Photothermal catalytic activity of combustion synthesized LaCo:XFe1- xO3 (0 ≤ x ≤ 1) perovskite for CO2 reduction with H2O to CH4 and CH3OH

Article abstract of DOI:10.1039/c7ra04879c

A range of LaCo_xFe_1-xO₃ perovskites with different Co-doping at the B-site were successfully synthesized via a sol-gel combustion route. Phase pure samples were obtained by calcination at 700 °C for 2 h. The morphology, crystal structure, surface area, band structures, oxygen vacancies and catalytic properties of each analog were characterized in detail. The band structures and oxygen vacancies of the catalysts were changed by adjusting the dopant concentration. The catalytic performance of the LaCo_xFe_1-xO₃ materials was characterized using water as a hydrogen source in the production of CH₄ and CH₃OH from CO₂. Under photothermal conditions, 350 °C with a visible light source equipped with 420 nm cut filter, it was found that x value will influence the total yield and the solar to CH₄ and CH₃OH energy conversion efficiency significantly. For x = 0.6 compound, the production of CH₄ and CH₃OH evolution can reach 437.28 and 13.75 μmol g^-1 in 6 h, which were 3.2 and 4.0 times that of LaFeO₃ under the same condition. The overall solar-to-methane efficiency and solar-to-methanol efficiency for LaCo_0.6Fe_0.4O₃ were 0.603%, 0.019% and for LaFeO₃ were 0.191%, 0.005% in the photothermal mode, respectively. The results show that the band gap energy is correlated with the photothermal activity and the LaCo_0.6Fe_0.4O₃ has the position of the CB and VB more suitable for CO₂ reduction. The CB and VB value of LaCo_0.6Fe_0.4O₃ were -0.258 and +1.422 V and CO₂ (-0.24 V for CB) can be reduced to methane under the conditions.

Full text of DOI:10.1039/c7ra04879c

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PAPER
Photothermal catalytic activity of combustion
synthesized LaCo Fe O (0 # x # 1) perovskite for
x
1ꢀx 3
Cite this: RSC Adv., 2017, 7, 45949
CO reduction with H O to CH and CH OH†
2
2
4
3
a
c
b
a
a
b
Lijuan Xu, Minh Ngoc Ha, Qiangsheng Guo, Lichao Wang, Yanan Ren, Na Sha*
ad
and Zhe Zhao*
A range of LaCo
x
Fe1ꢀx
O
3
perovskites with different Co-doping at the B-site were successfully synthesized
ꢁ
via a sol–gel combustion route. Phase pure samples were obtained by calcination at 700 C for 2 h. The
morphology, crystal structure, surface area, band structures, oxygen vacancies and catalytic properties of
each analog were characterized in detail. The band structures and oxygen vacancies of the catalysts
were changed by adjusting the dopant concentration. The catalytic performance of the LaCo Fe1ꢀxO
3
x
4 3
materials was characterized using water as a hydrogen source in the production of CH and CH OH
ꢁ
from CO
2
. Under photothermal conditions, 350 C with a visible light source equipped with 420 nm cut
filter, it was found that x value will influence the total yield and the solar to CH
4
and CH
3
OH energy
OH evolution
under the
same condition. The overall solar-to-methane efficiency and solar-to-methanol efficiency for
LaCo0.6Fe0.4 were 0.603%, 0.019% and for LaFeO were 0.191%, 0.005% in the photothermal mode,
respectively. The results show that the band gap energy is correlated with the photothermal activity and
the LaCo0.6Fe0.4 has the position of the CB and VB more suitable for CO reduction. The CB and VB
value of LaCo0.6Fe0.4 (ꢀ0.24 V for CB) can be reduced to
were ꢀ0.258 and +1.422 V and CO
methane under the conditions.
conversion efficiency significantly. For x ¼ 0.6 compound, the production of CH
4
and CH
3
ꢀ1
can reach 437.28 and 13.75 mmol g in 6 h, which were 3.2 and 4.0 times that of LaFeO
3
O
3
3
Received 6th May 2017
Accepted 14th September 2017
O
3
2
DOI: 10.1039/c7ra04879c
O
3
2
rsc.li/rsc-advances
7
quite attractive. A more environmentally robust solution is to
convert CO into raw chemical materials using solar energy.
2
Introduction
8,9
Global industrialization leading to rising energy demand has This approach has the dual advantages of addressing the CO
2
1
resulted in a substantial increase in CO
2
emissions worldwide.
emissions from factories, cars and other sources while
It is now broadly accepted that this increase is having a signi- providing by-products that are raw materials for future use as
2,3
10
cant impact on the environment. Finding a solution to this fuels.
problem has become a global research focus with many inves-
Photocatalysis is an emerging eld of catalysis that has
tigators using conventional physical capture and storage tech- developed over the last 20 years. Given the environmental
4,5
niques to control produced CO2.
However, there are impact of current energy consumption practices, photocatalysis
numerous drawbacks to this approach, such as low capture has the potential to provide a promising green and efficient
efficiency, high process cost, high energy consumption and alternative. A number of different semiconductor materials are
6
11–14
potential environmental hazards. Compared with this tradi- being used as photocatalysts.
The electrochemical splitting
tional physical method, chemical utilization is more economi- of water using TiO as a catalyst, published by Fujishima and
2
15
cally favorable and to convert CO
2
into hydrocarbon fuels was Honda in 1972, is regarded as the beginning of photocatalysis.
TiO has since been studied extensively owing to its efficiency,
2
a
stability, and low cost. But its wide band gap requires the use of
ultra-violet energy to initiate the reactions, resulting in its
School of Chemical and Environmental Engineering, Shanghai Institute of photocatalysis in the existence of some limitations. Photo-
School of Materials Science and Engineering, Shanghai Institute of Technology,
Shanghai 201418, China. E-mail: zhezhao@kth.se
b
Technology, Shanghai 201418, China. E-mail: shana8033@163.com
2 2
catalytic reduction of CO is a viable method for converting CO
c
Faculty of Chemistry, Hanoi University of Science, Vietnam National University, Hanoi
into chemical raw materials (carbon compounds such as inor-
ganic carbonates, methane and methanol) and has consider-
1
0000, Vietnam
d
Department of Materials Science and Engineering, KTH Royal Institute of Technology,
SE-100 44 Stockholm, Sweden
16,17
able signicance in carbon recycling.
Since Hiroshi reported
the photocatalytic reduction of CO into organic compounds
2
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra04879c
18
over semiconductor particles suspended in water, a new
1
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ꢁ
method combining photo, thermal, electrical and chemical was performed at 350 C and used a 300 W Xe lamp with a UV-
processes on the basis of photocatalysis has been developed by light lter (l > 420 nm). Taking a sample every hour and qual-
researchers. This approach improved the conversion rate for the itatively analyzed by GS-Tek (Echromtek A90) and quantitatively
reduction of CO2 to CO, C and O2 and H O to H and O2, analyzed by gas chromatograph equipped with a ame ioniza-
2
2
compared with conversion rates for photo or thermal catalysis tion detector (S1†). Finally, the catalytic performance of
19,20
alone.
Perovskites, which have the general formula ABO
LaCo
x
Fe1ꢀx
O
3
(x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1) are reported, with the
3
, are photothermal reduction of CO
2
with H O vapor to CH used to
2
4
materials of potential owing to their stability, magnetism, probe catalytic activity. The inuence of Co content on the
crystal structure, electrical conductivity and piezoelectric perovskite band gap, the CB and VB positions, oxygen vacancies
properties. These advantages give rise to applications in and resulting photothermal activity for the CO reduction are
2
2
1–23
24
magnetoresistance devices,
as solid electrolytes, as trans- revealed in detail.
25–28
29–32
ducers
and in catalysis.
Many perovskite materials have
good response in the ultraviolet region. Among them, transition
metal based LaFeO is a visible light active material, which is
suitable for the use of renewable solar energy as a light source to
excite electrons for the CO reduction. LaFeO has shown great Impurity phases within a crystal structure can have a substan-
3
Results and discussion
2
3
promising in terms of catalytic activity in the photocatalytic tial effect on the mechanical and electrical properties of
decomposition of water and the degradation of organic pollut- a material. The XRD patterns of LaCo0.4Fe0.6O nanoparticles
3
34–38
ants.
The efficiency of a material in harnessing solar energy prepared at different calcination temperatures are shown in
is related to its band gap, which determines the range of the Fig. 2a. Through comparison with the characteristic peaks of
incident photon band that can be absorbed and can further the perovskite LaFeO (Fig. 2b), we conclude that the Co-doped
3
39
3
affects the photocatalytic activity. For the typical ABO perov- samples have perovskite structures and contain no noticeable
skite, both A- and B-site can be substituted to modify the local impurities. The diffraction peaks display an increase in
41
electronic structure, band gap and oxygen vacancy concentra- intensity and narrowing in width with increasing calcination
33,40
tion in order to realize versatile properties.
In some case, the temperature, indicating that the perovskite crystallinity is
band gap of LaFeO can even be modied through variation of affected by temperature. For the samples calcined at 500 and
3
41
ꢁ
the calcination temperature. In this study, we try to modify the 600 C, the LaCo0.4Fe0.6O nanoparticles, which belong to the
3
band gap and the relative position of the valence band and the Pm 3ꢀ m space group, have a cubic perovskite structure. The
ꢁ
conduction band through partial substitution of Fe by Co at the a values of the perovskites calcined at 500 and 600 C were
B-site in LaFeO
of materials.
3
to investigate photothermal catalytic efficiency 3.9065 and 3.8912, respectively.
The diffraction peaks for samples calcined at 700, 800 and
ꢁ
In a typical experiment, the catalysts in accordance with 900 C suggest that these samples exhibit orthorhombic crystal
formula LaCo perovskites were synthesized via a sol– systems with an ABO type perovskite structure (JCPDS card no.
Fe1ꢀxO
x 3
3
gel combustion route and were calcined in air for 2 h at different 01-081-2301), which belong to the Pnma space group. The
ꢁ
temperatures between 500 and 900 C. Then the synthesized crystallite sizes determined by the Scherrer equation are 73.85,
samples were characterized using
a
range of scientic 34.73, 24.67, 31.59 and 59.54 nm for the samples calcined at
ꢁ
researching instruments (S1†). The whole experiment have been 500, 600, 700, 800 and 900 C, respectively. The small crystallite
carried out in a catalytic reactor as show in Fig. 1. The reaction sizes favored the surface reaction kinetic and electrical trans-
was in a gas tight circulation system, the volume of the reaction port properties of the catalysts. Thus calcination temperature of
ꢁ
system was about 130 mL. The evaluation of catalytic activity 700 C was chosen for all samples in the subsequent
2
Fig. 1 Photothermal catalytic reactor for CO reduction study.
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experiments in order to promise both small crystallite size and Table 2 The surface properties of the catalysts were examined by BET
surface area
reasonably good crystallinity.
The XRD patterns of LaCo Fe O (x ¼ 0, 0.2, 0.4, 0.6, 0.8
2
ꢀ1
x
1ꢀx
3
Ratio of Co doping (x)
Surface area (BET) m g
Pore size (nm)
and 1) nanoparticles prepared using the sol–gel combustion
ꢁ
method with calcination at 700 C for 2 h, are shown in Fig. 2b. 0.0
9.2853
7.3525
8.4393
9.7343
5.1724
2.7064
28.29821
44.57917
26.57818
21.50945
19.76648
73.00958
Analysis of the characteristic peaks revealed no signicant 0.2
0
0
0
1
.4
.6
.8
.0
impurity phases. As the partial substitution of Fe by Co
increases, we see that the peak intensity decreases and some of
the peaks become split, the diffraction peaks shi regularly to
the right, because of the greater strength of Fe–O bonding
23
compared with Co–O bonding. Additionally it was observed
that the perovskite structure is orthorhombic when the Co-
doping level is 0, 0.2 or 0.4, however the structures are
The SEM images of LaCo Fe O nanoparticles calcined at
x
1ꢀx
3
ꢁ
trigonal for crystals with Co-doping from 0.6 to 1. This suggests 700 C are shown in Fig. 3a–f. As would be expected as a result of
that Co-doping changes the lattice structure and x ¼ 0.4–0.6 sol–gel combustion synthesis, it can be seen that the samples
might be the region that two phases co-existed simultaneously, are porous materials. Comparing these micrographs with that
even we can't see that clearly from the XRD patterns. In addi- of a pure LaFeO sample, reveals subtle differences in particle
3
tion, the crystallite size determined by the Scherrer equation morphogenesis. Samples with a Co-doping of 0.2, 0.4 and 0.6
can be also affected by the Co concentration in LaCo Fe O as (Fig. 3b–d) exhibited increasingly rounded particles as the
x
1ꢀx
3
shown in Table 1.
The specic surface areas and pore sizes of the LaCo
amount of cobalt increased, and showed greater porosity. When
Co-doping is raised to 0.8 and 1, aggregation growth among the
Fe1ꢀxO
x 3
catalysts with the different substitutions (x) by using nitrogen particles is clearly observed, which is expected to lead to
adsorption–desorption isotherms at 77 K on a Micrometrics a reduction in catalytic activity. The SEM images also reveal that
ASAP 2020 HD88 systems, are listed in Table 2. Except for the the pores of the samples with Co-doping of 0.8 and 1 are iso-
3
pure LaCoO , the specic surface area of the samples changed lated. This contrasts with the samples with lower doping ratios
within a limited range and thus the catalytic performance for where the channels or pores are interconnected. Furthermore,
samples of x ¼ 0, 0.2, 0.4, 0.6 and 0.8 can be comparable if we their particle size is 100 nm. Consequently, we propose that the
believe that surface reaction dominated the most catalysis Co to Fe molar ratios of 0.2, 0.4 and 0.6 provide the most suit-
process.
able morphology for good catalytic activity. Ratios above 0.6
ꢁ
Fig. 2 (a) XRD of LaCo0.4Fe0.6
O
3
nanoparticles at different temperature, (b) LaCo
x
Fe1ꢀx
O
3
photocatalyst at different Co-doping at 700 C for 2 h.
Table 1 The XRD analysis results
Samples (x)
Crystal system
Orthorhombic
2-Theta (degree)
d(112) (nm)
Unit cell a, b, c (nm)
Crystallite size (nm)
0
0
0
.0
.2
.4
32.1818
32.3892
32.5459
0.277923
0.276191
0.274897
5.5601, 5.5651, 7.8562
5.5171, 5.5201, 7.8184
5.5022, 5.5410, 7.7713
32.7808
26.8103
24.6748
Samples (x)
Crystal system
Trigonal
2-Theta (degree)
d(110) (nm)
Unit cell a ¼ b, c (nm)
Crystallite size (nm)
0
0
1
.6
.8
.0
32.7522
32.8838
32.8977
0.273212
0.272149
0.272037
5.4610, 13.116
5.4378, 13.164
5.4441, 13.112
20.6059
56.8321
48.7331
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ꢁ
3
Fig. 3 SEM images of LaCo Fe1ꢀxO nanoparticle in different Co rate at (a) 0, (b) 0.2, (c) 0.4, (d) 0.6, (e) 0.8, (f) 1 at 700 C for 2 h.
x
promote crystallite growth, leading to larger particle size and support the SEM ndings. In addition, the TEM image in Fig. 4b
lower BET surface area.
More detailed information on LaCo0.6Fe0.4
morphology is presented in the TEM images in Fig. 4. The Fe0.4O
3
shows clear lattice spacing (d values) for the LaCo0.6Fe0.4
nanoparticle nanoparticles. The interplanar (110) spacing of the LaCo0.6
nanoparticle was determined to be 0.27 nm by TEM.
O
3
-
O
3
image in Fig. 4a shows that small nanoparticles with an average This corresponds with the (110) lattice spacing of the trigonal
size of 50–100 nm form larger agglomerations composed of phase of the perovskite crystal structure from the XRD analysis,
interconnected nanocrystal skeletons. These observations and is the same as XRD Rietveld structure renement results for
ꢁ
Fig. 4 (a and b) TEM images and (c) EDS images of LaCo0.6Fe0.4
O
3ꢀd prepared at 700 C for 2 h.
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LaSrCoFeO6ꢀd. A Bruker-AXS 133 eV XFlash 4010 Detector table is attributed to Fe .
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3+
42,46
The presence of doublets char-
3
+
3+
associated with the SEM was used to measure the elemental acteristic of Co 2p (Co ) and Fe 2p (Fe ) was observed for the
composition of the LaCo0.6Fe0.4O as well as the nature of the samples, together with two bands corresponding to O 1s. These
3
distribution. From the EDS spectrum and the elemental bands can be attributed to the lattice, surface and absorbed
47
mapping images in Fig. 4c, it can be seen that La, Fe, Co and O oxygen. The absorbed oxygen has been omitted from Table 4
dominate the composition of LaCo0.6Fe0.4 . These data with only the lattice and surface components used to calculate
support the uniform distribution of La, Fe, Co and O within the the relative amounts of the two types of oxygen. Interestingly, it
catalyst. has been reported that the surface oxygen component develops
The chemical state and surface species of the LaCo
catalysts were established by XPS analysis as shown in Fig. 5. nent decreases with decreasing iron content, indicating an
O
3
LaCo0.6Fe0.4O
3
48
x
Fe1ꢀxO
3
with the increase of oxygen vacancies, and the lattice compo-
44
The survey spectra (Fig. 5d) show peaks that are ascribed to La, increase in the oxygen vacancies, which can lead to asymmetry
Fe, Co, O and C. No peaks corresponding to other elements were of the main peak. It has also been reported that high-resolution
observed. Table 3 reports the binding energies of the LaCo
x
-
O 1s XPS spectra indicate that the peak area of surface oxygen
Fe1ꢀx
O
3
perovskites, which provide evidence of the coexistence peaks obviously decreased with increasing catalytic reaction
33
3
of at least two oxidation states for both the Co and Fe regions. times. Besides, the O 1s signal of the LaCo0.6Fe0.4O perovskite
Peak deconvolutions are shown in Fig. 5a and b. These obser- was shows as well. The photothermal catalytic activities for CO
2
20,42,43
vations agree with the literature.
both Fe and Co at the B-site were 2 and 3 . The cobalt region showed that the catalytic activity depends on Co /Co , Fe /
The oxidation states for conversion with H
2
O vapor to CH
4
over LaCo0.6Fe0.4
O
3
catalysts
+
+
3+
2+
3+
2
+
shows the typical Co 2p pattern with Co 2p3/2 and Co 2p1/2 as Fe (detail in Table 3).
shown in Fig. 5a, and also includes the respective shake-up The diffuse reectance spectra of LaCo Fe O perovskites
satellite peaks. The Co 2p3/2 binding energies attributed to prepared at different Co-doping molar ratios were measured
x
1ꢀx
3
2
+
3+
Co and Co were observed at 780 and 782 eV respectively. The and are shown in Fig. 6a. From the optical absorption spectra,
2
+
3+
Co 2p1/2 binding energy resolved into Co and Co contribu- we can see that the samples had a complete absorption edge.
43,44
tions centered at 794 and 796 eV, respectively.
peak tting of the LaCo catalyst was performed in the visible light region, therefore, the properties of the
according to the constraints of the Fe and Fe components samples can be tested using visible light driven methods.
x 3
The Fe 2p The results indicate that LaCo Fe1ꢀxO nanoparticles are active
Fe1ꢀxO
x 3
2
+
3+
45
and the respective shake-up satellites indicated by Liu. The
To estimate the optical band gap (E ) of the prepared
g
high binding energy of the second component of iron in the samples, we used acquired data in the following equation:
ꢁ
3
Fig. 5 The XPS spectra of (a) Co 2p, (b) Fe 2p, (c) O 1s and (d) survey spectra of LaCo Fe1ꢀxO (0, 0.2, 0.4, 0.6, 0.8, 1) catalyst prepared at 700 C
x
for 2 h.
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Table 3 XPS binding energies relative to the perovskites LaCo Fe1ꢀxO
3
x
Co 2p (eV)
Fe 2p (eV)
Catalyst (x)
Co 2p3/2(%)
Co 2p1/2(%)
Fe 2p3/2(%)
Fe 2p1/2(%)
2
+
+
2+
0.0
0.2
0.4
0.6
0.8
1
Co
Co
Co
710.03(66.22%)
711.85(33.78%)
709.90(68.96%)
711.79(31.04%)
710.13(70.92%)
712.46(29.08%)
710.28(69.93%)
713.41(30.06%)
709.66(62.32%)
711.72(37.68%)
723.51(66.50%)
725.53(33.50%)
723.41(70.01%)
725.64(29.99%)
723.33(71.43%)
724.80(28.57%)
723.34(70.45%)
724.64(29.55%)
722.83(59.45%)
724.09(40.55%)
3
3+
Co
779.70(71.09%)
82.27(28.91%)
779.67(69.20%)
80.69(30.80%)
779.59(65.92%)
80.73(34.08%)
779.24(66.66%)
80.25(33.34%)
779.54(72.42%)
80.51(27.58%)
794.66(72.34%)
796.05(27.66%)
794.68(70.08%)
796.15(29.02%)
794.59(66.32%)
796.15(29.02%)
794.49(66.67%)
796.16(33.33%)
794.75(70.34%)
796.37(29.66%)
7
7
7
7
2
+
+
2+
Fe
Fe
Fe
3
3+
7
Fe
Table
4
XPS binding energies relative to the perovskites The lowest calculated band gap energy of 1.68 eV was obtained
LaCo
x
Fe1ꢀxO
3
for x ¼ 0.6. The highest energy band gap of 2.07 eV was calcu-
lated for the sample with no Co-doping. These data demon-
O 1s (eV)
0.0 (%) 0.2 (%) 0.4 (%) 0.6 (%) 0.8 (%) 1.0 (%)
strate the potential to control the optical response of LaFeO
3
49
Lattice oxygen
Surface oxygen
Absorbed oxygen 7.49
Surface/lattice 0.70
54.34
38.17
52.95
40.28
6.77
51.49
41.53
6.98
46.49
47.84
5.67
46.17
46.58
7.25
45.97
45.34
8.69
perovskites by controlling the band gap, and support the
ndings of subsequent photocatalytic performance tests. Even
that Fe and Co both belongs to the typical transition metals,
0.76
0.81
1.03
1.01
0.99
3+
3+
Fe and Co are different in either E electron number or the
g
feasibility of reduction to 2+ valance state. So it is rational to
expect that the mixing ration between Fe and Co should inu-
m
(
ahn) ¼ K(hn ꢀ E
g
)
x
3
ence the band structure of LaCo Fe1ꢀxO .
The correlation between the electronic band gap energy of
a material and its photocatalytic activity is well understood.
Narrower band gap facilitates more visible light absorption and
so might increase the photocatalysis performance. In addition
to determining band gap it is also important to study the
conduction band (CB) and valence band (VB), and absolute
where a is the optical absorption coefficient, hn is the photon
energy, and K is a constant. The value of m depends on the
nature of the transition in a semiconductor. Using theoretical
calculations, the value of m was found to be 2 for this case,
which describes a direct transition type model. The band gap
2
energy of the semiconductor can be obtained by plotting (ahn)
against the photon energy hn and extrapolating the plot line as
represented in Fig. 6b.
Table 5 Band gap of each LaCo Fe O nanoparticle in different Co
x
1ꢀx
3
Table 5 shows the band gap energies determined for the rate (form Co ¼ 0 to 1.0)
LaCo
x
Fe1ꢀx
O
3
materials (x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1), which were
Co/Fe
(eV)
0.0
2.07
0.2
1.78
0.4
1.70
0.6
1.68
0.8
1.80
1.0
1.84
found to be 2.07, 1.78, 1.70, 1.68, 1.80 and 1.84 eV, respectively.
E
g
2
Fig. 6 UV-Vis diffuse reflectance spectra of LaCo
LaCo sample (b).
x
Fe1ꢀxO
3
nanoparticle in different Co rate (a), plots of (ahn) versus photon energy of
x
Fe1ꢀxO
3
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energies in relation to the reduction and oxidation potentials. Using the above results in combination with the band gap
The potential of the CB edges (ECB) can be measured using the energies obtained from the diffuse reectance spectra, the ob-
Mott–Schottky (MS) relationship. As can be seen in Fig. 7a–f, the tained values of EVB were 1.756, 1.491, 1.437, 1.422, 1.524, and
typical n-type semiconductor characteristics of all samples were 1.535 V, respectively. The EVB value for the sample with a Co-
revealed by observing the positive slopes of the MS plots. The doping of 0.6 was found to be the lowest in the series (1.422
E
CB of the samples was determined against a Ag/AgCl reference V). Photocatalytic performance of a material requires that the
electrode in aqueous solution. The ECB of LaCo nano- valence band electrons are excited to the conduction band into
particles with x values ranging from 0 to 1 were ꢀ0.314, ꢀ0.289, high-energy transition states, resulting in the formation of
x
Fe1ꢀxO
3
ꢀ
0.263, ꢀ0.258, ꢀ0.276 and ꢀ0.305 V, respectively. The VB electron–hole pairs. A less positive VB position makes this
potential (E ) values for the samples can be estimated excitation easier and enhances the redox potential of catalysts
VB
approximately using the following equation: EVB ¼ c ꢀ E + 2E , in photocatalytic processes. By considering the following half-
e
g
ECB ¼ E ꢀ E , where c is the electronegativity of the atom, E
equations:
VB
g
CB
is the CB edge potential, E
e
is the energy on the hydrogen scale
1
À
Á
þ
ꢀ
o
ox
H
2
O/ O
2
þ 2H þ 2e ;
E
¼ 0:82 V vs: NHE
(4.5 eV) and E
g
is the optical band gap of the semiconductor.
2
Fig. 7 Mott–Schottky plots of LaCo
LaCo nanoparticle (g).
x
3
Fe1ꢀxO nanoparticle in different Co rate from 0 to 1 (a–f). A schematic illustration of the band structure of
x
Fe1ꢀxO
3
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Table 6 Photothermal catalytic activity and physical properties of the
ꢀ
+
o
red
x 1ꢀx 3
LaCo Fe O perovskites
CO + 8e + 8H / CH + 2H O, (E ¼ ꢀ0.24 V vs. NHE)
2
4
2
The rst hour
production
It is possible to deduce that the VB values for all samples are
Production aer
Total yield
(%)
o
+
more positive than E (H
2
O/H ) and the CB values are more
). These results indicate that the
ꢀ1
ꢀ1
Sample (x)
(mmol g
)
6 h (mmol g
)
o
negative than E (CO
2
/CH
4
Produce
CH
4
CH
3
OH
CH CH
4
3
OH
4 3
CH + CH OH
reactions could be initiated with photogenerated carriers
electrons and holes) in the irradiated LaCo nano-
particles, leading to CH production through the adsorption of
(
Fe1ꢀxO
x 3
0.0
93.86
192.80
213.01
275.83
114.27
126.03
1.68
5.43
7.72
9.61
2.63
2.42
138.59
322.84
355.46
437.28
193.90
224.70
3.45
9.61
13.01
13.75
5.28
2.44732
5.72843
6.34893
7.77147
3.96264
3.42235
4
0.2
CO2 and H O. All samples are black in color indicating 0.4
2
a potential for efficient visible light absorption and higher 0.6
24
0.8
.0
photothermal catalytic efficiency. This is roughly consistent
with the photothermal catalytic performance data reported in
Fig. 8.
1
4.72
4 3
Fig. 8a and b show CH and CH OH generation and photo-
thermal catalytic activity at different Co-doping rates, evaluated
under photothermal conditions, 350 C and visible light irra-
diation for 6 h. Fig. 8a and b show a signicant catalytic activity
increase with Co-doping at the B-site, when compared with
(
conversion rate of CO
methane or methanol) efficiency (S2†). As shown in Table S2,†
the best overall CO
conversion rate can reach 7.77% for x ¼ 0.6
LaCo . Considering the fact that it is not possible to
2 4 3
to CH and CH OH) STM (solar-to-
ꢁ
2
x
Fe1ꢀxO
3
3
LaFeO perovskite. At the end of the 6 h reaction time, the
avoid gas leakage in our reactor (only simple sealing imple-
mented) at elevated temperature, it is rational to expect that
a higher total yield can be expected if a high pressure reactor
can be built in the future. The highest STM efficiency of CH4
methane yield for samples with different Co/Fe molar ratios was
in the order of 0.6 > 0.4 > 0.2 > 0.8 > 1 > 0, a similar sequence was
also observed for methanol yield. It is also clear from the data
for all samples that photothermal catalytic activity increased
over time. The production of CH and CH OH from CO and
and CH
.019% respectively. Unfortunately, there is not much reported
data can be used to compare the performance of our catalyst
3
OH for x ¼ 0.6 LaCo
x
3
Fe1ꢀxO can reach 0.6% and
4
3
2
0
H O, under photothermal conditions aer 6 h for x ¼ 0, 0.2, 0.4,
2
0
2
.6, 0.8 and 1, were 138.59, 322.84, 355.46, 437.28, 193.90,
24.70 mmol g and 3.45, 9.61, 13.01, 13.75, 5.28, 4.72 mmol g
51
without noble metal loaded. F. Saladin had reported an
accumulated CH
light irradiation with CO and H O as raw material and Degussa
P25 TiO as catalyst in a very similar photothermal catalysis
study.
ꢀ
1
ꢀ1
ꢀ1
ꢁ
4
yield of 24 mmol g at 200 C aer 6 h UV
respectively (see Table 6). Looking more closely at the best
2
2
performing Co-doped material based on the data described (x ¼
2
4 3
0.6), it is clear that CH and CH OH generation over LaCo0.6-
3 3
Fe0.4O catalyst are 3.2 and 4.0 times of that of pure LaFeO . It is
In conclusion, the catalytic activity determined for LaCo
x
-
also worth noting that two end compounds, pure LaFeO and
3
Fe1ꢀx
O
3
nanoparticles is consistent with the various character-
LaCoO , possess the lowest yield of CH and CH OH. These
3
4
3
ization results. More specically we can speculate that the
inuencing factors on catalytic activity may be correlated to
surface area, crystallite size, band gap and oxygen vacancies
based on the following observations. First the greater the
surface area, the greater the area for photon radiation and
catalytic reaction sites. Second, the small pores size can
improve their catalytic performance by increasing the
results demonstrate that partial substitution of Fe with Co at
the B-site leads to better catalytic performance under photo-
thermal conditions.
Energy conversion efficiency of photothermal reduction of
2 2 4 3 x 3
CO with H O vapor to CH and CH OH over LaCo Fe1ꢀxO
perovskites can be expressed more adequately by total yield
ꢁ
Fig. 8 CH
4
generation over the under visible light irradiation and heating condition at 350 C (a) and CH
3
OH generation over the under visible
ꢁ
light irradiation and heating condition at 350 C (b).
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Fig. 9 Schematic diagram of the combined photo- and thermal-catalytic reduction of CO
2
with H
2
O vapor to CH
4
in one system over
x
3
LaCo Fe1ꢀxO perovskites.
adsorption efficiency of the reactants. However, the character- (carrier). Considering Fig. 7g and the reported data, we can
ization results of the samples in these two aspects shows that conclude that the VB edge position of all LaCo
there is no signicant difference in our experiments. Further- compounds is more negative than that needed for the produc-
more, the smaller the crystallite size, the shorter the migration tion of CH . It is therefore believed that methane can be mainly
time of photoinduced charge carriers from the bulk to the produced through photocatalytic reduction of CO . However,
surface, meaning that they have chance to reach to surface the VB edge of all LaCo compounds is not negative
enough for the production of CH OH through photocatalytic
x
Fe1ꢀxO
3
4
2
Fe1ꢀxO
x 3
38
before recombination to improve the catalytic efficiency.
3
Lastly, we focus on the specic effects of band structure and reduction process. So we have to point out that the production
oxygen vacancies on photothermal activity. By adjusting the of small amount of CH OH in our study might be the result of
3
band structure of the material, the band gaps of the Co-doped thermal catalysis, the reaction process can be referred to that
samples became smaller and the positions of the CB and VB described in ESI S3.† (c) Most of the carriers will recombine and
became more suitable for CO photo reduction. We reported the energy produced will disperse out in the form of thermal
2
that the minimum band gap is 1.68 eV when Co content is 0.6. energy or photons. (d) At the same time, the carriers produced
This contributes to the effective separation process of electron– by electronic excitation migrate to the surface of the sample or
hole pairs and also helps to inhibit the recombination of reach the interface with further aid. Furthermore, the corre-
photoinduced charge carriers. And oxygen vacancies can facil- sponding active sites of oxidation and reduction are formed.
itate the separation of water and in the reaction process to Finally, the carriers which have been photogenerated and
19
2
improve the catalytic efficiency. Additionally, our observations transmitted to the surface of the catalyst react with CO and
+
50
may depend on the photo-thermal coupling effect which could
2 2
H O molecules to obtain O and H and various hydrocarbons.
have resulted from our experimental conditions which used In addition, methane, methanol and other hydrocarbons can be
+
2
simultaneous irradiating with visible light and heating. The generated by interacting CO with H and electrons, which
combination of all of these factors can be regarded as favorable according to the reduction of force can accept different
for photocatalytic reaction give the observed increase in the numbers of electrons. In summary, the photothermal catalytic
photocatalytic activity.
The reaction mechanism for the photothermal reduction of the band structure and reduced the band gap. These ndings
CO to CH with H O vapor over catalyst samples is shown in may be of importance for a variety of applications.
Fig. 9. The catalytic process involves ve steps (a)–(d): (a) the
rst is irradiation of the catalyst surface by a visible light source.
This process requires a large catalyst surface area for exposure Conclusions
to radiation from the sun. Since LaCo perovskite
activity of LaFeO was enhanced by Co-doping, which adjusted
3
2
4
2

x
Fe1ꢀxO
3
We successfully synthesized LaCo
.8, 1) nanoparticles using the sol–gel process by Co-doping to
achieve different molar ratios at the B-site of the ABO style
perovskite model. Their structure, phase components,
morphology, surface area, and electronic structure were inves-
tigated using XRD, SEM, TEM, BET and UV-vis characterization
methods, and their catalytic properties were investigated. Under
x
Fe1ꢀx
O
3
(x ¼ 0, 0.2, 0.4, 0.6,
surfaces can adsorb CO2 and H O molecules, the catalyst
surface is vital for developing its role in the photocatalytic
reduction of CO . (b) An electron of the valence band absorbs
energy from a photon greater than or equal to the band gap
energy, and transitions to a high energy state in the conduction
band, resulting in the formation of an electron–hole pair
2
0
3
2
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
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1
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