CO2 hydrogenation over acid-activated Attapulgite/Ce0.75Zr0.25O2 nanocomposite supported Cu-ZnO based catalysts

Article abstract of DOI:10.1016/j.mcat.2019.110499

A series of Cu-ZnO based catalysts supported on the Attapulgite/Ce_0.75Zr_0.25O₂ (ATP-CZO) nanocomposite prepared by different methods were synthesized using impregnation method for CO₂ hydrogenation to CO and methanol. The physicochemical properties and catalytic performance of these catalysts were investigated by N₂ adsorption/desorption, XRD, FE-SEM, TEM, N₂O chemisorption, XPS, H₂-TPR and CO₂-TPD techniques. Results showed that the ATP-CZO-SC-P400 nanocomposite prepared by polyethylene glycol (PEG 400) modified solution combustion (SC) method owned the best dispersion of CZO particles on the ATP-CZO composite. Due to the dispersing effect of PEG 400, the intensity of Ce_0.75Zr_0.25O₂ solid solution was enhanced, and the interaction between Ce-Zr-O oxides and ATP support was improved. Furthermore, the formation of strong basic sites and copper surface area of the catalyst were enhanced. As a result, the CZFK/ATP-CZO-SC-P400 catalyst showed the highest yield of methanol and CO from CO₂ hydrogenation. The intimate contact between metallic Cu sites and strong basic sites of ZnO, ATP-CZO composite and ZnO-CZO interfaces on the surface of catalyst was considered as an important factor. Attapulgite, with unique property, low cost and easy availability, is a potential catalyst support for CO₂ hydrogenation reaction.

Full text of DOI:10.1016/j.mcat.2019.110499

Molecular Catalysis 476 (2019) 110499
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
CO hydrogenation over acid-activated Attapulgite/Ce0.75Zr0.25O2
2
nanocomposite supported Cu-ZnO based catalysts
a,b,c,d
a,e
a,b,c,d
, Fen Penga,b,c,d, Lian Xionga,b,c,d
,
Haijun Guo
, Qinglin Li , Hairong Zhang
a,b,c,d
a,b,c,d
a,b,c,d,⁎
Shimiao Yao
, Chao Huang
, Xinde Chen
a
b
c
d
e
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, 510640, PR China
CAS Key Laboratory of Renewable Energy, Guangzhou, 510640, PR China
Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou, 510640, PR China
R&D Center of Xuyi Attapulgite Applied Technology, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Xuyi, 211700, PR China
Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, 215123, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of Cu-ZnO based catalysts supported on the Attapulgite/Ce0.75Zr0.25
O
2
(ATP-CZO) nanocomposite
CO
2
hydrogenation
prepared by different methods were synthesized using impregnation method for CO hydrogenation to CO and
2
Attapulgite
Ce0.75Zr0.25
methanol. The physicochemical properties and catalytic performance of these catalysts were investigated by N
2
2
O solid solution
adsorption/desorption, XRD, FE-SEM, TEM, N
2
O chemisorption, XPS, H
2
-TPR and CO -TPD techniques. Results
2
Solution combustion method
Methanol
showed that the ATP-CZO-SC-P400 nanocomposite prepared by polyethylene glycol (PEG 400) modified solution
combustion (SC) method owned the best dispersion of CZO particles on the ATP-CZO composite. Due to the
dispersing effect of PEG 400, the intensity of Ce0.75Zr0.25
2
O solid solution was enhanced, and the interaction
between Ce-Zr-O oxides and ATP support was improved. Furthermore, the formation of strong basic sites and
copper surface area of the catalyst were enhanced. As a result, the CZFK/ATP-CZO-SC-P400 catalyst showed the
highest yield of methanol and CO from CO hydrogenation. The intimate contact between metallic Cu sites and
2
strong basic sites of ZnO, ATP-CZO composite and ZnO-CZO interfaces on the surface of catalyst was considered
as an important factor. Attapulgite, with unique property, low cost and easy availability, is a potential catalyst
support for CO hydrogenation reaction.
2
1
. Introduction
According to the Global Carbon Budget 2018, the global CO
the fast hydrogenation of surface intermediates on catalyst surface with
a high hydrogen/carbon ratio, methanol and CO are much easier to be
produced than higher alcohols [18]. Unfortunately, because of the high
2
emissions are projected to go up in 2018 by more than 2% and reach to
7.1 Gt [1]. The increasing emissions bring a severe challenge for
governments to achieve the goal of limiting global warming to below
°C by 2020. Carbon dioxide (CO ) catalytic hydrogenation to fuels or
chemicals is considered as a feasible solution for reducing CO emission
2]. The desired products from CO hydrogenation include methanol
which was mainly produced over Cu-based catalysts [3–9], hydro-
thermodynamic stability and low reactivity of CO
2
molecule, the CO
2
3
conversion and aiming products selectivity are still hard to be im-
proved.
2
2
Cu/ZnO/Al
2
O catalyst has been applied to produce methanol in-
3
2
dustrially from CO and CO hydrogenation [19]. However, the un-
2
[
2
desired byproduct water molecule has a negative effect on the strong
hydrophilic characteristic of alumina, thus leading to the rapid Cu
sintering and serious catalyst deactivation [20]. In order to improve the
carbons through combination of CO reduction with the following Fi-
2
scher–Tropsch (FT) reactions over modified Fe-and Co-based FT cata-
lysts [10–13], and CO through reverse water-gas shift (RWGS) reaction
over bimetallic or carbide catalysts [2,14]. Higher alcohols refers to
catalytic activity of Cu-ZnO based catalyst, some oxide carriers like SiO
2
[21,22], TiO
tions [27,28] have been used to modify the catalyst by substitution or
addition. Especially, ZrO can improve the Cu dispersion and surface
basicity, thus increasing the catalytic activity and methanol selectivity
of Cu/ZnO catalyst [29–32]. In compare to Cu/ZnO/ZrO catalyst, the
2
[23], ZrO
2
[3,14,24,25], CeO
2
[26], or their combina-
C
2+ alcohols (C2+OH) can be also prepared from CO
2
hydrogenation
2
over some multi-functional catalysts, like K/Cu-Zn-Fe catalysts [15],
Mo-Co-K sulfide catalysts [16] and Pt/Co
3
O
4
catalysts [17]. Owing to
2
⁎
Corresponding author at: No.2 Nengyuan Road, Tianhe District, Guangzhou, 510640, PR China.
E-mail address: cxd_cxd@hotmail.com (X. Chen).
https://doi.org/10.1016/j.mcat.2019.110499
Received 18 April 2019; Received in revised form 9 June 2019; Accepted 1 July 2019
2468-8231/ © 2019 Elsevier B.V. All rights reserved.

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
partial replacement of ZrO
2
with CeO
2
leaded to the further enhance-
cata-
particles and Ce-Zr-O oxides. The effects of addition amount of PEG 400
ment of CO
2
hydrogenation properties over Cu/ZnO/Ce1−xZr
x
O
2
and probable CO
2
hydrogenation mechanism were also studied in de-
lyst [28]. Therefore, the Ce-Zr-O oxide is a potential carrier for mod-
ifying Cu-ZnO based catalyst, but the high cost of ceria and zirconium
materials brings a big challenge for its practical application.
tail.
2
. Experimental
Attapulgite (ATP, also known as palygorskite) is a natural hydrated
magnesium aluminum silicate nonmetallic mineral, having 1-D na-
norod-like crystal morphology and porous crystalline structure with
tetrahedral layers [33]. Due to the structural properties of big surface
area and pore volume and adjustable acid-base nature as well as
thermal stability, ATP has been extensively used as catalyst or catalyst
support in CO conversion [34–36], hydrogen production [37,38], bio-
mass and platform chemicals conversion [39–42], etc. Our previous
research [34] reported that the acid activation of ATP promoted the
decomposition of intergranular sticking substance and carbonate im-
purity which increased the surface area and pore volume. The removal
2.1. Catalyst preparation
The ATP/Ce0.75Zr0.25
O nanocomposite (ATP-CZO, for short) was
2
prepared by seven methods as showed in Supplementary material
(
Supplementary material, S1), including impregnation (IM), forward
coprecipitation (FCP), backward coprecipitation (BCP), parallel-flow
coprecipitation (PCP), hydrothermal (HT), Sol-Gel (SG) and solution
combustion (SC). These ATP-CZO supported Cu-ZnO based catalysts
with a molar ratio of Cu: Zn: Fe: K = 1:1:0.5:0.15 and Cu loading of
17.7 wt% were synthesized by impregnation method. Typically, the
2
+
3+
3+
of octahedral cations (Mg , Al
and Fe ) and the formation of
mixed aqueous solution containing Cu(NO
3
)
2
, Zn(NO
3
)
2
, Fe(NO
3
)
3
and
was
amorphous silica from the tetrahedral sheet were also enhanced which
could improve the interaction between catalyst active components and
ATP support [35]. Phongamwong et al. [21] found that a low amounts
of silica, around 0.5–1.5 wt%, promoted the interdispersion of metal
−
1
K
2
CO with a total metallic ions concentration of 5.5 mol L
3
prepared according to the theoretical molar ratio by deducting the in-
trinsic iron and potassium components of ATP support [35]. Then, 8 g
ATP-CZO powder (> 100 mesh) was added into the mixed solution.
After impregnation for 4.5 h, the catalyst precursor was dried in air at
oxides components in CuO-ZnO-ZrO
2
-SiO catalyst. Consequently, the
2
metallic copper surfaces area and surface basicity were increased, and
then the CO conversion, methanol selectivity and catalyst stability
were also improved. From this point of view, acid-activated ATP is also
a promising CO hydrogenation catalyst support because of geometric
3
93 K overnight, and then calcined in air at 623 K for 5.5 h. These
2
catalysts were named as CZFK/ATP-CZO and pelletized, crushed, and
sieved to 60–80 mesh before use.
2
structural effect. Li et al. [43] prepared a series of ATP/Ce1−xZr
O
x 2
nanocomposites by a homogeneous deposition method with hexam-
2.2. Catalytic performances evaluation and product analysis
ethylenetetramine as precipitator for the catalytic wet oxidation of
methylene blue. ATP/Ce0.8Zr0.2
2
O showed the maximum degradation
CO hydrogenation activity evaluation was carried out in a stainless
2
rate of methylene blue of 99% due to the significant interactions be-
tween Ce-Zr-O oxides and ATP support. Considering the low cost and
better textural properties of ATP, it is valuable to try out ATP/
steel tubular (internal diameter, 8 mm; length, 280 mm), continuous-
flow fixed-bed reactor [15]. Catalyst (60–80 mesh, 3 g) mixed with
quartz sand (40–60 mesh, 9 g) was loaded in the middle of reactor,
having a catalyst bed length of 100 mm. The reaction temperature was
monitored by a thermocouple inserted into the middle of catalyst bed.
Ce1−xZr
x
O
2
nanocomposite as the CO hydrogenation catalyst support.
2
For Cu-ZnO based catalyst in CO
2
hydrogenation reaction, the
0
mostly reported hydrogenation active sites are metallic copper (Cu )
Before CO
2
hydrogenation reaction, the catalyst was reduced at 573 K
+
and monovalent copper ion (Cu ) with a proper ratio [3,44]. The
for 4 h with a stream of pure H
2
under pressure of 1.5 MPa. After re-
surface area and dispersion of copper species [45] or the metal-support
interaction [26–28] or both of them [20,29,46] are crucial for affecting
the catalytic behavior of Cu-ZnO based catalysts. Bonura et al. [24]
proposed that an adequate balance between metal and oxide surface
sites was important to develop a high-performance catalyst. Simulta-
neously, the dispersion and particle size of active components are ex-
tremely relevant to the textural properties of support material which
can be largely affected by preparation method [47,48]. Therefore, the
effects of preparation method on the textures and surface properties of
duction, the reactor was decreased to reaction temperature and then the
reactant gas flow was introduced into the reactor. In order to reveal the
relationship between the catalytic performances and reaction condi-
tions, reaction temperature was increased from 503 K to 593 K by a
temperature controller. Reaction pressure was adjusted from 4.0 MPa to
8
.0 MPa with a backpressure regulator. The gas hourly space velocity
−1 −1
(
GHSV) was employed from 3000 h to 9000 h by changing the gas
flow rate. The related catalytic performances were showed in the
Supplementary material in detail. The gas and liquid products were
separated by a cold trap and analyzed by off-line gas chromatograph
ATP/Ce1−xZr
x
O nanocomposites need to be investigated in detail.
2
Besides catalyst support, some metallic promoters were adopted to
(
GC). The former was analyzed by a GC 9800 (Kechuang, Shanghai in
improve these catalytic crucial factors, like La, Ce, Nd, Pr, Mg, Mn
China; column: TDX-01, 1 m ×3 mm) equipped with a thermal con-
ductivity detector (TCD). The latter was analyzed by a GC 9900 (Jiafen,
Beijing in China; column: FFAP, 30 m ×0.25 mm ×0.25 μm) equipped
[
49,50]. Our previous study [51] investigated the effect of K content in
K/Cu-ZnO catalyst on CO hydrogenation performances and found that
2
K O played an important role in promoting Cu-ZnO interaction by ad-
2
with a hydrogen flame ionization detector (FID). The CO conversion
2
justing the transfer of surface oxide species. Subsequently, we added
medium concentration of Fe promoter into K/Cu-ZnO catalyst and
found that Fe could promote the reduction of catalyst and increase the
and products selectivity were calculated according to mass-balance
method and three duplicate experiments were conducted to obtain the
average with the error less than 5%. The space time yield (STY) and
synergistic effect of Cu-FeC dual-active sites for improving the se-
x
turnover frequency (TOF) of CH
to Dong et al. [3].
3
OH and CO were calculated according
lectivity of C2+ alcohols [15]. Therefore, Fe and K promoters are im-
portant components for modifying the Cu-ZnO based catalysts. In this
work, ATP/Ce1−xZr
x
2
O (x = 0.25) nanocomposite was prepared by
different methods and used as the support of Cu-ZnO based catalyst for
2.3. Characterization
CO hydrogenation. The textures and physicochemical properties of
2
ATP/Ce0.75Zr0.25
O
2
nanocomposite and these supported catalysts were
The detailed N adsorption/desorption, X-ray diffraction (XRD),
2
characterized. In order to improve the dispersion of copper species and
enhance the intimate contact of Cu-oxides dual sites, a modified solu-
tion combustion method using polyethylene glycol (PEG 400) as dis-
persing agent was adopted to promote the interaction between ATP
Field emission scanning electron microscope (FESEM), Transmission
electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS)
and Vis-Raman spectra are showed in Supplementary material
(Supplementary material, S2).
2

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
2
.3.1. Hydrogen-temperature programmed reduction (H
-TPR experiment was carried out in a U-tube quartz reactor using
ChemBET Pulsar-1 automated chemisorption analyzer
/Ar mixture as the reductive
2
-TPR)
ATP-CZO nanocomposites, implying the deposition of metallic oxides
H
2
onto ATP-CZO nanocomposites. CZFK/ATP-CZO-IM catalyst shows the
2
a
lowest SBET of 22.0 m /g but the largest average pore diameter (D
P
) of
(
Quantachrome, US) with a 10 vol.% H
2
24.1 nm, which is consistent with the lowest SBET of ATP-CZO-IM na-
nocomposite. Though the SBET and Vtotal of ATP-CZO-SC-P400(3) are
not the highest in those ATP-CZO nanocomposites, CZFK/ATP-CZO-SC-
gas. The samples (120 mg) were pretreated with Ar at 393 K for 60 min,
−1
and then reduced from 323 K to 1073 K at a heating rate of 10 K min
.
2
The hydrogen consumption was monitored by a TCD and quantified by
five pulses calibration.
P400(3) catalyst owns the highest SBET of 65.0 m /g and Vtotal of
3
0.236 cm /g. Due to the dispersing effect of PEG 400, the interaction
between ATP and CZO particles in ATP-CZO-SC-P400(3) nanocompo-
site had been improved, and CZO particles were uniformly distributed
on surface of ATP to enhance the orientation of ATP rodlike crystals
(Supplementary material, Fig. S2). As a result, the cation interchange
2
.3.2. CO
CO -TPD experiments were conducted in the same reactor as H
TPR detection. The samples were reduced at 573 K for 1 h using 10 vol.
/Ar mixture with a flow rate of 30 mL/min. After cooling to 323 K,
the catalysts were saturated in CO flow at 323 K for 1 h and then flu-
shed with Ar to remove the physical adsorbed molecules. Then, the
2
-temperature programmed desorption (CO
2
-TPD)
2
2
-
2
+
2+
3+
%
H
2
between ATP and metallic ions (Cu , Zn
and Fe ) might be hin-
2
dered, thus decreasing the entrance of metallic ions into ATP pores to
generate bulk metallic oxides. N O chemisorption was applied to detect
2
−1
desorption processes were performed with a heating rate of 10 K min
the dispersion (DCu) and surface area of metallic Cu (SCu) on surface of
CZFK/ATP-CZO-SC and CZFK/ATP-CZO-SC-P400(3) catalysts. The cal-
culated DCu and SCu according to these equations reported by Van Der
under Ar flow up to 1073 K and the desorbed CO
mass spectrometer.
2
were recorded by a
2
The number of surface metallic copper atoms of catalysts was de-
Grift et al. [52] increase from 4.4% and 30 m /g-Cu to 13.3% and
2
termined by N
Van Der Grift et al. [52]. Catalysts (105 mg) were first reduced for 2 h
according to the H -TPR procedure as mentioned above in a 10 vol.%
/Ar flow (30 mL min ) from 303 K to 573 K at 5 K min . The
amount of hydrogen consumption in the first TPR process was denoted
as X. And then, the reactor was purged with Ar to 303 K. A 20% N O/N
flow (30 mL min ) was introduced to oxidize surface copper atoms to
2
O oxidation at 323 K using the procedure described by
90 m /g-Cu, respectively. The results indicated that ATP-CZO-SC-
P400(3) nanocomposite with specific textures and morphology is more
beneficial for improving the dispersion of CuO than ATP-CZO-SC na-
nocomposite.
2
−
1
-1
H
2
2
2
3.2. XRD detection and morphology observation
−
1
Cu
2
O at 323 K for 1 h. Subsequently, the catalyst was flushed with Ar to
The XRD patterns of these ATP-CZO nanocomposites supported Cu-
ZnO based catalysts are shown in Fig. 1. The reflection peaks of paly-
remove the oxidant and cooled to 303 K. Finally another TPR experi-
ment was performed as the same as the first reduction. Hydrogen
consumption in the second TPR was denoted as Y. The surface area and
dispersion of Cu were calculated according to equations reported by
Van Der Grift et al. [52], as follows:
gorskite, quartz and Ce0.75Zr0.25
O solid solution as showed in the XRD
2
patterns of ATP-CZO nanocomposites (Supplementary material, Figs. S3
and S4) are still present with much lower peak intensity due to the
deposition of metallic oxides onto ATP-CZO composite. As shown in
Fig. 1, the diffraction peaks at 2θ = 35.7°, 38.9°, 49.0° and 2θ = 31.8°,
Reduction of all copper atoms:
3
4
4.4°, 36.3°, 47.5°, 56.6°, 62.9° are corresponded to CuO (JCPDS, No.
5-0937) and ZnO (JCPDS, No. 65-3411), respectively. The calculated
CuO + H
Reduction of surface copper atoms only:
O + H → 2Cu + H O, hydrogen consumption = Y
The dispersion of Cu and the area of surface Cu were calculated as:
D = (2 × Y/X) × 100%
2
→ Cu + H
2
O, hydrogen consumption = X
crystallite size of CuO and ZnO by Scherrer equation in Cu-ZnO based
catalysts are shown in Table 1. CZFK/ATP-CZO-PCP catalyst shows the
largest particle size of CuO, while CZFK/ATP-CZO-HT catalyst shows
the smallest particle size of CuO, followed by CZFK/ATP-CZO-SC cat-
alyst and CZFK/ATP-CZO-SC-P400(3) catalyst. The CZFK/ATP-CZO-IM
catalyst has the largest particle size of CZO and ZnO (Table 1) which is
ascribed to the agglormation of CZO oxide on surface of ATP (Supple-
mentary material, Fig. S5). However, the larger particle size of CuO and
ZnO for other catalysts is corresponded to the smaller particle size of
CZO oxide, implying that CuO and ZnO might be well dispersed on
ATP-CZO nanocomposite with much higher particle size of CZO oxide.
Furthermore, the diffraction peaks at 2θ of 33.2°, 35.7°, 54.2° and
Cu
2
2
2
S=2 × Y × Nav/(X × MCu × 1.46×1019) ≈ 1353 × Y/X (m /g)
2
where Nav = Avogadro’s constant, MCu = relative atomic mass of
1
9
copper (63.456 g/mol), 1.46 × 10 is the number of copper atoms per
square meter as the result of an average copper surface atom area of
0
.0711 nm2 [53].
weak diffraction peaks at 2θ of 35.5°, 62.7° are ascribed to Fe
JCPDS, No. 84-0307) and CuFe (JCPDS, No. 77-0010), respec-
tively. The formation of CuFe phase indicates the improvement of
2
O
3
(
O
2 4
3
. Results and discussion
O
2 4
Cu-Fe interaction, which played an important role in synthesizing
higher alcohols over binary Cu-Fe-based catalyst [35]. These fresh
CZFK/ATP-CZO catalysts were also observed by FE-SEM as shown in
Fig. 2. Most samples show similar block-like morphology with some
agglomeration. According to the particle size of CuO, ZnO and CZO
oxides (Table 1), these big blocks might be assigned to the aggregation
of ZnO. Since the metallic components are complex in these catalysts,
the dispersion position of metallic oxides is difficult to be clarified.
However, comparing to FE-SEM image of CZFK/ATP-CZO-SC catalyst
(Fig. 2g), CZFK/ATP-CZO-SC-P400(3) catalyst showed more uniform
pore structure. The agglomeration of metallic oxides (Fig. 2h) was also
relieved because of the dispersion effect of PEG 400. The detailed active
elements distribution of CZFK/ATP-CZO-SC and CZFK/ATP-CZO-SC-
P400(3) catalysts are displayed in Fig. 3. The distribution of Cu, Zn and
Fe is isolated each other on CZFK/ATP-CZO-SC catalyst, while Cu is
well distributed overlapping with the distribution of Zn and Fe on
3
.1. Textural properties
The textural properties of these ATP-CZO nanocomposites are lar-
gely affected by the preparation method (Supplementary material,
Table S1). For ATP-CZO-IM nanocomposite, the highest proportion of
microporous surface area (SMicro) of 17.4% in BET surface area (SBET
)
was obtained. The highest proportion of microporous volume (VMicro) of
.3% in total pore volume (Vtotal) was achieved over ATP-CZO-SC-
P400(3) nanocomposite. Differences in textures reflect differences in N
adsorption-desorption isotherms and BJH pore size distribution
3
2
(
Supplementary material, Fig. S1). The type IV isotherms with a dif-
ferent size of H3-type hysteresis loop are observed for all samples.
Furthermore, the textural properties of these ATP-CZO nanocomposites
supported Cu-ZnO based catalysts are shown in Table 1. It can be seen
that the SBET and Vtotal of those catalysts are much lower than them of
3

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
Table 1
Structural parameters of CZFK/ATP-CZO catalysts.
2
3
XRD – Crystallite size (nm)a
Sample
S
BET (m /g)
V
total (cm /g)
P
D (nm)
CZO
CuO
ZnO
CZFK/ATP-CZO-IM
CZFK/ATP-CZO-FCP
CZFK/ATP-CZO-BCP
CZFK/ATP-CZO-PCP
CZFK/ATP-CZO-HT
CZFK/ATP-CZO-SG
CZFK/ATP-CZO-SC
CZFK/ATP-CZO-SC-P400(3)
22.0
47.9
55.4
55.3
37.7
30.5
24.6
65.0
0.133
0.193
0.175
0.181
0.206
0.137
0.124
0.236
24.1
16.1
12.7
13.1
21.8
18.0
20.1
14.5
6.1
4.0
3.0
2.9
4.3
3.8
4.1
4.0
21.8
24.6
28.6
31.2
18.2
23.3
20.4
21.0
545.1
257.0
316.0
328.5
235.1
304.3
216.6
222.4
a
Calculated using Scherrer equation based on the reflection of (220) lattice plane at 2θ = 48.0° for CZO, (111) lattice plane at 2θ = 38.9° for CuO and (101)
lattice plane at 2θ = 36.3° for ZnO.
CZFK/ATP-CZO-SC-P400(3) catalyst. As suggested above, ZnO seems to
act as a physical support for dispersing CuO species [54], thus im-
proving the DCu and SCu on CZFK/ATP-CZO-SC-P400(3) catalyst.
3.3. XPS analysis
XPS patterns of Cu 2p3/2, Zn 2p and Fe 2p3/2 on fresh CZFK/ATP-
CZO-SC and CZFK/ATP-CZO-SC-P400(3) catalysts are shown in Fig. 4.
There are two peaks at 930 ˜ 938 eV assigned to the binding energy (BE)
of Cu (2p3/2), which are derived from the spin-obit doublet of Cu 2p
[
55]. Simultaneously, a couple of satellite peaks at higher BE of 938 ˜
9
48 eV is also observed. They are resulted from the shake-up transitions
by ligand-to-metal 3d charge transform [56]. The lower copper BE of
2
+
9
32 ˜ 934 eV is indicative of the Cu
A
in tetrahedral CuO (A) species
[
57], and the higher copper BE of 934 ˜ 937 eV is distributed to the
2
+
Cu
B
in octahedral composite metal oxides (B) [35]. The composite
copper oxide has also been detected by XRD with different intensity.
2
+
Fig. 1. XRD patterns of ATP-CZO nanocomposites supported Cu-ZnO based
catalysts. (a) IM, (b) FCP, (c) BCP, (d) PCP, (e) HT, (f) SG, (g) SC and (h) SC-
P400(3).
From Fig. 4, it is evident that the tetrahedral and octahedral Cu
ca-
tions are concomitant in these catalysts. From the high resolution scan
of Fe 2p3/2 on fresh CZFK/ATP-CZO-SC and CZFK/ATP-CZO-SC-
P400(3) catalysts as shown in Fig. 4, these peaks at BE of 710–712 eV
Fig. 2. FESEM images of Cu-ZnO based catalysts supported on ATP-CZO nanocomposites prepared from different method. (a) IM, (b) FCP, (c) BCP, (d) PCP, (e) HT,
f) SG, (g) SC and (h) SC-P400(3).
(
4

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
Fig. 3. EDS mapping of Cu, Zn, Fe and K on (a) ATP-CZO-SC and (b) ATP-CZO-SC-P400(3) nanocomposites supported Cu-ZnO based catalysts.
3
+
3+
and 712–714 eV are assigned to Fe
A
in Fe
2
O
3
4
phase and Fe
B
in
material, Fig. S6). CZFK/ATP-CZO-PCP catalyst shows the highest
2
+
2+
composite metal oxide phase (such as CuFe
2
O
spinel), respectively
proportion of Cu
A
/Cu
B
, indicating the presence of a large amount
3
+
[
35]. The BE of Fe
with the BE increase of Cu
For Zn 2p spectra, the BE of Zn (2p3/2) is 1022.3 eV and Zn (2p1/2) is
A
shows an increase by 0.5 eV, which is similar
of dispersed CuO phase. CZFK/ATP-CZO-HT catalyst has the highest
3+ 3+
2
+
A
on CZFK/ATP-CZO-SC-P400(3) catalyst.
proportion of Fe
B
/Fe
A
. This might be caused by different disper-
sion, location and size of metallic oxides on the ATP-CZO structure, and
the interactions between metallic oxides and ATP-CZO nanocomposite.
1
045.3 eV, which is the characteristic of ZnO [58]. The peak position
and peak intensity on CZFK/ATP-CZO-SC-P400(3) catalyst show a
slight increase relative to CZFK/ATP-CZO-SC catalyst. These results
2
+
implied that the shift of Cu
A
on CZFK/ATP-CZO-SC-P400(3) catalyst
3.4. The reducibility of the catalysts
to higher BE was related to the coordination of copper in the CuFe
2 4
. Further-
O
spinel and the stronger interaction between CuO and Fe
2
O
3
The investigation of the reduction behavior of the heterogeneous
catalyst is beneficial for the identification of active sites and the metal-
support interaction (MSI). TPR experiment of the ATP-CZO nano-
composite supported Cu-ZnO based catalysts was performed. As shown
in Fig. 5 and Table 2, two types of peaks are present on these reduction
curves, which located on the range of 540 ˜ 590 K (low-temperature)
more, the interaction between CuO and ZnO was also enhanced due to
the increase of DCu and SCu. The preparation method of ATP-CZO
composite has a large effect on the BE of Cu 2p3/2, Zn 2p and Fe 2p3/2
2
+
2+
3+
3+
and the proportion of Cu
A
/Cu
B
and Fe
A
/Fe
B
(Supplementary
5

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
Fig. 4. XPS patterns of Cu-ZnO based catalysts supported on ATP-CZO nanocomposites prepared from different method: (a) SC and (b) SC-P400(3).
and 698 ˜ 1029 K (high-temperature), respectively. Since ZnO and Ce-
Zr-O oxides are not reduced within this temperature range [46,59], the
low-temperature reduction peak is assigned to the reduction of highly
dispersed CuO to Cu: CuO+H
2
→Cu+H O [35,51]. CZFK/ATP-CZO-
2
PCP catalyst shows the highest reduction temperature of dispersed CuO
(586 K) with the maximum fraction (72%), which is related with its
6

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
Fig. 5. H
2
-TPR profiles of Cu-ZnO based catalysts supported on ATP-CZO nanocomposites prepared from different method. (a) IM, (b) PCP, (c) HT, (d) SG, (e) SC and
(
f) SC-P400(3).
smallest CZO size and the second largest ZnO size (Table 1). On the
contrary, CZFK/ATP-CZO-SC-P400(3) catalyst shows the lowest re-
duction temperature of CuO (540 K) with the minimum fraction (41%),
which are also lower than them of CZFK/ATP-CZO-SC catalyst. FESEM,
between CuO and ATP; (2) the intimate contact between CuO and other
oxides (Ce-Zr-O solid solution, ZnO and Fe
2
O ) for increased interac-
3
tions; (3) the improved dispersion of CuO upon ZnO sheets to form
much more Cu-ZnO interface. However, when x is 10, CuO particles
seem not to be closely connected with ZnO sheets on ATP-CZO-SC-
P400(10) nanocomposite but show much smaller crystallite size. This is
XRD, XPS and N O-adsorption results have proved that the dispersion of
2
CuO can be improved by using the PEG 400 modified ATP-CZO-SC
composite as catalyst support. In order to investigate the effects of
addition amount of PEG 400 on morphology and structure of ATP-CZO-
SC supported Cu-ZnO based catalyst, these CZFK/ATP-CZO-SC-P400(x)
catalysts were observed by TEM. As shown in Fig. 6, CuO particles are
dispersed onto the surface of ZnO and the Cu-ZnO interface is formed in
all catalysts. However, much more ZnO sheet are exposed on CZFK/
ATP-CZO-SC-P400(0) catalyst. The dispersion of metallic oxides was
because the mechanical strength and intensity of Ce0.75Zr0.25
O solid
2
solution in ATP-CZO-SC-P400(10) nanocomposite are much lower than
other ATP-CZO-SC-P400(x) nanocomposites, which can be due to the
strong heat release and more gases released in the combustion process
of PEG 400 [46].
Furthermore, apart from CZFK/ATP-CZO-IM and CZFK/ATP-CZO-
PCP catalysts, other catalysts display a shoulder peak at 608 ˜ 618 K
accompanying with the reduction peak of dispersed CuO. XPS results
improved by increasing x from 1 to 7. Combining with the above N
2
3
+
adsorption-desorption, XRD, SEM and XPS detections, the improvement
of dispersed CuO reduction on CZFK/ATP-CZO-SC-P400 catalyst can be
ascribed to the following three points: (1) the decrease of interaction
have shown that Fe
B
in composite metal oxide phase is present in all
catalysts (Fig. 4 and Fig. S6 in Supplementary material), thereby the
shoulder reduction peak can be assigned to the reduction of bulk CuO
Table 2
Reduction peak temperature and hydrogen consumption over CZFK/ATP-CZO catalysts.
Sample
Peak Ⅰ
Peak Ⅱ
Peaks Ⅲ, Ⅳ, Ⅴ, Ⅵ, Ⅶ
Tem. (K)a
Total Con.H2 (μmol gcat.−1)
Tem. (K)a
Con.H2 (%)b
Tem. (K)a
Con.H2 (%)b
Con.H2 (%)b
CZFK/ATP-CZO-IM
CZFK/ATP-CZO-PCP
CZFK/ATP-CZO-HT
CZFK/ATP-CZO-SG
CZFK/ATP-CZO-SC
CZFK/ATP-CZO-SC-P400(3)
583
586
568
576
570
540
3356(66%)
4467(72%)
2633(50%)
3355(59%)
3153(58%)
2107(41%)
–
–
735, 789, 971
1679(34%)
1698(28%)
1996(38%)
1706(30%)
1868(34%)
1947(37%)
5035
6165
5287
5701
5458
5202
–
–
769, 882
618
616
608
611
658(12%)
640(11%)
437(8%)
1148(22%)
718, 790, 830, 964
743, 792, 846, 968
725, 784, 840, 973, 1029
698, 791, 844, 971
a
b
Peak temperature.
Relative proportion of hydrogen consumption.
7

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
Fig. 6. TEM images of Cu-ZnO based catalysts supported on ATP-CZO-SC-P400(x) nanocomposites. (a) x=0, (b) x=1, (c) x=3, (d) x=5, (e) x=7 and (f) x=10.
and composite copper oxide (such as CuFe reduction to Cu and
2 4
O
Fe
2
O
3
[35,60]). N adsorption-desorption and SEM results (Table 1 and
2
Fig. S5 in Supplementary material) showed that the formation of bulk
metallic oxides were hindered due to the uniformly distribution of CZO
upon ATP support in CZFK/ATP-CZO-SC-P400 catalyst. As a result, the
obvious increase of H consumption involved in the shoulder peak in
2
contrast to CZFK/ATP-CZO-SC catalyst was caused by composite copper
oxides due to the improvement of interactions between CuO and other
metallic oxides. In addition, a high-temperature reduction peak at 698 ˜
1
029 K can be found on the reduction curves of all catalysts, which is
ascribed to the consecutive reduction of Fe
CZFK/ATP-CZO-HT catalyst shows the highest H
Fe reduction, and then is CZFK/ATP-CZO-SC-P400(3) catalyst.
2
O
3
→Fe
3
O
4
→FeO→Fe [60].
2
consumption of
2
O
3
Taken together, CZFK/ATP-CZO-SC-P400(3) catalyst is the sole sample
of which the H consumption of CuFe composite and subsequent
Fe is much higher than that of dispersed CuO, indicating the for-
mation of more CuFe and the enhancement of Cu-ZnO interaction.
2
2 4
O
2 3
O
O
2 4
Fig. 7. CO -TPD profiles of (a) ATP-CZO-SC-P400 nanocomposite, (b) CZFK/
2
3.5. Surface adsorption behavior of the catalysts
ATP-CZO-SC catalyst and (c) CZFK/ATP-CZO-SC-P400(3) catalyst.
In order to examine the basicity site of fresh CZFK/ATP-CZO-SC and
basic hydroxyl groups on the ATP and Ce-Zr-O oxide [3,61]. The HT
CZFK/ATP-CZO-SC-P400(3) catalysts, CO -TPD was performed to in-
vestigate the surface adsorption behavior of CO on these two catalysts.
For comparison, ATP-CZO-SC-P400(3) support was also studied. As
shown in Fig. 7, two broad CO desorption bands ranging from 353 and
73 K to 523 and 923 K (labeled as LT and HT peaks, respectively) can
be observed. The LT peak (α peak) of ATP-CZO-SC-P400(3) support
centered at 400 K is assigned to the interaction of CO with weakly
2
peak is constituted by a symmetrical peak at 695 K (β peak) containing
1
2
a feeble shoulder peak at 734 K (β peak). Qu et al. [61] found that the
2
CO desorption peak at 473 ˜ 673 K was corresponded to the bidentate
2
2
carbonate species which formed on the moderate basic sites of Ce-Zr
mixed oxides. Another HT peak at 823 ˜ 973 K was attributed to the
5
adsorption of CO on the strong basic sites (low-coordination oxygen
2
2
8

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
anions) [62]. Because of the interaction between Ce-Zr-O oxide parti-
cles and ATP on ATP-CZO-SC-P400(3) support, the bidentate carbonate
species is desorbed at much higher temperature. The strong basic sites
were decreased due to the loss of oxygen vacancies. As for CZFK/ATP-
CZO-SC and CZFK/ATP-CZO-SC-P400(3) catalysts, the weakly basic
hydroxyl groups was strongly diminished during the calcination process
of the catalyst, leading to the obvious decrease of desorption peak in-
tensity. However, the peak temperature was much higher which might
be related with the inner hydroxyl groups of the ATP support. In ad-
dition, the introduction of CZFK metallic particles did not affect ob-
viously on the desorption peak temperature and intensity of moderate
basic sites on ATP-CZO-SC-P400(3) composite. It was amazing that a
newly generated HT peak at 774 K on CZFK/ATP-CZO-SC catalyst (γ
peak) can be observed, which may be caused by the enhancement of
interaction between ZnO and Ce-Zr-O oxide [32]. As shown in Fig. 7,
CZFK/ATP-CZO-SC-P400(3) catalyst showed much higher desorption
peak temperature and intensity of γ peak than CZFK/ATP-CZO-SC
catalyst, implying the higher strength of ZnO-CZO interfaces active sites
direct CO
2
hydrogenation, but also generated by the hydrogenation of
CO intermediate [66]. CZFK/ATP-CZO-SG catalyst showed the highest
CH (non-desired product) selectivity of 30.3% while the lowest ROH
4
selectivity of 7.8%. The highest CO selectivity of 55.8% was obtained
over CZFK/ATP-CZO-HT catalyst, indicating that the lowest CuO par-
ticle size (Table 1) and the highest H
2
consumption of Fe
2
O reduction
3
(Table 2) play a decisive role in producing CO. For these ATP-CZO
composite without PEG 400 modification supported Cu-ZnO based
catalysts, the ROH selectivity over CZFK/ATP-CZO-FCP catalyst was the
maximum of 23.3%, and then was CZFK/ATP-CZO-BCP catalyst of
20.7% and CZFK/ATP-CZO-PCP catalyst of 18.7%. The downtrend was
consistent with the gradual decrease of crystallite size of CZO particles
but opposite to the increase of crystallite size of CuO and ZnO particles
(Table 1). However, other catalysts did not conform to the law. CZFK/
ATP-CZO-SC catalyst showed a smaller CuO particle size of 20.4 nm,
while the ROH selectivity was only 8.8% companying with the forma-
tion of 37.2% other oxygenates (marked as HOC). It was amazing that
the ROH selectivity was obviously increased to 17.5% over CZFK/ATP-
CZO-SC-P400(3) catalyst at the expense of the sharply decrease of HOC
selectivity. Since they have similar crystallite size of CuO, ZnO and CZO
particles, the increase of ROH selectivity can be due to much higher SCu
for CO adsorption and conversion. Since they have the same catalyst
2
composition, the improvement of total CO adsorbed can be attributed
2
to the enhancement of the interdispersion of mixed metal oxides (Figs.
S7 and S8 in Supplementary material). Simultaneously, the interaction
and DCu for H dissociation adsorption (Fig. 5 and Table 2) and higher
2
between metallic Cu and other metal oxides (ZnO, Fe
2
O
3
and
strength of ZnO-CZO interfaces active sites for CO adsorption and
2
Ce0.75Zr0.25
O
2
solid solution) after H
2
reduction was also improved,
conversion. At the same time, the proportion of methanol in mixed
alcohols was high up to 97.6% and the STY of methanol and CO also
showed the maxima over CZFK/ATP-CZO-SC-P400(3) catalyst (Fig. 8b).
which have been confirmed by BET, SEM, TEM, XPS and H -TPR as
2
mentioned above. Combining with these characterization results, it can
be concluded that the formed ZnO-CZO interfaces are the active sites for
In sum, on the purpose of improving the CO conversion and production
2
CO
2
adsorption and conversion, while metallic Cu is the hydrogenation
ability of methanol and CO over ATP-CZO composite supported Cu-ZnO
based catalysts, PEG 400 modified SC method is better choice for the
preparation of ATP-CZO carrier.
active site to adsorb H
thanol [32].
2
and facilitate H dissociation to produce me-
2
The effects of addition amount of PEG 400 on catalytic CO hy-
2
3
.6. Catalyst performances
.6.1. Evaluation of CO hydrogenation over different catalysts
drogenation performance over ATP-CZO-SC-P400(x) nanocomposites
supported Cu-ZnO based catalysts were also investigated. As shown in
3
2
Table 3, the CO
2
4
conversion shows a gradual increase. However, the
and ROH shows an opposite change which reflects the
CO
2
hydrogenation performances over those Cu-ZnO based catalysts
selectivity of CH
supported on ATP-CZO composite prepared from different methods
were evaluated. As shown in Fig. 8, the preparation method of ATP-
CZO composite has an important influence on catalytic performances.
competitive reaction route of CO hydrogenation. Moreover, CO se-
2
lectivity shows a consecutive decrease which seems like to contribute to
the increase of HOC selectivity. From the effects of the addition amount
of PEG 400 on the STY of methanol and CO formation over CZFK/ATP-
CZO-SC-P400(x) catalysts as shown in Fig. 9, the competitive produc-
tion of methanol and CO was present. With the increase of addition
amount of PEG 400, the DCu increased firstly and then decreased. Ac-
cording to the relationship between STY and DCu, the yield of methanol
was directly related with DCu and the CZFK/ATP-CZO-SC-P400(5) cat-
alyst showed the highest methanol yield due to the highest DCu. This
further proved that metallic Cu is the hydrogenation active site for
CZFK/ATP-CZO-PCP catalyst showed the lowest CO conversion of
2
1
0.5%, which was associated with its H -TPR behavior (the highest
2
reduction temperature of dispersed CuO and no reduction peak of
composite copper oxide), which implied the importance of Cu species
for improving catalytic activity. It’s well known that CO is thermo-
2
dynamically stable and difficult to be activated. The dispersed Cu
species are responsible for CO hydrogenation activity over Cu-ZnO
2
based catalysts [46]. Though the dispersion of CuO over CZFK/ATP-
CZO-SC-P400(3) catalyst was improved in contrast to CZFK/ATP-CZO-
adsorbing H
2
and facilitating H dissociation to produce methanol.
2
SC catalyst, which had been demonstrated by N
2
O adsorption, SEM,
Raman detection results showed that the intensity of Ce-Zr-O oxide
increased with the increase of x from 0 to 7 and then showed an obvious
TPR, TEM and XPS measurements, its catalytic activity is just a little
lower than that of CZFK/ATP-CZO-SC catalyst with the highest CO
conversion of 18.2%. Therefore, the SCu and DCu of Cu-ZnO based cat-
alyst was not the sole factor for determining CO hydrogenation ac-
tivity. Some other factors, like the intimate contact between CuO and
2
decrease when x was 10 (Fig. S9 in Supplementary material). CO -TPD
2
results showed that a higher strength of ZnO-CZO interfaces active sites
was generated in CZFK/ATP-CZO-SC-P400(3) catalyst. Therefore, the
increase of Ce-Zr-O oxide intensity is beneficial for improving the
2
other oxides (Ce-Zr-O solid solution, ZnO and Fe
2
O
3
) for increased in-
strength of ZnO-CZO interfaces active sites for CO adsorption and
2
teractions, and the improved dispersion of CuO upon ZnO sheet to form
much more Cu-ZnO interface should be taken into account for im-
proving the catalytic activity [63,64].
conversion. Consequently, CO conversion shows a gradual increase
2
with the increase of addition amount of PEG400 from x = 1 to x = 7,
but continuously increases when x = 10 which might be due to much
smaller CuO crystallite size (Fig. 6).
The products selectivity was also affected by the preparation
method of ATP-CZO composite (Fig. 8a). It can be seen that CO was the
To compare the catalytic efficiency produced by a unit site of a
copper atom on the surface of a catalyst, the turnover frequency (TOF)
which represents the number of CO and methanol produced on a unit
dominant product of CO hydrogenation over all catalysts under 593 K
2
via RWGS reaction (CO
2]. At the same time, CH
direct CO hydrogenation (CO
presence of FT element-Fe in the catalyst, which is called the metha-
nation of CO [65]. Mixed alcohols (marked as ROH, including me-
thanol-C OH and higher alcohols-C2+OH) cannot be only formed by the
2
+ H
also was generated with different levels by
+ 4H → CH + 2H O) due to the
2
→ CO + H O), an endothermic reaction
2
−1
[
4
site per second (s ), was calculated from the SCu for various catalysts.
Results from Fig. 9 showed that the TOF of methanol increased with
increased addition amount of PEG 400 until it reached the same max-
imum for CZFK/ATP-CZO-SC-P400(5) and CZFK/ATP-CZO-SC-P400(7)
catalyst; then, a decrease occurred with further increase in PEG 400
2
2
2
4
2
2
1
9

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
Fig. 8. CO
2
hydrogenation properties comparison of Cu-ZnO based catalysts supported on ATP-CZO nanocomposites prepared from different method. Reaction
conditions: T=593 K, P = 6.0 MPa, n(H
2
):nCO
2
= 3:1 and GHSV=5000 h−1
.
content. A maximum TOFMethanol of 1.3 × 10-3 is obtained with a
3.6.2. Structure-properties-catalytic performance correlations
CO hydrogenation to methanol has been extensively studied over
Cu-ZnO based catalysts, including different metallic oxides (Al O ,
-
3
minimum TOFCO of 1.9 × 10 . Gao et al. [31] found that the CO
2
2
conversion over Cu/Zn/Al/Zr catalysts is related to the exposed Cu
surface area and the dispersion of Cu, while the CH OH selectivity is
related to the distribution of basic sites on the catalyst surface. In this
2
3
3
CeO
2
, ZrO , etc.) or their composites supported and modified solid-state
2
catalysts. Some crucial factors for active sites, like a Cu step with a
work, the CO conversion over CZFK/ATP-CZO-SC-P400(x) catalysts
2
nearby Zn serving as adsorption site for oxygen-bound intermediates
did not show the same change trend with DCu and SCu due to some other
[19], a synergy of Cu and ZnO at the interface [67,68], and an adequate
0
parameters as mentioned below.
balance between metal (Cu ) and oxide surface sites (ZnO and ZrO
2
)
Table 3
Catalytic performance for CO
Catalyts
2
hydrogenation over ATP-CZO-SC-P400(x) nanocomposites supported Cu-ZnO based catalystsa.
CO
2
conversion
Products selectivity (C-mol%)
Alcohols distribution (wt%)
(
%)
CH
4
CO
ROHb
HOCc
C
1
OH
C
2+OH
CZFK/ATP-CZO-SC-P400(1)
CZFK/ATP-CZO-SC-P400(3)
CZFK/ATP-CZO-SC-P400(5)
CZFK/ATP-CZO-SC-P400(7)
CZFK/ATP-CZO-SC-P400(10)
12.4
13.9
14.0
15.7
17.7
25.7
14.4
10.2
11.5
16.6
62.2
44.6
36.4
36.5
33.0
7.4
4.7
92.3
96.3
97.0
97.0
90.2
7.7
3.7
3.0
3.0
9.8
17.3
25.3
21.4
12.6
23.7
28.1
30.6
37.8
a
b
c
= 3:1 and GHSV=5000 h−1
.
Reaction condition: T=593 K, P = 6.0 MPa, n(H
ROH stands for mixed alcohols.
2
):nCO
2
HOC stands for other oxygenates.
10

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
Fig. 9. Effect of the addition amount of PEG 400 in CZFK/ATP-CZO-SC-P400(x) catalysts on the dispersion of Cu (DCu) and STY of methanol and CO (a) and surface
area of Cu (SCu) and TOF of methanol and CO (b). Reaction conditions: T=593 K, P= 6.0 MPa, n(H
2
):nCO
2
=3:1 and GHSV=5000h−1
.
[
24], were proposed to form and stabilize the intermediates for me-
thanol synthesis. Metallic copper species acts as the crucial role for H
activation, of which the exposed surface area [46], dispersion, particle
high intrinsic activity toward CO
2
adsorption [2]. Two major reaction
2
mechanisms have been proposed for RWGS to CO. One is surface redox
mechanism which claimed that CO
2
is directly reduced into CO on the
0
+
size [69] and the ratio of Cu /Cu [3] can control the CO
2
conversion
surface of metallic copper [21,51]. The other is the formate decom-
and methanol selectivity. Oxides carriers plays the most important role
position mechanism, in which CO
2
is first hydrogenated into formate on
in adsorbing and activating CO
2
, of which the crystalline form [70] and
the metal site, followed by cleavage of the C]O bond to generate CO on
the metal/oxides interface site [2,71]. From this point of view, the es-
timate contact between metallic copper and oxides is very important,
which is similar to the synthesis of methanol.
basic sites [21,26,27] can control catalyst texture and exposed metal
surface, also affecting adsorption properties and catalyst functionality
of the Cu-ZnO system [28]. According to the mechanism of CO hy-
2
drogenation to methanol, the larger exposed copper surface area is fa-
vorable to provide the large interfacial contact between copper and
metal oxides [21], thus enhancing the formation and transformation of
formate intermediate species. Therefore, a metal-oxide dual site nature
on metal/oxides interface is crucial for the efficient synthesis of me-
In this study, the acid-activated ATP was combined with
Ce0.75Zr0.25
O solid solution to produce ATP-CZO composite by dif-
2
ferent methods. Characterization results show that the preparation
method of ATP-CZO composite has an important effect on its textures
and the characters of CZO nanoparticles which differ obviously in
texture, size and dispersion. In comparison, the ATP-CZO composite
produced by PEG 400 modified SC method showed much better dis-
persion of CZO particles and directionally arranged ATP rodlike crystals
thanol from CO
For the CO
2
2
hydrogenation.
hydrogenation to CO by RWGS reaction, CeO
2
is
commonly used to support active-metal because of its reducibility and
11

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
(
Figs. S2 and S5 in Supplementary material). The interaction between
interdispersion of mixed metal oxides and the interactions between
Ce-Zr-O oxides and ATP support was effectively enhanced [43], thus
alleviating the cation interchange between ATP and metallic ions
metallic Cu and other metal oxides (ZnO, Fe
2
O
3
and Ce0.75Zr0.25
O solid
2
solution) after H
2
reduction. The formation of CuFe
2
O phase could
4
2
+
2+
3+
(
Cu , Zn and Fe ). Then the crystallite size and intensity of Ce-Zr-
relieve agglomeration of Cu species to maintain lower particle size of
O oxides could be adjusted by changing the addition amount of PEG
00 (Figs. S4 and S9 in Supplementary material). Furthermore, as
shown in Fig. 3, ZnO and Fe were integrated with large crystallite
size on the surface of ATP-CZO composite. As a result, CuO was in-
timately contacted and uniformly distributed upon ZnO-Fe ag-
gregate to form CuO-ZnO interface and CuFe composite according
to XRD, XPS and H -TPR detections, implying the enhancement of in-
teractions between CuO and other oxides (ZnO and Fe ). Moreover,
the exposed copper surface area was also increased to provide much
more active sites for H adsorption and activation (Figs. 5 and 9). Bo-
nura et al. [28] investigated the effects of ZnO promoter on the struc-
metallic Cu where more dissociative adsorbed H provides a source of
2
4
atomic hydrogen for spillover onto these intimately contacted ZnO, Ce-
Zr-O solid solution and ZnO-CZO interfaces (Fig. 3) [3,67,70]. On the
other hand, the strong basic sites on ZnO, ATP-CZO composite and ZnO-
O
2 3
2
O
3
CZO interfaces could serve as the active sites for CO adsorption, acti-
2
2
O
4
vation and conversion into carbonate species, bicarbonate species and
formate species. The interactions between metallic Cu and ZnO, ATP-
CZO composite and ZnO-CZO interfaces favored to stabilize the inter-
mediates for further hydrogenation to produce methanol [21]. There-
fore, ZnO acted as a structural promoter for improving the dispersion of
metallic copper in the present catalyst and provided catalytic basic site
2
O
2 3
2
ture and CO
2
x
-hydrogenation functionality of Cu/CeO
2
, Cu/ZrO
2
and
for CO adsorption and activation. However, as exhibited in catalytic
2
Cu/Ce(1−x)Zr
O
2
(0 ≤ x ≤ 1) catalysts. ZnO acted as a promoter of both
performances, the CZFK/ATP-CZO-SC-P400 catalyst showed better
formation ability for CO by RWGS reaction than methanol. This can be
structural and catalytic functionality of the metal copper phase. The
CuZnO/Ce(1−x)Zr
highest catalytic activity and the increase of CeO
was beneficial to increase the methanol yield. That’s why the present
catalyst supported on the ATP/Ce0.75Zr0.25 nanocomposite with a
Ce/Zr ratio of 3/1 showed a lower catalytic activity and a higher CO
x
O
2
catalyst with a Ce/Zr ratio of 1/6 showed the
due to the promotion of C]O bond scission by reducible CeO
2
and
2
substitution by ZrO
2
proved later in a long-term stability study of catalyst.
O
2
3.6.3. Catalysts stability
The stability of CZFK/ATP-CZO-SC-P400(3) catalyst for CO
2
hy-
−1
selectivity. In future, the ATP/Ce(1−x)Zr
x
O
2
nanocomposite prepared
drogenation at 593 K, 6.0 MPa and 5000 h is shown in Fig. 11. It can
from PEG 400 modified SC method with much lower Ce/Zr ratio is a
promising carrier for supporting Cu-ZnO based catalysts to produce
be found that the CO conversion decreases slightly from 16.8% to 15%.
2
The products selectivity showed an obvious change. In the initial re-
methanol from CO hydrogenation.
2
action period of 8 h, the selectivity of HOC/ROH decays sharply to in-
As displayed in Fig. 10, CO and methanol could be simultaneously
synthesized depending on the structural properties of CZFK/ATP-CZO-
SC-P400 catalyst. On one hand, according to XRD (Fig. 1), XPS (Fig. 4)
crease the selectivity of CO and CH , which indicating the side reaction
4
for HOC production was restrained. Subsequently, the selectivity of
HOC/ROH decreases by 21.6% in following 22 h reaction which con-
and CO
2
-TPD result (Fig. 7), ZnO acted as a physical support for dis-
tributes to the selectivity increase of CO by 17.5% and CH by 4.1%.
4
persing CuO species. The FT element-Fe further improved the
The further increase of reaction time to 60 h did not change obviously
Fig. 10. Diagram of CO hydrogenation to CO and methanol over CZFK/ATP-CZO-SC-P400 catalyst.
2
12

H. Guo, et al.
Molecular Catalysis 476 (2019) 110499
Acknowledgments
This work was supported by the Science and Technology Program of
Guangzhou, China (201707010240), the Project of Guangdong
Provincial
Natural
Science
Foundation
(2018A030310126,
2
018A030313150), the Project of Jiangsu Province Science and
Technology (BE2018342), the tender project of HuaiAn
(
HAZC2018010036), the Key Research and Development Program of
Jiangsu Province (BE2016125) and the Project of Key Laboratory
Foundation of Renewable Energy, Chinese Academy of Science (CAS)
(
Y807jc1001).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.110499.
Fig. 11. Catalytic stability test of CZFK/ATP-CZO-SC-P400(3) catalyst for CO
2
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