Catalytic activity and stability of porous Co-Cu-Mn mixed oxide modified microfibrous-structured ZSM-5 membrane/PSSF catalyst for VOCs oxidation

Article abstract of DOI:10.1039/c4ra08769k

A porous cobalt-copper-manganese mixed oxide modified microfibrous-structured ZSM-5 membrane/PSSF (paper-like sintered stainless steel fibers) catalyst was fabricated by the incipient wetness impregnating method. Catalytic oxidation performances of VOCs i

Full text of DOI:10.1039/c4ra08769k

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Catalytic activity and stability of porous Co–Cu–
Mn mixed oxide modified microfibrous-structured
Cite this: RSC Adv., 2014, 4, 55202
ZSM-5 membrane/PSSF catalyst for VOCs
oxidation†
a
Huanhao Chen,ab Ying Yan, Yan Shaoa and Huiping Zhanga
*
A porous cobalt–copper–manganese mixed oxide modified microfibrous-structured ZSM-5 membrane/
PSSF (paper-like sintered stainless steel fibers) catalyst was fabricated by the incipient wetness
impregnating method. Catalytic oxidation performances of VOCs in single component (isopropanol or
ethyl acetate) and in binary mixtures (isopropanol and toluene, ethyl acetate and toluene) were
investigated over the microfibrous-structured ZSM-5 membrane/PSSF catalyst. The stability and
durability of the microfibrous-structured ZSM-5 membrane/PSSF catalyst for isopropanol oxidation were
also evaluated. The as-synthesized and tested (after being used at 260 ꢀC for 550 h) microfibrous-
structured ZSM-5 membrane catalysts have been characterized by a series of techniques including SEM,
EDS mapping, XRD, N2 adsorption–desorption, XPS and H2-TPR methods. Experimental results indicated
that the total destruction of isopropanol or ethyl acetate alone over the microfibrous-structured ZSM-5
membrane/PSSF catalysts can be achieved at a temperature of 280 ꢀC In the case of binary mixtures, the
total conversion temperatures for toluene are above 300 ꢀC The porous microfibrous-structured ZSM-5
membrane/PSSF catalyst possesses excellent reaction stability, demonstrating by a high catalytic activity
(>90%) during the 550 h long-term catalytic oxidation reaction of isopropanol. After being used at
260 ꢀC for 550 h, the structural and textural properties of the catalyst were changed slightly during the
long-term catalytic oxidation process.
Received 16th August 2014
Accepted 15th October 2014
DOI: 10.1039/c4ra08769k
www.rsc.org/advances
use of pellet shaped or powder catalysts.8–10 Several previous
investigations10–13 indicated that porous modied zeolite
1. Introduction
membranes as high efficiency catalyst can offer a much higher
catalytic activity and contacting efficiency.
Volatile organic compounds (VOCs) such as isopropanol
constitute a signicant fraction of the hazardous gaseous
pollutants due to their harmful effects to human health and
environmental safety.1–3 A low-temperature catalytic oxidation
technique has been recently identied as the most efficient
method to completely eliminate VOCs from air due to its low
operational cost and high efficiency.4–7 However, the
phenomena of relatively higher mass/heat transfer resistance,
and lower contacting efficiency as well as higher bed pressure
drop always exist in the traditional xed bed reactor due to the
Investigations of the catalytic oxidation performances for
VOCs in single component have been widely reported. However,
catalytic oxidation activity of catalyst for VOCs in single
component partially presents the catalyst application.14 There-
fore, it should be of interest to investigate the catalytic oxidation
performances of catalysts in the reaction of VOCs in multi-
component mixtures. Aguero et al.14 investigated the catalytic
combustion of VOCs in binary mixture (ethanol–ethyl acetate)
over MnOx/Al2O3 catalyst. It can be noted that there is a
competition for the adsorption sites between ethanol and ethyl
acetate. Catalytic oxidation behaviors of isopropanol and
o-xylene in binary mixture over basic zeolites catalysts have been
also studied by Beauchet et al.1,3 It has been conrmed that the
presence of isopropanol has promoting effects on the oxidation
of o-xylene.1 Abdullah et al.15 also investigated the catalytic
combustion of VOCs binary mixture (ethyl acetate–benzene)
over Cr-ZSM-5 catalysts, an inhibiting effect can be observed.
Therefore, it is very important to study the catalytic oxidation
activity of the catalysts for VOCs mixtures in the air stream.
aSchool of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, P. R. China. E-mail: yingyan@scut.edu.cn; Fax:
+86 2087111975; Tel: +86 2087111975
b
¨
Department of Chemical Engineering, University of Puerto Rico-Mayaguez Campus,
¨
Mayaguez, PR 00681-9000, USA
† Electronic supplementary information (ESI) available: Characterization results
of the as-synthesized and tested microbrous-structured ZSM-5 membrane/PSSF
catalyst;
catalytic
combustion
performances
of
isopropanol
over
microbrous-structured ZSM-5 membrane/PSSF catalyst and ZSM-5 powder
catalyst; catalytic combustion performances of isopropanol, ethyl acetate, and
toluene in single component over microbrous-structured ZSM-5
membrane/PSSF catalyst. See DOI: 10.1039/c4ra08769k
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It is also well known that the catalyst stability or durability in
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The morphologies of the catalysts were observed by scanning
the catalytic oxidation of VOCs is a key factor for the industrial electron microscopy (SEM, Hitachi S-3700N). All of the samples
applicability.16 The catalyst stability or durability in the catalytic were coated with an ultra-thin lm of gold to make them
oxidation of VOCs has been investigated in a lower extent than conductive before analysis. The EDS mapping images of cata-
the catalyst activity.17 Several pioneer works dealing with the lysts were also obtained by Energy dispersive X-ray spectrometer
stability of protonic zeolites (H-ZSM-5, H-MOR and H-BEA) in (EDS, Quantax, Bruker Co., Germany) coupled with the micro-
the catalytic oxidation of chlorinated VOCs were presented by scope chamber. X-ray diffraction (XRD) patterns of catalysts
Aranzabal et al.16,17 During the catalytic oxidation of VOCs over were recorded on a D8 Advance (Bruker Co.) diffractometer
porous catalysts, the catalyst deactivation may be caused by using Cu Ka radiation (40 kV, 40 mA) with 2q range of 5–80ꢀ.
several reasons including physical and chemical. According to The X-ray tube was operated at 40 kV and 40 mA. N2 adsorption–
Chatterjee et al.,16,18 during the catalytic oxidation of chlori- desorption isotherms of catalysts were tested using an ASAP
nated VOC over the protonic and metal exchanged zeolites, the 2020 (Micromeritics Instrument Co., USA) at 77 K. Before
formation of coke is the main reason for the catalysts deacti- measurements, all of the samples were out-gassed at 523 K for
vation. The thermal sintering has been also reported to be the 8 h. The X-ray photoelectron spectra (XPS) results were obtained
main factor for the deactivation of Ce/Zr mixed oxides by Dai by a Kratos Axis Ultra (DLD) spectrometer with an Al Ka
et al.19 and de Rivas et al.20 The formation of water during (1486.6 eV) radiation source operated at 15 kV and 10 mA. The
catalytic oxidation of VOCs has been also taken into account as binding energy (BE) of C1s peak at 284.6 eV was taken as a
main reasons for the catalysts deactivation in most research reference. Temperature programmed reduction (TPR) tests were
works.21–23 However, literature about the stability and durability performed on Quantachrom Automated Chemisorption
of modied microbrous-structured ZSM-5 membrane/PSSF Analyzer. A 50 mg of each sample was loaded into the reactor
catalyst employed in the oxidation of VOCs is not yet reported. and purged with 30 ml minÀ1 of helium at 300 ꢀC for 1 h to
The aim of this work is (1) to study the catalytic activity of eliminate contaminants, and then cooled down to room
microbrous-structured ZSM-5 membrane/PSSF catalyst for temperature. The temperature was increased to 700 ꢀC at a
VOCs alone or in binary mixtures oxidation; (2) to investigate heating rate of 10 ꢀC minÀ1 with owing of 10% H2 and 90% Ar.
the stability and durability of microbrous-structured ZSM-5
membrane/PSSF catalyst in the catalytic oxidation of VOCs.
As shown in Fig. 1, catalytic oxidation tests were measured in
a continuous ow experimental apparatus at atmospheric
pressure. The reactor based on Co–Cu–Mn (1 : 1 : 1)/ZSM-5
membrane/PSSF catalysts with bed height of 2 cm was con-
sisted of a 10 mm i.d stainless steel tube located inside an
electrical furnace. The vapor of VOCs in single component or
binary component was generated by passing air at a certain ow
rate through the generator. Each reaction temperature was kept
20 min until reaching the steady state of system and the data
were determined by using gas chromatographs (Agilent 7890A,
Palo Alto, CA) equipped with FID and TCD detectors for the
quantitative analysis of the reactants and products. The data
were the average of at least three measurements. In the present
work, although a small amount of intermediate species is
detected by GC at low temperature during the catalytic
2. Experimental
Porous microbrous-structured ZSM-5 membrane/PSSF
composites were prepared by the wet lay-up papermaking
process and secondary growth process according to our
previous reports.8,9 The Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/
PSSF catalysts with total metal loading of 28 wt% were prepared
by incipient wetness impregnation method. The as-synthesized
samples were dried at 100 ꢀC for 12 h and calcined at 350 ꢀC for
6 h. The detailed synthesis process of Co–Cu–Mn (1 : 1 : 1)/
ZSM-5 membrane/PSSF catalysts can be found in our previous
report.10
Fig. 1 A schematic diagram of the catalytic combustion test setup.
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oxidation process, the only nal products were CO2 and H2O VOCs molecules in the reactor was extended, and the contacting
and no other by-products were observed under the experimental efficiency was obviously enhanced because of a relatively larger
conditions.
catalyst bed height. In other words, for the 2 cm catalyst bed
height, VOCs molecules had sufficient time to contact with
active sites of catalyst due to much more catalyst active sites in
the reactor.
3. Results and discussion
3.1 Catalytic oxidation of single VOCs over Co–Cu–Mn
Catalytic oxidation performances for ethyl acetate alone over
the Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/PSSF catalysts with
a bed height of 2 cm were carried out using different inlet
(1 : 1 : 1)/ZSM-5 membrane/PSSF catalyst
Catalytic oxidation performances of VOCs in single component
(isopropanol, ethyl acetate) over the Co–Cu–Mn (1 : 1 : 1)/ZSM-5
membrane/PSSF catalysts were investigated. Firstly, the effects
of the bed height on catalytic oxidation performances of iso-
propanol were studied. Secondly, catalytic oxidation perfor-
mances of ethyl acetate in single component were measured
over the catalysts using different inlet concentrations and ow
rates. Catalytic combustion performances of isopropanol over
the microbrous-structured ZSM-5 membrane/PSSF catalyst
and ZSM-5 powder catalyst under the same experimental
conditions (4.7 mg LÀ1 of isopropanol in the feed gas, GHSV of
7643 hÀ1) are also shown in Fig. S7 (see ESI†). Moreover, cata-
lytic oxidation performance of toluene in single component over
the microbrous-structured ZSM-5 membrane/PSSF catalyst
under the experimental conditions (4.1 mg LÀ1 of toluene in the
feed gas, GHSV of 7643 hÀ1) is shown in Fig. S8 (see ESI†).
Catalytic oxidation performances of isopropanol were
studied over the Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/PSSF
catalysts with different bed heights (1 cm and 2 cm) using the
inlet concentration of 4.1 mg LÀ1 and GHSV of 7643 hÀ1, and
the conversion proles are presented in Fig. 2. Fig. 2 shows that
the T50% (the value of the temperature at conversions approach
50%) for isopropanol oxidation decrease clearly as the bed
height increased. However, the T90% values (the value of the
temperature at conversions approach 90%) for isopropanol
oxidation over the catalysts with bed height of 1 cm and 2 cm
are similar. The possible reasons are that the residence time of
concentrations (6.3–9.2 mg LÀ1) at a constant GHSV of 7643 hÀ1
.
Fig. 3 represents that the T50% and T90% for ethyl acetate
oxidation in relatively low inlet concentration (6.3–8.0 mg LÀ1
)
over the catalysts are similar. However, the T50% for ethyl acetate
oxidation in a high inlet concentration (9.2 mg LÀ1) increases
obviously. The possible explanations are that the unit catalysts
bed in a reactor offers limited active sites for ethyl acetate
Fig. 3 Catalytic oxidation performance of ethyl acetate over the
Fig. 2 Catalytic oxidation performances for isopropanol over the Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/PSSF catalysts at different
Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/PSSF catalysts with different inlet concentrations (6.3–9.2 mg LÀ1 of ethyl acetate in the feed
bed heights (4.1 mg LÀ1 of isopropanol in the feed gas, GHSV of gas, GHSV of 7643 hÀ1 in all cases), and different GHSV (GHSV of
7643 hÀ1 in all cases).
3822–11 466 hÀ1, 6.3 mg LÀ1 of ethyl acetate in the feed gas).
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oxidation, however, the treated ethyl acetate quantity in a unit
catalysts bed increases at a higher inlet concentration of
VOCs.11 Therefore, a decrease of catalytic oxidation conversion
of ethyl acetate can be observed at a higher inlet concentration.
Fig. 3(a) also indicates that catalytic conversion of ethyl
acetate over the catalysts can reach 100% below the temperature
of 280 ꢀC.
To investigate the effects of the GHSV on the catalytic
performance, the conversions of ethyl acetate in single
component over the Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/
PSSF catalysts with bed height of 2 cm were also measured
using different GHSV (3822–11 466 hÀ1) but at a constant inlet
concentration (6.3 mg LÀ1). As can be seen in Fig. 3(b), the T50%
and T90% for single ethyl acetate oxidation over the catalysts
increase slightly with increasing GHSV. The possible reasons
are that the residence time of ethyl acetate molecules on the
surface of catalysts was shortened at a relatively higher GHSV,
ethyl acetate molecules had insufficient time to contact with the
active sites of catalyst.11 However, complete conversion can be
obtained essentially under all GHSV values at the temperatures
above 250 ꢀC. The possible reasons are that the catalysts
possessed a higher contacting efficiency, lower mass/heat
transfer resistance as well as shorter diffusion path.11
Fig. 4 Catalytic oxidation conversion of VOCs in binary component
(isopropanol and toluene) over Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/
PSSF catalysts (5.8 mg LÀ1 of isopropanol and 0–10.1 mg LÀ1 of toluene
in the feed mixture gas, GHSV of 7643 hÀ1 in all cases).
kinetic diameter of isopropanol (0.47 nm) is smaller that of
toluene (0.58 nm).24 Similar phenomena have been also
observed by Burgos et al.24 during the catalytic oxidation of a
binary VOCs mixture (isopropanol and toluene) over the
Pt/Al2O3 catalysts. The similar phenomena for catalytic oxida-
tion of isopropanol and o-xylene in binary mixture have been
also reported by Beauchet.3 As can be seen in Fig. 5, the T50%
and T90% for isopropanol and toluene in binary mixture slightly
increase as the GHSV increased (3822–7643ꢀhÀ1), shiing to
higher temperatures by approximately 10–20 C.
3.2 Mixture effect of toluene
Catalytic oxidation behaviors of VOCs in binary component
(isopropanol–toluene, ethyl acetate–toluene) over the Co–Cu–Mn
(1 : 1 : 1)/ZSM-5 membrane/PSSF catalysts with bed height of
2 cm were also studied. Firstly, catalytic oxidation performances
of isopropanol (5.8 mg LÀ1) with the addition of gradual
concentration of toluene (0–10.1 mg LÀ1 of toluene) were
measured at a constant GHSV of 7643 hÀ1. Secondly, catalytic
oxidation performances of ethyl acetate (8.0 mg LÀ1) with the
addition of gradual concentration of toluene (0–10.1 mg LÀ1 of
toluene) were measured at a constant GHSV of 7643 hÀ1. Finally,
VOCs destruction in binary component (isopropanol and
toluene, ethyl acetate and toluene) was also performed using
different GHSV (3822–11 466 hÀ1).
Catalytic oxidation performances of VOCs in binary compo-
nent (8.0 mg LÀ1 of ethyl acetate plus 0–10.1 mg LÀ1 of toluene)
over the Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/PSSF catalysts
Fig. 4 shows that over the Co–Cu–Mn (1 : 1 : 1)/ZSM-5
membrane/PSSF catalysts, very low temperatures are needed
to destroy isopropanol alone (toluene: 0 mg LÀ1). We can notice
that the T50% for isopropanol oxidation alone over the catalysts
is 200 ꢀC and the total destruction can be obtained at the
temperature of 280 ꢀC It can be also observed in Fig. 4 that the
T50% and T90% for isopropanol oxidation over the catalysts
increase slightly when toluene is added (3.4–10.1 mg LÀ1).
Complete destruction of isopropanol in binary component can
be achieved below the temperature of 300 ꢀC However, the T50%
and T90% for toluene oxidation increase obviously as the
concentration of toluene increased, shiing to higher temper-
ꢀ
ꢀ
atures by about 10 C A higher temperature above 300 C was
needed to totally destroy toluene in binary mixture. The
possible reasons are that a competition phenomenon between
isopropanol and toluene exists on the same active sites of the
catalyst, and toluene possesses inhibiting effects on the iso-
Fig. 5 Catalytic oxidation conversion of VOCs in binary component
(isopropanol and toluene) over Co–Cu–Mn (1 : 1 : 1)/ZSM-5
membrane/PSSF catalysts (GHSV of 3822–11 466 hÀ1, 5.8 mg LÀ1 of
isopropanol and 5.6 mg LÀ1 of toluene in the feed mixture gas in all
propanol destruction.11 Another possible explanation is that the cases).
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with bed height of 2 cm were also investigated at a constant GHSV minimal inhibiting effects on the ethyl acetate destruction. We
of 7643 hÀ1 and were shown in Fig. 6. The effects of space velocity can also notice that the conversion curves of toluene are shied
on the catalytic behaviors were also studied using different GHSV slightly to higher temperatures as the inlet concentration of
(3822–11 466 hÀ1) but at a constant inlet concentration (8.0 mg LÀ1 toluene increased and the higher temperature above 300 ꢀC was
of ethyl acetate and 7.1 mg LÀ1 of toluene) and were shown in needed to completely destroy toluene in the binary mixture.
Fig. 7. Results in Fig. 6 and 7 show that ethyl acetate alone These results are in agreement with the observations reported
(toluene: 0 mg LÀ1) and in biꢀnary mixture is totally destroyed by Abdullah et al.15 during catalytic oxidation of a binary VOCs
below the temperature of 300 C It seems that the presence of mixture (ethyl acetate and benzene) over the Cr-ZSM-5 catalysts.
toluene (3.2–7.1 mg LÀ1) has no effect on the ethyl acetate It can be found that the destruction of ethyl acetate in the binary
conversion and that slightly variation of the T50% (shiing to mixture was easier than that of toluene, attributing to its
higher temperature by approximately 20 ꢀC) for ethyl acetate is nucleophilic property, smaller kinetic diameter as well as linear
observed when inlet concentration of toluene reached molecule type.11 Catalytic oxidation behaviors of the binary
10.1 mg LÀ1. The addition of toluene in the binary mixture had VOCs mixture (ethyl acetate and toluene) over the catalysts were
also carried out by changing GHSV from 3822 hÀ1 to 11 466 hÀ1
.
It can be clearly observed from Fig. 7 that a slight negative effect
of enhancing space velocity on VOCs conversions is obtained,
shiing the conversion curves to higher temperatures by
ꢀ
approximately 10 C.
3.3 Stability and durability of the Co–Cu–Mn (1 : 1 : 1)/ZSM-
5 membrane/PSSF catalyst
The reaction stability and durability tests for VOCs oxidation in
single component over the Co–Cu–Mn (1 : 1 : 1)/ZSM-5
membrane/PSSF catalysts were also investigated. The reaction
stability of catalyst for ethyl acetate oxidation (6.3 mg LÀ1 of
ethyl acetate) was investigated using the reaction temperature
of 260 ꢀC and GHSV of 7643 hÀ1. It can be observed in Fig. 8 that
the catalyst possesses excellent reaction stability for ethyl
acetate oxidation, and the conversions of ethyl acetate are
always remained above 95% during 50 h. The stability and
durability test of catalyst were further studied by following the
catalytic activity in terms of isopropanol conversion as a func-
tion of time-on-stream (550 h). Catalytic oxidation reaction was
carried out feeding 4.1 mg LÀ1 of isopropanol in air at the
Fig. 6 Catalytic oxidation conversion of VOCs in binary component
(ethyl acetate and toluene) over Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/
PSSF catalysts (8.0 mg LÀ1 of ethyl acetate and 0–10.1 mg LÀ1 of toluene
in the feed mixture gas, GHSV of 7643 hÀ1 in all cases).
ꢀ
reaction temperature of 260 C and the space velocity of 7643
hÀ1
. Fig. 8 shows that the Co–Cu–Mn (1 : 1 : 1)/ZSM-5
membrane/PSSF catalysts exhibited excellent stability, within
the rst 200 h, and relatively higher catalytic activity for iso-
propanol oxidation, remaining above 95%. Aer that, the
catalytic oxidation conversions of isopropanol present some
gradual decrease until the end of experimental run (550 h),
ranging between 95% and 90%.
3.4 Characterization of as-synthesized and tested catalyst
The reduction performances of the as-synthesized and tested
(aer being used at 260 ꢀC for 550 h) Co–Cu–Mn (1 : 1 : 1)/
ZSM-5 membrane/PSSF catalyst were investigated, and the TPR
curves are presented in Fig. 9. The TPR curve of as synthesized
catalyst presents a relatively sharp reduction peak at around
200 ꢀC, which could be attributed to the combined reduction of
Co–Cu–Mn mixed oxides. However, the TPR curve of tested
catalyst changed obviously aer being used at 260 ꢀC for 550 h,
charactering by the presence of two small reduction peaks. The
rst peak located at a higher temperature (around 212 ꢀC) is
similar to the single reduction peak of the as-synthesized cata-
lyst. The second reduction peak moves toward a much higher
Fig. 7 Catalytic oxidation conversion of VOCs in binary component
(ethyl acetate and toluene) over Co–Cu–Mn (1 : 1 : 1)/ZSM-5
membrane/PSSF catalysts (GHSV of 3822–11 466 hÀ1, 8.0 mg LÀ1 of
ethyl acetate and 7.1 mg LÀ1 of toluene in the feed mixture gas in all
cases).
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Fig. 8 Stability and durability tests for VOCs oxidation in single component with time-on-stream over the Co–Cu–Mn(1 : 1 : 1)/ZSM-5
membrane/PSSF catalysts: (a) 6.3 mg LÀ1 of ethyl acetate in the feed gas, reaction temperature of 260 ꢀC, reaction time of 50 h, GHSV of
7643 hÀ1; (b) 4.1 mg LÀ1 of isopropanol in the feed gas, reaction temperature of 260 ꢀC, reaction time of 550 h, GHSV of 7643 hÀ1
.
Fig. 9 H2-TPR profiles of the as-synthesized and tested Co–Cu–Mn
(1 : 1 : 1)/ZSM-5 membrane/PSSF catalysts (after being used at 260 ꢀC
for 550 h).
Fig. 10 XPS fully scanned spectra of the as-synthesized and tested
Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/PSSF catalysts (after being
used at 260 ꢀC for 550 h).
temperature at around 245 ꢀC. The possible reasons are that
some mixed metal oxides were partially reduced during the
long-term catalytic oxidation process. The XPS fully scanned
spectra of as-synthesized and tested (aer being used at 260 ꢀC
for 550 h) Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/PSSF catalyst
are shown in Fig. 10. It can be seen in Fig. 10 that XPS fully
scanned spectra of as-synthesized and tested catalysts are
similar, charactering by the presence of several dominant
peaks. The peaks of C 1s and O 1s can be observed at 284.6 eV
and 533 eV, respectively. The XPS spectra of Co 2p, Cu 2p3/2, Mn
2p and O 1s of the as-synthesized and tested catalysts are also
presented in Fig. 11, and the XPS analysis data are summarized
in Table 1. As can be seen in Fig. 11(a), the Co 2p spectra of as-
synthesized and tested catalysts present two main peaks located
at 775–790 and 790–800 eV, which could be ascribed to the Co
2p3/2 and Co 2p1/2. The Co 2p3/2 could be deconvoluted to two
components at 780 and 783 eV, corresponding to the Co3+ and
Co2+, respectively. The Co 2p1/2 can be also deconvoluted to two
components at 794 and 797.5 eV, which could be attributed to
the Co3+ and Co2+, respectively. As can be seen in Table 1, the
proportion of Co3+/Co2+ of as-synthesized catalyst (0.73) is
higher than that of tested catalyst (0.45). The possible reasons
are that most of Co3+ species was reduced to Co3+ species during
the long-term catalytic oxidation process, indicating that the
cobalt oxide species as catalytic active components play a role in
the catalytic oxidation process of VOCs. In the Cu 2p3/2 XPS
spectra of as-synthesized and tested catalysts, the peak located
at 928–938 eV could be deconvoluted into two main compo-
nents at 933.5 and 935 eV, attributed to Cu1+ and Cu2+,
respectively. As can be seen in Table 1, the as-synthesized
catalyst possesses a much higher Cu2+ content (the proportion
of Cu2+/Cu1+ is 1.15). However, aer being used at 260 ꢀC for 550
h, the Cu2+/Cu1+ of tested catalyst decreased obviously to 0.23.
These experimental results indicate that a lot of Cu2+ species
was reduced to Cu1+ species during the long-term catalytic
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Fig. 11 XPS analysis of the as-synthesized and tested Co–Cu–Mn (1 : 1 : 1)/ZSM-5 membrane/PSSF catalysts (after being used at 260 ꢀC for
550 h): (a) fitted Co 2p1/2 and Co 2p3/2 photoelectron peaks; (b) fitted Cu 2p3/2 photoelectron peaks; (c) fitted Mn 2p1/2 and Mn 2p3/2
photoelectron peaks; (d) fitted O 1s photoelectron peaks.
Table 1 Surface element compositions of the as-synthesized and tested catalysts (after being used at 260 ꢀC for 550 h)
a
Samples
Co3+/Co2+a
Cu2+/Cu1+a
Mn4+/Mn3+a
Oadsorbed/Olattice
Co–Cu–Mn (1 : 1 : 1)/ZSM-5/PSSF (as-synthesized)
Co–Cu–Mn (1 : 1 : 1)/ZSM-5/PSSF (tested in 550 h)
0.73
0.45
1.15
0.23
0.83
0.72
0.68
0.80
a
Calculated from the XPS results.
oxidation process. The possible explanations are that the that the actual ratio of Mn4+/Mn3+ for as-synthesized catalyst is
copper oxide species could be the dominant catalytic active 0.83. Aer being used at 260 ꢀC for 550 h, the ratio of Mn4+/Mn3+
components for isopropanol oxidation, playing the vital role in for tested catalyst (0.72) is slight lower than that of
the catalytic oxidation reaction. Fig. 11(c) clearly displays the as-synthesized catalyst, indicating that a small quotient of Mn4+
XPS spectra of Mn 2p for the as-synthesized and tested catalyst. species was reduced during the long-term catalytic oxidation
All the Mn 2p spectra present two main peaks located at process. The XPS spectra of O1s for as-synthesized and tested
640–645 and 650–655 eV, corresponding to the Mn 2p3/2 and Mn catalysts are also presented in Fig. 11(d). As can be seen in
2p1/2. The Mn 2p3/2 signal can be further deconvoluted to two Fig. 11(d), asymmetric two band structures can be observed, the
main components at 641.5 and 643 eV, attributed to Mn3+ and sharp peak at lower binding energy of about 529.8 eV is attrib-
Mn4+. The Mn 2p1/2 signal of each catalyst could be also uted to the lattice oxygen and the wide peak at higher binding
deconvoluted to two main components at 652.5 and 654 eV, energy of 532 eV is characteristic of the adsorbed oxygen. As can
corresponding to Mn3+ and Mn4+. It can be observed in Table 1 be seen in Table 1, the ratio of Oadsorbed/Olattice for as-
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synthesized catalyst is 0.68, indicating that the lattice oxygen is
the main presence form of oxygen in modied ZSM-5
membrane catalyst. However, the ratio of Oadsorbed/Olattice for
tested catalyst (0.8) is much higher than that of as-synthesized
catalyst. The XPS analysis results indicate that the Co and Cu
species in Co–Cu–Mn mixed oxides modied ZSM-5 membrane
catalysts are the dominating catalytic active sites for iso-
propanol oxidation, however, the Mn species offer small cata-
lytic activity for isopropanol oxidation.10 Moreover, the SEM
(Fig. S1†), EDS (Fig. S2†), XRD (Fig. S3†), N2 adsorption–
desorption (Fig. S4, Table S1†) characterization results of
modied ZSM-5 membrane catalyst and their detailed analysis
were also clearly presented in ESI.†
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4. Conclusion
Porous microbrous-structured Co–Cu–Mn (1 : 1 : 1)/ZSM-5
membrane/PSSF catalyst was developed for VOCs removal.
Catalytic oxidation performances of VOCs alone or in binary
component mixtures (isopropanol–toluene, ethyl acetate–
toluene) were evaluated at different inlet concentration and
space velocity. The stability and durability of catalyst were also
assessed feeding 4.1 mg LÀ1 of isopropanol in air, using the
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hÀ1. The as-synthesized and tested catalysts were characterized
by using SEM, EDS mapping, XRD, N2 adsorption–desorption,
XPS as well as H2-TPR techniques. Experimental results indi-
cated that the complete destruction of VOCs alone (isopropanol
or ethyl acetate) can be all achieved below the temperature of 300
ꢀC Isopropanol and ethyl acetate were found to be more reactive
than toluene in the catalytic oxidation of VOCs in binary
mixtures. Toluene in binary mixtures possessed slight inhibiting
effects on the destruction of isopropanol and ethyl acetate due to
the competitive adsorption phenomenon. The porous modied
microbrous-structured ZSM-5 membrane catalyst also
possesses excellent reaction stability, demonstrating by a high
catalytic activity (90%) during the 550 h long-term catalytic
oxidation reaction. Aer being used at 260 ꢀC for 550 h, the
structural and textural properties of catalyst were changed
slightly during the long-term catalytic oxidation process.
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Acknowledgements
We are grateful for the nancial support from the National
Natural Science Foundation of China (Grant no. 21176086 and
21376101).
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RSC Adv., 2014, 4, 55202–55209 | 55209