Visible light responsive Zeolite/WO3-Pt hybrid photocatalysts for degradation of pollutants in air

Article abstract of DOI:10.1016/j.apcata.2015.12.015

Adsorbent-photocatalyst hybrids based on WO₃ and Pt as cocatalyst immobilized on different type of zeolites were prepared. Their performance for the degradation of pollutants under UV and Vis light was analyzed. Two types of zeolites (ZSM-5 and Zeolite Y) with Si/Al ratios ranging between 5 and 280 were selected as hierarchical microporous materials acting as adsorbents. The incorporation of platinum on tungsten oxide was done by means of the photodeposition method. Pt-loaded WO₃ was dispersed on the zeolites and subjected to a lyophilization process. Their photocatalytic properties towards the abatement of representative pollutants e.g.; acetic acid, acetaldehyde and trichloroethylene, were analyzed under different operating conditions in batch and continuous flow photoreactors. Raw materials and hybrid photocatalysts were characterized by N₂ adsorption-desorption, XRD, UV-vis spectroscopy, XPS and TEM microscopy. The adsorption ability of the photocatalysts towards the selected pollutants was studied under dynamic conditions. The textural properties of the photocatalysts were not the main factor controlling their adsorption ability. The incorporation of WO₃-Pt on the zeolite results in Vis light responsive materials. Monoclinic WO₃ crystal phase was identify in all zeolitic materials. Large WO₃-Pt aggregates were detected on Zeolite Y (ca. 300 nm) with low Si/Al content in contrast with highly disperse phase for ZSM-5 (SiO₂/Al₂O₃ = 280) (ca. 100 nm). All WO₃-Pt hybridized zeolites showed photoactivity for the degradation of the studied pollutants under Vis light, improving the performance of bare WO₃-Pt. The results shown in this work reveal the influence of the hydrophobicity of the siliceous material for the adsorption of reaction intermediates or byproducts and as a consequence for the determination of the rates of CO₂ formation under static conditions. A F(Static) function, dependent of the Si/Al ratio and the BET area, to correlate the difference trend of the amount of CO₂ released produced by the hybrid composites, under static and dynamic conditions is proposed.

Full text of DOI:10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
Applied Catalysis A: General xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Visible light responsive Zeolite/WO –Pt hybrid photocatalysts for
3
degradation of pollutants in air
a
b
b
c
d
a
b
I. Jansson , K. Yoshiiri , H. Hori , F.J. García-García , S. Rojas , B. Sánchez , B. Ohtani ,
S. Suárez
a
a,∗
CIEMAT, División de Energías Renovables, FOTOAIR: Group of Analysis and Photocatalytic Treatment of Pollutant in Air, Avda. Complutense, 40, 28040
Madrid, Spain
b
Institute for Catalysis, Hokkaido University, Section of Catalytic Reaction Chemistry, Sapporo 001-0021, Japan
ICTS-CNME, Universidad Complutense de Madrid, 28040 Madrid, Spain
ICP-CSIC, Instituto de Catálisis y Petroquímica, C/ Marie Curie 2, 28049 Madrid, Spain
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Adsorbent–photocatalyst hybrids based on WO3 and Pt as cocatalyst immobilized on different type of
zeolites were prepared. Their performance for the degradation of pollutants under UV and Vis light was
analyzed. Two types of zeolites (ZSM-5 and Zeolite Y) with Si/Al ratios ranging between 5 and 280 were
selected as hierarchical microporous materials acting as adsorbents. The incorporation of platinum on
tungsten oxide was done by means of the photodeposition method. Pt-loaded WO3 was dispersed on
the zeolites and subjected to a lyophilization process. Their photocatalytic properties towards the abate-
ment of representative pollutants e.g., acetic acid, acetaldehyde and trichloroethylene, were analyzed
under different operating conditions in batch and continuous flow photoreactors. Raw materials and
hybrid photocatalysts were characterized by N2 adsorption–desorption, XRD, UV–vis spectroscopy, XPS
and TEM microscopy. The adsorption ability of the photocatalysts towards the selected pollutants was
studied under dynamic conditions. The textural properties of the photocatalysts were not the main fac-
Received 15 September 2015
Received in revised form 2 December 2015
Accepted 12 December 2015
Available online xxx
Keywords:
Visible Light
WO3–Pt
Zeolites
Adsorbent–photocatalyst hybrids
Air treatment
VOC
tor controlling their adsorption ability. The incorporation of WO –Pt on the zeolite results in Vis light
3
responsive materials. Monoclinic WO3 crystal phase was identify in all zeolitic materials. Large WO3–Pt
aggregates were detected on Zeolite Y (ca. 300 nm) with low Si/Al content in contrast with highly disperse
phase for ZSM-5 (SiO2/Al2O3 = 280) (ca. 100 nm). All WO3–Pt hybridized zeolites showed photoactivity
for the degradation of the studied pollutants under Vis light, improving the performance of bare WO3–Pt.
The results shown in this work reveal the influence of the hydrophobicity of the siliceous material for
the adsorption of reaction intermediates or byproducts and as a consequence for the determination of
the rates of CO2 formation under static conditions. A F(Static) function, dependent of the Si/Al ratio and
the BET area, to correlate the difference trend of the amount of CO2 released produced by the hybrid
composites, under static and dynamic conditions is proposed.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
Different approaches have been reported in literature for the
development of visible light active photocatalysts. Most attempts
are based upon the incorporation of foreign elements onto TiO2
including noble and transition metals such as Au, Ag, Cu, Pt or
Pd, cationic or anionic doping with non-metal elements such as
-C, -P, -S and -N. The coupling of semiconductors with TiO2 such
as titanates (FeTiO /TiO , NiTiO /TiO CoTiO /TiO ), mixed oxides
The development of Vis-responsive photocatalysts for air
purification is amongst the main goals in the research area of
photofunctional materials [1–3]. Due to its stability, nontoxicity,
cost effectiveness and photoactivity, titanium dioxide (TiO ) is the
2
most extensively studied photocatalyst [4,5]. However, TiO is only
2
3
2
3
2,
3
2
active under UV light, which is only about the 5% of the solar radi-
ation spectrum.
or sulfides (Fe O /TiO WO /TiO , CdS/TiO ), is another alterna-
2 3 2, 3 2 2
tive [3,6]. Tungsten oxide (WO ), with a band gap ranging from
3
2
.5 to 2.8 eV and its deeper valence band (+3.1 eV), can absorb
efficiently the visible light and as a consequence can be an interest-
ing alternative to TiO2 for the degradation of organic compounds
under Vis light. In addition, WO3 is a nontoxic semiconductor,
∗ Corresponding author. Fax:+34 913466037.
E-mail address: silvia.suarez@ciemat.es (S. Suárez).
http://dx.doi.org/10.1016/j.apcata.2015.12.015
0
926-860X/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
I. Jansson et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
2
resistance to photocorrosion effects and presents stable physico-
chemical properties [7]. The relative low energy of the electrons
in the WO3 conduction band, however, could hamper its capac-
ity to reduce oxygen. As a result, a build-up of electrons followed
by an increased incidence of recombination with holes leads to
a decrease in photocatalytic performance. Improvements in WO3
photoactivity, so as to counteract this limitation, may be achieved
by loading Pt nanoparticles on the WO3 surface. Abe et al. demon-
strated that Pt loading is an attractive approach to enhance the
photocatalytic properties of WO3 [8]. Pt metallic nanoparticles can
trap the photogenerated electrons from WO to reduce O to H O .
location of deposition of the active sites promoting the dispersion
of the catalytic phase [24,25].
In this study, a series of APHs based on Zeolite/WO –Pt hybrids
3
with five types of commercial zeolites have been prepared by
freeze-drying and used as photocatalysts. Zeolites exhibiting FAU
(Zeolite Y) and MFI (ZSM-5) structures and different SiO /Al O
2
2
3
ratios between 5 and 280 have been used for the synthesis of
the APHs. The photocatalysts were thoroughly characterized by N2
adsorption–desorption, UV–Vis spectroscopy, ICP–MS, XRD, XPS,
and TEM microscopy. The photocatalytic activity of the materials
for the degradation of acetic acid, acetaldehyde and trichloroethy-
lene in batch or continuous flow photoreactors was studied under
UV and Vis light irradiation. The effect of the zeolite structure
and the Si/Al ratio on the photocatalytic performance of the
3
2
2
2
Several studies to evaluate the photocatalytic performance within
the visible region of WO –Pt towards the abatement of different
3
pollutants have been reported [9–11]. The combination of WO and
3
Pt nanoparticles, where the Pt works as an electron acceptor, allows
Zeolite/WO –Pt hybrids is discussed.
3
O reduction multielectron reactions with a more positive potential
2
than for the single-electron reduction of oxygen. The reactions that
take place with multielectron reduction according to the following
equations [8]:
2
. Experimental
2.1. Photocatalyst preparation
O + 2H + 2e− → H O (aq) + 0.682 V
+
(1)
(2)
2
2
2
The WO -Pt photocatalytic phase was prepared by the pho-
3
O + 4H + 4e− → 2H O (aq) + 1.23 V
+
2
2
todeposition (PD) method using commercial WO3 (IV) powder
99.9% Kojundo Chemical Laboratory Co.) and hexachloroplatinic
acid (99.9% H PtCl 6H O, Wako). An CH OH/H O (5 vol.%) aqueous
(
The development of Adsorbent–Photocatalyst Hybrids (APHs)
2
6
2
3
2
based on the combination of a photocatalyst with an adsorbent
material, is an interesting approach to improving the photocat-
alytic properties of single semiconductors [6]. The combination of
TiO2 with adsorbent materials, results in APHs with superior pho-
tocatalytic activity because of the synergic effect of both materials’
properties. The synergistic effect was ascribed to the creation of a
contact interface between both solid phases allowing a continuous
transfer of pollutant species from the adsorbent to the active phase,
reducing the formation of undesirable reaction intermediates [12].
The appropriate selection of the support is a key factor to opti-
mize features such as the dispersion of the semiconductor on
the adsorbent and the textural properties of the final compos-
ite. Different types of adsorbents have been used with TiO2 in
order to improve those properties [13–20]. Amongst them, zeo-
lites are perfect host to combine with photoactive phase, due to
its high crystallinity and microporosity, UV–Vis light transparency,
large surface area, adsorption capacity, acidity and shape selectiv-
suspension containing the fine particulate WO and hexachloropla-
3
tinic acid was irradiated during 60 min with a 400-W high-pressure
mercury lamp (ꢀ > 290 nm Eikosha), under constant magnetic stir-
ring (1000 r.p.m) in an argon atmosphere. The temperature was
◦
kept at 25 C by a thermostatic water bath. Then, the suspen-
sion was centrifuged, and the solid recovered was washed with
methanol and distilled water several times. Finally, the WO –Pt
3
◦
powder sample was dried at 100 C overnight. According to the
literature, the optimum Pt loading was set to 0.5 wt.% [8].
Five commercially available zeolites from Zeolyst, Zeolite Y (a
FAU type zeolite with SiO /Al O = 5 and 80, producer’s codes:
2
2
3
CBV 600 and CBV 780) and ZSM-5 (a MFI type zeolite with
SiO /Al O = 23, 80 and 280, producer’s codes: CBV 2314, CBV 8014
2
2
3
and CBV 28014) were used without chemical modification as the
adsorbent phase of the APHs. Zeolite/WO –Pt hybrids were pre-
3
pared by mixing bare zeolite and WO –Pt powders as detailed
3
below. Zeolites powders (2 g) were suspended on distillate water.
ity properties [21]. Depositing WO –Pt onto microporous siliceous
3
WO –Pt particles (160 mg) were incorporated in the suspension
3
materials could improve the dispersion of the photocatalytic phase,
thus promoting the photocatalytic activity. To the best of our
knowledge, only one study of the properties of Pt-loaded WO3
nanoparticles deposited on a siliceous mordenite (MOR) zeolite
has been reported in the literature [22]. Studies about the influ-
ence of the zeolite structure or the Si/Al ratio on the photocatalytic
under constant magnetic stirring (1000 r.p.m) keeping the suspen-
sion under agitation for 120 min. Then, samples were frozen with
liquid N , removing water by freezing-drying (EYELA Freeze Dryer
2
◦
FDU-2100, P < 10 Pa, T = −80 C, t = 24 h). The final WO –Pt content
3
on the hybrid materials was of 7.5 wt.%. WO –Pt loading in the
3
composites have been analyzed by ICP, indicating a 0.03 wt.% of
Pt content on the final composite.
performance of WO /Pt–zeolite hybridized materials are lacking.
3
Freeze-drying is a process whereby solutions are frozen in a
cold bath and then the frozen solvents are removed via subli-
mation under vacuum, leading to formation of porous structures
2.2. Characterization techniques
[
23]. This technique, also known as lyophilization, has been widely
used to prepare porous materials for tissue engineering, biologi-
cal applications and pharmaceutical industry. In recent years, the
method of freeze drying has been explored as a unique route to
produce novel porous materials such as aerogels. One of the main
advantages of this method is the non-use of toxic organic solvents,
avoiding the use of large amounts of organic solvents, essential
for environmental-friendly applications. Moreover, the low sur-
face tension involved during the freeze drying process can maintain
the pore structure (particularly mesopores and micropores) which
would otherwise collapse due to the high capillary force or sur-
face tension during a conventional drying process. For catalyst
preparation, freeze-drying has been suggested to reduce precur-
sor solution mobility during drying and therefore to control the
The textural properties of the commercial zeolites and
of the Zeolite/WO –Pt hybrids were determined by N2
3
◦
adsorption–desorption at −196 C in a Micromeritics Asap 2420
◦
Analyser. Samples were outgassed overnight at 120 C to a pres-
2
sure of <1.33 × 10 Pa to ensure a clean dry surface, free from
any loosely bound adsorbed species. Micropore volume and sizes
were calculated by N2 adsorption at the low partial pressure
range (P/P = 0–0.1) using the Horvath–Kawazoe (H–K) method
0
and the differential pore volume plot for cylinder pore geometry
using the Saito–Foley approximation [26]. WO –Pt loading in the
3
composites have been analyzed by inductively coupled plasma mass
spectrometry ICP–MS NexION 300XX (PerkinElmer) of dispersions
of the ground catalysts in acid solutions.
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
I. Jansson et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
3
Table 1
Properties of the reference materials and the Zeolite/WO3–Pt composites at room temperature.
Sample
Adsorbent
SiO2/Al2O3a
WO3
wt.%)
BET area
Vpc
Mean pore size
Adsorption
C2HCl3
−1
(
(m2
g
)
VT
Micro
Meso
Micro
(nm)
Meso
CH3CHO
cm3
g
−1
)
−1
(␮mol g catal.)
(
Raw materials
Z1
Z2
Z3
Z4
Z5
WO3–Pt
Zeolite Y
Zeolite Y
ZSM-5
ZSM-5
ZSM-5
–
5
–
–
–
–
–
100
607
826
352
447
402
0.35
0.48
0.18
0.26
0.26
–
0.25
0.36
0.14
0.18
0.13
–
0.10
0.12
0.04
0.08
0.03
–
0.74
0.75
0.51
0.55
0.60
–
10
17
–
–
–
150
127
522
481
453
1
799
80
23
80
280
–
279
3,332
584
201
12
b
10
–
Zeolite/WO3–Pt composites
Z1/WO3–Pt
Z2/WO3–Pt
Z3/WO3–Pt
Z4/WO3–Pt
Z5/WO3–Pt
Zeolite Y
Zeolite Y
ZSM-5
ZSM-5
ZSM-5
5
7.5
7.5
7.5
7.5
7.5
574
762
342
420
365
0.30
0.47
0.13
0.24
0.20
0.19
0.29
0.09
0.17
0.17
0.11
0.18
0.04
0.07
0.03
0.50
0.50
0.40
0.42
0.42
10
14
–
–
–
50
19
382
256
212
592
184
3,189
302
116
80
23
80
280
a
SiO2/Al2O3 mole ratio.
From Ref. [50].
Vp: pore volume; VT: total pore volume.
b
c
Diffuse reflectance spectra of the solids were recorded on a
double beam UV–Vis spectrophotometer (PerkinElmer Lambda
50 UV/Vis) equipped with a diffuse reflectance accessory. The
reflectance data was converted to the absorption coefficient F(R∞)
according to the Kubelka–Munk equation:
Spectroscopy, XEDS. Samples were prepared by ultrasonically
dispersing the powders in butanol. One drop of the resulting sus-
pension was deposited onto a copper grid coated with a thin holey
amorphous carbon layer
6
2
.3. Photocatalytic activity tests
2
(
1 − R∞)
F (R∞) =
(3)
2
R∞
2.3.1. Photodegradation experiments in static conditions: batch
where F(R∞) is equivalent to the absorption coefficient.
reactor
The photocatalytic performance of WO3 and Pt-loaded WO3
The identification of the crystalline phases was conducted with
a PANalytical X’Pert PRO ultrafast detector X-ray diffractometer
samples was evaluated by following the photodegradation reac-
tion of acetic acid in water. The reaction was performed in ca. 30 mL
Pyrex glass tubes sealed with a rubber septum. 50 mg of the pho-
tocatalyst was suspended in a 5 mL aqueous solution of acetic acid
(5.0 vol.%). The photocatalysts were irradiated during 60 min under
continuous magnetic stirring with the same lamp described before
for the incorporation of platinum by the PD method. The produc-
tion of CO2 was evaluated with a gas chromatography equipped
with a TCD detector (Shimadzu GC-8A) using a Porapak-Q column.
The photodegradation of acetaldehyde (AcH) in air was also
studied. In this case, the materials (80 mg) were supported on a
borosilicate glass roughened plate (76 mm × 26 mm). The immobi-
lized photocatalysts was placed inside of a sealed cylindrical Pyrex
glass vessel (total volume 114 mL) and pretreated with UV light
during 12 h. A volume of 4.67 ␮mol of gaseous AcH was injected into
the vessel filled of ambient air, i.e., the concentration of AcH within
the reactor was of 1000 ppm. Once AcH was completely adsorbed
on the material (after ca. 240 min at dark conditions), the samples
were irradiated using either UV black light (fluorescent GE/Hitachi
(
XRD) with CuK␣ radiation operating at 45 kV and 40 mA. Bragg’s
◦
◦
◦
angles (ꢁ) between 20 and 80 were recorded at a rate of 0.02
per step and a count time of 50 s per step. Diffraction lines were
assigned to the corresponding crystalline phases by comparison
with Powder Diffraction Files (PDF) from the International Centre
for Diffraction Data (ICDD) Joint Committee on Power Diffraction
Standards (JCPDS).
X-ray photoelectron spectra (XPS) were acquired with a SPECS
customized system for surface analysis with an ultra-high-vacuum
−
10
(
UHV) chamber (base pressure: 1.5 × 10
mbar) equipped with
a non-monochromatic X-ray source XR 50 and a hemispherical
energy analyser PHOIBOS 150. X-ray MgK␣ line (1253.6 eV) was
used as excitation (operating at 200 W/12 kV), and the medium area
mode of the lenses was used for the detector. The energy regions
of the photoelectrons of interest were scanned at increments of
0.1 eV and fixed pass energy of 25 eV until an acceptable signal-to-
noise ratio was achieved. Atomic abundances were estimated by
calculating the integral of each peak, determined by subtraction of
the Shirley-type background and fitting of the experimental curve
to a combination of Lorentzian and Gaussian using the appropriate
sensitivity factors for each element [27]. Binding energies (± 0.2 eV)
were determined by setting the C 1s peak at 284.6 eV.
The scanning electron microscopy characterization was per-
formed in a JEOL JSM-7600F. The electron microscope is equipped
with an OXFORD INCA detector for X-ray Energy Dispersive
Spectroscopy, XEDS. Micrographs were recorded colleting the
backscattered electrons (COMPO mode) to ensure a proper identifi-
−
2
FL10BL-B) with 1 mW cm irradiance or a Vis lamp (fluorescent
National FL10ECW) with a cutoff UV filter. The evolution of the
CO2 concentration in the gas phase was evaluated with a GC-FID
equipped with a methanizer (Shimadzu GC-14B) and a Porapak-Q
column. The evolution of the concentration of AcH was measured
with a GC-FID (Shimadzu GC-14B) equipped with a Phenomenex
ZB-WAX column.
2.3.2. Photodegradation experiments under dynamic conditions:
continuous flow reactor
cation of the WO –Pt phase. Samples were prepared by depositing
3
a small amount of the powder onto a clean surface graphite sample
holder.
Transmission electron microscopy studies were carried out in a
JEOL JEM3000F, operated at 300 KV. Micrographs were recorded
with a multiscan CCD camera (GATAN). The microscope is attached
with an OXFORD INCA detector for X-ray Energy Dispersive
The photocatalytic performance of the hybrid composites
and the raw materials for the oxidation of selected model
VOC molecules, trichloroethylene (TCE) or acetaldehyde, was
also studied in a continuous flow photoreactor. Sample pow-
ders (30 mg) were immobilized on a glass borosilicate plate
(76 mm × 26 mm) and placed inside a continuous flow flat pho-
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
I. Jansson et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
4
Fig. 1. N2-adsorption–desorption isotherms for bare zeolites and Zeolite/WO3–Pt hybrids: (a) Zeolite Y: (᭿,ᮀ) Z1, (᭹,ꢀ) Z1/WO3–Pt, (ꢀ,ꢁ) Z2, (ꢁ,♦) Z2/WO3–Pt and (b)
ZSM-5: (᭿,ᮀ) Z3, (᭹,ꢀ) Z3/WO3–Pt, (ꢀ,ꢁ) Z4, (ꢁ,♦) Z4/WO3–Pt.
600 mL min 1 with concentrations of 25 ppm of C HCl3 or 15 ppm
−
toreactor (120 mm × 50 mm × 10 mm). The photoreactor was made
of stainless steel except for the side where a 37 cm2 borosilicate
glass window is placed [12]. After sample preparation, the materi-
als were pre-treated during 12 h under air atmosphere and UV-A
irradiation in order to remove water and weakly adsorbed organic
molecules at the material surface. Gas cylinders (Air Liquide) with
controlled compositions of C HCl /N (70 ppm) or CH CHO/N2
2
of CH CHO, respectively. The VOC/air gas mixture (TCE/air or
3
AcH/air) was introduced in the experimental set-up by-passing the
reactor. Once the VOC concentration was stable, the gas mixture
was passed through the reactor containing the sample. The adsorp-
tion process starts with a decrease of the VOC signal followed by a
progressive recovering until reaching initial value. The integration
of the area above the curve allowed to calculate the amount of VOC
adsorbed (Table 1). After the adsorption process reaches the equi-
librium, the samples were irradiated by two UV lights (8 W Philips)
resulting in an irradiance of 6.5 mW cm . In the reaction with the
Vis light (fluorescent OSRAM L 8W/954) a cutoff UV filter was used.
The gas-phase composition was continuously monitored by using
a FTIR Thermo-Nicolet 5700 spectrometer equipped with a tem-
perature controlled multiple-reflection gas cell (optical path 2 m)
2
3
2
3
(
250 ppm) were used to introduce the pollutants. The photocat-
alytic performance was evaluated in a continuous flow mode using
−
1
total gas flows of 100 mL min (3.8 s residence time) containing
−
2
1
0 or 25 ppm of the selected VOC.
The adsorption ability of the raw materials and of the
Zeolite/WO –Pt for each of the VOC molecules selected were mea-
3
sured. The experimental device was the same as the one used
−
1
for the photocatalytic tests with gas flows of 100 mL min
or
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
I. Jansson et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
5
a
b
Fig. 2. X-ray diffractograms of raw materials and Zeolite/WO3–Pt composites for (a) Zeolite Y and (b) ZSM-5 hybrid materials. Crystalline phases: (ꢀ) Zeolite Y, (᭹) ZSM-5
and (*) monoclinic WO3 phase.
◦
maintained at 110 C. The IR spectra were registered after accu-
where CVOC inlet and CVOC outlet were the concentrations of the VOC
at the reactor inlet and outlet, respectively.
−
1
mulation of 64 scans at a resolution of 4 cm . The evolution of
the reaction products during the reaction was monitored by fol-
lowing the evolution of the representative vibrational bands of
selected molecules as reported elsewhere [21]. VOC degradation
rate or reaction products formation rate was calculated considering
the amount of active phase. The conversion values were calculated
according the following equation:
3. Result and discussion
3
.1. Characterization of Zeolite/WO –Pt hybrids
3
The most relevant textural and physicochemical properties of
the raw materials and of the Zeolite/WO –Pt composites are shown
3
in Table 1. Commercial Zeolites Y (Z1 and Z2) with the FAU struc-
ture have micropores of around 0.75 nm and BET areas between
600 and 800 m g substantially greater than those of the ZSM-5
CVOC inlet − CVOC outlet
Conversion VOC (%) =
(4)
CVOC inlet
2
−1
Fig. 3. Diffuse reflectance spectra of Zeolites/WO3–Pt hybrid materials according to the Kubelka-Munk equation spectra. TiO2 spectra was included as a reference.
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
I. Jansson et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
6
zeolites. Mesopores in the range of 10–17 nm were also observed
in these materials, probably arising from the dealumination pro-
cesses conducted to obtaining the desired Si/Al ratio. The ZSM-5
(
Z3, Z4 and Z5) zeolites, with MFI structure, have micropores of
2
−1
.
around 0.5 nm and surface area values in the range of 400 m g
The BET area values obtained for the ZSM-5 zeolites used in this
2
−1
work ranged between 400 and 800 m g ; these values are indica-
tive of the presence of amorphous silica or alumina phases in the
commercial zeolites. As expected, the incorporation of ca. 7.5 wt.%
of WO3 with ca. 0.03-0.04 wt.% of Pt according to ICP-MS analysis
in the final composite, leads to a decrease of the BET area values for
all the samples as compared to the parent zeolites due to the incor-
poration of a low surface area material. In addition, the decreasing
BET values of the hybrid catalyst suggest that the deposition of the
WO –Pt particles results in the blockage of a fraction the microp-
3
ores of the siliceous materials. Fig. 1a and b shows representative N2
adsorption–desorption isotherms recorded for the composites and
for the raw materials, for zeolite Y and ZSM-5 composites, respec-
tively. According to the IUPAC classification, all zeolites show a Type
I isotherm characteristic of microporous solids [28]. The slightly
enhanced uptake of N2 at P/P0 values greater than 0.9 suggests
the presence of a small quantity of macropores. The isotherm also
exhibits a small degree of hysteresis, indicating the presence of
some mesopores and the possibility of capillary condensation. As
observed in these figures, zeolite Y based composites showed a high
volume of micropores and greater surface area values as compared
to those based on ZSM-5.
The amount of acetaldehyde and trichloroethylene adsorbed
under dynamic conditions by the raw materials, WO –Pt and
3
Fig. 4. W 4f core level spectra for Zeolite/WO –Pt hybrids.
3
Fig. 5. TEM micrographs for WO3–Pt photocatalyst: (a) Pt agglomerate, (b) Pt single spherical particle, (c) Pt elongated particle; (d) WO3–Pt particle size distribution histogram
based on TEM images.
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
I. Jansson et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
7
Fig. 6. SEM micrographs recorded using backscattered electrons for: (a) Z1/WO3–Pt, (b) Z3/WO3–Pt and (c) Z5/WO3–Pt.
Fig. 7. TEM micrographs obtained for: (a-c) Z1/WO3–Pt, (b-d) Z5/WO3–Pt. Inset figures in (a) and (b) correspond to MFI and FAU zeolite structures. Micrographs of the
WO3–Pt phase on: (c) Z1/WO3–Pt and (d) Z5/WO3–Pt.
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
I. Jansson et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
8
Zeolite/WO –Pt hybrids are reported in Table 1. Due to its low
Diffuse reflectance spectra of the solids are shown in Fig. 3.
As observed, the spectra show a significant increase of the light
3
surface area, WO –Pt showed poor adsorption ability for both the
3
aldehyde and the organochloride compound. On the other hand,
zeolites showed significantly greater adsorption ability, mostly
towards AcH adsorption. Electrostatically polarized or unbalanced
sites of zeolites can act as strong adsorption sites for organic
molecules, especially for polar molecules such as acetaldehyde
absorbance for the Zeolite/WO –Pt hybrids within the wavelength
range between 400 and 450 nm as compared to benchmark TiO2.
3
Note that Z1/WO –Pt has the lowest adsorption of light within
3
this range in the series. Being UV–vis transparent materials, WO3-
free zeolites record no UV–vis absorption response. The reflectance
[
29]. Notwithstanding of the nature of the adsorbed compound,
spectra of the zeolites containing WO –Pt exhibit a background
3
the adsorption ability of zeolites mainly influenced by two param-
eters: the Si/Al ratio and the zeolite structure. Thus, ZSM-5 zeolites
exhibit greater adsorption ability than Zeolite Y at the same Si/Al
ratio. Moreover, an inverse relationship between the Si/Al ratio
and the adsorption ability at dynamic conditions was observed.
Thus, irrespectively of the zeolite structure or the textural prop-
erties (BET area or total pore volume), less hydrophobic and acidic
zeolites tend to have a higher capacity to adsorb both VOC. Regard-
ing Zeolite/WO –Pt hybrid materials, the incorporation of WO –Pt
slightly higher than that of pure TiO2 probably due to the pres-
ence of Pt [36]. The absorption edge for the Zeolite/WO –Pt samples
3
shifts to ca. 450 nm. Assuming that the electronic structure of WO3
allows an indirect transition as TiO2 [37], a plot of the modified
Kubelka–Munk function vs. the energy of the exciting light affords
band gap energy of 2.55–2.60 eV for the Zeolite/WO –Pt photocata-
3
lysts except for Z5/WO –Pt with a band gap of 2.45 eV. These values
3
are consistent with the characteristic values reported for WO3
materials, which range from 2.40 to 2.80 eV [38]. Since all the cata-
3
3
leads to a reduction of the amount of adsorbed organic compounds.
This feature can be attributed to the low adsorption capacity of the
lysts contain the same amount of WO –Pt, ideally the absorbance in
3
the spectra should coincide with each other. The band gap redshift
observed with the Si/Al ratio, and specially relevant when com-
pared Z1 and Z5 based composites, could be related to the presence
of tungsten oxide species of different nature, anchored on the sur-
WO –Pt phase and the lower zeolite content on the final compos-
3
ite. Our results suggest that the textural properties, such as the
BET area and pore volume are not relevant factors to determine
the adsorption ability of the studied molecules in dynamic condi-
tions in contrast to other authors [30]. Takeuchi et al. reported that
face of strong acid sites. The deposit of the WO –Pt on the zeolites
3
leads to APHs capable of adsorbing light within the visible region,
revealing its potential application for degradation of pollutants
under visible irradiation conditions. The nature of tungsten oxide
species were investigated by XPS. Fig. 4 shows the W 4f core-level
region spectra for the zeolitic materials. All W-containing samples
show similar spectra in this B.E. region, with the 4f7/2 peak cen-
tred at ca. 35.0 eV indicative of the presence of oxidized W species,
probably as W6+ [10]. As observed, increasing the Si/Al ratio in the
zeolite shifts the B.E. of the W 4f7/2 peak from 34.8 eV to 35.2 eV.
This shift to higher B.E.s indicates the formation of more oxidized
tungsten species on the zeolites with stronger acid sites, probably
highly hydrophobic H-ZSM-5 with low Al O3 content was effec-
2
tive as an adsorbent for acetaldehyde molecules [31]. The Si/Al
ratio may modify the unit cell size and the void volume for VOC
adsorption. Thus, cell parameters were determined by XRD using
Cu K␣, ꢀ = 1.5406 A˚ refined by the Rietveld method [32], with the
Fullprof refinement program [33]. Z1 and Z2 crystal structures have
been refined using the zeolite cubic space group Fd-3m meanwhile
Z3–Z5 with a MFI zeolite structure with monoclinic P2 /n space
1
group. Z1 and Z2 presented a cell parameter of a = 24.4059(6) A˚
and a = 24.2728(11) A˚ respectively. The results indicate that the cell
parameter decreases with the Si/Al ratio increases. On the other
hand, differences in the cell parameters were not significant for MFI
as Al (WO ) . The shift of the band-gap observed in the UV–Vis
2 4 3
spectra (Fig. 3) could be related to the presence of these species.
According to the literature, the Pt 4f core-level region of metallic
platinum shows a peak at 70.9 eV. This peak overlaps with the very
intense peak of the Al 2 p appearing at ca. 74.0 eV [39]. Due to the
low concentration of Pt in the samples of ca. 0.03 wt.%, (significantly
lower than that of Al) it was not possible detect the presence of Pt
by means of the XPS analysis.
zeolites. For instance, the following cell parameters were obtained
for Z4: a = 19.9379(17) A˚ , b = 20.1940(21) A˚ , c = 13.4108(12) A˚ and
for Z5: a = 19.9236(9) A˚ , b = 20.1512(9) A˚ , c = 13.4041(6) A˚ . For Zeo-
lite Y the Si/Al ratio may influence the VOC adsorption ability of
the composites. Moreover, the difference in the amount of zeolites
between the raw materials and the composites should be con-
sidered in order to explain the lower adsorption ability of AHPs
materials.
Fig. 2 shows the XRD patterns of the samples under study.
Diffractions from the MFI or FAU phases are observed in the XRD
patterns recorded for the APHs prepared with Z1–Z2, or Z3–Z5,
respectively. The crystalline structure of the microporous materi-
Fig. 5 shows representative micrographs of the WO –Pt samples
3
prepared by the photodeposition method (before their incor-
poration onto the zeolites). The images clearly show that the
photodeposition method used in this study results in the forma-
tion of Pt nanoparticles of ca. 5 nm (Fig. 5b and c) [8]. However, a
small fraction of larger Pt particles of ca. 15–20 nm are also observed
(Fig. 5a). The particles are well crystallized and the XEDS exper-
iments clearly confirm that these particles are with no doubt Pt
als is not affected by the incorporation of WO –Pt onto the zeolites.
3
The monoclinic WO3 phase, the most stable tungstate oxide phase
◦
at room temperature, exhibits three characteristic peaks at 23.3 ,
2
(see Fig. 1 Supporting information). The average size of the WO –Pt
3
◦
◦
3.8 and 24.6 assigned to the (0 0 2), (0 2 0) and (2 0 0) planes,
domains is of ca. 160 nm (Fig. 5d). These values are in good agree-
ment with previously reported results for similar systems [40].
respectively [34]. These reflections are observed in the diffrac-
tograms of the Zeolite/WO –Pt samples (JCPDS No. 43-1035) [35]. It
The Zeolite/WO –Pt samples where characterized by SEM
3
3
should be noted that in the diffractograms of the samples contain-
ing the FAU phase, the peak at 23.3 contains contributions from
microscopy in order to obtain information about the morphology of
◦
the different zeolites and of the WO –Pt distribution on the zeolites.
3
both the WO3 and FAU crystalline phases. On the other hand, the
diffraction lines of the MFI phase overlap with the main diffraction
Representative images of Z1/WO –Pt, Z3/WO –Pt and Z5/WO –Pt
are shown in Fig. 6. Differences in the morphology, shape of the
3
3
3
lines of the WO phase. Nevertheless, a carefully observation of the
zeolite substrate and in the dispersion of the WO –Pt particles can
3
3
diffractograms shows an increase in the intensity of the peak at
be observed. White and bright WO –Pt particles on zeolite aggre-
gates of different size, depending of the type of zeolite and its
Si/Al ratio were observed. Evident differences in the morphology
3
◦
◦
2
3.8 and 24.6 due to the presence of the WO monoclinic phase.
3
A slight decrease in the intensity of the diffraction lines character-
istic of the zeolites after addition of 8 wt.% of Pt loaded WO3 was
also observed. Diffraction lines characteristic of Pt fcc phases are
not observed in the diffractograms probably due to the very low
concentration of Pt in the final catalysts.
of the zeolites can be appreciated. For instance, Z1/WO –Pt hybrid
3
is characterized by zeolite aggregates of sizes smaller than 1.0 ␮m
(Fig. 6a). In the case of ZSM-5 zeolite (Z5/WO –Pt) homogeneous
3
particles of ca. 1.0 ␮m can be observed (Fig. 6c). On the contrary,
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
I. Jansson et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
9
Fig. 8. Rate of CO2 generation for WO3 and WO3–Pt photocatalysts suspended in
aqueous acetic acid solution with a mercury lamp. Inset: time course of CO2 evolu-
tion. Operating conditions: VH2 O+CH3 COOH = 5 mL, [CH3COOH] = 5%vol., gcatal = 50 mg.
Fig. 9. Amount of CO2 generated during the degradation of acetaldehyde in
air measured in a batch reactor under UV (blue bars) and Vis (yellow bars)
light. Operating conditions: Reactor volume VR = 114 mL, [CH3CHO] = 1,000 ppm -ca
5 ␮mol, gcatal = 80 mg.
greater aggregates made of zeolite particles with mean size around
4
.0-8.0 ␮m were detected in Z3/WO –Pt (Fig. 6.b). SEM images also
3
show that the WO –Pt domains are better dispersed on the MFI
3
−
recorded with WO , 18 vs. 3 ␮mol h CO , respectively. Therefore,
3 2
1
type zeolites (Fig. 6b and c) than on the FAU (Fig. 6a) with low Si/Al
ratio ones, shown larger aggregates in the latter than in the for-
mer zeolites. Although the freezing-drying technique favored the
physical mixture between components, the acid sites of the zeo-
lites may promote the interaction between both components of
the hybrids, improving the dispersion of the photocatalytic phase
on the siliceous material.
theincorporationofPtnanoparticlesonWO phasehasa strongand
3
positive effect on the photoefficiency of WO3 in line with previous
reports [8,40]. According to photoacoustic spectroscopy measure-
ments, the photoexcited electrons in WO3 are reactive towards
O2 and that the reaction is accelerated by Pt loading. It has also
been proposed that the high activity of WO –Pt is likely due to
3
The samples were also analyzed by TEM. Fig. 7 shows rep-
the promotion of multielectronic O2 reduction with more positive
potential on Pt co-catalysts (Eq. (1)) rather than single electron
reduction (Eq. (2)) [8].
resentative micrographs for the WO –Pt loaded FAU and MFI
3
zeolites. The images corroborated the differences observed in the
morphology of the zeolites substrates and in the dispersion of
the active phase for materials prepared with Z1 (Fig. 7a) and Z5
Once the promotional effect of the addition of Pt to WO3 is
established, the photocatalytic performance of the Zeolite/WO –Pt
3
(
Fig. 7b). Zeolite Z5 presents a spherical shape, in marked con-
hybrids for the degradation of acetaldehyde in gas phase under UV
and Vis irradiation was evaluated. The tests were conducted in a
trast to the morphology observed for Z1 much more irregular with
the presence of some facets, where small crystals of zeolite can be
batch Pyrex reactor after adsorption of a fix amount of CH CHO on
3
identified. The formation of large WO –Pt particles above 300 nm
the material under static conditions. After AcH was totally adsorbed
on the sample under study at dark conditions (no signal detected
in the gas phase) the samples were irradiated. Then, the pho-
tocatalytic reaction started and the time course of CO2 amount
was analyzed. Fig. 9 shows the amount of CO2 detected at the
end of the reaction, when a CO2 constant value was attained, for
3
on Z1/WO –Pt can be observed. In contrast, mean particle sizes of
3
ca. 100 nm where obtained for Z5/WO –Pt and the other hybrid
3
materials analyzed. The ordered and crystalline microstructure of
zeolites Y and ZSM-5 was maintained after the incorporation of
the WO –Pt particles as clearly observed in Fig. 7a and b inset.
3
Finally, micrographs corresponded to WO –Pt particles on Z1 and
the five Zeolite/WO –Pt samples under UV and Vis conditions. All
3
3
Z5 hybrid material are shown in Fig. 7c and 7 d respectively. The
monoclinic WO3 crystal phase can be perfectly identify in both
cases with spherical Pt nanoparticles of around 5 nm on the surface.
Zeolite/WO –Pt samples showed photocatalytic activity under UV
3
irradiation. More importantly, all Zeolite/WO –Pt photocatalysts
3
are visible–light responsive materials. It should be noted that the
non-modified zeolites are inactive for the photodegradation of AcH.
The results indicate that WO –Pt particles are well dispersed over
3
ZSM-5 decreasing the particle size respect bare Pt-loaded WO3.
For photocatalysts based upon the FAU type zeolites, Z1/WO –Pt
3
shows a poor photoactivity. In line with the SEM and TEM results,
the poor performance can be ascribed to the low dispersion of
3.2. Photocatalytic activity study
the WO –Pt particles onto zeolite Z1, forming large aggregates
3
3
.2.1. Photocatalytic degradation of acetic acid and acetaldehyde
of around 300 nm. Moreover, this catalyst also exhibits the low-
est Vis light adsorption in the series. The production of CO2
with the photocatalysts based upon ZSM-5 follows the order:
Z3/WO –Pt > Z4/WO –Pt > Z5/WO –Pt. According to SEM and TEM
in a batch reactor
In an initial stage, the effect of Pt addition to WO3 was analyzed
by comparing the photocatalytic performance WO3 and WO –Pt
3
3
3
3
towards the degradation of acetic acid (CH COOH) in water using
results all ZSM-5 based hybrid APHs show similar WO –Pt disper-
3
3
a high-pressure mercury lamp. Fig. 8 shows the rate of CO2 forma-
sion, containing WO –Pt particle of ca. 100 nm and similar surface
3
2
−1
tion over WO3 and WO –Pt. Fig. 8 inset shows the time courses of
area values of around 350–400 m g . A careful examination of
adsorption data shown in Table 1 indicates a direct relationship
between the CO2 generated and the adsorption ability at dynamic
3
CO generation over both materials. A remarkable increasing in CO
2
2
generation rate is obtained with WO –Pt as compared to the value
3
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
I. Jansson et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
1
0
Fig. 10. Photocatalytic degradation of trichloroethylene in air under Visible irradiation for Zeolite/WO3–Pt hybrids: (
and ( ) COCl2 formation rate. Figure inset: C2HCl3 conversion. Operating conditions: Total flow, FT = 100 mL min [C2HCl3] = 10 ppm, residence time tr = 3.8 s, gcatal = 30 mg.
) Degradation rate of C2HCl3, ( ) CO2 formation rate
−
1
conditions of the APH. Our results indicate the lack of a direct cor-
relation between the adsorption ability and the textural properties
or the hydrophobicity of the zeolites. The final concentration of CO2
was lower than the expected for the complete photodegradation of
AcH (ca. 10 ␮mol) (CH CHO + 5/2O → 2CO + 2H O). Thus, the
for the photodegradation of TCE in a continuous fix bed reactor
with Zeolite/TiO2 hybrids, an opposite trend was observed [21]. In
fact, increasing the hydrophobicity of the zeolite, results in a lower
selectivity towards the formation of non-desirable product such as
COCl2. Considering the differences observed between the results
obtained in a batch or in a continuous reactor, the next step of
our research was to evaluate the photocatalytic performance of the
materials in a continuous fix photoreactor.
3
2
2
2
formation of partially oxidized species such as acetic and formic
acid has been proposed [41–43]. The results obtained in this work
suggest that under static conditions, far from the surface saturation
by the pollutant, the reaction intermediates or by products become
retained in more hydrophobic materials, reducing the amount of
CO2 detected in the gas phase. Brønsted acid sites on the zeolite
surface on ZSM-5 (23, 80) could not work as strong adsorption
sites for acetaldehyde, allowing the diffusion to the active sites
where the photocatalytic reaction take place. In our previous study
3
.2.2. Photocatalytic degradation of VOC in a continuous flow fix
reactor
In order to analyze the photoactivity of the samples under reac-
tion conditions as close as possible to the real application at low
resident times, the photocatalytic performance of the hybrid mate-
Fig. 11. Relationship between rate of CO2 formation in the degradation of acetaldehyde obtained in static and dynamic conditions for: (᭹) Z1/WO3–Pt, (ꢀ) Z2/WO3–Pt, (᭿)
Z3/WO3–Pt, (ᮀ) Z4/WO3–Pt and (♦) Z5/WO3–Pt.
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
I. Jansson et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
11
rials was evaluated at a constant and continuous air flow, using
trichloroethylene (TCE) as a model organochloride pollutant. Pre-
vious to the start of photocatalytic experiment, TCE was adsorbed
under dynamic conditions until the initial TCE inlet concentration
at by-pass mode was achieved. Fig. 10 shows the results obtained
the SiO /Al O ratio, then CO rate in a static system divided by the
2 2 3 2
(1/(Si O /Al O )) ratio and BET area may be compared with rate in
2
3
2
3
a flow system. In the later, the surface adsorption sites are saturated
during the course of reaction. As a consequence, the migration of
the pollutants to the active sites could be hindered in the zeolites
with the higher adsorption ability (Z3 > Z4 > Z5) at low residence
time. Considering these arguments, the CO2 release in the static
system (Fig. 2 Supporting information) should be compared with
rate in the flow system (Fig. 9) under UV irradiation using a F(Static)
function calculated as indicate in Eq. (5):
−
1
for the degradation of TCE at constant air flow of 100 mL min
with a 10 ppm inlet concentration of TCE using visiblelight. The
steady-state rates of TCE consumption and CO2 and COCl2 produc-
tion normalized to the mass of catalyst are shown in Fig. 10. As
observed, all samples present photoactivity under Vis irradiation
conditions; however, the TCE degradation rate of WO –Pt increases
3
SiO /Al O
3
BET area
2
2
significantly when deposited on the zeolites. Although WO –Pt
F (Static) = CO2 rate static ·
(5)
3
adsorb strongly in the Vis light region, the low surface area and the
poor AcH adsorption ability (Table 1) leads to poor photocatalytic
efficiency, as reported elsewhere [44]. Irrespectively of the zeolite
nature (FAU or MFI), it can be observed that the photocatalytic per-
formance of the final catalyst increases with the increasing Si/Al
Fig. 11 shows a representation of the F(Static) vs. the rate of
CO formation under dynamic conditions for the five zeolites stud-
ied. As observed, a linear relationship was attained for the different
zeolites. Thus, a correlation between the data obtained in static
and dynamic conditions should take into account these parame-
2
ratio in the zeolite. Z1/WO –Pt showed the lowest photocatalytic
3
ters. For instance, the ratio between SiO /Al O and BET area for
Z5/WO –Pt is of one order of magnitude higher than for Z3/WO –Pt
activity amongst the zeolite based APHs. Z5/WO –Pt records the
2
2
3
3
highest rate for C HCl degradation in the series. An increase of ca.
3
3
2
3
−
2
−1
−1
(0.767 vs. 0.067 g m ) resulting the most efficient photocatalyst
at dynamic conditions. The results clearly indicate the importance
of these factors in the interpretation of the results when experi-
ments at static (bath reactor with t∞ residence time) or dynamic
conditions (continuous flow reactor, short residence time) are per-
formed, explaining the dependency observed by others authors
5
␮mol min
g
compared to bare WO –Pt was obtained for
catal.
3
this composite. The inset to Fig. 10 shows the C HCl conversion
2
3
with all photocatalysts tested in this work. As observed, a TCE con-
version of around 30% is obtained with Z5/WO –Pt. Similar results,
3
with a TCE conversion of ca. 20% with Z5/WO –Pt were obtained by
3
using a TCE inlet concentration of 25 ppm. A further set of exper-
iments were performed by using AcH as the VOC molecule under
similar operating conditions (see Fig. 2 Supporting information).
[
31].
The results obtained in this work, indicate that WO –Pt phase is
3
on the external surface of the microporous material while pollutant
adsorption may take place in the internal and/or external surface
of the zeolite. The photocatalytic results show that the hybridation
The trend of the rate of CO production from AcH was the same than
2
those obtained using TCE. These results are promising since they
showed a good photoactivity of the samples at shorter residence
times (3.8 s) for the aldehyde and the organochloride compound.
Hitherto, most of the results with visible-light responsive photo-
catalysts reported in literature are obtained with batch reactor at
static conditions, i.e., at long contact times [9–11,22,42,45,46].
These results are in line with the results previously obtained
with Zeolite/TiO2 composites for the degradation of TCE under
similar reaction conditions [21]. In fact, it was proposed that the
zeolites with the MFI structure and with high Si/Al ratios are suit-
able to reduce the formation of non-desirable reaction products. It
should be noted that the activity data obtained under VOC flow and
under static conditions show opposite trends with respect to the
Si/Al ratio for ZSM-5. The results indicate an important influence of
the operating conditions in the behavior of the materials.
of the zeolite with the WO –Pt phase results in an enhancement of
3
the photocatalytic activity and CO release. Torimoto et al [16]. sug-
2
gested that adsorbed species can migrate to the active site where
the photocatalytic reaction takes place. Our previous studies with
SiMgOx/TiO2 composites, support the hypothesis of the key role of
OH-rich materials, are a source of OH- surface groups, keeping the
semiconductor active sites free from reaction intermediates when
both materials are in contact [12]. Thus, silicious materials acts as
a buffer of VOC, because of its adsorbent properties, keeping con-
stant VOC concentration in the vicinity of TiO2 active sites, and
the silicate structure allows the diffusion of non-desirable reac-
tion products with the subsequent reaction with OH groups of
the silicate, improving the CO2 release.
Oxidizing species photogenerated on titanium dioxide well-
defined microdomains, are capable of inducing the mineralization
of organic compounds anchored to inert materials [49]. Out- dif-
fusion of oxidizing species on the surface may induce a remote
photodegradation process. Siliceous microporous materials can
stabilize reactive intermediates in several photochemical reactions.
Reactive species such as OH or O2 , which are formed during the
photocatalytic process, may be stabilized with in the rigid frame-
work of the zeolite. Then, the adsorbed organic molecules can then
be efficiently reached by these reactive species. Moreover, oxygen
adsorption in the zeolite is essential to the electron accepting pro-
cess carry out on the photoactive phase. In line with our previous
results obtained for magnesium silicate, zeolite domains retain a
It is well known that hydrophobicity of zeolites increases with
the increasing SiO /Al O ratio in their structure. The adsorption
2
2
3
efficiencies of zeolites are largely affected by their size, charge
density and chemical composition of the cations in their porous
structures [47]. The porous structure of the zeolites is also a selec-
tive factor for the absorbed molecules. Taking into account the
adsorption values for both AcH and TCE, steric factors seems do
not play a significant role in the catalytic performance of the APHs
•
−
[
48]. According to our adsorption results, zeolites with low Si/Al
ratio and then higher number of acid sites, especially those with
the ZSM-5 structure, present higher adsorption ability for both pol-
lutants (TCE and AcH). In the static system, at conditions far from
the saturation of the surface by the pollutant, the kinetic might
be controlled by the diffusion of the VOC to the active site, and
therefore the photocatalytic activity may be dependent of the sur-
face area. Then, CO2 rate in the static system divided by surface
area should be compared with the rate in the flow system. More-
constant VOC concentration in the vicinity of WO –Pt active sites,
3
allowing the diffusion of non-desirable reaction products when
formed, and probably with the subsequent reaction with reactive
•
−
.
species such as OH or O2
over, the CO release is promoted on materials with high number of
2
active sites but less strong. The reaction intermediates seems to be
weakly adsorbed on less hydrophobic zeolites and as a result they
can easily reach the active sites thus promoting the release of CO2.
Assuming that the number of acid sites is inversely proportional to
4. Conclusions
The present results indicate that immobilization of WO –Pt
on zeolites (both FAU or MFI structure) is an effective strategy
3
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015

G Model
APCATA-15688; No. of Pages12
ARTICLE IN PRESS
I. Jansson et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
1
2
for the development of efficient visible light responsive photo-
catalysts. These hybrids materials showed photocatalytic activity
for acetaldehyde and trichloroethylene removal under UV and Vis
[9] J. Kim, C.W. Lee, W. Choi, Environ. Sci. Technol 44 (2010) 6849–6854.
10] M. Qamar, M.A. Gondal, Z.H. Yamani, Catal. Commun. 11 (2010) 768–772.
11] Y. Shiraishi, Y. Sugano, S. Ichikawa, T. Hiraia, Catal. Sci. Technol. 2 (2012)
[
[
400–405.
light. Hybridation of the WO –Pt with ordered microporous mate-
rials results in the promotion of the photocatalytic activity of bare
[12] S. Suárez, J.M. Coronado, R. Portela, J.C. Martín, M. Yates, P. Ávila, B. Sánchez,
Environ. Sci. Technol. 42 (2008) 5892–5896.
3
[
13] N. Takeda, T. Torimoto, S. Sampath, S. Kuwabata, H. Yoneyama, J. Phys. Chem.
C 99 (1995) 9986–9991.
WO /Pt. An inverse relationship of the VOC adsorption ability with
3
the SiO /Al O ratio was obtained independently of the zeolite
[14] H. Yoneyama, T. Torimoto, Catal. Today 58 (2000) 133–140.
[15] S. Suárez, T.L.R. Hewer, R. Portela, M.D. Hernandez-Alonso, R.S. Freire, B.
Sanchez, Appl. Catal. B 101 (2011) 176–182.
2
2
3
structure or the textural properties. Both VOCs studied are prefer-
entially adsorbed zeolites with higher aluminum content. The type
of the photocatalytic reactor (batch or continuous air flow) play
an important role in the results obtained. For ZSM-5, the rate of
[
16] T. Torimoto, S.S. Ito, S. Kuwabata, H. Yoneyama, Environ. Sci. Technol. 30
(1996) 1275–1281.
[17] T. Torimoto, Y. Okawa, N. Takeda, H. Yoneyama, J. Photochem. Photobiol. A
03 (1997) 153–157.
1
CO formation at static conditions correlate with adsorption ability,
2
[
[
18] N. Takeda, N. Iwata, T. Torimoto, H. Yoneyama, J. Catal. 177 (1998) 240–246.
19] A.D. Paola, E. García-López, G. Marcì, L. Palmisano, J. Hazard. Mater. 211–212
(2012) 3–29.
indicating that reaction intermediates or subproducts are retained
in more hydrophobic zeolites with stronger acid sites, limiting the
CO2 release to the gas phase. At the dynamic conditions of a con-
tinuous flow photoreactor, when the surface adsorption sites are
saturated by the pollutant, the VOC conversion and the amount
of CO2 released is promoted by more hydrophobic and less acidic
[
[
[
20] D. Kibanova, J. Cervini-Silva, H. Destaillats, Environ. Sci. Technol. 43 (2009)
1500–1506.
21] I. Jansson, S. Suárez, F.J. Garcia-Garcia, B. Sánchez, Appl. Catal. B 178 (2015)
100–107.
22] M. Takeuchi, H. Yamagawa, M. Matsuoka, M. Anpo, Res. Chem. Intermed. 31
(
2014) 23–40.
zeolites. The ratio between SiO /Al O and the BET area lead us to
2
2
3
[23] S.L. Nail, S. Jiang, S. Chongprasert, S.A. Knopp, Pharm. Biotechnol. 14 (2002)
281–360.
correlate the results obtained under static and dynamic conditions.
The hybrid system consisting of ZSM-5 with high SiO2 content and
[
24] T.M. Eggenhuisen, P. Munnik, H. Talsma, P.E. de Jongh, K.P. de Jong, J. Catal.
97 (2013) 306–313.
25] T. Vergunst, F. Kapteijn, J.A. Moulijn, Appl. Catal. A 213 (2001) 179–187.
2
WO –Pt phase, exhibited the best photocatalytic performance with
3
[
a 30% trichloroethylene conversion under visible light irradiation at
conditions of low residence time in a continuous flow photoreactor.
[26] X. Du, E. Wu, J. Phys. Chem. Solids 68 (2007) 1692–1699.
[27] C.D. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.H. Raymond, L.H. Gale, Suf.
Interface Anal. 3 (1981) 211–225.
[
[
[
28] G. Leofanti, M. Padovan, G. Tozzola, B. Venturelli, Catal. Today 41 (1998)
Acknowledgments
207–219.
29] K.D. Dubois, A. Petushkov, E. Garcia Cardona, S.C. Larsen, G. Li, J. Phys. Chem.
Lett. 3 (2011) 486–492.
30] M. Takeuchi, M. Hidaka, M. Anpo, J. Hazard. Mater. 237-238 (2012) 133–139.
The authors want to thank the Spanish Ministry of Economy
and Competitiveness for the financial support (project CTM 2011-
[31] M. Takeuchi, T. Kimura, M. Hidaka, D. Rakhmawaty, M. Anpo, J. Catal. 246
(2007) 235–240.
2
5093), Mss. Jansson for the Ph.D. grant (BES-2012-055758 and
[
[
[
32] H. Rietveld, J. Appl. Crystallogr. 2 (1969) 65–71.
33] J. Rodríguez-Carvajal, Physica B 192 (1993) 55–69.
34] P.M. Woodward, A.W. Sleight, J. Solid State Chem. 131 (1997) 9–17.
EEBB-I-1408837) and Dr. Suárez for the Ramón & Cajal Contract.
The Research Support Unit from the ICP-CSIC is acknowledged for
XRD and BET analysis.
[35] I.M. Szilágyi, J. Madarász, G. Pokol, P. Király, G. Tárkányi, S. Saukko, J. Mizsei,
A.L. Tóth, A. Szabó, K. Varga-Josepovits, Chem. Mater. 20 (2008) 4116–4125.
[36] S. Kim, S.J. Hwang, W. Choi, J. Phys. Chem. B 109 (2005) 24260–24267.
[37] A. Nakamura, S. Yamada, Appl. Phys. A 24 (1981) 55–59.
[38] C.G. Granqvist, J. Solar Energy Mater. Solar Cells 60 (2000) 201–262.
[39] L. Nie, A. Meng, J. Yu, M. Jaroniec, Sci. Rep. 3 (2013) 3215.
[40] M. Sadakane, K. Sasaki, H. Kunioku, B. Ohtani, W. Ueda, R. Abe, Chem.
Commun. (2008) 6552–6554.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2015.12.
015.
[41] I. Sopyan, M. Watanabe, S. Murasawa, K. Hashimoto, J. Photochem. Photobiol.
A 98 (1996) 79–86.
[
42] A. Murata, N. Oka, S. Nakamura, Y. Shigesato, J. Nanosci. Nanotechnol. 12
2012) 1–5.
References
(
[
43] T. Arai, M. Yanagida, Y. Konishi, Y. Iwasaki, H. Sugihara, K. Sayama, J. Phys.
Chem. C 111 (2007) 7574–7577.
[
[
1] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271.
2] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M.
Dunlop, J.W.J. Hamilton, J.A. Byrne, K. O’shea, M.H. Entezari, D.D. Dionysiou,
D.M. Wiles, Appl. Catal. B 125 (2012) 331–349.
[44] T. Arai, M. Horiguchi, M. Yanagida, T. Gunji, H. Sugihara, K. Sayama, Chem.
Commun. (2008) 5565–5567.
[45] M. Qamar, Z.H. Yamani, M.A. Gondal, K. Alhooshani, Solid State Sci. 13 (2011)
1748–1754.
[
3] M.D. Hernandez-Alonso, F. Fresno, S. Suarez, J.M. Coronado, Energy Environ.
Sci (2009) 1231–1257.
[46] Z. Wen, W. Wu, Z. Liu, H. Zhang, J. Li, J. Chen, Phys.Chem.Chem.Phys. 15 (2013)
6773–6778.
[47] Q. Wang, J. Luo, Z. Zhong, A. Borgna, Energy Environ. Sci. 4 (2011) 42–55.
[48] D. Bonenfant, M. Kharoune, P. Niquette, M. Mimeault, R. Hausler, Sci. Technol.
Adv. Mater. 9 (2008) 2–7.
[
[
4] O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33–177.
5] Eliminación de contaminantes por fotocatálisis heterogénea, in: M.A. Blesa, B.
Sánchez (Eds.), Ciemat Ed., Madrid, 2004, ISBN: 84-7834-489-6.
6] F. Fresno, R. Portela, S. Suarez, J.M. Coronado, J. Mater. Chem. A 2 (2014)
[
2
863–2884.
[49] H. Haick, Y. Paz, J. Phys. Chem. B 105 (2001) 3045–3051.
[50] T. Tomita, R. Abe, B. Ohtani, Chem. Lett. 40 (2011) 1405–1407.
[
[
7] G.R. Bamwenda, H. Arakawa, Appl. Catal. A 210 (2001) 181–191.
8] R. Abe, H. Takami, K. Murakami, B. Ohtani, J. Am. Chem. Soc. 130 (2008)
7
780–7781.
Please cite this article in press as: I. Jansson, et al., Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata.2015.12.015