Influence of element doping on La-Mn-Cu-O based perovskite precursors for methanol synthesis from CO2/H2

Article abstract of DOI:10.1039/c4ra07692c

Doped La-M-Mn-Cu-O based (M = Ce, Mg, Y, Zn) perovskite materials were prepared by the sol-gel method and characterized by XRD, N₂-adsorption, ICP-OES, SEM, TPR, N₂O-adsorption, XPS and TPD techniques. Upon the introduction of the fourth elements, all the samples keep the stable LaMnO₃ perovskite structure, and part of the copper species is separated from the perovskite lattice. More structure defects, lower reduction temperature and better low-temperature H₂ adsorption on the unit surface area are observed. In the application for methanol synthesis from CO₂/H₂, the Zn doped catalyst showed better performance which may because the strength of the weak basic sites play a significant role on methanol selectivity and the amount of H₂ adsorbed on the unit surface area is the key for CO₂ conversion. This journal is

Full text of DOI:10.1039/c4ra07692c

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PAPER
Influence of element doping on La–Mn–Cu–O
based perovskite precursors for methanol synthesis
from CO /H
Cite this: RSC Adv., 2014, 4, 48888
2
2
ab
a
c
a
a
ac
Haijuan Zhan, Feng Li, Peng Gao, Ning Zhao,* Fukui Xiao, Wei Wei
ad
and Yuhan Sun
Doped La–M–Mn–Cu–O based (M ¼ Ce, Mg, Y, Zn) perovskite materials were prepared by the sol–gel
method and characterized by XRD, N -adsorption, ICP-OES, SEM, TPR, N O-adsorption, XPS and TPD
techniques. Upon the introduction of the fourth elements, all the samples keep the stable LaMnO
perovskite structure, and part of the copper species is separated from the perovskite lattice. More
2
2
3
structure defects, lower reduction temperature and better low-temperature H
2
adsorption on the unit
2
, the Zn doped catalyst
Received 28th July 2014
Accepted 25th September 2014
surface area are observed. In the application for methanol synthesis from CO /H
2
showed better performance which may because the strength of the weak basic sites play a significant
DOI: 10.1039/c4ra07692c
2 2
role on methanol selectivity and the amount of H adsorbed on the unit surface area is the key for CO
www.rsc.org/advances
conversion.
x
the perovskite has been widely studied as the catalyst for NO ,
1
. Introduction
6
CH
CO
received much attention because of their important physical ronmental problem in the form of greenhouse effect and ozone
properties such as ferro-, piezo-, pyroelectricity, magnetism and depletion. For the sake of reducing the concentration of CO in
electro-optic effects. Perovskites were rst applied as catalysts atmosphere, various strategies have been implemented such as
x
, and CO conversion.
Perovskite-type oxides with the general formula of ABO
3
has
2
generated from fossil fuel combustion has led to envi-
2
1,2
1
7–10
for oxidation of carbon monoxide in 1952–1953 by Parravano.
capture, storage, and utilization of carbon dioxide.
An
For a typical ABO perovskite, the A-site is a larger rare earth important means of carbon dioxide utilization is the hydroge-
3
and/or alkaline earth cation and the B-site is a smaller transi- nation reaction. Among the products for carbon dioxide
tion metal cation. Fine dispersion of “metal on oxide” catalyst hydrogenation, methanol is considered as the most valuable
3
can be obtained by reduction of the perovskite oxides. The since it can be used as solvent, alternative fuel and raw material
advantages of perovskite materials could originate from the for synthesis of olens, aromatics or gasoline that derived from
11,12
ability to accommodate a variety of ions of different valence traditional petrochemical processes.
with appropriate radius that meet the tolerance factor
Many catalytic systems have been employed for methanol
pffiffiffi
3,4
synthesis from CO2 hydrogenation among which Cu-based
t ¼ ðrA þ rBÞ= 2ðrB þ rOÞ ð0:75\t\1Þ.
The
excellent
13
catalysts have been regarded as most effective. Different
tunable properties make perovskite potential candidate for
catalysis.
promoters have also been investigated such as Zn, Al, Mg, Mn,
14–16
0
0
B, Zr, Y, Ce,
which can improve the catalytic performance via
3
Recently, the perovskites with AA BB O structure have been
controlling the reactivity of the active site Cu phase by deter-
mining texture, exposure of the active sites and interaction
pattern with reagents, products and reaction intermediates.
used in catalysis since the substitution of A-site could inuence
the catalytic performance of B-site. The perovskites with special
properties can be obtained by advisable tailoring. At present,
5
17,18
Our previous works have found that the Cu-based catalysts
from perovskite precursors exhibit better methanol selectivity
compared with the other Cu-based catalysts due to the
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, South Taoyuan Road 27#, Taiyuan 030001, People's Republic
a+
19
of China. E-mail: zhaoning@sxicc.ac.cn; Fax: +86-351-041153; Tel: +86-351-4049612 appearance of Cu in the structure. The study for CO hydro-
b
University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China genation and CO
Center for Greenhouse Gas and Environmental Engineering, Shanghai Advanced skite oxides suggested that when the copper amount that
x 3
hydrogenation over LaMn1ꢀxCu O perov-
2
c
Research Institute, Chinese Academy of Sciences, Shanghai 201203, People's
substituted manganese was less than or equal to 50%, the
Republic of China
d
LaMnO could keep the perovskite structure and show prefer-
3
CAS Key Laboratory of Low-carbon Conversion Science and Engineering, Shanghai
able performance for methanol synthesis because of the inter-
Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203,
People's Republic of China
+
20
action of Cu and Mn. However, little work had been
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conducted on the inuence of the fourth metal elements doping
for the La–Mn–Cu–O perovskite catalysts. Therefore, in the
present work, the La–M–Mn–Cu–O (M ¼ Ce, Mg, Y, Zn) perov-
skites were prepared and characterized to obtain a clear insight
in the inuences of the fourth elements doping on the structure
and the catalytic performance. The methanol synthesis from
CO /H was chosen as the model reaction for the perovskite
2 2
materials to investigate the inuence of the structure changing
on the catalytic performance.
nCu ¼ 2Y
(2)
1
9
2
ꢀ1
)
S
Cu ¼ (nCu ꢂ N)/(1.4 ꢂ 10 ꢂ W)(m g
(3)
where DCu is the dispersion of Cu, W is the weight of the reduced
catalyst, SCu is the exposed copper surface area per gram cata-
lyst, nCu is molar number of copper, N is the Avogadro's
2
3
ꢀ1
19
constant (6.02 ꢂ 10 atoms mol ), and 1.4 ꢂ 10 is the
2
1,22
number of copper atoms per square meter.
The elemental composition was determined by inductively
coupled plasma optical emission spectroscopy (ICP-OES,
Thermo iCAP 6300), 100 mg of the each catalyst was dissolved
in 5% HCl solution and diluted. The reference solutions were
prepared with the metal nitrates or standard metal oxides used
in the catalyst preparation.
Investigations of the sample microstructure morphology
were performed on a FETXL30 S-FEG scanning electron micro-
scope (SEM) with an accelerating voltage of 10.0 kV.
2
. Experimental
2.1. Catalysts preparation
The La–M–Mn–Cu–O (M ¼ Ce, Mg, Y, Zn) perovskite-type oxides
were prepared by sol–gel method using citric acid as complex-
ing agent. The ratio for La, M, Mn, Cu is 0.8 : 0.2 : 0.5 : 0.5.
Adequate amounts of the precursor salts (La(NO
Mn(NO 50% solution; Y(NO $6H O; Cu(NO
Zn(NO $6H O; Ce(NO $6H O; Mg(NO $6H
citric acid were dissolved in deionized water at a molar ratio of
: 1 (metal cations: citric acid). The solution was heated to 353
K to remove the water, and then enhanced the temperature to
23 K until ignition. The resulting powder were nally calcined
3
)
)
3 2
3
$nH
$3H
2
O;
O;
3
)
2
,
3
)
3
2
2
3
)
2
2
3
)
3
2
3
)
2
2
O) along with
X-ray photoelectron spectroscopy (XPS) analyses were per-
formed over a Kratos XSAM800 spectrometer equipped with Al
Ka radiation (12 kV ꢂ 15 mA, hn ¼ 1486.6 eV) under ultrahigh
2
ꢀ
7
vacuum (10 Pa). The binding energies were calibrated inter-
4
nally by adventitious carbon deposit C(1s) with E ¼ 284.6 eV
b
under air at 673 K for 2 h and then at 1073 K for 4 h. The La–Mn–
Cu–O catalyst and Mg, Y, Zn, Ce doping catalysts were then
denoted as P, Mg–P, Y–P, Zn–P and Ce–P, respectively.
(
accuracy within ꢃ0.1 eV). Samples were treated under pure
hydrogen at 603 K for 2 h in the pre-treatment chamber before
transferred to the analysis chamber.
Temperature program reduction (TPR) measurements were
carried out in order to determine the reducible species and
performed in a U-tube quartz reactor. The samples (50 mg) were
2
.2. Characterization of catalysts
X-ray diffraction (XRD) analysis of powered samples was per-
formed using a PANalytica X'Pert Pro X-ray diffractometer with
Cu Ka radiation in the range of 10 < 2q < 90 and a scan step of
ꢀ1
purged with Ar (30 mL min ) at 423 K to remove physically
ꢁ
ꢁ
adsorbed water and then reduced in the ow of 5 vol% H + Ar
2
ꢁ
ꢀ1
0
.1 min
.
ꢀ1
ꢀ1
(
30 mL min ) at a heating rate of 5 K min up to 873 K.
Thermal conductivity detector (TCD) was used to monitor the
consumption of H
The adsorption property of H
2
measured by H temperature-programmed desorption (H -
The surface area of samples was determined by N adsorp-
2
tion–desorption at liquid nitrogen temperature 77.30 K, using a
Micromeritics Tristar 3000 instrument. Sample degassing was
carried out at 473 K prior to acquiring the adsorption isotherm.
The special areas were calculated from the isotherm according
to the Brunauer–Emmett–Teller (BET) method.
2
.
2
for the studied sample was
2
TPD). The catalyst was rst reduced at 603 K and cooled to 318 K
ꢀ1
to saturate for 60 min in H ow of 30 mL min for 2 h. Then
2
The dispersion of Cu (DCu) and exposed Cu surface area (SCu
)
ꢀ1
the catalyst was ushed with Ar ow (40 mL min ) to remove
were determined by dissociative N O adsorption and carried out
on Micromeritics AutoChem 2920 instrument. The catalysts
2
all physical adsorbed molecules. Aerward, the TPD experiment
ꢀ1
was started with a heating rate of 10 K min under Ar ow (40
ꢀ
1
(0.15 g) were rst reduced in 5% H
2
–Ar mixture (30 mL min
)
ꢀ1
mL min ), and the change of hydrogen signal was monitored
for 2 h at 603 K, and the amount of hydrogen consumption was
denoted as X. Then, the reduced samples cooled to 338 K and
isothermally purged with Ar for 30 min, aer which the sample
by a TCD and quantitatively calibrated by H
The basicity of the catalyst was measured by CO
temperature-programmed desorption (CO -TPD) was per-
2
pulses.
2
2
ꢀ
1
was exposed to N
metallic copper change into cuprous oxide (N
2
O (85 mL min ) for 1 h to ensure all the
2
O + Cu / N +
formed in the same way as that of H -TPD; the only difference
2
2
was that the desorbed CO2 was detected by a BALZER mass
Cu O). The samples were then ushed with Ar to remove the
2
spectrometer. CO peak area was quantitatively calibrated by
2
N O and cooled to room temperature. Finally, a pulse of pure H
2
2
injecting CO pulses.
2
was passed over the catalyst at 603 K. The surface Cu
2
O were
reduced in the pulse of pure H
was denoted as Y. The dispersion of Cu and exposed Cu
surface area of the catalyst were calculated by the eqn (1) (ref.
2
, and the amount of consumed
2
.3. Evaluation of catalysts
H
2
Activity measurements in the hydrogenation of CO
2
were
carried out in a continuous-ow, high-pressure, xed-bed
reactor. Catalyst (1.5 g, 40–60 mesh) diluted with quartz sand
(1) (60 mesh) was placed in a stainless steel tube reactor. Prior to
2
1) and eqn (3) (ref. 22)
2
Y
D
Cu
¼
ꢂ100%
X
2
reaction, the catalyst was reduced in pure H at a ow-rate of
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ꢀ
1
80 mL min
under atmospheric pressure. The reduction
temperature was programmed to increase from room temper-
ꢀ
1
ature to 603 K at a heating rate of 1 K min and maintained at
03 K for 8 h. The reactor was then cooled to room temperature.
Aer reduction, the activities of the catalyst samples in CO
hydrogenation process were determined under reaction condi-
6
2
ꢀ1
tions of 523 K, 5.0 MPa, n(H
The steady-state activity measurements were taken aer at least
4 h on the stream. Products were quantitative analyzed with
2
) : n(CO
2
) ¼ 3 : 1, GHSV ¼ 4000 h
.
2
gas chromatograph equipped with a TCD, and the columns
were TDX-01 for gas of H , CO and CO and the Propake-Q for
2
2
liquid of water and methanol. The CO2 conversion and
the carbon-based selectivity for CH OH, CO and CH were
calculated by an internal normalization method. The space
3
4
time yields (STYs) of CH
CH
the eqn (4):
3
OH, which gave the amounts of
OH produced per gram catalyst per hour, were dened as
3
W
T
ꢂXðCH
t ꢂ m
3
OHÞ
STYCH OH
¼
(4)
3
where W
X(CH OH) was the mass fraction of CH
time (h); m was the weight of catalyst (g).
T
was the total weight of CH
3
OH and H
3
OH; t was the reaction
2
O product (g);
3
3. Results and discussion
3.1. Textural and structural properties
The X-ray diffraction patterns of the fresh and reduced perov-
skites are presented in Fig. 1a and b respectively. It can be seen
that the LaMnO (JCPDS # 75-0440) are the main phase for all
3
ꢁ
samples. The diffraction peak at about 2q ¼ 32.5 shi towards
higher values with the addition of the fourth elements.
According to the Bragg formula, nl ¼ 2d sin q (where d denotes
the crystalline plane distance for indices (hkl), l is the X-ray
wave-length and q is the diffraction angle), the d value will
decrease when the ions with large radii are substituted by those
Fig. 1 XRD patterns of the calcined (a) and reduced (b) perovskite-
type catalysts: (,) LaMnO ; (C) CuO; (A) CeO ; (*) Cu.
3 2
3
+
3+
3+
˚
˚
˚
with small radii (1.36 A for La , 1.34 A for Ce , 1.08 A for Y ,
2
+
2+
2+
3+
˚
˚
˚
˚
0
.74 A for Zn , 0.73 A for Cu , 0.72 A for Mg , 0.65 A for Mn ),
which results in the shis of the diffraction peaks to higher
2
3,24
values.
The result implies that the fourth elements have
A-site and B-site of the perovskite oxide. However, doping with Y
2
+
strong inuences on La. Since the Mg is slightly smaller than
3+
in the sample has little inuence i.e. the characters of Y are
2
+
Cu , it might also have a complex effect on perovskite struc-
ture. For P sample, only LaMnO phase is observed which
means that all the copper either penetrates into the perovskite
3+
almost the same with La which then lead to little disturbance
3
for the catalyst structure. With the doping of Zn, the strongest
diffraction peak of LaMnO is observed which suggests that the
3
20
structure or well dispersed on the catalyst. With the addition
perovskite structure tends to perfection due to the similar
radius, valence state and electronegativity for Zn and Cu. In all
diffraction patterns, no phases that ascribe to Mg, Y or Zn is
observed while a new phase ascribed to CeO is found for the
2
sample of Ce–P which demonstrates that it is difficult for all the
ꢁ
ꢁ
of other ions, small peaks at 2q ¼ 35.6 and 38.9 assigned to
CuO (JCPDS # 89-5899) appear and the intensity of the diffrac-
3
tion peak of the LaMnO phase increase in the order of: Mg–P <
Ce–P < Y–P < P < Zn–P. The strength of the CuO peak is in the
order of: Y–P < Ce–P < Zn–P < Mg–P, which suggests the inu-
ence on the perovskite structure with the introduction of the
fourth elements. When Mg is introduced, the strongest
diffraction peak of CuO and the weakest diffraction peak of
Ce to enter the perovskite lattice which agrees with the
25
conclusion by Weng et al.
As shown in Fig. 1b, for all reduced samples, LaMnO phase
3
is still the main phase while the CuO phase disappears and the
Cu phase emerges which reveals that the reduction process does
not destroy the perovskite structure. However, the perovskite
structure has undergone some changes, e.g. the phase
LaMnO are found which implies the stronger impact of Mg for
3
the penetration of copper into the catalyst lattice as well as the
perfection of LaMnO
3
in comparison with other elements. As
Mg has the similar radius with Cu, it could have effect on both
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symmetry of all samples has changed except for P (Table 1). It is
interesting that the reduction process leads to ordered LaMnO3
phase only for Mg–P. The transition may occur in several steps
and the deviation from the cubic perovskite structure may
proceed from a simple distortion of the cubic unit cell, or an
3
enlargement of cubic unit cell, or a combination of both.
Moreover, the less particles shrink for Mg doped sample may
result from the electronic property of alkaline-earth metals.
The physicochemical properties of the calcined perovskite-
type catalysts are also summarized in Table 1. The Y–P and
the Zn–P possesses the largest and the lowest specic surface
area, respectively. Moreover, the exposed Cu surface area and
2
the Cu dispersion are measured by N O adsorption technique.
The largest copper surface area is observed for Y–P, neverthe-
less, the copper surface area cannot be measured for both P and
Ce–P. Since the surface copper may have strong inuence on the
Fig. 2 SEM images of the calcined catalysts (A) P, (B) Mg–P, (C) Y–P,
(D) Zn–P, (E) Ce–P.
26
2
activity for CO hydrogenation reaction, the lower copper
surface area may not favorable for the conversion of CO . The
2
fourth components, the binding energy decreases which indi-
cates that there are more electrons around oxygen. It is likely
that the fourth components transfer the electronic to the
oxygen. Obviously, the intensity of the peaks is different from
each other. The presence of surface adsorbed oxygen species
suggests the formation of oxygen vacancies in the defected
oxides. The increasing of the Oad/O ratio upon addition of
the fourth elements indicates that the increasing of the amount
of defect on perovskite oxide that is favorable for the activation
ICP results show that the real contents of the samples are
similar to the nominal values. For all catalysts, the experimental
lanthanum amount is lower than the theoretical value.
Fig. 2 shows the SEM images of the prepared catalysts. The
results show that all samples present as irregular granules.
Compared with other samples, the particle size is smaller for
Y–P and P. Particle agglomeration is observed for Mg–P and Ce–
P while not for Zn–P in spite of the large particles.
29
2ꢀ
of the catalyst. The binding energy of Mn 2p3/2 for MnO, Mn
and MnO are located at 640.6, 641.9 and 642.2 eV respectively.
As reported in other studies,
2 3
O
3
.2. The XPS investigations
2
30,31
the mean oxidation state of Mn
The reduced perovskite-type catalysts are analyzed by XPS, and
the binding energies (BE) of La 3d5/2, Mn 2p and O 1s are pre-
sented in Table 2. The La 3d5/2 peak at around 834.3 eV is close
to the value of pure lanthana at 834.4 eV, indicating that
lanthanum ions are present in the trivalent form. However,
with the addition of the fourth elements, the BE of La 3d5/2 shi
towards lower values which implies the increasing of the elec-
tron cloud density around La ions. It may due to the fourth
elements affect the transfer of the electrons of La to O since O
has the highest electronegativity value among all elements.
For O 1s, the lower binding energy at around 528.9–529.1 eV
can be ascribed to the lattice oxygen (O
at around 530.8–533.0 eV is assigned to the adsorbed oxygen
species (Oad) in the surface which contains hydroxyl (OH ),
ions at the surface layers is extremely difficult to detect by XPS.
However, the previous reports suggested that the BE difference
between Mn 2p3/2 and O 1s increase with about 0.6–0.7 eV for
3
+
4+
2
7
the change of the oxidation state between Mn and Mn . In
this study, as shown in Table 2, the BE difference is in the range
4
+
3+
of 112.3–113.0 eV which means a change of the Mn /Mn
32,33
ratio.
Since the binding energy of the Cu 2p band in the metal
3
/2
+
2
8
(932.6 eV) and in Cu (932.4 eV) are almost same, they can be
distinguished by different kinetic energy of the Auger Cu LMM
0
+
2+
2
ꢀ
15,28
line position in Cu (918.6 eV), Cu (916.7 eV) or in Cu (917.9
eV).
)
and the BE value
26,34
The Auger electron spectroscopies of Cu LMM of
ꢀ
reduced samples are shown in Fig. 3. The proles are decon-
voluted into two peaks. It can be seen that the majority of the
2
ꢀ
carbonate species (CO3 ) and molecular water. Doped with the
Table 1 The phase symmetry, lattice parameters and the physiochemical properties of the calcined perovskite-type catalysts
Size of LaMnO
crystallites ( A˚ )
3
Phase symmetry
Samples Calcined
a
2
ꢀ1
b
2
ꢀ1
Reduced
Calcined Reduced Elemental composition (ICP-OES)
S
BET (m g
)
Dispersion (%)
S
Cu (m g
)
P
Cubic
Orthorhombic Cubic
Cubic
Cubic
Cubic
Cubic
418
206
378
290
332
316
462
238
La0.84Mn0.51Cu0.50
La0.67Mg0.22Mn0.49Cu0.50
6.5
5.4
11.3
4.1
—
—
Mg–P
Y–P
Zn–P
Ce–P
0.9
3.8
0.7
—
1.2
4.6
0.9
—
Hexagonal
Orthorhombic 467
Orthorhombic 333
La0.67
Y0.23Mn0.47Cu0.50
La0.67Zn0.18Mn0.50Cu0.50
La0.68Ce0.19Mn0.49Cu0.50
7.2
a
b
Subscripts came from ICP results. Calculated from N
2
O dissociative adsorption.
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Table 2 The binding energy of La, Mn, O, the ratio of different oxygen species and relative surface concentration of metals measured by XPS
a
Binding energy (eV)
Relative surface concentration of metal (%)
2
Samples
P
La 3d5/2 (eV)
Mn 2p3/2 (eV)
642.2
O 1s (eV)
O
ad/O
La (%)
Mn (%)
Cu (%)
M (%)
835.1
834.0
834.4
834.1
834.3
529.2 (50.5%)
1.02
1.06
1.18
1.05
1.08
46.7 (50)
25.3 (40)
26.1 (40)
38.5 (40)
28.3 (40)
22.2 (25)
28.7 (25)
28.9 (25)
31.5 (25)
27.8 (25)
31.1 (25)
25.6 (25)
32.7 (25)
21.3 (25)
31.4 (25)
—
5
31.7 (49.5%)
529.1 (51.5%)
31.5 (48.5%)
529.0 (54.0%)
31.4 (46.0%)
529.1 (51.2%)
31.5 (48.8%)
529.2 (51.9%)
32.2 (48.1%)
Mg–P
Y–P
642.0
Mg: 20.4 (10)
Y: 12.3 (10)
Zn: 8.7 (10)
Ce: 12.5 (10)
5
641.3
5
Zn–P
Ce–P
641.5
5
641.8
5
a
Values in parentheses are nominal concentration normalized to the total metal content.
For Ce–P, six peaks located at 882.3, 888.4, 898.1, 900.7, 907.2
4
+
and 916.2 eV can be ascribed to Ce which is in accordance
37
with the XRD result of the Ce–P.
3.3. The reducibility of the prepared catalysts
The TPR proles of all samples are shown in Fig. 5. It can be
seen that the Y–P sample has two reduction peaks (denoted as
peak a and peak b) while other samples present only one peak
(denoted as peak b) which indicates that there is only one kind
of copper spices in the P, Mg–P, Zn–P and Ce–P samples. The
peak a at the lower temperature can be attributed to the
reduction of highly dispersed surface copper species, whereas
the peak at higher temperature is related to the reduction of the
38
bulk-like copper species. Upon introduction of the fourth
elements, the enrichment of the copper on the surface (Table 2)
Fig. 3 Cu LMM Auger electron spectroscopy of (a) P; (b) Mg–P; (c) Y– leads to lowered reduction temperature. Moreover, more Oad
P; (d) Zn–P; (e) Ce–P samples after reduction.
(Table 2) on the surface may also result in lowered reduction
temperature which conrms the function of the defection in the
perovskites. Y doping can increase the amount of the copper on
+
copper species exist as Cu for all samples except for the P the surface area which agrees with the N O chemisorption.
2
+
sample. The predominance of Cu in P is in accordance with the
The H2 consumption in the TPR experiment is listed in
report of Jia et al. and the results of XRD and N O-adsorption Table 3. The amount of H consumption is in the order of: Zn–P
20
2
2
0
mentioned above. The weak Cu peak could be the explanation > Y–P > P > Mg–P > Ce–P, which indicates that there are more
0
2
for the immeasurable of exposed Cu in the N O-adsorption copper species that can be reduced for Zn–P and Y–P.
results for Ce–P (Table 1).
The surface compositions and the nominal concentration of
the catalysts are also listed in Table 2. The enrichment of Cu
and depletion of La and Mn are observed for P. However, the Mn
enrichment is found with the doping of the fourth elements. As
Y and Ce entered the perovskite structure, it may lead to further
enrichment of Cu on the catalyst surface. For Zn–P, the lack of
both Zn and Cu on the surface may indicate that more Zn and
Cu enter the bulk of the sample. It is also found that Mg is
greatly enriched on the surface of Mg–P.
3
.4. H -TPD analysis
2
The H -TPD proles of the reduced catalysts are shown in Fig. 6.
In order to get more insight into the H -TPD results, the proles
are deconvoluted into two Gaussian peaks. According to the
literature, the lower peak around 400 K can be assigned to the
desorption of weakly molecularly adsorbed hydrogen on the Cu
sites. However, such desorption peak are not observed because
of less surface content of Cu . The desorption peak (peak a) at
lower temperature can be assigned to the desorption of disso-
ciative hydrogen on the surface copper species, while the higher
temperature peak (peak b) can be ascribed to desorption of
strongly adsorbed dissociative hydrogen on the bulk copper or
2
2
0
0
Fig. 4 shows the binding energy curves of the fourth
components. The peak at 49.4 eV is attributed to Mg–O
3
5
30
binding, 1021.8 eV is assigned to Zn–O binding. While the
1
36
56.8 eV and 158.6 eV may originate from the Y existed as 3+.
39–41
other metal oxides.
It can be seen that the desorption peak
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Fig. 4 XPS results of Mg 2p, Y 3d, Zn 3d and Ce 3d.
Table 3 The H
2
consumption amount in TPR and the H
2
desorption
amount in H -TPD experiment
2
ꢀ1
H
2
desorption (mmol g
)
2
H consumption
ꢀ1
a
b
Samples (mmol g
)
Peak a Peak b Total
H
2
-523
2
H -523
P
9.47
9.06
9.50
9.59
8.65
48.2
50.6
60.7
35.5
69.3
165.2 213.4 8.7
68.3 118.9 18.9
76.8 137.5 40.2
1.34
3.50
3.56
5.93
3.47
Mg–P
Y–P
Zn–P
Ce–P
46.8
82.3 24.3
51.6 120.9 25.0
a
b
H
2
desorption below 523 K.
2
H desorption below 523 K on per unit
ꢀ1
ꢀ2
are (mmol g
m ).
amount of hydrogen decreased. However, the desorption
amount of H at low temperature increased except Zn–P. It
2
implies that the doping of the fourth element is not favor the
Fig. 5 TPR profiles of the catalysts.
desorption of H
over, the H desorption on the unit surface area below 523 K
test temperature) increased for all the four components
samples.
2
, especially under high temperature. More-
2
(
2
of H at low temperature shis toward even lower temperature
with the introduction of the fourth components. However, the
temperature of the peak b is higher than that for methanol
synthesis, which means that these hydrogen might have little
effect on the reaction. In addition, the desorption peak of the H2
presents as a broad peak also suggests the strong spillover in
the perovskite catalysts.
The amount of H desorption over the pre-reduced mate-
2
rials is also listed in Table 3. With the introduction of the
fourth elements, the total and high temperature desorption
3
.5. CO -TPD analysis
2
Two desorption peaks (denoted as peak a and peak b) are
observed for all samples (Fig. 7) which could be assigned to
weak basic sites (around 400 K) and strong basic sites (around
6
00 K). The weak basic sites may be ascribed to the linear
adsorption of CO while the strong basic sites may result from
the bridge adsorption of CO With the introduction of the
2
41
2
.
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Table 4 The basicity calculated from CO
2
-TPD data
Adsorption type and distribution based in
a
2
CO -TPD data
Samples
Peak a
Peak b
Total
P
1.00 (383)
1.30 (396)
0.87 (387)
1.56 (401)
1.22 (385)
1.00 (614)
1.06 (583)
1.67 (595)
0.85 (596)
0.69 (581)
1.00
1.11
1.53
0.97
0.78
Mg–P
Y–P
Zn–P
Ce–P
a
The amount of basicity of P is assigned as 1.00 to compare with other
samplesandthevaluesinparenthesesarethedesorptiontemperature(K).
3
.6. Catalytic performance
The catalytic performances of the catalysts for CO
tion to methanol are summarized in Table 5. The main product
is methanol and the by-products are CO and H O derived from
It is found that the
2
hydrogena-
Fig. 6
2
H -TPD curves of the catalysts.
2
34,38,42
reverse water gas shi (RWGS) reaction.
CO conversion and methanol selectivity are improved for the
2
doped catalysts. With the introduction of Zn, both the CO2
conversion and the methanol selectivity increased greatly which
might due to the fact that CO
Cu –O–Zn.
2
could be activate on the site of
However, the addition of Ce leads to a slight
improvement for the catalytic performance. The relationship
between the CO conversion and the amount of H desorption
+
43,44
2
2
on unit surface area below 523 K (Table 3) is shown in Fig. 8. It
can be seen that the more H desorbed on the unit area, the
2
more CO conversion. Lower CO conversion may result from
2
2
lower copper content as well as lower surface area of copper in
the system. The results of Fig. 9 indicate that the strength of the
weak basic sites play an important role for methanol selectivity.
The formate theory may explain the relationship between the
2
catalytic performance and the H desorption as well as the
strength of the weakly basic sites in which the formaldehyde is
the intermediate species and the basic sites is the key factor for
methanol selectivity.
Fig. 7 CO
2
-TPD curves of the catalysts.
43
Previous studies have demonstrated that two active centers
were involved in the catalytic process of CO
2
hydrogenation for
fourth components, the peak a shis to higher temperature
which indicates the increasing of the strength of the weak basic
sites. The strength for the weak basic sites of the catalysts
increases in the order of: P < Ce–P < Y–P < Mg–P < Zn–P.
However, the peak b shis to lower temperature which reveals
Cu-based catalysts which undergo the formate intermediate
26,45
process,
2
i.e. CO was adsorbed on the surface of the metal
2
that the addition of the fourth elements decrease the basicity of Table 5 The performance for methanol synthesis from CO hydro-
a
genation over the reduced catalysts
the strong basic sites of the samples.
Doping with the fourth elements leads to the change of the
amount of the basic sites. The quantitative analysis for the CO -
2
Selectivity (C mol%)
2
CO conversion
TPD based on the relative area of the proles is listed in Table 4, Samples
in which the P sample is assigned as 1.00. For Mg–P, the
(%)
CH
3
OH
CO
CH
4
P
1.8
2.8
4.6
6.1
2.0
0.7
23.7
14.5
51.0
5.0
93.4
68.1
82.6
46.4
85.9
5.9
6.5
2.9
2.7
9.2
amount of all basic sites increases due to the alkalinity of Mg.
Although the amount of total basic sites and strong basic sites
for Y–P increases, the amount of weak basic sites decreases.
Mg–P
Y–P
Zn–P
Moreover, the amount of the weak basic sites increases for Zn–P Ce–P
and Ce–P samples despite the decrease of the amount of the
strong basic sites and total basic sites.
a
Reaction conditions: n(H
2
)/n(CO
2
) ¼ 3 : 1, T ¼ 523 K, P ¼ 5.0 MPa,
ꢀ1
GHSV ¼ 4000 h
.
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Scheme 1 Proposed reaction mechanism of CO
2
hydrogenation to
methanol over the perovskite catalysts.
the intermediate formed on other basic sites may produce CO.
The possible reaction mechanism for CO2 hydrogenation to
methanol over the perovskites catalysts can be described in
Scheme 1.
2
Fig. 8 The relationship between the CO conversion and the amount
2
of H desorbed on unit surface area below 523 K.
4. Conclusions
Doped La–M–Mn–Cu–O (M ¼ Ce, Mg, Y, Zn) catalysts derived
from perovskite-type precursors were prepared by sol–gel
method. Introduction of the fourth elements leads to the
3
separation of copper out of the LaMnO perovskite lattice and
thus produces more oxygen vacancies. Lower reduction
temperature and low temperature adsorption properties are
obtained with the increasing of defects for the doped samples.
For the reaction of CO hydrogenation to methanol, the CO2
2
conversion increases with increasing of the amount of absorbed
H
2
on the unit area under 523 K, while the methanol selectivity
increases upon increasing of the strength of the weak basic
sites. Both the H adsorption amounts on the unit area and
2
weak basic site strength are enhanced considerably as a result of
the introduction of Zn such that improved the catalytic perfor-
mance. Firstly, the Zn doped sample showed the largest H2
consumption in the TPR result which indicated the existence of
the most amount of reducible copper (the active site for the
reaction) in the sample. Secondly, the Zn doped sample
Fig. 9 The relationship between the selectivity for methanol and the
strength of the weak basic of the catalysts.
2
possessed the maximum amount of H desorption on the unit
surface area under 523 K. Thirdly, the samples possessed more
weakly basic sites which favored the methanol selectivity. The
Ce–P shows an iso-conversion compared with P due to the slight
oxides such as ZnO and ZrO
2 2
while the H was adsorbed on the
improvement of the H adsorption. However, with the slight
2
Cu sites. Then the activated species participated in the reaction
enhancement of the weak basic sites, the Ce–P showed almost
43,46,47
to produce the aim products.
During the reaction, either

ve times more selectivity towards methanol which implies that
methanol or by-product CO was produced from the interme-
diate, formate species, and the strength of the basic sites
decided the products selectivity. With the special perovskite
it is probably much easier to improve the methanol selectivity
for the perovskite catalytic system.
structure, in our study, CO
perovskite oxide carriers) to form activated CO
adsorbed and activated on the Cu sites resulting from the
2
could be adsorbed on the basic sites
*
(
2 2
and H may be
Acknowledgements
reduction of extra-perovskite and intra-perovskite copper This work was nancially supported by the Natural Science
species. The methanol selectivity is only related to the strength Foundation of China (21343012), the National Key Technology
of the weak basic sites. The hydrogen supplied from Cu sites Research and Development Program of the Ministry of Science
may spillover to the basic sites and react with the CO
2
* to and Technology (2013BAC11B00) and “Strategic Priority
form the formate intermediate. The formate species formed on Research Program-Climate Change: Carbon Budget and Related
the weak basic sites may lead to methanol production, while Issues” of the Chinese Academy of Sciences, Grant no.
XDA05010109, XDA05010110, XDA05010204.
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