Carbon dioxide hydrogenation to methanol over Cu/ZrO2/CNTs: Effect of carbon surface chemistry

Article abstract of DOI:10.1039/c5ra04774a

Methanol synthesis from CO₂ hydrogenation in a fixed-bed plug flow reactor was investigated over Cu-ZrO₂ catalysts supported on CNTs bearing various functional groups. The highest methanol activity (turnover frequency 1.61 × 10^-2 s^-1, space time yield 84.0 mg g_cat^-1 h^-1) was obtained over the Cu/ZrO₂/CNTs catalyst (CZ/CNT-3) with CNTs functionalized by nitrogen-containing groups and Cu loading only about 10.3 wt% under the reaction conditions of 260 °C, 3.0 MPa, V(H₂):V(CO₂):V(N₂) = 69:23:8 and GHSV of 3600 h^-1. The catalysts were fully characterized by N₂ physisorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), H₂-temperature-programmed reduction (H₂-TPR) and temperature-programmed desorption of H₂ (H₂-TPD) techniques. The excellent performance of CZ/CNT-3 is attributed to the presence of nitrogen-containing groups on the CNTs surface, which increase the dispersion of copper oxides, promote their reduction, decreases the crystal size of Cu, and enhances H₂ and CO₂ adsorption capability, thus leading to good catalytic performance towards methanol synthesis. This journal is

Full text of DOI:10.1039/c5ra04774a

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Page 1 of 38
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Carbon Dioxide Hydrogenation to Methanol over Cu-ZrO /CNTs:
2
Effect of Carbon Surface Chemistry
[
a]
*,[a,b]
[a]
[a]
[a]
Guannan Wang, Limin Chen,
Yuhai Sun, Junliang Wu, Mingli Fu, Daiqi
*
,[a,b]
Ye
[
a]
G.N. Wang, Prof. Dr. L.M. Chen, Y.H. Sun, Prof. Dr. J.L. Wu, Prof. Dr. M.L. Fu,
Prof. Dr. D.Q. Ye, School of Environment and Energy, South China University of
Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR
China. E-mail address: liminchen@scut.edu.cn (L.M. Chen), cedqye@scut.edu.cn
(D.Q. Ye); Tel: 86-20-39389592.
[
b]
Prof. Dr. L.M. Chen, Prof. Dr. D.Q. Ye, Guangdong Provincial Key Laboratory of
Atmospheric Environment and Pollution Control, South China University of
Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR
China
1

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Abstract
Methanol synthesis from CO
was investigated over Cu-ZrO
2
hydrogenation in a fixed-bed plug flow reactor
catalysts supported on CNTs bearing various
2
-2 -1
functional groups. The highest methanol activity (turnover frequency 1.61x10 s ,
-
1
-1
space time yield 84.0 mg·g (cat)·h ) was obtained over Cu-ZrO
CZ/CNT-3) with CNTs functionalized by nitrogen-containing groups and Cu loading
only about 10.3 wt% under the reaction conditions of 260 °C, 3.0 MPa,
2
/CNTs catalyst
(
-
1
V(H
characterized by N
diffraction (XRD),
temperature-programmed desorption of H
2
):V(CO
2
):V(N
2
) = 69:23:8 and GHSV of 3600 h . The catalysts were full
physisorption, X-ray photoelectron spectroscopy (XPS), X-ray
-temperature-programmed reduction (H -TPR) and
(H -TPD) techniques. The excellent
2
H
2
2
2
2
performance of CZ/CNT-3 is attributed to the presence of nitrogen-containing groups
on the CNTs surface, which increase the dispersion of copper oxides, promote their
2 2
reduction, decreases the crystal size of Cu, enhances H and CO adsorption capability,
thus leading to the good catalytic performance towards methanol synthesis.
Keywords: CO hydrogenation; Methanol synthesis; Carbon nanotubes; Surface
2
functional groups; Renewable resources
2

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Introduction
CO
an environmental problem but also becoming an economical and political one. Thus,
reducing CO emission is becoming more and more urgent. Recently, the
intergovernmental Panel on Climate Change (IPCC) has announced that the rising
2
emission issue is getting more and more sophisticated because it is not only
2
fossil fuel burning and deforestation are responsible for CO
2
concentration increasing
1
from pre-industrial (prior to 1750) levels of 280 ppm to 400 ppm today. It is
2 2
generally accepted that the conversion of CO to fuels can reduce CO emission
1
-7
efficiently and, simultaneously, alleviate our dependence on fossil energy. CO
2
conversion to methanol has been recognized as one of the most promising and
effective technology due to its indispensable position in chemical industry and
1,8,9
renewable energy field.
In chemical industry, the prominent role of methanol has
been extensively reviewed in methanol economy written by Nobel Laureate Olah et
1
0
al. In renewable energy, methanol is not only served as one of the most important
fuel alternatives but also as one of the most significant energy storage medium due to
its high energy density by both volume and weight as well as many other advantages
1
in comparison with compressed air, flywheels, batteries, supercapacitors etc.
Therefore, the research of CO
2
conversion to methanol is experiencing a
1
1-16
renaissance.
Compared with renewable energy-based direct CO conversion to
2
methanol, i.e. artificial photosynthesis, photoelectrochemical reduction,
electrochemical reduction etc., thermochemical routes are considered as the practical
1
,5,6
one.
3

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Various catalytic systems have been employed for methanol synthesis from CO
by catalytic hydrogenation (thermochemical) routes. Among them, Cu/ZnO and
Cu/ZrO based catalysts are the most commonly used ones. Cu/ZrO catalyst was
widely investigated due to its high stability and selectivity to methanol.
to the methanol production from synthesis gas, an equivalent amount of H
generates during the process of CO hydrogenation. Large amount of H O can not
only decrease methanol yield, but also accelerate the crystallization of Cu and ZrO
2
2
2
1
7-18
In contrast
2
O
2
2
2
,
1
7-18
which usually deactivates the catalysts rapidly.
Carbon materials have strong
hydrophobicity and can prompt the H O desorption from the surface soon after its
2
2
formation. Therefore, carbon materials can reduce the negative effects of H O and are
1
9-22
desirable support of the catalysts.
Carbon nanotubes (CNTs) are usually used as catalyst support for CO and CO
2
hydrogenation due to their high aspect ratio (length/diameter), specific surface area,
mechanical strength, rigidity, electric conductivity, thermostability and adjustable
2
0-24
23,24
surface chemistry.
Bao et al.
extensively investigated the confinement effects
of CNTs and revealed that the catalysts, such as FeN, confined in the channel of
CNTs are more active for CO hydrogenation than the catalysts dispersed on the CNT
2
0,21
exterior walls. Zhang et al.
CO/CO hydrogenation was remarkably improved over metal decorated CNTs
promoted Cu-ZnO-Al oxide and Co-Cu catalysts. The same group also developed
highly active Pd-ZnO/CNTs catalyst for CO
reported that the activity and alcohol selectivity in
2
2
O
3
2
2
2
hydrogenation to methanol. Their
research demonstrated that the ‘‘Herringbone-type (h-type)’’ CNTs supported
4

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Pd-ZnO catalyst is more active than ‘‘Parallel-type (p-type)’’ CNTs supported one.
2
This means the structure of CNTs plays important roles in CO hydrogenation to
methanol.
Besides, carbon surface chemistry also plays vital roles on the catalytic
2
5-27
properties.
Our previous research demonstrated that carbon surface chemistry can
influence catalyst particle size and support-metal interactions remarkably, then
modify the catalytic performance dramatically, especially for multi-component
28-31
catalytic systems. The similar phenomena were also reported by other researchers.
In general, carbon surface can prompt H O desorption due to its high hydrophobicity;
2
when functionalized with basic groups, it should further benefit to CO
well as H adsorption due to the highly dispersed catalyst formation. Therefore, in this
research, Cu-ZrO catalysts supported on CNTs functionalized with various surface
2
adsorption as
2
2
groups, such as nitrogen-containing groups (basic), oxygen-containing groups (acidic),
and few oxygen-containing groups (close to neutral) were investigated to elucidate the
effects of carbon surface chemistry on the catalytic properties of CO
2
hydrogenation
to methanol.
Experimental
Catalyst preparation
In this work, CNTs supports with different surface functional groups were
selected to prepare the supported Cu-ZrO catalysts. Two types of multi-walled
2
carbon nanotubes (original CNTs and nitrogen containing CNTs) with outer diameters
of 8-15 nm were supplied by Chengdu Institute of Organic Chemistry. Original CNTs
5

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were put in 68 wt% HNO
filtered and washed with de-ionized water, followed by drying at 100 C overnight.
The obtained sample was designed as CNTs-1. CNTs-1 was further calcined in N
3
at 140 ℃ for 14 h in an oil bath, and then the mixture was
2
stream at 600 ℃ for 6 h, the obtained sample was marked as CNTs-2. Nitrogen
containing CNTs was used directly without further purification and labeled as
CNTs-3.
The Cu/ZrO
2
/CNTs catalysts were prepared using a deposition-precipitation
/CNTs. The process was as the
method and the mass ratio was 10/40/50 for Cu/ZrO
2
following: A mixed aqueous solution containing calculated amounts of
Cu(NO ) ·3H O (AR grade, Guangzhou Chemical Reagent Factory) and ZrO(NO )
3
2
2
3 2
(
AR grade, Shanghai Aladdin), as well as the pre-determined amount of precipitant
-1
NaOH (0.2 mol·L ，AR grade, Sinopharm Chemical Reagent Co. Ltd.) aqueous
solution were simultaneously dropped into the glass container which contained a
calculated amount of the CNTs dispersion in water. The suspension was stirred
o
vigorously at 70 C, in order to maintain the pH value in the range of 7.0-7.5. After
o
precipitation, the solid was kept at 70 C for another 30 min under stirring, and then
aged at room temperature for 1 h. The obtained solid was washed with de-ionized
water and finally recovered by filtration. This process was repeated several times until
the pH of the filtrate was 7.0±0.2. The solid was subsequently dried at 100 ℃
overnight, calcined in pure N at 350 ℃ for 2 h in a tube furnace. The catalysts with
2
different CNTs supports were named as the CZ/CNTs-1, CZ/CNTs-2, CZ/CNTs-3,
respectively.
6

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Catalyst characterization
X-ray photoelectron spectroscopy (XPS) measurements were conducted on an
ESCALAB 250 spectrometer (Thermo Fisher Scientific) equipped with Mg Kα
radiation (hυ = 1253.6 eV). The energy resolution was 0.45 eV, the imaging spatial
resolution was 3 μm and the minimum analysis area was 15 μm. C1s binding energy
corresponding to graphitic carbon was set at 284.6 eV and used as reference to
position the other peaks. Fitting of the XPS spectra was essential for the determination
of the relative amount of oxygen functionalized carbon species on the surface by
using XPS PEAK4.1 software.
The specific surface area, pore volume and pore size were determined by N
2
adsorption/desorption isotherms at -196 ℃, using an ASAP 2020N Accelerated
Surface Area and Porosimetry system (Micromeritics Instrument Corp). Before the
analysis, all samples were degassed under vacuum at 300 ℃ for 4 h. The specific
surface area and pore size distribution were determined according to five point test
formula and BJH method, respectively.
The actual composition of metal elements in the catalysts was measured using an
OPTIMA 8000DV ICP-AES (Perkin-Elmer). A sample with 20 mg was pretreated by
nitrohydrochloric acid at 160 ℃ for 10 h.
X-ray diffraction (XRD) analysis were recorded on a D8 Advance X-ray
diffractometer (Bruker) with Cu Kα radiation (λ = 0.15418 nm, 40 kV, 40 mA) in the
2
θ range of 10-90°. The XRD results were analyzed by comparison with JCPDS
cards.
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2 2
Temperature-programmed reduction in H (H -TPR) was carried out with
Micromeritics AutoChem Ⅱ2920 instrument in a quartz U-tube reactor. Prior to the
reduction, the sample (ca. 100 mg) was pretreated in flowing Ar stream at 300 ℃ for
3
0 min and then cooled down to 60 ℃. Afterwards, a 10 vol.% H
2
/Ar flow (30
mL·min ) was passed over the sample at a heating rate of 10 ℃·min from 60 to 800
. The hydrogen consumption was monitored by a TCD quantitatively calibrated
-1
-1
℃
with a commercial CuO standard.
Temperature-programmed H
2
2
desorption (H -TPD) was performed at
Micromeritics AutoChem Ⅱ2920 system. The sample was first reduced in 5 vol%
-1
H /Ar at 30 mL·min at 240 ℃ for 6 h. After cooling down to 60 ℃ in He, the
2
-
1
2 2
catalyst was saturated with H in 10 vol% H /Ar at a rate of 30 mL·min for 60 min
-1
and then flushed with He flow at 30 mL·min to remove all physical adsorbed
molecules. Afterward, the TPD experiment was started with a heating rate of 5
-1
-1
℃
·min to 650 ℃ under He flow at 30 mL·min . The desorbed H
2
was detected by
an on-line TCD. Temperature-programmed CO desorption (CO -TPD) was similar to
2
2
2 2 2
that of H -TPD, except that the catalyst was saturated with CO in 80 vol% CO /Ar
and TPD experiment was conducted under Ar flow.
Catalyst Evaluation
CO hydrogenation to methanol was tested in a continuous-flow, high pressure,
2
fixed-bed reactor. Catalyst (500 mg) was placed in the center of a stainless steel tube
reactor and the other part was filled with quartz sand (40-60 mesh). Prior to the
2
reaction, the catalyst was calcinated in N at 350 ℃ for 2 h, then reduced in situ in 5
8

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vol% H
2
/Ar at 240 °C for 10 h. The reaction was conducted at a stationary state under
reaction conditions of 200-300 °C, 3.0 MPa, V(H
2
2 2
):V(CO ):V(N ) = 69:23:8 and
-1
-1
GHSV = 3600 mL·gcat ·h . The product mixtures were analyzed by an on-line gas
chromatograph (Model 2014C for Shimazu) equipped with dual detectors (TCD and
FID) and triplet columns. The TCD detector was equipped with 5A column and PQ
column (3.0 m length), used for the analysis of N
the FID detector was equipped with a capillary column of 30.0 m long, used for the
analysis of CH OH, CH , C and CH OCH . CO conversion was calculated by a
N2 internal standard analysis method. The carbon-based selectivity for the
hydrogenation products, CH OH, CO and hydrocarbons was calculated by an internal
normalization method. The specific calculation method can be found in the
2 2
(internal standard), CO and CO ;
3
4
2
H
6
3
3
2
3
2
2
reference. The TOF calculation was based on the diameter of Cu which was
calculated by Scherrer equation through XRD results. TOF was calculated through the
following equation:
TOF = μCO2·XCO2·MCu / (Vm·wCu·mcat·DCu
)
Where μCO2 is CO flow rate in the feed gas, XCO2 is CO conversion, MCu is molar
2
2
mass of Cu, Vm is standard molar volume of gas, w is mass percent of Cu, m is
Cu
cat
[
32]
catalyst mass, DCu is the dispersion of Cu, calculated through the diameter of Cu
.
Results and Discussion
Catalyst Performance
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All catalysts were prepared using deposition-precipitation method with the initial
mass ratio of 10/40/50 for Cu/ZrO /CNTs. Cu-ZrO /CNTs with the CNTs bearing
oxygen-, few oxygen- and nitrogen-containing groups were denoted as CZ/CNTs-1,
CZ/CNTs-2 and CZ/CNTs-3, respectively. Their catalytic properties for CO
hydrogenation to methanol are displayed in Figure 1. As shown, both CO conversion
and CO selectivity increase with the reaction temperature, while CH OH selectivity
2
2
2
2
3
decreases with the increase of reaction temperature. CH
3
OH yield exhibits a volcano

trend with temperature increase, and the maximum appears at 260 C for all of the
2 3
catalysts. Interestingly, all catalysts present similar CO conversion, CH OH
selectivity and CO selectivity when the reaction temperature is above 260 C. This is
mainly because CO hydrogenation to methanol is an exothermic reversible reaction
2
and it is close to equilibrium at high reaction temperatures. In sharp contrast, when the
reaction temperature below 260 C, CO
yield over the investigated catalysts follow the same decrease order: CZ/CNTs-3 >
CZ/CNTs-2 > CZ/CNTs-1. The highest CH OH yield was obtained over CZ/CNTs-3,
2 3 3
conversion, CH OH selectivity and CH OH
3
about 7.07%, which is about 34.7% and 18.4% higher than that of CZ/CNTs-1 and
CZ/CNTs-2, respectively. For comparison, the catalytic performance of 10 wt%
2
Cu/ZrO catalyst was also evaluated and presented in supporting information (Figure
S-1). It exhibits similar catalytic properties to CZ/CNTs catalysts but with evidently

lower CH
3
OH yield. The stability of CZ/CNTs-3 was also tested at 260 C, 3.0 MPa
-1
-1
and the GHSV of 3600 mL·gcat ·h , as displayed in Figure 2. It is clear that the
catalyst was slightly deactivated in the first 24 hours and then the catalytic
1
0

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performance was retained in the following test time. This indicates that the
introduction of CNTs indeed contributes to the catalyst stability.
In order to outstand the influence of CNTs on the catalytic performance, the
turnover-frequency (TOF, the number of CO
2
-molecule hydrogenated on unit site of
-1
exposed Cu per second, s ) and space time yield of methanol (STY, methanol yield
-1
-1
per gram catalyst per hour, mg·g ·h ) are calculated and also listed in table 1. The
TOF calculation was based on the particle size of Cu which was determined by XRD
results (the dispersion of Cu, DCu = 1/dCu). The catalytic performance of other
Cu-ZrO containing systems are also collected from the literature and listed in Table 1
2
3
2-40
for comparison.
The data (Table 1) show that, both TOF and STY values of the
investigated catalysts increase along the reaction temperature. At the same reaction
temperature, the TOF closely resembles each other except that of CZ/CNTs-3 at
2
20 °C; however, the STY value increases in the order of CZ/CNTs-1, CZ/CNTs-2
and CZ/CNTs-3. This suggests the high methanol yield may originate from large
amount of active sites at high reaction temperature. Due to the complexity of the
catalyst composition and the shortage of reaction data, many efforts have been made
to compare the current data with published ones in Table 1. The preliminary
conclusions can be drawn that the TOF of the most active catalyst in the current
research, CZ/CNTs-3, is obviously higher than most of those in the literature,
accompanied with comparably high STY value with less copper mass (10.3 wt%).
Although there are some catalytic systems, such as Cu(58.6)/ZrO2(41.4)
,
Cu /B O /ZrO , Cu /Ga O /ZrO and CuO /ZnO(17.4)/ZrO2(39.6), exhibit
(27)
2
3(3)
2(70)
(27)
2
3(3)
2(70)
(43)
11

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higher STY value than the investigated catalysts, it may be due to the paramount high
Cu loading, or the promotion effects of the third components and/or different catalyst
preparation methods. The study indicates that CZ/CNTs behaviors remarkably high
2
intrinsic CO hydrogenation activity. The origination of high catalytic activity and
STY values of the investigated catalysts will be discussed in details below.
Catalyst characterization
The surface chemistry of various CNTs samples was investigated by XPS.
High-resolution XPS C1s region deconvolution of all samples was carried out, as
shown in Figure 3. The C1s spectra can be resolved into C-C groups (BE = 284.6 eV),
C=C groups (BE = 285.2-285.5 eV), C-OH groups (BE = 285.9-286.5 eV), C-N
groups (BE = 286.7 eV), C=O groups (BE = 287.4-288.0 eV), COOH groups (BE =
289.0 eV) and shake-up satellite peaks due to π→π* transitions in aromatic rings (BE
2
8-29
>
290eV).
The calculated percentages of different types of carbon as well as
carbon, oxygen and nitrogen content are listed in Table 2. It is very clear that CNTs-1
has the highest amount of oxygen about 7.03%; CNTs-2 holds the lowest amount
about 1.89%; CNTs-3 possess the moderate amount of oxygen about 5.76%.
Compared with CNTs-2, CNTs-1 has slightly higher amount of C-OH, C=O and
COOH groups, especially less stable acidic COOH groups; moreover, it is negligible
over CNTs-2. This means heat treatment in inert atmosphere at high temperature
removes large amount of oxygen containing functional groups, especially less stable
acidic COOH groups. This suggests that it is possible to reveal the influence of
oxygen containing functional groups, especially COOH groups on the catalytic
1
2

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properties. In addition, compared with CNTs-1, CNTs-3 has nearly similar amount of
stable C-OH, C=O groups and less stable acidic COOH groups; therefore, it enables
the investigation of the effect of nitrogen-containing functional groups on the catalytic
properties. The deconvolution of XPS O1s spectra were also conducted and put in
supporting information (Figure S-2 and Table S-3). The results are in accordance with
C1s spectra.
In order to obtain clear information of nitrogen-containing groups on CNTs-3,
the deconvolution of N1s peak was conducted and displayed in Figure 4. According to
the previous publications, there are three types of nitrogen-containing functional
3
0
groups on the surface of CNTs-3. The most abundant one is ascribed to
pyridinic-type nitrogen with BE = 398.2 eV; the moderate abundance one is attributed
to various forms of quaternary nitrogen atoms with BE = 400.8 eV; the least one is
3
0
assigned to oxygenated nitrogen species with BE = 402.7 eV. All of these groups
are basic and prone to combine with catalyst precursors and adsorb acidic CO
2
.
However, their interactions with the catalyst precursors and adsorption
strength/capacity are quite different; furthermore, it is difficult to distinguish their
functions from each other. Thus, the total effects of basic nitrogen-containing groups
on the catalytic properties will be investigated.
The specific surface area, pore volume and pore size distribution of various
CNTs and the corresponding catalysts after calcination are listed in Table 3. It is
2
-1
found that the specific surface area for CNTs-2 (193 m ·g ) is obviously higher,
2
-1
2
-1
compared to the other two analogs (184 m ·g and 160 m ·g for CNTs-1 and CNTs-3,
1
3

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respectively). The pore volume of three CNTs does the same trend. Furthermore, after
loading metal, both surface area and pore volume of CZ/CNTs catalysts decreased,
accompanied with limited pore size decrease. The actual components of the catalysts
is measured by ICP-AES and also listed in Table 3. It can be seen that the Cu loadings
are 12.8%, 12.3% and 10.3 wt% for CZ/CNTs-1, CZ/CNTs-2 and CZ/CNTs-3,
2
respectively (Table 3). The obviously lower loadings of Cu and ZrO over CNTs-3
can be explained by both the small specific surface area and the surface functional
groups of CNTs-3. Generally, surface functional groups serve as anchoring sites for
metal precursors, which will influence metal loading. However, it cannot be ignored
that not all the anchoring of metal species is equally related to the functional groups.
Therefore, it cannot be easily deduced that there exists a quantitative correlation
between the metal loading and functional groups. Besides, the nitrogen
functionalization can change the surface chemistry of the CNTs, e.g., hydrophobicity,
polarity and proton affinity, which may influence the adsorption of the charged metal
4
1-42
precursor in solution either.
Hence, all aspects should be taken into consideration.
Figure 5 (a) shows the XRD patterns of CZ/CNTs-3 with different treatment
conditions during the preparation. As shown, both fresh and calcinated catalysts give
the characteristic diffraction of (002) plane of graphite-like tube-wall of the CNTs at
22
2
θ = 25.9° (JCPDS 26-1080); the diffraction peaks located at 2θ = 35.6 and 38.8°
can be ascribed to the diffraction of CuO (-111) and (111) planes, respectively
4
3
(
JCPDS 45-0937). The wide diffraction peak in the range of 30.5～32.0° comes
from the diffraction of amorphous ZrO (JCPDS 37-1484), which is consistent with
2
1
4

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2
the literatures that ZrO will appear obvious crystallization after calcination above 500
4
4
°
2
C. Meanwhile, the calcinated catalyst shows stronger CuO and ZrO diffraction
peaks compared with the fresh catalyst, which is possibly due to the loss of
crystallization water during the calcination, increasing the crystallinity of CuO and
2
ZrO . After reduction, CuO feature diffraction peaks disappear while new diffraction
peaks at 2θ = 43.1 and 50.1° are simultaneously observed. These two diffraction
peaks can be attributed to the diffraction of elemental Cu (111) and (200) planes,
4
3
respectively (JCPDS 65-9026). The XRD patterns of CZ/CNTs-1 and CZ/CNTs-2
catalysts displayed similar trend as presented in supporting information (Figures S-4
and S-5, respectively).
Figure 5 (b) exhibits the XRD patterns of different catalysts after reduction in H2.
It is obvious that all catalysts have very similar diffraction features with only different
diffraction intensity of Cu. Its diffraction intensity decrease in the following order:
CZ/CNTs-1 > CZ/CNTs-2 > CZ/CNTs-3, especially for the diffraction of Cu (200)
planes. The corresponding copper crystallite size was calculated through Scherrer
equation and they are 21.1, 18.8 and 15.5 nm, respectively. Normally, the crystallinity
and dispersion of Cu is in accordance with their copper crystallite size. Therefore, the
dispersion of Cu increases in the order of CZ/CNTs-1, CZ/CNTs-2 and CZ/CNTs-3.
XPS results demonstrate that the oxygen content of CNTs-2 is much lower than
that of CNTs-1, and negligible carboxyl groups were detected. Combined with XRD
results that the crystal size of Cu particles on CZ/CNTs-2 after reduction is smaller
than that of CZ/CNTs-1, it can be deduced that large amount of oxygen containing
1
5

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functional groups, especially carboxylic acid group, may promote the growth of Cu on
CNTs presumably due to its decomposition during heating/reduction in H , resulting
2
in big Cu particle size. In contrast, the oxygen content of CNTs-3 is much higher than
that of CNTs-2, while the crystal size of Cu particles on CZ/CNTs-3 is obviously
smaller than that of CZ/CNTs-2. This means that nitrogen containing functional
2
9
groups play vital roles on the high metal dispersion on CNTs. Chen et al. observed
the similar phenomena that Pd nanoparticles were stabilized on nitrogen-containing
CNTs with narrower size distribution when compared with oxygen containing CNTs.
4
5
Chen et al. also disclosed that smaller Ru particle could be obtained on nitrogen
doped CNTs than none-doped CNTs. This is presumably attributed to the existence of
nitrogen atoms on the surface of CNTs, which increases the number of anchorage
sites for the adsorption of the catalyst precursor salt, improving the nucleation of the
active phase, preventing the surface mobility and then decreasing the probability of
46
particle sintering during the thermal treatments. Therefore, it can be concluded that
large amount of acidic oxygen containing functional groups result in large Cu particle
size while basic nitrogen containing functional groups are beneficial to the formation
of small Cu particle size.
2
The H -TPR spectra of the calcinated catalysts are shown in Figure 6. Three
reduction peaks can be revealed in all catalysts in the temperature range of 100-200
1
9
°
C, this is in agreement with previous publication. The low temperature reduction
120-150 °C) can be assigned to the amorphous or highly dispersed CuO reduction, or
CuO species reduction to form Cu O then to metal copper species through gradual
(
2
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6

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4
7,48
DOI: 10.1039/C5RA04774A
High temperature reduction (150-180 °C) can be assigned to bulk-like
reduction.
47,48
CuO phase reduction.
In addition, the reduction of other metal oxides was
unobserved. The hydrogen consumption at different reduction stages is calculated by
using the Gaussian function for TPR curve fitting, and the results are displayed in
Table 4. Notably, the hydrogen consumption at low temperature reduction over these
catalysts decreases in the following order: CZ/CNTs-3 ＞ CZ/CNTs-2 ＞
CZ/CNTs-1, with the corresponding hydrogen consumption of 0.596, 0.274 and
-1
49
0
.261molH2·molCu , respectively. As pointed out by Avgouropoulos et al. , smaller
copper oxide particles are easier to be reduced, so the copper oxide particle size
increase in the above order; this is in accordance with our XRD results. Furthermore,
the reduction property of copper-based catalyst is affected by the interaction between
1
1,49-50
11
copper oxide and the carrier.
Chew et al. discovered that nanoparticles
supported on nitrogen-functionalized CNTs were easier to be reduced compared to
those on oxygen-functionalized CNTs. This promotion reduction effects was
explained by the fact that the metal cations preferably locate on N-containing sites on
the carbon surface, which favors the electron-donating from surface N species to
1
1,51
metal oxide.
Besides, as expected large amount of low temperature reduction
19
would inhibit the growth of metal Cu crystallite, resulting in high Cu dispersion.
These results indicate that CNTs with nitrogen species promote copper oxides
reduction and contribute to small Cu particles formation; however, CNTs with oxygen
species display negative effects on Cu dispersion.
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7

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The total hydrogen consumption sequence is consistent with that of their low
temperature hydrogen consumption. For CZ/CNTs-3 catalyst, the total hydrogen
-1
consumptions is even little higher than 1 molH2·mol Cu, which may be caused by the
adsorption of H on the surface of the catalysts. This further confirms that
2
nitrogen-containing functional groups can promote the reduction of copper oxides.
Hence, the presence of nitrogen species on CNTs result in large amount of highly
dispersed Cu precursors and deep reduction of copper oxides. In summary,
nitrogen-containing functional groups enhance the dispersion of copper oxides;
promote their reduction, leading to small Cu crystal size.
Figure 7 Shows the H
2
-TPD spectra of various catalysts. All catalysts contain a
o
low temperature desorption peak spanning from 160 to 200 C and a high temperature
o
desorption peak located around 320 C. Zhang et al. observed the similar H
2
1
9
desorption phenomena in their H
2
-TPD study. The former desorption can be
ascribed to the desorption of dissociative adsorbed atomic hydrogen from the
19,35,52
metallic Cu surface,
while the high temperature desorption represents the
desorption of strongly adsorbed hydrogen on the bulk of Cu particles or the
1
9,35,38,52
desorption from metal oxide surface
. As reported in the literature, pure ZrO
2
hardly adsorbs any hydrogen, but hydrogen can transport from Cu to ZrO
2
via
35,38,52
spillover.
Thus, the total desorption amount of hydrogen from these sites
reflects the dissociative adsorption capability of the catalysts. Then, the relative
area-intensities of the two desorption peaks for these catalysts were calculated and
2
listed in Table 5. The total amount of desorbed H from the catalysts decrease in the
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8

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order: CZ/CNTs-3 ≥ CZ/CNTs-2 > CZ/CNTs-1. This trend agrees well with the Cu
particle size. These results indicate that small Cu particles are favorable to the
disassociation of H
It is accepted that CO
L-H mechanism; so many efforts have been made to carry out CO
2
and possess large H
2
disassociation capacity.
interface follows a dual-site
-TPD of the three
2
hydrogenation on Cu/ZrO
2
2
5
3
catalysts. However, surface functional groups decompose to release CO and CO
2
27
when rising temperature; moreover, metal can catalyze the decomposition.
2
Importantly, CNTs release more amounts of carbon oxides than adsorbed CO at most
time. Unfortunately, it cannot distinguish the carbon oxides desorption from CNTs or
from adsorbed species on ZrO . Thus, it failed to get any useful information from
2
2 2
CO -TPD of the investigated catalysts. Fortunately, CO -TPD of the supports were
obtained and displayed in Figure 8. As shown, CNTs-3 exhibits obvious CO
2
o
desorption near 400 C while CO
2
desorption from CNTs-1 and CNTs-2 were
unobserved. This suggests that basic nitrogen containing functional groups can absorb
CO
Discussion
Since CO
worthy to investigate the reaction behavior far away from the equilibrium (below 260
2
.
2
hydrogenation to methanol is an exothermic reversible reaction, it is
o
o
C, here). Below 260 C, both CO
order of CZ/CNTs-3 > CZ/CNTs-2 > CZ/CNTs-1. Thus, the CH
in the same order. CO hydrogenation to methanol on Cu/ZrO
dual-site L-H mechanism through HCOO route,
2 3
conversion and CH OH selectivity decrease in the
3
OH yield decreases
2
2
interface follows a
3
5,53
involving hydrogenation and
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9

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basic oxide sites at the interfaces of metal and oxide phases. According to H
CO -TPD analysis, the high CO conversion observed on CZ/CNTs-3 may be due to
the largest dissociative adsorption capacity of H and CO adsorption capability over
2
- and
2
2
2
2
CNTs-3, which mainly originates from nitrogen containing functional groups on
CNTs surface.
Besides, previous publications reveal that the high CH
3
OH selectivity may
originate from the highly active Cu species and/or the properties of metal oxide
3
5,38,54-57
supports/additives, such as γ basic site, the surface oxygen vacancies etc.
For
54
example, Fujita et al. believed that the CH OH selectivity is related to the
3
preferential emergence of smooth surface Cu sites, such as Cu (100) and Cu (111).
55
+
3
Toyir et al. proposed that the active Cu species enhanced CH OH selectivity and
5
6
reduced CO selectivity. Jung et al. put forward that the high concentration of
methoxide species and formate intermediates on monoclinic zirconia contributed to
3
5,38
the high CH
to γ basic site on metal oxides. Wang et al. attributed the high CH
the synergistic effect between CuO and the surface oxygen vacancies of doped ceria.
3
OH selectivity. Guo et al.
revealed that CH
3
OH selectivity is related
57
3
OH selectivity to
2
1,58
Moreover, Zhang et al.
adsorption-activation of H
and CH
here, the higher CH
due to much larger dissociative adsorption capacity of hydrogen. The highest CH
selectivity of CZ/CNTs-3 may originate from both the large hydrogen dissociative
has proposed that the catalysts with high capacity for
2
will increase the rate of surface hydrogenation reactions
3
OH selectivity. Therefore, combined the current research with the literature,
OH selectivity of CZ/CNTs-2 compared with CZ/CNTs-1 may be
OH
3
3
2
0

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adsorption capability and the CO
2
adsorption capability, compared with CZ/CNTs-1
and CZ/CNTs-2.
Conclusions
CNTs with basic and acidic surface functional groups supported Cu-ZrO
2
2
catalysts have been prepared by deposition-precipitation method and tested for CO
hydrogenation to methanol. CNTs, functionalized with nitrogen containing groups,
supported Cu-ZrO2 catalyst (CZ/CNT-3) demonstrated high intrinsic activity
-2
-1
(
turnover frequency 1.61x10 s ) and comparably high space time yield (84.0
-1
-1
mg·g
·h ) with Cu loading level of only 10.3 wt% under the reaction conditions of
cat)
(
-1
3
2 2 2
.0 MPa, 260 °C, V(H ):V(CO ):V(N ) = 69:23:8, and GHSV = 3600 h . The
existence of nitrogen containing functional groups enhances the dispersion of Cu
oxides, promotes their reduction, then results in small Cu crystal size, leading to high
H
2
2 2 3
and CO adsorption capability, finally high CO conversion, CH OH selectivity
and CH OH yield. The existence of acidic oxygen containing functional groups is
3
2
responsible for negative effects on Cu dispersion, resulting in low CO conversion,
finally low CH OH yield.
3
Acknowledgements
This work is financially supported by the National Natural Science Foundation
of China (No. 21207039, 51108187, U1201231), the Natural Science Foundation of
Guangdong Province, China (Grant No. S2011010000737), Specialized Research
Fund for the Doctoral Program of Higher Education, China (20110172120017), and
2
1

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Scientific Research Foundation for Returned Scholars, Ministry of Education of
China (2012).
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8
0
2
2
1
1
5
0
5
0
5
CZ/CNT-1
CZ/CNT-2
CZ/CNT-3
(b)
CZ/CNT-1
CZ/CNT-2
CZ/CNT-3
(a)
70
60
50
40
30
20
10
200
220
240
260
280
300
200
220
240
260 o
280
300
o
Temperature / C
Temperature / C
7
7
6
6
5
5
4
.5
.0
.5
.0
.5
.0
.5
(d)
80
70
60
50
40
30
20
10
(
c)
4.0
CZ/CNT-1
CZ/CNT-2
CZ/CNT-3
3
.5
.0
CZ/CNT-1
CZ/CNT-2
CZ/CNT-3
3
2.5
.0
2
2
00
220
240
260
280
300
2
00
220
240
260
280
300
o
o
Temperature / C
Temperature / C
Fig. 1 The (a) CO
CH
conditions: V(H
2
3
Conversion, (b) CH OH Selectivity, (c) CO Selectivity and (d)
3
OH yield over the catalysts at different reaction temperatures. Reaction
-1
-1
2
2 2
):V(CO ):V(N ) = 69:23:8, p = 3.0 MPa, GHSV = 3600 mL·gcat ·h .
2
6

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6
5
4
3
2
1
0
0
0
0
0
0
0
CH OH Selectivity
3
CO Conversion
2
CH OH Yield
3
0
20
40
60
80
100
Time on stream / h
2 3
Fig. 2 Variation of CO conversion, CH OH selectivity and yield with reaction time
over CZ/CNTs-3. Reaction conditions: V(H
2
):V(CO ):V(N ) = 69:23:8, p = 3.0 MPa,
2 2
-1
-1
GHSV = 3600 mL·gcat ·h .
2
7

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(a)
(b)
2
sp C=C
2
sp C=C
3
sp C-C
3
sp C-C
-C-O
-
C-O
>C=O
>
C=O
-C-OO
*
*
282
284
286
288
B.E. / ev
290
292
294 280
282
284
286
288
290
292
294
B.E. / ev
(
c)
2
sp C=C
3
sp C-C
-
C-O
C-N
>
C=O
-
C-OO *
2
82
284
286
288
B.E. / ev
290
292
294
Fig. 3 High resolution XPS C1s spectra of (a) CNTs-1, (b) CNTs-2 and (c) CNTs-3.
2
8

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N2
N3
N1
396
398
400
402
404
B.E. / ev
Fig. 4 High resolution XPS N1s spectra of CNTs-3.
2
9