Elucidating structure-performance correlations in gas-phase selective ethanol oxidation and CO oxidation over metal-doped γ-MnO2

Article abstract of DOI:10.1016/S1872-2067(20)63551-3

Despite of considerable efforts on the MnO₂-based catalytic combustion, the different structural and component requirements of MnO₂ for gas-phase selective oxidation and complete oxidation largely remain unknown. By comparing four types of MnO₂ with different crystal structures (α, β, γ and δ), γ-MnO₂ was found to be the most efficient catalyst for both aerobic selective oxidation of ethanol and CO oxidation. The structural effect of γ-MnO₂ was further investigated by doping metal ions into the framework and by comparing the catalytic performance in the gas-phase aerobic oxidation of CO and ethanol. Among ten M-γ-MnO₂ catalysts, Zn-γ-MnO₂ showed the lowest temperature (160 °C) for achieving 90% CO conversion. The CO oxidation activity of the M-γ-MnO₂ catalysts was found to be more relevant to the surface acidity-basicity than the reducibility. In contrast, surface reducibility has been demonstrated to be more crucial in the gas-phase ethanol oxidation. Cu-γ-MnO₂ with higher reducibility and more oxygen vacancies of Mn²⁺/Mn³⁺ species exhibited higher catalytic activity in the selective ethanol oxidation. Cu-γ-MnO₂ achieved the highest acetaldehyde yield (75%) and space-time-yield (5.4 g g_cat^–1 h^–1) at 200 °C, which are even comparable to the results obtained by the state-of-the-art silver and gold-containing catalysts. Characterization results and kinetic studies further suggest that the CO oxidation follows the lattice oxygen-based Mars-van Krevelen mechanism, whereas both surface lattice oxygen and adsorbed oxygen species involve in the ethanol activation.

Full text of DOI:10.1016/S1872-2067(20)63551-3

Chinese Journal of Catalysis 41 (2020) 1298–1310
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Elucidating structure-performance correlations in gas-phase
selective ethanol oxidation and CO oxidation over metal-doped
γ-MnO
2
Panpan Wang, Jiahao Duan, Jie Wang, Fuming Mei, Peng Liu *
Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Hubei Key Laboratory of Material Chemistry and
Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Despite of considerable efforts on the MnO -based catalytic combustion, the different structural and
2
Received 3 December 2019
Accepted 25 December 2019
Published 5 August 2020
component requirements of MnO
remain unknown. By comparing four types of MnO
γ-MnO was found to be the most efficient catalyst for both aerobic selective oxidation of ethanol
and CO oxidation. The structural effect of γ-MnO was further investigated by doping metal ions into
the framework and by comparing the catalytic performance in the gas-phase aerobic oxidation of
CO and ethanol. Among ten M-γ-MnO catalysts, Zn-γ-MnO showed the lowest temperature (160
C) for achieving 90% CO conversion. The CO oxidation activity of the M-γ-MnO catalysts was found
to be more relevant to the surface acidity-basicity than the reducibility. In contrast, surface reduci-
bility has been demonstrated to be more crucial in the gas-phase ethanol oxidation. Cu-γ-MnO with
higher reducibility and more oxygen vacancies of Mn /Mn species exhibited higher catalytic ac-
tivity in the selective ethanol oxidation. Cu-γ-MnO achieved the highest acetaldehyde yield (75%)
and space-time-yield (5.4 g gcat h–1) at 200 °C, which are even comparable to the results obtained
by the state-of-the-art silver and gold-containing catalysts. Characterization results and kinetic
studies further suggest that the CO oxidation follows the lattice oxygen-based Mars-van Krevelen
mechanism, whereas both surface lattice oxygen and adsorbed oxygen species involve in the etha-
nol activation.
2
for gas-phase selective oxidation and complete oxidation largely
2
with different crystal structures (α, β, γ and δ),
2
2
Keywords:
2
2
MnO
2
°
2
Metal doping
Ethanol oxidation
Acetaldehyde
2
2+
3+
Catalytic CO oxidation
2
–1
©
2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1.
Introduction
tions, the crystal structure of MnO is suggested to play a cru-
2
cial role in determining the catalytic performance, with the
optimal crystal form varying by the different oxidations [4–7].
However, most studies for the gas-phase oxidation with crys-
Manganese dioxides (MnO ) have been widely used as het-
erogeneous catalysts for aerobic oxidations because of their
high availability, tunable crystal structures, mixed Mn valence
2
talline MnO are focusing on the catalytic combustion of CO [4],
2
and high mobility of lattice oxygen [1,2]. Diverse MnO
als are formed by octahedral MnO units shared with corners
or edges, which lead to various tunnel and layered structures
2
materi-
formaldehyde [5,8] and aromatics [7]; the reports on the aero-
bic selective oxidation are scarce. Therefore, it is highly desired
6
to develop efficient and practical MnO
2
catalysts for gas-phase
(
such as α-, β-, γ- and δ-MnO , etc.) [3]. For gas-phase oxida-
2
selective oxidation.
*
Corresponding author. Tel: +86-27-87543032; Fax: +86-27-87543632; E-mail: pengliu@hust.edu.cn
This work was supported by the National Natural Science Foundation of China (21673088, 21972050).
DOI: 10.1016/S1872-2067(20)63551-3 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 8, August 2020

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310
1299
Recently, gas-phase aerobic oxidation of ethanol to acetal-
dehyde (AC) has attracted wide attention, because this process
is increasingly competitive to the traditional AC production by
ethylene-based Wacker oxidation [9]. Although supported no-
ble metal catalysts can achieve moderate to high AC yield
Various
Cu(NO ·3H
Co(NO ·6H
Fe(NO ·9H
hyde (99%) and ethanol (> 99.8%) were provided by Si-
salts
O,
O, Ni(NO
O, La(NO
(KMnO
Zn(NO
·6H O, Ca(NO
·6H
4
,
)
MnSO
4
·H
2
O,
Mg(NO
O, Al(NO
(NH
4
)
2
S
2
O
8
,
3
)
2
2
3
2
·6H O,
2
3
)
2
·6H
2
O,
O,
3
)
2
2
3
)
2
2
3 2
) ·4H
2
3
)
3
·9H
2
3
)
3
2
3
)
3
2
O) with > 99% purity, acetalde-
[
10–13], noble-metal-free oxide catalysts are more promising
for practical application given their lower cost and easier re-
generation [14]. MnO catalysts have been used for gas-phase
nopharm Chemical Reagent Co., Ltd. CH
was purchased from Sigma-Aldrich.
3
CH OD (99 atom% D)
2
2
aerobic oxidation of ethanol [15–22]. However, most of reports
focused on the catalytic combustion of ethanol with excessive
2.2. Preparation of catalysts
oxygen [17–22]. Only silver-doped α-MnO
catalysts derived thereof were effective for the selective etha-
nol oxidation [16,23]. We have found that the unreducible met-
2
and the supported
α-MnO
KMnO under refluxing, as describing in our previous paper
[24]. β-MnO and γ-MnO were prepared by the hydrothermal
reaction of MnSO ·H O (3.38 g) and (NH (4.56 g) in de-
2
was prepared by oxidation of MnSO
4
·H
2
O with
4
2
2
al-doped α-MnO
2
(M-OMS-2), especially Na-OMS-2, could act as
4
2
4 2 2 8
) S O
stable and efficient catalysts for the selective ethanol oxidation,
achieving a comparable AC yield (~66% at 200 °C) to the
ionized water (100 mL), which was transferred into an auto-
clave (200 mL), then heated at 140 °C for 12 h and at 90 °C for
Ag-α-MnO
α-MnO , other crystalline MnO
studied in the selective ethanol oxidation. Moreover, it is still
unknown whether the phase structure effect of MnO on the
catalytic selective oxidation is different from that on the cata-
lytic complete oxidation. Undoubtedly, clarifying these points
2
(~71% at 230 °C) [24]. However, besides the
24 h, respectively [27]. δ-MnO
2
was synthesized from the oxi-
(1.2 g) in deionized
2
2
materials have not yet been
dation of MnSO ·H O (0.22 g) by KMnO
4
2
4
water (60 mL), which was hydrothermally heated at 220 °C for
24 h [4]. The resulting solid was filtered, washed and dried at
2
2+
2+
110 °C overnight. All metal-doped M-γ-MnO
2
(M = Cu , Zn ,
Mg , Co , Ni , Ca , Al , Fe , La ) catalysts were synthesized
by using the similar procedure for the undoped γ-MnO . The
metal ion dopant with M/(Mn+M) ratio of 10 mol% was initial-
ly added with MnSO ·H O to form a homogeneous mixture,
2+
2+
2+
2+
3+
3+
3+
will provide rational guidance for designing improved MnO
2
2
catalysts for selective oxidation.
In the present work, four types (α, β, γ and δ) of MnO
lysts were initially prepared and evaluated in the gas-phase
aerobic oxidation of ethanol and CO with one equivalent of O
Interestingly, γ-MnO achieved the highest AC yield (~65%)
and CO conversion (~100%) at 200 °C, which are apparently
higher than the results obtained by the other three MnO cata-
lysts. Because the catalytic oxidation capability of MnO can be
2
cata-
4
2
followed by the above procedure. All catalysts were calcined at
300 °C in air before use.
2
.
2
2.3. Characterization of catalysts
2
2
XRD patterns were collected on an Empyrean apparatus
modulated by metal doping [3,15,24–27], there are potential
opportunities to further improve the catalytic activity of
with Cu K
α
radiation (40 mA and 40 kV). The metal ratio of
was evaluated by X-ray fluorescence spectrometry
M-γ-MnO
2
γ-MnO
2
by framework modification with metal dopants. Cata-
(XRF-1800). The surface areas were analyzed by the BET
method using a Micromeritics ASAP 2420-4MP apparatus. SEM
micrographs were taken using a FEI Nova NanoSEM 450 mi-
croscope. TEM micrographs were collected on a FEI Tecnai G2
F30 electron microscope at an acceleration voltage of 300 kV.
XPS analysis was performed with an AXIS-ULTRA DLD-600W
lytic CO oxidation can be employed as a complete oxidation to
compare with the selective ethanol oxidation. To the best of our
knowledge, there are no reports on the aerobic selective oxida-
tion of ethanol or CO oxidation by using metal-doped γ-MnO
materials [28]. In this sense, it would be interesting to clarify
the difference in the structural and component requirements of
2
spectrometer with Al K irradiation. The binding energies were
α
γ-MnO
2
for the selective ethanol oxidation and CO oxidation.
adjusted by setting the C 1s peak of adventitious carbon at
284.5 eV.
2+
2+
2+
2+
2+
Therefore, various M-γ-MnO
2
(M = Cu , Zn , Mg , Co , Ni ,
2+
3+
3+
3+
Ca , Al , Fe , La ) catalysts were prepared by a one-pot hy-
drothermal method and evaluated in the two oxidations. The
The surface acidity and basicity of M-γ-MnO
studied with the temperature-programmed desorption (TPD)
of NH and CO on a Micrometrics AutoChem 2920II instru-
2
catalysts were
results strongly evidenced that Zn-γ-MnO
molar ratio of surface base/acid sites obtained the best activity
in the CO oxidation, whereas Cu-γ-MnO with the highest sur-
face reducibility achieved the highest AC yield (~75% at 200
C) in the selective ethanol oxidation. Cu-γ-MnO can show
2
with the highest
3
2
ment, respectively. Typically, 50 mg sample was loaded in a
quartz tube and heated in Ar at 350 °C for 2 h to diminish the
2
release of O
2
during TPD. After the sample was cooled to 100°C,
in Ar with a flow rate of 50 mL/min
°
2
0.3% NH in Ar or 10% CO
3
2
higher activity than previously reported noble-metal-con-
taining catalysts at lower temperature. Their kinetic behavior
were also studied.
were passed through the sample for 0.5 h. Then pure Ar was
used to flush the sample at 100 °C for 1 h, which can remove the
physically adsorbed NH
heated from 100 to 400 °C with a ramp rate of 10°C/min and a
flow rate of 20 mL/min Ar. The NH or CO evolution was mon-
itored by a thermal conductivity detector (TCD). The amount of
desorbed NH or CO was calculated by using the peak areas
and a calibration coefficient. H -TPR experiments were con-
3
or CO . After that, the sample was
2
2.
Experimental
3
2
2.1. Materials
3
2
2

1300
Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310
ducted on the same instrument as TPD. After the sample (20
mg) was pretreated at 300 °C in Ar for 1 h and cooled to room
Table 1
Structural and textural properties of various MnO samples.
2
temperature, it was treated with 10% H
2
in Ar at a flow rate of
Crystallite
size
(nm)
Crystal
radius
(nm)
A
BET
M/(Mn+M) b
(mol%)
10 mL/min and heated to 600 °C at a ramp rate of 10 °C/min.
a
c
Sample
2
(
m /g)
The H
2
consumptions were calculated by the peak area which
was recorded by TCD.
α-MnO
β-MnO
2
62
19
55
97
57
64
51
71
58
47
59
125
56
25.2
23.1
19.3
15.1
22.8
21.4
19.7
14.9
21.1
18.8
17.7
13.8
20.6
—
—
—
—
—
—
2
2.4. Catalytic reaction
γ-MnO
2
δ-MnO
2
—
—
Gas-phase aerobic oxidation of ethanol was performed on a
fixed-bed reactor at atmospheric pressure. The detailed pro-
cedure is similar to our previous report [24]. Briefly, 60 mg
Cu-γ-MnO
Zn-γ-MnO
Mg-γ-MnO
Co-γ-MnO
Ni-γ-MnO
2
2
0.9
0.7
0.8
0.8
1.0
0.3
3.1
9.7
0.3
0.087
0.088
0.086
2
0.079 (L)/0.089 (H)d
2
catalyst (80–100 mesh) mixed with 600 mg α-Al
mesh) was pretreated with O at 300 °C for 1 h. Then the pre-
heated gas mixture with 2.5 mL/min of ethanol in the gas phase
and a molar ratio of ethanol/O /N = 1/1/38 was passed by the
catalyst bed with the gas-hourly-space-velocity (GHSV) of
about 100000 mL gcat h–1. GC-FID and GC-TCD were simulta-
neously employed to analyze the reaction products online. In
all cases, the carbon balances closed at 100 ± 2%. Other catalyst
particle sizes and weights were checked to exclude the pres-
ence of diffusion and mass transfer limitations. Blank experi-
ments were also performed to exclude the activity contribution
O
2 3
(60–80
2
0.083
0.114
2
Ca-γ-MnO
2
Al-γ-MnO
Fe-γ-MnO
La-γ-MnO
2
0.068
0.069 (L)/0.079 (H)d
2
2
2
2
0.117
–1
a
b
Estimated by Scherrer equation using the strongest XRD peak. Cal-
c
culated by the XRF results. Shannon crystal radius of the doping metal
with six-coordination [29]. For low (L) and high (H) spin species.
d
δ-MnO
2
shows a distinct nanosheet-like shape.
(M = Cu , Zn , Mg ,
Co , Ni , Ca , Al , Fe , La ) catalysts were prepared by the
same “one-pot” hydrothermal method as the undoped γ-MnO
the urchin-like nanosphere morphology was largely retained
see Fig. 1 and Fig. S2), with different lengths and widths of the
nanorods. The average nanorod size of Fe-γ-MnO is below
, resulting in that the surface area of
2
+
2+
2+
Since various metal-doped M-γ-MnO
2
by the empty quartz reactor and α-Al
Catalytic CO oxidation was carried out in a fixed-bed reactor
with stainless steel tube (i.d. = 8 mm) under atmospheric pres-
2 3
O .
2+
2+
2+
3+
3+
3+
2
,
sure. Typically, 0.1 g M-γ-MnO (80–100 mesh) diluted with 0.5
2
(
g quartz sand (60–80 mesh) were loaded into the reactor. After
the catalyst was pretreated with dry air at 300 °C for 1 h and
cooled to the room temperature, a mixture of 1 vol% CO and 1
2
one-fifth that of the γ-MnO
2
2
the former (125 m /g) is much higher than the latter (55
vol% O
2
in N
2
was introduced as the reactants at a flow rate of
2
–1
–1
m /g). The surface areas of the other M-γ-MnO
2
catalysts de-
50 mL/min. The resulting GHSV is about 30000 mL gcat h .
2
2
creased from 71 m /g for Co-γ-MnO
As seen in Table 1, all the resulting M/(Mn + M) ratios were
lower than the theoretical 10 mol%, indicating that the dopant
2
to 47 m /g for Ca-γ-MnO .
2
The reaction temperature was monitored by a thermal couple
inside the catalyst bed and raised from room temperature to
250 °C by steps of 10–30 °C. The reactor was kept at each tem-
metal ions incapable of entering the framework of γ-MnO
removed by washing with water. It is known that the substi-
tuting degree of the octahedral MnO framework depends on
the coordination and the crystal radius (CR) of dopant ions
29,30]. Only six-coordinated metal ions with similar CR sizes
2
were
perature for 15 min to obtain experimental data at steady state.
The composition of the outlet gas was analyzed by GC (Fuli GC
2
9070) with a TDX-01 packed column. The carbon balance was
near 100% in all cases. Similarly, control and blank experi-
ments were performed to exclude the presence of diffusional
limitations and background activity, respectively.
[
to those of Mn3+ (0.072 nm for low spin species, 0.078 for high
spin species) and Mn4+ (0.067 nm) can be readily incorporated
in the octahedral MnO framework [27]. In this study, various
2
3.
Results and discussion
metal dopants with different CR sizes (Table 1) allow
six-coordination, thus different degree of substitution could be
3
.1. Preparation and characterization of catalysts
3+
expected. Evidently, doping with Fe , whose CR sizes (0.069
nm for low spin species and 0.079 nm for high spin species) are
The XRD patterns of α-MnO
2
, β-MnO
2
, γ-MnO
2
and δ-MnO
2
3+
4+
more close to the Mn /Mn species, resulted in the highest
Fe/(Mn + Fe) molar ratio (9.7 mol%). In contrast, doping La3+
rendered the lowest La/(Mn + La) ratio (0.3 mol%), as La3+ has
the largest CR size (0.117 nm).
(Fig. S1) are in good agreement with those of cryptome-
lane-type (JCPDS 42–1348), pyrolusite-type (JCPDS 24–0735),
nsutite-type (JCPDS 14–0644), and birnessite-type (JCPDS
8
0–1098) MnO
surface areas of the four types of MnO
The layered δ-MnO has the smallest crystallite size (~15.1 nm)
2
, respectively. The average crystallite sizes and
The XRD patterns of all M-γ-MnO
Clearly, the nsutite structure was basically retained after metal
doping, but some peaks for the doped M-γ-MnO shifted to
lower angles. The Ca-γ-MnO with low doping content (0.3
mol%) showed similar 2θ shift as the Fe-γ-MnO , suggesting
that the presence of dopant in both the mixture solution and
2
are shown in Fig. 2.
2
are listed in Table 1.
2
2
2
and the highest surface area (97 m /g), which is consistent
with previous reports [5,27]. As shown in Fig. 1, the SEM imag-
es of α-, β- and γ-MnO
2
2
2
show nanorod-like morphologies with
widths of 10–200 nm and lengths of 200–2000 nm, whilst the

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310
1301
Fig. 1. SEM images of four types (α, β, γ and δ) MnO
2
and representative M-γ-MnO samples.
2
the final framework may render partial loss of nsutite structure
250–500 °C attributed to reduction of (i) MnO
Mn ) and (ii) Mn to MnO [2,33], with the initial reduction
starting from 125–200 °C. Evidently, except for La-γ-MnO , all
M-γ-MnO samples showed lower temperature for the first
reduction peak than the undoped γ-MnO , which may be as-
cribed to the weakening effect of the metal dopants (other than
2
to Mn
3
O
4
(via
symmetry of γ-MnO
2
and some lattice expansion [31,32]. Fur-
2
O
3
O
3 4
thermore, as shown in Table 1, the crystallize sizes of
2
M-γ-MnO
2
estimated from the Scherrer equation using the
2
o
strongest (131) peak (2θ = 37.1 ) varied by the metal dopants,
2
with Cu-γ-MnO
(
(
2
showing the biggest average crystallize size
22.8 nm). The Fe-γ-MnO showed the smallest crystallize size
13.8 nm), which is in good agreement with its highest surface
3
+
2
La ) on Mn-O bonds [26,32]. These results point to the im-
proved reducibility of M-γ-MnO materials after metal doping.
Especially for Cu-γ-MnO , the first reduction peak was shifted
2
area and the smallest nanorod observed by SEM.
The reducibility of various M-γ-MnO samples was evaluat-
ed by H -TPR (Fig. 3A). There are two main peaks between
2
2
to 285 °C with the initial reduction starting as low as 125 °C,
which can be due to the surface reduction of adsorbed oxygen
species to form oxygen vacancies without framework degrada-
tion. This result suggests that doping with Cu2+ can greatly fa-
2
cilitate the reduction of γ-MnO
of γ-MnO is markedly retarded by doping with La . As shown
in Table 2, the measured H consumptions for all M-γ-MnO
samples (7.7–10.7 mmol/g) are lower than the theoretical val-
2
. On the contrary, the reduction
3+
2
2
2
Zn
Cu
Ni
ue (11.5 mmol/g) for the complete reduction of MnO
2
to MnO,
pointing to the mixed Mn valence in the framework.
The NH
3
-TPD and CO
2
-TPD profiles of various M-γ-MnO
2
are
shown in Fig. 3B and 3C, respectively. The corresponding
amount of surface acid/base sites is listed in Table 2. Clearly,
Co
Fe
the NH
3
/CO
2
-TPD profiles of the M-γ-MnO samples varied with
2
the metal dopants. The peak at lower temperatures (~130 °C)
could be ascribed to desorption at surface Brönsted acid sites
La
Al
(
Mn-OH and M-OH groups) for NH
3
-TPD and surface weak base
, OH ) species for CO -TPD. The higher tem-
-TPD and CO -TPD curves are at-
‒
‒
sites (such as O
2
2
Ca
perature peaks in the NH
3
2
tributed to the stronger Lewis acid sites derived from the de-
fective metal atoms [32,34] and the stronger base sites derived
from lattice oxygen species (such as metal-O2‒ pairs and defec-
tive O2‒ anions) [35], respectively. Compared with the undoped
Mg

-MnO2
, doping with Fe , La , Co and Al3+ resulted in a con-
3+
3+
2+
γ-MnO
siderable increase of surface acidity and basicity, especially for
stronger acid/base sites. Fe-γ-MnO with the highest specific
2
20
30
40
Theta (degree)
50
60
2
Fig. 2. XRD patterns of various M-γ-MnO
2
samples.
2

1302
Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310
A
C
B
H -TPR
NH -TPD
CO -TPD
2
133
3
2
285
130
Zn
Zn
Cu
Ni
Zn
125
Cu
Ni
Cu
Ni
Co
Fe
La
Co
Co
Fe
Fe
La
La
Al
Al
Al
Ca
Mg
Ca
Mg
Ca
Mg

-MnO2

-MnO2
-MnO2
1
00 150 200 250 300 350 400 450 500
100
-TPR (A), NH
150
200
250
300
100
150
200
250
o
300
o
o
Temperature ( C)
Temperature ( C)
Temperature ( C)
Fig. 3. H
2
3
-TPD (B) and CO
2
-TPD (C) profiles of various M-γ-MnO samples.
2
Table 2
Chemical and catalytic properties of various MnO
2
catalysts.
Basicity c
uptake a
Acidity b
(μmol/g)
T
90
d
Conv. e
(%)
Selec. e
(%)
TOF
f
STY g
(h )
H
2
Sample
α-MnO

1
1
(
mmol/g)
(μmol/g)
396
138
472
906
495
529
422
632
391
548
634
875
643
(°C)
225
275
185
190
170
160
185
205
200
175
195
180
240
(h )
2
8.9
43
16
49
95
43
40
43
75
45
50
69
87
80
58
52
65
45
75
68
62
70
72
71
60
56
49
99.3
99.6
99.4
49.3
99.6
96.0
99.3
99.4
99.5
99.5
99.2
93.8
99.1
4.6
3.9
4.8
3.4
5.6
5.1
4.6
5.2
5.4
5.3
4.5
4.2
3.6
2.2
2.0
2.4
0.8
2.8
2.5
2.3
2.6
2.7
2.7
2.2
2.0
1.8
β-MnO
2
9.5
γ-MnO
δ-MnO
2
10.7
8.5
2
Cu-γ-MnO
2
2
9.5
Zn-γ-MnO
9.8
Mg-γ-MnO
Co-γ-MnO
2
9.6
9.7
2
Ni-γ-MnO
2
9.2
Ca-γ-MnO
2
10.1
8.3
Al-γ-MnO
2
Fe-γ-MnO
2
7.7
La-γ-MnO
2
10.3
a
-TPR. b By NH
-TPD. c By CO
-TPD. d 90 stands for the temperature at which CO conversion reaches 90%. e Ethanol conversion and
T
Estimated by H
2
3
2
acetaldehyde selectivity at 200 °C. Turnover frequency (TOF) based on ethanol conversion, and given in molethanol molMn h . g Space-time-yield (STY)
of acetaldehyde in gAC gcat h .
f
‒1 ‒1
‒1
‒1
surface area possessed the highest surface acidity (87 μmol/g)
and basicity (875 μmol/g). For all samples, the surface basic
sites are more than the acidic sites by about an order of magni-
tude. This suggests that the basic oxygen species are predomi-
nant on the surface [24], which leads to the alkaline nature of
α-MnO
maintained > 99% except for δ-MnO
tivity ~40%) and ethyl acetate (selectivity ~10%) as
by-products. Above 200 °C, CO became the main by-product
for all catalysts. This activity order is totally different from pre-
vious report on the catalytic ethanol combustion [36], in which
2
< γ-MnO
2
, and the AC selectivity of various catalysts
2
, which yielded CO (selec-
2
2
the M-γ-MnO materials.
2
the optimal catalyst was α-MnO
2
followed by δ-, γ- and β-MnO
2
.
3
.2. Effect of crystal structure of MnO
2
on the catalytic
This confirms that there are different structure effects of MnO
2
performance
on the aerobic selective oxidation and complete oxidation of
ethanol. Furthermore, a different activity order was also ob-
We initially investigated the influence of crystal structure of
MnO on the catalytic performance for the aerobic ethanol oxi-
dation and CO oxidation with one equivalent of O (Fig. 4). In-
terestingly, the activity order in ethanol oxidation varied with
reaction temperature, but even so, γ-MnO exhibited the high-
served for CO oxidation, increasing with β-MnO
2
< α-MnO
showed the
2
<
2
δ-MnO ~ γ-MnO . Clearly, γ-MnO and β-MnO
2
2
2
2
2
lowest (185 °C) and highest (~275 °C) temperature for reach-
ing 90% CO conversion, respectively. The different activity
orders in the selective ethanol oxidation and CO oxidation fur-
2
est catalytic activity in the range of 125225 °C. As listed in
Table 2, the turnover frequency (TOF) and space-time-yield
ther point to the different structural requirements of MnO
the two oxidations. It is noteworthy that α- and δ-MnO were
reported to show higher activity than γ- and β-MnO in the pre-
2
in
2
(
STY) at 200 °C increased in the order of δ-MnO
2
< β-MnO
2
<
2

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310
1303
A100
B100
MnO2

MnO2
MnO2
MnO2
8
0
80
60
40
20
0


60
40
20
0
1
00 125 150 175 200 225
100 125 150 175 200 225
o
o
Temperature ( C)
Temperature ( C)
C100
D
150
8
0
MnO2
MnO2
60
40

MnO2
20

MnO2
0
50
100 150 200 250
100
200
300
400
500
o
o
Temperature ( C)
Temperature ( C)
Fig. 4. Ethanol conversion (A), acetaldehyde selectivity (B) and CO conversion (C) as a function of reaction temperature, and H
MnO catalysts with four crystal structures.
2
-TPR profiles (D) of
2
vious report on MnO
due to the different catalyst preparation, pretreatment and
reaction conditions. Undoubtedly, γ-MnO showed the highest
activity not only in the selective ethanol oxidation but also in
the CO oxidation in this study.
2
-catalyzed CO oxidation [4], which may be
mance of MnO
2
in the oxidation of ethanol and CO. Since the
surface acid-base and redox properties of the optimal γ-MnO
2
2
catalyst can be tuned by metal-doping, there are great oppor-
tunity to further improve the catalytic performance and to clar-
ify the different structural and component requirements of
Given the moderate surface area of γ-MnO
duced that the surface area was not the main factor determin-
ing the catalytic activity of MnO catalysts. According to the
-TPR results of the four catalysts (Fig. 4D), β-MnO and
δ-MnO showed lower temperature for the first reduction peak
~300 °C), but γ-MnO exhibited the lowest starting reduction
2
, it could be de-
M-γ-MnO for the selective ethanol oxidation and CO oxidation,
2
which would provide an interesting example for the compara-
tive study on these two oxidations using the same catalyst se-
ries [38].
2
H
2
2
2
(
2
3.3. Effect of metal doping in γ-MnO on the catalytic
2
temperature (~150 °C). These results imply that the catalytic
activity in ethanol oxidation depends not on the bulk reducibil-
performance
ity but on the surface reducibility of MnO
reducibility can render more oxygen vacancies for O
at lower temperature and benefit the oxidations [24,37].
Therefore, the drastic activity increase observed by β-MnO
above 175 °C could be primarily due to the formation of more
surface oxygen vacancies during ethanol oxidation. Although
2
. The higher surface
3.3.1. CO oxidation
2
activation
The CO conversions as a function of temperature over vari-
ous M-γ-MnO catalysts are shown in Fig. 5A and 5B. Almost all
catalysts achieved ~100% CO conversion below 250 °C. Obvi-
ously, all M-γ-MnO (except for La-γ-MnO ) showed higher
activity than the undoped γ-MnO below 150 °C, plausibly due
to the presence of more surface defect sites after metal doping
as suggested by the H -TPR results (Fig. 3A). To better compare
2
2
2
2
2
δ-MnO
2
also showed lower starting reduction temperature
~175 °C), its selectivity to CO dramatically increased above
75 °C. This may be ascribed to the presence of stronger sur-
(Table 2). Evidently, the sample with
the highest surface acidity and basicity (δ-MnO ) showed much
lower activity than that with moderate surface acidity and ba-
sicity (γ-MnO ) in the ethanol oxidation, whereas the former
(
2
2
1
the activity, T90 was used to identify the temperature at which
CO conversion reaches 90% over various catalysts. As listed in
Table 2, the T90 value decreased in the order of metal dopant La
face acid sites in δ-MnO
2
2
> Co > Ni > Al > γ ~ Mg > Fe > Ca > Cu > Zn, with Zn-γ-MnO
La-γ-MnO showing the lowest (160 °C) and highest (240 °C)
90, respectively. Fig. 5C and 5D show the Arrhenius plots and
the estimated apparent activation energy (E ) between room
temperature and 100 °C. The listed E values (13.726.0
2
and
2
2
showed comparable activity to the latter in the CO oxidation.
These results imply that both the surface reducibility and the
surface acid-base property can influence the catalytic perfor-
T
a
a

1304
Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310
A
B
100
-MnO2
100
-MnO2
Mg--MnO2
Ca--MnO2
Al--MnO2
Fe--MnO2
80
80
Co--MnO2
Ni--MnO2
La--MnO2
Cu--MnO2
60
Zn--MnO2
60
4
0
40
20
0
20
0
50
100
150
200
250
50
100
150
200
250
o
o
Temperature ( C)
Temperature ( C)
C
D
-1
-1
-2
k
k
-2

-MnO (E = 19.7 kJ/mol)
2 a

-MnO (E = 19.7 kJ/mol)
2 a
Mg--MnO (E = 19.0 kJ/mol)
2
a
-3
Fe--MnO (E = 16.3 kJ/mol)
Ca--MnO (E = 15.4 kJ/mol)
2
a
2
a
Co--MnO (E = 24.5 kJ/mol)
Al--MnO (E = 20.3 kJ/mol)
2
a
2
a
Ni--MnO (E = 23.1 kJ/mol)
La--MnO (E = 26.0 kJ/mol)
2
a
2
a
Cu--MnO (E = 14.5 kJ/mol)
Zn--MnO (E = 13.7 kJ/mol)
2
a
2
a
-3
-
4
2
.7
2.8
2.9
-13.0
3.1
3.2
2.7
2.8
2.9
000/T (K )
Fig. 5. CO conversion (A, B) as a function of the reaction temperature, and Arrhenius plots (C, D) for CO oxidation over various M-γ-MnO
3.0
3.1
3.2
-1
1000/T (K )
1
2
catalysts.
kJ/mol) are in good agreement with the previously reported E
values (1285 kJ/mol) for CO oxidation over metal doped
α-MnO catalysts [3]. Interestingly, the decreasing order of the
value is very similar to that of the T90, with Zn-γ-MnO
showing the lowest E and La-γ-MnO showing the highest E
Therefore, among various M-γ-MnO , Zn-γ-MnO is the most
and CO activa-
a
can significantly decrease the surface acid sites and increase
the base sites in the γ-MnO , and thus may facilitate the CO
2
2
adsorption and activation.
E
a
2
a
2
a
.
3.3.2. Ethanol oxidation
2
2
The catalytic performances of various M-γ-MnO
the gas-phase ethanol oxidation as the function of temperature
are shown in Fig. 7. Obviously, all M-γ-MnO catalysts showed
high AC selectivity (up to 99%) below 200 °C and a dramatic
selectivity decrease above 200 °C with CO as the predominant
by-product. It is evident that both reducible metals (such as
2
catalysts in
efficient catalyst for CO oxidation and enables O
2
tion at lower temperatures.
2
To establish the structure-performance relationship in the
M-γ-MnO -catalyzed CO oxidation, we attempted to relate the
catalytic activity with the structural and chemical properties of
the catalysts. However, the optimal Zn-γ-MnO is not the cata-
2
2
2+
2+
2+
2+
2+
2
Cu , Ni , Co ) and unreducible metals (such as Ca , Zn )
lyst with the highest surface area, reducibility, acidity or basic-
ity, implying that the CO oxidation activity is not determined by
one factor but influenced by the factors comprehensively. As
doped M-γ-MnO
2
catalysts can render higher activity than the
. As shown in Table 2, the ethanol conversion
undoped γ-MnO
2
at 200 °C increased in the order of metal dopant La < Fe < Al <
Mg < γ < Zn < Co < Ca < Ni < Cu, with Cu-γ-MnO and La-γ-MnO
listed in Table 2, the Zn-γ-MnO
2
showed the lowest surface acid
2
2
sites (40 μmmol/g), a moderate surface base sites (529 μm-
mol/g) but the highest molar ratio of surface base/acid sites
achieving the highest (75%) and lowest (49%) conversion,
respectively. The maximum AC yield at 200 °C was achieved by
(
13.2). In contrast, the La-γ-MnO
2
showed the lowest surface
Cu-γ-MnO (~75%), accompanied by the highest TOF (5.6
2
molethanol molMn h‒1) and STY (2.8 gAC
g
‒
1
cat h‒1). The STY value
was compared with that of the previously re-
ported catalysts for the gas-phase selective ethanol oxidation
(see Table S1). Evidently, the Cu-γ-MnO shows higher STY of
AC than the previously reported Na-OMS-2 (2.5 gAC
‒1
base/acid sites ratio (8.0). In an attempt to correlate the ratio
of surface base/acid sites with the catalyst activity, we com-
pared the orders of the ratio of surface base/acid sites and the
of Cu-γ-MnO
2
E
a
of CO oxidation (Fig. 6). Interestingly, the E
a
values decrease
2
g
cat h‒1 at
‒
1
with increasing the molar ratio of surface base/acid sites, sug-
gesting that the less acid sites and more base sites in the
M-γ-MnO
200 °C) [24] and the noble-metal-containing Ag-α-MnO
2
(2.5
would benefit the CO oxidation. Doping with Zn2+
g
AC
g
cat h‒1 at 230 °C) [16] and Au/MgCuCr
‒1
O
2 4
(2.8 gAC
g
cat
‒1
2

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310
1305
The activity order of M-γ-MnO
2
catalysts in ethanol oxida-
tion is clearly different from that in CO oxidation, confirming
that MnO -catalyzed different oxidations have different struc-
ture-performance relationships. Although the Fe-γ-MnO pos-
sesses the highest surface area, acidity and basicity among var-
ious M-γ-MnO catalysts, it shows even lower activity than the
undoped γ-MnO . Given that the Cu-γ-MnO shows the highest
reducibility and activity, whereas the La-γ-MnO shows the
La--MnO2
2
Co--MnO2
2
5
2
Ni--MnO2
Al--MnO2
2

-MnO2
2
2
Mg--MnO2
Fe--MnO2
Ca--MnO2
Cu--MnO2
Zn--MnO2
2
20
lowest reducibility and activity, it is reasonable to propose that
the surface reducibility is more important than the other prop-
erties of M-γ-MnO
tion of ethanol. As shown in Fig. 3A, the surface reduction of
Cu-γ-MnO started as low as 125 °C, implying that more surface
adsorbed oxygen species or oxygen vacancies are present in the
catalyst. It is known that the higher MnO reducibility and more
surface oxygen vacancies would benefit the O and ethanol
activations [22,37]. Accordingly, the enhanced catalytic per-
2
catalysts in the gas-phase selective oxida-
15
2
E
2
8
10
12
14
2
Mole ratio of surface base/acid sites
2+
2+
2+
2+
2+
Fig. 6. Correlation between E
face base/acid sites of various M-γ-MnO catalysts.
a
of CO oxidation and molar ratio of sur-
formance after metal (Cu , Ni , Ca , Co , Zn ) doping could
be mainly attributed to the increased surface oxygen vacancies
2
in the doped γ-MnO
3.4. Stability of M-γ-MnO
The TEM images of the Zn-γ-MnO catalyst before and after
use in the CO oxidation are shown in Fig. S3. Clearly, the nano-
rod-like morphology remained unchanged. The recycled cata-
lyst displayed the lattice fringes of (120) planes and (131)
planes with the d spacing distances of 0.39 nm and 0.24 nm,
respectively, which are in good agreement with the XRD re-
2
.
h‒ at 250 °C) [12], confirming the enhancement effect of metal
1
2
catalysts
doping in γ-MnO
expected, the E values (40.096.7 kJ/mol), evaluated from the
Arrhenius plots in the range of 100150 °C (Fig. 7E and 7F), are
in inverse proportion to the catalytic activity of M-γ-MnO . The
Cu-γ-MnO showed the lowest E (40.0 kJ/mol), which is much
lower than the undoped γ-MnO (58.8 kJ/mol) and the
Na-OMS-2 (63.7 kJ/mol) [24], suggesting that ethanol and oxy-
2
on gas-phase selective ethanol oxidation. As
a
2
2
2
a
2
gen are more prone to activation on Cu-γ-MnO
2
at lower tem-
sults for the fresh Zn-γ-MnO sample. It is worth noting that
2
peratures.
there are many defect sites at the surface and the edge of the
A100
C
E -2
-MnO2
100
Mg--MnO2
-3
80
Ca--MnO2
-MnO2
-4
Al--MnO2
80
Mg--MnO2
Ca--MnO2
60
La--MnO2
Zn--MnO2
-5
-6
-MnO (E = 58.8 kJ/mol)
k
2
a
Al--MnO2
Mg--MnO (E = 60.5 kJ/mol)
2
a
40
60
La--MnO2
Ca--MnO (E = 48.1 kJ/mol)
-7
2
a
Zn--MnO2
Al--MnO (E = 63.5 kJ/mol)
2
a
2
0
-8
La--MnO (E = 96.7 kJ/mol)
2
a
4
0
-9
Zn--MnO (E = 56.2 kJ/mol)
2 a
0
1
00
125
150
175o 200
225
100
125
150
175o 200
225
2.4
2.5
1000/T (K )
2.6
2.7
-1
Temperature ( C)
Temperature ( C)
B100
D
100
F
-MnO2
-2
Fe--MnO2
8
0
Co--MnO2
-MnO
2
-3
-4
-5
Fe--MnO2
Ni--MnO2
80
Co--MnO2
60
Cu--MnO2
k
Ni--MnO2
-MnO (E = 58.8 kJ/mol)
2 a
40
60
Cu--MnO2
Fe--MnO (E = 65.1 kJ/mol)
2 a
Co--MnO (E = 51.8 kJ/mol)
2
a
-
6
2
0
Ni--MnO (E = 45.1 kJ/mol)
2 a
4
0
Cu--MnO (E = 40.0 kJ/mol)
2
a
-7
0
1
00
125
150
175o 200
225
100
125
150
175o 200
225
2.4
2.5
1000/T (K )
2.6
2.7
-1
Temperature ( C)
Temperature ( C)
Fig. 7. Ethanol conversion (A, B) and acetaldehyde selectivity (C, D) as a function of the reaction temperature, and Arrhenius plots (E, F) for ethanol
oxidation over various M-γ-MnO catalysts.
2

1306
Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310
A Mn 2p
B O 1s
529.2
6
41.6
42.9
44.3
6
640.3
530.7
6
Spent Zn-MnO2
Spent Zn-MnO2
Fresh Zn-MnO2
Fresh Zn-MnO2
Spent MnO2
Spent MnO2
Fresh MnO2
Fresh MnO2
655
650
645
640
536 534 532 530 528
Binding Energy (eV)
Binding Energy (eV)
D O 1s
C Mn 2p
6
40.3
529.3
6
41.6
531.0
6
42.9
5
32.8
6
44.3
Spent Cu-MnO2
Spent Cu-MnO2
Fresh Cu-MnO2
Fresh Cu-MnO2
Spent -MnO2
Spent MnO2
Fresh MnO2
Fresh MnO2
5
36 534 532 530 528
655
650
645
640
Binding Energy (eV)
Binding Energy (eV)
Fig. 8. XPS spectra of the representative M-γ-MnO
2
catalysts before and after CO oxidation (A,B) and ethanol oxidation (C,D). (A,C) Mn 2p; (B,D) O 1s.
nanorods, which would be the active sites for the CO oxidation.
To clarify the redox property of M-γ-MnO in the CO oxidation,
the oxidation states of Mn and O in the fresh and spent
Zn-γ-MnO and γ-MnO catalysts were studied by XPS (Fig. 8A
degradation during CO oxidation, and the reaction may follow
the Mars-van Krevelen (MVK) mechanism starting from CO
activation on surface lattice oxygen with the consumed lattice
2
2
2
oxygen being replenished by O
Continuous gas-phase oxidation of ethanol was carried out
at 200 °C to evaluate the stability of M-γ-MnO catalysts (Fig. 9).
2
rapidly [3,42].
and 8B). The Mn 2p3/2 spectra can be deconvoluted into four
peaks locating at ca. 640.3, 641.6, 642.9 and 644.3 eV, which
2
2+
3+
4+
can be ascribed to Mn , Mn , Mn and a satellite, respectively
24,39,40]. Due to the partial decomposition of MnO to Mn
and Mn
upon calcination at 300 °C, the fraction of Mn2+ spe-
To our delight, the four representative catalysts can keep stable
without significant decrease in activity and selectivity for 30 h
on-stream. The AC selectivity was maintained over 99% for all
[
2
O
2 3
3 4
O
2+
cies (Mn /Mn ratio) in the fresh samples reached about 10%.
After the CO oxidation, the fractions of surface Mn2+ (~ 10%),
Mn3+ (~51%) and Mn4+ (~25%) species remained almost un-
changed. In line with this, the fractions of surface lattice oxygen
catalysts. The ethanol conversion of γ-MnO
decreased slightly from 65% to 64% and from 75% to 74%,
respectively. These results suggest that Cu-γ-MnO can serve as
2
and Cu-γ-MnO
2
2
a stable and efficient catalyst for the gas-phase selective oxida-
tion of ethanol. The TEM images of the fresh and recycled
(~529.2 eV) and adsorbed oxygen (~530.7 eV) species [33,41]
kept ~ 69% and ~ 30% before and after the reaction, respec-
tively. However, the surface O/Mn ratio increased notably from
Cu-γ-MnO catalysts (see Fig. S3) point to the unchanged na-
norod-like morphology with clear lattice fringes of (120) planes
and (131) planes, but many defects occur at the surface and
2
1
.65 to 1.96 for γ-MnO
2
and from 2.07 to 2.37 for Zn-γ-MnO ,
2
pointing to the enhanced ratio of surface base/acid sites in the
recycled catalysts. The higher surface O/Mn ratio for the fresh
edge. The XPS spectra of the fresh and spent Cu-γ-MnO
γ-MnO
catalysts (Fig. 8C and 8D) verify that the surface Mn2+
fraction significantly increased from 10% to 44% for the spent
γ-MnO and from 17% to 49% for the spent Cu-γ-MnO . More-
2
and
2
Zn-γ-MnO
2
is consistent with the NH
3
/CO -TPD results and can
2
account for its much higher activity than the undoped γ-MnO
2
2
2
at lower temperatures (< 200 °C). Based on the above charac-
terization and activity results, we can conclude that surface
acidity-basicity is more important than the reducibility of
over, the fractions of Mn3+ and Mn4+ species decreased some-
what, pointing to the partial reduction of surface Mn3+ and Mn4+
species to Mn2+ during ethanol oxidation. In line with this, the
surface lattice oxygen (~529.3 eV) fraction decreased from
M-γ-MnO
2
in the CO oxidation. The M-γ-MnO catalysts can
2
maintain their defect-enriched structure without framework
70% to 64% for the spent γ-MnO and from 68% to 65% for the
2

Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310
1307
was
and GHSV on the catalytic performance of Cu-γ-MnO
studied. Fig. 10A shows the effect of O
vol% ethanol concentration and a GHSV of 100000 mL gcat
2
100
2
concentration at 2.5
‒
1
‒
1
h . Evidently, Cu-γ-MnO
2
achieved 47% ethanol conversion at
, which is much higher than the pre-
200 °C in the absence of O
2
8
0
viously reported Na-OMS-2 (< 5% at 200 °C) [24], pointing to
the higher reactivity and mobility of the lattice oxygen in the
former case. The reaction rate and AC selectivity were almost
60
not influenced by O
was fed at 125 °C. These results suggest that the number of
surface oxygen vacancies for O activation keeps about the
same level at lower reaction temperature [43]. When more
than 2.5 vol% O was fed at 200 °C, the ethanol conversion in-
2
concentrations (1.2512.5 vol%) when O
2
2
Ethanol conversion
40

-MnO2
2
creased steadily, but the AC selectivity decreased drastically.
This can be explained by the presence of more Mn2+ defect sites
and oxygen vacancies at 200 °C, which would enhance the reac-
tivity of surface oxygen species and lead to complete oxidation
Cu--MnO2
La--MnO2
Zn--MnO2
20
AC selectivity
of ethanol to CO
The effect of ethanol concentration on the catalytic perfor-
mance of Cu-γ-MnO at 2.5 vol% O concentration and a GHSV
.
2
0
0
5
10
15
20
25
30
Time on stream (h)
2
2
of 100000 mL gcat h‒1 is shown in Fig. 10B. Clearly, the etha-
nol conversion decreased steadily from 18% to 6% at 125 °C
and from 90% to 52% at 200°C with the ethanol concentration
increasing from 1 vol% to 5 vol%, whilst the AC selectivity kept
stable (up to 99%). The reaction rates were nearly independent
of ethanol concentration at 125 °C. This kinetic behavior sug-
gest that the active sites at the catalyst surface can be almost
saturated with ethanol-derived intermediates (e.g., C H O-) at
‒
1
Fig. 9. Ethanol conversion and acetaldehyde selectivity as a function of
time on-stream using representative M-γ-MnO catalysts at 200 °C.
2
spent Cu-γ-MnO
cies (~531.0 eV) increased from 24% to 28% for the spent
γ-MnO and from 25% to 30% for the spent Cu-γ-MnO . The
more Mn2+ species and oxygen vacancies observed in the spent
Cu-γ-MnO is in good agreement with its higher surface reduci-
bility. Moreover, these observations confirm that increasing the
defective Mn2+ sites and oxygen vacancies in the γ-MnO
2
, whilst the fraction of surface oxygen vacan-
2
2
2
5
2
lower ethanol concentration (~2 vol%). Fig. 10C shows the
effect of GHSV on the catalytic behavior of the Cu-γ-MnO cata-
2
2
lyst at 200 °C. The GHSV was changed by the amount of cata-
framework could enhance the catalytic oxidation activity plau-
sibly by promoting the reactivity and mobility of active oxygen
species [24,37].
lyst, while keeping ethanol (2.5 vol%) and O (2.5 vol%) and
2
the total flow rate (100 mL h‒1). With the GHSV increasing from
50000 to 300000 mL gcat‒1 h‒1, the ethanol conversion de-
creased from 92% to 48%. Interestingly, the STY increased
with GHSV and achieved up to 5.4 gAC gcat‒1 h‒1, which is much
higher than that obtained by Na-OMS-2 (4.7 gAC gcat‒1 h‒1) un-
3
.5. Reaction kinetics for selective ethanol oxidation over
Cu-γ-MnO
2
der similar conditions [24] and is unequivocally the highest
value for the gas-phase selective ethanol oxidation over no-
ble-metal-free catalysts at 200 °C [14,44] . It needs to point out
that further optimization of the reaction conditions can im-
To further understand the reaction kinetics and mechanism
for the selective ethanol oxidation over M-γ-MnO , the influ-
ence of reaction conditions including reactant concentration
2
A
B
C100
1
00
100
80
60
40
20
0
6
4
2
0
1
0
0
0
.0
.8
.6
.4
1.0
0.8
0.6
0.4
0.2
0.0
8
6
4
2
0
0
0
0
0
80
60
40
20
0
AC selectivity
AC selectivity
o
o
125 C
125 C
o
o
2
00 C
200 C
Ethanol conversion
AC selectivity
Ethanol conversion
0.2
Ethanol conversion
0.0
0
2
4
6
8
10 12
1
2
3
4
5
50 100 150 3 200 250 300
-1
-1
O concentration (vol%)
Ethanol concentration (vol%)
GHSV ( 10 mL gcat h )
2
Fig. 10. Effect of O
2
concentration (A), ethanol concentration (B) and GHSV (C) on the catalytic performance of Cu-γ-MnO .
2

1308
Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310
prove the STY value to higher level.
Zn-γ-MnO shows the highest catalytic activity, which has been
2
As we proposed previously [12,24,45], the initial steps of
ethanol oxidation over reducible metal oxides may involve both
associated with its highest molar ratio of surface base/acid
sites. The stable oxidation state of surface Mn and oxygen spe-
the ethanol activation on surface lattice oxygen and the O
2
ac-
cies suggests that CO oxidation over M-γ-MnO
tice oxygen-based Mars-van Krevelen mechanism. In contrast,
Cu-γ-MnO shows the best catalytic activity and high stability in
2
follows the lat-
tivation on the reduced metal (e.g. Cu , Mn2+/3+)-derived oxygen
+
vacancies. Superoxide-type species (O
2
‒) is thought to serve as
2
the activated oxygen species, which can dissociate the hydroxyl
group of ethanol by a endothermic process [46,47]. To verify
selective ethanol oxidation, which have been attributed to its
highest surface reducibility and the presence of more
‒
2+
3+
the possible OH activation by O
2
species, ethanol molecule
OD) was employed to
Mn /Mn defects and oxygen vacancies for O
activation at lower temperature. The different struc-
ture-performance relationships for M-γ-MnO -catalyzed selec-
tive ethanol oxidation and CO oxidation would provide useful
2
and ethanol
deuterated at the hydroxyl group (C
2 5
H
probe the kinetic relevance of the elementary step involving
OH bond cleavage to form ethoxide during ethanol oxidation
2
on Cu-γ-MnO
undeuterated and deuterated ethanol (KIE) for C
C was measured to be 1.58, which confirms that the OH bond
activation is a kinetically relevant step for ethanol oxidation
over Cu-γ-MnO . This result is opposite to the literatures on
aerobic oxidation of ethanol over VO /Al [43] and
MoO /TiO [48], in which the OH bond cleavage is not kinet-
ically relevant but quasi-equilibrated. The normal KIE value
obtained by Cu-γ-MnO is comparable to that observed by
Na-OMS-2-catalyzed ethanol oxidation [24]. Therefore, the
reaction process of ethanol oxidation over Cu-γ-MnO not only
follows the MVK mechanism [19] but also involves the
‒-mediated OH and α-CH activations. Finally, we attribute
the higher activity of Cu-γ-MnO than Na-OMS-2 and other
metal doped γ-MnO catalysts in the gas-phase selective etha-
nol oxidation to the higher reducibility, which results in more
2
. The ratio of oxidative dehydrogenation rates for
guidelines for the design of improved MnO
selective gas-phase oxidation.
2
-based catalysts for
2 5
H OD at 125
°
Acknowledgments
2
x
O
2 3
The authors thank the Analytical and Testing Center of
Huazhong University of Science and Technology, and the Key
Laboratory of Material Chemistry for Energy Conversion and
Storage (Ministry of Education) for use of the facilities.
2
2
2
2
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Chin. J. Catal., 2020, 41: 1298–1310 doi: 10.1016/S1872-2067(20)63551-3
Elucidating structure-performance correlations in gas-phase
selective ethanol oxidation and CO oxidation over
metal-doped γ-MnO
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Panpan Wang, Jiahao Duan, Jie Wang, Fuming Mei, Peng Liu *
Huazhong University of Science and Technology
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[
金属掺杂γ-MnO 在乙醇气相选择氧化和CO氧化中的构效关系
2
*
王盼盼, 段嘉豪, 王 杰, 梅付名, 刘 鹏
华中科技大学化学与化工学院, 能量转换与存储材料化学教育部重点实验室,
材料化学与服役失效湖北省重点实验室, 湖北武汉430074
4
+
3+
2+
2
摘要: 二氧化锰(MnO )因具有制备简单、Mn /Mn /Mn 混合的金属价态、可调变的晶型结构(如α, β, γ, δ等)、丰富的表面
氧缺陷和活泼的晶格氧等特点, 在多相催化领域被广泛用作需氧氧化催化剂. MnO 催化气相氧化主要集中在催化燃烧或
2
完全氧化, 催化性能最佳的MnO
结构丰富、价廉易得的催化剂在气相选择氧化制高附加值化学品中面临着机遇和挑战. 乙醇气相选择氧化制乙醛符合绿
色和可持续性化学工业的发展需求, 被认为是替代传统高污染、高成本的乙烯瓦克氧化工艺的最佳选择. MnO 在催化乙醇
燃烧中应用广泛, 不同晶型MnO 中α-MnO 的活性最高. 我们已经报道了非变价金属离子掺杂α-MnO (M-OMS-2), 特别是
具有最强表面碱性和可还原性的Na-OMS-2在乙醇气相选择氧化中获得了高达66%的乙醛产率和与贵金属催化剂相接近
的催化效率. 然而, 不同晶型MnO 对乙醇气相选择氧化催化性能的影响, 以及选择氧化和完全氧化所需的MnO 晶型和结
构性质是否一致尚不清楚. 这些问题的阐明无疑能够指导更高性能MnO 选择氧化催化剂的设计合成.
本文首先制备了四种晶型(α, β, γ, δ)MnO , 并将其应用于气相乙醇选择氧化和CO氧化, 惊喜地发现γ-MnO
应中均表现出最高的催化活性, 这与之前报道的乙醇催化燃烧和CO氧化结果明显不同, 证明了MnO 的晶型结构和反应条
2 2
晶型因反应底物不同会有所差异. 由于其氧化催化活性较高、选择性难于控制, MnO 这类
2
2
2
2
2
2
2
2
2
在这两类反
2
2
件对其催化氧化性能有重要影响. 为了深入研究γ-MnO 在乙醇气相选择氧化和CO氧化中的构效关系, 通过一锅水热法制
2
+
2+
2+
2+
2+
2+
3+
3+
3+
备了金属掺杂的M-γ-MnO
2
(M = Cu , Zn , Mg , Co , Ni , Ca , Al , Fe , La ). 采用多种表征手段证明了金属掺杂能够
有效地调变γ-MnO 的结构组成、表面酸碱性和可还原性. 制得的M-γ-MnO 催化剂在CO和乙醇氧化中显示出不同的活性顺
2
2
2
序, 发现CO氧化中催化剂的酸碱性更加重要. 具有最高表面碱性/酸性位点摩尔比的Zn-γ-MnO 在CO氧化中活性最佳, 其

1310
Panpan Wang et al. / Chinese Journal of Catalysis 41 (2020) 1298–1310
表面锰物种和氧物种的氧化态在反应前后基本不变, 表明M-γ-MnO
2
上的CO氧化遵循基于晶格氧的Mars-van Krevelen机
理. 与之不同的是, 催化剂的可还原性是影响乙醇气相选择氧化性能的更主要因素. 具有最高表面可还原性的Cu-γ-MnO
2
2
+
3+
于200 °C获得了最高75%的乙醛产率和较好的催化稳定性, 这归因于其在较低温度下存在更多的Mn /Mn 缺陷位和氧空
位, 这有利于O 和乙醇的低温活化. 进一步的动力学研究表明, O 和乙醇浓度对反应速率影响不大, 而空速的增大能够大
2
2
幅度提高乙醛的时空收率. 同位素效应证明醇羟基断裂仍然是反应的速控步骤, 因此表面晶格氧和吸附氧物种均参与了
乙醇活化.
关键词: 二氧化锰; 金属掺杂; 乙醇氧化; 乙醛; CO催化氧化
收稿日期: 2019-12-03. 接受日期: 2019-12-25. 出版日期: 2020-08-05.
*
通讯联系人. 电话: (027)87543032; 传真: (027)87543632; 电子信箱: pengliu@hust.edu.cn
基金来源: 国家自然科学基金(21673088, 21972050).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).