CuO/TiO2 heterojunction composites: An efficient photocatalyst for selective oxidation of methanol to methyl formate

Article abstract of DOI:10.1039/c8ta09439j

This study demonstrates the synthesis of an efficient photocatalyst, CuO/TiO₂ heterojunction, for selective aerobic photo-oxidation of methanol to methyl formate. The CuO nanoparticles (size: 3.5 ± 1.0 nm) are particularly deposited at the {101} facet of anatase TiO₂ nanosheets to minimize recombination of photo-generated electrons/holes. The photocatalysts are characterized by different methods, including powder X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy, and hydrogen temperature-programmed reduction (H₂-TPR). The heterojunction catalysts exhibit an excellent photocatalytic performance under mild conditions (i.e., 25-45 °C and gas reactions under UV irradiation with a wavelength of 365 nm) for the selective photo-oxidation of methanol using 0.5 vol% oxygen (O₂) as an oxidizing agent. The 5 wt% CuO/TiO₂ photocatalyst exhibits a decent 95% methanol conversion with over 85% selectivity towards the formation of methyl formate. The by-product of the reaction is found to be merely CO₂, which can be readily eliminated from the product. The formation rate for the methyl formate is 10.8 mmol g^-1 h^-1 at 25 °C, which is considerably higher than that for the corresponding CuO and TiO₂ nanosheets as well as for conventional catalysts. The superior catalytic activity of the CuO/TiO₂ is associated with its unique electronic structure and the synergistic effects at the interface of the photocatalysts' components. The results of this study offer guidelines for design of a new synthetic strategy for preparation of efficient photocatalysts for selective oxidation of methanol to methyl formate and related reactions.

Full text of DOI:10.1039/c8ta09439j

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CuO/TiO heterojunction composites: an efficient
2
photocatalyst for selective oxidation of methanol
to methyl formate
Cite this: DOI: 10.1039/c8ta09439j
ab
a
a
a
a
a
Quanquan Shi, Guicheng Ping, Xiaojia Wang, Hui Xu,* Jingmei Li, Jiaqi Cui,
cd
a
b
Hadi Abroshan, †* Hongjing Ding and Gao Li
*
This study demonstrates the synthesis of an efficient photocatalyst, CuO/TiO
2
heterojunction, for selective
aerobic photo-oxidation of methanol to methyl formate. The CuO nanoparticles (size: 3.5 ꢀ 1.0 nm) are
particularly deposited at the {101} facet of anatase TiO nanosheets to minimize recombination of
2
photo-generated electrons/holes. The photocatalysts are characterized by different methods, including
powder X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis spectroscopy, X-ray
photoelectron spectroscopy (XPS), photoluminescence spectroscopy, and hydrogen temperature-
programmed reduction (H
2
-TPR). The heterojunction catalysts exhibit an excellent photocatalytic
ꢁ
performance under mild conditions (i.e., 25–45 C and gas reactions under UV irradiation with
a wavelength of 365 nm) for the selective photo-oxidation of methanol using 0.5 vol% oxygen (O
oxidizing agent. The 5 wt% CuO/TiO photocatalyst exhibits a decent 95% methanol conversion with
over 85% selectivity towards the formation of methyl formate. The by-product of the reaction is found to
be merely CO , which can be readily eliminated from the product. The formation rate for the methyl
formate is 10.8 mmol g
CuO and TiO nanosheets as well as for conventional catalysts. The superior catalytic activity of the
CuO/TiO is associated with its unique electronic structure and the synergistic effects at the interface of
2
) as an
2
2
ꢂ1 ꢂ1
ꢁ
at 25 C, which is considerably higher than that for the corresponding
h
2
Received 29th September 2018
Accepted 22nd December 2018
2
the photocatalysts' components. The results of this study offer guidelines for design of a new synthetic
strategy for preparation of efficient photocatalysts for selective oxidation of methanol to methyl formate
and related reactions.
DOI: 10.1039/c8ta09439j
rsc.li/materials-a
carbonylation and dehydrogenation processes of methanol at
elevated temperatures (>100 C) are applied for the production
Introduction
ꢁ
With the unprecedented scale of changes in the global energy of methyl formate. These methods, however, suffer from a lack
structure, the C
chemistry has gained overwhelming interest of selectivity that leads to the generation of a variety of by-
1
over the last decade or so as an important and reliable pathway products, such as formaldehyde, dimethoxymethane, dimethyl
1
to solve the energy crisis. Methyl formate (HCOOCH
3
), an ether, and carbon oxide, thereby posing a major challenge for
5–8
important organic synthesis intermediate in the C chemistry, separation steps.
1
is widely used in the chemical industry for the production of
dozens of chemical products (e.g., formic acid and formam- great advantages and considerable prospects for the large scale
Recently, photocatalytic oxidation of methanol has shown
2,3
ides). Methyl formate is also applied as a precursor in the synthesis of methyl formate due to the green and energy saving
9
manufacture of some pharmaceuticals and pesticides and as an nature of the pathway. The selective oxidation can provide the
4
additive to obtain several petroleum products. Traditionally, conversion of methanol to methyl formate at room temperature
with minimal formation of the by-product CO (e.g., <10%). For
2
a
example, Kominami et al. reported that methanol can selec-
College of Science, Inner Mongolia Agricultural University, Hohhot 010018, People's
tively become oxidized at room temperature and under UV
Republic of China. E-mail: yqfxxuhui@163.com
b
2
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese irradiation over anatase-TiO with 10% methanol conversion
10
Academy of Sciences, Dalian 116023, People's Republic of China. E-mail: gaoli@ and 91% selectivity towards the formation of methyl formate.
dicp.ac.cn; Fax: +86-411-82463017; Tel: +86-411-82463009
In follow-up studies, the noble metal nanoparticles (e.g., Au, Ag,
c
SUNCAT Center for Interface Science and Catalysis, Department of Chemical
and Pt) immobilized on the TiO were explored for the reac-
2
Engineering, Stanford University, Stanford, CA 94305, USA
11–14
tion.
Yang et al. reported that Au and Ag as well as their
d
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA
alloys supported on titania exhibit superior activity with 82%
methanol conversion and 85% selectivity for methyl formate
3
0332, USA. E-mail: hadi.abroshan@chemistry.gatech.edu
†
Present address: Georgia Institute of Technology.
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ꢁ
13
formation at 25 C. Despite the high performance of these pho- with a Barnstead NANOpure Diwater system. All glassware was
tocatalysts, a wide-spread implementation of the catalysts is thoroughly cleaned with aqua regia (37% HCl : HNO3
¼
limited due to the high cost or scarce supply of noble metals. In 3 : 1, vol%), rinsed with copious amounts of nanopure water,
this regard, Yang et al. synthesized Cu nanoparticles loaded on and then dried in an oven prior to use.
commercial P25 as an active photocatalyst for the oxidation of
methanol to methyl formate, achieving 65% methanol conversion
Materials preparation
ꢁ
with 55% selectivity for methyl formate at 25 C under UV irradi-
Anatase-TiO nanosheets were synthesized by a hydrothermal
15
2
2
ation. The deposition of copper nanoparticles on the TiO surface
20
method. Typically, 10 mL tetrabutyl titanate (Ti(OBu) ) was
4
forms a heterogeneous interface, which facilities electron transfer
mixed with 1.5 mL (40% w/w) HF in a Teon-lined autoclave (50
mL) under stirring at room temperature for 30 min. Then, the
from TiO to the copper nanoparticles. Nevertheless, the catalytic
2
activity is still substantially low for the large scale production of
methyl formate. One potential explanation for the low activity of
the catalyst is the random deposition of the Cu nanoparticles at
Teon-lined autoclave was sealed and hydrothermally treated at
ꢁ
1
80 C for 24 h. It was cooled down to room temperature
gradually. The obtained white precipitates were washed with
2
the TiO surface, leading to the presence of Cu nanoparticles at
ꢂ
0.1 M NaOH solution and distilled water to remove uoride (F )
facets that may facilitate the electron/hole recombination, and
ions on the TiO nanosheets. Finally, the white solids were dried
15
2
thus decrease the activity of the photocatalyst.
Photo-generated electrons and holes of TiO
ꢁ
at 80 C overnight.
2
nanomaterials
The CuO/TiO composite catalysts were prepared by a facile
2
can be directionally separated over different facets of the mate-
rials. Previous studies show that the {001} and {101} facets
intrinsically prefer to accommodate holes and electrons, respec-
15
reduction–deposition method. Typically, 1 g of the as-obtained
TiO nanosheet powder was ultrasonically dispersed in a certain
amount of Cu(NO solution (0.01 M) for 30 min. Next, an
aqueous solution of NaBH (Cu : NaBH
2
3 2
)
16
tively. Therefore, facet engineering has emerged as a robust
strategy to obtain an optimum ratio of different facets for the
high performance of catalysts. For example, a recent study by Yu
4
4
¼ 1 : 2.5, molar ratio)
was added to the suspension, followed by vigorous stirring at
room temperature for 24 h. The precipitate was collected,
washed thoroughly with water, and dried at 80 C overnight.
The catalysts were labelled ‘x’ wt% CuO/TiO , where ‘x’ repre-
sents the CuO weight percent. We examined the catalysts with
nominal CuO loadings of 0, 3, 5, 7 and 10 wt%.
2
et al. shows that the TiO activity in the photocatalytic reduction
ꢁ
of CO becomes maximum when 58% of the catalyst's surface is
2
16
2
terminated by the {001} facet. Formation of heterogeneous
interfaces via selective deposition of metal nanoparticles or metal
oxides on the {001} and {101} facets of TiO is shown to further
2
enhance the separation of photo-generated electrons and
17–19
Characterization
holes.
3 4
For instance, Pt nanoparticles and Co O oxides that
are selectively deposited on the {001} and {101} facets exhibit Powder X-ray diffraction (XRD) patterns were recorded with a D/
1
7
enhanced photocatalytic activity for water oxidation reactions.
Max-2500/PC diffractometer (Rigaku, Japan) using Cu Ka (l ¼
These results indicate that precise deposition of oxides over 0.154 nm) radiation operated at 40 kV and 200 mA. High reso-
specic facets can be systematically applied to considerably lution scanning electron microscopy (HRSEM) was carried out
reduce the recombination rate of photo-generated electrons/ on a Hitachi Hightechnology S-5500 with an operating voltage
holes during the operation of the catalysts.
Herein, we synthesize a novel CuO/TiO
of 40 kV. Transmission electron microscopy (TEM) images were
composite photo- recorded using a FEI Tecnai G Spirit microscope operated at
2
2
catalyst via a facile reduction–deposition method. In this study, 120 kV, and high resolution TEM (HRTEM) images were ob-
2
CuO nanoparticles with an average size of 3.5 ꢀ 1.0 nm are tained on a FEI Tecnai G F30S-Twin microscope operated at an
2
selectively loaded on the {101} facet of anatase-TiO nanosheets. accelerating voltage of 300 kV. The specimen was prepared by
2
The CuO/TiO shows an excellent performance in the photo- ultrasonically dispersing the sample in ethanol, and droplets of
catalytic oxidation of methanol to form methyl formate, i.e., the suspension were deposited on a carbon-enhanced copper
9
5% methanol conversion and >85% selectivity for the synthesis grid and then dried in air. UV-visible diffuse reectance spectra
ꢁ
of methyl formate at 25 C. A tentative reaction mechanism is were recorded in the range of 200–800 nm on a spectropho-
proposed. We demonstrate that the p–n heterojunction between tometer (JASCO-V550, Japan) using BaSO4 as the reference.
CuO and the electron-rich {101} facet of TiO
2
is signicantly Photoluminescence (PL) spectra were recorded at room
active towards the photocatalytic oxidation reaction under UV temperature on a Horiba Jobin Yvon LabRAM HR800 Raman
irradiation at a wavelength (l) of 365 nm.
spectrometer with a He–Cd laser (325 nm) as the light source.
Raman spectra were recorded on a Jobin-Yvon U1000 scanning
double monochromator with a single-frequency line at 532 nm
being used as the excitation source. X-ray photoelectron spectra
Experimental section
Chemicals
(XPS) were recorded on an ESCALAB MK-II spectrometer (VG
All chemicals were used as received without further purica- Scientic Ltd., UK) with Al K
a
radiation at room temperature. It
tion: tetrabutyl titanate (99.9%, Sigma-Aldrich), hydrouoric is worth noting that before the measurements, samples were
acid (40%, Adamas), sodium hydroxide (98%, Adamas), copper outgassed in the prechamber of the instrument at room
ꢂ12
nitrate hydrate (99%, Adamas), and sodium borohydride (98%, temperature up to a pressure of <10
bar. The charging effect
Adamas). Nanopure water (resistivity, 18.2 MU cm) was puried was corrected by adjusting the binding energy of C 1s to
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Cu(NO
3 2 2
) solution in the presence of TiO nanosheets using
NaBH as a reducing agent. The nal CuO/TiO products with
4
2
different CuO weight percentages (‘x’ wt% CuO/TiO ) were ob-
2
ꢁ
tained by centrifugation, dried at 80 C, and further character-
ized by XRD, TEM, XPS, and UV-vis and photoluminescence
spectroscopy as well as by the H
The phase composition and the crystallinity of the TiO
nanosheets and CuO/TiO composites are obtained by powder
XRD. Fig. 1a shows that the TiO nanosheets comprise only the
2
-TPR method.
2
2
2
anatase phase, evident from ve sharp and intense diffraction
peaks at 25, 38, 48, 54, and 55 that are attributed to the (101),
Scheme 1 The schematic diagram of the photocatalytic oxidation
process of methanol to form methyl formate.
ꢁ
(
(
004), (200), (105), and (211) crystal facets of pure anatase-TiO2
JCPDS 01-078-2486), respectively. Compared with the standard
card, the (200) diffraction peak is intensied, while the peak for
004) is weakened. These results indicate that the TiO particles
in our study have predominantly exposed {001} facets because
2
84.6 eV. The spectra were deconvoluted using the XPSPEAK
program by curve tting with mixed Gaussian–Lorentzian
peaks. Hydrogen temperature-programmed reduction (H -TPR)
(
2
2
of a higher growth along the [100] direction and a lower growth
was conducted on an Autochem II 2920 instrument (Micro-
meritics) equipped with a thermal conductivity detector. 50 mg
of the sample was pretreated with He (30 mL min ) at 50 C for
21,22
along the [001] direction.
For the CuO/TiO composites, no
2
diffraction peaks associated with metallic Cu, and CuO and
ꢂ1
ꢁ
Cu O oxides were detected, which is likely due to the low crys-
2
1
h. Aer cooling down to room temperature, a 5 vol% H
2
–N
2
2
tallinity and the high dispersion of CuO on the TiO surfaces.
ꢂ1
mixture (30 mL min ) was introduced, and the temperature
was increased to 800 C at a rate of 10 C min
The intensity of diffraction peaks decreased with an increase in
the CuO loading content, which is due to a shield effect from
ꢁ
ꢁ
ꢂ1
.
23
the CuO nanoparticles.
To further examine the existence of CuO at the surface of
TiO nanosheets, a set of CuO/TiO composites with different
Photocatalytic activity test
The photocatalytic oxidation of methanol to methyl formate was
performed in a self-designed continuous-ow aluminium alloy
reactor with a rectangular quartz window on the top. Scheme 1
shows a schematic diagram of the photocatalytic process. A
2
2
ratios of CuO : TiO2 is prepared using the same synthetic
protocol. The diffraction peaks of the TiO (200) are magnied
2
ꢁ
in the 2q range of 47.6–48.4 (Fig. 1b). For all the CuO/TiO
2
catalysts, the pattern is slightly broadened and shied to the
right, implying the presence of the additives (i.e., CuO
species). Fig. 1b shows that the use of a higher content of CuO
5
00 W high pressure mercury lamp (CEL-LAM500) with a wave-
length of 365 nm installed above the quartz window of the
reactor with a light intensity of 18.6 mW cm was employed as
the light source. The reaction temperature was set at 25–45 C
23
ꢂ2
ꢁ
leads to a lower intensity of the diffraction peak, evidence for
the presence of CuO at the TiO surface.
using cooling water circulation. 20 mg of the catalysts were
coated on the surfaces of a rectangular piece of glass. The entire
surface coated by the catalysts was fully exposed to light. A gas
mixture containing 1.0 v% methanol is taken in a ask, and x
v% O (where x ¼ 0.3, 0.5, 0.75, and 1.0) balanced with N at
2
TEM images show that the TiO particles exhibit a uniform
2
sheet-like structure with a side length of 40 nm and a thickness
of 6 nm (Fig. 2a). A top-view TEM image displays two crossed
ꢁ
lattice fringes of 0.19 nm with an interfacial angle of 90 that are
2
2
ꢂ1
ascribed to the {200} and {020} planes (Fig. 2b). This result
a ow rate of 20 mL min that is bubbled through the liquid
methanol in the ask is introduced into the reactor (see Scheme
implies the predominant exposure of the {001} facet of the TiO
2
.
A side-view TEM image shows the lattice spacings of 0.47 and
1
). The effluent was examined using an on-line Shimadzu
GC2014C equipped with a thermal conductivity detector (TCD)
and a ame ionization detector (FID). The formation rate of
15
methyl formate was calculated using
F ¼ rVCS/2mM
ꢂ1 ꢂ1
where F (mmol g
formation rate of methyl formate, concentration of methanol,
ow rate of feed gas, methanol conversion, methyl formate
h ), r, V, C, S, m, and M represent the

selectivity, weight of the catalyst, and molecular weight of
methanol, respectively.
Results and discussion
Fig. 1 XRD patterns of TiO
is worth noting that only the anatase-TiO
observable. The spectra in (a) are up-shifted for clarity. (b) Magnifica-
2
nanosheets and CuO/TiO
2
composites. It
2
diffraction peaks are
The CuO/TiO
2
heterojunction catalysts are synthesized via
a simple facile reduction–deposition protocol from an aqueous tion of the diffraction peaks at 2q ¼ 48ꢁ.
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Fig. 2 SEM and TEM images of (a–c) TiO
nanosheets is 3.5 ꢀ 1.0 nm. The insets in (c) and (g) show the schematic diagrams of the TiO
2
nanosheets and (d–h) CuO/TiO
2
composites. The size of CuO nanoparticles at the surface of TiO
nanosheets and the CuO/TiO composites.
2
2
2
ꢁ
2+ 26
0
.35 nm with an interfacial angle of 70 , corresponding to the Cu . We also note that the latter optical adsorption is another
{
101} facet (Fig. 2c). Our analyses indicate that the TiO nano- piece of evidence for the existence of CuO species in the CuO/
2
sheets consist of 77% {001} facet and 23% {101} facet exposed TiO
on the square and the isosceles trapezoidal surfaces, respec-
2
composites.
Furthermore, photoluminescence (PL) spectroscopy is
tively. Fig. 2d–f display the morphology of the TiO samples applied to examine the effects of the presence of CuO nano-
2
supporting the 5 wt% CuO. The CuO/TiO2 shows almost particles on the charge recombination rate in TiO nanosheets.
2
a similar size and morphology to the TiO
2 2
nanosheets before the The PL spectrum of TiO nanosheets presents the highest
CuO deposition. Fig. 2e and f show that the {101} facet of the emission intensity at 405 nm (Fig. 3b), consistent with
28
TiO
CuO nanoparticles with an average size of 3.5 ꢀ 1.0 nm, while of CuO/TiO
the {001} facet remained unchanged. The nanosphere-like plain TiO nanosheets. We note that a higher concentration of
2
nanosheets is decorated with a large number of spherical a previous study. The shape and position of the emission peak
2
composites are found to be similar to those of the
2
particles loaded over the {101} facet of the TiO show a lattice CuO leads to a lower emission intensity, indicating that the
2
spacing of 0.23 nm, corresponding to the {111} facet of CuO recombination of photo-generated charges (electrons/holes) is
27
(
Fig. 2g and h). Taking all of these results into account, we think suppressed. This result also reveals that a p–n heterojunction
that CuO nanoparticles are selectively anchored on the {101} is constructed at the interface of CuO (p-type semiconductor)
facet of TiO nanosheets. One possible reason for the selective and the electron-rich {101} facet of the TiO (n-type semi-
deposition of the CuO could be the more favorable adsorption conductor). A potential explanation for the suppressed charge
2
2
2
+
free energy of Cu species on the {101} facet than on the {001}
facet of TiO . Therefore, it would be worthwhile in the future to
2
investigate the details of atomic interactions between the CuO
and the TiO and their inuence on the selective deposition of
2
CuO on the {101} facet.
The optical properties of TiO
composites are investigated by UV-vis reectance spectroscopy
Fig. 3a). The TiO nanosheets exhibit a strong adsorption band
below 400 nm, corresponding to a band gap energy of 3.1 eV.
2 2
nanosheets and CuO/TiO
(
2
24,25
The addition of CuO nanoparticles distinctively alters the
adsorption spectrum of the system, with a new adsorption
feature appearing at 400–800 nm. The shied adsorption band
at 410–510 nm is in accordance with the charge transfer from
15
the valence to the conduction bands of CuO. The adsorption
Fig. 3 (a) UV-vis reflectance and (b) photoluminescence spectra of
2 2
band at 520–800 nm is associated with the d–d transition of the TiO nanosheets and CuO/TiO composites.
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recombination is the electron transfer from TiO
2
to CuO
nanoparticles at the interface, thus promoting the separation of
17–19
the photo-generated electrons and holes.
Fig. 4 shows the Raman spectra of TiO nanosheets and CuO/
2
2
TiO composites. The samples exhibit characteristic peaks of
the anatase-TiO
2
phase, in good agreement with a previous
16
ꢂ1
study. The E
g
peaks at 144 and 636 cm correspond to the
symmetric stretching vibration of O–Ti–O, and the B1g and A1g
ꢂ1
peaks at 394 and 514 cm correspond to the symmetric and
20
anti-symmetric bending vibrations of O–Ti–O, respectively. An
increase in the CuO concentration of the catalysts results in
a lower intensity of the peaks, which is primarily due to the high
dispersity of CuO nanoparticles over the TiO
2
nanosheet
29
surface, sharing the surface O entities.
X-ray photoelectron spectroscopy (XPS) is utilized to investigate
the valence states of CuO/TiO composites. Fig. 5a–c show the XPS
spectra of Ti 2p, O 1s, and Cu 2p for the 5 wt% CuO/TiO
2
2
30–33
composite.
The characteristic binding energies (BEs) at 458.5
and 464.2 eV correspond to Ti 2p and Ti 2p1/2, respectively,
3
/2
4+
30
attributed to Ti in anatase-TiO (Fig. 5a). The O 1s spectrum
2
consists of three types of oxygen species; the BE at 530.1 eV is
associated with the lattice oxygen (O
31.4 eV refers to the chemisorbed oxygen species (O
oxides, and the peak at 532.7 eV is ascribed to the hydroxyl groups (b) O 1s, and (c) Cu 2p. (d) H
composite.
a
) of TiO
2
or CuO, the BE at
5
b
) over the
Fig. 5 (a–c) XPS spectra of the 5 wt% CuO/TiO
2
composite: (a) Ti 2p,
2
-TPR profile of the 5 wt% CuO/TiO
2
31
on the oxide surface (Fig. 5b). Two typical peaks at 954.3 and
2
+
9
34.2 eV with a BE gap of 20.1 eV demonstrate the presence of Cu
+
32
species rather than Cu . It is worth noting that the BE peak at
2+
9
43.3 eV is a shakeup satellite peak of Cu , associated with an formate under UV irradiation (l ¼ 365 nm) in a temperature
9
ꢁ
unlled 3d shell of Cu (Fig. 5c), which is another piece of evidence range of 25–45 C. Fig. 6 shows that the TiO nanosheets yield
2
33
to conrm the presence of CuO in the CuO/TiO
2
composites.
37% CH OH conversion with 63% selectivity to form methyl
3
ꢁ
To further study the copper species of the photocatalysts, we formate at 25 C. An increase in the reaction temperature
ꢁ
conduct hydrogen temperature-programmed reduction (H
2
-
increases the methanol conversion (e.g., 57% at 45 C) and
TPR) over the 5 wt% CuO/TiO
2
composite. Fig. 5d presents three simultaneously decreases the selectivity for methyl formate
ꢁ
reduction peaks at 154, 171 and 301 C, corresponding to two (Fig. 6b, black line). We next consider the activity of the CuO/
34
different types of CuO species. The reduction peaks at 154 and TiO composites.
2
ꢁ
1
71 C are attributed to highly dispersed CuO species at the
Fig. 6 shows the catalytic performance of the CuO/TiO2
ꢁ
nanoscale, and the peak at 301 C corresponds to the reduction composites in terms of both the CH OH conversion and the
3
of CuO in a bulky state.
The CuO/TiO composites and TiO
methyl formate selectivity which are largely enhanced in
nanosheets are tested in comparison with those of TiO2 nanosheets. Among all the
2
2
2 2
the selective aerobic oxidation of methanol gas to methyl studied CuO/TiO composites, the 5 wt% CuO/TiO exhibits the
highest catalytic activity with 95% CH
3
OH conversion and
ꢁ
85.5% selectivity towards methyl formate formation at 25 C. It
is worth noting that the conversion rate over the 5 wt% CuO/
TiO remains almost the same regardless of the reaction
2
temperature; however, the selectivity drops as temperature
ꢁ
increases (e.g., 77% at 45 C). A lower selectivity at higher
temperature is mainly due to the over-oxidation process,
leading to the formation of the by-product CO
activities, including the conversion and the selectivity of the
CuO/TiO composites, decrease if the concentration of CuO
2
. The catalytic
2
becomes greater than 5 wt%. A higher loading of CuO that
exceeds that in the optimum and effective heterojunction
interface forms potential recombination sites for photo-
generated electrons/holes and hence results in a lower photo-
32
catalytic activity.
Previous studies indicate that it is plausible to consider the
ꢂ
Fig. 4 Raman spectra of TiO
2
nanosheets and CuO/TiO
2
composites. formation of superoxide species (O
) via surface electron
2
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2 2
Fig. 6 (a) Methanol conversion and (b) methyl formate (MF) selectivity as a function of temperature on TiO nanosheets and CuO/TiO pho-
tocatalysts. Reaction conditions: 20 mg catalysts and CH
3
OH : O
2
: N
2
¼ 1.0 : 0.5 : 98.5. (c) The photocatalytic performance of 5 wt% CuO/TiO
2
ꢁ
at 25 C in the presence of different O
2
concentrations. The methanol concentration is kept fixed at 1.0%, while the O
2
concentration is varied
from 0.3% to 1.0%.
35,36
transfer to adsorbed O2.
Therefore, we next investigate the shown in Fig. 8, the methanol conversion and the selectivity
effect of oxygen concentration on the catalytic performance of towards methyl formate throughout the reaction remain almost
ꢁ
the 5 wt% CuO/TiO at 25 C. The CH OH concentration is kept constant at ꢃ95% and ꢃ85%, respectively. At the end of the
2
3

xed at 1.0%, while the O
2
concentration is varied from 0.3 to reaction, the used catalyst is examined using transmission
1
.0%. The results show that in the presence of 0.3% O
CH OH conversion rate and the selectivity toward the formation size and morphology of the used catalyst exhibit a similar
of methyl formate are 90.6% and 71.4%, respectively. By- pattern to those of the fresh catalyst (Fig. 8, inset). These results
products of the reaction are found to be 15.4% CO and 3.2% indicate that the CuO/TiO composite is robust and holds
formaldehyde. As the oxygen concentration is increased to promise for application for prolonged periods of time.
.0%, the methanol conversion is gradually increased to 97.3% Finally, a tentative reaction mechanism for the selective
Fig. 6c). We also note that, as the O concentration is increased photo-oxidation of methanol to methyl formate over CuO/TiO2
2
, the electron microscopy. Analysis of TEM images shows that the
3
2
2
1
(
2
to 0.5%, the selectivity towards methyl formate increases to composites is proposed (Scheme 2). Irradiation of the catalysts
5.4% with the production of CO
as the only by-product by UV light (l ¼ 365 nm) excites electrons at the valence band
14.6%). A further increase in the O content is found to (VB) of TiO nanosheets to the conduction band (CB), forming
signicantly reduce the selectivity, e.g., 41.2% and 4.7% with electrons and holes, which later transfer to the {101} and {001}
.75% and 1.0% O , respectively. This is mainly due to the over- facets of TiO , respectively. The p–n heterojunction formed at
8
2
(
2
2
0
2
2
oxidation of methanol in the presence of excess oxygen to form the interface of CuO and the {101} facet of TiO nanosheets
2
CO as the main product. It is worth noting that no formalde-
2
hyde is detected as a by-product with an O concentration of
2
0
.5–1.0%. Therefore, we here choose 0.5% O
concentration for our further investigations.
Furthermore, we investigate the catalytic formation rate of
methyl formate over TiO nanosheets and CuO/TiO composites
2
as the optimal
2
2
at different reaction temperatures (Fig. 7). The formation rate of
methyl formate over TiO2 nanosheets slightly increases as
ꢂ1 ꢂ1
ꢁ
a function of temperature, reaching 4.0 mmol g
h
at 45 C.
The formation rate of methyl formate over all the CuO/TiO2
composites is found to be considerably higher than that over
the TiO
formation rate of 10.8 mmol g
substantially higher than those of previously reported photo-
2
catalysts (e.g., anatase-TiO , P25, Cu/TiO , Ag/TiO , Au/TiO ,
2 2
nanosheets. The 5 wt% CuO/TiO yields the highest
ꢂ1 ꢂ1
ꢁ
at 25 C (Fig. 7, blue bar),
h
2
2
2
10,11,15,37
and CuO/CuZnAl) (Table 1).
For completeness, we investigate the durability of the 5 wt%
CuO/TiO catalyst for the aerobic oxidation of methanol at Fig. 7 Formation rate of methyl formate (MF) over TiO2 nanosheets
2
ꢁ
and CuO/TiO2 photocatalysts as a function of temperature in the
methanol oxidation reaction. Reaction conditions: 20 mg catalysts and
2
5 C. The reaction conditions are the same as those noted in
Fig. 7, except that the reaction time is increased to 20 h. As
3
CH OH : O
2
: N
2
¼ 1.0 : 0.5 : 98.5.
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Journal of Materials Chemistry A
Table 1 Summary of reaction conditions and catalytic activities for the selective photo-oxidation of methanol into methyl formate over various
catalysts at room temperature
Temperature
( C)
CH
3
OH
Methyl formate
selectivity (%)
Formation rate
ꢁ
ꢂ1 ꢂ1
Entry
Catalyst
conversion (%)
(mmol g
h
)
Reference
1
2
3
4
5
6
7
Anatase-TiO
P25
2
25
25
25
25
25
25
25
10
27
65
75
65
80
95
91
56
55
80
75
60
84
1.5
1.8
4.4
7.3
5.9
10
13
15
13
13
35
Cu/TiO
2
Ag/TiO
Au/TiO
2
2
CuO/CuZnAl
5% CuO/TiO
5.8
10.8
2
-nanosheet
This work
the above processes are the rate-determining step for the
11–14
oxidation reaction.
The formaldehyde rapidly reacts with
the neighbouring adsorbed methoxyl to convert to methyl
formate. The synergistic effect (e.g., facet-induced and
heterojunction-induced effects on CuO/TiO
2
photocatalysts)
can strengthen the separation of photo-generated electrons and
holes (Scheme 2). Thus, the CuO/TiO2 composites exhibit
superior photocatalytic activity than the plain TiO nanosheets.
2
Conclusions
In summary, CuO/TiO2 heterojunction photocatalysts were
successfully synthesized through loading of CuO nanoparticles
over the electron-rich {101} facet of TiO
photocatalyst TiO
composites exhibited enhanced photocatalytic perfor-
mance in comparison to pure TiO nanosheets for the selective
2
nanosheets. The CuO/
Fig. 8 The durability test results of the 5 wt% CuO/TiO
2
2
in the methanol oxidation reaction. Reaction conditions: 20 mg
2
ꢁ
catalysts, 25 C, 20 h, and CH
3
OH : O
2
: N
2
¼ 1.0 : 0.5 : 98.5. Inset:
oxidation of methanol to methyl formate in a temperature range
TEM image of the used catalysts.
ꢁ
of 25–45 C under UV irradiation with a wavelength of 365 nm.
The higher catalytic activity was ascribed to the well-designed
CuO/TiO2 heterojunction photocatalysts, which remarkably
improved the separation rate of photo-generated electrons and
holes by synergistic effects at the interface of the catalyst
components. This study provides a new route for the design of
efficient photocatalysts for the selective oxidation of alcohol to
ester and related reactions.
works as a bridge to transfer the accumulated electrons at the
101} facet to the CuO surface. We note that the work function
of CuO (5.3 eV) is larger than that of the anatase TiO (4.7 eV),
and thus it is prudent to consider the electron transfer to the
CuO nanoparticles. An optimum separation efficiency of photo-
generated electrons and holes can be obtained through the
addition of the CuO, which is a key factor to improve the pho-
tocatalytic activity.
{
2
Conflicts of interest
The photo-generated holes are responsible for the selective
oxidation of the methanol to formaldehyde, which can be
There are no conicts to declare.
detected if the O
ratio of the reaction (e.g., CH
while, the photo-generated electrons on CuO surfaces can
2
concentration is below the stoichiometric
OH : O
3
2
¼ 0.3, Fig. 6c). Mean-
Acknowledgements
promote the oxygen dissociation to rell the oxygen vacancies at
Q. S. thanks the National Natural Science Foundation of China
(21665020), the Natural Science Foundation of Inner Mongolia
Autonomous Region (2018BS02004), and the Program of
Higher-level Talents of Inner Mongolia Agricultural University
the surfaces of the anatase-TiO . It is worth noting that both of
2
(NDYB2016-03) for nancial support, and BL14B and BL17B
beamlines of the National Facility for Protein Science (NFPS),
Shanghai Synchrotron Radiation Facility (SSRF) Shanghai,
China, for providing the beam time.
Scheme 2 Proposed reaction mechanism for the selective oxidation Notes and references
2 2
of methanol to methyl formate over TiO nanosheets and CuO/TiO
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