Boosting acetone oxidation efficiency over MnO2 nanorods by tailoring crystal phases

Article abstract of DOI:10.1039/c9nj04192c

Developing metal oxides with tailored crystal phases has become a research hotspot in environmental catalysis. In this work, three kinds of MnO₂ nanorods with different crystal phases, e.g. α-, β- and γ-MnO₂, have been prepared by a one-pot hydrothermal method with goals to explore their crystal-phase dependent catalytic performances for acetone elimination. The results attest that α-MnO₂ gave the optimal acetone oxidation activity as compared with β- and γ-MnO₂, completely achieving 100% acetone conversion and 100% CO₂ selectivity at 120 °C under the reaction conditions of acetone concentration = 1000 ppm, 20 vol% O₂/N₂ and WHSV = 90?000 mL g_cat^-1 h^-1. This superior activity of α-MnO₂ mainly originated from its unique crystal phase that resulted in the synergistic effect by combining the largest crystal tunnel size, the highly enhanced chemical nature originating from more Mn⁴⁺ cations, the highly improved low-temperature redox properties and the weakest Mn-O bond strength. Meanwhile, three kinds of MnO₂ nanorods also demonstrated strong long-term stability and good water tolerance for acetone elimination, showing good potential in practical applications.

Full text of DOI:10.1039/c9nj04192c

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Boosting acetone oxidation efficiency over MnO2
nanorods by tailoring crystal phases†
Cite this: New J. Chem., 2019,
43, 19126
Li Cheng, Jinguo Wang, * Chi Zhang, Bei Jin and Yong Men
Developing metal oxides with tailored crystal phases has become a research hotspot in environmental
catalysis. In this work, three kinds of MnO2 nanorods with different crystal phases, e.g. a-, b- and
g-MnO2, have been prepared by a one-pot hydrothermal method with goals to explore their crystal-
phase dependent catalytic performances for acetone elimination. The results attest that a-MnO2 gave
the optimal acetone oxidation activity as compared with b- and g-MnO2, completely achieving 100%
acetone conversion and 100% CO2 selectivity at 120 1C under the reaction conditions of acetone
h
À1. This superior activity of
À1
concentration = 1000 ppm, 20 vol% O2/N2 and WHSV = 90 000 mL gcat
a-MnO2 mainly originated from its unique crystal phase that resulted in the synergistic effect by
combining the largest crystal tunnel size, the highly enhanced chemical nature originating from more
Mn4+ cations, the highly improved low-temperature redox properties and the weakest Mn–O bond
strength. Meanwhile, three kinds of MnO2 nanorods also demonstrated strong long-term stability and
good water tolerance for acetone elimination, showing good potential in practical applications.
Received 13th August 2019,
Accepted 8th November 2019
DOI: 10.1039/c9nj04192c
rsc.li/njc
catalytic oxidation technique. It has been generally admitted
that noble metal catalysts are catalytically more active at low-
1. Introduction
Volatile organic compounds (VOCs), a major group of significant operating temperature and exhibit higher catalytic activities than
atmospheric pollutants, usually come from various industrial metal oxides for VOC elimination,1,7 but the high cost and rapid
processes, automobiles and municipal incineration, which not deactivation of noble metal catalysts inevitably limit their wide
only seriously harm human health but also threaten environ- applications. Therefore, great efforts have recently been devoted
mental quality due to their extremely toxic and malodorous to developing metal oxide catalysts, especially transition metal
nature.1–5 Driven by the more and more rigorous environmental oxides,8–14 which are expected to be a better alternative to replace
legislation of VOC emission, several end-of-pipe techniques, noble metal catalysts. Among various transition metal oxides,
e.g. adsorption, condensation, non-thermal plasma, thermal MnO2 is widely considered as one of the most effective candi-
incineration, biotechnology, photocatalysis and catalytic dates for VOC elimination, which has received extensive concern
oxidation,1–6 have been widely implemented for VOC elimination, of widespread applications in catalytically eliminating various
among which catalytic oxidation is generally believed to be one VOCs.3,5,15–19 Intrinsically, MnO2 commonly exists in different
of the most efficient and cost-effective techniques for VOC elimina- crystal phases, mainly including a-, b-, g- and d-MnO2 crystal
tion because it is energy saving, has high efficiency and does phases,15–25 in which the basic [MnO6] octahedral units are
not produce secondary pollutants.1–5 However, the vital chal- linked in different modes. Meanwhile, it is usually accepted that
lenge of such a technique is still to design and acquire highly the difference in crystal-phase structures can significantly affect
efficient catalysts.
the physicochemical properties, which in turn influence the
During the last few decades, two representative kinds of catalytic reactivity of MnO2. Accordingly, many studies have
catalysts, mainly including supported noble metals (Pt, Pd, Au, revealed the crystal-phase dependent catalytic reactivity of
Rh,. . .) and transition metal oxides (V, Mn, Co, Cu, Fe,...),1–5,7–12 MnO2 in the field of VOC elimination.15–19 For instance,
have been extensively developed for VOC elimination by using a Yu et al. reported that the catalytic activities of MnO2 with
different crystal phases for dimethyl ether oxidation distinctly
College of Chemistry and Chemical Engineering, Shanghai University of Engineering
Science, Shanghai 201620, P. R. China. E-mail: Jinguowang1982@sues.edu.cn;
Tel: +86 21 6787 4046
varied in the order of b- o g- o a-MnO2, fairly agreeing well with
their low-temperature reducibility order.15 Shangguan et al.
found that the catalytic activities of MnO2 with different crystal
phases for both acetaldehyde and benzene oxidations ranked in
the same sequence of b- o g- o a-MnO2, mainly attributed to
their different numbers of OH groups and VOC adsorption
† Electronic supplementary information (ESI) available: Schematic flow chart of
the reaction system for acetone oxidation, XPS surveys of MnO2 with different
crystal phases and activity data for different catalysts for acetone oxidation as in
previous studies. See DOI: 10.1039/c9nj04192c
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capacities together with their distinguishing oxygen mobility.16 2. Experimental section
Lu et al. revealed that the difference in the crystal phases of
MnO2 apparently affected the catalytic activity for propane
2.1. Catalyst preparation
All chemical reagents employed in this work were of analytical
grade and used as received without further treatment. Three
kinds of MnO2 nanorods with different crystal phases were
successfully fabricated by a one-pot hydrothermal process
according to previous studies with minor revisions.25,30 For
synthesizing a-MnO2, 7.5 mmol KMnO4 and 7.5 mmol NH4Cl
were completely dissolved in 80 mL distilled water, then poured
into a 100 mL Teflon-lined autoclave and kept at 200 1C for
24 h. Afterwards, the solid sediment was thoroughly washed
with distilled water and absolute alcohol several times, then
dried at 100 1C for 12 h, and finally calcinated at 350 1C for
3.0 h. Similarly, for preparing b-MnO2, 10 mmol MnSO4ÁH2O
and 10 mmol (NH4)2S2O8 were dissolved in 80 mL distilled
water and kept at 120 1C for 12 h. For fabricating g-MnO2,
20 mmol MnSO4ÁH2O and 20 mmol (NH4)2S2O8 were dissolved
in 80 mL distilled water and kept at 90 1C for 24 h.
oxidation and the catalytic activity arranged in the trend of
d- o b- o g- E a-MnO2, which was significantly associated with
their different exposed facets that influenced the active adsorp-
tion of propane on the surface of MnO2.17 Wang et al. studied
MnO2 with different crystal phases for toluene oxidation in
a combined plasma-catalytic process and disclosed that the
catalytic activity increased gradually in the order of b- o d- o
g- o a-MnO2, primarily ascribed to their different Mn–O bond
strengths and surface-chemisorbed active oxygen species.18
Hu et al. discovered that the catalytic activities of MnO2 with
different crystal phases for benzene oxidation ranked in the
sequence of d- o a- o b- o g-MnO2, which was mainly related
to their differences in the surface adsorbed oxygen species
ratio and low-temperature O2 desorption amount.19 Despite
the contribution of considerable efforts to explore the
crystal-phase dependent catalytic reactivity of MnO2 for VOC
elimination, the essential mechanism has not yet been clearly
elucidated and remains even partly contradictory. The reactivity
2.2. Catalyst characterization
order of MnO2 with different crystal phases for VOC elimina- The crystal phase characterized by the powder X-ray diffraction
tion is usually different from different researchers,15–19 and (XRD) technique was achieved on BRUKER D2 PHASER with
a similar phenomenon is also observed in other application CuKa radiation and 2y values ranging from 101 to 701 under the
fields, e.g. CO oxidation, NO oxidation, water oxidation, conditions of 40 kV and 30 mA. Raman spectra measured
catalytic ozonation, selective catalytic reduction of NOx with on HORIBA LabRAM HR Evolution were acquired using an
NH3, electrochemical oxygen reduction, electrochemical energy excitation wavelength of 532 nm at room temperature. Electron
storage and pollutant degradation in aqueous medium.20–29 paramagnetic resonance (EPR) analysis achieved on a JEOL
The possible reason is that previous studies of the crystal-phase JES-FA200 was conducted at 77 K. The morphological features
dependent catalytic reactivity of MnO2 for VOC elimination of catalysts captured by using a field emission scanning
normally accompany different morphological structures, electron microscope (FESEM) and a high-resolution trans-
exposed facets and preparation conditions. Hence, further mission electron microscope (HRTEM) were analysed on
studies are extremely urgent to reveal the essence of the Hitachi-S4800 and JEOL JEM-2100, respectively. The physical
crystal-phase dependent catalytic reactivity of MnO2 for VOC structural parameters of the catalysts, mainly including the
elimination.
specific surface area (SBET), pore diameter (DP) and pore volume
Herein, three kinds of MnO2 nanorods with different crystal (VP), were obtained from N2 adsorption–desorption isotherms
phases, e.g. a-, b- and g-MnO2, have been fabricated by a using Micromeritics ASAP 2460 at a temperature of 77 K.
one-pot hydrothermal method, aiming to reveal their crystal- Elemental compositions and chemical value states of catalysts
phase dependent catalytic performances for acetone elimination. analyzed using X-ray photoelectron spectroscopy (XPS) were
The results indicate that a-MnO2 presented the optimal determined on PerkinElmer PHI-5000C with AlKa radiation and
acetone oxidation activity in comparison with b- and calibrated with the reference of C1s = 284.6 eV. The reducibility
g-MnO2, completely achieving 100% acetone conversion and of the catalysts determined by using a hydrogen temperature-
100% CO2 selectivity at 120 1C under the reaction conditions programmed reduction (H2-TPR) technique was determined on
of acetone concentration = 1000 ppm, 20 vol% O2/N2 and Micromeritics Auto-chemisorbed II 2920 with a thermal conduc-
WHSV = 90 000 mL gcatÀ1 hÀ1. This superior activity of a-MnO2 tivity detector (TCD) to quantify the H2 consumed amount in the
was mainly associated with its unique crystal phase that temperature range of 100–350 1C, and 50 mg catalyst placed into a
resulted in the synergistic effect by integrating the largest U-shaped quartz tube was initially purged at 300 1C for 1.0 h in
crystal tunnel size, the highly enhanced chemical nature highly pure Ar, then cooled to 50 1C, and finally heated up to
originating from more Mn4+ cations, the highly improved 600 1C with a 10 1C minÀ1 ramp in a 30 mL minÀ1 10% H2/Ar
low-temperature redox properties and the weakest Mn–O bond stream. An oxygen temperature-programmed desorption (O2-TPD)
strength. Meanwhile, three kinds of MnO2 nanorods also test was performed in the same H2-TPR apparatus to quantify the
displayed strong long-term stability and good water tolerance desorption amount of oxygen species in the temperature range of
for acetone oxidation, showing good potential in practical 50–350 1C, and 50 mg catalyst was firstly pretreated at 350 1C for
applications. It can be believed that the findings from this 1.0 h in a 30 mL minÀ1 10% O2/He stream, then cooled to 50 1C,
study can deliver some new insights for guiding the rational and finally heated up to 900 1C with a 10 1C minÀ1 ramp in highly
design of high-efficient catalysts applied in VOC elimination.
pure He stream.
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2.3. Activity evaluation
and g-MnO2 (JCPDS Card No. 14-0644).18–23 The diffraction
peaks located at 2y values of 12.81, 18.11, 28.81, 37.51, 41.91,
49.91 and 60.31 were assigned to the (110), (200), (310), (211),
(301), (411) and (521) planes of a-MnO2 in the tetragonal phase,
the diffraction peaks centered at 2y values of 28.71, 37.31, 42.81,
56.71, 59.41 and 64.81 were well identified as the (110), (101),
(111), (211), (220) and (002) planes of b-MnO2 in the tetragonal
phase, and the diffraction peaks situated at 2y values of 22.41,
37.11, 42.61, 56.11 and 65.61 were fairly associated with the (120),
(131), (300), (160) and (421) planes of g-MnO2 in the ortho-
rhombic phase.23 All the diffraction peaks of three MnO2
catalysts corresponded to a single phase and no other peaks
ascribable to impurities were detected, indicating that they
were present in a highly pure single phase. Meanwhile, the
diffraction peaks of a- and g-MnO2 were much weaker and
broader than b-MnO2, implying that both of them had a lower
crystallization degree. Based on the principal (310) peak of
a-MnO2, the (110) peak of b-MnO2 and the (131) peak of
g-MnO2, their respective crystallite size has been calculated
using Scherrer’s equation as listed in Table 1. It can be clearly
observed that the crystallite size of b-MnO2 was much bigger
than those of a- and g-MnO2, especially g-MnO2, signifying its
high crystallization degree, which could be mainly attributed to
their different preparation processes.
The crystal structures in Fig. 2 show that three MnO2
catalysts with different crystal phases were constructed by
different interlinking modes of edge-sharing basic [MnO6]
octahedral units. For a-MnO2, one-dimensional structure was
constructed from the double chains of edge-sharing basic
[MnO6] octahedral units by forming (2 Â 2) and (1 Â 1) tunnels
that extended along the crystallographic E axis of a tetragonal
unit cell, and their corresponding sizes were 4.6 Â 4.6 Å and
2.3 Â 2.3 Å,15,18 respectively. b-MnO2 possessed a rutile-type
structure with tetragonal symmetry (P42/mnm), in which the
basic [MnO6] octahedral units build up strings of edge-sharing
octahedral units extending along the crystallographic c axis of a
tetragonal unit cell, and then linked with neighboring chains
by sharing common corners, finally leading to the formation
of (1 Â 1) tunnels with a size of 2.3 Â 2.3 Å in the crystal
structure of b-MnO2.18,20 For g-MnO2, the crystal structure was
recognized as a random intergrowth of pyrolusite (1 Â 1) tunnel
and ramsdellite (2 Â 1) tunnel structures with corresponding
sizes of 2.3 Â 2.3 Å and 4.6 Â 2.3 Å, respectively, in which
the basic [MnO6] octahedral units shared the edges and
corners.21,23 Therefore, g-MnO2 can be considered to be a most
complex and highly disordered material, which can be suffi-
ciently validated by its much weaker and broader diffraction
peaks of XRD patterns as shown in Fig. 1. Hence, it can be
speculated that the difference in the crystal structures of three
MnO2 catalysts resulted in different tunnel structures and sizes,
and the tunnel size of the three MnO2 catalysts distinctly varied
in the order of b- o g- o a-MnO2.
The catalytic activities of MnO2 nanorods with different crystal
phases for acetone oxidation were evaluated in a continuous
flow fixed-bed tubular quartz reactor (f = 6.0 mm) by the
temperature-programmed oxidation (TPO) reaction using
100 mg catalyst (40–60 mesh) at atmospheric pressure as
schemed in Fig. S1 (ESI†). Each TPO reaction runs from 50 1C
to 250 1C together with a K-thermocouple projecting into the
catalyst bed for guaranteeing the temperature raising ramp at
2.0 1C minÀ1. Feed gases, mainly including 1000 ppm acetone,
20 vol% O2 and major N2 as the balance gas, flowed through the
catalyst bed with a total stream of 150 mL minÀ1 corresponding
to a weight hourly space velocity (WHSV) of 90 000 mL gcatÀ1 hÀ1
.
To study the effect of WHSVs on catalytic activity, the total streams
of feed gases were changed into 300 mL minÀ1 and 4À010 mL minÀ1
corresponding to WHSVs of 180 000 mL gcat
hÀ1 and
À1
240 000 mL gcat
h
À1, respectively. To investigate the effect
of water on catalytic activity, 8.0 vol% and 16 vol% H2O vapor
generated by pumping distilled water into a gasification
chamber at 180 1C were separately introduced into the reaction
system along with feed gases. The concentrations of reactants
and products were analyzed online using a gas chromatograph
(Shimadzu GC-2014C) equipped with TCD and FID detectors.
The catalytic activities were determined by using the reaction
temperatures of T10, T50 and T90 corresponding to achieved
acetone conversions (Zacetone) of 10%, 50% and 90%, respectively,
where Zacetone was defined as the following equation: Zacetone
[(Cacetone-in À Cacetone-out)/Cacetone-in] Â 100%.
=
3. Results and discussion
3.1. Physicochemical characteristics
The XRD patterns in Fig. 1 reveal that three MnO2 catalysts
demonstrated distinctly different diffraction peaks, all of which
could be indexed to the respective pure crystal phase, e.g. a-MnO2
(JCPDS Card No. 44-0141), b-MnO2 (JCPDS Card No. 24-0735)
The Raman spectra in Fig. 3a further reveal the different
crystal structures and local Mn environments of three MnO2
catalysts in more detail. Generally, the Raman bonds in the
range of 200–500 cmÀ1 are associated with the Mn–O–Mn
Fig. 1 XRD patterns of MnO2 with different crystal phases.
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Table 1 Physical structural parameters of MnO2 with different crystal phases
Catalysts
Crystal phase
Crystallite size (nm)
Mn–O lengtha (Å)
SBET (m2 gÀ1
)
Vp (cm3 gÀ1
)
Dp (nm)
a-MnO2
b-MnO2
g-MnO2
Tetragonal
Tetragonal
Orthorhombic
20
27
16
1.98
1.88
1.91
60
16
46
0.12
0.027
0.15
26
17
17
a
The value of Mn–O length cited from ref. 18 and 20.
signifying the high mobility of surface oxygen species in
a-MnO2. b-MnO2 demonstrated two main peaks centered at
around 539 cmÀ1 and 667 cmÀ1. The peak at 539 cmÀ1
corresponded to the Mn–O stretching of the [MnO6] octahedral
unit and the peak at 667 cmÀ1 was indexed to the characteristic
A1g mode, indicating a well-developed rutile-type tetragonal
structure with a (1 Â 1) tunnel in b-MnO2.15,18 The g-MnO2
also displayed two major peaks situated at about 579 cmÀ1 and
669 cmÀ1. The former peak at 579 cmÀ1 suggested a well-
developed orthorhombic structure with a (2 Â 1) tunnel in
g-MnO2 and the latter peak at 669 cmÀ1 was ascribed to the
stretching mode of the Mn–O bond in the [MnO6] octahedral
unit.15,18 It should be noticed that no peaks located at less than
200 cmÀ1 can be observed in b- and g-MnO2, possibly implying
the weaker mobility of surface oxygen species as compared with
a-MnO2. Meanwhile, it can be noted that the full width at the
respective half-maximum values of the three MnO2 catalysts
ranked in the sequence of a- o g- o b-MnO2 corresponding
to the increased number or concentration of defective sites,
which seemingly contradicted with their variation in the
crystallization degree as revealed by XRD analysis. This
phenomenon can be interpreted by the fact that the Raman
technique is surface sensitive while the XRD technique is an
indication of bulk materials.31
Fig. 2 Crystal structures of MnO2 with different crystal phases.
To ascertain the chemical nature of defective sites in three
MnO2 catalysts, the EPR characterization has been employed
and the corresponding results have been shown in Fig. 3b.
Clearly, it can be seen that all of the three MnO2 catalysts
exhibited a sharp and strong EPR signal located at approxi-
mately g = 1.995, which was mainly related to the Zeeman
effect of the unpaired electrons trapped at the sites of oxygen
vacancies,32,33 distinctly attesting that the chemical nature
of defective sites in three MnO2 catalysts was due to oxygen
vacancies. To the best of our knowledge, the number or
concentration of oxygen vacancies can be generally propor-
Fig. 3 Raman spectra (a) and EPR spectra (b) of MnO2 with different
crystal phases.
bending vibrations in the MnO2 octahedral lattice while the tional to the peak intensity of the EPR signal according to
Raman bonds in the range of 500–700 cmÀ1 are related to the previous studies.32,33 It can be noted that the peak intensity
typical Mn–O stretching of the [MnO6] octahedral unit.15,17,18,20 of EPR signals increased gradually in the order of a- o g- o
The results in Fig. 3a show that a-MnO2 featured four major b-MnO2, indicating the increased number or concentration of
peaks located at approximately 188 cmÀ1, 389 cmÀ1, 580 cmÀ1 oxygen vacancies, which agreed well with the results of Raman
and 633 cmÀ1. The strong peak at 580 cmÀ1 was definitely analysis.
assigned to the deformation modes of the Mn–O–Mn chain in
The FESEM images in Fig. 4 demonstrate that the morpho-
the MnO2 octahedral lattice while the peak at 633 cmÀ1 was logical structures of three MnO2 catalysts were uniform nanorods
assigned to the Mn–O stretching modes, and both of them with different lengths and diameters. The respective lengths
distinctly indicated a well-developed tetragonal structure with a and diameters of the three MnO2 catalysts were calculated to be
(2 Â 2) tunnel in a-MnO2.15,17,18 The peak at 188 cmÀ1 of around 0.30–1.5 mm and 50–80 nm for a-MnO2, 0.20–2.5 mm
a-MnO2 represented an external mode that is derived from and 30–90 nm for b-MnO2, 0.50–2.0 mm and 15–60 nm for
the translational motion of basic [MnO6] octahedral units,17,18 g-MnO2, respectively. Meanwhile, their corresponding TEM
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Fig. 4 FESEM, TEM and HRTEM images of MnO2 with different crystal phases.
images also showed that the three MnO2 catalysts were present
in uniform nanorods, apparently exhibiting the consistent
morphology and corresponding size distribution of the three
MnO2 catalysts as disclosed by FESEM analysis. Moreover, the
distinct reflections with d-spacing values of 0.69 nm, 0.31 nm
and 0.24 nm in their respective HRTEM image for three MnO2
catalysts were assigned to the typical (110) plane of the tetra-
gonal a-MnO2 crystal phase, the principal (110) plane of the
tetragonal b-MnO2 crystal phase and the principal (131) plane
of the orthorhombic g-MnO2 crystal phase,17,30 obviously
attesting that a-, b- and g-MnO2 preferentially exposed the
(110), (110) and (131) facets, respectively.
The N2 sorption isotherms in Fig. 5 display that all of the
three MnO2 catalysts showed the typical type-IV N2 adsorption– Fig. 5 N2 adsorption–desorption isotherms (a) and pore size distribution
curves (b) of MnO2 with different crystal phases.
desorption isotherms along with a small H1 hysteresis loop in
the relative pressure (P/P0) range of 0.7–1.0, implying the
existence of mesoporous structures in three MnO2 catalysts
that are generally related to the slit-shaped pores according to different tunnel sizes as revealed by crystal structures in Fig. 2.
the classic IUPAC definitions.19,32,34,35 The slit-shaped pores It can be seen that the tunnel size of the three MnO2 catalysts
usually originated from the sheet-like or rod-like feature of increased gradually in the order of b- o g- o a-MnO2, agreeing
materials,32,34,35 coinciding well with the results of FESEM and well with the variation order in SBET. It can be noted that the
TEM analysis. Meanwhile, it can be found that no saturated larger the tunnel size is, the higher the SBET is. This fact can be
adsorption in a high P/P0 range of three MnO2 catalysts was mainly related to the size confinement of N2 molecules (3.05 Å)
achieved up to P/P0 = 1.0, signifying the existence of partial in a certain scope during the testing process of SBET
.
large mesopores.36,37 This speculation can be evidenced by
The surface chemical compositions and chemical valence
their respective pore size distribution curve in which a wide states of the three MnO2 catalysts have been studied by employing
pore size distribution could be observed in the range of an XPS technique as shown in Fig. 6. The survey spectra in Fig. S2
3.0–50 nm, mainly stemming from the aggregated voids by (ESI†) revealed that all of the three MnO2 catalysts only contained
the accumulation of the nanorod units. Based on N2 sorption Mn and O elements and no other impurities were detected,
isotherms, the surface area (SBET), pore volume (VP) and pore suggesting their high purity. The high-resolution spectrum of
size (DP) of the three MnO2 catalysts have been acquired by the Mn2p region in Fig. 6a demonstrated two spin–orbit doublets
using the classic BET and BJH models from the desorption with binding energies of around 641.7 eV and 653.4 eV corres-
branches, respectively. From Table 1, one can obviously see that ponding to Mn2p3/2 and Mn2p1/2,
18,19 respectively. The signals of
the SBET increased greatly in the order of b- o g- o a-MnO2, Mn4+ and Mn3+ cations can be observed after the decomposition
indicating that a-MnO2 had the highest SBET among the three of Mn2p spectra by using Gaussian fitting methods. Two distinct
MnO2 catalysts. This observation was mainly attributed to their peaks with binding energies of about 642.8 eV and 654.2 eV were
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molar ratio increased gradually in the sequence of a- o g- o
b-MnO2, obviously manifesting b-MnO2 with the highest number
or concentration of oxygen vacancies and also showing an inverse
trend to the variation order in the Mn4+/Mn3+ molar ratio, which
can be ascribed to the reason that the formation of oxygen
vacancies was mainly contributed by the presence of charac-
teristic Mn3+ cations. Meanwhile, the above results were in good
accordance with the characterization as revealed by Raman and
EPR analysis.
The reducibility of the three MnO2 catalysts has been
evaluated by using H2-TPR experiments as depicted in Fig. 7a.
Generally, the reduction of MnO2 often undergoes three-
stepwise reduction processes of MnO2 - Mn2O3 - Mn3O4 -
MnO.15,17–20 In this study, it should be emphasized that the
reduction products of the three MnO2 catalysts were green
powder, indicating that the final reduction products of the
three MnO2 catalysts were MnO. Clearly, b-MnO2 demonstrated
three distinct reduction peaks situated at about 327 1C, 385 1C
and 475 1C, which can be associated with the simultaneous
reductions of MnO2 - Mn2O3, Mn2O3 - Mn3O4 and Mn3O4 -
MnO,15,17,18 respectively. In contrast, the H2-TPR profiles of
a- and g-MnO2 were greatly different from that of b-MnO2.
a-MnO2 only exhibited two reduction peaks centered at around
264 1C and 325 1C. Similarly, g-MnO2 displayed two reduction
peaks located at approximately 325 1C and 485 1C. The
reduction process giving rise to these peaks of a- and g-MnO2
could not be elucidated in detail, but probably involved the
reduction of MnO2 - MnO with the simultaneous reductions
of Mn2O3 and Mn3O4.18,20 Interestingly, it should be noted that
the former peaks of a- and g-MnO2 shifted dramatically to the
lower temperature as compared with b-MnO2, unquestionably
signifying their enhanced low-temperature reducible ability.
The amount of H2 consumed below 350 1C has been acquired
by quantitatively calculating the reduction peaks in the H2-TPR
profiles of the three MnO2 catalysts as listed in Table 3. It can
be observed that the H2 consumed amount increased greatly
in the trend of b- o g- o a-MnO2, apparently indicating that
Fig. 6 XPS spectra of MnO2 with different crystal phases: Mn2p of (a) and
O1s of (b).
indexed to the typical Mn4+ cations while two major peaks with
binding energies of approximately 641.6 eV and 652.8 eV were
assigned to the characteristic Mn3+ cations.19 The presence of
Mn3+ cations is generally linked to the formation of oxygen
vacancies. To identify the origin of oxygen vacancies, the high-
resolution spectrum of the O1s region for three MnO2 catalysts in
Fig. 6b has been deconvoluted into two major peaks at binding
energies of around 529.9 eV and 531.7 eV corresponding to the
lattice oxygen (Olatt) species and surface chemisorbed oxygen
(Oads) species (e.g. O22À, O2À, OHÀ or OÀ),32–36 respectively.
Usually, the number or concentration of oxygen vacancies can
be normally reflected by the peak intensity of surface chemi-
sorbed oxygen (Oads) species according to previous reports.32,33
Interestingly, the peak intensity of Oads in Fig. 6b increased
slightly in the sequence of a- o g- o b-MnO2, indicating the
increased number or concentration of oxygen vacancies. Based on
the XPS analysis, the quantified surface chemical compositions
and chemical valence states of three MnO2 catalysts have been
calculated and are listed in Table 2. It can be seen that the
Mn4+/Mn3+ molar ratio increased gradually in the order of b- o
g- o a-MnO2, effectively attesting that a-MnO2 had the highest
number or concentration of Mn4+ cations, which was in good
accordance with previous studies.18,19 Taking into account the
fact that the Mn species with higher chemical value states were
more conductive to the oxidation reactions over a Mn-based
catalyst,14,38–40 it can be speculated that a-MnO2 will show higher
catalytic activity in acetone oxidation. However, the Oads/Olatt
Table 2 Surface elemental compositions and chemical value states of
MnO2 with different crystal phases
Mn2p envelope
O1s envelope
Catalysts Mn4+ (%) Mn3+ (%) Mn4+/Mn3+ Oads (%) Olatt (%) Oads/Olatt
a-MnO2 47
b-MnO2 37
g-MnO2 39
53
63
61
0.89
0.59
0.64
21
25
23
79
75
77
0.27
0.33
0.30
Fig. 7 H2-TPR (a) and O2-TPD (b) profiles of MnO2 with different crystal
phases.
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Table 3 Chemical structural parameters and special activities of MnO2
with different crystal phases
tunnel size (see Fig. 2 and Table 1) as compared with b- and
g-MnO2, which facilitated the enhanced adsorption and diffu-
sion of active oxygen species due to its less mass transfer and
diffusion resistance, thereby improving its adsorption and
activation potentials towards active oxygen species effectively.
Meanwhile, the average Mn–O bond length in MnO2 increased
slightly in the sequence of b- o g- o a-MnO2, which might also
account for the highest desorption amount of surface active
oxygen species of a-MnO2 due to the fact that the longer the
average Mn–O bond length is, the easier the Mn–O bond is
Consumption
(mmol gÀ1
Special
)
activity (1C)a
RS at 160 1C
Catalyst
H2
O2
T10
T50
T90
(mol mÀ2 sÀ1
)
a-MnO2
b-MnO2
g-MnO2
7.81
2.07
5.11
0.22
0.07
0.11
72
93
85
93
144
122
104
159
133
6.94 Â 10À6
6.53 Â 10À6
6.79 Â 10À6
a-MnO2 possessed the highest low-temperature reducibility. broken.18
This observation can be mainly attributed to the fact that
a-MnO2 contained more Mn4+ cations than b- and g-MnO2
since the reduction of Mn4+ - Mn2+ required much more H2
than the reduction of Mn3+ - Mn2+, fairly agreeing well with
the results of XPS analysis. Meanwhile, from Table 1, it can be
seen that the average Mn–O bond length in MnO2 ranked in the
order of b- o g- o a-MnO2.18,20 Usually, the longer the average
Mn–O bond length is, the easier the Mn–O bond can be
reduced. This might be also responsible for the highest low-
temperature reducibility of a-MnO2.
3.2. Catalytic performance
The catalytic performances of the three MnO2 catalysts have
been evaluated by using acetone oxidation as a model reaction
at atmospheric pressure ranging from 50 1C to 200 1C under
the reaction conditions of acetone concentration = 1000 ppm,
20 vol% O2/N2 and WHSV = 90 000 mL gcat
h
À1. Blank
À1
experiments revealed that no measurable acetone oxidation
can be found to take place without catalysts in the examined
temperature range, and only CO2 and H2O as the final products
can be detected over three MnO2 catalysts under the experi-
mental conditions employed. As shown in Fig. 8a, one can
clearly see that the acetone conversion over the three MnO2
catalysts increased dramatically along with the increasing
reaction temperature, albeit to their greatly distinguishing
degrees. Complete acetone conversion can be achieved at
reaction temperatures of approximately 120 1C, 190 1C and
150 1C for a-, b- and g-MnO2, respectively. The produced CO2
concentrations over the three MnO2 catalysts have been also
monitored and quantitatively analyzed as plotted in Fig. 8b.
Notably, the produced CO2 concentrations over three MnO2
catalysts also increased significantly along with the rise in the
reaction temperature regardless of their different degrees,
almost showing the same trend as the acetone conversion
curves. Combining the normalization of acetone conversion
The adsorption and activation potentials of the three MnO2
catalysts towards different oxygen species have been studied
by performing O2-TPD experiments as presented in Fig. 7b.
Obviously, one can see that all the MnO2 catalysts demon-
strated three kinds of desorption peaks with respect to oxygen
species as follows: the low-temperature desorption peak located
at less than 400 1C, the medium-temperature desorption peak
situated in the range of 400–600 1C and the high-temperature
desorption peak exceeding 600 1C, which were mainly assigned
to the desorption of chemisorbed surface active oxygen species
À
(e.g. O2, O2 and OÀ), the liberation of sub-surface lattice
oxygen (O2À
) and the release of bulk lattice oxygen
(O2À),17,19,41 respectively. Meanwhile, it can be found that the
desorption peaks below 600 1C of a- and g-MnO2, especially
a-MnO2, shifted slightly to the lower temperature in compar-
ison with b-MnO2, distinctly suggesting a-MnO2 with the high-
est adsorption and activation potentials towards surface active
oxygen species. Based on the O2-TPD profiles of the three MnO2
catalysts, the desorption amount of surface active oxygen
species below 350 1C has been quantitatively calculated and
is listed in Table 3. Clearly, it can be seen that the desorption
amount of surface active oxygen species increased significantly
in the order of b- o g- o a-MnO2, further confirming a-MnO2
with the highest adsorption and activation potentials towards
surface active oxygen species. However, the above result seems
to contradict the characterization as disclosed by EPR and
XPS analysis. Usually, the more the oxygen vacancies and the
surface chemisorbed oxygen (Oads) species of catalysts are, the
higher the adsorption and activation potentials of catalysts
towards surface active oxygen species are.42 a-MnO2 showed
the highest adsorption and activation potentials towards active
oxygen species although it had the relatively lowest number of
Fig. 8 Acetone conversion curves (a), the produced CO2 concentration
curves (b) and the corresponding T10, T50 and T90 (c) of MnO2 with
different crystal phases based on the same mass basis at WHSV =
oxygen vacancies and the surface chemisorbed oxygen (Oads
)
species as evidenced in Fig. 3b and 6b. This observation could
be possibly attributed to the reason that a-MnO2 had the largest
90 000 mL gcat hÀ1
.
À1
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and the produced CO2 concentration, the CO2/acetone molar distinctly different degrees, completely achieving acetone conver-
ratio was calculated to be around 2.97, which was very close to sion at 160 1C, 200 1C and 180 1C for a-, b- and g-MnO2, respectively,
the theoretical value of 3.0, attesting that acetone was comple- which demonstrated the same variation trend as the obtained
tely oxidized to CO2 and H2O under the present conditions. results as revealed in Fig. 8a and b. Meanwhile, as shown in
For intuitively comparing the catalytic performances of the Fig. 9c, it can be observed that the respective T10, T50 and T90 of
three MnO2 catalysts, the reaction temperatures of T10, T50 the three MnO2 catalysts also increased significantly in
and T90 corresponding to the achieved acetone conversion of the order of a- o g- o b-MnO2, further reconfirming a-MnO2
10%, 50% and 90%, respectively, are employed as the evalua- with the optimal acetone oxidation activity in this study. For
tion criteria for acetone oxidation. From Table 3 and Fig. 8c, guaranteeing the activity order more accurately, the acetone
it is obvious that the T10, T50 and T90 of the three MnO2 catalysts conversion rate at 160 1C over the three MnO2 catalysts has
ranked in the same sequence of a- o g- o b-MnO2, undeniably been calculated by using the activity data in Fig. 9a according to
showing that a-MnO2 gave the optimal acetone oxidation the equation of Rs (mol mÀ2 À1) = ZQCf/S, where Z, Q, Cf and
s
activity in this study. Meanwhile, from Table S1 (ESI†), it should S referred to the acetone conversion, the volumetric flow rate
be emphasized that the catalytic activity of a-MnO2 was also (mL minÀ1), the feed of acetone concentration (ppm) and the
much higher than those of many previously reported catalysts surface area of the catalyst (m2), respectively. As listed in
for acetone oxidation, mainly including noble metal-based, Table 3, it can be noticed that a-MnO2 showed the highest
Co-based, Fe-based, Cu-based, Ce-based, Ti-based and V-based acetone conversion rate among the three MnO2 catalysts,
catalysts together with some multi-component metal oxides, further validating a-MnO2 with the optimal acetone oxidation
which further confirmed the excellent acetone oxidation activity activity in this study.
of a-MnO2.
Considering the above results in Fig. 8 and 9 together with
Importantly, it should be pointed out that the activity data the Rs in Table 3, it can be concluded that a-MnO2 had the
in Fig. 8 were acquired by performing acetone oxidation over optimal acetone oxidation activity owing to the fact that
the three MnO2 catalysts based on the same catalyst mass basis the acetone oxidation activity increased gradually in the order
rather than simultaneously on the same surface area basis. of b- o g- o a-MnO2 in this work. Meanwhile, it can be said
Therefore, the effect of surface area on the acetone oxidation that the surface area of the three MnO2 catalysts did not play a
activity must be considered, mainly due to the fact that acetone crucial role in catalyzing acetone oxidation in this work. Hence,
oxidation as a typical gas–solid heterogeneous catalytic reaction the distinguishing acetone oxidation activities of the three
usually occurs at the binary-phase contact interface or MnO2 catalysts can be mainly correlated with their different
boundary.8,43–45 Accordingly, the activity evaluation of acetone crystal phases together with their caused different physico-
oxidation over the three MnO2 catalysts has been carried out on chemical properties as follows. Firstly, the three MnO2 catalysts
the basis of the same surface area by using b-MnO2 as a possessed different crystal phases, which resulted in different
reference: the SBET fixed at 1.6 m2 is equal to 100 mg of b-MnO2, crystal tunnel structures and tunnel sizes. The tunnel size of
26.7 mg of a-MnO2 and 34.8 mg of g-MnO2, respectively. As shown the three MnO2 catalysts increased gradually in the sequence of
in Fig. 9a and b, the acetone conversion and the produced CO2 b- o g- o a-MnO2, indicating that a-MnO2 possessed the
concentration of the three MnO2 catalysts also increased greatly relatively largest tunnel size. The largest tunnel size favored
along with the increasing reaction temperature irrespective of their the enhanced adsorption and diffusion of acetone molecules,
which in turn enlarged the contact interface or the contact
frequency of acetone molecule-catalyst and thus, effectively
improved the acetone oxidation activity of a-MnO2. Secondly,
the Mn4+/Mn3+ molar ratio (see Table 2) of the three MnO2
catalysts varied in the sequence of b- o g- o a-MnO2, attesting
that a-MnO2 contained the relatively highest amount of Mn4+
cations. As mentioned in previous studies,14,38–40 it is well
known that the Mn species with higher chemical value states
were more conductive to the oxidation reactions over a
Mn-based catalyst. Thus, the relatively highest amount of
Mn4+ cations in a-MnO2 facilitated its enhanced acetone oxida-
tion activity. Thirdly, the low-temperature reducibility and the
amount of surface active oxygen species (see Table 3) for three
MnO2 catalysts also increased gradually in the trend of b- o
g- o a-MnO2, validating the highest low-temperature redox
potential of a-MnO2, which also accounted for the enhanced
acetone oxidation activity of a-MnO2. Moreover, it has been
reported that the Mn–O bond strength of MnO2 catalysts
Fig. 9 Acetone conversion curves (a), the produced CO2 concentration
curves (b) and the corresponding T10, T50 and T90 (c) of MnO2 with
can play an important role in determining their catalytic
different crystalÀp1 hases based on the same surface area basis at WHSV =
performances according to previous reports.18,20 The longer
90 000 mL gcat hÀ1
.
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the average Mn–O bond length is, the weaker the Mn–O bond
The catalyst deactivation is a big challenge that urgently
strength is. It can be noted that the average Mn–O bond length requires addressing for practical applications. Meanwhile, it is
of the three MnO2 catalysts increased slightly in the order of well-known that water is often produced during the VOC
b- o g- o a-MnO2, suggesting the longest average Mn–O bond oxidation process and also has an adverse effect on the catalytic
length and the weakest Mn–O bond strength of a-MnO2. activity.43–45 Thus, the long-term stability and water tolerance
Accordingly, the Mn–O bond can be most easily broken in of catalysts for VOC oxidation are generally considered as two
a-MnO2 for acetone oxidation, which can also endow a-MnO2 important evaluation criteria for their potential in practical
with the improved catalytic performance. Therefore, it can be applications. Accordingly, the long-term stability over the three
concluded that all the above factors through the synergistic MnO2 catalysts for testing 48 h has been evaluated under the
effect simultaneously contributed to endow a-MnO2 with the reaction conditions of acetone concentration = 1000 ppm,
highest acetone oxidation activity.
20 vol% O2/N2 and WHSV = 90 000 mL gcatÀ1 hÀ1 at two selected
The effect of WHSV on the acetone oxidation activity over the different reaction temperatures corresponding to the acetone
three MnO2 catalysts has been investigated as shown in Fig. 10. conversions of approximately 60% and 100%, respectively.
Obviously, it can be seen that the acetone conversion over As shown in Fig. 11, it can be noticed that all of the three
the three MnO2 catalysts decreased dramatically and a much MnO2 catalysts demonstrated strong long-term stability at the
higher reaction temperature of three MnO2 catalysts was two selected different reaction temperatures, acetone conver-
required for completely achieving acetone conversÀio1n with sions being almost unchanged and stable at around 60% and
the increase in tÀh1e WHSV value from 90 000 mL gcat hÀ1 to nearly 100% in the entire catalytic oxidation test. The effect of
240 000 mL gcat
h
À1. In other words, the increase in the H2O vapor on the acetone oxidation activity over the three
WHSV value lead to the lower acetone conversion at the same MnO2 catalysts has also been studied by separately introducing
reaction temperature, which can be further validated by the 8.0 vol% and 16 vol% H2O vapor into the reaction system.
evidence that the respective T10, T50 and T90 of acetone conver- The experimental results in Fig. 11 revealed that the acetone
sion over the three MnO2 catalysts were increased to higher conversions at the selected different reaction temperatures
temperatures along with the increase in the WHSV value. These for reacting 12 h apparently decreased gradually from nearly
distinguishing acetone oxidation activities of the three MnO2 100% to around 95%, 91% and 82% for a-, b- and g-MnO2,
catalysts with respect to the WHSV value were mainly attributed respectively, when 8.0 vol% H2O vapor was introduced. Subse-
to the shorter retention time of acetone molecules in the quently, the acetone conversion further decreased to about
catalyst bed at a higher WHSV value,8,43–45 which significantly 78%, 87% and 79% for a-, b- and g-MnO2, respectively, when
shorten the contact time or reduced the contact frequency 16 vol% H2O vapor was introduced. This observation validated
of acetone molecule-catalyst and thus, decreased the acetone that the H2O vapor had a detrimental effect on the acetone
oxidation activity.
oxidation activity of the three MnO2 catalysts, which was mainly
due to the presence of competitive adsorption between the
Fig. 10 Acetone conversion curves and the corresponding T10, T50 and
T90 of MnO2 with different crystal phases at different WHSVs.
Fig. 11 Long-term stability test and the effect of H2O vapor on catalytic
activity for MnO2 with different crystal phases at WHSV = 90 000 mL gcatÀ1 hÀ1
.
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acetone molecule and H2O vapor on the surface of catalysts.43–45
Interestingly, it can be noticed that the acetone conversion over
the three MnO2 catalysts could immediately recover back to
almost 100% in several hours once the H2O vapor was inter-
rupted, indicating that three MnO2 catalysts had good tolerance
for H2O vapor at a certain content. However, it should be
emphasized that the acetone conversion of b-MnO2 (87%)
decreased much smaller than those of a- (78%) and g-MnO2
(79%) when 16 vol% H2O vapor was introduced, indicating its
relatively higher water tolerance, which could be possibly
ascribed to its higher stable crystal phase.
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In summary, three kinds of MnO2 nanorods with different
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lytic performances for acetone oxidation have been investigated
in detail. The results reveal that a-MnO2 gave the optimal
acetone oxidation activity as compared with b- and g-MnO2,
which was attributed to its unique crystal phase that resulted in
the synergistic effect by combining the largest crystal tunnel
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Acknowledgements
This work is supported by the Shanghai Talent Development
Foundation (Grant No. 2017076), the National Natural Science
Foundation of China (Grant No. 21503133), the Shanghai Key
Laboratory of Rare Earth Functional Materials (Grant No. 2018,
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