Three-dimensionally ordered macroporous Cr2O3?CeO2: High-performance catalysts for the oxidative removal of trichloroethylene

Article abstract of DOI:10.1016/j.cattod.2019.01.071

Three-dimensionally ordered macroporous (3DOM) CeO₂, 3DOM Cr₂O₃, 3DOM xCr₂O₃?CeO₂ (x (the weight percentage of Cr₂O₃) = 3.5, 5.5, and 8.0 wt%), and 5.5 wt% Cr₂O₃/3DOM CeO₂ samples were prepared using the polymethyl methacrylate (PMMA)-templating and incipient wetness impregnation methods, respectively. A number of techniques were used to characterize physicochemical properties of the materials, and their catalytic activities were evaluated for the oxidation of trichloroethylene (TCE). These samples possessed a good-quality 3DOM structure and a surface area of 35?47 m²/g. The 3DOM 5.5Cr₂O₃?CeO₂ sample performed the best (the temperature at TCE conversion = 90% = 255 °C at a space velocity of 20,000 mL/(g h)). Effects of water vapor and carbon dioxide on activity of the 5.5Cr₂O₃?CeO₂ sample were also examined. It is observed that partial deactivation induced by H₂O introduction of the 5.5Cr₂O₃?CeO₂ sample was reversible, while that induced by CO₂ addition was irreversible. Based on the activity data and characterization results, it is concluded that the good catalytic activity and thermal stability of 3DOM 5.5Cr₂O₃?CeO₂ was associated with its high adsorbed oxygen species concentration, good low-temperature reducibility, and strong interaction between Cr₂O₃ and CeO₂. We believe that the 3DOM 5.5Cr₂O₃?CeO₂ catalyst is promising in the application for oxidative removal of chlorinated volatile organic compounds.

Full text of DOI:10.1016/j.cattod.2019.01.071

Accepted Manuscript
Title: xThree-dimensionally ordered macroporous
Cr2O3nullCeO2: High-performance catalysts for the oxidative
removal of trichloroethylene
Authors: Xing Zhang, Yuxi Liu, Jiguang Deng, Xingtian
Zhao, Kunfeng Zhang, Jun Yang, Zhuo Han, Xiyun Jiang,
Hongxing Dai
PII:
S0920-5861(18)30982-9
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2019.01.071
CATTOD 11942
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
5 July 2018
2 January 2019
27 January 2019
Please cite this article as: Zhang X, Liu Y, Deng J, Zhao X, Zhang K, Yang J, Han Z,
Jiang X, Dai H, xThree-dimensionally ordered macroporous Cr2O3horbar;CeO2: High-
performance catalysts for the oxidative removal of trichloroethylene, Catalysis Today
(2019), https://doi.org/10.1016/j.cattod.2019.01.071
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Three-dimensionally
ordered
macroporous
Cr2O3CeO2:
High-performance catalysts for the oxidative removal of trichloroethylene
Xing Zhang, Yuxi Liu*, Jiguang Deng, Xingtian Zhao, Kunfeng Zhang, Jun Yang, Zhuo
Han, Xiyun Jiang, Hongxing Dai*
Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of Beijing on
Regional Air Pollution Control, Key Laboratory of Advanced Functional Materials,
Education Ministry of China, Laboratory of Catalysis Chemistry and Nanoscience,
Department of Chemistry and Chemical Engineering, College of Environmental and
Energy Engineering, Beijing University of Technology, Beijing 100124, China
* Corresponding authors.
E-mail addresses: yxliu@bjut.edu.cn (Dr. Y.X. Liu); hxdai@bjut.edu.cn (Prof. H.X. Dai)
Tel. No.: +8610-6739-6118; Fax: +8610-6739-1983.
Graphical_abstract
CO2
Trichloroethyle
HCl
Cl2
H2O
O2

The 3DOM xCr2O3CeO2 (x is the Cr2O3 weight percentage (wt%)) samples are prepared
using the polymethyl methacrylate-templating method. The good catalytic stability and
Cl-resistant ability of 3DOM 5.5Cr2O3CeO2 is associated with the strong interaction
between Cr2O3 and CeO2.
Research Highlights
 ► 3DOM xCr2O3CeO2 is prepared via the polymethyl methacrylate-templating
route.
 ► 3DOM 5.5Cr2O3CeO2 shows the best catalytic activity for trichloroethylene
oxidation.
 ► 3DOM 5.5Cr2O3CeO2 exhibits good catalytic stability and Cl-resistant ability.
 ► The strong Cr2O3CeO2 interaction is responsible for the good stability.
Abstract:
Three-dimensionally ordered macroporous (3DOM) CeO2, 3DOM Cr2O3, 3DOM
xCr2O3CeO2 (x (the weight percentage of Cr2O3) = 3.5, 5.5, and 8.0 wt%), and 5.5 wt%
Cr2O3/3DOM CeO2 samples were prepared using the polymethyl methacrylate
(PMMA)-templating and incipient wetness impregnation methods, respectively. A number
of techniques were used to characterize physicochemical properties of the materials, and
their catalytic activities were evaluated for the oxidation of trichloroethylene (TCE). These

samples possessed a good-quality 3DOM structure and a surface area of 3547 m2/g. The
3DOM 5.5Cr2O3CeO2 sample performed the best (the temperature at TCE conversion =
o
90 % = 255 C at a space velocity of 20,000 mL/(g h)). Effects of water vapor and carbon
dioxide on activity of the 5.5Cr2O3CeO2 sample were also examined. It is observed that
partial deactivation induced by H2O introduction of the 5.5Cr2O3CeO2 sample was
reversible, while that induced by CO2 addition was irreversible. Based on the activity data
and characterization results, it is concluded that the good catalytic activity and thermal
stability of 3DOM 5.5Cr2O3CeO2 was associated with its high adsorbed oxygen species
concentration, good low-temperature reducibility, and strong interaction between Cr2O3 and
CeO2. We believe that the 3DOM 5.5Cr2O3CeO2 catalyst is promising in the application
for oxidative removal of chlorinated volatile organic compounds.
Keywords: Three-dimensionally ordered macropore; Ceria; Chromia; Chromiaceria
composite oxide; Trichloroethylene oxidation.
1. Introduction
Chlorinated volatile organic compounds (CVOCs) are harmful to the environment and
human health [1]. Among the CVOCs, dichloromethane, 1,2-dichloroethane, chlorobenzene,
and trichloroethylene (TCE) have received much attention due to their high toxicity,
stability, and widespread applications in industry. Catalytic oxidation is one of the most
effective pathways for the abatement of CVOCs, in which supported noble metals and base
metal oxides are the most commonly used catalysts [25]. The severe nature of CVOCs

requires the catalysts to show good activity at low temperatures and good stability (resistant
to the poisoning of the formed chlorides). Therefore, it is highly desired to develop
high-performance and good-stability catalysts for the oxidative removal of CVOCs.
Although supported noble metal (e.g., Pt, Pd, Rh or Ru) catalysts show good activity and
high selectivity to CO2 and HCl in the oxidative destruction of CVOCs, they are expensive
and easy to be poisoned by the formed HCl and Cl2 as well as easily deactivated by carbon
deposition [6,7]. For example, Taralunga et al. [8] reported that the 1.2 wt% PtHY catalyst
was deactivated within 24 h of on-stream 1,2-dichlorobenzene oxidation and the
deactivation was due to the sintering of Pt. After having investigated the Ru-doped CeO2
catalysts for the combustion of chlorobenzene, Dai et al. [9] found that the Ru-CeO2
o
catalysts showed excellent performance (T90% < 250 C) and good stability (no significant
o
deactivation within 80 h of on-stream reaction at 275 C), which was ascribed to the rapid
removal of the inorganic chlorine species adsorbed on the active sites via the Deacon
process catalyzed by the RuO2 component.
Base metal oxides catalysts have been proposed as potential substitutes for noble
metal-based catalysts. Although base metal oxides are usually less active than noble metals,
they possess excellent redox properties [10] and are more resistant to deactivation. For
example, Zuo et al. [11] found that the CrCe (6 : 1)/Ti-PILC catalyst showed higher activity
(complete conversion of low-concentration chlorobenzene was achieved at ca. 250 oC) than
the Na-montmorillonite-supported catalysts, which was due to the fact that Ti-PILC
exhibited a significantly larger basal space and higher surface area and pore volume as
compared with Na-montmorillonite. Wang et al. [12] pointed out that the good performance

of Ce0.5Ti0.5O2 for 1,2-dichlorobenzene combustion has the highest apparent activity with
T90% about 375 °C, was due to the incorporation of Ti distorted the crystal structure and
thus increased greatly the acidity and the oxygen mobility at high temperature. Meanwhile,
Ti improved the stability of Ce1−xTixO2 catalysts through retarding the exchange of Cl for
basic lattice oxygen and hydroxyl groups. Among base metal oxide catalysts, CeO2-based
catalysts (e.g., CeO MnO [13–15], CeO ZrO [16–19], and Ce Pr O [20]) have been

1 x
2
x
2
2
x
2
extensively investigated due to their outstanding redox properties, oxygen storage capacity,
cheapness, environment benign nature, considerable oxidative activity in CVOCs oxidation,
and good resistance to Cl-poisoning. Among the Ce-based catalysts, pure ceria exhibited
fairly high catalytic activity for CVOCs oxidation [21,22]. Porous ceria with rich porosity,
high surface area, narrow pore-size distribution, and good reducibility is expected to
possess improved physicochemical properties [23–27]. In addition, chromium-based
catalysts exhibited high activity for the abatement of CVOCs [2830]. Nevertheless, the
use of this type of catalysts tends to be restricted due to formation of extremely toxic
residues (e.g., chromium oxychloride) at high temperatures and volatilization of the active
phase [31,32].
Although modified Cr-based catalysts have been applied to remove the CVOCs, there are
still some aspects to be improved, such as the type and structure of the catalyst support and
the optimized loading of Cr. Compared with the nonporous catalysts, the
three-dimensionally ordered macroporous (3DOM) catalysts exhibit better performance
owing to their higher surface areas and easy diffusion properties. Previously, our group
adopted the polymethyl methacrylate (PMMA)-templating and polyvinyl alcohol-protected

reduction strategies to generate a series of 3DOM-structured catalytic materials [3335],
and observed that most of them performed well in the oxidation of VOCs. In this work, we
report the preparation of 3DOM xCr2O3CeO2 and 3DOM CeO2-supported Cr2O3 using the
PMMA-templating and incipient wetness impregnation methods, respectively, and
investigated their catalytic properties for the oxidative removal of TCE. It is found that the
3DOM 5.5Cr2O3CeO2 sample showed good activity and stability for the addressed
reaction.
2. Experimental
2.1. Catalyst preparation
Well-arrayed colloidal crystal PMMA microspheres with an average diameter of 300 nm
were synthesized according to the procedure described elsewhere [36]. The 3DOM
xCr2O3CeO2 (x is the weight percentage (wt%) of Cr2O3) samples were prepared using the
PMMA-templating method [27]. In a typical preparation, 1.0 g of polyethylene glycol
(PEG)-10000, 4.34 g of Ce(NO3)36H2O, and 0, 0.13, 0.20, and 0.30 g of Cr(NO3)39H2O
were dissolved in 10.0 g of ethanol aqueous solution (40 wt%) at room temperature (RT)
under stirring for 4 h. After dissolution, 2.0 g of the PMMA template was soaked in the
above transparent solution for 6 h. After being filtered and dried at RT for 36 h, the
o
obtained powders were calcined in a N2 flow of 200 mL/min at a ramp of 1 C/min from
o
o
RT to 300 C and kept at this temperature for 3 h, then cooled to 50 C under the same
atmosphere, and finally in an air flow of 200 mL/min at a ramp of 1 oC/min from RT to 600
oC and maintained at this temperature for 5 h, thus obtaining the 3DOM xCr2O3CeO2 (x =
0, 3.5, 5.5, and 8.0) samples.

For comparison purposes, 3DOM Cr2O3 was also prepared using the above same method
with Cr(NO3)39H2O as chromium source. The 5.5Cr2O3/3DOM CeO2 sample was prepared
via the incipient wetness impregnation route. In a typical preparation, 0.50 g of 3DOM
CeO2 was impregnated with 5.0 mL of Cr(NO3)3ethanol aqueous solution (0.015 mol/mL)
for 30 min. After that, the mixture was dried at 80 °C for 12 h, and then calcined in an air
o
o
flow of 200 mL/min at a ramp of 1 C/min from RT to 600 C and maintained at this
temperature for 5 h.
2.2. Catalyst characterization
Physicochemical properties of the samples were characterized by means of techniques,
such as inductively coupled plasmaatomic emission spectroscopic (ICPAES), X-ray
diffraction (XRD), laser Raman, scanning electron microscopy (SEM), transmission
electron microscopy (TEM), high angle annular dark field and scanning transmission
electron microscopy (HAADFSTEM), N2 adsorptiondesorption (BET), X-ray
photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR),
and NH3 temperature-programmed desorption (NH3-TPD). The detailed characterization
procedures are described in the Supplementary material.
2.3. Catalytic activity evaluation
The catalytic activities of the samples were evaluated in a continuous flow fixed-bed
quartz microreactor (i.d. = 6 mm). To minimize the effect of hot spots, 50 mg of the sample
(4060 mesh) was mixed with 0.25 g of quartz sands (4060 mesh). Prior to the test, the
o
sample was treated in air (30 mL/min) at 350 C for 1 h. After being cooled to a given
temperature, the reactant gas mixture containing trichloroethylene (TCE) was passed

through the catalyst bed. The total flow rate of the reactant mixture (750 ppm TCE + 20
vol% O2 + N2 (balance)) was 16.7 mL/min and the space velocity (SV) was ca. 20,000
mL/(g h). After the reaction was stabilized for 1 h at a certain temperature, the reactants
and products were online analyzed by a gas chromatograph (GC-7890B, Agilent) equipped
with a flame ionization detector (FID) and a TCD, using a Agilent 19091J-413 HP-5 (30 m
in length) and G3591-81141 (silica gel), G3591-81142 (silica gel), and G3591-81022
(molsieve 5A, 2.44 m in length of each packing column). The TCD was used to analyze the
CO2 and CO concentrations, whereas the FID used to analyze the concentrations of the
organic compounds. The TCE conversion is defined as (cinletcoutlet)/cinlet × 100 %, where
the cinlet and coutlet are the inlet and outlet TCE concentrations in the feed stream,
respectively. In the case of water vapor introduction, 3.0 or 5.0 vol% H2O was introduced
o
by passing the feed stream through a water saturator at 23 and 34 C, respectively. In the
case of CO2 addition, 5.0 or 10 vol% CO2 was introduced from a CO2 cylinder with N2 as
balance gas. The balance of carbon throughout the catalytic system was estimated to be
99.5  1.5 %. The evolution of possible intermediates and products was checked online by
the Agilent GC-MS equipment.
Selectivities to HCl and Cl2 were measured by bubbling the outlet gas mixture through
a NaOH aqueous solution (12.5 mmol/L) for 0.5 h each time. The Cl2 concentration in the
bubbled solution was determined by chemical titration with ferrous ammonium sulfate
using N,N-diethyl-p-phenylenediamine as indicator, and the Cl concentration was
measured using an ion selective electrode.
3. Results and discussion

3.1. Crystal phase, morphology, pore structure, and surface area
Fig. 1 shows the XRD patterns of the 3DOM CeO2, 3DOM Cr2O3, 3DOM xCr2O3CeO2,
and 5.5Cr2O3/3DOM CeO2 samples. According to the XRD pattern (JCPDS PDF# 81-0792)
of the standard ceria sample, we deduce that the CeO2 in the 3DOM CeO2, 3DOM
xCr2O3CeO2, and 5.5Cr2O3/3DOM CeO2 samples possessed a cubic fluorite-type crystal
structure. By comparing the XRD pattern (JCPDS PDF# 84-1616) of the standard chromia
sample, one can realize that 3DOM Cr2O3 was of a rhombohedral Cr2O3 phase. All of the
Bragg diffraction peaks due to cubic CeO2 and rhombohedral Cr2O3 can be well indexed, as
indicated in Fig. 1e and f. Weak diffraction signals assignable to the Cr2O3 phase were
detected in the 3DOM 5.5Cr2O3CeO2, 3DOM 8.0Cr2O3CeO2, and 5.5Cr2O3/3DOM
CeO2 samples (Fig. 1b, c, and d); however, no diffraction signals of the Cr2O3 phase were
recorded in the 3DOM 3.5Cr2O3CeO2 sample (Fig. 1a), a result possibly due to the low
Cr2O3 content and good dispersion in this sample.
Raman spectra of the fluorite-type structures are sensitive to the crystalline symmetry
and related to the lattice oxygen vibrations. In order to further investigate the phase
structures of the samples, we recorded their Raman spectra, as shown in Fig. 2. The
characteristic Raman bands of 3DOM Cr2O3 were observed at 301, 383, 503, 533, and 590
1
1
cm . The most intense Raman band at 533 cm was assigned to the A1g symmetry, the
1
bands at 301 and 383 cm was due to the Eg symmetry, and the band with a medium
1
1
intensity at 503 cm and the band at 590 cm were assigned to the CrO stretching
vibration (F2g symmetry) [3741]. For the 3DOM CeO2, 3DOM xCr2O3CeO2, and
5.5Cr2O3/3DOM CeO2 samples, the Raman spectra were dominated by a strong band

1
1
centered at 449 cm and two less prominent broad bands centered at 320 and 580 cm .
1
The band at 449 cm was assigned to the defect space of oxygen vacancy in the CeO2
1
lattice, the band at 580 cm was ascribed to the non-degenerate LO mode of CeO2 due to
the relaxation of symmetry rules (which was connected with the oxygen vacancies, due to
1
the presence of Ce3+ ions in CeO2 lattice), and the band at 320 cm was attributed to the
displacement of oxygen atoms from their ideal fluorite lattice positions to the tetragonal
positions (also suggesting the existence of a defect structure). With increasing the Cr2O3
1
content, the band at 449 cm was weakened, indicating the decrease of the cubic
1
fluorite-type phase (c-phase); however, the band at 580 cm increased in intensity, which
indicates the increase of oxygen vacancy concentration [4244]. According to the literature
[44,45], oxygen vacancies could favor the redox reactions and thus promote deep oxidation
of CVOCs over the catalysts.
Fig. 3 shows the SEM images of the samples. It can be clearly observed that the 3DOM
CeO2 and 3DOM Cr2O3 samples displayed a high-quality 3DOM architecture (Fig. 3a and
b); with the doping or loading of Cr2O3, the 3DOM structure of the sample slightly
decreased in quality (Fig. 3ci). The periodic voids in the long-range ordered structure
were interconnected through open windows with a size of 120160 nm. The TEM images
(Fig. 4) of the samples confirm the formation of good-quality 3DOM structures. In order to
further examine the homogeneous dispersion of Cr2O3 and CeO2, we recorded the
HAADFSTEM and EDS elemental mapping images (Fig. 5) of the 3DOM
5.5Cr2O3CeO2 sample. Obviously, the Cr2O3 and CeO2 were uniformly distributed in the
3DOM 5.5Cr2O3CeO2 sample.

Fig. 6 shows the N2 adsorptiondesorption isotherms and pore-size distributions of the
3DOM CeO2, 3DOM Cr2O3, 3DOM xCr2O3CeO2, and 5.5Cr2O3/3DOM CeO2 samples.
Each sample exhibited a type II isotherm with an H3 hysteresis loop in the relative pressure
(p/p0) range of 0.81.0. There was also a small H2 type hysteresis loop in the p/p0 range of
0.2–0.8. The H3 hysteresis loop (which did not clearly show any adsorption plateau at p/p0
close to unity) was related to the existence of slit-shaped pores in the materials, which was
indicative of macropore formation [26]. The low-pressure portion of the almost linear
middle section of each isotherm (which could be attributed to the multilayer adsorption)
also suggests formation of macropores [46]. The H2 hysteresis loop in the p/p0 range of
0.20.8 (which was linkable to the capillary condensation taking place in mesopores)
indicates the presence of mesopores in the skeleton of 3DOM CeO2 [26,46,47]. In the
pore-size distribution, of each sample, there were mesopores (210 nm) and pores scattered
from 20 to140 nm (Table 1). Surface areas (40.343.6 m2/g) of the 3DOM xCr2O3CeO2
samples were slightly lower than that (47.3 m2/g) of the 3DOM CeO2 sample, but higher
than that (34.9 m2/g) of the 3DOM Cr2O3 sample (Table 1). It should be noted that the
5.5Cr2O3/3DOM CeO2 sample displayed the lowest surface area (27.6 m2/g), a result
possibly due to the blocking of macropores by part of the Cr2O3 entities. All of the samples
possessed a pore volume of 0.2100.233 cm3/g.
3.2. Surface element composition, metal oxidation, and oxygen species
XPS is an effective technique to investigate the surface element compositions, metal
oxidation states, and adsorbed oxygen species of a catalyst. The Ce 3d, Cr 2p3/2, O 1s XPS
spectra of the samples are illustrated in Fig. 7. There were two sets of signals,

corresponding to the Ce 3d3/2 and Ce 3d5/2 states; Ce 3d spectrum of each sample contained
the 3d3/2 feature at binding energy (BE) = 901.4, 902.9, 908.0, and 917.1 eV and the 3d5/2
feature at BE = 882.8, 884.8, 889.5, and 898.8 eV (Fig. 7A). The signals at BE = 884.8 and
902.9 eV were assigned to the surface Ce3+ species, whereas the others were ascribed to the
surface Ce4+ species [26,35,48]. This result suggests that cerium on the surface of each
sample existed in a mixed oxidation state of Ce3+ and Ce4+. The discrepancy in peak area
reflected the different Ce3+/Ce4+ molar ratios on the surface of the mixed oxide samples.
The existence of Ce3+ species indicates formation of oxygen vacancies in the CeO2 lattice,
which was beneficial for the generation of active oxygen species [35,49]. As shown in Fig.
7B, the asymmetric O 1s XPS signal could be decomposed to three components at BE =
529.6, 513.3, and 533.7 eV, ascribable to the surface lattice oxygen (Olatt), adsorbed oxygen
−
(Oads, e.g., O2 , O22− or O−), and carbonate or adsorbed water species [27,35], respectively.
The Oads/Olatt molar ratio decreased in the order of 3DOM 5.5Cr2O3CeO2 (0.51) > 3DOM
8.0Cr2O3CeO2 (0.45) > 3DOM 3.5Cr2O3CeO2 (0.41) > 5.5Cr2O3/3DOM CeO2 (0.40) >
3DOM Cr2O3 (0.39) > 3DOM CeO2 (0.38), and the 3DOM 5.5Cr2O3‒CeO2 sample
possessed the highest Oads/Olatt molar ratio (Table 2). Since the surface adsorbed oxygen
species could play an important role in deep oxidation of organics [33,35], it is expected
that 3DOM 5.5Cr2O3‒CeO2 sample would show good catalytic activity for TCE
combustion. The Cr 2p3/2 XPS spectrum of each sample could be decomposed into three
components at BE = 576.1, 576.6, and 578.5 eV (Fig. 7C), attributable to the surface
Cr(OH)3 or Cr2O3 [50], Cr3+ [51,52], and Cr6+ [53,54] species, respectively. The Cr6+/Cr3+
molar ratio increased from 0.26 to 0.35 with the rise in Cr2O3 doping from 3.5 to 8.0 wt%

(Table 2). It should be mentioned that the surface Ce4+ species might facilitate Cr3+
reduction at the Cr2O3CeO2 interfaces via 3Ce4+ + Cr3+  3Ce3+ + Cr6+, resulting in
increases in Ce3+ and Cr6+ species concentrations. According to the literature [32,55], Cr6+
species exhibited a strong oxidizing ability and the presence of Cr6+ species would be
beneficial for deep oxidation of CVOCs.
3.3. Low-temperature reducibility
H2-TPR technique was used to investigate the reducibility of 3DOM CeO2, 3DOM Cr2O3,
o
3DOM xCr2O3CeO2, and 5.5Cr2O3/3DOM CeO2 samples within 100–800 C, and their
profiles are illustrated in Fig. 8A. For the 3DOM CeO2 sample, there were two peaks at 465
o
and 698 C, in which the former was probably associated with the outmost layer reduction
of Ce4+ to Ce3+ and adsorbed oxygen species, whereas the latter was due to the reduction of
the inner Ce4+ to Ce3+ and lattice oxygen species [35]. After introduction of Cr2O3 to CeO2,
the initial reduction peak shifted to a lower temperature, indicating that the reducibility of
the sample was improved after Cr2O3 doping. Comparing to the 3DOM CeO2 sample, the
3DOM xCr2O3CeO2 and 5.5Cr2O3/3DOM CeO2 samples were more reducible at lower
temperatures. The reduction peaks of the 3DOM xCr2O3CeO2 samples shifted to lower
temperatures, with the 3DOM 5.5Cr2O3CeO2 sample possessing the lowest reduction
o
temperature of 284 C. By quantitatively analyzing the reduction peaks in the H2-TPR
profiles, we can obtain the H2 consumption of the 3DOM CeO2, 3DOM Cr2O3, 3DOM
xCr2O3CeO2, and 5.5Cr2O3/3DOM CeO2 samples, as summarized in Table 2. Apparently,
the H2 consumption of 3DOM 5.5Cr2O3CeO2 was higher than that of 3DOM CeO2.
Generally speaking, the initial (where less than 25 % oxygen in the sample is consumed for

the first reduction peak) H2 consumption rate can be used to evaluate the low-temperature
reducibility of a sample [56], and the results are shown in Fig. 8B. It is observed that the
initial H2 consumption rate (i.e., low-temperature reducibility) decreased in the order of
3DOM 5.5Cr2O3CeO2 > 3DOM 8.0Cr2O3CeO2 > 3DOM 3.5Cr2O3CeO2 > 3DOM
Cr2O3 > 5.5Cr2O3/3DOM CeO2 > 3DOM CeO2.
3.4. Catalytic performance
Fig. 9A shows the catalytic activities of the 3DOM CeO2, 3DOM Cr2O3, 3DOM
xCr2O3CeO2, and 5.5Cr2O3/3DOM CeO2 samples for TCE combustion. TCE could not be
completely oxidized over 3DOM CeO2, and only 31 % TCE conversion was obtained at
o
350 C. After the doping of Cr2O3, however, catalytic activity increased obviously. The
3DOM xCr2O3CeO2 samples performed much better than the 3DOM CeO2 or 3DOM
Cr2O3 sample. It is convenient to compare catalytic activities of the samples using the
reaction temperatures T10%, T50%, and T90% (corresponding to TCE conversion = 10, 50, and
90 %), as summarized in Table 3. Apparently, the catalytic activity decreased in the
sequence of 3DOM 5.5Cr2O3CeO2 > 3DOM 8.0Cr2O3CeO2 > 3DOM 3.5Cr2O3CeO2 >
3DOM Cr2O3 > 5.5Cr2O3/3DOM CeO2 > 3DOM CeO2, with the 3DOM 5.5Cr2O3CeO2
o
o
o
sample showing the highest activity (T10% = 153 C, T50% = 214 C, and T90% = 255 C at
SV = 20,000 mL/(g h)). The sequence in catalytic activity of these samples was basically
agreeable with those in Oads/Olatt molar ratio and low-temperature reducibility.
Previous works have reported that the oxidation of CVOCs over LaMn1+xO3 [57], Cr2O3
[58], Ru/Al2O3 [59], and MnxCe1−xO2/HZSM5 [60] obeyed a reaction mechanism of a
first-order toward CVOC concentration and a zero-order toward O2 concentration.

Therefore, it is reasonably supposed that TCE oxidation in the presence of excess oxygen
(TCE/O2 molar ratio = 1/1090) would follow a first-order reaction mechanism with respect
to TCE concentration (c): r = -kc = (-A exp(-Ea/RT)) c, where r is the reaction rate (mol/s),
1
k is the rate constant (s ), A is the pre-exponential factor, and Ea is the apparent activation
energy (kJ/mol). It is well known that the lower the apparent activation energy (Ea), the
easier the complete oxidation of an organic compound over the catalyst, and hence the
better is the catalytic performance. The discrepancy in Ea of the samples could reflect their
different catalytic activities. Fig. 9B shows the Arrhenius plots for the oxidation of TCE
conversion <20% over the samples and their Ea values are summarized in Table 3.
Apparently, the Ea increased in the sequence of 3DOM 5.5Cr2O3CeO2 < 3DOM
8.0Cr2O3CeO2 < 5.5Cr2O3/3DOM CeO2 < 3DOM 3.5Cr2O3CeO2 < 3DOM Cr2O3 <
3DOM CeO2, in rough agreement with the order in reaction rate. The 3DOM
5.5Cr2O3CeO2 sample exhibited the lowest Ea (81 kJ/mol).
In the catalytic elimination of CVOCs, there might be some by-products. We detected
the product mixture of TCE oxidation, and the results are shown in Fig. 10. Within the
detection limits of TCD and FID, C2Cl4 was the only by-product detected in the outlet gas
mixture during the TCE oxidation process. Over the 3DOM CeO2 sample, a low
concentration of C2Cl4 at Tmax (temperature at which the maximum concentration reached)
o
was detected and the maximum concentration was 30 ppm at Tmax = 330 C; over the
3DOM xCr2O3CeO2 and 5.5Cr2O3/3DOM CeO2 samples, however, very low
concentrations of the by-products were detected and the maximum concentrations were <
60 ppm. Moreover, the amount of by-products decreased with the rise in Cr2O3 doping and

was lower than 100 ppm over the Cr2O3-containing samples. Fig. S1 shows the NH3-TPD
profiles of the samples, and the relative amounts (Astrong/Aweak) of strong and weak acid sites
are summarized in Table S2. Obviously, the doping of Cr2O3 promoted formation of the
strong acidic sites, and the amount of strong acidic sites in 3DOM 5.5Cr2O3CeO2 was
more than that in 3DOM CeO2, suggesting that Cr2O3 doping could enhance the acidity of
the sample. The basic sites in the samples might be responsible for formation of the
by-products. The low amounts of the by-products formed over the 3DOM xCr2O3CeO2
and 5.5Cr2O3/3DOM CeO2 samples were related to the weak base strength (i.e., the strong
acid sites). CeO2 possesses unique Ce4+/Ce3+ redox property, strong interactions with
metals or metal oxides, and good ability for dissociating CCl bonds. However,
deactivation due to strong adsorption of Cl species produced during CVOCs oxidation
processes, high activity in chlorination, and low resistance to water of CeO2 limit its wide
industrial applications. It is well known that Cl tends to be adsorbed on oxygen vacancies
of CeO2. The doping of Cr2O3 to CeO2 could modify the chemical environment around
oxygen vacancies, prohibiting adsorption of Cl species. Part of the Cr3+ were incorporated
into the lattice of CeO2, whereas the other Cr2O3 were interacted with CeO2 to generate the
active Cr2O3CeO2 interfaces (formation of bonds like CrOCe). In addition, formation of
Cr6+ species with a strong oxidizing ability was beneficial for deep oxidation of CVOCs.
Furthermore, Brönsted acid sites and strong Lewis acid sites were enhanced by interaction
of Cr2O3 with CeO2. Since pure Cr2O3 and CeO2 were inactive for TCE combustion at 210
oC, it is likely that good catalytic performance of 3DOM Cr2O3CeO2 was associated with
the active interfaces between Cr2O3 and CeO2.

For total oxidation of TCE, the final products were CO, CO2, H2O, HCl, and Cl2. The
HCl selectivity (SHCl) over all of the samples are also shown in Table 3. It is observed that
all of the samples showed a high HCl selectivity, particularly over the 3DOM
5.5Cr2O3CeO2 sample. Since HCl is easier to be treated and less toxic than Cl2, a high
HCl selectivity would be favorable for the oxidation of TCE. Moreover, all of the samples
exhibited a more than 99 % CO2 selectivity. It should be pointed out that the estimated
carbon and chlorine balance was 95–97 and 89–94 %, respectively. In other words, small
amounts of carbon and chlorine were retained on the surface of the samples. It should be
noted that there might be no formation of chromium oxychloride in the 3DOM Cr2O3‒CeO2
o
samples for TCE combustion below 340 C since Cr2O3 strongly interacted with CeO2 to
generate a Cr2O3‒CeO2 solid solution and partial Cr3+ were incorporated into the lattice of
CeO2. The detailed situations need to be further investigated.
In order to examine the catalytic stability, we carried out the on-stream TCE oxidation
o
over the 3DOM 5.5Cr2O3CeO2 sample at 240 C and SV = 20,000 mL/(g h), and their
results are shown in Fig. 11. No significant loss in activity over 3DOM 5.5Cr2O3CeO2
was observed after 30 h of on-stream reaction. In other words, the 3DOM 5.5Cr2O3CeO2
sample was catalytically durable under the adopted conditions.
In order to better make a comparison on performance, Table 3 summarizes the TCE
reaction rates normalized by per gram of catalyst. Obviously, the TCE reaction rate at 250
oC decreased in the order of 3DOM 5.5Cr2O3CeO2 > 3DOM 8.0Cr2O3CeO2 > 3DOM
3.5Cr2O3CeO2 > 5.5Cr2O3/3DOM CeO2 > 3DOM CeO2, with the 3DOM 5.5Cr2O3CeO2
7
o
sample showing the highest TCE reaction rate (4.02 × 10 mol/(gcat s)) at 250 C. These

results confirm that factors, such as adsorbed oxygen species concentration, low
temperature reducibility, and strong interaction between Cr2O3 and CeO2, could influence
catalytic activity of the sample. There have been a number of works on the oxidation of
TCE over various catalysts in the literature [13,45,57,6166]. According to the results
o
7
reported in the literature, one can see that the TCE reaction rate at 250 C (4.02 × 10
mol/(gcat s)) over our 3DOM 5.5Cr2O3CeO2 sample was much higher than that (1.12 ×
7
7
10 mol/(gcat s)) over CeMn-HT-N6A4 [13], that (1.13 × 10 mol/(gcat s)) over 1.02 wt%
8
8
Ru/TiO2 (P25) [61], that (2.67 × 10 mol/(gcat s)) over Ce0.15Zr0.85O2 [62], that (4.08 × 10
8
mol/(gcat s)) over LaMn1.2O3 [57], that (4.31 × 10 mol/(gcat s)) over 4Ce1Cr-(NH4)2CO3
8
9
[45], that (9.76 × 10 mol/(gcat s)) over 3.5 wt% VOx/TiO2-SG [63], that (2.67 × 10
9
mol/(gcat s)) over 4.3 wt% Mn/H-ZSM-5 [64], that (3.49 × 10 mol/(gcat s)) over 0.42 wt%
9
Pd/Al2O3 [65], and that (7.05 × 10 mol/(gcat s)) over CoFeAlOx [66]. Therefore, our
3DOM 5.5Cr2O3CeO2 sample performed much better than most of the reported catalysts
for TCE oxidation. The 3DOM structure with a larger surface area was beneficial for
enhancement in adsorption and mass transfer of the reactant molecules, and the nanosized
skeleton surface was favorable for improvement in reducibility. Partial Cr3+ were
incorporated into the lattice of CeO2, resulting in an increase in oxygen vacancy density
and hence a rise in Oads concentration. The interaction between Cr2O3 and CeO2 could
generate a Cr2O3CeO2 solid solution, forming the active Cr2O3CeO2 interfaces.
Furthermore, the doping of chromia increased the amount of strong acid sites. Therefore,
the Oads species concentration, low-temperature reducibility, and strong interaction between

Cr2O3 and CeO2, and strong acidity were the factors influencing the catalytic performance
of 3DOM 5.5Cr2O3‒CeO2 for TCE combustion.
3.5. Effects of water vapor and carbon dioxide on activity
Due to the co-existence of TCE and H2O or CO2 in industrial waste gases, we examined
the effects of water vapor and carbon dioxide on activity of the 3DOM 5.5Cr2O3CeO2
sample at 270 oC and SV = 20,000 mL/(g h), and their results are shown in Fig. 12A and B,
respectively. It was reported that activity of a catalyst was negatively influenced by water
vapor introduction [27,67]. Therefore, it is highly desired to develop a water-resistant
catalyst for practical applications. In the presence of 3.0 vol% water vapor, TCE conversion
decreased by ca. 6 % after 6 h of on-stream reaction; a further rise in water vapor
concentration to 5.0 vol% led to a drop in TCE conversion by ca. 10 %. After water vapor
was cut off, TCE conversions were restored to their original values in the absence of water
vapor (Fig. 12A). This result means that partial deactivation induced by water vapor
addition was reversible. The inhibition of water was due to the competitive adsorption of
water and reactant molecules on the active sites in the 3DOM 5.5Cr2O3CeO2 sample.
Similar phenomena were also observed by other researchers [26,67]. Fig. 12B shows the
o
effect of CO2 addition on catalytic activity of 3DOM 5.5Cr2O3CeO2 at 270 C and SV =
20,000 mL/(g h). Introduction of 5.0 and 10.0 vol% CO2 to the reaction system led to ca. 4
and 6 % loss in TCE conversion. The negative effect of CO2 addition was due to the fact
that partial active sites in 3DOM 5.5Cr2O3CeO2 were covered by adsorption of carbonate
species formed during the on-stream reaction. After the used sample was activated in an O2
o
flow of 20 mL/min at 400 C for 1 h, TCE conversion slightly increased. This result

suggests that the formed carbonate species were not totally decomposed and part of the
active sites occupied by carbonate species were not recovered. Therefore, partial
deactivation due to CO2 addition was irreversible. Similar phenomena were also observed
in our previous investigations [68,69].
4. Conclusions
The 3DOM CeO2, 3DOM Cr2O3, 3DOM xCr2O3CeO2, and 5.5Cr2O3/3DOM CeO2
samples with a surface area of 3547 m2/g were prepared using the PMMA-templating and
incipient wetness impregnation methods, respectively. The ceria in 3DOM CeO2, 3DOM
xCr2O3CeO2, and 5.5Cr2O3/3DOM CeO2 was of cubic fluorite-type crystal structure,
whereas chromia in 3DOM Cr2O3, 3DOM xCr2O3CeO2, and 5.5Cr2O3/3DOM CeO2 was
of rhombohedral crystal structure. All of the samples displayed a good-quality 3DOM
architecture. It is found that an appropriate amount of Cr2O3 doped to CeO2 could enhance
the Oads species concentration and low-temperature reducibility of the 3DOM xCr2O3CeO2
sample. The 3DOM 5.5Cr2O3CeO2 sample showed the highest catalytic activity for TCE
oxidation (T10% = 153 oC, T50% = 214 oC, and T90% = 255 oC at SV = 20,000 mL/(g h)) and
good catalytic stability. Partial deactivation induced by water vapor addition was reversible,
whereas CO2 addition resulted in irreversible deactivation of 3DOM 5.5Cr2O3CeO2. It is
concluded that the good catalytic performance of 3DOM 5.5Cr2O3CeO2 was associated
with its Oads species concentration, low-temperature reducibility, and strong interaction
between Cr2O3 and CeO2. We believe that such 3DOM-structured chromiaceria materials
are promising catalysts for the oxidative removal of CVOCs.
Acknowledgements

This work was supported by the NSF of China (21677004) and National High
Technology Research and Development Program ("863" Program) of China
(2015AA034603).
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Captions of Figures and Tables
Fig. 1. XRD patterns of (a) 3DOM 3.5Cr2O3CeO2, (b) 3DOM 5.5Cr2O3CeO2, (c) 3DOM
8.0Cr2O3CeO2, (d) 5.5Cr2O3/3DOM CeO2, (e) 3DOM CeO2, and (f) 3DOM Cr2O3.
Fig. 2. Raman spectra of (a) 3DOM Cr2O3, (b) 3DOM 3.5Cr2O3CeO2, (c) 3DOM
5.5Cr2O3CeO2, (d) 3DOM 8.0Cr2O3CeO2, (e) 5.5Cr2O3/3DOM CeO2, and (f) 3DOM
CeO2.
Fig. 3. SEM images of (a) 3DOM CeO2, (b) 3DOM Cr2O3, (c, d) 3DOM 3.5Cr2O3CeO2,
(e, f) 3DOM 5.5Cr2O3CeO2, (g, h) 3DOM 8.0Cr2O3CeO2, and (f) 5.5Cr2O3/3DOM
CeO2.
Fig. 4. TEM images of (a) 3DOM CeO2, (b) 3DOM 3.5Cr2O3CeO2, (c) 3DOM
5.5Cr2O3CeO2, (d) 3DOM 8.0Cr2O3CeO2, (e) 5.5Cr2O3/3DOM CeO2, and (f) 3DOM
Cr2O3.
Fig. 5. HAADFSTEM and elemental mapping images of the 3DOM 5.5Cr2O3CeO2
sample.
Fig. 6. (A) N2 adsorptiondesorption isotherms and (B) pore-size distributions of (a)
3DOM 3.5Cr2O3CeO2, (b) 3DOM 5.5Cr2O3CeO2, (c) 3DOM 8.0Cr2O3CeO2, (d)
5.5Cr2O3/3DOM CeO2, (e) 3DOM CeO2, and (f) 3DOM Cr2O3.
Fig. 7. (A) Ce 3d, (B) O 1s, and (C) Cr 2p3/2 XPS spectra of (a) 3DOM 3.5Cr2O3CeO2, (b)
3DOM 5.5Cr2O3CeO2, (c) 3DOM 8.0Cr2O3CeO2, (d) 5.5Cr2O3/3DOM CeO2, (e) 3DOM
CeO2, and (f) 3DOM Cr2O3.

Fig. 8. (A) H2-TPR profiles and (B) initial H2 consumption rate versus inverse temperature
of (a) 3DOM CeO2, (b) 3DOM 3.5Cr2O3CeO2, (c) 3DOM 5.5Cr2O3CeO2, (d) 3DOM
8.0Cr2O3CeO2, (e) 5.5Cr2O3/3DOM CeO2, and (f) 3DOM Cr2O3.
Fig. 9. (A) TCE conversion as a function of temperature and (B) ln k versus inverse
temperature over (●) 3DOM CeO2, (○) 3DOM Cr2O3, (▲) 3DOM 3.5Cr2O3CeO2, (△)
3DOM 5.5Cr2O3CeO2, (■) 3DOM 8.0Cr2O3CeO2, and (□) 5.5Cr2O3/3DOM CeO2 at
SV = 20,000 mL/(g h).
Fig. 10. Concentrations of the C2Cl4 by-product formed over (●) 3DOM CeO2, (○)
3DOM Cr2O3, (▲) 3DOM 3.5Cr2O3CeO2, (△) 3DOM 5.5Cr2O3CeO2, (■) 3DOM
8.0Cr2O3CeO2, and (□) 5.5Cr2O3/3DOM CeO2 at SV = 20,000 mL/(g h).
Fig. 11. TCE conversion as a function of on-stream reaction time over 3DOM
5.5Cr2O3CeO2 at 240 oC and SV = 20,000 mL/(g h).
Fig. 12. Effects of (A) water vapor and (B) carbon dioxide on catalytic activity of 3DOM
5.5Cr2O3CeO2 at 270 oC and SV = 20,000 mL/(g h).

Table 1. BET surface areas, pore volumes, macro- and mesopore diameters of the
as-prepared samples.
Table 2. Surface element compositions and H2 consumption of the samples.
o
Table 3. Catalytic activities, HCl selectivity (SHCl), reaction rates at 250 C, and apparent
activation energies (Ea) at SV = 20,000 mL/(g h) of the samples.