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distributed on the surface of the TiO2 layer) to enhance the photo-
catalytic activity of the TiO2 photocatalyst. Thus, the synthesized Cu
and V single-doped TiO2 exhibited very high photocatalytic activ-
ity, even under visible light irradiation. More recently, numerous
studies have doped TiO2 with two different dopants (co-doping)
to synergistically improve the photocatalytic activity of the photo-
catalyst [12,13,20–22]. Therefore, in this study, we combine Cu and
V as co-dopants to synergistically enhance the photocatalytic
activity of TiO2 for CO2 conversion to produce valuable fuels under
visible light. In addition, the co-doping is expected to increase
defects or disorders in the TiO2 lattice. Paulino et al. reported that
the disorders in the TiO2 lattice influenced CO2 adsorption, activa-
tion, and dissociation processes [2]. Rodriguez et al. also reported
that the presence of defects on the TiO2 surface induced the forma-
tion of new adsorption sites to capture CO2, after which the elec-
trons stored in the oxygen vacancies were spontaneously
transferred to CO2, which in turn was finally reduced [23]. There-
fore, the co-doping is also expected to increase disorder in the
TiO2 lattice, leading to an increase in CO2 adsorption or photocat-
alytic reduction of CO2. In our previous studies, we investigated
the use of porous polyurethane (PU), a honeycomb structure mate-
rial, as a substrate to immobilize the Cu-doped TiO2 and V-doped
TiO2 photocatalysts [14,15,19]. This synthesized structure not only
overcame the disadvantages faced by the powder photocatalysts
but also increased the adsorption ability of the photocatalysts.
Therefore, in this study, we again used PU as the substrate to
immobilize Cu@V co-doped TiO2 (Cu@V-TiO2/PU) in order to
enhance the surface area and CO2 adsorption capacity and thereby
attain high photocatalytic activity and CO2 adsorption capacity to
overcome all the drawbacks of CO2 conversion by photocatalysis.
vanadium, and titanium in the synthesized photocatalysts. A Hita-
chi S4700 scanning electron microscope (SEM) was used to observe
the surface morphology of the Cu@V-TiO2/PU, which was coated
with Pt before SEM analysis to increase the conductivity of the
photocatalyst surface. A JEOL TEM-2010F system was used to
obtain transmission electron microscopy (TEM) and high-
resolution TEM (HR-TEM) images of the synthesized Cu@V-TiO2/PU.
The optical absorption ability of the shredded Cu@V-TiO2/PU
photocatalyst was characterized by an Evolution 300 spectropho-
tometer (UV-1700 Shimadzu). X-ray diffraction (XRD) spectra of
the Cu@V-TiO2/PU were obtained using a Bruker AXN model
equipped with
a Cu Ka radiation (k = 1.5418 Å) source and
operated at a scan rate of 0.02° sꢀ1 over the 2h range 10–80°.
2.3. Conversion experiments
Photocatalytic reduction of CO2 with H2O vapor was conducted
by a continuous system comprising a gas generator, reaction cham-
ber, and analyzer. The gas generator consisted of a cylinder of CO2
gas (99.99%), flow rate meters, and a humidifier. The reaction
chamber was a dark cover cask (50 ꢂ 25 ꢂ 50 cm) containing two
white light bulbs (EFTR 20EX-D, Kumho Co., Ltd.) and a reactor
(15 ꢂ 4 ꢂ 2 cm), the top and bottom parts of which were made of
quartz to allow easy passage of visible light (400 < k < 700 nm).
The reactor was placed in the center of the reaction chamber.
Two 20 W white bulbs were placed at the top and the bottom of
the reaction chamber to generate visible light in the range
400–700 nm with a power density of 0.05 W/cm2 for the photo-
catalysis. The dark and visible light conditions for photocatalysis
were achieved by turning the bulbs off and on, respectively. Before
the photocatalytic conversion experiments, the reactor containing
2 g (36 cm3) of the synthesized porous Cu@V-TiO2/PU was purged
three times with high-purity CO2 gas. Then 50 mL/min CO2 that
had been passed through water (303 K) was admitted to the
reactor. Thus, the residence time of CO2 in the reactor was 144 s
and the space velocity, calculated by dividing the volumetric flow
by the reactor volume, was 25 hꢀ1. The reactor temperature was
constant at 32 °C during the conversion experiments. To determine
2. Experimental
2.1. Photocatalyst preparation
We used a mixture of toluene, toluene-2,4-diisocyanate, and
anhydrous triethylamine to activate pristine PU to introduce iso-
cyanate groups onto its surface [14,15,19,24]. Titanium tetraiso-
gaseous products, a 100
the GC system, which consisted of
chromatograph (GC) equipped with a methanizer and a flame ion-
ization detector using packed column (Porapak 80/100
l
L sample was automatically injected into
propoxide and
c-aminopropyltriethoxysilane were used as
a
Varian CP-3800 gas
precursors for synthesis of amino titanosiloxane, which contains
amine groups (NH2). The isocyanated PU was immersed in the syn-
thesized amino titanosiloxane solution to promote the reaction
between the isocyanate groups of the isocyanated PU and the
NH2 groups of the amino titanosiloxane to form urea bonds, which
fixed the titanosiloxane on the PU surface. To synthesize Cu@V co-
doped TiO2/PU, a mixture of 0.1 M Cu(NO3)2 and 0.1 M NH4VO3
solution was added to the titanosiloxane fixed on PU. The obtained
material was cleaned by a 1 M oxalic acid solution. Finally, the
Cu@V-TiO2/PU was irradiated and calcined by UV-C light (60 W)
for 5 h under nitrogen at 200 °C. Seven different Cu@V-TiO2/PU
materials were synthesized by adjusting the added volumes of
Cu(NO3)2 and NH4VO3 solutions. The TiO2/PU ratio in the synthe-
sized materials was approximately 20 wt.%. The total weight ratios
of both dopants to TiO2 (total Cu/TiO2 and V/TiO2 ratios) were fixed
at 6 wt.%, and the weight ratio of each individual Cu and V dopant
was varied between 0 and 6 wt.% at intervals of 1 wt.%. The synthe-
sized materials were labeled xCu@yV-TiO2/PU, where x and y indi-
cate the weight ratios of Cu/TiO2 and V/TiO2, respectively.
a
Q
2 ꢂ 2 mm) at intervals of 20 min. The reaction start time (t0 = 0)
was assumed to be the time when the CO2 gas stream containing
H2O was admitted to the reactor. The production rate was calcu-
lated based on the total weight of synthesized catalyst (2 g).
3. Results and discussion
3.1. Material characteristics
3.1.1. Surface morphology and dopant states
Fig. 1 shows the SEM and elemental mapping of Ti, Cu, and V in
the selected areas in the SEM images of the synthesized materials.
In the TiO2/PU, TiO2 was smoothly immobilized on the PU surface
as a thin layer (Fig. 1A). The surfaces of the Cu-TiO2/PU, V-TiO2/PU,
and Cu@V-TiO2/PU materials were rougher than that of TiO2/PU
due to the presence of both Cu and V on the metal-doped photocat-
alysts (Fig. 1B–D). High-resolution XPS spectra with Gaussian mul-
tipeak shapes of Cu2p3/2 and V2p3/2 peaks were obtained to
indicate the elemental states of copper and vanadium in the syn-
thesized Cu@V-TiO2/PU (Fig. 2). The XPS spectra show that the
Cu2p3/2 peaks consist of two different peaks at 932.18 and
933.48 eV, corresponding to the binding energies of Cu2p3/2 in
the Cu+ and Cu2+ states, respectively [25,26]. The Cu+ and Cu2+ exist
2.2. Photocatalyst characterization
The synthesized Cu@V-TiO2/PU photocatalysts were analyzed
by a Thermo Fisher K-Alpha X-ray Photoelectron Spectrometer
(XPS) system. The obtained XPS spectra were fitted by Gaussian
multipeak shapes to characterize the elemental states of copper,