Evaluation of photoreduction performance of CO2 to CH4 with H2O over alkaline-earth-metal-based peroskite nanoparticles

Article abstract of DOI:10.1166/jnn.2017.14794

This work demonstrates the photo-reduction of carbon dioxide (CO₂) into methane (CH₄)using ATiO₃ (A = Ca, Sr, Ba) perovskite nanoparticles. ATiO₃ nanoparticles were successfully synthesized by a hydrothermal method. The obtained nanoparticles were characterized using XRD, UV-vis, PL and BET techniques to confirm their structure and property. The CO₂ photo-reduction was tested in a closed reactor with UV-lamp (6 Wcm^?2) irradiation. The gas products were analyzed using a gas chromatograph equipped with FID and TCD detectors. Through these test, the band gap and recombination between excited electrons and holes were confirmed in the order of CaTiO₃ > SrTiO₃ > BaTiO₃. Also, the surface area of the catalysts and CO₂ adsorption were observed in the order of CaTiO₃ < SrTiO₃ < BaTiO₃. And among the catalysts studied, BaTiO₃ showed the highest photo activity for CO₂ reduction performance to CH₄. The yield of CH₄ from the photo-reduction of CO₂ with H₂O was 6.5 μmol/g_cat for 8 h using 0.2 g of BaTiO₃.

Full text of DOI:10.1166/jnn.2017.14794

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 7351–7357, 2017
Copyright © 2017 American Scientific Publishers
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Printed in the United States of America
Evaluation of Photoreduction Performance of CO2 to
CH4 with H2O Over Alkaline-Earth-Metal-Based
Peroskite Nanoparticles
Byeong Sub Kwak and Misook Kang∗
Department of Chemistry, College of Science, Yeungnam University, Gyeongsan, Gyeongbuk, 38541, Republic of Korea
This work demonstrates the photo-reduction of carbon dioxide (CO2) into methane (CH4ꢀ using
ATiO3 (A = Ca, Sr, Ba) perovskite nanoparticles. ATiO3 nanoparticles were successfully synthesized
by a hydrothermal method. The obtained nanoparticles were characterized using XRD, UV-vis, PL
and BET techniques to confirm their structure and property. The CO2 photo-reduction was tested
in a closed reactor with UV-lamp (6 Wcm−2) irradiation. The gas products were analyzed using
a gas chromatograph equipped with FID and TCD detectors. Through these test, the band gap
and recombination between excited electrons and holes were confirmed in the order of CaTiO3 >
SrTiO3 > BaTiO3. Also, the surface area of the catalysts and CO2 adsorption were observed in the
order of CaTiO3 < SrTiO3 < BaTiO3. And among the catalysts studied, BaTiO3 showed the highest
photo activity for CO2 reduction performance to CH4. The yield of CH4 from the photo-reduction of
IP: 185.223.165.150 On: Mon, 01 Oct 2018 04:33:39
CO2 with H2O was 6.5 ꢁmol/gcat for 8 h using 0.2 g of BaTiO3.
Copyright: American Scientific Publishers
Keywords: Semiconductors, Optical Properties, Photoluminescence Spectroscopy.
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1. INTRODUCTION
Recently, perovskite with general formula of ABO3,
is one of the interested and attractive materials for pho-
tocatalysis. In the perovskite structure, we can not only
change the composition of A and B sites, but can also
change the stoichiometry by incorporating with cations
to improve the catalytic ability.9 The most commonly
used perovskite photocatalysts are CaTiO3, SrTiO3, and
BaTiO3, which A site is alkaline earth metal and B site
is Ti. Among them, SrTiO3 has been widely used for
photocatalytic hydrogen evolution and organic pollutants
decomposition because it has the most similar band gap
with TiO2.10 However, there are problems like wide band
gap and rapid recombination between excited electrons
and holes. In order to solve these problems, several
studies have been conducted as follows. The reported
perovskite type photocatalysts are including Mn/SrTiO3,11
Cu/SrTiO3,12 N-doped TiO2-SrTiO3,13 Fe2O3/SrTiO3,14
Using the fossil fuel for industrial development leads to an
increase of CO2 concentration and global warming. It is
important to find out the way to reduce CO2 concentration
in the future. One of the ways is the utilization of CO2.
There are representatively two ways for utilizing CO2:
conversion of CO2 into fuel and non-conversion use of
CO2. There are various ways like chemical,1 biochemical,2
photochemical3 and electrochemical4 methods to convert
CO2. Among them, photochemical method is more a
promising technology to reduce CO2 into fuel.5 Lots of
research works were carried out after the first report by
Inoue et al. on photoreduction of CO2 to produce HCOOH,
HCHO, CH3OH, and CH4.6 Various semiconductors were
synthesized and used as photocatalysts to reduce CO2.
Among semiconductors, TiO2 is the most widely known
and used for photocatalyst.7ꢀ8 However, the photo-catalytic
activity TiO2 is not sufficient owing to its wide band gap
and fast recombination of photo-generated electron–hole
pairs. Therefore, it is necessary to develop another material
in order to improve the activity.
SrTiO3:Cr/Ta/F,15
SrTiO3/HZSM-5,16
SrTiO3/TiO2/
H-Titanate nanofiber,17 SrTiO3:Rh/Sb,18 (La, Cr)-doped
SrTiO3,19 Pt/SrTiO3,20 Zn/SrTiO3, Zn/BaTiO3,21 Ag3PO4/
Cr-SrTiO3,22 g-C3N4-SrTiO3:Rh,23 (Ba, Sr)TiO3/TiO2 core/
shell,24 N-BaTiO3,25 BaTiO3@g-C3N4.26 However, studies
using these catalysts are mainly concentrated on organic
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 10
1533-4880/2017/17/7351/007
doi:10.1166/jnn.2017.14794
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Evaluation of Photoreduction Performance of CO2 to CH4 with H2O
Kwak and Kang
pollutants decomposition and hydrogen production, but
the application for CO2 photoreduction to useful chemical
is insufficient. In particular, alkaline earth metals are well
known and used in CO2 capture. Generally, CaO and
CaOH are used for CO2 capture, for example, CaO and
CaOH are dissolved in water and then CO2 was flowed
through the solution, finally, CaCO3 was precipitated.
This property can be advantageous to adsorb CO2 on the
surface of the catalysts and to produce usefuel chemical
by CO2 photoreduction. Therefore, in this study, we added
alkaline earth metal as a CO2 capturer at A site and Ti
ion at B site in order to improve the photo activity by the
synergistic effect between alkaline earth metal and Ti.
Herein, we study the synthesis, characterization and
photocatalytic performance of ATiO3 (A = Ca, Sr, Ba).
ATiO3 was synthesized by a hydrothermal method and the
photocatalytic performance was tested for the reduction of
CO2 with H2O under UV light. The features of prepared
catalysts were measured using X-ray diffraction (XRD),
N2 adsorption–desorption isotherm, UV-vis spectroscopy,
Photo-luminescence (PL), and CO2 adsorption techniques.
solution and stirred until the solution became homoge-
neous. Then, pH was adjusted to 12 using 3 M NaOH and
the mixture was stirred overnight. The final white solu-
tion was thermal treated in an autoclave at 453 K for 8 h.
The obtained precipitate was washed for several times with
distilled water, ethanol and dried at 343 K for 24 h.
2.2. Characterization
The structure and crystallinity of synthesized ATiO3 (A =
Ca, Sr, Ba) were confirmed using X-ray diffraction (XRD,
model MPD from PANalytical) using nickel-filtered CuKꢂ
radiation (40.0 kV, 30.0 mA). The BET surface area was
measured using a Belsorp II mini (BEL, Japan Inc.).
The UV-visible absorption spectra were obtained using
a SCINCO Neosys-2000 spectrometer with a reflectance
sphere. Photoluminescence (PL) profiles of the catalysts
were obtained using a SCINCO FluoroMate FS-2 at room
temperature using a He–Cd laser source at a wavelength
of 325 nm. CO2 absorption isotherms under static condi-
tions were obtained by means of a Belsorp II mini (BEL,
Japan Inc.) at 343 K.
2.3. Measurement of Photocatalytic Activity
2. EXPERIMENTAL DETAILS
The photocatalytic tests for the reduction of CO2 with H2O
were carried out in a photo-reactor. The photoreactor con-
sists of a quartz chamber with length of 7.50 cm, width of
1.04 cm and total volume of 7.00 cm3 (Fig. 2). To photore-
2.1. Synthesis of Photocatalysts
The schematic diagram for the synthesis of ATiO3 (A =
Ca, Sr, Ba) is shown in Figure 1. Calcium chloride
dehydrate (CaCl2 · 2H2O, 78.0IP%:,18W5a.2k2o3.P1u6r5e.1C50heOmni-: Mon, 01 Oct 2018 04:33:39
Copyright: American Scientific Publishers
duce of CO2, 0.2 g of catalysts was taken in the reactor
cal, Japan), strontium chloride hexahydrate (SrCl2 ·6H2O,
Delivered by Ingenta
chamber with 11.3 mg of double distilled water. UV-lamp
99.0%, Junsei Chemical, Japan), and barium chloride
dehydrate (BaCl2 ·2H2O, 98.5%, Junsei Chemical, Japan)
were used as A site precursors. Titanium isopropoxide
(Ti{OCH(CH3ꢁ2}4, TTIP, 98.0%, Junsei, Chemical, Japan)
was used as B site precursor. Each 0.05 mol of CaCl2 ·
2H2O, SrCl2 · 6H2O, and BaCl2 · 2H2O were dissolved in
double distilled water (400 mL) with continuous stirring.
After that 0.05 mol of TTIP was added to the above
(6 Wcm−2, 20 cm length× 2.0 diameter, Shinan, Korea)
emitting light at 365 nm was used to irradiate the cata-
lyst. Supercritical fluid-grade CO2 with a certified max-
imum of hydrocarbon less than 1 ppm was used as the
reactant. Before illumination, the CO2 gas was initially
introduced to purge into the reactor chamber. After that,
the reactor was closed, and then lamp was switched on to
Figure 1. Preparation of ATiO3 (A = Ca, Sr, Ba) catalysts.
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Kwak and Kang
Evaluation of Photoreduction Performance of CO2 to CH4 with H2O
Figure 2. The schematic diagram of experimental set up of a photo reactor for CO2 reduction.
start the experiment. The reaction temperature and pres-
sure were maintained at 303 K and 1 atm, respectively.
The gas products were analyzed using a gas chromatogra-
phy (iGC7200, DS Science, Republic of Korea) equipped
with thermal conductivity detector (TCD) and flame ion-
ized detector (FID).
56.15ꢀ and 65.74ꢀ.29 The main peaks of catalysts are
shifted to low angle when the size of A site ion increases.
The spacing between layers of atoms was calculated using
Bragg’s Law.
nꢅ = 2d sin ꢃ
(1)
The inter-planar distance d spacing of CaTiO3, SrTiO3
and BaTiO3 were found to be 2.71 Å, 2.76 Å and 2.83 Å
respectively. It is considered that the d spacing of catalysts
increase when the radius of A site ion increases.
3. RESULTS AND DISCUSSION
Figure 3 shows the XRD patterns of ATiO3 (A = Ca,
Sr, Ba). The peaks of CaTiO3 are observed at 2ꢃ =
23ꢄ08ꢀ, 32.98ꢀ, 40.91ꢀ, 47.26ꢀ, 57.07ꢀ and 69.12ꢀ, which
Figures 4 and 5 show the UV-visible absorbance spectra
and Tauc plots of ATiO3 (A = Ca, Sr, Ba). The absorption
IP: 185.223.165.150 On: Moonf, c0a1taOlycstts20ap1p8e0ar4s:3in3:3th9e range of 350 nm to 400 nm.
are assigned respectively to the (100), (110), (111),
Copyright: American Scientific Publishers
The absorption of BaTiO3 is shifted to a long wavelength
(200), (211), and (220) planes of CaTiO3.27 The peaks
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when compared to that of CaTiO3 and SrTiO3. The band
of SrTiO3 are appeared at 2ꢃ = 22ꢄ72ꢀ, 32.35ꢀ, 39.89ꢀ,
46.39ꢀ, 52.23ꢀ, 57.67ꢀ, and 67.66ꢀ and they are indexed
to the (100), (110), (111), (200), (210), (211), and (220)
planes of SrTiO3.28 The peaks related to the (100), (110),
(111), (200), (210), (211) and (220) planes of BaTiO3 are
observed at 2ꢃ = 22ꢄ13ꢀ, 31.51ꢀ, 38.87ꢀ, 45.07ꢀ, 50.74ꢀ,
gap energy was calculated using Tauc Eq. (2).30
ꢂhꢆ = Aꢇhꢆ −Ebgꢁ2
(2)
The band gap energy was estimated from the point of
contact of the tangents to the Tauc plots ((ꢂhꢆꢁ1/2 vs. hꢆ).
The band gap energies of CaTiO3, SrTiO3 and BaTiO3
were found to be 3.47 eV, 3.24 eV and 3.11 eV respec-
tively (Table I). When the number of alkali earth metal
increases, the low band gap energies were observed. It was
Figure 3. XRD patterns of catalysts.
Figure 4. UV-visible absorption spectra of catalysts.
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Evaluation of Photoreduction Performance of CO2 to CH4 with H2O
Kwak and Kang
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Figure 5. Tauc plots of catalysts: (a) CaTiO , (b) SrTiO3 and (c) BaTiO3.
3 Copyright: American Scientific Publishers
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confirmed that the experiment value is a similar to the
previously reported band gap of CaTiO3,31 SrTiO3,32 and
BaTiO3.33 It is attributed to effect of alkali earth metal
oxide in ATiO3 (A = Ca, Sr, Ba). The band gap energies of
CaO,34 SrO35 and BaO36 were 7.1 eV, 5.71 eV and 4.8 eV
respectively. The band gap of the CaTiO3 is 3.47 eV, which
is higher than the UV light source (365 nm, 3.4 eV) of
the reactor. Therefore, in this experiment, CaTiO3 is diffi-
cult to excite electron from VB to CB compared to other
catalysts. In addition, SrTiO3 and BaTiO3, which have
narrower band gap than CaTiO3, are expected to excite
more electrons and induce CO2 photoreduction with H2O
to CH4.
BaTiO3 and the strong intensity is confirmed for CaTiO3.
The result was revealed that the electron–hole recombina-
tion rate is lower when A site ion is Ba. It is attributed
to the interruption of recombination between excited elec-
trons and holes by extended interplanar distance.37
N2 adsorption and desorption isotherms and BET sur-
face area of ATiO3 (A = Ca, Sr, Ba) are shown in Figure 7
and Table I respectively. The surface area of the catalysts
means the site which can be used for the reaction with the
Figure 6 shows PL spectra of catalysts to understand
the recombination of electron and holes pairs. The cata-
lysts exhibit a strong PL signal about 468.1 nm. The PL
intensity of catalysts decreases while going down a group
from Ca to Ba. The smallest PL intensity is observed for
Table I. The band gap, BET surface area and CO2 adsorption values
of catalysts.
Measurement
CaTiO3
SrTiO3
BaTiO3
Band gap (eV)
3ꢄ47
3ꢄ1171
0ꢄ2986
3ꢄ24
3ꢄ3609
0ꢄ3690
3ꢄ11
3ꢄ5239
0ꢄ4462
BET surface area (m2/g)
CO2 adsorbed (cm3/g)
Figure 6. PL emission spectra of catalysts.
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Kwak and Kang
Evaluation of Photoreduction Performance of CO2 to CH4 with H2O
IP: 185.223.165.150 On: Mon, 01 Oct 2018 04:33:39
Figure 7. N2 adsorption–desorption isotherms of catalysts: (a) CaTiO , (b) SrTiO and (c) BaTiO .
Copyright: American Scientific Publishers
3
3
3
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feed gas, and the increase of the active site is one of the
important factors to improve the catalyst performance. The
isotherms are similar to type III curves and hence the pre-
pared catalysts are non-porous materials.38 However, the
hysteresis were observed, which is due to the bulk pore
between the particles. The BET surface area of catalysts
increases as going down a group from Ca to Ba. There is
no significant change in the surface area between the par-
ticles, however, it is a huge difference in that the length
of CO2 and H2O is 2.32 Å and 2.75 Å. Therefore it is
possible to expect better catalytic performance on BaTiO3
when compared to CaTiO3 and SrTiO3.
1.2943 cm3 g−1 for CaTiO3, SrTiO3 and BaTiO3 respec-
tively. As a result, it is clear that the addition of alkali earth
metals going down a group increases the interaction CO2
adsorption capacity. In addition, strong interaction between
BaTiO3 and CO2 seems to be a significant advantage for
CO2 photoreduction.
Methane yield was used to evaluate the performance of
the catalysts. Figure 9(a) summarizes the methane yield
The adsorption isotherms and the amount of CO2
adsorbed on ATiO3 (A = Ca, Sr, Ba) at 374 K are displayed
in Figure 8 and Table I. Generally, the difference of the
amount of adsorbed gas depending on the surface area of
the catalyst was cooured. However, the amount of adsorp-
tion differs because of the interaction between the feed
gas and the metal rely on the characteristics of the metal.
Therefore, CO2 adsorption is one of the important factors
for CO2 photoreduction to CH4. In this experiment, the
slop rapidness in the low-pressure region of CO2 adsorp-
tion isotherm indicates the strong adsorption between CO2
and monolayer of catalysts.39 The tendency of CO2 adsorp-
tion is similar with the BET surface area of catalysts. The
CO2 uptake was 0.2986 cm3
g
−1, 0.3690 cm3 g−1 and
Figure 8. CO2 adsorption isotherms of catalysts.
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Evaluation of Photoreduction Performance of CO2 to CH4 with H2O
Kwak and Kang
Figure 9. Photocatalystic CO2 reduction with H2O on ATiO3 (A = Ca, Sr, Ba). (a) Yield of CH4, (b) yield of O2.
over ATiO3 (A = Ca, Sr, Ba) photocatalysts. The methane
is just produced after light on (1 or 2 h). The initial activ-
ity for CH4 production was highest on the SrTiO3 cat-
alyst, but after 4 h reaction, the CH4 yield was higher
on the BaTiO3 than the other catalysts. The low initial
reaction rate on the BaTiO3 can be explained from CO2
adsorption experiments. The interaction between BaTiO3
and CO2 is higher than CaTiO3 and SrTiO3 in the above
methane yields of catalysts are in the order of BaTiO3 >
SrTiO3 > CaTiO3. The best photocatalytic performance
(about 6.5 ꢈmol/gcatal) is shown by BaTiO3. The mech-
anism for methane production from CO2 reduction is as
follows:40
BaTiO3 +hꢆ → BaTiO3ꢇecb− ꢁ+BaTiO3ꢇhvb+
4H O+8h+ → 8H+ +2O
ꢁ
(3)
(4)
(5)
IP: 185.223.165.150 On: Mon, 01 Oct 2018 04:33:39
2
2
analysis. This means that the adsorption area between
Copyright: American Scientific Publishers
CO2 +8H+ → 8e− → CH4 +2H2O
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catalysts surface and H2O, another reactant, is reduced.
Therefore, the yield of CH4 is low in the initial reac-
tion because the amount of H2 generated from H2O is
small. However, as the reaction continues, H2O was grad-
ually adsorbed to the site where CO2 was converted to
CH4 and CH4 was desorbed. It promotes the H2 produc-
tion and which leads to CH4 production. After 8 h, the
Total reaction equation
CO2 +2H2O → CH4 +2O2
(6)
The enhanced photocatalytic activity of BaTiO3 can be
explained as follows: Figure 10 shows the proposed reac-
tion mechanism for the photo-reduction of CO2 to CH4
on BaTiO3 catalyst. First, CO2 could be easily captured
by barium electron acceptor. The photo-exited electrons
on the catalysts surface are transferred to CO2 and then
carbon dioxide radical (ꢁCO2−) is produced. Water (H2O)
could be decomposed to hydrogen ions (H+) and hydroxyl
radical (ꢁOH) by holes ꢁon BaTiO3. The H+ is used for
reduction of CO2 from CO−2 to CH4. Barium has better
CO2 capture ability than calcium and strontium. These are
the reasons for the highest yield of CH4 production using
BaTiO3.
4. CONCLUSION
In this study, CaTiO3, SrTiO3 and BaTiO3 nano-particles
have been successfully synthesized using a hydrothermal
method. XRD analysis reveals that the obtained cata-
lysts have perovskite structure. Through the above anal-
ysis, the band gap and recombination between excited
electrons and holes were confirmed in the order of
Figure 10. The proposed mechanism for the photo-reduction of CO2
with H2O on BaTiO3.
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Kwak and Kang
Evaluation of Photoreduction Performance of CO2 to CH4 with H2O
CaTiO3 > SrTiO3 > BaTiO3. Also, the surface area of the
catalysts and CO2 adsorption were observed in the order
of CaTiO3 < SrTiO3 < BaTiO3. In the photocatalytic activ-
ity, BaTiO3 showed the best performance and the yield of
methane on BaTiO3 is about 6.5 ꢈmol/gcatal. The reason
that the highest photo activity was showed on the BaTiO3
is because the photochemical and physical properties are
suitable for CO2 photo reduction to CH4. First, many elec-
trons can be excited from VB to CB on the BaTiO3 than
the other catalysts owing to the narrowest band gap. Next,
many excited electrons on the BaTiO3 can be used in the
reaction for the CO2 photo reduction because the recom-
binations between excited electrons and holes are the least
than CaTiO3 and SrTiO3 according to PL results. Finally, a
large amounted of CH4 were generated on the BaTiO3 sur-
face from adsorbed CO2 and electrons. From these results,
the photo-reduction mechanism suggests that the enhanced
photo-reactivity of the BaTiO3 was achieved via CO2 cap-
ture and electron transfer between CO2 and BaTiO3 sur-
face. Further research needs to improve the efficiency,
because the CO2 photoreduction is one of the best ways
to convert CO2 into useful compounds.
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Received: 26 July 2016. Accepted: 23 January 2017.
J. Nanosci. Nanotechnol. 17, 7351–7357, 2017
7357