Aerobic oxidation of alcohols over Ru-Mn-Ce and Ru-Co-Ce catalysts: The effect of calcination temperature

Article abstract of DOI:10.1016/j.apcata.2017.02.006

Two ternary mixed oxides, Ru-Mn-Ce and Ru-Co-Ce, were prepared by a co-precipitation method and used in the aerobic oxidation of alcohols to corresponding aldehydes (ketones). Interestingly, different catalytic results were obtained when these compounds were calcined. The calcination temperature had an adverse effect on the catalytic performance of Ru-Mn-Ce catalysts, while being beneficial to the Ru-Co-Ce catalysts. To illustrate these effects, these materials were characterized using X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), Temperature-programmed reduction (TPR), Electron paramagnetic resonance (EPR) and other techniques. The data showed that ruthenium oxides were uniformly dispersed in the mixed oxides, and phase transformations occur after calcination. Mn₃O₄was transformed to MnO₂for the Ru-Mn-Ce catalyst, while CoO(OH) was transformed to Co₃O₄in the Ru-Co-Ce catalyst_.The interactions between ruthenium oxides and Co (Mn)-Ce mixed oxides of the former strengthened while the latter weakened. Calcination decreased the content of adsorbed oxygen and restricted oxygen transfer mechanism in the manganese system, while the opposite effect was observed with the cobalt-containing catalyst. Under optimal reaction conditions, various kinds of alcohols were transformed to corresponding aldehydes (ketones) in high yields over the Ru-Mn-Ce catalyst suggesting these ternary oxides are environmental friendly and economical catalytic systems.

Full text of DOI:10.1016/j.apcata.2017.02.006

Applied Catalysis A: General 535 (2017) 77–84
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Aerobic oxidation of alcohols over Ru-Mn-Ce and Ru-Co-Ce catalysts:
The effect of calcination temperature
Gui Liua, Junhua Liua,b,∗, Wenxiu Lia, Cheng Liua, Fang Wangc,∗, Junkai Hed,
Curtis Guildd, Jing Jind, David Krizd, Steven L. Suibd,∗
a College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China
b Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Key Laboratory of Biomedical Materials, College of Chemistry and
Materials Science, Nanjing Normal University, Nanjing 210023, China
c College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 210009, China
d Department of Chemistry, University of Connecticut, Storrs, CT 06269, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Two ternary mixed oxides, Ru-Mn-Ce and Ru-Co-Ce, were prepared by a co-precipitation method and
used in the aerobic oxidation of alcohols to corresponding aldehydes (ketones). Interestingly, differ-
ent catalytic results were obtained when these compounds were calcined. The calcination temperature
had an adverse effect on the catalytic performance of Ru-Mn-Ce catalysts, while being beneficial to the
Ru-Co-Ce catalysts. To illustrate these effects, these materials were characterized using X-ray diffrac-
tion (XRD), Fourier transformed infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS),
Temperature-programmed reduction (TPR), Electron paramagnetic resonance (EPR) and other tech-
niques. The data showed that ruthenium oxides were uniformly dispersed in the mixed oxides, and phase
transformations occur after calcination. Mn3O4 was transformed to MnO2 for the Ru-Mn-Ce catalyst,
while CoO(OH) was transformed to Co3O4 in the Ru-Co-Ce catalyst. The interactions between ruthenium
oxides and Co (Mn)-Ce mixed oxides of the former strengthened while the latter weakened. Calcination
decreased the content of adsorbed oxygen and restricted oxygen transfer mechanism in the manganese
system, while the opposite effect was observed with the cobalt-containing catalyst. Under optimal reac-
tion conditions, various kinds of alcohols were transformed to corresponding aldehydes (ketones) in
high yields over the Ru-Mn-Ce catalyst suggesting these ternary oxides are environmental friendly and
economical catalytic systems.
Received 26 October 2016
Received in revised form 6 January 2017
Accepted 4 February 2017
Available online 6 February 2017
Keywords:
Aerobic oxidation
Co-precipitation method
Mixed oxides
Oxidation of alcohols
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
[3–6]. However, these supported catalysts always face the loss of
the noble metals, which is a critical issue for the reusability of
Selective oxidation of alcohols to aldehydes (ketones) has been
considered as one of the most fundamental organic reactions, with
the products widely used in the manufacture of perfumes, agri-
cultural chemicals, and other fine chemicals [1–3]. Environmental
and economic concerns lead researchers to require clean oxida-
tion procedures that minimize byproducts and wastes, affected
by replacing the conventional stoichiometric methods with atom-
efficient catalytic protocols using molecular oxygen as the sole
oxidant. Recently many outstanding results have been reported
using noble metals (Ru, Pd, Au, etc.) as heterogeneous catalysts
catalysts.
Much attention has been given to mixed metal oxides (MMOs),
usually prepared by a one-pot synthesis method, e.g. the metal
doped vanadium phosphorus mixed oxides (M-VPOs) [7,8] and
metal doped molecular sieves [9,10]. In these systems, noble metals
are doped into the catalysts directly, forming a uniform compound
which minimizes synthesis and loss of the metals. Additionally,
noble metals also can be put into catalysts by a co-precipitation
method, generally uniform mixed oxide (hydroxide) catalysts.
These materials containing noble metals are formed when metal
precursor solutions are carefully mixed under vigorous stirring,
yielding solid precipitates [11,12]. For aerobic oxidative catalysts,
most researchers believe that excellent catalytic performance orig-
inates from highly dispersed metal components and rich oxygen
storage capacity, surface oxygen vacancies and defects, and some
∗
Corresponding author.
E-mail addresses: liujh2007@aliyun.com (J. Liu), wangfang2012@njtech.edu.cn
(F. Wang), steven.suib@uconn.edu (S.L. Suib).
http://dx.doi.org/10.1016/j.apcata.2017.02.006
0926-860X/© 2017 Elsevier B.V. All rights reserved.

78
G. Liu et al. / Applied Catalysis A: General 535 (2017) 77–84
Fig. 1. XRD patterns of catalysts (a) Mn-Ce, (b) Ru-Mn-Ce, (c) Ru-Mn-Ce (300), (d) Ru-Mn-Ce (500) and (e) Co-Ce, (f) Ru-Co-Ce, (g) Ru-Co-Ce (300), (h) Ru-Co-Ce (500).
mixed oxides consisting of cerium oxide, zirconium oxide and iron
oxide, have been used for oxidative reactions [13,14].
20 mW green laser (532.14 nm) without a filter and using a 600
grooves/mm grating. Temperature-programmed reduction (TPR) of
catalysts was carried out on a Micromeritics 2920 apparatus. Cata-
lyst samples (60 mg) were heated to 800 ◦C at a rate of 10 ◦C/min in
a H2-Ar (5:95) gas flow (50 cm3/min). The electron paramagnetic
resonance (EPR) spectra were recorded on a Bruker (EMXNano)
spectrometer, all spectra were recorded at a power of 0.3 mW, a
modulation amplitude of 4 G, and a modulation frequency of 100
KHz. Inductively coupled plasma optical emission spectroscopy
(ICP-OES) analysis was carried out using a Varian 720ES spectrom-
eter.
For MMOs, more active oxygen species can be produced due
to the synergistic interaction of different oxides, and they show
more excellent catalytic performance than unmixed ones [14,15].
To date, ternary mixed oxides containing noble metals are rarely
reported. In this article, Ru-Mn-Ce and Ru-Co-Ce ternary-mixed-
oxides were prepared by a co-precipitation method and were
investigated as catalysts for aerobic oxidation of alcohols. Inter-
estingly, the calcination temperature has an adverse effect on
oxidation of benzyl alcohol over Ru-Mn-Ce catalysts, while being
beneficial over the Ru-Co-Ce catalysts.
2.3. Catalytic studies
2. Experimental
A typical example for the oxidation of benzyl alcohol by the
Ru-Mn-Ce catalyst is as follows: A mixture of catalyst (0.05 g, Ru
0.03 mmol), benzyl alcohol (4 mmol) and benzotrifluoride (5 mL)
was stirred in a 100 mL round-bottom flask equipped with a con-
denser. All reactions were performed under an oxygen atmosphere
using an oxygen balloon. The reaction was initiated by immersing
the flask in the oil bath kept at the reaction temperature, and then
carried out with vigorous stirring for a certain time. Products were
analyzed by a standardized gas chromatograph (GC 9560) with a
SE-54 capillary column. For recyclability studies, the used catalyst
was washed with water, followed by drying at 110 ◦C, and then
subjected to the alcohol oxidation under the same conditions.
2.1. Catalyst preparation
A certain amount of Na2CO3 (0.5 M) and NaOH (1.5 M) was
slowly dissolved in 30 mL distilled water to produce solution A.
Solution B was composed by dissolving 2.2140 g of Ce (NO3)2·6H2O,
2.5000 g of Mn (CH3COO) (n Ce/n Mn = 0.5) and 0.317 g of RuCl3
2
in 20 mL distilled water. Solution A was then slowly dripped into
solution B under vigorous stirring. The resulting solid product was
aged in the mother liquor at room temperature for 24 h, and then
the dark brown product was filtered, washed with distilled water,
and dried at 110 ◦C for 10 h to obtain a black powder. The cata-
lysts were calcined at 300 ◦C and 500 ◦C, yielding materials referred
to hereafter as Ru-Mn-Ce, Ru-Mn-Ce (300) and Ru-Mn-Ce (500),
respectively. Ru-Co-Ce catalysts were prepared by a similar proce-
dure with Co(CH3COO)2·4H2O, the black powders obtained were
calcined at 300 ◦C and 500 ◦C, which are signed as Ru-Co-Ce, Ru-
Co-Ce (300) and Ru-Co-Ce (500), respectively.
3. Results and discussion
3.1. Catalyst characterization
Fig. 1 shows the XRD patterns of Mn-Ce, Co-Ce mixed oxides and
corresponding Ru-Mn-Ce and Ru-Co-Ce catalysts calcined at differ-
ent temperatures. As shown in Fig. 1(a–d), the diffraction peaks at
2␪ = 28.6, 33.1, 47.5 and 56.3◦ could be assigned to CeO2 (JCPDS: 43-
1002), the intensive and sharp diffractions at 2␪= 18.0, 28.9, 31.0,
32.3, 36.1, 36.4, 38.0, 44.4, 50.7, 58.5, 59.8 and 64.7◦ can be pri-
marily attributed to Mn3O4 (JCPDS: 24-0734), the peaks ascribed
to the phases of Mn3O4 and CeO2 are obvious and the peaks corre-
sponding to the RuO2 almost are invisible before the catalysts are
calcined. With calcination the peaks corresponding to the Mn3O4
are weaker and new peaks assigned to MnO2 (JCPDS: 50-0866)
appear, which indicates that most of the Mn3O4 has transformed
to MnO2 after calcination. Meanwhile, the peaks corresponding
to RuO2 (JCPDS: 40-1290) and CeO2 phases increased with the
higher calcined temperature (especially at 500 ◦C), suggesting the
RuO2 and CeO2 phases segregate and are isolated with the heat
2.2. Catalyst characterization
The morphologies of obtained catalysts were examined by
transmission electron microscopy (TEM) (FEI Talos F200X) and
scanning electron microscopy (SEM) (JSM-7600F). The crystals
of the catalysts were studied by X-ray diffraction (XRD) (X’Pert
PRO PANalytical). Specific surface area was measured by N2
adsorption–desorption experiments at 77 K in an ASAP 2010
instrument. The surface composition was determined by X-ray
photoelectron spectroscopy (XPS) (VG ESCALAB210) using a K-
Alpha-surface analysis system with monochromatized X-Rays.
FT-Infrared spectroscopy was performed on a Bruker Tensor 27
spectrometer. Raman spectra were recorded in a dispersive Horiva
Jobin Yvon LabRam HR800 Confocal Raman Microscope with a

G. Liu et al. / Applied Catalysis A: General 535 (2017) 77–84
79
Fig. 2. Raman spectra of catalysts (a) Ru-Mn-Ce; (b) Ru-Mn-Ce (500); (c) Ru-Co-Ce; (d) Ru-Co-Ce (500).
Fig. 3. TEM images of (a) Ru-Mn-Ce, (b) Ru-Mn-Ce (500), (c) Ru-Co-Ce, (d) Ru-Co-Ce (500) catalysts and SEM images of (e) Ru-Mn-Ce, (f) Ru-Mn-Ce (500), (g) Ru-Co-Ce, (h)
Ru-Co-Ce (500) catalysts.
Table 1
Characterization results for Ru-Mn-Ce and Ru-Co-Ce catalysts in the present study.
Catalyst
BET SA
(m2/g)
H2 consumption
(cm3/g)
Ru 3p3/2
(eV)
Mn 2p3/2
(eV)
Co 2p3/2
(eV)
Reduction temperature/ ◦
C
RuO2
/
MnOx
CoOx
/
Mn-Ce
76.2
84.7
30.1
(4.8 + 25.3)
101.8
/
/
/
/
201
385
122
240
Ru-Mn-Ce
462.8
639.7
641.5
644.2
639.7
641.5
643.8
640.6
644.5
/
80
/
/
/
(3.2 + 57.7 + 40.9)
Ru-Mn-Ce (300)
61.8
89.3
(2.9 + 39.6 + 46.8)
462.6
/
100
135
285
Ru-Mn-Ce (500)
Co-Ce
37.9
68.5
76.7
62.4
58.4
61.0
(2.7 + 58.3)
103.3
(21.5 + 81.8)
167.7
(5.9 + 41.2 + 120.6) 465.3
152.2
(5.6 + 38.9 + 107.7)
137.8
462.0
/
/
127
/
205
/
/
/
/
/
195
300
153
237
149
229
143
247
Ru-Co-Ce
462.5
/
/
/
778.5
782.3
105
106
113
Ru-Co-Ce (300)
Ru-Co-Ce (500)
462.9
462.9
778.8
782.5
778.7
781.5
(6.4 + 34.8 + 96.6)
treatment. For Ru-Co-Ce catalysts shown in Fig. 1(e–h), the peaks
corresponding to the CeO2 are more obvious than their Ru-Mn-
Ce partners, the diffraction peaks at 2␪ = 20.2, 38.9, 50.6 and 65.3◦
could be assigned to CoO(OH) (JCPDS: 07-0169). With calcination,
the CoO(OH) phase vanishes and a new Co3O4 phase appears, with
corresponding diffraction peaks at 31.3, 36.9, 59.4 and 65.2◦ (JCPDS:
42-1467), however, the isolation of RuO2 phase is not obvious.

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G. Liu et al. / Applied Catalysis A: General 535 (2017) 77–84
Fig. 4. XPS spectra of (A) Mn 2p, (B) O 1s of catalysts (a) Ru-Mn-Ce, (b) Ru-Mn-Ce (300), (c) Ru-Mn-Ce (500); (C) Co 2p, (D) O 1s of catalysts (d) Ru-Co-Ce, (e) Ru-Co-Ce (300),
(f) Ru-Co-Ce (500); (E) Ru 2p of (a) Ru-Mn-Ce, (b) Ru-Mn-Ce (300), (c) Ru-Mn-Ce (500), (d) Ru-Co-Ce, (e) Ru-Co-Ce (300) and (f) Ru-Co-Ce (500) catalysts.
Fig. 2 shows the Raman spectra of Ru-Mn-Ce and Ru-Co-Ce cat-
alysts before and after calcination. The 450 cm−1 peak corresponds
to the F2g Raman active mode of CeO2 fluorite structure, due to
the asymmetric breathing mode of the oxygen atoms around Ce4+
cations. This signal is markedly weaker for the catalyst calcined at
500 ◦C. None of the catalysts present any Raman peaks at 600 cm−1
,
suggesting the ceria is low in oxygen defects [16,17]. Two peaks
located at 575 and 650 cm−1 are present due to MnO2, and there

G. Liu et al. / Applied Catalysis A: General 535 (2017) 77–84
81
is an obvious increase in peak intensity when the catalyst was
calcined at 500 ◦C [18]. A signal at 525 cm−1 for Ru-Mn-Ce (500) cat-
alysts, corresponding to the Eg Raman active mode of RuO2 crystals,
suggests the formation of a RuO2 phase [19]. For Ru-Co-Ce catalysts
shown in Fig. 2(c–d), the F2g Raman active mode of the CeO2 flu-
orite structure appears in the uncalcined catalysts and disappears
after calcination to 500 ◦C. The signals at 470, 510 and 675 cm−1 are
attributed to the spinel structure of Co3O4 [20].
Fig. 3 shows TEM and SEM images of Ru-Mn-Ce and Ru-Co-Ce
catalysts before and after calcination. All the samples have platelet
morphologies with 200 nm sizes on average, with relatively low
aggregation. Both TEM and SEM images do not reveal a signifi-
cant difference before and after calcination for the morphology
of these four catalysts (Fig. 3a–h), although their phases segre-
gate. The size changes of Ru species are not obvious (black flakes),
which maintained at 5–8 nm before and after calcination. Mean-
while, the dispersion of Ru, Mn (Co), Ce and O atoms in the catalyst
was analyzed by STEM-elemental mapping (Fig. S1–S4). The map-
ping of catalysts before calcination indicated that each of the Ru,
Mn (Co) and O species were homogeneously dispersed. When the
Ru-Mn-Ce and Ru-Co-Ce catalysts were calcined, the elemental dis-
persion changed, with the concentration of Ce strengthening and
concentrations of Ru and Mn decreasing for Ru-Mn-Ce (500). In the
cobalt-containing samples, the concentration of Ce decreased and
concentrations of Ru and Co increased. This might be attributed to
their different interactions when the phases changed (See XPS Ru
2p section).
For both material systems the BET surface area decreases slightly
after calcination. The BET surface areas of Ru-Mn-Ce and Ru-Mn-
Ce (500) are 85 and 38 m2/g, respectively, and the Ru-Co-Ce and
Ru-Co-Ce (500) show surface areas of 77 and 58 m2/g, respectively,
this shows the Ru-Co-Ce catalysts have better resistance to high
temperature calcination. Fig.S5 shows the nitrogen adsorption-
desorption isotherms of the four catalysts, all of which are Type
H3 hysteresis loops, which are attributed to the aggregation of
nanoparticles giving rise to slit-shaped pores [21].
Fig. 4 shows the XPS spectra of Ru-Mn-Ce and Ru-Co-Ce
catalysts before and after calcination. Deconvolution peak fit-
ting was performed on Mn 2p3/2 spectra aiming to determine
the characteristic peaks of Mn species (Fig. 4A), including sur-
face Mn2+ (639.8–640.8 eV), Mn3+ (641.8–642.3 eV) and Mn4+
(644.0–644.5 eV) [22]. For the Ru-Mn-Ce catalyst, the spectrum
of Mn 2p is one asymmetrical signal which can be decomposed
into three components at BE = 639.7, 641.5 and 644.2 eV which
are assigned to the surface Mn2+, Mn3+, Mn4+ species, with Mn3+
species being the main species. The content of surface Mn3+ was
44.0% in the Ru-Mn-Ce Catalyst as synthesized. For the sample cal-
cined to 300 and 500 ◦C catalyst, the Mn was oxidized to Mn4+
species, with the content of surface Mn4+ reaching 52.2% and
73%, respectively. This result is consistent with the XRD analy-
sis showing that MnO2 becomes the main manganese phase after
calcination. The Co 2p peaks appear at 778.5 and 782.3 eV are cor-
responding to a significant amount of Co2+ or Co3+ for Ru-Co-Ce
catalysts (Fig. 4C), the valence state of the cobalt remained almost
unchanged after calcination. It has a 1.5 eV negative shift comparing
with the 780.0 eV reported for Ru/Co3O4 catalyst in the literatures
[23,24], which indicates a strong interaction between the Ru-Co-Ce
oxides.
Fig. 5. H2-TPR profiles of (a) Mn-Ce, (b) Ru-Mn-Ce, (c) Ru-Mn-Ce (300), (d) Ru-
Mn-Ce (500), (e) Co-Ce, (f) Ru-Co-Ce, (g) Ru-Co-Ce (300) and (h) Ru-Co-Ce (500)
catalysts.
Ce (300) and Ru-Mn-Ce (500) catalysts (Fig. 4D), the O 1s spectra can
be fit with three components and the content of Oads was 79.3, 50.9
and 42.6%, respectively (the content decreased after calcination).
The content of Oads was lower in the cobalt containing catalysts,
with 21.3, 31.5 and 57.6% for Ru-Co-Ce, Ru-Co-Ce (300) and Ru-
Co-Ce (500) catalysts (the contents increased after calcination).
Typically, a higher Oads concentration is considered beneficial for
improving catalytic performance as the surface adsorbed oxygen
can attack an organic molecule [27].
XPS analysis of the Ru component of the catalysts (Fig. 4E) is
performed on the Ru 3p3/2 located near 462.5 eV. For the Ru-Co-
Ce catalyst, the peak due to Ru 3p3/2 is composed of two doublets
(462.5 and 465.3 eV) with the former 1.0 eV higher than that of
bulk Ru (0) (461.5 eV), suggesting RuO2 is the dominant species
[28]. The peak centered at 466.5 eV can be assigned to RuO2·xH2O
[29], while the Ru 3p3/2 peak at 465.8 eV disappears and 462.5 eV
together moves to 462.9 eV for Ru-Co-Ce (300) and Ru-Co-Ce (500)
catalyst, which suggests stronger interactions exist between the
RuO2 and Co3O4-CeO2 mixed oxides after calcination. For the Ru-
Mn-Ce, Ru-Mn-Ce (300) and Ru-Mn-Ce (500) catalysts, the BEs of
Ru 3p3/2 shifted gradually from 462.8 to 462.0 eV, and intensity
of the peak is weaker, it is because that the manganese species
transforms from Mn3+ to Mn4+, and part of electrons flows to ruthe-
nium species, thus the interactions between the RuO2 and Mn-Ce
mixed oxides weaken when the Mn3O4 phase transforms to MnO2
and then MnO2 phase is isolated. Therefore, the BE of Ru 3p3/2 at
462.8–462.9 eV should be due to the strong interactions between
the RuO2 and other mixed oxides.
TPR experiments were carried out in order to study the redox
properties of Mn-Ce, Co-Ce mixed oxides and corresponding Ru-
Mn-Ce and Ru-Co-Ce catalysts calcined at different temperatures
(Fig. 5). The temperature at peak maximum and the integral H2
consumption in the temperature range 50–800 ◦C is summarized
in Table 1. The peak under 150 ◦C is assigned to the reduction
of RuO2 to Ru0 [29], however, the peak is not obvious, which is
might be due to its low content. The main reduction peaks of ceria
appear between 290 and 465 ◦C [30] (this peak is not obvious in
our catalysts) and the peak at 205–385 ◦C should be assigned to the
reduction of MnOx [31] (Fig. 5a–d). It is known cobalt-oxide reduc-
tion occurs in two steps-Co3+ to Co2+, between 200 and 300 ◦C,
and from Co2+ to Co0 at 325 ◦C [32]. For our Ru-Co-Ce catalysts,
the reduction temperature of the former decreased from 153 to
143 ◦C and the latter decreased from 300 to 247 ◦C with increas-
ing calcination temperature (Fig. 5e–h), obviously, the reduction
temperature shifted to a lower temperature as compared to pure
Co3O4 [24], indicating that the addition of Ru markedly facilitated
The spectra of O 1s of Ru-Mn-Ce and Ru-Mn-Ce (500) catalysts
are described in Fig. 4B. XPS of oxygen species are complicated for
mixed oxides, and peaks are generally deconvoluted into contri-
butions from surface lattice oxygen (Olatt, O2−), adsorbed oxygen
(Oads, O2−, O22−, or O−), and adsorbed OH groups or molecular
water on the surface of the catalysts (Osur). The O 1 s spectra at
BE = 528.0–529.5, 530.0–531.9, 532.8–534.6 eV are assigned to Olatt
,
Oads, and Osur species [25,26], respectively. For Ru-Mn-Ce, Ru-Mn-

82
G. Liu et al. / Applied Catalysis A: General 535 (2017) 77–84
Fig. 6. EPR spectra of samples at 300 K (a) Ru-Mn-Ce, (b) Ru-Mn-Ce (500), (c) Ru-Co-Ce and (d) Ru-Co-Ce (500) catalysts.
Table 2
Aerobic oxidation of benzyl alcohol over different catalysts. a
Entry
Catalyst
Solvent
Conversionb(%)
Benzaldehyde
Selectivity (%)b
1
2
3
4
5
6
7
8
Ru-Mn-Ce
Ru-Mn-Ce (300)
Ru-Mn-Ce (500)
Ru-Co-Ce
Ru-Co-Ce (300)
Ru-Co-Ce (500)
Ru-Mn-Ce
Ru-Mn-Ce
Ru-Mn-Ce
Ru-Mn-Ce
Mn-Ce
Benzotrifluoride
Benzotrifluoride
Benzotrifluoride
Benzotrifluoride
Benzotrifluoride
Benzotrifluoride
Toluene
N,N-dimethylformamide
Dimethyl sulfoxide
Acetonitrile
Benzotrifluoride
Benzotrifluoride
Benzotrifluoride
Benzotrifluoride
Benzotrifluoride
100
66.4
19.4
78.2
88.4
92.3
96.1
27.9
38.5
10.8
4.6
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
9
10
11
12
13
14
15
Co-Ce
Ru-Co3O4
Ru-CeO2
Ru-MnO2
7.2
57.2
44.8
44.6
a
Reaction conditions: catalyst 0.05 g (Ru: 0.03 mmol), benzyl alcohol 4 mmol, solvent 5 mL, oxygen balloon, 60 ◦C, 6 h.
Determined by GC.
b
the reduction of cobalt species. For our Ru-Mn-Ce catalysts, the
consumption was moderate with the higher calcination tempera-
ture for the Ru-Co-Ce catalysts. The decreased H2 consumption for
Ru-Mn-Ce catalysts after calcination may be due to reduced content
of adsorbed oxygen. The experimental results support the theory
we have proposed of the interactions change between ruthenium
oxides and Co (Mn)-Ce mixed oxides before and after calcination
in this system.
The transformation of phases after calcination also can be
approved by EPR spectra. As seen from Fig. 6, the sextet of hyperfine
lines of higher amplitude arises from Mn4+ species for Ru-Mn-Ce
(500) catalyst [35], which further testify that the MnO2 phase seg-
regates from the composite after calcination. However, we cannot
find the obvious signal of any Co2+/Co3+ species for Ru-Co-Ce (500)
catalyst; this illustrates no change of lone pair electron after calci-
nation.
reduction peaks at 122–285 ◦C should be due to the reduction of
MnO2 to Mn3O4 and the reduction peaks at 240–385 ◦C should
be due to the reduction of Mn3O4 to MnO, the reduction tem-
peratures shifted to a lower temperature after RuO2 mixed with
Mn-Ce oxides, this indicates that the addition of Ru markedly facil-
itates the reduction of manganese species. The peaks for Mn-Ce,
Ru-Mn-Ce, and Ru-Mn-Ce (300) are very weak; this shows the
MnO2 phase is less present in these catalysts. However, when the
catalyst was calcined at 500 ◦C, a high reduction peak appeared
at 205 ◦C, this indicates that MnO2 has become the dominant
phase in Ru-Mn-Ce (500) catalyst, which is in agreement with
other data [31]. Meanwhile, the reduction peaks of RuO2 phase
changed from 80 to 127 ◦C for Ru-Mn-Ce catalysts and from 105
to 113 ◦C for Ru-Co-Ce catalysts with the increasing calcination
temperature, respectively. Over all, whether they are Ru-Co-Ce or
Ru-Mn-Ce catalysts, the reduction temperature increases with the
higher calcination temperature, obviously, this experiment high-
lights different interactions between the components in mixed
oxides, which results in shifting of the reduction peaks. The stronger
the interaction between the oxides, the higher the initial temper-
ature of reduction [33,34]. Differently, as the MnO2 phase was
isolated for Ru-Mn-Ce (500) catalyst, the interaction between the
mixed oxides weakened. H2 consumption significantly increased
when RuO2 mixed with Co-Ce or Mn-Ce oxides. For the Ru-Mn-
Ce catalysts, the H2 consumption decreased significantly with the
increase of the calcination temperature, while the decrease of H2
3.2. Catalyst activity
The oxidation of benzyl alcohol was examined as a model reac-
tion over the four catalysts (Table 2). The Ru-Mn-Ce catalyst gave
100% conversion initially, however the activity decreases to only
19.4% when the Ru-Mn-Ce catalyst was replaced by Ru-Mn-Ce
(500). The Ru-Co-Ce catalyst gave 78.2% conversion initially, which
they improved to 92.3% when the calcined catalyst (500) was used
(entries 1–6, Table 2). A solvent study was run using different
solvents including toluene, N, N-dimethylformamide, dimethyl sul-
foxide, acetonitrile, and benzotrifluoride (entries 7–10, Table 2). Of

G. Liu et al. / Applied Catalysis A: General 535 (2017) 77–84
83
Table 3
Aerobic oxidation of different alcohols catalyzed by Ru-Mn-Ce catalyst.a
Entry
Time (h)
Substrate
Product
Conversion (%)b
Yield (%)b
1
4
4
100
98
100
98
2c
3
3
100
100
4
5
0.5
1
100
96
100
96
6
7
8
4
98
98
2
100
87
100
87
20
9
10
1
98
92
90
98
92
90
10
11
10
12
18
52
92
52
92
13
12
a
Reaction conditions: catalyst 0.3 g (Ru: 0.2 mmol), alcohol 4 mmol, benzotrifluoride 5 mL, oxygen balloon, 60 ◦C.
Determined by GC.
Recycled test (5th).
b
c
these reaction media tested, benzotriflouride performed the best
demonstrate recyclability of the catalyst, the catalyst was used
sequentially in five reactions and the Ru-Mn-Ce catalyst gave 98%
yield for the fifth use. Leaching of Ru was tested by hot filtration, and
induced coupled plasma techniques (ICP) revealed no ruthenium in
the filtrate.
of the studied reaction solvents, and was used in further tests.
To study the role of Ru, Mn-Ce and Co-Ce mixed oxides were
prepared by the same procedure, and the reaction yielded 4.6%
and 7.2% conversion of benzyl alcohol, respectively (entries 11–12,
Table 2). This indicated that the Ru component was responsi-
ble for the catalytic activity. To further illustrate the mixed-oxide
dependence on activity, binary Ru-Co3O4, Ru-CeO2 and Ru/MnO2
catalysts were prepared under the same conditions (entries 13–15,
Table 2), and these systems gave 57.2%, 44.8% and 44.6% conversion
of benzyl alcohol. This suggests that the use of binary oxides in addi-
tion to the ruthenium component yield a high oxidative catalytic
activity than the single component alone.
To further understand catalytic activity, the oxidation of various
alcohols was examined over the Ru-Mn-Ce catalyst using benzotri-
fluoride as solvent at 60 ◦C, and the results are shown in Table 3.
All the benzylic and allylic alcohols show high yields for the oxida-
tive dehydrogenation (entries 1–7, Table 3), transforming to the
corresponding aldehydes in higher than 96% yield. The primary
aliphatic alcohols were readily oxidized to aldehydes in high yields
with a longer reaction time, and aliphatic secondary alcohols had
higher catalytic performance than the primary ones, which shows
the promotion of the electron density of substrates in this reaction
(entries 8–9, Table 3). When the Ru-Mn-Ce catalysts were used in
the oxidation of heterocyclic alcohols, thiophene-2-methanol, and
furan-2-methanol could be oxidized to corresponding aldehyde
in short time, but pyridine-2-methanol showed low reactivity for
oxidative dehydrogenation, reaching only 52% yield with an 18 h
reaction time (entries 10–12, Table 3). Cyclohexanol was effec-
tively oxidized to cyclohexanone in 12 h (entry 13, Table 3). To
4. Discussion
The Ru-Mn-Ce catalysts showed poor performance after they
were calcined, while Ru-Co-Ce catalysts showed better perfor-
mance after they were calcined. For Ru-Mn-Ce catalysts, the
obvious loss of catalytic property is due to the change of phase.
All characterization techniques showed that the Mn3O4 phase was
transformed to MnO2 after the Ru-Mn-Ce catalyst was calcined at
500 ◦C, and this change of Mn valence results in a more loose distri-
bution (Fig.S1, S2). Through the formation of Mn-Ce mixed oxides
by the addition of Ce, a synergistic interaction between Mn and
Ce oxides can be interpreted as an oxygen transfer mechanism.
CeO2 may act as an oxygen reservoir, transferring oxygen from the
oxygen atmosphere to the Ru and MnOx, which realizes the effec-
tive activation of molecular oxygen [36,37]. However, this transfer
of oxygen is based on good-conditional Mn-Ce mixed oxides as a
pre-condition; we propose this oxygen transfer mechanism has
been blocked when MnO2 segregates from the composite due to
increased valence [38], this segregation also can be approved by
EPR and TPR data.
For Ru-Co-Ce catalysts, the increase of catalytic activity is due
to the change of the crystalline phase of cobalt. All characterization
techniques (XRD, Raman and XPS) show CoO(OH) was transformed
to Co3O4 after the Ru-Co-Ce catalyst was calcined at 500 ◦C. The

84
G. Liu et al. / Applied Catalysis A: General 535 (2017) 77–84
change of Co valence leads to a denser distribution (Figs. S3, S4)
opposite to the Ru-Mn-Ce catalysts, and stronger interactions were
obtained when the CoO(OH) phase was transformed to Co3O4. This
may strengthen synergistic interactions between Co and Ce oxides
(approved by the XPS and TPR test), further enhancing the oxy-
gen transfer mechanism. This conclusion also can be drawn from
the XPS O 1s data, as the contents of Oads for Ru-Mn-Ce catalysts
decrease after calcination, which may block oxygen transfer, while
the contents of Oads for Ru-Co-Ce and Ru-Co-Ce (500) catalysts
alternatively increase and intensify the oxygen transfer.
From the reaction results, the activity is principally due to the
RuO2, as evidenced by the lower conversions of the ruthenium-
free mixed oxides. The activation and transfer of molecular oxygen
to the ruthenium is likely due to the mixed oxides in the com-
posite. Highly dispersed RuO2 nanocrystals agglomerated due to
high-temperature calcination whether for Ru-Mn-Ce (500) or Ru-
Co-Ce (500) catalyst; however, the results obtained from them are
different. This difference is mainly due to phase separation of the
former leading to blockage of oxygen transfer and the agglomera-
tion of RuO2 is not obvious even if the catalyst is calcined (Figs. S2
and S4). This different phase separation also can be approved by
the change of BET surface area and H2 consumption when all the
catalysts are calcined.
References
[1] L.M.D.R.S. Martins, A.P.C. Ribeiro, S.A.C. Carabineiro, J.L. Figueiredoc, A.J.L.
Pombeiro, Dalton Trans. 45 (2016) 6816–6819.
[2] G.-J. ten Brink, I.W.C.E. Arends, R.A. Sheldon, Science 287 (2000) 1636–1639.
[3] D. Enache, et al., Science 311 (2006) 362–365.
[4] K. Yamaguchi, K. Mori, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc.
122 (2000) 7144–7145.
[5] K. Mori, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc. 126
(2004) 10657–10666.
[6] H. Wang, W. Fan, Y. He, J. Wang, J.N. Kondo, T. Tatsumi, J. Catal. 299 (2013)
10–19.
[7] J. Liu, F. Wang, Z. Gu, X. Xu, Chem. Eng. J. 151 (2009) 319–323.
[8] S. Sajip, J.K. Bartley, A. Burrows, M.T.S. Schulz, A. Tuel, J.C. Volta, C.J. Kiely, G.J.
Hutchings, New J. Chem. 25 (2001) 125–130.
[9] W. Zhan, Y. Guo, Y. Wang, X. Liu, Y. Guo, Y. Wang, Z. Zhang, G. Lu, J. Phys.
Chem. B 111 (2007) 12103–12110.
[10] X. Tang, J. Li, J. Hao, Catal. Commun. 11 (2010) 871–875.
[11] B. Qiao, A. Wang, M. Takahashi, Y. Zhang, J. Wang, Y. Deng, T. Zhang, J. Catal.
279 (2011) 361–365.
[12] E.M. Slavinskaya, R.V. Gulyaev, A.V. Zadesenets, O.A. Stonkus, V.I. Zaikovskii,
Yu. V. Shubin, S.V. Korenev, A.I. Boronin, Appl. Catal. B: Environ. 166–167
(2015) 91–103.
[13] Z. Miao, Y. Zhang, X. Pan, T. Wu, B. Zhang, J. Li, T. Yi, Z. Zhang, X. Yang, Catal.
Sci. Technol. 5 (2015) 1314–1322.
[14] P. Zhang, H. Lu, Y. Zhou, L. Zhang, Z. Wu, S. Yang, H. Shi, Q. Zhu, Y. Chen, S. Dai,
Nat. Commun. 6 (2015) 8446.
[15] J. Gaálová, P. Topka, L. Kaluzˇa, O. Solcová, Catal. Today 175 (2011) 231–237.
ˇ
[16] L. Ma, D. Wang, J. Li, B. Bai, L. Fu, Y. Li, Appl. Catal. B: Environ 148–149 (2014)
36–43.
[17] W.Y. Hernandez, M.A. Centeno, F. Romero-Sarria, J.A. Odriozola, J. Phys. Chem.
C 113 (2009) 5629–5635.
[18] C. Julien, M. Massot, R. Baddour-Hadjean, S. Franger, S. Bach, J.-P.
Pereira-Ramos, Solid State Ionics 159 (2003) 345–356.
[19] A. Korotcov, H.P. Hsu, Y.S. Huang, D.S. Tsai, K.K. Tiong, Crystal Growth Des. 6
(2006) 2501–2506.
[20] M. Rashad, M. Rüsing, G. Berth, K. Lischka, A. Pawlis, J. Nanomater. 714853
(2013) 1–6.
[21] K.S.W. Sing, Pure Appl. Chem. 54 (1982) 2201–2218.
[22] H. Li, D. Zhang, P. Maitarad, L. Shi, R. Gao, J. Zhang, W. Cao, Chem. Commun. 48
(2012) 10645–10647.
[23] F. Li, J. Chen, Q. Zhang, Y. Wang, Green Chem. 10 (2008) 553–562.
[24] C. Wang, C. Zhang, W. Hua, Y. Guo, G. Lu, S. Gil, A.G. Fendler, RSC Adv. 6 (2016)
99577–99585.
[25] A.F. Carley, M.W. Roberts, A.K. Santra, J. Phys. B: Chem. 101 (1997) 9978–9983.
[26] L.F. Liotta, F. Puleo, V. La Parola, S.G. Leonardi, N. Donato, D. Aloisio, G. Neri,
Electroanalysis 27 (2015) 684–692.
[27] S. Rousseau, S. Loridant, P. Delichere, A. Boreave, J.P. Deloume, P. Vernoux,
Appl. Catal. B: Environ. 88 (2009) 438–447.
[28] Y. Okawa, T. Masuda, H. Uehara, D. Matsumur, K. Tamur, Y. Nishihatad, K.
Uosaki, RSC Adv. 3 (2013) 15094–15101.
5. Conclusions
Two kinds of ternary mixed oxides, Ru-Mn-Ce and Ru-Co-Ce,
were prepared by a co-precipitation method and used in the
aerobic oxidation of alcohols to their corresponding aldehydes
(ketones). Based on the characterization data, the interactions
between ruthenium oxides and Mn-Ce mixed oxides weakened,
while the opposite was observed in the cobalt-containing sys-
tem after they were calcined. The decreased interactions between
Mn-Ce and ruthenium (oxide) decreased the content of adsorbed
oxygen and blocked oxygen transfer, while the heat treatment of
the Ru-Co-Ce catalyst increased the content of adsorbed oxygen
and promoted oxygen transfer. Under optimal reaction conditions,
various kinds of alcohols were oxidized with high yields over Ru-
Mn-Ce catalyst. The results also showed ruthenium oxide dispersed
in the mixed oxides plays a key role in this reaction. The interac-
tions between Ru oxide and Mn-Ce oxides as well as the higher
concentration of adsorbed oxygen are especially important in the
catalytic performance of these catalysts.
[29] N. Zhang, Y. Du, M. Yin, C. Guan, J. Feng, D. Li, RSC Adv. 6 (2016) 49588–49596.
[30] L.F. Liotta, G. Di Carlo, G. Pantaleo, A.M. Venezia, G. Deganello, Appl. Catal. B.
Environ. 66 (2006) 217–227.
[31] C. Zhou, F. Peng, H. Wang, H. Yu, J. Yang, X. Fu, Fuel Cells 11 (2011) 301–308.
[32] P. Gawade, B. Bayram, A.M.C. Alexander, U.S. Ozkan, Appl. Catal. B Environ.
128 (2012) 21–30.
[33] Y. Ji, Z. Zhao, A. Duan, G. Jiang, J. Liu, J. Phys. Chem. C 113 (2009) 7186–7199.
[34] T.M. Sankaranarayanan, A. Berenguer, C. Ochoa-Hernándeza, I. Moreno, P.
Jana, J.M. Coronado, D.P. Serranoa, P. Pizarro, Catal. Today 243 (2015)
163–172.
[35] A.G. Badalyan, P.P. Syrnikov, C.B. Azzoni, P. Galinetto, M.C. Mozzati, J. Rosa,
V.A. Trepakov, L. Jastrabik, J. Appl. Phys. 104 (2008) 033917.
[36] G. Qi, R.T. Yang, J. Chem. Soc. Chem. Commun. 84 (2003) 8–849.
[37] X. Tang, Y. Li, X. Huang, Y. Xu, H. Zhu, J. Wang, W. Shen, Appl. Catal. B: Environ.
62 (2006) 265–273.
Acknowledgements
This work is financially supported by the National Natural
Science Foundation of China (21303085), the Natural Science
Foundation of Jiangsu Province (BK20130901, BK20130930), the
Jiangsu Government Scholarship for Overseas Studies (JS-2014040,
JS-2014060), the Program to Cultivate Outstanding Young Key
Teachers of Nanjing Normal University and the Priority Academic
Program Development of Jiangsu Higher Education Institutions. SLS
acknowledges the support of the US Department of Energy, Office
of Basic Energy Sciences, Division of Chemical, Biological and Geo-
logical Sciences under grant DE-FG02-86ER13622.A000.
[38] Z. Ding, L. Li, D. Wade, E.F. Gloyna, Ind. Eng. Chem. Res. 37 (1998) 1707–1716.