Insight into composition evolution in the synthesis of high-performance Cu/SiO2 catalysts for CO2 hydrogenation

Article abstract of DOI:10.1039/c6ra02929a

High-performance catalysts are the key factor for CO₂ activation and conversion. In this paper, we systematically investigate the whole composition evolution process of Cu/SiO₂ catalysts for CO₂ hydrogenation. The precursor gradually evolved from amorphous Cu species to copper phyllosilicate, then to CuO with the increase of calcination temperature from ambient temperature to 700°C. With the evolution of the precursor, the corresponding species after reduction were gradually evolved from Cu⁰ to Cu⁺, then to Cu⁰ again. The ratios of Cu⁺/(Cu⁺ + Cu⁰) are positively correlated with the catalytic activities. The Cu⁺ species derived from copper phyllosilicate were demonstrated to be crucial in achieving high-performance Cu/SiO₂ catalysts for CO₂ hydrogenation. The composition evolution has been demonstrated to be an efficient strategy to synthesize high-performance catalysts. This strategy of composition evolution can be applied to the synthesis of other high-performance Cu-based heterogeneous catalysts.

Full text of DOI:10.1039/c6ra02929a

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Insight to composition evolution in the synthesis of high-
performance Cu/SiO2 catalysts for CO2 hydrogenation†
Received 00th January 20xx,
Accepted 00th January 20xx
Zhi-Qiao Wang,ab Zhong-Ning Xu,*ab Ming-Jian Zhang,a Qing-Song Chen,a Yumin Chen,a and Guo-
Cong Guo*a
DOI: 10.1039/x0xx00000x
High-performance catalysts are the key factor for CO2 activation and conversion. In this paper, we
systematically investigate the whole composition evolution process of Cu/SiO2 catalysts for CO2
hydrogenation. The precursor gradually evolved from amorphous Cu species to copper phylosilicate,
then to CuO with the increase of calcination temperature from ambient temperature to 700 °C . With
the evolution of the precursor, the corresponding species after reduction were gradually evolved
from Cu0 to Cu+, then to Cu0 again. The ratios of Cu+/(Cu+ + Cu0) are positively correlated with the
catalytic activities. The Cu+ species derived from copper phylosilicate were demonstrated to be the
crucial role to achieve high-performance Cu/SiO2 catalysts for CO2 hydrogenation. The composition
evolution has been demonstrated to be a efficient strategy to synthesize high-performance catalysts.
This strategy of composition evolution can be applied to the synthesis of other high-performance Cu-
based heterogeneous catalysts.
www.rsc.org/
It is well-known that the synthesis processes of
1. Introduction
heterogeneous catalysts contain multi steps, and the
compositions would undergo complicated changes.25-30 The
CO2 is one of the major “greenhouse gases”.1 Recently, CO2
emission along with the urbanization has become one of the
most important issues around the world, because the rapidly
increasing CO2 concentration in the atmosphere leads to global
climate changes (e.g., ocean acidification, global warming and
so on).2-5 Although natural photosynthesis can convert CO2 to
organic carbon, it is far from enough to consume so much CO2
in a short period with its increasing concentration.6 Hence,
finding a new way to achieve resource utilization of CO2 is
quite necessary and urgent.
composition evolution will be a efficient strategy to synthesize
high-performance catalysts.
In this paper, we systematically investigate the whole
composition evolution processes of Cu/SiO2 catalysts for CO2
hydrogenation. The whole synthesis processes were composed
of three steps: ammonia-evaporation, calcination and
reduction. Combining the powder XRD, quasi-in-situ XPS and
XAES, the whole composition evolution processes were
tracked in the three steps. Moreover, according to the catalytic
performance, calcination step was found to be the key step in
the whole synthesis process. In the calcination step, the
precursor gradually evolved from amorphous Cu species, to
copper phylosilicate, then to CuO with the increase of
calcination temperature from ambient temperature to 700 °C ,
which leads to the different ratios of Cu+/(Cu+ + Cu0) in the
final catalysts after reduction. The ratios of Cu+/(Cu+ + Cu0) are
positively correlated with the catalytic activities. The Cu+
species derived from copper phylosilicate were demonstrated
to be the crucial role to achieve high-performance Cu/SiO2
catalysts for CO2 hydrogenation.
Catalytic conversion of CO2 to high value-added chemicals,
such as methanol, formic acid, dimethyl carbonate, cyclic
carbonates and so on, was demonstrated to be an efficient
way to achieve resource utilization of CO2.7-15 CO2
hydrogenation reaction is one of the most important CO2
catalytic conversion reactions.16-21 Cu-based catalysts have
been widely used for CO2 hydrogenation.22-24 However, the
catalytic performance of Cu-based catalysts are still relatively
low. Thus, it is essential to develop high-performance Cu-
based catalysts for CO2 hydrogenation.
a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R.
China.
2. Experimental
2.1 Materials
b Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Chinese
Academy of Sciences, Fuzhou, Fujian 350002, P. R. China.
† E-mail: gcguo@fjirsm.ac.cn, znxu@fjirsm.ac.cn; Fax: +86 591 63173068; Tel: +86
591 63173336
The following materials were used for this experiment:
Cu(NO3)2·3H2O (AR, Aladdin Reagent Co., LTD, China), 28%
ammonia aqueous solution (Sinopharm Chemical Reagent Co.,
Electronic Supplementary Information (ESI) available: selectivity, BET, FT-IR, XPS,
H2-TPR. See DOI: 10.1039/x0xx00000x
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Ltd, China), silica gel (AR, Aladdin Reagent Co., LTD, China), and to a 10% H2/Ar mixture gas flowing at 30 mL/min,VaienwdArtthicelenOntlhinee
ultrapure water (18 MΩ·cm). All chemicals were used as sample was heated up to 300 °C at a ramDpOinIg: 1r0a.1t0e39o/fC160RA°0C2/9m29inA.
received without further purification. Glove box filled with The amount of H2 consumed was monitored by TCD.
high-pure argon was used for protecting the as-synthesized
samples and catalysts.
Transmission electron microscopy (TEM) was performed on
Titan F20 field-emission transmission electron microscopy at
low temperature (105 K) by liquid-nitrogen cooling.
2.2 Catalyst preparation
Quasi-in-situ X-ray photoelectron spectroscopy (XPS) and
X-ray auger electron spectroscopy (XAES) measurements were
performed with a Thermo Fisher ESCALAB 250Xi spectrometer
equipped with an Al anode (Al-Kα = 1486.6 eV). For the
Cu/SiO2 precursors, the measurements were performed
directly. For the reduced Cu-Si-T catalysts, a quasi-in-situ
reduction reactor installed in the Thermo Fisher ESCALAB
250Xi spectrometer chamber was used to reduce the Cu/SiO2
precursors. After reduction, the reduced Cu-Si-T catalysts were
directly transferred to analytical chamber to avoid exposure to
air. All samples were pressed into wafer for measurement.
Four Cu/SiO2 catalysts treated with different prepared
conditions were synthesized by ammonia-evaporation
method,31,32 and described as follows. 7.5 g of Cu(NO3)2·3H2O
was dissolved in 100 ml of ultrapure water, and 28% ammonia
aqueous solution was then dropped to form a copper
ammonia complex solution with pH value of 10–11. 15 g of
silica gel was subsequently added into the copper ammonia
complex solution and stirred for about 10 h. Then, the
suspension was heated at 70 °C to evaporate ammonia. The
evaporation process was not stopped until the pH value of the
suspension decreased to 6–7. The precipitate was washed with
deionized water and dried at 100 °C for 8 h. Then, the catalyst
2.4 Activity evaluation
precursors were calcined in air at 300 °C, 500 °C, and 700 °C The catalytic performance measurements of the reduced Cu-
for 5 h, respectively, and denoted as Cu-Si-O-T, where T Si-T catalysts for CO2 hydrogenation were carried out in a
represents the calcination temperature. The calcined continuous-flow, high-pressure and fixed-bed reactor. Before
precursors were reduced at 300 °C for 2 h under pure H2 reaction, the calcined Cu-Si-O-T precursors were reduced
atmosphere and the reduced Cu/SiO2 catalysts were denoted under pure H2 at a flow rate of 20 mL/min under atmospheric
as Cu-Si-T, where T represents the calcination temperature. pressure. The reduction temperature was programmed to
Moreover, the precursor without calcination was as
a
increase from room temperature to 300 °C and maintained at
reference, and denoted as Cu-Si-O-No calcination (Cu-Si-O-No 300 °C for 2 h. The reactor was then cooled to room
Cal.) After reduction at 300 °C under pure H2 atmosphere, the temperature. After reduction, the catalytic reaction was
corresponding reduced catalyst was denoted as Cu-Si-No carried out under the conditions of 3.0 MPa, V(CO2) : V(H2) = 1 : 4,
calcination (Cu-Si-No Cal.).
5% N2 as internal standard gas, and weight hour space velocity
of CO2 (WHSV) = 16 L·gcat-1·h -1. The composition of the raw
gases and products was monitored using an on-line Shimadzu
2.3 Catalyst characterization
GC-2014 gas chromatograph equipped with
conductivity detector and a flame ionization detector.
The conversion of CO2, the selectivity to CO and CH3OH,
and the weight-time yield (WTY) of CH3OH were calculated
using the following formulas:
a thermal
The Cu loading was analyzed by inductively coupled plasma
(ICP), which was carried out on Ultima2 plasma emission
spectrometer from Jobin Yvon.
The BET surface area was determined by N2 adsorption-
desorption at liquid nitrogen temperature (77 K), which
performed on Micromeritics ASAP 2020. The specific surface
area was calculated from the isotherms using the Brunauere
Emmette Teller (BET) method. Average pore size was
calculated by Barrett-Joyner-Halenda (BJH) equation in
desorption branch.
Conversion of CO2 (%) = ([CO2]in/[N2]in – [CO2]out/[N2]out
)
/([CO2]in/[N2]in) 100%

Selectivity to CO (%) = [(R-FCO

ACO)/(R-FCO2  ACO2)]

[(1 – CCO2)/CCO
]
2
The Fourier translation infrared (FT-IR) spectrum was
measured using
a
Perkin-Elmer Spectrum One FT-IR
spectrophotometer in the range of 1600–400 cm-1. The sample
were finely grounded, dispersed in KBr and pelletized.
Selectivity to CH3OH (%) = (R-FCH3OH
ACH3OH + R-FCH4 ACH4 (1 – SCO

ACH3OH)/(R-FCH3OH


)

)
The powder X-ray diffraction (PXRD) patterns were
collected with a Rigaku Miniflex 600 diffractometer at 40 kV
and 15 mA for Cu-Kα radiation (λ = 1.5406 Å ) with a scan
speed of 1 °/min in the 2θ range of 15–75 °.
Temperature-programmed reduction (TPR) was carried out
on Altamira Instruments (AMI-300) at atmospheric pressure.
Thermal conductivity detector (TCD) current was 75 mA, and
the detector temperature was 100 °C. The calcined Cu-Si-O-T
precursor was outgassed at 120 °C under Ar for 30 min. After
cooling to room temperature under Ar, the gas was switched
Selectivity to CH4 (%) = 1 – SCO – SCH3OH
Where [N2]in and [N2]out are the concentrations of N2 at the
inlet and outlet, [CO2]in and [CO2]out are the concentrations of
the CO2 at the inlet and outlet, respectively. A: the
chromatographic peak area; R-F: the relative correction factor;
SCO: selectivity to CO; SCH3OH: selectivity to CH3OH.
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Table 1 Physicochemical properties of the reduced Cu-Si-T catalysts
View Article Online
DOI: 10.1039/C6RA02929A
Catalysts
Cu loading
(%)
BET surface
Average pore
size (nm)
5.2
area (m2·g -1)
421.2
Cu-Si-No Cal.
Cu-Si-300
Cu-Si-500
Cu-Si-700
11.1
11.0
10.9
11.1
428.4
432.0
397.7
5.0
4.9
5.0
surface area plots (Fig. S3) of the reduced Cu-Si-T catalysts
were determined by N2 adsorption-desorption isotherms. And
corresponding average pore sizeswere calculated by BJH
equation in desorption branch.33 In Table 1, the BET surface
area of the reduced Cu-Si-T catalysts firstly increases from
421.2 to 432.0 m2·g -1, and then decreases to 397.7 m2·g -1 along
with the increasing of calcination temperature from ambient
temperature to 700 °C. The BET surface area of the Cu-Si-500
catalyst is higher than those of other reduced Cu-Si-T catalysts,
and the average pore size of the Cu-Si-500 catalyst is smaller
than those of other reduced Cu-Si-T catalysts, indicating that
calcination temperature affects the BET surface area and
average pore size, and the suitable calcination temperature
(500 °C) is of benefit to obtain catalyst with higher BET surface
area and smaller average pore size.
TEM images and the corresponding particle size
distribution diagrams of the reduced Cu-Si-T catalysts are
displayed in Fig. 2, respectively. The Cu nanoparticles are well
dispersed on the surface of SiO2 support for the four reduced
Cu-Si-T catalysts. And the average size of Cu nanoparticles for
the four reduced Cu-Si-T catalysts is close to each other (2–3
nm), which can exclude that the effect of particle size on the
difference of catalytic activities of the reduced Cu-Si-T
catalysts for CO2 hydrogenation.
Fig. 1 Conversions of CO2 over the reduced Cu-Si-T catalysts. Reaction condition:
。
0.3 g catalyst, 3.0 MPa and WHSV of 16 L·gcat -1·h -1.
3. Results and discussion
3.1 Catalytic activity
The performance of CO2 hydrogenation reaction over series of
the reduced Cu-Si-T catalysts with different calcination
condition were performed at different reaction temperature at
3.0 MPa and 16 L·gcat.-1·h -1. Products of CO2 hydrogenation
reaction over the reduced Cu-Si-T catalysts include CO, CH3OH
and CH4. The corresponding selectivities to CO, CH3OH and CH4
are presented in Fig. S1 (Details shown in ESI). The conversions
of CO2 over the reduced Cu-Si-T catalysts with different
reaction temperatures are shown in Fig. 1. When the reaction
temperature at 320 °C , the conversion of CO2 over the Cu-Si-
No Cal. catalyst is 12.6%. Along with the increasing of
calcination temperature from 300 to 500 °C, the conversions of
CO2 raise from 19.3% to 24.9%. When the calcination
temperature is further improved to 700 °C, the conversion of
CO2 over the reduced Cu-Si-700 catalyst obviously decreases,
suggesting that the conversion of CO2 over the reduced Cu-Si-
500 catalyst reaches the maximum value. Reaction
temperature at 280, 300 and 340 °C , there are similar trends.
The conversions of CO2 firstly climb up and then decline with
the increasing of calcination temperature. The trend of
catalytic activities is volcanic type at each reaction
temperature, which suggests that calcination temperature
would greatly affect the catalytic activities of Cu/SiO2 catalysts
synthesized by ammonia-evaporation method for CO2
hydrogenation. To explore the original effect of composition
evolution on the catalytic activity, a series of characterization
experiments were firstly performed on the reduced Cu-Si-T
catalysts.
3.2 Characterizations of the reduced Cu-Si-T catalysts
The physicochemical properties of the reduced Cu-Si-T
catalysts are shown in Table 1, respectively. Cu loadings
determined by ICP for the reduced catalysts are close to (11.0
± 0.1) wt%, which eliminates the influence of Cu loading on the
catalytic activity. N2 adsorption-desorption isotherms of the
reduced Cu-Si-T catalysts are presented in Fig. S2. The BET
Fig. 2 TEM images of the reduced Cu-Si-T catalysts: (a) Cu-Si-No Cal., (b) Cu-Si-
300, (c) Cu-Si-500, (d) Cu-Si-700. The insets show the corresponding size
distribution diagrams.
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Fig. 3 PXRD patterns of the reduced Cu-Si-T catalysts: (a) Cu-Si-No Cal., (b) Cu-Si-
300, (c) Cu-Si-500, (d) Cu-Si-700.;
Fig. 4 Quasi-in-situ Cu LMM XAES spectra of the reduced Cu-Si-T catalysts: (a) Cu-
Si-No Cal., (b) Cu-Si-300, (c) Cu-Si-500, (d) Cu-Si-700.
The PXRD patterns (Fig. 3a and 3d) of the reduced Cu-Si-No amorphous form in the Cu-Si-O-No Cal. precursor. In Fig. 5b, 5c,
Cal. and Cu-Si-700 catalysts show obvious Cu phases (2θ = and 5d, the diffraction peaks centered around at 30.8, 35.0,
43.30, 50.44, and 74.10 °, PDF#70-3039). In Fig. 3b, 3c and 3d, 57.5, and 62.4 ° are indexed to the phase of copper
the diffraction peak centered at 37.0 ° is indexed to Cu2O phyllosilicate (PDF#27-0188), indicating that calcination is of
phase (PDF#34-1354). Actually, the Cu and Cu2O phases co- benefit to form copper phyllosilicate. Compared with the
exist in all the reduced Cu-Si-T catalysts. Because of the standard peaks, the actual diffraction peaks of copper
relative less content of Cu or Cu2O in the corresponding phyllosilicate in the calcined Cu-Si-O-T precursors shift slightly
catalysts, their corresponding diffraction peaks are invisible. to higher degrees with the increasing of calcination
Quasi-in-situ XPS and XAES were further carried out to temperature, suggesting that the higher temperature
quantify the ratio of Cu+ and Cu0.
calcination treatment can decrease the cell volume of copper
The calcined Cu-Si-O-T precursors were reduced in a quasi- phyllosilicate due to the loss of crystalliferous water or
in-situ reduction reactor installed in the Thermo Fisher coordinated OH group.35 Furthermore, the phase of CuO (2θ =
ESCALAB 250Xi spectrometer chamber. After reduction, the 35.2, 38.4, 48.4 °) (PDF#44-0706) appeared in the calcined Cu-
reduced Cu-Si-T catalysts were directly transferred the Si-O-700 precursor (Fig. 5d), indicating that partial copper
analytical chamber to avoid exposure to air. The Cu2+ satellite phyllosilicate has been destroyed and converted to CuO after
peaks in the Cu 2p XPS spectra (Fig. S4) of the reduced Cu-Si-T calcination at 700 °C. In addition, FT-IR spectra of the calcined
catalysts are invisible, implying that the Cu2+ species have been and withour calcined Cu-Si-O-T precursors (Fig. S5) were also
completely reduced to Cu+ or Cu0 species. Cu LMM XAES measured. The δOH band at 670 cm-1 and the νSiO shoulder peak
spectra (Fig. 4) of the reduced Cu-Si-T catalysts were further at 1042 cm-1 suggest the presence of copper phyllosilicate in
carried out to distinguish and quantify the ratio of Cu+ and Cu0. the calcined Cu-Si-O-T precursors.34 However, considering the
The ratios of Cu+/(Cu+ + Cu0) in the reduced Cu-Si-T catalysts similar PXRD patterns of the calcined Cu-Si-O-300 and Cu-Si-O-
are 0.55, 0.70, 0.93, 0.74, respectively. The ratio of Cu+/(Cu+ + 500 precursors, why do the corresponding catalytic activities
Cu0) firstly increases and then decreases along with the show apparent difference?
changing of calcination condition.
Based on the above results, it can be concluded that
calcination temperature may obviously affect the catalytic
activities of the reduced Cu-Si-T catalysts by adjusting the
ratios of Cu+/(Cu+ + Cu0). However, how could calcination
temperature control the ratios of Cu+/(Cu+ + Cu0)? The calcined
Cu-Si-O-T precursors were further characterized in details
below.
3.3 Characterizations of the Cu-Si-O-T precursors
Firstly, the compositions of the calcined and withour calcined
Cu-Si-O-T precursors are identified by PXRD patterns (Fig. 5).
The board diffraction peak centered at 22 ° belongs to the
amorphous SiO2 support.34 In Fig. 5a, there is no other obvious
diffraction peak, indicating that the Cu species existed in the
Fig. 5 PXRD patterns of the calcined Cu-Si-O-T precursors: (a) Cu-Si-No Cal., (b)
Cu-Si-O-300, (c) Cu-Si-O-500 and (d) Cu-Si-O-700.
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Fig. 6 Cu 2p XPS spectra of the calcined Cu-Si-O-T precursors: (a) Cu-Si-No Cal., (b)
Cu-Si-O-300, (c) Cu-Si-O-500, (d) Cu-Si-O-700.
The Cu 2p XPS spectra of the calcined Cu-Si-O-T precursors
were further measured to analysis of the valence of copper
species in the calcined Cu-Si-O-T precursors (Fig. 6). The Cu
2p3/2 peaks at 933.0–936.0 eV and the corresponding satellite
peaks suggest that the oxidation state of Cu is +2 in all the
calcined Cu-Si-O-T precursors. After deconvolution processing
of Cu 2p3/2 peaks, it is obvious that binding energies (BE) of Cu
2p3/2 are at 935.9 and 934.5 eV, respectively, indicating that
there are two kinds of Cu2+ species in the calcined Cu-Si-O-T
precursors. And the Cu 2p3/2 peak at 935.9 eV should be
attributed to copper phyllosilicate, while that at 934.5 eV
should be ascribed to CuO or other Cu2+ species.33 Moreover,
based on the results of PXRD in Fig. 5a, Cu2+ species in the Cu-
Si-No Cal. precursor should be all amorphous state. Notably, it
is important to emphasize that the proportion of copper
phyllosilicate increases as the calcination temperature rising
from ambient temperature to 500 °C , while it decreases as the
calcination temperature increasing from 500 to 700 °C,
indicating that 500 °C is quite suitable for forming abundant
copper phyllosilicate, which is responsible for the high
percentage of Cu+ demonstrated by quasi-in-situ Cu LMM XAES
spectra in Fig. 4. The above results of PXRD and Cu 2p XPS
show that the calcined Cu-Si-O-T precursors were composed
with copper phyllosilicate and/or CuO/Cu2+ species. The
reduction temperature of copper phyllosilicate to Cu+ is lower
than that of CuO to Cu0.31 Furthermore, H2-TPR profiles of the
calcined Cu-Si-O-T precursors presented the reduction peaks
of the calcined Cu-Si-O-T precursors firstly shift to lower
temperature before 500 °C calcination and then to higher
temperature after 700 °C calcination (Fig. S6). Therefore, the
amount of copper phyllosilicate in the calcined Cu-Si-O-T
precursors firstly increases and then decreases with the
calcination temperature increasing from ambient temperature
to 700 °C can be proved by the results of PXRD, Cu 2p XPS and
H2-TPR characterizations.
Fig. 7 The relationships between conversion of CO2 and Cu+/(Cu+ + Cu0) over the
reduced Cu-Si-T catalysts at different reaction temperatures: (a) 340 °C , (b)
320 °C , (c) 300 °C and (d) 280 °C.
at different reaction temperature are presented in Fig. 7. It is
obvious that there is
a directly positively proportional
relationship between conversion of CO2 and Cu+/(Cu+ + Cu0),
indicating that the high performance of Cu/SiO2 catalysts is due
to the high ratio of Cu+/(Cu+ + Cu0)
. The proportion of copper
phyllosilicate in the calcined Cu-Si-O-T precursors, which is
determined by calcination temperature, greatly affects the
ratios of Cu+/(Cu+ + Cu0) after reduction. It is clear that
calcination temperature would control the ratios of Cu+/(Cu+ +
Cu0) by adjusting the proportion of copper phyllosilicate in the
calcined Cu-Si-O-T precursors, while the ratios of Cu+/(Cu+ + Cu0)
further determine the catalytic activities of the reduced Cu-Si-
T catalysts for CO2 hydrogenation.
4. Conclusion
In summary, the composition evolution from amorphous Cu
species to copper phyllosilicate and then to CuO driven by the
calcination temperature in the Cu-Si-O-T precursors was
elaborated in details. This composition evolution directly
results in different ratios of Cu+/(Cu+ + Cu0). Moreover, the
positive correlation between the ratios of Cu+/(Cu+ + Cu0) and
the catalytic activities of the reduced Cu-Si-T catalysts was well
established. Therefore, the whole correlation from the
composition evolution in the precursors and catalysts to the
catalytic performance was clearly demonstrated. The Cu+
Based on the results and discussion, the influence of Cu
loading and particle size on the catalytic activities has been
excluded. Remarkably, the relationships between conversion
of CO2 and Cu+/(Cu+ + Cu0) over the reduced Cu-Si-T catalysts
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for the synthesis of other high-performance Cu-based
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Acknowledgements
We gratefully acknowledge financial support from the NSF of
China (91545201, 21403237, 21303202, 21303203), the 973
Program (2013CB933200), and the NSF of Fujian Province
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DOI: 10.1039/C6RA02929A
Graphical Abstract
The Cu+ species derived from copper phylosilicate are the crucial role to achieve
high-performance Cu/SiO2 catalysts for CO2 hydrogenation.