Structural characteristics and catalytic performance of nanostructured Mn-doped CeO2 solid solutions towards oxidation of benzylamine by molecular O2

Article abstract of DOI:10.1016/j.materresbull.2016.12.028

This work reports a thorough investigation of nanosized Mn-doped ceria (Ce_0.7Mn_0.3O_2-δ, CM) as an efficient catalyst for oxidation of benzylamine under solvent-free conditions. These catalysts were prepared by a coprecipitation method followed by calcination at 773 and 1073?K. Effect of Mn doping was examined by a variety of characterization techniques. XRD results confirmed formation of single-phase Ce-O-Mn solid solution, and TEM studies showed nanosized nature of particles. BET surface area of CeO₂ was significantly enhanced after Mn incorporation. Raman, XPS and H₂-TPR results revealed that Mn cations in ceria lattice increase concentration of structural oxygen vacancies and reducibility of ceria. Among various catalysts, the CM calcined at 773?K exhibited a high conversion (～71%), product selectivity (～99.8%), and excellent stability. The better performance has been proved to be due to synergetic interaction between Ce and Mn ions thereby enhanced surface area, improved reducibility, and increased surface adsorbed oxygen species.

Full text of DOI:10.1016/j.materresbull.2016.12.028

Accepted Manuscript
Title: Structural characteristics and catalytic performance of
nanostructured Mn-doped CeO2 solid solutions towards
oxidation of benzylamine by molecular O2
Author: Agolu Rangaswamy Perala Venkataswamy Damma
Devaiah Singuru Ramana Benjaram M. Reddy
PII:
S0025-5408(16)32614-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.materresbull.2016.12.028
MRB 9070
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Accepted date:
30-8-2016
22-11-2016
22-12-2016
Please cite this article as: Agolu Rangaswamy, Perala Venkataswamy, Damma
Devaiah, Singuru Ramana, Benjaram M.Reddy, Structural characteristics and
catalytic performance of nanostructured Mn-doped CeO2 solid solutions towards
oxidation of benzylamine by molecular O2, Materials Research Bulletin
http://dx.doi.org/10.1016/j.materresbull.2016.12.028
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1
Structural characteristics and catalytic performance of nanostructured
Mn-doped CeO2 solid solutions towards oxidation of benzylamine by
molecular O2
Agolu Rangaswamya,b, Perala Venkataswamya,c, Damma Devaiaha,d, Singuru Ramanaa,b,
Benjaram M. Reddya,b,*
aInorganic and Physical Chemistry Division, CSIR – Indian Institute of Chemical Technology
Uppal Road, Hyderabad – 500 007, India
bAcademy of Scientific and Innovative Research, CSIR-IICT, India
cDepartment of Chemistry, Osmania University, Hyderabad - 500 007, India
dChemical Engineering Program, Biomedical, Chemical, and Environmental Engineering, University
of Cincinnati, Cincinnati, OH 45221-0012, USA
--------------------------------
* Corresponding Author: Tel: +91 40 2719 3510; Fax: +91 40 2716 0921.
E-mail addresses: bmreddy@iict.res.in; mreddyb@yahoo.com

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Graphical Abstract
96
NH2
100
80
60
40
20
0
71
30
N
1
2
3
Time(h)
CM-773
Research Highlights:
 Nanosized Mn-doped ceria solid solutions were synthesized by an economical
method.
 Synthesized catalysts were explored for benzylamine oxidation.
 Mn-doped ceria is more active than ceria and MnOx for benzylamine oxidation.
 The dopant induced structural changes were deeply discussed.
 Mn-doped ceria shows superior catalytic activity towards benzylamine oxidation.
ABSTRACT
This work reports a thorough investigation of nanosized Mn-doped ceria (Ce0.7Mn0.3O2-δ,
CM) as an efficient catalyst for oxidation of benzylamine under solvent-free conditions.
These catalysts were prepared by a coprecipitation method followed by calcination at 773 and
1073 K. Effect of Mn doping was examined by a variety of characterization techniques. XRD

3
results confirmed formation of single-phase Ce-O-Mn solid solution, and TEM studies
showed nanosized nature of particles. BET surface area of CeO2 was significantly enhanced
after Mn incorporation. Raman, XPS and H2-TPR results revealed that Mn cations in ceria
lattice increase concentration of structural oxygen vacancies and reducibility of ceria. Among
various catalysts, the CM calcined at 773 K exhibited a high conversion (~71%), product
selectivity (~99.8%), and excellent stability. The better performance has been proved to be
due to synergetic interaction between Ce and Mn ions thereby enhanced surface area,
improved reducibility, and increased surface adsorbed oxygen species.
KEYWORDS: A. Nanostructures; A. Oxides; C. X-ray diffraction; C. Raman spectroscopy;
D. Catalytic properties.
1 Introduction
The selective oxidation of amines to imines has been greatly attracted due to the versatile
applications of imines in the organic synthesis [1, 2]. For example, imines are the key
building blocks for the synthesis of nitrogen-heterocycles, fine chemicals, α-amino acids, α-
amino alcohols, fragrances, fungicides, pharmaceuticals, and agricultural chemicals [3–6].
Additionally, imines are important intermediates in a number of organic reactions, such as
addition, reduction, aziridination, β-lactamization, cyclization, condensations, and aza-Diels–
Alder reactions [7]. Significant development has been made in recent years in the synthesis of
imines, including direct synthesis of imines from amines, condensation of alcohols with
amines, self-condensation of amines with oxidants and secondary amines oxidation [8]. Most
of these reactions are achieved in solvent media which is conceivably the most active area for
green chemistry research. However, solvents have often accounted for the vast majority of
mass wasted in syntheses and processes. On the other hand, selective oxidation reactions are
performed with stoichiometric amounts of high valent inorganic oxidants, such as chromate
or permanganate, as well as with organic oxidants, like 2-iodoxybenzoic acid and N-tert-
butylphenylsulfinimidoylchloride [9–12]. Alternatively, researchers have focused on the
development of catalytic systems for the selective oxidation of amines into the corresponding
imines with green and atom-efficient oxidants, such as O2 and H2O2 [13]. Nevertheless,

4
achieving sufficient space time-yield with these more sustainable oxidants, while maintaining
high levels of selectivity, remains a challenge. The selective oxidation of amines has been
studied over a wide variety of heterogeneous catalysts based on noble metals. Despite the
excellent catalytic performance of noble metal-based catalysts, their high price due to the
limited resources and sensitivity to oxygen poisoning make their applications practically and
environmentally unfavourable. Therefore, the development of non-noble metal catalysts and
adequate catalytic oxidation performance is of crucial importance from both practical and
economical points of view [7, 14–17].
Recently, it has been reported that metal oxide based catalysts could be possible
alternatives for selective oxidation of amines [18]. Among the various oxide catalysts,
particular attention has been paid to CeO2 containing systems due to their unique
physicochemical properties, linked to high oxygen storage-release capacities via Ce4+/Ce3+
redox cycles [19]. In this regard, several CeO2-based catalysts were tested for oxidation of
benzylamines in earlier reports [16, 20]. An enhanced catalytic performance was observed
over the CeO2-based mixed oxides compared to the individual counterparts, which was
ascribed to better redox properties through synergistic interactions between the metal ions
[21, 22]. The high specific surface area, large amounts of oxygen vacancy defects and
abundant acid properties are found to be the key factors for the observed catalytic efficiency
of ceria-based catalysts [23-26].
In addition to CeO2-based catalysts, manganese-based oxides have also received
considerable attention in heterogeneous catalysis due to their high specific capacitance ready
availability of variable oxidation states, better redox properties, environmental compatibility,
low cost, and non-toxic nature [27-30]. The doping of metal oxide (e.g., MnOx) into the CeO2
is one of the promising ways to modify its physicochemical properties [31, 32]. CeO2-MnOx
mixed oxide displays smaller particle size, higher specific surface area, and more number of
oxygen vacancies compared with bare ceria, which is ascribed to the co-operation effect of
the cerium and dopant [31, 33-35]. Particularly, oxygen vacancies play an important role in
the adsorption and activation of gaseous phase oxygen in order to cooperate with the reactant
molecules in the oxidation reactions [36]. Therefore, doping of nanoscale manganese oxide
into ceria lattice may lead to unusual properties and exceptional catalytic activities.
In accordance with the above aspects, the present work aims at systematically exploring
the catalytic oxidation of benzylamine under eco-friendly reaction conditions over CeO2-
MnOx mixed oxides prepared by a simple coprecipitation method. The Ce/Mn mole ratio was

5
optimized and the effect of calcination temperature on the physicochemical properties and
catalytic performance was thoroughly investigated. Several characterization techniques such
as X-ray diffraction (XRD), inductively coupled plasma-optical emission spectroscopy (ICP-
OES), Brunauer–Emmett–Teller (BET) surface area, field emission scanning electron
microscope-energy dispersive X-ray analysis (FESEM-EDX), transmission electron
microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman
spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), temperature-programmed
reduction by hydrogen (H2-TPR), and temperature-programmed desorption by ammonia
(NH3-TPD) were employed to understand the effect of the MnOx doping on the
physicochemical properties of the prepared samples and its impact on benzylamine oxidation
activity.
2. Experimental Section
2.1 Catalyst Preparation
All of the chemicals used in this study were of analytical grade and used without further
purification. The optimized nanostructured Ce0.7Mn0.3O2-δ (CM, 7:3 mole ratio based on
oxides) oxide was prepared by a coprecipitation method through hydrolysis of metal nitrates
as precursors. In a typical procedure, appropriate amounts of Ce(NO3)2·6H2O (Aldrich, AR
grade, > 99.9%) and Mn(NO3)2·4H2O (Wako Pure Chemical, > 99.9%) were dissolved
separately in de-ionized water and mixed together with stirring to form a homogeneous
solution. Then the mixed solution was precipitated by drop-wise addition of aqueous
ammonia solution (NH4OH) over a period until the solution pH reached ~9. After room
temperature aging for 24 h, the resulting pale brown colored Ce-Mn was decanted, filtered
and washed several times with distilled water until free from anion impurities. In this method,
Ce and Mn ions in the synthetic solutions were completely precipitated by hydrolysis with aq.
ammonia, and therefore, the bulk molar ratio of Ce to Mn was consistent with those of the
metal nitrate precursors used in the preparation procedure. The composition of the mixed
oxides was confirmed by ICP-OES. The obtained precipitate was oven dried at 373 K for 5 h
and ground in an agate mortar. The dried powders were calcined at 773 K for 5 h at a heating
rate of 5 K min–1 under dry air. Finally, some portions of the finished catalyst were further
calcined at 1073 K for 5 h under dry air using the same heating rate to monitor thermal
stability. For comparison, the nanocrystalline single oxides, CeO2 and MnOx, were also
prepared by the same way and calcined at 773 K for 5 h. For convenient purpose, the as-

6
prepared Mn-doped CeO2 catalysts calcined at 773 and 1073 K were represented as CM-773
and CM-1073, respectively.
2.2 Catalysts Characterization
XRD patterns were acquired on an X’pert Pro MPD powder diffractometer (PANAlytical
Company) equipped with a nickel-filtered Cu Kα (0.15418 nm) radiation source and a
scintillation counter detector (SCD). The scattered intensity data were collected from 2θ of 2
to 80° by scanning at 0.01° steps with a counting time of 1 s at each step. Crystalline phases
were identified by matching with the International Centre for Diffraction Data Powder
Diffraction File (ICDD-PDF). The average crystallite size (D) of the samples was determined
with the help of Scherrer equation from line broadening and the lattice parameters (a) were
estimated through the formulae a = (h2+k2+l2)1/2* (λ/2sinθ) using the intensity of the most
prominent peak (111).
The elemental analysis was performed by ICP-OES to estimate the respective
concentrations of Ce and Mn. For ICP analysis, approximately 50 mg of the sample was
dissolved in a solution of 25 ml aqua regia and 475 ml distilled water. Then 10 ml of the
above solution was diluted to 250 ml. The measurements were performed on a Thermo Jarrel
Ash model IRIS Intrepid II XDL, USA.
The surface area and pore size distribution of the as-prepared samples were determined
by N2 adsorption-desorption isotherms at liquid N2 temperature (77 K) on a Micromeritics
(ASAP 2000) analyzer. Specific surface area and pore size distribution were calculated by
BET and Barrett-Joyner-Halenda (BJH) methods, respectively. Prior to the measurements,
the samples were pre-treated in a vacuum oven at 473 K for 2 h to remove any residual
moisture.
FESEM studies were performed by using a (FESEM, JEOL-7610F) electron microscope
to examine surface morphology, and the energy dispersive X-ray analysis (EDX) was used to
determine the elemental composition of samples with OXFORD Inca detector interfaced at 5
kV.
The TEM-HRTEM studies were made on a TECNAIG2 TEM microscope equipped with
a slow-scan CCD camera and at an accelerating voltage of 200 kV. Samples for TEM
analysis were prepared by crushing the materials in an agate mortar and dispersing
ultrasonically in ethyl alcohol. After well dispersion, a droplet was deposited on a copper grid

7
supporting a perforated carbon film and allowed to dry. The specimen was examined under
vacuum at room temperature.
Raman spectra were obtained at room temperature on a Horiba Jobin-Yvon HR 800
Raman spectrometer fitted with a confocal microscope and liquid-nitrogen cooled charge-
coupled device (CCD) detector. The line at 632 nm of Ar+ ion (Spectra Physics) laser was
used as an excitation source for the visible Raman spectroscopy. The laser was focused on the
sample under a microscope with the diameter of the analyzed spot being ∼1 μm. The
acquisition time was adjusted according to the intensity of Raman scattering. The
wavenumber values reported from the spectra are accurate to within 2 cm–1.
XPS analyses were performed using a PHI 5000 Versa probe (Ulvac-PHI) Spectrometer.
The X-ray source utilized was the Mg Kα (1253.6 eV) radiation. The analysis was done at
room temperature and the pressures were typically in the order of less than 10–8 Pa. The
charging of samples was corrected by setting the binding energy of the adventitious carbon
(C 1s) at 284.5 eV. The samples were out gassed overnight in a vacuum oven at 283 K before
XPS measurements. All binding energies were measured within a precision of ± 0.3 eV.
H2-TPR analysis was used for evaluating the reduction properties of prepared catalysts
using an automated Micromeritics AutoChem II-2720 instrument equipped with a thermal
conductivity detector (TCD). Approximately 30 mg of the sample mass was placed on top of
quartz wool in a U-shaped quartz reactor (Inner diameter 5 mm). The sample was pre-treated
at a rate of 10 K min–1 to 473 K under a pure He atmosphere at a flow rate of 30 mL min–1 for
30 min to clean the surface of the catalysts before TPR investigation. After cooling down to
room temperature, introducing the reduction agent of 5 vol% H2/Ar with a flow rate of 20 mL
min–1, the temperature was programmatically raised to 1073 K at a ramp of 5 K min–1,
keeping all the parameters unchanged. The outlet of the reactor was connected to a cold trap
to absorb the water molecules resulted by the reduction of the catalysts.
NH3-TPD data were collected on a Micromeritics Autochem 2910 Automated Catalyst
Characterization System. Each sample (ca. 100 mg) was initially pretreated through heating
at 473 K in an ultrahigh pure He stream (30 mL min–1) with a 2 h hold. After that, the furnace
temperature was lowered to 373 K, and the samples were then saturated with anhydrous NH3
(4% in He) at a flow rate of 30 mL min–1 for 1 h. The sample was flushed with 30 mL/min of
He for 2 h to remove weakly bound (physisorbed) NH3, after which the sample temperature

8
was reduced to 323 K. Once a stable baseline by thermal conductivity detector had been
achieved, the temperature was then ramped from 323 K at a rate of 5 °C/min to 973 K.
2.3 Catalytic Activity
The catalytic performance of the prepared samples was investigated for the oxidation of
benzylamine using an O2 balloon under solvent-free conditions. In a typical experiment, 0.2
mmol of substrate and 100 mg of catalyst were taken into a 10 mL round bottom flask and
stirred magnetically at required temperature for appropriate time. After completion of the
reaction, the catalyst was separated from reaction mixture by centrifugation. The products
were confirmed by GC-MS equipped with a DB-5 capillary column and a flame ionization
detector (FID). Samples were taken periodically during the reaction and analyzed by GC
equipped with BP-20 (wax) capillary column and a FID.
3
Results and Discussion
3.1 Characterization Studies
The synthesized samples were analyzed by powder XRD in order to know the phase purity.
An exemplary diffraction pattern of manganese doped ceria catalysts calcined at 773 and
1073 K are provided in Fig. 1. The diffraction patterns of CeO2 and MnOx samples calcined
at 773 K are also presented for comparison purpose. The pure MnOx exhibited the diffraction
patterns related to mixed phases of Mn2O3 (JCDS 41-1442) and Mn3O4 (PDF no. 24-0734)
without any impurity (as “α” and “β” marked in Fig. 1) [37]. Whereas, the pure ceria and
both the manganese doped ceria catalysts exhibit characteristic diffraction peaks of typical
cubic fluorite phase of ceria (JCPDS, No. 34-0394) [38]. Evidently, the absence of diffraction
peaks pertaining to manganese oxides, namely, MnO2, Mn2O3, and MnO indicates either all
the Mn ions have entered into the cubic ceria crystal forming solid solution or else the
manganese oxides are present in well dispersed state, which is beyond the instrument’s
detection limit [31, 39]. Moreover, it can be noted that the diffraction peaks of CM-773
sample are slightly shifted to higher Bragg angles, which can be related to a more significant
change of Ce-O environment by strong interaction with the adjacent MnOx under low
calcination conditions. The peak shift can be attributed to the fact that doping of smaller size
Mnx+ (Mn2+ = 0.083 nm; Mn3+ = 0.065 nm; Mn4+ = 0.053 nm) ions leads to contraction of the
ceria lattice as reported earlier [40]. Thus, lattice parameter of CM-773 exhibits a smaller
value compared to pure ceria (Table 1). The observed key features, such as peak shift,
variation in the lattice parameter and absence of XRD peaks pertaining to the dopant metal

9
oxides evidently confirm the formation of Ce-O-Mn solid solution in CM-773 catalyst. The
formation of Ce-O-Mn solid solution led to structural distortions in CeO2, which may
facilitate the formation of oxygen vacancies in the cubic lattice of ceria [41]. Compared with
the pure CeO2, the typical diffraction peaks of CM-773 catalyst are broadened and their
intensity is also basically decreased, demonstrating that the Ce-Mn mixed oxide has a much
smaller crystal size. Following the Scherrer equation, the crystallite size of as-prepared
catalysts are calculated by using the full width at half maximum of (111) line of cubic ceria
lattice and the results are summarized in Table 1. As expected, the crystallite size of CM-773
was 7.19 nm, which was smaller than other samples especially the single oxides (8.92 nm for
CeO2 and 32.4 nm for MnOx). For CM-1073 sample, peaks for not only cerium oxide but also
Mn oxide were observed (at 2θ = 55.6° corresponding to (440) plane of anatase Mn2O3),
which might be due to manganese oxide phase (i.e., Mn2O3) segregation at elevated thermal
treatments. This phenomenon indicates that the interaction between the two kinds of metal
oxides got weakened. Moreover, the peaks for CeO2 became sharper with increasing
calcination temperature due to improved crystallinity which in turn resulted in crystallite
growth. The chemical compositions of Ce and Mn in CM-773 and CM-1073 are calculated
by ICP-OES analysis. The extracted data are reported in Table S1. It is found that the
measured Ce and Mn concentrations are in good agreement with the nominal values. The
small difference observed may be due to inaccuracies in the preparation of the solution of
samples for ICP-OES analysis.
Fig. S1 shows the N2 adsorption/desorption isotherms of the CeO2 and MnOx calcined
at 773 K and Mn-doped ceria catalysts calcined at 773 and 1073 K. It can be noted that the
profile of the adsorption isotherms of all the samples is type IV according to the IUPAC
classification, demonstrating mesoporous character with a small pore size and good
homogeneity [42-43]. The formation of mesopores may provide more inner surface areas and
active sites for better contact between reactant and adsorbent. According to the P/P0 position
of reflection points shown in Fig. S1, the pore diameters were in the mesopore range. Further,
according to the IUPAC classification, the hysteresis of samples can be developed in the
multilayer range of physisorption isotherms and thus the hysteresis can be classified as H3
type. This type of hysteresis confirms the formation of non-uniform slit shaped pores through
the aggregation of the plate-like particles. Moreover, in case of Mn-doped ceria catalyst with
increasing calcination temperature from 773 to 1073 K, the hysteresis loop shifted to a higher
relative pressure (P/P0) range and the area of the hysteresis loops gradually became smaller.

10
It is generally accepted that the shift of the hysteresis loop to higher relative pressures
indicates the presence of larger pore diameters (Table 1). This is confirmed by the pore size
distribution (PSD) curves that were obtained from the desorption isotherms (Fig. S1 insets).
A quite narrow and unimodal pore size distribution was obtained for all the samples. The
figure also confirms that with increasing calcination temperature, the pore size increases,
accompanied by decrease in the surface areas as a result of pore destruction and particle
sintering. The textural properties including BET surface area, total pore volume and average
pore diameter of the samples studied in the present investigation are summarized in Table 1.
The surface areas of pure ceria and manganese oxide were found to be 41 and 28 m2 g–1,
respectively, which were less than that for the manganese incorporated ceria nanoparticles, in
particular with CM-773 (Table 1). The increase in the surface area of CM-773 may be due to
incorporation of Mnx+ into its lattice thereby inhibits crystalline growth of ceria and this
result is well matched with the XRD results [44]. It is well-known that the porous nano-
structured materials with a high surface area are suitable for catalytic oxidation reactions, due
to the high number of catalytically active centers per unit mass of substrate at their surface
[45]. Thus, N2 adsorption/desorption measurements suggests that CM-773 catalyst possesses
higher specific surface area and larger pore volumes, which are the advantage for the
catalytic oxidation of benzylamine.
To understand the surface morphology of CM-773 and CM-1073 catalysts, FE-SEM
studies were performed, and the results are shown in Fig. S2. The FE-SEM images of both
catalysts clearly indicate that the samples consist of spherical shaped nanosized particles with
some agglomeration of the primary nanoparticles. However, in the case of CM-1073 sample,
the size of the particles increases compared to CM-773 due to high temperature treatment.
The elemental analysis (EDX) pattern of CM-773 and CM-1073 is shown in C-D parts in Fig.
S2, which clearly indicate the presence of Ce, Mn and O as the chemical components in the
samples. No trace amount of other impurities could be seen in the detection limit of the EDX
which also supports the doping of Mn in the ceria lattice. Further, the EDS mapping of CM-
773 and CM-1073 nanostructures is shown in Fig. S3, which unambiguously confirms the
uniform distribution of Ce, Mn and O elements on the entire surface of the materials.
Fig. 2 shows TEM images of the CeO2, CM-773, and CM-1073 samples, respectively.
The shape of the CeO2 particles in pure CeO2 shows agglomerated quasi- spherical
morphology and they are in nonuniform nature. On the contrary, the CeO2-MnOx catalysts
show homogeneously distributed particles with a mixed morphology of spherical and

11
hexagonal. The magnified images in the inset in Figs. 2B and C show the nanoparticles with
hexagonal structure. It can be seen from Fig. 2A, that the particle sizes of CeO2 is about ~10-
11 nm, which is close to the value obtained from the Scherrer equation. In the case of CM-
773 and CM-1073, the TEM images show that the particle size is about ~6-9 and ~15-20 nm,
respectively. Further, the size of the nanoparticles in CM-773 and CM-1073 catalysts is fairly
evidenced by their histograms of the particle size distribution as calculated from the TEM
analysis, which are shown in Fig. S4. It is also obvious that the particle size of the CM-773
catalysts is smaller than that of CeO2 (~10-11 nm), which may be due to the formation of Ce-
O-Mn solid solution. Figs. 2D and E show the HRTEM images of CM-773 and CM-1073
with a continuous lattice, indicating the high crystalline nature of CeO2 nanostructures. A
lattice spacing of ~0.309 nm, which corresponds to the (111) plane of ceria, was measured
from the HRTEM images of both samples [46]. Meanwhile, the presence of lattice fringes
with a d-spacing of ~0.661 nm, which is related to the (440) plane of Mn2O3 nanoparticles
clearly visible in the CM-1073, suggesting that the segregation of manganese oxide from
solid solution at high thermal treatment [47]. We did not find any separate phase made from
MnOx in CM-773 which is also evident in our XRD analysis.
Raman spectroscopy appears to be a fruitful characterization method for ceria-based
materials, since it gives significant information about the crystal defects such as oxygen
vacancies. Fig. S5 shows the vis-Raman spectra of the as prepared samples at an excitation
wavelength of 632 nm. It can be clearly seen from the figure that the CeO2 and Mn-doped
1
CeO2 nanostructures exhibited a high intense band at around ~454-465 cm that corresponds
to a triply degenerate Raman-active phonon of the cubic CeO2 fluorite phase with the space
group Fm3m [48]. However, the broadened characteristic peak corresponding to CeO2 with a
visible shift (~11 cm−1) to lower wavenumber on the Raman spectrum of CM-773 catalyst, in
comparison to that of pure CeO2, can be readily observed, suggesting the existence of strong
interaction between Ce-O and Mn-O [49]. The red-shift for CM-773 is in agreement with the
observation of a smaller lattice parameter. Further, the appearance of a peak at about ~646
cm−1 in CM-773 is attributed to the formation of oxygen vacancies due to the contraction of
the lattice by the incorporation of Mn ions that favors the oxygen mobility thus improving the
redox properties. A similar observation was reported by Barros et al. for the CeO2–MnO2
catalyst system [50]. On the other hand, CM-1073 catalyst shows a slight displacement (~5
cm−1) of the band assigned to CeO2 with a sharpening and an improved symmetry, attributed
to the crystal size of the sample increasing at higher calcination temperature. This

12
phenomenon implied that a specific amount of Ce-Mn-O solid solution actually exists in the
CM-1073 catalysts even though the XRD peaks are not observed to shift to higher 2θ values
and a small amount of crystalline MnOx is observed. In addition to the F2g band, the CM-
1073 sample exhibited three bands at ∼312, 369 and 655.4 cm−1, indicating the presence of
Mn2O3 and Mn3O4 phases and it is also evident by XRD analysis (shown in Fig. 1) [51].
Interestingly, no MnOx peaks were noticed in the CM-773 sample, confirming the formation
of solid solution, and corroborating the XRD results very well.
To understand the effects of Mn ion doping in ceria on the surface chemical states and
identify the chemical status of the metal ions in the samples, XPS measurements were carried
out. The results of typical samples are shown in Fig. 3. The fitted XPS spectra of O 1s are
shown in Fig. 3A. The three resolved XPS peaks, which are based on the binding energy
from lowest to highest, are ascribed to the crystal lattice oxygen (O2−, denoted as Oa), surface
−
2−
adsorbed oxygen (O2 , O2 , O−, denoted as Ob) and chemisorbed water and/or carbonates
(denoted as Oc), respectively [52]. As shown in Table S2, the surface adsorbed oxygen
contents (Ob/Ototal) of CeO2, CM–773, and CM–1073 are 30.27, 36.11, and 33.67%,
respectively. This result clearly shows that CM-773 has much higher adsorbed oxygen
content than CeO2 and CM-1073. CM-1073 just has higher adsorbed oxygen content than
CeO2. It has been generally accepted that the surface adsorbed oxygen was more reactive
than the lattice oxygen in the oxidation reactions due to its higher mobility [53]. This meant
that the CM-773 catalyst might show better activity for the benzylamine oxidation than the
CeO2 catalyst. Additionally, compared with the undoped sample, the O 1s core level BE
values of the CM-773 are shifted to a lower energy region. This is assigned to the fact that the
Mnx+ ions are incorporated into the ceria crystal structure and there are strong interactions
between the MnOx ions and ceria.
Fig. 3B shows the XPS spectra of Ce 3d for CeO2-containing samples. The Ce 3d region
is composed of 5 spin-orbit doublets and these are commonly denoted by u and v, which
represent the 3d3/2 and 3d5/2 states, respectively, in the range 880–920 eV. The photoelectron
peaks were analyzed by Gaussian peak fitting. Ten peaks corresponding to the pairs of spin-
orbit doublets can be identified in the Ce 3d3/2 and 3d5/2 spectra, which is in good agreement
with other research [54]. Specifically, vo, v', uo and u' belong to Ce3+ species, whereas v, v'',
v''', u, u'' and u''' belong to Ce4+ species. As can be seen, the chemical valence of cerium on
the surface of our samples was a mixed valence state, and was mainly Ce4+ as well as small
fraction of Ce3+. The relative amount of surface Ce3+ could be determined by

13
Ce3+/(Ce3++Ce4+), where Ce3+ = (vo+v'+uo+u') and Ce4+ = (v+v''+v'''+u+u''+u''') and the results
are shown in Table S2 [55]. The comparatively higher amount of Ce3+ present in CM-773
nanoparticles (Table S2) can be attributed to the introduction of Mn that resulted in immense
conversion of Ce4+ to Ce3+ on the catalyst surface compared to that on CeO2 and CM-1073.
As a result of high-valent Ce3+ ion, oxygen vacancies are created which accumulates a large
number of adsorbed oxygen on the surface [56]. This observation is in good accordance with
the Raman and O 1s results and demonstrates that the surface adsorbed oxygen species have a
close relation with Ce3+ species concentration. Interestingly, the Ce 3d peaks of CM-773 are
slightly shifted to lower binding energy side in comparison to other samples. This is probably
due to the change in the chemical environment of Ce-O atoms which caused a change in the
coordination number and hence in the binding energies.
The Mn 2p XPS spectra of MnOx and Mn-doped ceria samples calcined at 773 and
1073 K temperatures are shown in Fig. 3C. Mn 2p spectrum is characterized by the doublet of
two spin-orbit components, namely, Mn 2p3/2 and Mn 2p1/2. The binding energies of Mn 2p3/2
and Mn 2p1/2 were ~641.6 and ~653.3 eV, respectively [57]. On the basis of the binding
energies of the Mn 2p main lines, as shown in figure, it is difficult to determine the oxidation
states of Mn cations because similar values can be obtained for most of the Mn oxides (e.g.
MnO, Mn2O3, and MnO2). Hence, the Mn 2p3/2 peak is fitted with a Shirley background and
Gaussian-Lorenz model functions, and three peaks at 641.5, 642.6 and 644. 3 eV can be
obtained, based on standard binding energies and previous literature [58, 59]. The observed
binding energies suggest the co-existence of Mn2+, Mn3+ and Mn4+ ions on the surface of
samples. The concentration of Mn4+/Mntotal in MnOx, CM-773, and CM-1073 are shown in
Table S2. It can be clearly seen from the Table S2 that the nanostructured CM-773 exhibits
higher valent Mn species on its surface than CM-1073, which allows it to be much superior
catalytic system for benzylamine reaction. From the above, although the same chemical
component in the two ceria-manganese samples, the different calcination temperatures result
in the diverse oxidation state for the surface Mn species, particular with Mn4+. Further, the
main peak of the Mn 2p3/2 state becomes broader and shifts to higher binding energy with
increasing calcination temperature, which may be attributed to the weaker interaction
between ceria and manganese oxides.
One of the other major factors having a noticeable effect on the catalytic performance of
a metal oxide catalyst is its redox property. Hence, the H2-TPR analysis was conducted on the
as-prepared samples to reveal their redox properties and the comparative profiles of all the

14
samples are illustrated in Fig. 4. It can be seen that the reduction of CeO2 occurs in two
consecutive steps at ~765 and 1065 K, which are due to the reduction of the surface active
oxygen, and the second peak corresponding to the reduction of bulk oxygen, respectively
[60]. The profile of pure MnOx shows a peak centered at ~603 K, which can be assigned to
the reduction of MnO2 to Mn3O4, whereas the peak at ~747 K is attributed to the
transformation of Mn3O4 to MnO [61]. When ceria is doped with manganese, the TPR
profiles of the mixed oxides showed three reduction peaks; the first peak (α) around 512–520
K was related to the MnO2 to Mn3O4 on the surface; the second peak (β) around 603–654 K
was assigned to the overlapping of the surface reduction of Ce4+ and the reduction of Mn3O4
to MnO; and the last peak (γ) at 1048-1055 was associated with the reduction of bulk ceria
[62]. When comparing with the individual oxides, the mutual promotion effect could improve
the reducibility of both CeO2 and MnOx by shifting reduction peaks toward lower
temperatures, in particular with surface reduction (β). This region of the profile is of
particular interest for low temperature catalytic oxidation reactions. The order of surface
reduction temperature is as follows: CM-773 < CM-1073 < CeO2. The decreased reduction
temperature in CM-773 attributed to the synergistic effect between the manganese and cerium
oxides through the formation of a Ce-O-Mn solid solution with maximum -Ce-O-Mn- bonds,
which can create oxygen defects and structural distortion [63]. Therefore, a higher
concentration of these surface defects can be expected for CM-773. This is in good
agreement with the results of the Raman and XPS analyses. It can be clearly seen that the
shift of bulk reduction peak in both CM-773 and CM-1073 samples is not pronounced which
probably indicates that Mn cations are mainly localized at the surface layers. However,
compared with pure ceria and CM-1073, the CM-773 shows this peak at lower temperature.
On the other hand, the CM-1073 sample shows an extra peak with low intensity (δ) at ~589 K
and this peak may be related to Mn2O3 phase, which is consistent with the XRD, Raman and
HRTEM results. Furthermore, it is evident that with an increase in calcination temperature,
the reduction curves shift gradually towards higher temperature which indicated that the
oxidation ability of the catalysts was decreased. The segregation of dopant cations of the
surface and domain boundaries of ceria is a well-known phenomenon at higher calcination
temperature; thereby interaction of Ce–Mn in the bulk is less thus making it hard to reduce
Ce4+ to Ce3+. The difference in the low temperature reducible properties on the two CeO2-
MnOx catalysts might lead to the distinction of their catalytic performances.

15
The identification and characterization of the acidic properties on the surface of metal
oxides is of significant fundamental interest in oxidation reactions. Temperature programmed
desorption is one of the most widely used and flexible techniques for characterizing these
properties on oxide surfaces, where NH3 was used as the probe molecule. Fig. S6 shows the
NH3-TPD curves of CM-773 and CM-1073 samples. As shown in Fig. S6, the shapes of the
NH3-TPD patterns of the both samples were similar with small differences and three
desorption peaks were observed between ~350-950 K due to the variability of adsorbed NH3
species with different thermal stabilities. Two broad peaks spanned in the temperature range
of ~375-535 and ~550-740 K were observed and these peaks could be assigned to successive
desorption of NH3 by weak and medium strength acidic sites, respectively. There is one
strong and broad peak at around ~835 K, which corresponds to strong acidic sites [64]. A
closer look at the figure revealed that as the calcination temperature increases, the intensity of
the NH3 desorption peaks decreases and shifted to higher temperature which indicates that the
surface acidic sites on the CM-1073 mixed oxide became weaker. Thus, we can conclude that
there are high amounts and uniform acidic centers on the CM-773 mixed oxide, which are
believed to be significantly beneficial to benzylamine oxidation reaction.
3.2 Catalytic activity studies
In order to determine the catalytic oxidation properties of synthesized catalysts, oxidation of
benzylamine was used as a model reaction and the reaction was carried out in the presence of
as prepared catalysts without solvent for 2 h at 393 K and atmospheric pressure. A graphical
illustration of the results obtained is given in Fig. 5. It can be clearly seen form the Fig. 5A
that approximately 8, 14, 54, and 71% of benzylamine conversions were obtained over the
CeO2, MnOx, CM-1073 and CM-773 catalysts, respectively. On the other hand, very low
benzylamine conversion (~3%) was noticed in the absence of catalyst under similar
conditions. This observation plainly indicates the importance of a catalyst for the oxidation of
benzylamine. Remarkably, a high selectivity of dibenzylimine (~99.7–99.8%) was found,
along with insignificant amounts of benzonitrile (~0.2–0.3%) for all the catalysts. The CeO2-
MnOx mixed oxides calcined at 773 and 1073 K showed an enhanced catalytic activity in the
reaction, compared to the pure CeO2 and MnOx oxides, which is ascribed to the formation of
a Ce-O-Mn solid solution with enhanced catalytic properties. Amongst the CeO2-MnOx
composite oxides, the CM-773 gave the best performance considering the conversion of
benzylamine. The better catalytic activity of CM-773 could be ascribed to the following
reasons; The XRD result indicated that Mn-doping significantly altered the lattice parameter

16
of the fluorite structure of CeO2 through the formation of pure Ce-O-Mn solid solution, and
formation of higher quantity of oxygen vacancies was observed from Raman study. The BET
results indicated that the reactants and products easily diffuse from the catalyst surface due to
the higher surface area of mesostructured CM-773. The incorporation of Mnx+ in the ceria
lattice increased the concentration of Ce3+ species. These Ce3+ species are paramagnetic in
nature which can interact with the molecular oxygen and further activate it for the oxidation
reaction. Hence, the increase in Ce3+ content and the presence of Mnx+ in the ceria lattice, as
evidenced from XPS results may further increase the lattice oxygen mobility and oxygen
chemisorption at the catalyst surface. TPR results show that Mn-doping lower the reduction
temperature due to the synergistic effect between Ce and Mn, whereas NH3-TPD results also
revealed that surface acidity of CeO2 increased with Mn doping. So, the current catalytic
performance tests suggested that the insertion of Mn into ceria could remarkably improve the
benzylamine oxidation.
Since the CM-773 catalyst showed a better performance, it was used for further
investigation of the effect of reaction conditions on performance. In order to recognize the
effect of temperature on the aerobic oxidation of benzylamine, we have performed the
reaction with different reaction temperatures from 313 to 393 K over the CM-773 using O2
under solvent-free conditions. The achieved results are shown in Fig. 5B. Obviously, the
reaction temperature exhibited a significant impact on the reaction. It can be clearly seen that
the conversion of benzylamine consistently increases from ~5, 12, 23, 55, and 71% with the
increase of reaction temperature from 313, 333, 353, 373, and 393 K, respectively. Notably
there was no significant variation in the dibenzylimine selectivity (99.7-99.8%) with the
reaction temperature. Further increasing the temperature from 393 to 403 K, we did not
observe significant variation in the conversion of benzylamine (75%) and dibenzylimine
selectivity (~99.5%). These interesting observations suggest that the temperature 393 K is the
optimum temperature and hence it was selected for further studies. The effect of reaction time
on the oxidation of benzylamine is demonstrated in Fig. 6A. Clearly, as the reaction
proceeded, the benzylamine conversion was increased. The achieved benzylamine
conversions are ~ 30, 71, and 96% for 1, 2, and 3 h of reaction times, respectively. On the
other hand, no considerable variation in the dibenzylimine selectivity (~99.7-99.8%) was
noted with the reaction time. Thus, the suitable reaction time was chosen as 2 h in this
investigation.

17
The effect of catalyst amount towards oxidation of benzylamine was studied by using
CM-773 from 20 to 100 mg and the obtained results are shown in Fig. S7. It can be clearly
notified that the conversion of benzylamine was gradually increased with respect to the
amount of catalyst and the high conversion of benzylamine was achieved at 100 mg of
catalyst but, no appreciable change in the dibenzylimine selectivity (~99.6-99.8%) was
observed. The enhancement in the conversion of benzylamine with the weight of catalyst
could be endorsed to availability of more number of active sites for contributing in the
reaction and enhancing conversion of benzylamine. Hence, the amount of catalyst showed
noteworthy effect in the benzylamine oxidation and the suitable catalyst amount was chosen
as 100 mg in the present case.
We also examined the scope of the CM-773 catalyst for the oxidative coupling of various
substituted benzylamines using molecular O2 under solvent-free conditions. The achieved
results are summarized in Table 2. In general, a variety of substituted benzylamines with
either electron-withdrawing or –donating groups can be selectively converted into the
respective imines using the CM-773. As expected, the CM-773 shows an outstanding
efficiency in converting the substituted benzylamines into the corresponding dibenzylimines.
This observation suggests that the electronic properties of the substituents show a diminutive
effect on the efficiency of the CM-773 in the benzylamine oxidation. In contrast, low amine
conversion was found for the o-chloro substituted benzylamine (69%) compared with the p-
chloro substituted benzylamine (76%, Table 2, entries 11-12). These results indicate that the
reaction is sensitive to the steric hindrance of the substituents. Despite the nature of the
substituent and its position on the benzylamine, almost 99.5-99.8% selectivity of the imine
product was achieved with the CM-773 catalyst. To our delight, we found the general
applicability of this conversion for secondary amine and cyclic secondary amines (Table 3,
entries 1-3).
Finally, the efficiency of CM-773 catalyst was studied up to 6 cycles to evaluate its
reusability and stability for the oxidation of benzylamine. The obtained results are presented
in Fig. 6B. The reaction conditions employed include benzylamine (0.2 mmol), catalyst
amount (100 mg), O2 pressure 1 atm, reaction time (3 h), and temperature 393 K. After
completion of each cycle, the catalyst was filter out from the reaction mixture by simple
centrifugation. The solid catalyst was washed several times with methanol to remove
impurities and dried at 393 K for 3 h. It can be concluded from the figure that there was no
significant decrease in the conversion and selectivity up to 5 cycles. Afterwards, a slight

18
variation in the catalytic activity was found which might be attributed to change in the
structural properties of the catalyst, which are responsible for exposing the less active sites on
the surface. Overall, the investigated catalyst has good stability during catalytic activity.
A possible mechanism was proposed for the aerobic oxidation of benzylamine to
dibenzylimine over the CM-773 (Scheme 1) [16, 65-66]. As shown in Scheme 1, an imine
intermediate can be formed from the oxidative dehydrogenation of benzylamine. This
unstable imine intermediate reacts with another benzylamine to form an aminal product,
which subsequently release NH3 molecule to give the dibenzylimine product. As shown in
Scheme 1, small amount of benzonitrile (0.1–0.5 %) was found in this study, which is due to
the consecutive oxidative dehydrogenation of the imine intermediate.
Conclusions
In summary, nanosized Mn-doped solid solution catalysts were successfully synthesized via
an easily performable coprecipitation method and evaluated their activity for the oxidation of
benzylamine using O2 as a green oxidant under solvent-free conditions. The doping of Mn
finely tunes the structural, textural and chemical properties of the pure cerium oxide. Among
all the catalysts, the CM-773 catalyst displays a remarkable improvement in terms of the
activity and stability compared to pure CeO2 and MnOx. Based on our characterization
results, it is believed that the enhanced benzylamine oxidation activity of CM-773 is
attributable to the presence of a large amount of oxygen vacancy defects, perhaps as a
consequence of the formation of Ce-O-Mn solid solution through the synergetic interaction
between ceria and MnOx in this catalyst. This conclusion is further supported by its superior
BET surface area, higher amount of surface adsorbed oxygen, low temperature reducibility
and high concentration of strong acidic sites. Various reaction parameters, such as reaction
time, temperature, catalyst amount and reusability were studied for oxidation of benzylamine
to optimize the reaction conditions. A remarkable observation noticed in this study is that the
CM-773 catalyst shows an outstanding performance in terms of benzylamine conversion
(~71%) and selectivity of dibenzylimine product (~99.8%) for 2 h at 393 K.
Supporting Information Available
Detailed information about the chemical compositions, the surface atomic concentrations, N2
adsorption–desorption isotherms, FESEM-EDX, EDX elemental mapping analysis, histogram
of the particle size distributions, Raman spectra, NH3-TPD profiles and the effect of CM-773
catalyst amount on the oxidation of benzylamine results of the investigated catalysts are

19
summarized in Table S1, Table S2, Figure S1, Figure S2, Figure S3, Figure S4, Figure S5
and Figure S6, Figure S7, respectively.
Acknowledgements
A.R and P.V gratefully acknowledge the University Grants Commission (UGC), New Delhi
for financial assistance under UGC-SRF and DSKPDF schemes, respectively.
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25
Captions for figures
Fig. 1. Powder XRD patterns of pure CeO2 and MnOx calcined at 773 K and Ce0.7Mn0.3O2-δ
(CM) samples calcined at 773 and 1073 K.
Fig. 2. TEM images pure CeO2 calcined at 773 K and Ce0.7Mn0.3O2-δ (CM) samples
calcined at 773 and 1073 K (A, B, C), and HRTEM images of Ce0.7Mn0.3O2-δ (CM) samples
calcined at 773 and 1073 K ( D, E) (inset: enlarged views of selected areas).
Fig. 3. (A) O 1s core level XPS spectra of pure CeO2 calcined at 773 K and Ce0.7Mn0.3O2-δ
(CM) samples calcined at 773 and 1073 K. (B) Ce 3d XP spectra of pure CeO2 calcined at
773 K and Ce0.7Mn0.3O2-δ (CM) samples calcined at 773 and 1073 K. (C) Mn 2p XP spectra
of pure MnOx calcined at 773 K and Ce0.7Mn0.3O2-δ (CM) samples calcined at 773 and 1073
K.
Fig. 4. H2–TPR profiles pure CeO2 and MnOx calcined at 773 K and Ce0.7Mn0.3O2-δ (CM)
samples calcined at 773 and 1073 K.
Fig. 5. (A) Selective oxidation of benzylamine over pure CeO2 and MnOx calcined at 773
K, and Ce0.7Mn0.3O2-δ (CM) samples calcined at 773 and 1073 K. Reaction conditions:
benzylamine (0.2 mmol), time (2 h), temperature (393 K), catalyst amount (100 mg), and O2
balloon. (B) Effect of temperature on the oxidation of benzylamine over CM-773: Reaction
conditions: benzylamine (0.2 mmol), time (2 h), catalyst amount (100 mg), and O2 balloon.
Fig. 6. (A) Effect of reaction time on the oxidation of benzylamine over CM-773: Reaction
conditions: benzylamine (0.2 mmol), temperature (393 K), catalyst amount (100 m g), and O2
balloon. (B) CM-773 catalyst recycling studies: Reaction conditions: benzylamine (0.2
mmol), time (2 h), temperature (393 K), catalyst amount (100 mg), and O2 balloon.
Scheme 1: A plausible reaction mechanism for oxidative coupling of benzylamine.

26
Fig. 1
(111)
CM-1073
CM-773
CeO2
27
28
29
30
31
32

Mn2O3(440)
*






MnOx



)
1
)
1
0
1
2
)
(
2
1
(
1
3
)
(
0
0
2
(
)
)
)
0
)
1
0
2
3
0
2
4
2
3
(
4
2
(
(
(
CM-1073
CM-773
*
CeO2
10
20
30
40
50
60
70
80
2 Theta (0)

27
Fig. 2
C
CeO2
A
B
CM-1073
CM-773
CM-1073
CM-773
E
D
CeO2 (111)
CeO2 (111)
0.309 nm
Mn2O3 (440)
0.309 nm
0.661 nm
CeO2 (111)
0.309 nm

28
Fig. 3
u0
Oa
v
B
u
Ce 3d
A
O 1s
u///
v/
v//
v0
u/
u//
v///
Ob
Oc
CM-1073
CM-1073
CM-773
CM-773
CeO2
CeO2
525.0
875 880 885 890 895 900 905 910 915 920 925
Binding energy (eV)
527.5
530.0
532.5
535.0
537.5
Binding energy (eV)
Mn 2p3/2
C
Mn 2p
Mn 2p1/2
CM-1073
Mn3+
Mn4+
Mn2+
CM-773
MnOx
640
645
650
655
660
Binding energy (eV)

29
Fig. 4
MnOx




CM-1073
CM-773
CeO2
200 300 400 500 600 700 800 900 1000 1100 1200
Temperature (K)