Multifunctional Pt/ZSM-5 catalyst for complete oxidation of gaseous formaldehyde at ambient temperature

Article abstract of DOI:10.1016/j.cattod.2015.03.043

For practical application in indoor low concentration HCHO control, the supported novel metal catalysts are expected to have large HCHO adsorption capacity, high chemical intermediates (i.e., formate species) storage ability, and rapid intermediates oxidization rate to CO₂ and H₂O under a low novel metal loading amount and ambient conditions. Herein, a highly efficient Pt/ZSM-5 (Si/Al = 350) catalyst for trace HCHO oxidation was developed by an impregnation method, followed by H₂ or HCHO solution reduction. In combination the high HCHO adsorption capacity and chemical intermediates storage ability of ZSM-5 with the good oxidation activity of dispersed Pt, Pt/ZSM-5 showed higher apparent activity for HCHO oxidation than the widely used Pt/TiO₂ (P25) under parallel preparation and test conditions. A high HCHO conversion above 95% with a (>) 100 h stable performance was obtained at 30 °C over 0.4 wt.% Pt/ZSM-5 reduced by HCHO solution. The mechanism leading to its high catalytic activity and stability was studied by in situ DRIFTS study.

Full text of DOI:10.1016/j.cattod.2015.03.043

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Contents lists available at ScienceDirect
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Multifunctional Pt/ZSM-5 catalyst for complete oxidation
of gaseous formaldehyde at ambient temperature
a
a,b,∗
a
a,b,∗
Huayao Chen , Zebao Rui
, Xuyu Wang , Hongbing Ji
a
Department of Chemical Engineering, School of Chemistry & Chemical Engineering, and The Key Lab of Low-Carbon
Chemistry & Energy Conservation of Guangdong Province, Sun Yat-Sen University, Guangzhou 510275, PR China
R&D Center of Waste-Gas Cleaning & Control, Huizhou Research Institute of Sun Yat-Sen University, Huizhou 516081, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
For practical application in indoor low concentration HCHO control, the supported novel metal catalysts
are expected to have large HCHO adsorption capacity, high chemical intermediates (i.e., formate species)
storage ability, and rapid intermediates oxidization rate to CO2 and H2O under a low novel metal loading
amount and ambient conditions. Herein, a highly efficient Pt/ZSM-5 (Si/Al = 350) catalyst for trace HCHO
oxidation was developed by an impregnation method, followed by H2 or HCHO solution reduction. In
combination the high HCHO adsorption capacity and chemical intermediates storage ability of ZSM-
Received 21 January 2015
Received in revised form 18 March 2015
Accepted 28 March 2015
Available online xxx
Keywords:
5
with the good oxidation activity of dispersed Pt, Pt/ZSM-5 showed higher apparent activity for HCHO
Formaldehyde
Catalytic oxidation
Platinum
oxidation than the widely used Pt/TiO2 (P25) under parallel preparation and test conditions. A high HCHO
◦
conversion above 95% with a (>) 100 h stable performance was obtained at 30 C over 0.4 wt.% Pt/ZSM-5
ZSM-5
reduced by HCHO solution. The mechanism leading to its high catalytic activity and stability was studied
In situ DRIFTS
by in situ DRIFTS study.
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
[8]. Recently, some common irreducible support material such as
-Al O and zeolite were also studied for HCHO complete oxidation
␥
2
3
Formaldehyde (HCHO) is one of the most concerning indoor
due to their low cost, thermal and chemical stability, high surface
area and amphoteric character, and some encouraging results were
obtained [8,9].
Meanwhile, the formaldehyde oxidation pathway and inter-
mediate over the supported novel metal catalysts have been
extensively studied using the in-situ DRIFTS experiment method
[3,5,6,8,10]. The major reaction intermediates were found as
formate and dioxymethylene species [11]. It was believed that com-
gaseous pollutants because it is often found in buildings and poses
a potential health risk to occupants even at a very low concentra-
tion level [1]. Significant efforts have been made to reduce indoor
HCHO emissions, especially on the catalytic oxidation of HCHO [2].
Previous studies by many groups indicated that supported noble
metal catalysts held high removal activity of HCHO, and their activ-
ities strongly depended on the catalyst supports and noble metals
employed [1]. Pt has been recognized as the most effective catalyst
formulation among other noble metal catalysts as reported ear-
lier, such as Pt/TiO2 [3–6] and Pt/MnOx–CeO2 [7] were active for
HCHO complete oxidation even at room temperature. In general,
reducible oxides have been used as the support of choice in past
studies because of their high concentrations of oxygen defects and
their ability to stabilize high dispersions of noble metal particles
plete oxidation of formaldehyde into water (H O) and CO required
2
2
the presence of oxygen [2]. While for the oxidation of HCHO to
formate intermediates, both surface oxygen [2,4–6,12] and surface
hydroxyls [8–10,13] have been taken as the major oxidant in the
literature. Especially, the design of surface hydroxyls rich catalysts
for efficient HCHO oxidation has been successfully tried by many
groups [8,10,13,14]. Liu et al. [14] has proven that •OH played a vital
role in oxygen supply in CO oxidation over Au/TiO2 using density
functional theory. Chen et al. [8] found that although there was no
active surface oxygen on ␥-Al O , surface hydroxyls had the ability
2
3
∗ Corresponding authors at: Sun Yat-Sen University, Department of Chemical
Engineering, School of Chemistry & Chemical Engineering, 135# West XIngang Road,
Guangzhou 510275, PR China. Tel.: +86 2084113653.
E-mail addresses: ruizebao@mail.sysu.edu.cn (Z. Rui), jihb@mail.sysu.edu.cn
H. Ji).
to partially oxidize HCHO into formate intermediates, which could
be further oxidized into CO2 and H O by nano-Au.
2
Despite the great progress made for the catalytic removal of
HCHO, there are still many obstacles for practical applications in
(
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920-5861/© 2015 Elsevier B.V. All rights reserved.
0
Please cite this article in press as: H. Chen, et al., Multifunctional Pt/ZSM-5 catalyst for complete oxidation of gaseous formaldehyde at
ambient temperature, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.043

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2
indoor air quality control. For example, the catalysts should be
proved to be effective (i.e., constant significant removal efficiency)
for a much longer time in practical application [2]. One issue need
to be considered is that the content of HCHO in indoor air is nor-
mally lower than several ppm. Thus, both the high adsorption
capacity and good catalytic activity are required for the practi-
cal control unit, since diffusion would become an important step
for the catalytic oxidation reaction at a low HCHO concentration.
Benefit from the understanding of the HCHO oxidation pathway,
Shi et al. [15] proposed a type of “storage-oxidation” cycling pro-
cess for the removal of HCHO in a single unit. In this process,
2. Experimental
2.1. Catalysts preparation
Pt/ZSM-5 catalyst was prepared by an impregnation method.
ZSM-5 (Si/Al = 350, XF NanoInc) particles was uniformly dispersed
into the H PtCl6 solution (5.4 mg/ml, Alfa Aesar) with an appropri-
2
ate volume to obtain a Pt loading amount of 0.4 wt.%. The sample
was then dried at 120 C over night to evaporate the solvent and
finally calcined at 400 C for 4 h with a heating rate of 10 C/min in
air. Two different reduction methods were used, i.e., H2 reduction
and HCHO solution reduction. The reduced catalysts were referred
◦
◦
◦
−
HCHO was first partially oxidized and stored as HCOO species
−
.
When the catalyst reached saturation, the stored HCOO species
to Pt/ZSM-5(H ) and Pt/ZSM-5(HCHO), respectively. For a typical
2
were completely oxidized into CO2 and H O by heating. Compared
with traditional “physical adsorption” followed by “catalytic oxida-
H reduction process, the as-prepared catalyst was reduced in a H
2
2
2
◦
stream (40 ml/min) at 300 C for 3 h. In a HCHO reduction process,
about 0.5 g as-calcined catalyst was mixed with 50 mL deionized
water and 5 mL HCHO solution (∼35 wt.% HCHO) for about 1 h under
tion” process, the competitive adsorption of H O with HCHO could
2
be limited due to the fact that HCHO was not simply being phys-
ically adsorbed but was undergoing chemical reaction during the
storage process. However, the separated operations on the “stor-
age (cooling)” and “oxidation (heating)” make the complexity of the
process, which can be excluded by using more active component
such as Pt [16,17] instead of Ag. Moreover, one need to take in mind
that the amount of HCHO adsorbed onto the surface of the catalyst
support is also a key factor in the storage process, since it is the ori-
gin of reaction intermediates such as formate and dioxymethylene
species [9,11].
◦
reflux condition and vigorous stirring at 70 C. Then, the suspension
◦
was separated and dried in air at 120 C for 6 h for use.
2.2. Catalysts characterization
The morphologies and metal nanoparticle size distribution of
the catalysts were observed by a JEOL 2100F transmission electron
microscopy (TEM). The phasepurity and crystal structure of the cat-
alystswere examined by X-ray diffraction (XRD, D-MAX 2200 VPC),
◦
Against this background, the aim of the present work is to design
a multifunctional catalyst for the treatment of trace HCHO in the
air, which can selectively trap HCHO molecules from the environ-
using monochromatic Cu K␣ radiation at a scanning rate of 10 /min
◦
and a step size of 0.02 . The Brunauer–Emmett–Teller (BET) sur-
face area, pore volume, and pore size distribution of the samples
were measured with a Micromeritics ASAP 2020 instrument using
adsorption of N2 at 77 K. Prior to adsorption analysis, the catalysts
-
ment, efficiently store the adsorbed HCHO as HCOO , and rapidly
oxidize the intermediates to CO2 and H O. ZSM-5 with a high Si/Al
2
◦
ratio was chosen as the support. ZSM-5, holding the regular peri-
odic structure with channels and cages and a large surface area, not
only exhibits physical adsorption properties in which hydrocarbons
are trapped within the zeolite pores [18,19], also exhibits chemi-
cal adsorption capabilities at points of unbalanced electric charges
resulting from the addition of Al, which can bond the hydrocar-
bons [20,21]. The infrared radiation (IR) study by Gora-Marek and
Datka [20] on the interaction of HCHO molecules with Co-ZSM-
were degassed in a flowing N2 at 300 C for 3 h. X-ray photoelec-
tron spectra (XPS) were recorded on a ESCALAB 250 spectrometer
(Thermo Fisher Scientific, Al K␣, hꢀ = 1486.6 eV) under a vacuum
−
7
of ∼2 × 10 Pa. Charging effects were corrected by adjusting the
main C 1s peak to a position of 284.8 eV. In-situ Diffuse Reflectance
Infrared Fourier Transformed Spectroscopy (DRIFTS) was carried
out on EQVINOX-55 FFT spectroscope apparatus (Bruker), equipped
with a diffuse reflectance accessory and a MCT detector. Finely
ground sample (ca. 10 mg) was placed in a ceramic crucible in the
in situ chamber. Prior to the test, the sample was heated in a flowing
5
suggested that HCHO molecules reacted with zeolitic hydroxyl
sites (i.e., Si OH Al groups) to form formate species which is
an important intermediate in catalytic oxidation [22]. Encouraged
with these properties, ZSM-5 support is expected to exhibit high
HCHO adsorption capacity and storage ability due to its high sur-
face area and zeolitic hydroxyl sites. However, recent study on the
Ag/ZSM-5 by Shi et al. [15] and Pd/ZSM-5 by Park et al. [9] for HCHO
oxidation revealed a poor activity. Many factors affected the prop-
erties and performance of ZSM-5, such as pore size and Al quantity
etc. [18,19]. It was reported that, with an increase in Si/Al ratio,
the total number of acid sites decreased [23] and the hydrophobic-
ity and organophilicity increased over ZSM-5 [24], which is benefit
◦
He at 120 C for 1 h to remove the adsorbed water and other spe-
cies. The total gas flow rate was 100 ml/min. HCHO was bubbled
into the chamber with He. The spectra under reaction conditions
−
1
were recorded after 64 scans with a resolution of 4 cm . The
for the selective adsorption of HCHO over H O and the interaction
2
0
.4% Pt/ZSM-5(H2)
between adsorbed HCHO with surface hydroxyl sites [25]. Thus,
ZSM-5 with a high Si/Al ratio of 350 was used. In order to rapidly
oxidize the stored formate intermediates, highly dispersed Pt was
chosen as the active component, which has been recognized as the
most effective catalyst formulation among other noble metal cat-
alysts as reported earlier [3–7]. Finally, a highly efficient Pt/ZSM-5
catalyst was developed by an impregnation (IM) method, followed
by H2 or HCHO solution reduction. In comparison with the widely
0.4% Pt/ZSM-5(HCHO)
ZSM-5
used Pt/TiO (P25), Pt/ZSM-5 showed higher activity under par-
2
allel preparation and test conditions. A HCHO conversion above
◦
9
0
5% with a (>)100 h stable performance was obtained at 30 C over
.4 wt.% Pt/ZSM-5 reduced by HCHO solution. The mechanism lead-
10
20
30
40
50
60
70
80
o
2
-theta ( )
ing to its high catalytic activity and stability was studied by in situ
DRIFTS study.
Fig. 1. XRD patterns of ZSM-5 and Pt/ZSM-5 samples.
Please cite this article in press as: H. Chen, et al., Multifunctional Pt/ZSM-5 catalyst for complete oxidation of gaseous formaldehyde at
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Table 1
After shaking for 5 s and staying for 15 min in the dark, HCHO con-
centration in the gas stream was then determined by measuring
light absorbance at 630 nm with a spectrophotometer (UV-240,
Shimadzu Co. Ltd., Japan). The conversion of HCHO was calculated
based on its concentration change. Each datum was measured in
triplicates, and the standard error at each point was evaluated and
presented with error bar in the HCHO conversion vs. tempera-
ture figure. HCHO equilibrium adsorption amount (Qe, mg/g) over
The textural properties of samples analyzed with nitrogen adsorption–desorption
method.
Samples
P25 ZSM-5 0.4%Pt/ZSM-5 (H2) 0.4%Pt/ZSM-5(HCHO)
51.1 313.6 326.1 320.1
BET surface area
2
(
m /g)
2
◦
the ZSM-5 or Degauss P25 (51.1 m /g) at 30 C was calculated by
outlet gas was collected to to monitor CO . The effluent species
were monitored with a mass spectrometry (MS; Hidden HPR 20).
The CO2 signal was measured at m/e = 44.
2
3
measuring the HCHO concentrations (mg/m ) of inlet gas (Cin) and
outlet gas(Cout) with the same setup for the activity evaluation, as
listed below.
t
ꢀ
2
.3. HCHO catalytic oxidation and adsorption test
Qv (Cin − Cout)
Qe =
dt
(1)
W
The catalytic oxidation of HCHO and adsorption test was per-
0
formed in a quartz tubular (i.d. =7 mm) fixed-bed reactor under
atmospheric pressure with a home-made setup. Approximately
3
here t (min) stands for the adsorption time, Qv (m /min) and W
(g) represent the total gas flow rate and the weight of the loaded
sample, respectively.
0
.2 g of the catalyst was packed in the reactor. A simulated air
stream (N /O = 4, 100 ml/min) containing ∼50 ppm HCHO and
2
2
water vapor (∼35% relative humidity) was introduced as the
reactants. Gaseous HCHO was generated by passing a stream of sim-
ulated air through a bubbler containing a HCHO solution (35 wt.%
HCHO). The gas hourly space velocity (GHSV) is 30,000 ml/(h g).
HCHO concentration in the reactant or product gas stream was
analyzed by phenol spectrophotometric method. The gas stream
containing trace HCHO was bubbled through 5 ml phenol reagent
3. Results and discussion
3.1. Structural properties
Fig. 1 displays the XRD patterns of the ZSM-5 and different ZSM-
5 supported Pt catalysts. All samples are structured with ZSM-5
phase. The characteristic peaks of PtO2 or Pt are too weak to be
detected during the XRD characterizations due to the low Pt content
in the catalysts. Table 1 lists the nitrogen adsorption analysis data
of the ZSM-5 support and the catalysts. The BET-area of Pt/ZSM-5
−
4
(
3
C H SN(CH )C: NNH ·HCl, Alfa Aesar) solution (1 × 10 wt.%) for
6
4
3
2
0 s to collect HCHO by absorption. Then, 0.4 ml (1 wt.%) ammo-
nium ferric sulfate (NH Fe(SO ) ·12H O, Tianjin Fuchen Chemical
4
4
2
2
Reagent Company) solution was added as the coloring reagent.
Fig. 2. TEM images and Pt nanoparticle size distribution of 0.4%Pt/ZSM-5(H2) (a and b) and 0.4%Pt/ZSM-5(HCHO) (c and d)
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2
(
HCHO) and Pt/ZSM-5(H ) are, respectively, 326 and 320 m /g, indi-
2
cating that the reduction method employed draw little effect on
the pore structure of the catalyst. Fig. 2 shows the TEM images
and Pt particle distribution of Pt/ZSM-5(H ) and Pt/ZSM-5(HCHO)
2
samples. It can be found that small and homogeneous Pt nanopar-
ticles are uniformly present over both samples. Approximately 500
Pt nanoparticles were calculated to obtain the histogram of metal
particle size distribution for each sample. The Pt nanoparticle size
Pt/ZSM-5(H ) has an average value of 12 nm, while it is 7.3 nm over
2
Pt/ZSM-5(HCHO). The results convince that the reduction process
has a significant effect on the structure of the supported Pt parti-
cles, which is consistent with our previous study on the effect of
reduction process on the properties of Pt/TiO2 catalysts [6]. Fig. 3
presents the XPS signal of Pt 4f over Pt/ZSM-5(HCHO) and Pt/ZSM-
5
(H ). Although the signal of Pt 4f is a bit weak due to the low Pt
2
loading amount and its well dispersion, the binding energy (BE) of
1.2 eV indicates Pt nanoparticles are in the metallic state over both
samples [26].
7
3.2. Catalysts activity test
The catalytic activities of ZSM-5, Pt/ZSM-5(H ) and Pt/ZSM-
2
5
(HCHO) are compared in Fig. 4a. The performance of 0.4(wt.)%
Pt/P25(H ) reported in our recent work [6], which has the same
2
preparation and activity evaluation conditions with Pt/ZSM-5(H ),
2
is also present for comparison. As shown, the Pt/ZSM-5(H ) is more
2
active than Pt/P25(H ). In the case of 0.4% Pt/ZSM-5(H ), HCHO con-
2
2
◦
◦
version is 29.2% at 30 C and increases to 76.9% at 110 C. While 0.4%
Pt/ZSM-5(HCHO) has a (>) 95% HCHO conversion during the tem-
perature range studied. As shown in Fig. 4b, HCHO conversion over
0.4% Pt/ZSM-5(HCHO) keeps above 95% even at room temperature
with a (>)100 h stable performance.
Fig. 4. (a) Dependence of HCHO conversion on reaction temperature for P25, blank,
3.3. In-situ DRIFT test
ZSM-5, 0.4%Pt/ZSM-5(H2), 0.4%Pt/P25(H2) and 0.4%Pt/ZSM-5(HCHO), and (b) Long
◦
term test for trace HCHO oxidation over 0.4%Pt/ZSM-5(HCHO) at 30 C.
In-situ DRIFTS study was carried out to compare the differ-
ent performance of ZSM-5, Pt/ZSM-5(H ) and Pt/ZSM-5(HCHO) for
2
−1
to the catalysts, respectively [27,28]. The band at 1630 cm
is
HCHO oxidation with respect to the behavior of adsorbed spe-
cies on the catalyst surface. The in situ DRIFT spectrum at 0 min
in He flow was measured and taken as a background for the fol-
lowing DRIFT spectra for each sample. Fig. 5 shows the dynamic
ascribed to combination/overtone band of T O fundamental lattice
ring vibration, or deformation vibrations of molecularly adsorbed
water [29]. The band at 1335 cm is an artifact attributed to spec-
−
1
−
1
ular reflectance [27]. While 1190 cm
is attributed to internal
changes in the DRIFTS spectra of the catalysts as a function of time
vibrations of framework TO4 tetrahedra. It is related to the HCHO
interaction with ZSM-5 inside the lattice [29]. The intensities of the
bands vary with exposing time.
◦
in a flow of O + HCHO + He at 30 C. After exposing the catalyst to
2
O + HCHO + He mixture gas, obvious bands appear at 1900, 1670,
2
−1
1
630, 1335 and 1190 cm for these samples. According to previous
studies, the bands at 1670 and 1900 cm can be attributed to C
A semi-quantitative analysis by calculating the integrated area
−
1
H
−
1
of the formate species band at 1670 cm is given in Fig. 5d. The
band intensity over the support ZSM-5 increases with increasing
exposure time and reach a steady level after 60 min of exposure,
indicating that HCHO can be chemically adsorbed and trans-
ferred into formate species over ZSM-5 even without the active
stretching and C O stretching vibration of formate species bonded
Pt 4f
200
71.2eV
0.4% Pt/ZSM-5(HCHO)
component. The band intensities over both Pt/ZSM-5(H ) and
2
Pt/ZSM-5(HCHO) are larger than that over ZSM-5 support during
the whole 60 min on stream time, indicating that Pt can facilitate
the chemical adsorption of HCHO. The band intensity over Pt/ZSM-
5
(H ) increases strongly during the first 30 min, and its increasing
2
speed slows down during the following 30 min exposure time. For
Pt/ZSM-5(HCHO), the band intensity increases fast during the first
15 min on stream time, and then decreases with further prolonging
the exposure time. The amount of formate species over the catalyst
surface is determined by two factors, i.e., the formation rate from
adsorbed HCHO and its decomposing rate. The overall effect of the
two factors can increase or decrease the formate species amount,
depending on their relative dominance. In combination with the
activity test in Fig. 4, we can see that both the formate species
formation rate and decomposing rate over Pt/ZSM-5(HCHO) are
0
.4% Pt/ZSM-5(H2)
72
70
68
66
Binding energy (eV)
Fig. 3. XPS spectra of the reduced 0.4%Pt/ZSM-5 catalysts for Pt 4f.
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a
1190
1900
1670
b
1900
1670 1630
1190
1630
1335
1335
6
3
1
0 min
0 min
5 min
6
0 min
3
0 min
1
5 min
5
min
5 min
3
2
1
min
min
min
3 min
2 min
1 min
2
200 2000 1800 1600 1400 1 1200 1000
2200 2000 1800 1600 1400 1200 1000
-
-1
Wavenumber /cm
Wavenumber /cm
c
1630
670
4
3
2
1
0
1
0
.4% Pt/ZSM-5(H2)
ZSM-5
d
1
190
1900
1335
0.4% Pt/ZSM-5(HCHO)
6
0 min
3
0 min
1
5 min
5
min
3
2
1
min
min
min
2
200 2000 1800 1600 1400 1200 1000
-1
0
10
20
30
40
50
60
Wavenumber /cm
Time (min)
◦
Fig. 5. In situ DRIFTS spectra of O2 + HCHO + He gas mixture adsorption at 30 C over (a) 0.4%Pt/ZSM-5(HCHO), (b) 0.4%Pt/ZSM-5(H2), (c) ZSM-5 and (d) intensity of the
−1
corresponding band at 1670 cm
.
the fastest among these catalysts, resulting to its higher initial for-
mate species amount followed by a fast decrease in its amount and
higher activity in comparison with those over ZSM-5 and Pt/ZSM-
given in Fig. 6d, one can expect that both the conversion of adsorbed
HCHO to formate species and the deep oxidation of formate spe-
cies to CO2 are fast over Pt/ZSM-5(HCHO), resulting to its excellent
performance for HCHO oxidation even at room temperature. The
5
(H ). After 60 min of exposure, the relative amount of accumulated
2
formate species over these catalysts has a sequence of Pt/ZSM-
fast band intensity decrease over Pt/ZSM-5(H ) accompanies with
2
5
(H ) > Pt/ZSM-5(HCHO) ≈ ZSM-5, which is inverse to the activity
significant increase in CO2 yield (Fig. 6e) and apparent HCHO oxi-
dation activity (Fig. 4a) with increasing temperature. These results
denote that the determining step for HCHO oxidation over Pt/ZSM-
2
sequence of Pt/ZSM-5(HCHO) > Pt/ZSM-5(H ). This phenomenon
2
indicates that the controlling step of the HCHO oxidation over these
catalysts is the formate species decomposing step, which is consis-
tent with the viewpoints in the literature for the TiO2 supported
Pt catalysts [3,5,6]. The considerable formate species amount over
these ZSM-5 based catalysts, especially over the pure ZSM-5 sup-
port, indicates their chemical storage ability for trace HCHO. This
behavior is quite different with the widely used TiO2 support [5,6],
over which the formate species formation is negligible during such
an exposure process. This is because the unshared pairs of elec-
trons in the carbonyl of HCHO have a strong affinity toward the
acidic sites of zeolite, and ZSM-5 in general contains a large num-
ber of acidic sites on their surfaces [20], leading to its higher storage
capacity. Such a difference might be an important reason for the
superior performance of Pt/ZSM-5(H ) over Pt/P25(H ).
5(H ) should be the deep oxidation of formate species, which can
2
be effectively promoted by raising reaction temperature. For ZSM-
5 support, although the amount of formate species decrease with
increasing temperature, no obvious change in CO yield is observed.
2
Considering its poor apparent activity in the temperature range
studied for HCHO oxidation (Fig. 4a), one can propose that the
decrease in the surface formate species may be due to the nega-
tive effect of a high temperature on the adsorption of HCHO over
ZSM-5.
3.4. Discussion
2
2
The reaction scheme for HCHO oxidation over TiO2 supported
noble metal catalysts has been extensively studied [3,5,6]. In our
recent work [5], we generally separated the process into two parts,
i.e., storage process and oxidation process. In the storage process,
HCHO was firstly chemisorbed and reacted with the chemisorbed
oxygen to form formate surface species and then decomposed
Fig. 6 shows in situ DRIFTS spectra of O + HCHO + He gas mix-
2
ture adsorption for 1 h at various temperatures over the ZSM-5
−
1
based catalysts. As shown, the band intensity of 1670 cm , which is
ascribed to the adsorbed formate species, decreases with increasing
temperature over these samples. A semi-quantitative comparison
of the band intensity change is presented in Fig. 6d. Fig. 6e presents
the corresponding effluent CO2 flux during the in situ DRIFTS tests.
For Pt/ZSM-5(HCHO), the band intensity and its change with tem-
perature are smallest among these samples. Meanwhile, there is
into adsorbed CO species and H O. In the oxidation process, the
2
adsorbed CO species reacted with O2 or chemisorbed oxygen to
produce gas phase CO , and the gas O was reactivated by the active
2
2
sites to form chemisorbed oxygen. The previous studies [28,30,31]
and the results obtained in this work showed that HCHO oxidation
over the ZSM-5 based catalysts generally followed this route. In
no obvious change in the CO yield with increasing temperature. In
combination with the activity test results and in situ DRIFTS spectra
2
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a
b
1670
900
1630
1
900 1670
1190
1335 1190
1630
1335
1
o
o
120 C
120 C
o
o
8
0 C
80 C
o
o
6
0 C
60 C
o
o
3
0 C
30 C
2
200 2000 1800 1600 1400 1200 1000
-1
2
200 2000 1800 1600 1400 1200 1000
Wavenumber /cm
-1
Wavenumber /cm
5
c
1900
d
0.4% Pt/ZSM-5(HCHO)
.4% Pt/ZSM-5(H)
1630
1335 1190
1670
0
2
4
3
2
1
0
o
ZSM-5
1
20 C
o
8
0 C
o
6
0 C
o
3
0 C
2
200 2000 1800 1600 1400 1200 1000
40
60
80
100
120
o
-1
Temperature ( C)
Wavenumber /cm
e
Time
Fig. 6. In situ DRIFTS spectra of O2 + HCHO + He gas mixture adsorption at various temperatures over (a) Pt/ZSM-5(HCHO), (b) 0.4%Pt/ZSM-5(H2), (c) ZSM-5, (d) intensity of
−
1
the corresponding bands at 1670 cm for ZSM-5 catalysts and (e) the corresponding effluent CO2 MS spectra during the in situ DRIFTS tests.
◦
3
Fig. 7. HCHO adsorption over ZSM-5 and P25 support at 30 C with an influent HCHO concentration of 45 mg/m : (a) Breakthrough curves of HCHO and (b) HCHO equilibrium
adsorption amount.
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this route, the storage of the HCHO as the surface formate species
in the storage process and its further oxidation in the oxidation pro-
cess were usually regarded as the rate-determining steps for HCHO
oxidation [3,5,32].
and oxidation activity are crucial for the performance of the cata-
lysts. The Pt/ZSM-5(HCHO) developed in this work possesses both
high HCHO storage ability and good oxidation activity, leading to
its exceptional performance for the complete oxidation of HCHO to
CO2.
In comparison with the widely used TiO2 catalyst, ZSM-5 has
a larger BET surface area, which is benefit for the adsorption of
gaseous HCHO. HCHO equilibrium adsorption amount (Qe, mg/g)
4. Conclusions
2
2
◦
over the ZSM-5 (313.6 m /g) and Degauss P25 (51.1 m /g) at 30 C
3
with an influent HCHO concentration of 45 mg/m were mea-
Herein, a strategy for the supported novel metal catalyst design
for practical application in indoor low concentration HCHO con-
trol was proposed. The supported catalysts are expected to have
large HCHO adsorption capacity, high chemical intermediates (i.e.,
formate species) storage ability, and rapid intermediates oxidiza-
sured and calculated. As shown in Fig. 7, ZSM-5 has a Qe of
1
0.9 mg/g, which is larger than that of P25 (7.3 mg/g). Furthermore,
in situ DRIFTS study proved that considerable chemical adsorp-
tion/transformation of adsorbed HCHO to surface formate species
over the ZSM-5 support were realized even at room tempera-
ture, which was negligible over the TiO2 support [5,6]. In other
words, ZSM-5 based catalysts have superior HCHO storage over the
P25 based catalysts in the storage process. However, the activity
of ZSM-5 support for HCHO oxidation is negligible in compari-
son with the supported Pt catalysts, indicating that the loading of
metallic Pt is crucial for the oxidation process. In addition, the per-
formance of the supported Pt catalysts is related to its structural
properties, which is influenced by the loading and reduction pro-
cess [4,6]. Highly-uniform dispersed metallic Pt nanoparticles with
smaller size are obtained for Pt/ZSM-5(HCHO)(∼7.3 nm) in compar-
tion rate to CO2 and H O under a low novel metal loading amount
2
and ambient conditions. In consideration with this strategy, a
highly efficient Pt/ZSM-5 (Si/Al = 350) catalyst was developed for
trace HCHO oxidation by an impregnation method, followed by H2
or HCHO solution reduction. In-situ DRIFTS study illustrated that
HCHO oxidation over Pt/ZSM-5 catalyst could be generally consid-
ered as a storage–oxidation process: HCHO was firstly chemisorbed
and stored as formate surface species and then oxidized into
adsorbed CO2 and H O. ZSM-5 support exhibited high adsorption
2
capacity and chemical intermediates storage ability due to its large
BET-area, special pore structure and rich surface hydroxyl sites.
In combination with the good oxidation activity of dispersed Pt,
Pt/ZSM-5 reduced by H2 showed higher apparent activity than
ison with Pt/ZSM-5(H )(∼12.0 nm). A semi-quantitative analysis
2
based on in-situ DRIFTS study indicates that Pt/ZSM-5(HCHO) is
more effective for both the formation of formate species and the
formate decomposition, so its activity for HCHO oxidation is higher.
These results indicate that metallic Pt with an appropriate particle
size is benefit for HCHO oxidation, which is consistent with liter-
the widely used Pt/P25 (H ) under parallel preparation and test
2
conditions. Holding dispersed metallic Pt nanoparticles with homo-
geneous and appropriate Pt particle size, a better performance was
obtained over 0.4 wt.%Pt/ZSM-5 reduced by HCHO solution with
a HCHO conversion of 95% and a (>)100 h stable performance. In
short, the proposed strategy and developed Pt/ZSM-5 catalyst are
expected to be applicable in indoor low concentration HCHO con-
trol unit design.
ature [4,6,33]. Although the dispersion of Pt over Pt/ZSM-5(H ) is
2
poorer than that over Pt/P25(H ), which has an average Pt particles
2
size of ∼7.0 nm, as calculated in our previous work [6], the concen-
trated HCHO and surface formate species around Pt nanoparticles
resulted from the adsorption and storage by ZSM-5 can promote
the HCHO oxidation reaction, leading to the excellent performance
of the Pt/ZSM-5 catalyst.
Acknowledgements
By now, the complete oxidation of HCHO over Pt/ZSM-5 cat-
alysts is generally illustrated in consideration both the reported
reaction mechanism and the data in this work, as shown in Fig. 8.
First, HCHO is selectively adsorbed and stored over the surface of
Pt/ZSM-5 as surface formate species. Then deep oxidization of the
This work was preliminarily supported by supported by
the National Natural Science Foundation of China (21425627),
the Science and Technology Plan Project (2013B090500029)
and Natural Science Foundation of Guangdong Province, China
(2014A030313135).
chemisorbed species with O or chemisorbed oxygen on the Pt sur-
2
face to produce gas phase CO2 happens. Both the storage ability
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Please cite this article in press as: H. Chen, et al., Multifunctional Pt/ZSM-5 catalyst for complete oxidation of gaseous formaldehyde at
ambient temperature, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.043