Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of formaldehyde and benzene

Article abstract of DOI:10.1016/j.cattod.2015.03.042

A catalytic process including formaldehyde oxidation and benzene "storage-oxidation" cycling for simultaneously removal of formaldehyde and benzene is proposed, by using HZSM-5 supported PtAg bimetallic catalysts. HZSM-5 with different crystal size supported PtAg bimetallic catalysts were synthesized with the investigation focusing on HCHO oxidation at room temperature, benzene storage capacity at room temperature, and benzene catalytic oxidation at elevated temperature. Crystal size of HZSM-5 zeolite not only influences the dispersion of active metals of PtAg, and therefore influence their catalytic properties for formaldehyde and benzene oxidation, but also has great effects on benzene storage capacity and the thermal stability of the stored benzene, which is directly related to the release of benzene at elevated temperature. The ratio of Pt/Ag has an opposite effect on benzene storage capacity and formaldehyde and benzene oxidation properties. Considering these factors, PtAg (Pt/Ag = 1/1) supported on nanoscale HZSM-5 catalysts were found to be suitable for the proposed cycling process, good carbon balances being obtained for each cycle without the production of secondary pollutants.

Full text of DOI:10.1016/j.cattod.2015.03.042

G Model
CATTOD-9561; No. of Pages11
ARTICLE IN PRESS
Catalysis Today xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Nanoscale HZSM-5 supported PtAg bimetallic catalysts for
simultaneous removal of formaldehyde and benzene
Yu Wanga,1, Chengyi Daib,1, Bingbing Chena, Yidi Wanga, Chuan Shia,∗, Xinwen Guob,∗
a Laboratory of Plasma Physical Chemistry, Dalian University of Technology, Dalian, People’s Republic of China
b State Key Laboratory of Fine Chemicals, PSU–DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology,
Dalian, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A catalytic process including formaldehyde oxidation and benzene “storage-oxidation” cycling for simul-
taneously removal of formaldehyde and benzene is proposed, by using HZSM-5 supported PtAg bimetallic
catalysts. HZSM-5 with different crystal size supported PtAg bimetallic catalysts were synthesized with
the investigation focusing on HCHO oxidation at room temperature, benzene storage capacity at room
temperature, and benzene catalytic oxidation at elevated temperature. Crystal size of HZSM-5 zeolite not
only influences the dispersion of active metals of PtAg, and therefore influence their catalytic properties
for formaldehyde and benzene oxidation, but also has great effects on benzene storage capacity and the
thermal stability of the stored benzene, which is directly related to the release of benzene at elevated
temperature. The ratio of Pt/Ag has an opposite effect on benzene storage capacity and formaldehyde
and benzene oxidation properties. Considering these factors, PtAg (Pt/Ag = 1/1) supported on nanoscale
HZSM-5 catalysts were found to be suitable for the proposed cycling process, good carbon balances being
obtained for each cycle without the production of secondary pollutants.
Received 28 October 2014
Received in revised form 26 March 2015
Accepted 27 March 2015
Available online xxx
Keywords:
Platinum
Silver
Formaldehyde
Benzene
Nanoscale HZSM-5
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Herein, we propose a new approach for HCHO and C6H6 simul-
taneous removal in the presence of H2O, which combines HCHO
Formaldehyde (HCHO) and Benzene (C6H6) are two of the most
common and most noxious indoor gaseous pollutants. They are
commonly emitted from materials used for the construction of
buildings, as well as from decorative materials. Long-term expo-
sure to indoor air containing HCHO and C6H6, even at the ppm level,
may cause adverse effects on human health. It is highly desirable
to simultaneously remove them from the indoor air.
Simultaneous removal of indoor air pollutants of formalde-
hyde (HCHO) and benzene (C6H6) remains challenging. Physical
adsorption appears to be promising, while the adsorption material
(activated carbon or zeolites is commonly used) cannot effectively
adsorb HCHO and C6H6 simultaneously in the presence of H2O
(which is inevitably present) [1], due to the quite differences in their
kinetic diameter and polarity. Moreover, it also has the difficulty in
regeneration of the saturated materials, which is accompanied with
the large amount of untreated VOC releasing again.
oxidation with a C6H6 “storage-oxidation” process. The catalysts
work under cyclic conditions of room temperature and an elevated
temperature. At room temperature, HCHO is oxidized to CO2 and
H2O, while C6H6 simultaneously stored on the catalyst. When the
catalyst reaches its saturation point for C6H6 storage, it is regener-
ated in situ by heating. During this phase, the activity of the catalyst
increases with elevation of the temperature, the stored C6H6 is
completely oxidized into CO2 and H2O, resulting in regeneration
of the catalyst. The regenerated catalyst can then be used for a new
cycle. Therefore, a catalyst suitable for such cyclic process should
possess not only good catalytic performance for HCHO oxidation,
and adequate C6H6 storage capacity at room temperature, but also
good activity for oxidation of the stored C6H6 into CO2 and H2O at
elevated temperature without release.
Nanoscale HZSM-5 zeolites are promising catalytic and adsor-
bent materials that have higher surface areas and reduced diffusion
path lengths relative to conventional microscale HZSM-5 zeolites
[2–7]. Song et al. [4] observed that the overall adsorption capac-
ity over HZSM-5 (15 nm) is approximately 50% greater than over
the HZSM-5 (>60 nm). Moreover, nanoscale molecular sieves are
proved to be better support than the microscale ones. Zhao et al.
[7] found that Pd could be dispersed into ordered 12 nm elliptical
∗
Corresponding authors at: Dalian University of Technology, Laboratory of Plasma
Physical Chemistry, Dalian, China. Tel.: +86 411 84986083/+86 411 84986134.
E-mail addresses: chuanshi@dlut.edu.cn (C. Shi), guoxw@dlut.edu.cn (X. Guo).
Author contributions. Yu Wang and Chengyi Dai contributed equally.
1
http://dx.doi.org/10.1016/j.cattod.2015.03.042
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Wang, et al., Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of
formaldehyde and benzene, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.042

G Model
CATTOD-9561; No. of Pages11
ARTICLE IN PRESS
2
Y. Wang et al. / Catalysis Today xxx (2015) xxx–xxx
particles on the nanoscale Al-HMS, while irregular larger particles
(20–50 nm) appear on the microscale Al-HMS, which contributes
to the better catalytic performance achieved over the nanoscale
Al-HMS for catalytic combustion of C6H6.
media was performed from a clear solution with the composition:
1 TEOS: 0.27 TPAOH: 0.0042 Al2O3: 37 H2O.
After mixing the initial reactants, the synthesis was performed
at 170 ◦C for 3 days. The product was collected by centrifugation
and dried overnight at 100 ◦C. Finally, the template was removed
by calcination in static air at 540 ◦C for 6 h. The microscale HZSM-
5 and nanoscale HZSM-5 were denoted as HZ-L (Large) and HZ-
S (Small), respectively. As a comparison, the purchase of HZSM-5
(SiO2/Al2O3 = 240, Nankai University, China) was denoted as HZ-M
(Medium).
A number of studies have been devoted to PtM bimetallic cata-
lysts (M = Fe, Pd, Ni, and Au) with enhanced catalytic performance
with respect to pure platinum [8–10]. Geometric and ligand effects
are reported for activity enhancement in the PtM bimetallic cat-
alyst. The geometric effects consist of the platinum morphology
(particle size or dispersion state), inter-atomic distance, coordina-
tion state, and lattice strain of platinum at the surface layers etc.,
which could be important, especially in bimetallic nanostructures
[11–13]. Ag exhibits a unique activity for many reactions, such as
ethylene epoxidation [14] and HCHO oxidation [15], especially for
C6H6 adsorption due to the ␲-complexation [16]. ␲-Complexation
is a subclass of chemical complexation. The unique characteris-
tics of the d orbitals in Ag or Ag+ ions enable them to form bonds
with unsaturated hydrocarbons in a nonclassical manner, which is
broadly referred to as ␲-complexation. This ␲-complexation was
seriously considered for olefin/paraffin separation and purification
by employing liquid solutions containing Ag or Ag+ ions [17,18].
Fan et al. [19] found a remarkable extension of breakthrough time
for C6H6 adsorption on Ag/HZSM-5 catalysts than HZSM-5 only,
which should be caused by the ␲-complexation adsorption of Ag
with C6H6.
Based on the above understanding, a series of HZSM-5 with
different crystal size supported PtAg bimetallic catalysts were syn-
thesized with the investigation focusing on HCHO oxidation at
room temperature, C6H6 storage capacity at room temperature, and
C6H6 catalytic oxidation at elevated temperature in order to assess
catalysts’ performance during the cyclic process. The crystal sizes of
HZSM-5 zeolite not only affect the dispersion of PtAg bimetal. But
also exhibit great influence on the desorption of the stored C6H6
during catalyst regeneration. On the other hand, the ratio of Pt/Ag
has opposite effect on C6H6 storage capacity and catalytic oxidation
of HCHO and C6H6. Therefore, comprehensive consideration on all
the factors mentioned above to achieve a catalyst suitable for this
cycling process to removal HCHO and C6H6 simultaneously is the
objective of the present paper.
2.1.3. Synthesis of PtAg/HZSM-5 samples
Pt and Ag catalysts with nominal metal loading of 1 wt% were
prepared by conventional incipient wetness impregnation using
ethanol solution of hexachloroplatinic acid, silver nitrate and HZ-S,
followed by drying at 120 ◦C for 12 h and then calcined at 500 ◦C
for 4 h in H2. The obtained catalyst was denoted as Pt1/HZ-S and
Ag1/HZ-S.
PtAg bimetal catalysts were prepared by the sequential wet-
ness impregnation. For example, HZSM-5 was impregnated with
ethanol solution of hexachloroplatinic acid according to the metal
loadings (0.25–0.75 wt%), followed by drying at 120 ◦C for 12 h, then
pretreated in H2 at 500 ◦C for 2 h, and finally impregnated with
ethanol solution of silver nitrate according to the metal loadings
(0.25–0.75 wt%), followed by drying at 120 ◦C for 12 h, then pre-
treated in H2 at 500 ◦C for 2 h. The obtained catalyst was denoted
as Pt0.5Ag0.5/HZ (including Pt0.5Ag0.5/HZ-L, Pt0.5Ag0.5/HZ-M, and
Pt0.5Ag0.5/HZ-S) and PtxAg1–x/HZ-S (including Pt0.75Ag0.25/HZ-S,
Pt0.5Ag0.5/HZ-S, and Pt0.25Ag0.75/HZ-S).
2.2. Characterization techniques
Scanning electron microscopy (SEM) images were obtained on
a Hitachi S-5500 instrument with an acceleration voltage of 3 kV.
Some samples were sputtered with a thin film of gold.
N2 isotherms at -196 ◦C were measured in a Quantachrome
QUADRASORB SI gas adsorption analyzer. Prior to the measure-
ment, the samples were degassed in vacuum at 300 ◦C for 10 h.
The Brunauer–Emmett–Teller (BET) method was applied to calcu-
late the total surface area, while the t-plot method was used to
discriminate between micro- and mesoporosity. In the t-plot, the
reported mesopore surface area (Smeso) consists of contributions
from the external surface of the particles as well as mesopores and
macropores.
2. Experimental details
2.1. Catalyst preparation
The loading of the catalysts was determined by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES, Optima
2000DV, USA).
X-ray powder diffraction (XRD) analysis was conducted using a
D/MAX-2400 equipment (Rigaku, Japan) with Cu K␣ radiation.
Transmission electron microscopy (TEM) images of the catalysts
were obtained on a JEM-2000 microscope transmission electron
microscope (JEOL, Japan).
XPS analysis was performed on a Leybold Max 200 spectrometer
(Leybold, Germany) using AlK␣ radiation as the excitation source,
XPS results were calculated from the corresponding areas of fitted
peaks done by XPSPEAK 4.1 with Shirley background.
TPO/TPD experiments were used to study the regeneration
behavior of the catalysts. After pretreatment with H2 for 120 min at
500 ◦C, the catalysts were saturated with a flow of 10 ppm C6H6/21%
O2/1.56% H2O/N2. Once the catalyst had been saturated, the feed
gas was switched to simulated air (flow rate of 100 ml/min) and
TPO performed by increasing the temperature to 500 ◦C at a rate
of 10 ◦C/min. TPD experiments followed the same procedure as the
TPO measurements except that the feed gas was inert gas (flow
rate of 100 ml/min). The gas products were analyzed by an online
mass spectrometer (MS, OmniStarTM Pfeiffer Vacuum, Germany)
2.1.1. Materials
The reagents used for the preparation are tetraethylorthosili-
cate (TEOS, analytical grade, Xilong Chemical Co., China), silicon
oxide, 30 wt% in H2O colloidal dispersion (SiO2, Haiyang Chemi-
cal Co., China), tetrapropylammonium hydroxide solution, 25 wt%
aqueous solution (TPAOH, Cairui Chemical Co., China), tetrapropyl
ammonium bromide (TPABr, analytical grade, Letai Chemical Co.,
China), aluminum chloride (AlCl3, analytical grade, Bodi Chemi-
cal Co., China), ethylamine, 65 wt% aqueous solution (EA, Tianjin
Guangfu Institute of Fine Chemicals Reagents, China), hexachloro-
platinic acid (Shenyang Chemical Reagent Company, AR) and silver
nitrate (Shanghai Chemical Reagent Company, 99.9%).
2.1.2. Synthesis of HZSM-5 samples
Microscale HZSM-5 samples was prepared in the TPABr-EA
hydrothermal system, using colloidal silica and aluminum chloride
as silicon and aluminum source, respectively. TPABr was used as
the template and aqueous ethylamine as the base. Molar composi-
tion of the gel was 1 SiO2: 0.15 TPABr: 0.0042 Al2O3: 1 EA: 17 H2O.
In contrast, the synthesis of nanoscale HZSM-5 samples in TPAOH
Please cite this article in press as: Y. Wang, et al., Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of
formaldehyde and benzene, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.042

G Model
CATTOD-9561; No. of Pages11
ARTICLE IN PRESS
Y. Wang et al. / Catalysis Today xxx (2015) xxx–xxx
3
Table 2
Textural properties of the samples.
300
250
200
150
HZ-S
HZ-M
HZ-L
Catalyst
Si/Al
Metalcontenta (wt%)
Particle
ratioa
diameterb (nm)
Pt
Ag
Pt0.5Ag0.5/HZ-L
Pt0.5Ag0.5/HZ-M
Pt0.5Ag0.5/HZ-S
Pt1/HZ-S
Pt0.75Ag0.25/HZ-S
Pt0.25Ag0.75/HZ-S
Ag1/HZ-S
119
121
120
120
120
120
120
0.49
0.51
0.49
0.97
0.74
0.22
–
0.48
0.50
0.50
–
0.23
0.76
1.01
15.6
9.7
4.5
–
–
–
–
100
0.0
a
Measured by ICP-OES.
From TEM.
b
0.2
0.8
1.0
0.4p/p00.6
equipped with a fast-response inlet capillary/leak diaphragm sys-
tem along with an infrared absorption spectrometer (IRAS, S710,
SICK–MAIHAK, Germany). Concentrations of C6H6 (m/z = 78) were
determined by MS, and the concentrations of CO and CO2 in the gas
phase were determined by IRAS.
Fig. 1. Nitrogen adsorption–desorption isotherms at −196 ◦C of the three HZSM-5
zeolites.
Table 1
Specific surface area and porosity characteristics of the HZSM-5.
a
b
c
Catalyst
SBET (m2/g)
Smeso (m2/g)
Vpore (cm3/g)
2.3. Catalytic measurements
HZ-L
HZ-M
HZ-S
389
374
485
3
14
45
0.17
0.21
0.46
Storage and oxidation experiments were performed in a contin-
uous flow fixed-bed quartz reactor at atmospheric pressure. 0.10 g
catalyst (40–60 mesh) was sandwiched between quartz wool layers
in the tube reactor. Before performing the experiment, the catalysts
were pretreated in flowing H2 at 500 ◦C for 2 h.
a
BET method.
t-Plot method.
b
c
p/p0 = 0.99.
Fig. 2. SEM images of HZSM-5 catalysts. (a) HZ-L, (b) HZ-M, and (c) HZ-S.
Please cite this article in press as: Y. Wang, et al., Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of
formaldehyde and benzene, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.042

G Model
CATTOD-9561; No. of Pages11
ARTICLE IN PRESS
4
Y. Wang et al. / Catalysis Today xxx (2015) xxx–xxx
All the feed gases used in this work were of high-purity grade
(99.99%). The gas flow rates were adjusted and controlled by mass
flow controllers. Gaseous HCHO was generated by flowing N2 over
paraformaldehyde (99%, Aldrich) in a thermostated bath. (The con-
centration of HCHO was controlled by adjusting the flow rate of N2
and the temperature of thermostated bath). Gaseous H2O was car-
ried into the gas stream by passing N2 through a bubbler in a water
bath at room temperature. The amount of water, expressed as the
relative humidity (RH) at 25 ◦C, was controlled by adjusting the
flow rate of N2, while keeping the total flow unchanged. The HCHO
and C6H6 concentrations used in HCHO and C6H6 oxidation were
ca. 80 and 120 ppm, respectively. The C6H6 concentration used in
C6H6 storage and cycling process was ca. 10 ppm. The total flow
rate was 100 ml/min, corresponding to a gas hourly space velocity
(GHSV) of 30,000 h−1. The concentrations of HCHO and C6H6 were
measured by converting them to CO2 in a homemade HCHO-to-CO2
converter (CuO-MnO2/␥-Al2O3 catalyst) at 500 ◦C and determining
the amount of CO2 formed by IRAS [20–22].
A
Pt
Ag
Pt0.5Ag0.5/HZ-S
Pt0.5Ag0.5/HZ-M
Pt0.5Ag0.5/HZ-L
Catalyst behavior for the total oxidation of HCHO and C6H6 into
CO2 and H2O was studied as a function of reaction temperature.
C6H6 storage capacities were determined at 25 ◦C. The examined
parameters, HCHO conversion to CO2 (XHCHO→CO , %), C6H6 con-
20
25
30
35
40
45
50
2 theta /o
version to CO2 (XC
, %), and C6H6 storage 2capacity (NC
,
H
6
H
→CO
B
6
6
6
mmol/gcat) are defined as2follows:
out
2
C
CO
XHCHO→CO2 (%) =
× 100%
(1)
(2)
in
C
HCHO
out
Ag
Pt
C
CO
2
XC
2 (%) =
→CO
× 100%
H
6
6
6 × CCin
H
6
6
Ag1/HZ-S
Pt0.25Ag0.75/HZ-S
Pt0.75Ag0.25/HZ-S
CCin F1t1
H
6
6
NC
=
(3)
H
6
6
Wcat
out
2
where C
is the CO2 concentration in the products and CHinCHO and
CO
CCin is the HCHO and C6H6 concentration in the feed gas, respec-
H
tivel6y. F1 is the total flow rate, t1 is the breakthrough time and Wcat.
is the catalyst weight. Breakthrough time was defined as the time
when outlet C6H6 concentration reached 1% of feed concentration.
For cycling process experiments, the storage phase was carried
out at 25 ◦C and the temperature was programmed to increase dur-
ing the oxidation phase. At the moment of C6H6 breakthrough (the
time at which the outlet C6H6 concentration reached 1% of the feed
concentration, equivalent to ca. 0.1 ppm), the HCHO and C6H6 gas
flow was switched off, and replaced by simulated air. At this point
the temperature was ramped to 500 ◦C at 10 ◦C/min and held there
for 10 min. During this oxidation phase the reactor effluent did not
pass through the converter but was instead directly analyzed by
MS and IRAS to detect CO, CO2 and C6H6. The examined parame-
ters, stored-C6H6 oxidation efficiency (stored-C6H6 to CO2, %) and
carbon balance (Bc, %) are defined as follows:
6
Pt1/HZ-S
20
25
30
35
40
45
50
2 theta /o
Fig. 3. XRD patterns of Pt0.5Ag0.5/HZ-S, -M and -L (A) and PtxAg1–x/HZ-S (B) catalysts.
and t2 are the total flow rate and oxidation period of the oxidation
phase, respectively.
Moreover, during the TPO of the saturated catalysts, the con-
centration (CCO2) and MS signal (I44) of CO2 were simultaneously
obtained on-line by the COx analyzer and the mass spectrometer,
respectively. Therefore, the concentrations of benzene (CC6H6) in
the outlet gas are calculated according to the following method.
The relationship of the mass signals at m/z = 44, 78, (I44, I78) and the
concentrations (CCO2, CC6H6) is shown in the following equations
[23]:
out
2
n
CO
Stored − C6H6 to CO2(%) =
× 100%
(4)
6 × nsCtored
H
6
6
npCOroduced + npCrOoduced + 6 × nCdesorption
H
6
2
6
Bc(%) =
× 100%
(5)
6 × nsCtored
CO
I44 = ꢀ44ꢁCO
ˇ
2 CCO
(6)
(7)
H
6
6
44
2
2
ꢀ
ꢀ
t
t
produced
produced
CO
where
n
=
2 F2CCO2 dt,
n
=
2 F2CCOdt,
C
H
0
0
I78 = ꢀ78ꢁC 6 ˇ78 6 CC
6
CO
2 F2C(C
2
H
6
H
6
ꢀ
6
t
ndCesorption
=
d dt, nsCtored = C(C
a F1t1, C(C6H6)a and
H
)
H
)
H
H
6
6
6
6
0
6
6
6
6
C(C6H6)d is the benzene concentration in the gas stream during
the storage phase and in the gas stream of the oxidation phase,
respectively. CCO and CCO2 are the concentrations of CO and CO2 in
the gas stream of the oxidation phase; F1 and t1 are the total flow
rate and the storage period of the storage phase, respectively; F2
where ꢀ44 and ꢀ78 are the detection constants of the mass spec-
trometer at m/z = 44 and 78, respectively; ꢁCO and ꢁC
are the
H
6
2
6
total ionization cross sections for CO2 and C6H6; ˇ4C4O and ˇ78
C
H
6
2
6
are the ratios of the partial ionization cross section of CO2 and
C6H6 generating the ion fragment with m/z = 44 and 78 to their
Please cite this article in press as: Y. Wang, et al., Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of
formaldehyde and benzene, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.042

G Model
CATTOD-9561; No. of Pages11
ARTICLE IN PRESS
Y. Wang et al. / Catalysis Today xxx (2015) xxx–xxx
5
Pt2+
Ag0
Ag 3d
Pt0
Pt 4f
Ag1/HZ-S
Pt1/HZ-S
Ag0
Pt0
Pt2+
Pt0.5Ag0.5/HZ-S
Pt0.5Ag0.5/HZ-S
82
80
78
76
74
72
70
68 382 380 378 376 374 372 370 368 366 364 362
B.E. /eV
B.E. /eV
Fig. 4. Pt 4f and Ag 3d spectra of the PtxAg1–x/HZ-S catalysts.
total ionization cross section. According to Eqs. (6) and (7), CC
obtained:
is
6
and Pt0.5Ag0.5/HZ-M catalysts, diffraction peaks appear at 2ꢂ = 38.1
and 39.3◦, which are assigned to Ag (1 1 1) and Pt (1 1 1) phases,
respectively (PDF#87-0597 and 88-2343). The intensities of the
diffraction peaks are much weaker for Pt0.5Ag0.5/HZ-M than
Pt0.5Ag0.5/HZ-L, indicating that Pt and Ag particles should exist in
the form of smaller particle sizes.
H
6
CO
ꢀ
44ꢁCO
ˇ
2 I78
I78
I44
2
44
CC
=
CCO = k1
CCO
2
(8)
H
6
6
2
C
H
6
6
ꢀ78ꢁC 6 ˇ78 I44
H
6
where k1 was calibrated to be 2.36 in this experiment.
Diffraction profiles of all the PtxAg1–x/HZ-S catalysts are sim-
ilar. Diffraction peaks assigned to Ag were not observed for
all PtxAg1–x/HZ-S catalyst. Diffraction peaks assigned to Pt for
Pt0.25Ag0.75/HZ-S and Pt0.5Ag0.5/HZ-S catalysts were not observed
as well, while diffraction peaks appear at 2ꢂ = 39.3◦, which are
assigned to Pt (1 1 1) phases were observed for the Pt0.75Ag0.25/HZ-S
and Pt1/HZ-S catalysts, implying that higher Pt concentration leads
to the growth of Pt particle during the preparation. Clearly, com-
pared with Ag1/HZ-S and Pt1/HZ-S catalysts, no shifts of Pt and Ag
diffraction angle can be seen as the Ag to Pt ratio increases. These
results indicate that there is no evident PtAg alloy formation [24].
To confirm the non-alloy structure of the PtAg bimetal catalysts,
XPS analyses of Ag1/HZ-S, Pt1/HZ-S and Pt0.5Ag0.5/HZ-S catalysts
were performed, the results being shown in Fig. 4. The Pt 4f, [25]
and Ag 3d [26] core level spectra of the catalysts were deconvo-
luted into several components and the results were listed in Table 3.
The typical binding energies Pt 4f7/2 at 73.0 and 71.8 eV on the
3. Results
3.1. Catalyst characterization
Fig. 1 shows N2 adsorption/desorption isotherms of the HZSM-
5 crystal with different sizes. With the particle size decreases, the
adsorption isotherms change from I type to II type because of
the increased intercrystal pores, while the external surface area
increases from 3 to 45 m2/g, and the pore volume increases from
0.17 to 0.46 cm3/g (Table 1). Table 2 summarizes the physical and
chemical properties of the samples. ICP results indicated that the
Si/Al ratios of the samples were about 120, and the precious metal
content of the catalysts was close to the theoretical value but not
completely the same. Such a deviation is acceptable in view of the
potential experimental error.
Fig. 2 displays the SEM images of the three HZSM-5 samples.
The sample of HZ-M has an irregular morphology, and its particle
size distributes from 1–2 ␮m. The SEM inspection of the samples of
HZ-S and HZ-L showed the presence of uniform crystals in all sam-
ples. Their size, however, was quite different. The average particle
size of HZ-S and HZ-L can be controlled at ca. 180 nm to ca. 18 ␮m,
respectively.
XRD patterns of the Pt0.5Ag0.5/HZ-S, -M and -L as well as
PtxAg1–x/HZ-S catalysts are shown in Fig. 3. All catalysts present
a series diffraction peaks at 20–35◦, which can be attributed to the
diffraction of HZSM-5 (PDF no. 44-0003). For the Pt0.5Ag0.5/HZ-L
Table 3
XPS data for the catalysts.
Catalyst
BE (eV)
Pt0
Surface atomic ratioa
Pt2+/Pt0
Pt2+
Ag0
Pt1/HZ-S
Pt0.5Ag0.5/HZ-S
Ag1/HZ-S
71.8
71.8
–
73.0
72.9
–
–
3.87
2.06
–
368.5
368.4
a
Calculated from the corresponding areas of fitted peaks done by XPSPEAK 4.1
with Shirley background.
Please cite this article in press as: Y. Wang, et al., Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of
formaldehyde and benzene, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.042

G Model
CATTOD-9561; No. of Pages11
ARTICLE IN PRESS
6
Y. Wang et al. / Catalysis Today xxx (2015) xxx–xxx
Pt1/HZ-S and Pt0.5Ag0.5/HZ-S catalysts were assigned to Pt2+ and
Pt0, respectively, suggesting that there are two valence states cor-
responding to ionic Pt2+ and metallic Pt0 [25]. The Ag 3d5/2 binding
energy of Ag1/HZ-S and Pt0.5Ag0.5/HZ-S catalysts was at 368.4 eV,
which could be attributed to the Ag0 [26], indicating that Ag0 is the
predominant specie on the Ag1/HZ-S and Pt0.5Ag0.5/HZ-S catalysts.
Compared with Ag1/HZ-S and Pt1/HZ-S catalysts, a shift of
Pt 4f and Ag 3d was not observed for Pt0.5Ag0.5/HZ-S, probably
because the Pt and Ag were unalloyed. Based on the decon-
volution results, the ratio of Pt2+ to Pt0 was calculated, as
shown in Table 3. From this, it is clear that the amount of Pt2+
species (Pt2+/Pt0 = 3.87) in the Pt1/HZ-S was much higher than
Pt0.5Ag0.5/HZ-S (Pt2+/Pt0 = 2.06), indicating that the surfaces in
Pt0.5Ag0.5/HZ-S were enriched in Pt0, as compared to the Pt1/HZ-S
when adding Ag, and gradually decreased with Pt2+ content. The
results indicate that there is no evident PtAg alloy formation but
Fig. 5. TEM images and particle size histograms of (a) Pt0.5Ag0.5/HZ-L, (b) Pt0.5Ag0.5/HZ-M, and (c) Pt0.5Ag0.5/HZ-S.
Please cite this article in press as: Y. Wang, et al., Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of
formaldehyde and benzene, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.042

G Model
CATTOD-9561; No. of Pages11
ARTICLE IN PRESS
Y. Wang et al. / Catalysis Today xxx (2015) xxx–xxx
7
they may some interaction between Pt and Ag to some extent
[24,27,28].
Noting that different HZ support lead to different size and mor-
phology of Pt and Ag particles, particle size and external surface
area of HZ support probably play a key role in the formation of Pt
and Ag particle. Smaller external surface area of HZ-L leads to the
aggregation and growth of Pt and Ag particle during the prepara-
tion. However, small and homogeneous Pt and Ag nanoparticles are
uniformly present on the outside edge of HZ-S, which was due to
the bigger external surface area of HZ-S. On the basis of related lit-
eratures [29,30], the relative small Pt and Ag particle size should
be a key factor for the catalytic oxidation activity of Pt0.5Ag0.5/HZ
catalyst.
TEM observations of the Pt0.5Ag0.5/HZ-S, -M and -L are shown
in Fig. 5. More than 200 metal particles were measured in order
to build the particle size histograms. It is found that the Pt and Ag
nanoparticles observed as dark spots are dispersed on the supports
external surface, while it is difficult to distinguish Pt from Ag. In
the case of the Pt0.5Ag0.5/HZ-L catalyst, broad particle size distri-
bution centered at 12–20 nm with few particles bigger than 22 nm,
the mean Pt and Ag particle size calculated from the particle size
distribution results being 15.6 nm. Pt0.5Ag0.5/HZ-M catalyst has a
much smaller particle size centered at 8–14 nm with few particles
bigger than 20 nm. While Pt0.5Ag0.5/HZ-S catalyst has the smallest
particle size centered at 4–6 nm with a mean Pt and Ag particle size
of 4.5 nm.
3.2. C6H6-TPD studies
To investigate the surface adsorption site on the catalysts
toward C6H6, TPD experiments of adsorbed C6H6 were conducted
and the results are displayed in Fig. 6. The Pt0.5Ag0.5/HZ-S, -M and
-L and PtxAg1–x/HZ-S catalysts exhibited a low-temperature C6H6
peak centered at ca. 184–213 ◦C, which corresponded to the same
desorption of C6H6 adsorbed on the HZ support (Fig. 6A). Besides
the adsorption on the pure HZ support, the addition of Ag to HZ
created a stronger adsorption site, likely due to ␲-complexation
A
HZ-L
Pt0.5Ag0.5/HZ-L
333
213
A
HZ-M
Pt0.5Ag0.5/HZ-M
100
324
197
80
60
40
184
HZ-S
Pt0.5Ag0.5/HZ-S
287
Pt0.5Ag0.5/HZ-L
Pt0.5Ag0.5/HZ-M
20
Pt0.5Ag0.5/HZ-S
0
0
20
40
60
80
100 120 140 160
100
200
300
400
500
Temperature / oC
o
Temperature / C
B
B
100
80
60
40
20
0
287
Ag1/HZ-S
Pt1/HZ-S
Pt0.25Ag0.75/HZ-S
Pt0.50Ag0.50/HZ-S
Pt0.75Ag0.25/HZ-S
Pt1/HZ-S
Pt0.75Ag0.25/HZ-S
Pt0.5Ag0.5/HZ-S
Pt0.25Ag0.75/HZ-S
Ag1/HZ-S
184
0
20 40 60 80 100 120 140 160 180 200
Temperature / oC
100
200
300
400
Temperature /oC
Fig. 7. Complete oxidation of HCHO to CO2 over the Pt0.5Ag0.5/HZ-S, -M and -L (A)
Fig. 6. C6H6-TPD of Pt0.5Ag0.5/HZ-S, -M and -L (A) and PtxAg1–x/HZ-S (B) catalysts.
and PtxAg1–x/HZ-S (B) catalysts.
Please cite this article in press as: Y. Wang, et al., Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of
formaldehyde and benzene, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.042

G Model
CATTOD-9561; No. of Pages11
ARTICLE IN PRESS
8
Y. Wang et al. / Catalysis Today xxx (2015) xxx–xxx
adsorption of Ag with C6H6, which required a much higher tem-
perature at ca. 287–333 ◦C for desorption.
3.3. Catalytic activity
For the Pt0.5Ag0.5/HZ-S, -M and -L catalysts, the low-
temperature desorption peak became much smaller after loading
Pt and Ag on the HZ-M and HZ-L zeolite. The results suggest that
the addition Pt and Ag has great influence on C6H6 adsorption on
the zeolite. From the BET and TEM results, it is known that the
larger of the HZ particle size, the less of the external surface area
and in turn the larger of the Pt and Ag particle size. Thus, the tunnel
entrance of HZ-L might be plugged by the Pt and Ag particle mostly,
which blocked the C6H6 to access into the tunnel, resulting in the
big loss of C6H6 adsorption on zeolite itself. On the other hand,
the peak due to C6H6 on ␲-complexation of Ag on Pt0.5Ag0.5/HZ-
M and -L shifted to higher temperatures than 300 ◦C compared
with that on Pt0.5Ag0.5/HZ-S, indicating an even stronger adsorp-
tion of C6H6 on Ag, which might be related to the bigger particle
size caused by the poor dispersion. For the PtxAg1–x/HZ-S catalysts,
the loading of Ag changed the adsorption of HZ-S significantly. With
increasing the Ag content, the low-temperature desorption peak of
C6H6 decreased, which indicates that C6H6 adsorbed on the HZ-S
sites was partially inhibited. Instead, a high-temperature desorp-
tion peak at 287 ◦C enhanced significantly and was assigned to the
desorption of C6H6 with ␲-complexation of Ag on HZ-S.
3.3.1. Catalytic oxidation of HCHO and C6H6
Fig. 7 compares the HCHO conversion to CO2 calculated from
Eq. (1) for the Pt0.5Ag0.5/HZ-S, -M and -L and PtxAg1–x/HZ-S cat-
alysts. Over Pt0.5Ag0.5/HZ-L and -M catalysts, HCHO conversion
increases monotonously with rise of reaction temperature, and
complete oxidation of HCHO occurs at 160 and 120 ◦C, respectively.
However, remarkably high HCHO conversion of 100% was obtained
over Pt0.5Ag0.5/HZ-S at room temperature. Moreover, the catalysts
remained highly active and stable during the tested hours. Note
that in the case of PtxAg1–x/HZ-S (Fig. 7B), 100% conversion of the
HCHO was converted over Pt1/HZ-S and Pt0.75Ag0.25/HZ-S at room
temperature. While HCHO conversion over Pt0.25Ag0.75/HZ-S is only
50% at 25 ◦C and reaches 100% at 120 ◦C. HCHO conversion is only
40% at 200 ◦C over Ag1/HZ-S catalysts.
Shown in Fig. 8 are the C6H6 conversions to CO2 calculated from
Eq. (2) obtained over Pt0.5Ag0.5/HZ-S, -M and -L and PtxAg1–x/HZ-
S catalysts as a function of reaction temperature from 50–500 ◦C.
In the case of Pt0.5Ag0.5/HZ-S, -M and -L catalysts (Fig. 8A),
Pt0.5Ag0.5/HZ-S was the most active catalyst, with 100% conversion
achievable at about 150 ◦C, while 100% conversion was achieved at
170 ◦C for Pt0.5Ag0.5/HZ-M. In contrast, the 100% conversion over
A
0.12
Non-supported HZ
Pt0.5Ag0.5/HZ
increased by 70%
A
0.10
0.08
0.06
0.04
0.02
0.00
100
Pt0.5Ag0.5/HZ-L
Pt0.5Ag0.5/HZ-M
Pt0.5Ag0.5/HZ-S
increased by 65%
80
60
40
20
0
increased by 33%
HZ-S
HZ-L
HZ-M
B
Pt content /%
0.50
100
120
140
160
180
200
1.00
0.75
0.25
0.00
Temperature / oC
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
B
PtxAg1-x/HZ-S
100
80
60
40
20
0
Pt1/HZ-S
Pt0.75Ag0.25/HZ-S
Pt0.5Ag0.5/HZ-S
Pt0.25Ag0.75/HZ-S
Ag1/HZ-S
50
100
150
200
250
300
350
400
450
0.00
0.25
0.50
0.75
1.00
Temperature / oC
Ag content /%
Fig. 8. Complete oxidation of C6H6 to CO2 over the Pt0.5Ag0.5/HZ-S, -M and -L (A)
Fig. 9. Comparison of C6H6 storage capacity over the Pt0.5Ag0.5/HZ-S, -M and -L (A)
and PtxAg1–x/HZ-S (B) catalysts.
and PtxAg1–x/HZ-S (B) catalysts.
Please cite this article in press as: Y. Wang, et al., Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of
formaldehyde and benzene, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.042

G Model
CATTOD-9561; No. of Pages11
ARTICLE IN PRESS
Y. Wang et al. / Catalysis Today xxx (2015) xxx–xxx
9
A
1200
200
Pt0.5Ag0.5/HZ-L
Pt0.5Ag0.5/HZ-M
Pt0.5Ag0.5/HZ-S
Pt0.5Ag0.5/HZ-L
Pt0.5Ag0.5/HZ-M
Pt0.5Ag0.5/HZ-S
60
40
20
0
1000
800
600
400
200
0
200
252
316
228
233
344
300
181
100
200
300
400
500
0
100
200
300
400
500
Temperature /oC
Temperature /oC
B
2000
1800
1600
1400
1200
1000
800
50
Pt1/HZ-S
165
181
Pt1/HZ-S
Pt0.75Ag0.25/HZ-S
Pt0.50Ag0.50/HZ-S
Pt0.25Ag0.75/HZ-S
Ag1/HZ-S
Pt0.75Ag0.25/HZ-S
Pt0.5Ag0.5/HZ-S
Pt0.25Ag0.75/HZ-S
Ag1/HZ-S
40
30
20
10
0
185
200
300
600
400
200
0
0
100
200
300
400
500
0
100
200
300
400
500
Temperature /oC
Temperature /oC
Fig. 10. Oxidation and release of the stored-C6H6 over the Pt0.5Ag0.5/HZ-S, -M and -L (A) and PtxAg1–x/HZ-S (B) catalysts.
Pt0.5Ag0.5/HZ-L was obtained at 190 ◦C. The effect of Pt/Ag ratio
on catalytic activity of PtxAg1–x/HZ-S was studied (Fig. 8B). Result
demonstrates that Pt1/HZ-S is an excellent catalyst for C6H6 oxida-
tion, with 99% conversion achievable at about 100 ◦C. The activity
of Ag1/HZ-S was the lowest, with a 100% conversion at as high
as 450 ◦C. This means Pt1/HZ-S and Ag1/HZ-S catalysts present
quite different catalytic properties for oxidation of C6H6. For all
PtxAg1–x/HZ-S catalysts, the C6H6 conversions increase along with
the increase of the Pt/Ag ratio, and 100% conversion were obtained
at ca. 190 ◦C over Pt0.25Ag0.75/HZ-S, which was remarkable lower
than Ag1/HZ-S.
to 0.056 mmol/gcat for the HZ-L, HZ-M, and HZ-S zeolites, respec-
tively (Fig. 9A). In comparison, the C6H6 storage capacity increased
to 0.020, 0.066 and 0.095 mmol/gcat for the Pt0.5Ag0.5/HZ catalysts,
respectively. The increase of storage capacity on Pt0.5Ag0.5/HZ can
be attributed to adsorption via ␲-complexation of Ag with C6H6
[16], while the degree of increase is greater in Pt0.5Ag0.5/HZ-S and
Pt0.5Ag0.5/HZ-M (70% and 65%) than in Pt0.5Ag0.5/HZ-L (33%). This
is consistent with the C6H6-TPD results, the dispersion of Pt and Ag
was related to pore blockage. Pore blockage would limit diffusion
of the C6H6 molecule into the HZ-L zeolite, caused a considerable
loss of storage capacity. HZ-M and HZ-S, on the other hand, have
Compared with the Pt0.5Ag0.5/HZ-L and Pt0.5Ag0.5/HZ-M, the
remarkable catalytic behavior shown by PtxAg1–x/HZ-S depends on
forming the PtAg particle into very small particles, on the basis of
TEM and XRD. This is because the massive metal and large particles
cannot chemisorb typical reactant molecules to any useful extent;
this only occurs when an adequate number of low-coordination
surface atoms are present, ideally on particles so small that they
lack full metallic character [31].
Table 4
Desorbed-C6H6, stored-C6H6 to CO2 and carbon balance calculated from TPO results
over the catalysts.
Catalyst
Desorbed-C6H6 (%)
Stored-C6H6 to CO2 (%)
Bc (%)
Pt1/HZ-S
0.9
1.9
3.5
18.8
31.9
7.7
99.9
97.0
94.5
80.5
65.4
87.5
80.1
101.4
99.2
98.1
99.5
97.6
95.5
91.2
Pt0.75Ag0.25/HZ-S
Pt0.5Ag0.5/HZ-S
Pt0.25Ag0.75/HZ-S
Ag1/HZ-S
Pt0.5Ag0.5/HZ-M
Pt0.5Ag0.5/HZ-L
3.3.2. C6H6 adsorption capacity at room temperature
Fig. 9 shows the C6H6 adsorption capacity at room temperature
of the Pt0.5Ag0.5/HZ-S, -M and -L and PtxAg1–x/HZ-S catalysts. The
total C6H6 adsorption of the experiment ranged from 0.015 to 0.040
10.8
Please cite this article in press as: Y. Wang, et al., Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of
formaldehyde and benzene, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.042

G Model
CATTOD-9561; No. of Pages11
ARTICLE IN PRESS
10
Y. Wang et al. / Catalysis Today xxx (2015) xxx–xxx
In a benzene molecule, the carbon atom is sp2 hybridized. Hence,
each carbon has three sp2 orbitals and another pz orbital. The six
pz orbitals in the benzene ring form the conjugative ␲ bond. The
pz orbitals also form the antibonding ␲* orbitals, which are not
occupied. When benzene interacts with the Ag metal, the ␲ orbitals
of benzene can overlap with the empty outer-shell s orbital of the
Ag metal to form a ꢁ bond. Moreover, it is possible that the vacant
antibonding ␲* orbital of benzene can overlap with the d orbitals in
the Ag metal, in a manner similar to the formation of the olefin-Cu+
bond [32].
larger external surface areas and smaller PtAg particle sizes; hence,
C6H6 adsorption was not limited in the measurements.
It was observed that the PtxAg1–x/HZ-S (0.75 ≥ x ≥ 0) catalysts
showed a gradual increase in storage capacity than the HZ-S sup-
port and Pt1/HZ-S (Fig. 9B), which can be attributed to C6H6
adsorption on the Ag by ␲-complexation [16]. The unique char-
acteristics of the d orbitals in Ag or Ag+ ions enable them to form
bonds with unsaturated hydrocarbons in a nonclassical manner,
which is broadly referred to as ␲-complexation [16]. The forma-
tion of these complexes was due to the two-way donor–acceptor
interactions. One is the charge donation from the filled ␲ orbital
of the ligand into the vacant s orbital of the metal and the other
is the back-donation from the d orbital of the metal to the vacant
␲* orbital of ligand. Theoretically, ␲-complexation can be formed
between all transition metal ions and unsaturated hydrocarbons.
However, due to the unique (n − 1)d10ns0 electron configuration,
Ag+ is easier to accept electron and donate electron at the same
time. Thus forms the more stable ␲-complexes. Moreover, the
results of molecular orbital calculations clearly demonstrate that
3.3.3. Oxidation and release of the stored-C6H6 during TPO
TPO experiments were conducted in a simulated air after C6H6
storage at 25 ◦C (10 ppm C6H6/21% O2/1.56% H2O/N2); the results
are summarized in Fig. 10A and B, and quantitative analyse for
TPO data was given in Table 4. As shown in Fig. 10A, for CO2 evo-
lution, the peaks at low temperatures (200 ◦C for Pt0.5Ag0.5/HZ-S,
228 ◦C for Pt0.5Ag0.5/HZ-M and 233 ◦C for Pt0.5Ag0.5/HZ-L) corre-
sponded to the oxidation of stored-benzene from the HZ support,
the peaks at higher temperatures (300 ◦C for Pt0.5Ag0.5/HZ-S, 316 ◦C
for Pt0.5Ag0.5/HZ-M and 344 ◦C for Pt0.5Ag0.5/HZ-L) were assigned to
the oxidation of stored-benzene from silver. For benzene evolution,
Ag+ cation would form stronger ␲-complexation bonds than Pt+[2]
.
These results confirmed the ␲ bonding of benzene with Ag is more
stable [32].
A
2500
20
5th
4th
2nd
1st
3rd
2000
1500
80
15
10
5
60
40
20
0
TPO
0
0
5
10
15
Time (h)
20
25
30
35
B
Stored-C H oxidized into CO2
6 6
HCHO oxidized into CO2
Carbon balance
100
95
90
85
80
1
2
3
4
5
Cycling process number
Fig. 11. (A) C6H6 and CO2 concentrations during five consecutive cycling process over Pt0.5Ag0.5/HZ-S catalyst. (B) Stored-C6H6 oxidation efficiency, HCHO conversion and
carbon balance during five consecutive cycling process over Pt0.5Ag0.5/HZ-S catalyst.
Please cite this article in press as: Y. Wang, et al., Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of
formaldehyde and benzene, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.042

G Model
CATTOD-9561; No. of Pages11
ARTICLE IN PRESS
Y. Wang et al. / Catalysis Today xxx (2015) xxx–xxx
11
upon raising the temperature, release of C6H6 (m/z = 78) was clearly
observed, the desorption rate reaching a maximum at 181 ◦C for
Pt0.5Ag0.5/HZ-S (3.5% of the stored-benzene was detected), 200 ◦C
for Pt0.5Ag0.5/HZ-M (7.7% of the stored-benzene was detected),
and 252 ◦C for Pt0.5Ag0.5/HZ-L (10.8% of the stored-benzene was
detected). These results indicated that some of the stored-benzene
desorbed at elevated temperatures but cannot be oxidized into CO2.
The case of PtxAg1–x/HZ-S catalysts were shown in Fig. 10B. A
sharp CO2 formation peak appeared in Pt1/HZ-S. This indicates that
most of the stored-benzene was catalytically oxidized into CO2 at
165 ◦C, only very weak desorption of C6H6 was detected (0.9%).
Two CO2 formation peak were detected, at 180–200 ◦C and ca.
300 ◦C over PtxAg1–x/HZ-S (0.75 ≥ x ≥ 0) catalysts. According to the
C6H6-TPD (Fig. 6B) and C6H6 adsorption capacity (Fig. 9B) results,
it is reasonable to ascribe the latter peak to the oxidation of C6H6
strongly adsorbed on silver, while the lower temperature peak cor-
responds to the oxidation of C6H6 adsorbed on the HZ-S support.
However, as the Ag to Pt ratio increases, significant release of C6H6
was unavoidable due to the lower activity of these catalysts for
C6H6 oxidation, 1.9%, 3.5%, 18.8% and 31.9% of the stored-benzene
was detected on Pt0.75Ag0.25/HZ-S, Pt0.5Ag0.5/HZ-S, Pt0.25Ag0.75/HZ-
S and Ag1/HZ-S, respectively. Comparing with the TPO results, it is
clear that only over PtxAg1–x/HZ-S (0.5 ≤ x ≤ 1) catalyst, it is possi-
ble to oxidize the stored-benzene into CO2 with trace-C6H6 release.
Overall, PtxAg1–x/HZ-S (0.5 ≤ x ≤ 1) catalysts are indicated to be
promising catalysts which is suitable for use in the cycling process.
in situ without the production of secondary pollutants (such as CO
and NOx).
4. Conclusions
Nanoscale HZSM-5 was a better support than the microscale
one on the catalytic oxidation of HCHO and C6H6 due to the better
dispersion of active bimetals of PtAg. Moreover, nanoscale HZSM-5
was a better benzene storage materials than the microscale one,
and the loading of Ag enhanced the storage capacity of nanoscale
HZSM-5 significantly as well. The Pt0.5Ag0.5/HZ-S catalyst exhibited
excellent activity and stability during 5 consecutive cycles, with
carbon balance higher than 97% in each cycle.
Acknowledgment
The work was supported by the National Natural Science Foun-
dation of China (Nos. 21073024 and 21373037).
References
[1] J. Rodríguez-Mirasol, J. Bedia, T. Cordero, J.J. Rodríguez, Sep. Sci. Technol. 40
(2005) 3113–3135.
[2] K. Wang, X. Wang, G. Li, Catal. Commun. 8 (2007) 324–328.
[3] B. Vogel, C. Schneider, E. Klemm, Catal. Lett. 79 (2002) 107–112.
[4] W. Song, R.E. Justice, C.A. Jones, V.H. Grassian, S.C. Larsen, Langmuir 20 (2004)
8301–8306.
[5] K. Wang, X. Wang, Microporous Mesoporous Mater. 112 (2008) 187–192.
[6] K. Wang, X. Wang, G. Li, Microporous Mesoporous Mater. 94 (2006) 325–329.
[7] W. Zhao, Y. Liu, L. Wang, J. Chu, J. Qu, Z. Hao, T. Qi, Microporous Mesoporous
Mater. 138 (2011) 215–220.
3.3.4. Catalytic activity of Pt0.5Ag0.5/HZ-S during the cycling
process
The cycling experiments involving “HCHO oxidation” and “C6H6
storage-oxidation” were performed using Pt0.5Ag0.5/HZ-S for 5 con-
secutive cycles (Fig. 11A). In the first cycle, during the six hours
of storage (feed gas: 80 ppm HCHO + 10 ppm C6H6/21% O2/1.56%
H2O/N2, GHSV = 36,000 h−1), no HCHO slip was detected. Instead,
80 ppm of CO2 was detected in the effluent gas, which corresponds
to the complete oxidization of HCHO into CO2 and H2O. Meanwhile,
no C6H6 release was observed except towards the end of the stor-
age phase. It should be noted that the observation of this C6H6 slip
was a consequence of our definition of C6H6 breakthrough time, in
this case the time at which the outlet C6H6 concentration reached
1% of the feed concentration (corresponding to 0.1 ppm), i.e., C6H6
breakthrough could be decreased or avoided entirely if a shorter
storage time were used. During the oxidation period, the temper-
ature was increased to promote catalytic oxidation of the stored
C6H6 to CO2 and H2O, resulting in the observation of two CO2 peak
at 200 and 300 ◦C which corresponded to the oxidation of stored-
C6H6 (as observed in Fig. 10). This occurred without the production
of secondary pollutants as evidenced by the absence of CO in the
effluent stream. The carbon balance calculated according to Eq. (5)
was 98.1%.
Considering the stability of the Pt0.5Ag0.5/HZ-S catalyst, as
shown in Fig. 11B, the stored-C6H6 oxidation efficiency and HCHO
oxidation efficiency kept stable values of ca. 95% and 100%, respec-
tively during the 5 cycles. Analysis of the reactor effluent during the
storage and oxidation steps during the 5 cycles confirmed that the
HCHO and stored-C6H6 were decomposed to CO2 and H2O. This is
supported by the excellent carbon balances obtained during cycling
(>97%). These preliminary results suggest that Pt0.5Ag0.5/HZ-S was
stable during the process. Moreover, the catalyst was regenerated
[8] Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Science 301 (2003) 935–938.
[9] R. Mu, Q. Fu, H. Xu, H. Zhang, Y. Huang, Z. Jiang, S. Zhang, D. Tan, X. Bao, J. Am.
Chem. Soc. 133 (2011) 1978–1986.
[10] H.S. Kim, T.W. Kim, H.L. Koh, S.H. Lee, B.R. Min, Appl. Catal., A: Gen. 280 (2005)
125–131.
[11] B. Hammer, J.K. Nørskov, in: H.K. Bruce, C. Gates (Eds.), Advances in Catalysis,
vol. 45, Academic Press, Davis, U.S.A., 2000, pp. 71–129.
[12] V. Stamenkovic, B.S. Mun, K.J.J. Mayrhofer, P.N. Ross, N.M. Markovic, J. Ros-
smeisl, J. Greeley, J.K. Nørskov, Angew. Chem. Int. Ed. 45 (2006) 2897–2901.
[13] A. Nilekar, Y. Xu, J. Zhang, M. Vukmirovic, K. Sasaki, R. Adzic, M. Mavrikakis,
Top. Catal. 46 (2007) 276–284.
[14] J.C. Dellamorte, J. Lauterbach, M.A. Barteau, Appl. Catal., A: Gen. 391 (2011)
281–288.
[15] C. Shi, B.-b. Chen, X.-s. Li, M. Crocker, Y. Wang, A.-m. Zhu, Chem. Eng. J. 200–202
(2012) 729–737.
[16] A. Takahashi, F.H. Yang, R.T. Yang, Ind. Eng. Chem. Res. 39 (2000) 3856–3867.
[17] R.B. Eldridge, Ind. Eng. Chem. Res. 32 (1993) 2208–2212.
[18] W.S.W. Ho, G. Doyle, D.W. Savage, R.L. Pruett, Ind. Eng. Chem. Res. 27 (1988)
334–337.
[19] F. Hong-Yu, S. Chuan, L. Xiao-Song, Z. De-Zhi, X. Yong, Z. Ai-Min, J. Phys. D: Appl.
Phys. 42 (2009) 225105.
[20] C. Shi, Y. Wang, A. Zhu, B. Chen, C. Au, Catal. Commun. 28 (2012) 18–22.
[21] Y. Wang, A. Zhu, B. Chen, M. Crocker, C. Shi, Catal. Commun. 36 (2013) 52–57.
[22] Y. Wang, X. Zhu, M. Crocker, B. Chen, C. Shi, Appl. Catal., B: Environ. 160–161
(2014) 542–551.
[23] X. Yang, Y. Shen, L. Bao, H. Zhu, Z. Yuan, React. Kinet. Catal. Lett. 93 (2008)
19–25.
[24] L. Chen, D. Ma, Z. Zhang, Y. Guo, D. Ye, B. Huang, ChemCatChem 4 (2012)
1960–1967.
[25] A. Dauscher, L. Hilaire, F. Le Normand, W. Müller, G. Maire, A. Vasquez, Surf.
Interface Anal. 16 (1990) 341–346.
[26] V.K. Kaushik, J. Electron Spectrosc. Relat. Phenom. 56 (1991) 273–277.
[27] A. Aristizábal, S. Contreras, N.J. Divins, J. Llorca, F. Medina, Appl. Catal., B: Envi-
ron. 127 (2012) 351–362.
[28] M. Schaal, A. Pickerell, C. Williams, J. Monnier, J. Catal. 254 (2008) 131–143.
[29] J.X. Peng, S.D. Wang, Appl. Catal., B: Environ. 73 (2007) 282–291.
[30] F. S¸ en, G. Gökag˘ac¸ , J. Phys. Chem. C 111 (2007) 5715–5720.
[31] G.C. Bond, D.T. Thompson, Catal. Rev. 41 (1999) 319–388.
[32] H.Y. Huang, J. Padin, R.T. Yang, J. Phys. Chem. B 103 (1999) 3206–3212.
Please cite this article in press as: Y. Wang, et al., Nanoscale HZSM-5 supported PtAg bimetallic catalysts for simultaneous removal of
formaldehyde and benzene, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.042