Insight into the Synergism between Copper Species and Surface Defects Influenced by Copper Content over Copper/Ceria Catalysts for the Hydrogenation of Carbonate

Article abstract of DOI:10.1002/cctc.201701384

Various Cu/CeO₂ nanorod catalysts with different copper contents were synthesized through a facile coprecipitation method for the hydrogenation of diethyl carbonate to methanol in a fixed-bed reactor. The methanol yield increased upon increasing the copper loading, but large excess amounts of copper led to deactivation of the catalyst at a high liquid hourly space velocity because of aggregation of the active metal. The 20Cu/CeO₂ and 30Cu/CeO₂ catalysts displayed superior catalytic performance with methanol space-time yields of 8.4 and 8.1 mmol g_cat ^?1 h^?1, respectively. The remarkably enhanced catalytic performance was mainly ascribed to a high Cu⁰ surface area, appropriate ratio of Cu⁺/Cu⁰, and more surface oxygen vacancies associated with Ce³⁺, which stemmed from a strong interaction between Cu and the CeO₂ support.

Full text of DOI:10.1002/cctc.201701384

Accepted Article
Title: Insight into the synergism between copper species and surface
defects influenced by copper content over Cu/CeO2 catalysts for
the hydrogenation of carbonate
Authors: Wei-Lin Dai, Huabo Li, Yuanyuan Cui, and Qianqian Liu
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Insight into the synergism between copper species and surface
defects influenced by copper content over Cu/CeO2 catalysts for
the hydrogenation of carbonate
Huabo Li, Yuanyuan Cui, Qianqian Liu and Wei-Lin Dai *[a]
Abstract: Various Cu/CeO2 nanorod catalysts with different copper
content were synthesized through a facile coprecipitation method for
the hydrogenation of diethyl carbonate to methanol in a fixed-bed
reactor. The methanol yield enhanced with increasing of copper
loading, but excessive amount of copper would lead to the
deactivation of catalyst at higher liquid hourly space velocity because
of the active metal aggregation. The 20Cu/CeO2 and 30Cu/CeO2
catalyst displayed superior catalytic performance with the methanol
space-time yield of 8.4 and 8.1 mmol·gcat−1·h −1, respectively. The
remarkably enhanced catalytic performance was mainly ascribed to
higher Cu0 surface area, appropriate ratio of Cu+/Cu0 and more
surface oxygen vacancies associated with Ce3+, which stemmed from
the strong interaction between Cu and the CeO2 support.
is extensively studied as support or catalyst for many reactions in
heterogeneous catalysis. Ceria has been investigated for
structure-sensitive reactions with different morphologies, which
can be directly catalyzed or assisted by active sites located at a
specific facet or edge.[5] Both theoretical and experimental
researches illustrated that CeO2 nano-structures with well-defined
{100} and {110} reactive planes exhibited enhanced catalytic
activity than CeO2 nanoparticles.[6] Tana et al. demonstrated that
the CeO2 nanorods with highly energetic {100} and {110} facets
showed excellent oxygen storage capacity and activity for CO
oxidation. Wang et al. reported the outstanding performance of
CeO2 nanorods for selective oxidation of o-xylene and clarified the
key role of surface concentration of oxygen vacancy clusters.[5a]
As well known, the migration of metal cations into the ceria
lattice will promote the redox behavior and catalytic activity
significantly owing to the formation of more oxygen vacancies. [7]
Furthermore, the shape dependence of ceria support for the
catalytic properties has been widely studied.[5b] Catalysts with Cu
loaded on CeO2 nanorods were more active and selective in N2O
decomposition reaction[8] and NO reduction by CO[6a]. Apparently,
this depended on the dispersion of the active copper species and
the interaction with ceria. Our previous work also indicated the
correlation between the enhancement of catalytic performance of
diethyl carbonate (DEC) hydrogenation to methanol and the
structural characteristics of CeO2 with different morphologies and
we found that the catalyst using CeO2 nanorods as support got
much higher intrinsic hydrogenation catalytic activity.[3a] However,
there is no comprehensive investigation about the role of copper
concentration in DEC hydrogenation to methanol, which should
never be neglected, as it significantly influences the interaction
between active Cu and CeO2 support, the molar ratio of surface
Cu+/Cu0 and surface defects concentration.[9] Hence, investiga-
tion showing the influence of metal concentration is urgently
hankered for a better understanding of CeO2 supported system.
We herein synthesized a series of Cu/CeO2 nanorods catalysts
and explored the role of copper content in the selective
hydrogenation of DEC, reaching a conclusion that well-dispersed
copper species and certain amount of surface Ce3+-Ov-Ce3+
defects are beneficial for the production of methanol. Moreover,
this type of copper-based ceria catalysts revealed an extended
application potential for the hydrogenation of carbonates.
Introduction
The transformation of chemically stable CO2 to methanol, which
is an important intermediate for present fuels and chemical
feedstock, has been intensively studied. While the direct
conversion of carbon dioxide to methanol remains a grand
challenge if considering the harsh reaction conditions, low CO2
conversion and methanol yield.[1] The hydrogenation of CO2
derivatives (carbonates, formates, carbamates, urea derivatives
and so on) compensates the high activation energy barrier for the
cleavage of the C=O bond in the conventional hydrogenation
process of CO2.[2] Recently, designing heterogeneous catalysts
for the hydrogenation of carbonates has been an important topic
in green and sustainable chemistry.[3] However, there are still few
investigations on continuous fixed-bed reactions being reported
in comparison with batch reactions about the hydrogenation of
carbonates.
Ceria (CeO2), as one of important rare-earth oxides, has
aroused widespread concern because of its specific physico-
chemical properties such as the outstanding redox ability, oxygen
storage/release capacity (OSC) associated with the formation of
oxygen vacancies, and low cost compared with noble metals.[4] It
[a]
H. Li, Dr. Y. Cui, Q. Liu, Prof. & Dr. W.-L. Dai
Department of Chemistry, Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials
Fudan University
Results and Discussion
Shanghai 200433, P. R. China
E-mail: wldai@fudan.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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Table 1. Physicochemical properties of various xCu/CeO2 catalysts with different copper content.
[a]
0[a]
[b]
[c]
[d]
SBET
Vpore
Dpore
(nm)
DCu
SCu
dCu
STYMeOH
TOFMeoH
(h-1)
Catalyst
CeO2
-1
(m2·g-1)
(cm3·g-1)
(%)
(m2·gcat
-
)
(nm)
(mmol·gcat-1·h-1)
-
86.3
72.0
76.7
69.4
69.4
57.6
0.55
0.18
0.23
0.16
0.23
0.21
25.2
-
-
-
9.8
80.7
71.2
54.0
36.9
17.4
10.5
23.1
35.0
47.9
33.9
-
6.2
7.7
7.8
0.68
0.74
0.78
1.04
0.83
2Cu/CeO2
5Cu/CeO2
10Cu/CeO2
20Cu/CeO2
30Cu/CeO2
11.8
9.1
-
30.8
39.3
45.9
15.1
14.4
8.4
8.1
[a] Cu dispersion and surface area of Cu0 determined by N2O titration. [b] Cu crystallite size calculated by the Scherrer formula. [c] Space-time
yield (STY). Reaction conditions: P(H2) = 2.5 MPa, H2/DEC = 190 mol/mol, Liquid hour space velocity (LHSV) of DEC = 1.3 h−1, T = 503 K. [d]
TOF: calculated as moles of MeOH formed in the initial 1 h per mole of surface copper calculated from a copper dispersion.
the average grain size increases from approximately 30.8 to 45.9
nm with the increasing of copper content from 10 to 30 wt.%
(Table 1). Almost no diffraction peak of any other phase of copper
species can be observed when the loading mass is lower than 10
wt.%, suggesting that too much copper species in the catalysts
will lead to the aggregation of the copper species.
Physicochemical properties of the catalysts
Table 1 lists the chemical compositions and textural properties
of xCu/CeO2 (x denotes the mass content of copper) nanorod
catalysts with different copper loading. The N2 adsorption-
desorption isotherms and pore-size distribution curves of samples
are illustrated in Figure S1. All the calcined xCu/CeO2 samples
exhibit a typical adsorption curve of Langmuir type IV isotherms
with H3-type hysteresis loops. The as-synthesized CeO2 has the
BET specific surface area of 86.3 m2/g, pore volume of 0.55 cm3/g,
and pore size at 25.2 nm. After the introduction of copper, the BET
surface area and pore volume decrease with copper particles
filling into the pores or the surface of the support. The broaden
pore size distribution of 30Cu/CeO2 could stem from the excess
amount of copper species, which varied with the porous structure.
The molar ratios of surface-to-bulk copper and concentrations of
cupreous surface have been treated as key factors affecting the
adsorption properties and catalytic performance of copper-based
catalysts.[10,11] The metallic copper dispersions and cupreous
surface areas of xCu/CeO2 determined by N2O titration method
are also shown in Table 1. The increase of copper content causes
the gradually reducing in the copper dispersion, while the exposed
copper surface areas reach maximum when the copper content is
20 wt.%.
Figure 1A shows the wide-angle XRD patterns of the calcined
catalysts. Distinct diffraction peaks of CuO at 35.5° and 38.7°
(PDF No.: 45-0937) are observed when copper content is higher
than 5 wt.% and the peaks intensity enhanced with the increasing
of copper content. All the patterns showing legible diffraction at
the positions of 28.6°, 33.1°, 47.5°, 56.3°, and 59.0° can be
ascribed to {111}, {200}, {220}, {311}, and {222} planes of the
cubic fluorite structure CeO2 (PDF No.: 34-0394), respectively.
After reduction (Figure 1B), a sharply diffraction peak at 43.3°
characteristic of face-centered cubic Cu {111} (PDF No.: 04-0836)
is observed while the peaks of CuO disappeared. The Cu
crystallite sizes calculated by the Scherrer formula indicates that
Figure 1. XRD patterns of xCu/CeO2 catalysts with different copper content
after calcination (A) and reduction (B).
Representative micrographs of the as-synthesized 5Cu/CeO2,
20Cu/CeO2 and 30Cu/CeO2 sample after reduction are illustrated
in Figure 2 (A, B and C) to demonstrate their morphology and
crystallite growth. For the Cu/CeO2 catalysts with low copper
content, Cu also exists as well-dispersed and permeated into the
CeO2 lattice forms which could not be distinguished due to the
poor contrast.[6a] The particles size significantly enlarges with the
increasing of copper loading, and aggregation of the species can
be clearly observed in the 30Cu/CeO2, which is agreement with
N2O titration and XRD results. The HRTEM image (inset of Fig.
2B) of the 20Cu/CeO2 catalyst illustrates the lattice fringe of 0.31
nm, which fit well with the {111} planes of cerianite,[3b] confirming
the maintenance of the crystalline CeO2 structure after the
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Figure 2. TEM images of the reduced catalysts: 5Cu/CeO2 (A), 20Cu/CeO2 (B), 30Cu/CeO2 (C).
incorporation of Cu. Meanwhile Cu {111} plane can be identified
with lattice fringe of 0.21 nm distinctly.[12] For the 30Cu/CeO2
catalysts, the morphology here seems reversed, the CeO2
nanorods are sheathed in the copper species, which is similar with
the results reported by Skårman.[12b]
To better understand the surface elemental composition and
chemical state, X-ray photoelectron spectroscopy (XPS) and X-
ray excited Auger electron spectroscopy (XAES) tests were
conducted. Figure 4A shows the Ce 3d, O 1s and Cu LMM spectra
of reduced catalysts. The peaks u1 and 1 correspond to the
3d104f1 electronic state of Ce3+ ions, while the peaks u, u2, u3, ,
2 and 3 are the main representative of the 3d104f0 state
corresponding to Ce4+.[13] The amount of Ce3+ increases gradually
from 12.7%, maximum at 18.4% then decreases to 17.8% (Table
2). The oxygen vacancies are generated to maintain the
electrostatic balance of Ce3+-containing fluorite ceria. To further
investigate the chemical state of O, O 1s spectra of different
catalysts are shown in Figure 4B. Three types of surface oxygen
species are identified by performing a peak-fitting deconvolution.
The peaks at 529.5, 531.7 and 533.6 eV can be ascribed to lattice
The real redox property of xCu/CeO2 catalysts was investigated
by temperature programmed reduction (TPR) method to reveal
the metal-support interaction and the results were shown in Figure
3. Generally, there is no H2 consumption for pure CeO2 phase
under the temperature lower than 500 K.[6a] Two obvious
reduction peaks at ~450 and ~470 K correspond to the well-
dispersed copper and the species interacting with CeO2 support
strongly (Cu-O-Ce), respectively. And the higher H2 consumption
around 470 K may contribute to large numbers of Cu2+ ions
incorporated into the lattice of CeO2 during calcination and some
of Ce4+ ions reduced at low temperature which agree with the XPS
results as discussed below.[12c] Shoulder peak at 505 K, which is
observed from the 20Cu/CeO2 and 30Cu/CeO2, is due to larger
and aggregated copper oxide that weakly interacts with ceria.
Hence, it can be deduced that the copper species are mostly well-
dispersed on the supports or in easily reduced bulk state with
lower copper content.
Figure 3. H2-TPR patterns of xCu/CeO2 catalysts with different copper content.
Figure 4. Ce 3d, O 1s XPS spectra and Cu LMM XAES spectra of the reduced
xCu/CeO2 catalysts with different copper content.
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oxygen (Oα), oxygen vacancies (Oβ), and other weakly bound and
chemisorbed oxygen species (Oγ). The ratio of Oβ to OT (OT = Oα+
Oβ+ Oγ) can be used to roughly estimate the concentration of
surface oxygen vacancies.[14] Notably, for 20Cu/CeO2 and
30Cu/CeO2, the γ-peak nearly disappeared, which shows that
20Cu/CeO2 and 30Cu/CeO2 own little amount of surface weakly
bound oxygen species. Further evidence is provided by FT-IR
characterization (Figure S2), and the peak between 3300 and
3600 cm−1 is attributed to stretching vibration of surface hydroxyl
(-OH).[15] The peaks are stronger for catalysts with lower copper
content, especially for 2Cu/CeO2 and 5Cu/CeO2. Generally, the
ratio of band intensity in LMM XAES can be representative for the
reduced copper species analysis on the catalyst surface due to
almost the same binding energy for Cu0 and Cu+ species. The Cu
LMM XAES spectra of working catalysts of xCu/CeO2 are
illustrated in Figure 4C, and peaks observed are deconvoluted
into two symmetrical peaks located at around ~916 and ~919 eV,
corresponding to Cu+ and Cu0 species, respectively. It can be
concluded from the deconvolution results that copper loading
amounts could significantly affect the distributions of surface Cu+
and Cu0, and the molar ratio of surface Cu+/Cu0 is gradually
enhanced with an increase of the copper loading and peaks at 20
wt.% (Table 2).
of DEC as main products, and the tail gas mainly consisting of
CO2 and CO which means the occurrence of side-reactions such
as methanation, decarbonylation and decomposition under the
reaction conditions. The carbon balance calculated maintained at
＞97%. Figure 5 presents the DEC conversion, MeOH selectivity
and MeOH yield based on LHSV. The 20Cu/CeO2 catalyst
displays the highest conversion in higher LHSV range compared
with other samples, indicating its superb catalytic activity, and the
2Cu/CeO2 exhibits the poorest catalytic activity. When we
consider MeOH selectivity carefully, the differences could also be
distinguished obviously. Although the DEC conversion on the
5Cu/CeO2 and 10Cu/CeO2 catalysts are close to 30Cu/CeO2, the
selectivity on the latter is higher and further resulting in superior
yield to MeOH. The sample of 20Cu/CeO2 show a superior yield
to MeOH of ＞75% even at a high LHSV of 1.2 h−1 compared to
other samples and it is quite stable during long-term stability test.
To gain further insight into the action mechanism of the copper
content, the STY and TOF values are estimated to acquire the
catalytic properties of the xCu/CeO2 catalysts. The 20Cu/CeO2
displays the highest STY (8.4 mmol·gcat-1·h -1) for MeOH at 503 K
and DEC LHSV of 1.3 h−1 (see Table 1) and TOF value, which
confirms the superior properties of 20Cu/CeO2. As shown in Table
S1, the severe reaction conditions of CO2 direct hydrogenation
reaction can be avoided to some extent by substitution of the
carbonates hydrogenation. Methanol formation rate of
20Cu/CeO2 catalyst is the highest among the reported Cu-based
catalysts for the hydrogenation of carbonates (Table S1).
Catalytic activity
The gas-phase hydrogenation of DEC as a probe reaction is
conducted to investigate the relevance between catalytic
performance and intrinsic structure of xCu/CeO2 catalysts.
Methanol and ethanol are formed in the hydrogenation process
Table 2. XPS parameters of the reduced xCu/CeO2 catalysts with different
copper content.
K. E.[b] (eV)
3+[a]
+[c]
CCe
(%)
XCu
(%)
Catalyst
Oβ/OT
Cu+
Cu0
2Cu/CeO2
5Cu/CeO2
10Cu/CeO2
20Cu/CeO2
30Cu/CeO2
12.7
13.2
17.1
18.4
17.8
0.37
0.37
0.29
0.39
0.36
915.8
916.1
916.2
916.0
916.1
919.3
918.8
919.0
919.1
919.2
43.5
46.7
50.4
66.2
42.2
[a] CCe3+=Ce3+/(Ce3++ Ce4+). [b] Kinetic Energy. [c] XCu+=Cu+/(Cu+ + Cu0).
Discussion
Usually, the catalytic performance of supported metal catalysts
is intrinsically associated with the microstructure of active metal
and supports, including active metal surface, metal-support
interactions and the contribution of support. In this work, Cu/CeO2
catalysts are prepared with different copper content, various of
characterization techniques are employed to obtain structural and
surface feature of the samples. As we know, the samples with
Figure 5. The conversion of DEC (A), selectivity to MeOH (B) and yield of MeOH
(C) over xCu/CeO2 catalysts with different copper content. Reaction condition:
P(H2) = 2.5 MPa, H2/DEC =190 (mol/mol), T= 503 K.
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lower copper content cannot supply sufficient active sites, which
can give explanation about the poorest activity of 2Cu/CeO2. The
20Cu/CeO2 catalyst with the maximum Cu0 surface shows the
best performance and the highest TOF value, while for
30Cu/CeO2, the MeOH yield is high at low LHSV (h−1) and
dramatically decreases with prolonging the reaction time or
increasing LHSV, which is mainly because of the weak interaction
between active metals and supports, further agglomeration of
active species due to the relatively small specific surface area
and lager copper particle size observed from the BET and XRD
results of previous paper. Similar results can be found in
literatures elsewhere.[16] The mole ratio of Ce3+/(Ce3++Ce4+) and
Cu+/(Cu+ +Cu0) on the basis of copper loading are shown in Figure
6. It is notable that the 20Cu/CeO2 catalyst shows the highest
involve in the dissociative process.[18] In the case of the xCu/CeO2
catalysts, the catalytic activities dramatically changed with the
number of Cu+ sites, which suggests that cuprous species acted
as efficient sites to motivate carbonyl group and that the relatively
higher amount of Cu+ led to the ideal catalytic activities.
It is generally accepted that the defect sites may significantly
impact the catalytic hydrogenation performance of catalyst,
among which oxygen vacancies are common characteristics.[19]
The surface Mn2+-Ov-Mn2+ defect of CuMn-x catalysts can
promote the hydrogenation of dimethyl succinate via interacting
with the carbonyl group and then weakening the C=O.[20]
Furthermore, Mudiyanselage et al. revealed that the surface Ce3+
ions and oxygen vacancies can act in the same way as the Cu+
species to stabilize the immediate products.[21] Herein, a plausible
simplified mechanism for DEC hydrogenation over Cu/CeO2
catalysts is proposed (Scheme 1).
3+
+
CCe and XCu value. Thus, the proper copper content in the
catalysts could significantly regulate the physicochemical
characteristics of the Cu/CeO2 catalysts.
Firstly, the oxygen vacancies neighbor with surface Ce3+ ions
coordinate with the oxygen atom of carbonyl group. Meanwhile,
Ce3+ species convert to Ce4+ via interacting with the C=O group,
thus resulting in the formation of C-O σ bond. Correspondingly,
In order to verify the influence of copper dispersion,
20Cu/CeO2-DM and 20Cu/CeO2-PG catalysts were synthesized
and characterized. From Table S2 we can see that the
20Cu/CeO2-DM shows the lowest copper dispersion and
methanol yield. The copper dispersion of the 20Cu/CeO2-PG
catalyst is 35.4%, which is close to the value of 20Cu/CeO2
(36.9%), whereas the catalytic activity of the latter is much higher
than that former. Therefore, we can give a preliminary conclusion
that copper dispersion plays a certain role in the catalytic
performance of catalysts, but is not the critical one. The surface
composition and valence state of copper species should be
further studied.
Scheme 1. Synergistic catalytic effect of Cu0 and CeO2 support (A), Cu+ and
CeO2 support (B).
adjacent oxygen vacancy is occupied by the O atom. Simultan-
eously, the Cu+ species also could offer sites for the stabilization
of carbonyl group. The H dissociated by Cu0 participates in the
next step, which plays important role in the cleavage of C-O bond,
producing ethanol, and aldehyde group interacted with oxygen
vacancy and Cu+ is left. And the transformation is finally
accomplished by the reduction of aldehyde group to methanol.
Figure 6. The mole ratio of Ce3+ and Cu+ over the reduced xCu/CeO2 catalysts
as a function of Cu content.
Previous studies have indicated that the synergy between Cu0
and Cu+ was also a crucial factor for the hydrogenation of
carbonates. It was well documented that the Cu0 sites would
dissociate adsorb hydrogen molecules, whereas surface Cu+ as
electrophilic specie may conduct the hydrogenation procedure by
cleaving the C=O bond via interaction with the electrons of oxygen
atom.[17] However, an excessive amount of Cu0 species might
induce the side reactions such as decomposition of the reaction
intermediates (ethyl formate, etc.), which probably results in the
generation of gas by-products during the hydrogenation of DEC,
while the substrate stabilized on the Cu+ species is not easy to
[2,3c,22]
In comparison, the significantly improved MeOH yield for
20Cu/CeO2, with the relatively higher maximum amount number
of Cu+ and Ce3+, is likely due to the stability of DEC and carbonyl
group by Cu+. Therefore, the efficiently enhanced catalytic
performance is attributed mainly to synergism between active
copper species and surface defects. In a word, Cu0 species are
responsible for the dissociation of hydrogen and following
activation of carbonyl groups. Briefly, Cu+ and surface Ce3+-Ov-
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Ce3+ defect sites promote the adsorption of the substrates and
stabilization of intermediate products.
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[3]
Conclusions
In summary, a series of Cu/CeO2 catalysts with different copper
content were synthesized by a facile co-precipitation method to
achieve the optimized catalytic performance. Among the as-
synthesized Cu/CeO2 samples, the 20Cu/CeO2 catalyst presents
superb activity for the vapor phase continuous hydrogenation of
DEC to methanol. Catalyst characterization revealed that the
efficiently enhanced catalytic performance is attributed mainly to
the synergism between active copper species and surface defects.
Cu0 species can activate hydrogen and carbonyl groups, and Cu+
and surface defects promote the adsorption of DEC and
stabilization of intermediate products. The loading of copper
influences the interaction between Cu and CeO2 support, further
resulting in the structural evolution. This work holds possibilities
for the achievement of methanol via indirect chemical utilization
of CO2 with an inexpensive and efficient copper-based catalyst.
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Experimental Section
CeO2 nanorods were synthesized by a hydrothermal method as reported
in Ref. [3a]. Then, Cu(NO3)2·3H2O was deposited on the CeO2 nanorods
using the Na2CO3 as precipitant to obtain the Cu/CeO2 catalysts.
Analogously, by adjusting the content of copper respectively, the same
procedures were carried out to synthesis other catalysts xCu/CeO2 (x
denotes the copper content). More experiments details were further shown
in supporting information.
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Keywords: Cu/CeO2 • Carbonate Hydrogenation • Copper
content • Surface defects • Synergism
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10.1002/cctc.201701384
ChemCatChem
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Table of Contents
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Huabo Li, Yuanyuan Cui, Qianqian Liu
and Wei-Lin Dai *
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Insight into the synergism between
copper species and surface defects
influenced by copper content over
Cu/CeO2 catalysts for the
Synergistic catalysis: Cu/CeO2 catalysts with proper copper content achieve the
superb activity for the vapor phase continuous hydrogenation of DEC to methanol.
The enhanced catalytic performance is attributed mainly to the synergism between
active copper species and surface defects of CeO2 support.
hydrogenation of carbonate
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