Photocatalytic reduction of NO with ethanol on Ag/TiO2

Article abstract of DOI:10.1007/s10562-010-0277-4

The effect of illumination on the surface interaction and the reaction between NO + C₂H₅OH was investigated on TiO₂ and Ag/TiO₂. By means of Fourier transform infrared spectroscopy the formation of an absorption band at 2,210 cm^-1 was observed on pure TiO₂, which was attributed to the NCO species. Similar measurements on Ag/TiO₂ produced another spectral feature at 2,170 cm ^-1, which was assigned to the vibration of NCO bonded to Ag. These absorption bands formed more easily on oxidized than on reduced surfaces. Study of the catalytic reduction of NO with ethanol showed that the illumination of the system induced the reaction even at room temperature. Graphical Abstract: Photo catalysis of NO + C₂H₅OH reaction on TiO₂ (T _R = 473 K) (A), 2% Ag/TiO₂ (T _R = 473 K) (B) and (T _OX = 573 K) (C). [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-010-0277-4

Catal Lett (2010) 135:16–20
DOI 10.1007/s10562-010-0277-4
Photocatalytic Reduction of NO with Ethanol on Ag/TiO2
•
Gy. Halasi A. Kecskemeti F. Solymosi
•
´
Received: 19 December 2009 / Accepted: 13 January 2010 / Published online: 28 January 2010
Ó Springer Science+Business Media, LLC 2010
Abstract The effect of illumination on the surface
interaction and the reaction between NO ? C2H5OH was
investigated on TiO2 and Ag/TiO2. By means of Fourier
transform infrared spectroscopy the formation of an
absorption band at 2,210 cm-1 was observed on pure TiO2,
which was attributed to the NCO species. Similar mea-
surements on Ag/TiO2 produced another spectral feature at
2,170 cm-1, which was assigned to the vibration of NCO
bonded to Ag. These absorption bands formed more easily
on oxidized than on reduced surfaces. Study of the catalytic
reduction of NO with ethanol showed that the illumination
of the system induced the reaction even at room
temperature.
formaldehyde and methane on pure and Rh-containing
TiO2 [3]. This feature also appeared in the photoinduced
activation of CO2 on the same samples [4]. In this short
paper we report the effect of illumination of the
NO ? C2H5OH reaction on pure and Ag-containing TiO2
catalyst. The increasing use of oxygenated organic com-
pounds, particularly ethanol, as fuel or additives for auto-
motive vehicles required the study of the reaction between
NO and ethanol. Ethanol was found to be extremely
effective for NOx reduction over Ag/Al2O3, which displays
high tolerances to water and SO2 [5–15]. By means of
FTIR spectroscopy two absorption bands were detected at
2,228–2,235 and 2,255–2,260 cm-1, which were attributed
to NCO species. As regards the location of NCO different
views were expressed. The first band was attributed to the
vibration of Ag–NCO, whereas the second one to that of
Al–NCO [5, 8]. Alternatively both bands were ordered to
Al–NCO [9, 10]. Based on the previous works on the
chemistry of NCO species on metals and supporting oxides
[16–19], it appeared more certain that both bands are due to
NCO attached to alumina [20].
Keywords Surface interaction between NO and ethanol ꢀ
Photoinduced formation of isocyanate ꢀ Photocatalytic
reduction of NO ꢀ Ag/TiO2 photocatalyst
1 Introduction
In the last decades extensive research has been performed
on photocatalytic reactions mainly using TiO2 as catalyst
or support [1, 2]. It appeared clearly that the semicon-
ducting properties of TiO2 play an important role in its
photocatalytic activity. In our laboratory we found that
changing the work function of TiO2 by doping influences
the rate of H2O ? CO2 reaction to yield formic acid,
2 Experimental
TiO2 was the product of Degussa (P 25, 50 m2/g). Ag/TiO2
samples were prepared by impregnation of titania in the
solution of AgNO3. The suspension was dried and pressed
into self-supporting wafers (30 9 10 mm * 10 mg/cm2)
for IR studies. For photocatalytic studies the sample was
sprayed onto the outer side of the inner tube from aqueous
suspension. The surface of the catalyst film was 168 cm2.
The catalysts were oxidized at 573 K and in certain cases
reduced at different temperatures in the IR cell or in the
catalytic reactor for 1 h. Ethanol was the product of
´
Gy. Halasi ꢀ A. Kecskemeti ꢀ F. Solymosi (&)
Reaction Kinetics Research Group, Chemical Research Centre
of the Hungarian Academy of Sciences, Department of Physical
Chemistry and Materials Science, University of Szeged,
P.O. Box 168, 6701 Szeged, Hungary
e-mail: fsolym@chem.u-szeged.hu
123

Photocatalytic Reduction of NO with Ethanol on Ag/TiO2
17
Sharlau with purity of 99.7%. NO was the product of
MESSER with purity of 99.8%.
spectra were recorded with a Biorad (Digilab. Div. FTS
155) instrument with a wavenumber accuracy of 4 cm-1
.
For FTIR studies a mobile IR cell housed in a metal
chamber was used. The sample can be heated and cooled to
150–200 K in situ. The IR cell can be evacuated to 10-5
Torr using a turbo molecular pumping system. The samples
were illuminated by the full arc of a Hg lamp (LPS-220,
PTI) outside the IR sample compartment. The IR range of
the light was filtered by a quartz tube (10 cm length) filled
with triply distilled water applied at the exit of the lamp.
The filtered light passed through a high-purity CaF2 win-
dow into the cell. The light of the lamp was focused onto
the sample. The output produced by this setting was
300 mW cm-2 at a focus of 35 cm. The maximum photon
energy at the sample is ca. 5.4 eV (the onset of UV
intensity from the lamp.) After illumination, the IR cell was
moved to its regular position in the IR beam. Infrared
All the spectra presented in this study are difference
spectra.
Photocatalytic reaction was followed in a thermostati-
cally controllable photoreactor, equipped with a 15 W
germicide lamp (type GCL307T5L/CELL, Lighttech Ltd.,
Hungary) as light source. This lamp emits predominantly in
the wavelength range of 250–440 nm. The reactor (vol-
ume: 970 mL) consists of two concentric Pyrex glass tubes
fit one into the other and a centrally positioned lamp. The
reactor is connected to a gas-mixing unit serving for the
adjustment of the composition of the gas or vapor mixtures
to be photolized in situ. The carrier gas was Ar, which was
bubbled through ethanol at room temperature. Afterwards
Argon containing *1.3% ethanol and 1.0% NO were
introduced in the reactor through an externally heated tube
Fig. 1 a Effects of illumination
time on the FTIR spectra of
TiO2 (TR = 473 K) in the
presence of NO ? C2H5OH
mixture: a, 5 min; b, 15 min; c,
30 min; d, 60 min; e, 90 min; f,
120 min. b Effects of the
reduction temperature of TiO2
on the integrated absorbance of
the 2,210 cm-1 band: a, 773 K;
b, 673 K; c, 573 K; d, 473 K
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Gy. Halasi et al.
avoiding condensation. The gas-mixture was circulated by
a pump. The reaction products were analyzed with a HP
5890 gas chromatograph equipped with PORAPAK Q and
PORAPAK S packed columns.
IR results for adsorbed ethanol the major bands at 2,972
and 2,870 cm-1 can be attributed to the vibrations of
ma(CH3), ms(CH3) and to ms(CH2), whereas the peaks at
1,074 and 1,051 cm-1 to the vibration of m(OC) of ethoxy
group [21, 22]. As a result of illumination the characteristic
absorption bands of ethoxy gradually attenuated and new
bands developed at 2,210 and 1,636 cm-1; their intensities
grew with the illumination time. Spectra registered on
titania reduced at 473 K are presented in Fig. 1a. All the
new bands remained unaltered after degassing the sample
at 300 K for 15 min. The intensity of the 2,210 cm-1 band
gradually decreased with the reduction temperature of
TiO2. In harmony with this, stronger band at 2,210 cm-1
was registered on oxidized sample. Control measurements
showed that only a very weak band appeared at 2,210 cm-1
3 Results and Discussion
3.1 FTIR Studies
Adsorption of NO ? ethanol gas mixture on TiO2
(TR = 473 K) at 300 K produced intense absorption bands
at 2,972, 2,931 and 2,870 cm-1 in the C–H stretching
region, and 1,381, 1,271, 1,151, 1,074 and 1,051 cm-1 in
the low frequency range. Taking into account the previous
Fig. 2 a Effects of illumination
time on the FTIR spectra
of Ag/TiO2 (TR = 473 K) in
the presence of NO ? C2H5OH
mixture. b The integrated
absorbance of the bands at
2,210 cm-1 (white circle) and
2,170 cm-1 (filled circle)
formed in the NO ? C2H5OH
reaction on Ag/TiO2
(TOX = 573 K)
123

Photocatalytic Reduction of NO with Ethanol on Ag/TiO2
19
at 300 K in the IR spectrum of pure titania without illu-
mination. The intensities of the 2,210 cm-1 band measured
on oxidized and reduced TiO2 are plotted in Fig. 1b. Based
on the previous results [16–19] there is no doubt the
2,210 cm-1 band is due NCO species formed on TiO2.
Same absorption band was identified following the disso-
ciative adsorption of HNCO on TiO2 [19]. The finding that
its generation is favored on oxidized surface may be con-
nected with the enhanced dissociation of ethanol and eth-
oxy on the oxidized TiO2 to produce reactive species for
the reduction of NO. The absorption band at 1,636 cm-1 is
assigned to the m(C=O) vibration of the acetaldehyde.
Same experiments were performed over 2% Ag/TiO2
catalyst. Introduction of NO ? C2H5OH mixture in the cell
in dark gave identical IR spectrum as in the case of pure
titania. Following the illumination, however, a new strong
feature appeared at 2,170 cm-1, in addition to the band at
2,210 cm-1, which was somewhat less intense than mea-
sured for pure TiO2. The new band was significantly larger
than that of at 2,210 cm-1, and it further grew with the
illumination time (Fig. 2a). As in the case of pure TiO2 the
oxidation of the sample favored and the reduction at high
temperature hindered its development. This spectral feature
is ordered to surface NCO attached to Ag particles. The
absorption band of isocyanate species bonded to Ag was
determined by the dissociative adsorption of HNCO on Ag/
SiO2 in similar way as the vibration of NCO bonded to Pt
metals was established [16–19]. Dissociation of molecu-
larly adsorbed HNCO on metal single crystal surfaces also
gave a vibration loss at 2,160–2,190 cm-1 [23, 24]. It is
important to note that the 2,170 cm-1 band was not
detected in the high temperature reaction of
NO ? C2H5OH supported Ag catalyst [5–15]. The possible
reason is that NCO formed on Ag spilt over the oxidic
supports after its production, where it was stabilized [20].
As regards the formation of NCO we can exclude the
step assumed in the case of supported Pt metals [16–18],
when adsorbed N formed in the dissociation of NO is
combined with CO
NðaÞ þ COðaÞ ¼ NCOðaÞ
ð1Þ
This is particularly true for pure TiO2. It is more likely that
the reactive enolic species produced in the photo-
dissociation of ethoxy
C2H5OðaÞ ¼ CH2 = CH ꢁ OðaÞ þ H2ðgÞ
ð2Þ
reacts with NO to give NCO, as was assumed for the
thermal reaction [12, 13].
3.2 Photocatalytic Studies
The decomposition of ethanol on oxidized and reduced
TiO2 in dark was hardly observable at 300 K. Same feature
was experienced for NO. Illumination of oxidized TiO2
Fig. 3 Photo catalysis of NO ? C2H5OH reaction on TiO2 (TR = 473 K) (a), 2% Ag/TiO2 (TR = 473 K) (b) and (TOX = 573 K) (c)
123

20
Gy. Halasi et al.
300 K. The rate of reaction was markedly enhanced
by deposition of Ag onto TiO2.
(TOX = 523 K), however, induced a slow consumption of
ethanol to yield acetaldehyde, H2 and smaller amount of
CH4, CO2 and CO. A slight photo-decomposition of NO
was also experienced over TiO2. When NO ? ethanol gas
mixture was photolyzed on TiO2 (TR = 473 K) we expe-
rienced a reaction between the two compounds as indicated
by the consumption of both reactants and the formation of
N2, N2O, acetaldehyde and—in trace quantity—H2, CO
and CH4. These compounds were produced at higher rates
on oxidized TiO2 (TOX = 573 K). Results are plotted in
Fig. 3a. Note that without illumination no reaction was
observed between the reactants. The photo-reduction of
NO with ethanol was markedly enhanced on Ag/TiO2
catalyst: the reaction occurred at somewhat faster rate on
oxidized than reduced Ag/TiO2 surfaces (Fig. 3b, c). The
main products were acetaldehyde, N2, CO, CH4 and H2O
(not determined), and the minor ones N2O and H2 indi-
cating the occurrence of the oxidative dehydrogenation of
ethanol
Acknowledgments This work was supported by the grant OTKA
under contact number NI 69327. The authors express their thanks to
´ ´
Prof. I. Dekany for the help of constructing the photoreactor.
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(i) Photolysis of NO ? C2H5OH gas mixture on pure
TiO2 led to the formation of an IR band of isocyanate
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123