Support morphology-dependent catalytic activity of the Co/CeO2catalyst for the aqueous-phase hydrogenation of phenol

Article abstract of DOI:10.1039/c9nj06226b

Herein, three Co/CeO₂catalysts with various support morphologies were prepared by Co₂(CO)₈decomposition at a low temperature of 180 °C on the ceria plane such as CeO₂nanocubes (c-CeO₂), nanorods (r-CeO₂), and nanopolyhedrons (p-CeO₂). The Co/r-CeO₂catalyst shows a much higher phenol conversion (82.5%) than Co/c-CeO₂(47.9%) and Co/p-CeO₂(24.7%) at 150 °C and 3 MPa H₂in water. We demonstrate that the less hydrophilic Co/r-CeO₂catalyst inhibits the adsorption of water and further promotes the adsorption of phenol. Moreover, the morphology effect and oxygen vacancies in different chemical environments of the support provide active sites for the dissociation and adsorption of phenol. The high concentration of oxygen vacancies exposed on the high active crystal plane leads to more efficient catalytic activity for the hydrogenation of phenol.

Full text of DOI:10.1039/c9nj06226b

NJC
View Article Online
View Journal
PAPER
Support morphology-dependent catalytic activity
of the Co/CeO2 catalyst for the aqueous-phase
hydrogenation of phenol†
Cite this:DOI: 10.1039/c9nj06226b
a
Jinzhi Lu,ab Zhanwei Ma, *a Xuemei Wei,ab Qinsheng Zhanga and Bin Hu
*
Herein, three Co/CeO2 catalysts with various support morphologies were prepared by Co2(CO)8
decomposition at a low temperature of 180 1C on the ceria plane such as CeO2 nanocubes (c-CeO2),
nanorods (r-CeO2), and nanopolyhedrons (p-CeO2). The Co/r-CeO2 catalyst shows a much higher
phenol conversion (82.5%) than Co/c-CeO2 (47.9%) and Co/p-CeO2 (24.7%) at 150 1C and 3 MPa H2 in
water. We demonstrate that the less hydrophilic Co/r-CeO2 catalyst inhibits the adsorption of water and
further promotes the adsorption of phenol. Moreover, the morphology effect and oxygen vacancies in
different chemical environments of the support provide active sites for the dissociation and adsorption
of phenol. The high concentration of oxygen vacancies exposed on the high active crystal plane leads to
more efficient catalytic activity for the hydrogenation of phenol.
Received 16th December 2019,
Accepted 1st April 2020
DOI: 10.1039/c9nj06226b
rsc.li/njc
CoOX@CN catalysts, and this indicated that the synergistic
1. Introduction
effect between Co3O4 and the in situ generated Co0 during the
The catalytic hydrogenation of phenol is a vital commercial reaction greatly promoted the reaction. Although the Co3O4
petrochemical process for the synthesis of KA oil and is also played the role for the adsorption and activation of phenol and
recognized as a prototypical model reaction for upgrading Co0 was responsible for hydrogen adsorption and dissociation,
the lignin-derived phenolic compounds to stable pyrolysis the resulting metal–oxide interface is highly complicated, and it
bio-oil.1,2 In particular, cyclohexanol as an important product is difficult to determine the inherent interactions between the
in the hydrogenation of phenol is widely used in the synthesis metal and the oxide support.
of nylon-6 and nylon 66, pharmaceuticals, plasticizers, surfac-
Recently, controlling the sample morphology with specific
tants, paints and industrial solvents.3–5 As a result, much effort crystal planes has been proven to be a feasible strategy for
has been devoted to the developments of highly selective and the governable surface structure and electronic features. For
efficient catalysts for phenol hydrogenation. For example, the example, ceria nanocrystals with uniform and well-defined
supported Pd,6 Pt,7 Rh8 and Ru9 catalysts showed very high planes, such as nanocubes ({100}), nanorods ({110} and {100}),
catalytic activity and selectivity for the hydrogenation of phenol. and nanopolyhedra ({111} and {100}), have been controllably
However, the venture for supply and volatile price of noble synthesized.12–14 Ideally, the CeO2(100) surface is terminated by
metals limited their practical applications. To address these an O layer. The CeO2(110) and (111) surfaces have both exposed
problems, the development of catalysts based on the low-cost Ce4+ and O atoms.15 Moreover, Nelson16 found that the use of
metals such as Fe, Co and Ni is promising for the phenol high-surface-area ceria as a support led to higher activity of phenol
hydrogenation reaction. For example, the bimetallic Ni–Co hydrogenation in comparison to the use of a low-surface-area one,
encapsulated in a N-doped carbon matrix was employed to and the mechanism of phenol adsorption and activation is related
catalyse the hydrogenation of phenol, which exhibited 499.9% to the ceria surface geometry. Inspired by the use of nano-building
cyclohexanol conversion and selectivity in isopropanol.10 blocks with the most amount of highly efficient planes, construc-
Wang et al.11 explored the hydrogenation of phenol on the ting the support for the catalyst with unique factors should be
promising for enhancing the catalytic hydrogenation activity of
phenol. However, to the best of our knowledge, the morphological
effect of the support on the hydrogenation of phenol over
supported Co catalysts has never been reported.
Herein, we effectively combine the ceria morphology effect
and cobalt by the Co2(CO)8 decomposition at a low temperature
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China.
E-mail: zhanweima@licp.cas.cn, hcom@licp.cas.cn; Fax: +86 931 8277088;
Tel: +86 931 4968258
b University of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj06226b of 180 1C on the ceria plane for constructing the Co/CeO2
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020

View Article Online
Paper
NJC
catalysts. The facet effect led to the strong interaction between ethyl acetate and n-heptane as the internal standard. The
the Co metal and the support. Moreover, the less hydrophilic content of the mixture was analyzed and the product was
Co/r-CeO2 catalyst can inhibit the adsorption of water and identified by GC-MS (Agilent Technologies 5975C and 7890A).
further promote the adsorption of phenol. And the high
2.5 Characterization of the catalysts
concentration of oxygen vacancies exposed on the high active
crystal plane leads to more efficient catalytic activity for hydro-
genation of phenol. The Co/r-CeO2 catalyst shows much higher
phenol conversion (82.5%) than Co/c-CeO2 (47.9%) and
Co/p-CeO2 (24.7%) at 150 1C and 3 MPa H2.
X-ray powder diffraction (XRD) (X’pert PANalytical, Dutch)
was used for the analysis of the crystalline structures of the
samples, using Cu Ka radiation (l = 1.54050 Å), 2y ranges were
101–901. Transmission electron microscopy (TEM) images were
obtained using a JEM-2010 with an acceleration voltage of
200 kV. X-ray photoelectron spectroscopy (XPS) analyses
were performed on an X-ray photoelectron spectrometer
(ESCALAB 250Xi) to detect the elemental compositions, and
the electron binding energy scale of all spectra was calibrated
using C 1s at 284.8 eV. UV-visible diffuse reflectance spectra
were obtained using a UV-2550 (Shimadzu) spectrometer with
BaSO4 as the reference. The FT-IR spectra of the samples were
recorded on a NEXUS 670 FT-IR spectrometer with KBr pellets
prepared by manual grinding. Static water contact angles
were measured using a DSA-100 optical contact angle meter
(Kruss Company, Ltd, Germany). The temperature-programmed
reduction (TPR) with H2 was performed on a TP-5080 charac-
terization instrument with a TCD detector. Prior to the TPR test,
the 50 mg sample was pretreated at 300 1C for 1 hour in an Ar
stream (27 mL minÀ1) and then cooled to room temperature. The
test was carried out by heating the sample (50 mg) from 25 1C
2. Experimental
2.1 Chemicals
All chemicals were purchased from Macklin Industrial Corpora-
tion in China. They were directly used without any further
purification. All gases used in the experiment were 99.999%
pure. The deionized water resistivity in all reactions was
18.25 MO cm.
2.2 Preparation of the support
The CeO2 supports with different morphologies were prepared
by a hydrothermal method.17 Firstly, Ce(NO3)3Á6H2O (6.96 g)
and NaOH (19.6 g) were dissolved in 5 mL and 35 mL of
deionized water, respectively. Secondly, the two solutions were
mixed without stirring and transferred directly to a 50 mL
Teflon-lined stainless steel autoclave. Then, the mixture was
heated at 100 1C and 180 1C for 24 hours to obtain r-CeO2 and
c-CeO2, respectively. After hydrothermal treatment, the preci-
pitate was washed with deionized water to pH = 7, then dried at
60 1C for 12 hours, and finally calcined at 450 1C for 4 hours in
air at a heating rate of 2.5 1C minÀ1. The p-CeO2 nanoparticles
were directly purchased.
to 900 1C in a H2/Ar (H2, 10%) mixture stream (30 mL minÀ1
)
with a linear heating ramp of 10 1C minÀ1. The temperature-
programmed reduction (TPO) with O2 was also performed on a
TP-5080 characterization instrument with a TCD detector, and the
test was carried out by heating the sample (50 mg) from 25 1C to
600 1C in a flow of O2/Ar (O2, 10%) mixture (30 mL minÀ1) with a
linear heating ramp of 10 1C minÀ1
.
2.3 Preparation of catalysts
All of the catalysts were prepared by a wetness impregnation
method18 and the cobalt loadings were 30 wt%. The prepara-
tion process is as follows: the CeO2 supports with different
morphologies (nanorods, nanocubes, and nanoparticles) were
stirred in a solution of cobalt carbonyl Co2(CO)8 in hexane at
room temperature for 12 h. Then a rotary evaporator was used
to remove the solvent at 30 1C. The sample was calcined at
180 1C and in an N2 atmosphere for 2 hours at a heating rate of
2.5 1C minÀ1. Finally it was cooled to room temperature in an
N2 atmosphere.
3. Results & discussion
The Co/CeO2 catalysts were fabricated by a two-step growth
process. Briefly, CeO2 supports with different morphologies were
achieved by the hydrothermal method by tuning the NaOH
concentration. Then, CeO2 supports with different morphologies
were immersed in the Co2(CO)8 hexane solution for adsorbing
and forming the Co(CO)x species, and decomposed at 180 1C
under a N2 atmosphere. Fig. 1 and Fig. S1 (ESI†) show the
transmission electron microscopy (TEM) images of the pure
CeO2 and Co/CeO2 catalysts. The r-CeO2 with a diameter of
8 Æ 2 nm and a length of 100–300 nm (Fig. 1a) can be evidently
2.4 Measurements of the catalytic activity
The hydrogenation reaction was carried out in a 50 mL observed in the figure, and the high resolution image (Fig. 1b)
stainless-steel high-pressure reactor. In general, phenol- shows a clear (200) lattice fringe with an interplanar spacing of
(p-methyl phenol or guaiacol) (250 mg), catalyst (100 mg) and 0.27 nm, which is parallel to the nanorod stretching direction,
water (25 mL) are loaded into an autoclave. Then the reactor indicating that r-CeO2 is mainly enclosed by the (200) planes and
was purged with H2 three times to remove air. Then the reactor preferentially grows along the (110) crystal orientation. c-CeO2 is
was charged with 3 MPa H2, and the reaction was run for displayed in Fig. 1d with average sizes of approximately
16 hours under stirring at 150 1C. After the reaction, the reactor 50–300 nm and lattice spacings of 0.27 nm corresponding to
was cooled to room temperature, and the remaining gas was the (100) facet (Fig. 1e). Fig. 1g shows that p-CeO2 has an irregular
carefully discharged. The reaction mixture was extracted with shape that mainly exposes the (111) planes. From Fig. 1c, 1f and 1i,
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020

View Article Online
NJC
Paper
becomes clear (Fig. S3b, ESI†), which suggested that the catalysts
after the reaction exhibit magnetic properties indicating in situ
generation of Co0 under the reaction conditions at 150 1C and
3 MPa H2. Moreover, the Co/r-CeO2 catalyst after H2 reduction
showed a similar catalytic activity (Table S1, ESI†). Therefore, the
in situ formation of Co0 could provide the real active sites in the
hydrogenation of phenol for activating hydrogen.
To illustrate the catalytic performance of the Co/CeO2 cata-
lysts, we select the hydrogenation of the phenol as a model
reaction and also investigate the hydrogenation of other phenol
derivatives. The hydrogenation of phenol reaction was carried
out under the reaction conditions of 150 1C and 3 MPa H2 using
water as a solvent. From Fig. 2, it is clearly observed that the
catalytic activity of Co/r-CeO2 (82.5%) is much higher than
those of Co/c-CeO2 (47.9%) and Co/p-CeO2 (24.7%) catalysts
in the phenol hydrogenation reaction. When the reaction
temperature was 130 1C, different cobalt-based catalysts also
showed the same tendency. However, the synthesized CeO2
(r-CeO2, c-CeO2 and p-CeO2) supports showed no activity and
the activity of phenol hydrogenation catalyzed by Co3O4 is not
high (26.5%) (Table S1, ESI†), indicating that the CeO2 support
has a great influence on the phenol hydrogenation reaction.
Table S1 (ESI†) also shows the results of the hydrogenation of
phenol derivatives using Co/r-CeO2 catalysts. As it can be seen,
the catalytic activity of Co/r-CeO2 for the hydrogenation of
guaiacol (40%) and p-cresol (56%) was lower than that for
phenol (82.5%). When the substitution of the aromatic ring
was varied, the conversions of the reactions were different,
which was because the presence of methyl and methoxy groups
makes it difficult to decrease the electron density on the
Fig. 1 TEM images of CeO2 supports and their corresponding catalysts,
(a and b) nanorods (r-CeO2), (d and e) nanocubes (c-CeO2), (g and h)
nanoparticles (p-CeO2), (c and f) Co/r-CeO2 and Co/c-CeO2, and
(i) Co/p-CeO2.
it can be noted that Co3O4 is formed on the r-CeO2, c-CeO2 and aromatic ring.
p-CeO2 supports, respectively. The results indicated that the decom-
In order to study the reasons for the difference in the
position of Co2(CO)8 at 180 1C under a N2 atmosphere led to catalytic activity, a series of characterization studies were
the reaction with the surface oxygen or lattice oxygen in CeO2. performed. As the specific crystal planes have different surface
Moreover, the XRD patterns of the cobalt catalysts on CeO2 electronic and structural features, the surface chemical states
supports with different morphologies are shown in Fig. S2 (ESI†). of Ce, O and Co elements in three Co/CeO2 catalysts were
The main feature peaks can be ascribed to the diffraction patterns investigated. Fig. 3 shows the high-resolution O 1s XPS spectra.
of the fluorite structure of CeO2 (JCPDS: 34-0394). However, the
diffraction peak of r-CeO2 was wider than those of p-CeO2 and
c-CeO2, which indicates the crystallinity is poor. The weak peaks at
around 36.71 were assigned to Co3O4 (JCPDS: 01-1152), which is in
accordance with the TEM results and the previous work.19 From the
TEM images and XRD patterns, we can see that no Co0 is present
on the catalysts. Considering that Co3O4 was the main metal phase
in the catalysts, which cannot activate hydrogen, we should uncover
the real active sites in the hydrogenation of phenol. To this end,
we collected the reacted catalysts and used XRD and XPS to analyze
the cobalt components in the different catalysts after the reaction.
As shown in Fig. S3 (ESI†), except for the known crystal planes of
CeO2, the characteristic peak appearing at 2y = 44.31 is attributed to
the Co0 species (JCPDS: 01-1254). This indicates that cobalt oxide is
reduced to Co0 species during the reaction, which is consistent with
the XPS of Co 2p (Fig. S4, ESI†) and the previous work.11 For further
verification, the used catalysts were dispersed in deionized water,
and the solution became cloudy (Fig. S3a, ESI†). When a magnet is
Fig. 2 Catalytic hydrogenation of phenol using different Co-based cata-
lysts at different temperatures.
placed next to it, the catalyst is sucked to one side, and the solution
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020

View Article Online
Paper
NJC
Fig. 4 UV-vis diffuse reflectance spectra of the CeO2 supports with
different morphologies and CeO2 adsorbed with phenol.
adsorption of water and further promote the adsorption of
phenol on the catalyst. Moreover, these results were also
supported by the reports on the construction of the hydrophilic
or hydrophobic CeO2 with different morphologies.21,22
Fig. 3 O 1s XPS spectra of Co/p-CeO2, Co/r-CeO2 and Co/c-CeO2
catalysts before (a–c) and after the reaction (d–f).
To deeply gain insight into the competitive adsorption of
The O 1s XPS spectra exhibit three peaks at 529.4 eV (OL), phenol and water on Co/CeO2, the effect of solvent on the
531 eV (OV), and 532.4 eV (OC), which are assigned to the lattice reaction was investigated (Table S2, ESI†). It can be obviously
oxygen bound to the metal cations, O2À in the oxygen-deficient noted that the solvent enormously affected the performance of
regions, and chemisorbed or dissociated oxygen species or OH, the Co/r-CeO2 catalyst. In n-hexane, the conversion of phenol
respectively.20 Interestingly, the amount of OC was much larger was 86.2%, which was the highest among the solvents. Ethanol
in Co/p-CeO2 and Co/c-CeO2 than that in Co/r-CeO2, which and i-propanol showed higher activity than water. These results
suggested that the chemisorbed or dissociated oxygen species further indicated that the less hydrophilic properties of the
or OH were more easily adsorbed on p-CeO2 and c-CeO2 than Co/r-CeO2 catalyst promoted competitive adsorption of phenol
r-CeO2. Moreover, in the FT-IR spectra (Fig. S5, ESI†), the peak on the facet of CeO2 in water. Ethanol and i-propanol can also
at 1628 cmÀ1 was indexed to H2O, and these results indicated adsorb on the catalyst, while the –OH group was much more
that the Co/p-CeO2 and Co/c-CeO2 catalysts can adsorb water, weakly bound to the CeO2 surface than the OHÀ of water.
which was coincident with the results of static water contact Additionally, the conversions of phenol in THF and dimethyl
angles measurement for Co-based catalysts (Fig. S6, ESI†), sulfoxide were 21.8% and trace, respectively, so it can be
where the contact angle of Co/r-CeO2 (43.71) catalysts is larger concluded that the activity of the catalyst was also related to
than those of Co/c-CeO2 (20.41) and Co/p-CeO2 (20.11) catalysts. the solvent polarity. Then, catalyst recycling experiments were
The above results illustrate that the Co/c-CeO2 and Co/p-CeO2 performed in hexane (Fig. S7, ESI†). The conversion decreased
catalysts were more hydrophilic than the Co/r-CeO2 catalyst. to 10% after the sixth cycle. Interestingly, the catalytic activity
Additionally, Ma et al.17 reported that the zeta potentials of could be fully restored after regeneration. Additionally, the
r-CeO2, c-CeO2, and p-CeO2 are different and relate to the FTIR spectra of the used Co/r-CeO2 catalyst showed strong
surface atom arrangement. Therefore, the competitive adsorp- peaks at 3508–3024 cmÀ1 assigned to adsorbed phenyl species
tion of phenol and water on Co/CeO2 catalysts could affect the and at 2865, 2806, 1403 and 1157 cmÀ1 assigned to the
activity of phenol hydrogenation.
cyclohexyl species (Fig. S8, ESI†). Consequently, the decreased
Moreover, the UV-vis diffuse reflectance measurements were activity in n-hexane may be ascribed to the adsorption of phenol
used to investigate the adsorption of phenol on CeO2 in water. and desorption of cyclohexanol, and the weaker polarity of
In Fig. 4, it can be clearly seen that there is a higher absorbance n-hexane is unfavourable for the desorption of cyclohexanol.
(lmax = 480 nm) in r-CeO2–phenol, which is an indication of In water, the catalytic activity of the regenerated catalyst was
higher amounts of phenoxy species. However, the absorbance close to that of the fresh catalyst and the morphologies were
peak of phenoxy species was much weaker in c-CeO2–phenol well maintained (Fig. S9, ESI†). The conversion mildly
and p-CeO2–phenol, which indicated that the hydrophilic decreased by about 6% after 8 runs due to the loss of the
Co/c-CeO2 and Co/p-CeO2 catalysts prefer to adsorb water, catalyst during the recovery. Therefore, the solvent could affect
which led to a decrease in the adsorption of phenol on these the adsorption of phenol and desorption of cyclohexanol on the
catalysts. As Co/r-CeO2 was less hydrophilic, it can inhibit the coordinatively unsaturated cerium cations or oxygen vacancies.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020

View Article Online
NJC
The number of surface oxygen vacancies can be roughly
Paper
estimated based on the ratio of OV/OL.23 As shown in Table S3
(ESI†), after the reaction, the ratio of OV/OL on the Co/r-CeO2
(0.9) catalyst is higher than that of Co/c-CeO2 (0.7) and
Co/p-CeO2 (0.65), which means that the Co/r-CeO2 catalyst
has more oxygen vacancies than Co/c-CeO2 and Co/p-CeO2.
As the Ce3+ is closely related to the oxygen vacancy, the Ce
species should be discussed. There are two types of cerium
oxides present, as shown in Fig. S10 (ESI†), Ce3+ and Ce4+. The
amount of Ce3+ is estimated to be 34%, 26.2% and 27.2%
for Co/r-CeO2, Co/c-CeO2 and Co/p-CeO2 (Table S3, ESI†),
respectively. The high concentration of Ce3+ ions on the surface
of the Co/r-CeO2 sample reflects the high concentration of Fig. 5 H2-TPR profiles of the CeO2 supports with different morphologies
surface oxygen vacancies. Numerous studies24 have shown
(a) and the supported Co catalysts (b).
that the dissociative adsorption of phenol mainly occurs on
the support, while the metal mainly activates the hydrogen.
After Co species loading, the reduction peak of the supports
Moreover, Nelson et al.16 reported that the adsorption and acti-
shifted to the low temperature region, due to the strong
vation of phenol occurs on the ceria support and the mecha-
interaction between the metal and the CeO2 support, which
nism is related to the ceria surface geometry. They suggested
greatly weakens the Ce–O bond at the contact with the metal
that coordinately unsaturated cerium cations near hydroxyl
and enhances its oxygen reduction performance.
groups may be the active sites for the reaction. Therefore,
In addition, the programmed temperature oxidation (TPO)
the concentration of oxygen vacancies depending on the mor-
of Co/CeO2 catalysts with three different morphological
phology of CeO2 is vital to the catalytic hydrogenation of
supports were also investigated (Fig. S12, ESI†). When the three
phenol. Considering that the concentration of surface oxygen
catalysts are heated from 0 1C to 600 1C, oxygen consumption
vacancies on Co/c-CeO2 and Co/p-CeO2 are similar, the differ-
peaks appear between 380 1C and 400 1C. The Co/c-CeO2 also
ences in their catalytic performance should be attributed to the
shows an oxygen consumption peak at around 61 1C, which
different facets on which they are exposed. The ideal CeO2(100)
could be attributed to the oxidation of CoOX. The intensity of
surface is a polar plane and its surface is terminated by an
the oxygen consumption peak is related to the oxygen vacancy
O layer. The CeO2(110) and (111) surfaces are nonpolar
and the Ce3+ concentrations. It can be seen that Co/r-CeO2 has
planes (charge neutral) and their surfaces have both exposed
the strongest oxygen consumption peak, followed by Co/c-CeO2
Ce4+ and O atoms (Fig. S11, ESI†). The oxygen vacancies on
and Co/p-CeO2, indicating that the oxygen vacancy and the Ce3+
different crystal planes are in different chemical environments,
concentrations are ranked in the order: Co/r-CeO2 4 Co/c-CeO2
resulting in different activities. Although the oxygen vacancies
4 Co/p-CeO2, which is in accordance with the XPS results.
on Co/p-CeO2 are more than those on Co/c-CeO2, p-CeO2
On the basis of the above results, the structure–activity
mainly exposes the (111) crystal plane, and its activity is worse
relationship between the Co/CeO2 catalysts and the hydrogena-
than that of the (100) crystal plane, resulting in weaker activity of
tion of phenol reaction in water could be ascribed to the
the oxygen vacancies on p-CeO2 than that on c-CeO2. Therefore, the
following reasons: firstly, the different facets of CeO2 have
phenol hydrogenation activity is related to the oxygen vacancies
varied surface structures and electronic features such as the
located in the chemical environment.
number of oxygen vacancies and chemical environments.
Furthermore, H2-TPR was used to explore the reduction
Moreover, the Co/c-CeO2 and Co/p-CeO2 catalysts were more
behaviour of the catalysts. In general, the reduction peak of
hydrophilic than the Co/r-CeO2 catalyst. The competitive
CeO2 can be divided into three temperature zones: 250–400 1C,
adsorption of phenol and water on the surface active sites of
400–600 1C and 600–900 1C, which represent the reduction of
CeO2 affects the dissociation and adsorption of phenol. The
surface oxygen species, subsurface oxygen species and bulk
metal Co nanoparticles were responsible for hydrogen adsorp-
lattice oxygen.25,26 Fig. 5 shows the H2-TPR profiles of the CeO2
tion and dissociation and further reaction for the adsorption
supports with different morphologies and the supported Co
and activation of phenol on the facet of CeO2.
catalysts. The peaks in high temperature regions (4600 1C) can
be attributed to the reduction of lattice oxygen of bulk CeO2.
The r-CeO2, c-CeO2 and p-CeO2 supports exhibit the reduction
peaks at 113 1C, 487 1C and 383 1C, respectively, which could be
Conclusions
attributed to the reduction of CeO2 surface oxygen. Interest- In summary, we immersed CeO2 supports with different morpho-
ingly, the surface oxygen reduction temperature of the support logies into Co2(CO)8 hexane solution and decomposed it at
clearly varied, which indicated the oxygen in the (100) plane 180 1C to form the Co/CeO2 catalysts. The obtained catalysts were
formed stronger chemical bonds with Ce than that in (110) and successfully applied for the hydrogenation of phenol into cyclo-
(111) planes. These results imply that the oxygen located on hexanol. The Co/r-CeO2 catalyst shows much higher phenol
the facets have different oxygen vacancy formation energies. conversion (82.5%) than Co/c-CeO2 (47.9%) and Co/p-CeO2
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020

View Article Online
Paper
NJC
9 V. Vinokurov, A. Glotov, Y. Chudakov, A. Stavitskaya, E. Ivanov,
P. Gushchin, A. Zolotukhina, A. Maximov, E. Karakhanov and
Y. Lvov, Ind. Eng. Chem. Res., 2017, 56(47), 14043–14052.
10 A. Li, K. Shen, J. Chen, Z. Li and Y. Li, Chem. Eng. Sci., 2017,
166, 66–76.
11 Z. Wei, Y. Li, J. Wang, H. Li and Y. Wang, Chin. Chem. Lett.,
2018, 29(6), 815–818.
12 K. Kaneko, K. Inoke, B. Freitag, A. B. Hungria, P. A. Midgley,
T. W. Hansen, J. Zhang, S. Ohara and T. Adschiri, Nano Lett.,
2007, 7(2), 421–425.
(24.7%) at 150 1C and 3 MPa H2. The synergistic effect between
the Co nanoparticles and the CeO2 morphology during the
reaction greatly promoted the reaction. Moreover, compared
with the (111) and (100) plane, exposing the (110) facets on the
r-CeO2 with more active oxygen vacancies and the less hydro-
philic properties enhances the dissociation and adsorption of
phenol. And this work confirms the importance of support
morphology on the hydrogenation of phenol using non-noble
metal based catalysts.
13 C. Pan, D. Zhang, L. Shi and J. Fang, Eur. J. Inorg. Chem.,
2008, (15), 2429–2436.
14 Y. Liu, Z. Li, H. Xu and Y. Han, Catal. Commun., 2016, 76,
1–6.
Conflicts of interest
There are no conflicts to declare.
15 H. F. Wardenga and A. Klein, Appl. Surf. Sci., 2016, 377, 1–8.
16 N. C. Nelson, J. S. Manzano, A. D. Sadow, S. H. Overbury and
I. I. Sowing, ACS Catal., 2015, 5(4), 2051–2061.
17 Z. Ma, S. Zhao, X. Pei, X. Xiong and B. Hu, Catal. Sci.
Technol., 2017, 7(1), 191–199.
Acknowledgements
This research was financially supported by the CAS ‘‘Light of
West China’’ Program.
18 L. Zhu, S. Shan, V. Petkov, W. Hu, A. Kroner, J. Zheng, C. Yu,
N. Zhang, Y. Li, R. Luque, C.-J. Zhong, H. Ye, Z. Yang and
B. H. Chen, J. Mater. Chem. A, 2017, 5, 7869–7875.
19 L. Zhu, Z. Yang, J. Zheng, W. Hu, N. Zhang, Y. Li,
C.-J. Zhong, H. Ye and B. H. Chen, J. Mater. Chem. A,
2015, 3, 11716–11719.
Notes and references
1 N. Yan, Y. Yuan, R. Dykeman, Y. Kou and P. J. Dyson, Angew.
Chem., Int. Ed., 2010, 49(32), 5549–5553.
2 K. L. Luska, P. Migowski, S. El Sayed and W. Leitner, Angew. 20 D. Liu, G. Li, F. Yang, H. Wang, J. Han, X. Zhu and Q. Ge,
Chem., Int. Ed., 2015, 54(52), 15750–15755. J. Phys. Chem. C, 2017, 121(22), 12249–12260.
3 S. Van de Vyver and Y. Roman-Leshkov, Catal. Sci. Technol., 21 G. M. Lyu, Y. J. Wang, X. Huang, H. Y. Zhang, L. D. Sun,
2013, 3(6), 1465–1479. Y. J. Liu and C. H. Yan, Nanoscale, 2016, 8(15), 7923–7932.
4 H. Zhang, A. Han, K. Okumura, L. Zhong, S. Li, S. Jaenicke 22 L. Hu, X. Song, X. Shan, X. Zhao, F. Guo and P. Xiao, ACS
and G. K. Chuah, J. Catal., 2018, 364, 354–365. Appl. Mater. Interfaces, 2019, 11(40), 37209–37215.
5 P. Makowski, R. D. Cakan, M. Antonietti, F. Goettmann and 23 Z. Hu, X. Liu, D. Meng, Y. Guo, Y. Guo and G. Lu, ACS Catal.,
M. M. Titirici, Chem. Commun., 2008, 999–1001. 2016, 6, 2265–2279.
6 G. Yang, J. Zhang, H. Jiang, Y. Liu and R. Chen, Appl. Catal., A, 24 X. Kong, Y. Gong, S. Mao and Y. Wang, ChemNanoMat, 2018,
2019, 588, 117306. 4(5), 432–450.
7 N. Singh, M. S. Lee, S. A. Akhade, G. Cheng, D. M. Camaioni, 25 J. Manuel Lopez, A. L. Gilbank, T. Garcia, B. Solsona,
O. Y. Gutierrez, V. A. Glezakou, R. Rousseau, J. A. Lercher
and C. T. Campbell, ACS Catal., 2019, 9(2), 1120–1128.
S. Agouram and L. Torrente-Murciano, Appl. Catal., B,
2015, 174, 403–412.
8 S. I. Fujita, T. Yamada, Y. Akiyama, H. Cheng, F. Zhao and 26 W. Mista, M. A. Malecka and L. Kepinski, Appl. Catal., A,
M. Arai, J. Supercrit. Fluids, 2010, 54(2), 190–201. 2009, 368(1-2), 71–78.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020