Facile fabrication of shape-controlled Co:XMnyOβ nanocatalysts for benzene oxidation at low temperatures

Article abstract of DOI:10.1039/c8cc00023a

We report a new strategy for the design and construction of Co_xMn_yO_β for C₆H₆ oxidation, a representative VOC. The nanostructures of Co_xMn_yO_β can be tuned from nanowires to needles, hollow/hierarchical microspheres and nanocubes. Moreover, different nanostructures of Co_xMn_yO_β show diverse textural/structural/surface properties, which in turn show strikingly different catalytic performances. Benefitting from its unique structural feature, nanocubic MnO₂ exhibits a superior to considerably high activity for C₆H₆ oxidation at low temperatures.

Full text of DOI:10.1039/c8cc00023a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Facile Fabrication of Shape‐controlled CoxMnyOβ Nanocatalysts for
Benzene Oxidation at Low Temperature
Xiuyun Wanga, Weitao Zhaoa, Tianhua Zhanga, Yongfan Zhanga, Lilong Jianga* and
Shuangfen Yinb
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report a new strategy to design and construction of CoxMnyOβ
for C6H6 oxidation, a representive VOC. The nanostructures of Cox
oxidation have been under vigorous investigation. Conclusions
on rate‐determining steps and kinetic parameters were made
MnyOβ can be tuned from nanowires to needles, hollow/hierarchi mainly based on the results of kinetic experiments. However, a
cal microspheres and nanocubes. Moreover, different nanostructu general picture for the oxidation of VOCs over Co and Mn
res of CoxMnyOβ result in diverse textural/structural/surface prop
oxides has not been established.4 Therefore, it is necessary to
erties, which in turn show strikingly different catalytic performanc elucidate the reaction pathways of VOCs oxidation over Co and
es. Benefitting from its unique structural feature, nanocubic MnO2 Mn oxides at a molecular level.
exhibits a superior to considerably high activity for C6H6 oxidation
at low temperature.
Herein, we report a new strategy to design of CoxMnyOβ
with controllable chemical compositions and nanostructures.
The nanostructures of CoxMnyOβ oxides can be tuned from
nanowires to needles, nanocubes, as well as to hollow and
hierarchical microspheres, via adjusting Mn/Co molar ratio.
Our studies demonstrate that different nanostructures of
CoxMnyOβ give rise to diverse textural/structural/surface
properties, resulting in different catalytic activity for benzene
oxidation. A detailed investigation on the catalyst structures
for the oxidation of benzene over the catalysts was conducted
by means of XANES, EXAFS, reaction kinetics and in situ
I
n the process of oil‐field exploitation, the by‐products volatile
organic compounds (VOCs) will be generally released into the
atmosphere or burned immediately due to the lack of the
relevant processing technology as well the uncontrollability of
oilfield associated gas.1 Supported noble metal catalysts are
highly reactive for catalytic combustion of VOCs, 2 while cheap
transition‐metal oxides (TMOs) including those of Co, Mn and
DRIFTS,
aiming
to
acquire
insight
into
the
3
Ni have attracted considerable attention. However, the
nanostructure−acꢀvity relaꢀonships. The obtained results can
help better understand the oxidation of VOCs over other
TMOs.
catalytic activity of the promising TMOs candidates is still not
4
satisfactory at low temperature. According to experimental
and theoretical results, it is reckoned that catalytic activities
are dependent on catalyst morphology, and there is a strong
relationship between the shape and properties of a catalyst.5, 6
Therefore, the design and controllable preparation of TMOs
catalyst is a key to achieve a considerably high catalytic activity
as that of noble metals.
Recently, a large number of researches have been focused
on the controlled preparation of Co and Mn oxides of various
morphologies.7 However, most of them have been confined to
nanowires, nanotubes and nanosheets rather than hierarchical
microspheres, arrow‐shaped particles and cubic structures.
Furthermore, the mechanistic aspects of catalytic VOCs
SEM images in Figure 1 show that the nanostructure of
CoxMnyOβ changes with the variation of Mn/Co molar ratio.
The morphology of Co3O4 is in the form of nanowires about
20–30 nm in diameter and 1–12 μm in length (Figure 1A–B).
With the introduction of Mn, the Co5Mn1O (Co‐Mn‐F) sample
shows nanowires that are interwoven together (Figure 1C–D),
and there is the detection of a needle‐like structure. With
increase of Mn/Co molar ratio, there is the generation of
nanoplates in Co3Mn1O (Figure 1E–F) and hollow micro‐
hemispheres in Co2Mn1O (Figure 1G), both with clear
boundaries between the particles. The radius of the latter is
about 400 nm (Figure 1H). The change of morphology as a
result of increase in Mn/Co molar ratio is attributed to the
reduction in terms of free energy. 8 For Co1Mn1O (Co‐Mn‐S),
there is the formation of flower‐like spheres made up of close‐
packed nanoplates (Figure 1I–J).
a. National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou
University, Fuzhou, Fujian 350002, China. E‐mail: jll@fzu.edu.cn.
b.College of Chemistry and Chemical Engineering, Hunan University, Changsha,
Hunan, 410082, China
Further increase of Mn/Co molar ratio to 2, there are
microballs composed of thicker nanoplates (Figure 1K–L).
When Mn/Co molar ratio is 3, there is the formation of
This work was financially supported by the national key research and
development program (2016YFC0203900, 2016YFC0203902), the National
Natural Science Foundation of China (21703037), and leading project of Fujian
Province (2017H0049)
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spheres < nanocubes, which probably be attributed to
nanocube MnO2 with narrow pore distribution (Fig. S2E).
Specifically, the T50% and T90% values of nanocube MnO2 are
Figure 1 SEM images of (A–B) Co3O4; (C–D) Co5Mn1O (Co‐Mn‐
F); (E–F) Co3Mn1O; (G–H) Co2Mn1O; (I–J) Co1Mn1O (Co‐Mn‐S);
(K–L)
Figure 2 (A) C6H6 conversions over Co3O4, Co‐Mn‐F, Co‐Mn‐S
and MnO2; (B) water‐resistant performance of MnO2 at 350 °C;
(C) Time on stream of C6H6 conversion over MnO2 at 500 °C;
(D) Arrhenius plots for C6H6 oxidation over Co3O4, Co‐Mn‐F, Co‐
Mn‐S and MnO2.
irregular blocks (Figure 1M–N). Finally, without Co
participation, there is the formation of regular nanocubes of
MnO2. A panoramic view (Figure 1O–P) reveals that the
average edge length of the MnO2 cubes is about 750 nm, and
the surface of the cubes is smooth. For better understanding
the morphology changes over CoxMnyOβ, the evolution
schematic is illustrated in Figure 1Q. Different nucleation
modes of CoxMnyOβ are presented in Figure S1 (The more
details see supporting information).
For the convenience of discussion, we use nanowires Co3O4,
needles Co‐Mn‐F, microspheres Co‐Mn‐S and nanocubic MnO2
of different nanostructures as representative catalysts. High‐
resolution TEM (HR‐TEM) image of MnO2 (Figure S2A–B) shows
that two kinds of lattice fringe directions of {101} and {220} are
observed with an interplanar spacing of 0.240 and 0.159 nm,
respectively. The HR‐TEM result (Figure S2D) suggests that the
{220} planes dominate the surface of Co3O4 nanowires. XRD
patterns (Figure S2F) of Co3O4 and Co‐Mn‐F show resolved
reflections that can be assigned to the Co3O4 phase with cubic
spinel‐type structure. The peaks of MnO2 can be well assigned
to tetragonal MnO2. It is well known that the textural
properties such as surface area and porosity are also
important for catalysis. As determined by N2 adsorption
measurement at 77 K, MnO2 displays the highest BET surface
area (78 m2/g), whereas Co3O4 possesses the lowest surface
area (25 m2 /g). In the case of Co‐Mn‐F and Co‐Mn‐S, they
show BET surface area of 49 and 55 m2/g (Table S1),
respectively. Additionally, ICP results (Table S2) suggest that
the Mn/Co atomic ratio of Co‐Mn‐F and Co‐Mn‐S are roughly
1/5.03 and 1/1.02, respectively, which are close to the
theoretical values.
171 and 207 °C, respectively. Such reactivity at a low
temperature over MnO2 is superior to that over supported Pd
catalysts.9 O2‐TPSR‐MS (Figure S4A) results suggest that C6H6 is
entirely oxidized to CO2 and H2O over MnO2. Besides, the
normalized initial reaction rate (Figure S4B) to BET surface
area indicate that the catalytic efficiency increased in the
following order: Co3O4 < Co‐Mn‐F< Co‐Mn‐S < MnO2, indicating
that the BET surface play an important role in the catalytic
performance. In the presence of 9.5 vol% water vapor, there is
a subtle decrease of C6H6 conversion from 99% to 97% at
350 °C (Figure 2B), which may be related to the competitive
adsorption of H2O and C6H6 molecules on the active sites.1
After cutting off the water supply, the C6H6 conversion is
rapidly recovered to 99%, demonstrating that MnO2 exhibits
excellent tolerance towards water vapor.
Moreover, the on‐stream experiment reveals a satisfactory
durability of 48 h at 500ꢁ°C (Figure 2C). Figure 2D displays the
Arrhenius plots of the catalysts, and the apparent activation
energies (Ea) are calculated and summarized in Table S1. The
results indicate that the Ea values in Co3O4 (91 kJ/mol) is much
higher than those of Co‐Mn‐F (65 kJ/mol), Co‐Mn‐S (63 kJ/mol)
and MnO2 (59 kJ/mol). SEM analysis was used to clarify the
microstructure of the catalyst after reaction. The
nanostructure is retained in the used MnO2 (Figure S5).
In order to study the effect of nanostructure evolution on
oxidation states of Mn and Co, we collected the X‐ray
absorption near‐edge spectroscopy (XANES) of Co3O4, Co‐Mn‐
S, Co‐Mn‐F, MnO2 and reference samples (Figure 3). Figure 3A‐
B suggests that the Mn valence states of the catalysts follow
the order of: MnO2 > Co‐Mn‐S > Co‐Mn‐F. After exposing MnO2
to C6H6 oxidation reaction for 15 h, the average oxidation state
of Mn shows no significant change (from 3.92 to 3.96/3.97)
(Figure 3C). Moreover, the Co K‐edge XANES spectra indicate
the co‐existence of +2 and +3 Co ions in Co‐Mn‐S, Co‐Mn‐F and
The catalytic oxidation of C6H6 was used as a probe reaction
to evaluate catalytic efficiency (Figure 2A). The temperatures
at a C6H6 conversion of 50% (T50) and 90% (T90), both are
important indicators of catalytic combustion activity, are
depicted in Table S1. The results suggest that catalytic
efficiency are significantly depended on the nano‐shapes, in
the order of nanowires < needle‐like structure < flower‐like
2 | J. Name., 2012, 00, 1‐3
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Co3O4, showing an average Co oxidation states of +2.86, +2.79
Moreover, extended X‐ray absorption fine structure (EXAFS)
and +2.67, respectively. These results suggest that the spectra indicate that FT peak positions (Figure 4) of MnO2, Co‐
dominant state of Co ions in Co‐Mn‐S and Co‐Mn‐F is +3. In Mn‐S, Co‐Mn‐F and Co3O4 are very close to each other (The
case of exposing Co‐Mn‐S to C6H6 oxidation reaction for 15 h, more details see supporting information), indicating that the
the average Co oxidation state increases from 2.79 to 2.92, chemical compositions and nanostructures have small
where percentage of Co3+ increases from 79 to 92% along with differences on their local atomic arrangement. Additionally,
decreases in the percentage of Co2+ to 8% (Figure 3F), the bond distance of Mn‐O was slightly elongated from 1.88 Å
indicating that the stabilization of Co2+ in the octahedral sites in MnO2 to 1.90Å in Co‐Mn‐S (Table S3), indicating the lattice
and they are easily oxidized to active Co3+ species in the C6H6 expansion and the relaxation of Mn‐O bond in the Co‐Mn
oxidation processes. On the basis of the above results, it can composite oxides.
be inferred that the Mn4+ and Co3+ are the main active sites in
the C6H6 oxidation processes over Co‐Mn oxides.
Figure 4 (A) Mn K‐edge EXAFS spectra, (B) relation of
interatomic distance in MnO2; (C) Co K‐edge EXAFS spectra
and (D) relation of interatomic distance in Co3O4.
Figure 3 (A) Normalized Mn K‐edge XANES spectra of reference
samples (Mn foil, MnO, Mn2O3 and MnO2) and catalysts; (B)
The energy position of main absorption of Mn K‐edge of
reference samples and catalysts; (C) Normalized Mn K‐edge
XANES spectra of fresh and used MnO2; (D) Normalized Co K‐
ede XANES spectra over reference samples [Co foil, CoO,
Co(NH3)6Cl6)] and catalysts; (E) The energy position of main
absorption of Co K‐edge of reference samples and catalysts; (F)
Normalized Co K‐edge XANES spectra of fresh and used Co‐
Mn‐S. (G) XPS Mn2p spectra; (H) XPS Co2p spectra, and (I) XPS
O1s spectra of catalysts.
The H2‐TPR (Figure S6) profile of Co3O4 shows three
reduction peaks. The peak at 353 °C can be assigned to surface
oxygen species, while the ones at 445 and 492 °C are
associated with the reduction of Co3+ to Co2+ and Co2+ to Co0,
respectively. 13As for MnO2, the reduction process could be
reasonably divided into two stages: (1) Mn4+ to Mn3+ and (2)
Mn3+ to Mn2+. 14 The reduction temperatures of Mn species in
MnO2 (290 and 413 °C) are lower than those of Co‐Mn‐S (302
and 482 °C) and Co‐Mn‐F (303 °C), suggesting better low
temperature reducibility and stronger lattice oxygen mobility
in MnO2.15 Another broad peak located at 450 °C in Co‐Mn‐F is
associated with the co‐reduction of Co3+ to Co2+ and Co2+ to
Co0. Furthermore, the amount of H2 consumption (8.2 mmol/g)
is smaller than the theoretical amount (11.5 mmol/g) required
for full reduction of MnO2 to MnO. The result implies the
existence of Mn species of lower valence (< +4), which is in
consistent with the Mn2p2/3 XPS and Mn K‐edge XANES results.
The O2‐TPD‐MS experiments help get insights into the
nature of the adsorbed O species and the mobility of the
corresponding surface species. As shown in Figure S7, oxygen
desorption peaks are observed in the cases of MnO2 (105, 479
and 499 °C), Co‐Mn‐S (560 °C) and Co‐Mn‐F (548 °C).
Meanwhile, the amount of surface active O‐ species in MnO2 is
larger than that in Co‐Mn‐S and Co‐Mn‐F (Table S4), plausibly
because the former can be efficiently reduced than the latter.
The active oxygen species are known to be electrophilic and
catalytically capable of breaking C‐H bonds. It is noticed that
Co3O4 only displays a wide desorption peak at 851 °C
associated with O2− (latt) species. Furthermore, the desorption
temperature of O2−(latt) in MnO2 (763 °C) is lower than that in
Co‐Mn‐F (807 °C) and Co‐Mn‐S (836 °C), implying that the O2‐
(latt) of MnO2 can be released more easily than those of Co‐
Mn‐F and Co‐Mn‐S. The relative easily release O2‐ (latt) in the
The surface oxidation states were investigated by XPS. XPS
spectra of Mn 2p3/2 indicate that the co‐existence of Mn3+
(642.0 eV) and Mn4+ (644.2 eV) species.10 Surface Mn4+/Mn3+
ratio (Table S2) in MnO2 (1.69) is higher than those of Co‐Mn‐S
(0.63) and Co‐Mn‐F (0.61), suggesting higher surface Mn4+
concentration in MnO2, which is beneficial for the promotion
of benzene oxidation activity at low temperature. The Co2p
XPS spectra (Figure 3H) of Co3O4, Co‐Mn‐F and Co‐Mn‐S
indicate that the co‐existence of Co2+ and Co3+ species.
11Meanwhile, structural defects can be correlated with the O
1s spectra shown in Figure 3I. The peak at 529.9–530.7 eV can
be assigned to lattice oxygen (Olatt), 1 while that at 531.1–531.5
eV to surface oxygen species (O2 ; O2 or O‐), resulting from
the adsorption of gaseous O2 into oxygen vacancies.12 The
Olatt/(Olatt+Oads) ratio of the catalysts follows the trend of Co‐
Mn‐S (41%) < Co‐Mn‐F (48%) < Co3O4 (55%) < MnO2 (58%),
suggesting that MnO2 displays more surface lattice oxygen
species.
‐
2‐
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MnO2 surface is considered to be the primary active species Meanwhile, a hydrogen atom leaves the cracked C6H6 and
for activating the C‐H bond.
stays as H+ on the surface and probably links to an oxide ion,
In situ DRIFTS experiments were performed to determine the giving rise to ‐OH. After adding 20%O2, the disappearance of
intermediate species present in the combustion of C6H6 over carboxylate species is more remarkable in the case of MnO2
the catalysts (Figure S8). The bands at 858–876 cm‐1 are compared to the cases of Co‐Mn‐S and Co3O4 (Figure S8). It is
observed in all the cases, and they are assigned to the because MnO2 possesses more surface active oxygen species
stretching vibration of metal‐oxide bonds. 16 The bands at 981– than Co‐Mn‐S and Co3O4. Finally, the carboxylates species can
1027 cm‐1 are related to adsorbed C6H6 species, as suggested be oxidized by active oxygen species to CO2 and H2O, as
by DFT calculations over MnO2 (101) surface (Figure S9). confirmed by the O2‐TPSR results. The results demonstrate
Notably, the intensity of the bands at 981–1027 cm‐1 in the that the lattice oxygen species participate in the formation of
case of MnO2 is much higher than that in the case of Co3O4 carboxylate species (MvK mechanism), while the surface active
(220) because the adsorption energy of C6H6 in the former (‐ oxygen species oxidize surface carboxylate species to the final
1.738 eV) is more negative than that in the latter (‐0.281 eV). products (L‐H mechanism).
For MnO2 (Figure S8A), the band at 1421 cm‐1 is characteristic
To summarize, Co‐Mn oxides with controllable chemical
of carboxylate species,17 generated as a result of ring opening compositions and nanostructures were successfully
during benzene oxidation, and this was confirmed through synthesized and investigated for the catalytic oxidation of
calculated FT‐IR vibration of carboxylate species over MnO2 benzene. The results demonstrate that the chemical
(101) (Figure S10). The bands at 1653 and 1364 cm‐1 are compositions and nanostructures of Co‐Mn oxides have
assigned to ring vibrations and acetate species, respectively. 18 significant influence on the structural defects, oxidation states,
Similarly, the bands at 1415–1419 and 1361–1376 cm‐1 are reducibility and surface oxygen species, resulting in different
also detected in Co‐Mn‐S (Figure S8B) and Co3O4 (Figure S8C). catalytic activities at relative low temperature. Moreover,
The weak band at 1541 cm‐1 observed over Co‐Mn‐S is nanocubic MnO2 exhibits superior catalytic activity for the
attributable to the stretching mode of symmetric carboxylate. oxidation of C6H6, and the reaction involves both MvK and L‐H
19 It should be noted that the band at 1541 cm‐1 is observed mechanisms, and the latter is more dominant. Our study
only in Co‐Mn oxides, which can be related to the withdrawal demonstrates that the nanostructure‐related catalytic
of electron density from Co towards Mn, resulting in decrease activities could provide guidance in the further development of
of π* back‐donation to the adsorbed C6H6 and hence increase easily prepared, scalable, and low‐cost catalysts for VOCs
of carboxylate stretching frequency.
oxidation based on TMOs and their derivatives.
During oxidation in the presence of 20%O2 at 150 °C, there
is the appearance of new bands at 3227–3313 cm‐1 associated
with C‐H stretching vibration. It is an indication that there is
strong interaction of adsorbed C6H6 with the Mn4+ species of
MnO2 (Figure S8D) and Co‐Mn‐S (Figure S8E). Interestingly,
there is near complete disappearance of the bands at 3313,
1653, 1421 and 1262 cm‐1 after the inflow of O2 for 20 min
over MnO2, suggesting the deep oxidation of surface
carboxylate species to CO2 and H2O, as confirmed in O2‐TPSR
analysis. However, such a phenomenon is not observed in the
cases of Co3O4 and Co‐Mn‐S even after prolonged exposure to
O2, indicating the catalytic performance of Co3O4 and Co‐Mn‐S
at 150 °C is poor. Besides, for MnO2 (Figure S8D) and Co3O4
(Figure S8F), the introduction of O2 does not result in obvious
increase of the carboxylate band intensity, suggesting that the
carboxylate species are generated mainly on surface Mn4+ and
Co3+ sites, and change little in the oxygen atmosphere. In
contrast, the introduction of O2 leads to an increase of
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