Cu/ZnO/SiO2 catalyst synthesized by reduction of ZnO-modified copper phyllosilicate for dimethyl ether steam reforming

Article abstract of DOI:10.1016/j.apcata.2017.04.013

Herein, we reduced the ZnO-modified copper phyllosilicate to synthesize a series of Cu/ZnO/SiO₂ catalysts (xCuZn/SiO₂) which were mixed with γ-Al₂O₃ for dimethyl ether steam reforming (DME SR). The structure and

Full text of DOI:10.1016/j.apcata.2017.04.013

Applied Catalysis A, General 540 (2017) 37–46
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Applied Catalysis A, General
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Research Paper
Cu/ZnO/SiO2 catalyst synthesized by reduction of ZnO-modified copper
phyllosilicate for dimethyl ether steam reforming
Xinlei Wanga, Kui Maa, Lihong Guoa,b, Ye Tiana,⁎, Qingpeng Chenga, Xueqin Baia,
Jingjing Huanga, Tong Dinga, Xingang Lia,⁎
a
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key Laboratory of Applied Catalysis Science and Engineering, School of Chemical
Engineering & Technology, Tianjin University, Tianjin 300072, PR China
b
School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450001, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Herein, we reduced the ZnO-modified copper phyllosilicate to synthesize a series of Cu/ZnO/SiO2 catalysts
(xCuZn/SiO2) which were mixed with γ-Al2O3 for dimethyl ether steam reforming (DME SR). The structure and
the formation of copper phyllosilicate were systematically characterized by nitrogen adsorption, XRD and IR.
The novel synthetic route led to the remarkably high dispersion of copper species and adjustable Cu0/Cu+ ratio
on the catalyst surface, resulting in the superior catalytic performance for DME SR. The amount of Cu0 and Cu+
sites on the catalyst surface was dramatically affected by ZnO loading, and the turnover frequency results
indicated that the 4CuZn/SiO2 catalyst (Cu/Zn = 4) with the Cu0/Cu+ ratio of 1 showed the highest catalytic
performance. The XPS, H2-TPR and TEM results revealed the existence of strong interaction between Cu species
and ZnO on the reduced 4CuZn/SiO2 catalyst, which suppressed the reverse water-gas reaction and prevented
the sintering of copper.
Hydrogen production
Dimethyl ether steam reforming
CO selectivity
Cu/SiO2
ZnO
1. Introduction
The overall reaction of DME SR is shown in Reaction (1), which
contains two successive endothermic reactions: DME hydrolysis to
The hydrogen fuel cell is expected as an environment-friendly
energy conversion device because of its high energy conversion
efficiency and low pollution [1,2]. However, the storage and transpor-
tation of hydrogen fuel is still a problem which restricts the application
of this technology [3]. One effective scheme to overcome the barrier is
to produce hydrogen onboard via some H2-rich materials. The catalytic
reforming is regarded as a promising technique for hydrogen produc-
tion onboard. Dimethyl ether (DME) is an attractive candidate of H2-
containing materials for the reforming because of many intrinsic merits
such as high energy density, high H/C ratio, non-toxic and less carbon
deposition formation during the reforming due to the absence of CeC
bond in its molecule [4,5]. In addition, the physical property of DME is
similar to the liquid petroleum gas (LPG), so the well-developed
infrastructure for LPG can also be used for DME. The reforming
techniques of hydrocarbons include partial oxidation (POX), autother-
mal reforming (ATR) and steam reforming (SR). Among all these
reforming modes, SR produces more hydrogen than POX and ATR
due to the utilization of the hydrogen in water. Thus, DME SR is
regarded as a promising way to produce H2 onboard for hydrogen fuel
cell.
methanol (Reaction (2)) and methanol steam reforming (MSR) to H2
and CO2 (Reaction (3)).
DME steam reforming:
CH3OCH3+ 3H2 O→ 6H2+ 2CO2
ΔHγθ= 135 kJ mol−1
(1)
DME hydrolysis:
CH3OCH3+ H2 O→ 2CH3OH
ΔHγθ= 37 kJ mol−1
(2)
MeOH steam reforming:
CH3OH + H2 O→ 3H2 + CO2
ΔHγθ= 49 kJ mol−1
(3)
An important side reaction is the reverse water-gas shift reaction (r-
WGS) (Reaction (4)).
Reverse water-gas shift reaction:
⁎
Corresponding authors.
E-mail addresses: tianye@tju.edu.cn (Y. Tian), xingang_li@tju.edu.cn (X. Li).
http://dx.doi.org/10.1016/j.apcata.2017.04.013
Received 13 February 2017; Received in revised form 16 April 2017; Accepted 21 April 2017
Available online 24 April 2017
0926-860X/ © 2017 Elsevier B.V. All rights reserved.

X. Wang et al.
Applied Catalysis A, General 540 (2017) 37–46
CO2+ H2 → CO + H2O
their catalytic performance for DME SR. The content of ZnO was
optimized, and the effect of ZnO loading on the Cu0 and Cu+ sites were
investigated. Several techniques such as BET, XRD, FT-IR, TEM, XPS,
TG, TPSR, CO2-TPD and TPR were adopted to investigate the catalysts.
ΔHγθ= 41 kJ mol−1
(4)
Thus, the bifunctional catalysts containing acidic (for DME hydro-
lysis) and metallic (for MSR) functions are generally required for DME
SR. However, too strong acid strength can cause a serious coke
deposition to decrease the catalytic performance and stability of the
catalysts, such as, zeolite [6]. Therefore, we chose γ-Al2O3 as a solid
acid catalyst because of its good activity and stability for DME
hydrolysis, and moderate acid strength which would result in only a
small amount of coke deposition [6–9]. Generally, the operation
temperature of MSR is 200–300 °C over Cu-based catalysts [10,11],
and alumina can hardly involve in the activation of CeO bond and
mainly serve as a support in alumina-containing MSR catalysts [12–14].
The γ-Al2O3 with weak Lewis acid-site requires the higher operation
temperature than 300 °C to activate CeO bond in DME hydrolysis
[9,15].
2. Experimental
2.1. Catalyst preparation
The Cu/SiO2 catalyst with 25 wt.% Cu loading was prepared by an
ammonia-evaporation method, according to previous reports [29].
5.70 g of Cu(NO3)2·3H2O (A.R., Aladdin Industrial Co.) was dissolved
in 100 mL of deionized water. 20 mL of 25 wt.% ammonia aqueous
solution (A.R., Aladdin Industrial Co.) was added and stirred for
30 min. Then, 18.0 g of silica sol (SiO2, 25 wt.%, Qingdao Ocean
Chemical Co., Ltd., China) was added to the copper ammonia complex
solution and stirred for 7 h. The initial pH of the suspension was 11–12.
All of the above operations were performed at room temperature. The
suspension was transferred in a water bath preheated at 90 °C to
evaporate ammonia. With the decrease of the pH value of the mixture,
the copper species were deposited on silica. When the solvent was
completely removed, the evaporation process was terminated. Then the
solid was washed with 500 mL of deionized water three times and dried
at 120 °C overnight.
For Cu-based catalysts, both Cu+ and Cu0 are proved to be the
active sites for MSR. Cu+ may promote the formation of HCOO− and
COO− species, while Cu0 may contribute to the formation of CH3O
species derived from CH3OH [16–19]. Nevertheless, under the high
operation temperature (> 300 °C), Cu species tend to aggregate and
then lose the catalytic activity [20].
The strategies to inhibit Cu particle growth include encapsulating
Cu species into well-defined inorganic shells or tubes [21], alloying
with a higher-melting point metal [22] and increasing the metal-
support interaction energy [23].
The Cu/SiO2 catalysts are often used for MSR [24–27]. In the
conventional Cu/SiO2 catalysts for MSR, the Cu crystallites were
generally large, the Cu species are easily reduced to Cu0, and the
amount of Cu+ is relatively small, which are responsible for the poor
catalytic performance. An ammonia-evaporation method is developed
to prepare Cu/SiO2 catalyst which has been used in several catalytic
reactions. Small CuO crystallites and copper phyllosilicates
[Cu2SiO5(OH)2] with a unique lamellar structure are formed by this
method, and they are reduced to a large amount of highly dispersed Cu0
and Cu+ species, respectively. More importantly, the formation of
copper phyllosilicate can significantly enhance the dispersion and
metal-support interactions [28,29]. Chen et al. [29] prepared the Cu/
SiO2 catalyst by the ammonia-evaporation method and investigated the
catalytic performance for the hydrogenation of dimethyl oxalate to
ethylene glycol. Gong et al. [30] also synthesized the Cu/SiO2 catalyst
via the ammonia-evaporation hydrothermal method and obtained
remarkable stability in synthesis of ethanol via syngas. They proposed
that the unique lamellar structure of phyllosilicate led to the strong
metal-support interaction which could inhibit the sintering of the Cu
nanoparticles. However, to the best of our knowledge, there is no
research focused on the Cu/SiO2 catalyst prepared by the reduction of
copper phyllosilicate for MSR and DME SR.
Generally, Zn(II) can improve the dispersion of Cu species [31,32].
Behrens et al. [33] proposed that ZnO functioned as a physical spacer
among Cu nanoparticles and improved the dispersion of copper. In
addition, Jansen et al. [34] proposed that after reduction at 400 °C,
most of Cu species on the Cu/ZnO/SiO2 catalyst surface were covered
with ZnO to avoid sintering. Scheur et al. [35] also revealed that a small
amount addition of ZnO prevented agglomeration of copper crystallites
in the Cu/ZnO/SiO2 catalyst.
Moreover, some researchers report that ZnO have a high CO2
selective for steam reforming [36,37]. The CO by-production in MSR
can be suppressed by introducing ZnO into silica-supported copper
catalyst [26]. The strong interaction between Cu species and ZnO is
thought to be beneficial for the MSR [38]. However, as far as we
known, there is no Cu/ZnO/SiO2 catalyst synthesized through the
reduction route of copper phyllosilicate for MSR and DME SR.
In this work, we synthesized a series of Cu/ZnO/SiO2 catalysts via
the reduction of ZnO-modified copper phyllosilicate and investigated
The ZnO-modified Cu/SiO2 catalysts were prepared by impregna-
tion. The dry Cu/SiO2 powder was immersed in a Zn(NO3)2 solution
and then oscillated in an ultrasonic washer for 30 min. Then the
mixture was evaporated at 80 °C with a rotary evaporation apparatus
and dried at 120 °C overnight. At last the catalyst precursor was
calcined in static air at 450 °C for 4 h. The final calcined sample was
denoted as xCuZn/SiO2 (x = 1, 2, 4, 6), where x represented the mass
ratio of Cu/Zn. The composite catalysts containing the xCuZn/SiO2 and
γ-Al2O3 (supplied by Tianjin Chemical Research & Design Institute,
S
BET = 189 m2 g−1) catalysts were prepared by mechanical mixing at
a fixed weight ratio of 2:1. The catalyst was pressed, grounded, and
sieved to obtain 40–60 mesh particles before the activity measurement.
The Zn/SiO2-IM catalyst was prepared by the impregnation method.
Briefly, silica sol was added to the Zn(NO3)2 solution under vigorous
stirring. Then the mixture was evaporated at 80 °C with a rotary
evaporation apparatus, dried at 120 °C overnight and calcined at
450 °C for 4 h. The contents of ZnO and silica equaled to that of the
4CuZn/SiO2 catalyst.
A traditional CuO/ZnO/Al2O3 (Cu:Zn:Al molar ratio = 6:3:1) cata-
lyst was prepared by coprecipitation to compare with the CuZn/SiO2
catalyst. Briefly, an aqueous solution of Na2CO3 was added dropwise
into the mixed aqueous solution of the metal nitrates [Cu(NO3)2
(0.6 M), Zn(NO3)2 (0.3 M), and Al(NO3)3 (0.1 M)] at 70 °C under
vigorous stirring until the pH reached 7. The obtained suspension was
aged for 2 h at the same temperature. The precipitate was filtered,
washed with deionized water, dried at 120 °C for 12 h, and calcined in
air at 500 °C for 4 h. The catalyst was denoted as CZA catalyst.
2.2. Evaluation of catalytic activity and stability
The catalytic activity of the xCuZn/SiO2 and CZA catalysts for DME
SR was evaluated in a conventional fixed-bed continuous flow reactor
containing 500 mg of catalyst under atmospheric pressure. Before the
reactions, all of the catalysts were reduced at 350 °C for 90 min in 10%
H2/N2 mixture gas. After the temperature of the catalyst bed was
dropped to 260 °C, the feed gas for DME SR containing 10% DME, 40%
N2 and 50% H2O (g) (S/C = 2.5) with a constant mass space velocity of
−1
−1
12,000 mL h
g
was supplied to the catalyst bed. The catalytic
cat
activity was tested at a series of temperatures with an interval of 20 °C.
The linear flow rate through the catalyst and the catalyst particle size
has been varied to exclude the external and internal mass transfer
limitation during the activity test. The feed and product were analyzed
38

X. Wang et al.
Applied Catalysis A, General 540 (2017) 37–46
using an on-line gas chromatograph (GC) (Agilent 7890A) equipped
with one hydrogen flame ionization detector (FID) and two thermal
conductivity detectors (TCD). The product from reactor was purified by
a U-tube filled with Mg(ClO4)2 to get rid of H2O before analysis. The
DME conversion and the selectivity to C1 species were defined as
follows:
isotherm.
The inductively coupled plasma optical emission spectroscopy (ICP-
OES) (VISTA-MPX, Varian) was used to determine the loading of copper
and zinc. Before the analysis, the samples were dissolved in a mixed
solution of HF and HBO3 and then prepared the solution with certain
concentration.
The thermal gravimetric (TG) analysis was carried out on
PerkinElmer Diamond TG/DTA instrument. About 6 mg sample was
a
F
− F
DME,out
DME,in
XDME
=
× 100%
F
DME,in
heated from room temperature to 900 °C under flowing air at a heating
rate of 10 °C min−1
.
F
Ci
SCi
=
× 100%
The H2 temperature-programmed reduction (H2-TPR) experiments
were performed to test the reducibility of the calcined catalyst on a
XianQuan TP-5079 instrument. 30 mg of powder sample was loaded
into quartz tube reactor and heated from ambient temperature to
600 °C (10 °C min−1) under an 8% H2/N2 mixture gas (30 mL min−1).
Before detected by TCD, the water vapor was removed by a glass tube
containing CaO and NaOH. The amount of hydrogen consumption was
determined by a TCD.
F
∑
C
i
FDME,in and FDME,out are the influent and effluent molar flow rates of
DME, respectively, and FCi is the effluent molar flow rate of C1-
containing products, including CH3OH, CH4, CO, and CO2.
Hydrogen yield was defined as a molar ratio of experimental
production to theoretical production of hydrogen per molar DME feed.
FH
1
6
2
The specific surface area of metallic Cu was measured by N2O
titration on a XianQuan TP-5079 instrument [39]. The sample was
YH
=
×
× 100%
2
F
DME,in
reduced at 350 °C for 30 min in 8% H2/N2 mixture gas (30 mL min−1
)
FH is the effluent molar flow rate of H2.
and then swept with N2 (30 mL min−1) for 30 min. Subsequently, 50%
N2O/N2 mixture gas (30 mL min−1) was introduced at 60 °C for 1 h,
ensuring that all the surface Cu atoms were completely oxidized. Then,
the H2-TPR experiment was carried out in an 8% H2/N2 mixture gas at
the heating rate of 10 °C min−1. The amount of chemisorbed oxygen
atoms on the Cu surface was determined from the area of H2-TPR peak.
The specific surface area is calculated assuming that the stoichiometry
of O:Cu is 1:2, and the concentration of Cu is 1.46 × 1019 atoms m−2
[40].
2 The stability tests of the Cu/SiO2 and 4CuZn/SiO2 catalysts were
conducted to investigate the deactivation behavior. It was carried out at
380 °C with the continuous feed gas which was the same as that for the
activity tests. These catalysts after stability tests were defined as the
spent catalysts.
2.3. Kinetics test
The kinetics test was performed on the same fixed-bed continuous
flow reactor as described in Section 2.2. The external and internal mass
transfer limitations were eliminated by varying the catalyst particle size
before the test. The tests were carried out at a certain temperature, at
which the DME conversion was less than 15%.
The X-ray photoelectron spectroscopy (XPS) analysis of the catalysts
was carried out on a Perkin-Elmer PHI 1600 ESCA system using Al Kα
radiation (1486.6 eV) as a X-ray source. The binding energies were
calibrated using C1s peak at 284.6 eV as the reference. The experi-
mental errors were within 0.2 eV.
Transmission electron microscopy (TEM) micrographs were ac-
quired on a JEOL-JEM-2100F electron microscope attached with an
X-ray energy-dispersive spectrometer (EDS), operated at 200 kV. The
sample was ultrasonically dispersed in ethanol for 30 min, and the
specimen was obtained by dropping a droplet suspension on a carbon
film supported on a nickel grid for TEM and EDS analysis.
The temperature-programmed surface reaction (TPSR) and the
temperature-programmed desorption of CO2 (CO2-TPD) experiments
were carried out on the TP-5079 instrument equipped with a mass
spectrometry (MS) (Hiden HPR20). For the TPSR experiment, 100 mg
of sample was loaded into the quartz tube reactor, and the sample was
heated to 350 °C and kept at 350 °C for 1.5 h in an 8% H2/N2 mixture
gas (30 mL min−1). After the catalyst bed cooled to room temperature,
the sample was flushed with pure helium for 1 h. Then, the 75% H2/
CO2 mixture gas (40 mL min−1) was introduced to the catalyst bed, and
the sample was heated to 400 °C (5 °C min−1). The effluent gas was
detected by MS. The pretreatment of the sample (100 mg) for the CO2-
TPD experiments was the same as that for TPSR. After the catalyst bed
cooled to 60 °C, pure helium flushed the sample for 1 h, and pure CO2
passed through the catalyst bed for another hour. Then, pure helium
flushed the residual CO2. The sample was heated to 400 °C by a
temperature program of 5 °C min−1, and the effluent gas was detected
by MS.
The turnover frequency (TOF) was calculated as follows:
XDME × CDME × 6.02 × 1023
TO F=
× SSACu +Cu × 1.46 × 1019
0
+
W
Cat
where XDME is the DME conversion; CDME is the DME molar flow in the
0
+
feeding gas; WCat is the weight of the loaded catalyst; and SSACu +Cu . is
the total specific surface area of Cu0 and Cu+
.
2.4. Catalyst characterization
The X-ray diffraction (XRD) measurements were performed on a D8
diffractometer (Bruker Company) using Cu Kα (λ = 0.15418 nm) as a
radiation source at room temperature. The voltage and current were
operated at 40 kV and 40 mA, respectively. The patterns were acquired
by the step scanning with a rate of 8° min−1 from 2θ of 10 to 80°. The
pre-reduced catalyst were carefully collected in highly pure N2 atmo-
sphere and sealed into a centrifugal tube before XRD analysis to
diminish the surface oxidation of metallic Cu. The X-ray broadening
technology was used to calculate the size of metallic Cu crystal by the
Scherrer equation.
The FT-IR spectra of the samples were recorded on a Nicolet Nexus
IR spectrometer in the range of 400–4000 cm−1. The powder samples
were dispersed in KBr and pressed into a very thin wafer for tests.
The N2 adsorption-desorption isotherms were determined by a
Quantachrome QuadraSorb SI instrument at the liquid nitrogen tem-
perature of 77 K. Before the measurement, the powder samples were
heated under vacuum at 300 °C for 6 h to remove the physical
adsorption gases on the surface. The specific surface area of the catalyst
was determined by the Brunner-Emmet-Teller (BET) method. The
Barrett-Joyner-Halenda (BJH) approach was used to calculate the
average pore diameter and pore volume based on the desorption
3. Results and discussion
3.1. Chemical composition, structure and texture of the catalysts
Table 1 gives the chemical compositions of the Cu/SiO2 and xCuZn/
SiO2 catalysts. The theoretical Cu loading is slightly higher than the
actual value because of the loss of the cupric ions during washing the
39

X. Wang et al.
Applied Catalysis A, General 540 (2017) 37–46
Table 1
The physicochemical property and chemical composition of the reduced xCuZn/SiO2
catalysts and Cu/SiO2 catalyst.
Catalysts
Cu
Zn
SBET
Vpore
dpore
(nm)
dCu (nm)b
loading
loading
(m2 g−1
)
(cm3 g−1
)
(wt.%)a
(wt.%)a
1CuZn/SiO2 18.91
2CuZn/SiO2 21.39
4CuZn/SiO2 22.89
6CuZn/SiO2 23.65
19.01
10.71
5.72
3.94
0
245
307
321
344
369
0.36
0.60
0.65
0.64
0.63
6.8
8.7
8.4
7.9
7.8
13.5
10.1
4.2
4.7
5.1
Cu/SiO2
24.76
a
Content of Cu and Zn determined by ICP-OES analysis.
Average size of Cu crystallites calculated from the XRD diffraction peaks at 2θ of
b
43.5° for Cu(111) based on the Scherrer equation.
Fig. 2. FTIR spectra of (a) 1CuZn/SiO2, (b) 2CuZn/SiO2, (c) 4CuZn/SiO2, (d) 6CuZn/
SiO2, (e) Cu/SiO2, (f) as-prepared 4CuZn/SiO2, (g) reduced 4CuZn/SiO2 and (h) SiO2.
emerges with the increase of ZnO loading. Toupance et al. found that a
part of copper phyllosilicate would start to decompose at 450 °C to form
CuO [43]. The XRD results indicate that the addition of ZnO promotes
the decomposition of copper phyllosilicate and leads to the formation of
the CuO crystallites. Although the diffracting peaks for CuO are not
found in the XRD patterns when the Cu/Zn mass ratio is higher than 2,
the CuO nanoparticles are also probably formed with a small size
beyond the XRD detection limit and high dispersion on the support.
Only 1CuZn/SiO2 catalyst presents the diffraction peaks at 2θ of 31.8°,
34.4°, 36.3°, 47.5°, 56.6° and 62.9° corresponding to ZnO, indicating
that ZnO crystallites are formed on the catalyst surface. No diffraction
peak belonging to ZnO is observed when the Cu/Zn mass ratio is higher
than 1, indicating the high dispersion of ZnO on the catalyst surface.
Fig. 1B shows the XRD patterns of the reduced Cu/SiO2 and xCuZn/
SiO2 catalysts. The peaks at 2θ of 43.5°, 50.4° and 74.0° are attributed to
the metallic Cu [44]. All of the catalysts show a weak and broad peak at
2θ of 36.6°, corresponding to Cu2O. Metallic Cu is formed from the
reduction of CuO, and Cu2O is formed from the reduction of copper
phyllosilicate [29,30]. The intensity of the diffraction peaks of the
metallic Cu in the Cu/SiO2, 6CuZn/SiO2 and 4CuZn/SiO2 catalysts has
little difference. However, the diffraction peaks of metallic Cu become
sharp and intense when the Cu/Zn mass ratio is lower than 4. The sizes
of the Cu crystallites were estimated from the XRD patterns of the
reduced catalysts by using Scherrer equation (see Table 1). The XRD
results of the reduced catalysts indicate that the content of ZnO has
little effect on the crystallite size of metallic Cu when the Cu/Zn mass
ratio is larger than 2, and further increasing the ZnO content would lead
to the growth of the metallic Cu crystallites.
Fig. 2 shows the Fourier-transform IR (FTIR) spectra of the catalysts.
In Fig. 2, the νSiO asymmetric stretching band at 1110 cm−1 is assigned
to SiO2. The δOH band at 663 cm−1 and the νSiO shoulder peak at
1040 cm−1 on the low frequency indicate that copper phyllosilicate is
formed [41]. To investigate the formation process of the copper
phyllosilicate, the as-prepared 4CuZn/SiO2 and the reduced 4CuZn/
SiO2 catalysts were also characterized by FTIR. The peaks of the as-
prepared 4CuZn/SiO2 catalyst appear at 1040 and 663 cm−1, suggest-
ing that the structure of copper phyllosilicate has been formed after
drying, and it still exists even after calcination at 450 °C for 4 h.
However, after reduction at 350 °C, the copper phyllosilicate disap-
pears, which is in accordance with the XRD results. All of the samples
exhibit the νSiO symmetric stretching band of SiO2 at 800 cm−1 [29].
However, only SiO2 shows the Si-O stretching vibrations in SiOH groups
at the band around 960 cm−1 [45,46]. Gong et al. has proposed that
copper phyllosilicate is formed through the reaction of Cu2+ complex
Fig. 1. XRD patterns of (A) calcined and (B) reduced catalysts. (a) 1CuZn/SiO2, (b)
2CuZn/SiO2, (c) 4CuZn/SiO2, (d) 6CuZn/SiO2 and (e) Cu/SiO2.
catalyst precursors, whereas the actual ZnO loading determined by ICP-
OES is close to the theoretical values.
Fig. 1A presents the XRD patterns of the calcined Cu/SiO2 and
xCuZn/SiO2 catalysts. The weak and broad diffraction peaks centered at
2θ of 30.8°, 35.0°, 57.5° and 62.4° are assigned to the copper
phyllosilicate phase [Cu2SiO5(OH)2] [41]. The diffraction peaks cen-
tered at 2θ of 35.6°, 38.8°, 48.7°, 58.4°, 61.8°, 66.2° and 67.9° are
assigned to CuO phase [42]. No diffraction peaks for CuO are observed
when the Cu/Zn mass ratio is higher than 2. However, the CuO phase
40

X. Wang et al.
Applied Catalysis A, General 540 (2017) 37–46
Fig. 3. N2 adsorption-desorption isotherms (A) and pore size distribution curves (B) of the
reduced (a) 1CuZn/SiO2, (b) 2CuZn/SiO2, (c) 4CuZn/SiO2, (d) 6CuZn/SiO2 and (e) Cu/
SiO2.
with the silanol groups of silica surface [30]. The FTIR results also come
to the same conclusion.
Figs. S1 and 3 show the nitrogen adsorption-desorption isotherms
and the corresponding BJH pore size distributions curves of the
calcined and reduced catalysts, respectively. The hysteresis loops of
the N2 adsorption-desorption isotherms indicate the mesoporous fea-
ture of the samples. The pores with the size of 3 nm belonging to the
interlayer pores of copper phyllosilicate, and they disappeared after
reduction. Table 1 gives the textural properties of the Cu/SiO2 and
xCuZn/SiO2 catalysts. The reduced Cu/SiO2 catalyst possesses
a
relatively high BET specific surface area of 369 m2 g−1. The specific
surface area decreases with the increase of ZnO loading. The ZnO is
assumed to tend to cover and block the pores leading to the decrease of
specific surface areas.
3.2. Catalytic performance
3.2.1. Steam reforming of dimethyl ether
Fig. 4 shows the catalytic performance of the Cu/SiO2, xCuZn/SiO2
and CZA catalysts for DME SR. The content of Zn has an obvious impact
on the catalytic performance. With the increase of ZnO loading, the
DME conversion increases and then decreases. The 4CuZn/SiO2 catalyst
exhibits the highest catalytic activity among the catalysts. The DME
conversion over the 4CuZn/SiO2 catalyst is 99.5% at 380 °C, which is
much higher than that of the traditional CZA catalyst and the CuZnAlO
catalysts prepared by other researchers [20,47]. The H2 yield curves
Fig. 4. DME conversion (A), H2 yield (B), CO2 selectivity (C) and CO selectivity (D) over
the Cu/SiO2, xCuZn/SiO2 and CZA catalysts.
shown in Fig. 4B are similar to the DME conversion. The C1 species
selectivity was also investigated. Methanol or other oxygenated com-
pounds are not detected according to the online analysis, and too low
selectivity of CH4 (< 1%) is ignored. Thus, CO2 and CO are the main C1
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X. Wang et al.
Applied Catalysis A, General 540 (2017) 37–46
surface areas (Table 2) [30]. The appropriate content of ZnO increases
the Cu0 specific surface area. The 4CuZn/SiO2 catalyst exhibits the
highest Cu0 specific surface area (27.2 m2 g−1). However, the Cu0
specific surface area gradually drops with the further increase of ZnO,
which could be ascribed to the growth of the metallic Cu crystallites.
The Cu+ specific surface area was calculated on the basis of the
intensity ratio between Cu0 and Cu+ by deconvolution of Cu LMM
XAES spectra. The Cu+ specific surface area decreases obviously with
the increase of ZnO loading. This tendency might be ascribed to the fact
that the addition of ZnO promotes the decomposition of copper
phyllosilicate, which is consistent with the results of XRD (see Fig. 1A).
The Cu0 and Cu+ species are all viewed as active sites for the
methanol steam reforming reaction [56]. The Cu+ species are favour-
able to the formation of the intermediates such as COO− and HCOO−
species [17]. Kulkarni and Rao reported that Cu0 species may promote
the formation of CH3O from methanol [57]. Tanaka et al. found that the
Cu0/Cu+ ratio determined the formation rate of the intermediates such
as HCOO− during the DME SR process [19]. We examined the effect of
Cu0/Cu+ on the turnover frequency (TOF) of DME in Fig. 6. The TOF of
DME increases continuously at the low Cu0/Cu+ ratio, reaches a
maximum value at Cu0/Cu+ = 1, and decreases with the further
increase of the Cu0/Cu+ ratio. Thus, we deduce that there is a
cooperative effect between Cu0 and Cu+. The optimal Cu0/Cu+ ratio
is close to 1. The amount of ZnO is the vital factor affecting the surface
Cu0/Cu+ ratio. Thus a method to modulate the surface Cu+ and Cu0
distribution of the Cu/SiO2 catalyst is provided in this work.
3.2.3. The interaction between Cu species and ZnO
Fig. 7 shows the XPS in the Zn 2p region of the catalysts. The Zn
2p3/2 peak centered at 1022.0 eV is attributed to Zn2+ in the calcined
xCuZn/SiO2 catalysts, which is larger than the value of Zn2+ in the
reference compound ZnO (1021.2 eV). The Zn 2p3/2 binding energy of
the Zn/SiO2-IM catalyst prepared by the impregnation method is close
to the calcined xCuZn/SiO2 catalysts, indicating that the peak shift of
Zn 2p3/2 in the calcined xCuZn/SiO2 catalysts results from the interac-
tion between the ZnO and SiO2 support. The Zn 2p3/2 peak of the
reduced Zn/SiO2-IM catalyst shifts slightly towards the lower binding
energy than that of the calcined Zn/SiO2-IM catalyst. However, the Zn
2p3/2 peak of the reduced 4CuZn/SiO2 catalyst shifts about 0.5 eV
towards the higher binding energy than that of the calcined 4CuZn/
SiO2 catalyst. This result indicates the strong interaction between the
Cu species and ZnO in the reduced catalysts. Since the binding energy is
usually relevant to the valence, the zinc ions in the reduced xCuZn/SiO2
should be electron-deficient in comparison with those in ZnO [26].
Fig. 8 shows the H2-TPR profiles of the catalysts with the different
Cu/Zn mass ratios. The ZnO-free Cu/SiO2 catalyst displays a sharp
reduction peak centered at 221 °C. For the calcined copper phyllosili-
cate, Van der Grift et al. [58] identified that the reduction temperature
for copper phyllosilicate to Cu+ was identical with the reduction
temperature for the well dispersed CuO to Cu0. The reduction peak of
the unmodified Cu/SiO2 catalyst was attributed to the collective
contribution of the reduction of the well-dispersed CuO to Cu0 and
the copper phyllosilicate to Cu+ [44]. The xCuZn/SiO2 catalysts show a
higher reduction temperature than the Cu/SiO2. The reduction tem-
perature of the catalysts gradually increases with the increase of ZnO
loading, which is probably due to the chemical interaction between Cu
species and ZnO. When the Cu/Zn mass ratio is lower than 2, the
reduction peak shifts toward the higher temperatures and the reduction
peak even splits to the shoulder peaks. According to the XRD results, the
large CuO crystallites exist on the 1CuZn/SiO2 and 2CuZn/SiO2
catalysts. Therefore, the high-temperature peak of the 1CuZn/SiO2
catalyst belongs to the reduction of the bulk CuO to the metallic Cu
[29]. However, the reduction peak of the bulk CuO in the 2CuZn/SiO2
catalyst is too small to be distinguished due to the overlap with the
other Cu species. A very weak reduction peak centered at 297 °C of the
Cu/SiO2 catalyst is attributed to the reduction of CuO, which has strong
Fig. 5. Cu 2p XPS spectra (A) and Cu LMM XAES spectra (B) of the reduced (a) 1CuZn/
SiO2, (b) 2CuZn/SiO2, (c) 4CuZn/SiO2, (d) 6CuZn/SiO2 and (e) Cu/SiO2 catalysts.
products. Fig. 4C and D shows that the CO2 selectivity decreases and the
CO selectivity increases with the increases of the reaction temperature
respectively. The catalyst without Zn shows the highest CO selectivity.
3.2.2. The effect of surface chemical states on the activity
The XPS in the Cu 2p region and the XAES in the Cu LMM region of
the reduced Cu/SiO2 and xCuZn/SiO2 catalysts are displayed in Fig. 5.
The peaks at about 932.7 and 952.6 eV are assigned to Cu 2p3/2 and Cu
2p1/2, respectively. According to the previous reports [29,48,49], the
Cu 2p3/2 binding energy of CuO is centered at around 933.5 eV, and the
binding energy of well-dispersed Cu2+ species interacting with silica is
found at about 934.9 eV [29]. In this work, the binding energy of Cu
2p3/2 centered at 932.7 eV and Cu 2p1/2 centered at 952.6 eV are
conventionally assigned to Cu+ and/or Cu0 species [50–52]. The
proportion of Cu0 and Cu+ species can be identified via the XAES
spectra [21,50,53,54]. In the Cu LMM XAES spectra of the reduced
catalysts, the asymmetry and broad peaks are observed and deconvo-
luted into two symmetrical peaks centered at around 914 and 918 eV,
corresponding to the Cu+ and Cu0 species, respectively. Based on the
deconvolution results, the content of ZnO greatly influences the molar
ratio of Cu0/Cu+ on the surface. With the decrease of the Cu/Zn ratio,
the molar ratio of Cu0/Cu+ was gradually increased (Table 2).
Cu0 specific surface area is deemed as a crucial factor to affect the
catalytic activity of copper-based catalysts [55]. The technology of
dissociative N2O chemisorption was used to determine Cu0 specific
42

X. Wang et al.
Applied Catalysis A, General 540 (2017) 37–46
Table 2
Surface Cu component of the reduced catalysts based on Cu LMM deconvolution.
a
b
c
Catalyst
Kinetic energy (eV)
Cu+
Auger parameter (eV)
Cu+
(Cu0/Cu+
)
SCu0 (m2 g−1
)
SCu+ (m2 g−1
)
Cu0
Cu0
1CuZn/SiO2
2CuZn/SiO2
4CuZn/SiO2
6CuZn/SiO2
Cu/SiO2
913.8
913.7
914.0
914.3
914.2
918.3
918.3
918.4
918.2
918.4
1846.5
1846.4
1846.7
1847.0
1846.9
1851.0
1851.0
1851.1
1850.9
1851.1
1.64
1.37
1.02
0.71
0.57
18.9
22.8
27.2
25.7
24.6
11.5
16.6
26.6
36.0
43.3
a
Intensity ratio of Cu0/Cu+ by deconvolution of Cu LMM XAES spectra.
b
c
Specific surface area of Cu0 calculated by N2O titration.
Specific surface area of Cu+ calculated based on the intensity ratio of Cu0/Cu+ by deconvolution of Cu LMM XAES spectra.
Fig. 6. Correlation of the TOF of DME with Cu0/Cu+
.
Fig. 8. H2-TPR profiles of the (a) 4CuZn/SiO2-RO, (b) 1CuZn/SiO2, (c) 2CuZn/SiO2, (d)
4CuZn/SiO2, (e) 6CuZn/SiO2 and (f) Cu/SiO2 catalysts.
the CuO particle size on the reduction temperature, the 4CuZn/SiO2-RO
catalyst was characterized by XRD (shown in Fig. S2). The particle size
of CuO in the 4CuZn/SiO2-RO catalyst calculated based on the Scherrer
equation is 4.5 nm, indicating that no large CuO particles are formed
after calcined at 450 °C for 4 h. Thus, the sizes of CuO particles are not
responsible for the higher reduction temperature of the 4CuZn/SiO2-RO
catalyst. The TPR profile of this catalyst displays two reduction peaks
below 300 °C. The low-temperature reduction peak (α) is attributed to
the highly dispersed CuO, which was generated by the oxidation of the
small metallic Cu and Cu2O particle during the calcination process. The
relatively high-temperature reduction peak (β) is assigned to the
reduction of CuO, which has interaction with ZnO. Interestingly, the
temperature of the reduction peak β is slightly higher than that of
4CuZn/SiO2 catalyst. Thus, the interaction between ZnO and Cu species
is responsible for the peak β with the high reduction temperature. We
deduce that the interaction between ZnO and Cu species is formed after
reduction, which is consistent with the results of the Zn XPS.
Fig. 9 shows the TEM image and EDS data to analyze the interaction
between the Cu species and ZnO. Four typical spots marked a, b, c and d
were chose to analyze the distribution of the Cu species and ZnO on the
reduced 4CuZn/SiO2 catalyst. The amplifying TEM image shows that
the lattice fringes corresponding to Cu (111) lattice plane emerges in
the spot a and c, indicating that the copper clusters exist at spot a and c,
which is not observed at spot b and d. The content of Si on the support
of the catalyst was adopted as the reference. The EDS data demonstrate
that the spot a has the largest amount of Cu species, and it also has the
largest amount of ZnO. The spot d has the least amount of Cu species,
and it also has the least amount of ZnO. No isolated Cu species or ZnO
was found by the EDS analysis. The change of Cu/Zn ratio is not too
large for the four spots. It proves that the Cu species have a strong
Fig. 7. XPS spectra in the Zn 2p region of the catalysts: (a) ZnO, (b) 1CuZn/SiO2, (c)
2CuZn/SiO2, (d) 4CuZn/SiO2, (e) 6CuZn/SiO2, (f) Zn/SiO2-IM, (g) reduced Zn/SiO2-IM
and (h) reduced 4CuZn/SiO2.
interaction with SiO2 matrix. However, the reduction peak gradually
disappears with the increase of ZnO, which indicates that the Zn species
weaken the interaction between the CuO and SiO2 support.
To further investigate the behavior of the interaction between the
Cu species and ZnO, we designed the experiment as follows. After
reduction at 350 °C, the 4CuZn/SiO2 catalyst was cooled to room
temperature at N2 atmosphere, and then was heated from room
temperature to 450 °C (10 °C min−1) and kept at 450 °C for 4 h under
a gas flow of air at a rate of 30 mL min−1. The final obtained sample is
named as the 4CuZn/SiO2-RO catalyst. In order to exclude the effect of
43

X. Wang et al.
Applied Catalysis A, General 540 (2017) 37–46
Fig. 9. EDS results of the reduced 4CuZn/SiO2 catalyst.
interaction with ZnO, which make the Cu/Zn ratio close at every
detected spot.
good thermal stability before 30 h. Thereafter the activity of the Cu/
SiO2 catalyst drops sharply, as compared with the 4CuZn/SiO2 catalyst.
In addition, as shown in Fig. S3, the 4CuZn/SiO2 catalyst also exhibits
higher catalytic activity and stability than CZA catalyst at 380 °C.
Generally, the coke deposition on the acid sites and the sintering of the
Cu species are the main reasons for the deactivation of the catalysts for
DME SR.
In this work, we selected γ-Al2O3 as a solid acid catalyst to
hydrolyze DME. After the stability test, the Cu/SiO2 and 4CuZn/SiO2
catalysts were characterized by TG technique in order to determine the
amount of the coke deposition. As shown in Fig. S4, the weight loss
below 100 °C is attributed to desorption of water or other species
adsorbed on the catalyst surface. The subsequent weight gain indicates
that the reduced Cu species are oxidized to oxides as the temperature
increases. The weight loss above 300 °C is related to the removal of
carbon deposition. After stability test, the amounts of coke deposition of
the two catalysts are similar and they are all below 4%. So the surface
coke deposition is not the main reason to determine the deactivation of
the two catalysts.
Fig. 12 shows the XRD patterns of the Cu/SiO2 and 4CuZn/SiO2
catalysts after the stability test. Based on the Scherrer equation, the size
of the Cu crystallites grew up from 5.1 nm to 20.3 nm in the Cu/SiO2
catalyst, while it grew up from 4.2 nm to 13.0 nm in the 4CuZn/SiO2
catalyst under the same condition. The morphology of the catalysts was
also examined by TEM images (Fig. S5). TEM images of the reduced Cu/
SiO2 and 4CuZn/SiO2 catalyst show that the Cu particle sizes are
smaller than 6 nm and are uniformly distributed on the SiO2 support.
TEM images of the spent catalysts reveal that ZnO-modified catalysts
can restrain the aggregation of the Cu crystallites and improve the
dispersion of Cu species. We speculate that the growth of the Cu
crystallites is the major reason for the deactivation of the two catalysts
3.2.4. The effect of the interaction between Cu species and ZnO on the
selectivity
Differently with the conversion of DME, all of the catalysts modified
with ZnO display a higher CO2 selectivity and a lower CO selectivity
than the Zn-free Cu/SiO2 catalyst. So, we suppose that the interaction
between Cu species and ZnO is responsible for the decrease in CO
selectivity. CO is regarded as a by-product of DME SR over the Cu based
catalyst by the r-WGS [20,59–61]. Purnama et al. proposed that the
main source of CO formation over CuO/ZnO/Al2O3 was the r-WGS
reaction [62]. To further understand the effect of the addition of ZnO
on CO selectivity, TPSR and CO2-TPD experiments were conducted on
the Cu/SiO2 and 4CuZn/SiO2 catalysts. Yoshihara et al. [63] proposed
that CO2 was adsorbed on metallic Cu in reverse water-gas shift
reaction. So, the MS signals of the effluent gases should divide by the
Cu0 specific surface areas of the catalysts (shown in Fig. 10). As shown
in Fig. 10A, the amount of CO increases with the increase of reaction
temperature. However, the 4CuZn/SiO2 catalyst exhibits much lower
CO production than the Cu/SiO2 catalyst. Fig. 10B depicts the CO2-TPD
of the two catalysts. The amount of CO2 desorption on the 4CuZn/SiO2
catalyst is smaller than that on the Cu/SiO2 catalyst. Thus, we deduce
that the interaction between ZnO and metallic Cu significantly affects
the amount of CO2 adsorption, and suppresses the r-WGS reaction. As a
result, the lower CO selectivity was obtained after the addition of ZnO.
3.3. Deactivation behavior
Fig. 11 shows the stability tests at 380 °C to evaluate the stability of
the Cu/SiO2 and 4CuZn/SiO2 catalysts. In Fig. 11, the catalysts exhibit
44

X. Wang et al.
Applied Catalysis A, General 540 (2017) 37–46
Fig. 12. XRD patterns of the (a) Cu/SiO2 and (b) 4CuZn/SiO2 catalysts.
4CuZn/SiO2 catalyst possesses much smaller Cu particles than the Cu/
SiO2 after the stability test, which indicates that moderate ZnO
modification stabilizes Cu species and mitigates the agglomeration of
Cu particles. So, the 4CuZn/SiO2 catalyst exhibits much higher thermal
stability than the Cu/SiO2 catalyst.
4. Conclusions
A novel ammonia evaporation followed by impregnation with zinc
nitrate method has been successfully applied to prepare a series of ZnO-
modified copper phyllosilicate catalysts. The unique lamellar structure
of the copper phyllosilicate results in a large amount of highly dispersed
Cu+ species which were formed on the reduced catalysts. The appro-
priate addition of ZnO shows a positive effect on the dispersion and the
valence distribution of copper species on the catalyst surface. Compared
with the traditional CZA catalyst, the xCuZn/SiO2 catalysts combined
with γ-Al2O3 exhibited significantly higher activity for the steam
reforming of dimethyl ether. Among all of the tested catalysts,
4CuZn/SiO2 catalyst exhibited the most excellent catalytic performance
with DME conversion of 99.5% and CO selectivity of 7.2% at 380 °C.
The superior catalytic performance of the catalyst is ascribed to the
highly dispersed copper species and the cooperation effect of Cu0 and
Cu+. It is also found that Cu0/Cu+ ratio is affected by the ZnO loading.
The molar ratio of surface Cu0/Cu+ is gradually decreased with the
increase of Cu/Zn. Moreover, the TOF results indicate that the catalyst
with an appropriate Cu0/Cu+ ratio of 1 displays the best catalytic
performance. The H2-TPR, Zn XPS and TEM results prove that ZnO as
an electron donor has a strong interaction with the Cu species, which is
helpful to restrain the reverse water-gas shift reaction and hinder the
sintering of the Cu species during the stability test.
Fig. 10. TPSR (A) and CO2-TPD (B) profiles of the Cu/SiO2 and 4CuZn/SiO2 catalysts.
Acknowledgements
This work is financially supported by National Natural science
foundation of China (No. 21476159, 21476160), the Natural Science
Foundation of Tianjin (No. 15JCZDJC37400, 15JCYBJC23000), the 973
program (2014CB932403), and the Program for Introducing Talents of
Discipline to Universities of China (No. B06006).
Fig. 11. Time-on-stream of the DME conversion over the Cu/SiO2 and 4CuZn/SiO2
catalysts in DME SR.
during the stability test. For the highly dispersed Cu/SiO2 catalyst, the
interaction between Cu and the silica support plays an important role in
stabilizing the small Cu crystallites, whereas the frequent transition of
copper valence and crystallite phase probably weaken and destroy this
interaction, and finally induce the aggregation of Cu particles [52]. The
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.apcata.2017.04.013.
45

X. Wang et al.
Applied Catalysis A, General 540 (2017) 37–46
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