Highly dispersed cobalt oxide nanoparticles on manganese oxide nanotubes for aerobic oxidation of benzyl alcohol

Article abstract of DOI:10.1016/j.catcom.2019.105763

Hybrid Co₃O₄/MnO₂ nanotube-based catalysts were prepared by a simple hydrothermal synthesis method. The physico-chemical properties of Co₃O₄/MnO₂ catalyst were then studied by different characterization techniques, namely, SEM, TEM and HR-TEM, XRD, BET surface area, XPS and H₂-TPR. The hybrid catalyst showed superior catalytic performance toward benzyl alcohol oxidation than pure MnO₂ nanotubes and Co₃O₄ nanoparticles. The uniform dispersion of Co₃O₄ nanoparticles, good redox behaviour, the variable oxidation states of manganese and cobalt (Mn^3+/4+ or Co^3+/2+) as well as the abundance of active surface oxygen species were responsible for such a high catalytic activity.

Full text of DOI:10.1016/j.catcom.2019.105763

Accepted Manuscript
Highly dispersed cobalt oxide nanoparticles on manganese oxide
nanotubes for aerobic oxidation of benzyl alcohol
Velma Ganga Reddy, Deshetti Jampaiah, Anastasios Chalkidis,
Ylias M. Sabri, Edwin L.H. Mayes, Suresh K. Bhargava
PII:
DOI:
Article Number:
Reference:
S1566-7367(19)30225-0
https://doi.org/10.1016/j.catcom.2019.105763
105763
CATCOM 105763
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
26 April 2019
17 July 2019
25 July 2019
Please cite this article as: V.G. Reddy, D. Jampaiah, A. Chalkidis, et al., Highly dispersed
cobalt oxide nanoparticles on manganese oxide nanotubes for aerobic oxidation of benzyl
alcohol, Catalysis Communications, https://doi.org/10.1016/j.catcom.2019.105763
This is a PDF file of an unedited manuscript that has been accepted for publication. As
a service to our customers we are providing this early version of the manuscript. The
manuscript will undergo copyediting, typesetting, and review of the resulting proof before
it is published in its final form. Please note that during the production process errors may
be discovered which could affect the content, and all legal disclaimers that apply to the
journal pertain.

ACCEPTED MANUSCRIPT
1
Highly dispersed cobalt oxide nanoparticles on manganese oxide nanotubes
for aerobic oxidation of benzyl alcohol
Velma Ganga Reddy,a Deshetti Jampaiah,*a Anastasios Chalkidis,a Ylias M. Sabri,a Edwin L.
H. Mayes,b Suresh K. Bhargava*a
a. Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Science,
RMIT University, GPO Box 2476, Melbourne, VIC 3001, Australia
b. RMIT Microscopy and Microanalysis Facility (RMMF), RMIT University, GPO Box
2476, Melbourne, VIC 3001, Australia
Corresponding author.
*Email: jampaiah.deshetti@rmit.edu.au
*Email: suresh.bhargava@rmit.edu.au

ACCEPTED MANUSCRIPT
2
Abstract: Hybrid Co3O4/MnO2 nanotube-based catalysts were prepared by a simple
hydrothermal synthesis method. The physico-chemical properties of Co3O4/MnO2 catalyst
were then studied by different characterization techniques, namely, SEM, TEM and HR-
TEM, BET surface area, H2-TPR and XPS. The hybrid catalyst showed superior catalytic
performance toward benzyl alcohol oxidation than pure MnO2 nanotubes and Co3O4
nanoparticles. The uniform dispersion of Co3O4 nanoparticles, better redox behaviour, the
variable oxidation states of manganese and cobalt (Mn3+/4+ or Co3+/2+) as well as the
abundance of active surface oxygen species were responsible for such a high catalytic
activity.
Keywords: Hybrid Co3O4/MnO2; Benzyl alcohol oxidation; High dispersion; Nanotubes

ACCEPTED MANUSCRIPT
3
1. Introduction
The selective oxidation of alcohols to aldehydes and carboxylic acids is of critical
importance to fine chemicals production. At industrial scale, the oxidation processes were
conducted for decades to oxidize toxic organic solvents by deploying hazardous oxidants at
least in stoichiometric amounts [1]. While the gas-phase oxidation of simple alcohol
molecules has been well practiced, the limitations due to requirements for adequate volatility
as well as thermal stability of reactants and products, have directed research to liquid
catalytic oxidation processes at mild conditions (temperatures < 150°C, low oxygen
pressures) [2].
Benzaldehyde has been extensively used in plastic, pharmaceutical, and agrochemicals
industry [3]. It has been traditionally produced by the hydrolysis of benzyl chloride or
vapor/liquid oxidation of toluene [4, 5]. At present these processes have little industrial
importance and have a major drawback of low conversion to achieve high selectivity.
Alternatively, the production through the liquid-phase aerobic catalytic oxidation of benzyl
alcohol by oxygen using solid catalysts, presents an environmentally sustainable alternative
route [6]. Typically, Pd, Pt, and Au metals supported on various metal oxides such as TiO2,
Al2O3, MgO, and CeO2, have been tested as catalysts, exhibiting promising behavior in terms
of both conversion and selectivity [7, 8]. For example, Enache et al. synthesized Au-Pd/TiO2,
which showed excellent catalytic activity at temperatures between 100-160 °C under oxygen
pressures in the range of 1-10 bar [9]. However, the deployment of such catalysts at industrial
level is rather unfeasible due to the scarcity and consequently prohibitive cost of noble metals
[10].
In 2001, Son et al. reported the successful catalytic oxidation of alcohols in the liquid
phase using MnOx as catalyst and O2 from air as oxidant [11]. According to the authors, the

ACCEPTED MANUSCRIPT
4
oxidation could be described by a redox mechanism according to which, Mn4+ reduces to
Mn3+ and then it is re-oxidized by O2. The same group further showed that the oxidation
followed a Mars-van Krevelen type mechanism with the lattice oxygen of the manganese
octahedral molecular sieves (OMS-2) participating in the reaction and being replenished by
the fluid-phase O2 [12]. In fact, one of the crystal phases of MnO2, alpha-phase MnO2,
consists of 2x2 (4.6x4.6 Å) tunnels constructed from double chains of octahedra [MnO6] is a
promising catalyst for oxidation reactions [13]. The tunnel cavity, moderately acidic sites as
well as easy release of lattice oxygen of MnO2 have rendered it promising for various
applications [14]. This was confirmed by Su et al. [15] who showed that among the different
manganese oxides, α-MnO2 exhibits a superior catalytic activity in the oxidation of benzyl-
alcohol by O2 under microwave radiation. It is believed in general that the Mn oxidation
state, local symmetry and active site-support interaction, play a defining role in the catalytic
activity.
Apart from manganese, cobalt oxides (i.e. Co3O4) have been evaluated in the selective
oxidation of benzyl alcohol [16]. Co3O4 nanoparticles were homogeneously dispersed on
activated carbon by Zhu et al. [17] and showed ~100% benzyl alcohol conversion as well as
87.3% aldehyde selectivity at 80 °C and atmospheric pressure, with the catalyst exhibiting no
significant activity loss even after 4 cycles. Octahedral Co3O4 nanocrystals have been utilized
by Teng et al. [18], and the incorporation of nanoparticles on alumina [19] and hydrotalcite
[20] has resulted in promising results with respect to conversion, selectivity, stability and
recyclability. Interestingly, Kamimura et al. [21] prepared cobalt doped MnO2, which showed
up to 89% yield of benzaldehyde at 110 °C under oxygen atmosphere. This verifies, that Co
could function as a promoter for the redox behavior of Mn3+/Mn4+ in the presence of oxygen,
which is responsible for the conversion of benzyl-alcohol to benzaldehyde.

ACCEPTED MANUSCRIPT
5
Herein, we report the employment of Co3O4-decorated α-MnO2 nanotubes for the liquid-
phase benzyl-alcohol oxidation in the presence of O2. The aim of the study was to exploit the
synergy between manganese and cobalt oxides, as well as take advantage of a potentially
favorable α-MnO2 morphology, that could significantly boost the oxidation process. Further,
the prepared catalysts were thoroughly characterized to enable the elucidation of the
oxidation mechanism.
2.
Experimental section
2.1
Catalyst synthesis and characterization
α-MnO2 nanotubes were prepared hydrothermally via the reduction of KMnO4. A
solution of 0.66 g KMnO4, 1.5 mL HCl and 75 mL deionized water (DI-H2O) was stirred at
room temperature (RT) to form a homogeneous mixed solution. The solution was then
transferred to a Teflon-lined stainless-steel autoclave and kept at 150 °C for 10 h. After it was
cooled down naturally to RT, the reaction product was centrifuged and vacuum dried
overnight. Following the synthesis of α-MnO2 nanotubes, the decoration with Co3O4
nanoparticles was carried out via a low-temperature hydrothermal method. 5.82 g
Co(NO3)3·6H2O (source of Co), 6 g CO(NH2)2 (use as template), 0.5 g α-MnO2 as well as
150 ml DI-H2O and 50 ml CH3CH2OH were added into a solution that was strongly stirred at
80 °C for 2 h. After the process was completed, the product was centrifuged, vacuum dried
overnight and finally calcined at 350 °C under air for 2 h with a heating ramp of 2 °C min-1.
With the decorating process, Co3O4 was successfully coated on MnO2 nanotubes (represented
as Co3O4/MnO2) with a 20 wt.%, which was validated by MP-AES analysis. Materials and
characterization details are discussed in supporting information.
2.2
Catalytic activity test

ACCEPTED MANUSCRIPT
6
In a typical reaction, 1 mmol of benzyl alcohol, 25 mg of Co3O4/MnO2 catalyst (equals to
291 µmoles (62 µmoles Co+ 229 µmoles Mn)) and 10 mL of solvent were added to a 50 mL
round bottom flask with a reflux condenser. For compassion purpose, we have used 291
µmoles of Mn (equals to 25.32 mg of MnO2) and Co (equals to 51.51 mg of Co3O4) for the
same reaction. The oxygen gas was supplied to the reaction system with a 10 mL/min flow
rate. The reaction was carried out at different temperatures and times. After the reaction, the
catalyst was separated by undergoing centrifugation and was analyzed by gas
chromatography and mass spectrometry.
3. Results and discussion
3.1 Catalyst characterization
The decoration of Co3O4 nanoparticles on MnO2 nanotubes was examined by scanning
electron microscopy (SEM). Fig. 1 shows the morphologies of MnO2 and Co3O4/MnO2
catalysts, confirming that the MnO2 particles possessed a nano-tubular shape. The average
length and diameter of the MnO2 nanotubes was calculated 3±2 µm and 60±20 nm,
respectively. Fig. 1 clearly shows that the Co3O4 nanoparticles (25±5 nm) were well
dispersed on the surface of MnO2 nanotubes. For comparison, pure Co3O4 nanoparticles size
was determined from TEM and shown in Fig. S1. It was found that the average particle sizes
of Co3O4 was 30±2 nm. Moreover, the energy dispersive X-Ray spectrometry (EDS)
mapping (Fig. S2) confirmed the presence of Mn, Co, and O elements on the surface of
Co3O4/MnO2 catalyst.

ACCEPTED MANUSCRIPT
7
Fig. 1. SEM images of MnO2 and Co3O4/MnO2 catalysts at two different magnifications.
The transmission and high-resolution electron microscopy (TEM and HR-TEM) images
of MnO2 nanotubes are displayed in Fig. S3(a-c), respectively. As it can be observed in Fig.
S3a-b, MnO2 comprises of a nanotubular morphology. Further, in Fig. S2c, the calculated
lattice d-spacing of 0.69 nm may be attributed to (110) crystal plane of alpha-MnO2 phase
[22]. Fig. 2 shows the TEM, HR-TEM, and fast Fourier transform (FFT) images of hybrid
Co3O4/MnO2 catalyst. The Co3O4 nanoparticles were well-dispersed on the surface of MnO2
nanotubes and it was further confirmed that an interface was formed between the Co3O4
nanoparticles and MnO2 nanotubes. The corresponding HR-TEM image displays the
interplanar d-spacing values of 0.28 nm and 0.69 nm, which are in good agreement with the
(220) and (110) crystal planes of Co3O4 and MnO2 phases, respectively [22, 23]. Also, the
corresponding FFT image shows the coexistence of MnO2 and Co3O4 phases in the hybrid
catalyst.

ACCEPTED MANUSCRIPT
8
Fig. 2. TEM, FFT, and HR-TEM images of Co3O4/MnO2 catalyst.
Powder X-ray diffraction (XRD) profiles of MnO2, Co3O4/MnO2, and Co3O4 are shown in
Fig. 3. The diffraction lines appeared for pure MnO2 and Co3O4 catalysts can be attributed to
the pure phase of alpha-MnO2 and Co3O4, respectively [14, 24]. As expected, for the
Co3O4/MnO2 catalyst, there are two peaks at 32 and 38° along with MnO2 diffraction peaks,
which can be attributed to the Co3O4 phase, indicating the formation of hybrid Co3O4/MnO2.

ACCEPTED MANUSCRIPT
9
# Co3O4
Co3O4
#
* MnO2
#
#
#
#
#
#
#
#
*
Co3O4/MnO2
*
*
*
*
*
#
#
*
*
*
# *#
*
*
*
#
*
*
*
*
*
MnO2
*
*
*
*
*
*
*
*
10 20 30 40 50 60 70 80
2 Theta (o)
Fig. 3. Powder XRD pattern of MnO2, Co3O4/MnO2, and Co3O4 catalysts.
The N2 sorption isotherms of MnO2, Co3O4/MnO2, and Co3O4 catalysts are displayed in
Fig. S4. All catalysts manifested a type III isotherm with the surface of pure MnO2 nanotubes
being 33.3 m2 g-1. This was decreased to 25.8 m2 g-1 upon the decoration with Co3O4
nanoparticles.
X-ray photoelectron spectroscopy (XPS) was employed to determine the oxidation states
and surface atomic ratios of elements in the developed catalysts. Fig. S5a shows the O 1s
XPS spectra of MnO2, Co3O4/MnO2, and Co3O4 catalysts. The surface atomic ratios of Mn,
Co, and O for the catalysts are displayed in Table S2. All catalysts displayed three types of
oxygen species that were attributed to lattice oxygen (OA, 529.5˗529.7 eV), surface adsorbed
oxygen species (OB, 531.4˗531.9 eV), and oxygen associated with surface hydroxyl or
carbonate species (OC, 532.6˗533.7 eV) [25]. Among the three catalysts, the hybrid
Co3O4/MnO2 catalyst showed the highest concentration of surface atomic oxygen (47.14%)
species, followed by the pure MnO2 (31%) and Co3O4 (24.32%) catalysts. Many reports have
suggested that the presence of these surface-active oxygen species offer facile oxidation

ACCEPTED MANUSCRIPT
10
reaction sites [26, 27]. Hence, the as-prepared Co3O4/MnO2 catalyst could be expected to
exhibit an enhanced catalytic performance when compared to its pure counterparts. Fig. S5b
shows the Mn 2p spectra of MnO2 and Co3O4/MnO2 catalysts. Pure MnO2 displayed one peak
attributable to Mn4+ (642.2 eV), confirming that Mn4+ was the main valance state of
manganese in MnO2 [28]. Mn 2p XPS spectrum of Co3O4/MnO2 was deconvoluted into two
components. These could be attributed to Mn4+ (642.0 eV) and Mn3+ (640.2 eV) oxidation
states, respectively [29]. The co-existence of Mn3+ and Mn4+ ions means that the oxygen
vacancies are generated due to the process of charge compensation in MnO2 lattice [30]. The
Co 2p XPS spectra of Co3O4 and Co3O4/MnO2 catalysts are shown in Fig. S5c. The two
peaks appearing at 779.8 eV and 781.7 eV may be attributed to Co 2p3/2 and Co 2p1/2,
respectively, indicating in this way that Co ion existed in two different oxidation states (+3
and +2) [29, 30]. It is postulated that the presence of variable oxidation states (Mn4+/Mn3+
and Co3+/Co2+) along with the oxygen deficiency could enhance catalytic performance in
Co3O4/MnO2 catalyst.
Hydrogen-temperature programmed reduction (H2-TPR) was used to investigate the
redox properties of the as-prepared catalysts. H2-TPR patterns of MnO2, Co3O4/MnO2, and
Co3O4 catalysts are shown in Fig. S6. Pure MnO2 showed a broad peak, centered at 362 °C,
which can be attributed to the reduction of MnO2 to MnO with Mn2O3 and Mn3O4 as the
intermediates, which was similar with other reported MnO2 systems [31, 32]. Besides, pure
Co3O4 catalyst exhibits broad peak at 379 °C, which can be attributed to the reduction of
Co3O4 to CoO and metallic Co [33]. Interestingly, the hybrid Co3O4/MnO2 showed the
reduction peak of MnO2 at 309 °C, whereas the reduction of cobalt oxide peak in between
350 to 550 °C. The observed low temperature peaks of hybrid Co3O4/MnO2 catalyst
compared to its counterparts can be referred to the synergistic interaction between Co3O4 and
MnO2.

ACCEPTED MANUSCRIPT
11
3.2
Catalytic activity results
The catalytic activities of MnO2, Co3O4/MnO2, and Co3O4 were tested in benzyl alcohol
oxidation under the presence of O2. Table 1 illustrates the influence of reaction time in regard
to various catalysts’ performance towards benzyl alcohol oxidation. Initially, the benzyl
alcohol was chosen for a model substrate and the oxidation reaction was performed at room
temperature for 6 h in the absence of catalysts and solvents. However, no conversion of
benzaldehyde from benzyl alcohol was observed. The same reaction was then conducted
under the same conditions except that gaseous O2 was introduced to the reaction at
atmospheric pressure. The conversion of benzyl alcohol approached 1.9% with 81% of
selectivity. Further, to determine the temperature effect, the reaction was carried out at 100
°C under air, with the product conversion increasing to 4.1%.
Table 1
Optimization of the reaction conditions and catalyst selection for benzyl alcohol oxidation.
Entry
Catalyst
Solvent Oxidant Time
(h)
Temp.
(°C)
Conv. (%) Select. (%)
1
2
Absence
Absence
Absence
Absence
Absence
DMF
N2
6
6
6
6
6
6
6
6
6
6
rt
-
-
Air
Air
Air
Air
Air
Air
Air
Air
Air
rt
1.9
4.1
58
51
89
84
81
74
93
81
85
89
74
95
92
90
88
99
3
Absence
100
100
100
100
80
4
α-MnO2
5
Co3O4
DMF
6
Co3O4/MnO2
Co3O4/MnO2
Co3O4/MnO2
Co3O4/MnO2
Co3O4/MnO2
DMF
7
ACN
8
Toluene
THF
100
100
100
9
10
1,4-
Dioxane

ACCEPTED MANUSCRIPT
12
11
Co3O4/MnO2
1,4-
Air
12
100
93
99
Dioxane
The reactions using pure MnO2 and Co3O4 as well as Co3O4/MnO2 were carried out in
DMF solvent for 6 h at 100°C in air. As shown in Table 1, the product conversion was raised
to 58, 51, and 89%, respectively, indicating that the conversion and formation of
benzaldehyde from benzyl alcohol with Co3O4/α-MnO2 was more efficient than the
respective pure catalysts i.e., MnO2 and Co3O4. No additional conversion was observed by
increasing the reaction time from 6 to 12 h. Furthermore, it was reported that the polarity and
dipole moment of the solvents can play pivotal role in oxidation reaction [34]. Thus, aprotic
polar and non-polar solvents are used to determine the solvent effect on the oxidation
reaction. Different solvents like acetonitrile, 1,4-dioxane, toluene, and THF were screened
out to determine possible effects when using Co3O4/α-MnO2 at 100 °C for a 6 h reaction
period. It was observed that the maximum conversion (93%) of benzaldehyde was found in
the presence of medium polarity with low dipole moment having 1,4-dioxane, with 99%
selectivity.
Table 2. Effect of functional groups on conversion and selectivity for benzyl alcohol
oxidation.
S.No
R
Conversion (%) Selectivity (%)
1
2
3
4
5
6
H
OMe
93
94
95
96
80
80
99
99
99
99
99
99
3,4-OMe
3,4,5-OMe
F
Cl

ACCEPTED MANUSCRIPT
13
7
NO2
77
99
In catalytic reactions, the required amount of catalyst may also play a pivotal role
especially on large scale industrial processes. In our study, no significant changes were
detected by increasing the catalyst amount from 50 to 100 mg. This indicates that the lowest
amount of catalyst loading was enough to carry out the benzyl alcohol oxidation reaction
under the selected conditions. Furthermore, the effects of functional groups were investigated
by introducing various substituents i.e., 4-OMe, 3,4-OMe, 3,4,5-OMe, F, Cl and NO2 under
the same reaction conditions (Table 2). The conversion increased in the presence of electron
donating groups, while it decreased when the electron withdrawing groups were present.
Therefore, it can be argued that the electron donating groups such as methoxy favored the
oxidation reaction. The repeatability of catalyst was investigated by using the Co3O4/MnO2
catalyst 4 times. In each cycle, the catalyst was washed with 1,4-dioxane and dried at 80 ºC
and reused for the next cycle. The results reported in Fig. 4 show that the catalyst exhibited
80% conversion and 73% selectivity after 4 cycles. Further, the leaching experiment was
conducted by testing the filtered solution for benzyl alcohol oxidation after each cycle. The
solution did not show any activity, which confirms that the as-prepared hybrid catalyst was
not leached and stable during the reaction. The stability of used Co3O4/MnO2 catalyst was
validated by further characterization (XRD and SEM). As shown in Fig. S7, the XRD pattern
of used Co3O4/MnO2 catalyst is similar to fresh catalyst, which confirms that the crystallinity
of material is not changed. The morphology of Co3O4/MnO2 (Fig. S8a-b) indicates that the
nanotubular morphology was not perturbed during the reaction.

ACCEPTED MANUSCRIPT
14
100
% Conversion
% Selectivity
80
60
40
20
0
1
2
3
4
Number of cycles
Fig. 4. Recyclability of Co3O4/MnO2 catalyst for benzyl alcohol oxidation.
To further elucidate the reaction mechanism over Co3O4/MnO2 catalyst, XPS analysis
was performed. The XPS spectra of the used catalyst was shown in Fig. S5. The calculated
surface atomic ratios of Mn4+/Mn3+, Co3+/Co2+, and OB+OC/OA for the used Co3O4/MnO2
catalyst are compared with the fresh catalyst. As shown Table S2, the ratio of Mn4+/Mn3+
decreased after benzyl alcohol oxidation, whereas the ratio of Co3+/Co2+ increased. It
indicates that the redox reaction between Mn and Co (Mn4+ + Co2+ ↔ Mn3+ + Co3+) was
benefited benzyl alcohol oxidation. Further, the decreased in ratio of OB+OC/OA indicates that
the surface oxygen also participated in the reaction process. Based on these results, the
plausible reaction mechanism of benzyl alcohol oxidation by Co3O4/MnO2 to benzaldehyde
was illustrated by the schematic presented in Fig. 5. First, the benzyl alcohol molecules are
absorbed on the surface of Co3O4/MnO2 due to the presence of abundant surface-active
oxygen species. Then, the presence of surface oxygen species leads to the activation of
molecular oxygen (O2) due to the redox reaction between Mn and Co. Consequently, the

ACCEPTED MANUSCRIPT
15
absorbed benzyl alcohol molecules transform into benzaldehyde and H2O by the active
oxygen species on Co3O4/MnO2. Moreover, Mn(OH)2 undergoes rapid oxidation in the
presence of O2 to give back MnO2 [11]. Thus, the hybrid catalyst becomes reusable for
application in subsequent reaction cycles. During the reaction, the ability to activate O2 is
critical for the catalytic activity. As discussed in the characterization section, the hybrid
Co3O4/MnO2 catalyst manifested uniform dispersion of Mn, Co, and O elements, oxygen
deficiency, good redox behaviour, abundant surface oxygen species, and variable oxidation
states (Mn3+/4+ and Co2+/3+), which are all resulted due to the synergistic interaction between
Co3O4 and MnO2.
Fig. 5. The plausible reaction mechanism for benzyl alcohol oxidation over Co3O4/MnO2
catalyst.
4.
Conclusion
In summary, we have reported that highly dispersed Co3O4 nanoparticles on the surface
MnO2 nanotubes may act as an efficient catalyst for benzyl alcohol oxidation. The hybrid
Co3O4/MnO2 catalyst was able to oxidize different functional groups containing benzyl

ACCEPTED MANUSCRIPT
16
alcohol with excellent conversion and selectivity efficiency. Moreover, the hybrid catalyst
displayed good repeatability and stability after the reaction. The highest catalytic
performance was reported in the absence of soft oxidants such as TBHP and H2O2, clearly
demonstrating the green nature of the catalyst. The surface oxygen species, better redox
behaviour, and variable oxidation states of Co3O4/MnO2 hybrid catalyst, were responsible for
its enhanced activity as compared to its pure constituents.
Acknowledgement
The authors acknowledge the facilities, and the scientific and technical assistance of the
RMIT Microscopy & Microanalysis Facility (RMMF), a linked laboratory of Microscopy
Australia.
Appendix A. Supplementary data
Supplementary data to this article can be found online.
Supplementary data
Supplementary material
References
[1] R.A. Sheldon, I.W.C.E. Arends, A. Dijksman, Catal. Today, 57 (2000) 157-166.
[2] T. Mallat, A. Baiker, Chem. Rev., 104 (2004) 3037-3058.
[3] F. Brühne, E. Wright, Ullmann's Encyclopedia of Ind. Chem., (2011).
[4] J. Xu, N. Liu, L. Zou, F. Cheng, X. Shen, T. Liu, H. Lv, R. Khan, B. Fan, Asian J. Org.
Chem., 8 (2019) 261-264.
[5] D.D. Mal, S. Khilari, D. Pradhan, Green Chem., 20 (2018) 2279-2289.
[6] X. Feng, P. Lv, W. Sun, X. Han, L. Gao, G. Zheng, Catal. Comm., 99 (2017) 105-109.
[7] S.E. Davis, M.S. Ide, R.J. Davis, Green Chem., 15 (2013) 17-45.
[8] M. Lakshmi Kantam, K. Mahendar, S. Bhargava, J. Chem. Sci., 122 (2010) 63-69.
[9] D.I. Enache, J.K. Edwards, P. Landon, B. Solsona-Espriu, A.F. Carley, A.A. Herzing, M.
Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science, 311 (2006) 362.
[10] R. Ciriminna, V. Pandarus, F. Béland, Y.-J. Xu, M. Pagliaro, Org. Process Res. Dev., 19
(2015) 1554-1558.
[11] Y.-C. Son, V.D. Makwana, A.R. Howell, S.L. Suib, Angew. Chem. Int. Ed., 40 (2001)
4280-4283.
[12] V. Makwana, J Catal, 210 (2002) 46-52.

ACCEPTED MANUSCRIPT
17
[13] A. Chalkidis, D. Jampaiah, P.G. Hartley, Y.M. Sabri, S.K. Bhargava, Fuel Process.
Technol., 193 (2019) 317-327.
[14] D. Jampaiah, V.K. Velisoju, P. Venkataswamy, V.E. Coyle, A. Nafady, B.M. Reddy,
S.K. Bhargava, ACS Appl. Mater. Interfaces, 9 (2017) 32652-32666.
[15] Y. Su, L.-C. Wang, Y.-M. Liu, Y. Cao, H.-Y. He, K.-N. Fan, Catal. Commun., 8 (2007)
2181-2185.
[16] M. Li, X. Fu, L. Peng, L. Bai, S. Wu, Q. Kan, J. Guan, ChemistrySelect, 2 (2017) 9486-
9489.
[17] J. Zhu, K. Kailasam, A. Fischer, A. Thomas, ACS Catal., 1 (2011) 342-347.
[18] Y. Teng, L.X. Song, L.B. Wang, J. Xia, J Phys Chem C, 118 (2014) 4767-4773.
[19] J. Albadi, A. Alihosseinzadeh, M. Jalali, M. Shahrezaei, A. Mansournezhad, Mol. Catal.,
440 (2017) 133-139.
[20] D. Qiao, C. Xu, J. Xu, Catal. Commun., 45 (2014) 44-48.
[21] A. Kamimura, Y. Nozaki, M. Nishiyama, M. Nakayama, RSC Adv., 3 (2013) 468-472.
[22] C.M. Julien, A. Mauger, Nanomaterials, 7 (2017) 396.
[23] Y. Wang, Z.X. Huang, Y. Shi, J.I. Wong, M. Ding, H.Y. Yang, Sci. Rep., 5 (2015) 9164.
[24] D. Jampaiah, T. Srinivasa Reddy, V.E. Coyle, A. Nafady, S.K. Bhargava, J. Mater.
Chem. A, 5 (2017) 720-730.
[25] X. Yang, X. Yu, M. Jing, W. Song, J. Liu, M. Ge, ACS Appl. Mater. Interfaces, 11
(2019) 730-739.
[26] D. Jampaiah, S.J. Ippolito, Y.M. Sabri, J. Tardio, P.R. Selvakannan, A. Nafady, B.M.
Reddy, S.K. Bhargava, Catal. Sci. Technol., 6 (2016) 1792-1803.
[27] H. Sun, Z. Liu, S. Chen, X. Quan, Chem. Eng. J., 270 (2015) 58-65.
[28] Y. Liu, J. Xu, H. Li, S. Cai, H. Hu, C. Fang, L. Shi, D. Zhang, J. Mater. Chem. A, 3
(2015) 11543-11553.
[29] V. Tripkovic, H.A. Hansen, T. Vegge, ChemSusChem, 11 (2018) 629-637.
[30] Q. Li, T. Odoom-Wubah, Y. Zhou, R. Mulka, Y. Zheng, J. Huang, D. Sun, Q. Li, Ind.
Eng. Chem. Res., 58 (2019) 2882-2890.
[31] G. Zhu, J. Zhu, W. Jiang, Z. Zhang, J. Wang, Y. Zhu, Q. Zhang, Appl. Catal. B., 209
(2017) 729-737.
[32] S. Liang, F. Teng, G. Bulgan, R. Zong, Y. Zhu, J. Phys. Chem. C, 112 (2008) 5307-
5315.
[33] Ž. Chromčáková, L. Obalová, F. Kovanda, D. Legut, A. Titov, M. Ritz, D. Fridrichová,
S. Michalik, P. Kuśtrowski, K. Jirátová, Catal. Today, 257 (2015) 18-25.
[34] S. Narayanan, J.J. Vijaya, S. Sivasanker, L.J. Kennedy, A. Ariharan, J. Porous Mater.,
21 (2014) 633-641.

ACCEPTED MANUSCRIPT
18
Highlights:
 Co3O4 nanoparticles dispersed MnO2 nanotubes were fabricated by hydrothermal
synthesis
 An interface is observed between Co3O4 and MnO2 in the hybrid catalyst
 Co3O4/MnO2 exhibited low temperature redox behaviour due to synergistic interaction

Surface oxygen species and variable oxidation states are responsible for superior activity