Three-dimensional TiO2/CeO2 nanowire composite for efficient formaldehyde oxidation at low temperature

Article abstract of DOI:10.1039/c4ra13906b

We developed a low-cost and high-performance TiO₂/CeO₂ nanowire-based catalyst for efficient catalytic volatile organic compound oxidation at low temperature. The TiO₂/CeO₂ nanowires yield a remarkable HCHO conversion efficiency of 60.2% at a low temperature of 60°C and have excellent catalytic stability as well as good activity for toluene oxidation.

Full text of DOI:10.1039/c4ra13906b

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Three-dimensional TiO2/CeO2 nanowire composite
for efficient formaldehyde oxidation at low
temperature†
Cite this: RSC Adv., 2015, 5, 7729
Received 5th November 2014
Accepted 10th December 2014
Yongchao Huang, Haibo Li, Muhammad-Sadeeq Balogun, Hao Yang, Yexiang Tong,
*
*
Xihong Lu and Hongbing Ji
DOI: 10.1039/c4ra13906b
www.rsc.org/advances
We developed
a low-cost and high-performance TiO2/CeO2 show excellent catalytic performance for HCHO oxidation at
nanowire-based catalyst for efficient catalytic volatile organic relatively low temperatures. For example, complete oxidation of
compound oxidation at low temperature. The TiO2/CeO2 nanowires HCHO over noble metals/metal oxide-based catalysts occurs
yield a remarkable HCHO conversion efficiency of 60.2% at a low above 150 ꢀC over Ru/CeO2,19 Ag/MnOx–CeO2,20 above 50–100 ꢀC
temperature of 60 ꢀC and have excellent catalytic stability as well as over Au/CeO2,21 and above 80 C over Au/FeOx and Pt/TiO2.14
ꢀ
17
good activity for toluene oxidation.
Recent studies have shown that HCHO can be completely
oxidized on Au/CeO2–Co3O4 18 and Pt/TiO2 13,14 catalysts near/at
room temperature. Liu et al. developed a kind of macroporous
Au/CeO2–Co3O4 catalyst that can completely oxidize HCHO at a
low temperature of 39 ꢀC.22 Nie et al. reported a NaOH-modied
Pt/TiO2 catalyst for room-temperature catalytic oxidation of
HCHO.7 However, the scarcity and expensive nature of noble
metals severely hinder their practical application as catalysts.
The development of alternate low-cost and effective catalysts for
HCHO oxidation at room/low temperature is highly desirable.
In this study, we focused on the development of a high-
performance and non-precious metal catalyst based on one-
dimensional (1D) low cost metal oxide nanowires. In compar-
ison to bulk materials and nanoparticles, nanowires can
provide a larger interfacial area and shorter diffusion path for
active species, and thus hold great promise for catalysts. Here,
we report a novel TiO2/CeO2 nanowire composited catalyst for
low-temperature combustion of HCHO without any precious
metal. The synergistic effects from CeO2, TiO2, and 1D structure
enable the TiO2/CeO2 nanowires to possess superior catalytic
activity and stability for HCHO oxidation at low temperature. It
The ever-increasing demands for human health have stimu-
lated extensive attention on the quality of indoor air because
people usually spend more than 80% of their time in houses
and offices.1–3 Formaldehyde (HCHO), a typical volatile organic
compound (VOC), is a major indoor air pollutant that is
released from the urea–formaldehyde insulation nishing
materials, particle board, sealants and oil paint.4–6 Long term
exposure to HCHO may cause serious health problems, such as
irritation of eyes and the respiratory tract, headache, pneu-
monia, and even lung and nasopharyngeal cancer.7 Numerous
efforts have been devoted to removing the indoor HCHO, and
some strategies have been proposed in the recent years.8–10 One
of the most effective methods is to convert HCHO into CO2 and
H2O over catalysts via low-temperature thermal catalytic
oxidation in terms of its low cost, environmentally friendly
reaction conditions and energy saving.11 To improve the effi-
ciency and reduce the reaction temperature of the thermal
catalytic HCHO oxidation, considerable attention has focused
on the design and synthesis of high-performance catalysts.
Noble metal-based catalysts, such as supported Pt,7,12–14 Pd15,16
and Au17,18 on transition metal oxides containing Mn, Ce, Ti and
Cu have been extensively explored as catalysts and proven to
ꢀ
could convert 60.2% of HCHO at a low temperature of 60 C,
which is not achieved by any non-precious metal based catalysts
at such a low temperature. The as-prepared TiO2/CeO2 nanowire
composite exhibited excellent catalytic stability as well as good
activity for toluene oxidation. Furthermore, the catalytic activity
of the TiO2/CeO2 nanowire composite could be remarkably
enhanced aer loading 1 wt% Pt nanoparticles (NPs), which was
able to completely oxidize HCHO to CO2 and H2O at room
Department of Chemical Engineering, MOE of the Key Laboratory of Bioinorganic and
Synthetic Chemistry, School of Chemistry and Chemical Engineering, The Key Lab of
Low-carbon Chemistry, Energy Conservation of Guangdong Province, Sun Yat-Sen
University, Guangzhou 510275, People's Republic of China. E-mail: luxh6@mail.
sysu.edu.cn; jihb@mail.sysu.edu.cn; Fax: +86 2084112245
ꢀ
temperature (20 ꢀC) and toluene at about 250 C.
Fig. 1a presents the growth process of the 1D TiO2/CeO2
nanowire catalyst. First, free-standing TiO2 nanowires were
grown on exible carbon cloth substrate by a seed-assisted
† Electronic supplementary information (ESI) available: Synthetic details,
experimental details and additional descriptions, gures. See DOI:
10.1039/c4ra13906b
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spacing of 0.32 nm, which is in agreement with the d-spacing of
(110) planes of rutile TiO2 (Fig. 1f). These data disclose that the
TiO2/CeO2 nanowires are not core–shell nanowires but
composites of TiO2 nanowires and CeO2 nanowires. Further-
more, X-ray photoelectron spectroscopy (XPS) analysis further
conrms that the composition of the composite nanowires is
TiO2 and CeO2 (Fig. S3†). All these results reveal the successful
fabrication of TiO2/CeO2 nanowire composite.
Catalytic oxidation of HCHO was carried out to evaluate the
catalytic performance of the TiO2/CeO2 nanowire catalysts. For
comparison, pristine TiO2 nanowires and CeO2 nanowires were
also studied. Pristine CeO2 nanowires were directly grown on
carbon cloth by the same electrodeposition method with TiO2/
CeO2 nanowires (Experimental section, ESI†). SEM and XRD
analyses conrm that cubic CeO2 nanowires with a diameter of
150 nm were uniformly coated on the carbon cloth surface
(Fig. S4†). Fig. 2a shows the conversion efficiency of HCHO to
CO2 as a function of temperature over the TiO2 nanowires, CeO2
nanowires and TiO2/CeO2 nanowires. By analyzing these data,
several features are worth noting. First, the gradual increase in
the conversion efficiency with increasing reaction temperature
Fig. 1 (a) Schematic diagram illustrating the growth process of TiO2/
CeO2 nanowires on carbon cloth. (b) SEM and optical images of the as-
prepared TiO2/CeO2 nanowires on carbon cloth. (c) TEM image of
TiO2/CeO2 nanowires. (d) SAED pattern and (e) HRTEM image of CeO2
nanowire that recorded from CeO2 nanowire in (c). (g and f) HRTEM
image and SAED pattern of TiO2 nanowire that recorded from TiO2
nanowire in (c).
hydrothermal method reported elsewhere (see the Experimental
section).23 Scanning electron microscopy (SEM) images revealed
that the carbon bers were uniformly covered with TiO2 nano-
wires (Fig. S1a†). The average diameter of these as-grown TiO2
nanowires is about 100–200 nm and their lengths are around 2
mm (Fig. S1b†). An X-ray diffraction (XRD) spectrum collected
for TiO2 nanowires conrms the tetragonal structure of the
rutile TiO2 (JCPDF # 65-0192) (Fig. S2†). Then, CeO2 nanowires
were deposited onto the TiO2 nanowires by anodic electrode-
position at different time variation (15–120 min) (with opti-
mized catalytic performance at 90 min) (Experimental section,
ESI†). Aer CeO2 electrodeposition, the white color of TiO2 lm
turned into light yellow-white (bottom in Fig. 1b). SEM studies
reveal that the morphology of nanowires was mostly preserved
but distributed more closely (Fig. 1b), suggesting the CeO2
nanowires were grown between the spaces of TiO2 nanowires.
The successful growth of CeO2 nanowires was conrmed by
XRD spectra (Fig. S2†). Besides the diffraction peaks of rutile
TiO2 (JCPDS # 88-1175), the diffraction peaks of cubic CeO2
(JCPDS # 65-2975) are clearly observed, showing the presence of
CeO2 in TiO2/CeO2 nanowires.
Transmission electron microscopy (TEM) analyses were
conducted to study the detailed microstructures of TiO2/CeO2
nanowires. Fig. 1c displays a typical TEM image of TiO2/CeO2
nanowires, from which the TiO2 nanowires and CeO2 nanowires
can be clearly identied. The surface of TiO2 nanowires is very
smooth while the surface of CeO2 nanowires is relatively rough.
Selected-area electron diffraction (SAED) analyses reveal that
the CeO2 nanowire has poly-crystalline structure (Fig. 1d) and
TiO2 nanowire has single crystalline structure (Fig. 1g). Fig. 1e is
a high-resolution TEM (HRTEM) image of the CeO2 nanowire
(Fig. 1c), again conrming the poly-crystalline nature of CeO2
nanowire. The measured lattice fringe spacing is about 0.31 nm,
which is consistent with the d-spacing of (111) planes of cubic
CeO2 (JCPDS # 65-2975). A HRTEM image collected from the
TiO2 nanowire reveals clear lattice fringes with a lattice fringe
Fig. 2 (a) Catalytic performance of HCHO over TiO2 nanowires, CeO2
nanowires and TiO2/CeO2 nanowires as a function of temperature
under the following conditions: HCHO concentration ¼ 50 ppm, 25
vol% O2, N2 as balance gas, hourly space velocity (GSHV) ¼ 30 000 mL
hꢁ1 ꢁ1. (b) The comparison of HCHO conversion efficiencies of our
g
TiO2/CeO2 nanowires with the recently reported catalysts.18,24,27,29–33
(c) Catalytic performance of HCHO over TiO2/CeO2 nanowires at 60
ꢀC as a function of time.
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indicates that HCHO is more easily oxidized over these samples
at high temperature. The HCHO conversion efficiency of the
TiO2/CeO2 nanowires is much higher than the pristine TiO2
nanowires and CeO2 nanowires in the entire temperature
windows, conrming the superior catalytic activity of the TiO2/
CeO2 nanowires. Second, at a low temperature of 20 ꢀC,
approximately 20% of HCHO could be converted over the TiO2/
CeO2 nanowires, while only 3% and 4% for pristine TiO2
nanowires and CeO2 nanowires, respectively. Third, with the
Fig. 3 (a) H2-TPR profiles of the TiO2 nanowires, CeO2 nanowires and
TiO2/CeO2 nanowires. (b) CV curves of the TiO2 nanowires, CeO2
nanowires and TiO2/CeO2 nanowires obtained at a scan rate of 100
ꢀ
reaction temperature increased to 60 C, the TiO2/CeO2 nano-
wires yielded a remarkably higher HCHO conversion efficiency
of 60.2%, whereas the TiO2 and CeO2 nanowires only achieved
9.5% and 18.8%, respectively, at the same temperature. More-
over, such low reaction temperature for 60% HCHO conversion
obtained for TiO2/CeO2 nanowires is substantially lower than
the values recently reported for most of non-precious cata-
lysts24–27 and some precious catalysts,14,17 such as 3D-Co3O4 (110
ꢀC),24 and MnO2/cellulose (120 ꢀC),27 MnOx–CeO2 (80 ꢀC),25
MnxCo3ꢁxO4 (65 ꢀC),26 Au/CeO2 (70 ꢀC),17 1% Rh/TiO2 (75 ꢀC).14
Fig. 2b compares the HCHO conversion efficiencies of our TiO2/
CeO2 nanowires with other reported catalysts at low tempera-
ture ranging from 20 to 80 ꢀC. Signicantly, the HCHO
conversion performances of the TiO2/CeO2 nanowires at low
temperatures are substantially higher than most reported no-
precious catalysts, and even comparable to the previously
reported precious catalysts, such as Au/CeO2 17,21 and Ag/MnOx–
CeO2.20 All the above results fully validate that the TiO2/CeO2
nanowires possess signicantly high catalytic activity for HCHO
oxidation at low temperature.
mV sꢁ1
.
ꢀ
CeO2, while the high-temperature reduction peak at 708 C is
ascribed to the lattice oxygen of CeO2.28 Reduction of TiO2 alone
is more difficult and essentially no TPR peak was observed from
30 to 750 ꢀC. Importantly, signicant enhancement in the
reducibility was observed for the TiO2/CeO2 composite sample.
A new reduction peak (marked by green dash rectangle)
occurred in the range from 200 to 350 ꢀC, which is attributed to
the synergistic effect of CeO2 and TiO2 nanowires. This reduc-
tion peak makes the major contribution to HCHO oxidation in
comparison with the other two reduction peaks in the high
ꢀ
ꢀ
temperature region around 560 C and 708 C. In general, the
lower temperature of the corresponding desorption peak
ꢀ
centered at the 200–350 C range indicates that it is easier to
generate surface active oxygen species, which may offer a higher
catalytic activity in oxidation reactions. The lowering of reduc-
tion temperature implies that the presence of CeO2 helps to
weaken the surface oxygen on TiO2/CeO2 nanowires, and
therefore improves the reducibility of the catalyst.
Beside the conversion efficiency, the long-term stability of
the catalyst is also very important for their practical applica-
tions. To evaluate the catalytic stability of the TiO2/CeO2
nanowires, the catalytic oxidation performance of HCHO over
Cyclic voltammogram (CV) curves of the pristine TiO2
nanowires, CeO2 nanowires and TiO2/CeO2 nanowires collected
at a scan rate of 100 mV sꢁ1 in HCHO (40%) aqueous electrolyte
are displayed in Fig. 3b. All the samples showed approximately
rectangle-like shapes, revealing the electric double layer
capacitance characteristic. The substantially higher current
density of the TiO2/CeO2 sample over the pristine TiO2 and
CeO2 samples show that it possesses larger surface area. The
specic surface area of the pristine TiO2, CeO2, and TiO2/CeO2
samples is about 18.3, 24.4 and 36.7 m2 gꢁ1, respectively. Given
the experimental results above, two possible reasons are
proposed to explain the signicantly enhanced catalytic
performance of the TiO2/CeO2 nanowires. First, a good inter-
facial contact between CeO2 and TiO2 nanowires would form
legitimately, which could allow HCHO to be absorbed easily. In
addition, CeO2 is rich in oxygen vacancy defects and has a large
oxygen storage capacity, which is benecial for the HCHO
catalytic oxidation. Second, the free-standing 1D nanowires
grown on carbon cloth not only offer a large surface area for
surface reactions, but also enable the fast transport of species
and extend the reaction sites from the surface to the subsurface
of the catalysts. In order to study the optimizational ratio of the
CeO2 on the TiO2 nanowires, we deposited CeO2 on the TiO2
nanowires at different durations and studied their catalytic
performance, which are shown in Fig. S6.† Signicantly,
ꢀ
TiO2/CeO2 nanowires on stream at 60 C for 100 h is shown in
Fig. 2c. Impressively, the TiO2/CeO2 nanowires exhibited a
remarkable long-term catalytic stability with only less than 3%
decrease in HCHO conversion efficiency aer 100 h. Fig. S5a†
displays the TG curve of TiO2/CeO2 nanowires performed in air
ow from 35 to 900 ꢀC. Less than 2% of mass loss was observed
for the TiO2/CeO2 nanowires below 600 ꢀC, indicating the
excellent thermal stability of TiO2/CeO2 nanowires between 35
ꢀ
to 600 C. SEM observations reveal that the morphology of the
TiO2/CeO2 nanowires was preserved aer 100 h long-test
(Fig. S5b†). Additionally, XRD (Fig. S5c†) and XPS (Fig. S5d†)
survey spectra conrm that there were no obvious changes in
the phase and chemical composition of TiO2/CeO2 nanowires
aer testing for 100 h. Thus, the remarkable stability of the
TiO2/CeO2 nanowires is due to their excellent thermal strength,
morphology and phase stability as well as exible free-standing
carbon cloth, which could offer high mechanical stability.
To gain insights into the reasons for the excellent catalytic
performance of TiO2/CeO2 nanowires, the H2-temperature-
programmed reduction (TPR) analysis was carried out. Fig. 3a
compares the H2-TPR proles of the TiO2, CeO2 and TiO2/CeO2
samples. The pristine CeO2 nanowires exhibited two reduction
peaks, and the low-temperature reduction peak located at 485
ꢀC is attributed to the reduction of surface capping oxygen of
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depositing CeO2 on the TiO2 nanowires for 60 and 90 min gave wt% Pt/TiO2 (100% at 40 C)43 and slightly lower than those of
closely related performance.
LaMnO3 (100% at 200 ꢀC)44 and 6.5-Au/meso-Co3O4 (100% at
ꢀ
Our as-prepared TiO2/CeO2 nanowires also have excellent 190 ꢀC).45 Furthermore, the Pt/TiO2/CeO2 catalyst also
catalytic activity for toluene combustion. Fig. 4 shows the showed remarkably enhanced catalytic activity compared to the
conversion efficiency of toluene (1000 ppm) to CO2 as a function TiO2/CeO2 catalyst toward toluene oxidation that could achieve
ꢀ
of temperature over the TiO2/CeO2 nanowires at the gas hourly 50%ꢀtoluene conversion at 120 C and complete conversion at
space velocity ¼ 60 000 mL hꢁ1 gꢁ1. It is worth pointing out that 250 C.
CO2 and H2O were the only products. Signicantly, the TiO2/
CeO2 nanowires catalyst could convert 50% of the toluene at 170
Conclusions
ꢀ
ꢀC and achieved complete toluene conversion at above 300 C.
ꢀ
The present temperatures of TiO2/CeO2 sample at 170 C and
In summary, we have demonstrated the feasibility of exible 3D
TiO2/CeO2 nanowires as a new and high-performance catalyst
for low-temperature thermal catalytic oxidation of HCHO. The
as-prepared TiO2/CeO2 nanowires exhibited superior catalytic
activity that could convert 60.2% of HCHO to CO2 and H2O at a
ꢀ
300 C for 50% and 100% of toluene conversion, respectively,
are substantially lower and comparable to the values obtained
from previously reported catalysts at similar reaction condi-
tions, such as wire-like MnO2 (50% at 225 ꢀC),34 MnO2–KIT6
(50% at 203 ꢀC),35 NiO–CTAB-2 (50% at 256 ꢀC),36 LaMnO3 (50%
at 193 ꢀC),37 mesoporous LaFeO3 (50% at 200 ꢀC),38 KxMnO2
nanospheres (50% at 209 ꢀC),39 Ag–Mn/SBA-15 (50% at 202 ꢀC),40
ꢀ
low temperature of 60 C, which is the best performance ever
reported for non-precious metal based catalysts at such a low
temperature. Additionally, the TiO2/CeO2 nanowires had an
outstanding long-term cycling stability without any decay of its
catalytic activity aer 100 h, and also showed a good catalytic
activity toward toluene oxidation. Furthermore, the TiO2/CeO2
nanowires were proved to be excellent supports for noble metals
to achieve VOCs oxidation at lower temperatures. Aer loading
with 1% Pt NPs, the Pt/TiO2/CeO2 nanowires were able to
completely oxidize HCHO at room temperature (20 ꢀC) and
toluene at about 250 ꢀC. This work offers a new insight to design
low-cost and high-efficiency catalysts for low-temperature
thermal catalytic oxidation of VOCs.
Eu1ꢁxSrxFeO3 (50% at 278 C).41
ꢀ
In addition to being used as catalyst, the free-standing 1D
structure of TiO2/CeO2 nanowires also enables them to serve as
a good support for noble metals to achieve VOC oxidation at
lower temperatures. Here, we examined the feasibility of TiO2/
CeO2 nanowires to support Pt nanoparticles for HCHO and
toluene oxidation. Pt nanoparticles with a mass loading of
about 1% were prepared on the surface of the TiO2/CeO2
nanowires by a reduction method (Experiment section, ESI†). A
SEM image shows that the smooth surface of TiO2/CeO2 nano-
wires became relatively rough aer Pt nanoparticle coating
(Fig. S7a†). The TEM observation clearly shows that well crys-
talline Pt nanoparticles were uniformly coated on the nanowire Acknowledgements
surface, which is further conrmed by the EDS elemental
We acknowledge the nancial support of this work by the
mapping (Fig. S7b†). As expected, the Pt/TiO2/CeO2 nanowires
achieved complete HCHO conversion at room temperature,
which could convert more than 99% of HCHO to CO2 at 20 ꢀC.
This present efficiency is substantially higher than or compa-
rable to the values of the recently reported 1 wt% Pt/TiO2
catalyst (14.3% at 20 ꢀC),14 2 wt% Pt/nest-like MnO2 catalyst
(24.3% at 20 ꢀC),32 Au/Co3O4–CeO2 (75.2% at 25 ꢀC),18 Pt/g-Al2O3
(100% at 25 ꢀC),12 Pd–Mn/Al2O3 (100% at 90 ꢀC),15 Pt/f-SiO2
(100% at 25 ꢀC),42 1 wt% Pd/TiO2 (95.5% at 25 ꢀC),16 1 wt% Na–1
Natural Science Foundations of China (21036009, 21273290 and
91323101), the Research Fund for the Doctoral Program of
Higher Education of China (20120171110043) and the Young
Teacher Starting-up Research program of Sun Yat-Sen
University.
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Fig. 4 Catalytic performances of HCHO and toluene over TiO2/CeO2
nanowires and Pt/TiO2/CeO2 nanowire samples as a function of
temperature.
7732 | RSC Adv., 2015, 5, 7729–7733
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