Highly efficient Cu-based catalysts via hydrotalcite-like precursors for CO2 hydrogenation to methanol

Article abstract of DOI:10.1016/j.cattod.2016.02.004

A series of Cu–Zn–Al–Zr precursor materials are prepared by coprecipitation at different pH values (6.0–11.0) and treated under hydrothermal condition. Zincian malachite is formed as the main phase at the low pH of 6.0 and 7.0, and is replaced by hydrotalcite-like phases with increasing the pH. After calcination and reduction of precursors, Cu/ZnO/Al₂O₃/ZrO₂ catalysts are obtained and tested for methanol synthesis from CO₂ hydrogenation at the reaction temperature of 463?K. With increasing pH, the Cu particle size first increases until pH 9.0 and then decreases. Compared with the sample resulting from well-crystallized zincian malachite (pH 7.0), the catalysts derived from phase-pure hydrotalcite-like precursors (pH?≥?9.0) exhibit lower BET specific surface area and lower specific Cu surface area. In addition, due to the smaller of Cu particle size and the stronger interaction among Cu and ZnO, the catalytic activity for the Cu/ZnO/Al₂O₃/ZrO₂ catalysts via the hydrotalcite-like precursors is higher than that for the catalysts derived from zincian malachite precursors at low reaction temperature. A maximum CH₃OH yield of 0.087?g?gcat^?1?h^?1 with the CO₂ conversion of 10.7% and the CH₃OH selectivity of 81.8% at 463?K and 5.0?MPa is obtained over the Cu/ZnO/Al₂O₃/ZrO₂ catalyst prepared at pH 9.0.

Full text of DOI:10.1016/j.cattod.2016.02.004

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Highly efficient Cu-based catalysts via hydrotalcite-like precursors for
CO2 hydrogenation to methanol
Shuo Xiaoa,b, Yanfei Zhanga,b, Peng Gaoa,∗, Liangshu Zhonga, Xiaopeng Lia,
Zhongzheng Zhanga, Hui Wanga,∗∗, Wei Weia,c, Yuhan Suna,c
a CAS Key Lab of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, No.99 Haike Road,
Zhangjiang Hi-Tech Park, Shanghai 201210, China
b School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China
c School of physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Cu–Zn–Al–Zr precursor materials are prepared by coprecipitation at different pH values
(6.0–11.0) and treated under hydrothermal condition. Zincian malachite is formed as the main phase
at the low pH of 6.0 and 7.0, and is replaced by hydrotalcite-like phases with increasing the pH.
After calcination and reduction of precursors, Cu/ZnO/Al2O3/ZrO2 catalysts are obtained and tested for
methanol synthesis from CO2 hydrogenation at the reaction temperature of 463 K. With increasing
pH, the Cu particle size first increases until pH 9.0 and then decreases. Compared with the sam-
ple resulting from well-crystallized zincian malachite (pH 7.0), the catalysts derived from phase-pure
hydrotalcite-like precursors (pH ≥ 9.0) exhibit lower BET specific surface area and lower specific Cu
surface area. In addition, due to the smaller of Cu particle size and the stronger interaction among
Cu and ZnO, the catalytic activity for the Cu/ZnO/Al2O3/ZrO2 catalysts via the hydrotalcite-like pre-
cursors is higher than that for the catalysts derived from zincian malachite precursors at low reaction
temperature. A maximum CH3OH yield of 0.087 g gcat−1 h−1 with the CO2 conversion of 10.7% and the
CH3OH selectivity of 81.8% at 463 K and 5.0 MPa is obtained over the Cu/ZnO/Al2O3/ZrO2 catalyst prepared
at pH 9.0.
Received 11 September 2015
Received in revised form 1 January 2016
Accepted 1 February 2016
Available online xxx
Keywords:
Carbon dioxide hydrogenation
Methanol synthesis
Hydrotalcite-like structure
Copper particle size
Copper-based catalyst
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
application to CO2 hydrogenation. Especially, at low reaction tem-
perature, their behavior is the deactivation of Cu-based catalysts
Catalytic conversion of CO2 to useful chemicals and fuels is
a promising route that may offer a solution to the issues of
greenhouse gas control and fossil fuel substitution. Methanol is a
common feedstock for several important chemicals and can be used
as a fuel additive or clean fuel. It can also be converted to high-
octane gasoline, aromatics, ethylene, propylene as well as other
useful petrochemicals which now are derived from crude oil [1,2].
In this context, hydrogenation of CO2 to methanol has attracted
much attention from both industry and academia.
The Cu/ZnO/Al2O3 catalyst, used for methanol synthesis from
syngas in industry, is also widely investigated for CO2 hydrogena-
tion to methanol [3–7]. However, the low activity of industrial
Cu/ZnO/Al2O3 catalysts creates major barriers toward direct
due to the high activation energy of hydrogen dissociation [8,9]. It
has been suggested that the high activity for CO2 hydrogenation
was generated by the presence of surface defects of metallic Cu
surface which could reduce the activation energy of hydrogen dis-
sociation [10,11]. In addition, rich surface defects can be achieved
by decreasing the size of Cu nanoparticles with a simultaneous high
dispersion. Great efforts have been made to decrease the copper
particle size for Cu/ZnO-based catalysts by addition of various pro-
moters such as Zr, Ti, Ga, Y, Ce and B [5,12–14]. The influences
of synthesis conditions and pre-treatment process on the copper
crystallite size of Cu/ZnO-based catalysts were also investigated
[15–18].
With the use of Cu–Zn–Al hydrotalcite-like (HTl) precursors,
small Cu clusters are generated as the active sites for methanol syn-
thesis from syngas or CO2 hydrogenation [19–22]. The composite
oxides derived from hydrotalcite-like compounds (HTlcs) possess
homogeneous dispersion of metal at an atomic level, enhanced syn-
ergetic effects among different elements, strong basic properties
∗
Corresponding author. Fax: +86 21 20608003.
Corresponding author.
∗∗
E-mail addresses: gaopeng@sari.ac.cn (P. Gao), wanghh@sari.ac.cn (H. Wang).
http://dx.doi.org/10.1016/j.cattod.2016.02.004
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Xiao, et al., Highly efficient Cu-based catalysts via hydrotalcite-like precursors for CO2 hydrogenation
to methanol, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.004

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and high stability against sintering [23–25]. Moreover, by virtue of
the wide versatility of HTlcs composition and architecture, metal
nanoparticles with high dispersion, high density, and good thermal
stability for catalysis can be fabricated via reduction of HTl precur-
sors [26]. In our previous work, we presented Cu/ZnO/Al2O3/ZrO2
catalysts with promising catalytic performance in the CO2 hydro-
genation to methanol obtained from HTl precursors [19,27,28], and
found that the catalytic activity decreased with significant decrease
of the yield of HTl phase in precursors [19]. HTlcs are convention-
ally synthesized through coprecipitation methods. However, such
process suffers from a product with a low crystallinity. Especially,
much amorphous phases were formed with the introduction of
large radius of metal cations (such as Y3+, Ce3+ and Zr4+) into HTl
structures [14,19]. The product crystallinity can be affected by var-
ious reaction parameters (pH, temperature, concentration and rate
of added solutions, and the aging temperature and time) and/or
post-synthesis operations (for example, aging of the precipitate)
[29]. A substantial improvement of the product crystallinity can be
achieved by hydrothermal treatment and the hydrothermal treat-
ment was applied to improve the crystallinity of many kinds of
HTlcs [29–31].
(a)
(b)
Furthermore, the typical hydroxy carbonate precursor phases
for industrial Cu/ZnO/Al2O3 catalysts are zincian malachite,
aurichalcite, or a phase mixture of zincian malachite and aurichal-
cite [32]. The crystal phase of the precursor was mainly depended
on the metal composition, for example, high Al content was
favourable for the formation of HTl phase [28] and zincian mala-
chite or aurichalcite phases were formed as a main phase at the
low Al content [17]. Al2O3 has been pointed out to work as the
promoter to increase the stability and the activity [33], thus the Al
content will affect the catalytic performance for CO2 hydrogena-
tion to methanol. So far, few comparative studies on the influences
of different precursors with the same metal composition on the
catalyst activity exist.
In the present work,
a series of Cu–Zn–Al–Zr precursors
(Cu2+:Zn2+:Al3+:Zr4+ = 2:1:1.2:0.1) were synthesized by coprecip-
itation method at different pH values (6.0–11.0) and treated under
hydrothermal conditions at 293 K. The Cu/ZnO/Al2O3/ZrO2 cata-
lysts were then obtained by calcination and reduction of precursors
and tested for methanol synthesis from CO2 hydrogenation at
463 K. The main focus is to study the effect of pH conditions on
physicochemical properties of precursor materials, related mixed
oxides, and reduced samples. In addition, the effects of pH of
preparation on the catalytic performance were investigated for the
structure–activity relationship. This work also allowed us to inves-
tigate how different precursors can affect the catalytic activity for
CO2 hydrogenation to methanol.
(c)
2. Experimental
2.1. Preparation of catalysts
Fig. 1. XRD patterns of (a) uncalcined materials, (b) calcined samples and (c) reduced
Cu/ZnO/Al2O3/ZrO2 catalysts preapred at various pH conditions. (ꢀ) Zincian mala-
chite, (Cu,Zn)2(OH)2CO3; (ꢀ) Cu; (ꢀ) Cu2O; (᭹) CuO; (᭿) ZnO.
Cu/ZnO/Al2O3/ZrO2
catalysts
with
co-
Cu2+:Zn2+:Al3+:Zr4+ = 2:1:1.2:0.1 were synthesized by
a
precipitation method at 293 K. Typically, two aqueous solutions, a
solution of the metal nitrates and a mixed solution of NaOH and
Na2CO3 precipitant, were added dropwise to 200 mL of deionized
water under vigorous stirring. The mixed solution was conducted
at pH range of 6.0–11.0. The product was then transferred into the
Teflon coated stainless steel autoclave for hydrothermal treatment
at 373 K for 16 h under autogenous water vapor pressure. After the
hydrothermal treatment, the precipitate was filtered and washed
with deionized water. The filter cakes were dried overnight at
353 K. The samples were calcined in air at 773 K for 5 h, and the
obtained samples were crushed and sieved to particles in the
range of 40–60 mesh. The as-obtained precursors with different
pH were denoted as HT-n (n is pH during precipitation), and the
corresponding calcined and reduced samples were denoted as
CHT-n and RHT-n, respectively.
2.2. Characterization of catalysts
The chemical composition of the calcined samples was deter-
mined by X-ray fluorescence (XRF) spectroscopy (PW2404 X-ray
wavelength dispersive spectrometer).
Please cite this article in press as: S. Xiao, et al., Highly efficient Cu-based catalysts via hydrotalcite-like precursors for CO2 hydrogenation
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Fig. 2. SEM images of Cu–Zn–Al–Zr precursor materials (a) HT-6, (b) HT-7, (c) HT-8, (d) HT-9, (e) HT-10, (f) HT-11.
The surface area of samples was determined by N2
adsorption–desorption at liquid nitrogen temperature 77.30 K,
using a TriStar II 3020 instrument. Sample degassing was carried
out at 473 K prior to acquire the adsorption isotherm. BET isotherm
and BJH model were used to obtain specific surface area and pore
volume, respectively.
Powder X-ray diffraction (XRD) patterns were recorded on a
Rigaku Ultima 4 X-ray diffractometer with Cu K␣ radiation in the
range of 5◦ < 2ꢁ < 75◦.
work [14]. Before measurements the catalysts (0.1 g) were reduced
at 573 K in 5% H2/Ar mixture (30 mL min−1) for 2 h. The reduced
samples cooled to 303 K and isothermally purged with Ar for
30 min. Then, the sample was exposed to N2O (85 mL min−1) for 1 h
to ensure complete oxidation of surface metallic copper. Finally, a
pulse of pure H2 was passed over the catalyst at 573 K and the sur-
face Cu2O was reduced in the pulse of pure H2. SCu was calculated
from the amount of hydrogen consumption (nH2). In addition, the
dispersion of Cu (DCu) was calculated by Eq. (1).
The morphology of the samples was investigated using a ZEISS
SUPRA 55 scanning electron microscope (SEM) with an accelerating
voltage of 2.0 kV.
(2nH2 × MCu/W) × 100%
DCu
=
× 100%
(1)
X
Thermogravimetric mass spectrometry (TG–MS) analysis was
performed by using a STA449-QMS thermal analyzer. Samples were
heated up to 1023 K with linear temperature program at a heating
rate of 10 K min−1 in continuous flow of synthetic air (30 mL min−1).
Transmission electron microscope (TEM) and high resolution
transmission electron microscope (HRTEM) were performed on
a JEM 2100 transmission electron microscope with a field emis-
sion gun operating at 200 kV to investigate the dimensions and the
structural details of the reduced catalysts.
where nH2 was molar number of consumed H2. DCu was the disper-
sion of Cu, MCu was relative atomic mass (63.546 g mol−1), W was
the weight of the catalyst, and X was the composition of Cu (wt.%)
determined by X-ray fluorescence spectroscopy.
X-ray photoelectron spectroscopy (XPS) and Auger electron
spectroscopy (XAES) were performed over a Quantum 2000 Scan-
ning ESCA Microprobe instrument equipped with Al K␣ radiation
(12 kV, 4 mA, hꢂ = 1486.6 eV) under ultrahigh vacuum (10−7 Pa).
Prior to each test, the calcined sample was reduced in pure hydro-
gen at 553 K for 6 h, and XPS measurements were recorded with
the exclusion of air contact after reduction. The binding energies
The exposed Cu surface area (SCu) was determined by dissocia-
tive N2O adsorption and carried out on Micromeritics AutoChem
2920 instrument using the procedure described in our previous
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were calibrated internally by adventitious carbon deposit C (1s)
with Eb = 284.8 eV (accuracy within 0.1 eV).
100
95
90
85
80
75
70
65
510 K
HT-6
513 K
Temperature program reduction (TPR) was conducted in a U-
tube quartz reactor on a Micromeritics ChemiSorb 2920 with a
thermal conductivity detector (TCD). 50 mg of sample placed in
a U-tube quartz reactor was first degassed under flowing argon
(30 mL min−1) at 423 K for 1 h and cooled down to room tempera-
ture. Then the sample was reduced at a rate of 5 K min−1 up to 873 K
in a flow of 5%H2/Ar (30 mL min−1) mixture.
488 K
469 K
376 K
323
423
523
623
723
823
823
923
923
1023
HT-7
100
95
90
85
80
75
70
65
2.3. Evaluation of catalysts
642 K
648 K
403 K
Activity measurements in the hydrogenation of CO2 were car-
ried out in a fixed-bed reactor. Catalyst (1.5 mL, 40–60 mesh)
diluted with 1.5 mL quartz sand (40–60 mesh) was placed in a stain-
less steel tube reactor. Prior to reaction, the catalyst was reduced
in pure H2 at a flow-rate of 80 mL min−1 under atmospheric pres-
sure. The reduction temperature was programmed to increase from
room temperature to 553 K and maintained at 553 K for 6 h. The
reactor was then cooled to room temperature. After reduction,
the activities of the catalyst samples in CO2 hydrogenation pro-
cess were determined under reaction conditions of 463 K, 5.0 MPa,
H2/CO2/N2 = 73/24/3, GHSV = 4000 h−1. The products were quanti-
tatively analyzed using a Shimadzu GC-2010C gas chromatograph
equipped with a thermal conductivity detector (TCD, TDX-01 col-
umn) for gas of H2, N2, CO, CH4 and CO2, and Porapak-Q for liquid
of H2O and CH3OH. The CO2 conversion and the carbon-based
selectivity for CH3OH and CO were calculated by an internal nor-
malization method. The yield of CH3OH, which gave the amounts
of CH3OH produced per gram catalyst per hour, was defined as Eq.
(2):
506 K
546 K
523
323
423
623
723
1023
HT-8
100
95
90
85
80
75
70
65
436 K
513 K
536 K
904 K
650 K
412 K
860 K
823
649 K
723
323
423
523
623
923
1023
HT-9
100
95
90
85
80
75
70
65
856 K
442 K
WT × X(CH3OH)
911 K
Y(CH3OH) =
(2)
t × m
521 K
502 K
where WT was the total weight of CH3OH and H2O product (g);
X(CH3OH) was the mass fraction of CH3OH; t was the reaction time
(h); m was the weight of catalyst (g).
450 K
3. Results and discussion
323
423
437 K
523
623
723
823
923
1023
3.1. Textural and structural properties of the precursor materials
100
95
90
85
80
75
70
65
HT-10
925 K
862 K
The powder XRD patterns of Cu–Zn–Al–Zr precursors at differ-
ent pH conditions are depicted in Fig. 1a. The zincian malachite
[rosasite, (Cu,Zn)2(OH)2CO3] was formed as a main phase at the
low pH of 6.0 and 7.0, and was replaced by hydrotalcite-like (HTl)
phases with increasing the pH. For HT-8, both zincian malachite
and HTl phases can be detected, and HTl phase was the main phase.
Hydrotalcite alone was selectively produced at the high pH, above
8.0. The patterns of HT-9, HT-10, and HT-11 samples exhibited
typical reflections of HTl structure: sharp and symmetrical peaks
for (0 0 3), (0 0 6), (1 1 0) and (1 1 3), and broad and asymmetrical
for (0 0 9), (0 1 5) and (0 1 8), respectively [34,35]. In addition, the
narrow peaks indicated that hydrotalcites were present as a well-
defined crystalline phase, and the well-defined (1 1 0) and (1 1 3)
diffraction peaks were also observed at 60◦ and 61◦, suggesting a
homogeneous dispersion of various cations in the hydroxide layers
[34]. As pH increases, from 8.0 to 11.0, the intensity of the reflec-
tions increased and the line width decreased, corresponding to an
increase in crystallinity.
512 K
541 K
523
448 K
323
423
623
723
723
823
923
1023
100
95
90
85
80
75
70
65
458 K
828 K
HT-11
938 K
457 K
536K
525K
323
423
523
623
823
923
1023
Temperature (K)
The fibrous morphology of HT-6 and HT-7 precursors was
observed by using scanning electron microscopy (SEM), as pre-
sented in Fig. 2a and b, respectively, and the fibers size for
HT-7 (1–3 ␮m) was much larger than that for HT-6 (0.5–1.3 ␮m).
As shown in Fig. 2c–f, the expected platelet-like morphology of
hydrotalcite-like compounds (HTlcs) can be observed for HT-8,
Fig. 3. Thermogravimetry mass spectrometry (TG-MS) profiles of Cu–Zn–Al–Zr pre-
cursors in synthetic air. (red line) H2O, (green line) CO2, (black line) mass loss (%). For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.
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HT-9, HT-10 and HT-11 samples. In addition, the platelets were
well-developed and intercrossed with each other, and most of the
edges were rounded at low pH. With increasing pH from 8.0 to
11.0, both the diameters and thicknesses of platelets increased sig-
nificantly and the dispersion of platelets decreased. For the HT-8
precursor, minor amounts of fibers with thick dimension can also
be found, which is consistent with the XRD result.
calcination at 773 K. It can be seen that the other samples upon
thermal decomposition maintained morphology similar to that of
the precursors. For CHT-7, the nano-structuring of homogeneous
particles and the generation of pores can be found in Fig. 5b. The
calcined CHT-8, CHT-9, CHT-10 and CHT-11 samples were com-
posed of platelet-like particles, suggesting that the derived oxides
remained the morphological characteristic of HTl precursors.
The textural properties of CHT-n samples prepared at differ-
ent pH values are summarized in Table 1. The BET specific surface
area (SBET) of CHT-6 was much lower than those of other samples
due to the formation of compact particles during calcination as a
result of the much lower thermal stability of the HT-6 precursor.
CHT-7 exhibited the highest SBET, and it decreased with increas-
ing pH, indicating the metal oxides derived from well-crystallized
zincian malachite possessed higher SBET compared to HTl precur-
sors. For the samples via phase-pure HTlcs, the decline of SBET with
increasing pH can be ascribed to the decrease of the dispersion of
platelets.
Calcination of the precursors was simulated by thermogravi-
metric (TG) measurements in synthetic air together with the
evolved H2O and CO2 curves detected upon heating by using mass
spectrometry (MS), and the resulting curves for all samples are
shown in Fig. 3. HT-6 and HT-7 samples exhibited the same phase
composition and more X-ray amorphous phases existed in HT-6, as
judged by the diffraction patterns. In the whole temperature range,
the behavior was quite similar and three H2O emission peaks and
two peaks belongs to CO2 can be observed, while all peaks shifted
to lower temperatures remarkably as pH decreases, from 7.0 to 6.0,
which revealed the much lower thermal stability of HT-6 and the
structural differences between the two precursors. In addition, the
dehydroxylation and decarboxylation of the zincian malachite (HT-
6 and HT-7) were completed around 773 K. With further increase of
pH from 8.0 to 11.0, the typical 3-step decomposition of HTlcs was
observed [21,28]. First, the peaks around 450 K can be attributed to
the elimination of physically adsorbed H2O and CO2 molecules and
evaporation of interlamellar water molecules. The second weight
loss at 473–773 K was ascribed to dehydroxylation of the HTl layer
and loss of carbonate of the interlayer space. For HT-8 sample, the
second weight loss peak can be also due to the decomposition of
zincian malachite phase. Moreover, CO2 formation above 823 K was
observed, which can be assigned to the stabilizing “high temper-
ature carbonate” formed during the former steps of the thermal
decomposition of the HTl structure. This carbonate species, which
possessed strong interfacial interaction and stable grain bound-
aries, were trapped between segregating CuO particles and other
metal oxides during calcination of precursors [21,36].
3.3. Textural and structural properties of the reduced samples
XRD patterns obtained for reduced and passivated CHT-n sam-
ples are displayed in Fig. 1c. The calcined samples reduced in a
10% H2/Ar flow at 553 K for 6 h, and passivation was carried out
at 298 K in a flow of 1% O2/Ar for 60 min. As shown in Fig. 1c, the
XRD diffraction peaks (2ꢁ = 35.6◦ and 38.8◦) of CuO disappeared in
all catalysts except of RHT-6, while the peaks (2ꢁ = 43.2◦, 50.2◦, and
74.1◦) attributed to metallic Cu emerged. For RHT-6, apart from
metallic Cu, the diffraction peaks of CuO can be still observed and
poorly crystallized Cu2O can also be found after reduction at 553 K,
indicating that the amount of reducible copper species for the RHT-
6 sample was much lower than that for the other samples below
553 K. The intensity of diffraction peaks of ZnO decreased markedly
with increase of pH, and the peaks due to ZnO cannot be observed
when pH ≥ 8.0, except for the sample prepared at pH 11.0. Com-
pared with RHT-6 and RHT-7, rather weak peaks due to a poorly
crystallized ZnO phase were detectable in the XRD patterns of RHT-
11. These results revealed that the ZnO dispersions in catalysts via
HTl precursors were better than those in samples derived from zin-
cian malachite. In addition, the Cu crystallite size (dCu) calculated
by the Scherrer equation decreased markedly with increasing pH,
while the average Cu particle size increased when pH > 9.0, and the
minimum dCu was 6.5 nm for RHT-9 (Table 1). After reduction of
CHT-n catalysts, dCu was larger than corresponding dCuO for RHT-6
and RHT-7 derived from zincian malachite, suggesting that sinter-
ing of copper particles occurred upon heat treatment. Compared
with reduced samples derived from zincian malachite, the dCu for
the samples via HTl precursors was much lower, especially for RHT-
9 (only 6.5 nm). These results indicated that the HTl structure can
hinder the growth of Cu particle size during the calcination and
reduction.
The transmission electron microscopy (TEM) and high resolu-
tion transmission electron microscopy (HRTEM) images illustrate
the typical microstructure of the reduced CHT-n catalysts. The RHT-
7 sample comprised the much compact particles (Fig. 5a). After
the reduction of CHT-9 and CHT-11 at 553 K for 6 h, the samples
still maintained the overall plate-like morphology (Fig. 5b and c),
while abundant dispersed small dots were observed throughout
the plates. On platelets of RHT-9, spherically shaped and well crys-
talline of Cu could be noticed from HRTEM images and no indication
for individual separated oxide particles were found (Fig. 5d). The
average diameter of Cu particles for RHT-9 was around 5–9 nm,
in agreement with the result from XRD analysis (Table 1). In addi-
tion, the Cu particles were partly embedded in the remaining metal
oxide matrix resulting in close interfacial contact of Cu particles and
continuous Cu depleted oxide. Therefore, the extrapolation of the
3.2. Textural and structural properties of the calcined samples
The results of the elemental analysis of the calcined samples are
included in Table 1. The Cu2+:Zn2+:Al3+:Zr4+ atomic ratios deter-
mined from X-ray fluorescence spectroscopy (XRF) were in close
agreement, within experimental errors, with the nominal compo-
sitions (2:1:1.2:0.1) taken for the catalysts preparations, indicating
the complete precipitation of the metallic nitrates for all sam-
ples, except for CHT-6. Only at the acidic condition (pH 6.0), the
determined ratio of CHT-6 was significantly deviated from the nom-
inal data, for example, with lower Al3+/Zn2+ and Zr4+/Zn2+ ratios
and a higher Cu2+/Zn2+ atomic ratio, suggesting that more cop-
per nitrates were precipitated in the solution compared to other
metallic nitrates at pH 6.0. XRD patterns of the calcined samples are
shown in Fig. 1b. The peaks in CHT-6 XRD pattern were assigned
to CuO and ZnO with good crystalline. The crystallinity of the cal-
cined samples decreased significantly with increasing pH from 6.0
to 9.0, while the intensity of the reflections increased remarkably
with further increase of pH. When pH ≥ 8.0, only the diffraction
peaks belonging to a CuO phase were detected and there were no
peaks that can be attributed to ZnO. In addition, the CHT-9 pattern
showed low signal-noise ratio and the weakest peak intensity, and
the larger half-peak width indicated the relatively small crystalline
size of CuO compared to other calcined samples. The CuO crystallite
size (dCuO) was also calculated by the Scherrer equation, and dCuO
decreased significantly with increasing pH until pH 9.0 and then
increased (Table 1).
SEM images of the calcined samples are shown in Fig. 4. The
zincian malachite fibers prepared at pH 6.0 transformed into
the spherically shaped and compact particles (Fig. 5a) during
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Table 1
Structure and composition of calcined samples prepared at different pH.
c
c
Sample
Cu2+:Zn2+:Al3+:Zr4+
atomic ratioa
Crystallite sizes (nm)b
CuO
SBET
SCu
DCu
Cu
(m2 g−1
)
(m2 g−1
)
(%)
CHT-6
CHT-7
CHT-8
CHT-9
CHT-10
CHT-11
2.72:1:0.84:0.06
2.25:1:1.12:0.09
2.21:1:1.24:0.10
2.28:1:1.19:0.08
2.12:1:1.14:0.08
2.29:1:1.12:0.08
23.9
14.0
13.1
8.8
12.2
16.9
28.0
14.6
11.7
6.5
8.1
13.8
28
112
79
68
59
46
10.1
29.2
21.9
19.4
16.9
16.6
2.74
8.85
7.00
6.07
5.45
5.16
a
Determined by X-ray fluorescence spectroscopy.
Determined from full width at half maxima of CuO (1 1 1) or Cu (1 1 1) XRD peak.
Calculated from N2O dissociative adsorption.
b
c
Fig. 4. SEM images of calcined samples (a) CHT-6, (b) CHT-7, (c) CHT-8, (d) CHT-9, (e) CHT-10, (f) CHT-11.
interaction among Cu and ZnO for RHT-9 was stronger than that for
RHT-7. For the RHT-11, apart from Cu particles, the separated ZnO
particles were formed according to the XRD analysis, thus more
agglomerated particles on the platelets could be observed (Fig. 5c).
The reactive N2O adsorption on reduced samples (Table 1) was
to determine the exposed copper surface area (SCu) and disper-
sion of copper (DCu). It was found that the variations of SCu and
DCu were consistent with the trend of the SBET. Compared with the
RHT-7 derived from well-crystallized zincian malachite, SCu for the
catalysts (RHT-9, RHT-10 and RHT-11) via phase-pure HTlcs was
much lower. The Cu particles were embedded in oxidic matrix for
RHT-9, while were well-separated from ZnO for RHT-7 as illustrated
by TEM and XRD, thus the lower Cu surface area can be accessible
to the gas phase for the sample resulting from the HTl precursor.
3.4. XPS investigations on reduced samples
XPS analysis was performed to determine the surface chemical
states and compositions of reduced RHT-n catalysts. As shown in
Fig. 6a, XPS spectrum of RHT-6 exhibited broad peaks for Cu2p3/2
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Fig. 5. TEM images of RHT-n samples after reduction at 553 K for 6 h: (a) RHT-7, (b) RHT-9, (c) RHT-11, and (d) HRTEM images of the RHT-9 sample.
at 933 eV and 942 eV as parent and satellite peak, respectively, and
XPS peaks were found at around 953 eV and 962 eV for Cu2p1/2 core
electrons and satellite excitations. The emergence of the satellite
structure indicated that some Cu2+ species were still present on
the surface of RHT-6 after reduction at 553 K for 6 h [37]. For other
reduced samples, the spectra consisted of two peaks at around 932
and 952 eV which were mainly ascribed to Cu 2p3/2 and Cu2p1/2
peaks of Cu0, respectively [38]. The absence of satellite peaks
around 942 and 962 eV revealed that the Cu2+ species for the sam-
ples prepared at pH ≥ 7.0 were reduced completely after reduction
at 553 K. In addition, the Zn 2p3/2 peak at 1020.9–1021.4 eV can be
attributed to the Zn2+ species on the catalyst surface (Fig. 6b).
The binding energies (BE) of Cu 2p3/2, Zn 2p3/2 and Al 2p bands
and surface compositions of the RHT-n catalysts are listed in Table 2.
It was found that both the BE of Cu 2p3/2 and Zn 2p3/2 bands
increased gradually with increasing pH. Obviously, these BE for
the catalysts (pH ≥ 8.0) mainly derived from HTl precursors were
higher than those for the RHT-7 sample resulting from zincian
malachite. Additionally, the Al 2p core-level was located at BE
around 74.0 eV for all the reduced samples and the BE of Zr 3d3/2
and Zr 3d5/2 changed slightly with increase of pH (Fig. 6c). These
results illustrated that there existed interaction between Cu and
ZnO and this interaction was weakened at low pH of 7.0, which can
be ascribed to lower dispersion of ZnO. Relative surface concentra-
tions of metal the catalysts, as determined by XPS, are compared in
Table 2 with the bulk compositions measured by XRF. For RHT-6,
the surface contents of all metals were similar to the bulk compo-
sitions. However, the surface was significantly depleted of Cu for
other reduced samples, whereas Zn was enriched. In addition, the
Zr accumulated preferentially on the surface (an increase of three
to five times compared to the data measured by XRF). It was also
clearly seen that the surface Cu content and the surface Cu/Zn ratio
raised first and then decreased with increasing pH from 7.0 to 11.0
and the maximums were found for RHT-9.
3.5. The reducibility of Cu/ZnO/Al2O3/ZrO2 catalysts
In order to obtain a better understanding of the reduction
process, CHT-n samples were carefully analyzed by using TPR mea-
surements (Fig. 7). The CHT-6 and CHT-7 samples derived from
zincian malachite presented one peak of hydrogen consumption.
The reduction peak of CHT-6 was around 575 K, while the profiles of
other samples showed no H2 consumptions at temperatures above
565 K, which suggested that the reducibility of CHT-6 was much
lower than those of other calcined samples, is consistent with the
XRD and XPS results. The CHT-8, CHT-9 and CHT-10 samples via
HTl precursors possessed the bimodal TPR profiles. Kühl et al. [21]
suggested that the reduction of the Cu/ZnO/Al2O3 catalyst result-
ing from a Cu–Zn–Al hydrotalcite-like compound proceeded in two
steps through a kinetically stabilized Cu+ intermediate due to the
strong interaction between Cu2+ species and Zn–Al–oxide. More-
over, many studies claimed that the reduction of CuO to metallic Cu
passed through an intermediate Cu+ species [33,37]. Therefore, this
can explain the pronounced shoulder of the TPR signal for CHT-8,
CHT-9 and CHT-10. In addition, the position of the second reduction
peak for CHT-9 was much lower than those for CHT-8 and CHT-
10, which can be attributed to the smaller CuO particles. CHT-11
exhibited three reduction peaks and the first peak centered at 489 K
which was closed to the position of the reduction peak for CHT-7.
According to the XRD and TEM analysis of CHT-11, a part of CuO
particles were well-separated from ZnO for CHT-11, therefore, the
reduction behavior of these CuO particles was similar to that of
CHT-7.
3.6. Catalytic performance
Activity data of various Cu/ZnO/Al2O3/ZrO2 catalysts in
methanol synthesis from CO2 hydrogenation are summarized in
Table 3. Methanol and CO are the only carbon-containing prod-
ucts at low reaction temperature (463 K). The catalytic activity for
Cu/ZnO/Al2O3/ZrO2 catalysts via HTl precursors was better than
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Table 2
XPS results for the reduced Cu/ZnO/Al2O3/ZrO2 catalysts prepared at different pH.
Sample
Binding energy (eV)
Relative surface concentration of metal (at.%)a
Cu/Zn
Cu 2p3/2
Zn 2p3/2
Al 2p
Cu
Zn
Al
Zr
RHT-6
RHT-7
RHT-8
RHT-9
RHT-10
RHT-11
933.1
932.0
932.3
932.4
932.4
932.3
1021.3
1021.3
1021.7
1021.8
1021.8
1021.6
74.5
74.3
74.3
74.4
74.4
74.3
52.3 (58.9)
34.9 (50.5)
38.1 (48.6)
39.6 (50.2)
29.4 (48.9)
28.3 (51.2)
24.9 (21.6)
35.0 (22.4)
37.3 (21.9)
31.5 (22.0)
39.1 (23.0)
44.1 (22.4)
18.4 (18.1)
24.3 (25.1)
17.6 (27.2)
22.8 (26.1)
23.0 (26.3)
21.4 (24.7)
4.3 (1.3)
5.8 (2.0)
7.0 (2.3)
6.2 (1.7)
8.5 (1.8)
6.2 (1.7)
2.10
1.00
1.02
1.26
0.75
0.64
a
Values in parentheses are concentration normalized to the total metal content measured by XRF.
that for the catalysts derived from zincian malachite precursors. In
addition, both CO2 conversion and CH3OH yield took on a volcanic
trend with increasing pH. The CO2 conversion over RHT-6 was much
lower than those over other catalysts. Most researchers proposed
that the Cu0 species were the predominant active sites for methanol
synthesis from CO2 hydrogenation [10,38–40]. As discussed above,
the CHT-6 sample was not reduced completely at 553 K and three
kinds of copper species (Cu, Cu+, Cu2+) were present after reduc-
tion, which might result in the much lower activity for RHT-6. A
maximum CO2 conversion of 10.7% with the CH3OH yield of 0.087 g
gcat−1 h−1 was obtained over the catalyst prepared at pH 9.0. Under
the similar reaction conditions, the CO2 conversions for other types
of the Cu/ZnO-based catalysts were around 6% [41,42]. Therefore,
the Cu/ZnO/Al2O3/ZrO2 catalysts via HTlcs exhibited excellent per-
formance for CO2 hydrogenation. For all catalysts, expect for RHT-6,
the CH3OH selectivity was very high, above 80%, because the low
temperature was favorable for the reaction of methanol synthesis
[28,43,44]. As well known, there are two important competitive
reactions in CO2 hydrogenation to methanol. The first one is the
synthesis of methanol (3) and the second one is the reverse water
gas shift (RWGS) reaction (4). As shown in Reaction (3), the syn-
thesis of methanol is an exothermic reversible reaction and its
equilibrium constant increased upon declining of reaction tem-
perature. In addition, the methanol synthesis reaction has a lower
apparent activation energy compared to RWGS reaction [8,45],
which indicates that the increase of methanol relative production is
higher than that of CO with decreasing reaction temperature. How-
ever, the CH3OH selectivity for RHT-6 was much lower than those
for other catalysts, only around 50%. Some researchers suggested
that the monovalent copper species could promote CO production
during the RWGS reaction [6,46]. Consequently, the low CH3OH
selectivity for RHT-6 might be attributed to the presence of Cu+
species.
conversion for the various Cu/ZnO/Al2O3/ZrO2 catalysts at 463 K are
presented in Fig. 8a. It was found that the CO2 conversion increased
linearly with decreasing dCu, while there seemed no relationship
between the CO2 conversion and SCu. Although RHT-7 exhibited
the highest SCu, the CO2 conversion for RHT-7 was lower than
those for the catalysts prepared at pH ≥ 8.0, and the CO2 conversion
decreased with the decrease of SCu for the catalysts via phase-
pure HTl precursors, proving that a high Cu surface area was an
important but not the only influencing parameter. Natesakhawat
et al. [38] suggested that smaller crystallites had larger numbers of
open planes, edge/defect sites containing coordinately unsaturated
atoms which were typically more reactive than fully coordinated
species. Behrens et al. [10] also claimed that high activity for the
Cu-based catalyst was generated by the presence of steps at the Cu
surface. These results indicated that the catalytic activity for CO2
hydrogenation at low reaction temperature was mainly depended
on the particle size of Cu.
To discern the role of copper particle size in the CO2 hydro-
genation to methanol, we attempted to correlate the turnover
frequency (TOF), which represents the number of CO2-molecule
hydrogenated on unit site of exposed copper atom per second
(s−1), with the dCu. Results from Table 3 showed that the TOF
value increased with increasing pH until it reached a maximum of
7.34 × 10−3 for RHT-9 and then decreased. As illustrated in Fig. 8b
the TOF value declined with increasing dCu over these catalysts,
providing direct evidence that methanol synthesis from CO2 hydro-
genation was structurally sensitive. However, the relationship was
not linear, and the TOF for RHT-7 was much lower than that for RHT-
11 though the difference between dCu of these two samples was
pretty small, suggesting that the TOF value for CO2 conversion was
not simply dependent on the crystallite size of Cu. Many researchers
believed that the synergistic interaction among Cu and ZnO was
necessary to ensure high catalytic activity in methanol synthe-
sis from CO2 hydrogenation [38,39,49–51]. In our case, compared
with the catalysts resulting from HTl precursors, the interaction
between Cu and ZnO for the catalyst derived from zincian malachite
was much weaker from XRD, XPS and HRTEM analysis, leading to
decrease the catalytic activity. Therefore, RHT-9 exhibited the best
catalytic performance for CO2 hydrogenation to methanol due to
the minimum of copper particle size and the efficient interaction
among Cu and ZnO.
CO2 + 3H2 ꢁ CH3OH + H2O ꢀH298K,5MPa = −40.9kJmol−1
(3)
CO2 + H2 ꢁ CO + H2O ꢀH 298K,5MPa = 49.8kJmol−1
(4)
According to the reported mechanistic studies, the copper par-
ticle size (dCu) and the exposed copper surface area (SCu) played an
important role for the catalytic performance of copper-based cat-
alysts [14,47,48]. Variations of dCu and SCu as a function of the CO2
Table 3
The catalytic performance for methanol synthesis from CO2 hydrogenation over RHT-n catalysts.
Sample
CO2 conversion
(%)
Selectivity (C-mol%)
CH3OH
CH3OH yield
TOF × 103
(s−1
)
CO
(g gcat−1 h−1
)
RHT-6
RHT-7
RHT-8
RHT-9
RHT-10
RHT-11
2.6
6.9
8.5
10.7
9.0
7.3
49.5
81.1
80.6
81.8
81.6
81.3
50.5
18.9
19.4
18.2
18.4
18.7
0.011
0.055
0.068
0.087
0.074
0.059
2.52
3.26
5.26
7.34
6.66
4.96
Reaction conditions: T = 463 K, P = 5.0 MPa, GHSV = 4000 h−1, H2/CO2/N2 = 73/24/3 (molar ratio).
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9
489 K
511 K
Cu 2p
RHT-11
(a)
538 K
522 K
CHT-11
467 K
470 K
469 K
484 K
RHT-10
CHT-10
CHT-9
515 K
537 K
RHT-9
RHT-8
CHT-8
CHT-7
RHT-7
RHT-6
575 K
928 932 936 940 944 948 952 956 960 964
Binding energy (eV)
CHT-6
373
473
573
673
773
Temperature (K)
Zn 2p
(b)
Fig. 7. TPR profiles of the CHT-n samples preapred at various pH conditions.
RHT-11
RHT-10
RHT-9
30
25
20
15
10
5
30
25
20
15
10
5
(a)
RHT-8
RHT-7
RHT-6
1016 1018 1020 1022 1024 1026 1028 1030
Binding energy (eV)
3d5/2
Zr 3d
(c)
3d3/2
RHT-11
RHT-10
2
3
4
5
6
7
8
9
10
11
CO2 conversion (%)
RHT-9
RHT-8
RHT-7
RHT-6
(b)
7
6
5
4
3
2
175 177 179 181 183 185 187 189 191 193
Binding energy (eV)
Fig. 6. (a) Cu 2p, (b) Zn 2p and (c) Zr 3d XPS of the reduced RHT-n catalysts.
4. Conclusion
A
series
of
Cu–Zn–Al–Zr
precursor
materials
(Cu2+:Zn2+:Al3+:Zr4+ = 2:1:1.2:0.1) were prepared by coprecip-
itation at different pH values (6.0–11.0) and treated under
hydrothermal conditions. Zincian malachite was formed as the
main phase at the low pH, and both zincian malachite and HTl
phases can be detected at pH 8.0. In addition, hydrotalcite alone
was selectively produced at the high pH, above 8.0. After calci-
nation and reduction of precursors, Cu/ZnO/Al2O3/ZrO2 catalysts
were obtained and tested for CO2 hydrogenation to methanol at
the reaction temperature of 463 K.
4
8
12
16
20
24
28
dCu (nm)
Fig. 8. (a) Variations of the copper particle size and the exposed copper surface
area as a function of CO2 conversion, and (b) the relationship between the TOF
and the copper particle size for the Cu/ZnO/Al2O3/ZrO2 catalysts prepared at var-
ious pH conditions. Reaction conditions: T = 463 K, P = 5.0 MPa, GHSV = 4000 h−1
,
H2/CO2/N2 = 73/24/3 (molar ratio).
Compared with the material resulting form well-crystallized
zincian malachite (pH 7.0), the BET specific surface area and Cu
surface area for samples derived from phase-pure hydrotalcite-like
precursors (pH ≥ 9.0) were lower, while smaller Cu particle size and
stronger interaction between Cu and ZnO could be obtained. With
increasing pH from 6.0 to 11.0, the Cu particle size first increased
until pH 9.0 and then decreased, and both the CO2 conversion and
TOF took the same trend. Additionally, the Cu/ZnO/Al2O3/ZrO2
catalysts via hydrotalcite-like compounds exhibited better
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Please cite this article in press as: S. Xiao, et al., Highly efficient Cu-based catalysts via hydrotalcite-like precursors for CO2 hydrogenation
to methanol, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.004