Capping experiments reveal multiple surface active sites in CeO2 and their cooperative catalysis

Article abstract of DOI:10.1039/c9ra02353d

Understanding of surface active sites (SAS) of CeO₂ is crucial to its catalytic applications. In the present study, we have employed capping experiments, DFT calculations, and spectroscopic characterization to study pristine CeO₂ catalyst. We find that multiple SAS coexist on the CeO₂ surface: oxygen vacancies as redox sites and the coordinately unsaturated Ce cations near the oxygen vacancies and the neighboring oxygen ions as Lewis acid-base sites. Dimethylsulfoxide (DMSO), pyridine, and benzoic acid are utilized to cap the redox sites, Lewis acid sites, and base sites, respectively. Selective capping on the redox site does not have much effect on the acid-base catalysis, and vice versa, indicating the distinct surface proximity and independent catalysis of these SAS. We draw attention to a relationship between the well-known redox sites and the surface Lewis acid and Lewis base pairs on CeO₂ surface, which are responsible for driving various heterogeneous catalytic reactions.

Full text of DOI:10.1039/c9ra02353d

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PAPER
Capping experiments reveal multiple surface active
sites in CeO and their cooperative catalysis†
2
Cite this: RSC Adv., 2019, 9, 15229
a
b
b
b
b
Xiaoning Ren, Zhixin Zhang, * Yehong Wang,
Jianmin Lu, Jinghua An,
b
a
a
a
Jian Zhang, Min Wang,
Xinkui Wang
and Yi Luo
Understanding of surface active sites (SAS) of CeO
study, we have employed capping experiments, DFT calculations, and spectroscopic characterization to
study pristine CeO catalyst. We find that multiple SAS coexist on the CeO surface: oxygen vacancies as
2
is crucial to its catalytic applications. In the present
2
2
redox sites and the coordinately unsaturated Ce cations near the oxygen vacancies and the neighboring
oxygen ions as Lewis acid–base sites. Dimethylsulfoxide (DMSO), pyridine, and benzoic acid are utilized
to cap the redox sites, Lewis acid sites, and base sites, respectively. Selective capping on the redox site
does not have much effect on the acid–base catalysis, and vice versa, indicating the distinct surface
proximity and independent catalysis of these SAS. We draw attention to a relationship between the well-
Received 28th March 2019
Accepted 8th May 2019
DOI: 10.1039/c9ra02353d
known redox sites and the surface Lewis acid and Lewis base pairs on CeO
responsible for driving various heterogeneous catalytic reactions.
2
surface, which are
rsc.li/rsc-advances
catalyse different reaction pathways to the complex reaction
Introduction
21–26
products.
It is generally considered that selective oxidation
Identifying and understanding the location and the nature of of organic compounds almost exclusively involves redox cycles
the surface-active sites (SAS) for a given catalyst is an oen- during which lattice oxygen oxidizes the organic molecule
1
–5
sought goal in the eld of catalysis, and is helpful for the leaving an oxygen vacancy that is healed by O
2
, i.e. the so called
27,28
2ꢀ
rational design of catalysts. The complexity of a heterogeneous Mars–van Krevelen (MVK) mechanism.
catalyst, however, makes this a challenging objective to achieve. in CeO
Lattice oxygen (O
)
2
9
2
is the active species in the oxidation of hydrocarbon
Common lab techniques using electron, photon, or X-ray as and the surface oxygen vacancies act as “activation sites” for the
probes are employed to detect the SAS and to establish the oxygen. On the other hand, most authors agree that the low-
structure–performance relationship, such as scanning coordinated Ce cations or oxygen vacancy act as Lewis acid
tunneling microscopy (STM), in situ infrared spectroscopy (IR), sites and the neighbouring oxygen ions behave as Lewis base
6–9
30–33
and edge X-ray absorption ne structure (EXAFS). But these sites in the catalytic cycle.
And the quantitative relations
methods usually offered ex situ information on a catalyst between the properties and activity or selectivity in Lewis acid–
3
4,35
surface, which is far from realistic conditions. A more reliable base catalysis reactions have also been proposed.
way is to use probing methods to study SAS.
Thus, one
10–13
This requires remaining uncertainty in these studies is the natural difference
the careful selection of suitable probe molecules and reactions, between the Lewis acidic sites and oxygen vacancies. Besides,
together with in situ characterization techniques. Versatile structural proximity and the relation between the Lewis acid–
14
probe molecules (or capping agents) with unique structures base sites and redox sites are still ambiguous.
and adsorption site can be selectively adsorbed on catalytic SAS
Capping experiments with suitable probe molecules may be
sites, and thus provide information of SAS at the molecular a promising approach to uncover this ambiguous veil, because
level.
these capping experiments are usually simple and reliable to
Cerium oxide (CeO ) is a multifunctional catalyst consisting distinguish the multiple SAS over the catalyst surface. Suitable
2
of three types of SAS: redox sites, Lewis acid sites, and Lewis capping agent can block one of SAS and have no effect on the
1
5–20
base sites.
These three sites are able to synergistically others. Therefore, it may make a positive or negative effect on
28
the resulting activity/selectivity of a certain probing reaction.
For examples, Tamura and Tomishige et al. have reported that
-cyanopyridine interacted with the Lewis acid sites of CeO and
enhanced its Lewis base property and the corresponding cata-
a
State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of
Technology, Dalian 116024, Liaoning, China
2
2
b
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, lytic performance. Addition of Br progressively decreases the
Liaoning, China. E-mail: zhangzhixin@dicp.ac.cn
36
2
reactivity of Au/TiO catalyst for the water-gas shi (WGS)
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra02353d
5
10,37
reaction and CO oxidation.
1,10-Phenanthroline has served
1
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as a capping agent to block the catalytically active sites of Ru-
based catalyst for the benzene hydrogenation. Recently, we a typical reaction, the catalyst was added into a screw-capped
Oxidative coupling of acetophenone with benzaldehyde. In
12
have reported several work on the redox and acid–base prop- glass pressure vessel with a stirring bar. A solution of aceto-
2
2,38–41
erties of CeO2,
their single or dual properties, respectively.
we study the multiple SAS of CeO via the capping experiments, to the set value in an oil bath. The calculation of conversion and
DFT calculations, and co-adsorption IR spectroscopy. Depend- selectivity is based on acetophenone.
ing on these experiments, the location and correlation of the
Oxidation of benzyl alcohol. The catalytic reactions were
redox and Lewis acid–base sites of CeO have been proposed. conducted in a screw-capped glass pressure vessel. Typically,
and related the catalytic performance with phenone (0.5 mmol) and benzaldehyde (0.75 mmol) in 2 mL
17,30–33
In this paper, solvent was added. The reactor was tightly screwed and heated
2
2
Unlike previous studies on a single active site of CeO under benzyl alcohol (0.75 mmol), catalyst (100 mg), p-xylene (2 mL),
2
ideal conditions, our study herein provide a global under- and a magnetic stir bar were loaded into the reactor. The reactor
standing of multiple active sites of CeO catalyst under working was sealed and placed in an oil bath preheated to the desired
2
conditions.
temperature for the reaction.
Hydrolysis of 1,3-dioxane. Typically, 1,3-dioxane (1.5 mmol),
catalyst (100 mg), water (2.0 mL), and a magnetic stir bar were
loaded into a Teon-lined autoclave reactor. The reactor was
sealed and placed in a preheated mantle at the desired
temperature. Six parallel reactions could be conducted in order
to achieve the same reaction conditions. Each reaction was
continued for the desired time before sampling for analysis.
All the product's mixtures were analyzed by gas chromatog-
raphy (GC, Agilent 7890A) and gas chromatography-mass
spectrometry (GC-MS) using an Agilent 7890A/5975C instru-
ment equipped with an HP-5MS column (30 m in length,
Experimental section
Chemicals and reagents
All chemicals were of analytical grade, purchased from Aladdin
Chemicals, and used without further purication.
Preparation of CeO
The CeO sample was prepared by a conventional precipitation
method, which was reported by our group. Briey, 5.0 g of
Ce(NO $6H O was dissolved in 100 mL of Millipore-puried
water (18 mU cm), and the solution was adjusted to pH ¼
1.0 by the addition of NH OH (3.4 M) under magnetic stirring
2
catalysts
2
3
8
3
)
3
2
0.25 mm in diameter). Authenticated samples were utilized to
quantify the reactants and products.
1
4
DFT calculation settings. The periodic plane wave based
at room temperature. The resulting gel mixture was washed
density functional theory program VASP (Vienna Ab Initio
ꢁ
with pure water, dried in an oven at 120 C for 12 h, and
42,43
Simulation Package)
has been employed to perform all the
ꢁ
ꢀ1
calcined at 550 C in air (50 mL min ) for 4 h.
DFT calculations. The electron–ion interactions are described
by the projector-augmented wave method (PAW), which is
a frozen core all-electron method using the exact shape of the
General characterizations of CeO catalysts
2
44
valence wave functions instead of pseudo-wave functions. The
exchange correlation energy has been calculated within the
generalized gradient approximation by the Perdew–Burke–Ern-
Pristine CeO2 was characterized by powder X-ray diffraction
conducted with a PANalytical X-Pert PRO diffractometer using
Cu-Ka radiation at 40 kV and 20 mA. The sample was scanned
45,46
ꢁ
ꢁ
zerhof formulation (GGA-PBE).
The kinetic energy cutoff of
over a 2q range of 10 to 80 . The morphology of pristine CeO
was examined by transmission electron microscopy (TEM, JEM-
000). The defect structure and oxygen vacancies of CeO were
determined by Raman spectroscopy recorded on a micro-
Raman spectrometer (Renishaw) equipped with CCD
2
plane-wave basis sets was xed to 500 eV in all calculations.
2
4
2
6
2
2
s 2p electrons of O and 5s 5p 4f5d6s of Ce are explicitly
2
2
47
taken into consideration. We used DFT + U corrections to
48
describe the CeO with the value of U ꢀ J ¼ 4.5 eV for Ce 4f
2
a
orbitals.
detector using a He/Ne laser with a wavelength of 532 nm.
This level of theory computes the lattice constant of a ¼
˚
5
.495 A for bulk CeO
2
, in reasonable agreement with the
Catalytic reactions and product analyses
49
˚
experimental value (a ¼ 5.411 A). This lattice constant has
Oxidative coupling of aniline with benzyl alcohol. In a typical been used to construct a periodic CeO (111) p (3 ꢂ 3) slab with
2
reaction, the catalyst was weighed into a screw-capped glass 9 atomic layers (which are equal to 3 stoichiometric layers)
pressure vessel containing a stir bar. The required amounts of separated by a vacuum layer of 15 A˚ to eliminate interactions
alcohol (0.5 mmol) and amine (0.6 mmol), pre-dissolved in between the slab and its periodic images. In total, there are 27
a solvent (2 mL), were added and the vessel was lled with Ce atoms and 54 O atoms in the slab. The bottom 4 atomic
oxygen, sealed, and heated to the desired temperature in an oil layers are xed to their optimized bulk conguration during all
bath with stirring.
computations, and the top 5 atomic layers and surface inter-
Transformylating amine with DMF. The catalytic reactions mediates are fully relaxed. All atomic coordinates of the
were conducted in a screw-capped glass pressure vessel with adsorbates and the atoms in the relaxed layers are optimized to
ꢀ
1
a stirring bar. Typically, benzylamine (1.5 mmol), N,N-dime- a force of <0.03 eV A˚ on each atom. All self-consistent eld
ꢀ
5
ꢀ1
thylformamide (DMF, 2 mL), and catalyst (100 mg) were placed calculations are converged to 1 ꢂ 10 kJ mol . Brillouin zone
in the reactor. The reactor was then immersed in an oil bath
preheated at the desired reaction temperature.
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integration is performed using a 3 ꢂ 4 ꢂ 1 MonkhorstꢀPack the redox sites of CeO
2
with the assistance of Lewis acid sites.
The reaction consists of the oxidation of benzyl alcohol and
grid and a Gaussian smearing of 0.05 eV.
FT-IR of co-adsorption over CeO . The co-adsorption of condensation between aniline and benzaldehyde. The oxidation
2
pyridine and methanol was carried out on a Bruker Tensor 27 requires the redox sites, and the condensation is favourable and
instrument in absorbance mode. The sample was pressed into will be promoted by the Lewis acid sites. We here selected the
a self-supporting disk (13 mm diameter) and placed in reaction as a model reaction to test the redox property of CeO .
2
a homemade IR cell attached to a closed glass-circulation In the absence of the capping agents, the reaction smoothly
ꢁ
system. Prior to the adsorption, the sample disk was pre- proceeded at 60 C to give 99% conversion of benzyl alcohol
ꢁ
treated by heating at 400 C for 1 h in H
2
stream and then with 94% selectivity of imine and 6% selectivity of benzaldehyde
ꢁ
ꢀ3
cooled to 30 C, meanwhile evacuated to <10
Pa. Aer (Table 1, entry 1). Conversions of benzyl alcohol decreased to
a spectrum was collected, the sample disk was exposed to 8% and 66% aer adding 0.58 mmol (¼ mole of the catalyst
pyridine vapor. IR spectra of the chemisorbed pyridine were added) of DMSO and DMF, respectively. Correspondingly, the
ꢁ
ꢁ
ꢁ
recorded aer evacuation at 30 C and 150 C for 0.5 h, selectivities of benzaldehyde increased to 95% and 61%,
respectively. Aer desorption of pyridine at 150 C, the meth- respectively, indicating that the condensation between benzal-
anol vapor was introduced to the sample disk. IR spectra of the dehyde and aniline is also affected, which may be caused by the
chemisorbed methanol were also recorded aer evacuation at steric effect of –CH
3
in DMSO and DMF on the Lewis acid sites
ꢁ
ꢁ
3
0
C and 150 C for 0.5 h, respectively. All spectra were over CeO
2
(Table 1, entries 2, 3). Furthermore, the reaction was
ꢁ
collected aer cooling to 30 C. The spectra were also recorded completely suppressed in solvent of DMSO or DMF (Table 1,
22,40
aer adsorbed methanol and then pyridine in the same entries 5, 6). This result is consistent with our previous work,
manner.
which have found the oxidative coupling of benzyl alcohol with
aniline or acetophenone did not occur when the DMSO or DMF
was used as solvent. These results reveal that DMSO and DMF
Results and discussion
can inhibit the redox cycling of CeO
2
and have an impact on
Lewis acid sites over CeO . In addition, DMSO or DMF could be
2
The redox and acid–base properties of CeO2 are important
parameters that provide an opportunity to activate complex
organic molecules and to generate the target products selec-
used as in situ capping agent at the redox site over the CeO
2
surface.
21,50
Furthermore, methanol dissociates on the CeO
surface to
2
tively.
2
Pristine CeO is employed in several acid–base catal-
16
form methoxy species, which can also adsorb on the oxygen
ysis reactions, such as the dehydration of alcohol, the
alkylation of aromatic compounds, ketone formation and
aldolization,
water-gas shi,
6
2–64
51
vacancy
and may affect the catalytic activity of oxidative
catalyst.
5
2,53
54
55
coupling. Methanol also acts as capping agent of CeO
2
and redox reactions, such as CO oxidation,
5
6,57
41,58
It can be seen that a small amount of methanol has little effect
on the catalytic performance (98% conversion of benzyl alcohol
with 96% selectivity of imine, Table 1, entry 4). However, when
the reaction was conducted in the solvent of methanol, the
catalytic activity was inhibited (24% conversion of benzyl
and alcohol oxidation.
In this study,
several probe reactions, including oxidative coupling of aniline
with benzyl alcohol for redox and acid sites, transformylating
amine with DMF for acid and base sites, oxidative coupling of
acetophenone with benzaldehyde for redox and base sites,
oxidative of benzyl alcohol for redox sites, and hydrolysis of 1,3-
dioxane for acid sites, were selected to detect the SAS of CeO
according to the selective capping of SAS.
2
Table 1 Oxidative coupling of aniline with benzyl alcohol over CeO
catalyst
2
Before these probe reactions, the general structure infor-
a
mation of pristine CeO was examined by the TEM, XRD, and
2
Raman spectroscopy (see ESI, Fig. S1†). CeO nanopolyhedrons
2
with about 10 to 20 nm were observed in the TEM image
ꢁ
ꢁ
ꢁ
(
5
Fig. S1a†). The diffraction peaks at 2q ¼ 28.7 , 33.2 , 47.5 , and
ꢁ
5.6 were corresponded to the crystal planes (111), (200), (220),
5
9
and (311) of CeO
2
with the uorite-type structure (Fig. S1b†).
and
Sel. (%)
The Raman spectrum shows the defect structure of CeO
2
b
c
Entry Solvent
Capping agents
Conv. of 1 (%)
3
4
the relative concentration of oxygen vacancies was about 0.052
calculated by curve-tting based on the peak areas of 462 and
1
2
3
4
5
6
7
p-Xylene
p-Xylene DMSO
p-Xylene DMF
p-Xylene CH OH
3
DMSO
DMF
—
>99
8
94
5
39
96
6
ꢀ1
5
90 cm , which have been ascribed to the Raman-active F
95
61
4
2
8,60
g
3
66
98
<1
<1
24
vibrational mode and the signal of oxygen vacancies
(Fig. S1c†).
—
—
—
N.D. N.D.
N.D. N.D.
98
CH
3
OH
2
Inhibit the redox sites of CeO
catalysis
2
and conduct the acid–base
a
Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), CeO (100 mg),
2
ꢁ
b
solvent (2 mL), 60 C for 12 h, in 0.1 MPa oxygen. 0.58 mmol of
capping agent. The conversions and selectivities were determined by
61
40
Tamura's work and our previous study have shown that the
c
oxidative coupling of aniline with benzyl alcohol occurred on GC based on benzyl alcohol. N.D. ¼ not detection.
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alcohol), but has minor effect on Lewis acid sites and results in Table 2 Transformylation of aniline (or benzyl amine) with DMF over
a
2
CeO catalyst
98% selectivity of imine (Table 1, entry 7). The different
behaviour between DMSO, DMF, and CH OH could be ascribed
3
the different binding affinity of DMSO, DMF, CH OH on CeO .
3
2
The adsorption energy of these capping agents over CeO
2
may
elucidate this difference and will be discussed in the following
text (see DFT calculations).
TEM images (Fig. S2†) show that the morphologies of CeO
before (Fig. S2a†) and aer treatment with DMF (Fig. S2b†) or
Entry
1
Solvent
Amines
Conv. (%)
Yield (%)
2
DMF
DMSO
Aniline
Aniline
Aniline
Benzyl amine
Benzyl amine
Benzyl amine
14
20
18
25
18
17
14
20
18
25
18
17
DMSO (Fig. S2c†) under the similar conditions with the above
b
2
3
ꢁ
c
reaction (100 mg of CeO in 2 mL of solvent, 60 C for 12 h, in
CH OH
2
3
0.1 MPa oxygen). Irregular CeO2 nanoparticles with similar
4
DMF
DMSO
CH OH
3
b
5
morphologies were observed in these three samples. Slight
aggregation of nanoparticles was observed aer the treatment
with DMF or DMSO.
c
6
a
Reaction conditions: 1.5 mmol amines, CeO
00 C, 4 h, in Ar. The conversions and yields were determined by GC
2
(100 mg), DMF (2 mL),
ꢁ
1
XPS analysis was used to determine the oxidation states of
b
based on amines and formamides, respectively. DMSO (2 mL),
various species in the CeO
2
treated in DMSO or DMF under the
ꢁ
c
ꢁ
3
mmol DMF, 100 C, 4 h. CH OH (2 mL), 3 mmol DMF, 100 C, 4 h.
3
similar conditions. As shown in Fig. S3a,† the survey XPS
spectra of the CeO , CeO -DMF (CeO treated in DMF), and
2
2
2
CeO -DMSO (CeO treated in DMSO) indicate the presence of Ce
2
2
amine) with DMF (only as substrate) was conducted in DMSO or
CH OH, the conversions of amines reached 20% (18%) and 18%
17%), respectively, with excellent selectivities (>99%) of corre-
sponding formamides (Table 2 entry 2, 3, 5, 6). According to the
discussions of the above paragraph, the redox sites on CeO2
and O. The Ce 3d spectra of these three samples were shown in
Fig. S3b† and the complex spectra were deconvoluted into 10
components with the assignment as showed in Fig. S4,† in
3
(
3
+
which v
0
, v
2
, u
0
, and u
, v , v
2
peaks are ascribed to the Ce ions, and
4
+
the rest peaks (v
1
3
4
, u
1
, u
3
, and u
4
) are attributed to the Ce
The content of Ce near the surface zone were semi-
quantitatively calculated based on the following equation:
surface were blocked in DMF, DMSO, or CH
indicate that the acid–base catalysis by CeO
3
OH. These results
could proceed
65,66
3+
ions.
2
67
when the redox cycling was inhibited.
À
þÁ
3
c Ce
Inhibit the Lewis acidic sites of CeO
base catalysis
2
and conduct the redox–
A
v0 þ Av2 þ Au0 þ Au2
¼
Av0 þ Av1 þ Av2 þ Av3 þ Av4 þ Au0 þ Au1 þ Au2 þ Au3 þ Au4
Pyridine strongly reacts with the acidic sites of the catalyst, and
usually acts as a probe molecule to determine the acidity of the
72–74
3
+
catalyst.
In this work, pyridine was used as the capping
The calculation results show that the content of Ce in these
3
+
agent to inhibit the Lewis acidic sites of CeO . In the presence of
2
three samples was in the range of 30–35%. The Ce content of
5% pyridine, the hydrolysis of 4-methyl-1,3-dioxane was
CeO treated in DMF (33%) or DMSO (35%) is slight higher than
2
remarkably suppressed (the 1,3-butanediol yield was only 7%,
that of pristine CeO (30%), indicating the slight reduction of
2
38
4
+
3+
which was 51% without pyridine) (Table 3, entries 1 and 2).
These results indicate that pyridine can block the Lewis acidic
site via the interaction of pyridine with Ce cation sites. The
oxidative coupling of acetophenone with benzyl alcohol usually
catalyzed by bifunctional catalysts, which consist of redox sites
for alcohol dehydrogenation and basic sites for the aldol
Ce to Ce during the treatment. This result is similar with the
68,69
literature,
which reported that the DMF and DMSO are
capable to reduce Au, Ag, Cu metal ions to the corresponding
metallic nanoparticles. However, the difference of catalytic
performance in p-xylene (Table 1, entry 1) and DMSO (or DMF)
(Table 1, entries 5 or 6) is obvious, indicating another factor may
22,75,76
condensation of benzaldehyde with acetophenone.
When
affect the activity of the CeO in the oxidative coupling reaction.
2
70,71
the reaction was performed in the presence of pyridine, the
acetophenone conversion and the selectivity of the target
product were almost the same as the conversion and selectivity
According to the literature,
the metal oxide can be capped
with DMSO via the oxygen atom coordinated with metal ions on
the surface of nanoparticles. We speculate that the oxygen atom
of DMSO or DMF could strongly adsorb on the oxygen vacancy
and block the redox cycle. The adsorption states and energies of
22
without the addition of pyridine. (Table 3, entries 3, 4). These
results show that the redox and basic sites are still effective even
the Lewis acidic sites are capped.
2
DMSO and DMF over CeO can support our speculation and
discussed in the following text (see DFT calculations).
Transformylation of amines with DMF requires the bifunc-
tional acid–base sites as proved by our previous work. The N-
Inhibit the Lewis basic sites of CeO and conduct the redox
and acid catalysis
2
39
phenylformamide and benzyl amide were obtained with 14% The aldol condensation of benzaldehyde with acetophenone
77
and 25% yields in the transformylation of aniline and benzyl was usually catalyzed by base catalysts. Benzoic acid can
78
amine with DMF (as solvent and substrate), respectively (Table poison basic site and was used as the capping agent for CeO2
, entries 1, 4). Even if the transformylation of aniline (benzyl catalyst. Fresh CeO treated with benzoic acid (denoted as CeO -
2
2
2
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a
Table 3 4-Methyl-1,3-dioxane hydrolysis and oxidative coupling of However, aldol condensation did not occur over the CeO
-BA
Fig. 1a). This result is further proved by our preceding work, in
2
b
2
benzyl alcohol with acetophenone over CeO catalyst
(
which the oxidative coupling of acetophenone with benzyl
alcohol was completely suppressed in the presence of benzoic
22
acid. These results reveal that the benzoic acid can poison the
Diol Sel.
(%)
Yield of diol basic site on the CeO
surface by post-treatment or in situ
2
c
d
Entry
Capping agent
Conv. (%)
(%)
capping. It should be noted that aldol condensation requires
fresh benzaldehyde or clean benzaldehyde distilled carefully, or
1
2
—
Pyridine
51
7
>99
>99
51
7
2
the reaction does not catalyze by the CeO catalyst; since it is
facile for the oxidation of benzaldehyde to benzoic acid even if
in the absence of catalyst.
38
Oxidation of benzyl alcohol and hydrolysis of 1,3-dioxane
were utilized to test the redox and Lewis acidic property of fresh
CeO and CeO -BA. CeO -BA catalyst shows a comparable
catalytic performance with fresh CeO , indicating that the redox
sites and Lewis acidic sites over the surface of CeO are unaf-
fected by benzoic acid (Fig. 1 and Tables S2 and S3†).
2
2
2
Sel. (%)
2
c
e
Entry Capping agent
Conv. (%)
6
7
4
8
2
f
f
3
4
—
Pyridine
39 (43)
37 (31)
96 (93) 2 (1)
99 (92) — (—) — (8)
— (6)
2
1
a
Reaction conditions: 4-methyl-1,3-dioxane (0.16 mL, 1.5 mmol), CeO
2
ꢁ
38
b
(0.1 g), water (1.0 mL), pyridine (0.075 mmol), 100 C, 4 h.
5 (0.5
Inhibit the redox sites and Lewis acidic sites of CeO and
ꢁ
2
mmol), 1 (0.75 mmol), CeO
2
(100 mg), p-xylene (2 mL), 100 C, for
c
d
conduct the base catalysis
1
2 h, under air. 0.58 mmol of capping agent.
The conversions
e
based on 4-methyl-1,3-dioxane were determined by GC-MS. The
We speculate that Lewis acidic sites of CeO2 are next to its
GC-GS. Data in parenthesis indicate the conversion and selectivity oxygen vacancies. According to the discussion above and
conversions based on acetophenone consumption were determined by
f
22
based on benzyl alcohol consumption.
previous work, the pyridine adsorbed on the Lewis acidic
72–74
sites.
If a substituent located on the 2-position of the pyri-
dine would affect the oxygen vacancy, it may affect the Lewis
acidic sites and redox sites simultaneously, so it would suppress
the catalytic performance involved oxidation reactions. 2-Cya-
nopyridine, which has a cyano group at the ortho-position of the
pyridine ring, was selected as a capping agent to block the Lewis
acidic sites and redox sites simultaneously. The oxidative
coupling of benzyl alcohol with aniline was chosen as a probe
reaction to detect the shielding effect (inhibit the redox sites
BA) would only expose redox and acid sites. FT-IR spectra
Fig. S5†) of fresh CeO and CeO -BA have shown the presence
of benzoic acid adsorbed on the surface of CeO . CO -TPD
Fig. S6†) of fresh CeO and CeO -BA have also proved that the
concentrations of basic sites decrease from 85.6 to 24.8 mmol
(
2
2
2
2
(
2
2
ꢀ1
g
aer the fresh CeO treated with benzoic acid.
2
The fresh CeO gave an increasing conversion of acetophe-
2
and Lewis acidic sites of CeO and conduct the base catalysis).
2
none for the aldol condensation (Fig. 1a and Table S1†).
Table 4 Effect of the amount of 2-cyanopyridine (2-CN-Pyr) on the
a
synthesis of imine
Sel. (%)
b
c
d
Entry 2-CN-Pyr/CeO
2
Density of 2-CN-Pyr Conv. (%)
3
4
1
2
3
4
5
6
7
8
1/1
1/4
5800
1450
145
<1
<1
<1
23
53
67
97
>99
—
—
—
99
99
98
97
97
>99
>99
>99
1
1
2
1/40
1/50
1/60
1/80
1/160
1/320
Fig.
2 2
1 Time-on-stream profiles over fresh CeO and CeO -BA.
116
Reaction conditions: (a) 5 (0.5 mmol), 4 (0.75 mmol), catalyst (100 mg),
p-xylene (2 mL), 150 C, under Ar. The conversions based on 5
consumption were determined by GC; (b) 1 (0.75 mmol), catalyst (100
mg), p-xylene (2 mL), 150 C, 1 atm air. The conversions based on 1
ꢁ
96.7
72.5
36.3
18.1
ꢁ
3
3
consumption were determined; (c) 10 (1.5 mmol), catalyst (100 mg),
ꢁ
2
H O (2 mL), 150 C. The conversions based on 10 consumption were
a
Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), CeO
2
(0.58 mmol, 100
determined. The CeO -BA was fresh CeO treated by benzoic acid in
2
2
ꢁ
b
mg), p-xylene (2 mL), 60 C, for 12 h, under oxygen. The molar ratios (n/
and then centrifugation, washed with ethanol for three times, and n) of 2-CN-Pyr/CeO2. Unit: mmol
c
ꢀ1
d
O
2
g
.
The conversions and
ꢁ
dried at 100 C.
selectivities based on 1 consumption were determined by GC.
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In the presence of 2-cyanopyridine, the reactions were sup-
pressed completely until the mole amount of 2-cyanopyridine
was less than 1/40 of the added CeO (Table 4, entries 1–3). The
2
conversion of benzyl alcohol increased gradually from 23% to
9
9% as the decrease of 2-cyanpyridine amount from 116 to 18.1
ꢀ
1
mmol g (Table 4, entries 4–8). 2-Cyanopyridine inhibited the
oxidation reaction and blocked the redox sites, indicating that
the Lewis acidic sites were adjacent to the oxygen vacancies
(redox sites). Thus, 2-cyanopyridine can be used as capping
agent for redox sites and Lewis acidic sites of CeO , meanwhile
2
its Lewis basic sites are not affected and can promote the base-
catalysed reaction, such as transesterication of methyl
benzoate with 1-hexanol, Knoevenagel condensation from
benzaldehyde and ethyl cyanoacetate, and hydromethoxylation
Fig. 3 Optimized structures and adsorption energies (eV) of DMSO,
2
DMF, and methanol (MeOH) adsorption on CeO (111) with one oxygen
vacancy: (a) DMSO, (b) DMF, and (c) MeOH.
36
of acrylonitrile, reported by the Tamura and Tomishige.
On the other hand, based on the amount of 2-cyanopyridine,
the concentration of oxygen vacancy was estimated and the
value was in the range of 145 to 116 mmol g . This result also
and their adsorption energies (DE) are shown in Fig. 3a–c.
Adsorption energy (DE) is dened as the electronic energy of the
adsorption system relative to that of the isolated system. Thus,
a negative value means an exothermic process. Taking DE into
consideration, DMSO and DMF prefer the adsorption through O
over the oxygen vacancy. The adsorption of DMSO is higher
than that of DMF (ꢀ1.68 vs. ꢀ1.51 eV). This can be attributed to
the higher basicity of O in DMSO than that in DMF. This result
indicates DMSO and DMF can block the redox sites, and the
effect of DMSO is higher than that of DMF, which is in agree-
ment with the catalytic results (Table 1, entries 2, 3; 8 vs. 66%
conv.) DMSO and DMF also have an effect on the catalytic
performance (different selectivities) of Lewis acid sites over
CeO2 (Table 1, entries 2, 3), also indicating that the nearest
neighbour between the redox sites and Lewis acid sites. MeOH
ꢀ
1
indicated that 2-cyanopyridine adsorption on CeO catalyst can
2
be used to evaluate oxygen vacancy dispersion and the
concentration of redox sites/Lewis acidic sites of CeO .
2
The locations of SAS of CeO
sites)
2
(redox sites and Lewis acid–base
The above results imply the independent catalytic functions of
the SAS over CeO surface, even if one of the SAS was poisoned,
the catalytic roles of others were not inhibited. Based on the
above discussion, the locations of SAS of CeO have shown in
2
2
Fig. 2. The oxygen vacancies work as the redox sites via the MVK
mechanism. Coordinatively unsaturated cerium sites next to
the surface oxygen vacancies functioned as Lewis acid sites and
the neighbouring oxygen ions behaved as Lewis base sites in the
3
can be dissociated to form –H and –OCH , and the –OCH3
adsorbs on an oxygen vacancy via the O atom. The adsorption
30–33
energy of MeOH (ꢀ0.39 eV) is lower than that of DMSO (ꢀ1.68
eV) and DMF (ꢀ1.51 eV). These different adsorption energies
can explain the different binding affinity of DMSO, DMF, MeOH
catalytic cycle. This speculation is similar to the literature
2
and gives a clear picture for the SAS of CeO .
2
on CeO and the corresponding catalytic performance in the
presence of these three capping agents (Table 1).
DFT calculations
In order to verify the above hypotheses, the adsorption states of
Benzaldehyde (BD) was also compared to that of benzoic acid
DMSO, DMF, methanol (MeOH), benzaldehyde (BD), and ben- (BA) on (111) surface of CeO . The optimized structures of BD
2
zoic acid (BA) on CeO (111) were examined by DFT calculations. and BA and their adsorption energies (DE) are shown in Fig. 4a
2
First, the adsorption states (top view and side view) of DMSO
and DMF were compared with that of MeOH on (111) surface of
CeO , which is the most stable plane among the three low index
2
planes. The optimized structures of DMSO, DMF and MeOH
Fig. 4 Optimized structures and adsorption energies (eV) of benzal-
2
dehyde (BD) and benzoic acid (BA) adsorption on CeO (111) with one
2
Fig. 2 A schematic diagram of the location of SAS of CeO .
oxygen vacancy: (a) BD and (b) BA.
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and b. Taking DE into consideration, BA prefers the adsorption on-top, doubly bridging or triply bridging. Because of the presence
3+
4+
through the two O atoms near an oxygen vacancy (ꢀ2.89 eV). of oxygen vacancies, two kinds of Ce cationic sites (Ce /Ce ) exist.
And the adsorption of BD is endothermic (0.16 eV), indicating Thus, six kinds of adsorbed species coexisted on the surface of
the weak adsorption of BD on CeO . These results imply that the CeO and have been identied by IR spectroscopy (Fig. 6). In these
2
2
BD is not the poison for the basic sites over CeO
2
and the BA is six adsorbed sites, methoxy species adsorbed on the triply bridging
3+
the real poison during aldol condensation of benzyl alcohol/ of Ce (III-B), i.e. adsorbed on the oxygen vacancy. Because it is
benzaldehyde with acetophenone. difficult to discriminate between n(CH ) bonds according to the
Pyridine and CO adsorption on the CeO can probe the different methoxy coordinations, n(OC) bond was used to distin-
locations of the Lewis acid and base sites on the CeO surface. guish the distinctive cationic adsorption sites.
The adsorptions of pyridine and CO on the CeO (111) with one Pyridine molecule is one of the most frequently used probes to
3
2
2
2
2
2
35,72,73,79
oxygen vacancy have also been computed based on the DFT characterize the acidic properties of solid surfaces.
Accord-
calculations. Several adsorption models of pyridine and CO on ing to the literature of pyridine adsorption on metal oxides, coor-
2
CeO
2
(111) with one surface oxygen vacancy have been con- dinative pyridine on Lewis acid sites shows bands around 1445
ꢀ
1
39
structed, and the corresponding adsorption energies have been and 1610 cm . Based on our previous work and other
35,38
calculated. The top and side views of pyridine and CO
adsorption on CeO (111) with one oxygen vacancy were pre- the Lewis acid sites (on-top sites of Ce ) were observed around
sented in Fig. 5. It can be seen that pyridine is adsorbed on the 1440 and 1595 cm [Fig. 7A-a and A-b].
111) crystal face of CeO , which is optimized to occur at the Ce If the pyridine adsorbed on the on-top sites of Ce rstly, the
site next to the oxygen vacancy with ꢀ1.34 eV (Fig. 5a), indi- methoxy species only adsorbed on the on-top (I-A), doubly (II-A) or
2
studies,
2
for CeO , bands assignable to coordinative pyridine on
3+
2
ꢀ
1
3+
(
2
4+
3+
cating the Ce site next to the oxygen vacancy is the Lewis acid triply (III-A) bridged to Ce sites and the triply bridged of Ce
site. In addition, CO is adsorbed on the O site next to the Ce sites/oxygen vacancies (III-B). Conversely, if methanol adsorbed on
sites with ꢀ0.58 eV, which is next to the oxygen vacancy the SAS of CeO rstly, which will occupy almost all the SAS, the
Fig. 5b), indicating the O ion site near the oxygen vacancies is pyridine adsorbed will be inhibited. In this manner, the oxygen
the Lewis base site. These results also conrmed the locations vacancies and Lewis acidic sites would be identied. Take into
2
2
(
of SAS over CeO
experiments.
2
surface proposed based on the capping account this speculation, co-adsorption of pyridine and methanol
was conducted. In order to amplify the signal of adsorbed species
3+
ꢁ
on Ce , the CeO samples were reduced at 400 C under H stream
2
2
ꢀ
1
(
30 mL min ) before the adsorption, and the sample was denoted
by CeO -H. Firstly, FT-IR spectra of species formed from the
introduction of pyridine and then methanol on the CeO -H was
Probing multiple SAS of CeO
methanol and pyridine
2
via the co-adsorption FT-IR of
2
2
Methanol and pyridine have served as adsorption probe molecule recorded (Fig. 7A). Bands assignable to coordinative pyridine on
3+
2
2
using the FTIR to reconrm the locations of SAS over CeO surface. the on-top sites of Ce were observed for CeO -H (1440 and
ꢀ
1
ꢁ
Methanol is a smart probe molecule and has been applied to 1595 cm ) (Fig. 7A-a, asterisks). These bands were stable at 150 C
62–64
ꢀ3
study the surface sites of CeO -based catalysts.
Its dissociative under the vacuum (1.2 ꢂ 10 Pa) (Fig. 7A-b, asterisks). Once the
2
adsorption on the CeO surface to form methoxy species requires methanol adsorbed on the sample, these two bands were overlaid.
2
62
the cation/anion site pairs (i.e. Lewis acid–base pairs), and the as- In the vibration range of C–O, bands assigned to I-A, II-A, III-A, and
formed methoxy structure is dependent on the coordinative envi- III-B were observed (Fig. 7A(c and d)), indicating that the methoxy
ronment of the surface Ce sites and the presence of oxygen species could adsorbed on the oxygen vacancies in the presence of
63
vacancies. Thus, methanol may be a promising probe molecule pyridine. It also reveals that the diverse locations of oxygen
to study the difference and location of the Lewis acid–base pairs vacancies and Lewis acidic sites.
and the redox sites (oxygen vacancies) of CeO
2
. According to these
2
When methanol was introduced to the CeO -H sample
62–64
literature,
methoxy species adsorbed on the Ce sites are either before the adsorption of pyridine, all of the n(OC) bonds of
methoxy species adsorbed on CeO were observed (Fig. 7B(a–d)).
2
Fig. 5 Optimized structures and adsorption energies (eV) of pyridine
a) and CO (b) on CeO (111) with one oxygen vacancy.
62–64
(
2
2
Fig. 6 CH
3
OH adsorbed on the CeO
2
(according to the literature
).
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21706250, 21711530020, and 21721004)
and supported by the “Strategic Priority Research Program of
the Chinese Academy of Sciences” Grant No. XDB17000000,
XDB17020300, and the Department of Science and Technology
of Dalian under the contract 2017RQ114.
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