Photodecomposition of NO by sol-gel TiO2 catalysts under atmospheric conditions: Effect of the method on the textural and morphologic properties

Article abstract of DOI:10.1016/j.jallcom.2006.08.334

Photodecomposition of nitric oxide (NO) in the open air by sol-gel TiO₂ catalysts with different solvents and thermal treatments was studied. The sol-gel TiO₂ catalysts showed high surface areas, rich anatase phases, small particle sizes and bandgap-energy changes. The photoactivity was performed in an insulated chamber with 50 ppm (NO) and a 365-UV light. The sol-gel TiO₂ catalysts (which were activated at 200 and 500 °C and prepared by using two different solvents) showed high NO decomposition under UV light illumination. Taking into account the aforementioned results, these catalysts could be applied for the NO decomposition under atmospheric conditions.

Full text of DOI:10.1016/j.jallcom.2006.08.334

Journal of Alloys and Compounds 434–435 (2007) 788–791
Photodecomposition of NO by sol–gel TiO2 catalysts under
atmospheric conditions: Effect of the method on the
textural and morphologic properties
R. Carreraa,b, A.L. Vazqueza,b, E. Arcea, M. Moran-Pinedab, S. Castilloa,b,∗
´
a Departamento de Ingenier´ıa Metalu´rgica, ESIQIE-IPN, AP 75-876, Me´xico D.F., Mexico
b Programa de Ingenier´ıa Molecular, Instituto Mexicano del Petro´leo, Eje La´zaro Ca´rdenas 152, C.P. 07730, Me´xico D.F., Mexico
Available online 17 October 2006
Abstract
Photodecomposition of nitric oxide (NO) in the open air by sol–gel TiO2 catalysts with different solvents and thermal treatments was studied.
The sol–gel TiO2 catalysts showed high surface areas, rich anatase phases, small particle sizes and bandgap-energy changes. The photoactivity was
performed in an insulated chamber with 50 ppm (NO) and a 365-UV light. The sol–gel TiO2 catalysts (which were activated at 200 and 500 ◦C and
prepared by using two different solvents) showed high NO decomposition under UV light illumination. Taking into account the aforementioned
results, these catalysts could be applied for the NO decomposition under atmospheric conditions.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Nanostructured titania; Sol–gel method; Anatase; Photodecomposition
1. Introduction
and at the same time, offers alternative applications as films
or coverings on a grant variety of surfaces or substrates [6].
Application in oxidation and photocatalytic decomposition of
atmospheric contaminants, are mainly focused on its textural
and morphological properties, synthesis of nanostructured TiO2
withahighsurfaceareaandacontrolledandstablephasecompo-
sition, with the aim of reaching higher photocatalytic activity at
lowertemperatureandclosertotheUVlightvisibleregion [7–9].
As for the atmospheric contaminants, it is of strategic impor-
tance to control the nitrogen oxides (NOx) coming from the
combustion of fossil fuels in a great variety of combustion
systems [10]. One way of reducing the NOx under outdoors
conditions is by photoassisted reactions with the possibility of
controlling simultaneously reactive organic gases (GOR) also
known as volatile organic compounds (VOC’s) [11]. In this
work, TiO2 powder was produced with particle size from 12 to
7 nm, high surface area (>120 m2/g), and higher anatase phase
purity (>90%), for its evaluation in decomposition and photo-
catalytic oxidation of NOx in a UV light interval as close as
possible to the visible spectrum and if possible in this region
(solar light), and with a longer useful life.
Nanostructured materials offer promising opportunities for
improved and tailored properties for application in environ-
mental catalysis [1]. TiO2 is a very interesting material due
to its properties and diverse applications. Currently, TiO2 is
an interesting nanoparticle semiconductor material, especially
because of its application in photoassisted reactions in the pro-
cess of reduction or elimination of water contaminants, and the
auto cleaning (anti-bacterial, anti-soot), and more recently in
the treatment of contaminant gases present in the air and in
medicine [2]. As a catalytic support, TiO2 has a low surface
area (≈40 m2/g), and the crystalline phase with higher photo-
catalytic activity with respect to rutile is the anatase due to their
energy band structures [3].
The synthesis of nanostructured materials like TiO2 by wet
chemistry routes is a powerful way to obtain reproducible homo-
geneous samples. The control of stable particle size and crys-
talline phase obtained by the sol–gel method is one of the
most versatile, reproducible procedures to obtain high reactivity,
small particles and large surface areas [4,5]. Due to its especial
characteristics, TiO2 can be used in photocatalytic processes,
2. Experimental
∗
Corresponding author.
E-mail address: scastill@imp.mx (S. Castillo).
TiO2 catalysts were prepared by the sol–gel method in a three neck glass
reactor at atmospheric pressure. Titanium(IV) isopropoxide was used as
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.08.334

R. Carrera et al. / Journal of Alloys and Compounds 434–435 (2007) 788–791
precursor and the hydrolysis was carried out with 2-propanol and ethanol
789
and finally with deionized water. The mixture was kept under reflux and
uniform stirring until the gel was formed. In postgelation, the product was
dried at 70 ◦C during 12 h and calcined 6 h at 200 and 500 ◦C. The sol–gel
catalysts were labeled as follows: TiO2-P-200 ◦C (prepared with 2-propanol
and calcined at 200 ◦C), TiO2-P-500 ◦C (prepared with 2-propanol and calcined
at 500 ◦C), TiO2-E-200 ◦C (prepared with ethanol and calcined at 200 ◦C) and
TiO2-E-500 ◦C (prepared with ethanol and calcined at 500 ◦C).
The specific area was determined by the BET method through the nitrogen
adsorption isotherms at −196 ◦C. The volume and pore size distribution were
calculated from BET isotherms by the BJH method (Barret, Joyner, Halenda), in
a Micrometrics ASAP-2000 equipment. For X-ray diffraction analysis, Siemens
500 was used with an anode copper tube and integrated software and the compo-
nents of each catalyst were determined. The band gap of forbidden energy was
determined through visible ultraviolet spectroscopy. Once the diffuse reflectance
spectra were obtained, a numerical method was applied to obtain the Eg values
[12]. The particle size of the TiO2 crystals was obtained with a transmission
electronic microscope JEOL 100 (X) STEM with resolution ranging from 2 to
˚
5 A capable of working at 100 kV.
The catalysts were analyzed in the middle infrared by FT-IR Brucker
equipment. The samples were prepared in potassium bromide tablets with an
approximate ratio of 10 parts of KBr to 1 of sample. The photoactivity was
performed in an insulated chamber with 50 ppm (NO) and 365-UV light. The
photocatalytic evaluation of the catalysts decomposing the NO was performed
with infrared spectroscopy with IFS66V/s equipment with a MCT detector
(Mercury–Tellurium–Cadmium) with high sensibility, resolution of 0.5 cm−1
and optic step length of 25 cm.
Fig. 1. XRD pattern for TiO2-P-200 ◦C (A) and TiO2-E-500 ◦C (B) sol–gel
catalyst.
3. Results and discussion
According to the BET results, the sol–gel catalysts that
showed higher surface areas, higher pore volume and less mean
pore diameter were the ones that were calcined at 200 ◦C (TiO2-
P-200 ◦C and TiO2-E-200 ◦C) (Table 1). X-ray analysis of the
four sol–gel catalysts showed five characteristic peaks for the
anatase phase and in a minor signal for the rutile phase, except
for the catalyst that was prepared with ethanol and calcined
at 500 ◦C (TiO2-E-500 ◦C), where pure anatase was obtained
(Fig. 1). Table 1 shows that through sol–gel method, TiO2 cata-
lysts with nanometric size were obtained which is fundamental
to get a high photocatalytic activity in NO photodecomposition.
Among the sol–gel catalysts with small crystal size calcined at
200 ◦C, the one that showed the smaller size was prepared with
ethanol (TiO2-E-200 ◦C) (Fig. 2).
According to the method used in this work, the fit-plot-yield-
bandgap values could change; this change could be related to
Quantum-Size effects through the TiO2 crystalline phases and
particle size [13]. The forbidden-energy band of the commer-
cial TiO2 anatase phase is 3.2 eV with a particle size of 39 nm.
As for the diffuse reflectance spectra for the four sol–gel cat-
alysts, the Eg value varied from 3.05 to 3.95 eV, so, it can be
said that the sol–gel method is alternative to modify this param-
Fig. 2. TEM-micrograph for TiO2-E-200 ◦C sol–gel catalyst.
eter and therefore photocatalytic activity [14]. Fig. 3 shows the
Ephot for the TiO2-P-200 ◦C, this catalyst showed the higher Eg
value (3.95 eV), a small particle size (12 nm) and an anatase
phase with a minor rutile proportion (Fig. 1) (Table 1). The
FT-IR spectra for the four sol–gel catalysts show the follow-
ing bands: OH group vibration with the Ti atoms at 3641 cm−1
;
enlargement vibration of the bond in the Ti–O–Ti structure at
1122 cm−1; OH vibrational flexion at 1630 cm−1 (typical band
of the physisorbed water in the material), which is of greater
interest for photocatalytic purposes.
Table 1
Characterization of TiO2 sol–gel catalysts
Surface area (m2g −1
)
Pore volume (cm3/g−1
)
Mean pore diameter (A)
Crystal size (nm)
Photon energy, hν (Eg)
˚
Catalysts
TiO2-P-200 ◦C
TiO2-P-500 ◦C
TiO2-E-200 ◦C
TiO2-E-500 ◦C
189
60
212
72
0.17
0.11
0.25
0.16
36
74
48
91
12
33
10
30
3.95
3.15
3.05
3.1

790
R. Carrera et al. / Journal of Alloys and Compounds 434–435 (2007) 788–791
Fig. 5. Selectivity to NO2 formation in sol–gel catalyst.
Fig. 3. Plot of (hν)2 ␣2 vs. Ephot for TiO2-P-200 ◦C sol–gel catalyst.
these are the precursors of further dissociative reactions between
the adsorbed O2 and of bulk oxygen vacancies, where the O
species may attack adsorbed NO to yield NO2 at the proposed
reaction conditions.
Through the application of the sol–gel method and thermal
treatments, it was possible to modify the textural and morpho-
logic properties of synthesized catalysts, for instance, in two of
them, which were activated at a low temperature (200 ◦C), the
minor particle size (10 and 12 nm), stabilization of the anatase
phase and changes on its bandgap (Ephot), were obtained. The
properties mentioned above could be related to the activity and
selectivity showed by the sol–gel catalysts, since to a high NO
concentration (50 ppm), the NO decomposition was higher than
80%. Particularly, the TiO2-E-200 ◦C showed a high NO decom-
position (>80%) during the first 30 min, as well as a low selectiv-
ity towards NO2 formation (Figs. 4 and 5). The TiO2-E-200 ◦C-
photocatalytic activity could be related to the particle size and
the anatase phase. Both characteristics could have exerted an
effect on the forbidden-energy bandgap (3.95 eV), which can
improve the photoinduced electron transfer to adsorbed NO as
a result of the migration of electrons and holes to the surface of
the bulk TiO2 sol–gel [15].
4. Conclusions
The handling of the sol–gel variables permitted the synthesis
of titania nanoparticles with improved textural and morphologic
properties such as: high specific area, purity, stabilization of the
anatase phase and modification of bandgap energy (Eg), espe-
cially those that were treated at a low temperature (200 ◦C).
These properties offer a high photocatalytic activity over NO
decomposition and a minor NO2 selective formation even at
a high NO concentration (50 ppm). The sol–gel catalysts acti-
vated at a low temperature can be considered as a good option
for application to reduce the NO indoors and under atmospheric
conditions with UV light assistance.
Acknowledgments
With respect to selectivity towards the NO2 formation in
sol–gel catalysts activated at a high temperature (TiO2-P-500 ◦C
and TiO2-E-500 ◦C), it could be due to surface defect sites since
We are indebted to the IMP-Molecular Engineering Program
and CONACyT for its financial support. The authors want to
´
thank Technician Rufino Velazquez Lara for his assistance and
technical support in this work.
References
[1] N.L. Stock, J. Se´ller, K. Vinodgal, P.V. Kamat, Environ. Sci. Technol. 34
(2000) 1747.
[2] F.D. Ollis, H. Al-Ekabi, Photocatalytic Purification and Treatment of Water
and Air, Elsevier Science, Amsterdam, 1993.
[3] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 54.
[4] J. Zang, T. Ayusawa, M. Minagawa, K. Kinugawa, M. Matsuoka, M. Anpo,
J. Catal. 198 (2001) 1.
[5] A. Fujishima, K. Hashimoto, T. Watanabe, TiO2 Photocatalysis Fundamen-
tal Applications, Published by BKC, Inc., Tokyo, Japan, 1999.
[6] A. Fujishima, K. Hashimoto, Y. Kubota, J. Surf. Sci. Soc. Jpn. 16 (1995)
188.
[7] X. Bokhimi, A. Morales, O. Novaro, T. Lo´pez, E. Sa´nchez, R. Gomez, J.
Mater. Res. 10 (1995) 2778.
[8] X. Bokhimi, O. Novaro, R.D. Gonzalez, T. Lo´pez, O. Chimal, M. Asomoza,
R. Go´mez, J. Solid State Chem. 144 (1999) 349.
Fig. 4. Dependence of NO concentrations on the time in sol–gel catalyst.

R. Carrera et al. / Journal of Alloys and Compounds 434–435 (2007) 788–791
791
[9] J. Yu, X. Zhao, Mater. Res. Bull. 36 (2001) 97.
[10] K. Wark, C.F. Warner, Contaminacio´n del Aire, Origen y Control, ed.
Limusa 5a Edicio´n, Me´xico, 1995, pp. 44–82.
[12] E. Sanchez, Y. Lopez, Mater. Lett. 25 (1995) 271–275.
[13] K.M. Reddy, S.V. Manorama, A.R. Reddy, Mater. Chem. Phys. 78 (2002)
239–245.
[11] J. Pasel, V. Speer, C. Albrecht, F. Richter, H. Papp, Appl. Catal. B: Environ.
25 (2000) 105.
[14] J. Yu, X. Zhao, Q. Zhao, Mater. Chem. Phys. 69 (2001) 25.
[15] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995) 735–758.