Routes of formation and composition of NOx complexes adsorbed on palladium-promoted tungstated zirconia

Article abstract of DOI:10.1016/j.molcata.2005.11.022

Surface species obtained during the adsorption of NO and NO/O₂ coadsorption at room temperature on Pd-free (WZ) and Pd-promoted tungstated zirconia (Pd/WZ) are identified by means of in situ FT-IR spectroscopy. The WZ and Pd/WZ samples have a tetragonal structure and contain randomly distributed mesoporous phase. Dispersed palladium(II) species are present in two different environments: (i) Pd²⁺ ions, which have only Zr⁴⁺ ions in their second coordination sphere and (ii) Pd²⁺ ions, which are linked to both zirconium and tungsten ions via oxygen bridges. On the Pd/WZ sample, NO undergoes oxidation to various NO_x species depending on the temperature. The compounds formed at room-temperature oxidation are adsorbed N₂O₃ and products of its self-ionization, NO⁺ and NO₂^-. In this process W(VI) is involved, being reduced to W(IV). At high temperature N₂O₃ decomposes, restoring the WO species. Under these conditions, NO undergoes oxidation to NO₂ by the Pd(II) ions, which are reduced to Pd(I). The nitrosyls of Pd(I) display high thermal stability and do not disappear upon evacuation at 623 K. During NO/O₂ coadsorption on the Pd/WZ catalyst at room temperature, the amounts of surface nitrates and NO₂/N₂O₄ formed in the gas phase are significantly lower than those observed under identical conditions in the presence of tungstated zirconia. It is concluded that promotion of tungstated zirconia with palladium(II) suppresses the oxidation of NO by molecular oxygen at room temperature due to the elimination of acidic protons involved in the process.

Full text of DOI:10.1016/j.molcata.2005.11.022

Journal of Molecular Catalysis A: Chemical 247 (2006) 88–98
Routes of formation and composition of NO complexes adsorbed on
x
palladium-promoted tungstated zirconia
∗
Margarita Kantcheva , Ilknur Cayirtepe
Laboratory for Advanced Functional Materials, Department of Chemistry, Bilkent University,
06800 Bilkent, Ankara, Turkey
Received 21 September 2005; received in revised form 14 November 2005; accepted 16 November 2005
Available online 27 December 2005
Abstract
Surface species obtained during the adsorption of NO and NO/O
2
coadsorption at room temperature on Pd-free (WZ) and Pd-promoted tungstated
zirconia (Pd/WZ) are identified by means of in situ FT-IR spectroscopy. The WZ and Pd/WZ samples have a tetragonal structure and contain
2
+
randomly distributed mesoporous phase. Dispersed palladium(II) species are present in two different environments: (i) Pd ions, which have only
4
+
2+
Zr ions in their second coordination sphere and (ii) Pd ions, which are linked to both zirconium and tungsten ions via oxygen bridges. On
the Pd/WZ sample, NO undergoes oxidation to various NO species depending on the temperature. The compounds formed at room-temperature
oxidation are adsorbed N and products of its self-ionization, NO and NO
high temperature N decomposes, restoring the W O species. Under these conditions, NO undergoes oxidation to NO
which are reduced to Pd(I). The nitrosyls of Pd(I) display high thermal stability and do not disappear upon evacuation at 623 K. During NO/O
coadsorption on the Pd/WZ catalyst at room temperature, the amounts of surface nitrates and NO /N formed in the gas phase are significantly
x
+
−
2
O
3
2
. In this process W(VI) is involved, being reduced to W(IV). At
by the Pd(II) ions,
2
O
3
2
2
2
2 4
O
lower than those observed under identical conditions in the presence of tungstated zirconia. It is concluded that promotion of tungstated zirconia
with palladium(II) suppresses the oxidation of NO by molecular oxygen at room temperature due to the elimination of acidic protons involved in the
process.
©
2005 Elsevier B.V. All rights reserved.
Keywords: NO adsorption; NO/O2 coadsorption; NOx adsorbed species; FT-IR spectroscopy; WO3–ZrO2; Pd/WO3–ZrO2
1
. Introduction
the Brønsted acid sites of the zeolite are needed to keep palla-
dium(II) highly dispersed and active [4,5,10–20] although the
structure of the palladium species (isolated Pd2 ions [10–15]
versus highly dispersed PdO [16–20]) is still matter of discus-
sion.
The use of zeolite-based catalysts for the CH4-SCR of NO
presents important drawbacks due to their low thermal stability
and deactivation by water or SO2. It has been shown that sim-
ple oxides with strong acidity, such as sulfated and tungstated
zirconia, possess the ability to stabilize highly dispersed Pd(II)
species analogous to the acidic zeolites [11,12,21–23]. Low-
loading palladium catalysts supported on sulfated and tungstated
zirconia exhibit catalytic properties in the CH4-SCR of NO
under dry conditions comparable to those of zeolitic catalysts
and they show improved selectivity in the presence of H2O and
SO2 [21–24].
+
Reduction of NO with CH4 in the presence of oxygen (CH4-
SCR) is an attractive strategy for control of the exhaust from
gas engines or turbines fueled with natural gas. Since the first
report of Li and Armor [1] on Co-exchanged ZSM-5 catalysts,
various zeolites containing transition and non-transition metals
have been shown to be active in this reaction [2]. Palladium-
exchanged H-ZSM5 and H-Mor catalysts are more attractive
becausetheyappeartobelesssensitivetosteam[3–7]. Nishizaka
and Misono [8,9] first reported a high activity of Pd-exchanged
H-ZSM-5 catalysts and indicated the direct involvement of pro-
tonic acid sites in both palladium dispersion and the mechanism
of the CH4-SCR of NO. Nowadays, it is widely accepted that
∗
Tungstated zirconia is thermally stable [25] and emerges
as a good candidate for the replacement of zeolitic materials.
Corresponding author. Tel.: +90 312 2902451; fax: +90 312 2664579.
E-mail address: margi@fen.bilkent.edu.tr (M. Kantcheva).
1
381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.11.022

M. Kantcheva, I. Cayirtepe / Journal of Molecular Catalysis A: Chemical 247 (2006) 88–98
89
Maximum activity in the CH4-SCR of NO is observed on
zirconia catalysts containing up to 16–20 wt.% WO3 and
TiCl3, SnCl2 and NH4SCN as chromogenic agents [33,34]. The
WZ sample contained 18.6 wt.% WO3.
0
.1–0.17 wt.% Pd [21–24]. Tungstated zirconia is usually pre-
The zirconia was synthesized by the same method [32], with-
out the addition of ammonium metatungstate solution. The final
carbonization temperature was 873 K. After that the material
was calcined at 773 K for 5 h.
pared by impregnation of hydrated zirconia with ammonium
metatungstate, followed by calcination in the 873–1100 K range
[
25–29]. High density of strong acid sites can also be achieved by
simultaneous coprecipitation with ammonia from solutions con-
taining zirconium oxychloride and ammonium metatungstate
Palladium-promoted tungstated zirconia was prepared
impregnating the WZ sample with a solution of Pd(NO3)2·2H2O
(Merck-Schuchardt) ensuring 0.1 wt.% of nominal palladium
content. The sample was dried and calcined at 773 K for 5 h. For
this sample the notation Pd/WZ is used.
[
30].
Dispersedpalladiumspeciesontungstatedzirconiahavebeen
−
1
identifiedbytheappearanceoftwobandsat1868and1821 cm
in the IR spectra of adsorbed NO [22,23]. Similar bands have
been observed on Pd-H-ZSM-5 and Pd-H-Mor and have been
attributed to nitrosyls formed on isolated Pd2 ions [3,5,16,17].
Other authors [31] have concluded, based on low-temperature
CO adsorption, that the oxidation state of palladium deposited
on tungstated zirconia is +3.
The physico-chemical characteristics of the materials are
summarized in Table 1.
+
2.2. Surface area measurements, X-ray diffraction and
DR-UV–vis spectroscopy
Since the dispersion of Pd(II) is favored by non-microporous
materials [12], in this study we used Pd-free and Pd-promoted
tungstated zirconia containing mesoporous phase. By means of
insituFT-IRspectroscopywehaveidentifiedthesurfacespecies,
obtained during the adsorption of NO and NO/O2 coadsorp-
tion on these materials. The samples have been characterized by
XRD and DR-UV–vis spectroscopy. The localization of the Pd2
species on the surface of tungstated zirconia has been studied
by room-temperature adsorption of CO. The interaction of the
surface NOx complexes with methane can throw a light about
the potentials of Pd-promoted tungstated zirconia containing
mesoporous phase as a catalyst for the reduction of NO and
the mechanism of the reaction. The results of this study will be
reported separately.
The BET surface areas of the samples (dehydrated at 523 K)
were measured by nitrogen adsorption at 77 K using a Monosorp
apparatus from Quantachrome. XRD analysis was performed on
a Rigaku Miniflex diffractometer with Ni-filtered Cu K␣ radia-
tion under ambient conditions. Crystallite sizes were calculated
from the peak broadening of principal peaks with the Scherrer
formula [36]. DR-UV–vis spectra were obtained under ambi-
ent conditions with a fiber optic spectrometer AvaSpec-2048
(Avantes) using WS-2 as a reference.
+
2.3. FT-IR spectroscopy
The FT-IR spectra were recorded on a Bomem MB 102
FT-IR spectrometer equipped with a liquid-nitrogen cooled
−
1
MCT detector at a resolution of 4 cm (128 scans). The self-
supporting discs were activated in the IR cell by heating for 1 h
in a vacuum at 773 K, and in oxygen (13.3 kPa, passed through a
trap cooled in liquid nitrogen) at the same temperature, followed
by evacuation for 1 h at 773 K. A specially designed transmis-
sion IR cell (Xenonum Scientific, USA) equipped with BaF2
windows allowed recording of the spectra at elevated tempera-
tures. The sample holder of the cell can be moved up and down
relative to the light beam, giving the possibility of recording the
gas phase spectrum. All of the FT-IR spectra presented (except
those in Fig. 3A) were obtained by subtracting the spectra of the
activated samples from the spectra recorded.
2
. Experimental
2
.1. Sample preparation
Tungstated zirconia (notation WZ) was prepared by copre-
cipitation of aqueous solutions of ZrOCl2·8H2O (Merck)
and ammonium metatungstate (Aldrich) with ammonia using
polyvinyl alcohol (Aldrich) as a template according to a proce-
dure described in detail in the literature [32]. The gel obtained
was dried at 473 K for 24 h in air. The dried material was heated
for 2 h at 673 and 923 K in an inert atmosphere (carbonization
steps) followed by calcination at 773 K for 5 h. After fusion with
KHSO4 and dissolution in deionized water, the tungsten con-
tent was determined spectrophotometrically at λ = 402 nm using
The CO (99.95%, BOC) used was passed through a trap
cooled by liquid nitrogen before admission to the IR cell. The
purity of the NO gas was 99.9% (Air Products).
Table 1
Physico-chemical characteristics of the samples
2
WO3a (wt.%)
b
WO3 (nm2)
c
Sample
BET surface area (m /g)
Pd (wt.%)
WO3 coverage (monolayer)
ZrO2
WZ
Pd/WZ
96
153
145
–
18.6
18.6
–
–
0.1
–
2.6
2.7
0.5
0.5
a
Analytical content.
b
c
Nominal content.
Estimated by using the maximum packing density of planar WO3 species of 0.21 g WO3/100 m [35].
2

9
0
M. Kantcheva, I. Cayirtepe / Journal of Molecular Catalysis A: Chemical 247 (2006) 88–98
Fig. 1. Powder X-ray diffraction patterns of the calcined samples ZrO2, WZ and
Pd/WZ (m: monoclinic; t: tetragonal).
Fig. 2. UV–vis diffuse reflectance spectra of the calcined samples.
Incorporation of palladium does not modify the spectrum of
tungstated zirconia in the region of charge transfer transitions
3
. Results and discussion
(
Fig. 2), most probably because this absorption originates from
3
.1. Structural characterization of the samples
the bulk phase of the sample. The band at 450 nm in the spectrum
of the Pd/WZ sample is attributed to the d–d transition of either
isolated Pd ions linked to surface oxygen atoms of the support,
or small Pd(O)n2 entities [38]. Shimizu et al. [39] observed a
similar band at 460–480 nm in the absorption spectrum of Pd-H-
Fig. 1 shows the XRD patterns of the catalysts studied.
2+
The zirconia sample is a mixture of tetragonal and monoclinic
phases. The stable crystalline form of ZrO2 is monoclinic. How-
ever, the presence of residual carbonates (see below) leads to
stabilization of a tetragonal phase. For this sample no meso-
porous phase is detected. The average crystallite size is 7.7 nm.
The WZ and Pd/WZ samples are predominantly tetragonal and
contain a randomly distributed mesoporous phase, which is evi-
+
2
+
Mor. This band was attributed to d–d transitions of isolated Pd
ions coordinated to the cation exchange sites of the zeolite. It can
be concluded that tungstated zironia prepared by coprecipitation
is able to stabilize dispersed palladium(II) in a manner similar
to the acidic zeolites.
◦
dent by the peak at 2θ = 1.8 [37]. This peak became lower in
intensity and somewhat broader after the introduction of palla-
dium indicating an increase in the pore size distribution relative
to the WZ sample. The average crystallite size for both WZ and
Pd/WZ samples is 4.9 nm. The mesopores collapse after calci-
nation at 973 K.
The DR-UV–vis spectra obtained for tungstated zirconia and
Pd-containing sample are shown in Fig. 2. The absorption spec-
trum of tungstated zirconia exhibits a strong band at 300 nm
attributed to charge-transfer transitions in the W–O–W units
3.2. FT-IR spectra of the activated samples and adsorption
of CO
Fig. 3A shows the FT-IR spectra of the activated samples. The
spectrum of ZrO2 in the OH-stretching region displays a pair of
−
1
−1
bands at 3770 and 3745 cm , strong absorption at 3665 cm
−1
and a broad band between 3500 and 3000 cm . According to
the literature [40], the band at 3770 cm is attributed to terminal
−
1
−1
[
26]. Using the approach of Iglesia et al. [26,27], the width of
OH groups due to monoclinic zirconia. The band at 3745 cm
the band gap, E0, determined from a linear extrapolation to zero
absorption in a plot of (αhν) versus hν (α, absorbance; hν, pho-
ton energy) has a value of 3.38 eV. This value is similar to that of
ammonium metatungstate and reveals the presence of polymeric
tungstate domains of intermediate size [23,25–28].
arises from bibridged OH groups of tetragonal zirconia. The
2
−1
band at 3665 cm is assigned to tribridged OH groups of mon-
−1
oclinic zirconia. The broad band between 3500 and 2750 cm
is attributed to H-bonded tribridged hydroxyls. The adsorption
of CO (5.33 kPa) on this sample at room temperature does not

M. Kantcheva, I. Cayirtepe / Journal of Molecular Catalysis A: Chemical 247 (2006) 88–98
91
Fig. 3. (A) FT-IR spectra of the activated samples. (B) FT-IR spectra obtained upon adsorption of CO (5.33 kPa) for 45 min at room temperature. The spectra WZ
and Pd/WZ are gas phase corrected.
4
+
The spectrum of the Pd/WZ catalyst in the ν(OH) stretching
lead to the formation of Zr –CO species (Fig. 3B). The rea-
son for this could be the presence of residual carbonates (strong
region differs from that of the WZ support. There are no bands
4+
−
1
arising from isolated Zr –OH groups and the absorption in the
region of H-bonded hydroxyls is almost completely eroded. This
indicates that during the deposition process, Pd2 ions replace
the protons of the surface hydroxyls. In addition, the incorpora-
tion of palladium causes both a shift in the position of the W O
bands in the 1600–1150 cm region, Fig. 3A) which complete
the coordination sphere of the exposed Zr4 sites. The carbonate
species appear during the calcination of the carbonized material
and cannot be removed by a prolonged activation procedure.
The spectrum of the WZ sample (Fig. 3A) contains a sharp
+
+
−
1
−
1
fundamental band to 1010 cm , and a decrease in its intensity.
This behavior can be explained by assuming that some of the
band at 3752 cm corresponding to bibridged OH groups of
tetragonal zirconia [40]. A broad band in the 3650–2500 cm
−
1
2+
Pd ions coordinate to the W O species.
region reveals the presence of H-bonded hydroxyls. In the W O
stretching region (see also the inset in Fig. 3A) a single band
at 1007 cm with an overtone vibration [41] at 2009 cm is
detected. In the case of the WO3–ZrO2 system with similar WO3
loading prepared by impregnation of hydrated zirconia with
The spectrum of CO adsorbed on the Pd/WZ sample (Fig. 3B)
shows that the amount of cus Zr4 ions decreases consider-
ably after the deposition of palladium. The bands at 2160 and
+
−
1
−1
−
1
2
122 cm develop at the time of CO exposure and reach maxi-
−
1
mum intensity after 45 min. These bands do not disappear com-
pletely upon dynamic evacuation at room temperature. Since
Pd(II) undergoes reduction in a CO atmosphere at room tem-
ammonium metatungstate, two bands at 1021 and 1014 cm
are reported [28,42,43]. These bands are assigned to two dif-
ferent oxo species. The presence of a single fundamental W O
band and a single overtone band indicates that the WZ sample
contains only one type of W O groups. It can be concluded
that during the coprecipitation, better dispersion is achieved,
leading to the formation of more uniform WOx species. As pro-
posed by Scheithhauer et al. [28,42], the unresolved absorption
−
1
perature [31], the bands at 2160 and 2122 cm are attributed
+
to Pd –CO carbonyls in two different environments. The weak
−
1
feature at 1888 cm detected upon evacuation corresponds to
bridging CO adsorbed on metallic palladium [16]. It can be
assumed that as a result of replacement of the protons of the
isolated OH groups at 3752 cm (Fig. 3A, spectrum WZ) there
are Pd2 ions linked to zirconium(IV) through oxygen atoms.
These palladium species do not have tungstate groups in close
proximity and upon CO adsorption give rise to the carbonyl band
−
1
−1
at 925 cm can be assigned to W–O–Zr linkages.
+
CO does not form complexes with coordinatively unsaturated
+
(
cus) W6 ions [28,42]. Thus the band at 2195 cm observed
−1
upon adsorption of CO on the WZ sample (Fig. 3B) is attributed
−
1
4+
at 2122 cm . Due to the charge-withdrawing WOx species, the
Pd ions coordinated to the W O groups possess increased
to Zr –CO carbonyls. The modification of zirconia with WOx
species alters the basicity of the oxide ions leading to significant
decrease in the amount of residual carbonates and appearance
of coordinatively unsaturated Zr sites upon activation. The
adsorption of CO causes perturbation of the W O stretching
band. This is evident by the shift of the overtone vibration.
2+
−1
electrophilicity and produce the carbonyl band at 2160 cm
.
ThelocationofthePd2 ionsinthelattercasecanbedescribedby
+
4+
the model shown in Scheme 1. The proposed structure accounts
4+
for both a lowering of the number of cus Zr , and a decrease in

9
2
M. Kantcheva, I. Cayirtepe / Journal of Molecular Catalysis A: Chemical 247 (2006) 88–98
Scheme 1.
the intensity of the W O band. Coordinatively unsaturated Zr4+
sites are created by dehydroxylation during the activation of the
sample.
nitrate is resolved at 1243 cm . The spectrum of NO adsorbed
−1
on the ZrO2 sample contains features that differ from those
observed on monoclinic zirconia [44,46]. This could be associ-
atedwithdifferencesinthemorphology, degreeofhydroxylation
and presence of impurities.
3
.3. Adsorption of NO
The adsorption of NO (1.34 kPa) on the WZ sample at room
temperature produces a more complex spectrum than that of
zirconia (Fig. 4A). Despite the presence of species that are com-
mon for both the WZ sample and tungstated zirconia obtained by
impregnation of Zr(OH)4·xH2O with ammonium metatungstate
The spectrum of NO (1.34 kPa) adsorbed on zirconia at room
−
1
temperature is shown in Fig. 4A. The intense band at 1196 cm
is assigned to the ν(NO) stretching mode of anionic nitrosyl,
−
4+
NO [44]. Since Zr ions are considered to be irreducible under
these conditions, it is proposed that these species are formed by
(
19 wt.%ofWO3), theinterpretationofthespectrumofadsorbed
disproportionation of NO on surface O2− sites:
NO in this study differs from that proposed for the impregnated
material [47]. The assignment of the bands takes into account
their development with the time of NO exposure, behavior upon
evacuation at room temperature and the fact that tungsten(VI)
does not form nitrosyls [48–50]. All of the bands appear in
the spectrum immediately after the introduction of NO to the
IR cell except the absorption at 1950 cm . They increase in
intensity with the time of NO exposure except those at 2243
and 1215 cm . The spectrum obtained after 30 min is shown in
Fig. 4A. The sharp, intense band at 2243 cm and the absorp-
2
4
NO + 2O − → NO3− + 3NO−
(1)
−
According to the ν3 spectral splitting [45], the NO3 species
−
1
are of two types: bidentate nitrate at 1601 cm correspond-
ing to the ν(N O) mode, and monodentate nitrate at 1495 cm
νas(NO2)) and 1296 cm (νs(NO2)) modes. Most probably the
−
1
−1
−
1
(
−
1
νas(NO2) stretching vibration of the bidentate nitrate is covered
−
−1
−1
by the strong NO band at 1196 cm . This assumption is con-
firmed by the behavior of the bands upon evacuation (Fig. 4B):
−
1
tion at 1215 cm correspond to adsorbed N2O and belong to the
ν(NN) and ν(NO) stretching vibrations, respectively [47]. The
broad band at 2131 cm is typical of adsorbed NO species and
−
1
the absorption at 1196 cm decreases in intensity, and the low-
−
1
+
frequency component of the split ν3 vibration of the bidentate
Fig. 4. (A) FT-IR spectra of NO (1.33 kPa) adsorbed for 30 min at room temperature on the samples studied. The gas phase spectra are subtracted. (B) After evacuation
for 15 min at room temperature. The negative bands marked by asterisks are due to residual carbonates, which are removed by the NOx species.

M. Kantcheva, I. Cayirtepe / Journal of Molecular Catalysis A: Chemical 247 (2006) 88–98
93
belongs to the ν(NO) mode [47]. The band at 1950 cm 1, which
−
NO, a negative band develops at 3750 cm (due to altered
−1
4+
is absent from the spectrum at the beginning of the process,
Zr –OH groups) accompanied by a positive absorption in the
3500–3000 cm region. The intensities of these bands increase
is assigned to NO adsorbed on Zr4 sites with increased elec-
+
−1
trophilicity caused by induction effect from adjacent electron-
with dynamic evacuation of NO.
−
1
withdrawingNOx species[47]. Theshoulderatabout1915 cm
Another possibility for formation of N2O3 is through dispro-
portionation of NO with N2O as a concomitant product [51].
This process does not require the presence of redox sites and if
it takes place, adsorbed N2O should appear also in the case of
zirconia. Since this species was not detected on the latter sam-
ple (Fig. 4A), it is assumed that N2O observed on the WZ is
formed by a different process, e.g. adsorption of NO on oxygen
vacancies [52].
The spectrum of NO (1.34 kPa) adsorbed on the Pd/WZ cat-
alyst at room temperature displays features similar to those
observed for the tungstated zirconia (Fig. 4). However, some
differences should be noted:
−
1
and the weak absorption centered at 1327 cm are attributed
to the ν(N O) and νs(NO2) modes of adsorbed N2O3, respec-
tively [44,45,51]. The νas(NO2) stretching vibration appears
−
1
usually at 1590–1550 cm [45] and overlaps with the band
−
1
at 1601 cm . The assignment of the latter absorption is pro-
posed below. The weak band at 1817 cm can be attributed
to NO adsorbed on reduced tungsten species. According to the
literature [49,50], W –NO nitrosyls possess ν(NO) stretching
vibrations at 1843–1856 cm . These species disappear upon
evacuation. In contrast, the W ions form stable W (NO)2
dinitrosyls characterized by a pair of bands at 1765–1785 and
691–1700 cm [48–50]. Although the band at 1817 cm dis-
−
1
5+
−
1
4+
4+
−
1
−1
1
appears upon evacuation (Fig. 4B, spectrum WZ), it cannot be
attributed to W –NO species, because it is positioned below
1
. In agreement with the literatures [3,5,16,17,22,23], the bands
at 1863 and 1812 cm are due to NO adsorbed on Pd(II)
5+
−1
−
1
1
840 cm . For this reason, we assign this band to the ν(NO)
sites in two different environments. The intensities of the
4+
−
stretchingmodeofW –NOnitrosylthathasNOx speciesinits
coordination sphere, i.e. to the complex ON–W –NOx (most
2+
Pd –NO bands do not change with the time of NO exposure.
4+
−
By analogy with the results of CO adsorption, it is proposed
−
probably x = 2, see below). The electron-withdrawing NOx
−1
2+
that the band at 1812 cm represents NO adsorbed on Pd
species increase the electrophilicity of the W4+ ion. This causes
4+
sites, which have only Zr ions in their second coordination
sphere. The band at 1863 cm is attributed to the ν(NO)
stretching mode of NO adsorbed on Pd ions with enhanced
decrease in the ␲ contribution to the bond of adsorbed NO, i.e.
−1
4+
4+
weaker W –NO bond. It is assumed that the W ions appear
in the following process:
2+
Lewis acidity due to an induction effect from adjacent WOx
NO + W6+
2−
→ N2O3 + W4+
species.
2
O
(2)
−1
2
. Most probably, the absorption at 1817 cm assigned to the
The adsorbed N2O3 can undergo self-ionization according to the
reaction [51]:
ν(NO) stretching mode of the mixed nitrosyl-nitrato complex
of W4 is covered by the stronger Pd –NO nitrosyl bands
+
2+
and cannot be detected.
. The N2O3 (ν(N O) at 1925 cm , νas(NO2) at 1560 cm
+
−
N2O3 ꢀ NO + NO2
(3)
−1
−1
3
−
1
−
and νs(NO2) at 1280 cm ) and the bidentate nitrito
species with νas(NO2) and νs(NO2) modes at 1220 and
The NO2 species formed in reaction (3) are identified as biden-
tate nitrito species characterized by the absorption at 1184 cm
due to the νs(NO2) mode [45]. The νas(NO2) mode most proba-
−
1
−
1
1
171–1175 cm , respectively, resist evacuation. The stabi-
−1
lization of the adsorbed N2O3 on the surface of the Pd/WZ
catalyst can be explained by the fact that this sample does
not contain isolated OH groups (see Fig. 3A). This behavior
confirms the occurrence of the process described by Eq. (4).
bly is superimposed on the band at 1215 cm , which is assigned
to the adsorbed N2O. Under dynamic evacuation (Fig. 4B) the
bidentate nitrito species disappear together with the adsorbed
+
N2O3 and NO ions. This confirms the occurrence of equilib-
rium (3), and supports the proposed assignment of the bands. In
the spectrum taken under these conditions, the bands at 1605,
The spectra of the tungstated materials display negative bands
−
1
−1
−1
1
450 and 1425 cm show increased intensities (Fig. 4B). The
at 1009 cm and positive absorption at 978 cm due to per-
turbation of the wolframyl groups. The gas phase spectra taken
in the presence of the WZ and Pd/WZ samples do not contain
bands attributable to N2O3. Most probably, N2O3 is present on
the surface as condense phase, or its concentration in the gas
phase is low and cannot be detected under the conditions of the
existing path length of the IR cell.
latter two bands are attributed to ν(N O) modes of two types
of monodentate nitrito species [45]. The concomitant ν(NO)
−
1
stretching vibrations fall in the 1100–1050 cm region [45]
and are covered by the strong ν(W O) band due to the perturbed
tungstate species. In order to explain the growth of the bands at
−
1
1
605, 1450 and 1425 cm observed upon dynamic evacuation,
the following interaction between the adsorbed N2O3 and the
surface hydroxyls is assumed:
Table 2 summarizes the assignments of the IR bands of the
NOx compounds obtained during the adsorption of NO at room
temperature.
−
−
N2O3 + 2OH → H2O + 2NO2
(4)
The band at 1601 cm 1 is assigned to the δ(H2O) mode of
adsorbed water. This assignment is supported by the fact that
in the OH-stretching region (not shown) in the presence of
−
3.3.1. High-temperature adsorption of NO
Fig. 5A shows the spectrum of the Pd/WZ catalyst taken after
adsorption of NO (1.34 kPa) for 15 min at room temperature

9
4
M. Kantcheva, I. Cayirtepe / Journal of Molecular Catalysis A: Chemical 247 (2006) 88–98
Table 2
Assignments of the FT-IR bands observed upon room-temperature adsorption of NO on the samples studied
−
Sample
ZrO2
NOx species
Band position (cm 1)
Mode
−
NO3 (bidentate)
1601, 1243
1495, 1296
1196
ν(N O), νas(NO2)
νas(NO2), νs(NO2)
ν(NO)
−
NO3 (monodentate)
−
NO
WZ
N2O (ads.)
2243, 1215
2131
1950
ν(NN), ν(NO)
ν(NO)
ν(NO)
+
NO
ON–Zr4+–NOx
N2O3 (ads.)
1915, 1517 1327
1817
ν(N O), νas(NO2), νs(NO2)
ν(NO)
NO–W +–NO2
4
−
H2O (ads.)
1605
1450, 1425
1184
δ(H2O)
ν(NO)
νs(NO2)
−
NO2 (monodentate nitrito, two types)
−
NO2 (bidentate nitrito)
Pd/WZ
N2O (ads.)
NO
2240
2105
ν(NN)
ν(NO)
+
N2O3 (ads.)
1925, 1560, 1280
1863, 1812
1220, 1171
ν(N O), νas(NO2), νs(NO2)
ν(NO)
νas(NO2), νs(NO2)
2
+
Pd –NO (two types)
−
NO2 (bidentate, nitrito)
−
(
spectrum a), followed by heating the closed IR cell for 15 min
in spectrum b (Fig. 5A) is attributed to adsorbed NO3 species.
ThisexperimentalfactrevealsthatoxidationofNOhasoccurred.
at 623 K (spectrum b). The latter treatment causes all of the
NOx species observed at room temperature to disappear. Conse-
quently, the altered WOx species are almost completely restored.
This behavior can be explained by assuming that at high temper-
atures, reaction(2)shiftstotheleft. Thisassumptionissupported
by the fact that N2O3 is stable at low temperature [53].
The gas phase spectrum recorded at 623 K (Fig. 5B, spectrum
b) contains NO2 in addition to the NO. Some amount of N2O
is also detected. This was already present at room temperature
2+
The strong perturbation in the region of the Pd –NO nitro-
syls suggests that in this process, the Pd ions are involved.
The weak bands at 1863 and 1816 cm corresponding to the
Pd –NO nitrosyls have disappeared, and considerably stronger
bands at 1837 and about 1790 cm (shoulder) are detected
instead. These two bands are assigned to the Pd –NO species
[54], which originate from the Pd –NO nitrosyls observed
at room temperature. This assignment is justified by the fact
that under the same experimental conditions, all of the NOx
2+
−
1
2+
−
1
+
2+
−
Fig. 5B, spectruma). Basedonthis, theweakbandat1613 cm
1
(
Fig. 5. (A) FT-IR spectra of the Pd/WZ sample taken after adsorption of NO (1.33 kPa) at room temperature for 15 min (a), after heating of the closed IR cell for
5 min at 623 K (b), and subsequent evacuation for 15 min at 623 K (c), then heating the closed IR cell for 15 min at 723 K (d), at 773 K (e) and after cooling down to
1
room temperature (f) followed by evacuation for 15 min at room temperature (g). The gas phase spectra are subtracted. The negative bands marked by asterisks are
due to residual carbonates, which are removed from the surface by the NOx species formed. (B) Gas phase spectra recorded at room temperature (a) and 623 K (b).

M. Kantcheva, I. Cayirtepe / Journal of Molecular Catalysis A: Chemical 247 (2006) 88–98
95
Fig. 6. (A) FT-IR spectra of NO (1.34 kPa) adsorbed at room temperature for 10 min (ZrO2 and WZ samples) and 30 min (Pd/WZ sample), and after subsequent
introduction of O2 (2.66 kPa) for 30 min. The spectra are gas phase corrected. (B) Gas phase spectra recorded in the presence of WZ and Pd/WZ samples.
complexes formed on the WZ support by room-temperature
adsorption of NO disappear. No absorption bands which can be
attributed to nitrosyls of reduced tungsten species are detected.
+
The Pd –NO nitrosyls display high thermal stability and
resist the evacuation at 623 K (Fig. 5A, spectrum c). Increas-
ing the temperature of the closed IR cell to 723 K (Fig. 5A,
spectrum d) leads to desorption of the nitrate species, and then
at 773 K (Fig. 5A, spectrum e) to loss of NO adsorbed on the
+
Pd sites. Cooling to room temperature (Fig. 5A, spectra f and g)
+
causes the reappearance of Pd –NO bands with reduced intensi-
ties relative to those in spectrum c. This indicates that additional
oxidation of NO to NO2 has occurred, leading to the formation
−
1
of nitrate species at 1620, 1584 and 1456 cm
.
3
.3.2. Coadsorption of NO and O2
Fig. 6A shows the spectra of the samples studied after
the adsorption of 1.34 kPa of NO and subsequent addition
of 2.66 kPa of O2 to the IR cell at room temperature. This
causes significant changes in the spectra of preadsorbed NO.
−
In the case of ZrO2, the NO species disappear and the inten-
sities of the nitrate bands with maxima at 1630, 1238 and
−1
−1
1
012 cm increase. The band at 1750 cm reveals the pres-
ence of adsorbed N2O4 ([46], see Fig. 6B). The weak band at
−1
2
230 cm which is detected also in the spectrum taken dur-
ing the room-temperature evacuation (Fig. 7) is attributed to
+
adsorbed NO species [44–46]. The Zr–OH hydroxyls at 3740
−
1
and 3693 cm are perturbed giving rise to positive absorption
in the region of H-bonded OH groups. The gas phase spec-
trum obtained in the presence of zirconia contains bands due
Fig. 7. FT-IR spectra obtained upon dynamic evacuation of pre-adsorbed NOx
species for 15 min (ZrO2 and WZ) and 30 min (Pd/WZ) at ambient temperature.
For the experimental conditions of NO/O2 adsorption see Fig. 6.
−
1
−1
to N2O4 (νas(NO) at 1747 cm and νs(NO) at 1264 cm ) and
−
1
NO2 (νas(NO2) at 1617 cm ) [53]. The weak absorption at

9
6
M. Kantcheva, I. Cayirtepe / Journal of Molecular Catalysis A: Chemical 247 (2006) 88–98
−
−
1
1
the Brønsted acid sites of the tungstated zirconia in the forma-
tion of NO species is supported by the fact that the replacement
2
1
224 cm belongs to the ν(NN) mode of N2O, whereas that at
876 cm correspondsto the ν(NO)stretchingof NO [53]. NO2
+
of protons of the surface hydroxyls (isolated and H-bonded) by
and N2O4 are formed by the reaction of NO with O2 in the gas
phase (2NO + O2 = 2NO2/N2O4). The intensities of the bands of
the gas phase spectrum taken in the presence of zirconia are iden-
tical to those of the spectrum of NO + O2 obtained in absence of
ZrO2. This indicates that the ZrO2 sample does not catalyze the
oxidation of NO by molecular oxygen at room temperature. The
adsorbed species resist room-temperature evacuation (Fig. 7).
It is known from the chemistry of NO2 [55] that NO2 dis-
proportionates in basic solutions to nitrate and nitrite ions with
formation of water. Similar process (Eq. (5)) has been proposed
to take place upon interaction of NO2 with oxides containing
basic OH groups such as titania (anatase) [56–58] and mono-
clinic zirconia [44,46]:
2+
+
Pd ions causes significant decrease in the intensity of the NO
band detected under the same conditions (see below). It should
be pointed out that we cannot discriminate between the pertur-
4+
bation of the Zr –OH groups on tungstated zirconia due to their
involvement in the NO2 disproportionation according reaction
(5) and the perturbation caused by the water molecules produced
in reaction (7).
The weakly adsorbed species on the surface of the WZ
sample, which disappear after evacuation at room temperature
−
1
(
(
Fig. 7, spectrum WZ), are identified as N2O4 (1747 cm ), NO2
−
1
4+
−1
∼1620 cm ), Zr –NO (∼1930 cm ) and nitrate species
−1
most probably monodentate (1580 and 1278 cm ). The inten-
+
sity of the NO band decreases significantly, which is accom-
−
−
2
OH + 3NO2(1.5N2O4) → 2NO3 + NO + H2O
Theformationofwatermoleculeshasbeenconfirmedbyadsorp-
tion of NO2 on deuterioxylated TiO2 [56].
Another possibility for the formation of nitrate species is
through self-ionization of NO2 on surface Lewis acid–base pairs
according to the reaction [51,56,57]:
(5)
panied by lowering of the intensities of the negative band at
−
1
3670 cm
and the positive absorption in the region of H-
bonded hydroxyls. This behavior can be explained assuming
that nitrosonium ions are formed by reaction (7) and during
the dynamic evacuation the reverse of the reaction is favored.
−
1
Most probably the negative band at 3670 cm
belongs to
+
−
W–OH groups. Recombination of NO and NO3 species
to N2O4 during the evacuation can also contribute to the
decrease in the amount of adsorbed NO ions. The bands at
NO2(N2O4) → NO3− + NO
The nitrosonium ion, NO , chemisorbed on titania (anatase)
and monoclinic zirconia gives rise to absorption band at
206–2210 cm [44,46,56–59].
The low intensity of the band attributed to the NO species
adsorbed on the ZrO2 sample used in this study leads to the
conclusion that the surface nitrates are obtained mainly by dis-
proportionation of NO2 with the involvement of the isolated
hydroxyls at 3740 and 3693 cm (reaction (5)).
The spectrum of NO and O2 coadsorbed on the WZ sam-
ple (Fig. 6A) displays similar features to those of impregnated
WO3/ZrO2 [47]. The strong bands in the 1650–1200 cm
region correspond to surface nitrates. The band at 1747 cm
+
(6)
2
+
+
−1
2
604, 1975, 1905 and 1760 cm , observed under these con-
ditions are due to combination modes of the nitrate species
−
1
2
−1
[
44]. The pair of bands at 1975 and 1905 cm reveals the
+
−
presence of bridged NO3 species and corresponds to the
split νs(NO2) + δ(ONO) mode, whereas the bands at 2604
−1
(
ν(NO) + νs(NO2) mode) and 1760 cm (high-frequency com-
ponent of the split νs(NO2) + δ(ONO) mode) belong to bidentate
nitrates.
−
1
In contrast to the zirconia and WZ samples, the amount of
NOx species on the Pd/WZ is considerably lower (Fig. 6A). The
gas phase spectrum (Fig. 6B) is identical to that recorded in
the presence of zirconia, which indicates that the promotion of
tungstated zirconia with palladium(II) suppresses the oxidation
of NO by molecular oxygen at room temperature. This suppres-
sion is caused by the elimination of acidic protons involved in
the NO oxidation. Formation of NO2 during the catalytic reduc-
tion of NO with methane is considered to be an important step
in the reaction mechanism [8–10,15,23,39]. We are of the opin-
−1
−
1
is due to adsorbed N2O4, whereas the intense absorption at
−1
+
2
175 cm correspondstoadsorbedNO species[47]. Theunre-
−
1
4+
solved absorption at about 1930 cm is assigned to Zr –NO
nitrosyl that has nitrate ions in its vicinity [47].
Compared to zirconia, the amounts of the gaseous species
Fig. 6B) and NOx complexes adsorbed on the WZ sample
Fig. 6A) are greater. This fact indicates that the Brønsted acid-
ity favors the oxidation of NO to NO2, which is in agreement
with the results of other reports [8,10]. The intense band due
(
(
2+
ion that the ability of Pd ions to oxidize NO to NO2 at high
temperatures (shown above) could ensure sufficient amount of
NO2 under the reaction conditions despite the decrease in the
+
to adsorbed NO species reveals the occurrence of surface reac-
2+
+
Brønsted acidity. Reduction of Pd in Pd-H-ZSM-5 to Pd ions
upon exposure to NO leading to formation of NO2 was observed
by Descorme et al. [3] using IR spectroscopy and in the XAFS
measurements of Okumura et al. [19].
tion (6). Another possibility for the formation of nitrosonium ion
is the reaction of NO and NO2 with the Brønsted acid sites of
tungstated zirconia via process observed to take place on acidic
zeolites [60]:
The fact that the hydroxyl content of the Pd/WZ sample is
very low (see Fig. 3A) suggests that the nitrates species (bands
+
NO + NO2 + 2H ꢀ H2O + 2NO+
The water produced in this process can perturb the iso-
lated hydroxyls (negative bands at 3740 and 3670 cm ) by
H-bonding leading to appearance of positive absorption that
(7)
−
1
in the 1650–1400 and 1300–1200 cm regions) are produced
mainly via reaction (6). The broad absorption with maximum
−
1
−1
+
at 2160 cm is due to the ν(NO) stretching mode of the NO
species. The intensity of this band is much lower than that on
−
1
2+
extends below 3000 cm . The possibility for involvement of
the WZ sample. The Pd –NO nitrosyls give rise to the bands at

M. Kantcheva, I. Cayirtepe / Journal of Molecular Catalysis A: Chemical 247 (2006) 88–98
97
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