Decahedral anatase titania particles immobilized on zeolitic materials for photocatalytic degradation of VOC

Article abstract of DOI:10.1016/j.cattod.2016.11.041

Decahedral anatase titania particles (DAPs) immobilized on zeolites (zeolite/DAP hybrids), have been prepared by the first time. DAPs have been synthesized from titanium chloride (TiCl₄) through a gas-phase reaction process by rapid heating and quenching. DAPs with well-defined {101} and {001} facets were synthetized and dispersed onto the zeolites. The incorporation of an 8wt% of DAPs into the zeolites has been achieved by means of the freezing-drying technique. Two types of zeolites (ZSM-5 and zeolite Y) with different SiO₂/Al₂O₃ contents have been selected as adsorbents. The photocatalytic properties for the degradation of representative volatile organic compounds (VOC), i.e. acetaldehyde, formaldehyde or trichloroethylene were analyzed under batch and continuous flow reactors. Raw materials and composites were characterized by SEM, N₂ adsorption-desorption, UV–vis spectroscopy, XRD, zeta potential and the amount of VOC adsorbed was measured at dynamic conditions. Zeolite/DAP hybrids combine the textural and crystalline properties of zeolite and DAPs phases. The zeolite type (crystal structure, SiO₂/Al₂O₃ ratio and surface net charge) play a relevant role in the dispersion of DAPs. Zeolite/DAP composites show better photocatalytic performance than those prepared with commercial TiO₂ due to the presence of {001} facets in the former_. The composites prepared with hydrophobic ZSM-5 and DAPs, are the most versatile materials for photooxidation of aldehydes and organochloride compounds, with substantially less formation of non-desirable reaction products respect to those based on benchmark TiO₂.

Full text of DOI:10.1016/j.cattod.2016.11.041

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Contents lists available at ScienceDirect
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Decahedral anatase titania particles immobilized on zeolitic materials
for photocatalytic degradation of VOC
a
b
b
a
b
a,∗
I. Jansson , K. Kobayashi , H. Hori , B. Sánchez , B. Ohtani , S. Suárez
a
CIEMAT, Renewable Energy Division, FOTOAIR: Group of Analysis and Photocatalytic Treatment of Pollutants in Air, Avda. Complutense, 40, 28040,
Madrid, Spain
Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2016
Received in revised form 6 November 2016
Accepted 21 November 2016
Available online xxx
Decahedral anatase titania particles (DAPs) immobilized on zeolites (zeolite/DAP hybrids), have been
prepared by the first time. DAPs have been synthesized from titanium chloride (TiCl4) through a gas-
phase reaction process by rapid heating and quenching. DAPs with well-defined {101} and {001} facets
were synthetized and dispersed onto the zeolites. The incorporation of an 8wt% of DAPs into the zeolites
has been achieved by means of the freezing-drying technique. Two types of zeolites (ZSM-5 and zeolite
Y) with different SiO2/Al2O3 contents have been selected as adsorbents. The photocatalytic properties for
the degradation of representative volatile organic compounds (VOC), i.e. acetaldehyde, formaldehyde or
trichloroethylene were analyzed under batch and continuous flow reactors. Raw materials and compos-
ites were characterized by SEM, N2 adsorption-desorption, UV–vis spectroscopy, XRD, zeta potential and
the amount of VOC adsorbed was measured at dynamic conditions. Zeolite/DAP hybrids combine the tex-
tural and crystalline properties of zeolite and DAPs phases. The zeolite type (crystal structure, SiO2/Al2O3
ratio and surface net charge) play a relevant role in the dispersion of DAPs. Zeolite/DAP composites show
better photocatalytic performance than those prepared with commercial TiO2 due to the presence of
Keywords:
Photocatalysis
Decahedral anatase particles
Zeolites
Adsorbent–Photocatalyst Hybrids
Air treatment
VOC
{
001} facets in the former. The composites prepared with hydrophobic ZSM-5 and DAPs, are the most
versatile materials for photooxidation of aldehydes and organochloride compounds, with substantially
less formation of non-desirable reaction products respect to those based on benchmark TiO2.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
TiO2 particles with a low density of defects is a promising strat-
egy to improve the photocatalytic performance [5]. It is reported
in the literature that TiO2 {001} facets are more reactive towards
the dissociative adsorption of reactant molecules than {101} ones
[6,7]. Decahedral anatase titania particles (DAPs) exhibit a high
fraction of {101} facets along with two additional square {001}
facets. As a result, DAPs show a great potential as photocatalysts for
the degradation of organic pollutants [5,8]. The preparation of DAPs
can be performed by hydrothermal reactions using fluorine species
as shape-control reagents [9–11]. The drawback of this method,
however, is the strong adsorption of the shape-control reagents
at the surface of the synthesized particles which severely affects
the photodegradation efficiency [12]. To synthesize anatase TiO2
crystals with {001} facets without using fluorine species, the crys-
tal growth should to be confined within the kinetically controlled
regime under non-equilibrium conditions. These conditions can be
achieved by conducting the synthesis at high-temperature using
rapid heating and quenching processes in gas-phase reaction [13].
Adsorbent-photocatalyst hybrid materials (APHs), are bifunc-
tional materials based on the combination of an adsorbent and a
Heterogeneous photocatalysis is one of the most promising
technologies for air decontamination [1]. TiO2 is the main semi-
conductor component used as photocatalyst because is a non-toxic,
abundant and inexpensive material endowed with high photocat-
alytic efficiency and chemical stability [2]. Despite these properties,
TiO2 still presents limitations affecting its photocatalytic perfor-
mance. On one hand, it is only active under UV light, and therefore
it can only use about a 5% of the total solar radiation spectrum
[
3]. On the other hand, when the activation of the photocatalyst
occurs, a high fraction of the electrons promoted from the valence
band to the conduction band become recombined, reducing the
photocatalytic efficiency greatly [4]. In this regard, it has been
reported that the recombination process occurs at grain bound-
aries and crystalline defects. Therefore, the synthesis of crystalline
∗ Corresponding author.
E-mail address: silvia.suarez@ciemat.es (S. Suárez).
http://dx.doi.org/10.1016/j.cattod.2016.11.041
0
920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: I. Jansson, et al., Decahedral anatase titania particles immobilized on zeolitic materials for photocatalytic
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(75 mL min 1, 180 C) was introduced as the carrier gas for TiCl
−
◦
semiconductor. These materials have demonstrated enhanced pho-
tocatalytic properties for the degradation of pollutants in air [14].
By combining the individual properties of both materials, a syn-
ergetic effect is developed during the photocatalysis with AHPs
4
−
1
vapor. Then, Ar/TiCl4 gas and oxygen (800 mL min ) preheated to
a fixed temperature, were mixed in the reaction zone of the quartz
reactor heated and the oxidation reaction takes place, according to
Eq. (1).
[
15,16]. Thus, the adsorbent captures the pollutants and facili-
tate their transport to the interface with the semiconductor phase.
The appropriate combination of the properties of both materi-
als allows an improvement in the photocatalytic performance. An
increase in the reaction rate along with a decrease of the forma-
tion of secondary products, i.e., higher degree of mineralization,
has been reported in the literature [14]. When the semiconductor
is combined with the adsorbent, an improvement in the textu-
ral properties of the final material, this is, higher BET area and
pore volume is usually reported [17–19]. Moreover, depending
on the physico-chemical properties of the adsorbent, the promo-
tion of the electron-hole separation, the incorporation of hidroxyl
species, the control of the deactivation processes or the absorp-
tion of visible light, can be promoted. Finally, adsorbents can also
act as binders providing optimal rheological properties, allowing
the formation of shaped materials and facilitating the separation
of the photocatalyst from the fluid [19]. Zeolites are microporous
aluminosilicates-based materials commonly used as commercial
adsorbents and catalysts. These aluminosilicates are characterized
by high crystallinity, large surface area, high adsorption capacity,
acidity and shape selectivity [20–22]. These properties place zeo-
lites as ideal adsorbents to improve the photoefficiency of TiO2 for
the degradation of pollutants.
TiCl + O2 → TiO + 2Cl2
The part of the reactor heated was wrapped with a platinum foil
into an infrared furnace at 1,200 C. Once the reaction was finished,
(1)
4
2
◦
the reactor was rapidly quenched. Then, the DAPs synthetized were
collected by filtration and washed [8]. Finally, water was removed
by freeze-drying under vacuum for 24 h (EYELA Freeze Dryer FDU-
◦
2100, P < 10 Pa, T = –80 C).
2.2. Preparation of zeolite/TiO2 hybrids
Five types of commercial zeolites from Zeolyst, two zeolites Y
with the FAU structure and SiO /Al O ratio 5–80 (CBV 600 and
CBV 780) and three ZSM-5 with the MFI structure and SiO /Al O
ratio 23–280 (CBV 2314, CBV 8014 and CBV 28014), were selected
as adsorbents.
2
2
3
2
2
3
The incorporation of TiO -DAPs onto the zeolite was performed
2
by means of the freezing-drying technique. 160 mg of DAP were
added to a suspension of 2 g of zeolite in water and the mixture
was stirring for 2 h. Next, the suspension was frozen with liquid N2
and water was removed by freezing-drying under the same con-
ditions as described in the previous section. In agreement with
previous studies conducted with zeolite/TiO2 hybrids wherein the
optimum mass of zeolite and titanium was evaluated for the max-
imum photocatalytic performance, the amount of TiO2 was set at
8wt%. [18].
The photocatalytic properties of zeolites/TiO hybrids have been
2
already published in the literature [23–26]. Also, studies related
to the synthesis and features of DAPs are numerous [8,12,13,27].
Nevertheless, to our knowledge, the effect of the addition of small
amounts of decahedral anatase particles to zeolitic structures has
not been reported hitherto. Moreover, most of the photocatalytic
results reported in the literature concerning the photocatalytic per-
formance of DAPs are obtained in batch reactor where a fix amount
of the pollutant is introduced in the reactor and forced to react
with the photocatalyst [12,13]. Long residence times are generally
required for the complete removal of the pollutant. However, these
reaction conditions are far from real applications, where the con-
tact time between the pollutant and the photocatalyst active sites
issort and treatment of a high gas volume is required.
In this study, decahedral anatase titania particles were pre-
pared by a gas-phase process using TiCl4 as a precursor and
immobilized on zeolites for the abatement of VOC in gas phase.
These hybrids, hence forward referred to as zeolite/DAP hybrids
were prepared by the freezing drying technique. Furthermore,
zeolite/TiO2 composites based on commercial TiO2 were prepared
for comparison purposes. The photocatalytic properties of the
synthesized materials for the degradation of aldehydes (acetalde-
hyde and formaldehyde), common pollutants of indoor air and an
organochloride compound such as trichloroethylene, were evalu-
ated in bath and in a continuous flow reactor at low contact time.
The influence of the adsorption ability, the textural properties, the
nature of the VOC molecule and the zeolite type (zeolite Y vs.
ZSM-5) in the physico-chemical and photocatalytic properties was
investigated.
Commercial photocatalyst TiO2 (G5, Millenium/Cristal) was
used as a reference sample. The hybrids materials were prepared
by mechanical mixing of the commercial titania and the zeolites in
nitric acid. Then, the samples were extruded and treated thermally
◦
at 500 C for 3 h.
2.3. Characterization techniques
Brunauer-Emmett-Teller (BET) specific surface area and pore
volume of the samples was determined from nitrogen adsorption
◦
at −196 C in a Micrometrics Asap 2020 Analyser. Samples were
◦
2
outgassed overnight at 120 C and pressure < 1.66 10 Pa. Micro-
pore volume and sizes were estimated by N2 adsorption at low
partial pressure range (P/P = 0-0.1) using the Horvath-Kawazoe
0
(H-K) method and the Saito and Foley approximation for cylindri-
cal pore geometry. Crystalline and non-crystalline composition of
samples was analyzed by powder X-ray diffraction (PXRD) using
a Rigaku SmartLab diffractometer with CuK␣ radiation operat-
ing at 40 kV and 30 mA. NiO (0.050 g, Wako Pure Chemical) was
selected as an internal standard and mixed with 0.200 g of sample.
◦
Bragg’s angle between 10 and 80 were recorded at scanning rate
◦
−1
◦
of 1.0 min and step of 0.008 . The acquired diffractograms were
analyzed using the software installed in the controlling personal
computer (PDXL including a RIETAN-FP Rietveld analysis pack-
age). On the assumption that NiO is 100% crystalline, i.e., without
non-crystalline part, crystalline anatase and rutile contents were
calculated based on the results of Rietveld analysis [28]. Zeta poten-
tial measurements were performed with a Zetasizer Nano ZS90
with a MPT-2 autotitrator. Experiments were carried out using
2
. Experimental
−
3
2
.1. Synthesis of decahedral anatase titania particles
Decahedral anatase titania particles (DAPs) were prepared by a
15 mg of 1 ␮m powder samples suspended in 100 mL of 10
M
KCl solution, adjusting the pH value with 0.2 M and 0.02 M KOH
and HCl solutions. Each curve was recorded at least three times to
ensure reproducibility. The Diffuse reflectance spectra of the solids
were obtained using a UV–vis/DRS technique on a JASCO V-670
spectrophotometer equipped with a PIN-757 integrating sphere
gas-phase process in a quartz tubular reactor using titanium(IV)
−
1
chloride (TiCl ) as the TiO2 precursor. Liquid TiCl4 (2.5 mL h
4
during 3 h) was conducted into a vaporizer vessel where argon
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2
−1
accessory for diffuse reflectance spectra acquisition. The reflectance
data was converted with the Kubelka-Munk equation to absorp-
tion coefficient F(R∞) and the band gap energy was evaluated by
the Tauc plot [29]. The morphology of DAP and the dispersion of
the DAP on the zeolites were investigated by scanning electron
microscopy JEOL JSM-7400F at 5 kV.
between 600 and 800 m g with pore size of 0.75 nm. On the other
hand, ZSM-5 showed lower BET area than zeolite Y, ranging around
2
−1
300–400 m g and 0.5 nm mean pore size. The zeolites exhibit the
typical profile for type I isotherms, characteristic of microporous
materials with a hysteresis H4 indicative of slit-shaped pores, in
accordance with the IUPAC classification [32]. These data fits with
those found in the literature, for similar FAU and MFI zeolite struc-
tures [33]. DAPs present lower surface area than that of commercial
2.4. Photocatalytic activity tests and adsorption ability
TiO , with mesoporosity mainly generated due to interparticular
2
separation. The specific surface area of the TiO -DAPs can be con-
2
The photocatalytic activity of the DAPs and the APHs for the
trolled by varying the TiCl concentration in the feed gas during the
4
degradation of three VOC molecules, namely acetaldehyde (AcH),
formaldehyde (FMD) or trichloroethylene (TCE) was measured in
air under UV-A irradiation conditions.
synthesis step. In a previous study, Sughisita et al., reported that the
photocatalytic activity of TiO -DAPs varies with the surface area
2
2
−1
being maximum for the catalyst with a BET area of 12 m g [13].
As a consequence, we have synthesized similar particles following
that synthetic approach for this study. Taking into account that the
zeolite is the main component (92wt%) of the zeolite/DAP compos-
ites, the textural properties of zeolites determine the properties
of the composites. The textural properties of the hybrids prepared
The photodegradation of acetaldehyde was evaluated in a batch
reactor consisting of a cylindrical Pyrex glass vessel sealed with a
septum (114 mL, 120 mm length and 35 mm diameter). The materi-
als (80 mg) were supported on a borosilicate glass roughened slide.
Previous the photocatalytic test, samples were pretreated with UV-
A light during 12 h. Gaseous acetaldehyde (4.67 ␮mol) was injected
into the chamber filled with ambient air. The AcH concentration in
the reactor was of 1,000 ppm. Once acetaldehyde was completely
adsorbed on the material at dark conditions (ca. 240 min), the sam-
ples were irradiated with UV black light (fluorescent GE/Hitachi
with commercial TiO are similar to the ones of the zeolite/DAPs but
2
with a slight large BET area and porosity values due to the inherent
properties of TiO -G5.
2
The adsorption ability of the raw materials and the hybrids
towards different VOCs molecules was studied under dynamic
conditions (Table 1). Commercial zeolites exhibit high adsorp-
−
1
FL10BL-B 10W), 1 mW cm irradiance for 60 min. The evolution of
the amount of AcH reacted and CO2 produced in the reaction were
measured by GC-FID (Shimadzu GC-14B) using a Phenomenex ZB-
WAX column in the former and a GC equipped with a methanizer
and a Porapak-Q column in the later [30].
tion capacity for CH CHO and HCHO. However, the organochloride
3
adsorption ability is lower as compared to the aldehydes. The
zeolites with unbalanced sites or electrostatically polarised are
especially relevant for the adsorption of polar molecules, due the
formation of strong adsorption sites [34,35]. The adsorption of
the three model molecules under study on the zeolite/DAP com-
posites, independently of the nature of the zeolite (MFI or FAU
The photocatalytic performance for the oxidation of FMD and
TCE was studied in a continuous flow reactor. The flat photore-
actor was made of stainless steel (external dimensions: 120 mm
x 50 mm x 10 mm) except for one size where a borosilicate glass
structure) follows the same pattern: the higher the SiO /Al O
2
2
2
3
window (37 cm ) was placed. The samples (30 mg) were immobi-
ratio, the lower the adsorption ability. The presence of Al-OH, Si-O-
Al or Al-O-Al surface groups promotes the adsorption of the organic
lized on a borosilicate glass slide and place inside the reactor. The
materials 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 surface. A gas mixture of TCE or FMD
with air was prepared using gas cylinder of C HCl /N (Air Liquide,
compounds. It is well stablished that the SiO /Al O ratio defines
2
2
3
the hydrophilic-hydrophobic character of zeolites and their sorp-
tive properties [25,36,37]. In spite of the lower surface area of the
2
3
2
2
−1
DAPs (14 m g ), the results show an unexpected high adsorption
ability for aldehydes, especially acetaldehyde. In the case of TiO2,
the amount of aldehyde adsorbed is somehow related to the sur-
face area of the materials. Noguchi et al. found differences between
the decomposition rates of formaldehyde and acetaldehyde and
ascribed them to differences in their adsorption strengths [38].
On the other hand, the ability of TiO2 to adsorb TCE is low and
this molecule tends to become adsorbed on microporous materials
especially in MFI structures, with 0.5 nm mean pore size. Finally,
the mixture with an 8wt% DAPs, combine the individual properties
of the raw materials, resulting in a diminution of the BET area of
the composites, the blocking of a small portion of the micropores
and a modification of the amount of VOC adsorbed.
7
0 ppm) or HCHO/N2 (50 ppm) and compressed air free of water
and CO . The flow rate was controlled by using electronic mass flow
2
controllers. The photocatalytic tests were performed using two UV
lamps (Philips TL8 Actinic BL) with a maximum emission at 365 nm.
−
2
The irradiance at the photocatalyst surface was 6.5 mW cm . The
photocatalytic performance was evaluated varying the total air gas
−
1
flow between 500 and 900 mL min (residence time 0.77–0.43 s)
with a FMD or TCE concentration of 15 ppm and 25 ppm, respec-
tively. The gas-phase composition was continuously monitored by
a FTIR spectrometer (Thermo Nicolet 57000) equipped with a tem-
perature controlled multiple reflection gas cell optical path 2 m
[
31]. The adsorption ability of the zeolite/DAP samples was stud-
ied at dynamic conditions in same experimental system using for
the photodegradation of FMD and TCE as described elsewhere. The
experiment at the batch conditions were repeated seven times to
ensure reproducibility of the results. On the other hand, due to the
stability of the operating conditions at the continuous flow reactor,
experiments were repeated two times.
The DAPs and zeolite/DAP samples were analyzed by scanning
electron microscopy. Fig. 1a shows SEM images of DAPs with a
mean particle size around 100 nm. As observed in the micrograph,
pure decahedral particles are the most abundant particles (marked
with a green circle). However, particles exhibiting decahedral shape
but with noticeable defects on the facets, (e.g., mounds in the {001}
facets), aggregates and broken decahedral particles and others par-
ticles with a shape different from decahedral geometry (regular or
irregular) can be also observed as previously reported [8]. Particles
with mounds or deposits on the {001} facets were detected (see
Fig. 1a inset). The appearance of this type of morphology depends
on the synthesis conditions. Shorter reaction time at lower tem-
peratures with concentrated titania favors the formation of mound
in the {001} facets. The formation of new crystals on the surfaces
3
. Result and discussion
3
.1. Characterization of zeolite/TiO -DAPs samples
2
Table 1 collates the main textural properties of the commercial
TiO , DAPs, zeolites and the zeolite/DAP hybrids. The pore diame-
2
ter, pore volume and specific surface area were determined using
N2 adsorption-desorption isotherms. Zeolites Y present a BET area
Please cite this article in press as: I. Jansson, et al., Decahedral anatase titania particles immobilized on zeolitic materials for photocatalytic
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Table 1
Properties of raw materials and zeolite/DAP hybrids at room temperature.
Sample
Material
SiO2/Al2O3a
BET
Area
Vpb
Mean pore size
Adsorption
HCHO
VT
micro
meso
micro
meso
CH3CHO
C2HCl3
m2 g-1
cm3 g-1
nm
␮mol g-1catal.
Raw materials
TiO2-DAP
TiO2-G5
Z1
Z2
Z3
Z4
Z5
TiO2
TiO2
-
-
5
80
23
50
280
14
0.17
0.39
0.35
0.48
0.18
0.26
0.26
-
-
0.17
0.26
0.10
0.12
0.04
0.08
0.03
-
-
100
7/13
10
17
-
773
973
799
279
3,332
584
201
443
1,940
882
319
4,051
803
12
18
c
152
607
826
352
447
402
-
-
-
-
-
0.25
0.36
0.14
0.18
0.13
0.74
0.75
0.51
0.55
0.60
150
127
522
481
453
-
-
473
Zeolite/DAP Hybrids
Z1/DAP
Z2/DAP
Z3/DAP
Z4/DAP
Z5/DAP
Zeolite Y
Zeolite Y
ZSM-5
ZSM-5
ZSM-5
5
476
725
298
377
392
0.34
0.50
0.18
0.26
0.18
0.21
0.28
0.13
0.14
0.14
0.14
0.22
0.05
0.12
0.04
0.74
0.75
0.51
0.55
0.60
10
15
-
-
-
568
205
2,212
768
258
92
28
3,906
784
318
113
30
470
373
327
80
23
80
280
a
SiO2/Al2O3 mole ratio.
Vp: pore volume obtained for N2 adsorption-desorption isotherm.
Data obtained for sample heat treated at 500 C.
b
c
◦
Fig. 1. SEM micrographs for TiO2-DAPs and zeolite/DAP hybrids: (a) TiO2-DAPs, (b) Z2/DAP, (c) Z3/DAP and (d) Z5/DAP.
of existing particles (surface oxidation pathway) or transforma-
tion of DAPs to crystals containing high-index facets could explain
the formation of these deposits. The freezing-drying technique to
incorporate DAPs on the zeolitic materials, keep the typical mor-
of around 400–700 nm with similar DAPs dispersion than Z2/DAP.
The morphology of the Z5 (ZSM-5 SiO /Al O 280) Fig. 1d, is char-
2
2
3
acterized by irregular zeolites particles with considerable higher
particle size than the others zeolites, ranging from 700 nm to 1 ␮m
and a large dispersion of DAPs can be appreciated. Therefore, the
results seem to indicate that ZSM-5 with high SiO /Al O improves
phologies of raw DAPs. The commercial TiO particles are spherical
2
and have lower particle size than DAPs, around 10–20 nm, form-
ing irregular clusters, in line with previous reports [19]. Fig. 1b–d,
show representative SEM images for Z2/DAP, Z3/DAP and Z5/DAP
samples, respectively. The DAPs structure in the APHs is main-
tained with a main particle size of 100 nm and particles can be
distinguished easily from the zeolite phase. Fig. 1b shows zeolite
Y particles of sizes between 400 and 700 nm, mixed with decahe-
dral anatase particles. The Z3/DAP containing ZSM-5 zeolite with
a SiO /Al O ratio of 23 (Fig. 1c), is also characterized by crystals
2
2
3
the dispersion of TiO2 respect to the others zeolites, favouring the
adsorption of UV irradiation for a higher fraction of titania particles
decorating the surface. Thus, the selection of the zeolite with the
adequate composition and crystal structure plays a relevant role in
the control and improvement of the dispersion of DAPs.
Fig. 2 shows the X-ray patterns for DAPs and zeolite/DAP
hybrids. TiO –anatase phase (JCPDS 89–4921) is the main TiO2
2
phase detected in the samples. TiO -rutile phase (JCPDS 87-0920) is
2
2
3
2
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Fig. 3. Amount of CO2 generated during the acetaldehyde photodegradation in air:
(
ꢀ) commercial TiO2, (ꢀ) TiO2-DAPs, ( ) Z1/DAP, ( ) Z2/DAP, ( ) Z3/DAP, (
)
Fig. 2. X-ray diffractograms of TiO2-DAPs and zeolite/DAP hybrids: (ᮀ) TiO2-anatase
and (᭹) TiO2-rutile.
Z4/DAP and ( ) Z5/DAP. Dashed lines: theoretical curves assuming a first order
reaction. Operation conditions: reactor volume: V = 114 mL, [CH3CHO] = 1,000 ppm-
ca. 5 ␮mol, gTiO2 = 6.4 mg.
shift observed for TiO -DAPs, was related to the present of {001}
also observed as a minority component. The characteristic diffrac-
tion lines for the MFI (Z3, Z4 and Z5) and FAU (Z1 and Z2) zeolites
2
facets.
along with the peaks for TiO -anatase phase are observed in the
2
diffractograms of the zeolite/DAP hybrids. As deduced by the well-
defined, very intense and small breadth diffraction peaks, the
zeolites are highly crystalline [39]. The intensity of the diffraction
3.2. Photocatalytic activity of hybrid materials
Firstly, the photocatalytic activity of the zeolite/DAP hybrids
and of the reference titania materials was studied in a batch reac-
tor. Fig. 3 shows the rate of CO2 formation during acetaldehyde
peaks of the zeolite phases decreases after the mixing with TiO due
2
to a dilution effect [40]. Peaks from the TiO -rutile crystal phase are
2
not detected in the hybrid materials. Quantitative determination of
the ratio of {001}/{101} facets areas are, at present, rather difficult
due to possible heterogeneity of particles. Aspect-ratio estimation
seems one of possible comprehensive methods to calculate the pro-
portion of {101}-plane exposure. Particle Aspect Ratio (PAR) was
evaluated, for convenience, as a ratio of crystallite size (depth) ver-
decomposition as function of the irradiation time with TiO -DAPs
and zeolite/DAPs. Irrespectively of the catalysts under study, CO2
formation increases with reaction time.
2
The mineralization (total oxidation) of one mole of acetaldehyde
produces two moles of CO2 (CH CHO + 5/2O → 2CO + 2H O). As
3
2
2
2
observed in Fig. 3, the amount of CO2 obtained during the miner-
tical to {001} and {101} lattice planes (d
and d101, respectively)
alization of CH CHO is lower than the stoichiometric value except
0
04
3
calculated using Scherrer’s equation [41]; the higher the aspect
ratio is, the higher is the proportion of {101}-plane exposure. Thus,
the PAR parameter was estimated to be 0.8-0.9, indicating that
more than 80% of total surface corresponds to {101} facets for the
decahedral particles prepared in this study. These data are in line
with those reported in the literature [41]. According to XRD, DAP
are well crystalized (c.a. > 90%). The main crystal phase detected
was TiO -anatase (c.a. 87.9%) and a minor proportion of TiO -rutile
with Z5/DAP. This behaviour indicates that intermediate species
are formed with the other catalysts or that the reaction products
remain adsorbed on the composites [43]. Thus, the Z5/DAP hybrid is
the only one endowed with enough activity to mineralize the initial
amount of AcH. Pure TiO -DAPs reaches a CH CHO conversion of
2
3
73% meanwhile only a 22% was attained with the commercial TiO2.
Any of the microporous materials tested showed photocatalytic
activity for the degradation of this compound for themselves.
The variation of the zeta potential with the pH for suspension
of commercial TiO2 and DAPs was analyzed (Figure not shown).
The isoelectric point (IEP) indicates the pH at which the positive
2
2
(
5.9%). The amorphous phase content at the selected synthesis con-
ditions was lower than 5.0%. Anatase titania particles showed a
mean crystal size of ca. 67 nm comparing to only 15 nm for com-
+
−
mercial TiO . Larger DAPs showed high stability due to the presence
(−OH2 ) and negative (−O ) surface charges are in average. TiO -
2
2
−
−
of OH2 and OH adsorbed groups [8].
G5 and DAPs suspension present an IEP point at pH of 5.9 and 6.5
respectively. These results suggest that the average acid strength of
UV–vis diffuse reflectance spectra of DAPs, zeolite/DAP compos-
ites and commercial TiO2 as a reference was evaluated (Figure not
shown). The spectra for the zeolites were featureless indicating the
lack of absorption. On the other hand, the hybrids containing TiO2-
DAPs showed an absorption edge at around 380 nm. The band gap
was calculated by plotting the modified Kubelka–Munk function,
surface hydroxyl is in the order of TiO -G5 > DAP probably related to
2
the presence of {001} facet on the later [44]. All the studied zeolites
present an isoelectric point at pH lower than two, except zeolite Z1
which exhibits an isoelectric point at pH = 7. The preparation of the
hybrid was performed in water at pH between 6.5 − 7. At this pH,
near the isoelectric point of both materials, with the same positive
and negative net charges, the interaction between TiO2 and zeolite
Y Z1 is prevented, explaining the low photoactivity of the Z1/DAP.
The apparent rate constants for each photocatalyst have been
calculated (Fig. 3 inset). The value for the synthetized DAP was
0
.5
(
F(R∞)E) versus the energy of the exciting light, determining the
band gap [42]. The bang gap energy value was between 3.0 for DAP
based materials and 3.2 eV for the ones based on TiO -G5, consis-
tent with the characteristic value reported in the literature for TiO2
2
[
4]. The UV absorption band gap of TiO2 is affected by the exposed
−1
−1
facets following the order: {010} > {101} > {001} [7]. Thus, the blue
twice than that of commercial TiO (2.16 s vs. 1.14 s ). It is worth
2
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Fig. 4. Photocatalytic degradation rate of formaldehyde in air with UV-A irradia-
tion for TiO2 and zeolite/TiO2 hybrids: (filled bars) Zeolite/TiO2-G5 and (hatched
−
1
bars) zeolite/TiO2-DAP samples. Operating conditions: total flow F = 900 mL min
HCHO] = 15 ppm, tr = 0.43 s (residence time) and gcatal = 30 mg.
,
[
nothing to remark the values obtained for ZSM-5 based materials
−
1
with high SiO2 content, where this value increases up to 6.6 s
.
Moreover, the experimental data have been fitted to the theoretical
data, assuming a first order integrated rate equation, which relates
the apparent rate constant (k) with the concentration of AcH (Eq.
(
2)):
C = C (1 − exp (−kt))
(2)
0
Where C is the amount of CO2 produced and C0 is the initial
concentration of acetaldehyde and t is the reaction time (dash
lines in Fig. 3). The theoretical curves fit well with the experi-
mental points, indicating that the photocatalytic degradation of
acetaldehyde depends linearly on the amount of acetaldehyde. This
behaviour is commonly observed for gas-phase reactions at low
concentration of reactants. Nevertheless, the adsorption and the
diffusional processes on the composites, should be considered in
order to explain the reaction rate results obtained. Since DAPs sur-
face area is relative low, the higher rate constants for zeolite/DAP
composites may show that the adsorption/diffusion of acetic acid
on the zeolites but not on DAPs, enhances the reaction rate.
Fig. 5. Photocatalytic degradation of trichloroethylene in air with UV-A irradi-
ation for TiO2 and zeolite/TiO2 hybrids: (a) Degradation rate of C2HCl3: (Filled
bars) zeolite/TiO2-G5, (hatched bars) zeolite/DAPs and (b) Relationship between
COCl2 selectivity and adsorption ability: (
amount of C2HCl3 adsorbed. Operating conditions: total flow F = 700 mL min
) COCl2 selectivity and (filled bars)
−
1
,
[
C2HCl3] = 25 ppm, tr = 0.56 s residence time and gcatal. = 30 mg.
lowing equation HCHO + O → CO + H O, the total oxidation of one
2
2
2
mole of HCHO produces one mole of CO . Fig. 4 shows the degra-
2
As observed, zeolite/DAP photocatalysts show higher AcH min-
dation rate of formaldehyde, for all catalysts. As observed, the rate
eralization than commercial TiO , except Z1/DAP. ZSM-5 with
of HCHO degradation on TiO -DAPs was 2.4 times higher than that
2
2
high silica content and high dispersion of the titania, enhances
the photocatalytic activity of the bare DAPs, reaching conversions
around 95%. In good agreement with previous reports, the photo-
catalytic activity increases with the hydrophobic character of the
zeolite, especially for those with the MFI structure [18]. Shape-
controlled anatase titanium(IV) oxide particles were synthetized
by hydrothermal treatment of peroxo titanic acid (PTA) solution
with polyvinyl alcohol as a shape-control reagent and tested in the
photodegradation of acetaldehyde [12]. A 60% of acetaldehyde con-
version was achieved after 100 min reaction time. Sugishita et al.
studied the photodegradation of DAPs for AcH removal, in a similar
experimental set up to the ones used in this study [13]. Authors
report a 40% AcH conversion after 60 min of reaction time. The
hybrid of our study based on ZSM-5 with only an 8wt% of DAPs
allows more than 90% AcH conversion at similar reaction time.
In order to test the catalytic properties of the zeolite/DAP under
harsh conditions the degradation of HCHO was studied. For this
purpose, a continuous air flow was fed through the photocatalytic
reactor, without recirculation, changing the total gas flow from 500
on TiO -G5. DAPs with a PAR ratio of 0.8-0.9 result in a two-fold
2
increase of the VOC photodegradation rate with respect to that
of benchmark TiO2 under both static and dynamic conditions. As
observed in Fig. 4, the HCHO degradation follows the same trend
as observed before for the CH CHO degradation with the Z1-Z5
3
based photocatalysts. The hybrid photocatalysts prepared with DAP
showed notably better photocatalytic performances than those
based on commercial TiO2 specially for Z5/DAP. The improvement
of the photocatalytic efficiency was related to the high crystallinity
and low density of defects of DAPs with {001} facets [5]. The pres-
ence of {001} facets promotes a selective migration and separation
of electrons and positive holes and enhanced the photocatalytic
oxidation paths in combination with a most thermodynamically
stable {101} facets [45,46].
Finally, the composites were tested in the photodegradation of
an organochloride compound such as trichloroethylene (TCE). The
photocatalytic degradation pathway of this molecule is complex,
resulting in the formation of a large quantity of reaction intermedi-
ates, along with non-desired products. In addition to the formation
of CO2 and HCl, TCE degradation also leads to the formation of
−
1
−1
to 900 mL min (GHSV = 4,600 − 8,000 h ). According to the fol-
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Fig. 6. Scheme for aldehyde (left) or organochloride (right) photooxidation with zeolite/DAP hybrids. Zeolitic material (
) and DAPs particles (
).
significant amounts of harmful products such as dichloroacetyl
chloride (DCAC), COCl2 and CO [47]. Fig. 5a shows the degradation
rate of TCE with the hybrids materials prepared with commer-
cial TiO2 and DAP. Again, the hybrids containing DAPs are more
efficient for the removal of TCE than those prepared with commer-
cial titania, in spite of the different surface area. The differences
between the photocatalytic performance of DAPs and commercial
TiO2 is accentuated when the zeolite is included in the photocat-
alytic system, this is, degradation rates are higher when the zeolite
the zeolite and the direct and indirect diffusion of the pollutant
and reaction products from the zeolite to TiO -DAPs active sites
2
and from the titania to the adsorbent. Moreover, the presence of
{001} facets and the high crystallinity of the TiO -DAPs reduces the
2
electron and hole recombination reactions, thus improving pho-
tooxidation pathways.
Two different mechanisms may be proposed depending on the
nature of pollutant to be treated (Fig. 6): For aldehydes (FMD or
AcH) which are adsorbed on both phases, TiO and zeolitic material,
2
is mixed with TiO . The activity pattern described for the effect
and no reaction products others than CO2 are produced, the role of
the microporous material is buffer the access of the aldehyde to the
active sites. Then, the photocatalytic process occurs, involving the
2
of the SiO /Al O ratio for the oxidation of HCHO and CH CHO
2
2
3
3
is also observed during the degradation of TCE. The conversion of
−1
•
•−
TCE with Z5/DAP at 700 mL min is 63% while TiO -DAP reaches
radical species (OH and O2 ) generated on DAPs {101} and {010}
2
only a 50% conversion. Fig. 5b shows the selectivity to COCl which
facets (Fig. 6 left).
2
along with CO2 are the main products detected and the amount
of TCE adsorbed on the hybrid catalysts. We have also observed
that the photocatalysts prepared with the zeolites with higher sil-
ica contents result in both higher degradation rates and a 12%
For the organochloride compound, which is preferentially
adsorbed on the microporous materials, a migration of the pollutant
from the zeolite to the TiO active sites is produced, where the pho-
2
tocatalytic process occurs (Fig. 6 right). Moreover, non-desirable
reaction products may migrate again into the zeolitic structure,
where they are either retained or suffer further hydrolysis reac-
tion. Migration of radical species generated on the titania active
sites for long distances on siliceous materials, has been reported
in the literature [48]. Thus, this reaction pathway cannot be rule
out. Further experiments are being carried out in our laboratory to
elucidate the different reaction pathways.
lower production of COCl with respect to that obtained with DAPs.
2
The results indicate an inverse relationship between the selectivity
to COCl2 and the adsorption capacity. The photocatalytic proper-
ties of TiO -DAPs in the degradation of this molecule have not
2
been reported in the literature. Previous results obtained in our
group with hybrid materials based on sepiolite (a natural mag-
nesium silicate) and commercial TiO2 indicate at least four times
higher reaction rate for zeolite/DAP respect to the magnesium sil-
icate based material [15]. In the case of Z5/DAP composite, values
4. Conclusions
−
1
−1
−1 −1
vs. 0.41 ␮mol s g
catal.
of 5.10 ␮mol s
g
for Sep/TiO2 at
catal.
−
1
7
00 mL min
were obtained. Beside the intrinsic properties of
DAPs with well-defined {101} and {001} facets, prepared by gas
DAPs, the combination of the adsorption properties of the zeolitic
structures, along to its hydrophobic nature and the improvement
of the dispersion of semiconductor on the microporous material,
are key factors to explain the results.
phase reaction at high temperature, were immobilized on differ-
ent types of zeolites by the freezing-drying technique. Zeolite/DAP
hybrids are efficient photocatalysts for the degradation of different
types of aldehydes and organochloride compounds in gas phase
both in batch and continuous flow reactors. Zeolite/DAP hybrids,
combine the intrinsic properties of the raw materials, resulting in
low BET area of the composites, the blocking of a small fraction
of micropores and a modification of the amount of VOC adsorbed,
preserving the zeolite and DAPs morphology and crystallinity. The
nature of the zeolite has a strong influence on the adsorption
properties and on the dispersion of DAPs. The isoelectric point
of microporous material is a key parameter that determines the
dispersion of titania particles.
Our results indicate a direct relationship between the amount of
VOC adsorbed and aluminium content of the microporous material
and the formation of undesirable products. Takeuchi et al. reported
a similar effect and concluded that if the strength of adsorption of
the reactant molecules with the zeolite is strong, they cannot dif-
fuse toward the TiO2 surface whereas molecules weakly adsorbed
on the zeolite surface can easily diffuse toward the TiO2 surfaces,
resulting in an enhancement of the photocatalytic reactivity [26].
Therefore, to select the adequate adsorbent to be combined with
the TiO2 it is important to consider the adsorption ability and the
strength of the interaction, so that the diffusion of the adsorbent
towards the semiconductor is not impeded. Hybrid materials pre-
pared with ZSM-5 with high silicon content combined to DAPs
allow an improvement of the VOC photodegradation rate favouring
the mineralization process. The improvement of the photocatalytic
degradation of the VOC with zeolite/DAPs is related to the combi-
nation of a moderate adsorption strength of the gas molecule onto
DAPs with a particle aspect ratio of 0.8-0.9 result in a two-fold
increase of the VOC photodegradation reaction rate with respect
to that of benchmark TiO2 under both static and dynamic condi-
tions. Zeolite/DAP hybrids have higher photocatalytic properties
than those prepared with commercial TiO . The amount of VOC
2
adsorbed is promoted on the high aluminium containing zeolites.
Nevertheless, hybrid materials prepared with zeolites with high
silicon content, especially ZSM-5, lead to an enhancement of the
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photodegradation rate favouring the mineralization process. The
improvement of the photocatalytic efficiency with zeolite/DAPs is
related to the combination of a moderate adsorption strength of
the VOC onto the zeolite and the direct and indirect diffusion of the
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Please cite this article in press as: I. Jansson, et al., Decahedral anatase titania particles immobilized on zeolitic materials for photocatalytic
degradation of VOC, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.041