Role of oxygen vacancies and Mn4+/Mn3+ ratio in oxidation and dry reforming over cobalt-manganese spinel oxides

Article abstract of DOI:10.1016/j.mcat.2019.110704

Spinel type cobalt-manganese oxides Mn_xCo_3-xO₄ (x = 0; 0.05; 0.10; 0.15) were prepared by co-precipitation method, from cobalt and manganese salts in the presence of ammonium carbonate. X-ray diffraction (XRD), texture measurements (BET/BJH), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for their characterization. Their catalytic behavior in ethanol combustion, dry reforming with CO₂ and dehydrogenation was investigated. The results showed that the cubic structure of Co₃O₄, corresponding to spinel network, is not disturbed by adding manganese. Results obtained revealed that Mn_0.15Co_2.85O₄ catalyst, with the highest content of Mn⁴⁺ cations in the octahedral sites and oxygen defects, display superior catalytic activity compared to the others. The apparent activation energies, calculated from Arrhenius plots, are as follows: E_a combustion < E_a non-oxidative dehydrogenation < E_a dry reforming, showing that cobalt-manganese spinel type oxides are good candidates for combustion.

Full text of DOI:10.1016/j.mcat.2019.110704

MolecularCatalysisxxx(xxxx)xxxx
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Role of oxygen vacancies and Mn4+/Mn3+ ratio in oxidation and dry
reforming over cobalt-manganese spinel oxides
Gheorghiţa Mitrana,*, Shaojiang Chenb, Dong-Kyun Seob,*
a Laboratory of Chemical Technology and Catalysis, Department of Organic Chemistry, Biochemistry & Catalysis, University of Bucharest, 4-12, Blv. Regina Elisabeta,
030018, Bucharest, Romania
b School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287-1604, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
Spinel type cobalt-manganese oxides MnxCo3-xO4 (x = 0; 0.05; 0.10; 0.15) were prepared by co-precipitation
method, from cobalt and manganese salts in the presence of ammonium carbonate. X-ray diffraction (XRD),
texture measurements (BET/BJH), Fourier transform infrared spectroscopy (FT-IR), Scanning electron micro-
scopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for their characterization. Their catalytic
behavior in ethanol combustion, dry reforming with CO2 and dehydrogenation was investigated. The results
showed that the cubic structure of Co3O4, corresponding to spinel network, is not disturbed by adding man-
ganese. Results obtained revealed that Mn0.15Co2.85O4 catalyst, with the highest content of Mn4+ cations in the
octahedral sites and oxygen defects, display superior catalytic activity compared to the others. The apparent
activation energies, calculated from Arrhenius plots, are as follows: Ea combustion < Ea non-oxidative
dehydrogenation < Ea dry reforming, showing that cobalt-manganese spinel type oxides are good candidates
for combustion.
Co-Mn spinel oxides
Ethanol combustion
Ethanol dry reforming
Ethanol dehydrogenation
1. Introduction
The catalysts used commonly for C1-C4 alcohols combustion are
based on noble metals such as Pt, Pd, Rh, Ir supported on TiO2 [4],
The use of advanced biofuels such as alcohols is advantageous,
provided that volatile compounds like aldehydes, with harmful effects
to human health, do not release by their combustion. The alcohols have
been intensively studied as biofuels. Among them, methanol and
ethanol were the most used as alternative fuels for internal combustion
engines. Currently, the dominant liquid fuel is ethanol, which having
the advantage to be obtained from renewable resources is used espe-
cially as fuel for transportation [1]. Ethanol, unlike to gasoline, is an
oxygenated fuel by whose combustion no nitrogen oxides are emitted.
Another fuel obtained from biomass is isopropanol, which is more so-
luble in diesel fuel than ethanol, making its use being more sustainable
alternative [2].
Generally, the addition of oxygenates compounds such as C1-C4 al-
cohols in gasoline prevent the ignition, increasing the octane number
and also decreasing the toxicity of the exhaust gases [3]. According to
the European Standard EN 228:2000 for the addition of alcohols in
diesel fuel, the composition corresponds to 3% methanol, 5% ethanol
and 10% isopropanol and isobutanol. Moreover, from their combustion,
volatile compounds such as aldehydes and ketones, with harmful effects
to human health, should not be released into the atmosphere.
bimetallic compounds Pt–Rh, Rh–Au, Rh–Pd, and Pt–Pd over CeO2 [5],
Rh-Ce [6], Pt, Rh, Ag oxides [7].
The noble metal based catalysts have a number of disadvantages
including their high price, are partially deactivated as a result of coke
deposition, and additionally, they are more active for dehydrogenation
despite combustion.
Therefore, their replacement by mixed oxides, mainly perovskites
type has been investigated. The advantages of these catalysts are re-
sistance to poisoning and the low price. It is known that, the catalytic
performances of these oxides are directly correlated with their surface
area, the pore structure, the nature of oxygen species present on the
surface and their reducibility [8]. These factors are remarkably influ-
enced by the preparation method. Synthesis of ordered porous structure
especially nanoporous one, lead to increased surface area. Another
important factor to take into consideration in the combustion reaction
is the catalyst stability [9], which has been studied to a lesser extent
than the catalytic activity.
Catalysts based on Co3O4 and Mn3O4 on different supports have
been intensively studied for the combustion of organic volatile com-
pounds [10–14]. Cobalt oxides display higher oxidation potential
⁎ Corresponding authors.
E-mail addresses: geta.mitran@chimie.unibuc.ro (G. Mitran), DSeo@asu.edu (D.-K. Seo).
https://doi.org/10.1016/j.mcat.2019.110704
Received 22 September 2019; Received in revised form 28 October 2019; Accepted 28 October 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:GheorghiţaMitran,ShaojiangChenandDong-KyunSeo,MolecularCatalysis,
https://doi.org/10.1016/j.mcat.2019.110704

G. Mitran, et al.
MolecularCatalysisxxx(xxxx)xxxx
Fig. 1. N2 adsorption–desorption isotherms and pore size distribution of MnxCo3-xO4 (x = 0, 0.05, 0.1, 0.15).
Table 1
Textural properties of Co-Mn oxides.
a
Sample
SBET
Vmicro
Vmeso
dmeso
(nm)
Cell parameter (a) (Å)
(m2/g)
(cm3/g)
(cm3/g)
Co3O4
21
27
37
47
0.0004
0.0005
0.0009
0.0007
0.09
0.20
0.22
0.23
–
8.06
8.08
8.06
8.07
Mn0.05Co2.95O4
Mn0.1Co2.9O4
Mn0.15Co2.85O4
34.5
30.3
20.5
a
BJH Desorption average pore diameter.
relative to manganese oxides, but manganese has the advantage to
transfer more electrons [15]. The reaction mechanism and kinetic
parameters for catalytic VOC oxidation are currently intensively stu-
died; a general conclusion regarding combustion over cobalt and
manganese oxides has not been established.
Ethanol dry reforming with CO2 is an interesting task from socio-
environmental considerations, related to the reduction of greenhouse
gases emissions. This reaction has many advantages such as the feed-
stock is renewable, biodegradable and easily transportable and in ad-
dition, the reaction is thermodynamically favorable at relatively lower
temperatures (∼ 300 °C) [16,17].
Fig. 3. FT-IR spectra of MnxCo3-xO4 samples.
The studied reactions are:
Dry reforming: C2H5OH + CO2 → 3CO + 3H2 ΔH298 = 297 kJ/mol
Dehydrogenation: C2H5OH → CH3CHO + H2 ΔH298 = 71 kJ/mol
Combustion: C2H5OH + 3O2 → 2CO2 + 2H2O ΔH298 = −1271 kJ/mol
(2)
(3)
(1)
Fig. 2. (a) XRD patterns of MnxCo3-xO4 spinel oxides, (b) the variation of crystallite sizes and elastic strain with respect to manganese atomic composition.
2

G. Mitran, et al.
MolecularCatalysisxxx(xxxx)xxxx
2. Experimental
2.1. Catalysts preparation
Cobalt-manganese mixed oxides, with formula MnxCo3-xO4 (x = 0,
0.05, 0.1, 0.15) were synthesized by co-precipitation method. In this
scope, (CH3COO)2Co·4H2O and (CH3COO)2Mn·4H2O were dissolved in
water and homogenized by vigorous stirring for 30 min. Then, an excess
of ammonium carbonate, in aqueous solution was added in drop over
the previous solution, under stirring. The obtained pink precipitate was
filtered, thoroughly washed with distilled water, dried at 60 °C for 24 h
and calcined at 500 °C (5 °C/min) for 6 h.
2.2. Catalysts characterization
The crystalline phases present in the samples were determined with
a PANalyticalX’Pert Pro MRD diffractometer using a Cu Kα radiation
(λ = 1.78897). Data recording was made in the 2θ range of 5–80° with
a step size of 0.0251°.
The nitrogen adsorption-desorption isotherms, measured by
a
Micromeritics ASAP 2020, were used for the specific surface area cal-
culation using Brunauer–Emmett–Teller (BET) mode. The total pore
volume, the micropore and mesopores volume were calculated by a
BJH (Barrett–Joyner–Halenda) method. The samples were degassed
under vacuum at 200 °C for 8 h prior to measurements.
Scanning electron microscopy, using an XL-30 Environmental SEM,
was used for morphological information about samples.
The X-ray photoelectron (XPS) measurements were obtained using a
VG-220IXL spectrometer with a monochromated Al Kα radiation
(1486.6 eV, line width 0.8 eV) and energy resolution of 0.4 eV. The
binding energy of C(1 s) at 284.8 eV was used as a reference. Spectra
were deconvoluted using a CASAXPS software with the accuracy of
0.2 eV using the Shirley background.
Information about metal–oxygen bonds were obtained from Fourier
transform infrared (FT-IR) using a Bruker IFS 66 V/S spectrometer,
equipped with a diamond attenuated total reflectance (ATR) and
spectra were recorded in the range of 400–4000 cm−1 and a resolution
of 4 cm−1
.
2.3. Catalytic activity test
The catalytic activity studies were carried out in a continuous-flow
fixed bed reactor (8 mm i.d.) at atmospheric pressure. Temperature in
the catalyst bed was monitored by electronic control. Ethanol vapors
were generated using air as a carrier gas passed through a saturator,
with a total flow rate of 50 ml/min at the inlet of the reactor. The
catalyst amount was 0.2 g, the ethanol concentration 145 ppm and the
temperature varied between 100–300 °C.
Ethanol dry reforming was perform at a CO2 flow rate of 50 ml/min
and ethanol flow rate of 0.15 ml/min (molar ratio 1:1), while for the
ethanol dehydrogenation a flow of 50 ml/min N2 was passed through a
saturator with ethanol.
Fig. 4. SEM images of MnxCo3-xO4 samples.
Table 2
The EDX composition of MnxCo3-xO4 spinel oxides.
Catalyst
Wt (%)
Co
At (%)
Co
Mn
O
Mn
O
The reactant and the reaction products (after a stabilization time of
30 min) were analyzed using a GC K072320 Thermo-Quest gas chro-
matograph equipped with a FID detector. CO2 analysis was performed
using a Thermo Finnigan Gas-Chromatograph equipped with a thermal
conductivity detector (TCD) and an alumina column. The ethanol
conversion was calculated as the percentage of the ethanol which re-
acted, after reaching the stationary regime, by the following relation:
Co3O4
73.14
71.66
70.81
69.72
–
26.86
27.09
26.35
26.88
42.51
41.47
41.43
40.45
–
57.49
57.75
56.79
57.43
Mn0.05Co2.95O4
Mn0.1Co2.9O4
Mn0.15Co2.85O4
1.26
2.83
3.4
0.78
1.78
2.12
The aim of the present work is to investigate the catalytic com-
bustion of ethanol, dry reforming with CO2 and dehydrogenation, over
mixed oxides based on cobalt and manganese and to study the influence
of Mn/Co ratio over the structure and catalytic activity.
X(%) = [(Ai − Af)/Ai] × 100
where: ‘Ai’ and respectively ‘Af’ represent the ethanol chromato-
graphic peaks areas before and after catalytic reaction.
3

G. Mitran, et al.
MolecularCatalysisxxx(xxxx)xxxx
Fig. 5. XPS spectra of MnxCo3-xO4 samples (a-Co 2p, b- Mn 2p, c-O 1s).
Table 3
The surface composition from XPS.
Catalyst
At (%)
Co
Co ratio
Mn ratio
O (%)
OL1
Mn/Co
Mn
O
Co3+/Co2+
Mn3+/Mn4+
OL2
OV
EDX
XPS
Co3O4
43.9
42
43.1
41.3
–
56.1
56.9
55.2
55.1
0.92
1.44
4.29
1.40
–
52.5
53.4
51.3
46.4
–
47.5
43.9
44.6
44.7
0
0
Mn0.05Co2.95O4
Mn0.1Co2.9O4
Mn0.15Co2.85O4
1.1
1.7
3.6
1.27
0.78
0
2.7
4.1
8.9
0.02
0.04
0.05
0.03
0.04
0.09
3. Results and discussion
P/P0 = 0.9 and this sample has a low porosity with a very broad pore
size distribution.
3.1. Characterization of catalysts
The specific surface area for Co3O4 is 21 m2/g. The specific surface
area is increased at 27 m2 /g, 37 m2 /g and respectively 47 m2 /g by
manganese adding, the increase being directly proportional to its
quantity.
N2 adsorption-desorption isotherms of Co-Mn spinel type oxides,
shown in Fig. 1, were plotted in order to determine the specific surface
area and the pore distribution. All the samples with manganese ex-
hibited type-IV isotherms, characteristic for mesoporous materials. The
capillary condensation takes place at high relative pressure, respec-
tively 0.75 - 0.8, corresponding to multilayer adsorption and capillary
condensation in mesopores. The average pore size decreases with the
manganese content increasing, from 34.5 to 20.5 nm (Table 1) and the
size distribution is broad (Fig. 1). Co3O4 sample reveals type II isotherm
with H3 hysteresis loop, characteristic of macroporous materials. The
volume of adsorbed nitrogen over this sample is much lower than the
volume adsorbed over the others, the capillary condensation occurs at
The XRD patterns of synthesized Co3O4 and MnxCo3-xO4 powders
are depicted in Fig. 2a. All lines observed in figure are indexed to a
cubic structure of Co3O4. No characteristic lines of CoO, Co2O3 and,
respectively, manganese oxides are present, which indicates a very
good incorporation of manganese into the spinel network of Co3O4. The
reflections at 2θ = 22, 36.6, 43.2, 45, 52.6, 65.7, 70.3 and 77.6° are
attributed to the typical diffraction lines, corresponding to a face-cen-
tered-cubic phase of Co3O4 spinel [18]. These were attributed to the
diffraction of (111), (220), (311), (222), (400), (422), (511), and (440)
crystal planes. The value of cell parameter “a” (Table 1) does not
4

G. Mitran, et al.
MolecularCatalysisxxx(xxxx)xxxx
Fig. 6. Catalytic performances in ethanol combustion over MnxCo3-xO4 systems (a) and Arrhenius plots (b) (145 ppm of ethanol, 0.2 g catalyst, total flow rate 50 mL/
min).
the cations located in octahedral position [20]. The last peak, located at
515 cm−1, is attributed to Mn3+(4+)-O bond in octahedral position,
whose intensity increased with the increase of manganese amount,
therefore, it can be concluded that Mn cations are predominantly
placed in octahedral positions.
The FT-IR spectra are in good agreement with XRD patterns which
confirm the presence of a pure spinel structure.
The morphology of all samples is shown in Fig. 4, and EDX com-
position in Table 2. For manganese modified samples, an agglomeration
of particles can be seen clearly, and also, the morphology of micro-
sphere particles is very uniform, with a very narrow distribution, re-
vealing flake-like spherical structures. From EDX composition has been
evidenced that, only for the last sample, the atomic ratio between Co
and Mn is equal with the experimental one. For other samples the Co/
Mn ratio is slightly lower than for the last sample. In addition, the Co/O
ratio is very close to the experimental one.
The XPS spectra of all samples are shown in Fig. 5, while the XPS
composition compared with EDX is shown in Table 3. All the binding
energies (BEs) were corrected using C1 s as internal reference (set at
284.8 eV). The Co 2p3/2 spectrum presents two spin–orbit doublets,
characteristic to Co2+ (781.2–781.9 eV) and Co3+ (779.9–781.3 eV)
and two shakeup satellites (785.7–789.7 eV) [15,21]. The characteristic
peak of Co3+ is shifted to higher values of energy in the presence of
manganese, revealing that cobalt ions share the octahedral positions
Fig. 7. Correlation between ethanol conversion (300 °C), lattice strain and
Mn4+ content.
with manganese ions, leading to an asymmetry in the bond arrange-
ment and an extra pressure on the Co3+ ions. The Mn 2p3/2 spectrum
presents a peak located at 641.6 eV characteristic to Mn3+ [22], that
has been observed only in the samples with 0.05 and 0.1 Mn, and a
peak, in the range of 643.4–643.5 eV, corresponding to Mn4+ (in all the
samples). The spectrum of O1 s could be fitted into two peaks, at
change by addition of manganese, into the spinel structure of cobalt
oxide.
The average crystallite size, calculated using Scherrer equation, has
value of 19 nm (Co3O4), 18 nm (Mn0.05Co2.95O4), 17 nm (Mn0.1Co2.9O4)
and 15 nm (Mn0.15Co2.85O4), respectively.
529.1–531.3 eV, attributed to lattice oxygen (OL1
)
and at
530.4–532.4 eV, characteristic for oxygen-deficient regions as under-
coordinated lattice oxygen, O−, (OV) [23]. In the samples with man-
ganese, an additional peak at 529.3–530.9 eV, attributed to manganese
oxide lattice oxygen (OL2), is evidenced and its area increased at in-
creasing in manganese loading. The interaction between Co and Mn
leads to a change in coordination environment of oxygen and as a re-
sult, the O1 s peaks are shifted toward higher binding energy at in-
creasing in manganese loading. The presence of surface oxygen va-
cancies plays a very important role in heterogeneous catalysis, being
the active oxygen species, essential in redox reactions [24]. The atomic
composition of elements in the subsurface layer, the ratio between Co
and Mn ions and atomic ratio from XPS and EDX are listed in Table 3. In
the samples with 0.05 and 0.1 Mn the atomic ratio Mn/Co is close to the
For the purpose of crystal lattice deformations evaluation, the
elastic strain of all catalysts has been calculated with the formula
E = β/2·cos θ [19], where β is the full-width at half maximum (FWHM),
and θ is the diffraction angle, and the results compared with the crys-
tallite size are shown in Fig. 2b. The elastic strain is directly propor-
tional to amount of manganese and inversely proportional to crystallite
size. The replacement of cobalt ions with manganese ions leads to
network defects such as micro-straining in the lattice.
Fig. 3 displays the FT-IR spectra of MnxCo3-xO4 samples. Three
distinctive peaks are observed. The first, at 657 cm−1, that could be
attributed to the stretching vibration of Co2+-O bond, with the cations
placed in the tetrahedral position. The second peak, located at
553 cm−1, is attributed to the stretching vibration of Co3+-O bond, for
5

G. Mitran, et al.
MolecularCatalysisxxx(xxxx)xxxx
Fig. 8. Effect of temperature on C2H5OH, CO2 conversions and products composition (a), Arrhenius plot for ethanol dry reforming (b) on Mn0.15Co2.85O4 catalyst
(CO2 flow rate 50 ml/min, ethanol 0.15 ml/min, molar ratio 1:1, 0.2 g catalyst).
Fig. 9. Effect of temperature on ethanol conversion and products selectivities (a), Arrhenius plot of ethanol non-oxidative dehydrogenation (b) over Mn0.15Co2.85O4
catalyst (145 ppm of ethanol, 0.2 g catalyst, total flow rate 50 mL/min).
stoichiometric value, whereas for the last sample the manganese oxides
are concentered in the subsurface layer.
and Co3O4 sample. The temperature of T50 (for a conversion of 50%)
was selected as a parameter to evaluate the activity toward the ethanol
oxidation. According to this parameter, the activities in ethanol com-
bustion
are
as
follows:
Co3O4
(218 °C) < Mn0.05Co2.95O4
3.2. Catalytic activity
(193 °C) < Mn0.1Co2.9O4 (182 °C) < Mn0.15Co2.85O4 (167 °C).
At the increasing of Mn/Co ratio, an increase in Mn4+ content on
the surface (XPS), in line with activity, has been observed. The presence
of Mn4+ cations and oxygen vacancies is important since they act as
active sites for ethanol combustion. The alcohol molecule is adsorbed
on Mn4+ sites that are more easily reduced to Mn3+ than Co3+ could be
reduced to Co2+. At replacement of Co3+ cations in the octahedral
position with Mn4+ cations, the oxygen contribute to the charge com-
pensation (by reducing of Mn4+ to Mn3+), leading to emergence of the
O2− vacancy that is subsequently filled with O2 from air [25]. The
placement of Mn3+ and Mn4+ cations in octahedral positions, as seen
in the FT-IR spectra, provide more active catalysts, knowing that the
cations in octahedral positions are more reactive than those in tetra-
hedral positions.
The apparition of oxygen vacancies, evidenced by increase of the
lattice strain (XRD) with manganese content, is determined by the
oxygen release. There is a good correlation between ethanol conversion,
Mn4+/ Mn3+ ratio and lattice strain, as can be seen in Fig. 7. The
presence of Mn4+ cations on the surface, at the same time with the
presence of lattice defects, represents the essential factors for ethanol
3.2.1. Ethanol combustion
Cobalt-manganese oxides are nonstoichiometric oxides whose oxi-
dation state changes almost continuously. Lattice oxygen may reduce
the oxidation state of Mn and Co cations (from Co3+ to Co2+ and from
Mn4+(3+)to Mn2+) and leads to apparition of oxygen vacancies, which
could be the active sites for oxidation reactions. Oxygen species, ex-
tracted from the lattice, also could act as oxidizing agents.
We discussed the potential applications of these catalysts in in-
dustry, taking into account their catalytic behavior in the combustion of
volatile compounds. In this regard, the catalytic performances of cobalt-
manganese oxides, with different Mn/Co molar ratios, were evaluated
in ethanol combustion; the results are shown in Fig. 6(a). The curves of
conversion as a function of reaction temperature were used for the
catalytic activity assessment. The main products of reaction are CO2
and H2O. The selectivity to acetaldehyde not exceeds 5%; therefore the
catalytic activity was assessed by conversion to CO2.
At the same time, we also evaluated the effects of Mn doping on the
Co3O4 activity. The catalyst with higher manganese content presents
better catalytic activity than catalysts with lower manganese content
6

G. Mitran, et al.
MolecularCatalysisxxx(xxxx)xxxx
The hydrogen and carbon monoxide composition increased with
increasing the reaction temperature. A similar behavior has been ob-
served by Bahari [29,30] in ethanol dry reforming over Ce-promoted
Ni/Al2O3 and over La-Ni/ Al2O3 catalysts. The H2/CO ratio is very close
to the stoichiometric value of 1:1, the highest H2/CO ratio (1.18) has
been observed at 250 °C, probably due to the parallel reaction as
ethanol dehydrogenation [31].
The apparent activation energy, from Arrhenius plot (Fig. 8b), of
ethanol dry reforming reaction, has been determined to be 58 kJ/mol,
much higher compared to the ethanol combustion, which means that in
dry reforming, high temperatures are required for the catalyst activa-
tion.
3.2.3. Non-oxidative dehydrogenation of ethanol
In order to investigate the role of O2 in combustion and CO2 in dry
reforming, we studied the ethanol conversion using N2 as diluent gas,
on the same catalyst. The ethanol conversion and products selectivities
are shown in Fig. 9a. The ethanol conversion is continuously increasing
starting with the temperature of 100 °C up to 300 °C. Acetaldehyde,
hydrogen, ethylene, water, methane and carbon monoxide are the re-
action products, obtained from dehydrogenation, dehydration and
decarbonylation of ethanol, respectively. The acetaldehyde selectivity
reaches a maximum at 250 °C, while ethylene and methane selectivities
have a slight decrease with the reaction temperature increasing.
The apparent activation energy of ethanol dehydrogenation is
40 kJ/mol, a value comparable to that obtained by Shan (45–49 kJ/
mol) [32] in non-oxidative dehydrogenation of ethanol on Ni-Cu based
catalysts.
As observed from Fig. 10 and from the values of apparent activation
energies, the combustion proceeds under much milder conditions than
dehydrogenation and dry reforming.
The apparent activation energies increase in the following order: Ea
combustion < Ea non-oxidative dehydrogenation < Ea dry reforming,
which means that the catalytic activity is decreased from combustion to
dehydrogenation and dry reforming, respectively, as expected from
thermodynamic data of (1)-(3) reactions.
Fig. 10. Ethanol conversion in combustion, dry reforming and dehydrogenation
over Mn0.15Co2.85O4 spinel oxide.
combustion.
Our results are in agreement with those reported by other re-
searchers in alcohols combustion. Chen [9] noticed that Co–Cu–Mn
(1:1:1)/ZSM-5 membrane/PSSF catalysts are active for isopropanol (T50
at 200 °C), ethyl-acetate and toluene combustion. Bellal et all [26] also
found that mixed iron-cobalt spinel oxides are active for ethanol com-
bustion (T50 is reached at 260–300 °C).
The catalytic performances in ethanol combustion could be eval-
uated from the activation energies. The apparent activation energies,
calculated from the slopes of Arrhenius linear plots, (Fig. 6b) are, as
follows: 48 kJ/mol for Co3O4, 43 kJ/mol for Mn0.05Co2.95O4, 37 kJ/mol
for Mn0.1Co2.9O4 and respectively 36 kJ/mol for Mn0.15Co2.85O4. Si-
milar values of the activation energy were obtained by Bellal [26] in the
ethanol combustion over cobalt-iron spinel oxides.
Our results are comparable to the literature data on ethanol com-
bustion [33] over cobalt-iron mixed oxides, where the ethanol con-
version reaches 100% at temperatures above 300 °C; on ethanol dry
reforming with CO2 [34] using Cu-Ce-Zr-O catalysts, the ethanol con-
version exceeding 50% only at temperatures above 550 C; and on
ethanol dehydrogenation [35] over copper stabilized in Beta zeolite,
the ethanol conversion having values of 30–70% at temperatures of
200–350 °C.
3.2.2. Dry reforming of ethanol with carbon dioxide
In the same time, ethanol dry reforming, in scope of the syngas
production, represents an interesting method, since it uses as raw ma-
terials a renewable such as ethanol and undesirable such as CO2. Syngas
production, from methane and ethanol reforming with CO2, is an im-
portant method through which the greenhouse gas reduction occurs
[27]. Between the two methods, ethanol reforming is more attractive
because the feedstock is renewable, nontoxic, easily transportable and
in addition, energy consumption is lower compared to methane re-
forming.
In this scope, on the best catalyst of the studied series, namely
Mn0.15Co2.85O4, dry reforming of ethanol with carbon dioxide has been
carried out and the results are shown in Fig. 8a. The influence of re-
action temperature on the catalytic performance, in dry reforming of
ethanol, has been investigated, using a stoichiometric CO2: ethanol
ratio and temperature range from 200 °C to 450 °C. The ethanol and
carbon dioxide conversions are improved significantly with increasing
temperature; the ethanol conversion exceeds CO2 conversion. Dry re-
forming reaction takes place in parallel with ethanol dehydrogenation
to acetaldehyde, at temperatures below 350 °C.
4. Conclusions
The MnxCo3-xO4 spinel oxides have been prepared by coprecipita-
tion and investigated in combustion of ethanol, dry reforming with CO2
and in ethanol dehydrogenation.
The substitution of Co with Mn, in the cubic phase of spinel struc-
ture, leads to the appearance of network defects as micro-straining in
the lattice (oxygen vacancies), observed from evaluation of lattice de-
formation where the elastic strain increases with Mn content increasing.
The Co3+ cations are substituted by Mn3+ and Mn4+ cations in octa-
hedral sites as can be observed from FT-IR. Moreover, a change of
oxygen coordination environment, as a result of the interaction be-
tween Co and Mn, has been evidenced by the O1 s peak shifting toward
higher binding energy with manganese loading increasing.
The presence of Mn4+ cations on the surface and the presence of
lattice defects represent the essential factors for ethanol conversion in
combustion and dry reforming.
The surface mobility of oxygen species and the cations redox ability
are the key factors, in ethanol dry reforming, as it was observed by Kawi
[17] and Lee [28]. Our catalyst presents the both features, especially
the redox ability of Mn4+↔Mn3+(2+) and Co3+↔Co2+ cations.
The catalyst is efficient for all three reactions, but in dry reforming
and dehydrogenation, a high temperature is required for catalyst acti-
vation.
7

G. Mitran, et al.
MolecularCatalysisxxx(xxxx)xxxx
Declaration of Competing Interest
There are no conflicts to declare.
References
586–592.
[19] A. Nirmalesh Naveen, S. Selladurai, Physica B 457 (2015) 251–262.
[20] R. Tholkappiyan, A. Nirmalesh Naveen, S. Sumithra, K. Vishista, J. Mater. Sci. 50
(17) (2015) 5833–5843.
[21] Y. Wang, L. Guo, M. Chen, C. Shi, Catal. Sci. Technol. 8 (2018) 459–471.
[22] N. Li, Y. Tian, J. Zhao, J. Zhang, J. Zhang, W. Zuo, Y. Ding, Appl. Catal. B: Environ.
214 (2017) 126–136.
[23] S. Zhao, F. Hu, J. Li, ACS Catal. 6 (2016) 3433–3441.
[24] W. Zhao, Y. Zhang, X. Wu, Y. Zhan, X. Wang, C.T. Au, L. Jiang, Catal. Sci. Technol. 8
(2018) 4494–4502.
[1] J. Swana, Y. Yang, M. Behnam, R. Thompson, Bioresour. Technol. 102 (2011)
2112–2117.
[2] B.R. Kumar, S. Saravanan, Renew. Sustain. Energy Rev. 60 (2016) 84–115.
[3] S.A. Karpov, Chem. Technol. Fuels Oils 44 (4) (2008) 31–35.
[4] V.P. Santos, S. Carabineiro, P. Tavares, M. Pereira, J. Órfão, J. Figueiredo, Appl.
Catal. B: Environ. 99 (2010) 198–205.
[25] Y. Yu, T. Takei, H. Ohashi, H. He, X. Zhang, M. Haruta, J. Catal. 267 (2009)
121–128.
[26] Y.H. Bellal, N.Z. Mahzoul, I. Lounas, A. Benadda, R. Benrabaa, A. Auroux,
L.M. Boukhobza, A. Djadoun, J. Mol. Catal. A: Chem. 426 (2017) 97–106.
[27] D. Cao, F. Zeng, Z. Zhao, W. Cai, Y. Li, H. Yu, S. Zhang, F. Qu, Fuel 219 (2018)
406–416.
[28] D.W. Lee, C.Y. Yu, K.H. Lee, J. Phys. Chem. C 112 (2008) 5136–5140.
[29] M.B. Bahari, N.H.H. Phuc, B. Abdullah, F. Alenazey, Dai-Viet N. Vo, J. Environ.
Chem. Eng. 4 (2016) 4830–4838.
[30] M.B. Bahari, N.H.H. Phuc, F. Alenazey, K.B. Vu, N. Ainirazali, D.Viet N. Vo, Catal.
Today 291 (2017) 67–75.
[31] A. Zawadzki, J.D.A. Bellido, A.F. Lucrédio, E.M. Assaf, Fuel Process. Technol. 128
(2014) 432–440.
[5] H. Idriss, Platin. Met. Rev. 48 (3) (2004) 105–115.
[6] J.R. Salge, G.A. Deluga, L.D. Schmidt, J. Catal. 235 (1) (2005) 69–78.
[7] D. Kulkarni, I.E. Wachs, Appl. Catal. A: Gen. 237 (2002) 121–137.
[8] Y. Liu, J. Deng, S. Xie, Z. Wang, H. Dai, Chin. J. Catal. 37 (2016) 1193–1205.
[9] H. Chen, Y. Yan, Y. Shao, H. Zhang, RSC Adv. 4 (2014) 55202–55209.
[10] P. Mountapmbeme Kouotou, Z.-Y. Tian, U. Mundloch, N. Bahlawane, K. Kohse-
Höinghaus, RSC Adv. 2 (2012) 10809–10812.
[11] N. Bahlawane, E. Fischer Rivera, K. Kohse-Höinghaus, A. Brechling, U. Kleineberg,
Appl. Catal. B: Environ. 53 (2004) 245–255.
[12] Y. Hammiche-Bellal, N. Zouaoui-Mahzoul, I. Lounas, A. Benadda, R. Benrabaa,
A. Auroux, L. Meddour-Boukhobza, A. Djadoun, J. Mol. Catal. A: Chem. 426 (2017)
97–106.
[32] J. Shan, N. Janvelyan, H. Li, J. Liu, T.M. Egle, J. Ye, M.M. Biener, J. Biener,
C.M. Friend, M. Flytzani-Stephanopoulos, Appl. Catal. B: Environ. 205 (2017)
541–550.
[13] Y. Yan, L. Wang, H. Zhang, X. Zhang, Sep. Purif. Technol. 175 (2017) 213–221.
[14] H. Chen, H. Zhang, Y. Yan, Chem. Eng. Sci. 111 (2014) 313–323.
[15] J. Li, S. Xiong, X. Li, Y. Qian, Nanoscale 5 (2013) 2045–2054.
[16] K.J. Puolakka, A.O.I. Krause, Catal. Lett. 116 (2007) 87–93.
[17] S. Kawi, Y. Kathiraser, Front. Energy Res. 3 (13) (2015) 1–17.
[18] Q. Zhang, C. Chen, M. Wang, J. Cai, J. Xu, C. Xia, Nanoscale Res. Lett. 6 (2011)
[33] Y. Hammiche-Bellal, A. Benadda, L. Meddour-Boukhobza, S. Barama, A. Djadoun,
A. Barama, Catal. Commun. 42 (2013) 62–67.
[34] Q. Chen, W. Cai, Y. Liu, S. Zhang, Y. Li, D. Huang, T. Wang, Y. Li, J. Saudi Chem.
Soc. 23 (2019) 111–117.
[35] D. Yu, W. Dai, G. Wu, N. Guan, L. Li, Chin. J. Catal. 40 (2019) 1375–1384.
8