Catalytic oxidation of ethyl acetate and toluene over Cu-Ce-Zr supported ZSM-5/TiO2 catalysts

Article abstract of DOI:10.1039/c6ra06421c

Copper-cerium-zirconium catalysts loaded on ZSM-5/TiO₂ (denoted as CCZ/Z and CCZ/T) were prepared and characterized in this investigation and the catalytic behaviors of VOCs over the catalysts were determined. It is shown that the low-temperature activities of ethyl acetate and toluene oxidation over the CCZ/T catalyst are obviously higher than that over the CCZ/Z catalyst. Compared with CCZ/Z, the force between the metal oxides and the support TiO₂ is weaker and TiO₂ can provide some lattice oxygen. The greater number of oxygen vacancies, together with more Cu⁺ species in CCZ/T increases the mobility of atomic oxygen anions, which can inhibit by-product formation and enhance catalytic activity effectively. Moreover, the larger pore size of CCZ/T is beneficial for the diffusion of reactants and products. The analysis of intermediate species and the Mars-van Krevelen mechanism well explain the superior oxidation performances of ethyl acetate and toluene on CCZ/T. Both CCZ/Z and CCZ/T show superior stability for the catalytic removal of ethyl acetate and toluene at their complete conversion temperatures, which contribute to the positive effects of the Ce-Zr promoters on the copper species and support.

Full text of DOI:10.1039/c6ra06421c

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Catalytic oxidation of ethyl acetate and toluene over Cu-Ce-Zr
supported ZSM-5/TiO2 catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Dou Baojuan1, Li Shumin2, Liu Deliang2, Zhao Ruozhu2, Liu Jingge2, HaoQinglan2*, Bin
Feng3*
DOI: 10.1039/x0xx00000x
Copper-cerium-zirconium catalysts loaded on ZSM-5/TiO2 (denoted as CCZ/Z and CCZ/T, respectively) were
prepared and characterized in this investigation and the catalytic behaviors of VOCs over the catalysts were
determined. It is shown that the low-temperature activities of ethyl acetate and toluene oxidation over CCZ/T
catalyst are obvious higher than that over CCZ/Z catalyst. Compared with CCZ/Z, the force between the metal
oxides and the support TiO2 is weaker and TiO2 can provide a part of lattice oxygen. The more oxygen vacancies,
together with more Cu+ species in CCZ/T increasing the mobility of atomic oxygen anions, can inhibit the by-product
formation and enhance the catalytic activity effectively. Moreover, larger pore size of CCZ/T is beneficial to the
diffusion of reactants and products. The analysis of intermediate species and the Mars-van Krevelen mechanism well
explained superior oxidation performances of ethyl acetate and toluene on CCZ/T. Both CCZ/Z and CCZ/T show
superior stability for catalytic removal of ethyl acetate and toluene at their complete conversion
temperatures,contributing to the positive effects of the Ce-Zr promoters on the copper species and the support.
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1 Introduction
250ꢀ500 oC) are required, preventing the formation of
Volatile organic compounds (VOCs) are a series of undesirable intermediate product [9ꢀ15]. Commonly used
important environmental pollutants originated from catalysts for VOCs removal are noble metals and transition
petroleum refineries, solvent cleaning, fuel storage and metal oxides. Noble metals have higher activity compared
motor vehicles [1ꢀ3]. VOCs not only cause terrible with transition metal oxides. However, attentions have
environmental problems like photochemical smog, but also been given to transition metals due to the limited
are great threats to the human health due to their availability and high cost of noble metals. A common
carcinogenic, mutagenic, and teratogenetic nature [4ꢀ6]. In feature of these materials is the presence of multiple
addition, VOCs are criminal for the formation of oxidation states of transition metal in the structure,
secondary particulate matters in the atmosphere, resulting in the ability of the cation to undergo reversible
contributing the haze formation [7]. Therefore, it is superior oxidation and reduction under reaction. Compared with
desirable to completely convert VOCs into CO2 and H2O other transition metal oxides, CuO is one of most active
[8]
.
catalysts for various VOCs elimination [16ꢀ20]. Even so, the
Catalytic oxidation is one of the most efficient and catalytic activity of transition metal oxides is still not as
environmentally friendly technologies for VOCs removal. high as the noble catalysts. Great works were taken to
More importantly, low temperatures (generally around improve the catalytic performance by modifying catalyst
support, adding promoters and forming solid solution of
catalysts. For example, CeO2 and/or ZrO2 promoter was
a.
College of Marine & Environmental Sciences, Tianjin University of Science &
Technology, Tianjin 300457, China;
intensively studied to improve the oxygen storage capacity,
b.College of Chemical Engineering & Materials Sciences, Tianjin University of
Science & Technology, Tianjin 300457, China;
redox properties, thermal stability and catalytic
performances in VOCs elimination [21ꢀ23]
.
c. State Key Laboratory of High-Temperature Gas Dynamics, Institute of
Mechanics, Chinese Academy of Science, Beijing 100190, PR China.
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However, the unsupported of metal oxidation differences between ethyl acetate and toluene degradation
catalysts display poor thermal stability, low specific over CCZ/Z and CCZ/T catalysts, in an attempt to for
surface area and weak mechanical strength, severely complete oxidation of VOCs by examining the effect of
limiting them in practical industrial applications. ZSMꢀ5/TiO2 support, and to develop more efficient
Fortunately, the function of catalytic support can offset this catalyst formulations for catalytic oxidation of VOCs.
[25]
defect [24]. Bin et al.
found that the CO selfꢀsustained
2 Experimental
combustion was achieved over the CuꢀCe/ZSMꢀ5 catalyst
with the CO concentration ≥5 vol.%, due to the formation
of Cu2+ ions incorporated into cerium oxides, which are
more reducible than the copper clusters on the ZSMꢀ5
support. Previous study demonstrated that CuꢀZr/ZSMꢀ5
catalysts exhibited excellent catalytic activity for selective
catalytic reduction (SCR) of NO [26]. Currently, Ce/Zr
2.1 Catalyst preparation
H/ZSMꢀ5 with an atomic Si/Al ratio of 25 was
supplied by Nankai University, and the TiO2 support is
pure anatase titanium dioxide obtained commercially from
Degussa. The supported catalysts were prepared by a
currently wet impregnation. Copper nitrate, cerium nitrate
and zirconium nitrate were dissolved in deionized water
and mixed with 20 g HꢀZSMꢀ5/TiO2 powder at room
temperature until water evaporation. The resulting
promoted copper/ZSMꢀ5 catalysts was found to have
[27]
excellent activity and stability for VOCs oxidation
.
o
Besides, the TiO2 supported metal oxides have been
widely used for the past few decades. No characteristic
peaks associated with either copper, cerium/zirconium
oxides are observed at the CuꢀCeꢀZr/TiO2 catalysts,
indicating that the metal oxides species are formed in the
nanometer size range and well dispersed on the surface of
precursor was dried at 105 C for 24 h in air and then
o
calcined at 550 C for 4 h. The copper content of two
catalysts were fixed at 4 wt%, and the molar ratio of
Cu/(Ce+Zr) is 1:1. The catalysts were denoted as CCZ/Z
and CCZ/T. In order to evaluate the catalytic activity,
catalyst pellets were pressed by pressure of 20 MPa, then
granulated and screened to a size of 20ꢀ40 meshes.
TiO2 support, which exhibited high NH3ꢀSCR activity in
[28]
NOx abatement over a wide temperature ranges
.
2.2 Catalyst characterization
However, the use of Ce/Zr promoted copper/TiO2 catalysts
in the VOCs oxidation process has seldom been reported.
In particular, the comparison of catalytic behavior and
reaction pathway of VOCs between CuꢀCeꢀZr/ZSMꢀ5 and
CuꢀCeꢀZr/TiO2 is of great significance and should be
further explored.
N2 adsorption/desorption isotherms were measured at
ꢀ196 oC using an AutosorbꢀIqꢀMP instrument
(Quantachrome). The samples were degassed at 300 oC for
4 h before measurements. BET surface area of samples
was obtained according BrunauerꢀEmmettꢀTeller (BET)
method, and the microporous pore size distribution was
achieved by HorvathꢀKawazoe (HK) theory and the
mesoporous pore size distribution was calculated by
Barrett–Joyner–Halenda (BJH) equation. Xꢀray diffraction
(XRD) patterns were carried out in a XDꢀ3ꢀautomatic
In the present work, toluene and ethyl acetate, which
are two types of typical saturated aliphatic hydrocarbons
and aromatics vast existed in industrial processes, were
adopted as the probe pollutants to evaluate the catalytic
performances over the CuꢀCeꢀZr metal oxides supported
on ZSMꢀ5/TiO2. The correlations between the catalysts
activity and structural characteristic, dispersion, reduction
adsorption/desorption behaviors are investigated with
extensive characterizations. The focus is to explore the
(PERSEE)
equipped
with
multiꢀcrystal
Xꢀray
diffractometer using Cu Kα in the 2θ range of 5° to 80°
(scanning rate of 4o/min) and a step size of 0.02o.
Ultravioletꢀvisible diffuse reflectance spectra (UVꢀvis DR)
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were recorded in the range of 200ꢀ1000 nm at room
temperature by a Perkin Elmer Lambda 750 UVꢀvis
spectrophotometer with an integration sphere diffuse
reflectance attachment. Xꢀray photoelectron spectroscopy
(XPS) experiments were recorded on a PerkinꢀElmer PHIꢀ
1600 ESCA spectrometer using a Mg Kα Xꢀray source.
The binding energy (BE) was calibrated based on the line
position of C 1s (285 eV). Fourier transform infrared
(FTIR) spectra of the catalysts in KBr pellets were scanned
in the range between 4000 and 400 cmꢀ1 on a TENSOR27
(GermanyꢀBruker). Temperatureꢀprogrammed reduction
experiments with hydrogen (H2ꢀTPR) were investigated by
PCAꢀ140 instrument (Bolider) with a thermal conductivity
detector (TCD). 200 mg of each sample was first
pretreated in a quartz Uꢀtube under an Ar stream (30
mL/min) from room temperature to 500 oC for 60 min at a
rate of 20 ºC/min. The samples were then purged from
100 °C to 800 °C at 10 °C/min in 5% H2/Ar with a flow
rate of 50 mL/min. Temperatureꢀprogrammed desorption
of ammonia (NH3ꢀTPD) was also carried out on the above
PCAꢀ140 instrument (Bolider). Prior to adsorption
experiment, the sample (~200 mg) was first pretreated
under the same condition of H2ꢀTPR. The adsorption step
was performed by admitting 5%NH3/Ar (50 mL/min) at
100 ºC to saturation. Subsequently, the sample was
exposed to a flow of Ar for 30 min at 100 ºC in order to
remove reversibly and physically bound ammonia from the
catalysts surface. Finally, desorption was carried out from
100 ºC to 800 ºC at a rate of 10 ºC/min.
bed. The gas flow rates of the two branches were metered
by mass flow controllers.
In each test, 0.8 g of the catalyst (20ꢀ40 meshes) was
placed in the middle of the fixedꢀbed and the total flow
rate was 400 mL/min, corresponding to a space velocity of
24,000 hꢀ1, and the concentration of ethyl acetate and
toluene in the air feed was 1000 ppm and 750 ppm,
respectively. The fixedꢀbed temperature was first raised to
80 oC with the gas stream passing and stabilized for 30 min.
The effluent gas composition was analyzed by a gas
chromatograph (GCꢀ7900, Shanghai Tianmei Co., China)
equipped with FID (HPꢀ5) for analysis of VOCs and byꢀ
products quantitatively.
3 Results and discussion
3.1 Textural properties
Shown in Fig. 1A and B are the N2 adsorption and
desorption isotherms and the corresponding pore size
distributions of CCZ/Z and CCZ/T catalysts, respectively.
CCZ/Z and CCZ/T catalysts display typical I and Ⅳ shape
isotherms with the P/Po of material according to the
IUPAC classification, respectively. The pore size
distribution curve of CCZ/Z exhibit one single narrow
peak centered at 0.4ꢀ0.8 nm (Fig. 1B), suggesting the good
homogeneity of the micropore, while the pore size of
CCZ/T (6ꢀ120 nm) is more higher than that of the CCZ/Z.
In addition, the specific surface area of CCZ/Z catalyst
(339 m2/g) is much higher than that of CCZ/T (49 m2/g).
B
A
CCZ/T
2.3 Catalyst tests
0
20 40 60 80 100 120 140 160
The catalytic performance tests were investigated in a
continuousꢀflow fixedꢀbed reactor at the atmospheric
pressure, consisting of a stainless steel tube that was filled
with the catalyst. The streams with VOC were produced by
bubbling air through the VOC saturators. And then the gas
containing VOC was further diluted with another air
stream to required concentration before reaching the fixedꢀ
CCZ/T
CCZ/Z
CCZ/Z
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
0.0
0.2
0.4
0.6
0.8
1.0
Pore width (nm)
Relative pressure (p/p )
o
Fig. 1 N2 adsorption/desorption isotherms curves (A) and pore size
distributions (B) of CCZ/Z and CCZ/T
3.2 XRD measurement
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The XRD powder patterns of the CCZ/Z and CCZ/T formation of well dispersed copper species on the surface
compared with the support of ZSMꢀ5 and TiO2 are shown of ZSMꢀ5. The result is in conformity with the
in Fig. 2. CCZ/Z shows the typical diffraction peaks at characterization results obtained from XRD analysis. The
2θ=7.5°, 8.1°, 23.1° and 24.5°, representing (011), (200), absorption band at 700ꢀ1000 nm is related with the
(051) and (303) planes of crystal structures, respectively transitions of Cu2+ in octahedral oxygen configuration
[26]. The main diffraction peak of crystalline CCZ/T more or less tetragonally distorted, corresponding to the
appears at 2θ= 25.3°, 38.4°, 48.1° and 54.0°, and these CuO phase [13]
diffraction angles are consistent with the fluorite structure For CCZ/T, five adsorption peaks at 201, 250, 350 nm,
.
of TiO2 (101), (004), (112), (200) and (105) planes of and two broad bands at 200ꢀ550 and 500ꢀ1000 nm were
crystal structures, respectively [28]. No diffraction peaks discovered by peakꢀfit processing. The adsorption peak at
327 nm corresponding to the O2ꢀ→Ce3+ charge transition
attributed to metal oxides are observed, which suggests
that the copper, cerium and zirconium species are
homogeneously dispersed on the ZSMꢀ5 and TiO2 supports.
The above results indicate that the catalysts still keep the
carried structure after wet impregnation.
was clearly observed. It is reported that the existence of
Ce3+ in CeO2 implies the formation of an oxygen vacancy
[5]. Obviously, the Ce3+contents in CCZ/T catalyst is higher
than that in CCZ/Z sample, suggesting that the former
provides more oxygen vacancies than the latter. The
position and intensity of those adsorptions are in direct
linear relationship with the mount of surface electron,
since the enhanced concentration of the oxygen vacancies
in composite oxides system forms a defect band that
further changes the energy of conduction band [23]. It can
be seen from Fig.3 that the peak intensity of adsorption
band and the position for CCZ/T are obviously higher than
that for CCZ/Z, further indicating that CCZ/T possesses
more oxygen vacancies.
CCZ/Z
ZSM-5
CCZ/T
TiO
2
10
20
30
40
50
60
70
80
2 Theta (o)
Fig. 2 XRD diffraction spectra for CCZ/Z, CCZ/T and the supports
of ZSMꢀ5 and TiO2
266
3.3 UV-vis spectroscopy
327
201
UVꢀvis diffuse reflectance spectra of the CCZ/Z and
CCZ/T samples are plotted in Fig. 3. Both CCZ/Z and
CCZ/T catalysts present two adsorption bands at 200ꢀ500
and 600ꢀ1000 nm, respectively. For CCZ/Z, the two
adsorption bands can be well fitted by five peaks. The peak
at 200 nm can be attributed to oxygenꢀtoꢀmetal charge
transfer related to the Cu+/Cu2+ ions [13], and the peak at
284 nm is assigned to O2ꢀ→Ce4+ charge transfer transitions
and inter band transitions [30]. The weak peak at 460 nm
can be assigned to the transitions of Cu2+ in tetragonal
oxygen configuration [13,31], which can be attributed to the
389
815
813
CCZ/T
284
203
461
CCZ/Z
600
300
450
750
900
Wavelength (nm)
Fig. 3 UVꢀvis diffuses reflectance spectra of the CCZ/Z and CCZ/T
catalysts
3.4 XPS analysis
The CCZ/Z and CCZ/T catalysts were analyzed by
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XPS to obtain surface compositions and chemical states, as CCZ/T catalysts for O 1s are shown in Fig. 4B, after
shown in Fig. 4 and Table 1. Fig. 4A depicts the XPS deconvolution into two different oxygen species.
spectra in the Cu 2p region for the CCZ/Z and CCZ/T According to the literature, the higher binding energy at
catalysts. CCZ/Z sample is characterized by two main 531.6 eV is attributed to the lattice oxygen (Olatt)
peaks of Cu 2p1/2 (952.5ꢀ955 eV) and Cu 2p3/2 (930ꢀ935 associated with copper, cerium and zirconium metal oxides,
eV), along with the shakeꢀup satellite peaks centered at while the lower binding energy at 529.6 eV is ascribed to
937.5ꢀ947.5 eV[4,5]. According to relevant studies [4,5,26,27,29], absorbed oxygen (Oads [25,27,28]. The quantitative result (see
)
Cu 2p3/2 binding energy at ca. 933ꢀ934.2 eV, in Table 1) reveals that the Olatt/(Olatt+ Oads) ratio of CCZ/T
combination with the appearance of shakeꢀup peaks are catalyst is markedly higher than that of the CCZ/Z,
typical characteristics of Cu2+. Lower binding energy at ca. indicating that TiO2 provides some lattice oxygen [28]
.
932ꢀ933 eVare characteristics of further reduced copper
species, mainly Cu+. These results suggest that the
Table 1 Surface species of CCZ/Z and CCZ/T catalysts.
Catalysts
CCZ/Z
CCZ/T
Cu2+/Cu+
Olatt/(Olatt+ Oads)(%)
[4,5,27]
coexistence of Cu+ and Cu2+ ions in CCZ/Z catalyst
.
2.96
ꢀ
0.14
0.52
While lower binding energy at ca. 932ꢀ933 eV and the
absence of the shakeꢀup peak are characteristics of further
reduced copper species, mainly Cu+. For CCZ/T catalyst, it
is evident that the copper is mostly in a Cu+ oxidation state
(see Table 1).
3.5 FT-IR analysis
Functional groups of the samples were determined
using FTꢀIR, and the spectra of the fresh and used catalysts
are shown in Fig. 5. For fresh CCZ/Z, the peaks located at
513, 819 and 859 cmꢀ1 represent the asymmetrical
stretching vibration of SiꢀOꢀSi and the band at around 949
cmꢀ1 is due to the asymmetrical stretching vibration of
SiO4. The feature peaks of fresh CCZ/T at low 1000 cmꢀ1
are discernible being ascribed to flexural vibration of OꢀTiꢀ
O, and the peak at 1079 and 1127 cmꢀ1 are attributed to the
characterized vibration of TiꢀO. Compared with the fresh
CCZ/Z, the presence of the absorption peak at 1061 cmꢀ1
after removal of ethyl acetate is attributable to the CꢀOH
bond deformation vibration, illustrating that generation of
ethanol on the catalyst surface [4,27]. However, no new
spectrum was detected for the used CCZ/T with ethyl
acetate removal proved that no intermediate species over
the catalytic surface. The band in the range of 860ꢀ800 cmꢀ
Cu2p
3/2
A
Cu2p
1/2
CCZ/T
shake-up
CCZ/Z
960
950
940
930
Binding energy (eV)
B
O
latt
O
ad
CCZ/T
CCZ/Z
540 538 536 534 532 530 528 526 524 522 520
Binding energy (eV)
1
and 800ꢀ750 cmꢀ1 are assigned to the feature adsorption
Fig.4 XPS narrow spectra of Cu 2p (A) and O 1s (B) for
CCZ/Z and CCZ/T catalysts
peaks for monoꢀsubstituted phenyl. With respect to the
used CCZ/Z for toluene removal, the peak at 1665 cmꢀ1 is
The high resolution XPS spectra of CCZ/Z and
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assigned to stretching vibration of C=O, and the band in reduction of the copper species dispersed on the TiO2
o
the range of 541ꢀ451 cmꢀ1 is ascribed to the flexural support, and the γ peak (440 C) is due to the interactions
vibration of C=O, suggesting the formation of between CuO and TiO2, and oxygen reduction on the TiO2
benzaldehyde on the catalyst surface [34]. The feature peaks surface [27]. Upon the addition of zirconium and cerium,
at around 1719, 1266 and 947 cmꢀ1 are attributed to the the position of reduction peak for CCZ/T presents no shift,
flexural vibration of ꢀOH coupling with the stretching implying the weak force between metal oxides and the
vibration of C=O, indicating the generation of benzoic acid support TiO2. Moreover, partial copper species have been
on the surface of CCZ/T [34]
.
incorporated into the vacant sites of zirconium oxides or
cerium oxides to form coordinated surface structure with
the capping oxygen [30,31], corresponding to the β peak (270
oC).
451
541
795
1665
CCZ/Z-Toluene
1061
949
CCZ/Z-Ethyl acetate
CCZ/Z
513
859
819
α
γ
β
1266
1127
855
1719
947
CCZ/T-Toluene
CCZ/T
Cu/T
CCZ/T-Ethyl acetate
CCZ/T
949
859
1079
515
532
α
β
2500
2000
1500
1000
500
-1)
Wavenumber
(cm
γ
CCZ/Z
Cu/Z
Fig.5 FTꢀIR profiles of fresh and used for CCZ/Z and CCZ/T
catalysts
100 200 300 400 500 600 700 800
(o
3.6 H2-TPR
Temperature
C)
Fig. 6 H2ꢀTPR profiles of CCZ/Z and CCZ/T catalysts
Table 2 H2 consumption of CCZ/Zand CCZ/Tcatalysts
H2 consumption (mol·gꢀ1)
The reducibility of the samples was explored by H2ꢀ
TPR studies. Fig. 6 displays the H2ꢀTPR results obtained
over CCZ/T and CCZ/T, together with ZSMꢀ5/TiO2
supported copper catalysts (Cu/Z and Cu/T). As shown in
Fig. 6, the Cu/Z catalyst exhibits three main reduction
peaks, denoted as α, β and γ. The α peak is attributed the
reduction of well dispersed copper species on the ZSMꢀ5
support, and the β peak is associated with the reduction of
the copper oxide adhering to the external surface of zeolite
crystallites, whereas the γ peak is generally proposed as
the reduction of the bulk copper oxide [32,33]. Compared
with Cu/Z, α and β peaks of CCZ/Z shift to lower
temperature, moreover, the γ peak disappears with the Ce
and Zr incorporation. When copper, cerium and zirconium
species are loaded, the surface shell reduction is facilitated,
and those peaks can be slightly shifted to lower
temperatures. Cu/T catalyst exhibits two reduction peaks.
The α peak (200 oC) is generally resulted from the
Catalysts
Cu/Z
αꢀpeak
23.3
βꢀpeak
14
γꢀpeak
23.8
ꢀ
Total
61
CCZ/Z
Cu/T
35.0
38.9
ꢀ
73.9
17.6
42.5
14.3
3.3
CCZ/T
21.3
5.8
15.4
The total amounts of H2 consumption are summarized
in Table 2. It is worth noting that the amount of H2
consumption of α, β peaks for CCZ/Z and CCZ/T catalysts
are increasing with the incorporation of the Ce and Zr. The
disappearing of H2 consumption of γ peak for CCZ/Z is
due to the incorporation of zirconium and cerium into the
catalyst promoting the dispersion of the copper species. In
this study, the total amount of H2 consumption increases
from 61 to 73.9 mol•gꢀ1 with the incorporation of the Ceꢀ
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Zr into the Cu/Z. The total amount of H2 consumption of zirconium.
CCZ/T catalyst is significantly higher than that of Cu/T,
suggesting that the Cu species have been incorporated into
vacant sites of cerium oxides to form coordinated surface
structure. Combining with XRD results, Cu and Zr ions
A
B
CCZ/Z
ZSM-5
CCZ/T
incorporated in the cubic lattice of Ce form
a
TiO
homogeneous CuꢀCeꢀZrꢀO solid solution [27]. It is also
worth mentioning that the increment of total amount of H2
consumption for CCZ/T is obviously more than that for
CCZ/Z, indicating that support TiO2 provides some
oxygen species.
2
200
400
600
800
200
400
600
(o
C)
800
(o
C)
Temperature
Temperature
Fig. 7 NH3ꢀTPD profiles of CCZ/Z and CCZ/T catalysts
3.8 Catalyst performances
The investigation of catalytic performances of CCZ/Z
and CCZ/T were conducted in a fixedꢀbed reactor for the
removal of ethyl acetate and toluene. Fig. 8 (A, B) shows
the catalytic behavior of the CCZ/Z and CCZ/T catalysts
toward ethyl acetate and toluene oxidation. It can be seen
that the more active catalyst is CCZ/T with full
combustion of ethyl acetate and toluene being achieved at
258 and 280 oC respectively. For the catalytic oxidation of
ethyl acetate over CCZ/T, the onset temperature (T10) is as
3.7 Temperature-programmed desorption of
ammonia
The acid properties of the catalytic materials were
analyzed by TPD of ammonia. The NH3ꢀTPD profiles of
CCZ/Z, CCZ/T, and the support ZSMꢀ5 and TiO2 are
shown in Fig. 7. As shown in Fig. 7A, the NH3ꢀTPD
profile of ZSMꢀ5 displays two desorption peaks. The lowꢀ
temperature peak is assigned to weakly bound NH3, and
the highꢀtemperature peak is attributed to the NH3
adsorbed on the strong acid sites of SiꢀOHꢀAl. Upon
incorporation of copper, cerium and zirconium, the uptake
of weakly bound NH3 decreases, moreover, the desorption
peak of strong acid sites moves to the high temperature,
o
low as 102 oC which is lower 58 C than unsupported Cuꢀ
Ce1ꢀxSmxOδ catalysts with T10 of 160 oC[4], and the
complete conversion temperature is lower 22 oC compared
with the CuꢀCe/SiO2 (280 oC)[24]. The T90 of toluene
removal over CCZ/T catalyst is 259 oC which is lower than
that of the most reported catalysts [5]
.
which mainly originates from metal oxide nanoclusters[26]
.
From the results of characterizations, it can be
concluded that the pore size distribution is obviously the
important factor deciding the catalytic activity. Compared
with CCZ/Z, the larger pore size assigning to mesoporous
and macropore in CCZ/T is beneficial for the adsorption,
desorption and diffusion of VOCs. In addition, the
presence of plentiful oxygen vacancy on CCZ/T surface
facilitates the activation of more oxygen molecules to
active oxygen adspecies, consequently enhancing its
catalytic activity.
As shown in Fig. 7B ， the NH3ꢀTPD profile of TiO2
presents one broad desorption peak in the temperature
o
o
range of 50ꢀ400 C.The desorption peak at 80ꢀ200 C is
associated with weakly bound NH3, and the desorption
o
peak around 400 C is assigned to NH3 bound to strong
acid sites. The desorption peak of strong acid sites moves
to the low temperature with incorporation of copper,
cerium and zirconium for CCZ/T catalyst, suggesting that
the low temperature is easy to strip the adsorbate after
adding metal oxides. Additionally, it can be seen that the
intensity of total acidity, which is lower for pure TiO2,
significantly increases upon addition of copper, cerium and
Combined with results of XPS, CCZ/T with mostly in
the Cu+ state on the support surface exhibits better
catalytic behavior. The Cu+ can generates more ionꢀdefects
on the support surface because of charge compensation in
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metal oxidation, and it can improve the catalytic activity results obtained from FTꢀIR analysis. It can be seen that
consequently. Ce3+ and Cu species are indicative of the the variation of the ethanol yield for ethyl acetate over
redox equilibrium (Ce4++Cu+←→Ce3++ Cu2+) shifting to CCZ/Z initially increases remarkably, and then reach a
right, which increase the mobility of lattice oxygen. On the maximum before a rapid decrease and even disappear with
o
other hand, according to the XPS and H2ꢀTPR results, the further increase of 250 C. Combined with the results of
support of TiO2 provides a part of lattice oxygen. FTꢀIR, it can be seen from Fig.9B that very small amount
Moreover, the Cu and Zr ions incorporated in the cubic of benzaldehyde and benzoic acid are formed at lower
lattice of Ce form a homogeneous CuꢀCeꢀZrꢀO solid temperature during the toluene removal over CCZ/Z and
solution, allowing a mass of oxygen vacancies, increasing CCZ/T respectively. However, the intermediates of
mobility of atomic oxygen anions, which may accelerate benzaldehyde and benzoic acid can be further completely
o
the ability of stored/released oxygen for CCZ/T catalyst.
oxidized with the temperature higher than 250 C. Even
though the types of intermediate products for CCZ/Z and
CCZ/T catalysts are different, the results exhibit the same
trend, which initially increases remarkably, and then
100
T
90
Ethyl acetate
80
60
40
20
0
CCZ/Z
CCZ/T
T
50
o
reaches a maximum at 180 C before decrease and even
o
disappear at 255 C. It is worth mentioning that the yield
T
of benzaldehyde is slightly higher than benzoic acid at the
same reaction temperature. It can be explained by the
results from UVꢀvis that CCZ/T possesses more oxygen
vacancies, increasing mobility of atomic oxygen anions,
which may inhibit of intermediate product formation in
10
A
300
100
150
200
250
(o
C)
Temperature
100
80
60
40
20
0
T
90
Toluene
CCZ/Z
CCZ/T
favor of CO2.
10
A
8
T
50
6
T
10
Ethyl acetate
B
300
4
2
0
CCZ/Z
CCZ/T
100
150
200
250
(o
C)
Temperature
Fig. 8 Conversion of CCZ/Z and CCZ/T catalysts in the oxidation of
ethyl acetate (A) and toluene (B) as a function of temperature
100
150
200
250
300
(o
Temperature
C)
A small portion of ethyl acetate can be converted to
intermediate species such as ethanol, acetaldehyde and
acetic acid at low temperatures. Fig. 9 shows the
intermediates yield of catalytic oxidation of ethyl acetate
(Fig. 9A) and toluene (Fig. 9B) over CCZ/Z and CCZ/T as
a function of reaction temperature. In this study, ethanol is
a unique intermediate product in the removal of ethyl
acetate over CCZ/Z (Fig. 9A). However, it should be noted
that no intermediate was discovered for catalytic oxidation
of ethyl acetate on CCZ/T, which is in conformity with the
B
0.3
0.2
0.1
0.0
Toluene
CCZ/Z
CCZ/T
100
150
200
250
300
350
(o
Temperature
C)
Fig. 9 Yield of intermediate as a function of reaction temperature for
ethyl acetate (A) and toluene (B) over CCZ/Z and CCZ/T catalysts
8 | J. Name., 2016, xx, xx-xx
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The proposed reaction pathway of catalytic oxidation owing to more lattice oxygen partially provided by TiO2
of ethyl acetate and toluene is displayed in Fig. 10. The and the weak force between the support and the metal
catalytic oxidation reactions are processing with the oxides, which can oxidize more VOCs molecules rapidly.
electronic transfers among the CuꢀCeꢀZr based catalysts,
reactants and intermediate species formed. For most
catalytic oxidation occurring over CuꢀCeꢀZr based
catalysts, the Marsꢀvan Krevelen mechanism well
explained the oxidation reaction [35], where the organic
molecules are primarily oxidized by the lattice oxygen of
metal oxides. The Marsꢀvan Krevelen mechanism assumes
that in the first step (1) the oxygen from the catalyst
oxidizes the reactant from the gas phase producing the
reduced active site on the catalyst surface and oxidation
products which immediately leave the catalyst surface. In
the second step (2) the catalyst is reoxidized by oxygen
from the gas phase[1]. Firstly, a little portion of a gas phase
VOCs can be oxidized to CO2 and H2O by the oxygen
from the support on account of the excellent oxygen
storage capability of CeO2 and activated absorbed oxygen,
while a majority of VOCs are oxidized by lattice oxygen
releasing from the catalysts oxides. Simultaneously,
oxygen vacancies on the catalyst surface are released and
the metal centers (copper and cerium species) are reduced.
Secondly, the oxygen vacancies are supplemented by
oxygen from the gas phase when the catalyst is reꢀoxidized.
In this study, it is deduced that the activation of ethyl
acetate and toluene over CCZ/Z and CCZ/T catalysts occur
by abstraction of the H atoms from the weakest CꢀH bonds,
Fig. 10 Schematic diagrams of VOCs oxidation mechanism over
CCZ/Z (A) and CCZ/T (B) catalysts
100
80
Ethyl acetate
60
CCZ/Z
CCZ/T
40
20
0
A
0
10
20
30
40
50
60
Time on stream (h)
100
80
60
40
20
0
Toluene
CCZ/Z
CCZ/T
with a simultaneous reduction of the surface sites and
B
0
10
20
30
40
50
60
[1,5]
successive formation of the surface hydroxide ions
.
Time on stream (h)
The active oxygen preferentially extract the H atoms from
the weakest CꢀH bonds in ethyl acetate and toluene
molecules, therefore more lattice oxygen species in the
catalyst can facilitate the oxidation of ethyl acetate and
toluene. In addition, the presence of oxygen vacancy
promotes the activation of oxygen molecules to active
oxygen species. In comparison to CCZ/Z (Fig. 10A),
CCZ/T (Fig. 10B) provides more reactive oxygen species
Fig. 11 Stability of CCZ/Z and CCZ/T catalysts at their complete
conversion temperatures as a function of time on stream
The stability of CCZ/Z and CCZ/T catalysts at
complete conversion temperature are displayed in Fig. 11.
In the Fig.11A, at the reaction temperature of complete
conversion it is observed that the CCZ/Z and CCZ/T could
remain high stability on the ethyl acetate removal of
around 99.7% without noticeable activity loss throughout
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the 60 h test. Similarly, the conversion of toluene over catalyst stability effectively.
CCZ/Z and CCZ/T are always remained above 99% at the
o
temperature of complete conversion (320 C for CCZ/Z
Acknowledgements
and 280 oC for CCZ/T, respectively), as shown in Fig 11B.
We gratefully thank the financial support from the National
Natural Science Foundation of China (21307088).
The obtained results demonstrate that the catalyst stability
could be improved by support. Furthermore, augment of
the amount and the kinds of surface mobile oxygen due to
the addition of CeꢀZr promoters on the support can
effectively suppress the catalyst deactivation.
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Schematic diagrams of VOCs oxidation mechanism over CCZ/Z (A) and CCZ/T (B) catalysts