Cu/SiO2 catalysts prepared by hom- and heterogeneous deposition-precipitation methods: Texture, structure, and catalytic performance in the hydrogenolysis of glycerol to 1,2-propanediol

Article abstract of DOI:10.1016/j.cattod.2011.08.038

Cu/SiO₂ catalysts prepared by homogeneous deposition- precipitation (Hom-DP) and heterogeneous deposition-precipitation (Het-DP) methods have been systematically characterized focusing on the effect of precipitation manner during catalyst preparation. It is found that the texture, structure and composition of the dried, calcined and reduced catalysts were largely affected by the precipitation manner. Based on characterizations and previous findings, the copper species on the dried, calcined, and reduced catalysts as well as on the catalysts after work were assigned. Due to a homogeneous precipitation manner, the catalyst prepared by Hom-DP method presented a much higher dispersion, a smaller copper particle size and thus a larger copper surface area than those of its counterpart prepared by Het-DP method. Unexpectedly, catalytic activity tests in the hydrogenolysis of glycerol showed that the catalyst prepared by Het-DP method inversely surpassed the former catalyst, mainly due to a larger number of copper species in the Het-DP catalyst was prereduced to active Cu⁰ sites, which also presented remarkably higher stability during aqueous phase glycerol reaction as compared to those in the former catalyst. In addition, the selectivity to 1,2-propanediol product is found to be affected by both the precipitation agent applied and the structure of the catalysts.

Full text of DOI:10.1016/j.cattod.2011.08.038

Catalysis Today 183 (2012) 42–51
Contents lists available at SciVerse ScienceDirect
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Cu/SiO2 catalysts prepared by hom- and heterogeneous deposition–precipitation
methods: Texture, structure, and catalytic performance in the hydrogenolysis of
glycerol to 1,2-propanediol
Zhiwei Huanga,b, Fang Cuia,b, Jingjing Xuea, Jianliang Zuoa, Jing Chena,b,∗, Chungu Xiaa,b,∗
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
b Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 April 2011
Received in revised form 22 August 2011
Accepted 22 August 2011
Available online 25 September 2011
Cu/SiO2 catalysts prepared by homogeneous deposition–precipitation (Hom-DP) and heterogeneous
deposition–precipitation (Het-DP) methods have been systematically characterized focusing on the effect
of precipitation manner during catalyst preparation. It is found that the texture, structure and composi-
tion of the dried, calcined and reduced catalysts were largely affected by the precipitation manner. Based
on characterizations and previous findings, the copper species on the dried, calcined, and reduced cata-
lysts as well as on the catalysts after work were assigned. Due to a homogeneous precipitation manner,
the catalyst prepared by Hom-DP method presented a much higher dispersion, a smaller copper particle
size and thus a larger copper surface area than those of its counterpart prepared by Het-DP method. Unex-
pectedly, catalytic activity tests in the hydrogenolysis of glycerol showed that the catalyst prepared by
Het-DP method inversely surpassed the former catalyst, mainly due to a larger number of copper species
in the Het-DP catalyst was prereduced to active Cu0 sites, which also presented remarkably higher sta-
bility during aqueous phase glycerol reaction as compared to those in the former catalyst. In addition,
the selectivity to 1,2-propanediol product is found to be affected by both the precipitation agent applied
and the structure of the catalysts.
Keywords:
Cu/SiO2 catalyst
Deposition–precipitation method
Glycerol
Hydrogenolysis
Propanediol
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
hydrogenolysis of glycerol represents a low cost and green route for
1,2-PDO [4–22]. Noble metals such as Ru, Rh and Pt are extensively
Catalytic conversion of renewable biomass to fuels and com-
modity chemicals becomes more and more important because of
the worldwide energy and environmental problems. Glycerol is
one of the top-12 building block chemicals that can be derived
from plant sources [1]. In addition, the production of biodiesel by
the transesterification of vegetable oils and animal fats makes large
quantities of glycerol available as a reaction co-product, ca. 10 wt%
of the biodiesel produced. The availability and low cost of glycerol
make it a promising feedstock for producing a wide range of value-
added chemicals [2,3]. 1,2-Propanediol (1,2-PDO) is an important
commodity chemical widely used for polyester resins, pharma-
ceuticals, foods, cosmetics and functional fluids, etc. Currently,
1,2-PDO is mainly produced by the hydration of propylene oxide
from non-renewable petroleum derived propylene [2]. The pro-
duction of 1,2-PDO from renewable glycerol by selective catalytic
used in the hydrogenolysis of glycerol because of their high reac-
tivity [7–10,13,16,19]. Nevertheless, these catalysts often promote
excessive C–C cleavage, resulting in a poor selectivity to 1,2-PDO.
Copper is known for its poor activity for C–C bond cleavage and high
efficiency for C–O bond hydro-dehydrogenation [4,5,23]. Thus, as
a less expensive alternative, copper is found to be potentially a
good catalyst for the hydrogenolysis of glycerol. For example, over
a copper-chromite catalyst Dasari et al. [5] converted glycerol to
1,2-PDO with 85% of 1,2-PDO selectivity at 55% of glycerol conver-
sion under a mild condition of 1.4 MPa and 473 K. More recently,
Cu–ZnO [20,21], Cu–Al2O3 [12,14] as well as Cu/ZnO/Al2O3 [11]
catalysts are widely investigated at temperatures between 453
and 523 K due to their attractive performance in glycerol reaction.
In our previous work, we found that Cu/SiO2 catalysts pre-
pared by both impregnation and precipitation–gel (PG) methods
showed high selectivity towards 1,2-PDO in the hydrogenolysis of
glycerol with the catalyst prepared by the latter method exhib-
ited much higher activity and better stability [24]. In addition,
we have also found that the residual sodium in the Cu/SiO2 cata-
lysts prepared by PG method has profound effects on the structure
and catalytic performance in the hydrogenolysis of glycerol of the
∗
Corresponding authors at: State Key Laboratory for Oxo Synthesis and Selective
Oxidation, Lanzhou Institute of Chemical Physics, CAS, Lanzhou 730000, China.
Tel.: +86 931 4968089; fax: +86 931 4968129.
E-mail addresses: chenj@lzb.ac.cn (J. Chen), cgxia@lzb.ac.cn (C. Xia).
0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.08.038

Z. Huang et al. / Catalysis Today 183 (2012) 42–51
43
catalyst [25]. Deposition–precipitation (DP) technique is a sim-
ple and convenient method widely used for the preparation of
dispersed metal nanocatalysts [26–30]. It involves the suspen-
sion of a support material in the solution of a metal salt and the
precipitation of the metal ions onto the support. Due to a differ-
ent precipitation manner, this method can be divided into two
parts: one is homogeneous deposition–precipitation (Hom-DP),
in which metal ions were homogeneously precipitated by slowly
hydrolysis of urea [26] or evaporation of ammonia [27,28]; and
the other is heterogeneous deposition–precipitation (Het-DP), in
which metal ions were quickly precipitated by an aqueous base
solution [29,30]. Despite numerous researches on the prepara-
tion of dispersed nanocatalysts by DP method, few studies were
focused on the differences in the structure and catalytic property
of the catalysts prepared by different DP methods. Additionally,
the catalytic performance of the Cu/SiO2 catalysts prepared by
DP method in the glycerol hydrogenolysis remains open to study.
Accordingly, this work was undertaken with the aim of elucidat-
ing the differences in the structure and catalytic performance of
the Cu/SiO2 catalysts prepared by two different DP methods: one
is the ammonia evaporation-based Hom-DP method, the other
is the sodium hydroxyl precipitation-based Het-DP method. The
structures of the catalysts were characterized by N2 adsorption,
N2O chemisorption, infrared spectroscopy (FTIR), X-ray pow-
der diffraction (XRD), simultaneous thermogravimetric/differential
thermogravimetric analysis (TG–DTG), X-ray photoelectron spec-
troscopy (XPS), temperature-programmed reduction with H2
(H2-TPR), and transmission electron microscope (TEM). The rela-
tion between the catalytic performance for glycerol hydrogenolysis
and the structure of the Cu/SiO2 catalysts were discussed.
in a Nicolet Nexus 870 FTIR spectrometer. Simultaneous thermo-
gravimetric/differential thermogravimetric (TG/DTG) experiments
were performed on a Perkin-Elmer Pyris Diamond TG/DTA instru-
ment in the temperature range 303–1273 K in N2 atmosphere at a
temperature ramp of 10 K/min.
The X-ray powder diffraction (XRD) of the samples was carried
out on a PANalytical X’pert Pro Diffractometer using nickel filtered
Cu K␣ radiation with a scanning angle (2ꢀ) of 15–80◦ and a voltage
and current of 40 kV and 30 mA. XRD patterns were also obtained
during in situ reduction in 5% H2/Ar at a flow rate of 50 mL/min
up to 723 K. In these experiments the temperature was ramped
between measurements at a rate of 10 K/min with a 5-min pause at
each temperature before recording the pattern. The mean crystal-
lite size of Cu was calculated by Scherrer equation according to the
Cu(1 1 1) peak. X-ray photoelectron spectra (XPS) were obtained
using a VG ESCALAB 210 spectrometer equipped with an Mg K␣ X-
ray radiation source (hꢁ = 1253.6 eV) and a hemispherical electron
analyzer. All binding energies were calibrated using the Si2p peak
at 103.4 eV as the reference.
Temperature-programmed reduction (TPR) measurements
were carried out in a quartz U-tube reactor with 20 mg of sam-
ple used for each measurement. The samples were first pretreated
at 473 K under He flow for 1 h and then samples were reduced with
20% H2/Ar at a flow rate of 50 mL/min and the temperature was
increased from 303 K to 773 K at a ramping rate of 10 K/min. H2 con-
sumption was continuously monitored by a thermal conductivity
detector (TCD).
The dispersion of the catalysts was determined by dissociative
N2O chemisorption [31]. The catalysts were first reduced at 773 K
with 20% H2/Ar at a flow rate of 50 mL/min for 1 h. After cooling
to 363 K in a He flow, the reduced samples were exposed to a 5%
N2O/N2 mixture (50 mL/min) for 0.5 h. Finally, the samples were
cooled to room temperature to start another TPR run with 20%
H2/Ar at a flow rate of 50 mL/min and a ramping rate of 10 K/min
to 573 K. The copper metallic surface area, average copper particle
size and dispersion were calculated by assuming 1.4 × 1019 copper
atoms per m2 and a molar stoichiometry N2O/Cus = 0.5, where
the symbol Cus means the copper atoms on the surface
[31,32].
The BET surface area measurements were performed on a
Micromeritics ASAP 2010 instrument at liquid nitrogen temper-
ature. Prior to measurements, the samples were degassed at 473 K
overnight. Transmission electron microscopic (TEM) investiga-
tions were carried out using a JEM2010 electron microscope at
200 kV.
2. Experimental
2.1. Catalyst preparation
Hom-DP
catalyst
was
prepared
by
homogeneous
deposition–precipitation of the mixture of aqueous cupram-
monia complex and silica sol. Briefly, a calculated amount of
colloidal aqueous silica solution (SiO2, 40.0 wt%, Guangzhou
Renmin Chemical Plant, China) was added to the solution of
cuprammonia complex (pH around 11). After stirring at room
temperature for 3 h, the mixture was subsequently heated to 353 K
and kept at this temperature for 20 h to evaporate ammonia. Then,
the slurry of the gel was filtered, washed with hot distilled water,
dried at 393 K overnight, and calcined at 723 K under air for 3 h.
Het-DP catalyst was prepared starting with a suspension of dried
silica gel powder (Qingdao Haiyang Chemical Co., China, 442 m2/g,
<100 mesh) in a solution of Cu(NO3)2 (0.5 mol/L). After stirring
at room temperature for 3 h, Cu(NO3)2 solution was exclusively
precipitated by adding an aqueous solution of NaOH (4 mol/L) till
pH > 10. Then the temperature was raised to 353 K and kept at this
temperature for 4 h. The resulted catalyst precursor was washed
with distilled water until pH was 7 and then dried and calcined
under the same conditions as that of Hom-DP catalyst. The metal
loadings of the catalysts in terms of metal oxide were determined
by X-ray fluorescence spectroscopy (XRF, PANalytical Magix PW
2403). Pure CuO reference was prepared by precipitating Cu(NO3)2
(0.5 mol/L) with the solution of NaOH (4 mol/L) and aged at 353 K for
4 h; after washing to pH = 7, the filter cake was dried and calcined
as those of the silica containing samples.
2.3. Catalytic activity testing
The glycerol reaction was carried out in a 200 mL stainless steel
autoclave at a stirring speed of 400 rpm. Prior to the reaction,
the catalysts were prereduced by H2 stream at 553 K for 3 h. The
standard reaction was carried out under the following reaction
conditions: 6.4 MPa of initial H2 pressure, 80 g of 80 wt% glycerol
aqueous solution, 4 g of reduced catalyst, 12 h. After being purged,
the reactor was heated to the reaction temperature, and the H2
pressure was increased to about 9.0 MPa and maintained during the
reaction. The liquid phase products were analyzed by using a gas
chromatograph with a SE-54 capillary column (50 m × 0.32 mm)
and a flame ionization detector. The gas products were analyzed
by using a gas chromatograph (Porapak Q column (4 m × 3 mm))
equipped with a TCD. Products were also identified on a HP
6890/5793 GC-MS with a DB-5MS column. The detected liquid
products were methanol, ethanol, 2-propanol, 1-propanol, acetol,
ethylene glycol (EG), 1,2-PDO and 1,3-PDO, and a typical gas chro-
matogram of these products is shown in Fig. 1. The detected gas
products were small amount of CH4 and CO2. Conversion of the
2.2. Catalyst characterization
Fourier transform infrared (FTIR) spectra were recorded at room
temperature on powdered samples using the KBr wafer technique

44
Z. Huang et al. / Catalysis Today 183 (2012) 42–51
Fig. 1. Gas chromatogram of the hydrogenolysis reaction products: (a) methanol; (b) ethanol; (c) 2-propanol; (d) 1-propanol; (e) acetol; (f) EG; (g) 1,2-PDO; (h) 1,3-PDO; (i)
glycerol.
glycerol was calculated on the basis of the following equation:
These findings suggests that Cu2(OH)3NO3 species in this sample
completely decomposed to CuO after calcination while the struc-
ture of copper phyllosilicate was mainly preserved. In contrast,
for calcined Het-DP sample, the ꢂSiO band at 1027 cm−1 shifted to
1097 cm−1 and the broad ıOH band at ∼670 cm−1 almost disap-
peared, indicating that copper phyllosilicate in this sample largely
decomposed into silica and CuO. In addition, the shoulder peak
at 690 cm−1 presented in the dried Het-DP sample completely
disappeared, suggesting the full decomposition of Cu(OH)2. The for-
mation of CuO after calcination in this sample is also confirmed by
XRD pattern given below.
conversion(%)
sum of C-based moles of all products
sum of C-based moles of reactant and all products
=
×100
The selectivity of the products was calculated based on a carbon
basis:
C-based moles of specific products
sum of C-based moles of all products
selectivity(%) =
× 100
Fig. 3 illustrates the TG–DTG curves of the dried precursors of
Hom-DP and Het-DP catalysts, which revealed that the decompo-
sition of both samples occurred in three main weight loss steps.
The first weight loss step at temperatures below 400 K for both
samples can be attributed to the removal of weakly bonded water,
3. Results and discussion
3.1. Structure properties
FTIR experiments were carried out to study the structure and
composition of the dried and calcined samples prepared by differ-
ent DP methods. For dried Hom-DP sample (Fig. 2(A)), the apparent
absorption bands at approximately 1120, 800, and 476 cm−1 are
assigned to the different vibration modes of the Si–O bonds in the
amorphous SiO2 [33,34]. The shoulder peak at around 1046 cm−1
on the low-frequency side of the ꢂSiO band at 1120 cm−1 together
with the presence of the weak ıOH band at around 670 cm−1 indi-
cates the presence of copper phyllosilicate [33]. In addition, a small
absorption at 1384 cm−1 related to NO3 groups is also observed
in the profile of the dried Hom-DP sample, suggesting that a small
amount of Cu2(OH)3NO3 is also present [33]. For dried Het-DP sam-
ple the broad bands at around 1028 and 670 cm−1 can be ascribed
to the ꢂSiO and the ıOH band of copper phyllosilicate (Fig. 2(C)),
respectively. It should be noted here that an attenuated shoul-
der peak at around 690 cm−1 characteristic of the ıOH vibration of
Cu(OH)2 is also observed [33]. More evidence for the presence of
Cu(OH)2 in the dried Het-DP sample will be given below. Cu(OH)2 is
reported to form in the dried catalyst precursor prepared by ammo-
nia evaporation-based Hom-DP method analogy to this work [28].
Therefore, the formation of a small amount of Cu(OH)2 in the dried
Hom-DP sample can not be excluded.
After calcination at 723 K, the absorption band associated with
NO3 groups in Hom-DP sample disappeared, and at the same time
both the ıOH band at ∼670 cm−1 and the ꢂSiO shoulder at 1046 cm−1
associated with copper phyllosilicate slightly decreased (Fig. 2(B)).
Fig. 2. FTIR spectra of: (A) dried Hom-DP sample; (B) calcined Hom-DP sample; (C)
dried Het-DP sample and (D) calcined Het-DP sample.

Z. Huang et al. / Catalysis Today 183 (2012) 42–51
45
Fig. 4. XRD patterns of: (A) dried Het-DP sample; (B) calcined Het-DP sample; (C)
dried Hom-DP sample; (D) calcined Hom-DP sample; (E) used Het-DP sample and
(F) used Hom-DP sample.
the aging and drying processes. The diffraction peaks of CuO fur-
ther intensified after calcination (Fig. 4(B)). The apparent feature
of dried Hom-DP sample was the broad and diffused diffraction of
amorphous silica at around 2ꢀ = 22◦ (Fig. 4(C)). In addition, the weak
and diffuse diffraction peaks at ca. 31.2◦ and 35.8◦ suggest the pres-
ence of copper phyllosilicate with poor crystallinity [28,33]. No big
difference was seen after calcined at 723 K, suggesting the high sta-
bility of copper species in Hom-DP sample (Fig. 4(D)). After glycerol
reaction at 473 K, sharp and intensified diffraction peaks of metal-
lic copper were seen in the used Hom-DP catalyst (Fig. 4(F)). In
contrast, the diffraction peaks of Cu0 in the used Het-DP catalyst
are much lower and broader (Fig. 4(E)). These findings suggest that
copper species in Hom-DP catalyst were seriously aggregated to
much larger particles (21.0 nm) during glycerol reaction as com-
pared to Het-DP catalyst, which is 10.8 nm (almost one half that
of used Hom-DP catalyst). In addition, a broad diffraction peak at
around 2ꢀ = 36.8◦ attributed to Cu2O is also observed in both sam-
ples, and its diffraction is more intense for Hom-DP catalyst than
for Het-DP catalyst, indicating the presence of a larger amount of
Cu2O phase in the former catalyst.
In situ XRD diffraction patterns were taken during the reduction
of the calcined samples. At temperature below 473 K, no obvious
change was observed in the XRD pattern of Hom-DP sample
(Fig. 5(A)). With increasing temperature up to 473 K, broad and
clear diffraction peaks characteristic of metallic Cu appeared. The
diffraction peaks of copper slightly intensified with no distinct
increase in the amplitude of the diffraction peaks even up to 723 K.
The in situ reduction profiles of calcined Het-DP sample showed
that this sample is stable at temperature below 448 K (Fig. 5(B)).
After reduction at 473 K, a weak but observable peak appeared
in between the diffraction peaks of CuO at 2ꢀ = 35.5 and 38.6◦, it
may associate with the presence of Cu2O, more evidence for the
presence of this species will be given below. When the tempera-
ture raised up to 498 K, the diffraction peaks associated with Cu0
appeared with no additional CuO and Cu2O phases detectable,
indicating that copper species were mainly reduced to Cu0 at this
temperature. Note that the full presence of the diffractions of Cu0
for Hom-DP sample occurred at a temperature around 25 K lower
than Het-DP sample, suggesting that the reduction of the majority
of copper species in the calcined Het-DP occurred at temperature
a bit higher than Hom-DP sample. Only slightly intensified diffrac-
tion peaks of Cu0 were seen with further increasing reduction
temperature to 723 K. It should be noted here that although there
Fig. 3. TG–DTG curves of the dried precursors of Hom-DP (A) and Het-DP (B).
while the second weight loss step in the range 400–950 K is likely
associated with the decomposition of different copper species (cop-
per phyllosilicate and Cu(OH)2 in the dried Het-DP sample; copper
phyllosilicate, Cu(OH)2 and Cu2(OH)3NO3 in the dried Het-DP sam-
ple) and the dehydration of silanol groups in the samples. The
obvious weight loss peak observed in the DTG curve of Het-DP sam-
ple at around 447 K is associated with the decomposition of copper
hydroxide in the dried precursor of this sample, as the decom-
position of which usually occurs in the range of 413–473 K [35].
The decomposition extent of copper phyllosilicate is reported to
depend on the preparation conditions and the thermal pretreat-
ment. Toupance et al. [36] revealed a partial decomposition of
copper phyllosilicate when being calcined at 723 K, while Yurieva
et al. [37] reported that the copper phyllosilicate prepared by urea
hydrolysis-based Hom-DP method is stable up to a calcination tem-
perature of 900 K and then decomposed to CuO and silica after
calcination at 1023 K. In addition, the decomposition of CuO to Cu2O
is reported to occur at a high temperature of around 1100 K [35].
Thus, it is reasonable that the third profound weight loss of Hom-
DP sample, which is composed of two overlapped processes as
indicated from the DTG curves, is associated with the full decompo-
sition of copper phyllosilicate to CuO and silica (at 1079 K) and the
degradation of CuO to Cu2O (at 1126 K). Similarly, the broad weight
loss peak of Het-DP sample in the range of 1000–1173 K is possi-
bly attributed to the collective contribution of the decomposition
of copper phyllosilicate and CuO.
XRD diffraction patterns were taken for both the dried and cal-
cined samples prepared by different DP methods. For dried Het-DP
sample (Fig. 4(A)), weak and diffused diffractions characteristic of
CuO have already appeared at 2ꢀ = 35.5 and 38.7◦ (JCPDS 05-0661).
It seems that a part of Cu(OH)2, which is formed by precipitation of
Cu(NO3)2 with NaOH, is unstable and decomposed to CuO during

46
Z. Huang et al. / Catalysis Today 183 (2012) 42–51
Fig. 6. XPS spectra of calcined and used Hom-DP and Het-DP catalysts: (A) Cu2P
spectra and (B) Cu LLM auger kinetic energy of (a) calcined Hom-DP; (b) used Hom-
DP; (c) calcined Het-DP and (d) used Het-DP.
from 6.1 to 7.0 nm, respectively, with increasing reduction temper-
ature from 498 to 723 K.
Fig. 5. In situ XRD patterns taken during reduction of calcined Hom-DP (A) and
calcined Het-DP (B).
XPS experiments were carried out to determine the oxidation
state of copper as well as the chemical compositions of the sam-
ples. The high values of the binding energy (BE) (>936.0 eV) along
with the presence of the characteristic shakeup satellite peaks sug-
gest that the copper oxidation state is +2 in both samples (Fig. 6(A))
[38]. The BE of pure CuO prepared by precipitation was deter-
mined at 934.0 eV. Thus, the relatively large positive BE shift of the
Cu2p core level for the DP samples is indicative of a charge trans-
fer from the metal ions towards the support matrix [39]. Van Der
Grift et al. [26] found that no copper (II) oxide but copper phyl-
was no detection of the diffraction of Cu2O in the XRD patterns
of both samples after reduction to temperatures above 498 K, it
does not mean that copper species in both samples have been fully
reduced to Cu0 at this temperature, because Cu2O species with high
dispersion and/or small amount are likely to be X-ray amorphous.
Both Hom-DP and Het-DP catalysts showed remarkably high
stability during the reduction process as the crystallite sizes of cop-
per for Hom-DP and Het-DP just increased from 4.2 to 4.7 nm and

Z. Huang et al. / Catalysis Today 183 (2012) 42–51
47
Table 1
XPS analysis of CuO/SiO2 catalysts prepared by different methods.
Catalyst
Position of
Cu/Si atoms ratio
Cu2p3/2 (eV)
Bulk
XPS
Hom-DP
Het-DP
Fresh
After work
Fresh
936.0
932.9
936.1
932.6
0.331
–
0.339
–
1.275
0.374
0.913
0.329
After work
losilicate was present in the calcined CuO/SiO2 samples prepared
by urea hydrolysis-based Hom-DP method by XPS, as the BE of the
latter species is 2.0 eV higher than that of the former. Chen et al.
[28] also ascribed the higher BE of the CuO/SiO2 samples prepared
by ammonia evaporation-based Hom-DP method analogy to this
work at higher evaporation temperature to the presence of copper
phyllosilicate. Therefore, the high BE of both Hom-DP and Het-DP
samples can also be ascribed to the formation of copper phyllosili-
cate, the presence of which was confirmed by FTIR analysis (Fig. 2).
The XPS surface Cu/Si atom ratios on Het-DP and Hom-DP samples
are 2.7 and 3.8 times higher than their corresponding bulk ratios,
respectively, suggesting copper species on both samples are largely
enriched on the surface [40].
After work, the BE of Cu2p3/2 core levels of both catalysts
reduced to 932.6–932.9 eV and the characteristic shakeup satellite
lines of Cu2+ disappeared, indicating that copper became reduced
(Cu+ or Cu0). Because the BE values of Cu+ and Cu0 are almost
identical, the distinction between these two species present on
the catalyst surface is feasible only through the examination of
the Auger spectra [34,41]. Two overlapping Cu LMM Auger kinetic
energy peaks centered at about 918.3 and 916.3 eV are observed in
the used Hom-DP and Het-DP catalysts (Fig. 6(B)), characterizing
the coexistence of both Cu0 and Cu+ on the surface. As can be seen
from the Auger kinetic energy profiles of the used catalysts, the peak
at 916.3 eV is more apparent for Hom-DP catalyst, while the peak
at 918.3 eV is more prominent for Het-DP catalyst. These findings
suggest that Cu+ species was the major species in the used Hom-DP
catalyst while Cu0 was the major species present in the used Het-DP
catalyst [29,41]. It is reported that when copper is highly dispersed
and in intimated contact with the supports, Cu2p3/2 energies, par-
ticularly Cu Auger parameters, may deviate significantly from the
corresponding bulk values [29]. The large negative shift of Cu LMM
Auger kinetic energy for both calcined DP catalysts implied the dif-
ference in copper coordination. After reaction, a more profound
decrease of Cu/Si ratio for the used Hom-DP catalyst (from 1.275 to
0.374) as compared to Het-DP catalyst (from 0.913 to 0.329) was
seen (Table 1), suggesting a more serious aggregation of copper in
the former catalyst during glycerol reaction. The above findings are
in good agreement with the XRD results shown in Fig. 4(E and F).
From the above characterizations, we know that due to a differ-
ent precipitation manner, different copper species are present in
both the dried and calcined samples. In addition, grafted Cu2+ ions,
which are formed by electrostatic adsorption of CuII cations with
the negatively charged silica particles [33] and cannot be tracked
by the above characterizations, may also selectively form in the
DP samples. In Cu/SiO2 catalyst prepared by Het-DP, the pH of the
solution is increased from around 3 to above 10 with the drop-
ping of NaOH. At a low initial pH, the silica surface is positively
charged. Thus, it is not favorable for CuII cations to adsorb on silica
surface to form grafted Cu2+ ions. With increasing pH, CuII cations
were precipitated to Cu(OH)2 nanoparticles. Cu(OH)2 nanoparti-
cles would further react with the silica surface to form copper
phyllosilicate in a high pH solution [33]. Therefore, both Cu(OH)2
and copper phyllosilicate coexist in the dried Het-DP, besides CuO
(which is formed by the decomposition of Cu(OH)2 during high
temperature aging and drying). Differently, in the catalyst prepared
Fig. 7. H2-TPR profiles of calcined Hom-DP sample (A); calcined Het-DP sample (B)
and the reference sample pure bulk CuO (C).
by Hom-DP, the initial pH is around 11, the silica surface is nega-
tively charged, favoring the adsorption of copper cations. With the
decrement of the pH of the suspension by evaporation of ammonia,
Cu(OH)2 may form and it would further react with the silica sur-
face to form copper phyllosilicate. With further decreasing of pH,
copper nitrate hydroxide may also form, as which is reported to pre-
cipitate at 7 < pH < 8.8 [33]. Thus we suggest that on dried Cu/SiO2
samples prepared by Hom-DP method, besides the presence of cop-
per nitrate hydroxide (identified by FTIR) and copper phyllosilicate
(identified by FTIR and XRD), there are a small amount of highly
dispersed Cu(OH)2 and grafted Cu2+ ions. After calcination, both
CuO, which comes from the decomposition of Cu(OH)2 and copper
phyllosilicate, and undecomposed copper phyllosilicate coexist in
Het-DP sample. For calcined Hom-DP sample, there are three kinds
of copper species: copper phyllosilicate, isolated-grafted Cu2+ ions
and a small amount of highly dispersed CuO (originating from the
full decomposition of Cu(OH)2 and Cu2(OH)3NO3, and the partial
decomposition of copper phyllosilicate).
Fig. 7 shows the TPR profiles of the calcined Hom-DP and Het-DP
samples along with the reference unsupported bulk CuO. As can be
seen, Hom-DP catalyst is reduced in a much wide range from 410
to 642 K with a main reduction peak centered at 500 K and a broad
shoulder peak centered around 570 K. Similar to Hom-DP catalyst,
Het-DP sample also showed two reduction peaks with the main
peak present at 509 K and the shoulder peak at 558 K. Differently,
unsupported pure CuO showed a good symmetric hydrogen con-
sumption peak centered around 490 K, which is 10–20 K lower than
that of the main peak of DP samples, showing clearly the inhibit
effect of the silica particles in the reduction of Cu2+ species.
As it is well known, dispersed CuO with small particle sizes can
be more facilely reduced to Cu0 than did the bulk CuO with larger
sizes [38,42]. In contrast, dispersed and surface interacted Cu2+
species are reported to be more difficult to reduce than bulk CuO
[43,44]. Thus, the above mentioned inhibiting effect of the silica
particles in the reduction of the DP samples would probably be due

48
Z. Huang et al. / Catalysis Today 183 (2012) 42–51
Fig. 8. TEM images of calcined and reduced samples (A) calcined Hom-DP sample; (B) calcined Het-DP sample; (C) reduced Hom-DP sample; and (D) reduced Het-DP sample.
to the presence of the surface interacted Cu2+ species, that is copper
phyllosilicate in calcined Het-DP sample, copper phyllosilicate and
grafted Cu2+ ions in calcined Hom-DP sample [28]. Therefore, it is
reasonable that the main reduction peak of the CuO/SiO2 samples
is attributed to the collective contribution of stepwise reduction of
dispersed CuO to Cu0 (from in situ XRD results) and the partially
reduction of surface interacted Cu2+ species to Cu+; while the shoul-
der peak in the high temperature range is ascribed to the reduction
of supported Cu+, which is derived from the stepwise reduction of
surface interacted Cu2+ species. Thus, a small proportion of Cu2O
with high dispersion tracked by H2-TPR is inferred to present in the
in situ XRD reduction experiments of both samples, and it would
still exist after the full presence of Cu0 diffraction peaks.
above 330 m2/g with the calcined Hom-DP much higher, which
is 366 m2/g. The pore volume of both catalysts is in the range of
0.72–0.85 cm3/g. The copper dispersion of the catalysts obtained
from N2O chemisorption for Hom-DP catalyst is also higher than
that of Het-DP catalyst, which is in consistent with the results
obtained from XRD and XPS. As a result of copper dispersion, Hom-
DP catalyst presented a much higher copper surface area and a
lower copper particle size than those of the Het-DP catalyst.
Fig. 8 shows the TEM images of the calcined and reduced sam-
ples prepared by different DP method. In calcined Hom-DP sample
(Fig. 8(A)), randomly oriented filandrous species, which can be
assigned to copper phyllosilicate according to literature [28] and
the above characterizations, along with a few dark particles with
much small sizes assignable to CuO are observed. The highly diffu-
sive ring pattern in the select area electron diffraction (SAED) of this
sample reveals an amorphous structure. For calcined Het-DP sam-
ple, dark gray particles with quasi-spherical shape can be assigned
The textural properties of both calcined catalysts including the
specific surface area, average pore diameter, metal dispersion,
metal surface area, and copper particle size are summarized in
Table 2. Both catalysts showed high specific BET surface areas of
Table 2
The texture of the calcined samples prepared by Hom-DP and Het-DP methods.
Catalyst
CuOa (wt%)
BET surface areab (m2/g)
VPb (cm3/g)
Cu-dispersionc (%)
Cu-metal area c (m2/g)
Copper particle sizec (nm)
Hom-DP
Het-DP
30.5
31.0
366
334
0.72
0.85
24.6
16.2
165.1
109.6
4.1
6.2
a
Measured by XRF.
BET method.
Calculated from dissociative N2O adsorption.
b
c

Z. Huang et al. / Catalysis Today 183 (2012) 42–51
49
Fig. 9. Catalytic activity and selectivity of the reduced Hom-DP (A) and Het-DP (B)
catalysts as a function of temperature. Reaction conditions: 80% aqueous solution
of glycerol 80 g, total pressure 9.0 MPa, reduced catalyst 4 g, 12 h (propanols = 1-
propanol + 2-propanol).
Fig. 10. Effect of catalyst amount on the glycerol reaction over reduced Hom-DP (A)
and Het-DP (B) catalysts. Reaction conditions: 80% aqueous solution of glycerol 80 g,
total pressure 9.0 MPa, 473 K, 12 h (propanols = 1-propanol + 2-propanol).
1,2-PDO for Hom-DP catalyst was 1.8–4.3% higher than that of Het-
DP catalyst; at temperature above 473 K, the selectivity to 1,2-PDO
for Hom-DP sharply decreased to a value of 5% lower than that of
Het-DP catalyst. With increasing temperature, both the selectivity
to EG and propanols smoothly increased, and a sharp increase of
propanols byproducts from 2.3% at 473 K to 8.8% at 493 K was seen
over Hom-DP catalyst. The formation of a large number of extensive
hydrogenolysis products of propanols explained the sharp decrease
of 1,2-PDO selectivity of Hom-DP catalyst at 493 K [7]. The space-
time-yield (STY) of 1,2-PDO for Het-DP and Hom-DP catalysts are
calculated to be 2.6 and 2.2 g1,2-PDO gCu−1 h−1 at 473 K, respectively.
Although the STY values of Het-DP and Hom-DP catalysts are lower
than that of the copper based state-of-the-art catalysts for the
same reaction reported by Bienholz et al. over a CuO/ZnO prepared
by an oxalate gel method [20], which is 9.8 g1,2-PDO gCu−1 h−1 at
the same reaction temperature, they are higher than those of the
CuO/ZnO prepared by coprecipitation [45] and Cu/Al2O3 catalyst
prepared by impregnation method [14], which produce 1,2-PDO
with STYs of 0.8 and 1.9 g1,2-PDO gCu−1 h−1, respectively.
to CuO, which exhibit a narrow size distribution of 2–9 nm with an
average diameter of around 6 nm. The inset in Fig. 8(B) shows a cor-
responding SAED pattern of this sample, in which a few scattered
spots are occasionally observed, characterizing of highly dispersed
or amorphous samples with poor crystalline.
After reduction at 553 K, the filandrous morphology of the
copper phyllosilicate in Hom-DP was to some extent retained
(Fig. 8(C)). In addition, a few dark particles with an average
diameter of 6 nm due to the presence of Cu0 are also observed
in this sample. Differently, many black quasi-spherical particles
attributable to metallic Cu emerged in the reduced Het-DP sam-
ple (Fig. 8(D)). The particles showed a wider size distribution of
3–16 nm with an average size of about 9 nm. The metallic Cu par-
ticle sizes observed by TEM are larger than the crystallite sizes
derived from in situ XRD, indicating the polycrystalline nature of
the metallic Cu particles.
3.2. Catalytic performance
Fig. 10 shows the effect of catalyst amount on glycerol reaction
over the prereduced Hom-DP and Het-DP catalysts at 473 K. As can
be seen, the conversions of glycerol increased 4 to 5 fold with the
increase of catalyst amount from 1 wt% to 10 wt% for both catalysts,
and the higher the amount of the catalyst, the larger the activity of
Het-DP catalyst is over Hom-DP catalyst. The selectivity to 1,2-PDO
for Hom-DP catalyst slightly decreased from 97.8% to 92.4%, while a
large decrease of 1,2-PDO selectivity from 96.0% to 86.4% was seen
The catalytic activity and selectivity of both catalysts as a
function of temperature are given in Fig. 9. As can be seen, the
conversion of glycerol for both catalysts steadily increased
with increasing temperature, with the activity of Het-DP
catalyst 4–12% higher than that of Hom-DP catalyst at each
temperature point investigated. The selectivity to 1,2-PDO is a
little bit complex. At temperatures below 473 K, the selectivity to

50
Z. Huang et al. / Catalysis Today 183 (2012) 42–51
Table 3
Studies of the reuse of Hom-DP and Het-DP catalysts.
Catalyst
Hom-DP
Het-DP
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
First time
Second time
Third time
22.1
11.7
6.1
98.0
97.8
98.4
35.0
30.1
21.6
93.7
95.2
96.0
Reaction conditions: 80% aqueous solution of glycerol 80 g, 453 K, total pressure 9.0 MPa, reduced catalyst 4 g, 12 h.
over Het-DP catalyst with increasing catalyst concentration. The
increase in conversion with the increase in catalyst concentration
could be due to the more availability of active sites for the reaction
which also leads to the excessive degradation of glycerol reactant
and hydrogenation of 1,2-PDO product to lower alcohols.
amount of Cu2O species are inferred to exist in the 553 K prereduced
Hom-DP sample than in Het-DP sample. Previously, we reported
that the Cu0 formed during the prereduction treatment and/or gen-
erated in situ during the reaction is the active site for glycerol
reaction, while the Cu+ species formed during the prereduction
treatment might be catalytically inactive in glycerol reaction [24].
Thus, in the present work, the presence of a larger amount of Cu2O
species in Hom-DP sample would also to some extent result in the
lower activity of this sample as compared to Het-DP sample.
In our previous work [25] we found that the incorporating of
Na is one of the main reasons for the degradation of glycerol to
EG byproduct over copper catalysts prepared by precipitation with
sodium-containing bases. In the present work, a higher selectiv-
ity to 1,2-PDO product and a lower selectivity to EG byproduct are
found over Hom-DP catalyst than over Het-DP catalyst at tempera-
ture below 473 K, probably due to the free of sodium residual in the
former catalyst. In addition, a sharp decrease of 1,2-PDO selectiv-
ity of Hom-DP catalyst from 473 K to 493 K due to the formation of
a large number of extensive hydrogenolysis products of propanols
was seen. As known from the above XRD and XPS characterizations,
the structure of Hom-DP catalyst significantly changed during the
aqueous phase glycerol reaction, and small copper particles in the
reduced Hom-DP catalyst profoundly sintered to large aggregates.
In contrast, the structure of Het-DP catalyst was reasonably stable.
Thus, the sharp decrease of 1,2-PDO selectivity due to extensive
hydrogenolysis would be aroused by the profound structure change
of Hom-DP catalyst during the aqueous phase glycerol reaction.
To study the recycle ability as well as the deactivation behavior
of the catalysts, both catalysts were also used to carry out the recy-
cle experiments at 453 K (Table 3). After reaction, the freshly used
catalysts were filtrated and recharged into the autoclave together
with the fresh reactant to perform the next run. After recycle serious
loss of activity was observed over Hom-DP catalyst, as nearly one
half of the initial activity was lost (decreased from 22.1% to 11.7%).
Het-DP catalyst presented moderately good stability, its activity
just declined from 35.0% to 30.1% after the second run. The activity
of both catalysts further decreased after the third run, however, the
activity of Het-DP catalyst is still maintained almost equal to that of
the fresh Hom-DP catalyst, showing a higher stability of the former
catalyst. The selectivity to 1,2-PDO for Hom-DP catalyst maintained
at around 98.0% without big change after three-run catalytic cycles,
whereas a slight increase of selectivity was seen for Het-DP catalyst
after recycle (Table 3).
From the above characterizations, we know that the catalyst
prepared by Hom-DP method exhibited a higher dispersion, a lower
copper domain and thus a larger copper surface area than those of
the catalyst prepared by Het-DP method. Nonetheless, the situa-
tion is opposite after reaction. XRD characterization of the used
samples (Fig. 4(E and F)) showed that the particles of Cu0 in Hom-
DP catalyst significantly sintered to much large aggregates, while
those particles in Het-DP were reasonably stable. XPS results also
revealed a more serious aggregation of copper in the Hom-DP cat-
alyst after reaction (Table 1). The poor stability of copper in the
prereduced Hom-DP catalyst during the aqueous phase glycerol
hydrogenolysis reaction would be the major reason for the infe-
rior performance (lower activity, poorer recycle stability) of this
catalyst. After reaction, the leaching of Cu2+ as measured by atomic
absorption spectrometry (AAS) is 13.0 ␮g/g for Hom-DP catalyst
and 2.8 ␮g/g for Het-DP catalyst. In our previous report [25] we
found that the leaching of Cu2+ during glycerol reaction could
inhibit the activity of the Cu/SiO2 catalyst prepared by PG method.
Thus, the much serious leaching of Cu2+ would also reduce the activ-
ity of the Hom-DP catalyst, and be another reason for the lower
activity of this catalyst.
4. Conclusions
The present work demonstrated that the precipitation manner
in the Cu/SiO2 catalysts prepared by DP method has wide effects on
the texture, structure and composition of the dried, calcined and
reduced catalysts. The copper species on the dried, calcined and
reduced Cu/SiO2 catalysts prepared by Hom-DP and Het-DP meth-
ods as well as on the catalysts after work were assigned based on
characterizations and literature works. Due to a homogeneous pre-
cipitation manner, the Hom-DP catalyst presented a much higher
dispersion, a smaller copper particle size and thus a larger copper
surface area than those of the Het-DP catalyst. Nonetheless, cat-
alytic tests in the hydrogenolysis of glycerol showed that the latter
catalyst inversely surpassed the former catalyst. The better perfor-
mance of Het-DP catalyst as compared to Hom-DP catalyst is mainly
due to a larger number of copper species in the former catalyst
was prereduced to active Cu0 sites, which also showed remarkable
higher stability during the reaction. Moreover, the selectivity to
1,2-PDO product is found to be affected by both the precipitation
agent applied and the structure of the catalysts. It is preferable to
produce Cu/SiO2 catalyst for glycerol reaction by Het-DP method,
however, the catalytic performance of this catalyst still needs fur-
ther improvement.
From the above H2-TPR characterizations (Fig. 7), we know that
it requires a high temperature of above 615 K for calcined Het-DP
sample and 642 K for calcined Hom-DP sample to be fully reduced to
Cu0. Thus, under a low prereduction temperature of 553 K, a propor-
tion of hard-reducible copper species in both samples would just be
inadequately reduced to Cu+ species rather than Cu0. In our previ-
ous work [24], we found that Cu+ species is not likely formed during
glycerol reaction but rather formed during the prereduction treat-
ment. In this work, the Cu2O species detected in the used samples
by both XRD and XPS is probably formed by the aggregation of the
highly dispersed Cu2O in the inadequately reduced catalysts dur-
ing the high temperature glycerol reaction. Based on the above XRD
(Fig. 4(E and F)) and XPS (Fig. 6) characterizations, a larger amount
of Cu2O species are found in the used Hom-DP catalyst. Thus, a large
Acknowledgments
The authors gratefully acknowledge the technical assistance
provided by Ms. L. He and Ms. L. Gao, and the financial support

Z. Huang et al. / Catalysis Today 183 (2012) 42–51
51
from the National Natural Science Foundation of China (Grant
20973187).
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C. Montassier, G. Pérot (Eds.), Stud. Surf. Sci. Catal., Elsevier, Amserdam, 1988,
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