IR spectroscopic study of NOx adsorption and NOx-O2 coadsorption on Co2+/SiO2 catalysts

Article abstract of DOI:10.1039/a703955g

Adsorption of nitrogen oxides (NO, NO₂) and their coadsorption with oxygen on Co²⁺/SiO₂ samples has been investigated by IR spectroscopy with a view to elucidating the mechanism of selective catalytic reduction (SCR) of NO_x with hydrocarbons. A Co²⁺/SiO₂ sample synthesized by ion exchange is characterized by a highly dispersed cobalt and a very weak surface acidity: CO is adsorbed only at low temperature (100 K) forming Co²⁺ - CO carbonyls [v(CO) = 2180 cm^-1]. Adsorption of NO on Co²⁺/SiO₂ leads to the formation of Co²⁺(NO)₂ dinitrosyl complexes (1872 and 1804 cm^-1) which are decomposed upon evacuation. Adsorption of NO₂, as well as coadsorption of NO and O₂, produce NO₂ species weakly bound to the support (a band at 1681 cm^-1) and N₂O₄ (a band at 1744 cm^-1 with a shoulder at 1710 cm^-1), the latter being adsorbed reversibly on both the support and the Co²⁺ ions. In the second case N₂O₄ is transformed into surface monodentate nitrates of Co²⁺ (a band at 1550-1526 cm^-1) and partly into bridged nitrates (a band at ca. 1640 cm^-1). The monodentate nitrates are stable with respect to evacuation up to 125°C and act as strong oxidising agents: they are reduced by NO, even at room temperature, and by methane at 100°C. In the latter case, organic nitro-compounds and isocyanate groups are registered as reaction products (probably intermediate compounds in SCR). The surface species obtained after NO and NO₂ adsorption on Co²⁺/SiO₂ prepared from cobalt acetate (active SCR catalyst) are essentially the same as those observed with the ion-exchanged sample. No monodentate nitrates, however, are formed during NO₂ adsorption on a Co²⁺/SiO₂ sample synthesized by impregnation with cobalt nitrate, which accounts for the lack of activity of this sample in the SCR.

Full text of DOI:10.1039/a703955g

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IR spectroscopic study of NO adsorption and NO –O coadsorption
on Co2‘/SiO catalysts
x
x
2
2
Boyan Djonev, Boyko Tsyntsarski, Dimitar Klissurski and Konstantin Hadjiivanov*
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, SoÐa 1113,
Bulgaria
Adsorption of nitrogen oxides (NO, NO ) and their coadsorption with oxygen on Co2`/SiO samples has been investigated by
2
2
IR spectroscopy with a view to elucidating the mechanism of selective catalytic reduction (SCR) of NO with hydrocarbons. A
x
Co2`/SiO sample synthesized by ion exchange is characterized by a highly dispersed cobalt and a very weak surface acidity: CO
2
is adsorbed only at low temperature (100 K) forming Co2`wCO carbonyls [l(CO) \ 2180 cm~1]. Adsorption of NO on
Co2`/SiO leads to the formation of Co2`(NO) dinitrosyl complexes (1872 and 1804 cm~1) which are decomposed upon
2
2
evacuation. Adsorption of NO , as well as coadsorption of NO and O , produce NO species weakly bound to the support (a
2
2
2
band at 1681 cm~1) and N O (a band at 1744 cm~1 with a shoulder at 1710 cm~1), the latter being adsorbed reversibly on both
the support and the Co2` ions. In the second case N O is transformed into surface monodentate nitrates of Co2` (a band at
2
4
2
4
1550È1526 cm~1) and partly into bridged nitrates (a band at ca. 1640 cm~1). The monodentate nitrates are stable with respect to
evacuation up to 125 ¡C and act as strong oxidising agents: they are reduced by NO, even at room temperature, and by methane
at 100 ¡C. In the latter case, organic nitro-compounds and isocyanate groups are registered as reaction products (probably
intermediate compounds in SCR). The surface species obtained after NO and NO adsorption on Co2`/SiO prepared from
2
2
cobalt acetate (active SCR catalyst) are essentially the same as those observed with the ion-exchanged sample. No monodentate
nitrates, however, are formed during NO adsorption on a Co2`/SiO sample synthesized by impregnation with cobalt nitrate,
2
2
which accounts for the lack of activity of this sample in the SCR.
HC-SCR with respect to NO and the high order of the reac-
tion towards hydrocarbons.9 Now, the opinion that oxidized
NO ad-species are the active oxidizers in HC-SCR is gaining
popularity.8,10h13,19 This indicates that knowledge of the
nature of surface compounds appearing on the catalysts
during NO adsorption and NOÈO coadsorption is of
1
Introduction
SCR is one of the most attractive possibilities for environ-
mental protection from nitrogen oxides. The process has
found industrial application with immobile NO sources,
x
ammonia being used as a reducing agent.1,2 However, a new
2
2
technology has been discovered3,4 and developed:5h33 selec-
special importance for establishing the mechanism of the
process. Unfortunately, investigations of the adsorption
tive catalytic reduction of NO with hydrocarbons (HC-SCR).
x
This process is very promising because it may be applied to
motor vehicles and may permit the replacement of ammonia
species of NO are very restricted in number13,17,20,34h38 and
2
only a few studies consider surface compounds formed during
in immobile NO sources by hydrocarbons.
A characteristic feature of SCR is that it proceeds in the
NOÈO coadsorption.12,13,20h22,29 However, in these cases
x
2
the interpretations of the di†erent authors are quite contradic-
presence of oxygen. Under anaerobic conditions the catalyst
activity is either low4,5 or the nitrogen oxide is reduced to
ammonia.6 It is assumed that the role of oxygen is to keep the
surface clean5,7 and oxidized6,8 and to convert NO to reactive
tory, as is typical of each new Ðeld of science. Thus, a band at
2133 cm~1 has been ascribed by di†erent authors to
NO ,20,39
NO d`,40,41
or
NO `.18,37
Many
2
2
2
authors10h12,18,20,22,39,41,42 attribute bands stable towards
evacuation in the region 1620È1550 cm~1 to nitrite complex-
es, while others13,17,21,34h36,38,43h45 have noted that nitrites,
as a rule, absorb at lower frequencies, and the bands observed
should belong to surface nitrates.
surface compounds such as NO ,8,9 nitrites10,12 or
2
nitrates.10,12,13
Regardless of the fact that the key characteristics of
HC-SCR are already known, there are still a number of prob-
lems that need elucidation. Thus, according to Delahay et
al.14 and Lentmaier et al.15 the activity of the catalysts correl-
ates with their Lewis acidity. Hadjiivanov et al.,13 however,
have established that Cu-ZSM-5 is characterized by a negligi-
ble surface acidity and suppose that this favours desorption of
the SCR reaction products. Armor16 has paid attention to the
low acidity of Ga-containing catalysts. The mechanism of
HC-SCR is also debatable. Some authors5 are of the opinion
that the process is based on NO decomposition to nitrogen
and oxygen. The absence of correlation between the activities
of the di†erent catalysts in the SCR reactions and the NO
decomposition is one of the main reasons for rejecting this
mechanism. According to other concepts,17,18 the formation
of carbonaceous surface deposits or partially oxidized hydro-
carbon derivatives interacting with NO is of special impor-
tance. However, these hypotheses contradict the zero order of
An important stage in the development of SCR is the dis-
covery of Armor et al.23 that NO reduction on Co-ZSM-5
x
can be performed with methane. More recently, it was estab-
lished that cobalt deposited on other supports,20,24 as well as
catalysts containing another active phase (e.g. Rh`, Ni2`,
Mn2` and Ga3`, but not Cu2` ions) are also active in this
reaction.5,16,25 Then the question arises: why is methane an
e†ective reducing agent with some catalysts but feeble with
others? According to Beutel et al.,41 surface nitrites (IR band
ca. 1526 cm~1) appearing on Co-ZSM-5 and not on Cu-ZSM-
5 are responsible for the oxidation of methane. Another inter-
esting question associated with cobalt catalysts is the e†ect of
the support on their activity. Armor et al.25 reported that
oxide-supported cobalt is not efficient in SCR with CH .
4
Other investigations26,27 have shown that both Al O and
2
3
SiO can be successfully used as cobalt supports in SCR of
2
J. Chem. Soc., Faraday T rans., 1997, 93(22), 4055È4063
4055

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NO with C hydrocarbons, but, unfortunately, the reduction
adsorption apparatus with a residual pressure of 10~5 Torr.
Before the adsorption measurements the pellets were activated
by calcination for 1 h at 400 ¡C and evacuation for 1 h at the
same temperature. Low-temperature CO adsorption was
carried out in a specially constructed cell equipped with a
dosing cock, allowing the introduction of deÐnite adsorbate
portions to the sample.
The speciÐc surface area was measured by low-temperature
nitrogen adsorption according to the conventional BET
method.
x
3
with methane has not been studied. Inaba et al.27 have found
that the activity of CoO/SiO catalysts in SCR with propene
2
depends strongly on the method of preparation. Thus, they
established that catalysts, prepared by impregnation with
cobalt acetate, showed a good activity, whereas a sample pre-
pared using cobalt nitrate was completely inactive. These
authors concluded that highly dispersed cobalt ions are the
active sites of the reaction. A similar conclusion has been
arrived at by Hamada et al.26 in studies of HC-SCR on
CoO/Al O .
Chemical analysis was performed by Ñame atomic absorp-
tion spectrophotometry using a PYE-UNICAM SP-1950
apparatus.
2
3
In this paper, an attempt was made to elucidate the e†ect of
cobalt dispersion on the interaction of NO , O and methane
x
2
over silica-supported cobalt. We studied the adsorption of
nitrogen oxides and their coadsorption with oxygen on a
3
Results
Co2`/SiO ion-exchanged catalyst with a high cobalt disper-
2
sion, and determined the stability of the observed adsorption
The Co /SiO sample was investigated in detail, whereas the
ie
2
species and their reactivity with respect to some reaction pro-
ducts (water), possible intermediates (CO) and the reagents of
HC-SCR (NO, O , CH ). Methane was chosen as a reducing
silica support and the impregnated catalysts were studied for
comparison purposes.
2
4
agent because of the absence of data on its behaviour in SCR
3.1 SiO
2
on Co2`/SiO catalysts. Some experiments were also carried
2
3.1.1 Background spectrum and hydroxy group coverage.
out with two impregnated Co2`/SiO samples prepared, one
2
The IR spectrum of the activated SiO support exhibits three
from acetate (showing a good SCR activity), and the other
2
broad bands within the 2000È1600 cm~1 region. According to
from nitrate (totally inactive in SCR) using, in both cases, the
procedure described by Inaba et al.27
data from the literature6,13 they characterize overtones of
lattice vibrations (1990 and 1626 cm~1) and a combination
frequency (1872 cm~1). Self absorption determines the so-
called “cut-o†Ï of the sample at ca. 1350 cm~1: below this
frequency the sample is opaque and no spectra of surface
compounds can be detected. In the region of the OH-
stretching modes there is a narrow band with a maximum at
3740 cm~1 which characterizes terminal groups of the
SiwOH type.47
2
Experimental
2.1 Sample preparation
Commercially available SiO (aerosil) with a speciÐc surface
2
area of 336 m2 g~1 was used as a support. A Co2`/SiO
2
sample (denoted further on as Co /SiO ) was synthesized by
ion exchange from a 0.75 M Co3` solution. The latter was
prepared by adding ammonia to a Co(NO ) É 6H O solution
ie
2
3.1.2 Adsorption of CO, NO and H O. Owing to the lack
2
of Lewis acidity on silica,48 CO is not adsorbed on the SiO
2
3 2
2
so as to achieve a Ðnal total ammonia concentration of 12.5
wt.%. During this reaction, the Co2` ions were oxidized to
Co3`.46 To increase the amount of reaction, 30% H O [ca.
sample. Accordingly, adsorption of NO does not lead to for-
mation of surface nitrosyl complexes. This is in agreement
with the results of Ghiotti et al.47 who have reported that NO
is adsorbed on aerosil only at low temperatures. Under 5 Torr
2
2
12 ml (g Co)~1] was also added. Silica was dispersed in the
Co3` solution [0.2 (g SiO ) ml~1] and the mixture was stirred
2
water vapour, the spectrum of SiO exhibits an intense band
for 1 h. To ensure equilibrium with the initial cobalt concen-
2
at 1626 cm~1, due to d(H O) vibrations of molecularly
tration, the suspension was Ðltered and the precipitate was
treated in a similar way. After a further 1 h the solid phase
was Ðltered, abundantly washed with water and dried in air at
120 ¡C. The cobalt content in the sample was, according to
chemical analysis data, 1.01 wt.%.
2
adsorbed water. However, 10 min evacuation leads to the
complete disappearance of this band and restoration of the
initial spectrum.
3.1.3 Adsorption of NO . The admission of NO (16 Torr)
Two other Co2`/SiO samples were prepared, as described
2
2
2
to an activated SiO sample produces two main bands with
elsewhere,27 by incipient wetness impregnation of silica: one
2
maxima at 1686 and 1745 cm~1 (Fig. 1). Both bands decrease
with Co(NO ) and the other with Co(CH CO ) . Both
3 2
3
2 2
in intensity with the equilibrium pressure and disappear after
evacuation. Note that the band at 1686 cm~1 is less sensitive
to the pressure changes, i.e. it characterizes a more stable
samples contained, nominally, 5 wt.% cobalt. These samples
will be denoted as Co /SiO and Co /SiO , respectively.
nitr
2
ac
2
surface species. Subsequent NO adsorption (16 Torr) causes
the appearance of a spectrum almost identical with Fig. 1(a),
2.2 Gases and reagents
2
i.e. the surface has not been modiÐed by the adsorptionÈ
AR oxygen (99.6, Merck), carbon monoxide (99.5, Merck) and
methane (99.995, Messer, Griesheim) were additionally puri-
Ðed prior to adsorption by passing through a liquid-nitrogen
trap. AR nitrogen oxide (99.0, Merck) was puriÐed by frac-
tional distillation. Nitrogen dioxide was synthesized by
desorption of NO .
2
The band at 1745 cm~1 can be attributed to weakly
adsorbed N O .13,18,39,40,42 This molecule has antisymmetric
2
4
NO stretching modes with two components: an intense in-
phase band at 1758 cm~1 and a low-intensity band at 1709
cm~1 which corresponds to out-of-phase vibrations.49 Indeed,
our spectra also show a shoulder at ca. 1715 cm~1. Li et al.20
have assigned the pair of bands at 1751 and 1705 cm~1 to
covalent N O . However, we think that this interpretation is
thermal decomposition of AgNO as described earlier.35 It
2
was puriÐed by addition of oxygen, its excess being evacuated
after 24 h, while the NO was frozen.
2
2.3 Methods
2
5
not quite reliable because the bands at 1744 and 1710 cm~1
IR spectroscopy studies were carried out with a Specord M-80
apparatus at a spectral resolution of 1 cm~1. Self-supporting
pellets were prepared from the samples and heated directly in
an IR cell. The latter was connected with a vacuumÈ
appear in an atmosphere of NO (which is easily dimerized)
2
and disappear almost completely after evacuation.
The band at 1686 cm~1 also characterizes a reversibly
bound surface species and it seems logical to assign it to
4056
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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surface, there are cobalt ions, we studied low-temperature CO
adsorption. It is known that at low temperatures very weak
Lewis acid sites can be monitored by CO.54 The introduction
of a portion of CO (2 lmol), at ca. 100 K, to the pellet leads to
the appearance in the IR spectrum of a symmetric band with a
maximum at 2180 cm~1 (Fig. 2) which characterizes
Co2`wCO complexes.53,55 The high frequency and low sta-
bility of the complexes indicate that, in this case, the character
of the cobaltÈcarbon bond is mainly r and/or electrostatic.
The introduction of a second CO portion only enhances the
band intensity without causing a frequency shift. After the
third CO portion this band does not change, i.e. the adsorp-
tion sites have practically all been occupied. With increase in
the amount of CO introduced into the cell, the spectrum also
displays a shoulder with a maximum at 2156 cm~1 which,
according to data from the literature,54 is due to CO H-
bonded to hydroxy groups. Gradual heating of the sample to
room temperature results, initially, in the disappearance of the
band of hydrogen-bound CO and then the band at 2180 cm~1
decreases in intensity, to disappear completely at room tem-
perature. Here again no dependence of l(CO) on coverage is
observed, i.e. the band maximum remains unchanged at 2180
cm~1. This implies absence of interaction between the
adsorbed CO molecules and is evidence that the Co2` sites
are isolated. Thus, the CO adsorption experiments show
unambiguously that, on the sample surface, (i) there are iso-
lated Co2` ions with a very weak electrophility and (ii) there
are neither cobalt ions in a lower oxidation state nor metal
cobalt.
Fig. 1 IR spectra of NO adsorbed on a SiO support. Equilibrium
2
2
pressure of 16 (a), 9 (b), 5 (c) and 3 (d) Torr NO , after evacuation (e)
2
and after subsequent introduction of 16 Torr NO (f).
2
3.2.3 Adsorption of NO. During NO adsorption (10 Torr),
a pair of bands with maxima at 1884 and 1802 cm~1 appear
in the IR spectrum of the Co /SiO sample (Fig. 3). According
adsorbed NO . The NO vibrations in the gas phase are
ie
2
2
2
to data from the literature18,20,22,38,40,42,44,47,48,54 these
found at somewhat lower frequencies (1624 cm~1).50 It is
bands are due to the asymmetric and symmetric NO vibra-
known that NO is characterized by the presence of a free
2
tions of dinitrosyl complexes of the type Co2`(NO) . The
electron situated on an antibonding orbit.49 For that reason,
2
decrease in the equilibrium pressure of NO is accompanied by
ionization of the molecule leads to an increase in the NwO
a simultaneous drop in the bandsÏ intensity, which does not
bond order and, as a result, to a rise in the l frequency up to
3
2392È2360 cm~1. Similarly to the case of CO,48 coordination
of NO accompanied by partial electron transfer would
2
enhance the absorption frequency. Indeed, NO adsorption
on acid hydroxy groups of zeolites (showing an IR band at
2
3615 cm~1) produces a surface compound which has been
assigned by
a
number of authors to NO d`,40,41 or
2
NO `,18,37 and is characterized by a band at 2133 cm~1. On
2
the basis of the above analysis we have assigned the band at
1686 cm~1 to NO adsorbed with partial charge transfer and
2
probably with the participation of SiwOH groups.
3.2 Co /SiO
ie
2
3.2.1 Background spectrum and hydroxy group coverage.
The activated Co /SiO sample is pale blue in colour and its
ie
2
IR spectrum scarcely di†ers from that of the support. This
indicates that the terminal silanol groups have not partici-
pated in the ion exchange with the cobalt ions. Probably the
active sites of the process are hydrogen-bonded hydroxy
groups, which are desorbed under the conditions of sample
activation.
3.2.2 CO adsorption. CO is one of the most used probe
molecules for precise determination of the Lewis acidity.48
Adsorption of CO (equilibrium pressure 32 Torr) at room
temperature on an activated Co /SiO sample does not lead
ie
2
to new bands in the IR spectrum. This indicates that the
cobalt ions are either not on the surface or are characterized
by a very weak electrophilicity and form no complexes with
CO at room temperature. In the latter case they may be Co2`
or Co3` ions51,52 because the Co` and Cod` ions form stable
carbonyls.53 In order to establish whether, on the sample
Fig. 2 IR spectra of CO adsorbed on Co /SiO . Introduction of 2
ie
2
(a), 4 (b), and 14 (c) lmol of CO at 100 K and time evolution of the
spectrum during temperature increase (d)È(f).
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
4057

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Fig. 3 IR spectra of NO adsorbed on Co /SiO . Equilibrium pres-
Fig. 4 IR spectra of NO adsorbed on Co /SiO . Equilibrium pres-
ie
2
2
ie
2
sure of 20 (a), 10 (b), 5 (c), 2.5 (d) and 1 (e) Torr of NO and after a brief
evacuation (f).
sure of 16 (a), 9 (b), 5 (c) and 3 (d) Torr NO , increase in pressure to
2
16 Torr NO (e) and after evacuation (f). Spectrum (g) is taken after
2
subsequent introduction of 30 Torr NO into the IR cell.
a†ect the positions of the maxima. Both bands disappear after
evacuation, which evidences the low stability of the dinitro-
syls. A very weak feature at ca. 1865 cm~1 is visible at low
equilibrium pressures, which might be attributed tentatively to
the absorption of Co2` mononitrosyls. This interpretation is
in agreement with the weak electrophility of Co2`, on our
sample, which presupposes weak interaction with NO and the
frequency of the nitrosyls formed being close to that of
gaseous NO. Peculiarities of NO adsorption on our sample
are the relatively low absorption frequencies and the low sta-
bility of the dinitrosyls. For the sake of comparison one may
use the results of Li and Armor20 who have established that
dinitrosyls on Co-ferrierite (bands at 1900 and 1815 cm~1) are
stable, even after evacuation at 300 ¡C.
e†ects, NO (16 Torr) was again admitted to the cell. As a
2
result, the 1540 cm~1 band intensity exhibited a small addi-
tional increase, whereas the bands at 1744 and 1709 cm~1
displayed a much weaker intensity than those in the initial
spectrum Fig. 4(a) and should perhaps correspond to N O
2
4
adsorbed on the support.
After a 10 min evacuation, the band at 1540 cm~1 is
observed together with a low-intensity band at ca. 1640 cm~1.
The latter also characterizes an activated adsorption form and
appears only after prolonged contact of the sample with NO .
2
We assume this band to be due to bridged nitrates. The results
obtained evidence that N O adsorbed on Co2` ions from the
2
4
Co /SiO surface is transformed, with time, into new activat-
ie
2
ed adsorption species which are stable with respect to evac-
uation at room temperature.
3.2.4 Adsorption of NO . Adsorption of NO (equilibrium
2
2
pressure 16 Torr) on the Co /SiO sample (Fig. 4) is accom-
ie
2
panied by formation of the adsorption species characteristic of
3.2.5 Coadsorption of NO and O . To elucidate the mecha-
2
the SiO support: the bands at 1744 cm~1 (with a shoulder at
nism of SCR, it is important to study the surface compounds
2
ca. 1710 cm~1) and 1681 cm~1 have already been interpreted
resulting from coadsorption of NO and O . From a ther-
2
as being due to adsorbed N O and NO . However, the band
modynamic viewpoint, NO is unstable in the presence of
2
4
2
at 1744 cm~1 has a much stronger intensity than the analog-
oxygen and should be completely converted into NO .8 Since
2
ous band observed on SiO (note the di†erent scales of Fig. 1
the oxidation of nitrogen oxide is a three-molecule reaction
2
and 4), which indicates that, in addition to the support, part of
(2NO ] O ), this process is slow, especially at low NO con-
2
the N O occupies Co2` ions. Moreover, the spectrum also
centrations.
2
4
contains an intense band with a maximum at 1540 cm~1,
which obviously characterizes compounds formed with the
participation of cobalt ions. This band is attributed to mono-
dentate nitrates. Additional support of this interpretation will
be proposed in the Discussion section. It is worth noting that,
The addition of 3 Torr oxygen to Co /SiO preliminary
ie
2
situated in an atmosphere of 20 Torr NO leads to the almost
complete disappearance of the bands at 1884 and 1802 cm~1
characterizing Co2`(NO) dinitrosyls (Fig. 5). Simultaneously,
2
a broad band with a maximum at 1550 cm~1 becomes visible.
after NO adsorption, the sample acquires a purple nuance
The admission of an additional amount of oxygen (32 Torr) to
the sample leads to a ca. twofold increase in intensity of the
band at 1550 cm~1 and a shift of its maximum to 1536 cm~1.
In addition, an intense band at 1744 cm~1, due to adsorbed
N O , appears, as well as a weak band with a maximum at
2
which may indicate either change in the coordination state of
Co2` ions or their oxidation to Co3`.
The decrease in the NO equilibrium pressure causes a drop
2
in intensity of the bands at 1744 and 1681 cm~1, while the
2
4
1540 cm~1 band intensity grows. In order to establish whether
1640 cm~1, which has already been associated with bridged
nitrates. The 1744 cm~1 band intensity decreases a little with
a concomitant slight increase in intensity of the 1536 cm~1
the development of this band is associated with a decrease in
the NO equilibrium pressure or is, rather, due to kinetic
2
4058
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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3.3.3 Interaction with NO. The interaction of the surface
nitrates with NO is presented in Fig. 4(g). Admission of 30
Torr NO to a sample with preadsorbed nitrates leads to a
signiÐcant decrease in intensity of the 1540 cm~1 band and to
a shift of its maximum to 1526 cm~1. No other surface com-
pounds (in particular dinitrosyls) are formed. These results
conÐrm that only part of the nitrates suffice to block NO
adsorption on cobalt ions.
3.3.4 Interaction with methane. The interaction of the
surface nitrates with methane is of great interest because CH
is a selective reducing agent of nitrogen oxides over zeolite
4
supported Co catalysts. Interaction with methane (10 Torr) for
30 min at 100 ¡C with a sample containing preadsorbed
nitrates leads to a drastic intensity drop of the band at 1535
cm~1 (Fig. 6). In addition, a series of new bands emerge. A
weak band at 1625 cm~1 characterizes d(H O) vibrations and
2
evidences the presence of adsorbed water, obviously formed as
a result of methane oxidation. A band at 1558 cm~1 can, on
the basis of data from the literature,12,17,33 be attributed to
organic nitro-compounds. A weak and broad band with a
maximum at 2207 cm~1, assigned to surface iso-
cyanates,30,31,56 is detected in the higher-frequency region.
These isocyanates are coordinated to cobalt ions, since iso-
cyanates on silica manifest a band at considerably higher fre-
quencies, namely at 2300 cm~1.56
Since the surface nitrates are thermally stable at 125 ¡C, the
results obtained show clearly that even at 100 ¡C, these com-
pounds are reduced by methane. Both organic nitro-species
and isocyanates are intermediates in this reaction.
Fig. 5 IR spectra of NO (20 Torr) adsorbed on Co /SiO (a), after
ie
2
introduction of 3 (b) and 32 (c) Torr of O into the IR cell and after
2
evacuation (d)
3.3.5 Interaction with water. As water is an important SCR
product, we have studied its e†ect on the surface nitrates. The
admission of water vapour (5 Torr) to the Co /SiO sample
causes the appearance of an intense band at 1626 cm~1 evi-
dencing the presence of adsorbed water on the support, and
possibly on the cobalt ions (Fig. 7). Moreover, the nitrate
band. After evacuation, the band at 1744 cm~1 disappears,
while the remaining two bands do not change, i.e. the stable
ie
2
surface compounds being formed during NOÈO
2
coadsorption are the same as those observed after adsorption
of NO . It is interesting that blocking of the NO adsorption
2
sites occurs on attaining about half of the maximum concen-
tration of the surface nitrates, characterized by a band at
1550È36 cm~1.
3.2.6 Coadsorption of NO and O . In order to check pos-
2
2
sible formation of additional surface compounds in a strongly
oxidizing medium, we have also studied the coadsorption of
NO and O on the sample. The results do not di†er from
2
2
those on NO adsorption.
2
3.3 Reactivity of the surface nitrates on Co /SiO (band at
ie
2
ca. 1540 cm—1)
The surface nitrates needed for the experiments have been
obtained after coadsorption of NO and O or adsorption of
2
NO on the Co /SiO sample, followed by evacuation.
2
ie
2
3.3.1 Thermal stability. Evacuation at 100 or 125 ¡C
induces no changes in the band at 1540 cm~1. After a 10 min
evacuation at 150 ¡C, however, the band decreases ca. twofold
in intensity and disappears completely after evacuation at
200 ¡C, i.e. the monodentate nitrates are decomposed com-
pletely at this temperature.
3.3.2 Interaction with oxygen. Admission of oxygen (10
Torr) to a sample with preadsorbed surface nitrates leads to
no changes in the spectrum. Changes have not been observed,
either, after heating the sample at 100 ¡C under oxygen. The
intensity of the nitrate bands, however, decreases after heating
at 150 ¡C in an O atmosphere, which is probably due to their
partial decomposition. Bands characterizing new surface com-
pounds have not been detected.
Fig. 6 IR spectra of surface species obtained after adsorption of
2
NO (15 min, 16 Torr, followed by evacuation) on Co /SiO sample
2
ie
2
(a) and after interaction with CH (10 Torr) at 100 ¡C for 10 min (b)
4
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
4059

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Adsorption of NO (16 Torr equilibrium pressure) on
2
Co /SiO leads to the formation of the already observed
nitr
2
reversibly bound species, namely N O (1744 and 1710 cm~1)
2
4
and NO (1680 cm~1). The strongly adsorbed compounds,
2
however, di†er totally from those observed with the Co /SiO
ie
2
sample: two bands with maxima at 1597 and 1380 cm~1 are
visible after evacuation (Fig. 8). The band at 1380 cm~1 is
typical of symmetric nitrates, whereas the band at 1597 cm~1
may be due either to water produced by replacement of
surface CowOH groups by nitrate anions, or to some kind of
bidentate nitrates. A very broad feature in the 1550È1450
cm~1 region might evidence a negligible amount of mono-
dentate nitrates on the sample surface. These results clearly
demonstrate that no monodentate nitrates are characteristic
of the surface of the Co /SiO sample, i.e. the latter can be
nitr
2
formed on highly dispersed Co2` ions only.
3.5 Co /SiO : adsorption of NO
ac
2
x
After calcination, the sample colour turns from pink to blue,
deÐnitely deeper in colour than Co /SiO . This colour is also
ie
2
preserved after activation. NO adsorption on Co /SiO leads
ac
to formation of the same nitrosyls observed with Co /SiO ,
ie
2
2
the respective bands having a higher intensity.
The spectra registered after NO (16 Torr) adsorption on
2
Co /SiO are very similar to those observed with the
ac
2
Co /SiO sample (see Fig. 8). The only di†erences are that (i)
ie
2
the bands due to surface nitrates (1526 cm~1) and NO (1676
2
cm~1) are more intense with Co /SiO , whereas (ii) bridged
Fig. 7 IR spectra obtained after coadsorption of NO (15 Torr) and
(5 Torr) on Co /SiO followed by evacuation (a), introduction of 5
ac
2
nitrates (1640 cm~1) are not detected with this sample and (iii)
the N O band (1745 cm~1) is almost absent from the spec-
O
2
ie
2
Torr of water vapour (b), evacuation at room temperature (c) and at
100 ¡C (d) and after interaction with CO (10 Torr) at 100 ¡C for 10
min (e)
2
4
trum. This is associated with the small free surface area of the
silica support, resulting from the high cobalt concentration.
This observation supports the previously made assumption
that all of the N O molecules adsorbed on dispersed cobalt
2
4
ions are converted into monodentate nitrates.
band at 1539 cm~1 is shifted to 1464 cm~1. Evacuation of the
sample at room temperature leads to a pronounced drop in
the 1626 cm~1 band intensity (due to water desorption from
the support) and a back shift of the band from 1464 to 1504
cm~1. After evacuation at 100 ¡C, the band at 1626 cm~1 van-
ishes (due to water desorption from cobalt sites) and the initial
spectrum is almost restored. The maximum of the nitrate
band is at 1548 cm~1 and the intensity is slightly lower. These
results show that the nitrates are, in general, stable under
The results obtained demonstrate that the Co /SiO
nitr
2
sample is characterized by a surface chemistry very di†erent
from that of the Co /SiO and Co /SiO samples.
ac
2
ie
2
water vapour but adsorbed H O molecules strongly a†ect the
2
position of their IR band. This observation is of importance
for the identiÐcation of the monodentate nitrates in a humid
atmosphere.
3.3.6 Interaction with CO. In order to check the oxidation
properties of surface nitrates, we also studied their interaction
with CO. It is known that CO is not a selective reducing agent
in HC-SCR but probably belongs to the intermediate com-
pounds of the reaction.6,13 Carbon monoxide coming into
contact with a sample having preadsorbed nitrates causes no
changes in its IR spectrum. After 10 min heating at 100 ¡C
(Fig. 7) the intensity of the band at 1548 cm~1 decreases
slightly, which is an indication of interaction of a small frac-
tion of the nitrates with CO.
3.4 Co /SiO : adsorption of NO
x
nitr
2
After activation, the colour of the Co /SiO sample turns
nitr
2
black, which suggests the formation of a separate cobalt oxide
phase. NO adsorption leads to the appearance of a very weak
band at 1802 cm~1 due to the l (NO) of Co2` dinitrosyls. The
s
respective l (NO) mode was not detected because of its low
as
intensity and overlap with one of the silica background bands.
Fig. 8 IR spectra of the species obtained after NO adsorption (16
2
The low intensity of the dinitrosyl bands evidences the poor
cobalt dispersion on the sample.
Torr, 2 h, followed by evacuation) on Co /SiO (a), Co /SiO (b) and
ie
2
ac
2
Co /SiO (c)
nitr
2
4060
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

View Article Online
compounds with NO. The nitrogen oxide might be oxidized
by nitrates but not by nitrites since the formal oxidation
Discussion
The compounds formed on our Co /SiO sample after NO
number of nitrogen in NO and NO~ is 2 and 3, respectively.
ie
2
x
2
adsorption are either reversibly or irreversibly adsorbed. The
Ðrst type of species are dinitrosyls, appearing during NO
adsorption, and weakly bound NO and N O , observed in a
An interesting peculiarity of the monodentate nitrates is the
ability for two such anions to be bound to one cobalt cation.
The formation of the second nitrate anion, however, is more
2
2 4
NO atmosphere. The dinitrosyls are directly coordinated to
difficult. During coadsorption of NO and
O
(a sub-
2
2
Co2` ions. In the presence of oxygen these species are easily
stoichiometric amount), Co(NO ) surface nitrates are formed
3 1
converted into surface cobalt nitrates. Note that NO is
quickly. They are characterized by a relatively high absorp-
2
adsorbed on the support alone because its adsorption is not
suppressed by the formation of nitrates. Part of the N O ,
however, is located on the support, and the other part on the
cobalt ions. Evidently, the NO stretchings of this molecule are
not sensitive enough to be used for distinguishing between the
adsorption sites. When adsorbed on the cobalt ions, N O is
tion frequency (1550 cm~1). In the presence of excess oxygen
or during NO adsorption, N O is formed and slowly trans-
2
4
2
2 4
formed into a second nitrate ion bound to the cobalt, the Ðnal
compound being Co(NO ) . In this case both nitrate ions
3 2
manifest one band at lower frequencies: 1540È1526 cm~1.
Additional information on the nitrate structures may be
obtained from the results on their interaction with water. In
principle, water adsorption would weaken the bonding of the
nitrate ion with the surface. As a result, the absorption fre-
quency of the nitrates would decrease. It has been established
that water adsorbed on cobalt ions leads to a red shift of the
nitrate ion band by ca. 35 cm~1, this shift increasing by an
additional 40 cm~1 in the presence of water adsorbed on the
support. These results support the idea that the nitrates are
bonded not only to cobalt but also to silicon ions:
2
4
transformed into surface nitrates.
The strongly adsorbed species are characterized by absorp-
tion bands at 1640 and 1540 cm~1. Both types of compound
are stable during evacuation and represent activated surface
species formed with the participation of cobalt ions. Later, we
shall concentrate on strongly bound NO adsorption forms.
On the basis of the absorption frequency, the band at 1640
cm~1 might characterize NO strongly bound to a cobalt ion.
However, the band develops slowly, whereas (NO )
be an activated adsorption species. Bands at ca. 1600 cm~1,
appearing after NO adsorption on oxide surfaces, may also
x
2
cannot
2 ads.
OwNwO
2
=
be due to water formed as a result of replacement of surface
OwSiwOwCowO
OH groups by nitrates.35,36 This interpretation is not very
probable in our case for two reasons: (i) water adsorbed on
our sample manifests a band at 1626 cm~1 and (ii) the pres-
ence of adsorbed water should cause a signiÐcant shift of the
band at 1540 cm~1 to lower frequencies, which is, in fact not
observed.
Coordination of an NO ~ anion to a cationic surface site
3
requires either a simultaneous bonding of a positively charged
particle in the vicinity (to prevent the zero charge), or oxida-
tion of the cation. Since no other species have been formed
simultaneously with the nitrates, it seems reasonable to
propose that Co2` ions are oxidized to Co3` during NO
adsorption:
Another peculiarity of the band at 1640 cm~1 is its low
intensity, in spite of the sample treatment, which indicates that
the formation of the corresponding surface compounds is
limited by the sample structure. Taking into account the sta-
bility of the adsorption species and the high absorption fre-
quency, we assign this band to bridged nitrates.34h36,38
Indeed, pairs of cobalt ions have a very low concentration on
our sample surface because the cobalt on it is highly dis-
persed. The lack of bridged nitrates on the surfaces of the
NO -precovered Co /SiO and Co /SiO samples conÐrms
x
Co2`(NO) ] O ] Co3`wNO ~ ] NO
(1)
2
2
3
or
Co2`wN O ] Co3`wNO ~ ] NO
(2)
2
4
3
This supposition is supported by the colour change of the
sample and the fact that Co2` ions can be easily oxidized.
The results obtained show a signiÐcant di†erence in the
2
ac
2
nitr
2
that the formation of such complexes requires a speciÐc
arrangement of the Co2` ions.
mechanism of NO adsorption on cobalt and copper contain-
x
ing systems. Coadsorption of NO and O on Cu-ZSM-5 leads
2
The main type of activated compounds obtained during
both NO adsorption and NOÈO coadsorption on our
to the successive formation of adsorbed N O , N O and
2
3
2 4
NO ~,13 i.e. the nitrates are formed as a result of dispro-
2
2
3
Co /SiO and Co /SiO catalysts (characterized by dispersed
portionation of N O adsorbed on Cu2` ions. The quick for-
ie
2
ac
2
2
4
cobalt) manifest a band at 1550È1526 cm~1. Obviously, these
species are formed with the participation of cobalt ions, since
they are not found on the pure support. Consider now the
basis of interpretation of the band at 1550È1526 cm~1.
Adelman et al.10 have reported a similar band (1526 cm~1)
mation of nitrates on Co2`/SiO is probably a result of the
2
dimeric adsorption form of NO, in contrast to the monomeric
adsorption form of NO on the Cu2` ions from Cu-ZSM-5.
The presence of Co2`(NO) compounds leads evidently to the
2
instant formation of adsorbed N O which, according to reac-
2
4
after coadsorption of NO and O on a Co-ZSM-5 catalyst
tion (2), is rapidly converted into Co(NO ) species.
The monodentate nitrates are very strong oxidising agents.
2
3 1
and ascribed it to surface nitrite compounds. Note that this
interpretation is only based on the relatively low absorption
frequency. However, inorganic nitro and nitrito compounds
have much lower frequences.44,49,50 The free nitrate ion is
They are reduced by NO but are obviously in equilibrium
with the gas mixture: in the presence of oxygen the equi-
librium is shifted towards formation of surface nitrates. We
are of the opinion that the results on the interaction of the
monodentate nitrates with methane are important. This inter-
action proceeds even at 100 ¡C and two types of compounds,
probably intermediate in HC-SCR, have been observed:
organic nitro-compounds and isocyanates. Tanaka et al.33
have also observed organic nitro-species as SCR interme-
diates. According to them, these compounds are converted to
characterized by l at 1380 cm~1, this vibration being split
3
with decreasing symmetry lowering the symmetry.50 Thus,
bridged and bidentate nitrates are characterized by bands at
1650È1550 and 1300È1170 cm~1 while the monodentate
nitrates have frequencies at 1530È1480 and 1290È1250
cm~1.57 Unfortunately, the lower frequency is within the
range of the supportÏs own absorbance and cannot be detected
in our case. However, a number of authors have reported
bands at ca. 1550 cm~1 together with bands at ca. 1250 cm~1,
N
after oxidation by NO . Ukisu et al.32 reported that iso-
2
x
cyanates are formed at higher temperatures than organic
nitro-compounds, which suggests that the latter species are
converted into isocyanates. Thus, the following simpliÐed
scheme of the process can be proposed:
after NO adsorption on oxides, and have assigned them to
2
surface nitrates.13,18,34h36,38,40,45 In our case, this interpreta-
tion is strongly supported by the interaction of the surface
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
4061

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that of Co /SiO : surface monodentate nitrates are the prin-
ie
2
cipal irreversibly bound compounds formed upon NO
x
adsorption. On the contrary, adsorption of NO on
2
Co2`/SiO prepared from nitrates (non-active SCR catalyst)
2
leads to the formation of strongly bound symmetric (1380
cm~1) and bidentate (1597 cm~1) nitrates. No monodentate
nitrates are formed on this sample. It is proposed that the
ability of supported cobalt catalysts to form monodentate
nitrates, favoured by a high cobalt dispersion, would deter-
mine their activity in the HC-SCR.
This work was supported by the Bulgarian National Research
Foundation (Projects X-486 and X-534).
In the above scheme only the steps derived from our results
are shown. To establish other intermediate states, e.g. to
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ie
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ie
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ie
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ie
2
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ie
2
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J. Chem. Soc., Faraday T rans., 1997, V ol. 93
4063