Cylindrical shaped ZnO combined Cu catalysts for the hydrogenation of CO2 to methanol

Article abstract of DOI:10.1039/c9ra00658c

Hydrogenation of CO₂ to chemicals is of great importance in the reduction of greenhouse gas emission. And the interaction and/or the boundary between Cu and ZnO played a crucial role in the performance of the Cu-ZnO catalyst for CO₂ hydrogenation to methanol. In this work, cylindrical shaped ZnO was first synthesized via controlled hydrothermal precipitation of Zn(CO₂CH₃)₂·2H₂O, and Cu was further deposited on ZnO via in situ reduction in aqueous solution. Characterizations indicated that the crystallization degree of ZnO decreased with the increasing content of Cu, while the exposed surface area of Cu exhibited a volcano shaped curve. It was found that the cylindrical shaped ZnO combined Cu catalysts were active for the hydrogenation of CO₂, and the space time yield of methanol reached 0.50 g_-MeOH (g_-cat h)^-1 at H₂/CO₂ = 3, 240 °C, 3.0 MPa, and 0.54 mol (g_-cat h)^-1, but the methanol selectivity decreases with the reduction of the (002) polar plane of ZnO. The conversion of CO₂ and methanol selectivity were discussed with the detected exposed Cu surface area and the number of oxygen vacancies.

Full text of DOI:10.1039/c9ra00658c

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Cylindrical shaped ZnO combined Cu catalysts for
the hydrogenation of CO2 to methanol
Cite this: RSC Adv., 2019, 9, 13696
Hong Lei,
and Xiaoliang Feng
Ruheng Zheng, Yeping Liu, Jiacheng Gao, Xiang Chen
*
Hydrogenation of CO2 to chemicals is of great importance in the reduction of greenhouse gas emission.
And the interaction and/or the boundary between Cu and ZnO played a crucial role in the performance
of the Cu–ZnO catalyst for CO2 hydrogenation to methanol. In this work, cylindrical shaped ZnO was
first synthesized via controlled hydrothermal precipitation of Zn(CO2CH3)2$2H2O, and Cu was further
deposited on ZnO via in situ reduction in aqueous solution. Characterizations indicated that the
crystallization degree of ZnO decreased with the increasing content of Cu, while the exposed surface
area of Cu exhibited a volcano shaped curve. It was found that the cylindrical shaped ZnO combined Cu
catalysts were active for the hydrogenation of CO2, and the space time yield of methanol reached 0.50
g-MeOH (g-cat h)ꢀ1 at H2/CO2 ¼ 3, 240 ꢁC, 3.0 MPa, and 0.54 mol (g-cat h)ꢀ1, but the methanol selectivity
decreases with the reduction of the (002) polar plane of ZnO. The conversion of CO2 and methanol
selectivity were discussed with the detected exposed Cu surface area and the number of oxygen vacancies.
Received 25th January 2019
Accepted 29th April 2019
DOI: 10.1039/c9ra00658c
rsc.li/rsc-advances
Karelovic et al. found that Cu/ZnO catalysts prepared by
combustion exhibited higher activity than those prepared via
1. Introduction
conventional coprecipitation methods, and the activity related
to the contact between Cu and ZnO.20 Valant et al.21 reported
that the activity of Cu/ZnO for methanol production changed
with the content of Zn showed a volcano-like prole. Similar
studies also conrmed that synergetic effect between Cu and
ZnO could be responsible for the catalytic activity of Cu/ZnO
catalysts in the synthesis of methanol from CO2 hydrogena-
tion.7,9,22–28 Tsang et al.7 disclosed that morphology of ZnO had
a signicant effect on its interaction with Cu in the hydroge-
nation of CO2 to methanol. Plate-like ZnO showed a strong
interaction with Cu, which leaded to high selectivity of meth-
anol. Previous work in our laboratory also found that lament-
like ZnO supported Cu catalyst was highly active for CO2
hydrogenation,11 which might be attributed to that lament-like
ZnO possessed large polar plane, and the interaction between
Cu and ZnO was strong. At the same time, Khanna et al.29
developed a wet chemical reduction method to prepare nano-
sized Cu particles, in which a reduction reaction occured
between Cu2+ and reducing agent. This method is simple, easy
in control and can prepare metallic Cu without further
reduction.
The Paris Agreement, that dealing with greenhouse gas emis-
sion mitigation, was negotiated by representatives of 195
countries at the end of 2015. 194 members had signed and
ratied the treaty in 2016. Among the greenhouse gases, the
concentration of CO2 in the atmosphere increased from
280 ppm in 1750 to 400 ppm in 2015. For this reason, an
approach for efficient reduction of CO2 has attracted the
attention of scientists all over the world.1–3 CO2 recycling is one
of the promising ways that will not only reduce CO2 concen-
tration but also produce some valuable chemicals like meth-
anol, which is a key chemical feedstock and an alternative
fuel.4,5
In published works, it was popularly accepted that Cu/ZnO-
based catalysts were active for the hydrogenation of CO2 to
methanol,6–11 and the activity of Cu-based catalysts depended
mainly on the dispersion of Cu. However, the latest achieve-
ments disclosed that the interfacial area between Cu and ZnO
played a crucial role for the activity and stability of the Cu/ZnO
catalyst in methanol synthesis.12–14 Cu/ZnO catalysts are usually
prepared by co-precipitation using precipitation agents con-
taining sodium,8–11,15–18 while Kondrat et al. pointed out that
residual sodium ions were a potential catalyst poison.19 There-
fore, the development of novel and effective techniques for
preparing Cu/ZnO-based catalysts is of great importance.
In this work, cylindrical shaped ZnO was synthesized via
controlled hydrothermal precipitation of Zn(CO2CH3)2$2H2O,
and Cu was further deposited on ZnO via the in situ reduction of
Cu2+ with L-ascorbic acid in aqueous solution. The physico-
chemical properties of prepared Cu/ZnO were characterized by
X-ray diffraction (XRD), scanning electron microscope (SEM), N2
adsorption, N2O chemisorption techniques, Raman spectra, X-
College of Chemical and Material Engineering, Quzhou University, Quzhou 324000,
China. E-mail: 28826589@qq.com; Fax: +86-570-8015112; Tel: +86-570-8026546
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ray photoelectron spectroscopy (XPS), H2 and CO2 chemisorp- coprecipitation.11 Aer hydrogen reduction at 270 C for 2 h,
tion, H2-TPD, and CO2-TPD techniques. The relationship the catalyst was identied as CA-CC.
between the physiochemical properties of Cu/ZnO and its
ꢁ
performance in hydrogenation of CO2 was discussed.
2.4. Characterization
The composition of prepared catalysts was examined by
inductively coupled plasma-atomic emission spectrometry
(ICP, Plasma-Spec-II spectrometer), and the actual copper
2. Experimental section
2.1. Synthesis of cylindrical shaped ZnO
12.00 g of Zn(CO2CH3)2$2H2O (Aladdin, 99.99%) was dissolved
in deionized water (240 mL), and then 7.32 g of diethylenetri-
amine (Aladdin, 99%) was added under vigorous stirring. Aer
diethylenetriamine was dissolved, the solution was transferred
into aꢁ500 mL polytetrauoroethylene-lined autoclave and kept
at 97 C for 24 h, and then cooled to room temperature. The
precipitate was ltered and washed thoroughly with deionized
content in prepared catalysts was conrmed and summarized
in Table 1.
XRD analysis was carried out on a Rigaku D/MAX-2500
diffractometer with a scanning angle of 15–85ꢁ by Cu Ka
radiation. SEM images of these catalysts were obtained using
Leo Series VP 1430 microscope operated at 10 kV. Aer
samples were pretreated at 250 ꢁC for 4 h in vacuum, N2
adsorption was measured at ꢀ196 ꢁC using an ASAP 2010
analyzer. Surface area was calculated according to the BET
method, which used 0.164 nm2 as the cross-sectional area of
the nitrogen molecule. The metallic copper surface areas
(SCu) were measured using a N2O chemisorption method as
described elsewhere.30 The surface area of copper was
calculated by assuming that the surface density of atomic
copper was 1.46 ꢂ 1019 Cu atoms per m2 and molar stoichi-
ometry of N2O/Cu was 0.5. Raman analysis was performed on
a Jobin Yvon Labram HR800 spectrometer. XPS measurement
was performed using a Mg Ka ray as excitation source (hn ¼
1253.6 eV) on a PerkinElmer PHI 5000C system. Surface
contamination of C 1s peak (284.6 eV) was used as internal
standard calibration binding energy.
ꢁ
water to remove excess precursor and dried at 60 C for 12 h,
ꢁ
and further calcined in air at 450 C for 2 h.
2.2. Synthesis of Cu
20 mmol of Cu(CO2CH3)2$H2O (Alfa Aesar, 99.9%) was dis-
solved in 50 mL of deionized water. The resulting solution was
heated to 70 ꢁC in an oil bath with magnetic stirring. A 60 mL
of L-ascorbic acid (Alfa Aesar, $99%) aqueous solution (1.0 M)
was added dropwise into the ask to reduce Cu2+ under
vigorous stirring. The black precipitate was ltered and
ꢁ
washed with ethanol, dried in vacuum at 60 C for 12 h and
stored in desiccator (in N2).
2.3. Deposition of Cu on ZnO
H2 (or CO2) chemisorption measurements were performed
ꢁ
Cu(CO2CH3)2$H2O aqueous solution was prepared by dissolv-
ing 20 mmol of Cu(CO2CH3)2$H2O in 50 mL of deionized water
and then the as-prepared cylindrical shaped ZnO (10 mmol)
was added. The resulting suspension solution was heated to
70 ꢁC in an oil bath with magnetic stirring. A 60 mL of L-
ascorbic acid aqueous solution (1.0 M) was added dropwise
into the ask to reduce Cu2+ under vigorous stirring. Aer
reaction, the black precipitate was ltered and washed with
at 30 C using a Micromeritics Autochem 2920 II apparatus.
0.30 g of the catalyst was loaded between two quartz wool
balls into a quartz U-shaped tube reactor. The measurements
were carried out by pulses injection (0.551 mL) of 10% H2/Ar
(or 10% CO2/He) until saturation. Chemisorbed and phys-
isorbed H2 (or CO2) consumption was monitored by
a thermal conductivity detector (TCD), which was denoted
Atotal. The sample was then degassed with Ar (or He) for
10 min to evacuate physisorbed H2 (or CO2). Physisorbed H2
(or CO2) consumption was measured by another series of H2
(or CO2) pulses under the same conditions, which was
denoted Aphys. The chemisorbed H2 (or CO2) was calculated
as follows:
ꢁ
ethanol. The solid was dried in vacuum at 60 C for 12 h and
stored in desiccator (in N2). The prepared catalyst was desig-
nated as CZ-2. CZ-1, CZ-3 and CZ-4 were also prepared in the
same procedure (L-ascorbic acid/Cu2+ ¼ 3, molar ratio) with
different copper acetate amount, in which 1, 3 and 4 mean Cu/
Zn molar ratio. For comparison, the Cu/ZnO catalyst (with
a Cu/Zn atomic ratio of 2) was prepared by carbonate
Achem ¼ Atotal ꢀ Aphys
Table 1 Physicochemical properties of the ZnO, Cu and CZ catalysts
Cu/(Cu + Zn)a (mol%)
SBET (m2 g-cat
)
dCu (nm)
SCub (m2 g-cat
)
ꢀ1
ꢀ1
Catalysts
ZnO
CZ-1
CZ-2
CZ-3
CZ-4
CZ-CC
Cu
0
7.8
23.7
35.5
26.3
21.2
38.8
3.3
—
9.6
7.3
13.2
14.1
10.5
67.1
—
49.2
66.3
74.4
79.6
66.9
100.0
8.2
12.3
9.6
7.4
10.9
3.1
a
b
Determined by ICP method. Determined by N2O chemisorption method.
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where the unit of A is mol gꢀ1
.
1. It can be found that the crystallite size of Cu was 67.1, 9.6, 7.3,
H2 or CO2 temperature-programmed desorption (H2-TPD or 13.2, 14.1 and 10.5 nm in Cu, CZ-1, CZ-2, CZ-3, CZ-4 and CZ-CC,
CO2-TPD) of catalysts were performed on Micromeritics respectively, which indicated that ZnO could promote the
Autochem 2920 II apparatus. H2 or CO2 stream was introduced dispersion of Cu. Another interesting observation was that the
for adsorption (30 min) at room temperature. Aer adsorption, diffraction intensity of ZnO decreased gradually with the
the samples were ushed with Ar (H2-TPD) or He (CO2-TPD)
stream (30 mL minꢀ1) for 30 min to remove weakly adsorbed
H2 or CO2, and then they were heated from 20 to 700 ꢁC at
a rate of 10 ꢁC minꢀ1. The desorbed H2 or CO2 was detected by
TCD.
2.5. Catalyst reactions
The activity of the catalyst was tested in a continuously owing
xed-bed reactor (MRCS8004). 1.0 g of the catalyst diluted with
quartz sand (both in 20–40 mesh) was charged into a stainless
steel tubular reactor. Then, CO2 (99.9%) and H2 (99.9%) was
introduced. The reaction conditions used in the activity test
were 240 ꢁC, 3.0 MPa, 0.54 mol (g-cat h)ꢀ1 and molar feed
composition of CO2/H2 ¼ 1/3. All post reactor lines and valves
were heated to 150 ꢁC to prevent product condensation. The
products were analyzed online by gas chromatography (Agilent
6820). Methanol was measured by a ame ionization detector
with a Porapak-Q column and other gaseous products were
analyzed by a thermal conductivity detector with a Carbosieve
column. The conversion and selectivity values were calculated
as the average of three different analyses performed aer 2
hours of ow operation.
3. Results and discussion
3.1. Structure of the catalysts
Cu content in prepared catalysts was determined in ICP and
summarized in Table 1. The real content of Cu was quite close
to the theoretical values, indicating that the composition of CZ
catalysts via in situ chemical reduction could be precisely
controlled.
Fig. 1a shows the XRD patterns of cylindrical shaped ZnO
prepared by the hydrothermal synthesis method and Cu ob-
tained with chemical reduction method. All diffraction peaks of
ZnO could be ascribed to wurtzite ZnO, and no other impurity
peaks was observed. These results indicated that mainly well
crystallized ZnO was formed in the hydrothermal reaction. It
can be found that the diffraction lines of Cu appeared at 2q ¼
43.1ꢁ and 50.2ꢁ. These data conrmed that the Cu2+ was
reduced to metallic Cu by L-ascorbic acid in the aqueous
solution.
Fig. 1b and c show the XRD patterns of CZ catalysts with
different Cu/ZnO molar ratios obtained by in situ reduction
reaction and carbonate coprecipitation. The diffraction peak of
Cu (111) was weakened and broadened when Cu/Zn molar ratio
was less than 2, which implied that crystallite size of Cu was
small. In contrast, the diffraction peaks of Cu enhanced when
the molar ratio Cu/Zn increased to 4, which meant that the
crystallite size of Cu would be large. Similar phenomena were
also reported by Lee et al.31 The average crystallite size of Cu was
Fig. 1 XRD patterns of cylindrical shaped ZnO, Cu and CZ catalysts. (a)
calculated using Scherrer's equation and summarized in Table Cylindrical shaped ZnO and Cu; (b) CZ catalysts; (c) Cu (111).
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increase of Cu content. The diffraction peak of polar (002) plane The diameter of cylindrical shaped ZnO is about 7 mm. Fig. 2b is
of ZnO disappeared and only wide and weak diffraction peaks of the image of Cu prepared by chemical reduction method. It
nonpolar (001) and (101) planes were observed over CZ-4, which reveals that the shape of Cu is sphere-like with diameter of 3
suggested that the polarity and crystallite size of ZnO decreased mm. Interestingly, when ZnO was used as carrier for heteroge-
with the increase of Cu content. The diffraction peak of (002) neous nucleation and growth of Cu, different shapes of Cu were
plane of ZnO over CZ-CC was also weak, which suggested that observed, ranging from sphere to irregular shape (Fig. 2c–e).
the polarity of ZnO was also weak. Zhu et al.32 found that plate- The result demonstrates that there is strong interaction
like ZnO possessed mostly polar (002) faces contained more between Cu and ZnO. The particle sizes of Cu over CZ catalysts
oxygen vacancies, while rod-like ZnO had mostly nonpolar (101) are also smaller than pure Cu, implying that ZnO can promote
faces contained less oxygen vacancies.
the dispersion of Cu.
Fig. 2a shows the SEM image of ZnO sample prepared by
Fig. 2c–e are the images of CZ-1, CZ-2 and CZ-4 prepared by
controlled hydrothermal precipitation of Zn(CO2CH3)2$2H2O. a in situ chemical reduction reaction. An increase in Cu content
Fig. 2 SEM images of cylindrical shaped ZnO, Cu and CZ catalysts. (a) Cylindrical shaped ZnO; (b) Cu; (c) CZ-1; (d) CZ-2; (e) CZ-4.
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leaded to a change in particle size of Cu. The minimum particle BET surface area and exposed Cu surface area. It can be also
size was obtained for CZ-2, and then an increase in particle size found from Table 1 that CZ-2 prepared by in situ chemical
of Cu appeared for CZ-4. This result was in accordance with the reduction reaction has larger exposed Cu surface areas than
result of XRD. Furthermore, it can be seen from Fig. 2c–e that that obtained by carbonate coprecipitation, which correlated
the particle size of cylindrical shaped ZnO decreased with the well with Cu crystallite size.
increase in the Cu content and ZnO was coated with large
amount of Cu over CZ-4. The result also correlated well with the obtained by in situ chemical reduction reaction. Fig. 4a reveals
result of XRD. that the binding energy value of O 1s for CZ-1 (528.9 eV) is lower
Fig. 4 shows the XPS date of CZ-1, CZ-2 and CZ-4 catalysts
Fig. 3 shows Raman spectra of catalysts with different copper than that of CZ-2 (529.7 eV) and CZ-4 (530.2 eV), implying that
loading. The Raman shi at around 323 cmꢀ1 belonged to the electrons are easier to be excited from ZnO over CZ-1 and ZnO
E
2H–E2L phonon mode of ZnO, and another peak at around over CZ-1 has more oxygen vacancies.11 From Fig. 4a, it is found
428 cmꢀ1 was attributed to the high frequency optical phonon that O 1s XPS band can be decomposed into low binding energy
mode (E2H) of ZnO.33 The characteristic peaks of Raman spectra peak and high binding energy peak, which are attributed to
indicated that ZnO was in the form of wurtzite structure. Fig. 3 lattice oxygen (Olatt) and oxygen vacancies (Ovac), respec-
also conrmed that the position, width and intensity of E2H–E2L tively.35,36 With the increase of copper content, the Ovac/Olatt
and E2H peaks changed obviously with the increasing content of molar ratio decreases gradually. The result indicates that CZ-1
Cu. The position of peak shied to low wave number, width of has more oxygen vacancies. The binding energy value of Cu
peak broadened and its intensity became small, which indi- 2p3/2 of CZ catalysts is shown in Fig. 4b. It can be found that the
cated that Cu contacted strongly with ZnO.33,34
binding energy value of Cu 2p3/2 of CZ-1 is lower than that of CZ-
The BET surface area and exposed Cu surface area in pure 2 and CZ-4 catalysts. The [O] terminated polar (002) facet is
Cu, ZnO and CZ catalysts were summarized in Table 1. The known to be electron richer than those of nonpolar facets (i.e.
exposed Cu surface area of CZ catalysts was higher than that of 100) that contain equal number of cations and anions.7 The
pure Cu, which indicated that ZnO could dispersed Cu. It can be
also found that surface area of Cu/ZnO catalyst prepared by in
situ chemical reduction reaction passed its maximum (35.5 m2
g-catꢀ1, in CZ-2) and then decreased slightly with the increasing
content of Cu, and the detected surface area of exposed Cu
exhibited in a similar tendency. However, both BET surface area
and exposed Cu surface area reduced progressively with high Cu
loadings, which could be due to copper agglomeration at Cu/Zn
molar ratio > 2. CZ-2 had the largest BET surface area (35.5 m2 g-
ꢀ1) and the largest exposed Cu surface area (12.3 m2 g-catꢀ1).
cat
These results showed that the Cu/Zn ratio could inuence the
Fig. 4 XPS patterns of CZ catalysts. (a) O 1s in CZ catalysts; (b) Cu 2p3/2
in CZ catalysts.
Fig. 3 Raman spectra of CZ catalysts.
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XRD result has demonstrated that reection of polar (002) facet hydrogen spillover,21,38 which leads a strong adsorption. When
of ZnO reduces with the increase of Cu content, implying that the ZnO content increases, a volcano-type prole is observed
electrons are easier to transfer from ZnO over CZ-1 to Cu.
showing a maximum of chemisorbed hydrogen for CZ-2. This
Fig. 5a shows that when the Cu content increases, the shape is characteristic of a synergetic effect where the combi-
amount of chemisorbed CO2 decreases, and bare Cu does not nation of Cu and Zn permits a signicant increase of adsorbed
adsorb CO2. The observation indicates that adsorbed CO2 hydrogen. This evolution matches exposed Cu surface area. The
amount is inversely proportional to the Cu content since the investigation of CO2 and H2 chemisorption suggests that both
addition of Cu decreases the basicity of the catalyst and there- Cu and ZnO are active sites.
fore is not in favor of the adsorption of acidic CO2. However,
Fig. 6 shows the H2-TPD patterns of CZ-1, CZ-2 and CZ-4
this decrease is slower than what can be expected from a linear catalysts obtained by in situ chemical reduction reaction. For
trend (Fig. 5a, dotted lines). This indicates that the basicity of comparison, the CZ-CC catalyst obtained by carbonate copre-
ZnO in catalysts has been increased with increase of Cu content, cipitation was also investigated. It is found that all samples
which is in agreement with the amount of oxygen vacancies (the display a H2 desorption peak in the range of 120–300 ꢁC, which
Lewis basicity is increased in the absence of vacancies). is assigned to the desorption of atomic hydrogen adsorbed on
Hydrogen chemisorption experiments are shown in Fig. 5b. It the surface of metallic Cu sites.39 Another strong H2 desorption
can be found that hydrogen is weakly chemisorbed on pure Cu peak located in the range of 450–600 ꢁC is also discovered for all
since Cu is known to quickly decompose and recombine samples. It represents the desorption of strongly-adsorbed
hydrogen.37 However, ZnO can adsorb atomic H through hydrogen on the ZnO surface through spillover from Cu to
Fig. 6 H2-TPD profiles of the CZ catalysts.
Fig. 5 Chemisorbed CO2 (a) and hydrogen (b) as a function of Cu
content in the catalyst.
Fig. 7 CO2-TPD profiles of the CZ catalysts.
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ZnO.39 From Fig. 6, it is found that the maximum H2 desorption with the increase of Cu content. CZ-2 catalyst exhibited the
amount is obtained for CZ-2 with increase in Cu content. Under largest exposed Cu surface area.
the same Cu/Zn molar ratio, the H2 desorption amount CZ-2 is
larger than that over CZ-CC. This result matches exposed Cu methanol selectivity can be observed with the increase in Cu
surface area. content. The value of methanol selectivity over CZ-4 was 48.3%,
Furthermore, it is noteworthy that a remarkable decrease in
Fig. 7 shows the CO2 desorption proles of CZ-1, CZ-2, CZ-4 which decreased by about 47% relative to ZnO (91.2%). This
and CZ-CC catalysts. From CO2-TPD results, it can be observed result was consistent with the reduction of oxygen vacancies.
that all proles can be deconvoluted into two Gaussian peaks, Hydrogen was spilled on ZnO and ZnOx (oxygen vacancies) by
which can be assigned to the weakly-adsorbed CO2 on the weak hydrogen spillover phenomenon9 where CO2 was adsorbed. We
basic sites and strongly-adsorbed CO2 on the strong basic sites, supposed that atomic H adsorbed on ZnO reacted with CO2 to
respectively. The CO2 adsorption capacity order of CZ catalysts produce CO, while atomic H adsorbed on ZnOx reacted with
as follows: CZ-1 > CZ-CC > CZ-2 > CZ-4. The result indicates that CO2 to synthesize methanol.
the addition of Cu decreases the basicity of the catalyst. CZ-CC
Table 2 shows the effect of Cu loading on TOF value of
has stronger basicity than CZ-2 under the same ZnO content, methanol over CZ catalyst. The TOF value of methanol
which can be attributed to fewer oxygen vacancies.
decreased with the increase of Cu/Zn molar ratio. These results
indicated that the CZ catalyst was a structural sensitive catalyst
for methanol synthesis from CO2 hydrogenation. Tsang et al.7
proposed that Cu/ZnO catalyst had double active centers: one
was Cu, and another was the oxygen vacancy on the interface of
the Cu–Zn. In our experiment, the increase of Cu loading led to
the decrease of TOF value of methanol, which was attributed to
reduction of oxygen vacancy between Cu–Zn interface leading to
the decrease of methanol selectivity. In view of the above, we
presented that the mechanism of CO2 hydrogenation over Cu/
ZnO catalysts could be described as follows: rstly, the
3.2. Catalytic performance
The catalytic activity and selectivity of these catalysts for CO2
hydrogenation to methanol are summarized in Table 2. Meth-
anol and CO are the only carbon-containing products under the
reaction conditions. It can be seen that the conversion of CO2
increased with increasing Cu/Zn molar ratio from 0 to 2.
However, when the Cu content was increased further, the
conversion of CO2 decreased. If bare Cu was used, no CO2
conversion was observed. These results indicated that bare Cu
did not adsorbed CO2. CZ-2 catalyst exhibited the best activity
for CO2 hydrogenation with a 17.8% conversion of CO2. The
calculated space time yield (STY) of CZ-2 reached maximum of
0.50 g-MeOH (g-cat h)ꢀ1, while STY decreased to 0.18 g-MeOH (g-cat
h)ꢀ1 over CZ-4. The conversion of CO2 and STY decreased to
13.2% and 0.28 g-MeOH (g-cat h)ꢀ1 over CZ-CC. Under the same
reaction conditions, STY was 0.24 g-MeOH (g-cat h)ꢀ1 over the CZ
catalyst prepared by conventional carbonate co-precipitation
method.11 Gao et al.39 reported that STY was 0.34 g-MeOH (g-cat
h)ꢀ1 over the Cu/ZnO/Al2O3 prepared by the co-precipitation
method (reaction conditions: H2/CO2 ¼ 3, T ¼ 250 ꢁC, P ¼
5.0 MPa, GHSV ¼ 0.54 mol (g-cat h)ꢀ1). As well known, for Cu-
based catalysts, a larger exposed Cu surface area results in
a higher catalytic activity.40,41 The amount of chemisorbed
hydrogen increases with increase of exposed Cu surface area,
which leads to higher CO2 conversion. At the same time, the
results of N2O chemisorption measurements suggested that the
Fig. 8 Stability of CZ-2 for the CO2 hydrogenation process. (Reaction
conditions: H2/CO2 ¼ 3, T ¼ 240 ꢁC, P ¼ 3.0 MPa, GHSV ¼ 0.54 mol (g-
exposed Cu surface area increased rstly and then decreased cat h)ꢀ1).
Table 2 Performance for the hydrogenation of CO2 over ZnO, Cu and CZ catalystsa
Catalysts
CO2 conversion (%)
CH3OH selectivity (%)
STY (g-MeOH (g-cat h)ꢀ1
)
TOF of CH3OH ꢂ103 (sꢀ1
)
ZnO
CZ-1
CZ-2
CZ-3
CZ-4
CZ-CC
Cu
0.2
11.9
17.8
12.6
8.7
91.2
71.5
64.7
54.2
48.3
49.9
0.0
0.01
0.37
0.50
0.30
0.18
0.28
0.00
—
17.0
15.4
11.7
9.4
9.5
0.0
13.2
0.0
a
Reaction conditions: H2/CO2 ¼ 3, T ¼ 240 ꢁC, P ¼ 3.0 MPa, GHSV ¼ 0.54 mol (g-cat h)ꢀ1
.
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hydrogen was adsorbed on the Cu surface and dissociated into
hydrogen atoms. Secondly, hydrogen atoms arrived at the ZnO
and ZnOx surface on the interface of Cu–Zn by hydrogen spill-
over effect to react with CO2 adsorbed on ZnO and ZnOx. Finally,
methanol was generated on ZnOx through a series of interme-
diate, and CO was produced on ZnO.
The most efficient CZ-2 catalyst was selected for a stability
test, and the result was presented in Fig. 8. The conversion of
CO2 and the selectivity of methanol decreased slightly during
the continuous 100 h, indicating that the CZ catalysts prepared
by in situ chemical reduction reaction involved impregnation of
cylindrical shaped ZnO with Cu2+ aqueous solution had a stable
catalytic performance for CO2 hydrogenation to methanol.
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Conflicts of interest
There are no conicts to declare.
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21 A. Le Valant, C. Comminges, C. Tisseraud, C. Canaff,
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Acknowledgements
This work was nancially supported by Basic Public Welfare
Research Project of Zhejiang (grant number LGG18B020003), 22 T. Witoon, T. Permsirivanich, W. Donphai, A. Jaree and
Science and Technology Project of Quzhou (grant number
M. Chareonpanich, Fuel Process. Technol., 2013, 116, 72–78.
2016Y005), National College Students Innovation Project of 23 J. Schumann, M. Eichelbaum, T. Lunkenbein, N. Thomas,
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China (grant number 201711488017).
M. C. A. Galvan, R. Schlogl and M. Behrens, ACS Catal.,
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