Oxygen-vacancy-promoted catalytic wet air oxidation of phenol from MnO: X-CeO2

Article abstract of DOI:10.1039/c7ra04037g

Catalytic oxidation can be effectively promoted by the presence of oxygen vacancies on the catalyst surface. In this study, the effect of oxygen vacancies on the catalytic wet air oxidation (CWAO) of phenol was investigated with CeO₂ and MnO_x-CeO₂ as catalysts. CeO₂ and MnO_x-CeO₂ catalysts with different amounts of oxygen vacancies were obtained via hydrothermal methods and applied for the CWAO of phenol. It was found that CeO₂ and MnO_x-CeO₂ nanorods were much more active than the cubic nanorods. The physicochemical properties of the samples were characterized by TEM, XRD, BET, XPS, and H₂-TPR techniques. The results revealed that the presence of oxygen vacancies in CeO₂ and MnO_x-CeO₂ catalysts could increase the oxidizing ability of the catalysts surface. The addition of Mn could greatly improve the adsorption ability of CeO₂ and more efficiently oxidize phenol and its intermediates. The synergy between Mn and Ce could further improve the catalyst redox properties and produce a larger amount of active oxygen species, which is the reason why MnO_x-CeO₂ nanorods are the most active catalysts among the catalysts investigated in this study.

Full text of DOI:10.1039/c7ra04037g

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Oxygen-vacancy-promoted catalytic wet air
oxidation of phenol from MnOx–CeO2†
Cite this: RSC Adv., 2017, 7, 27079
a
a
a
a
a
a
*
Changjian Ma, Yaoyao Wen, Qingqing Yue, Anqi Li, Jile Fu, Nouwei Zhang,
Hengjun Gai,b Jinbao Zhenga and Bing H. Chen
*
a
Catalytic oxidation can be effectively promoted by the presence of oxygen vacancies on the catalyst
surface. In this study, the effect of oxygen vacancies on the catalytic wet air oxidation (CWAO) of phenol
was investigated with CeO2 and MnOx–CeO2 as catalysts. CeO2 and MnOx–CeO2 catalysts with different
amounts of oxygen vacancies were obtained via hydrothermal methods and applied for the CWAO of
phenol. It was found that CeO2 and MnOx–CeO2 nanorods were much more active than the cubic
nanorods. The physicochemical properties of the samples were characterized by TEM, XRD, BET, XPS,
and H2-TPR techniques. The results revealed that the presence of oxygen vacancies in CeO2 and MnOx–
CeO2 catalysts could increase the oxidizing ability of the catalysts surface. The addition of Mn could
greatly improve the adsorption ability of CeO2 and more efficiently oxidize phenol and its intermediates.
The synergy between Mn and Ce could further improve the catalyst redox properties and produce
a larger amount of active oxygen species, which is the reason why MnOx–CeO2 nanorods are the most
active catalysts among the catalysts investigated in this study.
Received 9th April 2017
Accepted 8th May 2017
DOI: 10.1039/c7ra04037g
rsc.li/rsc-advances
proper catalysts.8,9 Clearly, developing a suitable catalyst
essential for CWAO processes.
1 Introduction
Among the various catalysts investigated for the CWAO of
phenol in the reported literature, the cerium-incorporated
manganese oxide composite (MnOx–CeO2) catalyst has been
identied as one of the most promising catalysts.10 It was re-
ported that MnOx–CeO2 is an effective catalyst for the CWAO of
ammonia, poly(ethylene)glycol, acetic acid, pyridine, etc.11,12
Arena reported a new “redox-precipitation” route to synthesis
the MnOx–CeO2 catalyst resulting in improved catalytic
activity.13 It was also reported that formic and acetic acids are
the common intermediates during the CWAO of phenol, and
that the adsorption of formic and acetic acids was driven by
electrostatic interactions with the MnOx–CeO2 surface.14,15 The
surface reaction between the adsorbed intermediates and the
activated oxygen species is the rate-determining step.16
Furthermore, it was reported that the interaction between Mn
and Ce is benecial for the CWAO of ammonia,11 where
a physical mixture of Mn2O3 and CeO2 was far less active than
that of Mn/Ce catalysts. The above information shows the
merits of MnOx–CeO2 catalysts in the eld of CWAO; however,
less attention has been paid to the physicochemical properties
of MnOx–CeO2 catalysts, such as the effect of oxygen vacancies
on the CWAO of phenol.
Phenol and its derivatives can be found as by-products from
a variety of sources such as chemical, petrochemical, pharma-
ceutical and coal chemical industries.1,2 Since phenol is highly
toxic to aquatic life, it would result in a severe impact on water
resources if phenol-contaminated streams are discharged
without treatment.3 Biotreatment is oen applied in water
treatment; however, phenol can inhibit the activity of microbes,
causing biotreatment to be impractical for phenolic waste-
water.4 Catalytic wet air oxidation (CWAO) has been shown to be
an effective and environmentally friendly way to remove
refractory organic compounds, such as dyes, phenol, and suc-
cinic acid as normally no additions other than oxygen/air are
needed for the process.5–7 Under an oxygen or air atmosphere at
relatively elevated temperatures and pressures, refractory
organic compounds can be completely oxidized into CO2 and
H2O or partially converted to other less toxic compounds by
aDepartment of Chemical and Biochemical Engineering, National Engineering
Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, College of
Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, PR
China. E-mail: zhnw@xmu.edu.cn; chenbh@xmu.edu.cn
Our previous study showed that O2 activation ability can
directly affect the performance of CO oxidation.17 The activation
of oxygen can form superoxo (O2À) and/or peroxo (O22À) species,
which are more oxidative than molecular oxygen (O2). Active
oxygen species can be formed on the surface of ceria in propane
bQingdao University of Science and Technology, Qingdao 266042, Shandong, China
† Electronic supplementary information (ESI) available: HRTEM images of CeO2
110 surface, XRD patterns of MnOx catalysts; phenol and TOC conversions on
MnOx–CeO2 nanorods and nanocubes at various temperatures; phenol and TOC
conversions on MnOx–CeO2 nanorods and nanocubes at various pressures. See
DOI: 10.1039/c7ra04037g
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oxidation passing via a Mars and van Krevelen mechanism.18 stainless autoclave and heated up to 100 C for 24 h. Aer the
Molecular oxygen prefers to ll the ceria oxygen vacancies to solution was cooled to room temperature, the precipitates were
produce activated oxygen species.19 High oxygen vacancy separated by ltration and washed with deionized water several
concentrations can also assist the cycle transition between Ce3+ times. The obtained precipitates were dried at 60 ꢀC overnight,
and Ce4+ ions (Ce3+ 4 Ce4+), which is the key for many catalytic and then calcined at 400 ꢀC for 4 h. The procedure for the
ꢀ
activities.
synthesis of CeO2 nanocubes was exactly the same as for CeO2
There are several ways to create oxygen vacancies. In the nanorods, except the hydrothermal condition was at 180 ꢀC for
literature, both ceria (110) and (100) surfaces have been re- 24 h.
ported as having superior oxygen storage ability than the (111)
2.1.2.2 Synthesis of MnOx–CeO2 nanorods and nanocubes.
surface.20 However, the ceria (100) surface contains less oxygen MnOx–CeO2 nanorods and nanocubes were synthesized
vacancies than the (110) surface.21 This property of the ceria through a modied method reported in the literature.24,25 To
(110) surface causes the improved catalytic activity for CO obtain MnOx–CeO2 nanorods, 0.3957 g (50 mol% of Mn with
oxidation and soot combustion.22,23 Doping various metals into respect to Ce) of MnCl2$4H2O and 0.868 g of Ce(NO3)3$6H2O
ceria is another way to create oxygen vacancies. As Mn and Ce were dissolved into 5 mL and 10 mL of deionized water,
have a tendency to form solid solutions,24,25 doping Mn into the respectively. Then the two solutions were mixed and stirred for
ceria lattice can greatly increase oxygen mobility and also help 10 min. In another container, 14 g of NaOH was dissolved into
to produce oxygen vacancies on a catalyst's surface.26 When the 35 mL of deionized water. Then, the NaOH solution was mixed
atomic ratio of Mn : Ce is less than 1, all the MnOx–CeO2 with the metal precursor solution. Aer stirring for 30 min, the
catalysts possess the uorite cubic structures of CeO2.27 Thus, slurry was transferred into a 100 mL Teon-lined stainless
ꢀ
doping 50% Mn atoms into CeO2 is expected to produce the autoclave and heated up to 120 C for 24 h. Upon leaving the
maximum amount of oxygen vacancies.
solution to cool down to room temperature, the precipitates
In order to study the effect of oxygen vacancies on the CWAO were separated by ltration and washed by deionized water
of phenol, we synthesized CeO2 nanorods and nanocubes using several times. The precipitates were then dried at 60 ꢀC over-
hydrothermal methods. To further conrm the effect of oxygen night and calcined at 400 ꢀC for 4 h. MnOx–CeO2 nanocubes
vacancies on the CWAO of phenol, 50% Mn in a molar ratio was were synthesized using the same procedures as for the MnOx–
doped into CeO2 nanorods and nanocubes to form MnOx–CeO2 CeO2 nanorods, except the hydrothermal condition was at
ꢀ
nanorods and nanocubes. Characterizations, including XRD, 180 C for 24 h.
H2 temperature-programmed reduction (H2-TPR), XPS, and
2.1.2.3 Synthesis of MnO2 catalysts. MnO2 catalysts were
TEM, were performed to link the physicochemical properties prepared by a modied method according to the literature re-
and catalytic performance. The results revealed that the oxygen- ported by Zhu et al.29 In a typical experiment, 1.00 g of KMnO4
vacancy-related properties as well as the synergy between Mn and 0.50 g of 50 wt% Mn(NO3)2 solution were added into 64 mL
and Ce are the key for the high activities in the CWAO of phenol. of deionized water. Aer stirring for 10 min, the mixed solution
was transferred into a 100 mL Teon-lined stainless autoclave
and kept at 160 ꢀC for 12 h. Aer the solution was cooled to
2 Experimental
2.1 Preparation of the catalyst
room temperature, the precipitates were washed and dried at
ꢀ
100 C for 12 h.
2.1.1 Chemicals. Cerium(III)
nitrate
hexahydrate
2.1.2.4 Synthesis of Mn3O4 catalysts. Mn3O4 catalysts were
(Ce(NO3)3$6H2O), manganese(II) chloride tetrahydrate (MnCl2- synthesized by adding 3.22 g of 50 wt% Mn(NO3)2 solution and
$4H2O), phenol (C6H6O), potassium permanganate (KMnO4), 1.78 g of MnCl2$4H2O into 10 mL of deionized water. In another
manganous nitrate, 50% solution (Mn(NO3)2), and acetonitrile container, 100 mL of 3 M NaOH solution was prepared, and
(C3H3N) were purchased from Sinopharm Chemical Reagent then mixed with the metal precursor solution. Aer stirring for
Co. Ltd. (Shanghai, China); sodium hydroxide (NaOH) and 1 h, the precipitates were washed using deionized water and
acetic acid glacial (CH3COOH) were purchased from Guang- dried at 100 ꢀC for 12 h.
dong Guanghua Sci-Tech Co. Ltd. (Shantou, China). The water
used in all the experiments was deionized water produced using
the Millipore Milli Q system. All the reagents were used as
2.2 Catalyst characterization
received.
2.1.2 Material synthesis
X-ray diffraction (XRD) patterns for the cerium-based catalysts
were collected on a Rigaku X-ray diffractometer and Cu Ka
2.1.2.1 Synthesis of CeO2 nanorods and nanocubes. CeO2 radiation (35 kV and 20 mA) was used as the X-ray source. The
nanorods and nanocubes were prepared through a method samples were scanned in the range of 10ꢀ to 80ꢀ at a scanning
similar to that reported in the literature.28 To obtain CeO2 rate of 10ꢀ minÀ1. X-ray photoelectron spectra (XPS) were ob-
nanorods, 1.736 g of Ce(NO3)3$6H2O was dissolved in 10 mL of tained from the PHI Quantum 2000 Scanning ESCA Microprobe
deionized water and stirred for 10 min. In a separate container, and the monochromatic Al Ka radiation (1846.6 eV) was used as
19.2 g of NaOH was dissolved in 70 mL of deionized water, and the X-ray source. The binding energies of all the spectra were
stirred until the solution turned into a transparent color. The calibrated with the reference to the C 1s peak at 284.6 eV. N2
above two solutions were mixed and stirred for 30 min. Then, adsorption–desorption isotherms were performed at 77 K using
the mixed solution was transferred into a 100 mL Teon-lined a Micromeritics ASAP 2020 instrument. All the samples (300
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mg) were degassed at 200 ꢀC for 2 h in vacuum before the nanorods contain more oxygen vacancies than CeO2 nanocubes.
measurements started. The specic surface areas were calcu- The TEM results also indicate that the ceria (110) surface has
lated by the Brunauer–Emmett–Teller (BET) method. Hydrogen a tendency to form more oxygen vacancies than the (100)
temperature-programmed reduction (H2-TPR) was recorded surface, which is consistent with previous studies.20,21
using a thermal conductivity detector (TCD). Typically, 80 mg of
The TEM images of MnOx–CeO2 nanocubes and nanorods
sample was pretreated in pure argon (Ar) at 200 ꢀC for 2 h with are shown in Fig. 2. The average diameter of the MnOx–CeO2
a ow rate of 40 mL minÀ1. Aer the temperature was cooled nanocubes is about 17 nm, and the exposed surface is
down to 50 ꢀC, the gas was switched to 5% H2/Ar with a ow rate conrmed to be the (100) surface, where the inter-planar space
of 40 mL minÀ1. The temperature was then raised to 850 C at is 0.27 nm. Fig. 2(c) and (d) are the typical images of MnOx–
ꢀ
a ramp of 10 ꢀC minÀ1. Transmission electron microscopy CeO2 nanorods, which have an average diameter of 10 nm. The
(TEM) was performed on the Tecnai F30 with an acceleration predominant facets for the MnOx–CeO2 nanorods are the (111)
voltage of 300 kV. The specimen was prepared by dispersing the and (100) surfaces. The surface of MnOx–CeO2 nanorods was
sample in ethanol solution and sonicating for 20 min. The also found to be “rougher” than the surface of MnOx–CeO2
dispersed sample was dripped on a carbon-coated copper grid nanocubes, which implies that MnOx–CeO2 nanorods have
and dried at room temperature. The catalyst composition was more surface reconstructions and oxygen vacancies. The exis-
determined by inductively coupled plasma mass spectrometry tence of oxygen vacancy clusters can promote the transition
(ICP-MS).
from Ce4+ to Ce3+, which generates more active oxygen species.30
This property of CeO2 and MnOx–CeO2 nanorods is expected to
play an important role in the CWAO of phenol.
2.3 Catalytic activity measurements
In order to conrm the chemical compositions of the MnOx–
CeO2 catalysts, line-scan analysis was conducted (Fig. 3). For
both MnOx–CeO2 nanocubes and nanorods, Mn and Ce follow
the same pattern, which suggests that they may have formed
solid solutions. ICP-MS analysis was performed to further verify
the elemental compositions of the MnOx–CeO2 nanocubes and
nanorods. The results show that both MnOx–CeO2 nanocubes
and nanorods contain 48 mol% of Mn. Therefore, it was proven
that MnOx–CeO2 nanocubes and nanorods have similar chem-
ical compositions.
The XRD patterns of CeO2 and the MnOx–CeO2 nano-
materials are presented in Fig. 4. All the catalysts have the
uorite cubic structures of CeO2 (PDF # 34-0394), in spite of
there being some minor peaks of Mn3O4 (PDF # 18-0803) found
for the MnOx–CeO2 nanocubes. This indicates that less Mn has
doped into the ceria lattice for MnOx–CeO2 nanocubes. To
clarify this phenomenon, magnied (111) diffraction peaks of
MnOx–CeO2 nanomaterials with reference to CeO2 nanorods are
illustrated in Fig. 5.
The CWAO process was operated in a 100 mL temperature-
programmed autoclave reactor with a magnetic-driven stirring
mixer. A typical experiment was carried out as follows: 70 mL of
phenol solution (1000 ppm) and 0.28 g of catalyst (4 g LÀ1) were
added into the reactor, and the reactor was purged using N2 to
remove the gaseous oxygen. The reactor was allowed to heat to
the required temperature (100–180 ꢀC) and the rst sample was
taken to evaluate the amount of phenol adsorbed. The desired
amount of air (0.5–3.5 MPa) was then charged into the reactor,
meanwhile, the time counting started. Samples were withdrawn
periodically to monitor the progress of the reaction. The stirring
speed was kept at 800 rotation per minute (rpm) at all times.
The collected samples were ltered and the phenol concentra-
tions were analyzed by Agilent 1100 high performance liquid
chromatography (HPLC) instrument using
a Dionex C18
column. The wavelength of the UV detector was set to 254 nm.
Acetonitrile and 0.2 wt% acetic acid solution were used as the
mobile phases with a total ow rate of 1 mL minÀ1. The reaction
evolutions of phenol's intermediates were also analyzed by
HPLC according to the methods reported by Yang.9 Total
organic carbon (TOC) was measured by a Shimadzu TOC-L
analyzer.
3 Results and discussion
3.1 Structural characterization
The TEM images of CeO2 nanomaterials are displayed in Fig. 1.
The crystal size for CeO2 nanocubes and nanorods are 30 and
10 nm in diameter, respectively. The exposed surfaces for CeO2
nanorods are the (110) and (100) surfaces, rather than the (100)
surface for CeO2 nanocubes. A clearer CeO2 nanorods (110)
surface image is also provided in Fig. S1.† It is obvious that
CeO2 nanorods have more “dark spots” and the surface seems
“rougher” than that of the CeO2 nanocubes.
A similar
phenomenon was also reported by Li, and it can be explained
that the rougher surface of CeO2 nanorods is due to the formed
Fig. 1 TEM images of the CeO2 nanocubes (a and b) and nanorods (c
oxygen vacancy clusters.19 Thus, it could be expected that CeO2 and d).
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Fig. 2 TEM images of the MnOx–CeO2 nanocubes (a and b) and
nanorods (c and d).
Fig.
4
XRD patterns of the CeO2 (a) and MnOx–CeO2 (b)
nanomaterials.
nanorods. This reveals that CeO2 and MnOx–CeO2 nanorods
have a higher degree of lattice defects than the corresponding
cubic morphology.8,28 This is consistent with the TEM results
discussed earlier. The diffraction peaks of CeO2 and MnOx–
CeO2 nanorods are also relatively broader compared to the CeO2
and MnOx–CeO2 nanocubes, suggesting that CeO2 and MnOx–
CeO2 nanorods have a smaller crystal size and would result in
a higher specic surface area. This was further conrmed by the
BET measurements, which show that the BET areas of CeO2 and
MnOx–CeO2 nanorods were 91.5 and 76.7 m2 gÀ1, respectively.
The BET areas of CeO2 and MnOx–CeO2 nanocubes were 15.4
and 39.2 m2 gÀ1, respectively. From the data shown above, it is
apparent that the specic surface area was affected by the
catalyst particle size. Generally, the smaller the particle size is,
the larger BET surface area the catalyst possesses. As shown in
Fig. 1 and 2, the particle size is increased in the following order:
CeO2 nanorods (10 nm) z MnOx–CeO2 nanorods (10 nm) <
MnOx–CeO2 nanocubes (17 nm) < CeO2 nanocubes (30 nm).
Therefore, the specic surface area is negatively related to the
particle size: CeO2 nanorods (91.5 m2 gÀ1) > MnOx–CeO2
nanorods (76.7 m2 gÀ1) > MnOx–CeO2 nanocubes (39.2 m2 gÀ1) >
CeO2 nanocubes (15.4 m2 gÀ1). Furthermore, as shown in Fig. 4,
CeO2 nanorods exhibited weaker XRD patterns than MnOx–
CeO2 nanorods. This indicated that CeO2 nanorods possessed
poorer crystallinity and a smaller crystal size, although similar
diameters were observed by TEM over the two catalysts. The
poorer crystallinity and smaller crystal size enabled CeO2
nanorods to have a larger BET surface area than MnOx–CeO2
nanorods.
Fig. 3 Line-scan analyses of the MnOx–CeO2 nanocubes (a and b) and
nanorods (c and d).
It was found that the diffraction peak for MnOx–CeO2
nanorods had shied to a higher Bragg angle compared with
MnOx–CeO2 nanocubes. The Bragg diffraction of each (111)
peak was recorded, and each corresponding lattice parameter ɑ
was calculated (Fig. 5(b)). Indeed, the lattice parameter of the
MnOx–CeO2 nanorods shrunk from 0.541 nm to 0.537 nm,
whereas MnOx–CeO2 nanocubes only decreased to 0.539 nm.
This further justies that MnOx–CeO2 nanorods have formed
a more homogeneous solid solution. Furthermore, the ionic
radii of Mn ions were reported as follows: Mn4+: 0.053 nm,
Mn3+: 0.065 nm, Mn2+: 0.083 nm.26 The ionic radii of cerium
ions were: Ce4+: 0.097 nm, Ce3+: 0.114 nm. Thus, if more Mn is
doped into Ce for the MnOx–CeO2 nanorods, the average ionic
radius is expected to be smaller than that of MnOx–CeO2
nanocubes, which might cause a decrease in the lattice
parameters.
Moreover, Fig. 4 also illustrates that CeO2 and MnOx–CeO2
nanocubes have greater intensity than the corresponding
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3.2 Valence state and redox properties
The XPS spectrum of Ce 3d is displayed in Fig. 6. Typically, 10
peaks were deconvoluted into pairs of spin orbit doublets. Two
peaks at 916.6 and 898.2 eV are attributed to the 3d3/2, which
correspond to the Ce4+ states. Two peaks at 903.4 and 885.2 eV
are assigned to the 3d5/2, which are the Ce3+ states. The other six
peaks at 900.7, 882.5, 907.3, 888.5, 899.2, and 880.9 eV are the
satellite peaks due to the “shake down” type processes.31 These
results agree with what we found in the literature.32 The relative
portion of Ce3+ and Ce4+ valence states are summarized in Table
1. It was found that CeO2 nanorods have 6% more Ce3+ ions
than CeO2 nanocubes, whereas MnOx–CeO2 nanorods contain
11% more Ce3+ ions than MnOx–CeO2 nanocubes. The oxygen
vacancy concentration is positively related to the concentration
of Ce3+ ions, because oxygen vacancies can reduce the ther-
modynamic barrier for the transition from Ce4+ to Ce3+ 30 Thus,
.
the XPS results are consistent with the XRD and TEM results,
which helps to explain why CeO2 and MnOx–CeO2 nanorods
have rougher surfaces.
The Mn 2p spectra of the MnOx–CeO2 catalysts are shown in
Fig. 7. The three peaks of Mn 2p3/2 are located at 640.8, 641.8,
and 643.9 eV, corresponding to Mn2+, Mn3+, and Mn4+, respec-
tively.33–35 The Mn 2p1/2 peak is centered at 653.3 eV, with
a splitting energy of 11.45, based on the Mn 2p3/2. By estimating
the area of each Mn species, Table 1 shows that MnOx–CeO2
nanorods contain a larger portion of higher valence Mn species,
whereas the corresponding Mn4+, Mn3+, and Mn2+ ion abun-
dances are 23%, 62%, and 15%, respectively. Conversely,
MnOx–CeO2 nanocubes prefer lower valence Mn species, and
the corresponding portions of Mn4+, Mn3+, and Mn2+ are 14%,
36%, and 50%, respectively.
Fig. 5 Magnified (111) diffraction peaks (a) and the effect of Mn doping
on cerium lattice parameters (b).
It was also found that the Ce3+ concentration was suppressed
when Mn was added. This effect was also reported by Larachi,
where the addition of Mn caused an increase in surface Ce4+
concentration.36 This phenomenon can be explained by the fact
that electrons are transferred from Ce to Mn, which can
decrease the concentration of Ce3+ and increase the concen-
tration of Mn2+ or Mn3+. Compared to MnOx–CeO2 nanorods,
the Ce3+ concentration of MnOx–CeO2 nanocubes was sup-
pressed to a higher extent, and a much greater fraction of Mn2+
was observed. This also suggests that more electrons are
transferred from Ce to Mn for MnOx–CeO2 nanocubes.
Noted that the Mn3+/Mn2+ that are incorporated into the
CeO2 lattice sites also contribute to the oxygen vacancies
formation. MnOx–CeO2 nanocubes contained more Mn2+
species, but showed lower oxygen vacancies concentration than
MnOx–CeO2 nanorods. Two possible reasons were responsible
for the lower concentration of oxygen vacancies over MnOx–
CeO2 nanocubes. First, some part of the Mn was not incorpo-
rated into the CeO2 lattice for the MnOx–CeO2 nanocubes.
Mn3O4 was detected by XRD, as shown in Fig. 4(b). It is well
known that Mn3O4 is composed of both Mn3+ and Mn2+.37,38
Some Mn3+/Mn2+ species come from the phase-segregated
Mn3O4. These lower valence Mn species must not contribute
to the formation of oxygen vacancies, since they were not in the
CeO2 lattice. Second, when Mn is doped into CeO2, electrons
Fig. 6 Ce 3d core level XPS spectra of the CeO2 and MnOx–CeO2
catalysts.
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Table 1 Relative abundance of the different species obtained from the XPS data for each catalyst
Catalyst label
Ce3+ (%)
Mn4+ (%)
Mn3+ (%)
Mn2+ (%)
Oactive (%)
CeO2 nanocubes
CeO2 nanorods
MnCeOx nanocubes
MnCeOx nanorods
32
38
19
30
—
—
14
23
—
—
36
62
—
—
50
15
16
20
18
26
Fig. 7 Mn 2p core level XPS spectra of the MnOx–CeO2 catalysts.
Fig. 9 H2-TPR results of the CeO2 (a) and MnOx–CeO2 (b) catalysts.
Table 2 Summary of the catalytic activity of different metal oxides on
the CWAO of phenol. Phenol concentration: 1000 ppm; catalyst
loading: 4 g LÀ1; temperature: 140 ꢀC; pressure: 2 MPa air; time: 1 h
Phenol conversion
(%)
TOC conversion
(%)
Catalyst label
CeO2 nanocubes
CeO2 nanorods
MnO2
Mn3O4
MnCeOx nanocubes
MnCeOx nanorods
8
6
27
79
70
100
100
23
67
62
97
98
species was detected by XPS, the MnOx–CeO2 nanocubes still
exhibited a lower concentration of oxygen vacancies. In the case
of MnOx–CeO2 nanorods, electron transfer between Mn and Ce
was also observed. However, the electron transfer was less
signicant than that over MnOx–CeO2 nanocubes. Aer the
addition of Mn, the Ce3+ fraction was still up to 30%. Addi-
tionally, most of Mn was incorporated into the CeO2 lattice for
the MnOx–CeO2 nanorods, as proved by the XRD results. Thus,
Fig. 8 O 1s core level XPS spectra of the CeO2 and MnOx–CeO2
catalysts.
were transferred from Ce to Mn and the Ce3+ fraction was
decreased from 32% to 19%. Some oxygen vacancies over Mn3+
/
Mn2+ were produced at the expense of those over Ce3+ sites.
Therefore, although a higher concentration of low valence Mn
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Fig. 11 Phenol (a) and TOC (b) conversions on MnOx–CeO2 nanorods
and nanocubes. Concentration of phenol: 1000 ppm; catalyst loading:
4 g LÀ1; temperature: 140 ꢀC; pressure: 2 MPa air.
Fig. 10 The CWAO of phenol using CeO2 nanorods and nanocubes,
(a) phenol conversion, (b) TOC conversion. Concentration of phenol:
1000 ppm; catalyst loading: 4 g LÀ1; temperature: 140 ꢀC; pressure:
2 MPa air.
reduction peak of CeO2 nanomaterials relates to the removal of
surface capping oxygens, while the high temperature reduction
peak is related to the bulk oxygen.42,43 It was observed that CeO2
nanorods are able to remove surface oxygen at 100 ꢀC, while
Mn3+/Mn2+ might have contributed to the formation of oxygen
vacancies for the MnOx–CeO2 nanorods.
Fig. 8 shows the O 1s spectra for the CeO2 and MnOx–CeO2
nanomaterials. The peaks appearing at 528.9 and 530 eV are the
lattice oxygen of the CeO2 and MnOx–CeO2 catalysts.39 The peak
at 531.4 eV is the adsorbed active oxygen species, the peroxo
ꢀ
CeO2 nanocubes start to reduce at 150 C. This indicates that
CeO2 nanorods have more reducible surface Ce4+ ions
compared with that of CeO2 nanocubes, which is attributed to
the oxygen vacancies formed on the CeO2 nanorods surface,
because oxygen vacancies can boost the transition from Ce4+ to
Ce3+.
À
species (O22À), and the superoxo species (O2 ).40 The remaining
peak at 533 eV is attributed to the adsorbed surface hydroxyl
and water species. For the CeO2 catalysts, CeO2 nanorods
possess 20% surface active oxygen, where CeO2 nanocubes have
16%. It is reported that oxygen vacancies can promote the
formation of Ce3+, and that Ce3+ can induce the formation of
active oxygen species.41 Thus, the cause of the higher active
oxygen percentage for CeO2 nanorods might be due to the
oxygen vacancies formed on the CeO2 nanorods surface. A
similar trend was also observed for the MnOx–CeO2 catalysts,
where MnOx–CeO2 nanorods contained 8% more active surface
oxygen than MnOx–CeO2 nanocubes. It is also reported that
active oxygen species can be created at the interface between the
CeO2 lattice and MnOx.26 It is reasonable that more active
oxygen species are found in MnOx–CeO2 nanorods, due to the
synergy of Mn/Ce (more –MnO–Ce– bonds) and the higher Ce3+
concentration.
Doping Mn into the Ce lattice can improve the reducibility of
the catalyst surface,27 which can clearly be observed in Fig. 9(b).
The reduction peak of MnOx–CeO2 nanorods starts at 50 ꢀC,
while MnOx–CeO2 nanocubes start to reduce at 150 ꢀC. Four
reduction peaks were obtained for each MnOx–CeO2 catalyst.
ꢀ
For MnOx–CeO2 nanorods, the peak at 200 C is attributed to
the reduction of MnO2 to Mn2O3. The second peak at 272 ꢀC is
assigned to the reduction of Mn2O3 to Mn3O4. The third peak at
360 ꢀC is the mixed reduction of Mn3O4 to MnO and surface
Ce4+ to Ce3+, while the high temperature reduction peak at
4+
410 C is the reduction of the bulk Ce to Ce3+
.
ꢀ
35,44
The reduction peaks for MnOx–CeO2 nanocubes followed
a similar pattern as that of the MnOx–CeO2 nanorods. It was
found that MnO2 was rst reduced to Mn2O3 at 220 ꢀC, and then
Mn2O3 was reduced to Mn3O4 at 315 ꢀC. The third peak at
415 ꢀC is due to the reduction of Mn3O4 to MnO. The reduction
of surface Ce4+ was shied to a high temperature around 500 ꢀC
The redox properties of the CeO2 and MnOx–CeO2 catalysts
were studied by H2-TPR experiments, and the results are dis-
played in Fig. 9. It is widely accepted that the low temperature
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composites. In order to study the effect of oxygen vacancies on
the CWAO of phenol, CeO2 nanorods and nanocubes were
carefully investigated. The adsorption experiments showed that
CeO2 nanorods could absorb 5% of phenol, whereas CeO2
nanocubes could only absorb 1%. Fig. 10 illustrates the reaction
proles of phenol and TOC conversions for CeO2 nanomaterials
at 140 ꢀC and 2 MPa air. It was found that CeO2 nanocubes were
almost inert toward the CWAO of phenol, whereas phenol and
total organic carbon (TOC) conversions were 11% and 8%,
respectively. Conversely, CeO2 nanorods were more active than
CeO2 nanocubes, whereas 51% of phenol conversion and 45%
of TOC removal were achieved.
Compared to CeO2 nanomaterials, MnOx–CeO2 catalysts give
very rapid phenol oxidation. MnOx–CeO2 nanorods could
completely convert phenol and remove 98% of TOC within
30 min, at 140 ꢀC and 2 MPa air (Fig. 11). The comparable
activity for MnOx–CeO2 nanocubes could only be achieved aer
60 min. Furthermore, the adsorption experiment showed that
MnOx–CeO2 nanorods could absorb 15% of phenol, whereas
MnOx–CeO2 nanocubes only absorbed 7%. It was seen that the
addition of Mn could enhance the adsorption ability of CeO2
nanorods by more than 3 times. This is important, since
adsorption is the primary step for the CWAO of phenol. More-
over, it was reported that high oxidation states of manganese
oxide can also behave as active centers for phenol oxidation.36
Thus, it could be postulated that the addition of Mn helped to
adsorb phenol and its intermediates, then Mn4+ was reduced
and the adsorbed organic compounds were oxidized.
Fig. 12 Evolution of the small-molecule intermediates in the CWAO of
phenol over MnOx–CeO2 nanorods (a) and nanocubes (b).
Tests on the evolution of the small-molecule intermediates
for MnOx–CeO2 catalysts were conducted, and the results are
shown in Fig. 12. There are 4 major small-molecule interme-
diates: formic acid, acetic acid, oxalic acid, and hydroquinone.
Other intermediates, including polymeric products, are also
reported in the literature,45 and can be calculated by subtracting
the concentration of the remaining TOC and small-molecule
intermediates. The initial TOC value for phenolic solution was
about 850 ppm. For MnOx–CeO2 nanorods, the TOC conversion
was 98%, and the remaining TOC was calculated to be 17 ppm.
The total organic carbon concentration of formic acid, acetic
acid, oxalic acid, and hydroquinone was also converted, and
turned out to be 15 ppm. Thus, it implies that the small-
molecule intermediates are responsible for 88% of the
remaining TOC for MnOx–CeO2 nanorods. On the other hand,
the small-molecule intermediates are only responsible for 32%
of the remaining TOC for MnOx–CeO2 nanocubes. This means
MnOx–CeO2 nanocubes have converted more phenol into large-
molecule intermediates. In other words, MnOx–CeO2 nano-
cubes are less oxidative than MnOx–CeO2 nanorods.
due to the lower concentration of Ce3+ and the electron transfer
from Ce to Mn. Compared to the reduction peaks of MnOx–
CeO2 nanorods, all of the reduction peaks for MnOx–CeO2
nanocubes have shied to higher temperatures. This is
reasonable since more Mn has entered the Ce lattice for MnOx–
CeO2 nanorods, which causes the formation of –Mn–O–Ce–
bonds. It was reported that –Mn–O–Ce– bonds is more reduc-
ible than –Ce–O– or –Mn–O– bonds.26 The presence of –Mn–O–
Ce– bonds effectively lowered the overall reduction temperature
for MnOx–CeO2 nanorods.
It is interesting that MnOx–CeO2 nanorods also possess more
compacted reduction peaks than MnOx–CeO2 nanocubes. This
might be due to the greater abundance of Mn4+ and Mn3+ of
MnOx–CeO2 nanorods, because higher oxidation states of Mn
are easier to be reduced. Another possible explanation is that
oxygen vacancies in MnOx–CeO2 nanorods could also promote
the transition of Mn from 4 + to lower valence states, since it is
able to lower the thermodynamic barrier for Ce4+ / Ce3+.
The MnOx–CeO2 catalysts were also examined by comparing
their catalytic activities at various temperatures (Fig. S3†). The
results show that the catalytic activity of MnOx–CeO2 nanorods
and nanocubes are both positively related to the reaction
3.3 Catalytic wet air oxidation of phenol
The catalytic performance of various oxides were investigated temperature. MnOx–CeO2 nanorods and nanocubes exhibit
and the results are displayed in Table 2. It was found that similar catalytic activity at 100 and 180 ꢀC. However, MnOx–
cerium and manganese monoxides (Fig. S2†) exhibit much CeO2 nanorods become much more active than MnOx–CeO2
lower activities for the CWAO of phenol than the MnOx–CeO2 nanocubes at 140 ꢀC. This can be due to the high reducibility of
catalysts, which proves the effectiveness of Mn/Ce oxide
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MnOx–CeO2 nanorods surface, which play a more important
role at moderate temperature.
References
In order to study the effect of oxygen partial pressure in the
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The authors would like to thank the nancial supports from the
National Key Technology Support Program of China (No.
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