FTIR spectroscopic characterization of NOx species adsorbed on ZrO2 and ZrO2-SO42-

Article abstract of DOI:10.1021/jp013593o

The nature of the NO_x species produced during NO adsorption at room temperature and during its coadsorption with oxygen on pure and sulfated zirconia was studied via in situ FTIR spectroscopy. NO adsorption at room temperature and sulfated zirconia occurred through disproportionation, leading to the formation of nitrous acid; water molecules; nitro species; and anionic nitrosyls, NO^-. The latter species were stable on the surface of zirconia, while on the sulfated sample, they were readily oxidized by the SO₄^2- groups. The process of NO disproportionation was favored by wet surfaces and occurred with the participation of the hydroxyl groups of zirconia. The coadsorption of NO and O₂ on pure zirconia led to the formation of monodentate, bidentate, and bridged nitrate species. On sulfated zirconia, no monodentate NO₃^- species were formed, and the total surface concentration of the NO₃^- ions was lower. The nitrate species obtained on the sulfated zirconia were thermally less stable than those on pure zirconia.

Full text of DOI:10.1021/jp013593o

J. Phys. Chem. B 2002, 106, 3941-3949
3941
2-
FTIR Spectroscopic Characterization of NOx Species Adsorbed on ZrO2 and ZrO2-SO4
Margarita Kantcheva* and Erkan Z. Ciftlikli
Department of Chemistry, Bilkent UniVersity, 06533 Bilkent, Ankara, Turkey
ReceiVed: September 20, 2001; In Final Form: January 7, 2002
The nature of the NOx species produced during the adsorption of NO at room temperature and during its
coadsorption with oxygen on pure and sulfated zirconia has been investigated by means of in situ FTIR
spectroscopy. The adsorption of NO on both samples occurs through disproportionation leading to the formation
of nitrous acid; water molecules; nitro species; and anionic nitrosyls, NO-. A mechanism for the formation
of these adsorption forms is proposed. The NO- species are stable on the surface of zirconia, whereas on the
2-
sulfated sample, they are readily oxidized by the SO4 groups. The process of NO disproportionation is
2-
favored by wet surfaces and occurs with participation of the tribridged (ZrO2) and terminal (ZrO2-SO4
)
hydroxyl groups. Coadsorption of NO and O2 on pure zirconia leads to the formation of various kinds of
nitrate species. The presence of sulfate ions reduces the amount of surface nitrates and decreases their thermal
stability. An analysis of the combination bands of the nitrate species shows that this spectral region can be
used for structural identification of bidentate and bridged nitrates.
Introduction
been proposed.15 To the best of our knowledge, no IR data on
the coadsorption of NO and O2 on sulfated zirconia are available.
Recent reports have shown that transition metal oxides
supported on zirconia and sulfated zirconia are promising for
the selective catalytic reduction (SCR) of NO with NH31,2 and
hydrocarbons.2-8 Delahay et al.5,6 demonstrated that the promo-
tion of Cu/ZrO2 (an effective catalyst in the SCR of NO with
decane) by sulfate ions widens the temperature range of
selectivity toward dinitrogen. To understand the effect of
sulfation in the mechanism of NO reduction, it is important to
obtain information on the nature and stability of the NOx species
adsorbed on the surface of pure and sulfated zirconia. Infrared
(IR) spectroscopy is one of the most powerful tools for the in
situ characterization of adsorbed NOx species on catalytic
materials.9 However, only a few studies9 have appeared employ-
ing this technique for the identification of surface NOx species
produced during the adsorption of NO and during its coadsorp-
tion with oxygen on pure and sulfated zirconia.
In this study, FTIR spectroscopy has been used to identify
and determine the thermal stability of surface NOx compounds
on pure and sulfated zirconia. An attempt is made at under-
standing the processes leading to the formation of NO and NO/
O2 adsorption forms.
Experimental Section
Sample Preparation. Zirconia was prepared by hydrolysis
of ZrCl4 (Merck, for synthesis) with a concentrated (25%)
solution of ammonia. During the dissolution of ZrCl4 in water
(0.3 mol of ZrCl4/L) and addition of ammonia solution, the
temperature was kept below 343 K. The precipitate was washed
thoroughly with deionized water to remove the chloride ions,
dried at 393 K for 12 h, and calcined for 4 h at 773 K. The
BET surface area was 69 m2/g, and according to XRD, the
substance was predominantly monoclinic (94%).
In general, the interaction of NO with the surface of zirconia
results in the formation of small amounts of NOx surface
compounds. Some authors10-12 have reported that nitrosylic
species cannot form on pure zirconia, whereas others13-15 have
observed coordination of NO to Zr4+ sites. Mononitrosyl species
have been found on sulfated zirconia.11 The adsorption of NO
on pure10-15 and sulfated zirconia11 at room temperature gives
rise to bands in the nitrito-nitrato region. However, interpreta-
tions of the bands in this region are quite contradictory. A typical
example is the band at 1180-1190 cm-1, which has been
assigned to nitrite10,13,15 or nitrate12 species. Greater amounts
of NOx species (mainly nitrates and nitrites) on ZrO2 are
observed when NO and O2 are coadsorbed at room tempera-
ture.12,15 In the identification of NOx species adsorbed on
zirconia, the use of bands in the combination region can be
helpful.12 The adsorption of NO12 and NO/O2 coadsorption cause
alterations in the hydroxyl groups of zirconia,12,15 and formation
of chelated or bridged species of the type (O2NO-H)δ- has
Sulfated zirconia was prepared by impregnation of the
calcined material with a solution of (NH4)2SO4. The nominal
content of the sulfate ions was 3.8 wt %. The sulfatation of
zirconia led to an increase in the content of the tetragonal phase
(10%). The BET surface area of this material was 83 m2/g.
Surface Area Measurements and X-ray Diffraction. The
BET surface areas of the samples (dehydrated at 523 K) were
measured by nitrogen adsorption at 77 K using a MONOSORP
apparatus from Quanto Chrome. XRD analysis was performed
on a Rigaku Miniflex diffractometer with Ni-filtered Cu KR
radiation under ambient conditions. The crystallographic phase
composition was calculated using the method of Toraya et al.16
Infrared Spectroscopy. The FTIR spectra were recorded on
a Bomem MB 102 FTIR (Hartman & Braun) spectrometer
equipped with a liquid-nitrogen-cooled MCT detector at a
resolution of 4 cm-1 (128 scans). A specially designed IR cell
allowed registration of the spectra at ambient temperature and
catalyst activation at higher temperatures. The cell was con-
nected to a vacuum/adsorption apparatus. Self-supporting disks
(0.044 g/cm2) were used for the FTIR studies. These specimens
* To whom correspondence should be addressed. Phone: +90 (312) 290
2451. Fax: +90 (312) 266 4579. E-mail: margi@fen.bilkent.edu.tr.
10.1021/jp013593o CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/21/2002

3942 J. Phys. Chem. B, Vol. 106, No. 15, 2002
Kantcheva and Ciftlikli
The bands at 1540, 1495, 1363, 1327, and 1015 cm-1 are
due to bidentate carbonates.18,20 Such residual surface species
are often observed on zirconia and cannot be removed by high-
temperature activation. The sharp band at 1380 cm-1 and the
broad, complex absorption with a maximum at 1000 cm-1
observed on the sulfated sample are assigned to the split ν3
mode, whereas the weaker band at 930 cm-1 is attributed to
the ν1 mode of multidentate SO42- groups coordinated to Zr4+
ions.21 The magnitude of splitting (approximately 380 cm-1) is
typical of highly covalent sulfates.21
The partially deuterioxylated ZHD sample, in addition to the
absorption in the OD region, exhibits a band at 1660 cm-1
,
which is probably due to a carboxylate structure. For the sulfated
sample, the absorptions at 2860 and 2740 cm-1 reveal the
presence of formate species.20 Most probably, the formate and
carboxylate species are produced during the activation of the
samples by interaction of CO and CO2, respectively (formed
as products of the decomposition of the residual carbonates),
with the sample surface. Jung and Bell20 showed that, in the
formation of formate species from CO adsorbed at 523 K on
zirconia, the surface hydroxyls are involved, and the extent of
their participation depends on their acidity, being higher in the
case of sulfated zirconia.21 An alternative interpretation of the
weak bands at 2860 and 2740 cm-1 (observed only in the case
of activated ZS sample) is to attribute them to first overtones
of the band with a maximum at 1380 cm-1 due to the ν(SdO)
mode, which is not resolved in the fundamental region.
Figure 1. FTIR spectra of the samples studied in the (A) OH-stretching
region and (B) carbonyl region of CO (666.6 Pa) adsorbed at room
temperature.
were activated by heating for 1 h in a vacuum at 673 K and in
oxygen (133 kPa) at the same temperature, followed by
evacuation for 1 h at room temperature (standard activation). Z
denotes the zirconia sample that was subject to the standard
activation procedure, whereas the designation ZS is used for
the sulfated material. The partially deuterioxylated sample was
obtained by introduction to the activated zirconia of water vapor
(173.3 Pa) containing 50% deuterium, followed by evacuation
at room temperature for 15 min and heating in a vacuum for 1
h at 623 K. The sample treated in this way is denoted ZHD.
The hydroxylated sample of zirconia (notation ZH) was prepared
by the same procedure using water vapor with normal isotopic
content. The spectra of the activated samples (taken at ambient
temperature) were used as a background reference. The spectra
of the samples that were subject to treatment at elevated
temperatures were recorded after the IR cell had been cooled
to room temperature. All of the spectra presented (except those
in Figure 1) were obtained by subtraction of the corresponding
background reference.
Figure 1B shows the IR spectra of adsorbed CO (666.6 Pa)
at room temperature on the Z and ZS samples. Two overlapping
bands centered at 2201 and 2193 cm-1 are observed for the
pure zirconia, which correspond to two types of Zr4+-CO
carbonyls.22 For the sulfated sample, there is a high-frequency
shift of both carbonyl bands: the band at 2193 cm-1 detected
on the pure zirconia is shifted to 2200 cm-1 (∆ν ) 7 cm-1),
and the band at 2201 cm-1 to 2209 cm-1 (∆ν ) 8 cm-1). This
indicates that modification of the surface by sulfate ions causes
an increase in the strength of the Lewis acid sites as a result of
the presence of charge-withdrawing SO42- groups.21,23 It should
be pointed out that, in the case of the sulfated sample, the total
population of coordinatively unsaturated Zr4+ sites has de-
creased. Obviously, there are sulfate groups directly coordinated
to the Zr4+ sites making them inaccessible for CO adsorption.
The computer peak fittings were performed using the
minimum number of peaks and fixed peaks positions based on
the original spectra. The sum of the squares of the deviations
between the experimental points and fit values was less than
Adsorption of NO on Zirconia at Room Temperature.
Most of the bands observed during the contact of NO (1.07
kPa) with the surface of the zirconia samples with different
degrees of hydroxylation or after partial deuterioxylation,
develop with time (the spectra are not shown). For all of the
samples studied, a negative band at 3650-3630 cm-1 (2696
cm-1 in the case of the ZHD sample) is observed indicating
alteration of bridged OH groups. The intensity of this band
reaches a maximum approximately 20 min after the introduction
of NO to the IR cell and then starts gradually to decrease. At
the same time, a broad band in the 3500-3000/2700-2300
cm-1 region due to H/D-bonded OH/OD groups develops and
increases steadily in intensity.
10-4
.
The CO (99.95%, BOC, Ankara) used was passed through a
trap cooled by liquid nitrogen before admission to the IR cell.
The purity of NO gas was 99.9% (Air Products).
Results and Discussion
FTIR Spectra of the Activated Samples and Adsorbed CO.
Figure 1A shows the FTIR spectra of the pure and sulfated
zirconia, which were subjected to different activation procedures.
The spectrum of the Z sample in the OH-stretching region is
similar to that reported by Guglielminotti17 and is characterized
by a weak band at 3750 cm-1, a sharp and strong band at 3650
cm-1, and a broad absorption with a maximum at 3475 cm-1
.
The spectra of adsorbed NO (1.07 kPa) at room temperature
for a period of 60 min on samples Z, ZH, and ZHD are shown
in Figure 2. The intensities of the bands at 2285, 2244, and
1912 cm-1 do not change with time and appear less intense in
the case of wet samples (compare spectra a-c in Figure 2).
After evacuation for 15 min at room temperature, the bands at
2285 and 2244 cm-1 decrease strongly in intensity, whereas
the band at 1912 cm-1 disappears from the spectrum. This
behavior is illustrated in the spectrum of sample Z (Figure 2,
According to the literature data,18-20 the former two bands are
assigned to terminal (3750 cm-1) and bridged OH groups (3650
cm-1) coordinated to three Zr atoms. The broad band centered
at 3475 cm-1 is attributed to H-bonded tribridged hydroxyls.
Modification of zirconia with SO42- ions (sample ZS) leads
to a decrease in the population of bridged hydroxyl groups. This
indicates that sulfate ions have replaced some of the three-
coordinated OH groups.

2-
NOx Species Adsorbed on ZrO2 and ZrO2-SO4
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3943
Figure 2. FTIR spectra of NO (1.07 kPa) adsorbed at room temperature
for a period of 60 min on samples (a) Z, (b) ZH, and (c) ZHD and (d)
after evacuation of sample Z at room temperature for 15 min.
Figure 4. Correlation between the integrated absorbances of the species
in the 1200-1100 cm-1 region produced during NO adsorption (1.07
kPa, room temperature) at various times on sample ZH.
Figure 3. Results of the curve-fitting procedure applied to the FTIR
spectrum in the 1270-1050 cm-1 region taken after 15 min of NO
adsorption (1.07 kPa, room temperature) on the ZH sample.
Figure 5. FTIR spectra of NO (1.07 kPa) (a) adsorbed on sample ZH
at room temperature for 60 min, (b) followed by evacuation for 15
min at room temperature, (c) after subsequent addition of 1.33 kPa of
oxygen, and (d) after heating of the closed IR cell for 10 min at 573
K.
spectrum d), which shows that the evacuation also leads to the
disappearance of the shoulder at 1230 cm-1 of the band at 1186
cm-1. Judging from this behavior, the bands at 2285, 2244, and
approximately 1230 cm-1 are attributed to N2O12,14,15,24 probably
adsorbed on two different surface sites. The absorption at 1912
cm-1 is assigned to Zr4+-NO species.13-15 All other bands grow
with time in the presence of gaseous NO, and their intensities
increase with increasing degree of sample hydroxylation (com-
pare spectra a and b in Figure 2). Partial deuterioxylation
(sample ZHD) leads to a lowering of the intensity of the band
at 1605 cm-1 compared to that in the Z and ZH samples and
the appearance of an additional absorption at 1403 cm-1 (Figure
2, spectrum c). From this result, the bands at 1605 and 1403
cm-1 are assigned to δ(H2O) and δ(HOD), respectively, of
adsorbed water.25 The corresponding ν(OH/OD) stretching
modes are observed in the 3500-3000/2700-2300 cm-1 region.
The band at 1186 cm-1 (Figure 2, spectra a-c) has a higher
intensity in the case the of wet samples (ZH and ZHD). This
band has a complex shape. The results of curve fitting of the
spectrum taken after 15 min of NO adsorption on the ZH sample
in the 1270-1050 cm-1 region are shown in Figure 3. The weak
bands at 1235 and 1220 cm-1 are due to ν(NO) stretching modes
of adsorbed N2O (two types).12,14,15,24 Curve fitting was applied
to all of the spectra taken within a period of 60 min of NO
adsorption, and the dependence of the integrated absorbance of
the band at 1186 cm-1 on the integrated absorbances of the
bands at 1680, 1605, 1325, and 1140 cm-1 was examined. In
all of these cases, no linear correlation was found, which
indicates that this band corresponds to one NOx species. Figure
4 displays the plots of the integrated absorbances of the bands
at 1680, 1605, 1186, and 1140 cm-1 versus the integrated
absorbance of the band at 1325 cm-1 at various times of NO
adsorption. The linear relationship between the intensities of
the bands at 1680, 1605, 1140, and 1325 cm-1 suggests either
that they belong to one surface species, or that the species
characterized by the corresponding bands are produced in a
common process. The bands at 1680 and 1140 cm-1 most
probably characterize one surface species because their intensi-
ties change almost in parallel. On the basis of the literature
data,12,24,26 they are assigned to the ν(NdO) and δ(NOH) modes,
respectively, of cis-HNO2 formed on the surface of zirconia.
Taking into account that the band at 1605 cm-1 is due to
adsorbed water molecules, it is concluded that water and the
species at 1325 cm-1 are produced simultaneously by one and
the same reaction. The latter band is attributed to superimposi-
-
tion of the νas(NO2) and νs(NO2) modes of NO2 nitro
species.9,25,27
Figure 5 shows the spectrum of adsorbed NO (1.07 kPa) on
the ZH sample for 60 min (spectrum a) followed by evacuation
for 15 min at room temperature (spectrum b). Oxygen (1.33
kPa) was added to the sample treated in this way (spectrum c).
This procedure does not affect the intensity of the bands in the
2500-1000 cm-1 region. However, the band at 1186 cm-1

3944 J. Phys. Chem. B, Vol. 106, No. 15, 2002
Kantcheva and Ciftlikli
disappears after heating the closed IR cell (without evacuation)
for 10 min at 573 K, and a series of new bands in the 1650-
1200 cm-1 region is observed instead (Figure 5, spectrum d).
The strong band at 1568 cm-1 is typical of surface bidentate
nitrate species.9,27 This species can be produced by the dispro-
portionation of nitrous acid, which is known to be unstable on
heating. Indeed, the band at 1680 cm-1 due to the ν(NdO) mode
of HNO2 disappears from the spectrum. The products of HNO2
disproportionation, HNO3, H2O, and NO, give rise to nitrate
species (1568 cm-1) and adsorbed water molecules (shoulder
at approximately 1630 cm-1). In the oxygen atmosphere, the
NO evolved can be oxidized to NO2 and then adsorbed as nitrate
species. The bands at 1460 and 1400 cm-1 correspond to
ν(NdO) modes of monodentate nitrito species.27 Probably, the
nitro species (band at 1325 cm-1), which are obtained during
the NO adsorption, are not affected by the treatment with
oxygen. It is known that nitro compounds are stable in oxygen
atmosphere.28 Finally, the species that are characterized by the
band at 1186 cm-1 and disappear after oxidation contribute to
the formation of surface nitrate and nitrite compounds. This
indicates that this adsorption form contains nitrogen in a low
oxidation state and undergoes oxidation under the conditions
described.
Figure 6. FTIR spectra of NO (2.0 kPa) (a) adsorbed on sample Z at
room temperature for 30 min, (b) after subsequent introduction of 4.27
kPa of oxygen, and (c) after evacuation for 15 min at room temperature.
Coadsorption of NO and O2 on Zirconia at Room
Temperature. Spectrum b in Figure 6 was obtained after the
subsequent introduction of 4.27 kPa of oxygen into the IR cell
containing 2.0 kPa of NO (spectrum a in the same figure taken
30 min after NO adsorption). It should be noted, that im-
mediately after the admission of oxygen, the color of the gas
phase changed to brown, indicating that a considerable amount
of NO2 was formed. This causes drastic changes in the spectrum
of preadsorbed NO (Figure 6, spectrum b). The bands at 1186
and 1140 cm-1 disappear, and two unresolved and intense bands
in the 1700-1150 cm-1 region appear. A poorly resolved
absorption at 1040 cm-1 is also detected. In the 2250-1700
cm-1 region a broad, complex absorption is observed. In
addition, weak bands at 2845, 2800, 2610, 2450, and 1755 cm-1
are detected. In the OH region, the negative band at 3675 cm-1
due to the altered, bridged OH groups increases in intensity
and is accompanied by an enhancement in the positive absorp-
The formation of the surface species observed during the
adsorption of NO on the surface of zirconia and the changes in
their surface concentrations with time can be explained assuming
the following reaction
-
2NO + OH- f HNO2 + NO- {OH } NO- + NO2- + H2O
(1)
According to this process, NO undergoes disproportionation with
participation of the bridged OH groups of zirconia to cis-HNO2
and anionic nitrosyl, NO-. The band at 1186 cm-1 is assigned
to the latter species. The nitrous acid produced partially
dissociates to nitro species (1325 cm-1), evolving protons, which
recombine with surface hydroxyls to form water molecules or
recombine with coordinatively unsaturated oxide ions. The latter
process leads to partial restoration of the consumed hydroxyls.
This can explain the observed gradual decrease in the intensity
of the negative band at 3650 cm-1 in the later stages of NO
adsorption. The experimental data show that the occurrence of
reaction 1 is favored by wet surfaces.
tion at 3600-3000 cm-1
.
The strong absorption in the 1700-1150 cm-1 region is due
to the formation of surface nitrates and is attributed to the ν3
spectral splitting of adsorbed NO3- ions differing in their types
of coordination.9,13,27 The band at 1040 cm-1 is assigned to the
νs(NO2) mode.9,13,27 The nitrate species can form by dispropor-
tionation of NO2 with the participation of the surface OH groups
The proposed interpretation of the band at 1186 cm-1
disagrees with its assignments to surface nitrites10,13,15 and
monodentate nitrates12 offered in the literature. Anionic nitrosyl
species have been observed when NO is adsorbed on alkaline
earth metal oxides such as MgO29 and CaO,30 on lanthanide
2OH- + 3NO2 f 2NO3- + H2O + NO
(2)
The appearance of a negative band at 3675 cm-1 and the
broad positive absorption in the 3600-3000 cm-1 region
supports the occurrence of this process. Similar interaction was
observed to occur during the adsorption of NO2 on titania
(anatase).33,34 Another possibility for the formation of nitrate
species involves disproportionation of NO2 on surface Lewis
acid-base pairs according to the reaction28,33
metal oxides such as CeO2 and La2O3,27,32 and on TiO2.28
31
The evacuation for 15 min at room temperature of all of the
samples studied causes a slight increase in the intensities of
the bands at 1680, 1605, 1325, and 1186 cm-1 due to adsorbed
HNO2, H2O, NO2-, and NO- species, respectively (see Figure
2, spectra a and d, and Figure 5, spectra a and b). Enhancement
in the intensity of the H-bonded OH groups is also observed,
which is accompanied by a decrease in the intensity of the
negative band due to alteration of the bridged hydroxyls at 3650
cm-1 (Figure 2, spectra a and d). These changes most probably
are caused by transformation of the nitrosyls coordinated to the
Zr4+ ions according to reaction 1.
Finally it should be pointed out that adsorbed N2O is observed
often upon adsorption of NO on zeolites and oxides.9 In the
case of zirconia, it can be produced by the adsorption of NO
on an oxygen pair of vacancy sites as proposed for La2O3 by
Huang et al.27
2NO2 f NO3- + NO+
(3)
The N-O stretching frequency of nitrosonium ion adsorbed
on surfaces is found at 2290-2102 cm-1 9,28,33-35
Spectrum b
.
in Figure 6 displays a broad, poorly resolved absorption at
approximately 2210 cm-1, which loses intensity after evacuation
at room temperature for 15 min and shifts to 2240 cm-1
.
However, the behavior of the band at 2240 cm-1 under
evacuation at higher temperatures (see below) demonstrates that
it is not associated with adsorbed NO+ ions.
The complex band in the 2200-1700 cm-1 region with a
maximum at 1970 cm-1 (Figure 6, spectrum b) falls in the

2-
NOx Species Adsorbed on ZrO2 and ZrO2-SO4
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3945
spectral region typical for the coordination of NO on strong
Lewis acid sites. The assignment of the feature at 1970 cm-1 is
made by taking into account the behavior of the weak bands at
1980 and 1905 cm-1 that are observed after evacuation at room
temperature. These bands resist high-temperature vacuum treat-
ment (see below) and are not associated with nitrosylic species.
The band at 1970 cm-1 appears in the spectrum when large
amounts of surface nitrates are present. For this reason, in
accordance with the literature data,12 we assign this band to
Zr4+-NO nitrosyls that have NO3- species in their coordination
sphere, i.e., to the complex (ON)-Zr4+-(NO3-). The absorption
at 1930 cm-1 can be attributed to a second (ON)-Zr4+-(NO3
)
-
species or to adsorbed N2O3. Because evacuation causes
simultaneous disappearance of the band positioned at ap-
proximately 1330 cm-1, we prefer to assign the features at 1930
and 1330 cm-1 to ν(NdO) and νs(NO2) modes, respectively,
of adsorbed N2O3. This species forms during NO and O2
coadsorption on various oxide systems and is removed easily
by evacuation at room temperature.9 The N2O3 molecule
adsorbed on zeolites produces a ν(NdO) stretching vibration
in the 1930-1880 cm-1 region, the νas(NO2) modes appear at
1590-1550 cm-1, and the modes due to νs(NO2) are detected
at 1305-1290 cm-1 9 The shoulder at 2040 cm-1, which
.
disappears after evacuation at room temperature, is tentatively
δ+
assigned to NO2
.
Figure 7. FTIR spectra obtained after heating of sample Z containing
preadsorbed NOx species for 15 min in a vacuum at various temper-
atures. This spectrum (298 K) corresponds to spectrum c in Figure 6.
After removal of the gaseous phase by evacuation at room
temperature, the bands in the nitrate region are better resolved
(Figure 6, spectrum c). The changes in the two major band
envelops in the 1700-1000 cm-1 region suggest that, most
probably, some desorption/decomposition of the species has
occurred. It can be seen that the bands at 2610 and 2450 cm-1
appear in the spectrum with reduced intensity. The bands at
2845 and 2800 cm-1 are covered by the absorption due to
H-bonded OH groups, and their behavior under these conditions
cannot be followed. In addition, two weak bands at 1980 and
1905 cm-1 are detected.
at 1415 and 1330 cm-1, which are more visible after treatment
at 623 K, are attributed to monodentate nitro compounds.27
The weak bands at 2845, 2800, 2610, 2450, 2240, 1980, 1905,
and 1755 cm-1 (Figures 6 and 7) are due to combination modes
of the nitrates species.24 These features are detected during NO/
O2 coadsorption, which results in the formation of a large
amount of surface nitrate species. This assignment is supported
by the experiments showing the thermal stability of nitrate
species. The bands in the 3000-1700 cm-1 region follow the
behavior of the fundamental nitrate bands. Combination bands
at 2610, 2580, and 2520 cm-1 due to different kinds of nitrates
on the surface of zirconia and the overtone band at 2444 cm-1
of nitrito species were also reported by Hadjiivanov.12
It is difficult to propose an unambiguous assignment for the
combination bands of the species in the nitrate region because
the fundamental bands below 1000 cm-1 cannot be detected.
We start with the spectrum obtained after evacuation for 10
min at 673 K, which is characterized by the lowest concentration
of surface nitrates. The spectrum exhibits bands corresponding
to the ν3 spectral splitting of two types of bidentate nitrates:
1590 [ν(NdO)] and 1222 cm-1 [νas(NO2)] and 1555 [ν(Nd
O)] and 1222 cm-1 [νas(NO2)]. The νs(NO2) stretching vibration
is at 1045 cm-1. From the inset in Figure 7, it can be seen that
the band at 2600 cm-1 with a shoulder at 2580 cm-1 corresponds
to a binary combination between the ν(NdO) vibrations at 1590
and 1555 cm-1 and the νs(NO2) mode at 1045 cm-1. The
combinations between the νas(NO2) mode at 1222 cm-1 and
the νs(NO2) mode at 1045 cm-1 give rise to the absorption with
a maximum at 2250 cm-1. This band has a high-frequency
shoulder positioned at about 2295 cm-1. This implies that the
fundamental band at 1222 cm-1 should contain a weaker
component at approximately 1260 cm-1, which is the contami-
nant band of that at 1590 cm-1. The existence of an unresolved
absorption in the region of the band at 1222 cm-1 can explain
the relatively high intensity of the latter relative to that of the
bands at 1590 and 1555 cm-1. The band at 2430 cm-1 is the
first overtone of the band at 1222 cm-1. Again, this band has a
An attempt to assign the bands in the nitrate region was made
by following the changes in the spectra obtained under vacuum
treatment at elevated temperatures. Study of the thermal stability
of the adsorption forms produced during interaction of zirconia
with the NO/O2 mixture (Figure 7) shows that the majority of
species formed at room temperature are still present after heating
in a vacuum for 10 min at 623 K. (The spectra were recorded
after the IR cell had been cooled to room temperature). The
changes in the shapes and intensities of the nitrate bands indicate
that in the temperature range 298-398 K, rearrangement rather
-
than decomposition of the NO3 species has occurred. For
example, the band at about 1480 cm-1 is not visible after heating
at 398 K, but the intensities of the nitrate bands at 1635, 1580,
and 1520 cm-1, together with that of the complex band at 1280-
1200 cm-1, have increased. An increase in the intensities of
the weak bands in the 3000-1700 cm-1 region is also detected.
Noticeable decomposition of the nitrate species begins at 623
K. After evacuation at 673 K, the absorption in the 3000-1700
cm-1 region becomes very weak, and the bands in the nitrate
region appear with strongly reduced intensity. The negative band
in the OH region shows that the hydroxyl groups are not
completely restored and that some of them are lost during high-
temperature evacuation. From the results of the thermal stability
and the magnitude of the ν3 spectral splitting,9,13,27 the nitrate
bands can be assigned to bridged (1640-1620 and 1216-1200
cm-1), bidentate (1602-1590 and 1260-1240 cm-1, 1580-
1545 and 1222-1210 cm-1, 1520 and 1280 cm-1), and
monodentate (1480 and 1295 cm-1) species. The weak bands

3946 J. Phys. Chem. B, Vol. 106, No. 15, 2002
Kantcheva and Ciftlikli
TABLE 1: Assignments of the FTIR Bands Observed
during Adsorption of 1.07 KPa of NO and Its Coadsorption
with O2 (NO:O2 ) 1:2) at Room Temperature on Zirconia
NOx species
N2O (ads)
band positions (cm-1
)
modes
2285, 2244
1230
1912
1970
1930
1330
1680
1140
3550-3000
1605
1325
1186
1640-1620
1216-1200
1040
ν(NN)
ν(NO)
ν(NO)
ν(NO)
Zr4+-NO
ON-Zr4+-(NO3
N2O3
-
)
ν(NdO)
νs(NO2)
cis-HNO2 (ads)
ν(NdO)
δ(NOH)
H2O (ads)
ν(OH)
δ(HOH)
NO2- (nitro)
NO-
νas(NO2), νs(NO2)
ν(NO)
NO3- (bridged)
ν(NdO)
νas(NO2)
Figure 8. FTIR spectra of NO (1.07 kPa) adsorbed on sample ZS at
room temperature for various times. The spectra are taken (a)
immediately after NO admission, (b) after 12 min, (c) after 60 min,
and (d) after evacuation for 15 min at room temperature.
νs(NO2)
2845, 2800
∼2240
1980, 1900-1905
1602-1590, 1580-1545, 1520 ν(NdO)
1260-1240, 1222-1210, 1280 νas(NO2)
1040
2600, 2580
2470, 2430
2295, 2250
1755, (1708)
ν(NdO) + νas(NO2)
νas(NO2) + νs(NO2)
νs(NO2) + δ(ONO)
NO3- (bidentate)
for structural identification of bidentate and bridged nitrates.
The proposed application of the combination region12 for
distinguishing between nitrato and nitrito surface compounds
on the basis of the presence (or absence) of an overtone band
at 2440 cm-1 requires further evidence.
νs(NO2)
ν(NdO) + νs(NO2)
2νas(NO2)
νas(NO2) + νs(NO2)
νs(NO2) + δ(ONO)
νas(NO2)
NO3- (monodentate) 1480
1295
The absorption bands observed on zirconia after exposure to
either NO or a NO/O2 mixture are summarized in Table 1.
Adsorption of NO at Room Temperature on Sulfated
Zirconia. The evolution of the spectra of adsorbed NO (1.07
kPa, room temperature) on the ZS sample with time is shown
in Figure 8. As in the case of the unpromoted sample, the bands
at 2290, 2240, and about 1230 cm-1 are due to adsorbed N2O,
whereas the weak broad band at 1950 cm-1 is assigned to NO
adsorbed on Zr4+ ions that have sulfate groups in the vicinity.11
This absorption appears at higher frequency because of the
increased acidity of the adsorption site caused by the electron-
νs(NO2)
high-frequency shoulder at about 2470 cm-1, which confirms
the assumption that there is an unresolved absorption at
approximately 1260 cm-1. This interpretation disagrees with
the conclusion made by Hadjiivanov12 that an analogous band
observed at 2440 cm-1 is due to the first overtone of a band at
1220 cm-1 associated with nitrito species, which are formed
during thermal decomposition of surface nitrates on zirconia.
The weak bands at 1980, 1900, and 1755 cm-1 (see Figures
6 and 7) probably are due to combinations between the νs(NO2)
(1045 cm-1) and δ(ONO) (in-plane bending) modes. This
assignment is based on the fact that the combination band
2-
withdrawing SO4 species. The absorption at 1912 cm-1
corresponds to the N-O stretching vibration of NO coordinated
to Zr4+ sites that are not perturbed by sulfate groups. The bands
due to adsorbed N2O and Zr4+-NO nitrosyls reach saturation
within a period of 6 min. All other bands continue to grow up
to 45 min of NO adsorption.
-
[νs(NO2) + δ(ONO)] of free NO3 ion appears in the 1800-
1700 cm-1 region.24,25,36 Upon coordination in bulk compounds,
the δ(ONO) mode splits into two bands giving rise to two
[νs(NO2) + δ(ONO)] combination bands.36 The maximum
separation between the latter is 66 cm-1 for bidentate nitrate
ion and 26 cm-1 for monodentate species.36 Thus, it can be
proposed that the pair of bands at 1980 and 1900 cm-1 is
A noticeable difference between the pure and sulfated zirconia
is that, in the latter case, the altered OH groups are at 3750
cm-1, i.e., terminal hydroxyls are involved in interactions with
NO, leading to the development of a positive absorption between
3600 and 3000 cm-1. The difference in reactivity of the hydroxyl
-
associated with [νs(NO2) + δ(ONO)] modes of bridged NO3
species (spectral splitting of 80 cm-1), whereas the band at 1755
cm-1 and probably that at 1708 cm-1 (see the inset in Figure
7) are indicative of the presence of bidentate NO3- ion (spectral
splitting of approximately 50 cm-1). The latter absorption is
masked by the band at 1630-1600 cm-1 and can be detected
after heating the sample in a vacuum at 673 K. Under these
conditions, the broad absorption with a maximum at 2800 cm-1
and the pair of bands at 1980 and 1900 cm-1 have disappeared,
and the spectrum in the fundamental nitrate region contains
bands associated with bidentate nitrates. It can be concluded
that the bands at 2845 and 2800 cm-1 (Figures 6 and 7) are
due to a combination between the ν(NdO) (ca. 1640-1620
cm-1) and νas(NO2) (ca. 1210 cm-1) modes of the bridged
nitrates. Finally, a combination between the νas(NO2) and
νs(NO2) vibrations of the bridged nitrates should contribute to
2-
groups on zirconia in the presence or absence of SO4 ions
can be explained by assuming that sulfate ions replace some of
the topmost bridged hydroxyl groups and that the remaining
ones are not involved in disproportionation of NO. As in the
case of pure zirconia, the bands at 1680 and 1610 cm-1, due to
adsorbed HNO2 and H2O, grow with time. A weakly resolved
component at approximately 1555 cm-1 develops as well. The
increase in intensity of the negative band at 1400 cm-1, due to
terminal SdO bonds, indicates that a perturbation of the sulfate
groups occurs. A broad, poorly resolved absorption in the 1390-
1200 cm-1 region and a band at 1120 cm-1 grow with the extent
of NO exposure. A similar set of bands has been observed upon
adsorption of water on sulfated zirconia21 and is assigned to
split ν3 mode of bidentate sulfato complex of C2V symmetry.
Obviously, the water molecules produced upon NO adsorption
affect the structure of the original sulfate groups. In contrast to
the adsorption of NO on pure zirconia, no band at 1186 cm-1
is detected.
the absorption at 2240 cm-1 when these NO3 species are
present on the surface of zirconia, i.e., in the 298-623 K
temperature range of evacuation (Figure 7). These considerations
show that the combination region of the spectra can be used
-

2-
NOx Species Adsorbed on ZrO2 and ZrO2-SO4
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3947
Figure 10. FTIR spectra of NO (2.0 kPa) (a) adsorbed on sample ZS
at room temperature for 30 min, (b) after subsequent introduction of
4.27 kPa of oxygen, and (c) after evacuation for 15 min at room
temperature.
Figure 9. FTIR spectra of NO (1.07 kPa) adsorbed on sample ZS for
60 min followed by evacuation for 15 min at room temperature and
(a) after subsequent addition of 1. 33 kPa of oxygen and (b) after heating
of the closed IR cell for 10 min at 573 K. Spectrum c was obtained by
the subtraction of spectrum b from spectrum a.
tion (spectrum d in Figure 8) does not cause any changes.
Heating of the closed IR cell for 10 min at 573 K leads to the
disappearance of the bands at 2244 and 1680 cm-1 due to
adsorbed N2O and HNO2, respectively (Figure 9, spectrum b).
New bands at 1545 and 1360 cm-1 are detected. Spectrum c in
Figure 9 (obtained by the subtraction of spectrum b from
spectrum a) illustrates the changes described. In addition, it
shows the existence of two negative bands at 1245 and 1160
Evacuation for 15 min at room temperature (Figure 8,
spectrum d) leads to an increase in the intensity of both positive
and negative bands in the 1700-1000 cm-1 region, which is
accompanied by an enhancement of the intensities of the
negative band at 3750 cm-1 and the positive absorption in the
region of the H-bonded OH groups. The bands due to adsorbed
N2O (2290 and 2240 cm-1) appear with reduced intensity, and
the absorption at 1912 cm-1 due to Zr4+-NO species disappears.
The band at 1950 cm-1 shifts to 1960 cm-1 and resists
evacuation, which confirms that it is associated with NO
adsorbed on strong Lewis acid sites.
cm-1. The band at 1545 cm-1 can be assigned to nitrate NO3
-
species. The negative bands at 1245 and 1160 cm-1 are due to
ν(N-O) and δ(NOH) modes of decomposed N2O and HNO2,
respectively. It is known that N2O is also an unstable compound
and decomposes rapidly to N2 and O2 when heated. Probably,
the nitrate species produced are localized in the vicinity of the
sulfate groups. This causes a perturbation of the latter, as
evidenced by an additional increase in the intensity of the
negative band at 1400 cm-1 and the appearance of a positive
band at 1360 cm-1. The suggestion that the nitrate species affect
the sulfate groups is supported further by the experiments of
It is difficult to propose a straightforward interpretation of
the experimental results obtained during the adsorption of NO
on sulfated zirconia, because of the existence of overlapping
bands in the 1500-1000 cm-1 region arising from simultaneous
alteration of the sulfate groups and formation of NOx adsorbed
species. The fact that adsorbed water and nitrous acid are
detected implies that a process analogous to that described by
reaction 1 occurs in the case of the ZS sample. However, no
absorption that can be assigned to an anionic nitrosyl, NO-, is
observed. It can be proposed that NO- species also form in
this case but are oxidized fast by sulfate ions, for example, to
-
NO/O2 coadsorption where large amounts of NO3 ions are
formed (see below). However, the increase in intensity of the
positive band at 1370 cm-1 is greater than the corresponding
decrease in intensity of the band at 1400 cm-1 due to alteration
of sulfate groups (Figure 10, spectrum c). It can be assumed
-
-
NO3 and NO2 species according to the reactions
2-
that treatment with oxygen causes oxidation of SO3 species
2-
NO- + 2SO42- f NO3- + 2SO3
(4)
(see reaction 4). The restored sulfate groups can have localiza-
tion on the zirconia surface that differs from that of the original
NO- + SO42- f NO2- + SO3
2-
2-
SO4 species. This difference contributes to the absorption at
1360 cm-1. It is also possible that there is a band contaminant
to that at 1545 cm-1, which is superimposed to that at 1360
-
The vibrational modes of the free SO3 ion are below 1000
cm-1
.
cm-1 25
.
In bulk compounds,25 if coordination occurs through
the sulfur atom, the νas(S-O) mode shifts to 1100-1050 cm-1
.
Coadsorption of NO and O2 at Room Temperature on
Sulfated Zirconia. The addition of 4.27 kPa of oxygen (Figure
10, spectrum b) to the IR cell containing NO (2.0 kPa) for 30
min (spectrum a in the same figure) causes a significant
enhancement of the absorption bands in the whole region. The
strong absorption in the 1650-1500 and 1355-1200 cm-1
regions arises from nitrate species analogous to those formed
during NO/O2 coadsorption on pure zirconia. However, their
amount is considerably smaller than in the Z sample (compare
spectra b in Figures 6 and 10). This gives rise to very weak
bands in the combination region of the nitrates.
In the case of O bonding, this mode splits into two and falls in
the 900-850 cm-1 region. Because of their poor quality, the
spectra below 1050 cm-1 are not informative, and the formation
2-
of coordinated SO3 species cannot be proven. However, the
shoulder at approximately 1555 cm-1 (Figure 8) can be assigned
to nitrate species. In the case of NO adsorption on pure zirconia,
this band is not observed (Figure 2), which can support the
-
occurrence of reaction 4. The NO2 species (nitro and nitrito)
exhibit characteristic bands in the 1400-1000 cm-1 27
but if
,
formed, these bands cannot be observed because of overlap with
the absorption due to perturbed sulfate groups.
Upon contact of the ZS sample with NO/O2 mixture, the band
at 1680 cm-1 due to adsorbed HNO2 disappears (Figure 10,
spectrum b). The negative band in the OH-stretching region
The addition of 1.33 kPa of oxygen (spectrum a in Figure 9)
at room temperature to the sample evacuated after NO adsorp-

3948 J. Phys. Chem. B, Vol. 106, No. 15, 2002
Kantcheva and Ciftlikli
broadens, indicating that additional Zr4+-OH groups are
involved in the adsorption process. Two strong bands with
maxima at 3570 and 3380 cm-1 are detected, which are
attributed to νas(OH) and νs(OH) modes, respectively, of
adsorbed water molecules, produced according to the process
already described for the Z sample (see reaction 2). The δ(H2O)
mode falls in the region of the strong nitrate band at 1620 cm-1
.
The intensity of the negative band at 1400 cm-1 (due to altered
2-
SO4 groups during NO adsorption) has also increased. This
can be explained by assuming a perturbation of some of the
original sulfate groups caused by the NO3- species formed. The
electronegative nitrate species affect the topmost SO42- groups
through an induction effect, which causes a shift of the band
due to the ν(SdO) mode to lower frequency (a positive
absorption at approximately 1355 cm-1, more clearly visible
in spectrum c obtained after evacuation).
The broad absorption with a maximum at 1930 cm-1 (Figure
10, spectrum b) can be attributed to adsorbed N2O3 as in the
case of the Z sample. The band at 2240 cm-1 broadens and
shifts to 2228 cm-1. It can be proposed that the latter absorption
is due to the superimposition of bands associated with adsorbed
N2O and combination modes of the nitrate species. After
evacuation for 15 min at room temperature (Figure 10, spectrum
c), a band positioned at 2240 cm-1 is detected again. This
absorption is assigned to a residual amount of N2O and
Figure 11. FTIR spectra obtained after heating of sample ZS containing
preadsorbed NOx species for 15 min in a vacuum at various temper-
atures. This spectrum (298 K) corresponds spectrum c in Figure 10.
-
combination mode of NO3 species. The weak band at 1750
cm-1 (Figure 10, spectra b and c) is also due to a combination
mode. Evacuation causes the disappearance of the band at 1930
cm-1 assigned to adsorbed N2O3. This is accompanied by the
loss of intensity and narrowing of the complex band at 1650-
1500 cm-1, which indicates that the low-frequency shoulder at
about 1530 cm-1 is associated with N2O3 [νas(NO2) mode24].
The band at 1120 cm-1 due to water-perturbed sulfate groups
shifts to 1145 cm-1 after addition of oxygen and does not change
significantly in intensity after evacuation at room temperature.
The νs(NO2) mode of the nitrate species should appear between
1100 and 1000 cm-1 but is not observed because of the
enhancement of the absorption in this region after the admission
of oxygen.
residual carbonate species (see Figure 1, spectrum ZS) that
undergo decomposition during the vacuum heat treatment, it is
difficult to follow the behavior of the band at 1400 cm-1
.
Evacuation at 673 K causes complete desorption of the nitrate
species and restoration of the original hydroxyl groups.
The inset in Figure 11 shows the combination region of the
fundamental nitrate bands in the spectrum obtained after
evacuation at 363 K. As illustrated above, the bands in the
combination region can help in determining the type of
coordination of surface nitrates. The presence of a relatively
symmetric band at 2615 cm-1 indicates that there is only one
-
type of bidentate NO3 species with the fundamental band at
1580 cm-1 [ν(NdO) mode]. Taking into account that the former
band represents a sum of the ν(NdO) and νs(NO2) modes of
bidentate nitrates, the position of their νs(NO2) vibration can
be estimated as approximately 1040-1050 cm-1. The poorly
resolved band at 1750 cm-1 also confirms the existence of
bidentate nitrates. The pair of bands at 1964 and 1892 cm-1
Heating the sample at 363 K in a vacuum (Figure 11) results
in a small decrease in intensity of the nitrate bands and the
almost complete disappearance of the band at 2240 cm-1. As
shown above, a similar absorption arises from a combination
between the νas(NO2) mode of the nitrates (at approximately
1250 cm-1) and the corresponding vibrations due to the νs(NO2)
mode. However, after evacuation at 363 K, the absorption at
1280-1250 cm-1 decreases in intensity by less than 50%,
whereas the decrease in intensity of the band at 2240 cm-1 is
approximately 10-fold (see the inset in Figure 11). This result
confirms the suggestion that the absorption at 2240 cm-1 is due
mainly to strongly held N2O, which desorbs or decomposes after
heating at 363 K. The intensities of the bands at 1280 and 1145
cm-1 and that of the negative band at 1400 cm-1 decrease,
which indicates that restoration of the perturbed sulfate groups
has occurred. Heating under a vacuum at 523 K causes a
decrease in intensity of the band at 1620 cm-1, which is
accompanied by the disappearance of the hydrogen-bonded OH
groups in the 3500-2750 cm-1 region. This behavior is
consistent with the suggestion that the band at 1620 cm-1 is
due to nitrate species and adsorbed water molecules. It seems
that the bands at 1280 and 1145 cm-1 are not present after this
treatment and the sulfate groups are almost completely restored.
However, because of the appearance of negative bands at 1520,
1450, and 1330 cm-1 (indicated by asterisks) corresponding to
-
supports the presence of bridged NO3 species (fundamental
band at 1620 cm-1). The absorption at 2260 cm-1 corresponds
to a binary combination between the νas(NO2) mode at 1200
cm-1 and νs(NO2) vibration (estimated at approximately 1080
cm-1) of the bridged nitrates. The band at 2118 cm-1 probably
is associated with a combination between the component at 1145
cm-1 of the split ν3 mode and the ν1 mode of the perturbed
sulfate groups. The latter vibration is at approximately 990
cm-1 21
, and it was not observed by us. By using the estimated
values of νs(NO2) modes of the nitrate species, the band at 2190
cm-1 could be interpreted as a sum between the νs(NO2) mode
of nitrato and the ν3 component at 1145 cm-1 of sulfato species.
The first overtone of the band at 1250 cm-1 due to nitrato
species should appear between 2500 and 2400 cm-1. However,
the absorption in this region is very weak and obscured by the
negative band due to ambient CO2 and, therefore, cannot be
used in the assignment.
On the basis of these considerations, the nitrate species formed
upon NO/O2 coadsorption on the ZS sample are identified as

2-
NOx Species Adsorbed on ZrO2 and ZrO2-SO4
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3949
TABLE 2: Assignments of the FTIR Bands Observed
during Adsorption of 1.07 KPa of NO and Its Coadsorption
with O2 (NO:O2 ) 1:2) at Room Temperature on Sulfated
Zirconia
and the Scientific and Technical Research Council of Turkey
(TU¨ BITAK), Project TBAG-1706.
References and Notes
NOx species
N2O (ads)
band positions (cm-1
)
modes
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G.; Prinetto, F. Studies in Surface Science and Catalysis; Hightower, J.
W., Delgass, N. W., Iglesia, E., Bell, A. T., Eds.; Elsevier Science
Publishers: Amsterdam, 1996; Vol. 101, p 691.
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V.; Occhiuzzi, M.; Pepe, F. Appl. Catal. B 1999, 21, 141.
(3) Hamada, H.; Kintaichi, Y.; Tabata, M.; Sasaki, M.; Ito, T. Chem.
Lett. 1991, 2179.
(4) Pasel, J.; Speer, V.; Albrecht, C.; Richter, F.; Papp, H. Appl. Catal.
B 2000, 25, 105.
(5) Delahay, G.; Ensuque, E.; Coq, B.; Figue´ras, F. J. Catal. 1998,
175, 7.
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Today 1998, 42, 117.
(7) Chin, Ya-H.; Alvarez, W. E.; Resasco, D. E. Catal. Today 2000,
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2290, 2240
1230
1912
1950-1960
1930
1530
1680
3550-3000
1610
1620
1200
2260
1964, 1892
1580
1250
2615
1750
ν(NN)
ν(NO)
ν(NO)
ν(NO)
Zr4+-NO
ON-Zr4+-(SO4
N2O3
2-
)
ν(NdO)
νas(NO2)
cis-HNO2 (ads)
H2O (ads)
ν(NdO)
ν(OH)
δ(HOH)
NO3- (bridged)
ν(NdO)
νas(NO2)
νas(NO2) + νs(NO2)
νs(NO2) + δ(ONO)
ν(NdO)
NO3- (bidentate)
νas(NO2)
ν(NdO) + νs(NO2)
νs(NO2) + δ(ONO)
bridged (1620 and 1200 cm-1) and bidentate (1580 and 1250
cm-1) nitrates.
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13, 5588.
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Table 2 illustrates the absorption bands observed during NO
adsorption and its coadsorption with O2 on sulfated zirconia.
Conclusions
The adsorption of NO at room temperature on pure and
sulfated zirconia occurs through disproportionation, leading to
the formation of nitrous acid; water molecules; nitro species;
and anionic nitrosyls, NO-. The latter species are stable on the
surface of zirconia, whereas, on the sulfated sample, they are
2-
readily oxidized by the SO4 groups. The process of NO
disproportionation is favored by wet surfaces and occurs with
the participation of the hydroxyl groups of zirconia. These are
tribridged OH groups for pure zirconia and terminal OH groups
for sulfated sample. The anionic nitrosyls are stable in an oxygen
atmosphere at room temperature but are oxidized at 573 K. If
they are in contact with a NO/O2 mixture at room temperature
they transform into nitrato species.
(22) Bolis, V.; Morterra, C.; Volante, M.; Orio, L.; Fubini, B. Langmuir
1990, 6, 695.
(23) Morterra, C.; Bolis, V.; Cerrato, G.; Magnacca, G. Surf. Sci. 1994,
307-309, 1206.
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(25) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds. Part B: Applications in Coordination, Orga-
nometallic, and Bioinorganic Chemistry, 5th ed.; John Wiley & Sons: New
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K. G. Phys. Chem. Chem. Phys. 1999, 1, 2441.
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29.
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2, 2803.
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Soc., Faraday Trans. 1 1974, 70, 729.
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A.; Cataluna, R. J. Chem. Soc., Faraday Trans. 1995, 91, 1679.
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Langmuir 1994, 10, 464.
The coadsorption of NO and O2 on pure zirconia leads to
the formation of monodentate, bidentate, and bridged nitrate
-
species. On sulfated zirconia, no monodentate NO3 species
are formed, and the total surface concentration of the NO3- ions
is lower. In addition, the nitrate species obtained on the sulfated
zirconia are thermally less stable than those on pure zirconia.
Analysis of the combination bands of the nitrate species shows
that this spectral region can be used for the structural identifica-
tion of bidentate and bridged nitrates. Bridged nitrates produce
combination bands at 2845-2800 cm-1 and a pair of bands at
1980-1960 and 1900-1890 cm-1. Bidentate nitrates can be
distinguished by the appearance of combination bands in the
region of 2600 cm-1 and a pair of bands at 1755 and 1700 cm-1
.
(34) Kantcheva, M. M.; Bushev, V. P.; Hadjiivanov, K. I. J. Chem. Soc.,
Faraday Trans. 1992, 88, 3087.
(35) Givan, A.; Loewenschuss, A. J. Chem. Phys. 1989, 90, 6135.
(36) Lever, A. B. P.; Mantovani, E.; Ramaswamy, B. S. Can. J. Chem.
1971, 49, 1957.
Acknowledgment. This work was financially supported by
the Bilkent University, Research Development Grant for 2001,