Impact of the Oxygen Vacancies on Copper Electronic State and Activity of Cu-Based Catalysts in the Hydrogenation of Methyl Acetate to Ethanol

Article abstract of DOI:10.1002/cctc.201900413

Reducible oxides supported copper-based catalysts have been widely used in ester hydrogenations due to their excellent catalytic performance. However, the role of surface oxygen vacancies is still unclear. Here, we fabricated four copper-based catalysts u

Full text of DOI:10.1002/cctc.201900413

Accepted Article
Title: Impact of the Oxygen Vacancies on Copper Electronic State and
Activity of Cu-Based Catalysts in the Hydrogenation of Methyl
Acetate to Ethanol
Authors: Yushan Xi, Yue Wang, Dawei Yao, Antai Li, Jingyu Zhang,
Yujun Zhao, Jing Lv, and Xinbin Ma
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Impact of the Oxygen Vacancies on Copper Electronic State and
Activity of Cu-Based Catalysts in the Hydrogenation of Methyl
Acetate to Ethanol
Yushan Xi, Yue Wang*, Dawei Yao, Antai Li, Jingyu Zhang, Yujun Zhao, Jing Lv and Xinbin Ma
Abstract: Reducible oxides supported copper-based catalysts have
been widely used in ester hydrogenations due to their excellent
catalytic performance. However, the role of surface oxygen
vacancies is still unclear. Here, we fabricated four copper-based
catalysts using different shaped CeO2 nanocrystals as supports for
the hydrogenation of methyl acetate (MA) to ethanol. The catalytic
activities significantly changed depending on the morphology of
supports in the order of rod > cube > spindle > octahedron, which
was in line with the trend of the formation energy of oxygen
vacancies on the corresponding exposed lattice planes. Combined
with the results of chemisorption and in situ FTIR experiments, it is
demonstrated that the oxygen vacancies are not the primary active
sites for MA hydrogenation, whereas they could significantly affect
the electronic state of copper species. Under reduced conditions, the
mobile oxygens could be released from the lattice and form lots of
oxygen vacancies, which could strongly interact with copper particles
and benifit for the generation and stablization of Cu+ species. Thus,
increasing the oxygen mobility of supports could effectively increase
the amount of surface Cu+ species and enhance the catalytic activity
for MA hydrogenation.
sites in vapor-phase chemoselective hydrogenation reactions of
esters to alcohols accounting for their selective hydrogenolysis
of C-O bonds.[2b] Recently, a synergistic catalysis mechanism
has been proposed between Cu0 and Cu+ species in MA
hydrogenation: while Cu0 dissociatively adsorbed H2, Cu+
stabilized the methoxy and acyl species.[3] In our previous work,
the balancing effect of two active species was revealed. When
the accessible metallic Cu surface area was below a certain
value, the catalytic activity of hydrogenation linearly increased
with the increasing Cu0 surface area, whereas it was primarily
affected by the amount of Cu+ surface area.[3b] Thus, besides
enhancing copper dispersion, how to improve the amount of Cu+
surface area requires great efforts to study. Cu-based catalysts
supported on the CeO2,[4] TiO2 and ZrO2 or modified by
LaOx,[7] MnOx[8] and ZnO[3a,9] additions have been reported to
perform excellent activity. Notably, all of these supports or
additions are reducible oxides, which may generate oxygen
vacancies in the reduction condition.[10] Li and co-workers
proposed that surface oxygen vacancies of CuMnAlOx mainly
contributed to the carbonyl group adsorption of esters.[8]
However, it remains controversial about the precise role of the
reducible oxides, especially about the role of the surface oxygen
vacancies in MA hydrogenation.
CeO2, as a typical defective oxide, has been widely used in
catalysis, owing to its unique oxygen storage capability (OSC)
and excellent redox property resulting from the rich oxygen
vacancies.[11] In hydrogenation reactions, the abundant H2 can
easily induce the generation of oxygen vacancies,[12] which may
has a great impact on the active species distribution and the
catalytic activity. With recent advances in nanomaterial
synthesis, it has been discovered that the oxygen vacancies are
highly influenced by the morphology of ceria.[13] Thus, applying
shape-controlled CeO2 nonacyrstals as supports is a possible
way to manipulate the oxygen vacancy concentration in
[5]
[6]
Introduction
With the reducing crude oil reserves and increasing
demands for energy resources, the development of alternative
routes for valuable products synthesis from syngas, which can
be derived from coal, natural gas or biomass, has attracted a lot
of attentions. Ethanol is a versatile feedstock and a promising
fuel alternative or additive.[1] In particular, ethanol is widely
blended into the gasolines, which significantly reduced the
emissions of CO, particulates and NOx. Recently, a new route of
ethanol synthesis from syngas via dimethyl ether carbonylation
and methyl acetate (MA) hydrogenation has been proposed (Eq.
1-2)[2] and shown high potential in industry. Since the
hydrogenation of MA is one of the crucial steps, the design and
fabrication of catalyst, and the deep understanding of the
structure-function relationship are worth to be put in great efforts.
metal/CeO2 catalysts for
a
deep understanding of the
relationship between the oxygen vacancy concentration and the
catalytic behavior.
Herein, we synthesized CeO2 rods, cubes, octahedrons and
spindle-like nanocrystals which were reported to dominantly
expose {110}, {100} or {111} facets respectively,[14] and applied
them as the supports to prepare Cu/CeO2 catalysts. The
reduced catalysts presented significant support-morphology-
dependent activities in MA hydrogenation. After detailed study of
the copper electronic state and the MA adsorption process, we
demonstrated the role of oxygen vacancies in MA hydrogenation,
which may bring new thoughts in further design of catalysts.
CH3OCH3 + CO = CH3COOCH3
CH3COOCH3 + 2H2 = C2H5OH + CH3OH
(1)
(2)
Copper species are considered as excellent catalytic active
[a]
Yushan Xi, Dr. Yue Wang*, Dawei Yao, Antai Li, Jingyu Zhang, Dr.
Yujun Zhao, Dr. Jing Lv and Pro. Xinbin Ma
Key Laboratory for Green Chemical Technology of Ministry of
Education, Collaborative Innovation Centre of Chemical Science
and Engineering, School of Chemical Engineering and Technology,
Tianjin University, Tianjin 300072, China
E-mail: yuewang@tju.edu.cn
Results and Discussion
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Textual properties and catalytic performance
positions in H2 temperature programmed reduction (H2-TPR)
profiles of catalysts (Figure S2) as well. In addition, the
extremely weak peak at 36.4o ascribed to Cu2O (JCPDS 65-
0328) is only observed in 10Cu/CeO2-Spindle, and no obvious
diffraction peaks of other crystal faces of Cu2O can be observed,
which implies that the Cu+ species are probably highly dispersed
in the reduced Cu/CeO2 catalysts.[17]
To investigate the role of supports with different facets
exposed, the morphologies of reduced Cu/CeO2 catalysts should
be first defined. Transmission electron microscopy (TEM)
images are shown in Figure 1. It is noticeable that the CeO2 rods,
cubes, octahedrons and spindle-like nanocrystals were
successfully synthesized and their shapes and crystal planes
were perfectly maintained during the following procedures of
catalyst preparation. We can clearly see the staggered
distributed nanorods with a growth direction along [110] from
Figure 1a. The light points on the crystalline surfaces indicate
the existence of many lattice defects. The TEM images in Fig 1c
and 1e display that the obtained nanocubes and octahedrons
possess pretty perfect morphology with smooth surfaces and
little lattice defects, implying the good crystallinity of them. From
their corresponding fast Fourier transform (FFT) patterns (insets
of Figure 1d and 1f), the predominately exposed planes of cubes
and octahedrons can be defined as (100) and (111), respectively.
As shown in Figure 1g, the spindle-like CeO2 nanocrystals have
been obained. The ends of them are sharp and the edges are
curving, meaning that a large amount of defected sites exists.
The FFT analysis suggests that the spindle-like crystals display
clear (111) planes.[14] X-ray diffraction (XRD) patterns of the
CeO2 nanoparticles (Figure S1) were collected as well. In Figure
S1, all the peaks of CeO2 nanparticles can be ascribed to a
Fm3m cubic fluorite structure (JCPD04-0593), and the peaks of
Ce(OH)3 are absent indicating the good purity of the as-prepared
CeO2. Compared with the cube and octahedron, the peaks of
rod and spindle are weak and wide, which shows the poor
crystllinity, i.e. lots of lattice defects existing,[14] which is
consisted with the TEM images. Thus, the specific lattice planes
are exposed by different nanocrystals as we designed.
The copper species were loaded on CeO2 crystals by
ammonia evaporation method. The copper contents were
determined by inductively coupled plasma optical emission
spectrometer (ICP-OES). It is shown all catalysts have a similar
copper loading, which is close to the designed value of 10 wt.%
(Table S1). From TEM images (Figure 1) and XRD patterns of
the reduced 10Cu/CeO2 samples (Figure 2), it is demonstrated
that through ammonia evaporation method and the following
reduction process, the structures of CeO2 nanoparticles were
well remained. As shown in the white circle of Figure 1, the
metallic Cu nanoparticles supported on CeO2 can be seen
clearly after reduction. The contrast fluctuations shown in the
bulk of Cu particles are moiré fringes that arise owing to partial
overlapping with other particles.[15] No obvious sintering of
copper can be seen in the TEM images. The average sizes of
copper particles were counted and listed in Table 1. In Figure 2,
the diffraction peaks at 2θ angles of 43.3o and 50.4o are
attributed to metallic Cu (JCPDS 04-0836). The sizes of Cu
nanoparticles can be calculated using the Scherrer formula
(listed in Table 1). The sizes calculated by XRD are slightly
larger than those counted from the TEM images, which is
possibly caused by the presence of some small copper particles
such as the white circled ones in Figure 1.[16] The results suggest
that the sizes of Cu0 in the four samples are basically identical
(about 20 nm), which is confirmed by the similarity of the peak
Figure 1. TEM images and their FFT of the selected area in the red box and
the corresponding structural models (insets of the left ones) of the reduced
10Cu/CeO2 catalysts with different morphologies: a) and b) Rod; c) and d)
Cube; e) and f) Octahedron; g) and h) Spindle. The Cu nanoparticles are
shown in the white circle.
The catalytic performance tests of the reduced Cu/CeO2
catalysts were carried out at 488 K, 2.5 MPa with the H2/MA of
80 in feed. As shown in Table 1, even with the similar particle
size of metallic copper, the catalytic activities for MA
hydrogenation varied significantly depending on the morphology
of the supports, which decreased in order of rods > cubes >
spindle-like > octahedrons nanocrystal as supports. By-products
include methanol, ethyl acetate (EA), acetaldehyde, dimethyl
ether, etc. Remarkably, 10Cu/CeO2-Rod catalyst exhibits the
best catalytic performance. The MA conversion of the catalysts
supported on CeO2 nanorods is over 5 times higher than that of
supported on octahedrons. What the substantial structure is to
induce these differences among catalytic performances needs to
be further investigated. Consisting with other reports,[3b,18] the
selectivity of ethanol changes parallel to the MA conversion. As
the MA conversion increased, the selectivity of ethanol
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increased and the selectivity of EA decreased. It is because the
faster hydrogenation of EA to ethanol greatly enhanced the
ethanol selectivity at higher MA conversions.[18b,19]
To study the contribution of oxygen defects to the reaction,
the texture factors such as pore structure should be first
excluded. N2 adsorption-desorption isotherms were measured to
calculate the specific surface areas and the pore structures of
the catalysts. As shown in Figure S3, there is a hysteresis loop
in the isotherms of all samples demonstrating the mesoporous
structure in all materials. For spindle-like sample, the pores
mainly centered at about 4 nm but there are also some
micropores, which consisted with the reported literature.[14]
Moreover, the pore sizes of all samples distributed in a relatively
wide range. Combined with the few defects observed in
nanocrystals, it could be deduced most pores are generated by
the accumulation of nanocrystals, which would not hinder the
reactants diffusion.[20] It is also worthy noticed that the specific
surface areas (Table S1) of rods and spindles are around 100
m2/g and those of cubes and octahedrons are much smaller.
The four catalysts possess parallel mesoporous structures, but
the specific surface areas of some of them exit great distances.
However, even with the similar BET surface areas, the catalytic
activity of 10Cu/CeO2-Rod is still over 3 times higher than that of
spindles. Thus, to study the critical factors on catalytic
performance, the active sites distribution, contribution and their
formation process should be primarily discussed.
Figure 2. XRD patterns of the reduced 10Cu/CeO2 catalysts with different
morphologies.
Table 1. Copper contents, particle sizes of Cu/CeO2 catalysts and the catalytic performances for MA hydrogenation.
10Cu/CeO2
Cu
(wt.%)
contents[a]
Crystallite size (nm)
MA conversion[c]
(%)
Selectivity (%)
XRD[b]
18.3
19.3
17.7
22.8
TEM
17.1
16.7
16.4
19.8
Ethanol
91.4
EA
Others[d]
0.2
Rod
9.5
9.8
9.7
9.8
93.0
35.5
17.3
25.8
8.4
Cube
61.4
38.5
42.3
40.6
0.1
Octahedron
Spindle
57.6
0.1
59.2
0.1
[a] Metal loading was determined by ICP-OES. [b] Diameter of Cu particles calculated from XRD data by Scherrer equation. [c] Reaction conditions: catalyst
weight = 0.5 g, P = 2.5 MPa, T = 488 K, H2/MA = 80, and WLHSV(MA) = 0.5 h-1. [d] Others: CH3CHO, C2H6O, etc.
reduction process, leading to an inaccurate result. Here, we
Characterization of surface copper species and oxygen
vacancies
used a combined method of H2-TPR, N2O-CO titration, X-ray
photoelectron spectra (XPS) and Auger electron spectroscopy
(AES) to determine the amounts of surface sites (Figure S6),[8,21]
which detailed procedures are described in the Experimental
Section and the results are shown in Table 2. Two peaks at
932.0 eV and 951.8 eV are shown in Cu 2p XPS (Figure 3a),
which are typically assigned to the binding energy of Cu 2p3/2
and Cu 2p1/2 respectively. There are no satellite peaks at 942-
944 eV, demonstrating the absence of Cu2+ species after
reduction.[22] In this case, we can use the asymmetrical and
broad peaks in the Cu LMM AES (Figure 3b) to distinguish Cu0
and Cu+ species, which peaks centred at binding energy of
568.7 eV and 572.1 eV respectively. Thus, the specific surface
areas of Cu0 and Cu+ sites can be calculated combined with the
Although the particle sizes of metallic copper are similar
among all catalysts, the Cu+ species would form using CeO2 as
supports. And the amount of surface oxygen vacancies would be
influenced by the morphology of nanocrystals[13a], as well as may
be boosted during reduction with the presence of copper.[10]
Thus, it is necessary to quantitatively measure the surface areas
of copper species and oxygen vacancies in the reduced
Cu/CeO2 catalysts.
In literatures, N2O titration is uesd as a typical method for
characterization of metallic copper dispersion and surface
areas.[3b] However, for the reducible supports, the oxygen
vacancies would participate in the N2O oxidation and H2
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results obtained from N2O-CO titration (listed in Table 2).[22] The
amounts of surface Cu0 species are similar among the catalysts,
consisted with the XRD and TEM results, while the amounts of
surface Cu+ species change a lot. It is demonstrated that the
surface area of Cu+ supported on the nanorods is much larger
than that supported on other ceria nanocrystals.
Table 2. Surface areas of copper species and oxygen vacancies in the reduced Cu/CeO2 catalysts.
[a]
10Cu/CeO2
SCu
XCu(I)[b] (%)
SCu(0)[c] (m2/g)
SCu(I)[c] (m2/g)
SOv[d] (m2/g)
ACu(I)[e] (area)
(m2/g)
Rod
46.0
19.9
9.4
87.6
73.9
38.3
47.3
5.7
5.2
5.8
4.9
40.3
14.7
3.6
73
10
52
37
22.6
8.7
Cube
Octahedron
Spindle
2.8
9.3
4.4
3.2
[a] Assuming that the Cu+ and Cu0 species occupy identical areas, the total surface area of Cu species is calculated from the CO uptake in N2O-CO titration.
[b] Cu+/(Cu+ + Cu0) was calculated from the Cu LMM AES. [c] The surface area of Cu0 and Cu+ species are calculated combining the N2O-CO titration and
AES results. [d] The surface area of oxygen vacancies is determined combining the N2O-CO titration and N2O-TPR. [e] Integral area under the peaks of Cu+-
CO species in the FTIR spectra of CO adsorption after evacuation.
catalytic performance in ester hydrogenations, where Cu0
facilitated the H2 decomposition while Cu+ sites adsorbed the
methoxy and acyl species.[3b,22] Recently, Li and co-workers[8]
proposed that the synergy between active Cu0 species and
surface Mn2+-Ov-Mn2+ defect structures, rather than Cu+ species,
was responsible for the high catalytic efficiency of CuMnAlOx for
dimethyl succinate hydrogenation. It was demonstrated that the
surface vacancies could strongly interact with the carbonyl group,
thus weakening the C=O and promoting the hydrogenation. But
in the work of Ru/CeO2 for CO2 methanation, there is no positive
correlation between the methanation activity and the
concentration of oxygen vacancies.[24] Thus, whether the oxygen
Figure 3. a) Cu 2p XPS spectra, b) Cu LMM AES spectra and c) in situ FTIR
of CO adsorption spectra of the reduced 10Cu/CeO2 catalysts with different
morphologies.
vacancies act as active sites needs further investigation.
Catalytic performance was correlated with the surface areas
of Cu0, Cu+ species and oxygen vacancies respectively (shown
in Figure 4) to reveal the role of oxygen vacancies in MA
hydrogenation. The surface areas of Cu0 species among the four
In situ Fourier transform infrared (FTIR) of CO absorption
catalysts are similar, which are negligible for discussion. It is
are used to verify the obtained results of surface sites. For the
displayed that the catalytic activity is linearly increased with the
Cu0-CO and Cu2+-CO bonds are very weak as reported and no
increasing amount of surface Cu+ species, indicating that the
Cu2+ species existed after reduction, the obtained peaks in
surface Cu+ species should play a key role in enhancing the
Figure 3c could be attributed to CO adsorbed on Cu+ species.[23]
catalytic activity for MA hydrogenation, whereas the scatters in
The band at 2110 cm-1 could be assigned to CO adsorbed on
the correlation plot between activity and SOv suggests that the
associated Cu+ species while the broad band around 2177 cm-1
surface oxygen vacancies are not the main factor of affecting the
is attributed to isolated ones.[3b] The experiments were carried
catalytic performance.
out using CeO2 nanocrystals as blank samples as well (Figure
To further illustrate the contributions of surface Cu+ and
S4). There is no obvious peak at 2050-2250 cm-1. After
oxygen vacancies in MA hydrogenation, in situ FTIR of MA
calculation, it is demonstrated that the integrated areas of the
adsorption was performed with 10Cu/CeO2-Rod and the
Cu+-CO peaks (ACu(I)) is linearly changed with SCu(I) (Figure S5),
corresponding support respectively. Figure 5a and 5b are the
which verified the correction of characterization results of
FTIR spectra of the two samples collected during the process of
surface Cu species.
MA adsorption. The bands at 1778 and 1760 cm-1 are assigned
to the C=O stretching in gaseous MA, and the bands at 1255
The role of oxygen vacancies in MA hydrogenation
A lot of papers have demonstrated that the cooperation of
Cu0 and Cu+ active species is the key to achieve excellent
and 1247 cm-1 are ascribed to the hydroxyl bending of MA in gas
phase.[25] These four intense bands appeared immediately after
the samples were exposed to MA, and the intensity decreased
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with time due to the adsorption and dissociation of MA. It can be
seen that the intensities of bands at ~1440, 1374 and 1600 cm-1,
which are related to the C-H bending, O-C-O stretching and
C=O stretching modes of adsorbed MA or acetyl species
respectively, were increased gradually according to the trend
over time. These changing trends happened in both 10Cu/CeO2-
Rod and CeO2-Rod samples, indicating that MA can be
adsorbed on both catalyst and support, and some of them may
be dissociated to methoxy and acyl species. The results suggest
the surface oxygen vacancies could indeed enhance the
adsorption and dissociation of MA. However, the difference of
abilities to adsorb and activate MA between the two samples
appeared after evacuation. Compared to 10Cu/CeO2-Rod, the
spectrum of CeO2-Rod after evacuation displayed two very weak
and board bands at 1633 and 1400 cm-1, which are assigned to
the asymmetric and symmetric stretching vibrations of acetyl
group respectively.[26] Thus, based on the above investigation, it
can be concluded that although the surface oxygen vacancies
are conductive to MA adsorption and dissociation, compared to
Cu+ species, its ability is too weak to be the key factor for
influencing the catalytic performance. In addition, the TOF value
Cu0 is normalized, which is consistent with the above analysis.
Considering the big difference of Cu+ species among the four
catalysts, the role of oxygen vacancies may reflect in affecting
the cooper electronic state distribution.
of Cu0 species shows a linear positive correlation with SCu(I)
.
Figure 4. Correlation of MA conversion with the amounts of a) Cu0 sites, b)
Cu+ sites and c) oxygen vacancies on the surface of reduced Cu/CeO2
catalysts.
(Figure S7) It is indicated that the surface Cu+ is the main factor
affecting catalytic performance when the influence of surface
Figure 5. In situ FTIR spectra of MA adsorption of the reduced 10Cu/CeO2-Rod and CeO2 nanorods: a) and b) the spectra during MA adsorption process; c) the
final spectra after evacuation, which was collected when the spectrum no longer changed.
We propose the role of oxygen vacancies of CeO2 support is
mainly interacting with Cu particles and benefiting for Cu+
species generation and stabilization. In the reductive conditions,
the mobile oxygens can be released from the lattice and form
lots of oxygen vacancies. The oxygen mobility (OM) of the CeO2
nanocrystals was measured to follow the order of rods > cubes >
spindle-like > octahedrons (Table S2). Given the certain facets
they exposed, the results are consistent with the reports that the
calculated oxygen vacancy formation energies are displayed in
order of (110) < (100) < (111).[13a,27] It is observed by TEM that
the spindle-like nanocrystals have much more lattice defects
than the octahedrons, which can explain the OM difference
between them. The significant effect of oxygen mobility of
supports on the distribution of Cu species is demonstrated in
Figure 6. The amounts of Cu+ sites are linearly increased with
the increasing OM of CeO2 nanocrystals, which strongly
supported our opinion. Thus, the catalytic performance for MA
hydrogenation is significantly enhanced by the increasing
amount of surface Cu+ sites, which are generated and stabilized
by the strong interaction between Cu species and the oxygen
vacancies of supports.[28] Concerning the oxygen vacancies,
their formation energies are dominantly affected by the exposed
facets, which are dependent on the morphology of CeO2
nanocrystals. The CeO2 nanorods, which exposed crystal planes
of {110} with the lowest oxygen vacancy formation energy,
presented the highest activity in MA hydrogenation.
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autoclave and heated for 24 h at 373 K (453 K) to get nanorods
(nanocubes). After filtered, washed and dried, the precipitates were
calcined at 673 K for 4 h. For the synthesis of CeO2 nanooctahedrons
and nanospindles, the procedures were basically the same as described
above except the chemicals and hydrothermal treatment conditions. For
nanooctahedrons,
2
mmol Ce(NO3)3·6H2O and 0.02 mmol
Na3PO4·12H2O were used and the following hydrothermal treatment was
at 433 K for 12 h. 2.4 mmol Ce(NO3)3·6H2O and 6.4 mmol carbamide
were used to fabricate nanospindles and the autoclave was heated at
393 K for 8 h.
The Cu/CeO2 catalysts were synthesized by the ammonia evaporation
method. First, 2.5 g CeO2 nanocrystals was added to 100 mL of
deionized water and 23 mL aqueous ammonia solution and stirred at 303
K for 1 h. After heated to 353 K, a solution of 1.056 g Cu(NO3)2·3H2O
was added to the suspension and then started the ammonia evaporation.
This process was terminated when the pH value decreased to 6-7. The
precipitate was filtered, washed, dried and calcined at 673 K for 4 h, and
then tableted, crushed and sieved to 40-60 meshes.
Figure 6. The effect of oxygen mobility of CeO2 nanocrystals on the surface
Cu+ species area of reduced catalysts.
Conclusions
Catalyst characterization
In summary, the Cu-based catalysts supported on shape-
controlled CeO2 nanocrystals were fabricated and exhibited
significantly support-morphology dependent catalytic activity in
MA hydrogenation. The catalytic performance increases linearly
with the increasing amount of surface Cu+ sites, and the
amounts of Cu+ species are found to be dominantly affected by
the mobile oxygen vacancy concentration of CeO2 supports,
which is associated with the crystal facets exposed. Moreover,
the contribution of surface oxygen vacancies in reaction was
investigated as well. Although the surface oxygen vacancies
could adsorb and dissociate MA, compared to Cu+ species, its
ability is too weak to be the key active species in the
hydrogenation reaction. The significant role of oxygen vacancies
is reflected in the strong interaction with copper particles, which
benefits for the Cu+ generation and stabilization. These results
may provide new possibilities for the rational design of catalyst
according to a deep understanding of the contribution of
supports to the active sites distribution and the catalytic activity
in hydrogenation reactions.
N2 adsorption-desorption analysis was measured by Micromeritics Tristar
3000. The specific surface area and pore size distribution was calculated
by Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda
(BJH) method respectively. Transmission electron microscopy (TEM)
images were obtained by Philips TECNAI G2 F20 system electron
microscope with field emission gun. Inductively coupled plasma optical
emission spectrometer (ICP-OES, Varian Vista-MPX) was applied to test
Cu content of catalysts. Powder X-ray diffraction (XRD) were conducted
on Rigaku C/max-2500 diffractometer with a graphite-filtered Cu Kα
radiation (λ = 1.5406 Å).
X-ray photoelectron spectra (XPS) and Auger electron spectroscopy
(AES) were carried out on PHI 1600 ESCA (PE Company) with an Al Kα
X-ray radiation source (hν = 1486.6 eV). We pressed catalysts to thin
disks and reduced them in a flow of H2 at 523 K for 4 h. After cooling to
room temperature, the samples were transferred to a holder immediately
in a glove-box filled with N2 and then outgassed in chamber, which
guaranteed the states of surface Cu species in the reduced catalysts
were measured correctly. The binding energies were calibrated using the
C 1s peak at 284.6 eV as the reference. The experimental errors were
within ±0.2 eV.
Combining H2-TPR, N2O oxidation and CO pulse chemisorption, the
surface areas of copper species and oxygen vacancies in the reduced
Cu/CeO2 catalysts were obtained using a Micromeritics Autochem II 2920
apparatus.[8,21] The sample was reduced with 10%H2/Ar at 523 K, when
the Cu2+ turned to Cu0 or Cu+ species and oxygen vacancies generated
in CeO2. After cooling down to 363 K in Ar flow, the sample was exposed
in N2O for 1 h to oxidize surface Cu0 to Cu+ species, as well as the
surface oxygen vacancies. Following, a typical CO chemisorption was
conducted at 323 K, which results reflected the total amount of surface
copper species. Thus, the surface areas of copper species (SCu(0) or SCu(I)
m2/g) can be easily calculated depending on the Cu+/(Cu0+Cu+) ratios
(XCu(I), %) from Cu LMM AES. Based on the above results, the amounts
of surface oxygen vacancies in the reduced catalysts can be determined
by combining the N2O oxidation and H2-TPR results.[8] Instead of the
typical CO chemisorption after the N2O oxidation in the above procedure,
a H2-TPR was carried out to reduce the surface Cu+ species and the
mobile oxygen atoms with a temperature ramp from 323 K to 523 K and
held for 1 h. In this way, the amount of surface oxygen vacancies in the
reduced catalysts could be calculated by subtracting the total amount of
Experimental Section
Chemicals
Ce(NO3)2·6H2O (99.99%, AR), Cu(NO3)2·3H2O (≥ 99%), aqueous
ammonia solution (25-28 wt.%) and carbamide (≥ 99.3%, AR) were
purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China).
NaOH (≥ 96.0%, AR) and Na3PO4·12H2O (≥ 99.6%, AR) were obtained
from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China).
All chemicals were used without any treatment or purification.
,
Catalyst preparation
All of the CeO2 nanocrystal supports were prepared using the
hydrothermal method.[14] The procedures of preparing CeO2 nanorods
and nanocubes are listed as follows. 4 mmol Ce(NO3)3·6H2O and 480
mmol NaOH were dissolved in deionized water respectively then mixed
together and stirred for 30 min. The suspension was transferred into
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10.1002/cctc.201900413
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surface copper species obtained previously from the H2-TPR
consumption.
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We gratefully acknowledge supports from the National Natural
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Keywords: heterogeneous catalysis • hydrogenation • copper-
based catalyst • ethanol • CeO2 • oxygen vacancy
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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The catalytic activity changed
significantly depending on the
morphology of supports in order of rod
> cube > spindle > octahedron, which
is in line with the trend of the
Yushan Xi, Yue Wang*, Dawei Yao,
Antai Li, Jingyu Zhang, Yujun Zhao, Jing
Lv and Xinbin Ma
Page No. – Page No.
formation energy of oxygen vacancies
on the corresponding exposed lattice
planes. The oxygen vacancies are not
the primary active sites for MA
hydrogenation, whereas they could
effectively increase the amount of
surface Cu+ species and enhance the
catalytic activity for MA hydrogenation.
Impact of the Oxygen Vacancies on
Copper Electronic State and Activity
of Cu-Based Catalysts in the
Hydrogenation of Methyl Acetate to
Ethanol
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