Efficient Fenton-like La-Cu-O/SBA-15 catalyst for the degradation of organic dyes under ambient conditions

Article abstract of DOI:10.1039/c4ra00917g

Fenton-like La-Cu-O/SBA-15 catalyst is highly efficient for dye degradation, due to a synergistic effect between La-Cu-O and SBA-15, in which La-Cu-O is the active site of the reaction and SBA-15 acts as a gallery to adsorb and transport the organic dyes from solution to the surface of La-Cu-O.

Full text of DOI:10.1039/c4ra00917g

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Efficient Fenton-like La–Cu–O/SBA-15 catalyst for
the degradation of organic dyes under ambient
conditions†
Cite this: RSC Adv., 2014, 4, 12601
Received 1st February 2014
Accepted 21st February 2014
Ping Xiao, Hailong Li, Tao Wang, Xuelian Xu, Jinlin Li and Junjiang Zhu*
DOI: 10.1039/c4ra00917g
www.rsc.org/advances
Fenton-like La–Cu–O/SBA-15 catalyst is highly efficient for dye area of these oxides is low and the interaction between the
degradation, due to a synergistic effect between La–Cu–O and SBA- substrates and the active sites (metal cations) is limited.
1
5, in which La–Cu–O is the active site of the reaction and SBA-15 acts
In this work, in order to increase the surface area and
as a gallery to adsorb and transport the organic dyes from solution to improve the catalytic efficiency of perovskite-type oxide cata-
the surface of La–Cu–O.
lysts, we attempted to synthesize them on a high-surface-area
mesoporous silica SBA-15 support. In detail, we synthesized a
9
La–Cu–O mixture, which are composed of perovskite-type oxide
La CuO and a few amount of CuO, on a SBA-15 support,
2 4
3 2 4
Perovskite-type oxides with ABO or A BO structure are one
class of promising materials in heterogeneous catalysis because
of their structural features, such as exchangeable cations at A
and B positions, controllable oxidation state of the B-site cation,
dened as La–Cu–O/SBA-15. This way the La–Cu–O can be well
dispersed and more active sites can be exposed, relative to that
of the bulk one. Furthermore, because of the large surface area
and pore volume, SBA-15 shows good adsorption capacity for
Rhodamine B (RhB) and thus has the potential to transport the
adsorbed RhB to the active sites, guaranteeing the sufficient
supply of RhB to the active sites and reaching the objective of
improving the catalytic efficiency.
1
and generation of oxygen vacancies. They can be applied to gas
or solid reactions carried out at high temperatures, or liquid-
phase reactions carried out at low temperatures, due to their
2
highly thermal and hydrothermal stability. So far, these oxides
have not gained applications in practice and their catalytic
efficiency needs to be improved before the possibility of
industrialization.
Fig. 1A and B show the X-ray diffraction patterns of the
samples conducted at small- and wide-angles. At small angles,
One application of perovskite-type oxides is to catalyze the
3
oxidative degradation of organic dyes in aqueous solution,
which are discharged from industrial plants and are destructive
4
to aquatic life. In this regard, these oxides can be classied as
5
“
Fenton-like” catalyst according to the description of Walling.
6
Compared to the classic homogeneous Fenton catalysts, the
perovskite-type catalyst has wide working pH range, and is more
operable in practice because of the facile separation and
7
recovery of catalyst from industrial wastewater. However, the
oxidative efficiency of these catalysts is low and the use of
ultrasonic and/or UV light irradiation is essential to accelerate
8
reaction rate for better catalytic performance, since the surface
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs &
Commission Ministry of Education, South-Central University for Nationalities,
Wuhan 430074, China. E-mail: ciaczjj@gmail.com
Fig. 1 (A) Small-angle XRD patterns for SBA-15 and La–Cu–O/SBA-
15; (B) wide-angle XRD patterns for La–Cu–O/SBA-15 and bulk La–
†
Electronic supplementary information (ESI) available: Experimental section,
additional characterizations, activity tests with reaction time and that for
different dyes, and the HPLC-MS analysis on the intermediate products. See
DOI: 10.1039/c4ra00917g
2
Cu–O; (C) N physisorption isotherms and (insert) the corresponding
pore size distribution for La–Cu–O/SBA-15; (D) TEM image for La–
Cu–O/SBA-15.
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the peak intensity is largely attenuated and the peak position is exposure of copper active sites; and on the other hand that is
slightly shied to higher angles from SBA-15 to La–Cu–O/SBA- more crucial is to adsorb and transport RhB from solution to the
1
1
5, indicating that the long-range order of the channels of SBA- surface of La–Cu–O, accelerating the reaction rate and improving
5 is decreased and the unit cell of SBA-15 is shrunk. Whereas, the catalytic activity. Namely, there has a synergistic effect
10
the reserved (100), (110) and (200) faces (see the insert picture) between La–Cu–O and SBA-15, in which La–Cu–O is the active
suggest that the porous structure of SBA-15 is not destroyed site of reaction and SBA-15 acts as a gallery to adsorb and
aer the loading of La–Cu–O. At wide angles, the predominant transport RhB from the solution to the surface of La–Cu–O,
peaks of La–Cu–O/SBA-15 appeared almost at the same position where the reaction occurs and CO
2
is produced, Scheme 1.
as those of La–Cu–O. By referring to the standard PDF card of Besides, considering that the real amount of La–Cu–O in the
copper compounds, we inferred that the La–Cu–O contains La–Cu–O/SBA-15 (loading: 55.6 wt%) is 0.056 g, while that for
mainly of La CuO (signal #) and few amount of CuO (signal the bulk La–Cu–O is 0.1 g, and the fact that SBA-15 is inactive to
2
4
4
d,11
*).
The peak intensity of La–Cu–O/SBA-15 was largely the reaction, this means that 0.056 g supported La–Cu–O is
attenuated relative to that of La–Cu–O, which could be that the more active than 0.1 g bulk La–Cu–O. Moreover, recalling that
La–Cu–O mixture is well dispersed and has poorer crystal- the total amount of RhB converted over La–Cu–O/SBA-15
12
linity, and/or a dilution effect of the SBA-15 support.
physisorption isotherms (Fig. 1C) and TEM image while that over bulk La–Cu–O contains only the RhB in the
Fig. 1D) of La–Cu–O/SBA-15 (more pictures can be found in solution as it shows no adsorption capacity for RhB. This
contains both the pre-adsorbed RhB and those in the solution,
N
2
(
Fig. S4†) conrms that the matrix structure of SBA-15 is main- suggests that the real ability of La–Cu–O/SBA-15 to RhB oxida-
tained and demonstrates that the La–Cu–O mixture is highly tion should be better than what observed in Fig. 2A. Overall, La–
dispersed on the surface of SBA-15, suggesting that the La–Cu– Cu–O/SBA-15 can show far high efficiency for dye degradation
O/SBA-15 composite is successfully prepared. With this struc- relative to the bulk La–Cu–O.
ture, it can be imaged that the organic dyes adsorbed on the
pores of SBA-15 could be transported to the surface of La–Cu–O, out an additional measurement: aer the oxidation reaction, the
and subsequently oxidized by H . The good adsorption ability catalyst was ltered and added to a new batch of RhB solution, to
To ensure that the pre-adsorbed RhB is oxidized, we carried
2 2
O
of SBA-15 to organic dyes would ensure the sufficient supply of test if it could adsorb RhB again from the solution. Indeed, we
organic dyes to the surface of La–Cu–O, improving the reaction found that the concentration of this new RhB solution decreased
rate and thus catalytic efficiency.
aer addition of the used catalyst, conrming that there have
To verify the above speculation, we rst measured the refreshed space in the used La–Cu–O/SBA-15 and part of pre-
capacity of La–Cu–O, SBA-15 and La–Cu–O/SBA-15 for RhB adsorbed RhB was consumed. By calculation, the used La–Cu–O/
ꢀ
1
adsorption. Results indicate that SBA-15 and La–Cu–O/SBA-15 SBA-15 shows capacity of 0.0036 mmol g for RhB adsorption,
ꢀ
1
exhibit 0.017 and 0.0076 mmol g adsorption capacity for which is nearly half of the fresh one, conrming that the
RhB, while negligible value is observed for La–Cu–O. This adsorbed RhB is indeed oxidized in the reaction.
implies that the supply of RhB from solution to the surface of
La–Cu–O/SBA-15 should be better than that to the surface of activity of a mechanically mixed “La–Cu–O + SBA-15” sample for
bulk La–Cu–O. RhB oxidation, nding that although its activity is lower than
In justifying the transport capacity of SBA-15, we tested the
As expected, La–Cu–O/SBA-15 showed far higher activity for that of La–Cu–O/SBA-15, is far better than that of bulk La–Cu–O
RhB oxidation than La–Cu–O, and no appreciable activity was (see Fig. 2A and B). This is interesting as SBA-15 itself is hard to
detected on SBA-15, Fig. 2A, indicating that SBA-15 is hard to catalyze RhB oxidation, while the physically mixed sample
catalyze the reaction while it can largely improve the catalytic shows enhanced activity relative to the bulk La–Cu–O. The
performances of La–Cu–O. Considering that SBA-15 is a porous reason could be that when this mixture is added to and stirred
material and has strong ability to RhB adsorption, it is suggested in the RhB solution, some of SBA-15 powders are suspended
that the contribution of SBA-15 in the reaction, on one hand, is to and attached to the La–Cu–O mixture, forming a new composite
act as a support to improve the dispersion of La–Cu–O or the (let dene as La–Cu–O%SBA-15), which is similar to La–Cu–O/
SBA-15 except that the interaction between La–Cu–O and SBA-15
Fig. 2 (A) and (B) RhB conversion measured on the specified samples
and at different molar ratios of RhB to catalyst; (C) reusability of La–
Cu–O/SBA-15 for RhB oxidation; “R1” means that the activity was
ꢁ
obtained from the used catalyst re-treated at 500 C for 2 h. Data in (A)
and (B) were measured at 60 min, while those in (C) were taken at 30 Scheme 1 Proposed reaction route for the oxidation of RhB with
min. The molar ratio of RhB to catalyst is 0.0045, if not specified.
2 2
H O over La–Cu–O/SBA-15.
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In order to test if RhB is completely converted into CO , that
is the mineralization of RhB, we measured the volume of CO2
evolved during the reaction, Fig. 3. Results show that although
some amounts of small organic molecules (Fig. S12†) exist at
reaction time of 1 h, where the colour of RhB solution is faded,
they can be completely converted into CO
h. Besides, blank experiment (no RhB is added to the solution
with otherwise identical reaction conditions) shows that H is
hardly decomposed into O over the catalyst. This indicates that
H O can be mostly used for RhB oxidation and the gas evolved
2
at reaction time of
3
2 2
O
2
2
2
2 2
Fig. 3 Volume of CO and O evolved from the real and blank
from the reaction is due to the yield of CO degraded from RhB.
experiments. Reaction conditions: molar ratio of RhB to catalyst is
.077, H amount: 1 mL.
2
0
O
2 2
Finally, we also tested the applicability of La–Cu–O/SBA-15 to
other organic dyes including reactive brilliant red X-3B, direct
scarlet 4BS and methylene blue (MB), showing that the sample
is weaker. Hence, RhB will mobilize or be transported to the is the most active for RhB, meanwhile also shows considerable
surface of La–Cu–O through SBA-15, and thus an increased activities for MB, X-3B and 4BS, Fig. S9.† Moreover, like other
4
d,13
activity is observed. This conrms that SBA-15 can absorb and perovskite catalysts
La–Cu–O/SBA-15 also endures wide
transport RhB from solution to the surface of La–Cu–O. working pH range (pH ¼ 2–10) for the reaction, owing to the
Otherwise, La–Cu–O%SBA-15 should behave as the bulk La–Cu– buffer function of the active compounds, see Fig. S10 and S11.†
O, which is obviously contrary to the experimental results.
In summary, we reported that Fenton-like La–Cu–O/SBA-15
As demonstrated above that La–Cu–O is composed of is a highly efficient catalyst for the oxidative degradation of
La CuO and CuO, we thus prepared La CuO /SBA-15 and CuO/ organic dyes, in particular for RhB, which can be fully oxidized
2
4
2
4
SBA-15 and tested their activities for RhB oxidation, Fig. 2B, to into CO
clarify which phase predominates the reaction. At molar ratio of tions. This is the best perovskite-type catalyst reported in liter-
RhB to catalyst equals to 0.0045, the RhB conversion measured ature for RhB degradation using H as oxidant to the best of
over CuO/SBA-15, La CuO
/SBA-15 and La–Cu–O/SBA-15 is 65%, our knowledge. The high efficiency is due to a support effect by
00%, 100%, indicating that La CuO
is more crucial to the increasing the exposure of copper active sites and, more crucial,
2
at reaction time of 3 h in the present reaction condi-
2 2
O
2
4
1
2
4
reaction than CuO. To differentiate the oxidation ability of to a synergistic effect between La–Cu–O and SBA-15, in which
La CuO /SBA-15 and La–Cu–O/SBA-15, we increased the molar La–Cu–O is the active site and SBA-15 acts as a gallery to
2
4
ratio of RhB to catalyst to 0.077 and found that the former transport the organic dyes from solution to the surface of La–
shows lower activity than the latter (66% and 86%, respectively), Cu–O. The high catalytic efficiency, wide working pH ranges,
suggesting that the La–Cu–O mixture is more favorable for the strong resistance to deactivation as well as the facile separation
reaction than La
more copper active sites exposed in La–Cu–O (molar ratio of La/ industrial catalyst for oxidative degradation of organic dyes.
Cu is 1) than that in La CuO (molar ratio of La/Cu is 2).
2 4
CuO . The reason might be that there have from aqueous solution enable La–Cu–O/SBA-15 to be a potential
2
4
Compared to previously reported perovskite catalysts, the
herein La–Cu–O/SBA-15 shows better turnover (TOF) for RhB
Acknowledgements
oxidation, meanwhile requires fewer amount of H O , Financial supports from the National Science Foundation of
2
2
Table S1.† This demonstrates that La–Cu–O/SBA-15 is indeed an China (21203254), and the Scientic Research Foundation for
efficient catalyst for RhB degradation. The strong ability of La– Returned Scholars, Ministry of Education of China (BZY11055)
Cu–O/SBA-15 to RhB oxidation is further conrmed by applying are gratefully acknowledged.
it to high concentrated RhB solution. It is found that even at
molar ratio of RhB to catalyst equals to 0.077, the RhB conver-
sion can be reached up to 95% at reaction time of 3 h, Fig. S6.†
Notes and references
Reusability tests indicate that an extent of ca. 14% decrease
in RhB conversion is observed aer the h run, but the activity
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