Influence of Ni species on the structural evolution of Cu/SiO2 catalyst for the chemoselective hydrogenation of dimethyl oxalate

Article abstract of DOI:10.1016/j.jcat.2011.03.006

A novel family of heterogeneous Cu-Ni/SiO₂ catalysts with appropriate metal ratios displayed outstanding selectivity to methyl glycolate (96%) and to ethylene glycol (98%) in the chemoselective gas-phase hydrogenation of dimethyloxalate. The chemical states of nickel species were found to have a strong influence on the structural evolution of the catalysts and correspondent catalytic behaviors. The selectivity to the two products could be tuned by modulating the chemical states of nickel species. It is shown that oxidative nickel species are helpful in improving the dispersion of copper species because of the enrichment of copper on the surface of the nickel species, thus enhancing the catalytic activity and selectivity to ethylene glycol. The selectivity to methyl glycolate could be greatly improved by the Cu-Ni bimetallic catalyst. An 83% yield of methyl glycolate and a 98% yield of ethylene glycol could be obtained over the bimetallic Cu-Ni catalyst and the NiO-modified catalyst, respectively.

Full text of DOI:10.1016/j.jcat.2011.03.006

Journal of Catalysis 280 (2011) 77–88
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Influence of Ni species on the structural evolution of Cu/SiO2 catalyst
for the chemoselective hydrogenation of dimethyl oxalate
⇑
Anyuan Yin, Chao Wen, Xiaoyang Guo, Wei-Lin Dai , Kangnian Fan
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel family of heterogeneous Cu–Ni/SiO2 catalysts with appropriate metal ratios displayed outstand-
ing selectivity to methyl glycolate (96%) and to ethylene glycol (98%) in the chemoselective gas-phase
hydrogenation of dimethyloxalate. The chemical states of nickel species were found to have a strong
influence on the structural evolution of the catalysts and correspondent catalytic behaviors. The selectiv-
ity to the two products could be tuned by modulating the chemical states of nickel species. It is shown
that oxidative nickel species are helpful in improving the dispersion of copper species because of the
enrichment of copper on the surface of the nickel species, thus enhancing the catalytic activity and selec-
tivity to ethylene glycol. The selectivity to methyl glycolate could be greatly improved by the Cu–Ni
bimetallic catalyst. An 83% yield of methyl glycolate and a 98% yield of ethylene glycol could be obtained
over the bimetallic Cu–Ni catalyst and the NiO-modified catalyst, respectively.
Received 5 January 2011
Revised 2 March 2011
Accepted 7 March 2011
Available online 13 April 2011
Keywords:
CuNi/HMS
Dimethyl oxalate
Ethylene glycol
Methyl glycolate
Hydrogenation
Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction
Ni-based bimetallic catalysts containing Cu exhibit significantly
different catalytic activity and selectivity than Ni monometallic
Because of their superior catalytic performance, bimetallic cat-
alysts have wide applications in catalytic reforming [1], selective
hydrogenation [2], and selective alcohol oxidation [3]. The intro-
duction of a second metal is an important approach to tailoring
the electronic and geometric structures of the catalysts to enhance
their activity and modulate their selectivities for products [4–8].
Recent investigations have demonstrated that the structure, chem-
ical composition, and surface chemical state of bimetallic catalysts
play a critical role in determining their catalytic properties. By
combining both experimental studies and theoretical calculations,
many fundamental investigations have been carried out in an ef-
fort to correlate the electronic properties of bimetallic surfaces
with chemical reactivities [9–12]. Establishing the connection be-
tween the catalytic activity and selectivity of a catalyst and its
physical and electronic structure is crucial for the design of new
catalysts.
Great efforts have been made to improve the catalytic activity of
Cu-based catalysts by introducing a second metal, such as Ag [13],
Au [14], and Pd [15], most of which are noble metals. The chemical
state of Cu was the same as in the solely Cu-based catalyst due to
the chemical stability of the noble metals. It is found that Ni and Cu
both have fcc structures with lattice parameters 3.51 and 3.61 Å,
respectively. They can form a continuous solid solution of cupro-
nickel binary alloys crystallized in a cubic close packed lattice.
catalysts [16–20]. However, some controversial results for Ni–Cu
alloy catalysts have been reported. The water–gas shift activity in
the presence of CO2 [17] and the catalytic hydrogenation activity
for ethylene hydrogenation [18,19] could be greatly enhanced by
the introduction of Cu into the Ni catalyst. The addition of Cu to
Ni/Al2O3 also increases the selectivity to 1-butene [16]. Also, the
addition of Cu favors CO formation in CO2 hydrogenation [20]
and could dramatically enhance the generation of carbon fibers
from the decomposition of hydrocarbons [21]. Changes in the cat-
alytic performance induced by adding Cu to Ni can be caused by
changes in the electronic properties of the homogeneous alloy par-
ticle, the formation of active surface structure, or the preferential
segregation of one metal to the surface, or by a combination of
all these effects [16–21]. However, the addition of copper to nickel
foil significantly reduced the catalytic activity for styrene hydroge-
nation and hydrogenolysis of ethane to methane [22,23]. The de-
crease in catalytic activity was explained by changes in the
electronic properties of nickel upon the addition of copper [22].
The original ability of the bimetallic catalyst to influence the cata-
lytic behavior still faces a great challenge to elucidate. Effects of the
introduction of Cu into the nickel catalyst have been extensively
studied during the past few years; however, the influences of the
modification with Ni on the copper catalyst have been less consid-
ered. In the present work, systematic investigations were carried
out to elucidate the effects of Ni on the Cu based catalysts.
Chemoselective gas-phase hydrogenation of dimethyl oxalate
(DMO) to corresponding methyl glycolate (MG) and ethylene gly-
⇑
Corresponding author. Fax: +86 21 55664678.
E-mail address: wldai@fudan.edu.cn (W.-L. Dai).
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.03.006

78
A. Yin et al. / Journal of Catalysis 280 (2011) 77–88
col (EG) has great academic and industrial significance owing to
their highly added value as building blocks [24]. In this context,
catalytic methods leading to high-quality grade MG and EG via
the design of the catalysts are particularly attractive for applica-
tions in the polymer and fine chemical industries. Previous study
showed that higher yields of MG and EG could be obtained by con-
trolling the reaction temperature via the versatile Ag/SiO2 catalyst
[25]. However, the large amount of noble metal used limited its
further application in industry. Developing a much cheaper and
highly efficient catalyst is therefore economically and environmen-
tally benign.
(AE). In a typical procedure, 7.625 g of Cu(NO3)2ꢀ3H2O and a certain
amount of Ni(NO3)2ꢀ6H2O were dissolved in 172 ml of deionized
water. Then 23 ml of 28% ammonia aqueous solution was added
and stirred for 0.5 h at 333 K. A certain amount of HMS was added
to the above solution and stirred for another 4 h. The initial pH of
the suspension was 11–12. The suspension was then preheated at
363 K to evaporate ammonia and decrease of pH and consequently
deposit copper and nickel species on silica. When the pH value of
the suspension decreased to 7–8, the evaporation process was ter-
minated. The filtrate was washed with deionized water and dried
at 393 K overnight, followed by calcination at 723 K for 4 h. The
catalyst precursor was denoted as CuxNi/HMS, where x represents
NiO loading. The catalysts reduced under 5% H2/Ar flow at 543 K
for 4 h were correspondingly denoted as CuxNi/HMS-re. For exam-
ple, Cu3Ni/HMS is composed of 20 wt% Cu, 3 wt% NiO, and 77 wt%
HMS.
It is commonly accepted that the catalyst surface plays the main
role in the catalytic reactions and the composition of the surface
may not be the same as that of the bulk due to the differences in
the volatility of nickel and copper [26]. The d-band vacancies the-
ory was suggested to account for the binding energy changes of
nickel and copper upon alloying in X-ray photoelectron spectros-
copy (XPS) [27]. Moreover, it was recognized that, in addition to
d-band vacancies, other factors, such as surface composition, must
also be taken into account when the properties of nickel–copper
catalysts are investigated. Chemoselective hydrogenation of DMO
is a two-step continuous hydrogenation reaction, and EG could
be obtained by hydrogenation of DMO via MG. From a thermody-
namic viewpoint, it is easy to get EG due to the much larger equi-
librium constant than that of the first-step hydrogenation reaction.
Therefore, how to control the product distributions via designing
and structuring the catalyst was a great challenge. In previous
studies [28–33], Cu/HMS and Cu/Al–HMS catalysts applied in the
hydrogenation of DMO were systematically investigated in relation
to the optimization of the catalyst preparation parameters and
preparation methods and the active center discussion. A high yield
of EG could be obtained under optimized conditions. Introduction
of nickel species into the Cu/SiO2 catalyst would help not only to
understand deeply the origin of the catalyst itself, but also to sup-
ply a reference for catalysis in the hydrogenation of ester.
In the present work, we report a novel family of ternary Cu–Ni–
Si catalysts with outstanding MG and EG selectivity in the gas-
phase hydrogenation of DMO, obtained by adjusting the surface
chemical state of nickel species. The design comprised the incorpo-
ration of components with complementary functions into the het-
erogeneous catalyst: copper as the basic hydrogenation metal,
optimized amounts of nickel as a promoter to increase the disper-
sion of copper species, and silica as the support of exposed copper
and nickel sites.
For comparison, the Cu/HMS catalyst with 20 wt% Cu loading,
denoted as 20Cu/HMS, and the Ni/HMS catalyst with 3 wt% NiO,
denoted as 3Ni/HMS, were also prepared by the AE method.
2.1.3. Bimetallic CuxNiy/HMS
The bimetallic CuNi/HMS catalyst precursors were prepared by
a chemical reduction deposition method involving the reduction of
two precursors with KBH4 under Ar flow. HMS was impregnated
with the desired amount of aqueous solution of Cu(AC)2ꢀ3H2O
and Ni(AC)2ꢀ6H2O; then KBH4 solution (0.05 M) was added drop-
wise to the mixture. The metal-loaded powder was then dried in
an oven under Ar at 383 K for 16 h, and calcined at 723 K for 4 h.
Finally, the sample was reduced at 673 K with 5% H2/Ar for 4 h
and was denoted as CuxNiy/HMS, where x and y stand for the mole
ratio of Cu/Ni. The total bimetal loading was controlled at 5 wt%.
2.2. Catalyst characterization
The BET surface area (SBET) was measured using N2 physisorp-
tion at 77 K on a Micromeritics Tristar 3000 apparatus. The pore
size distributions were obtained from the desorption isotherm
branch of the nitrogen isotherms using Barrett–Joyner–Halenda
(BJH) method. The powdered X-ray diffraction (XRD) patterns were
collected on a Bruker D8 Advance X-ray diffractometer using nick-
el-filtered Cu Ka radiation (k = 0.15418 nm) with a scanning angle
(2h) of 10°–90°, a scanning speed of 2° minꢁ1, and a voltage and
current of 40 kV and 40 mA, respectively. Elemental analysis of
Cu and Ni loading was performed using ion-coupled plasma (ICP)
atomic emission spectroscopy on a Thermo Electron IRIS Intrepid
II XSP spectrometer. Transmission electron microscopy (TEM)
and energy-dispersive X-ray spectrometry microanalysis (EDX)
were performed on a JEOL JEM2011 instrument.
2. Experimental
2.1. Catalyst preparation
Temperature-programmed reduction (TPR) profiles were ob-
tained on a Tianjin XQ TP5080 autoadsorption apparatus. A 50-
mg sample of the calcined catalyst was outgassed at 473 K under
Ar flow for 2 h. After the sample was cooled to room temperature
under Ar flow, the in-line gas was switched to 5% H2/Ar, and the
2.1.1. HMS
Mesoporous siliceous HMS was prepared according to a well-
established procedure delineated by Tanev and Pinnavaia [34] using
tetraethyl orthosilicate (TEOS) as silica source and dodecylamine
(DDA) as template agent. Typically, the HMS materials were pre-
pared by dissolving 5.04 g of DDA in 53.33 g of deionized H2O and
39.42 g of ethanol under vigorous stirring before adding 21.39 g of
TEOS dropwise. The solution mixture was then stirred at 313 K for
0.5 h. The resulting gel was aged for 18 h at ambient temperature
to afford the crystalline templated product. After that, the resulting
solid was recovered by filtration, washed with deionized water, and
dried at 373 K, followed by calcination at 923 K for 3 h.
sample was heated to 1073 K at a ramping rate of 10 K minꢁ1
.
The H2 consumption was monitored by a TCD detector. The metal-
lic Cu surface area was measured by decomposition of N2O at 363 K
using a pulsed method, with N2 as the carrier gas [35]. The con-
sumption of N2O was also detected by a TCD detector. The specific
area of metallic copper was calculated from the total amount of
N2O consumption with 1.46 ꢂ 1019 copper atoms per m2. XPS
experiments were carried out with a Perkin–Elmer PHI 5000C ESCA
system equipped with a hemispherical electron energy analyzer.
2.1.2. CuxNi/HMS
The Mg Ka (hm = 1253.6 eV) anode was operated at 14 kV and
Nickel-modified Cu/HMS catalysts containing 20 wt% copper
loading were prepared by the ammonia evaporation method
20 mA. The carbonaceous C1s line (284.6 eV) was used as a refer-
ence to calibrate the binding energies (BEs).

A. Yin et al. / Journal of Catalysis 280 (2011) 77–88
79
2.3. Activity measurements
or copper silicate was generated during the synthesis procedure.
The N2 adsorption–desorption isotherms and BJH pore size distri-
butions of the calcined catalysts are presented in Fig. 1. All the
samples exhibited Langmuir type IV isotherms with an H1-type
hysteresis loop, corresponding to a typical large-pore mesoporous
material with a 1D cylindrical channel. Capillary condensation of
nitrogen with uniform mesopores occurred, causing a sudden steep
increase in nitrogen uptake in the characteristic relative pressure
(P/P0) range of 0.3–0.5 for all the samples investigated, implying
a typical mesoporous structure with uniform pore diameter. Be-
cause HMS exhibited a uniform hexagonal array of mesopores con-
nected by smaller micropores, the broad hysteresis loop in the
isotherms was an indication of long mesopores, which limited
the emptying and filling of the accessible volume.
It is worth noting that all the physisorption curves of the nickel-
containing samples exhibited the same shape, except for the Cu/
HMS sample, which suggests the effect of nickel on the textural
structure of the catalysts. The pore size distribution curve calcu-
lated from the desorption branch of the isotherms (Fig. 1B) exhib-
ited a bimodal distribution at 2.5 and 3.8 nm. Compared with the
Cu/HMS sample, another kind of larger pore was generated, indi-
cating that redissolution and reprecipitation might happen during
the synthetic procedure. To address this issue, an experiment
exposing the support to the same pH conditions during the catalyst
synthesis was carried out and N2-sorption characterization was
also conducted. The BET surface area, average pore volume, and
average pore diameter are also listed in Table 1. It is interesting
to find that a redissolution and reprecipitation process has hap-
pened during the synthesis procedure, and the BET surface area
and average pore volume decreased from 970 to 220 m2 gꢁ1 and
from 0.90 to 0.53 cm3 gꢁ1, respectively, while the average pore
diameter increased from the original 3.88 to 9.05 nm. In addition,
for the nickel-modified Cu/HMS samples, the main pore diameter
was 3.8 nm (not the original 2.5 nm), indicating that the textural
structure of the catalyst was determined not only by the copper
species but also by the nickel species. The 3Ni/HMS catalyst
showed, besides the main pores at 3.8 nm, the presence of smaller
pores at about 2.5 nm that probably corresponded to the porosity
provided by the laminar structure of nickel phyllosilicate, which
was also observed in Ni/SBA-15 catalyst prepared by the deposi-
tion–precipitation method [36]. In line with the above assumption,
the hysteresis of the nitrogen adsorption isotherm is enlarged due
to the porosity contribution of the lamellar hydrosilicates, which
also contributes to the surface area and average pore diameter val-
The gas-phase hydrogenation of DMO was performed in a con-
tinuous-flow fixed-bed microreactor, which has been described in
detail elsewhere [28]. Prior to the catalytic measurement, the cat-
alysts were activated by in situ reduction with 5% H2/Ar. In this
study the following reaction conditions were used: H2/DMO molar
ratio 100, temperature (T) 473 K, total pressure (P) 2.5 MPa, and
room-temperature liquid hourly space velocity (LHSV) of DMO
ranged from 0.2 to 1.8 hꢁ1
.
To eliminate internal diffusion, catalysts with different grain
diameters were used to evaluate the effect of diffusion. The exper-
imental results showed that the conversion of DMO did not vary
with the change of grain diameter, which indicated that no diffu-
sion effect could happen when the grain diameter of the catalyst
was less than 20 mesh. Also, since the H2/DMO molar ratio was
about 100, and there was 85% methanol in the feed flow, no exter-
nal diffusion effect was observed.
The heat transport effect was also considered. The total heat of
reaction of the two-step hydrogenation is ꢁ37.41 kJ/mol at 478 K.
Although the heat of reaction is large, the adiabatic temperature
rise is rather low at very high H2/ester ratios (100–120). In the
integral reactor of our lab, the amounts of catalyst are 1–5 g, and
the horizontal and vertical temperature difference is within
0.5 K. By regulating the temperature of the fixed-bed layer via
the temperature-controlled instrument, the reaction heat could
be eliminated quickly in the equalizing zone and the maximum
temperature difference was no more than 1.0 K.
3. Results and discussion
3.1. Characterization of Ni modified samples
3.1.1. Structural evolution and textural properties of the catalysts
The physicochemical properties of the synthesized CuxNi/HMS
catalysts are summarized in Table 1. Compared with the HMS sup-
port, the SBET of the 3Ni/HMS sample decreased more obviously
than that of the Cu/HMS sample with 20 wt% copper loading,
although the nickel loading was far lower than the copper loading,
which indicated that the effect of nickel on the textural structure
was greater than that of copper. This phenomenon was more pro-
nounced on the nickel-modified Cu/HMS samples. However, the
SBET of the samples rose from 226 to 361 m2 gꢁ1 as the amount
of nickel increased from 1 to 5 wt%, suggesting that nickel silicate
Table 1
Physicochemical properties of as-synthesized CuxNi/HMS catalysts.
Ni/Cua (mol/mol)
SBET (m2 gꢁ1
)
Vpore (cm3 gꢁ1
)
Dpore (nm)
TOFc (hꢁ1
)
SEG (%)
d
b
ꢁ1
SCu ðm2
g
Þ
Catalyst
Catal
HMS
–
–
0
970
220
556
226
296
361
337
482
556
239
1005
0.90
0.53
0.36
0.39
0.38
0.46
0.29
0.84
0.36
0.57
1.30
2.88
9.05
2.20
2.76
3.93
4.26
4.39
6.27
2.20
7.20
4.60
–
–
–
HMSe
Cu/HMS
11.6
13.7
21.6
14.6
8.2
–
0.5
9.3
13.7
2.6
8.6
19.7
2.4
2.2
–
0.2
1.9
38.8
21
28
35
18
17
–
–
–
–
Cu1Ni/HMS
Cu3Ni/HMS
Cu5Ni/HMS
Cu7Ni/HMS
3Ni/HMS
Cu/HMSf
Cu/HMSg
Cu/Al–HMSh
0.054
0.162
0.270
0.378
–
–
–
–
a
b
c
Determined by ICP–AES analysis.
Cu metal surface area determined by N2O titration method.
TOF (DMO molecular reacted per mol surface Cu per hour) at LHSV = 1.7 hꢁ1 and 473 K.
d
e
f
EG selectivities of different Ni loading catalysts at the 50% DMO conversion.
Exposing the HMS to the same conditions encountered during the catalyst synthesis.
Ref. [28].
Ref. [31].
Ref. [33].
g
h

80
A. Yin et al. / Journal of Catalysis 280 (2011) 77–88
2.5 nm
(A)
(B)
3.8 nm
Cu/HMS
3Ni/HMS
3Ni/HMS
Cu1Ni/HMS
Cu3Ni/HMS
Cu1Ni/HMS
Cu3Ni/HMS
Cu5Ni/HMS
Cu7Ni/HMS
Cu5Ni/HMS
Cu7Ni/HMS
Cu/HMS
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
1
2
5
10
20 30 40 50
Pore Diameter (nm)
P/P
0
Fig. 1. (A) N2 adsorption–desorption isotherms and (B) BJH pore size distribution of the calcined catalysts.
ues. A concrete proportion of nickel phyllosilicate could be ob-
tained by deconvolution of the TPR profile of 3Ni/HMS sample.
To understand the nature of the copper and nickel species after
calcination and reduction at 543 K for 4 h under 5% H2/Ar, XRD
characterizations were carried out on the calcined and reduced cat-
alysts. Fig. 2 shows the XRD patterns of the calcined and reduced
catalysts. As can be seen from Fig. 2A, except for the Cu7Ni/HMS
sample, no obvious diffraction peak attributed to copper and nickel
species could be observed except a broad diffraction peak at 2h
around 22° coming from amorphous silica, which indicated that
the copper and nickel species were highly dispersed on the silica
support. When the nickel content was higher than 5 wt%, diffrac-
tion peaks at 35.5° and 38.7° assigned to CuO (JCPDS05-0661)
could be observed, but no diffraction peaks attributed to nickel
species could be detected, which suggests that the introduction
of the proper amount of nickel species could not affect the disper-
sion of copper species and the existence of copper species might
enhance the dispersion of nickel species. The formation of nickel
phyllosilicate might be another factor in the highly dispersed nick-
el species, which is in accord with the result reported by Burattin
et al. [37].
signed to the NiO phase (JCPDS78-0643) could be observed. In
addition, nickel silicate species could be observed, which indicated
that redissolution and reprecipitation might occur to form silicate.
Because of the high resistance to sintering of the nickel oxide par-
ticles before reduction and the metal particles during reduction,
the average metal particle size calculated from the Scherrer equa-
tion is 7.2 nm [37]. For nickel-modified Cu/HMS samples, two obvi-
ous diffraction peaks at 37° and 44.3° attributed to NiO and Ni or
Cu, respectively, could be observed and no detectable silicate spe-
cies could be found, which indicated that the interaction between
metallic (Cu and Ni) species and silica was affected by the amount
of nickel. It is worth noting that the 2h values of NiO, metallic Cu,
and Ni are almost the same; thus it is difficult to distinguish among
NiO, Cu, and Ni at 2h = 44.3°. Moreover, a copper and nickel oxide
solid solution might be formed due to the similar fcc structures
with similar lattice parameters. As a conclusion, the reduced cata-
lysts are composed of metallic Cu species, NiO species, or metallic
nickel species.
3.1.2. Redox properties of the catalysts
The reducibility of the complex oxides revealed the intimate
metal–support interaction in these compounds. Fig. 3 shows the
H2-TPR profiles of the series of calcined samples. For the 3Ni/
Fig. 2B presents the XRD patterns of the reduced catalysts. For
3Ni/HMS-re catalyst, distinct diffraction peaks at 37° and 44.3° as-
CuO
♦
(A)
(B)
SiO
NiO &Cu
NiSiO
2
•SiO
♦
•
2
♦
♦
Cu7Ni/HMS
3
♦
♦
Cu5Ni/HMS
Cu3Ni/HMS
3Ni/HMS-re
Cu7Ni/HMS-re
Cu5Ni/HMS-re
♦
Cu1Ni/HMS
Cu3Ni/HMS-re
Cu1Ni/HMS-re
Cu/HMS-re
Cu/HMS
3Ni/HMS
10 20 30 40 50 60 70 80 90
2 Theta (Degree)
10 20 30 40 50 60 70 80
2 Theta (Degree)
Fig. 2. XRD patterns of catalysts modified with different nickel amounts: (A) after calcination; (B) after reduction with 5% H2/Ar at 573 K for 4 h.

A. Yin et al. / Journal of Catalysis 280 (2011) 77–88
81
persed copper species, the mediated reduction temperature
(518 K) could be attributed to the reduction of copper species sur-
rounded by Si or Ni species (through O2ꢁ linkage), and the highest
reduction temperature (530 K) might originate from the reduction
of copper and nickel oxide solid solutions. By deconvolution of the
H2 consumption peak, it could be found that the proportion of the
higher reduction species increased as the increase of the nickel
contents. For the Cu7Ni/HMS sample, an additional reduction peak
appeared at ca. 552 K, which could be attributed to the reduction of
bulk NiO.
518
530
648
499
691
613
552
3Ni/HMS
Cu7Ni
Cu5Ni
Based on the XRD results, we know that the Ni content has a
strong influence on the stabilization of the copper nanoparticles.
No obvious diffraction peaks assigned to CuO could be observed
except for the Cu7Ni/HMS sample, which indicated that the intro-
duction of nickel species could hinder the aggregation of copper
species by increasing the surface area through forming nickel phyl-
losilicates. After reduction, the copper species dispersed homoge-
neously on the surface of the support. Thus, the exposed metallic
Cu surface area could be greatly improved from 11.6 to
21.6 m2 gꢁ1. Combined with the relevant characterizations, a sche-
matic model of the structure evolution of the copper species with
the increasing of the nickel loading is given in Scheme 1. Without
the introduction of the nickel species, the copper species was
highly dispersed on the surface of the silica support. However, it
was easy to aggregate during the reaction due to the weak interac-
tion between copper and silica support. After modification with
1 wt% Ni, two kinds of copper species appeared, highly dispersed
copper and segregated copper clusters. With further increased
nickel content, an optimized dispersion of copper species could
be obtained. In addition, the interaction between the copper spe-
cies, the silica support and the interface in the support, the copper
species, and the nickel species could be further enhanced via the
highly dispersed nickel species. However, excess introduction of
nickel would lead to the aggregation of part of the nickel oxide,
which blocked the active copper sites.
Cu3Ni
Cu1Ni
Cu/HMS
350 400 450 500 550 600 650 700 750
Temperature (K)
Fig. 3. TPR profiles of catalysts modified with different nickel amounts.
HMS sample, the temperature at which the maximum reduction
peak (648 K) is substantially lower than that for the reduction of
unsupported pristine NiO (703 K) [38]. This phenomenon illus-
trated that there was interaction between the nickel species and
the silica support. After deconvolution of the peaks contained in
the envelope of the TPR pattern and taking into account the nickel
phyllosilicate and the nickel species that strongly or weakly inter-
acted with silica, the wide reduction peak could be divided into
three parts: nickel phyllosiliacate, highly dispersed nickel species
with stronger interaction with the support, and bulk nickel species
with weak interaction with the support, positioned at 613, 648,
and 691 K, respectively.
3.1.3. Surface chemical states of the catalysts
The use of XPS intensity ratios of metal catalysts to the support
provides information regarding the dispersion degree of the active
metal on the support. The results of the XPS intensity ratios of M/Si
(M = Cu, Ni) as a function of nickel content are presented in Fig. 4.
As shown in Fig. 4A, the surface content of copper increased with
increased amount of nickel, indicating that the introduction of
nickel species was beneficial for the surface enrichment of copper
species. The surface Cu/Si ratio exhibited exponential growth with
the increase of nickel content over the calcined samples, especially
when the nickel content was higher than 3 wt%, which suggested
that the copper species were easier to segregate on the surface of
the sample after calcination. After reduction, however, the surface
content of copper species obviously decreased in comparison with
that of the calcined samples and the surface Cu/Si ratio was almost
proportional to the nickel content, which meant that the pretreat-
ment of H2 could alter the surface distribution of copper and nickel
For catalysts modified with different amounts of nickel, besides
the main reduction peak at 530 K, there was a shoulder peak at ca.
499 K, which was attenuated for the Cu3Ni/HMS sample, and be-
came more obvious at higher or lower nickel contents. The broad
reduction peak indicated strong interaction between the metals,
which might form a solid solution. It is known that the radius of
Ni2+ is smaller than that of Cu2+, and the eventual incorporation
of Ni2+ into the CuO framework causes a slight contraction of the
lattice. By comparison with the reference data for 3Ni/HMS and
Cu/HMS samples, the feature at 530 K is attributed to the reduction
of the copper oxides to metallic copper, while the peak at higher
temperature is assigned to the reduction of nickel oxide. The pres-
ence of nickel shifts the reduction peak positions of copper species
and the presence of copper also alters those of nickel species. Nag-
hash et al. [38] also observed CuO reduction at low temperature in
CuNi/Al2O3 catalysts. Bridier and Perez-Ramirez [39] observed the
same phenomena on the Cu–Ni–Fe catalyst. In addition, the maxi-
mum reduction temperature of nickel species was greatly de-
creased (even lower than 570 K) due to the existence of copper,
which was in good agreement with the results in the literature
[38].
The reduction of NiO is also catalyzed by metallic copper [38],
as pure nickel oxide is typically reduced at above 700 K. On the
other hand, the maximum reduction temperature of copper was in-
creased from 518 to 530 K by the introduction of nickel, which
might result from the interaction between copper and nickel.
Based on the XRD characterizations, the lowest reduction temper-
ature (499 K) could be attributed to the reduction of highly dis-
Scheme 1. Schematic model for the variation of Cu species with increased nickel
content.

82
A. Yin et al. / Journal of Catalysis 280 (2011) 77–88
0.25
0.38
0.36
0.34
0.32
0.30
0.28
0.26
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
(A)
(B)
Calcined samples
Reduced samples
0.20
0.15
0.10
0.05
0.00
Calcined samples
Reduced samples
1
2
3
4
5
6
7
1
2
3
4
5
6
7
Nickel content (wt%)
Nickel content (wt%)
Fig. 4. Intensity ratios of M/Si based on XPS results: (A) Cu/Si ratios before or after reduction; (B) Ni/Si ratios before or after reduction.
species. Different from that for CuNi/SiO2 samples, the surface Cu/
Si ratio could be enhanced on the Cu/SiO2 sample after being re-
duced with H2 [29]. In addition, the surface nickel content
(Fig. 4B) of the reduced samples was also lower than that of the
calcined samples. To further elucidate the distributions of the sur-
face metals, the bulk and surface Ni/Cu ratios in the calcined and
reduced samples as a function of nickel content are presented in
Fig. 5. The surface Ni/Cu ratios, especially in the reduced samples,
were much higher than those of the bulk, which indicates that the
surface enrichment of nickel occurred after the introduction of
nickel. The enrichment of nickel became more obvious when the
samples with higher nickel content (more than 3 wt%) were pre-
treated under a reducing atmosphere. It might be the nickel enrich-
ment on the surface that greatly improved the dispersion of the
copper species, tremendously enhancing the catalytic hydrogena-
tion performance.
Fig. 6 shows the Cu2p XPS spectra of the calcined and reduced
samples. Typically, the Cu2p3/2 BE of CuO is found at ca. 933.5 eV
[40], and this value of the supported copper phyllosilicate is
934.9 eV [41]. Fig. 6A shows that the copper in all the calcined
samples existed in the oxidation state of Cu2+, as evidenced by
the Cu2p3/2 peak at 933–936 eV and the 2p to 3d satellite at
942–945 eV characteristic of Cu2+ with electron configuration of
3d9 [42]. The intensities of the shake-up lines relative to the main
core level of both the Cu2p3/2 and 2p1/2 levels varied as a function
of nickel content. The shake-up intensities denoting oxidative cop-
per species on the surface were relatively more intense at higher
nickel content. The variation of the Cu2p3/2 BE values with the
nickel content is an indication of the formation of different copper
species in the calcined samples. The peak fitting of the Cu2p3/2 core
level spectra revealed two BE states at 934.4 and 936.0 eV, which
were assigned to the copper species interacted with silica and nick-
el, respectively. The XPS spectra of the reduced samples are illus-
trated in Fig. 6B. Compared with the calcined samples, the Cu2p3/
1.4
BE of the reduced catalysts was shifted to ca. 932.7 eV, and the
2
2p to 3d satellite disappeared due to the reduction of Cu2+ to Cu0
and/or Cu+. Besides, the BE of Cu2p3/2 shifted to lower values with
increased the nickel content, indicating the electronic effect was
more obvious as the nickel content increase.
1.2
Ni/Cu-bulk
Ni/Cu-Calcined
Ni/Cu-Reduced
1.0
Fig. 7 shows the Ni2p XPS spectra of the calcined and reduced
catalysts. Fig. 7A shows that all the calcined samples exhibited a
main peak with a satellite at higher BE. For divalent nickel com-
pounds, the main peak corresponded to the final state of 2p53d9L
(L means a ligand hole) and the satellite peak to a 2p53d8 final
state. Compared with the bulk Ni 2p3/2 (854.9 eV), the main peak
of the calcined samples appeared at higher BE, which indicated the
much stronger interaction between the nickel species and the silica
support. In addition, the spectrum of the main peak showed two
overlapped peaks at low (857.6 eV) and high (858.9 eV) BE values,
which could account for the changes in linking arrangements of Ni
cations within the lattice in the composite catalyst. Thus Ni2+ ions
surrounded by Cu2+(through O2ꢁ linkage) as second neighbors
showed a much higher BE than isolated Ni2+ ions surrounded by
other Ni2+ cations. Shape changes in XPS spectra were observed
with the increase of the nickel content, suggesting that nickel is
in different valence states as the amount increases. After reduction,
the BE value of the nickel species decreased compared with that in
0.8
0.6
0.4
0.2
0.0
1
2
3
4
5
6
7
Nickel content (wt%)
Fig. 5. Surface intensity ratios of Cu/Ni based on XPS results and bulk intensity
ratios of Cu/Ni.

A. Yin et al. / Journal of Catalysis 280 (2011) 77–88
83
Cu2p
934.4
Cu2p
(A)
(B)
932.7
Cu7Ni/HMS-re
Cu5Ni/HMS-re
Cu3Ni/HMS-re
Cu1Ni/HMS-re
Cu/HMS-re
Cu7Ni/HMS
936.0
Cu5Ni/HMS
Cu3Ni/HMS
Cu1Ni/HMS
Cu/HMS
970 965 960 955 950 945 940 935 930 925
Binding Energy (eV)
970
960
950
940
930
920
Binding Energy (eV)
Fig. 6. Cu2p photoelectron spectra of catalysts modified with different nickel amounts: (A) after calcination; (B) after reduction.
(A)
Ni2p
858.9
Ni2p
855.9
852.7
857.6
(B)
Cu7Ni/HMS
857.3
Cu7Ni/HMS-re
Cu5Ni/HMS
Cu5Ni/HMS-re
Cu3Ni/HMS
Cu1Ni/HMS
Cu3Ni/HMS-re
Cu1Ni/HMS-re
3Ni/HMS-re
3Ni/HMS
880
870
860
850
840
890 880 870 860 850 840
Binding Energy (eV)
Binding Energy (eV)
Fig. 7. Ni2p photoelectron spectra of catalysts modified with different nickel amounts: (A) after calcination; (B) after reduction.
the calcined samples. No characteristic peak with BE 852.7 eV,
3.2. Characterization of bimetallic CuNi samples
3.2.1. Structural evolution of the catalysts
attributed to the metallic nickel species, could be observed, imply-
ing that all the nickel species exhibited oxidized chemical states on
the surface of the catalysts.
To investigate the effect of metallic nickel on catalytic perfor-
mance, different bimetallic CuNi/HMS catalysts were also prepared
by the chemical reduction deposition method. Fig. 9 shows the
XRD patterns of bimetallic catalysts with different Cu/Ni molar ra-
tios. As a comparison, the XRD pattern of the reduced Cu/HMS
sample is also presented in Fig. 9. No obvious diffraction peaks ex-
cept the one at 2h = 22° could be observed for most of the samples,
indicating that all the bimetallic species were highly dispersed.
However, for Cu3Ni1/HMS, two peaks positioned at 2h = 43.9° and
51.9° were observed, which corresponded to the (1 1 1) and
(2 0 0) lattice of the Cu, Ni, or Cu–Ni alloy. Since Cu and Ni have
the same fcc structure and almost the same lattice constant, we
cannot distinguish copper–nickel alloy from a mixture of the
monometallic phases from the XRD patterns alone. However,
according to the literature results [38], we believe that nanoparti-
cles of Cu–Ni alloy were formed after the reduction pretreatment.
In combination with the TPR results, the XRD peaks presented here
To investigate the surface Cu and Ni chemical environments of
the catalysts during the reaction, the XPS spectra of Ni2p and Cu2p
for the postcatalyst of Cu3Ni/HMS was also performed (see the
supporting information). It was found that no obvious differences
in the BE values and surface ratios of Cu/Si, Ni/Si, and Ni/Cu could
be observed, which indicated that the surface chemical states and
surface chemical compositions of the Cu3Ni/HMS catalyst were rel-
atively stable during the whole reaction.
The evolution of the different electronic parameters of copper
and nickel with the amount of the nickel content can be observed
in Fig. 8. As in the previously described experiment for the calcined
and reduced samples, most of the changes in the BE occurred for
relatively low content of nickel species. The difference in BE values
among samples with different nickel content indicated that the
electron transfer between copper and nickel species could happen
during the catalyst calcination and the reduction processes.

84
A. Yin et al. / Journal of Catalysis 280 (2011) 77–88
cle size calculated from the peak width of the (1 1 1) reflection for
936.5
936.0
935.5
935.0
934.5
934.0
933.5
933.0
932.5
932.0
Cu2p3/2
Cu3Ni1/HMS is estimated as 8.4 nm. It is clear that the particle size
of the Cu–Ni alloy becomes smaller with increasing Ni content,
which is also confirmed by the TEM observations.
Calcined samples
Reduced samples
Fig. 10 shows a typical TEM image for a CuNi/HMS sample with
an atomic ratio Cu/Ni = 1:1. One can see that the metal particles are
uniformly dispersed throughout the support and show a narrow
particle-size distribution with an average particle size of
2.7 0.5 nm (not shown here). Fig. 10 also (on the right) shows
the EDX of the sample with Cu/Ni ratio 1/1, demonstrating the
coexistence of Cu and Ni. The Cu/Ni ratio is measured to be 50.7/
49.3, which is in excellent agreement with the initial ratio as
added.
0
1
2
3
4
5
6
7
858.0
857.8
857.6
857.4
857.2
857.0
856.8
Ni2p3/2
Calcined samples
Reduced samples
3.2.2. Redox properties of the catalysts
To further elucidate the interaction between copper and nickel
species and the distribution behavior of the bimetallic species, TPR
characterizations were carried out for the bimetallic samples pre-
treated under air. After pretreatment at 673 K under static air,
the alloy samples could be oxidized homogeneously and a uniform
solid solution was formed; thus, under the reduced conditions, one
symmetric hydrogen consumption peak can be observed. Fig. 11
shows the TPR results of the oxidized alloy samples. For compari-
son, the monometallic supported HMS samples were also evalu-
ated. The reduction temperature of the nickel-supported sample
(920 K) is substantially higher than that of the copper-supported
one (530 K), which indicates that the copper species were reduced
more readily than nickel species. After the introduction of copper,
the higher reduction peak disappeared and a lower symmetry peak
was observed at a relatively lower reduction temperature, imply-
ing that the two metallic species were homogeneously mixed.
With increased nickel content, the reduction peak temperature de-
creased, showing that the copper species could remarkably en-
hance the reducibility of the mixed oxide catalysts. However, the
separated reduction peaks appeared at lower temperature and
higher temperature at the same time when the molar ratio of Cu/
Ni reached 1/3, indicating almost no interaction between copper
and nickel species. In other words, the copper or nickel species
are distributed on the support separately or as clusters. Differences
in peak temperatures between Cu1Ni1/HMS and Cu3Ni1/HMS sam-
ples might result from the different interaction between copper
and nickel species. By adjusting the Cu/Ni molar ratio, different bi-
metal samples could be obtained.
0
1
2
3
4
5
6
7
Nickel content (wt%)
Fig. 8. Evolution of the Cu2p3/2 and Ni2p3/2 BEs for increasing nickel amounts.
5Cu3Ni1/HMS
5Cu1Ni1/HMS
5Cu1Ni3/HMS
5Cu/HMS
10
20
30
40
50
60
70
80
90
2 Theta (Degree)
Fig. 9. XRD patterns of bimetallic catalysts with different molar ratios of Cu/Ni.
[Comp: change horizontal axis label from 2h (°) to 2h (°)].
should be ascribed to alloys of Cu–Ni or metallic Cu or Ni. One can
see that the XRD peaks become more obvious with increasing cop-
per content in the samples. Using the Scherrer equation, the parti-
3.2.3. Surface chemical states of the catalysts
Fig. 12 shows the Cu2p and Ni2p XPS spectra of the bimetallic
catalysts. The BE of Cu2p3/2 core levels was in the range 932.2–
Fig. 10. TEM image of bimetallic Cu1Ni1/HMS sample and EDX analysis of individual particles.

A. Yin et al. / Journal of Catalysis 280 (2011) 77–88
85
in the electron density of supported particles with respect to the
bulk metal or oxide, and alloying and/or chemical interaction with
the support. Similar changes in the electronic characteristics of
oxide particles and layers deposited on the substrates of other me-
tal oxides were also attributed to the dispersion degree of the
deposited phase (assuming this decrease in the amount of depos-
ited material). The linking arrangement of nickel in the solid solu-
tion with other nickel or silica cations accounted for the large
changes in BEs at the high end of the spectra.
920
510
5Ni/HMS
900
516
5Cu1Ni3/HMS
537
5Cu1Ni1/HMS
3.3. Catalytic activity and stability
530
5Cu3Ni1/HMS
5Cu/HMS
3.3.1. Effect of NiO loading on the catalytic performance of DMO
hydrogenation
300 360 420 480 540 600 660 720 780 840 900 9601020
Temperature (K)
DMO hydrogenation reaction was carried out to study the cata-
lytic properties of NiO-modified Cu/HMS catalysts, and the conver-
sion and selectivity as a function of LHSV are shown in Fig. 13.
Because of the much higher dispersion of copper species and the
promotion effect of nickel oxide, the nickel oxide modified cata-
lysts presented much better catalytic performance than Cu/HMS
in the hydrogenation of DMO. Even at a LHSV of 1.0 hꢁ1, which is
significantly higher than those reported in the literature [28–31],
nickel-modified Cu/HMS with nickel content of 3 wt% showed
100% conversion for DMO hydrogenation with 98% selectivity to
EG. Nevertheless, only 88% of DMO conversion with an obviously
low selectivity (34%) toward EG was obtained over Cu/HMS cata-
lyst at the same LHSV. In addition, almost no conversion could be
detectable over Ni/HMS catalyst. All these results indicated a syn-
ergetic effect over the nickel-modified copper catalysts and the
high efficiency of the nickel–copper containing catalysts in DMO
hydrogenation. Along with the increase of the nickel content, the
yield of EG exhibited a volcanic variation trend, and the highest
yield could be obtained over the catalyst with Ni content of
3 wt%. The reason for the significant decrease in catalytic perfor-
mance over catalysts with nickel content higher than 3 wt% as
compared to the Cu/HMS catalyst most probably originates from
the dramatic decrease of active sites on the surface due to the seg-
regation of nickel oxide on the surface, which blocks the active
copper species. Compared with literature reports [28,31], the nick-
el-modified catalyst exhibited much higher reaction activity than
the catalysts prepared by impregnation and ion exchange.
As we know, Cu/SiO2 catalyst has been widely applied in many
gas-phase hydrogenation reactions, such as hydrogenation of di-
methyl maleate to 1,4-butanediol [43], hydrogenation of glycerol
to 1,2-propanediol [44], and selective hydrogenation of cinnamal-
dehyde [45]. The metallic Cu surface area at the same active site
was compared. It was found that the nickel-modified catalyst
exhibited much higher Cu surface area than the above-mentioned
catalysts.
Fig. 11. TPR profiles of bimetallic catalysts with different ratios of Cu/Ni.
852.7
855.8
Cu2p
932.9
Ni 2p
5Cu3Ni1/HMS
5Cu Ni3/HMS
1
5Cu1Ni1/HMS
5Cu1Ni3/HMS
5Cu Ni1/HMS
1
5Cu Ni1/HMS
3
970 960 950 940 930 920
Binding Energy (eV)
890
880
870
860
850
Binding Energy (eV)
Fig. 12. XPS spectra of bimetallic catalysts: (A) Cu2p3/2; (B) Ni2p3/2
.
932.6 eV, indicating that metallic copper existed on the surface of
the catalysts. The difference in BE might originate from the differ-
ent electronic interactions between copper and nickel species. In
addition, the BE of Ni2p3/2 was mainly at 852.7 eV, with a shoulder
peak in a higher band, which might result from the partial oxida-
tion of nickel species during the operation. On the basis of the
XPS characterization results, the chemical states of the copper
and nickel species were both metallic. It was worth noting that
no obvious difference could be observed in BE for either metal as
a function of bimetallic composition. However, compared to the re-
duced monometallic Cu and Ni catalysts, an obvious difference in
BE for either metal could be observed. For reduced monometallic
Cu/HMS, the BE of Cu is 932.6 eV, while for bimetallic CuNi/HMS,
the BE of Cu is 932.8 eV; for reduced monometallic Ni/HMS, the
BE of Cu is 855.9 eV, while for bimetallic CuNi/HMS, the BE of Ni
is 852.7 eV. The increase in BE of Cu and the reduction in BE of
Ni for bimetallic CuNi/HMS compared to the monometallic catalyst
might be related to changes in unfilled d-band electron holes aris-
ing from the charge transfer from Cu to the adjacent Ni.
3.3.2. Effect of composition of bimetallic CuNi on the catalytic
hydrogenation of DMO
Based on the characterizations of the nickel-modified catalysts,
the surface chemical state of nickel is not metallic but oxidative.
The oxidative nickel species played an important role in improving
the activity in the hydrogenation of DMO and the selectivity to EG.
To further elucidate the role of chemical states of nickel in catalytic
performance, bimetallic CuNi catalysts were prepared by the
chemical reduction deposition method. Interestingly, MG, the
first-step hydrogenation product, was the main product over the
bimetallic catalysts. This finding was totally different from those
for the monometallic copper catalysts. The catalytic performance
over different bimetallic catalysts is summarized in Table 2. Under
the reaction conditions specified in the caption of Table 2, up to
96% selectivity to MG with 86% conversion of DMO could be ob-
tained over the bimetallic catalyst with Cu/Ni ratio 1:1. Although
Characterization of the chemical state of an element on a sup-
port by XPS has traditionally been carried out by comparing their
BEs with those of the bulk. For example, for copper or copper oxide
supported phases, BE changes have also been attributed to the size
of the deposited metals or oxides. Several reasons for BE changes
have been invoked: variation of the average coordination number
of particle atoms due to a high surface-to-volume ratio, decrease

86
A. Yin et al. / Journal of Catalysis 280 (2011) 77–88
100
90
80
70
60
50
40
30
20
10
0
Cu/HMS
100
90
80
70
60
50
40
30
20
10
0
Cu1Ni/HMS
Cu3Ni/HMS
Cu5Ni/HMS
Cu7Ni/HMS
3Ni/HMS
Cu/HMS
Cu1Ni/HMS
Cu3Ni/HMS
Cu5Ni/HMS
Cu7Ni/HMS
3Ni/HMS
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
-1
-1
LHSV (h )
LHSV (h )
Fig. 13. Effect of the LHSV of DMO on the catalytic performance of the nickel-modified catalysts. Reaction conditions: 2.5 MPa H2, 473 K, and H2/DMO ratio 100 (mol/mol).
has a strong influence on the selectivity of the unsaturated alde-
hyde hydrogenation. The great difference in selectivity for hydro-
genation of DMO might be due to the different surface structures
mentioned above, modified with transitional metal or oxide on
the surface or subsurface of the catalyst. Phase segregation is ob-
served when the Cu/Ni ratio reaches 1/3, and Ni bleeds out from
CuNi and the Cu content in the remaining nanoparticle increases
accordingly. This phenomenon can be explained by the following
considerations. (1) The reduction of the Cu precursor is faster than
that of the Ni precursor. Therefore, it is easier and quicker to form
reduced Cu-containing species in the early stage of the reduction
process. (2) Cu and CuO have lower Tamman temperatures than
Ni and NiO; thus on the silica surface, Cu species diffuse faster than
Ni species, which leads to rapid formation of Cu crystals. (3) The
slower diffusion of the Ni species to the Cu-rich nuclei leads to
the formation of particles with a Cu-rich core and Ni-rich shell. Be-
cause of the Ni segregation on the surface of the Cu species, the
number of active sites greatly decreased; thus the catalytic hydro-
genation activity was reduced compared with that of the homoge-
nous bimetallic Cu–Ni samples.
It is well known to us that monometallic Ni is active in many
hydrogenation reactions such as CO2 hydrogenation [52], liquid
phase citral hydrogenation [53], and hydrogenation of o-, m-, and
p-xylene [54] due to its strong ability to dissociate hydrogen and
adsorb C@O. However, monometallic Ni is not active in the hydro-
genation of DMO, which might result from the strong dimethyl
oxalate adsorption on the surface of Ni, which poisons the surface.
If metallic Cu exists on the surface of catalysts such as CuNi/HMS,
on one hand, it could dissociate hydrogen molecules to hydrogen
atoms; on the other hand, it could react with adsorbed DMO mol-
ecules to form the product. As for EG or MG, this might depend on
the adsorption morphology of DMO. If Ni adsorbs only one C@O
bond, then MG will be obtained; however, if Ni adsorbs two C@O
bonds, then EG will be obtained. As for the concrete adsorption,
it is beyond our research in the present work.
Table 2
The catalytic performance of bimetallic catalysts with different ratios of Cu/Ni in gas-
phase hydrogenation of DMOa.
TOFc (hꢁ1
)
b
ꢁ1
SCu ðm2
g
Þ
Catalyst
T (K) CDMO (%) SMG SEG (%)
Catal
Cu3Ni1/HMS 493
Cu1Ni1/HMS 493
Cu1Ni3/HMS 493
Cu/HMS
Ni/HMS
89
100
27
88
0
60
86
67
14
0
40
14
33
86
0
8.2
6.1
3.6
11.6
–
7.0
10.6
4.8
4.9
–
493
493
a
Reaction condition: 2.5 MPa, H2/DMO ꢃ 100 (mol/mol), and LHSV of DMO
0.2 hꢁ1
.
b
Cu metal surface area determined by N2O titration method.
Turnover frequency (DMO molecular reacted per mol surface Cu per hour) at
c
LHSV = 0.2 hꢁ1 at 493 K.
the conversion of DMO could be improved with higher reaction
temperature, the selectivity to MG would decrease, accompanied
by a corresponding increase in the selectivity to EG. The bimetallic
composition was found to have a strong influence on the catalytic
performance in the hydrogenation of DMO. Excessive introduction
of nickel or copper would greatly decrease the activity of the cata-
lyst, which indicated that the proper surface composition was nec-
essary to obtain high DMO conversion and MG selectivity.
In the literature reports on bimetallic Ni–Cu systems, surface
segregation of Cu is predominantly reported [46], justified on the
basis of thermodynamics. Because of the lower heat of sublimation
(i.e., lower surface free energy) of Cu compared with Ni, Cu is prone
to occupy the surface sites of the bimetallic particles [46]. How-
ever, surface segregation of Ni has been reported in polymer sup-
ported NiCu nanoparticles [47] and has also been observed by
in situ TEM over Ni–Cu/TiO2 [48]. In the present work, Ni and Cu
were uniformly distributed within the particle when the Cu/Ni ra-
tio was higher than 1:1, indicating the formation of homogeneous
Ni–Cu solid solution nanoparticles. The synergetic effects gener-
ated by the Ni–Cu solid solution greatly changed the reaction
dynamics, which made MG the main product. The selectivity to
MG depends critically on the Cu/Ni ratio; higher nickel content
leads to decreased selectivity and DMO conversion. It has been re-
ported that the bimetallic surface structure including bimetallic
composition, the binding energy, and the bonding morphologies
played a crucial role in selective hydrogenation reactions especially
for C@O and C@C hydrogenation [49–51]. The presence of 3d tran-
sitional metal on the surface or subsurface due to different binding
energies and bonding morphologies with the adsorbed substrate
Various mechanisms can be considered in which the nickel spe-
cies influence the catalytic properties of the deposited metal. An-
other possibility would be direct participation of the support
material in the catalytic reaction. The special active sites might
form at the metal–support perimeter, which might be able to coor-
dinate and activate different functional groups in the DMO mole-
cule. As described in the Experimental section, the Cu–Ni
bimetallic catalysts could be formed homogeneously by tuning
the Cu/Ni ratio. XPS results showed that when nickel was dispersed

A. Yin et al. / Journal of Catalysis 280 (2011) 77–88
87
in copper, changes in the electronic properties of nickel resulted in
the displacement of BEs to lower values, depending on the copper
content. The change of the electrical properties might lead to a
change in the catalytic behavior.
The quantitative evaluation of each factor with respect to elec-
tronic variations between the Ni and Cu or Ni and Si can be done by
comparing the BE value changes with the quantum–mechanical
bond structure of M–O–Si at the interface. This information and a
quantum–mechanical description of M–O–Si bonds at the inter-
faces in terms of covalence, and also the density of charge distribu-
tion, would be necessary to completely illustrate the observed
changes.
selectivity of EG. By using AE, oxidative nickel-modified Cu/SiO2
catalyst could be obtained and exhibited excellent catalytic perfor-
mance in the synthesis of EG. A 100% DMO conversion and 98%
selectivity to EG could be obtained at LHSV 1.0 hꢁ1 over Cu3Ni/
HMS catalyst. Using the chemical reduction deposition method, a
bimetallic catalyst was prepared and also exhibited good catalytic
hydrogenation performance in the synthesis of MG, which was
completely different from that of the oxidative nickel-modified
catalyst. By adjusting the molar ratio of Cu to Ni, nearly 96% selec-
tivity to MG with 86% conversion of DMO could be achieved. The
great difference in catalytic behavior might lie in the different sur-
face compositions induced by the nickel species.
3.3.3. Effect of Ni chemical state on the catalytic behavior for
hydrogenation of DMO
3.3.4. Catalytic stability
The long-term stability of catalysts is important for gas-phase
DMO hydrogenation to MG and EG from both academic and indus-
trial viewpoints. To investigate the stability of the NiO-modified
Cu/HMS catalyst, comparisons of catalytic activity and selectivity
to EG as a function of reaction time for Cu3Ni/HMS and Cu/HMS
catalysts are displayed in Fig. 14A. The Cu3Ni/HMS catalyst
showed excellent performance in both catalytic activity and selec-
tivity to EG, which was stable for 150 h. In contrast, obvious deac-
tivation of nickel-free Cu/HMS catalyst was observed within 45 h
under identical reaction conditions. To investigate the stability of
the bimetallic CuNi/HMS catalyst, comparisons of catalytic activity
and selectivity to MG as a function of reaction time for Cu1Ni1/HMS
and Cu/HMS are shown in Fig. 14B. The Cu1Ni1/HMS catalyst exhib-
ited outstanding catalytic performance, which was stable for 50 h.
However, evident deactivation of the monometallic Cu/HMS cata-
lyst could be observed within 12 h under identical reaction condi-
tions. The stability of Cu3Ni/HMS and Cu1Ni1/HMS could be
attributed to the stable surface chemical environments, including
surface chemical compositions and surface species structures,
which could be determined from the XPS results by comparing
the chemical environments of the reduced catalysts and the post-
catalyst. XPS characterizations were carried out for the postcatalyst
by taking the following factors into consideration. First, the sub-
strates (DMO, MG) are oxygenates, which are presumably ad-
sorbed via the carbonyl oxygen atoms on the catalyst surfaces
under working conditions. Second, methanol is not only the carrier
that has been used to supply DMO (and MG) to the catalysts, but
also a stoichiometric co-product of both reactions. Third, notwith-
standing the strongly reducing reaction conditions used, the pres-
ence of these oxygenates on the surfaces of the working catalysts
may result in significant changes in the Cu/Ni valence state distri-
butions relative to those measured for the catalysts in the pristine
Hydrogenation of DMO to EG via MG is a two-step continuous
hydrogenation process. From the thermodynamic point of view,
it is inclined to generate EG due to the much higher equilibrium
constant of the second step than that of the first one. Therefore,
high EG selectivity could be obtained in the context of high hydro-
genation conversion. Our previous studies showed that silica-sup-
ported copper catalysts with highly dispersed active copper species
exhibited high efficiency in the synthesis of EG from the hydroge-
nation of DMO [29]. Thus, highly efficient catalysts could be de-
signed to have higher dispersion of copper species and much
stronger interaction between the active sites and the support.
Introduction of aluminum into the silica support provided both
greater copper dispersion and stronger interaction with the sup-
port; however, acid sites were also generated, which was favorable
to the dehydration reaction [33]. How to design a highly efficient
catalyst without generating acidity is still a great challenge, which
lies in the dynamical control of the product distribution, and could
be achieved by nanocasting highly active and chemoselective
catalysts.
By introducing nickel species through different synthetic proce-
dures, several copper catalysts modified with nickel species having
different surface chemical states were prepared and exhibited high
conversion and chemoselectivity. The introduced amount of nickel
species was found to have a profound effect on the chemoselectiv-
ity of the hydrogenation of the DMO reaction. To establish the real
effect of Ni loading on EG selectivity for CuxNi/HMS series, SEG was
compared at the same DMO conversion (about 50%) and listed in
Table 1. As can be seen from Table 1, the EG selectivity is a function
of Ni loading at the same DMO conversion at ꢃ50%. The maximum
EG selectivity could be obtained via the Cu3Ni/HMS catalyst, which
indicates that the Ni content indeed had a great influence on the
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
Cu3Ni/HMS
Cu3Ni/HMS
Cu/HMS
Cu1Ni1/HMS
Cu1Ni1/HMS
Cu/HMS
Cu/HMS
Cu/HMS
(A)
(B)
0
10
20
30
40
50
0
30
60
90
120
150
Time on stream (h)
Time on stream (h)
Fig. 14. Catalytic performance of catalysts as a function of reaction time (A) over the Cu3Ni/HMS and Cu/HMS catalysts; (B) over the Cu1Ni1/HMS and Cu/HMS catalysts.
Reaction conditions: (A) 2.5 MPa H2, 473 K, and H2/DMO ratio 100 (mol/mol). LHSV = 1.0 hꢁ1; (B) 2.5 MPa H2, 493 K, and H2/DMO ratio 100 (mol/mol). LHSV = 0.2 hꢁ1
.

88
A. Yin et al. / Journal of Catalysis 280 (2011) 77–88
[5] T. Bligaard, J.K. Nørskov, J. Rossmeisl, C.H. Christensen, Nat. Chem. 1 (2009) 37.
[6] J.K. Nørskov, C.H. Christensen, Science 312 (2006) 1322.
[7] J.K. Nørskov, Nature 414 (2001) 405.
[8] F. Besenbacher, I. Chorkendorff, B.S. Clausen, B. Hammer, A.M. Molenbroek, J.K.
Nørskov, I. Stensgaard, Science 279 (1998) 1913.
state, i.e., immediately after reduction but prior to exposure to the
substrates, which is particularly relevant in view of the well-
known extreme sensitivity of metallic Ni to oxidation. As can be
seen from the spectra (see the supporting information), no obvious
differences in the BE values and surface ratios of Ni/Cu, Cu/Si, and
Ni/Si from the reduced catalysts could be observed, which indi-
cated that the surface chemical state and surface chemical compo-
sition of the Cu3Ni/HMS and Cu1Ni1/HMS catalysts were relatively
stable during the whole reaction. It was the stable surface chemical
environments that made sure that good activity and selectivity
could be maintained in the long term.
[9] A. Cao, G. Veser, Nat. Mater. 9 (2010) 75.
[10] A.Q. Wang, C.M. Chang, C.Y. Mou, J. Phys. Chem. B 109 (2005) 18860.
[11] X.Y. Liu, A.Q. Wang, X.F. Yang, T. Zhang, C.Y. Mou, D.S. Su, J. Li, Chem. Mater. 21
(2009) 410.
[12] P. Strasser, S. Koha, J. Greeley, Phys. Chem. Chem. Phys. 10 (2008) 3670.
[13] J.C. Dellamorte, J. Lauterbach, M.A. Barteau, Catal. Today 2 (2007) 182.
[14] S. Kameoka, A.P. Tsai, Catal. Today 132 (2008) 1.
[15] M.T. Schaal, A.Y. Metcalf, J.H. Montoya, J.P. Wilkinson, C.C. Stork, C.T. Williams,
J.R. Monnier, Catal. Today 123 (2007) 142.
[16] M. Kang, M.W. Song, T.W. Kim, Can. J. Chem. Eng. 80 (2002) 63.
[17] T. Huang, S. Jhao, Appl. Catal. A Gen. 302 (2006) 325.
[18] R.J. Best, W.W. Russel, J. Am. Chem. Soc. 76 (1954) 838.
[19] M.K. Gharpurey, P.H. Emmett, J. Phys. Chem. 65 (1961) 1182.
[20] Y. Liu, D.Z. Liu, Int. J. Hydrogen Energy 24 (1999) 351.
[21] Y. Li, J. Chen, L. Chang, Y. Qin, J. Catal. 178 (1998) 76.
[22] K.C. Khulbe, R.S. Mann, Catal. Rev. Sci. Eng. 24 (1982) 311.
[23] J.H. Sinfelt, J.L. Carter, D.J.C. Yates, J. Catal. 24 (1974) 28.
[24] Y. Sun, H. Wang, J.H. Shen, H.C. Liu, Z.M. Liu, Catal. Commun. 10 (2009) 678.
[25] A.Y. Yin, X.Y. Guo, W.L. Dai, K.N. Fan, Chem. Commun. 46 (2010) 4348.
[26] W.M.H. Sachtler, G.J.H. Dorgelo, R. Jongepier, J. Catal. 4 (1965) 100.
[27] S. Huffner, G.K. Werthein, J.H. Wernika, Phys. Rev. 138 (1973) 14511.
[28] A.Y. Yin, X.Y. Guo, W.L. Dai, H.X. Li, K.N. Fan, Appl. Catal. A Gen. 349 (2008) 91.
[29] A.Y. Yin, X.Y. Guo, W.L. Dai, K.N. Fan, J. Phys. Chem. C 113 (2009) 11003.
[30] A.Y. Yin, X.Y. Guo, W.L. Dai, K.N. Fan, Acta Chim. Sin. 67 (2009) 1731.
[31] A.Y. Yin, X.Y. Guo, W.L. Dai, K.N. Fan, ChemCatChem 2 (2010) 206.
[32] A.Y. Yin, X.Y. Guo, W.L. Dai, K.N. Fan, Appl. Catal. A Gen. 377 (2010) 128.
[33] A.Y. Yin, X.Y. Guo, W.L. Dai, K.N. Fan, J. Phys. Chem. C 114 (2010) 4348.
[34] P.T. Tanev, T.J. Pinnavaia, Science 267 (1995) 865.
4. Conclusions
High yields of MG and EG could be achieved over nickel–mod-
ified copper catalysts by tuning the nickel surface chemical states.
An 86% yield of MG and a 98% yield of EG could be obtained over
Cu1Ni1/HMS and Cu3Ni/HMS, respectively. The chemical states of
the nickel species were found to have a great influence on the cat-
alytic behavior. Copper-based catalysts promoted by oxidative
nickel species were favorable for EG synthesis due to the enhanced
dispersion of copper species, while MG could be obtained che-
moselectively over the bimetallic CuNi catalyst, which might result
from the synergetic effects of the metallic copper and nickel
species.
[35] J.W. Evans, M.S. Wainwright, A.J. Bridgewater, D.J. Young, Appl. Catal. 7 (1983)
75.
[36] G.R. Rodolfo, R. Jorge, N. Ruben, L. Rosario, M. Florentino, Catal. Today 107–108
(2005) 926.
Acknowledgments
[37] P. Burattin, M. Che, C. Louis, J. Phys. Chem. B 104 (2000) 10482.
[38] A.R. Naghash, T.H. Etsell, S. Xu, Chem. Mater. 18 (2006) 2480.
[39] B. Bridier, J. Perez-Ramirez, J. Am. Chem. Soc. 132 (2010) 4321.
[40] A. Gervasini, M. Manzoli, G. Martra, A. Ponti, N. Ravasio, L. Sordelli, F.
Zaccheria, J. Phys. Chem. B 110 (2006) 7851.
[41] C.J.G. van der Grift, P.A. Elberse, A. Mulder, J.W. Geus, Appl. Catal. 59 (1990)
275.
[42] F. Raimondi, K. Geissler, J. Wambach, A. Wokaun, Appl. Surf. Sci. 189 (2002) 59.
[43] L.F. Chen, P.J. Guo, Z.L. Jun, M.H. Qiao, W. Shen, H.L. Xu, K.N. Fan, Appl. Catal. A
356 (2009) 129.
We thank the Major State Basic Resource Development Program
(Grant 2003CB 615807), the NNSFC (Project 20973042), the Re-
search Fund for the Doctoral Program of Higher Education
(20090071110011) and the Science and Technology Commission
of Shanghai Municipality (08DZ2270500) for financial support.
Also, we express our sincere gratitude to one of the reviewers for
important and constructive suggestions.
[44] L. Huang, Y.L. Zhu, H.Y. Zheng, M.X. Du, Y.W. Li, Appl. Catal. A Gen. 349 (2008)
204.
Appendix A. Supplementary material
[45] B.M. Reddy, G.M. Kumar, L. Ganesh, A. Khan, J. Mol. Catal. A Chem. 247 (2006)
80.
[46] J.H. Sinfelt, Acc. Chem. Soc. 10 (1977) 15.
[47] S. Wu, C. Zhu, W. Huang, Chin. J. Polym. Sci. 14 (1996) 76.
[48] P. Li, J. Liu, N. Nag, P.A. Crozier, J. Catal. 262 (2009) 73.
[49] L.E. Murillo, A.M. Goda, J.G. Chen, J. Am. Chem. Soc. 129 (2007) 7101.
[50] S. Laref, F. Delbecq, D. Loffreda, J. Catal. 265 (2009) 35.
[51] L.E. Murillo, C.A. Menning, J.G. Chen, J. Catal. 268 (2009) 335.
[52] G.D. Weatherbee, C.H. Bartholomew, J. Catal. 68 (1981) 67.
[53] U.K. Singh, M.A. Vannice, J. Catal. 199 (2001) 73.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jcat.2011.03.006.
References
[1] C. Burda, X.B. Chen, R. Narayanan, M.A. El-Sayed, Chem. Rev. 105 (2005) 1025.
[2] A. Hugon, L. Delannoy, J.M. Krafft, C. Louis, J. Phys. Chem. C 114 (2010) 10823.
[3] C. Della Pina, E. Falletta, M. Rossi, J. Catal. 260 (2008) 384.
[54] M.A. Keane, J. Catal. 166 (1997) 347.
[4] J.A. Rodriguez, D.W. Goodman, Science 257 (1992) 897.