Photocatalytically reducing CO2 to methyl formate in methanol over ag loaded SrTiO3 nanocrystal catalysts

Article abstract of DOI:10.1007/s10562-012-0876-3

Ag-SrTiO₃ nanocrystal photocatalysts were prepared by hydrothermal synthesis method for reducing CO₂ to methyl formate (MF) in methanol via ultraviolet irradiation. CO₂ was reduced to MF by electrons on conduction band of the photocatalyst. In order to obtain a high rate of MF evolution, we researched and optimized the preparation procedure of Ag-SrTiO₃ by changing the Ag dosage, hydrothermal temperature and time. The as synthesized photocatalystswere characterized with X-ray diffraction, UV-Vis absorption spectra, transmission electron microscope, N ₂ sorption analysis at 77 K and activity evaluation. A catalyst of 7 wt% Ag on SrTiO₃ with hydrothermal synthesis at 150 °C and 22 h was found to exhibit the highest photocatalytic activity inMF formation rate of 3,006 μmol/(h g cat), which was more remarkable than that of pristine SrTiO₃.

Full text of DOI:10.1007/s10562-012-0876-3

Catal Lett (2012) 142:1202–1210
DOI 10.1007/s10562-012-0876-3
Photocatalytically Reducing CO2 to Methyl Formate in Methanol
Over Ag Loaded SrTiO3 Nanocrystal Catalysts
•
Dandan Sui Xiaohong Yin Hongzhi Dong
•
•
•
Shiyue Qin Jingshuai Chen Wanlin Jiang
•
Received: 6 April 2012 / Accepted: 15 July 2012 / Published online: 2 August 2012
Ó Springer Science+Business Media, LLC 2012
Abstract Ag–SrTiO3 nanocrystal photocatalysts were pre-
pared by hydrothermal synthesis method for reducing CO2 to
methyl formate (MF) in methanol via ultraviolet irradiation.
CO2 was reduced to MF by electrons on conduction band of
the photocatalyst. In order to obtain a high rate of MF evo-
lution, weresearchedandoptimizedthepreparationprocedure
of Ag–SrTiO3 by changing the Ag dosage, hydrothermal
temperature and time. The as synthesized photocatalysts were
characterized with X-ray diffraction, UV–Vis absorption
spectra, transmission electron microscope, N2 sorption anal-
ysis at 77 K and activity evaluation. A catalyst of 7 wt% Ag
on SrTiO3 with hydrothermal synthesis at 150 °C and 22 h
was found to exhibit the highest photocatalytic activity in MF
formation rate of 3,006 lmol/(h g cat), which was more
remarkable than that of pristine SrTiO3.
in the atmosphere, such as converting CO2 to dissolved car-
bonate in sea water, capturing CO2 and transferring it to the
bottom of the sea in a supercritical state [1]. Meanwhile, the
exploration of new energy is urgent, due to the increasing lack
of fuel resources. The solar energy has become a feasible way
to solve the problems by using the sustainable energy of
sunlight to capture and convert CO2 in atmosphere to valuable
chemicals, which will prohibit global warming and sustain-
able energy shortage simultaneously. Photocatalysis is dom-
inated by photocatalysts, such as semiconductor of metal
oxideslikeTiO2, CdS, ZrO2, ZnO, andMgO, whichhavebeen
studied. Among them, nanostructured TiO2 is one of the most
promising photocatalytic materials in terms of its cost and
stability [2–6]. Meanwhile, the research is also focus on the
design and preparation of perovskite nanostructured photo-
catalysts due to their ferroelectricity, superconductivity, high
dielectric constant, and thermal stability. Strontium titanate
(SrTiO3) has a typical Perovskite structure, as a potential
photocatalyst, SrTiO3 and metal-loaded SrTiO3 have been
effectively utilized inwater decomposition anddegradation of
dye compounds under ultraviolet (UV) radiation [7–13].
In this paper, we used the hydrothermal method to
synthesize the Ag-loaded SrTiO3 nanocrystal photocata-
lysts, where Ag site acted as the electron acceptor to avoid
the recombination of electrons and holes, and result in
effective extension of the life-span of the electron–hole
pairs. The as prepared photocatalysts enhanced the photo-
catalytic activity remarkably in reducing CO2.
Keywords Photocatalysis ꢀ Reduction of CO2 ꢀ
Ag–SrTiO3 ꢀ Hydrothermal synthesis ꢀ Methanol ꢀ
Methyl formate
1 Introduction
The combustion of the fossil fuel has led to a sharp increase in
CO2 emission, and then resulted in worsening greenhouse
effect, which has attracted the worldwide attention. Many
effortshavebeenmadetoreducetheamountofcarbondioxide
D. Sui ꢀ X. Yin (&) ꢀ H. Dong ꢀ W. Jiang
School of Chemistry and Chemical Engineering,
Tianjin University of Technology, Tianjin 300384, China
e-mail: xiaohongyin@hotmail.com
2 Experimental
2.1 Preparation of Photocatalysts
S. Qin ꢀ J. Chen
Ag-loaded SrTiO3 powder was prepared by hydrothermal
synthesis. All reagents were used as received from Tianjin
School of Chemical Engineering and Technology,
Tianjin University, Tianjin 300072, China
123

Photocatalytically Reducing CO2 to Methyl Formate
1203
k is the wavelength (nm). The specific surface area of the
SrTiO3 photocatalysts was measured and calculated by a
BET apparatus (APP V-Sorb 2800P) by nitrogen sorption
at 77 K using the Brunauere–Emmette–Teller (BET)
model. Mean pore size and distribution were determined
by the Barrett–Joyner–Halenda (BJH) approach using the
desorption data.
Benchmark Chemical Reagent Co., Ltd of China without
further purification. The Ag–SrTiO3 was synthesized as
follows: strontium nitrate [Sr(NO3)2, 99.5 %] and silver
nitrate (AgNO3, 99 %) were mixed according to the mass
ratio Sr:Ag = 1 - X:X, (X = 0, 1, 3, 5, 7, 9 %, respec-
tively). First, the solid mixture was dissolved in 30 mL
deionized water under magnetic stirring; Second, 2.4 mL
tetrabutyl titanate was mixed with 30 mL absolute ethyl
alcohol to form a clear solution; Thirdly, a certain amount
of sodium hydroxide was added to 30 mL deionized water
with magnetic stirring; the next step was to mix 14 mL
oleic acid with 30 mL absolute ethyl alcohol via magnetic
stirring; Finally, all of the above solution was transferred
into a 170 mL of sealed Teflon-lined stainless-steel auto-
clave, which would be kept at 150 °C for 12 h and then
cooled down to room temperature. The solid product was
collected by centrifugal, washing with acetic acid, absolute
ethyl alcohol and deionized water for three times respec-
tively, and finally drying at 80 °C under vacuum.
2.3 Photocatalytic Activity Evaluation
The photocatalytic reduction of CO2 over the pristine
SrTiO3 and Ag–SrTiO3 were evaluated in a batch slurry
reactor showed in Fig. 1. A 250 W high-pressure mercury
lamp was employed as UV light, whose main wavelength
was 365 nm and 2800 lW/cm2 illumination intensity
reached the surface of the slurry. The photocatalytic
activity evaluation was performed at 25 °C. To keep the
reaction temperature constant, cooling water was recircu-
lated through the jacket in outside of the quartz glass
reactor. For each batch 20 mg photocatalyst was dispersed
into 20 mL methanol by using a magnetic stirrer. CO2
passed through the methanol at a flow rate of 100 mL/min
for 30 min to approach the absorption equilibrium before
irradiation. After ultraviolet light irradiation for 6 h, the
product solution was qualitatively analyzed by GC–MS
(Agilent 5975C) and quantitatively analyzed by gas chro-
matograph equipped with a hydrogen flame detector (FID).
A ReactIR 15 (Mettler-Toledo, Switzerland) reaction
analysis system equipped with a light fiber and Dicomp
(diamond composite) insertion probe was used to collect
FT-IR spectra of CO2 in liquid. For comparison, the blank
test was also carried out by irradiating the methanol
without photocatalyst, or with photocatalyst in dark, the
products were undetectable. This implied that the reaction
depended on catalyst and light irradiation, indicating the
reaction was a photocatalytic reduction process.
2.2 Characterization of Catalysts
The crystal structure of the pristine and Ag-loaded SrTiO3
powders were characterized by a Rigaku D X-ray powder
diffractometer with Cu Ka radiation scanning from 10° to
80° at a scan rate of 2°min-1 and operating conditions of
40 kV and 150 mA. The SrTiO3 crystallite size (D) was
estimated by the Debye–Scherrer equation:
Kk
D ¼
b ꢀ cos h
where K is the Scherrer constant (0.89), k is the wavelength of
the X-ray radiation (0.15418 nm for CuKa), b is the full width
half maximum (FWHM) of the diffraction peak measured at
2h, and h is the diffraction angle. The photocatalyst mor-
phologies and the existence of the loaded Ag were observed by
a high-resolution transmission electron microscope (HRTEM:
Tecnai G2 F20) at 200 kV. UV–Vis absorption spectra of the
SrTiO3 photocatalysts were obtained by using a UV–Vis
diffuse reflectance spectroscopy (Shimadzu UV-2550) in
wavelength of 200–800 nm at room temperature with BaSO4
as a reference. Afterwards, the diffuse reflectance spectroscopy
was used to estimate the band gap energy (Eg, eV) by using the
Tauc equation:
3 Results and Discussion
3.1 The XRD Analysis of SrTiO3 and Ag–SrTiO3
Photocatalysts
The crystal structures of synthesized SrTiO3 and 7 wt%
Ag–SrTiO3 were analyzed by using XRD spectroscopy. As
shown in Fig. 2, all the characteristic peaks, which refer to
the single phase of cubic perovskite SrTiO3, are in good
agreement with the perovskite SrTiO3 (JCPDS 35-0734,
Cubic). Compared with the pristine SrTiO3, an additional
phase was identified to be the Ag (JCPDS 04-0783, Cubic)
on the Ag-loaded SrTiO3 photocatalyst. The peaks at
2h = 38.116°, 44.277°, 64.426° and 77.472° belong to the
1=2
ðahmÞ ¼ Kðhm ꢁ EgÞ
wherehis the Planck’s constant, misthefrequencyofvibration,
a is the absorption coefficient, Eg is the band gap (eV), K is the
proportional constant. The unit for hm is eV (photon energy),
and hm was determined by using the equation:
1240
hm ¼
k
123

1204
S. Dandan et al.
Fig. 1 Setup of photocatalytic reaction
Fig. 2 X-ray diffraction patterns of a SrTiO3, b 7 wt% Ag–SrTiO3
characteristic peaks of Ag, which confirm that the Ag was
loaded on the surface of perovskite SrTiO3 and did not
affect the crystallinity of the synthesized SrTiO3 photo-
catalysts. The crystallite size of 7 wt% Ag–SrTiO3 was
calculated from the crystal plane (110) of SrTiO3 diffrac-
tion peak by using the Debyee–Scherrer equation, which
Table 1 Textural properties of pristine and 7 wt% Ag-loaded SrTiO3
from XRD and UV–Vis DRS analysis
Photocatalyst Lattice Lattice
planea
Lattice
Crystallite Egb
parametera spacinga size of
(eV)
(A)
SrTiOa3
(nm)
˚
was nearly 19.8 nm, and the lattice spacing was 2.7776 A.
The contrasting data are listed in Table 1. Compared with
the pristine SrTiO3, there was negligible change in the
Lattice Parameter for addition of Ag. The Lattice Param-
eter of SrTiO3 measured from X-ray diffraction patterns
was consistent to the JCPDS card. Furthermore, according
to the UV–Vis DRS spectra results, the addition of Ag on
the SrTiO3 surface can enhance the light absorption ability
and increase the band gap wavelength (k) of the synthe-
sized SrTiO3 photocatalyst, leading to the decrease in the
band gap energy from 2.85 to 2.28 eV (Table 1).
Pristine
SrTiO3
110
110
0.3929
0.3931
2.7759
2.7776
16.0
19.8
2.85
2.28
7 wt% Ag–
SrTiO3
a
From XRD analysis
b
From UV–Vis DRS spectra analysis
In Fig. 3i, a TEM image of Ag–SrTiO3 photocatalyst
showed that a few black particles of silver was dispersed on
the surface of the SrTiO3. The particle size of Ag–SrTiO3
is approximately 20 nm, which is quite in agreement with
the XRD analysis. Figure 3j showed a HRTEM image of
the same sample. The lattice plane of SrTiO3 (110) with
3.2 The SEM, TEM and HRTEM Observation
of Ag–SrTiO3 Photocatalyst
˚
lattice spacing of 2.757 A were clearly displayed, which
Morphologies of as-synthesized Ag–SrTiO3 photocatalyst
synthesized at different hydrothermal temperature and for
different hydrothermal time were observed by SEM
(Fig. 3). When the temperature reached 150 °C, the crystal
was well crystallized and dispersed with uniform grain size
(Fig. 3b). But at 170 °C, the crystal formed a structure of
agglomeration, resulted in a low specific surface area and
decrease in photocatalytic activity (Fig. 6c). SEM images
Fig. 3(f–h) showed a partial collapse of crystal structure for
crystallization period with a hydrothermal time for 28 h,
which led to an extremely low specific surface area of
0.71 m2/g (see BET result from Table 2).
were in accordance with the XRD analysis (shown in
Table 1). It was clearly shown that the interface between
SrTiO3 and Ag changes continuously by forming an
intergradation between SrTiO3 and Ag. As indicated by the
white loop, the interface between SrTiO3 and Ag is con-
junct well. The intergradation of the interface is beneficial
in the transfer of the photogenerated electrons from the
interior of SrTiO3 to Ag in the reaction process [14, 15],
and extend the life-span of the electron–hole pairs. The
intergradation between the two particles has been found to
be crucial for improving the photocatalytic activity.
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Photocatalytically Reducing CO2 to Methyl Formate
1205
3.3 The UV–Vis DRS Analysis of SrTiO3
and Ag-Loaded SrTiO3 Photocatalysts
shown in Fig. 5. The isotherms showed a type IV IUPAC
pattern with a hysteresis loop, demonstrating that the
pristine SrTiO3 and 7 wt% Ag–SrTiO3 consist of well-
developed mesopores (2.0 nm \ dp B 50 nm) in their
assembled frameworks. From the BJH pore diameter dis-
tributions, as shown in Fig. 5 inset, it is evident that the Ag
loaded SrTiO3 possessed a similar pore diameters with the
pristine SrTiO3. The N2 absorption–desorption analysis
results showed that the Ag loading increased the specific
surface area and total pore volume, and possessed a similar
mean pore diameter to the pristine SrTiO3. The BET spe-
cific surface area of the Ag loaded SrTiO3 samples were
investigated as summarized in Table 2. We focused on the
Ag loaded SrTiO3 with a much larger surface area than the
pristine SrTiO3 particles. As for the specific surface area of
the Ag loaded SrTiO3 samples on different synthesis
temperature, when the temperature reached 150 °C, the
surface area of the catalyst became the largest, which is
possibly because the crystal synthesized at 150 °C are well
crystallized and dispersed with uniform grain size, which is
in well agreement with the SEM analysis, as shown in
Fig. 3b. As shown in Fig. 6c for the photocatalytic activi-
ties analysis, the Ag–SrTiO3 prepared at 150 °C showed
the highest activity. Therefore, 150 °C is considered the
optimum hydrothermal temperature. The surface area of
the crystal decreased as the Ag dosage increased below
5 wt%, and increased at 7 and 9 wt%, as shown in Table 2.
Furthermore, the specific surface area of the 7 wt% Ag–
SrTiO3 crystal increased as the hydrothermal time
increased below 22 h, but the specific surface area sharply
decreased when the time increased to 28 h, possibly due to
a collapse of crystal structure for crystallization period with
a hydrothermal time for 28 h. The result is quite in
accordance with the SEM analysis (shown in Fig. 3h). The
photocatalyst with high specific surface area exhibited the
enhanced activity, which is ascribed to more reaction sites
arising from high specific surface area, and the efficient
transition of the photogenerated electron–hole pairs to the
surface. For all that, the relatively small specific surface
area for our samples suggested that surface area could not
be the decisive factor for its high activity.
Figure 4a showed the typical diffuse reflection spectra for
the SrTiO3 and Ag-loaded SrTiO3 samples. Compared with
the pristine SrTiO3, Ag-loaded SrTiO3 samples have a strong
absorption in the visible light region, which is almost as
strong as that in the UV region. This is attributed to the
surface plasmon resonance of Ag nanoparticles deposited on
SrTiO3 particles [16–18]. Generally speaking, the surface
plasmon absorption band of metal nanoparticles can be
influenced by many factors such as particle size, particle
shape, particle size distribution and surface charge density
etc. The Ag nanoparticles deposited on SrTiO3 particles have
a large number of different shapes and diameters so that their
plasmon oscillations cover a wide range of frequencies [19,
20]. Furthermore, surface plasmon absorption peak becomes
broader and stronger with the increasing of hydrothermal
time. It is the broader and stronger SPA peak that leads to an
extension of the light absorption of the diffuse reflectance
spectra of SrTiO3. The presence of plasmon absorbance and
the negligible change in the lattice parameters (as shown in
Table 1) suggest that most of the metal exists as zerovalent
metal deposits instead of substituting Sr or Ti sites within the
perovskite structure [10]. This indicates that the main portion
of the Ag added was present as zerovalent Ag. The absorp-
tion intensity of the diffuse reflectance spectra of Ag–SrTiO3
is dependent on the synthesis times, which give rise to an
extension of the light absorption with the increase of syn-
thesis times. Theoretically speaking, when the absorption
intensity via UV–Vis analysis increases, the generation of
electron–hole pairs increases, resulting in the photocatalyst
exhibiting a higher photocatalytic activity.
As shown in Fig. 4b, the intercept of the tangent to the
square root of the Kubelka–Munk functions (ahm)1/2 versus
the photon energy (hm) plots would give an estimation of
the band gap for indirect bandgap semiconductors such
as SrTiO3. When the tangent lines are extrapolated to
a1/2 = 0, the band gaps of pristine SrTiO3 and 7 wt%
Ag–SrTiO3 on hydrothermal time for 10, 16, 22 and 28 h
are indicated to be 2.85, 2.28, 2.28, 1.79, and 1.12 eV
respectively. The band gap energy of pristine SrTiO3 is
larger than that of 7 wt% Ag loaded SrTiO3. And it is
evident that the band gap energies of Ag–SrTiO3 on dif-
ferent hydrothermal time decreased with the increasing
hydrothermal time. Thus, 7 wt% Ag–SrTiO3 might absorb
light in a larger wavelength region up to visible light.
3.5 Photocatalytic Activities of SrTiO3 and Ag-Loaded
SrTiO3 Photocatalysts for Reduction of CO2
The Photocatalytic activities of as-synthesized Ag-loaded
SrTiO3 samples during photocatalytic reduction of CO2
under irradiation were evaluated. For the sake of the
improvement of photocatalytic activity, the optimization of
the operational parameters for the CO2 reduction system is
important. In our work, the effects of Ag dosage, time and
temperature of hydrothermal synthesis on the photocata-
lytic activity were investigated. As shown in Fig. 6a,
3.4 The BET Specific Surface Area Analysis of SrTiO3
and Ag-Loaded SrTiO3 Photocatalysts
The porous structure of the pristine SrTiO3 and 7 wt% Ag–
SrTiO3 had been identified by the N2 sorption isotherms, as
123

1206
S. Dandan et al.
Fig. 3 SEM images of 7 wt%
Ag–SrTiO3 samples: a–e the
samples synthesized at 140,
150, 160, 170 and 180 °C for
10 h, f–h the samples
synthesized for 16, 22 and 28 h
at 150 °C, i TEM image of
7 wt% Ag–SrTiO3 (150 °C,
10 h), j HRTEM image of
7 wt% Ag–SrTiO3 (150 °C,
10 h)
123

Photocatalytically Reducing CO2 to Methyl Formate
1207
Table 2 The surface area of Ag-loaded SrTiO3 photocatalysts
Photocatalyst
Dosage of Ag (wt%) loaded on SrTiO3 at
150 °C for 10 h
7 wt% Ag–SrTiO3, 150 °C
synthesis time (h)
7 wt% Ag–SrTiO3, 10 h synthesis
temperature (°C)
0
1
3
5
7
9
10
16
22
28
140 150
160
170 180
BET specific surface area 1.85 18.88 12.10 7.11 13.81 10.68 13.81 14.46 23.78 0.71 7.16 13.81 10.81 3.46 9.20
(m2/g)
Fig. 5 N2 sorption isotherms and pore diameter distributions (inset)
of the pristine and the 7 wt% Ag-loaded SrTiO3 photocatalysts
recombination center, resulting in a negative activity effect
[7, 21, 22] .
The effect of hydrothermal time and hydrothermal
temperature on photocatalytic activity was shown in
Fig. 6b, c. In Fig. 6b, for the hydrothermal time from 10 to
22 h, the increase of hydrothermal time led to an increase
in the crystallinity and the specific surface area [8], which
resulted in the positive effect on photocatalytic activity.
However, for the hydrothermal time from 22 to 28 h, the
Fig. 4 a UV–Vis DRS spectra of pristine SrTiO3 and 7 wt% Ag–
SrTiO3 for different hydrothermal time. b Plots of (ahm)1/2 versus
photon energy (hm) for pristine SrTiO3 and 7 wt% Ag–SrTiO3 on
different hydrothermal times
increase in the hydrothermal time led to a slight increase in
the crystallinity but an abrupt decrease in the specific
surface area. In this time range, the decrease of the pho-
tocatalytic activity possibly due to the negative effect of
the specific surface area might surpass the positive effect of
crystallinity. The effect of hydrothermal temperature can
be divided into three sections, as shown in Fig. 6c. Firstly,
for the hydrothermal temperature from 140 to 150 °C, the
increase hydrothermal time led to an increase in the crys-
tallinity and the specific surface area, which resulted in the
positive photocatalytic activity. Secondly, for the hydro-
thermal temperature from 150 to 170 °C, the increase in
the hydrothermal temperature led to a slight increase in the
crystallinity but a decrease in the specific surface area. In
this time range, the photocatalytic activity possibly due to
increasing loaded Ag from 0 to 7 wt% gave rise to an
increase in the MF evolution rate. In our experimental
range, the increase of the surface area of the Ag particles
with increasing loaded Ag could result in a positive evo-
lution rate of MF. As the Ag increased from 1 to 5 wt%,
the specific surface area decreased abruptly in spite of the
increasing MF evolution rate. Nevertheless, a further
increase in the Ag loading (higher than 9 wt%) led to a
decrease in rate of MF evolution, which should be ascribed
to the excess Ag agglomeration acting as the electron–hole
123

1208
S. Dandan et al.
The photocatalytic activities of the Ag loaded SrTiO3
photocatalysts strongly depended on the light absorbance
of SrTiO3 powders, the Ag loading on SrTiO3, the hydro-
thermal synthesis condition, which resulted in corre-
sponding variation in the specific surface area and
crystallinity, affecting their photocatalytic activities.
For monitoring the conversion of CO2 in methanol, we
employed the on line ReactIR to collect FT-IR spectra of
CO2 in liquid phase by using a AgX optic probe inserted
into the reactor. After CO2 bubbled through the methanol
solution the IR spectra are shown in Fig. 7. From the 3D
plots in Fig. 7a, we noted that CO2 at 2,342 cm-1 was
captured. The peak area of CO2 versus time in Fig. 7b
clearly showed the conversion of CO2. In the presence of
ultraviolet illumination, the CO2 was obviously consumed,
but turning off the light, the concentration of CO2 remained
almost stable. It is, therefore, noteworthy that CO2 was
directly involved in the reaction.
3.6 Mechanism of Photocatalysis
The mechanism for photocatalytic reduction of CO2 to
methyl formate in methanol was shown in Fig. 8. When the
SrTiO3 is irradiated by UV light of suitable wavelength
with sufficient photonic energy (hm), photo-generated
electron–hole pairs were created, part of the excited elec-
trons and holes recombined together and radiated out heat,
while rest part would react with the reactants near the
SrTiO3 surface. It was certain electrons transmitting to the
surface of Ag particle that avoided their recombination
with holes, resulting in effective extension of the life-span
of the electron–hole pairs. In the process, the Ag particle
acted as electron acceptor. The potential energy of con-
duction band of SrTiO3 is -1.26 V, which is more negative
than that of CO2/formic acid or formaldehyde, and the
potential energy of valence band of 2.14 V is more positive
than that of methanol oxidation [23, 24]. The excited
electrons reacted with the CO2 adsorbed on the catalyst
surface to produce formic acid or formaldehyde, at mean
time, the excited holes reacted with the CH3OH to produce
formaldehyde and formic acid. The methyl formate product
can be generated through the esterification of formic acid
and methanol and the dimerization of formaldehyde via
Tishchenko reaction.
Fig. 6 a The rate of MF evolution and specific surface area of Ag
loaded SrTiO3 (a hydrothermal temperature of 150 °C, a hydrother-
mal time of 10 h) on different Ag loading wt%. b The rate of MF
evolution and specific surface area of 7 wt% Ag–SrTiO3 (a hydro-
thermal temperature of 150 °C) on different hydrothermal time. c The
rate of MF evolution and specific surface area of 7 wt %Ag–SrTiO3
(a hydrothermal time of 10 h) on different hydrothermal temperature
In
a comparison experiment, 7 wt% Ag–SrTiO3
(150 °C, 22 h) was used to investigated the effect of CO2.
A test bubbling the solution with N2 instead of CO2 was
performed. It turned out that, in the presence of CO2, MF
generation rate was 3006 mol/(h g cat), while in the pres-
ence of N2, MF generation rate was 925 mol/(h g cat).
When CO2 was substituted for N2, MF was generated
through the dimerization of formaldehyde via Tishchenko
reaction, but the amount was less than one-third of that in
the negative effect of the specific surface area might sur-
pass the positive effect of crystallinity. Thirdly, for the
hydrothermal temperature from 170 to 180 °C, the increase
hydrothermal temperature resulted in an increase of the
crystallinity and the specific surface area, which led to the
enhanced photocatalytic activity.
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Photocatalytically Reducing CO2 to Methyl Formate
1209
Fig. 7 a 3D plots of on line
ReactIR spectra for the reaction,
(b) peak area of CO2 at
2,342 cm-1
methanol to methyl formate, the highest MF evolution rate
reached 3,006 lmol/(h g cat).
Acknowledgments We gratefully acknowledge financial support
by the National Natural Science Foundation of China (NSFC)
(No. 21176192) and the Tianjin natural science foundation (No.
12JCZDJC29400).
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