Modern Research in Catalysis, 2013, 2, 136-147 Published Online October 2013 (
Adsorption of CO and NO on Ceria- and Pt-Supported
TiO2: In Situ FTIR Study
Zeinhom M. El-Bahy1,2
1Chemistry Department, Faculty of Science, Taif University, Taif, Saudi Arabia
2Chemistry Department, Faculty of Science, Al-Azhar University, Cairo, Egypt
Received June 19, 2013; revised August 8, 2013; accepted September 2, 2013
Copyright © 2013 Zeinhom M. El-Bahy. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Pt/TiO2, Ce/TiO2 and binary system PtCe/TiO2 catalysts were prepared by impregnation method and the structural
properties of these catalysts were investigated by means of XRD, CO-TPD and UV-vis diffuse reflectance spectroscopy.
As investigated by XRD, the composition of the prepared samples anatase and rutile phases with higher amount of ana-
tase phase and its particle size was in the range of 19 - 22 nm. The band gap also decreased from 3.1 to 2.85 after addi-
tion of metal to TiO2. The adsorption and interaction properties of NO and/or CO gases were monitored using an in situ
FTIR technique. The intensity and position of the infrared peaks were strongly dependent on the composition of the
catalyst. In presence of Pt, the main oxidative reductive products of (NO + CO) are CO2 and NCO complex. The forma-
tion of NCO depends on not only the presence of platinum in the catalyst but also the presence of Lewis acid sites
which is Ti4+ in this study. However, the interaction between NO and CO gases increased in presence of CeO2. The op-
timum Ce content in PtCe/TiO2 was 0.1% (Ce/TiO2) at which the maximum peak intensity was observed for NCO and
Keywords: In Situ FTIR; NO Reduction; CO Oxidation; TPD-CO; Platinum-Cerium
1. Introduction
The importance of environmental gas monitoring and
controlling is now recognized as an important area to
diminish the hazardous chemical vapors present beyond
specified levels. CO and NO are known to be extremely
harmful to the human body and also a main cause of air
pollution since they are two of the most hazardous prod-
ucts released in car exhausts [1]. The best employed
methods for eliminating NO and CO are the catalytic re-
ductive and oxidative mechanisms, respectively. Both
CO and NO are used as common probe molecules in sur-
face science to obtain fundamental information about the
gas-surface interactions, adsorption sites, and reactive
dynamics on variety of metals [2]. One of the most im-
portant reactions in automobile exhaust catalysis is the
reaction between NO and CO over metal oxide surfaces:
 
gg 2
  (1)
Rhodium, platinum and palladium are mostly used to
catalyze the reaction in Equation (1) [3]. A Pt/TiO2 sam-
ple has been studied by adsorption and co-adsorption of
CO + NO [4]. The activity of Rh supported on the ceria
doped titania was investigated and it was found that the
presence of ceria favors the Rh dispersion [5]. González
et al. [6] reported that Pt supported on Ce-modified TiO2
support exhibited better activity for water gas shift reac-
tion than those corresponding to individual CeO2 and
TiO2-supported catalysts. These catalysts were prepared
by using metal/Ti ratios of 0.5 for Pt/Ti and 0.005 - 0.07
for Ce/Ti. It will be important to prepare Ce-promoted
Pt/TiO2 catalyst and test it for NO + CO adsorption and
interaction as an oxidative/reductive catalytic method for
harmful gas removal.
In situ IR spectroscopy study of adsorbed probe mole-
cules, especially of CO, is very useful for the characteri-
zation of solid surfaces and gives a unique possibility to
characterize the coordination state and electrophilic pro-
perties of accessible sites [7]. Activity measurements,
when coupled with the physicochemical characterization
of catalysts suggest that the modifications in the surface
reducibility of the support play an essential role in the
enhancement of activity and stability observed when Pt-
modified TiO2 was promoted with CeO2. Hence, testing
the gas interaction over the reduced catalyst should be
The purpose of the present work is to investigate the
opyright © 2013 SciRes. MRC
Z. M. EL-BAHY 137
effect of cerium oxide addition on the structure and ad-
sorption properties of titania-supported platinum catalyst.
CO and/or NO were admitted to the prepared catalysts
and the interaction of admitted gases over the catalysts
surface was monitored by in situ FTIR spectroscopy
technique. X-ray diffraction (XRD), Temperature Pro-
grammed Desorption (TPD) of CO and Diffuse reflection
spectra (UV) were used for solid characterization.
2. Experimental
2.1. Catalyst Preparation
Analytical grade reagents, Pt(NH3)4Cl2·H2O,
Ce(NO3)3·6H2O and TiO2 (Degussa P25, known as a
commonly used photocatalyst), were employed as start-
ing materials in this study. The metal supported TiO2
nanoparticles were prepared by the incipient wetness
impregnation method. TiO2 (4 g) was placed and stirred
in water to form a homogenous suspension. A proper
amount of an aqueous solution of Pt2+, Ce3+ and (Pt2+ +
Ce3+) was added to TiO2 suspension to form the required
Pt, Ce, and (Pt + Ce)/TiO2 ratios, respectively. The molar
ratio Pt/Ti was fixed at 1% and Ce/Ti was changed in the
range of 0.05% - 1%. The Pt, Ce and (Pt + Ce) supported
TiO2 precursors slurry were refluxed at 80˚C with con-
tinuous stirring for 3 h. After impregnation the water was
allowed to evaporate using rotary evaporator and the re-
sidues were dried at 110˚C over night. The obtained sol-
ids were then calcined at 500˚C in air for 3 h with ramp
rate of 2˚C /min. The samples were referred to as Pt/TiO2,
Cex/TiO2 and PtCex/TiO2 for Pt, Ce and PtCe-supported
titania, where x denotes to the Ce/Ti ratio.
2.2. Catalysts Characterizations
X-ray diffraction (XRD) technique was used to deter-
mine the bulk crystalline structures of the prepared cata-
lysts. The XRD patterns were obtained with an x-ray
diffractometer, D8 Advance (Bruker axs), with a Cu Kα
radiation source (30 kV and 20 mA) in the 2θ range of
20˚ - 60˚. The average crystallize size (D) of the obtained
powders was calculated by Hall-equation-Scherrer’s for-
mula D = 0.9λ/βcosθ [8]; where λ represents the x-ray
wavelength (1.54 Ǻ), θ is the Bragg’s angle and β (in
radians) is the pure full width of the fraction line at half
of the maximum capacity.
Temperature Programmed Desorption (TPD) experi-
ments were carried out using CO gas in a fixed-bed re-
actor system. Before TPD measurements, the catalyst (30
mg) was treated at 500˚C for 1 h in air. Then the heated
sample was evacuated for 1 h at the same temperature
under vacuum (104 torr). The sample was then cooled
under the same pressure until room temperature and then
an amount of CO gas with partial pressure of 7 torr was
added and left to equilibrate for 30 min. The sample was
then degassed for 1 h at room temperature. The degassed
CO molecules (m/z = 28) were monitored by a gas de-
sorption analyzer (ANELVA, M-QA100TS) equipped
with a quadruple mass analyzer in a high-vacuum cham-
ber of 7.5 × 109 torr range. TPD profiles were recorded
by linear heating of the samples from 25˚C to 750˚C at
constant ramp rate of 5˚/min.
The UV-vis diffuse reflectance spectra of various sam-
ples in the 700 - 200 nm range were obtained using a
Jasco V-570 (serial number, C29635) spectrophotometer
equipped with a diffuse reflectance attachment, using
BaSO4 as a reference.
2.3. Gas Adsorption and Catalytic Activity
In situ FTIR spectra of CO and/or NO adsorption (7 torr
of each) were recorded using JASCO FTIR-660 Plus.
Briefly, 15 - 20 mg/cm2 self-supporting pellets of the ca-
talyst powder were prepared and treated directly in the
purpose-made IR quartz cell equipped with CaCl2 win-
dows. The IR cell was connected to a static vacuum-ad-
sorption system with a residual pressure below 104 torr.
The samples were heated at 500˚C for 2 h in air followed
by 1 h evacuation at same temperature. Then the cata-
lysts were cooled at room temperature (RT) and at this
point, background spectrum was recorded. CO and/or
NO gas mixtures were then admitted. The gases were
used in equal partial pressure of 7 torr of each (CO:NO;
1:1). After gas admission, the cell was left to equilibrate
for 20 min at RT after which the spectrum was recorded
again. The former spectrum (before gas admission) was
subtracted from the latter one (after gas admission) and
will be discussed below. To test the adsorption at higher
temperature, the temperature of the IR cell, including the
sample wafer and the gas mixture, was raised to the de-
sired temperature. It was kept at such temperature for 20
min after which the IR cell was cooled to RT and then
the spectrum was recorded, and kept to test the effect of
the addition of trace amount of water vapor, it was added
in the gas mixture with a ratio of (CO + NO + H2O;
3. Results and Discussions
3.1. Catalyst Characterization
3.1.1. X-Ray Diffr ac tion
XRD was usually used for identification of the crystal
phase and estimation of the ratio of the anatase to rutile
as well as crystallite size of each phase. Figure 1 shows
the XRD patterns of titania and metal supported TiO2
catalysts. The patterns proved the presence of both ana-
tase and rutile phases in all samples. The peaks at 2θ =
26.1 (101), 38.6 (112), 48.8 (200) and 55˚ (211) in the
spectrum of all samples are easily identified as the crystal
Copyright © 2013 SciRes. MRC
of anatase form [PDF# 84 - 1286], whereas the XRD
peaks at 2θ = 28.2 (110), 36.9 (101), 42 (111) and 55.9˚
(211) are easily taken as the crystal of rutile form [PDF#
88 - 1175]. In addition, small peak at 2θ = 37.9˚ due to
titanate (H2Ti4O9·1.9H2O) [PDF# 39 - 0040] was ob-
served and increased in samples containing Pt. The pat-
terns did not show any peaks for loaded metal oxides,
indicating that these metal oxides were well dispersed in
all cases. The XRD intensities of the anatase peak at 2θ =
26.1˚ and the rutile peak 2θ = 28.2˚ were also analyzed to
determine the percentage of anatase in the samples from
the respective integrated XRD peak intensities using the
following equation [9]: Χ (%) = 100/(1 + 1.265IR/IA),
where X is the weight percentage of anatase in the sam-
ple; IA represents the intensity of the anatase peak at 2θ =
26.12˚ and IR is that of the rutile peak at 2θ = 28.2˚. The
patterns in Figure 1 and the data listed in Table 1 show
that most of the structure of the prepared samples is ana-
tase form. It could be seen that the presence of other
metal oxides such as Pt or Ce oxides increased the ana-
tase ratio. These data demonstrated that the employed
metals could inhibit the transformation from anatase to
rutile. These observations are in good agreement with
previous reports [10]. The particle size of the prepared
20 25 30 35 40 45 50 55 60
2 theta, degree
Counts, au
A= Anatase; R = Rutile; T = Titanate
Figure 1. X-ray powder diffraction patterns of parent TiO2
and Pt and Ce-supported TiO 2 catalysts.
Table 1. Crystallite size of parent TiO2 and Ce- and Pt-
supported TiO2 catalysts.
Phase % by XRD Crystallite size, nm
Anatase Rutime Anatase, Rutile
gap, ev
TiO2 72.6 27.4 19.9 25.1 3.1
PtTiO2 80.0 20.0 21.5 27.2 2.95
Ce0.1TiO2 72.3 27.7 20.6 25.7 2.85
PtCe0.05TiO2 76.0 24.0 17.8 24.9 -
PtCe0.1TiO2 82.3 17.7 22.6 34.9 2.9
PtCe1TiO2 74.6 25.4 19.4 26.7
solids was calculated using Scherrer’s equation. Table 1
shows the average crystallite size for both anatase (2θ =
25.9˚, 38.5˚ and 48.7˚) and rutile (2θ = 28.1˚, 36.7˚ and
55.9˚) phases. The particle size was found to be in the
range of 19 - 22 nm for anatase phase while it was 25 -
35 nm for rutile phase.
3.1.2. Temperature Programmed Desorption (TPD)
The TPD spectra after CO adsorption of pure TiO2 and
the prepared catalysts are shown in Figure 2. The mag-
nitude of the desorption maxima differs somewhat be-
tween the metal-free and supported TiO2 samples. The
CO peak maximum for pure TiO2 is observed at ap-
proximately 540˚C. This profile changed after loading
of Pt and Ce to TiO2 surface. In case of Pt/TiO2, three
CO desorption peaks can be distinguished at 175˚C,
560˚C and 695˚C. However, Ce0.1/TiO2 showed one CO
desorption peak at 615˚C. This indicates that CeO2 does
not show any CO adsorption which in accordance with
other reports [11], though it caused the shift of CO de-
sorption peak to higher temperature compared to that of
Ce-free TiO2. Bimetallic PtCe0.1/TiO2 sample showed
two desorption peaks at 125˚C and another one at
560˚C. The CO desorption temperatures of Ce0.1/TiO2
and Pt/TiO2 are anomalously high comparing to the de-
sorption values of TiO2 i.e. these samples have higher
molecular CO desorption energy due to the creation of
strong basic sites. The strong basic sites of Pt/TiO2 at-
tenuated in presence of Ce in Pt/Ce0.1/TiO2. Such at-
tenuation may be due to site-blocking effect of the other
metal ion (Ce) which occupies adsorption sites for CO
and suppresses CO chemisorptions [12]. The data show-
ed that the apparent area under peaks for Pt/TiO2 is larger
than that for PtCe0.1/TiO2 indicating that the dispersion of
Pt in the former is higher than that in the later [11].
3.1.3. UV-vis Diffuse Reflectance
Diffuse reflection spectra of pure TiO2 and metal sup-
Ms signal intensity, au
Temperature, ˚C
0 100 200 300 400 500 600 700
Figure 2. CO-TPD profiles of parent TiO2 and the prepared
Copyright © 2013 SciRes. MRC
Z. M. EL-BAHY 139
ported catalysts are shown in Figure 3. Generally, ana-
tase-type TiO2 crystalline has a strong absorption edge
below 380 nm [13]. From the presented profiles, the pre-
pared samples showed mainly anatase structure which is
consistent with XRD patterns. In addition, after mounting
metal ions to the surface of TiO2, the slopes of the ab-
sorption edges slightly changed compared with pure TiO2.
The band gap energy was estimated with the Kubelka-
Munk method using diffuse reflectance spectra [14]. Af-
ter calculations, unpromoted titania has a typical band
gap of 3.1 ev, whereas the band gap magnitude of the
metal supported titania decreased slightly; that is 2.95,
2.9 and 2.85 ev for Pt/TiO2, PtCe0.1/TiO2 and Ce0.1/TiO2,
respectively, as listed in Table 1. These results disclose
one crucial fact that the employed metals interact with
TiO2 and hence the band gap changed.
3.2. Adsorption of CO on the Prepared Samples
The results of CO adoption (7 torr) over the pure TiO2
and metal loaded TiO2 at RT in the range of 2400 - 1100
cm1 are presented in Figure 4. Since the absorptions
regions of Ptn+-CO and Ti4+-CO carbonyls may overlap,
it was important to study the adsorption of CO over
metal free TiO2. Adsorption of CO on TiO2 sample, Fig-
ure 4(a), leads to the appearance of two bands, at 2205
and 2189 cm1 which are produced by carbonyl com-
plexes of two types of coordinatively unsaturated sites
(c.u.s.) Ti4+ cations (α- and β-Ti4+, respectively) [15]. The
β-Ti4+ sites are the highly unsaturated sites due to kinks
and corners [5]. It was reported that the IR bands in the
1600 - 1200 cm1 is mainly due to the formation of large
number of different adsorbed carbonate species. In addi-
tion, formate (HCOO-) species can be excluded because
they are formed at relatively high temperatures (227˚C)
[16]. Consequently, the existence of different bands in
the region of 1600 - 1220 cm1 (1356, 1435 and 1533
cm1) under the present adsorption conditions may
Absorbance, au
PtC e
P t/TiO
Wavelength, nm
210 260 310 360 410 460
Figure 3. UV-vis diffuse reflectance of parent TiO2 and the
prepared catalysts.
2200 2000 1800 1600 1400 1200
2205 2187
2205 2187
Wavenumber, cm
Absorbance, au
Figure 4. FTIR spectra after admission of CO (7 torr) over
parent TiO2 and Pt and Ce-supported TiO2.
be taken with a higher degree of confidence to νas(COO-)
and νs(COO-) with a geometry change of these species
[16]. This indicates that CO can be converted to carbon-
ate species over TiO2 even at RT. Indeed, the formation
of carbonate has been previously confirmed spectro-
scopically on surfaces of various oxides as a result of the
adsorption of CO on coordinatively unsaturated Lewis
acid-base pair sites (Mn+-O2), similar to the results re-
ported for CO adsorption over ZrO2 [17], PrO2 [16] or
PtY [18].
To reveal the effect of Ce and Pt loading on TiO2 sur-
face, the in-situ FTIR of CO adsorption on the samples
Ce0.1/TiO2, Pt/TiO2 and PtCe0.1/TiO2 was performed and
the results are presented in Figures 4(b)-(d) as well. In
case of Pt/TiO2, the peaks characteristic of Ti4+ at 2189
and 2205 cm1 were observed besides three peaks at
2082, 2096 and 2128 cm1 which are attributed to line-
arly adsorbed Pt0-CO, Pt+-CO and Pt2+-CO, respectively
[18,19]. The adsorption peaks at 1276, 1362 and 1467,
1690 cm1 are also related to the presence of carbonate
species which are formed on Ptn+-O2 or Ti4+-O2 sites. A
peak at 1737 cm1 was observed which is due to car-
boxylate (COO) species [20].
For Ce0.1/TiO2 catalyst, there were only two peaks at
2187 and 2205 cm1. It was reported in literature that,
two weak peaks at 2177 and 2156 cm1 were found for
CO adsorbed on ceria after evacuation at liquid nitrogen
temperature [21]. However, our adsorption temperature
is much higher than 196˚C, so it is expected to not ob-
serve any of CO adsorption peaks on ceria. Consequently,
the peaks at 2187 and 2205 cm1 are characteristic of CO
adsorption on β- and α-Ti4+ sites, respectively. This is
also in agreement with the work of Chen et al. [22]. This
spectrum also shows peaks characteristic of CO oxida-
tion products such as (-COO) at 1735 cm1 and carbon-
ate species at 1700, 1694, 1467, 1384 and 1273 cm1.
Copyright © 2013 SciRes. MRC
Finally, with respect to PtCe0.1/TiO2 spectrum, only
one peak due to CO adsorption on β-Ti4+ was observed at
2192 cm1, however, the CO adsorption peak at 2205
cm1 characteristic of α-Ti4+ was not observed. Addition-
ally, the sample presented small peaks at 2101 and 2140
cm1 which are due to Pt+-CO and Pt2+-CO, respectively.
It is noticed that, these peaks are located at higher fre-
quencies than those in Pt/TiO2. This shift may be due to
the presence of Ce4+ which decreases the electron density
on Ptn+ species. It is noteworthy to mention that, bridged
Pt2(CO) species were not observed in all the samples in
CO adsorption experiments. This phenomenon means
that Pt particles are mostly isolated with high dispersion
over the surface and this is in good accordance with
XRD data. The spectrum of CO adsorption on PtCe0.1/
TiO2 showed mostly the same carbonate like species in
the frequency range of 1800 - 1100 cm1.
3.3. Adsorption of NO + CO on the Prepared
The spectra (not shown) collected after exposure of the
prepared catalysts to NO gas (7 torr) did not lead to the
formation of any nitrosyls complexes adsorbed on the
metal loaded TiO2. This was not consistent with the pre-
vious literature [23] and it may be due to the low con-
centration of admitted NO gas and the relatively high
adsorption temperatures.
3.3.1. In Situ FTIR Results of NO and CO
Co-Adsorption on TiO2 Cat alys t
Figure 5 shows the spectra of TiO2 (2400 - 1100 cm1)
after admission of gas mixture of CO + NO (7:7) at dif-
ferent temperatures in the range of RT to 200˚C. At RT,
it was observed in the spectrum that, two peaks at 2187
and 2350 cm1 were detected after 20 min of gas expo-
Absorbance, au
Wavenumber, c
2200 2000 1800 1600 1400 1200
Figure 5. FTIR spectra after admission of gas mixture (NO
+ CO; 7:7) on TiO2 at different temperatures.
sure. Based on previous literature reports, the former
band is assigned to CO adsorbed on β-Ti4+ cations and
the latter one is attributed to CO2 [4-6]. These results
indicate that some fraction of the adsorbed CO molecules
undergo oxidation to CO2 upon the contact with gas
phase NO through TiO2 surface even at room tempera-
Additionally, a peak characteristic of carboxylate spe-
cies (COO) was noticed at 1736 cm1 [20]. Carbonate
species were detected at several positions such as 1700
cm1, however, the corresponding asymmetric νas(COO)
peaks for these species are detected at 1470 and 1442
cm1. Different types of carbonates species exhibited
peaks at 1700, 1531, 1385 and 1271 cm1 [24]. The peak
at 1695 and 1470 cm1 may be due to carbonate species
[24] or νs and νas of NH4
+ [25,26]. However, we could not
notice any vibrations in the range of 2600 - 3000 cm1
due to adsorbed NH3. Therefore, the absorption peaks at
1695 and 1470 cm1 are due to carbonate species. When
the temperature of the infrared cell containing wafer was
increased to 100 and 200˚C while keeping the same gas
mixture CO + NO, the same group of bands was noticed
with an increase in the intensity of the peaks characteris-
tic of CO oxidation products. This indicates the increase
of the possibility of CO oxidation and NO reduction with
increasing the wafer temperature. It is noticed that, after
admission of NO gas together with CO, the peak at 2205
cm1 which is due to α-Ti4+-CO was not observed. This
may be due to the increase in the coverage of strongly
adsorbed carbonate species resulting from CO oxidation.
The peak at 1653 cm1 is due to δ(HOH) mode of ad-
sorbed molecular water [27].
3.3.2. In Situ FTIR Results of NO and CO
Co-Adsorption with Pt/TiO2 Catalyst
After recording the spectrum of the sample prior admis-
sion of gas mixture, the IR spectra of CO + NO (7:7)
adsorption over platinum supported TiO2 in the range of
2400 - 1100 cm1 at different temperatures (RT - 200˚C)
are shown in Figure 6. After 20 min of admission of gas
mixture at RT, the spectrum presented peaks due to
α-Ti4+-CO at 2205 cm1, β-Ti4+-CO at 2187 cm1 and the
peak at 2077 cm1 assigned to singleton frequency of CO
molecules linearly adsorbed on-top metallic Pt atoms
(Pt0-CO) [28]. These results indicate that some fraction
of the Pt2+ species, which are initially present in the sam-
ple, underwent reduction to metallic Pt at RT in presence
of CO gas.
Other peaks characteristic of the CO oxidation prod-
ucts such as physisorbed CO2 at 2350 cm1 and a group
of peaks characteristic of (-COO) species at 1737 cm1
and the peaks at 1693, 1582, 1461, 1386 and 1276 cm1
were recorded and attributed to carbonate species ad-
sorbed on different sites or in different geometry [16].
Copyright © 2013 SciRes. MRC
Z. M. EL-BAHY 141
Wavenumber, cm
Absorbance, au
2200 2000 1800 1600 1400 1200
1472 1226
2028 1736
Figure 6. FTIR spectra after admission of gas mixture (NO
+ CO; 7:7) on Pt/TiO2 at different temperatures.
The intensity of these peaks increases with increasing
temperature from RT to 200˚C.
In addition to the oxidation products of CO, the ap-
pearance of a new small peak at 2236 cm1 was noticed
in the collected spectra (at RT) in this case, Figure 6. It
should be mentioned that, the peak at 2236 cm1 was not
observed when only one reactant, either CO or NO, was
admitted to the catalyst surface. It was only observed
when CO + NO were present simultaneously. Also, it
was not observed when CO + NO were admitted to Pt-
free catalysts. Based on previous literature reports [29],
this band can be assigned to the asymmetric vibration of
a surface NCO species as presented in Equation (3). Al-
though, the mechanism of NCO formation is not clear
until now, the assumption in Equation (3) is supported by
the presence of physisorbed CO2 [3]:
O2CO NCOCO  (3)
2NCO2O-SN2CO -S (4)
CO NONCO (5)
According to previously published data, the formation
of isocyanate requires the presence of platinum group
metals in the catalyst [30-32]. In addition, the process of
NCO formation is typically followed by the migration of
these species from the metal sites to the support which
means that NCO may be adsorbed on Ti4+ sites. The
formation of NCO may lead to the formation of CO2 via
the reaction with the oxygen of oxide species (Equation
(4)) or with the reaction with NO molecules (Equation
(5)) [29].
Elevating the temperature of the FTIR cell to 100˚C
and 200˚C leads to the increase of CO2 peak at 2350 cm1,
Figure 6. An increase of the intensity of the peak at 2080
cm1 was detected besides a development of a new peak
at 2028 cm1. These two peaks can be most probably
assigned to symmetric and asymmetric stretching, re-
spectively, of gym-dicarbonyl species (CO-Pt0-CO). At
200˚C, the peak at 2236 cm1 became very intense and
centered at 2215 cm1. Besides the possibility of the
presence of absorption peak characteristic of α-Ti4+-CO
(2205 cm1), the shape of this peak suggests that at least
three overlapping components are present which indi-
cates that several types of adsorbed species on Pt/TiO2
catalyst surface are located in this wide peak. The first
component can be distinguished at 2187 cm1 which was
attributed to CO adsorption on β-Ti4+. The second com-
ponent is centered at 2215 cm1 which is attributed to
isocyanate species (NCO) [30]. The last component is
located at 2235 cm1 and can be assigned NCO ad-
sorbed on different sites or to nitrous oxide (N2O) [33].
N2O may be formed either by reduction of NO by CO
Equations (6) and (7) or by disproportionation process as
Equation (8). It was reported that, N2O is easily formed
on Ti3+ and Ce3+ sites formed after reduction of the cata-
lyst by H2 gas [5]. In this work, this band was not ob-
served after admitting NO + CO to the reduced catalyst
as will be shown later on. This indicates that the peak at
2235 cm1 is most probably assigned to NCO. Close in-
spection of Figure 6, it can be inferred that, the peak
assigned to physisorbed CO2 gas (2350 cm1) increases
in parallel with NCO (2216 cm1) species. This may
point to the formation of NCO through reduction (Equa-
tions (3)-(5)) rather than disproportionation (Equation
(8)). The presence of other NO-derived species at Pt/
TiO2 can be inferred from the small peaks at 1226 and
1197 cm1. For example, the peak at 1200 cm1 could be
associated with a chelating nitrite species (NOx) [15].
 (6)
 (8)
It is well known that, isocyanate, nitrous oxide or ni-
trogen dioxide are proposed to be connected to the reac-
tion mechanism of NO-CO reaction [34]. This result was
also noticed by Keiski et al. [3] since they detected an
increase of isocyanate complex peak after heating at
300˚C. They reported that, the formation of NCO surface
complexes over CeO2 catalyst required that the surface of
the catalyst was first occupied by NO. In our case, the
formation of NCO species did not require NO admission
prior to CO. However it was formed just after NO + CO
gases are admitted together at relatively low temperature
3.3.3. In Situ FTIR Results of NO and CO
Co-Adsorption with Ce0.1/TiO2 Catalyst
In situ FT-IR spectra (2400 - 1100 cm1) of the co-ad-
sorption of NO and CO over Ce0.1/TiO 2 catalyst at RT
Copyright © 2013 SciRes. MRC
was shown in Figure 7. After exposure of NO + CO
mixtures to the catalyst surface and equilibrate for 20
min, the peaks at 2189 cm1 which are due to CO adsorp-
tion over β-Ti4+ was observed. Other peaks characteristic
of CO oxidation products were detected at 1740, 1700,
1465, 1362 and 1276 cm1. The peak at 1740 cm1 is
attributed to carboxylate (COO) species. The other
peaks are due to carbonate species adsorbed at different
sites or adsorbed in different geometry [16]. With the
stepwise increasing temperature from RT to 200˚C, the
intensity of these peaks slightly increases with a parallel
slight increase of the peak at 2350 cm1 which is due to
physically adsorbed CO2. Since NCO was not observed
at all, it can be concluded that CO2 was formed due to the
oxidation of CO with lattice oxygen. Moreover, NO oxi-
dation product was detected at 1622 cm1 due to bridged
nitrate species [27]. A peak at 1653 cm1 was detected
and assigned to δ(HOH).
By careful examination of the spectra in Figure 7, it
can be noticed that, there were some differences between
the spectra obtained after admission of NO + CO gas
mixture to Pt-containing and Ce-containing catalysts as
The α-Ti4+-CO adsorption peak disappeared.
The Ptn+-CO adsorption peaks disappeared.
An oxidation product of NO was observed at 1617
cm1 which is assigned to NO2 [3] and a peak at 1360
cm1 due to nitrate species [15,35].
Due to the absence of platinum group metals, the
spectra did not show any peaks characteristic of NCO
species [29].
3.3.4. In Situ FTIR Results of NO + CO
Co-Adsorption with PtCe0.1/TiO2 Catalyst
Figure 8 reports the evolution of the adsorbed gaseous
Absorbance, au
Wavenumber, cm1
2200 2000 1800 1600 1400 1200
2189 1467
Figure 7. FTIR spectra after admission of gas mixture (NO
+ CO; 7:7) on Ce0.1/TiO2 at different temperatures.
Absorbance, au
Wavenumber, cm
2200 2000 1800 1600 1400 1200
2187 2134
2234 2205 2100
Figure 8. FTIR spectra after admission of gas mixture (NO
+ CO; 7:7) on PtCe0.1/TiO2 at different temperatures.
species under the reaction conditions from RT to 300˚C
over the bimetallic Ce- and Pt-supported on TiO2
(PtCe0.1/TiO2) catalyst. At RT, the spectrum shows peaks
at 2189 and 2205 cm1 which are characteristic of CO
adsorbed over Ti4+ sites as previously mentioned. Other
bands at 2134 and 2100 cm1 are due to carbonyl com-
plexes formed with the participation of platinum cations
Pt2+-CO and Pt+-CO, respectively [36-38]. The spectrum
shows also a small peak at 2234 cm1 due to NCO spe-
The spectrum showed also peaks due to CO and NO
interaction over the PtCe0.1/TiO2 catalyst surface. CO
Oxidation products were observed as strong peak at 2350
cm1 due to physically adsorbed CO2. Other peaks at
1737 cm1 due to (COO) species and 1700, 1465 and
1273 cm1 due to carbonate species adsorbed on different
sites were observed. Some NO oxidation derivatives
were detected as well at 1581 cm1 due to bidentate
and at 1206 cm1 due to [15].
It is clear that there are drastic changes in the spectra
with elevation of the temperature of sample wafer and IR
cell including adsorbed CO on Ptn+ sites and CO + NO
oxidation reduction derivatives. These changes can be
summarized as:
The peaks characteristic of Pt0-CO increased indicat-
ing further reduction of Ptn+ sited with CO at high
A new peak at 2028 cm1 appeared. This peak was
detected (small intensity) in case of Pt/TiO2 catalyst.
Hence it can be assigned to CO adsorbed on Pt0 sites.
This also ensures the possibility of Ptn+ reduction in
presence of CO at high temperature.
Increase of CO2 peak on the expense of other CO
oxidation products which means that CO2 is the pre-
dominant CO oxidation product at high temperature.
Copyright © 2013 SciRes. MRC
Z. M. EL-BAHY 143
Moreover, this data points to the fact that, carbonate
species are the intermediates in CO oxidation to CO2.
The peak centered at 2220 cm1 may be composed of
three components at 2187 cm1 (due to β-Ti4+-CO), a
peak at 2220 and 2235 cm1 (due to NCO species).
CO absorption peak at α-Ti4+ may be one of these
components too.
Increase of the peak characteristic of NCO with a
simultaneous decrease of each of the peaks at 1360
cm1 (3
NO ), 1737 cm1 (COO) and 1700, 1465 and
1273 cm1 may point to the reaction of these species
to form NCO and CO2. However the exact mecha-
nism of these reactions is not exactly known. It was
reported in literature that, the addition of cerium to
Rh/TiO2 [5] and Rh/Al2O3 [39,40] enhances the NO
dissociation rate at quite low temperatures and sup-
presses the N2O formation. Consequently, isocyanate
is the dominant product that arises after the co-ad-
sorption of NO + CO gas to the PtCe/TiO2.
Examining the results of NO + CO adsorption over
Pt/TiO2 and Ce0.1/TiO2, Figure 8, points to the fact that,
NCO can be formed only on the surface of Pt/TiO2 but
not on Ce0.1/TiO2 surface. This is in good agreement with
the previously reported data [3]. In general, it is believed
that NCO formation occurs on Lewis acid sites via the
reaction of CO and nitrogen atoms that is formed by NO
dissociation. It has also been suggested that the process
of NCO formation is typically followed by the migration
of these species from the metal sites to the support where
such species are more stable [28]. Additionally, investi-
gating Figures 6 and 8, it is observed that, the intensity
of the peaks characteristic of CO2, NCO are higher in
Figure 8 (PtCe0.1/TiO2). It can be concluded that, Ce
enhanced the formation of NCO and CO2. This may be
due to the availability of more Lewis acid sites which is
necessary to form NCO [28]. It will be interesting to
eliminate the Lewis acid sites by reducing one of the
employed catalysts and check the possibility of the for-
mation of isocyanate species.
3.3.5. In Situ FTIR Results of NO + CO
Co-Adsorption with Reduced PtCe0.1/TiO2
It is well known that, the capability of the oxidized and
the reduced forms of platinum are different towards the
CO or NO adsorption and interaction on its surface.
Therefore, PtCe0.1/TiO2 was reduced by H2 gas at 400˚C
for 30 min in static vacuum system in presence of liquid
nitrogen trap. The reduced catalyst PtCe0.1/TiO2 was ex-
posed to NO + CO gas mixture at RT and left for 20 min
before spectrum measurements. The changes in the spec-
tra (2400 - 1100 cm1) as a function of wafer temperature
were recorded and shown in Figure 9. Notably, no peak
absorptions belonging to Ti4+-CO were noticed even after
2200 20001800 1600 14001200
2030 2084
1603 1465 1383
Absorbance, au
Wavenumber, cm
Figure 9. FTIR spectra after admission of gas mixture (NO
+ CO; 7:7) on reduced PtCe0.1/TiO2 at different tempera-
raising the IR cell temperature to 100˚C, however, two
components were observed at 2199 and 2215 cm1 due to
CO adsorption on Ti4+ sites and NCO, respectively, at
200˚C. Similar to NO + CO adsorption over reduced
Rh/Al2O3 [3], gym-dicarbonyl species were observed at
2084 and 2030 cm1 after admission of gases at RT.
These peaks are characteristic of symmetric and asym-
metric stretching of C-O bond, respectively, of adsorbed
CO on Pt sites. The absorption peak related to asymmet-
ric C-O bond (2030 cm1) disappears, while the intensity
of symmetric C-O bond (2084 cm1) increased with blue
shift to lower wavelength (2079 cm1) indicating the de-
crease of CO concentration. Increasing the wafer tem-
perature to 200˚C leads to an increase of the absorption
peak at 2345 cm1 which is characteristic of physisorbed
CO2. The appearance of CO2 was accompanied by a de-
crease of gym-dicarbonyl species which points out to the
transformation of CO to CO2.
In the low wavelength region (1800 - 1100 cm1), dif-
ferent peaks were attributed to CO oxidation species
were observed at 1738 cm1 due to carboxylate species
(COO) and the peaks at 1700, 1582, 1472, 1383 and
1276 cm1 were recorded and attributed to carbonate spe-
cies adsorbed on different sites. Additionally, the peak at
1655 cm1 may be due to H2O vibration or 3
. The
progress of both 1655 and 1224 cm1 confirms the as-
sumption that these peaks are related to the presence of
asymmetric bicarbonate species (3
HC ) [41] and their
intensity increase by increasing the wafer temperature.
Besides the CO oxidation products, NO reduction de-
rivatives were also observed. The small broad peak cen-
tered at 2657 cm1 (not shown) is attributed to phy-
sisorbed ammonia (NH3). This peak did not change even
with increasing the IR cell temperature. The peaks at
Copyright © 2013 SciRes. MRC
1696 and 1472 cm1 were also observed and they are
assigned to the νs and νas vibrations, respectively, of
4 ions [25,26]. A new peak at 1178 cm1 was no-
ticed after admission of the gas mixture at RT to the re-
duced catalyst surface. According to Liu et al. [35], the
peak at 1178 cm1 is due to anionic nitrosyle (NO) spe-
cies. With increasing temperature to 100˚C and 200˚C,
the intensity of this peak increased with a red shift to
higher wavelength (1183 cm1). It is noteworthy to men-
tion that, NH3 and NO were detected only after admis-
sion of the gas mixture (NO + CO) over reduced PtCe0.1/
TiO2 catalyst but not over Pt/TiO2, Ce0.1/TiO 2 or as pre-
pared PtCe0.1/TiO2. These data confirms that, at RT, the
formation of NCO (as reductive derivative) is preferable
over as-prepared Pt-containing catalysts while NH3 or
NO is preferable over reduced catalysts.
It is clear that, all the absorption peaks characteristic
of C containing species are shifted to lower wavelength
with raising temperature. However, the peak at 1692
cm1, which was detected at 100˚C and 200˚C, can be as-
signed to() rather than () species.
It is noteworthy to mention that, the absorption peak
related to Pt0-CO increased with increasing temperature
to 100 and 200˚C. Also, two peaks at 2345 and 2200
cm1 were observed at 200˚C. These peaks are attributed
to physisorbed CO2 and NCO species, respectively. The
increase of the absorption peak characteristic of Pt0-CO
and the increase of the peak characteristic of CO2 may
indicate that carbonate species formed at RT may be re-
duced once again to CO and forms Pt0-CO or they may
be oxidized further to CO2 which is detected at 200˚C.
These results confirm the assumption that
is an intermediate in the oxidation of CO to CO2. Fur-
thermore, reduction of NO with CO (NO* + CO*
NCO* + O*) on the reduced catalysts surface occurs only
at relatively high temperature (200˚C) on the reduced
surface. Though, this reaction takes place at RT in the
Pt-containing as-prepared catalysts.
It was mentioned earlier that the presence of Pt sites
are essential to the formation of NCO. The absence of
both the absorption peaks characteristic of Ti4+-CO and
NCO in the temperature range of RT - 100˚C provides
clear evidence that Lewis acid sites (most probably Ti4+)
is essential for the stabilization of isocyanate species.
Nevertheless, NCO species formed at Pt sites can spill
onto the support surface where they are accumulated and
stabilized [29,30].
The rational increase of the peak characteristic of
3 with the decrease of 4 may indicate that
bicarbonate species are formed by the interaction be-
tween species and
43 3
3.3.6. In Situ FTIR Results of NO + CO
Co-Adsorption with PtCe0.1/TiO2 Catalyst in
Presence of Water Vapor
Addition of trace amount of water vapor increased the
activity towards the CO oxidation in presence of Pt and
decreased the oxidation temperature as well [18,42-44].
Moreover, the gas exhaust usually contains water vapor
and then it will be valuable to add trace amount of H2O
vapor to the gas mixture to detect its effect on the NO +
CO interaction. Figure 10 shows the spectra taken after
20 min of admission of the gas mixture (NO + CO + H2O;
7:7:1) over PtCe0.1/TiO2 to shade light on the effect of
introducing water vapor during NO reduction by CO
(more realistic conditions). After adsorption of the gas
mixture at RT, several peaks were observed due to CO
adsorption over Pt+-CO and Pt0-CO at 2094 and 2072
cm1, respectively. A new peak at 1840 cm1 due to
bridged Pt0-CO-Pt0 was observed [4]. This indicates that
the presence of water vapor leads to the decrease of Pt
particles dispersion.
With increasing temperature, the intensity of the peak
at 2092 cm1 decreases and the peak at 2072 cm1
slightly shifted to lower frequency and even gained some
intensity. This phenomenon is, however, most probably
due to some reduction of cationic platinum to metal dur-
ing the CO desorption. Also, at 200˚C, small intensity
peaks were observed at 2350 cm1 indicating the forma-
tion of physically adsorbed CO2. Inspecting of Figure 10,
isocyanate species NCO (2240 - 2200 cm1) could not be
observed in presence of water which means that the in-
teraction of gas mixture (NO + CO) do not proceed in the
pathway to form NCO even at high temperature. The
small intensity peaks characteristic of CO2 may be
formed by direct CO oxidation with lattice oxygen. It is
clear that the Ti4+ sites were not observed except very
Wavenumber, c
Absorbance, au
2200 2000 1800 1600 1400 1200
100ºC 1850
2068 1738
1700 1696
Figure 10. FTIR spectra after admission of gas mixture (NO
+ CO + H2O; 7:7:1) on PtCe0.1/TiO2 at different tempera-
Copyright © 2013 SciRes. MRC
Z. M. EL-BAHY 145
weak peak at 2180 cm1 after raising IR cell temperature
to 200˚C. This may be due to the competitive adsorption
of H2O and CO over Ti4+ sites. The unavailability of Ti4+
and the absence of isocyanate species confirm the as-
sumption that, Ti4+ sites (Lewis acid) are necessary as
hosts for NCO species after their formation over Pt sites.
The spectra were also studied in the frequency range
of 1800 - 1100 cm1. Different peaks characteristic of CO
oxidation products were observed at 1738 cm1 for car-
boxylate (COO), 1554 and 1225 cm1 for bicarbonate
(3) and (1700, 1611 and 1383 cm1) for
species were detected. Finally, some NO reduction prod-
ucts were also observed at 1696 and 1465 cm1 for 4
and 1183 cm1 for NO. It can be seen that, by increasing
the wafer temperature, the peak characteristic of NO is
red shifted to low wavenumber and the intensity of the
peaks characteristic to CO + NO oxidative reductive
products increased.
3.3.7. Effect of Ce Rati o on t h e in Situ FTIR Results
of NO + CO Co-Adsorpt ion with PtCex/TiO2
The influence of the amount of Ce in PtCex/TiO2 on the
NO + CO adsorption and interaction on the catalyst sur-
face was studied using the same above-mentioned ex-
perimental conditions, except that the ratio of Ce/TiO2
was changed in the range of 0.05% - 1%. For the sake of
simplicity, Figure 11 shows the spectra recorded after
admission of the gas mixture and heating the wafer at
200˚C in the range of 2400 - 2000 cm1 since the spectra
did not show significant difference below 2000 cm1
(now shown) in the CO oxidation products region. The
spectra recorded after admission of gases to Pt/TiO2 was
added for comparison. This spectrum Figure 11(a)
shows different peaks characteristic of linear Pt0-CO
(2077 cm1), CO absorption peak on β-Ti4+ (2187 cm1),
NCO species adsorbed on different sites (2217 and 2236
cm1) and finally physisorbed CO2 (2350 cm1). Addition
of Ce with 0.05 and 0.1% did not change the peak posi-
tions however it caused drastic changes in the intensity of
all the peaks since the peaks intensity was maximum in
PtCe0.1/TiO2 sample. Increasing Ce content to 0.5% leads
to the decrease of the peak intensity and all the peaks
(except CO absorption peak on β-Ti4+) disappeared after
raising the Ce content to 1%. As previously discussed,
according to Di Monte et al. [33], the formation of iso-
cyanate requires the presence of platinum group metals
in the catalyst. The decrease of NCO and CO2 with in-
creasing Ce and the vanish of the adsorption peaks of
Ptn+-CO indicate that Ce atoms may block Pt sites at
Ce/TiO2 ratio more than 0.1% and it totally mask Pt sites
when Ce/TiO2 ratio is 1%. Consequently, neither NCO
nor CO2 could be formed. As a result, the optimum ratio
of Ce in Pt/TiO2 to reduce NO and oxidize CO is being
Wavenumber, cm
2400 2300 2200 2100 2000
Absorbance, au
2350 2187
Figure 11. FTIR spectra after admission of gas mixture (NO
+ CO; 7:7) on PtCex/TiO2 at different temperatures. (a) x =
0; (b) x = 0.05; (c) x = 0.1; (d) x = 0.5; and (e) x = 1.
It is noticed that, NCO was not detected after admis-
sion of NO + CO over (PtCe1/TiO2, Ce0.1/TiO2 and TiO2)
and also after admission of (NO + CO + H2O) over
PtCe0.1/TiO2. In the former case, we could not observe
any peak due to Ptn+-CO and in the later case we could
not observe any peak due to Ti4+-CO. However, we ob-
served NCO after admission of (NO + CO) over Pt/TiO2
and PtCe0.1/TiO2. In these cases, we observed both of
Ptn+-CO and Ti4+-CO as well. From forgoing results, it is
concluded that the presence of the mutual sites Ptn+ and
Ti4+ is crucial for the formation of NCO as a result of NO
reduction. Additionally, the activity of PtCe0.1/TiO2 was
higher than Pt/TiO2, indicating that Ce enhances the re-
action towards NO reduction and forms NCO.
4. Conclusion
Platinum and cerium supported TiO2 catalysts were suc-
cessfully prepared by incipient wetness impregnation
method in the particle size range of 19 - 22 nm. Different
reductive/oxidative species were observed after adsorp-
tion of NO and/or CO gasses. The presented results in-
stantly allowed us to estimate the chemisorbed species
after the admission of NO + CO gas mixture and their
interactions on Pt and Ce-containing catalysts supported
on TiO2. The formation of both NCO and CO2 was par-
alled indicating that their formation belongs to the same
mechanism which is not certain until now. It was found
that, the presence of both Pt sites and Ti4+ is important
for NO reduction and NCO formation. Addition of Ce
increased the formation of both NCO and CO2 and it was
optimum Ce/TiO2 ratio of 0.1%.
Copyright © 2013 SciRes. MRC
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