A novel microreaction strategy to fabricate superior hybrid zirconium and zinc oxides for methanol synthesis from CO2

Article abstract of DOI:10.1016/j.apcata.2020.117507

The synergetic interaction of solid solutions plays important roles in the reaction efficiency of CO₂ hydrogenation, which is used to produce methanol. The effects of material mixing, impregnation, traditional coprecipitation, and microreaction synthesis on the structural and catalytic properties of ZnO/t-ZrO₂ hybrid oxides were investigated. Although the impregnation effect caused a certain electronic structural modulation at the Zn/Zr interface, the pore structure blocking effect and low number of oxygen vacancy defects leaded to a low catalytic reaction efficiency. The solid solution structure enhanced the Zn/Zr interfacial interaction and increased the electron binding energy. More importantly, many oxygen vacancy defects were formed, which promoted CO₂ activation. However, the solid solution structure was more affected by the preparation method. The solid solution formed from the microreaction exhibited excellent catalytic activity, thermal stability and regeneration performance due to a more uniform solid solution structure and abundant oxygen vacancy defects. The CO₂ conversion rate, methanol selectivity, and methanol space-time yield (STY) under the conditions of H₂/CO₂ = 3:1, 320 °C, 3 MPa, GHSV = 12,000 ml g^?1 h^?1 reached 9.2 %, 93.1 %, and 0.35 g_MeOH h^?1g_cat^?1, respectively.

Full text of DOI:10.1016/j.apcata.2020.117507

AppliedCatalysisA,General595(2020)117507
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
A novel microreaction strategy to fabricate superior hybrid zirconium and
zinc oxides for methanol synthesis from CO2
Xiuxiu Wanga,b,c,1, Yizhou Wanga,b,c,1, Chunliang Yanga,b,c, Yun Yia,b,c, Xiaodan Wangb,c
,
Fei Liua,b,c,*, Jianxin Caoa,b,c,*, Hongyan Pana,b,c
a School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou, 550025, PR China
b Key Laboratory of Green Chemical and Clean Energy Technology, Guizhou University, Guiyang, Guizhou, 550025, PR China
c Engineering Research Center of Efficient Utilization for Industrial Waste, Guizhou University, Guiyang, Guizhou 550025, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The synergetic interaction of solid solutions plays important roles in the reaction efficiency of CO2 hydro-
genation, which is used to produce methanol. The effects of material mixing, impregnation, traditional copre-
cipitation, and microreaction synthesis on the structural and catalytic properties of ZnO/t-ZrO2 hybrid oxides
were investigated. Although the impregnation effect caused a certain electronic structural modulation at the Zn/
Zr interface, the pore structure blocking effect and low number of oxygen vacancy defects leaded to a low
catalytic reaction efficiency. The solid solution structure enhanced the Zn/Zr interfacial interaction and in-
creased the electron binding energy. More importantly, many oxygen vacancy defects were formed, which
promoted CO2 activation. However, the solid solution structure was more affected by the preparation method.
The solid solution formed from the microreaction exhibited excellent catalytic activity, thermal stability and
regeneration performance due to a more uniform solid solution structure and abundant oxygen vacancy defects.
Microreaction synthesis
ZnO/t-ZrO2 hybrid oxide
Synergetic interaction
Oxygen vacancy defect
CO2 hydrogenation
The CO2 conversion rate, methanol selectivity, and methanol space-time yield (STY) under the conditions of H2/
CO2 = 3:1, 320 °C, 3 MPa, GHSV = 12,000 ml g−1 h−1 reached 9.2 %, 93.1 %, and 0.35 gMeOH h−1gcat
respectively.
,
−1
1. Introduction
°C. As the reaction temperature increases, the CO2 conversion rate in-
creases, but the methanol selectivity and thermal stability of the cata-
lyst decrease because the metallic active components in the catalysts
are prone to sinter and rapidly deactivate. In recent years, with the
successful development of solid solution structures, such as ZnO-ZrO2
and InO-ZrO2 oxides [11,12], thermally stable CO2 conversion has been
achieved in catalytic systems operated above 300 °C. It is generally
believed that CO2 is activated by oxygen vacancies after being adsorbed
by basic sites. H2 undergoes a hydrogen overflow effect due to the
surface charge of the metal atom and migrates to CO2* to form a
HCOO* reactive intermediate. Then, continuous hydrogenation proto-
nates H3CO* to form methanol [13,14]. It was found that the special
interaction significantly impact the catalytic performance of CO2 hy-
drogenation to methanol. The formation of many oxygen vacancy de-
fects in the structure plays a crucial role in the catalytic adsorption
conversion of CO2 and the formation of HCOO* active intermediates
[15–22].
Large amounts of CO2 have been released into the atmosphere,
causing increasing environmental and ecological problems and resource
utilization [1]. Open-bond reduction and constant-valence com-
pounding are the two primary pathways for the chemical conversion of
CO2 to carbon-containing products [2]. The CO2 molecule is relatively
stable, and a relatively high energy barrier must be overcome in order
to activate it. High-energy hydrogen, derived from sustainable energy
resources including solar energy and wind energy [3,4], can be used to
reduce CO2 to methanol, which not only recycles C and reduces CO2
emissions, but also alleviates the dependence of methanol production
on non-renewable resources, such as coal and natural gas.
Many industrial catalysts for methanol synthesis via CO2 conversion
have been developed, such as Cu-based [5–8] (Cu/ZnO/Al2O3, Cu/
ZrO2, Cu/ZnO, Cu/ZnO/ZrO2) and noble metal Pd- and Pt-based cata-
lysts (GaPd2/SiO2, PtCo/CeO2) [9,10]. However, these catalysts are
only suitable for low-temperature reactions in the range of 220−270
Therefore, it is necessary to further explore the influence of
⁎ Corresponding authors at: School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou, 550025, PR China.
E-mail addresses: ce.feiliu@gzu.edu.cn (F. Liu), jxcao@gzu.edu.cn (J. Cao).
1 These authors contributed equally to this work and should be considered co-first authors.
https://doi.org/10.1016/j.apcata.2020.117507
Received 5 January 2020; Received in revised form 1 March 2020; Accepted 2 March 2020
Availableonline02March2020
0926-860X/©2020ElsevierB.V.Allrightsreserved.

X. Wang, et al.
AppliedCatalysisA,General595(2020)117507
interfacial interaction of Zn/Zr on the hydrogenation of CO2 to me-
thanol, and establish structure-activity relationships of complex oxides.
In this work, ZnO/t-ZrO2 composite oxides were prepared by a micro-
reaction synthesis method in a membrane separation microreactor.
Such a design takes advantage of the unique high mixing efficiency and
high mass and heat transfer characteristics of microreaction synthetic
technology. The effects of compound mixing, impregnation, traditional
coprecipitation, and microreaction synthesis on the structural and cat-
alytic properties of composite oxides were investigated and compared.
The determination of the most synergetic interaction for CO2 hydro-
genation to methanol are expected to improve the catalytic reaction
efficiency of the process and enable the industrialization of the reaction
process.
precipitating agent was added dropwise to the zinc-zirconium mixed
solution at a rate of 3 mL/min−1, and the reaction was carried out with
magnetic stirring at 80 °C for 2 h. The product precursor was aged at 80
°C for 2 h, and cooled to room temperature. It was successively washed
three times with water and ethanol, dried at 80 °C for 10 h, and calcined
at 500 °C for 3 h to obtain the zinc-zirconium hybrid oxide, which was
labelled ZnO/t-ZrO2-CP.
2.1.3. Incipient wetness impregnation
Zinc-zirconium hybrid oxide was prepared by an incipient wetness
impregnation method. A Zn(NO3)2·6H2O solution with concentration of
0.6 mol/L was prepared using a 1:6 M ratio of Zn/Zr. Equal volumes of
the prepared Zn(NO3)2·6H2O solution and 200 mesh t-ZrO2 with water
absorption of 0.5 g/mL were impregnated at room temperature for 12
h, dried at 80 °C for 10 h, and calcined at 500 °C for 3 h to obtain the
zinc-zirconium hybrid oxide, which was labelled ZnO/t-ZrO2-IM.
2. Experimental
2.1. Methods
2.1.4. Physical blending method
2.1.1. Microreaction synthesis
Using a 1:6 M ratio of Zn/Zr, 0.1 g of ZnO and 0.9 g of t-ZrO2 were
weighed and thoroughly mixed to obtain a physically blended sample,
which was labelled ZnO/t-ZrO2-PB.
Zinc-zirconium hybrid oxide was prepared by a microreaction
synthesis method. A microreactor device was used, and the technical
route is shown in Fig. 1. The dimensions of the microreactor were 6 cm
× 5.5 cm × 3 cm, and the diameter of microsieve tunnel was 0.22 nm.
The microreactor consisted of two stainless-steel plates and a stainless-
steel microfiltration membrane. The microreactor was divided into a
dispersed phase channel and a continuous phase channel by the mi-
crosieve. Zn(NO3)2·6H2O (0.73 g) and Zr(NO3)4·5H2O (6.34 g) in a Zn/
Zr molar ratio of 1:6 were dissolved in deionized (DI) water to prepare a
0.3 mol/L mixture as the dispersed phase. (NH4)2CO3 (3.07 g) was
dissolved in DI water to make a 0.1 mol/L solution as the continuous
phase. The microreaction synthesis was carried out at 80 °C, and the
above-mentioned dispersed phase and continuous phase were injected
into the microreactor by a parallel flow pump at a flow rate of 20 mL/
min. The precursor of the reaction product was aged at 80 °C for 2 h,
cooled to room temperature, successively washed three times with DI
water and ethanol, dried at 80 °C for 10 h, and then calcined at 500 °C
for 3 h to obtain the zinc-zirconium hybrid oxide, which was labelled as
ZnO/t- ZrO2-MR.
2.2. Catalyst characterization
The crystal phase composition and lattice structure of the samples
were analyzed using an X’Pert Powder X-ray polycrystal diffractometer
(PANalytical, Netherlands) with CuKα as the radiation source, a tube
voltage of 40 kV, a tube current of 40 mA, a scanning range of 2θ = 5 -
90°, a scanning speed of 5°·min−1, and a step size of 0.02°. The valence,
binding energy, surface element content, and oxygen vacancies of
samples were analyzed by a K-Alpha Plus X-ray photoelectron spec-
trometer (Thermo Fisher, USA), with Al-Kα radiation (1486.6 eV) and a
power of 150 W. The sample’s element content was analyzed by
Malvern Panalytical Company’s Zetium multi-function X-ray fluores-
cence spectrometer with a power of 4 kW. The microstructure of sam-
ples was analyzed by an IGMA + X-Max 20 scanning electron micro-
scope (ZEISS, Germany) with an accelerating voltage of 30 kV. The
microstructure of the sample was analyzed and characterized by a
FeiTitan 80 200 kV field emission transmission electron microscope
with a test voltage of 200 kV. The surface acidity and alkalinity of
samples were analyzed by an AutoChem II2920 automatic temperature
programmed chemical adsorption instrument (Micromeritics, U.S.A.).
The N2 flow rate was 40 ml·min−1, the filling amount was 80 mg, and
the desorption heating rate was 15 °C min−1. H2-TPR: the redox per-
formance of samples was analyzed and characterized by an AutoChem
II2920 automatic temperature-programmed chemical adsorption in-
strument (Micromeritics, U.S.A.) with the following parameters: Ar
purging at a flow rate of 30 ml/min, heating at a rate of 10 °C/min to
150 °C after preheating for a 30 min, cooling to 50 °C, purging for 60
min with 10 % H2 and 90 % Ar atmosphere, heating to 700 °C at a rate
of 5 °C/min. The TPR signal data was recorded. O2-TPO: nitrogen at-
mosphere was purged at a flow rate of 30 ml/min, and a heating rate of
10 °C/min to 200 °C for a 30 min preheating, cooled to 50 °C in a 2% O2
/ 98 % He atmosphere, purged until the baseline was stable, heated to
800 °C at a rate of 10 °C/min, and then the TPO signal data was re-
corded. The specific surface area and pore structure parameters of
samples were analyzed by an ASAP 2020(M) automatic surface area
micropore analyzer (Micromeritics, U.S.A.). Samples were degassed at
250 °C for 3 h, and the nitrogen adsorption/desorption temperature was
77 K. The specific surface areas of samples were calculated using the
BET equation, and the pore size distributions of samples were calcu-
lated using DFT theory, P/P0 = 0.995. Oxygen vacancies in samples
were also analyzed and characterized using a Shimadzu UV-3600 plus
UV–vis-NIR spectrophotometer. The test wavelength range was 200
nm–800 nm.
To prepare the sole t-ZrO2, the dispersed phase in the microreaction
preparation process was changed to Zr(NO3)4·5H2O solution.
To prepare the sole ZnO, the dispersed phase in the microreaction
preparation process was changed to Zn(NO3)2·6H2O solution.
2.1.2. Traditional liquid phase coprecipitation method
Zinc-zirconium hybrid oxide was prepared by a liquid phase co-
precipitation method. Using a Zn/Zr molar ratio of 1:6, 0.73 g Zn
(NO3)2·6H2O and 6.34 g Zr(NO3)4·5H2O were dissolved in DI water to
prepare a 0.3 mol/L mixed solution. (NH4)2CO3 (3.07 g) was dissolved
in DI water to prepare
a 0.1 mol/L precipitant solution. The
Fig. 1. Experimental microreactor device for preparing ZnO/t-ZrO2-MR nano-
particles.
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X. Wang, et al.
AppliedCatalysisA,General595(2020)117507
2.3. Catalyst evaluation
Fig. 2. It can be seen from Fig. 2a that the mixed sample consisted of
two phases of ZnO and ZrO2. ZnO was a hexagonal wurtzite (corre-
sponding to PDF card number 76-0704), and ZrO2 was a tetragonal
phase (corresponding to PDF card number 79–1769). The two-phase
diffraction peak of the physically mixed sample was consistent with the
standard cards, indicating that the two phases were only physically
mixed, and no special chemical interaction occurred. Compared with
the physically mixed sample, the intensities of the ZnO diffraction peaks
of the impregnated sample were attenuated, indicating that the ZnO
component in the impregnated sample was highly dispersed in the
zirconia. The diffraction peaks generally shifted to lower angles, in-
dicating that there might be an interaction between these two compo-
nents in the impregnated sample. Interestingly, the diffraction peaks of
the ZnO crystal phase were not observed in the samples prepared by the
conventional coprecipitation and microreaction synthesis methods. The
diffraction angles corresponding to the (101) and (110) crystal planes of
ZrO2 crystals shifted to higher angles, indicating that the prepared
samples had unique solid solution structures. This was mainly due to
the Jahn-Teller elongation effect [23]. When ZnO component was dis-
solved into ZrO2, the Zr-O bond was shortened, and the corresponding
interplanar spacing was reduced, resulting in an increase in the dif-
fraction angle. Furthermore, the peak shifts of the (101) and (110)
planes of the ZrO2 crystals in the microreaction samples were more
pronounced than those in the conventional co-precipitated samples.
This indicated to some extent that the ZnO/t-ZrO2-MR samples formed
more solid solution defects. This was also confirmed by the UV–vis
near-infrared absorption spectroscopy results (Fig. S1). The absorption
peaks in the spectra of ZnO/t-ZrO2-MR samples were significantly red
shifted. The red shift was mainly caused by the smaller Zn2+ replacing
the larger Zr4+, which decreased the band gap, further indicating the
existence of many solid solution defects [24,25]. Therefore, the mi-
croreaction synthetic route provided strong mixing efficiency and high
mass transfer with its unique microchannels. Combined with the con-
stant pH reaction system during the process, this provided a relatively
uniform and rapid path for the formation of ZnO component in the
initial precipitated product.
Catalyst performance was evaluated in a pressurized fixed bed re-
actor. Catalyst (1.0 g) was packed in the constant temperature zone of a
fixed-bed stainless steel reaction tube (the upper and lower parts were
filled with quartz wool). After leak detection was performed, reaction
gases CO2 and H2 were fed into the reactor, with V(H2): V(CO2): V (N2)
= 72:24:4, a reaction pressure of 3 MPa, 320 °C, and a gas hourly space
velocity GHSV of 12,000 ml g−1 h−1. CH4 and CO2 in the reaction
product were measured using a GC9560 gas chromatograph equipped
with a TCD detector. The CH4 and CH3OH in the reaction product were
measured by a GC9560 gas chromatograph using an FID detector.
The normalization method was used to calculate the CO2 conver-
sion, methanol space-time yield (STY), selectivity of methanol, carbon
monoxide, methane and dimethyl ether [11]. All data was recorded
after a 3 h reaction time.
X (CO2)
fCO ACO + i(fCH ACH + fCH OH ACH OH + 2fCH OCH ACH OCH
)
4
3
3
3
4
3
3
3
=
fCO ACO + fCO ACO + i(fCH ACH + fCH OH ACH OH + 2fCH OCH ACH OCH
)
2
4
3
3
3
2
4
3
3
3
S (CH3OH)
i * fCH OH ACH OH
3
3
=
fCO ACO + i(fCH ACH + fCH OH ACH OH + 2fCH OCH ACH OCH
)
4
3
3
3
4
3
3
3
i * fCO ACO
S (CO) =
fCO ACO + i(fCH ACH + fCH OH ACH OH + 2fCH OCH ACH OCH
)
4
3
3
3
4
3
3
3
i * fCH ACH
4
4
S (CH4) =
fCO ACO + i(fCH ACH + fCH OH ACH OH + 2fCH OCH ACH OCH
)
4
3
3
3
4
3
3
3
S CH3OCH3
i * 2fCH OCH ACH OCH
3
3
3
3
=
fCO ACO + i(fCH ACH + fCH OH ACH OH + 2fCH OCH ACH OCH
)
4
3
3
3
4
3
3
3
Fig. 2b shows the lattice stress curves of different samples calculated
by the Williamson-Hall equation [26,27]. The slope of the curve re-
presents the strength of the interaction between the zinc and zirconium
components. The figure shows that the slope of the curve formed by the
impregnated ZnO/t-ZrO2-IM sample increased compared with the
mixed ZnO/t-ZrO2-PB sample, although a larger slope was observed in
the solid solution structure sample. Compared to the traditional co-
precipitation sample ZnO/t-ZrO2-CP, the slope of the curve of ZnO/t-
ZrO2-MR prepared by microreaction synthesis was the largest, in-
dicating that the zinc-zirconium composition of the sample had the
strongest interaction and many defects. This can also be further con-
firmed by X-ray photoelectron spectroscopy of different catalysts
fCH TCD ACH
fCH FID ACH
TCD
FID
4
4
i =
4
4
GHSV
22.4
STY =
× V%(CO2) × X (CO2) × S CH3OH × M CH3OH
2.4. Hybrid oxide structure properties
The XRD patterns and corresponding lattice stress curves of zinc-
zirconium hybrid oxides prepared by different methods are shown in
Fig. 2. Crystalline structure of ZnO/t-ZrO2-MR, ZnO/t-ZrO2-CP, ZnO/t-ZrO2-IM and ZnO/t-ZrO2-PB catalysts. (a) XRD patterns of various catalysts. (b) Lattice stress
curves of various catalysts.
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X. Wang, et al.
AppliedCatalysisA,General595(2020)117507
Fig. 3. XPS spectra of ZnO/t-ZrO2-MR, ZnO/t-ZrO2-CP, ZnO/t-ZrO2-IM and ZnO/t-ZrO2-PB catalysts.
(Fig. 3). The binding energies of the solid solution samples prepared by
the microreaction synthesis and traditional coprecipitation methods
were larger than those of the samples prepared by the impregnation
method. This indicates that Zn2+ in the solid solution more strongly
interacted with Zr4+. In the solid solution samples, further comparison
revealed a larger Zn/Zr binding energy in the ZnO/t-ZrO2-MR sample.
The outermost electrons of Zr shifted toward Zn during the complex
impregnation and solid solution in the Zn/Zr composite, which de-
creased the density of the outermost electron cloud of Zr and increased
its electron binding energy. However, ZnO component can undergo
isomorphous substitution in the ZrO2 precursor during solid solution
processing, and the interaction of the two-phase elements was further
enhanced. Comparing the Zn/Zr ratio from the XPS surface and XRF
bulk phase spectra of different catalysts (Table S1), the increase in the
electron binding energy of the solid solution prepared by microreaction
synthesis was mainly attributed to better solid solution dispersibility of
ZnO component. The transmission electron microscopy (TEM) images
and scanning element distribution by scanning electron microscopy
(SEM) test of traditional coprecipitation and microreaction synthesis
samples are shown in Fig. 4. From the TEM images and particle size
distributions, ZnO/t-ZrO2-MR had smaller grain size and more cen-
tralized distribution compared with ZnO/t-ZrO2-CP. The high-resolu-
tion lattice fringe TEM patterns and SEM element distribution of the
samples further indicated that the ZnO component of ZnO/t-ZrO2-MR
presented a better dispersibility, and the solid solution structure of the
sample was more uniform.
desorption peak from 300−780 °C was attributed to β-oxygen, which
was mainly lattice oxygen [29]. Compared with ZnO/t-ZrO2-CP, the β-
oxygen desorption peak of ZnO/t-ZrO2-MR shifted to a lower tem-
perature and the desorption was larger, indicating that the solid solu-
tion had a better desorption capacity and more oxygen vacancies. The
H2-TPR analysis of different samples (Fig. S2) showed that the reduc-
tion temperature of ZnO/t-ZrO2-MR shifted to lower temperatures,
which further confirmed that the solid solution had a strong ability to
generate, migrate, and transform active oxygen.
Different preparation methods greatly influenced the interaction of
the Zn/Zr composite interface. The physically-mixed ZnO/t-ZrO2-PB
was only a simple physical mixture, and no special chemical interaction
existed. The impregnated sample ZnO/t-ZrO2-IM prepared by Zn im-
pregnation on t-ZrO2 as the carrier formed a particular interaction,
which caused a change in the electronic structure properties of the
sample. A larger electronic structure modulation was observed in the
solid solution sample, and the special synergetic interaction formed
changed the Zn/Zr electron binding energy, and generated oxygen va-
cancy defects. Compared with ZnO/t-ZrO2-CP, ZnO/t-ZrO2-MR had a
more uniform ZnO component distribution, which resulted in the lar-
gest structural property modulation range and the most oxygen vacancy
defects formed. This can also be confirmed from the CO2 adsorption
characteristics data of different solid solution samples, as shown in
Fig. 7. It is not difficult to see that ZnO/t-ZrO2-MR showed a larger CO2
adsorption capacity and exhibited a potential CO2 activation ability.
Although all composite samples prepared by different preparation
methods were mesoporous, ZnO/t-ZrO2-MR had the largest specific
surface area of 38.65 m2/g and a pore volume of 0.084 m3/g (Fig. S3
and Table S2). This was related to the excellent mixing efficiency and
mass and heat transfer characteristics of the microreaction process. The
constant pH during the reaction promoted the effective collision of
colloidal particles and easily controlled nucleation. Localized mixing
tended to exist in the traditional coprecipitation method, and the un-
stable pH of the reaction system during the process caused particle
agglomeration, which was not conducive to nucleation control.
The O1s XPS spectra of different samples are shown in Fig. 5. The
peak at a binding energy of 529−530 eV was attributed to lattice
oxygen (OLatt), and the binding energy at 530−532 eV was attributed
to defective oxide or surface oxygen (OSur) [28]. Combined with the
OSur /(OLatt + OSur) data in Table 1, OSur /(OLatt + OSur) in ZnO/t-ZrO2-
MR was 49 %, which was significantly higher than other samples, in-
dicating that the sample had more oxygen vacancy defects. O2-TPO
analysis of different solid solution samples are shown in Fig. 6. The
desorption peak from 50−200 °C was attributed to α-oxygen, which
was mainly oxygen adsorbed on surface oxygen vacancies. The
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X. Wang, et al.
AppliedCatalysisA,General595(2020)117507
Fig. 4. TEM and scanning element distribution (SEM) images of ZnO/t-ZrO2 binary. (a, e) TEM images of ZnO/t-ZrO2-CP and ZnO/t-ZrO2-MR. (c, g) HRTEM images
of ZnO/t-ZrO2-CP and ZnO/t-ZrO2-MR. (b, f) Particle size distribution of ZnO/t-ZrO2-CP and ZnO/t-ZrO2-MR. (d, h) Scanning element distribution of ZnO/t-ZrO2-CP
and ZnO/t-ZrO2-MR.
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X. Wang, et al.
AppliedCatalysisA,General595(2020)117507
Fig. 5. XPS spectra of O1s in various catalysts.
Table 1
XPS results of ZnO/t-ZrO2-MR, ZnO/t-ZrO2-CP, ZnO/t-ZrO2-IM and ZnO/t-
ZrO2-PB catalysts.
Catalyst
Binding energy (eV)
OSur /(OLatt + OSur) (100%)
OLatt
OSur
ZnO/t-ZrO2-MR
ZnO/t-ZrO2-CP
ZnO/t-ZrO2-IM
ZnO/t-ZrO2-PB
529.54
529.56
529.32
529.64
531.03
531.07
531.29
531.27
49
43
39
37
Fig. 7. CO2-TPD profiles of various catalysts Fresh ZnO/t-ZrO2-MR and ZnO/t-
ZrO2-CP.
3. Catalytic performance
In order to understand the relationship between composite sample
structural properties and the CO2 hydrogenation to methanol, the CO2
conversion rate, methanol selectivity, and methanol space-time yield
(STY) of different ZnO/t-ZrO2 hybrid samples were investigated under
the conditions of H2/CO2 = 3:1, 320 °C, 3 MPa, GHSV = 12,000 ml
g−1 h−1, and the results are shown in Fig.8. Carbon monoxide and
dimethyl ether were examined as the main by-products apart from
methane (Table S3). The figure clearly shows that ZnO/t-ZrO2-IM had
the worst catalytic performance, with the lowest CO2 conversion rate,
methanol selectivity, and methanol STY. From the above analysis,
Fig. 6. O2-TPO profiles of ZnO/t-ZrO2-MR and ZnO/t-ZrO2-CP.
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X. Wang, et al.
AppliedCatalysisA,General595(2020)117507
Fig. 8. Catalytic performance of ZnO/t-ZrO2-MR, ZnO/t-ZrO2-CP, ZnO/t-ZrO2-IM and ZnO/t-ZrO2-PB catalysts.
compared to the physically mixed sample, the Zn/Zr interfacial inter-
action formed by the impregnation of ZnO component on the surface of
t-ZrO2 changed the electronic properties of the sample. However, it can
be seen from the catalytic results that these structural properties did not
favor the hydrogenation of CO2 to methanol. Another major reason for
the low catalytic performance was that the impregnation process
tended to partially block the pores in the samples (Fig. S3 and Table
S2). In contrast, the Zn/t-ZrO2 solid solution produced a new synergetic
interaction by isomorphous substitution. The presence of this special
solid solution caused the crystal phase diffraction peaks to shift to a
higher angle and the diffraction plane spacing of the solid solution
structure to decrease. In the impregnated sample, the diffraction peaks
of the crystal phase shifted toward a lower angle, and the spacing of
some diffraction crystal planes was increased. At the same time, the
solid solution structure resulted in many oxygen vacancy defects, which
promotes CO2 activation according to some reports [30–32]. The
modulation of different structural properties produced by the impreg-
nation and solid solution methods led to two extreme structural changes
and catalytic results. Compared with the physically mixed samples, the
solid solution structure exhibited a high catalytic reaction efficiency,
while the impregnated sample exhibited a low catalytic performance.
More importantly, in the prepared solid solution samples, ZnO/t-ZrO2-
MR showed better catalytic results, and achieved a CO2 conversion,
methanol selectivity, and methanol STY of 9.2 %, 93.1 %, and 0.35
gMeOH h−1 gcat−1, respectively.
Table 2
XPS results of the spent ZnO/t-ZrO2-MR and ZnO/t-ZrO2-CP catalysts.
Catalyst
Binding energy (eV)
OSur /(OLatt + OSur
)
(100%)
OLatt
OSur
Spent ZnO/t-ZrO2-MR-320
529.71
529.80
529.78
529.81
531. 11
531.49
531. 35
531. 55
44
35
42
33
℃
Spent ZnO/t-ZrO2-CP-320
℃
Spent ZnO/t-ZrO2-MR-400
℃
Spent ZnO/t-ZrO2-CP-400
℃
spent samples after 30 h on stream not only at 320 °C but at higher
temperature of 400 °C also. All of these results indicated that the spent
ZnO/t-ZrO2-MR still had more oxygen vacancy defects.
To test the regenerative performance of catalysts, we annealed the
spent ZnO/t-ZrO2-MR at 500 °C in air for 3 h to subtract any potential
surface species. The regenerated catalyst is examined for the reaction at
320 °C and 400 °C respectively. Fig. 10 shows no noticeable decrease of
catalytic performance after regeneration, which could be attributed to
the retrieve of oxygen vacancy and little change in crystalline structure
from the XPS shown in Fig. 11 and Table 3, and CO2-TPD and XRD
results shown in Fig. S7-S8.
From the above analysis, the excellent catalytic performance of this
sample was closely related to its more uniform solid solution structure
and more oxygen vacancy defects. It can thus be concluded that the
large number of oxygen vacancy defects generated by the special sy-
nergetic interaction that formed in the solid solution was the primary
reason for the significant improvement in the catalytic performance of
such samples. However, when comparing the solid solution preparation
methods, the microreaction synthesis method has incomparable tech-
nical advantages over the traditional coprecipitation method. The solid
solution structure in the sample prepared by microreaction synthesis
method was more uniform, resulting in more abundant oxygen vacancy
defects and a superior catalytic performance for the hydrogenation of
CO2 to methanol.
4. Conclusions
Different preparation methods significantly affected the phase
structure properties of Zn/Zr hybrid interfaces and the catalytic hy-
drogenation of CO2 to methanol. The physical mixing method produced
only two phases without the formation of any chemical interaction. The
sample prepared by impregnation had a different electronic structure at
the Zn/Zr interface due to the impregnation of ZnO component on the
surface of mesoporous t-ZrO2. However, ZnO component impregnation
tended to block pores, and its low number of oxygen vacancy defects
caused the impregnated sample to have a low catalytic efficiency. The
solid solution effect increased the binding energy of the Zn/Zr interface
and enhanced the interfacial interaction. The formed solid solution also
produced many oxygen vacancy defects and exhibited excellent cata-
lytic performance during CO2 activation. More importantly, the mi-
croreaction synthesis method had incomparable technical advantages
during solid solution preparation. Its unique microchannels provided an
excellent mixing efficiency and high mass and heat transfers. Combined
with the constant pH value in the reaction system, it provided a rela-
tively more uniform and rapid reaction path for ZnO component.
Therefore, a more uniform solid solution structure and more abundant
oxygen vacancy defects were obtained, which played crucial roles in the
high reaction efficiency of methanol synthesis by CO2 hydrogenation.
The catalyst obtained via microreaction synthesis exhibited excellent
thermal stability and regeneration performance.
It was worth mentioning that the ZnO/t-ZrO2-MR exhibited ex-
cellent thermal stability as well from the further experimental analyses
of spent samples. XRD, XPS, CO2-TPD and H2-TPR results of samples
after 30 h on stream at 320 °C and 400 °C are shown in Fig. S6-S9 and
Table 2. Compared with spent ZnO/t-ZrO2-CP, it is obvious from XRD
analysis that a larger offset degree and slope (Fig. S4) were observed in
the spent ZnO/t-ZrO2-MR. The O1s XPS spectra of spent samples are
shown in Fig. 9. Combined with the OSur /(OLatt + OSur)data in Table 2,
OSur /(OLatt + OSur) in ZnO/t-ZrO2-MR was 44 %, which was slightly
lower than fresh catalyst (49 %) but significantly higher than spent
ZnO/t-ZrO2-CP sample (35 %). Similar results could be obtained from
CO2-TPD (Fig. S5) and H2-TPR (Fig. S6) analyses. A larger CO2 ad-
sorption capacity and lower reduction temperatures tended to exist in
spent ZnO/t-ZrO2-MR. Interestingly, the same tendency occurred over
7

X. Wang, et al.
AppliedCatalysisA,General595(2020)117507
Fig. 9. XPS spectra of O1s of the (a) spent ZnO/t-ZrO2-MR, (b) spent ZnO/t-ZrO2-CP catalysts.
Table 3
XPS results of the Regenerated ZnO/t-ZrO2-MR-320 ℃ and ZnO/t-ZrO2- MR-
400 ℃ catalysts.
Catalyst
Binding energy (eV)
OSur /(OLatt + OSur
)
(100%)
OLatt
OSur
Regenerated ZnO/t-ZrO2-MR-
320 ℃
529.84
529.92
531. 18
531. 48
48
47
Regenerated ZnO/t-ZrO2-MR
-400 ℃
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Fig. 10. Catalytic regenerative performance of ZnO/t-ZrO2-MR.
Fig. 11. XPS spectra of O1s of the (a)Regenerated ZnO/t-ZrO2-MR-320 ℃ and (b) Regenerative ZnO/t-ZrO2-MR-400 ℃ catalysts.
8

X. Wang, et al.
AppliedCatalysisA,General595(2020)117507
Acknowledgments
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This work was supported by National Natural Science Foundation of
China (NO. 21666007), Scientific and Technological Innovation Talents
Team Project of Guizhou Province (NO. 20185607), One Hundred
Person Project of Guizhou Province (NO. 20165655), Science and
Technology Project of Guizhou Province (NO. 20175788 and
20185781), Natural Science Foundation of Guizhou Province (NO.
20177260) and Science & Technology Support Plan Project of Guizhou
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