Efficient Photocatalytic Reduction of CO2 Using Carbon-Doped Amorphous Titanium Oxide

Article abstract of DOI:10.1002/cctc.201800476

CO₂-related solar to chemical conversions have gained extensive interest due to the great concerns on renewable energy utilization. Here, we have demonstrated a new synthetic route to C-doped amorphous titanium oxide using a facile citric acid

Full text of DOI:10.1002/cctc.201800476

Accepted Article
Title: Efficient Photocatalytic Reduction of CO2 Using Carbon-Doped
Amorphous Titanium Oxide
Authors: Peng Wang, Guoheng Yin, Qingyuan Bi, Xieyi Huang,
Xianlong Du, Wei Zhao, and Fu-Qiang Huang
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ChemCatChem
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Efficient Photocatalytic Reduction of CO
2
Using Carbon-Doped
Amorphous Titanium Oxide
Peng Wang,[a,b] Guoheng Yin, Qingyuan Bi, Xieyi Huang, Xianlong Du, Wei Zhao, and Fuqiang
[a]
[a]
[a]
[c]
[a]
Huang*[a,b,d]
2
Abstract: CO -related solar to chemical conversions have gained
The photocatalytic reduction of CO , which employs renewable
2
extensive interest due to the great concerns on renewable energy
utilization. Here, we have demonstrated that the C-doped amorph-
ous titanium oxide was made by using a facile citric acid assisted
solar to stimulate the process of catalytic CO2 reduction to
valuable chemicals, is considered a promising approach to
resolve global warming and energy crisis.[3] However, CO
2
2
sol-gel method for efficient photocatalytic reduction of CO . The
molecules are of high stability and chemical inertness with a
closed-shell electronic configuration and linear geometry.[4] The
synthesized amorphous material exhibits a mesoporous structure
with high specific surface area and a significantly narrowed band
gap of 2.1 eV which are crucial for solar light harvesting and
addition of one electron can contribute to the loss of symmetry
and the increased repulsion for CO
result in the high energy of the Lowest Unoccupied Molecular
Orbital (LUMO) as well as the very low electron affinity of CO
molecule.[4c] Therefore, the adsorption and chemical activation of
CO molecules are quite difficult but crucial for photocatalysts.
Due to the abundance, non-toxicity and high resistance to
photocorrosion, the crystallized TiO has been the most widely
used semiconductor for photocatalytic CO
reduction.[3d,4c]
2
molecule, which would
2
adsorption/chemical activation of CO for energy transformation. The
amorphization, mesoporous structure, and the band structure of the
C-doped samples were also successfully tuned by controlling the
2
o
annealing temperatures. The optimized catalyst annealed at 300 C
shows the highest specific surface area, favorable visible-light
2
2
response as well as the considerable performance for CO photored-
uction. Moreover, the further treatment of Al reduction can induce
numerous surface oxygen vacancies on the amorphous sample and
thus efficiently restrain the recombination of photogenerated carriers.
Of significant importance is that the Al-reduced catalyst achieves
2
2
However, its large band gap (3.03.2 eV) and the low carrier
mobility restrain the utilization for solar energy.[ In addition, the
limited specific surface area, the lack of surface reaction sites
and the difficulty for nonmetallic element (such as C, N, and B)
doping seriously restrict the improvement of photocatalytic
5]
excellent performance with the space-time yield of CH
4
and CO of
–1
–1
–1
–
4
.1 and 2.5 μmol g
h
for solar light, and 0.53 and 0.63 μmol g h
1
for visible light, respectively. This sample is also stable for
transformation.
photocatalytic CO
2
performance.[5b,c,6] Low Packing Factor (PF) of amorphous TiO
2
makes the heavy non-metal doping possible, which can distinctly
change the band structure and result in the greatly enhanced
light absorption as well as the high surface reaction activity.[7]
More importantly, the scattering and recombination of photogen-
erated carriers could be well restrained by the existence of
Introduction
With the increasing use of fossil fuels by human beings, the
concentration of CO in the atmosphere has reached an
2
o 2
numerous oxygen vacancies (V with
).[8] The amorphous TiO
astonishing degree that many serious environmental issues,
including sea-level rising, abnormal climate and land
desertification, are becoming much more terrible.[1] The physical
and chemical strategies for the fixation and utilization of
high specific surface area and large amount of surface reaction
sites can be easily obtained through various facile preparation
methods.[9] However, there is few study on the amorphous
2
titanium oxides for photocatalytic CO reduction.
greenhouse gas CO
2
have attracted ever-increasing attention.[2]
2
Many ways can be used to synthesize amorphous TiO , such
as the conventional hydrolysis route,[
10]
ultrasonic method,[11]
chemical precipitation,[ solvothermal process,[13] and sol–gel
12]
‡
‡
[14]
[
a]
P. Wang, G. Yin, Dr. Q. Bi, X. Huang, Dr. W. Zhao, Prof. Dr. F.
Huang
synthesis.
Among these strategies, the sol–gel route is well
applied for its advantage in lowering heat temperature to obtain
tailored particles for photocatalysis materials. It was previously
reported that citric acid can be widely employed as a kind of
organic additive for the sol–gel synthesis of metal oxides.
Experiments show that citric acid was not only used as a
chelating agent and carbon source, but also as the dispersant
and stabilizing agent to prevent the aggregation of nanoparticles.
Generally, carbon doping can significantly enhance the
State Key Laboratory of High Performance Ceramics and Superfine
Microstructure, Shanghai Institute of Ceramics, Chinese Academy of
Sciences, Shanghai 200050 (China)
E-mail: huangfq@mail.sic.ac.cn
[
b]
c]
P. Wang, Prof. Dr. F. Huang
School of Physical Science and Technology, ShanghaiTech
University, Shanghai 200031 (China)
[
Dr. X. Du
Key Laboratory of Interfacial Physics and Technology, Shanghai
Institute of Applied Physics, Chinese Academy of Sciences,
Shanghai 201800 (China)
[15]
conductivity and stabilize the structure of titanium oxide.
2
Herein, we prepared a C-doped amorphous TiO (CAT) with
[d]
Prof. Dr. F. Huang
relatively high specific surface area and a mesoporous structure
via a facile citric acid-assisted sol–gel method combined with a
simple annealing treatment. The obtained powders annealed in
air atmosphere at different temperatures are denoted as CAT-X,
where X represents the annealing temperature. The C-dopant
Beijing National Laboratory for Molecular Sciences and State Key
Laboratory of Rare Earth Materials Chemistry and Applications,
College of Chemistry and Molecular Engineering, Peking University,
Beijing 100871 (China)
[‡]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
2
obviously changed the band structure of amorphous TiO , and
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ChemCatChem
10.1002/cctc.201800476
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the engineered CAT-300 displayed excellent visible light
response properties. Furthermore, with Al reduction treatment,
oxygen vacancies and Ti3+ were successfully introduced into the
In order to further investigate the effect of citric acid
during the preparation process, the X-ray diffraction (XRD)
analysis of T-300 and CAT-300 was conducted. As the
patterns depicted in Figure 2a, the T-300 sample with no
addition of citric acid is well crystallized as anatase phase
after annealing at 300 oC for 4 h, while CAT-300 is
amorphous without visible diffraction signals. Citric acid
acting as a chelating agent and dispersant not only
prevents the aggregation of nanoparticles, but also
amorphous TiO
carriers were also weakened greatly. Combined with the high
ability for CO adsorption and chemical activation, the Al-
reduced CAT-300 sample exhibited considerable photocatalytic
2
, and the scattering and recombination of
2
2
performance for CO reduction under both solar and visible-light
irradiation.
stabilizes precursors and inhibits the crystallization of TiO
matrix. As shown in Figure 2b, CAT-300 gives a pore size
of about 2 8 nm, which is consistent with the TEM results
Figure 1). Insert in Figure 2b shows the N adsorption–
2

Results and Discussion
(
2
desorption isotherms of as-prepared samples. There is a
characteristic type-IV isotherm with clear hysteresis loop
locating at 0.451.0 (P/P ) in CAT-300, indicative of the
0
We found the addition of citric acid showed great effect on
the composition and structure of samples during the
preparation process. T-300, the sample calcined at 300 C
o
typical mesoporous feature. It seems that the porosity in
CAT-300 is dominant with the interparticle form with a small
fraction of intraparticle analogue. Note that CAT-300
possesses a higher specific surface area of 73 m g ,
which is more than 7 times of that of T-300 (Table S1).
without the addition of citric acid, as reference material was
also prepared. The detailed morphology and structure of T-
3
00 and CAT-300 were characterized by scanning electr-
on microscope (SEM), transmission electron microscopy
TEM), and high-resolution (HR)TEM analysis with selected
2
1
(
area electron diffraction (SAED) patterns, as shown in
Figures 1 and S1. It is indicated that T-300 shows a simple
quasi-spherical structure with multifarious particle size
distribution of 20100 nm (Figure 1a). While CAT-300
exhibits a fractal porous structure and the uniform particle
size (Figure 1b and S2) is smaller than that of T-300,
indicating that citric acid is beneficial for the formation of
2
small nanoparticles of amorphous TiO . The pore size of
CAT-300 is several nanometers. There is a well-resolved
lattice fringe with a plane distance of 0.35 nm attributed to
the crystal anatase (101) in T-300, as the HRTEM image
depicted in Figure 1c.[16] By contrast, the CAT-300 is
amorphous and shows highly disordered lattice orientation
(Figure 1d).
Figure 2. (a) XRD patterns, (b) pore size distributions. Inserts in (b) are N
2
adsorption–desorption isotherms. (c) FTIR and (d) UV–VIS–NIR diffuse
reflectance spectra of T-300 and CAT-300. Inserts in (d) are Tauc plots
calculated by using Kubelka-Munk equation against photon energy.
The bonding characteristics of functional groups in T-300 and
CAT-300 are identified by Fourier transform infrared (FTIR)
spectroscopy. The spectra are depicted in Figure 2c and S3.
The peaks at 3440 and 1620 cm–1 are ascribed to the stretching
vibrations of surface water molecules (e.g., hydroxyl groups and
molecular water).[17] No signals of –CH
3 2
, –CH , or –CH bonds
are observed, indicating that T-300 and CAT-300 are free of
organic species.[18] Figure 2d shows the diffuse reflectance
spectra of T-300 and CAT-300 samples. The absorption edge at
the wavelength of ~390 nm for T-300 is attributed to the intrinsic
Figure 1. TEM and HRTEM images of (a and c) T-300 and (b and d) CAT-300.
Insets in (c) and (d) show the corresponding SAED patterns.
2
bandgap absorption of crystalline anatase TiO . By contrast, the
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ChemCatChem
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light absorption edge of CAT-300 gives obvious red shift along
with the enhanced light absorption, indicating that the band stru-
cture is significantly changed. Tauc plots calculated by using
Kubelka-Munk equation against photon energy are also shown
in Figure 2d.[19] And CAT-300 gives a narrowed band gap of
We further explored the influence of annealing temperat-
ures on the properties of series amorphous CAT samples.
As can be seen in Figure 4a, the amorphous TiO
2
gradually
crystallized to anatase TiO
2
when the annealing temperat-
o
ure rises to 400 C. However, the citric acid was not fully
pyrolyzed at the annealing temperature of 200 oC.
Therefore, CAT-200 shows a relatively low specific surface
area and less surface active sites (Figure 4b and c). Once
the annealing temperature rising to 400 oC, the higher
temperature not only cause the collapse of the mesoporous
framework and decrease the specific surface area, but also
2.1 eV, which is much lower than 3.4 eV of T-300 sample.
X-ray photoelectron spectroscopy (XPS) was employed
to investigate the chemical states of C, Ti, and O species in
T-300 and CAT-300. As shown in Figure 3a, two broad peaks at
284.6 and 288.4 eV are observed in both samples. The C 1s
peak at 284.6 eV is usually assigned to adventitious elemental
carbon.[20] The shoulder peak at 288.4 eV indicates the existe-
nce of carbonate species on the surface of T-300 and CAT-
2
induce amorphous TiO to undergo crystallization. The
diffuse reflectance spectra of CAT samples reveal that the
C content gradually decreases and the band structure
3
00.[20] Noted that a new shoulder peak at around 282.0 eV
attributed to Ti–C bond arises in CAT-300. This Ti–C bond
probably result from the substitution of the lattice oxygen atoms
by carbon atoms.[20a,b] Figure 3b demonstrates the correspon-
ding XPS Ti 2p spectra of T-300 and CAT-300 with similar
shape. The two peaks at 458.5 and 464.3 eV are typical for
2
returns to the intrinsic band gap of anatase TiO with the
annealing temperature rising to 500 oC, as depicted in
Figure 4d. These results suggest that the narrowed band
gap of CAT samples is mainly caused by the C dopant.[20]
Ti 2p3/2 and Ti 2p1/2 of anatase TiO
2
, respectively.[21] The
XPS O 1s spectra are also provided in Figure 3c. The doped
C is bonded with Ti to form Ti–C bonds, which has a crucial
effect to the electronic arrangement of the neighboring O
atoms and further leads to the spin splitting of O 1s XPS
peak. Thus, the new shoulder peak at around 527.9 eV
arises in CAT-300 mainly caused by the C dopant.[22] To
investigate the stability and the oxidation-tolerance ability of
CAT-300, the thermogravimetric (TG) measurements in air
atmosphere were carried out. As shown in Figure 3d, the
2
mass loss before 120 °C is due to the volatilization of H O
adsorbed on the surface of T-300 and CAT-300. With the
temperature increasing to 450 °C, the further mass loss of
CAT-300 with 3.9 wt% is attributed to the burning of doped
carbon. By contrast, T-300 shows no visible change.
2
Figure 4. (a) XRD patterns, (b) N adsorption–desorption isotherms, (c) pore
size distributions, and (d) UV-VIS-NIR diffuse reflectance spectra of CAT
samples.
Figure 5 shows the SEM images of series CAT samples.
When the annealing temperature at 200 °C, the citric acid
was only partially carbonized, which may cause the
aggregation of particles and thus result in relatively larger
particle size (> 100 nm) for CAT-200. With the annealing
temperature increasing, citric acid was totally carbonized
and the remained carbon was also successfully doped into
2
the amorphous TiO . Therefore, CAT-300 shows mesopor-
ous structure with particle size of about 20 nm. However,
the amorphous particles would crystallize again and grow
o
larger when the temperature further rose to 400 C, which is
consistent well with the above XRD and TEM results.
Upon establishing that the amorphous TiO
2
catalysts
showed enhanced visible light absorption, we sought to
explore the potential applications in photocatalytic reduction
Figure 3. XPS data of (a) C 1s, (b) Ti 2p, and (c) O 1s, and (d) TG curves of
T-300 and CAT-300.
2
of greenhouse gas CO . Control experiments show that
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Figure 5. SEM images of CAT samples with different treatment temperatures.
there were no carbon-containing compounds generated without
CO
the generated products only derived from the substrate of CO
The results are shown in Figure 6. The mainly evolved
products are methane, H and CO. Although T-300 can
catalyze the CO transformation under solar light irradiation,
2
being charged, which indicates that the carbon source of
2
.
2
2
series C-doped samples except CAT-200 exhibit obvious
increase on photocatalytic activity. Among them, the
engineered CAT-300 shows the highest performance for
Figure 6. Photocatalytic activity of CO
2
reduction over T-300 and CAT
samples under solar- or vis-light irradiation. Reaction conditions: 50 mg
catalyst, 2 bar CO , 6 mL H O, 5 h.
2
2
2 4
CO reduction with the space-time yield (STY) of CH and
CO of 2.9 and 2.1 μmol g–1 h–1 (Figure 6a and Table S2),
respectively. Both values are four and two times higher than
that of T-300 catalyst under identical reaction conditions.
to be successful for the generation of numerous V
further investigated the introduction of V
and Ti3+ species
into amorphous TiO by Al reduction strategy. It is widely
accepted that the defects like V
and/or Ti3+ can efficiently
,[23] we
o
o
CAT-300 also displays a high selectivity of 67% to CH
4
2
generation. CAT-200 gives significant decrease of
a
o
performance probably due to the blocking effect on active
sites by the residual organic carbonaceous from the
partially carbonized of citric acid in preparation process.
With the annealing temperature increasing, the crystallizat-
2
ion of amorphous TiO and the loss of the doped carbon
can result in the attenuation of surface active sites as well
as the weakened light absorption. Therefore, CAT-400 and
4
CAT-500 show gradually decreased STY of CH and CO. It
is worth noting that the CAT samples can display a
considerable visible-light activity which is much lower than
that of solar stimulated, and CAT-300 is also the best on
the same conditions, as depicted in Figure 6b. The O
generation during the photocatalytic reduction of CO with
O was also observed. Take CAT-300 under full solar
irradiation for example, the STY of O
is 8.1 μmol g–1 h–1,
which is similar to that of theoretical rate of O formation
[(H formation rate)/2 + (CO formation rate)/2 + 2 × (CH
formation rate)]) of ca. 8.7 μmol g–1 h–1.
Having established that CAT-300 catalyzed the
photoinduced reduction of CO under solar or even visible-
light irradiation, and bearing in mind that Al reduction
method for metal oxides (especially TiO ) has been proven
2
2
H
2
2
2
(
2
4
2
Figure 7. (a) EPR, (b) UV-VIS-NIR diffuse reflectance, and (c) PL spectra, and
d) CO −TPD profiles of T-300, CAT-300, and RCAT-300.
(
2
2
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+
−
[8c]
accelerate the fast separation and transportation of photog-
separation of light-excited h −e pairs. It is concluded that
+
−
[8c,d]
enerated hole−electron (h −e ) pairs,
large number of mobile oxygen species being able to react
with stable CO
molecules.[24] Based on our previous
and thus provide a
the RCAT-300 sample can exhibit a much lower recombina-
+
−
tion rate of h −e under light irradiation which is crucial for
2
its photocatalysis applications. Moreover, the temperature-
studies,[23] the CAT-300 sample was further subjected from
Al reduction via two-zone method, and the obtained catalyst
was denoted as RCAT-300. As a highly sensitive technique
to detect paramagnetic species contain-ing unpaired
electrons, electron paramagnetic resonance (EPR) has
been usually employed to probe the existence of Ti3+ and
programmed desorption of CO
yed to investigate the adsorption ability for CO
CAT-300, and RCAT-300 samples. As depicted in Figure
7d, the total amount of CO adsorbed on the amorphous
TiO is much superior to that of crystallized T-300 due to the
(
CO
−TPD) test was emplo-
2
2
2
on T-300,
2
2
high specific surface area and disordered properties,
indicative of the more active sites on catalyst surface. After
V
o
.[25] As depicted in Figure 7a, a strong EPR signal at g =
2
.003 assigned to V combining with one electron induced
o
o
Al reduction treatment, the induced V further accelerates
by surface Ti3+ species arises in RCAT-300.[25] By contrast,
there is no obvious signal for T-300 and CAT-300. The UV-
VIS-NIR diffuse reflectance spectra provided in Figure 7b
indicate that RCAT-300 exhibits a significantly enhanced
the adsorption and activation of CO
shows the best adsorption ability for CO
,[27] thus RCAT-300
2
2
molecules.
Compared with CAT-300 catalyst, RCAT-300 displays
significantly enhanced performance for photocatalytic
visible-light response after Ti3+ and V
to the generated intermediate states caused by Ti3+ or V
and thus further change the band structure.[26]
induced, which is due
reduction of CO
under both solar- and vis-light irradiation,
and
o
2
o
as shown in Figure 8a and Table S2. The STY of CH
4
CO achieves 4.1 and 2.5 μmol g–1 h–1 for solar light, and 0.53
and 0.63 μmol g–1 h–1 for visible light, respectively. Emphasized
that the photocatalytic activities of RCAT-300 are comparable
with the previously reported materials under the identical
reaction conditions (Table S3).[23c,27a,28]
The photoluminescence (PL) emission measurements
were further conducted to reveal the behavior of
+
−
photogenerated h −e pairs, as shown in Figure 7c. The PL
peak intensity of CAT-300 is much stronger than that of T-
3
00 may due to the disordered amorphous structure of the
former. Interestingly, the Al reduction treatment for RCAT-
00 can induce the generation of numerous V
and/or Ti3+
which act as electron sinks and thus effectively facilitate the
We further explored the reusability of RCAT-300 under solar-
light irradiation. To our delight, the RCAT-300 shows relatively
3
o
2
stable for photocatalytic CO transformation, especially after the
2nd reuse on CH formation, as depicted in Figure 8b. For the
4
used RCAT-300 catalyst, a comprehensive characterization of
XRD, TEM, XPS, and EPR illustrated in Figure 9 indicates that
the sample still shows an amorphous structure but with little
generation of crystallized phase. The XPS C 1s spectrum
(
Figure 9c) demonstrates that the Ti–C bond is highly stabilized
during the photocatalytic process. In addition, the RCAT-300 still
provides an obvious signal at g = 2.003 (Figure 9d), indicative of
Figure 8. (a) Photocatalytic performance and (b) recycling of CO
2
reduction
Figure 9. (a) XRD, (b) TEM, (c) XPS C 1s, and (d) EPR data of the used
over RCAT-300. Reaction conditions: 50 mg catalyst, 2 bar CO
h.
2
, 6 mL H O, 5
2
RCAT-300.
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ChemCatChem
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the existence of numerous Vo and/or Ti3+ in the used RCAT-300.
the measurement. The specific surface area of the samples was
calculated using the Brunauer–Emmett–Teller (BET) method with the
0
adsorption data at the relative pressure (P/P ) range of 0.05–0.2. The
Conclusions
total pore volumes were estimated at P/P0 = 0.99. The pore size
distribution (PSD) curves were calculated from the adsorption branch
using Barrett-Joyner-Halenda (BJH) model. The prepared materials were
pressed into tablets with KBr powder and then detected by FTIR (Perkin
Elmer, USA) with the scanning range from 400 to 4000 cm–1. SEM
images were obtained by Hitachi-S4800. A JEOL 2011 microscope
operating at 200 kV equipped with an EDX unit (Si(Li) detector) was used
for the TEM investigations. A JEM 2100F electron microscope operating
at 200 kV equipped with an EDX unit (Si (Li) detector) was used for the
HRTEM investigations. The samples for electron microscopy were
prepared by dispersing the powder in ethanol and applying a drop of very
dilute suspension on carbon-coated grids. UV-VIS-NIR DR spectra of the
In this work, we have demonstrated that the C-doped
amorphous titanium oxide with high specific surface area
and mesoporous structure was designed and prepared via
a facile citric acid assisted sol-gel method for efficient
2
photocatalytic reduction of CO . The C dopant can
obviously narrow the band gap and thus result in an
excellent solar and even visible-light response. The feature
of amorphous structure and high specific surface area can
greatly accelerate the adsorption and chemical activation of
2
stable CO molecules. Furthermore, the Al reduction treat-
ment can also induce numerous surface oxygen vacancy
for the engineered amorphous sample and thus efficiently
facilitate the separation and transportation of photogener-
ated carriers, and significantly enhance the photocatalytic
performance. These findings can provide a new insight into
the application of amorphous titanium oxide in catalysis,
solids were recorded at room temperature on
a
Hitachi U4100
as
2
samples was attained by the
Spectrometer equipped with an integrating sphere and using BaSO
4
reference. The solar absorption of TiO
equation (1):[29]
A =
(1)
2
especially for CO transformation.
Where A is the solar absorption, T is the reflectance of the sample, S is
the solar spectral irradiance (W m–2 nm–1), λ is the wavelength (nm), and
the (1T)·S represents the sample absorption of solar spectral irradiance.
XPS data were recorded with a Perkin Elmer PHI 5000C system
Experimental Section
equipped with
a
hemispherical electron energy analyzer. The
Chemicals and materials. All chemicals used are analytical grade
spectrometer was operated at 15 kV and 20 mA, and a magnesium
anode (Mg Kα, hν = 1253.6 eV) was used. The C 1s line (284.6 eV) was
used as the reference to calibrate the binding energies (BE). EPR
spectra were investigated using a Bruker EMX-8 spectrometer at 9.44
GHz and 300 K. PL spectra were excited at 320 nm using a fluorescence
spectrophotometer of MODEL Fluoro Max-3 (Horiba) at room
temperature. TG measurements were conducted on a Netzsch STA
without further purification. Titanium (IV) isopropoxide ((Ti(OCH(CH
3 2 4
) ) ,
9
7%) and citric acid (C , 99%) were purchased from Sigma Aldrich.
6 8 7
H O
Al was supplied by Sinopharm Chemical Reagent Co., Ltd (SCRC).
2
Catalyst preparation. The C-doped mesoporous amorphous TiO was
synthesized by a sample and reliable method as follows: 1 mL of
Titanium (IV) isopropoxide (TTIP) was dissolved in 50 mL of ethanol
under slow stirring at room temperature, which was marked as solution A.
Then, 0.1 g of citric acid and 50 mL of ethanol were dissolved in 50 mL of
deionized water, which was denoted as solution B. Later, solution A was
subsequently added dropwise into solution B with vigorous stirring to
form a homogeneous mixture. After stirring for 2 h, the mixture was
centrifuged, washed with ethanol and deionized water several times,
followed by freeze drying overnight, resulting in the formation of a white
powder. The obtained powder was respectively calcined under air
atmosphere at 200, 300, 400, and 500 °C for 4 h with a heating rate of
4
49C TG–DSC thermoanalyzer. The flow rate of the carrier gas (air) was
0 mL min–1. The temperature was raised from room temperature to 800
3
o
C with a ramp rate of 10 C min–1. CO
o
2
–TPD was measured to evaluate
the ability of CO
2
adsorption for the catalyst. Prior to adsorption of CO
2
,
o
2
00 mg of sample was pretreated with Ar at 200 C for 2 h and then
o
2
cooled to 50 C. At this temperature, sufficient CO was injected to attain
adsorption saturation, followed by charging with Ar (30 mL min–1) for 2 h.
o
The temperature was then raised from 50 to 600 C with a ramp rate of 5
oC min–1 to desorb CO
2
. The desorbed CO
2
gas was detected by a
thermal conductivity detector (TCD).
5
2
°C min–1. The as-prepared products were correspondingly named CAT-
00, CAT-300, CAT-400, and CAT-500. The sample without the addition
Photocatalytic
CO
2
reduction
measurement.
Photocatalytic
of citric acid was also prepared by the same procedure, and further
calcined under air atmosphere at 300 °C for 4 h, denoted as T-300.
experiments of CO
2
2
reduction with H O were conducted in a stainless
autoclave reactor (100 mL) with a quartz window on the top (Figure S4).
The full-solar light irradiation was from a 300 W Xe lamp (Aulight CEL-HX,
Beijing) and the power of the light source was calibrated to AM 1.5 by a
NREL-calibrated Si cell (Oriel 91150). The Vis-light was attained using a
light-reflector of 400~780 nm and the reflectivity was larger than 95%. 6
mL of deionized water was firstly added into the reactor. Later, 50 mg of
catalyst was ultrasonically dispersed in 1 mL of deionized water and
The preparation of Al-reduced CAT-300 (denoted as RCAT-300) was
conducted in a two-zone furnace, in which Al and CAT-300 powders
were placed separately, and then evacuated to a pressure of < 0.5 Pa.[23]
After that, the Al chamber was heated up to 800 oC, and then the CAT-
3
00 was heated to 300 oC for 2 h. After the treatments of annealing and
pressure release, the RCAT-300 sample was carefully collected.
2
drop-coated on a glass sheet (area of 4 cm ), which was placed on the
catalyst holder in the upper region of the reactor. Then the autoclave was
sealed, and the internal air was degassed quickly and completely using
Catalyst characterization. XRD characterization of the samples was
carried out on a German Bruker D8 Advance X-ray diffractometer using
the Ni-filtered Cu Kα radiation at 40 kV and 40 mA. Nitrogen adsorption–
desorption isotherms were measured at 196 oC on a Micromeritics
ASAP 2460 analyzer. Samples were degassed at 200 °C for 24 h prior to
high-purity CO
2
(99.99%, no other carbon-containing compounds
detected) for twenty times at room temperature and charged with CO
2
gas to the pressure of 2 bar. The stirrer was started (500 rpm) when the
light was irradiated. After 5 h, the autoclave was placed in cool water and
the gas was carefully released. The gaseous mixture was qualitatively
and quantitatively analyzed using a gas chromatograph (GC) Agilent
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10.1002/cctc.201800476
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7
820A equipped with a TDX-01 column connected to a TCD coupling
with a HP-5 capillary column connected to a flame ionization detector
FID). The liquid product was also analyzed using the GC.
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n = PV/RT
(2)

[
[
[
[
5]
6]
7]
8]

750;
Where n is the mole of the products (mol); P is the standard atmospheric
,
44,
3
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STY = 10 n/mt
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
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2
4
3

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+
+ 2H+
H
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O + 2h → 1/2O
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(4)
(5)
(6)
(7)
(8)
+
−
CO
2
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+ 8H + 8e → CH + 2H
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O
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4 4 2
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[
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FULL PAPER
P. Wang, G. Yin, Q. Bi, X. Huang, X. Du,
W. Zhao, and F. Huang*
Page No. – Page No.
Efficient Photocatalytic Reduction of
2
CO Using Carbon-Doped Amorphous
Titanium Oxide
Carbon-doped amorphous titanium oxide with unique feature of narrowed band gap
and numerous surface oxygen vacancy made by using a facile procedure achieves
an efficient solar to chemical conversion for viable catalytic transformation of CO .
2
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