Formation and reactivity of isocyanate (NCO) species on Ag/Al2O3

Article abstract of DOI:10.1039/a801849i

The formation of the reactivity of isocyanate species have been studied over Ag/Al₂O₃ by IR spectroscopy and mass spectrometry. Adsorbed C_xH_yNO_z and NO₃^- species are produced by reaction among NO, O₂ and C₃H₆ at room temperature. Thermal decomposition of adsorbed C_xH_yNO_z species leads to the formation of two types of NCO species (NCO on Ag and NCO on Al₂O₃) above 423 K. These NCO species are thermally stable in vacuum at 673 K, while adsorbed NO₃^- species decompose completely. The NCO species are highly reactive toward NO + O₂ at room temperature, being converted into N₂, CO₂, CO and a small amount of N₂O. The NCO species are less active in NO or O₂ alone than in the mixture of NO and O₂. Thus, excess oxygen added in the NO reduction by C₃H₆ plays an important role in the formation of adsorbed C_xH_yNO_z species and in the reaction of adsorbed NCO with NO. It is suggested that the formation of adsorbed C_xH_yNO_z and adsorbed NCO is essential for the progress of the NO reduction with C₃H₆ in the presence of O₂ under the present experimental conditions.

Full text of DOI:10.1039/a801849i

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Formation and reactivity of isocyanate (NCO) species on Ag/Al O
2 3
Satosi Sumiya,a Hong He,a Akira Abe,a Nobutsune Takezawab and Kiyohide Yoshidaa
a Research and Development Division, Riken Corporation, 4-14-1 Suehiro, Kumagaya, Saitama,
360-8522, Japan
b Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido
University, W est 8, North 13, Sapporo, Hokkaido 060, Japan
The formation of the reactivity of isocyanate species have been studied over Ag/Al O by IR spectroscopy and mass spectrom-
2
3
etry. Adsorbed C H NO and NO ~ species are produced by reaction among NO, O and C H at room temperature. Thermal
x
y
z
3
2
3 6
decomposition of adsorbed C H NO species leads to the formation of two types of NCO species (NCO on Ag and NCO on
x
y
z
Al O ) above 423 K. These NCO species are thermally stable in vacuum at 673 K, while adsorbed NO ~ species decompose
2
3
3
completely. The NCO species are highly reactive toward NO ] O at room temperature, being converted into N , CO , CO and
2
2
2
a small amount of N O. The NCO species are less active in NO or O alone than in the mixture of NO and O . Thus, excess
2
2
2
oxygen added in the NO reduction by C H plays an important role in the formation of adsorbed C H NO species and in the
reaction of adsorbed NCO with NO. It is suggested that the formation of adsorbed C H NO and adsorbed NCO is essential for
the progress of the NO reduction with C H in the presence of O under the present experimental conditions.
3
6
x
y
z
x
y
z
3
6
2
ground in an agate mortar into a wafer of ca. 20 mg cm~2.
Prior to the experiment, the sample was heated at 573 K in
vacuum (10~5 Torr) for 30 min and then oxidized in 50 Torr
Introduction
Catalytic reduction of lean NO with hydrocarbons has been
investigated on a number of catalysts.1h8 Various hydrocar-
bons were used as reducing agents for the NO reduction. The
reduction of NO is greatly enhanced in the presence of O .3
However, the activity of the catalyst is largely suppressed in
x
of O at 573 K for 30 min followed by brief pumping at room
2
temperature. All IR measurements were carried out at room
x
temperature. The reactivity of adsorbed species formed in the
reaction was also evaluated over 0.4 g of Ag/Al O catalyst in
x
2
2
3
the presence of water vapor and SO .4 Prominent catalysts
a closed vessel, and the gaseous, products using 15NO as a
2
for the reduction of lean NO have been developed.
x
reactant were analyzed by mass spectrometry.
In recent IR studies, surface species such as isocyanate,9h13
(N O ) ,11 CN,12,13 nitro14 and nitrite species14 have been
x
y z
Results and Discussion
observed on various supported catalysts in the reduction of
lean NO . Isocyanate species (wNCO) were detected on
Preparation of NCO on Ag/Al O
x
2
3
alumina-supported Cu and Ag catalysts in the reaction among
Fig. 1 shows FTIR spectra of Ag/Al O subjected to various
NO, hydrocarbons and O .9,10 It was suggested that NCO
2
3
2
treatments in O , a mixture of C H and NO, or in a mixture
species were produced through the decomposition of
2
3 6
of C H , NO and O . When Ag/Al O is exposed to a
“precursorsÏ, C H NO complexes, which were formed in reac-
3
6
2
2 3
x
y
z
mixture of C H (11 Torr) and NO (14 Torr) at room tem-
tions among NO, O and hydrocarbons.9 N and CO were
3
6
2
2
2
perature, weak absorption bands are discerned at 2860, 2920
formed in the reaction between NCO species and NO on
Cu/Al O .9 The reduction of NO and the formation of NCO
2
3
x
species are suppressed in the reaction among NO, hydrocar-
bons and O in the presence of water vapor.15 These results
2
suggest that NCO species were important intermediates in the
lean NO reduction. However, no experiments were carried
x
out for the reaction of NCO with NO in the presence of O ,
2
although excess oxygen is essential for lean NO reduction.
x
In the present paper, the formation and the reactivity of
NCO species on Ag/Al O are studied by IR spectroscopy
2
3
and mass spectrometry, and the role of O in the reduction of
2
lean NO is also discussed.
x
Experimental
An alumina-supported silver catalyst (denoted Ag/Al O ) was
2
3
prepared by an impregnation method employed in previous
experiments.4 Silver loading was 5.0 wt.%. FTIR spectra of
adsorbed species were obtained using a Nicolet Magna 550
spectrophotometer with a resolution of 2 cm~1. An IR cell
made of Pyrex glass was Ðtted with a sample holder, an elec-
Fig. 1 FTIR spectra of Ag/Al O after (a), oxidation treatment, (b),
2
3
exposure to a mixture of NO (14 Torr) ] C H (11 Torr) at room
3
6
temperature, (c), exposure to a mixture of C H (11 Torr) ] NO (14
3
6
tronic heater and BaF windows. The sample holder can be
Torr) ] O (15 Torr) at room temperature, (d), heating (c) at 423 K in
2
2
moved from the heater section to the window section in the
the gas mixture for 15 min, and (e), heating (d) at 573 K in the gas
cell. The IR sample was prepared by pressing catalyst powder
mixture for 15 min
J. Chem. Soc., Faraday T rans., 1998, 94(15), 2217È2219
2217

View Article Online
and 2960 cm~1, as shown in Fig. 1(b). According to Shep-
pard,16 the bands are assigned to CwH stretching modes for
adsorbed C H . When O is added to the mixture of NO and
broad IR band at ca. 2240 cm~1 reported above consists of
two types of IR bands, at 2230 and 2260 cm~1.
A weak band for CN species appears at 573 K in the pres-
ence or the absence of a mixture of NO, C H and O . In Fig.
3
6
2
C H , various absorption bands appear in the region 1200È
3
6
3
6
2
2000 cm~1 as shown in Fig. 1(c). The band observed at 1655
1(d), it was shown that the weak and broad band at ca. 2240
cm~1 for NCO surface species appeared at 423 K. The band
disappeared at 573 K when the Ag/Al O was heated in the
cm~1 is ascribable to organic compounds. C H NO , formed
x
y
z
by reaction among NO, O and C H .9 Other bands can be
2
3
6
2 3
assigned to physisorbed NO (1880 cm~1)9,17 and NO ~
presence of a mixture of NO, C H and O [Fig. 1(e)]. In
3
3
6
2
(1580, 1300 cm~1) localized on Al O surface.9,17h20 Tanaka
and Okuhara previously assigned the IR band at ca. 1655
contrast to the NCO species, the CN species was stable at 573
K in the presence of a mixture of NO, C H and O , suggest-
2
3
3
6
2
cm~1 to an NwO stretching mode of RONO in lean NO
ing that CN species were less active toward a mixture of NO,
C H and O than the NCO species. The mechanism of the
x
z
reduction on Pt/SiO .14 Thus, we conclude that the C H NO
2
x
y
3
6
2
species are organic nitrites formed on the catalyst surface.
formation of the CN species is not clear at present. Bell et
al.25 recently proposed that CN species were formed by
decomposition of CH NO in NO reduction by CH . Hence,
Tanaka and Okuhara also found an IR band at 1565 cm~1
and assigned it to RNO .14 Haneda et al. recently observed
2
3 4
it is probable that the C H NO species are involved in the
an IR band at 1580 cm~1 by reaction among NO, O and
2
x
y
z
C H over Ag/TiO -ZrO and assigned it to RNO .21 Fig.
1(d) and (e) show IR spectra for Ag/Al O heated in a mixture
of NO, C H and O at 423 K and 573 K for 15 min, respec-
formation of CN species in the present experiments.
3
6
2
2
2
2
3
Reaction of NCO/Ag/Al O with O , NO or a mixture of NO
3
6
2
2
3
2
tively. The band at 1655 cm~1 decreases at 423 K, while a
weak and broad band resulting from NCO species22 appears
at ca. 2240 cm~1. After heating to 573 K, the IR spectrum
changes drastically. The IR band at ca. 2240 cm~1 diminishes
while a weak band for CN species12,13,17 is newly detected at
2166 cm~1.
and O
2
Surface NCO species (AgwNCO and AlwNCO) were pre-
pared on heating in a vacuum at 673 K after exposing
Ag/Al O to a mixture of NO (14 Torr), C H (11 Torr) and
2
3
3 6
O (15 Torr) at room temperature. The reactivity of the NCO
2
species was then tested with O , NO or a mixture of NO and
O . Fig. 3 shows the variation of the NCO species in the pres-
ence of O on Ag/Al O at various temperatures. The bands
for AgwNCO and AlwNCO species decrease greatly in
intensity after heating to 523 K. Heating to 523 K results in
the disappearance of the NCO species. By mass spectrometric
measurements, it was found that CO and N were produced
Fig. 2 shows the IR spectra observed after evacuation of a
mixture of NO (14 Torr), C H (11 Torr) and O (15 Torr) at
2
2
3
6
2
room temperature and then heating stepwise to various tem-
peratures for 15 min. For the sample heated to 373 K for 15
min, bands appear in the region 1200È2000 cm~1 as shown in
Fig. 2(a). The absorption band at 1655 cm~1 decreases slightly
at 423 K, and a weak band appears at 2230 cm~1. After
heating to 523 K, the IR spectra change drastically. The IR
band at 1655 cm~1 almost disappears and a new IR band
grows at 2260 cm~1 on a shoulder of the band at 2230 cm~1
as shown in Fig. 2(d). The absorption bands at 1580 and 1300
2
2 3
2
2
in a molar ratio of ca. 2 : 1, suggesting that the reaction 2NCO
] O ] 2CO ] N took place.
2
2
2
In a similar fashion, the reactivity of adsorbed NCO was
inspected in the presence of NO. No reaction occurred below
573 K. The absorption bands for AgwNCO and AlwNCO
disappeared around 623 K.
cm~1 for NO ~ species still remained. The IR band at 2230
3
cm~1 was assigned to NCO species bound to Ag, AgwNCO,
Fig. 4 shows the interaction between adsorbed NCO species
(AgwNCO and AlwNCO) prepared at 673 K and a mixture
while the band at 2260 cm~1 was assigned to AlwNCO
species.22h24 Hence, NCO species preferably occupy Ag sites
and then Al O sites. Heating to 623 K leads to the decompo-
of NO (14 Torr) and O (15 Torr). When the NCO species is
2
2
3
exposed to the mixture of NO and O , the bands ascribed to
sition of NO ~ and the growth of intense IR bands at 2230
2
3
AlwNCO and AgwNCO species decrease markedly even at
and 2260 cm~1 [Fig. 2(f)]. The peak intensities for these NCO
species further increase on heating at 673 K for 15 min, while
room temperature (298 K). A band remains at 2245 cm~1 with
a weak shoulder at ca. 2230 cm~1. These results suggest that
the position of the band for AlwNCO species shifts to 2245
cm~1 in the course of the reaction in the presence of NO and
the bands for NO ~ at 1580 and 1300 cm~1 decrease greatly.
3
This suggests that AgwNCO and AlwNCO species are ther-
mally more stable than NO ~ species in a vacuum. Hence,
3
O . The IR bands attributed to AlwNCO and AgwNCO
these NCO species were selectively produced on heating the
2
sample in a vacuum at 673 K. These results suggest that a
Fig. 3 FTIR spectra of the NCO species on Ag/Al O . (a) NCO
2
3
species on Ag/Al O after heating to 673 K for 15 min in a vacuum,
2
3
Fig. 2 FTIR spectra of Ag/Al O after evacuating a mixture of NO
(b) after exposure to 15 Torr of O at 373 K for 15 min, (c) after
heating at 423 K in 15 Torr of O for 15 min, (d) after heating at 473
K in 15 Torr of O for 15 min, (e) after heating at 523 K in 15 Torr of
2
3
2
(14 Torr) ] C H (11 Torr) ] O (15 Torr) at room temperature and
3
6
2
2
heating to various temperatures in a vacuum for 15 min; (a) 373 K, (b)
423 K, (c) 473 K, (d) 523 K, (e) 573 K, (f) 623 K, (g) 673 K
2
O
for 15 min.
2
2218
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

View Article Online
were produced on Ag/Al O by reaction among NO, O and
2
3
2
C H at room temperature. The NCO species (AgwNCO and
3
6
AlwNCO) were formed by the thermal decomposition of
C H NO . The NCO species were thermally stable up to 673
K in a vacuum. These species were highly reactive in a
x
y
z
mixture of NO and O at room temperature, being converted
2
to N , CO , CO and small amounts of N O. Two important
2
2
2
contributions of O to the reduction of NO by C H were
2
x
3 6
suggested in the present experiments; (1) the presence of
oxygen is essential for the formation of organic compound
C H NO intermediates for NCO species, and (2) oxygen pro-
motes the reaction of NCO with NO. On the basis of the
above results, the importance of the formation of C H NO
x
y
z
x
y
z
precursors and NCO species was stressed.
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Fig. 4 FTIR spectra of the NCO species on Ag/Al O . (a) NCO
2
3
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2
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4
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6
7
8
2
2
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2
2
presence of a mixture of NO and O was extremely rapid as
2
compared with that in a vacuum, or in the presence of O or
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2
9
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2
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2
2
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2
3
expressed as,
NO ] O ] C H ] C H NO ]
2
3
6
x
y
z
NO, O2
NCO(a) ÈÈÈÈ N ] CO ] CO
2
2
It was shown that the organic compounds, C H NO , were
readily produced on the Ag/Al O in a mixture of NO, O
x
y
z
2
3
2
and C H at room temperature [Fig. 1(c)]. The organic com-
3
6
pounds underwent thermal dissociation at ca. 600 K in a
vacuum to yield NCO species. Because of facile formation of
C H NO and high reactivity of NCO species in the presence
x
y
z
of NO and O , we conclude that the NCO formation is a rate
2
determining step of the NO reduction with C H in the pres-
3
6
ence of O under the present experimental conditions.
2
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Conclusion
The reduction of NO with C H was carried out over
3
6
Ag/Al O in the presence of oxygen by the use of IR spectros-
2
3
copy and mass spectrometry. Adsorbed C H NO species
Paper 8/01849I; Received 6th March, 1998
x
y
z
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
2219