Enhanced catalytic activity of hierarchically macro-/mesoporous Pt/TiO2 toward room-temperature decomposition of formaldehyde

Article abstract of DOI:10.1039/c4cy01712a

Hierarchically macro-/mesoporous Pt/TiO₂ is prepared by hydrothermally treating the tetrabutyl titanate (Ti(OC₄H₉)₄) precipitates in pure water, followed by a combined NaOH-assisted NaBH₄-reduction deposition of Pt nanoparticles (NPs) on the TiO₂ surface. The catalytic activity toward the catalytic decomposition of formaldehyde (HCHO) vapor in air at ambient temperature is evaluated. The prepared Pt/TiO₂ catalyst exhibits excellent catalytic activity and recyclability, and 90.6% HCHO with an initial concentration of ca. 265 ppm can be completely decomposed within 60 min, which is significantly higher than that using commercial-grade Degussa P25 TiO₂ with an equal Pt-loading amount (56.2% HCHO decomposition). The high catalytic activity can be mainly attributed to the hierarchically macro-/mesoporous structures of the TiO₂ support with large specific surface area and optimal pore size. These characteristics not only enhance the adsorption of HCHO molecules, as well as facilitate the diffusion and transport of reactants and products, but also promote the dispersion of Pt NPs with small particle size inside and outside the pores. This study demonstrates that hierarchically macro-/mesoporous Pt/TiO₂ can serve as an efficient catalyst for indoor HCHO decomposition and will provide new insights into the environmentally benign design and preparation of low-cost and high-performance catalysts for indoor air purification. This journal is

Full text of DOI:10.1039/c4cy01712a

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Enhanced catalytic activity of hierarchically
macro-/mesoporous Pt/TiO2 toward room-
temperature decomposition of formaldehyde
Cite this: DOI: 10.1039/c4cy01712a
a
b
c
c
ad
*
*
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Lifang Qi, Wingkei Ho, Jinlong Wang, Pengyi Zhang and Jiaguo Yu
Hierarchically macro-/mesoporous Pt/TiO2 is prepared by hydrothermally treating the tetrabutyl titanate
IJTiIJOC4H9)4) precipitates in pure water, followed by a combined NaOH-assisted NaBH4-reduction deposi-
tion of Pt nanoparticles (NPs) on the TiO2 surface. The catalytic activity toward the catalytic decomposition
of formaldehyde (HCHO) vapor in air at ambient temperature is evaluated. The prepared Pt/TiO2 catalyst
exhibits excellent catalytic activity and recyclability, and 90.6% HCHO with an initial concentration of ca.
265 ppm can be completely decomposed within 60 min, which is significantly higher than that using
commercial-grade Degussa P25 TiO2 with an equal Pt-loading amount (56.2% HCHO decomposition). The
high catalytic activity can be mainly attributed to the hierarchically macro-/mesoporous structures of the
TiO2 support with large specific surface area and optimal pore size. These characteristics not only enhance
the adsorption of HCHO molecules, as well as facilitate the diffusion and transport of reactants and prod-
ucts, but also promote the dispersion of Pt NPs with small particle size inside and outside the pores. This
study demonstrates that hierarchically macro-/mesoporous Pt/TiO2 can serve as an efficient catalyst for
indoor HCHO decomposition and will provide new insights into the environmentally benign design and
preparation of low-cost and high-performance catalysts for indoor air purification.
Received 22nd December 2014,
Accepted 29th January 2015
DOI: 10.1039/c4cy01712a
www.rsc.org/catalysis
photocatalytic oxidation, plasma catalytic oxidation, and bio-
technical decomposition, have been studied.9–14 Conventional
Introduction
Indoor air quality is crucial for human health because people
spend more than 80% of their time in an indoor environ-
ment.1 HCHO emitted from buildings and furnishing mate-
rials, plastic cements, paintings, and other consumer prod-
ucts is regarded as one of the main indoor air pollutants.2–8
Long-term exposure to HCHO with concentrations exceeding
safe limitations can cause serious health problems, including
irritation of the eyes, throat, and skin, as well as headache,
respiratory tract infection, pneumonia, and even cancer.2–8 To
eliminate HCHO from indoor air, several purification tech-
niques, such as adsorption (including physical and chemical),
adsorption has been proven efficient for HCHO elimination,
but the adsorbents used usually have limited capacities and
need regular replacement. Photocatalytic and plasma cata-
lytic oxidation can effectively purify indoor chemicals. How-
ever, they need light or extra apparatus, and harmful by-
products may be generated during the purification process.
Biotechnical decomposition is an environmentally benign
method for HCHO degradation, but its rate is not sufficiently
high, and the life cycles of the microorganisms involved
are still a problem. Thermal catalytic oxidation (TCO) has
recently attracted considerable attention because of its envi-
ronmentally friendly reaction conditions and energy-savings
consideration, and it achieves complete oxidation of HCHO
to CO2 and H2O. Various TCO catalysts have been investi-
gated for HCHO removal, including metal oxides (Ti, Mn,
Ce, and Cu) and supported noble metal (Pt, Au, Pd, Rh, and
Ag) catalysts.1,15–20 Compared with low-cost metal oxides,
the supported noble metal catalysts were proven to possess
better catalytic performance and durability at relatively low
temperatures, even at room temperature. For example,
Au/Co3O4–CeO2, Pt/TiO2, Pd/TiO2, and Pt/MnO2 exhibit
excellent catalytic activities toward HCHO catalytic oxidation
at room temperature.1,15–20 However, developing a highly
a State Key Laboratory of Advanced Technology for Material Synthesis and
Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070,
PR China. E-mail: jiaguoyu@yahoo.com; Fax: 0086 27 87879468;
Tel: 0086 27 87871029
b Department of Science and Environmental Studies and Centre for Education in
Environmental Sustainability, The Hong Kong Institute of Education, Tai Po,
N.T. Hong Kong, PR China. E-mail: keithho@ied.edu.hk
c State Key Joint Laboratory of Environment Simulation and Pollution Control,
School of Environment, Tsinghua University, Beijing, 100084, PR China.
E-mail: zpy@tsinghua.edu.cn
d Department of Physics, Faculty of Science, King Abdulaziz University,
Jeddah 21589, Saudi Arabia
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effective catalyst that can completely oxidize HCHO at room
temperature at high concentrations is necessary in practical
applications.
HCHO oxidation.17 Nie et al. also reported that the catalytic
activity of the Pt/TiO2 (P25) catalyst could be enhanced by
NaOH-modification because of the introduction of additional
surface hydroxyl groups, the increased adsorption capacity
toward HCHO, and the formation of larger mesopores and
macropores.1 In addition, Nie et al. firstly reported the fluo-
ride poisoning phenomenon of Pt/FTiO2-NS (surface-fluori-
nated anatase TiO2 nanosheets with dominant {001} facets)
for the decomposition of HCHO, which resulted from the
blocking of the Pt catalytic active sites by the strong interac-
tions between Pt and highly electronegative fluoride.26 How-
ever, these studies have not mentioned the important effects
of the structure of the supports on the catalytic activities of the
catalysts. The combination of hierarchical macro-/mesoporous
structures with large surface area in the supports is a new
approach to enhance the catalytic activity of the catalysts. In
this study, we reported the synthesis of a novel and high effi-
cient Pt/TiO2 catalysts with hierarchically macro-/mesoporous
structure and large Brunauer–Emmett–Teller (BET) specific
surface area without using any template or additive. The
specific structure and high specific surface area of the
support would contribute to the efficient mass transport
and the high dispersion of Pt NPs simultaneously and
finally promote the catalytic activity of the Pt/TiO2 catalysts.
The prepared catalysts exhibit a higher activity than com-
mercial P25 TiO2 with an equal Pt-loading amount for HCHO
oxidation at ambient temperature. To our knowledge, this
study is the first to report the preparation and catalytic
activity of hierarchically macro-/mesoporous Pt/TiO2 for
HCHO decomposition.
For heterogeneous solid–gas catalytic reaction, the TCO
efficiency of a catalyst is strongly affected by the intrinsic
properties of the active components (for example, the kind,
particle size, and dispersion of the noble metals on sup-
ports), as well as the effects of mass transport. The latter
highly correlates with the diffusion (intra- or inter-particles)
of the reactants and products. As to supported noble metal
catalysts, the physical properties of the supports significantly
influence their TCO efficiency from the following aspects. On
the one hand, the particle size and dispersion of the active
noble metals are affected by the loading methods as well as
the morphology, structure, and specific surface area of the
supports. Supports with a large specific surface benefit the
formation and dispersion of small noble metal particles,
which are crucial for high TCO activity.18,19 On the other
hand, the rate of mass transfer in TCO catalysis is partly
determined by the morphology and structure of the sup-
ports.19,21 Both theoretical and experimental studies have
shown that the surface roughness degree of pores strongly
influences the rate of mass transfer in the pores, particularly
through surface diffusion.16 The fabrication of hierarchically
ordered macro-/mesoporous materials on multiple scales has
recently attracted considerable attention because of their
high specific surface area, micro- and mesoporosity, and the
accessible diffusion pathways of macroporous networks.
Hierarchically porous structures are practically applied in the
following: separation, absorption, and catalysis; optical, bio-
materials engineering, and controlled drug delivery devices;
and energy storage and conversion.13,22–24 Our previous stud- Experimental
ies confirmed that hierarchically porous TiO2 has high photo-
Materials
catalytic activity for the degradation of gaseous acetone.22,23
All chemicals were of analytical grade and purchased from
Our group found that hierarchically porous NiIJOH)2/SiO2
composites can efficiently adsorb HCHO in air at room tem-
perature, and the adsorption rate and capacity far exceed
those of commercial zeolite and active carbon.13 In designing
highly effective catalysts for gaseous HCHO oxidation, sup-
ports with hierarchically porous structures are needed to
improve the catalytic activity. These structures are necessary
because hierarchically porous structure and high specific sur-
face area can incorporate or stabilize the active components,
such as noble metals, and provide more space for the diffu-
sion of gas molecules, which play an important role in
enhancing HCHO decomposition.
Recently, Pt/TiO2 catalysts have attracted a lot of attention
due to their high catalytic activities toward decomposition of
HCHO at room temperature. For example, Huang et al. inves-
tigated the influences of the oxidation state, support and size
of Pt particles, and the operating parameters for room tem-
perature oxidation of HCHO.18,25 Zhang et al. reported a
novel alkali-metal-promoted Pt/TiO2 catalyst for ambient
destruction of HCHO.17 They found that the addition of
alkali-metal ions could open a new low-temperature reaction
pathway, significantly promoting the catalytic activity for
Shanghai Chemical Reagent Factory of China without further
purification. Deionized (DI) water was used in the whole
experiment.
Sample preparation
In a typical synthesis, 20 mL of TiIJOC4H9)4 was added
dropwise to 200 mL of distilled water without stirring, and
the mixture was then sealed and incubated at ambient tem-
perature for 24 h. The resulting precipitates were filtered,
repeatedly rinsed with distilled water, and dried in a vacuum
oven at 80 °C for 10 h. The dried sample was denoted as T25.
Subsequently, 1.0 g of T25 and 150 mL of distilled water were
mixed in a Teflon-lined autoclave with a capacity of 200 mL,
followed by hydrothermal treatment of the mixture at 180 °C
for 24 h. After the hydrothermal reaction, the obtained pre-
cipitates were filtered, repeatedly rinsed with distilled water,
and finally dried in a vacuum oven at 80 °C for 10 h. The
obtained sample was labeled as T180. The effects of the
morphology and structure of the catalyst on the catalytic
activity for HCHO oxidation were confirmed by grinding
T180 into fine powder using an agate mortar to destroy the
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macrochannels (or hierarchically macro-/mesoporous struc-
tures). The obtained sample was labeled as T180G.
Scientific, USA). The sample was initially placed on a sample
cell within an atmospherically controllable chamber and sub-
sequently pretreated with a flow of dry air at 150 °C for 1 h
prior to analysis. The analysis was performed under a flow
(30 mL min−1) of gas mixture (78 ppm HCHO/O2) at ambient
temperature. The spectra were obtained from 4000 cm−1 to
1000 cm−1 with a resolution of 4 cm−1 and an accumulation
of 32 scans. The background spectrum was subtracted from
each spectrum. The BET surface area (SBET) of the samples
was analyzed by nitrogen adsorption using a Micromeritics
ASAP 2020 nitrogen adsorption apparatus (USA). All pow-
dered samples were degassed at 150 °C prior to nitrogen
adsorption measurements. SBET of the samples was deter-
mined by a multipoint BET method using the adsorption
data within the relative pressure IJP/P0) range of 0.05–0.3. The
pore-size distributions were evaluated using the adsorption
data by the Barrett–Joyner–Halenda (BJH) method, assuming
a cylindrical pore model.27 The amount of the adsorbed
nitrogen under a relative pressure IJP/P0) of 0.97 was used to
determine the pore volume. The porosity was evaluated using
Pt/TiO2 catalysts were prepared using a combined NaOH-
assisted impregnation of TiO2 with Pt precursor and NaBH4
reduction process. In a typical synthesis, 0.5 g of T25, T180,
and T180G powders were separately dispersed in 10 mL of
1.28 mM H2PtCl6 aqueous solution without stirring. After
impregnation for 0.5 h, 2.5 mL of a mixed aqueous solution
containing NaBH4 (0.1 M) and NaOH (0.5 M) was rapidly
added to each suspension with slow stirring. After reacting
for 0.5 h, the suspensions were evaporated and dried in an
oven at 80 °C. The obtained samples were denoted as T25P,
T180P, and T180GP. The effects of the morphology and struc-
ture on HCHO decomposition were also investigated. T180P
was ground into fine powder, and the obtained sample was
denoted as T180PG. Commercial Degussa P25 TiO2 (P25) is
one of the most popular commercial TiO2 and usually
selected as a reference. In this study, P25P was also prepared
under the same conditions except that 0.5 g of P25 powder
was used instead of the previously mentioned TiO2 powder.
The nominal content of Pt (RPt) in the Pt-supported catalysts
was 0.5 wt%. The detailed experimental conditions for the
preparation of all the samples are listed in Table 1.
the following simplified formula: Porosity = Vp × 100%/(Vp
+
1/ρ), where Vp and ρ are the measured pore volume and den-
sity of titania, respectively.
Characterization
Catalytic activity test
The X-ray diffraction (XRD) patterns were obtained using a
D/Max-RB X-ray diffractometer (Rigaku, Japan) with Cu Kα
irradiation at a scan rate of 0.05° 2θ s−1. Transmission
electron microscopy (TEM) and high-resolution transmission
electron microscopy (HRTEM) images were obtained using a
JEM-2100F electron microscope (JEOL, Japan). Scanning
electron microscopy (SEM) images were obtained utilizing a
JSM-6510 microscope (JEOL, Japan). X-ray photoelectron
spectroscopy (XPS) measurements were performed using an
ESCALAB210 spectrometer (VG Scientific, UK) with Mg Kα
source and charge neutralizer. All binding energies were
referenced to the C 1s peak at 284.8 eV of the surface adventi-
tious carbon. Differential thermal analysis-thermogravimetry
The room-temperature thermal catalytic activity for HCHO
oxidation was performed in a dark organic glass box covered
with a layer of aluminum foil on its inner wall at 25 °C. A 5 W
fan was placed at the bottom of the reactor with a volume
of ca. 4.4 L (17 × 16 × 16 cm3) throughout the reaction pro-
cess. In a typical experiment, 0.1 g of catalyst was initially dis-
persed at the bottom of a glass Petri dish with a diameter of
9 cm. Subsequently, the dish containing the sample was
placed at the bottom of the reactor and then covered with a
glass slide. The reactor was sealed and injected with a certain
amount of condensed HCHO solution (38%). The concentra-
tions of gaseous HCHO, CO2, and H2O were detected online
using a Photoacoustic IR Multigas Monitor (INNOVA air Tech
Instruments Model 1412). Prior to the catalytic reaction, the
dish containing the sample was covered to ensure the isola-
tion of the catalyst from HCHO until the adsorption/desorp-
tion equilibrium within the reactor was reached and until the
concentration of HCHO vapor was stable. The initial concen-
tration of the HCHO vapor was controlled at about 265 5 ppm.
During the catalytic reaction, a near 1 : 1 ratio of the pro-
duced CO2 to the decomposed HCHO was observed, and
the HCHO concentration steadily decreased with increasing
reaction time. Each experiment was performed for at least 60
min. Removal efficiency (η), denoting removal percentage of
HCHO after reaction for 1 h, was used to evaluate the perfor-
mance of the samples. η was calculated using the equation:
η = {[HCHO]initial − [HCHO]final} × 100%/[HCHO]initial, where
[HCHO]initial is the HCHO concentration after adsorption/
desorption equilibrium and [HCHO]final is the HCHO concen-
tration after reaction for 1 h. Specific removal efficiency (ηs),
(DTA-TGA) was performed using
a DTG-60H analyzer
(Shimadzu, Japan), and about 10 mg of sample was heated at
5 °C min−1 until 860 °C in air. The in situ diffuse reflectance
infrared Fourier transform (DRIFT) spectra were obtained
with a Nicolet 6700 FT-IR spectrometer (Thermo Fisher
Table 1 Experimental conditions for sample preparation
Samples
Treatmenta
RPt
Grinding
Composition
T25
T180
T25P
T180P
P25P
T180G
T180GP
T180PG
No
H
0
0
No
No
No
No
No
Yes
Yes
Yes
TiO2
TiO2
Pt-loading
H & Pt-loading
Pt-loading
H
H & Pt-loading
H & Pt-loading
0.5
0.5
0.5
0
0.5
0.5
TiO2/Pt
TiO2/Pt
TiO2/Pt
TiO2
TiO2/Pt
TiO2/Pt
a
H denotes hydrothermal treatment.
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denoting the removal efficiency per unit surface area of the
catalyst, was also provided to see the difference in the perfor-
mance among the studied samples. ηs was calculated using
the equation: ηs = η/IJSBET × m), where m is the mass of the
catalyst.
formation of brookite. Fig. 1 shows that T25P and T180P
have similar diffraction patterns to those of T25 and T180,
suggesting that the Pt-loading has no evident influence on
the phase structure and composition of TiO2. The average
crystallite sizes of anatase and brookite phases in T180 and
T180P are 7.6 and 10.1 nm for T180, and 7.7 and 9.9 nm for
T180P (see Table 2). These values were calculated using
Scherrer's equation (d = Kλ/B cos θ, where d is the crystal size,
B is the width in radians of the peak, K is equal to 0.89, θ is
the Bragg angle, and λ is the X-ray wavelength) from the main
diffraction peaks of anatase (101) and brookite (121). This
result confirms that Pt-loading cannot alter the crystallite
sizes of TiO2. Considering that the colors of T25 and T180
change from white to gray after Pt-loading, no Pt diffraction
peak (the strongest peak at 2θ = 39.8°) is observed in the case
of T25P and T180P because of the low loading amounts,
small particle sizes, and good dispersion of Pt NPs.18
H2PtCl6 adsorption test
The H2PtCl6 adsorption test was performed at ambient tem-
perature in air. About 50 mg of the as-prepared sample was
dispersed in 10 mL of 0.32 mM H2PtCl6 aqueous solution in
a 25 mL beaker. After impregnating for a certain amount of
time, the solution was filtered to measure the concentration
change of H2PtCl6 by recording the changes in the absorption
peak intensities at 258 nm using a UV–vis spectrophotometer
(UV-2550, Shimadzu, Japan).
Results and discussion
Structure and morphology
Nitrogen adsorption–desorption isotherms and the corre-
sponding pore-size distribution curves for T25, T180, T25P,
and T180P are presented in Fig. 2. For the samples before
hydrothermal treatment (T25 and T25P), the isotherms con-
sist of a combination of types I and IV with two notably dis-
tinct regions according to the Brunauer–Deming–Deming–
Teller (BDDT) classification (Fig. 2a). At low relative pressure,
the isotherms exhibit high adsorption, indicating the pres-
ence of micropores (type I).27 However, at the high relative
pressure range of 0.4–0.6, the isotherms are type IV and have
a small hysteresis loop, suggesting the presence of a small
amount of mesopores (2–50 nm).27 Furthermore, the hystere-
sis loop is type H2, indicating the existence of ink-bottle
pores with narrow necks and wider bodies.27 Hydrothermal
treatment has a significant influence on the isotherms of the
samples. After hydrothermal treatment (T180 and T180P), the
isotherms are classical type IV (BDDT classification) and have
a larger hysteresis loop (type H2) at a relatively higher pres-
sure range of 0.6–0.8 than those of T25 and T25P, indicating
the presence of ink-bottle mesopores (2–50 nm) with larger
pore volume and pore sizes.23,27 This result may be related to
the enlargement of the crystallite size and the enhancement
of crystallization (see Fig. 1 and Table 2). Further observation
shows that Pt-loading affects the isotherms of the samples.
Compared with T25 and T180, T25P and T180P show lower
absorption in the relative pressure range of 0.4–1.0, indicat-
ing their smaller pore volume. The pore-size distributions of
the samples are further confirmed through their correspond-
ing pore-size distribution curves in Fig. 2b. Before hydrother-
mal treatment, T25 and T25P display a narrow pore-size dis-
tribution in the micro- and mesoporous regions (≤4 nm) with
a peak pore diameter of ca. 3 nm. After hydrothermal treat-
ment, the pore-size distributions of T180 and T180P shift to
the right with a sharp peak at 6 nm. The N2-absorption volume
of the samples decreases after Pt-loading, which is consistent
with the results in Fig. 2a. All physical properties of the sam-
ples are listed in Table 2. The as-prepared amorphous samples
show larger specific surface areas (387 and 380 m2 g−1 for
T25 and T25P, respectively) than the hydrothermally treated
XRD was used to investigate the effects of hydrothermal treat-
ment and Pt-loading on the phase structure and crystallite
size of the as-prepared samples. Fig. 1 shows the XRD pat-
terns of T25, T180, T25P, and T180P. The sample before
hydrothermal treatment (T25) had an amorphous phase
because of the incomplete hydrolysis of TiIJOC4H9)4 at room
temperature and the large amounts of the residues of un-
hydrolyzed alkyls in the precipitate, which prevent crystalliza-
tion by adsorbing on the surface of TiO2 NPs. After hydro-
thermal treatment (T180), phase transformation occurred
and the main diffraction peaks were readily indexed to ana-
tase TiO2 [JCPDS, no. 21-1272; space group, I41/amd (141)]. A
minor peak appears at 2θ = 30.7°, corresponding to the (121)
plane diffraction of the brookite phase TiO2 [JCPDS, no. 29-
1360; space group, Pcab (61)]. The appearance of the brookite
phase can be ascribed to the neutral-pH environment
throughout the synthesis, which is favorable for the
Fig. 1 XRD patterns of T25, T25P, T180 and T180P.
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Table 2 Physical properties and catalytic activities of as-prepared samples
Crystalline sizeb
[nm]
SBET
Pore volume
Peak pore diameterc
[nm]
Porosity
[%]
Sample
Phasea
[m2 g−1
]
[cm3 g−1
]
η [% h−1
]
ηs [% h−1 m−2
]
T25
T180
T25P
T180P
P25P
T180G
T180GP
T180PG
Am
A, B
Am
A, B
A, R
A, B
A, B
A, B
—
387
158
380
135
41
158
174
129
0.26
0.30
0.25
0.26
0.18
0.31
0.14
0.26
3.0
6.0
2.9
6.0
70.0
5.3
3.4
6.2
50.0
53.5
49.0
50.0
40.6
54.7
35.0
49.7
50.9
37.2
76.5
90.6
58.4
26.5
50.6
62.6
1.3
2.4
2.0
6.7
14.3
1.7
2.9
4.9
A: 7.6; B: 10.1
—
A: 7.7; B: 10.2
A: 22.2; R: 43.9
A: 7.6; B: 10.1
A: 7.7; B: 10.2
A: 7.7; B: 10.2
a
b
Am, A, B and R denote amorphous, anatase, brookite and rutile, respectively. The crystalline size of TiO2 was determined by XRD using
Scherrer's equation (d = Kλ/B cos θ, where d is the crystal size, B is the width in radians of the peak, K is equal to 0.89, θ is the Bragg angle, and
c
λ is the X-ray wavelength from the main diffraction peaks of anatase (101) and brookite (121)). Peak pore diameter denotes the diameter of
the pores at the maximum of pore-size distribution curves.
samples (158 and 135 m2 g−1 for T180 and T180P, respec-
tively). These changes are mainly due to the increase in crys-
tallite sizes after hydrothermal treatment. After Pt-loading,
the specific surface area and pore volume of the samples
slightly decrease.
Therefore, the macroporous structures of the samples were
directly observed using SEM. Fig. 3a and b show that T25
and T180 exhibit a worm-like macroporous framework with
continuous walls consisting of closely packed aggregates of
particles (ranging from 0.4 μm to 1.5 μm). The sizes of the
3D continuous macropores and their wall thicknesses are ca.
6–12 μm and ca. 0.5–2.0 μm, respectively. These 3D continu-
ous macroporous channels may serve as ideal transport
routes for introducing gas molecules into the interior space
of the samples. In addition to the worm-like macropores, a
significant amount of small aggregated pores composed of
several small interconnected spherical particles with sizes of
0.8–1.5 μm also exists in the sample. Therefore, SEM and N2
adsorption analyses reveal that T25 and T180 exhibit the hier-
archically porous structure of a macro-/mesoporous system.
TEM and HRTEM were used to study the microstructures and
crystallization of the hierarchically macro-/mesoporous Pt/
TiO2 sample (T180P). Fig. 3c presents the typical TEM image
of T180P. The primary particle size is about 8 nm, which is
in agreement with the value of the crystallite size determined
by XRD (7.7 nm) (Table 2). The findings also indicate that a
significant number of mesopores formed from the aggrega-
tion of primary particles (or crystallites). Fig. 3d and e pres-
ent another TEM image at higher magnification and an
HRTEM image of T180P, respectively. These two images con-
firm the presence of Pt NPs, as well as their particle size and
dispersion status on the TiO2 surface. Fig. 3d shows that sev-
eral small Pt NPs with average diameters of ca. 1–3 nm are
uniformly deposited on the TiO2 surface with high dispersion
because of their high electron density and good immobiliza-
tion on the surface of the TiO2 support. Fig. 3e shows observ-
able lattice fringes, which allow the identification of the crys-
tallographic spacing and indicate that T180P is well
crystallized. Fringes of 0.352 nm match the (101) crystallo-
graphic plane of anatase TiO2, and the lattice fringes of 0.227
nm match the crystallographic plane of Pt (111). The energy
dispersive X-ray spectroscopic analysis (Fig. 3f) further con-
firms that T180P is composed of Ti, O, and Pt elements origi-
nating from the Pt-loaded TiO2 sample itself.
N2-adsorption–desorption analyses could not provide
macroporous information on the as-prepared samples.
Fig. 2 Nitrogen adsorption–desorption isotherms (a) and corresponding
pore-size distribution curves (b) of T25, T25P, T180 and T180P.
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Fig. 3 SEM images of T25 (a) and T180 (b) samples; TEM (c, d) and HRTEM (e) images of T180P; TEM-EDX spectrum (f) of T180P.
XPS analysis
energies of ca. 458.4 (Ti 2p), 529.8 (O 1s), 70.6 (Pt 4f), 62.4
(Na 2s), and 284.8 eV (C 1s). Na species originate from the
residual Na+ because of the use of NaOH and NaBH4 reac-
tants. C species originate from the adventitious hydrocarbon
from the XPS instrument itself. The high-resolution XPS spec-
trum of the Pt 4f region obtained from the surface of T180P
is shown in Fig. 4a. The binding energies corresponding to
The surface elemental compositions of T180P and its chemi-
cal status were analyzed by XPS, and the results are shown in
Fig. 4. The XPS survey spectrum of T180P (not shown here)
indicates the presence of Ti, O, Pt, Na, and C elements, and
their corresponding photoelectron peaks appear at binding
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groups is favorable for the enhancement of the catalytic
activity for HCHO decomposition.1
DTA-TGA analysis
The DTA-TGA curves of T180P (Fig. 5) are similar to those
previously reported for mesostructured materials.30 The deep
endothermic peak below 100 °C and the broad exothermic
peak at ca. 250 °C can be ascribed to the dehydration, car-
bonization, and combustion of the organic species in the
sample. The continuous exothermic process from 400 °C to
800 °C is due to the oxidation of the residual organic compo-
nents as well as the simultaneous crystallization and phase
transformation of the sample from amorphous to anatase
and rutile. The TGA curve of T180P shows three weight-loss
regions. The first stage is observed from room temperature to
100 °C, and the weight loss is ca. 7.9%, which can be attrib-
uted to the dehydration and evaporation of volatile organic
species. The second stage is observed from 100 °C to 400 °C,
and the weight loss is ca. 6.5%. These results can be attrib-
uted to the combustion and carbonization of the remaining
organic species in the sample. The third stage is observed
from 400 °C to 800 °C, and the weight loss is ca. 3.6%, which
is associated with the oxidation of residual carbon and evapo-
ration of chemisorbed water. The DTA-TGA curves of T25,
T180, and T25P are similar to that of T180P, except for the
different weight losses (not shown here). The weight losses
from room temperature to 800 °C of these samples follow the
order: T25P (24.9%) > T180P (18.1%) > T25 (15.7%) > T180
(12.9%). T180 and T180P exhibit less weight loss compared
with T25 and T25P, respectively. This result is attributed to
hydrothermal treatment, which can enhance the hydrolysis of
the unreacted alkoxides in the amorphous samples as well as
accelerate their restructure and phase transformation from
amorphous to anatase (see Fig. 1). This phenomenon ulti-
mately results in the liberation of the organic species in the
samples. The organic species can be easily removed by wash-
ing, leading to the relatively low amounts of the organic com-
ponents in T180 and T180P and having less weight losses
Fig. 4 High-resolution XPS spectra of Pt 4f (a) and O1s (b) for T180P.
Pt 4f7/2 and Pt 4f5/2 of T180P are 70.6 and 73.9 eV, respec-
tively, indicating metallic Pt. The 3.33 eV difference between
the binding energies of the Pt 4f7/2 and Pt 4f5/2 peaks is also
characterized by the metallic Pt 4f states. This result is con-
sistent with the TEM results (Fig. 3c–e). Usually, the binding
energies of Pt 4f7/2 for Pt0, Pt2+, and Pt4+ are around 71.2,
72.4, and 74.2 eV, respectively. Compared with the binding
energy of Pt 4f7/2 for the bulk metallic Pt0 (71.2 eV),28 the
negative shift of Pt 4f7/2 of T180P is caused by a strong
metal–support interaction,1,18 resulting in the transfer of
electrons from TiO2 to Pt. For the catalytic oxidation of
HCHO, metallic Pt provides more reactive sites than the
other Pt oxidation states, such as PtO and PtO2.18 The high-
resolution XPS spectrum of the O1s region of T180P is
shown in Fig. 4b. The O1s region has two peaks. The main
peak is attributed to the lattice oxygen of TiO2 (Ti–O–Ti),
and the minor peak is assigned to the oxygen of the hydroxyl
group (Ti–OH).18,29 This finding is attributed to T180P pre-
pared under alkaline conditions, resulting in more hydroxyl
groups chemisorbed on the TiO2 surface. As previously
reported, the presence of surface hydroxyl groups can facili-
tate HCHO adsorption, and high concentration of hydroxyl
Fig. 5 DTA-TGA curves of T180P.
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than those in T25 and T25P. In addition, T25P and T180P
exhibit more weight loss than T25 and T180. As previously
mentioned, the Pt-loading treatment in NaOH aqueous solu-
tion leads to the chemisorption of the hydroxyl groups from
the solution onto the catalyst surface.1 The adsorbed hydroxyl
groups are gradually removed with increasing heating tem-
perature, leading to the larger weight losses of T25P and
T180P than those of T25 and T180.
(see Fig. 6b). Table 2 shows that η for T180P, T25P and
P25P are 90.6, 76.5 and 58.4% h−1, respectively. These results
indicate that T180P exhibits higher catalytic activity for
HCHO oxidation than T25P and P25P at ambient tempera-
ture. According to the previous literature, the highest cata-
lytic activity for HCHO decomposition over the Pt/TiO2 cata-
lysts under similar experimental conditions was reported by
Nie et al.1 In their study, 1 wt% Pt/TiO2 (P25) showed the
highest catalytic activity (HCHO concentration decreases
from ca. 253 to 15 ppm and the corresponding ΔCO2 reached
Evaluation of catalytic activity
ca. 379 ppm after reaction for 1 h) with an η of ca. 94.1% h−1
,
Fig. 6 compares the catalytic activities in the HCHO oxidation
against the as-prepared samples. Fig. 6a shows that the
HCHO concentration decreases with increasing reaction time
for each sample, and the HCHO concentration of T180P
decreases more rapidly than those of T25P and P25P. For
instance, the HCHO concentrations of T25P, T180P, and
P25P decrease after 1 h from ca. 265 to 62.5, 25.1, and 116.0
ppm, respectively. Accordingly, the CO2 concentration increases
(ppm, ΔCO2, which is the difference between the initial and
reaction CO2 concentrations) of T25P, T180P, and P25P are
about 267.5, 407.2, and 182.7 ppm within 1 h, respectively
slightly exceeding that of T180P. However, the mass of the
catalyst used in their experiment was triple to that in our
study. This comparison also confirms the high catalytic activ-
ity of T180P. It can also be seen from Fig. 6 that ΔCO2 for
each sample is higher than the corresponding decrease in
the HCHO concentration. It is due to the desorption of
several HCHO molecules from the reactor surface during
the experiment and their subsequent oxidation to CO2.1 To
explain the observed concentration difference, HCHO adsorp-
tion test was performed on the corresponding supports
(T25, T180, and P25), and the results are shown in Fig. 7. The
Fig. 6 Concentration changes of HCHO (a) and ΔCO2 (b) as a function
Fig. 7 Concentration changes of HCHO (a) and ΔCO2 (b) as a function
of reaction time on T25P, T180P, and P25P.
of reaction time on P25, T25 and T180.
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HCHO adsorptions on the above samples are significantly
rapid within the initial 5 min, and then gradually decelerate,
resulting in equilibrium within ca. 1 h (Fig. 7a). In contrast,
the CO2 concentrations are almost unchanged throughout
the process, suggesting that the HCHO molecules are mainly
adsorbed on the aforementioned samples (Fig. 7b). As shown
in Table 2, η for T25, T180 and P25 are 50.9, 40.6, and
37.2% h−1, respectively, following the same order as in SBET
:
T25 (387 m2 g−1) > T180 (158 m2 g−1) > P25 TiO2 (52 m2 g−1). The
specific surface area and HCHO adsorption capacity of the
catalysts are important factors for the HCHO decomposition
reaction because HCHO adsorption on the surface of the cat-
alysts is the first step for HCHO oxidation. Moreover, the
large specific surface area can offer more active adsorption
sites, which benefits the HCHO adsorption. Noble metals are
also well dispersed in the small species on the support mate-
rials with large specific surface area.31 Zhang et al.31 reported
that the small particle size of noble metals is crucial for a
better activity because more active sites are accessible. There-
fore, the low catalytic activity for HCHO oxidation on P25P
may be partly ascribed to its small specific surface area and
low HCHO adsorption capacity. However, T25P with the larg-
est specific surface area has a lower catalytic activity than
T180P, suggesting that the specific surface area is not the
determining factor for the catalytic oxidation of HCHO; this
finding has also been confirmed in several previous stud-
ies.7,13,19 In addition to specific surface area, other factors,
including the morphology, pore size, crystal phase, exposure
of the different superficial functional groups and crystal
facets and the dispersion and size of the supported noble
metals, also affect the HCHO oxidation performance.15–20,31–34
A comparison of ηs permits understanding of the difference
in the catalytic activity of the samples. As shown in Table 2, ηs
for T180P, T25P and P25P are 6.7, 2.0 and 14.3% h−1 m−2,
respectively. The observed difference in ηs also indicates that
apart from the surface area, other factors may play different
roles in promoting the catalytic activity of the catalyst. Table 2
shows that the peak pore diameters of T180 (support of
T180P) and T25 (support of T25P) are 6.0 and 3.0 nm, respec-
tively. Zhang et al.31 reported that nanocomposites with
large pore size and metal oxide nanocrystals can readily
load active components, such as gold, on the surface of the
metal oxide nanocrystals through a deposition–precipitation
method. In the present study, T180 with relatively large pore
size may contribute to the better dispersion of the Pt NPs
than T25. Fig. 8 presents the UV–vis spectral changes in the
H2PtCl6 aqueous solution as a function of impregnation time
in the presence of T25 and T180. The maximum absorption
peak of H2PtCl6 was selected to monitor the adsorption pro-
cess. With increasing impregnation time, the maximum
absorption peaks of T25 and T180 at 258 nm gradually
decrease; however, the absorption peak of T180 more rapidly
decreases than that of T25. Particularly, the absorption
peaks of T25 and T180 decrease from around 4 to 2.52 and
0.19 after 12 min, respectively. This result indicates that
T180 has higher H2PtCl6 adsorption capacity than T25. For
Fig. 8 UV–vis spectral changes of H2PtCl6 aqueous solution as a
function of impregnation time with T25 (a) and T180 (b).
Pt-loading experiment, the adsorption of H2PtCl6 on the
surface of the support is the first step, and a good H2PtCl6
adsorption capacity may result in well-dispersed Pt NPs after
reduction. Fig. 3d shows that the Pt NPs with diameters of
1 nm to 3 nm are well dispersed both inside and outside the
pores of T180. In contrast, only a small number of Pt NPs on
T25P with diameters of 5 nm to 10 nm are observed in the
TEM image (not shown here). Further observation shows
that these Pt NPs form large agglomerates (not shown here).
Therefore, the higher catalytic activity of T180P than that
of T25P can be mainly attributed to its larger pore size and
lead to highly dispersed Pt NPs, which provide more active
sites for HCHO oxidation.35 The phase structure of the TiO2
support may also play an important role in the HCHO cata-
lytic oxidation on the supported noble metal catalysts. This
study suggests that the anatase TiO2 support has higher cata-
lytic activity for HCHO oxidation than amorphous TiO2,
which has been widely proven by studies on photocatalysis.
The peculiar hierarchically porous structure of T180P should
be considered to rationalize sufficiently its high catalytic
activity.
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T180 was ground into fine powder to destroy the macro-
for Pt-loading because they contribute to the good dispersion
of Pt NPs.
porous networks or the hierarchically macro-/mesoporous
structures (T180G) and then loaded with Pt NPs using the
same method (T180GP). This was conducted to further verify
the effects of the hierarchically macro-/mesoporous struc-
tures on the catalytic activity of TiO2 samples. For compari-
son, T180PG was also prepared by grinding T180P into fine
powder (see Experimental section). Fig. 9a presents the con-
centration changes of HCHO on T180GP and T180PG as a
function of reaction time. Compared with T180P, the HCHO
concentrations on T180GP and T180PG decrease more rapidly
within the initial 5 min, become significantly slower in the
subsequent 55 min, and then finally decrease from ca. 265 to
124.4 and 84.1 ppm, respectively. Accordingly, the increases
in the CO2 concentrations on T180GP and T180PG reach 60.9
and 242.2 ppm, respectively (Fig. 9b). Table 2 shows that η
for T180GP and T180PG are 50.6 and 62.6% h−1, respectively,
lower than that of T180P (90.6% h−1). These results suggest
that the absence of the hierarchically macro-/mesoporous
structures would result in an evident decrease in the catalytic
activity of the catalyst. T180GP notably has a lower catalytic
activity than T180PG, further confirming that the presence of
the hierarchically macro-/mesoporous structures is necessary
According to the preceding results and those of previous
studies,22–24 hierarchically macro-/mesoporous structures are
beneficial to the catalytic activity enhancement for HCHO
decomposition. This finding can be understood through the
following aspects. First, the hierarchically macro-/meso-
porous structure, high total pore volume, and large specific
surface area of T180 (support of T180P) contribute to the
HCHO adsorption on the catalyst surface. Second, the spe-
cific hierarchically macro-/mesoporous structure of T180,
along with its relatively large specific surface area and pore
sizes, favors the dispersion of active Pt NPs (Fig. 3d and e)
because more loading sites and spaces are provided for the
Pt NPs; these loading sites and spaces have significant effects
on the HCHO catalytic oxidation.17,19 Third, such structures
combine large pores (providing more space for the diffusion
of gas molecules and enabling high gas flow rates) with small
pores (ensuring high substrate–gas contact) in a connected
network.23 This combination facilitates the diffusion of gas
into the active sites, enhancing their contacts and ultimately
resulting in the decomposition of HCHO. Finally, the favor-
able (or close) contact between Pt and TiO2 and the synergis-
tic effect between the Pt NPs and the TiO2 nanostructure may
also serve as essential roles in the catalytic decomposition of
HCHO. In fact, several studies reported the synergistic effects
between noble metal NPs and supports. Yu et al.19 reported
that the high catalytic activity for HCHO decomposition on
Pt/nest-like MnO2 can be ascribed to the high dispersion and
small size of Pt NPs as well as the synergistic effect between
Pt NPs and MnO2 nanostructure. Tang et al.34,36 demon-
strated the synergistic effect between noble metal NPs and
supports by investigating the catalytic activities for HCHO
oxidation on Ag/MnOx–CeO2 and Pt/MnOx–CeO2. Shen
et al.35,37 found that different morphologies and pore struc-
tures of CeO2 supports have notable effects on the catalytic
activity for HCHO decomposition on Au/CeO2. They also
reported that the interaction between gold species and CeO2
supports enhances the catalytic activity of Au/CeO2. In the
present study, the high catalytic activity of T180P can be
ascribed to the following factors, as previously discussed:
hierarchically macro-/mesoporous structure, optimal pore
size and large specific surface area of its support (T180),
small size and good dispersion of Pt NPs, and favorable (or
close) contact between Pt and TiO2 and synergistic effect
between them. The hierarchically macro-/mesoporous struc-
ture enables the diffusion and transport of reactants (HCHO
and O2 molecules) and products (H2O and CO2 molecules) as
well as promotes the dispersion of Pt NPs with small particle
size inside and outside the pores.
The reaction mechanism of the HCHO catalytic oxidation
on T180P was investigated using in situ diffuse reflectance
infrared Fourier-transform (DRIFT) spectra (Fig. 10) at ambi-
ent temperature. After exposing T180P to HCHO/O2 for
2 min, two bands [around 3500 cm−1 for νIJOH) and 1670 cm−1
for δIJH–O–H)] appear, indicating the formation of H2O
Fig. 9 Concentration changes of HCHO (a) and ΔCO2 (b) as a function
of reaction time on T180PG and T180GP.
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Fig. 11 Concentration changes of HCHO as a function of reaction
time on the recycled T180P catalyst in seven repeated tests within 7
days (once a day).
Fig. 10 Dynamic changes in the in situ DRIFT spectra of T180P under
a flow of HCHO/O2 at ambient temperature.
during the HCHO exposure. The intensities of the two bands
gradually increase with increasing reaction time because
HCHO is further oxidized and more H2O is generated. After
10 min, formate species [2830, 2807, and 2703 cm−1 for
νIJCH); 1610, and 1435 cm−1 for νasIJCOO); 1360 and 1320 cm−1
for νsIJCOO); 1215 cm−1 for νIJCO)] and dioxymethylene species
[1500 cm−1 for δIJCH2) and 1100 cm−1 for νIJCO)] form and
become dominant with increasing HCHO exposure
time.1,17,38–42 Based on the above results and those of
previous studies, the underlying mechanism for the HCHO
catalytic oxidation on T180P is proposed.1,17 First, HCHO and
O2 are adsorbed onto the surface of T180P. The adsorbed
HCHO is then oxidized to dioxymethylene species, followed
by formate species. No band assigned to HCHO is observed
because of the rapid reaction rate of HCHO oxidation. The
formed formate species gradually decomposes into CO spe-
cies (the rate-determining step), which finally react with O2
to produce gas phase CO2. The stability and recycling per-
formance of the catalysts are essential factors for practical
applications. To investigate the stability of the as-prepared
hierarchically macro-/mesoporous Pt/TiO2 for HCHO decom-
position, room-temperature oxidation on T180P was repeated
seven times within 7 days (once a day), and the results are
shown in Fig. 11. The oxidation rate of HCHO and the gener-
ation rate of CO2 (not shown here) in seven repeated cycles
do not evidently change compared with those obtained in the
first cycle. This result suggests that the prepared T180P pos-
sesses stable and efficient catalytic performance. In addition,
the prepared hierarchically macro-/mesoporous Pt/TiO2 cata-
lyst in the present study exists in the form of large particles
with porous structure; these larger particles help avoid sev-
eral problems because using powdered catalysts often
involves serious agglomeration and loss of fine powder dur-
ing the reaction.31 The prepared hierarchically macro-/meso-
porous Pt/TiO2 catalysts are promising for the decomposition
of indoor HCHO because they are easily synthesized, low
cost, and possess excellent catalytic performance.43
Conclusions
A hierarchically macro-/mesoporous Pt/TiO2 catalyst was eas-
ily synthesized by the hydrothermal treatment of TiIJOC4H9)4
precipitates in pure water, followed by a combined NaOH-
assisted impregnation of TiO2 with Pt precursor and NaBH4
reduction. The prepared T180P catalyst shows excellent cata-
lytic activity for gas-phase HCHO decomposition and the
removal efficiency reaches 90.6% h−1, which is much higher
than those of T25P (76.5% h−1), P25P (58.4% h−1), T180GP
(50.6% h−1) and T180PG (62.6% h−1). According to the litera-
ture and our experiments, the high catalytic activity of T180P
is because of the combined hierarchically macro-/mesoporous
structure with a large specific surface area and macro-/mesopores
of the TiO2 support. The synergistic effect between Pt NPs
and TiO2 nanostructures and the favorable (or close) contact
between them also contributed to the aforementioned effect.
The presence of the hierarchically macro-/mesoporous struc-
ture is a key factor for the enhancement of the catalytic activ-
ity because such structures can enhance the HCHO adsorp-
tion on the TiO2 surface, facilitate the diffusion and transport
of gaseous reactants and products, and promote the disper-
sion of Pt NPs. The present study may provide new insights
into the design and fabrication of cost-effective, high-perfor-
mance, and environmentally benign catalysts for indoor air
purification.
Acknowledgements
This study was partially supported by the 863 Program
(2012AA062701), the 973 Program (2013CB632402), NSFC
(21433007, 51320105001 and 51272199), the Fundamental
Research Funds for the Central Universities (2014-VII-010),
Deanship of Scientific Research (DSR) of King Abdulaziz Uni-
versity IJ90-130-35-HiCi) and PSFC (2013M542078 and
2014T70749). This research was financially supported by the
research grant of Early Career Scheme (ECS 809813) from the
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Research Grant Council, Hong Kong SAR Government,
Dean's Research Fund-Early Career Researchers (04022), and
Research Equipment Grant (REG-2).
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