placed on the 16c site (0, 0,
6
1
surface. Thus, the pyrochlore is a favourable candidate for 0) and the 48f site (0.3186, 1/8, 1/8), respectively. The corner-
6
improving the oxidation reactions. To our knowledge, photo- sharing SbO octahedron creates a three-dimensional open
2
+
catalysts with the pyrochlore structure have been applied to the framework (the inset of Fig. 1(b)). The Sr cation (A site cation)
ꢀ
degradation of benzene. However, the superiority of this struc- occupies the highly symmetrical position (32e, 3m) which is in
1
0
ture has not been revealed.
6
the cage formed by the SbO octahedron (Fig. S2†). The formula
2
A series of strontium antimonate photocatalysts (denoted as of SSO-8 is H1.23Sr0.45SbO3.48 (Rwp ¼ 3.50%, R
p
¼ 2.93%, c ¼
+
SSO-pH) are prepared using potassium pyroantimonate and 1.895, Table S1†), and H is added to maintain electrical
strontium chloride as precursors at pH ¼ 4–9 by a simple neutrality. The XRD patterns of other samples are shown in
hydrothermal method (see the Experimental Section in the Fig. S3a.† The variation of peak strength corresponding to the
12
ESI†). The transmission electron microscopy (TEM) of the SSO-8 (111) plane illustrates the different occupancy of the A site.
shows that the sample consists of mainly 11–13 nm particles The content of the oxygen adsorbed on the surface of SSO-8 is
Fig. 1(a)). The clear lattice fringe (0.581 nm) corresponding to also displayed in Fig. S1.† The binding energy of 530.04, 531.31
the (111) planes throughout each nanoparticle and the clear and 532.41 eV corresponds to the lattice oxygen, adsorbed
spot array of the FFT pattern suggest the single crystalline oxygen and surface hydroxyl, respectively. Thus, the adsorbed
nature of these nanoparticles. We then determine the Sr/Sb oxygen on the surface of SSO-8 is abundant.
(
13
ratio of SSO-8 using an X-ray photoelectron spectroscope (XPS)
Photocatalytic oxidation of 312 ppm gaseous benzene at
and an energy dispersive spectrometer (EDS) equipped on a certain gas hourly space velocity (GHSV) is used as the probe
a scanning electron microscope. The Sr/Sb ratio obtained from reaction. The photocatalytic activity of samples synthesized at
EDS and XPS is 0.43/1 and 0.48/1, respectively. The X-ray different pH values is shown in Fig. S4,† and the photo-
diffraction (XRD) pattern of the SSO-8 (Fig. 1(b)) indicates that it catalytic activity of the samples is related to their specic
11
crystallizes in the pyrochlore structure (ICSD-72097). The surface area (Fig. S3b†) and SSO-8 is the best sample. Fig. 2
ꢀ
structure adopts space group Fd3m (227) and has a cubic lattice shows that the conversion and mineralization ratios (C
R
and
M ) of the photocatalytic oxidation of benzene over SSO-8 are
R
ꢀ
1
up to 100% and 74% (GHSV ꢁ 3000 h ), which are about 10
and 5 times higher than those of P25. The quantum yield (QY)
on the SSO-8 and P25 is calculated to be ca. 29.7% and 1.32%,
respectively (Table S3 and 4†, the radiant illuminance of the
light source is shown in Fig. S5†). To evaluate the stability,
photocatalytic oxidation of benzene is measured at different
ꢀ
1
GHSVs (4285.7–11538.5 h ) for 42 h (Fig. S6†). When the
GHSV is increased to 11538.5 h , the C and M of benzene
ꢀ
1
R
R
still remained at 37.7% and 74% aer 42 hours indicating
a good stability of photocatalytic activity of SSO-8. In addition,
the same XRD and XPS patterns before and aer the reaction
suggest the stable structure of SSO-8 (Fig. S7†). From Table
S1,† ZnSn(OH)
photocatalysts. To evaluate the photocatalytic activity of the
SSO-8 further, we select the ZnSn(OH) and b-Ga as the
6 2 3
and b-Ga O are considered as excellent
6
2 3
O
Fig. 1 (a) TEM image of SSO-8 (inset: the FFT pattern). (b) XRD pattern Fig. 2 The C
R
and M
R
of C
H
6 6
over SSO-8 and P25 (GHSV ꢁ 3000
ꢀ1
of SSO-8 (inset: the open framework of the corner-sharing SbO
6
).
h ).
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reference. As shown in Fig. S8,† the SSO-8 still exhibits the the indirect band gap of SSO-8 is 4.1 eV. The MottꢀSchottky
best photocatalytic activity. (M–S) plots show that the conduction band (CB) position is at
The inuence of the O2 concentration in atmosphere on ꢀ0.3 V vs. standard hydrogen electrode (SHE). Therefore, the CB
SSO-8 is investigated (Fig. 3(a) and S9(a) and(b)†). The CR of and VB positions of SSO-8 are suitable for the production of cOH
ꢀ
ꢀ
benzene over SSO-8 drops with the decreased O
The variation of M is different from the C
conditions (O : N
¼ 0 : 100), the M
while it grows up to 74% when the O
2
concentration. (cOH/H
2
O 2.27 V vs. SHE, pH ¼ 7) and O
2
c (O
2
/O
2
c ꢀ0.28 V vs.
ꢀ
R
R
. Under anaerobic SHE, pH ¼ 7) (Fig. S11†).The relative content of cOH and O
2
c
2
2
R
of benzene is only 15%, is determined by the ESR spin trapping technique. When
2
concentration increases to 5,5-dimethyl-1-pyrroline N-oxide (DMPO) is used as a spin
ꢀ
only 3%. A similar control experiment is conducted over the P25 trapping agent, the DMPO/O
2
c
transfers to DMPO/cOH in
14
(
Fig. 3(a) and S9(c) and(d)†), and both the C and M of benzene water quickly. Therefore, 5-tertbutoxycarbonyl-5-methyl-1-
R R
decrease with the drop of the O concentration. P25 does not pyrroline N-oxide (BMPO) is selected as a spin trapping agent,
2
ꢀ
show photocatalytic activity when the O concentration is 3%. because both of the BMPO/O c and DMPO/cOH are stable in
These results indicate that (1) the photocatalytic activity is water. In this case, the origin of O c and cOH is considered to
2
2
1
5
ꢀ
2
ꢀ
+
affected greatly by the concentration of O
tion efficiency of O
investigate the role of O
system as the origin of oxygen instead of O
2
, and (2) the utiliza- be e /O
2
and h /–OH, respectively. Fig. 3 (b) and (c) illustrate the
ꢀ
2
over SSO-8 is better than that over P25. To ESR spectra of O
further, H
2
c
and cOH formed during photoexcitation of
O is introduced into the SSO-8 and P25. No ESR signal is observed for samples without
(Fig. S10†). In this irradiation or photocatalysts. Upon irradiation, a four-line
2
2
2
case, holes can be captured on the hydroxyl groups to form spectrum with relative intensities of 1 : 2:2 : 1 can be observed,
hydroxyl radicals (cOH), and the consumed hydroxyl groups will which is the adduct formed between BMPO and the hydroxyl
ꢀ
be compensated by the decomposition of H O into two hydroxyl radical (BMPO/cOH) or superoxide (BMPO/O c ). Since the
2
2
ꢀ
groups at the oxygen vacancy on the surface of the photo- BMPO/cOH and BMPO/O
2
c
have overlapping ESR spectra,
ꢀ
catalyst. Under anaerobic conditions, P25 does not show pho- whether the ESR signal in part comes from the O
2
c
should be
tocatalytic activity whether the water vapor is introduced into determined. Dimethyl sulfoxide (DMSO, 12%) and superoxide
ꢀ1
the system or not. In the case of SSO-8, 10% benzene has been dismutase (SOD, 600 U ml ) are used to quench the cOH and
ꢀ
15,16
decomposed in the dry N
2
ow, and the addition of water vapor
O
2
c , respectively.
The scavenging effects of DMSO and SOD
decreases the CR slightly. Although the MR soars to 100% as on the ESR signals from both photocatalysts are investigated.
soon as the water vapor is introduced into the system, it When the photocatalysts are irradiated by UV light, we found
ꢀ
decreases sharply as time goes by. Therefore, –OH/cOH and that the total amount of cOH and O c from SSO-8 is larger than
2
holes play a limited role during the oxidation of benzene that from P25. The addition of SOD leads to signicant reduc-
ꢀ
without O
2
.
tion of O
2
c
and the residual signal can be assigned to the
The reactive oxygen species (ROS) such as hydroxyl radicals BMPO/cOH adduct, which indicates that (1) SSO-8 produces
ꢀ ꢀ
2 2 2
cOH) and superoxide radicals (O c ) are the products of carrier more O c and cOH than P25, and (2) O not only improves the
capture, which are indispensable for further photochemical separation of e and h , but also acts as the main source of ROS.
reactions. The UV-Vis absorption spectrum of SSO-8 reveals that The addition of DMSO for scavenging of cOH illustrates the
same results.
(
ꢀ
+
To investigate the adsorption of oxygen further, we analysed
2 2
the ROS by O temperature programmed desorption (O - TPD).
The oxygen adsorption proceeds by the following procedure: O
2
ꢀ
ꢀ
2ꢀ
17
(
ad) /O
Fig. 4(a), the O
desorption peaks corresponding to desorption of O
2
c
(ad) /O (ad) /O
(lattice). As shown in
2
-TPD proles of SSO-8 and P25 show three
ꢀ
ꢂ
c (#180 C),
2
ꢀ
ꢂ
O
(#500 C) and lattice oxygen at higher temperatures, respec-
ꢂ
tively. The peak at 300 C in the prole of SSO-8 and the peak at
4
ꢂ
ꢀ
50 C in the prole of P25 are recognized as the O (ad). Since the
ꢀ
ꢀ
electrophilic reaction between O or O c and benzene is bene-
2
cial for the total oxidation of benzene, the large desorption
ꢀ
amount and the lower desorption temperature of O (ad) on the
SSO-8 mean the higher oxidation ability of SSO-8. In addition,
the irradiation during the pretreatment process increases the
ꢀ
desorption amount of O (ad) indicating that the production of
ꢀ
O
(ad) can be a photochemical process.
Water desorption proles during the O -TPD process are
2
recorded simultaneously with the oxygen desorption proles
Fig. 4(b)). The water peak area of SSO-8 is larger than that of
P25 indicating much more water desorption from SSO-8 during
the O -TPD process. Notably, the highest water peak is at the
(
R R
Fig. 3 (a) The C and M of benzene in different atmospheres over
ꢀ1
SSO-8 and P25 (GHSV ꢁ 4000 h ). (b) The ESR signals generated from
SSO-8. (c) The ESR signals generated from P25. All of the samples are
irradiated by UV light (centered at 254 nm).
2
ꢂ
ꢀ
same temperature (300 C) as the O (ad) peak, and the decrease
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site allows the oxygen ion in the 48f site to move to the 8b site,
7
leaving an oxygen vacancy in the 48f site (step 1). When a water
molecule is absorbed on the 48f oxygen vacancy of SSO, it
decomposes into two surface hydroxyl groups quickly (step 2).
Step 3 oen takes place on a proton conductor. To verify the
proton conductivity of SSO-8, impedance spectra of SSO and P25
are recorded in a H
conductivity of P25 shows no difference between the H
sphere and vacuum, while the conductivity of SSO-8 in the H
2
atmosphere and vacuum (Fig. S14†). The
2
atmo-
2
atmosphere is higher than that in vacuum. This conrms that
the protons can diffuse into the bulk of SSO-8, and the proton-
conducting properties contribute to forming more OH groups
by transferring protons via single OH bond formation in the
2
ꢀ
9
crystal (particularly with O ions on 48f sites). Thus, SSO-8 has
a highly hydroxylated surface, which is suitable for O adsorp-
2
tion. In steps 4, 5 and 6, the oxygen adsorbs on the surface
ꢀ
hydroxyl group and forms superoxide. Then, the O or cOH with
stronger oxidation ability will be formed via a series of reactions
with superoxide (Scheme 1).
In summary, we have prepared a series of antimonate pyro-
chlores by a simple hydrothermal method. Photocatalytic
oxidation of benzene is chosen as the probe reaction. When the
ꢀ
1
GHSV is 3000 h , the conversion and mineralization ratios of
photocatalytic benzene oxidation over H1.23Sr0.45SbO3.48 are up
to 100% and 74%, which are about 10 and 3 times higher than
those of P25. Then, we demonstrated that the photooxidation of
benzene over the antimonate pyrochlore depends on the effi-
ciency of oxygen usage greatly, and the highly hydroxylated
surface caused by the proton conductivity of the pyrochlore
plays a key role for the improvement of the adsorption of
oxygen. We envision that this discovery will lead to further
development of highly efficient photocatalysts for photooxida-
tion of benzene.
Fig. 4 (a) The O
2
-TPD profiles of SSO-8 and P25. (b) The desorption of
2
H O from SSO and P25 during the O
2
-TPD process.
ꢂ
ꢂ
of the pretreatment temperature from 180 C to 80 C leaves
more oxygen species and hydroxyl groups on the surface
(Fig. S12†). These results suggest the proportional relationship
between surface hydroxyl groups and adsorbed oxygen. Thus,
the PCO activity will be increased in the water vapor, which is Acknowledgements
conrmed by Fig. S13.†
This work was nancially supported by the NNSF of China
21173047 and 21373049).
From the above discussion, it is clear that the large amount
of surface-adsorbed O is a key factor for good photocatalytic
(
2
activity. A detailed description about why SSO-8 is suitable for
the absorption of O is as follows. As shown in Scheme 1, SSO-8
adopts a pyrochlore structure which is non-stoichiometric
according to its formula. The intrinsic oxygen vacancy in the 8b
2
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