Low temperature chlorobenzene catalytic oxidation over MnOx/CNTs with the assistance of ozone

Article abstract of DOI:10.1039/c4ra16687f

Manganese oxide supported on carbon nanotubes (MnO_x/CNTs) were prepared by an impregnation method and their catalytic oxidation performances of chlorobenzene (CB) with the assistance of ozone were investigated. The experimental results indicated a synergistic effect between the MnO_x/CNTs catalyst and ozone promotion for CB destruction, which promoted CB oxidation at temperatures between 80 °C and 280 °C. Especially at temperatures below 120 °C, both CB conversion and CO₂ selectivity above 95% were achieved with an apparent activation energy of 15.0 kJ mol^-1. Moreover, MnO_x/CNTs showed good stability and resistance to chlorine poisoning, which was demonstrated by stable catalytic activity for a long-term CB catalytic oxidation with 2300 ppm ozone up to 240 h.

Full text of DOI:10.1039/c4ra16687f

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Low temperature chlorobenzene catalytic
oxidation over MnOx/CNTs with the assistance of
Cite this: RSC Adv., 2015, 5, 15103
ozone†
a
b
a
a
a
*
Dongdong Jin, Zhiyuan Ren, Zhaoxia Ma, Fu liu and Hangsheng Yang
Manganese oxide supported on carbon nanotubes (MnOx/CNTs) were prepared by an impregnation
method and their catalytic oxidation performances of chlorobenzene (CB) with the assistance of ozone
were investigated. The experimental results indicated a synergistic effect between the MnOx/CNTs
catalyst and ozone promotion for CB destruction, which promoted CB oxidation at temperatures
between 80 ꢀC and 280 ꢀC. Especially at temperatures below 120 ꢀC, both CB conversion and CO2
selectivity above 95% were achieved with an apparent activation energy of 15.0 kJ molꢁ1. Moreover,
MnOx/CNTs showed good stability and resistance to chlorine poisoning, which was demonstrated by
stable catalytic activity for a long-term CB catalytic oxidation with 2300 ppm ozone up to 240 h.
Received 19th December 2014
Accepted 22nd January 2015
DOI: 10.1039/c4ra16687f
www.rsc.org/advances
Normally, noble metals,5 perovskites6 and transition metal
oxides7 are major candidates for catalyst optimization. Noble
1. Introduction
metals generally show high activity for the destruction of vola-
tile organic compounds (VOCs) at relatively low temperatures,8
however, they are not ideal catalysts due to high cost and severe
deactivation from chlorine poisoning.9 Perovskites are activated
only at temperatures above 500 ꢀC which lead to high energy
consumption.10 Therefore, many researches are focused on
transition metal oxides (supported or unsupported), such as
MnO2, V2O5, Cr2O3 and so on.11–18 Among them, MnOx based
catalyst is one of the most studied catalysts for CB catalytic
oxidation, which exhibits excellent activity at temperature
between 220 and 300 ꢀC, and shows great resistance to chlorine
poisoning.19 However, the operation temperature is still high
and there is a strong need to further reduce the catalytic
oxidation temperature.
Polychlorinated dibenzodioxins and polychlorinated dibenzo-
furans (PCDD/Fs, also dened as dioxins), which are mainly
produced unintentionally in high temperature processes such
as waste incineration and metallurgical industries, are a group
of unintentionally persistent organic pollutants (UP-POPs)
[Stockholm convention, http://chm.pops.int/TheConvention/
ThePOPs/The12InitialPOPs/tabid/296/Default.aspx].
These
compounds are very toxic, carcinogenic, and environmentally
persistent. The emission of dioxins has been one of the most
focused topics in global environmental problems. Stringent
environmental regulations and standards are established to
limit dioxins emission.1 A series of techniques have been
developed for dioxins removal including adsorption, thermal
combustion, and catalytic oxidation.2 Among them, catalytic
oxidation is the most promising technology which can
completely oxidize dioxins into CO2, H2O and HCl (or Cl2) at
relatively low temperatures. Catalytic oxidation can also be used
to remove very dilute pollutants (<1%) which cannot be ther-
mally combusted without fuel addition efficiently. These
distinct energy and economic efficiencies have attracted great
attentions.3 By the way, chlorobenzene (CB) is normally selected
as model molecular in laboratory to optimize catalysts used for
dioxins catalytic oxidation.4
Ozone, as a strong oxidizer, can react with most organic
compounds at low temperature (even at room temperature).
Recently, several reports show that adding ozone into the
catalytic oxidation process of VOCs can dramatically reduce the
operation temperature and activation energy (Ea). For example,
ozone can promote the catalytic oxidation of VOCs at temper-
atures below 200 C.20,21 Even at room temperature, a benzene
ꢀ
conversion of approximately 80% is reported over MnOx/Al2O3
catalyst with the assistance of ozone.22,23 But the low CO2
selectivity is still a main barrier for its possible application to
promote VOCs catalytic oxidation.22 At the same time, carbon
nanotubes (CNTs),24 a novel nanomaterial, is reported to be an
excellent VOCs adsorbent,25,26 and the addition of CNTs into
metal oxide catalysts promotes the CB oxidation evidently.27
Previous studies indicated that the surface of CNTs could be
modied by metal and metal oxide nanoparticles with good
stability, according to which, it is expected that MeOx/CNTs are
aState Key Laboratory of Silicon Materials, Department of Materials Science and
Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, China. E-mail:
hsyang@zju.edu.cn; Fax: +86-571-87951404; Tel: +86-571-87951404
bForeign Economic Cooperation Office, Ministry of Environmental Protection China,
No. 5 HouyingfangHutong, Xicheng District, Beijing 100035, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra16687f
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also stable in the presence of ozone.28 We also nd that ozone conductivity detector (FITD) was used to measure CB (SE-54
promoted CB catalytic oxidation at a wide temperature range column) and CO2 (porapakq-Q column) concentrations with
ꢀ
ꢀ
between 100 C and 300 C over CuO/CNTs, and CNTs in the an experimental error less than 5%.
catalyst showed a good stability in the presence of ozone.29
In this study, the catalytic activity was expressed herein in
In this study, the performance of CB catalytic oxidation over terms of CB conversion, CO2 selectivity and CB complete
MnOx/CNTs with the ozone assistance was studied, and our conversion:
results indicated that with the assistance of ozoneꢀ, CB could be
CB conversion (%) ¼ (CBinlet ꢁ CBoutlet) O CBinlet
(1)
completely degraded at temperature as low as 80 C.
CO2 selectivity (%) ¼ CO2output O [6 ꢂ (CBinlet ꢁ CBoutlet)] (2)
CB complete conversion (%) ¼ CO2output O (6 ꢂ CBinlet
2. Experimental section
2.1 Catalyst preparation and characterization
)
¼ CB conversion ꢂ CO2 selectivity (3)
MnOx/CNTs composite was prepared by impregnation method.
At rst, 10.0 g multi-walled carbon nanotubes (Shenzhen
Nanoport Co. Ltd) were puried in 69% HNO3 at 60 ꢀC for 3 h,
followed by ltrated and washed with deionized water to a pH
value of 7. Then, 350 mL aqueous solution of Mn(CH3COO)2 was
used to impregnate the puried CNTs under vigorous stirring at
50 ꢀC for 12 h. Finally, the suspension liquid was dried at 100 ꢀC
3. Results and discussion
3.1 XRD and SEM analysis
The XRD patterns of fresh and used MnOx/CNTs were shown in
Fig. 1. Diffraction peaks corresponding to CNTs and MnOx
(MnO2 and Mn2O3)31 were detected, indicating that the struc-
ture of CNTs remained and different valence states of Mn
species existed on CNTs. Typical SEM images with low, medium
and high magnication in Fig. 2 showed the morphology of
fresh (a–c) and used (d–f) catalysts (another low magnication
SEM image of used catalyst is shown in ESI†). It could be
observed that MnOx was dispersed on the surface of multi-
walled CNTs. Some aggregation of MnOx particles could also
be found, which reduced the BET surface area (SBET) of catalyst
to 133.5 m2 gꢁ1, while the SBET of pristine CNTs was measured
to be 270 m2 gꢁ1. Besides, there was no signicant difference
between fresh and used MnOx/CNTs in XRD and SEM results,
indicating the good stability of catalysts.
ꢀ
and then calcined at 450 C for 2 h in N2 so that MnOx/CNTs
with a theoretical MnOx loading of 15.0 wt% can be obtained.
The real Mn loading was measured to be 7.13 wt% by X-ray
energy dispersive spectroscopy (EDX; EDAX Genesis-4000) and
the BET surface area of catalyst was calculated as 133.5 m2 gꢁ1
by N2-physisorption (BET; Quantachrome Autosorb-1-C). X-ray
powder diffraction (XRD; PANalytical X'Pert PRO) with Cu Ka1
radiation (l ¼ 0.15406 nm), scanning electron microscopy
(SEM; Hitachi S-4800), X-ray photoelectron spectroscopy (XPS;
Thermo ESCALAB-250) were also used for the characterization
of fresh and used catalysts (aer being used at 120 ꢀC for 240 h
under high O3 concentration of 2300 ppm). The X-ray source
was an Al Ka radiation, and all binding energies were referenced
to the peak at 284.8 eV attributed to C 1s.
Moreover, H2-temperature-programmed reduction (H2-TPR)
experiments were performed using 100 mg of each catalyst. The
samples were pretreated under a N2 gas ow at 200 ꢀC for 1.5 h.
3.2 XPS analysis
In order to further investigate the catalysts, XPS spectra of fresh
and used MnOx/CNTs were collected and tted by XPSPEAK 4.1
TPR experiments were performed at
a heating rate of
10 ꢀC minꢁ1 under a mixed ow of 5% H2 in argon at a ow rate
of 40 mL minꢁ1. The H2-TPR data were recorded using an on-
line gas chromatograph equipped with a thermal conductivity
detector.
2.2 Catalytic activity characterization
2.5 g of catalyst was uniformly coated on 7 aluminum plates
(4 cm ꢂ 10 cm), which was then inserted into a xed bed ow
reactor for CB oxidation test, and the distance between two
plates is approximately 3 mm, and the geometry of the reactor is
shown in ESI,† which has also been shown in our previous
study.30 CB was carried by N2 using a saturator maintained at 40
ꢀC, and a mixture of 20% O2 + 80% N2 was used as reactant gas
to control a CB concentration of 30–500 ppm. Ozone was
generated using 20% O2 + 80% N2 by an ozone generator, which
could produce a maximum O3 concentration of 4000 ppm under
a gas ow of 1000 sccm. The reactant stream was preheated to
80 ꢀC before entering into the reactor. A gas chromatograph (GC
9790) online apparatus equipped with ame ionization thermal Fig. 1 XRD patterns of fresh and 240 h tested MnOx/CNTs.
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can be seen in Table 1, the ratio of Mn4+/Mn3+ of fresh catalyst
(4.03) was higher than that of used catalyst (1.97), indicating
that the surface Mn4+ species were reduced to Mn3+ species
during the long-term reaction process even with O3 assistance.
Thus it can be inferred that Mn species in MnOx/CNTs catalyst
were involved in CB oxidation.32 Fig. 4b showed the XPS spectra
of O 1s for the fresh and used catalysts. In all spectra, O 1s
spectra could be deconvoluted into three peaks at 529.7, 531.4
and 533.0 eV, corresponding to the lattice oxygen (Olatt, O2ꢁ),
2ꢁ
adsorbed oxygen (Oads, e.g., O radicals, O2ꢁ, O2 or Oꢁ), and
adsorbed OH groups or molecular water (Osur), respectively.33 As
calculated in Table 1, aer being used for CB catalytic oxidation
ꢀ
at 120 C for 240 h under high O3 concentration of 2300 ppm,
the atomic concentration of Olatt decreased from 2.75 at% to
1.85 at%, which was in good agreement with reduction of the
Mn4+/Mn3+ratio shown in Fig. 4a. In contrast, the atomic
concentrations of Oads and Osur increased from 4.17 at% to 5.81
at% and 1.24 at% to 1.99 at%, respectively, which were derived
from O3 decomposition. In fact, the increase of Oads was sug-
gested to be helpful for its catalytic activity34,35 and the existence
of CNTs in catalyst was reported as an excellent CB adsor-
bent.25,26,36 Therefore, Oads species were ready to react with the
adsorbed CB molecules thoroughly, thus promoting the cata-
lytic oxidation of CB.
Fig. 2 SEM images of (a)–(c) fresh; and (d)–(f) 240 h tested MnOx/
CNTs.
3.3 TPR analysis
To investigate the reducibility of the catalyst, H2-TPR proles
were tested and shown in Fig. 5. For fresh catalyst, the H2-TPR
ꢀ
spectrum showed two peak centered at 360 and 500 C. These
two peaks could be ascribed to the two-step reduction of MnO2.
The rst step corresponded to the reduction of MnO2 to Mn3O4
and the second step represented the further reduction of Mn3O4
to MnO.37 Moreover, the overlapped peak at 500 C might also
ꢀ
contain the reduction of Mn2O3 to Mn3O4 in the rst step and
further reduction to MnO.38 Compared with fresh catalyst, the
peak attributed to Mn4+ reduction in spectrum obtained from
the used catalyst was weaker, which was in agreement with the
XPS results shown in Fig. 4a.
3.4 Catalytic activity characterization
Fig. 6 presented the CB oxidation efficiency over the MnOx/
CNTs. For reactions in the absence of O3, the catalyst was
almost inactive below 240 ꢀC, which was consistent with the
high-temperature reduction peak at 360 ꢀC in the TPR proles.
Fig. 3 XPS fully scanned spectra of fresh and 240 h tested MnOx/
CNTs.
ꢀ
soware. The XPS fully scanned spectra presented in Fig. 3 also
showed the similar results between fresh and used catalysts.
Moreover, signal of chlorine could hardly be detected in the
used catalyst, indicating no Cl accumulation on the surface of
the used catalyst, which suggested the great resistance to
chlorine poisoning.
The XPS spectra of Mn 2p and O 1s were presented in Fig. 4,
and the XPS analysis data were summarized in Table 1. Four
peaks centered at 640.7, 642.0, 652.4, and 653.7 eV were
observed in Mn 2p XPS spectrum (Fig. 4a), among which peaks
centered at 640.7, 652.4 eV could be attributed to Mn3+ and
peaks centered at 642.0, 653.7 eV could be attributed to Mn4+. As
The CB conversion was less than 20% even at 240 C and the
ꢀ
CO2 selectivity was also as low as 30% and 55% at 80 C and
ꢀ
240 C, respectively. When O3 was added, the CB removal effi-
ciency increased remarkably. As was shown in Fig. 6a, the CB
conversion reached 80% and 95% at 80 ꢀC under O3 concen-
tration of 513 ppm and 2300 ppm, respectively. CO2 selectivity
also enhanced with O3. Even at 80 ꢀC, the CO2 selectivity
increased to 44% under 50 ppm CB and 513 ppm O3. Particu-
larly under high O3 concentration of 2300 ppm, a CO2 selectivity
of almost 100% was achieved at 80 ꢀC. Besides, with the
increase of CB concentration, both CB conversion and CO2
selectivity decreased due to the deciency of O3. At the same
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Fig. 4 XPS analysis of fresh and 240 h tested MnOx/CNTs: (a) fitted Mn 2p1/2 and Mn 2p3/2 photoelectron peaks; (b) fitted O 1s photoelectron
peaks.
Table 1 Surface element compositions of the fresh and 240 h tested MnOx/CNTs calculated from the XPS results
Samples
Mn (at%)
O (at%)
Olatt (at%)
Oads (at%)
Osur (at%)
Mn4+/Mn3+
Fresh MnOx/CNTs
Used MnOx/CNTs
1.64
1.54
8.16
9.65
2.75
1.85
4.17
5.81
1.24
1.99
4.03
1.97
obtained by O3 only was also shown. The CB complete conver-
sion of O3 promoted catalysis was higher than the sum of CB
complete conversion over catalyst only and O3 only, indicating
the synergistic effect of the catalyst and O3 for CB destruction.
Fig. 6 also showed that under high O3 concentration of
2300 ppm, a CB complete conversion up to 95% could be ach-
ꢀ
ieved at 80 C, suggesting the possibility to completely oxidize
CB to CO2 at very low temperature.
3.5 Kinetic study
Similar to our previous study,29 in this study, the CB oxidation
could be considered to follow the Langmuir–Hinshelwood
(L–H) mechanism. The reaction rate of CB oxidation can be
expressed as eqn (4):40
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
kKCBCCB KO CO
3
3
Fig. 5 H2-TPR profiles of fresh and 240 h tested MnOx/CNTs.
ꢁr ¼
(4)
À
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiÁ
2
1 þ KCBCCB
þ
KO CO
3
3
where r is the reaction rate (ppm sꢁ1), k is the rate constant, KCB
time, increasing the reaction temperature also induced the
decrease of catalyst activity, and CO2 selectivity reduced to 78%
at 240 ꢀC even at high O3 concentration of 2300 ppm, which
could be attributed to the quick combination of O radicals to
O2.39 By the way, in this study, no signal of CO was detected
throughout our test, while some trace fragments of organic
materials could be detected from gas chromatographer.
However, at the present stage, the determination of these
intermediate products from CB oxidation is still a challenge for
us.
and KO are the equilibrium constants for CB and O3 adsorp-
3
tions, and CCB and CO are the concentrations of the CB and
3
O3.
Considering that the concentrations of CB and O3 are nor-
mally low (ppm), eqn (4) can be further simplied to eqn (5):
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢁr ¼ kKCBCCB KO CO
(5)
3
3
From the dependence of k on the CB and O3 concentrations
presented in Fig. 8a, it was evident that the experimental data
matched well with the modeled reaction route for CB catalytic
oxidation. The apparent activation energy (Ea) was evaluated to
be approximately 15.0 kJ molꢁ1 from Arrhenius plot between
reaction temperatures and k shown in Fig. 8b, which was in
Since the concentration of VOCs in ue gases was usually
very low, Fig. 7 showed dependence of CB complete conversion
on O3 concentration and temperature under low CB concen-
tration of 50 ppm. For comparison, CB complete conversion
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Fig. 6 Temperature dependent of CB catalytic oxidation over MnOx/CNTs with and without O3.
good agreement with recent published results.29,40 In the
absence of ozone, previous studies revealed that the Ea was
48 kJ molꢁ1 over manganese oxide for the CB catalytic oxida-
tion,20 suggesting that the activation energy for the oxidation
reaction was signicantly reduced when O3 instead of O2 was
applied.
According to the above mentioned analyses, the O3 promo-
tion of CB oxidation over MnOx/CNTs could be described as
follows: MnOx was highly dispersed on CNTs which could
selectively adsorb both CB and O3; O3 decomposed on the
catalyst surface which produced adsorbed oxygen radicals
(O radicals or Oads); oxygen radicals could efficiently react with
adsorbed CB through L–H route. In order to completely oxidize
CB into H2O, CO2, and HCl (or Cl2), the strong CB adsorptivity of
CNTs, which prolonged the residence time of CB on the cata-
lysts surface thus prolonging the interaction time between CB
and O3,36 should be essential, or else, the CO2 selectivity would
be low.22,23 Similar to O3 promoted ethanol and ethane oxida-
tion,41 catalytic decomposition of ozone was also closely
Fig. 7 CB complete conversion under conditions of 50 ppm CB and
O3. For comparison, the oxidation of CB by 513 ppm O3 only was also
presented.
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the poorest activity.42,43 In the case of CB oxidation over MnOx/
CNTs, O3 molecules decomposed fast even at low temperature,
so the maximal oxygen radical concentration was achieved at
relatively low temperature below 120 ꢀC. While in the case of
reaction over CuO/CNTs, O3 decomposition was delayed and the
maximal oxygen radicals' concentration was obtained above
240 ꢀC. Therefore, it seemed that the optimal temperature of
catalytic reaction with O3 could be adjusted by modifying
property of metal oxide catalysts.
4. Conclusion
In summary, we found that O3 efficiently promoted CB catalytic
oxidation over MnOx/CNTs at only 80 ꢀC and CB conversion and
CO2 selectivity above 95% can be achieved. These phenomena
could be attributed to the synergistic effect of O3 decomposition
and CB oxidation during catalytic reaction. Our ndings can be
extended to investigate the effect of p-type and n-type oxide on
chlorobenzene catalytic oxidation with ozone and provide
possible application of O3 promotion for unintentionally
persistent organic pollutants catalytic oxidation over semi-
conduct metal oxides at low temperature.
Acknowledgements
This work was supported by the GEF project of Environmentally
Sustainable Management of Medical Wastes in China, Global
Environment Facility (Project no. GF/CPR/07/X02), the National
Natural Foundation of Zhejiang Province, China (Grant no.
Z4080070), and Zhejiang Province Environmental Protection
Science Research Plan of China (2011B14).
Fig. 8 Dependence of r on O3 concentration over MnOx/CNTs at
120 ꢀC (the inset is the plot of apparent k) and Arrhenius plot of CB
catalytic oxidation over MnOx/CNTs with the assistance of O3.
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