Preparation and characterization of W/γ-Al2O3 and Pd-W/γ-Al2O3 catalysts from organometallic precursors. The catalytic activity for NO decomposition

Article abstract of DOI:10.1016/S1381-1169(98)00130-7

The photochemical reaction of W(CO)₆ with triphenylphosphine (PPh₃) in the presence of γ-Al₂O₃ and Pd/γ-Al₂O₃ has been used to prepare W/γ- Al₂O₃ and Pd-W/γ-Al₂O₃ catalysts. Adsorbed mono- and disubstituted W species have been identified by FTIR spectroscopy. There is evidence of the adsorption of W(CO)(6-x) L(x) species on both the alumina and the Pd surface. After thermal decomposition and reduction at 573 K the catalysts have been characterized by FTIR spectroscopy of adsorbed NH₃, CO and NO. The retention of W and P suppresses the Lewis acidity of the alumina support. On Pd-W/γ- Al₂O₃, the W is present in a partially reduced state in close association with Pd. This interaction modifies the chemisorptive properties of NO relative to those of the monometallic Pd and W catalysts. In line with these observations the Pd-W/γ-Al₂O₃ catalyst presents an enhanced activity for NO decomposition at 473 K.

Full text of DOI:10.1016/S1381-1169(98)00130-7

Ž
.
Journal of Molecular Catalysis A: Chemical 137 1999 287–295
Preparation and characterization of Wrg-Al2O3 and
Pd–Wrg-Al2O3 catalysts from organometallic precursors.
The catalytic activity for NO decomposition
a
b
b
a,)
A.M. Sica , J.H.Z. Dos Santos , I.M. Baibich , C.E. Gigola
a
Planta Piloto de Ingenierıa Quımica, 12 de Octubre 1842, 8000 Bahıa Blanca, Argentina
Instituto de Quımica, UFRGS, AÕ. Bento GoncalÕes 9500, 91540-970 Porto Alegre, Brazil
´
´
´
b
´
Received 9 October 1997; accepted 13 April 1998
Abstract
Ž
.
Ž
.
The photochemical reaction of W CO with triphenylphosphine PPh3 in the presence of g-Al2O3 and Pdrg-Al2O3
6
has been used to prepare Wrg-Al2O3 and Pd–Wrg-Al2O3 catalysts. Adsorbed mono- and disubstituted W species have
been identified by FTIR spectroscopy. There is evidence of the adsorption of W CO
Ž
.
L x species on both the alumina and
6yx
the Pd surface. After thermal decomposition and reduction at 573 K the catalysts have been characterized by FTIR
spectroscopy of adsorbed NH3, CO and NO. The retention of W and P suppresses the Lewis acidity of the alumina support.
On Pd–Wrg-Al2O3, the W is present in a partially reduced state in close association with Pd. This interaction modifies the
chemisorptive properties of NO relative to those of the monometallic Pd and W catalysts. In line with these observations the
Pd–Wrg-Al2O3 catalyst presents an enhanced activity for NO decomposition at 473 K. q 1999 Elsevier Science B.V. All
rights reserved.
Keywords: Metal carbonyls; Alumina; Surface species; Characterization; NO decomposition
w x
1. Introduction
that of Rh containing catalysts 2 . In order to
overcome this limitation, palladium catalysts
Recently there has been a growing interest in
the use of Pd as the only active metal for
exhaust catalysts, due to its low cost as com-
pared with Pt and Rh. Previous studies have
shown that Pd can catalyze the reduction of
promoted with metal oxides such as MnOx,
MoO3 and WO3 have been proposed 3–5 . In
principle W, in the reduced state, is quite active
w
x
w x
for NO decomposition 6 , but it is difficult to
obtain cations in a low oxidation state when the
metal is well-dispersed and isolated on the alu-
mina surface. However it is expected that the
close proximity of Pd and W may facilitate the
partial reduction of WO3. Using inorganic pre-
cursors, catalysts with adequate metal–metal
contact could be prepared but it requires the use
of high metal loadings. Consequently W and
w x
nitric oxide 1 to N2 in addition to its well
known activity for hydrocarbons and carbon
monoxide oxidation. However in the presence
of hydrocarbons the nitric oxide removal capac-
ity of Pd under reducing conditions is inferior to
)
Corresponding author.
1381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S1381-1169 98 00130-7

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A.M. Sica et al.rJournal of Molecular Catalysis A: Chemical 137 1999 287–295
2
Ž
.
Mo volatilization occur under oxidizing condi-
and g-Al2O3 Rhone-Poulenc, 206 m rg . The
solid and the solution were left in contact for 24
h at room temperature. After impregnation the
liquid was removed and the solid dried in N2.
Subsequently the solution was calcined in air at
573 K for 2 h and then reduced in flowing
hydrogen at the same temperature. Using this
palladium catalyst and the pure alumina sup-
port, Pd–Wrg-Al2O3 and Wrg-Al2O3 cata-
lysts were obtained. In order to carry out the
w x
tions 7 . A better approach to prepare Pd–W
catalysts may be the use of organometallic pre-
cursors. It is known that they allow the prepara-
tion of well-dispersed, low valence species with-
w
Ž
. x
out high thermal treatments. Using W CO ,
6
w x
Kazusaka and Howe 8 prepared W on alumina
catalysts, but the maximum metal loading was
about 0.3%. Upon activation at 2008C and ad-
sorption of NO they were able to observe a pair
4
of IR bands assigned to Wq NO species. An
photochemical reaction between W CO and
Ž
.
w
Ž
.6x
2
improved procedure to deposit W, Mo and Cr
PPh3 under inert atmosphere the Schlenk tech-
nique was used. The g-Al2O3 was first evacu-
ated at 723 K for 1 h and then cooled in Ar. For
the Pdrg-Al2O3 catalyst a 1 h hydrogen treat-
ment at 573 K was performed before cooling to
room temperature. In a typical experiment, 1 g
of solid was added to a pentane solution of
W CO 40 cm , 6.7% M and mixed continu-
ously for 30 min. The amount of W added was
enough to obtain a 5 wt.% loading and the
.6x
W CO to PPh3 molar ratio was 1r2. The
photochemical reactions were performed in an
Ar atmosphere using a Philips HPL-N 125 W
UV lamp fitted into a Pyrex cold finger. The
transmitted UV wavelength was limited by the
Pyrex glass to 220 nm. In accordance with
w x
on alumina has been recently introduced 9 and
is based on the photochemical reaction of
w
Ž
.
6x
Ž
.
M CO
MsCr, Mo, W with PPh3. The
Ž
.
substitution of CO in W CO for PPh3 ligands
6
allows the formation of mono- and disubstituted
species that interact more strongly with the
alumina support than pure hexacarbonyls.
Moreover the irradiation time could be used as a
preparation variable to adjust the metal loading.
As mentioned before, our aim is to stabilize
reduced W on the Pd particles, to observe the
effect of the second metal on the activity of Pd
for NO decomposition. We expect that a good
Pd–W interaction may develop either during the
W adsorption process or following the removal
of the ligands. In this work the surface species
formed on Pdrg-Al2O3 by the photochemical
3
Ž
. Ž
.
6
w
Ž
Ž
.
w x
previous studies 9 the slurry was irradiated
during 6 h to ensure a maximum loading of W.
In order to observe the reaction progress small
liquid samples were withdrawn to be analyzed
by FTIR. After reaction the liquid solution was
removed, the solid washed several times with
pentane, evacuated for 1 h and finally stored
under Ar. The contents of Pd, W and P were
w
Ž
.6x
reaction of W CO with PPh3 have been
identified by FTIR and 13C NMR analysis and
compared with those present on the pure sup-
port. Upon thermal treatment to decompose the
substituted carbonyls the resulting mono and
bimetallic catalysts have been characterized by
the FTIR spectra of adsorbed NH3, CO and NO.
Finally the NO decomposition reaction has been
used to investigate the influence of W on the
catalytic activity of Pd.
Ž
.
Ž .
determined by AAS Pd and W and ICP P .
Adsorbed species on the catalyst precursors
were identified by FTIR spectroscopy. Infrared
experiments were performed in a Nicolet 20
DXB instrument at 4 cmy resolution. Catalyst
1
samples of approximately 30 mg were pressed
to form transparent disks of 10 mm in diameter
that were mounted in a heated metal holder. The
holder was placed in the beam path of a stain-
less steel cell sealed with CaF2 windows, and
coupled to a vacuum system for evacuation to
2. Experimental
The palladium on alumina catalyst was pre-
pared by the wet impregnation technique using
a benzene solution of palladium acetylacetonate

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A.M. Sica et al.rJournal of Molecular Catalysis A: Chemical 137 1999 287–295
289
10y Torr. It was possible to perform heat
The flow rate was adjusted at 30 cm3rmin.
Prior to reaction the catalysts were reduced in
situ at 473 K. Gas chromatographic separation
of reactant and products was achieved by means
6
treatments up to 573 K, to dose small amounts
of CO, NO and NH3 through a leak valve or
maintain a steady flow of H2 or He. A MKS
Baratron Type 170 M allows pressure measure-
ments in the 0.1–10 Torr range.
The FTIR characterization of the Pd–Wrg-
Al2O3 and Wrg-Al2O3 catalysts precursors was
first performed on samples evacuated at room
Ž
of two Porapak Q columns. One 1r8 in.=6
.
mts was operated at 263 K for the separation of
NO and N2; another 1r8 in.=2.4 mts at 313
Ž
.
K allowed the identification of N2O.
Ž
temperature an empty cell was used as a back-
.
ground . Subsequently the samples were heated
3. Results and discussion
in He up to 573 K to decompose the tungsten
carbonyls; a FTIR spectrum was taken under
this condition to verify the absence of surface
species. After reduction in flowing hydrogen at
the same temperature the samples were cooled
in flowing He to room temperature. An evacu-
ated background spectrum was then taken for
adsorption studies. In typical experiments adsor-
3.1. Identification of adsorbed carbonyls on g-
Al2O3 and Pdrg-Al2O3
FTIR analysis of solid samples washed with
pentane have shown that fairly stable adsorbed
species remained on the surface. Strong bands
1
in the 1800–1950 cmy region, corresponding
Ž
.
bates CO, NO and NH3 were dosed at pres-
sures of 1 to 5 Torr and the samples exposed to
the gas phase for 2 to 5 min prior to spectra
measurement. Scanning time was about 1 min.
to monosubstituted and disubstituted carbonyls,
was the relevant feature observed on g-Al2O3.
This situation is clearly observed in Fig. 1. A
w
Ž
.6x
small amount of W CO
after washing, as seen by the band located at
remains on Al2O3
Finally the cell was evacuated to 10y Torr for
5
1
1985 cmy T1u mode, Oh symmetry . Bands
Ž .
2 min and a new spectrum was taken. Gas
pressures, contact time and temperatures were
well-controlled in order to facilitate the compar-
ison of results for different samples.
that appear at 1946 cmy A1 qE and 2072
1
ax
Ž
.
1
rad
cmy
A1
are due to monosubstituted
Ž
.
The 13C-NMR spectrum of the species ex-
tracted from the alumina surface with CH2Cl2
was obtained in CDCl3 solution on a Varian
Gemini VXR 200 MHz instrument and exter-
nally referred to TMS. The spectrum was
recorded after 20 000 scans at room temperature
with a pulse width of 458 and pulse delay of 1.5
w
Ž
.3x
s. The relaxing agent Cr acac was added to
overcome the long T1 relaxation time of the
carbonyl resonance.
Catalytic experiments were carried out in a
flow reactor mounted in an electric furnace.
Samples of 0.1 g were packed in a 1r4 in. SS
tube with a thermocouple placed at the reactor
entrance. The NO decomposition reaction was
studied as a function of time at 473 K and 573
K, using a feed mixture containing 340 ppm of
Ž .
a
Fig. 1. FTIR Spectra of adsorbed carbonyl species on g-Al2O3
Ž .
Ž
.
NO in a He background Matheson, certified .
and Pdrg-Al2O3 b .

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290
A.M. Sica et al.rJournal of Molecular Catalysis A: Chemical 137 1999 287–295
Table 1
w
Ž
.
x
W CO L species. On the other hand the bands
5
Composition of catalysts precursors prior to thermal treatments
located at 1868, 1884, 1898, 1919 and 2019
Ž
.
Ž
.
Ž
.
Pd wt.%
W wt.%
P wt.%
cmy correspond to W CO L2 species. Ei-
1
w
Ž
.
x
4
g-Al2O3a
Wrg-Al2O3
Pd–Wrg-Al2O3
y
y
y
0.48 0.27
0.54
0.21
b
1
Ž
.
ther the band at 1884 cmy or the one at 1898
2.2
1.8
cmy is due to trans- W CO L2 Eu mode,
1
w
Ž
.
x Ž
6
0.78
.
D4 symmetry . The other four bands corre-
spond to A1, A1, B1 and B2 modes of cis-
h
a PPh3 adsorption in the absence of W CO
.
Ž
.
6
b Numbers in parentheses give the P content after the He and H2
treatments.
w
Ž
.
x
Ž
.
W CO L2 C2v , going from low to high
4
wavenumbers. The trans species are occluded
by the more abundant cis species. Consequently
a NMR analysis was performed for the species
extracted from the Wrg-Al2O3 sample. The
NMR spectrum of the tungsten carbonyl com-
strictly compared due to differences in the sup-
port area.
The Lewis acid sites on the alumina surface
may also be occupied by PPh3, taking into
account that it is a strong base. In other words
we cannot exclude a competition of PPh3 and
substituted carbonyls for the adsorption sites. A
clear FTIR evidence of PPh3 adsorption on
g-Al2O3 was found when the photochemical
reaction was carried out in the absence of
w
Ž
.6x
pounds showed bands related to W CO
Ž
.
w
Ž
.
x Ž
191.1 ppm , W CO PPh3 doublet at 197.2
5
ppm, Jc yPs6.9 Hz due to the carbonyls cis
.
w
Ž
. Ž
.
x
to the phosphine ligand , cis- W CO PPh3 2
4
Ž
two triplets, one at 205.4 ppm, Jc yPs9.6
Hz related to the CO’s trans to the ligand and
the other at 203.6 ppm, Jc yPs7.3 Hz due to
.
Ž
.
the CO’s cis to the phosphines . The higher
W CO ; an intense band was present at 1438
6
intensity of the triplet at 203.6 ppm is supposed
cmy1. The subsequent ICP analysis confirmed
w
Ž
.
Ž
to be due to the presence of trans- W CO
the presence of 0.48% by weight of P see
2
Ž
.
x
.
Ž
.
PPh3 2 . The band integration showed a trans
Table 1 . In the presence of W CO , the P
6
to cis ratio of 0.4, confirming the predominance
uptake of g-Al2O3 was slightly higher; 0.54%.
Consequently the P content arises mainly from
adsorption of PPh3 on the support surface al-
though the ligands from mono and disubstituted
tungsten carbonyls may become an additional
source of P.
The spectra of adsorbed species was quite
different on Pdrg-Al2O3. As seen in Fig. 1 the
bands were less intense, and mainly due to
disubstituted species that seems to be more
stable. Therefore the amount of W expected was
less than that on g-Al2O3. However the values
in Table 1 in fact indicate that the W loading
was slightly lower, 1.8%, while the major dif-
ference was in the amount of P; it decreased
from 0.54% to 0.21%.
of cis species.
It is well-known that metal carbonyls become
attached to Alq Lewis acid sites through the
3
w
x
oxygen atom in the CO ligands 10 . When
these ligands are replaced by PPh3, the electron
density in the metal increases and consequently
the species coordinated to surface sites are more
stable. An interesting experimental observation
is that only monosubstituted species were de-
tected in solution. Apparently the adsorption of
monosubstituted carbonyls facilitates the elimi-
nation of CO ligands not involved in bonding,
which in turn may be replaced by PPh3 ligands
to form the disubstituted species. The 2.2% W
loading on g-Al2O3, shown in Table 1, indi-
cates that PPh3 ligands facilitate the attachment
of carbonyls to the support surface. It is relevant
to recall that a maximum W loading of about
The appreciable decrease of intensity in the
FTIR spectra of adsorbed carbonyls and the
reduced P uptake on Pdrg-Al2O3, relative to
those on g-Al2O3, may imply that the presence
of Pd limits the sites where attachments of
substituted metal carbonyls and PPh3 take place.
w x
0.3% was obtained by Kazusaka and Howe 8
when the hexacarbonyl was adsorbed from the
vapor phase, although the values cannot be

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A.M. Sica et al.rJournal of Molecular Catalysis A: Chemical 137 1999 287–295
291
Information about the relative concentration and
strength of Lewis acid sites was obtained by
inspection of the spectra of adsorbed NH3.
As seen in Fig. 2, the presence of Pd slightly
alters the band due to the symmetric deforma-
have observed that catalyst samples examined
after a reaction time of 2 h, showed more
intense IR bands than those in Fig. 1. We
recognize that additional studies are needed to
describe the palladium–hexacarbonyl interac-
tion in more detail. No information has been
found on the reactivity of supported group-VIII
metals with hexacarbonyls compounds.
tion vibration of NH3 adsorbed on g-Al2O3; on
Pdrg-Al2O3 the band at 1267 cmy 11 , is
broader. On the basis of this result a diminished
interaction of PPh3 and carbonyl species with
the Lewis acid sites of Pdrg-Al2O3, relative to
those on g-Al2O3, should not be expected. Con-
sequently the large decrease in intensity of ad-
sorbed carbonyls, the slight variation in the W
content and the decrease in P content are diffi-
cult to explain. Our interpretation is based on
the assumption that substituted carbonyls inter-
act with both the alumina support and the
1
w
x
Following the addition of W significant alter-
ations were observed on the spectra of adsorbed
NH3. It is seen in Fig. 2, that on Wrg-Al2O3
the characteristic band experiences a marked
decrease in intensity and it is shifted to a lower
y
1
Ž
.
IR frequency 1234 cm , an indication that
only weak Lewis acid sites remain on the sur-
face. This behavior is due to W and P content
interacting with exposed Al3 sites. Moreover,
q
w
Pd surface. In addition we suspect that W-
as Bronsted acid sites are not formed it is
reasonable to conclude that the elements remain
in a low valence state. On Pdrg-Al2O3 the
addition of W and P also reduced the concentra-
tion of acid sites but the frequency maintained
Ž
.
x
CO yx L x species adsorbed on the metal lose
6
the ligands during the long irradiation time and
consequently only a fraction of the attached W
contribute to the adsorption spectra. This is in
accordance with a lower P content on Pd–
Wrg-Al2O3. Supporting this interpretation we
y
1
Ž
.
its value 1273 cm . These results are in line
with a lower amount of W and P on the Al2O3
surface. Regarding the effect of P, it is impor-
tant to state that the contents given in Table 1
have been determined prior to thermal treat-
ments in He and H2. The elimination of PPh3
occurs during the hydrogen activation step at
573 K, leading to lower levels of P on the
reduced samples. This behaviour was verified
when g-Al2O3 exposed to PPh3 was heated in
He and H2. The P content decrease from 0.48%
Ž
.
to 0.27% Table 1 and the PPh3 band at 1438
1
cmy was strongly attenuated. In the presence
of Pd the elimination of PPh3 should be more
w
x
effective 12 .
3.2. Adsorption of CO and NO
After a heat treatment in He at 573 K, a
spectra ratioed with the initial empty cell back-
ground indicated the absence of residual car-
bonyl species on both, Wrg-Al2O3 and Pd–
Wrg-Al2O3. These samples and that of Pdrg-
Al2O3 were then characterized by FTIR spec-
Ž .
Fig. 2. FTIR Spectra of NH3 adsorbed on g-Al2O3 a , Pdrg-
Ž . Ž . Ž .
Al2O3 b , Pd–Wrg-Al2O3 c and Wrg-Al2O3 d .

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A.M. Sica et al.rJournal of Molecular Catalysis A: Chemical 137 1999 287–295
troscopy of adsorbed CO and NO. Following a
reduction in hydrogen at 573 K, the room tem-
perature adsorption of CO on Wrg-Al2O3 gave
no significant IR bands while NO adsorption
1
only shows weak bands at 1554 cmy and 1235
1
cmy that are attributed to nitrite and nitrate
species on the alumina support. Surface science
w
x
studies 6,13 have shown that NO adsorbs dis-
sociatively on W at room temperature. In addi-
w
x
tion Yan et al. 14 have demonstrated that
Wrg-Al2O3 catalysts reduced at high tempera-
5
q
and Wq4 sites
Ž
.
ture )773 K exposed W
that are able to absorb CO and NO. Kazusaka
w x
and Howe 8 performed an IR study of ad-
sorbed NO on Wrg-Al2O3 prepared from
Ž
.
Ž .
a and
Fig. 4. FTIR Spectra of NO adsorbed on Pdrg-Al2O3
Ž .
W CO . Samples were prepared in situ to
6
Pd–Wrg-Al2O3
b
maintain a low valence state of W. In this way
1
they were able to observe bands at 1795 cmy
and 1685 cmy that were assigned to dinitrosyl
1
species adsorbed on W4q. Taking into account
the preparation method used in the present study
we also expected to obtain partially reduced W
on g-Al2O3 catalysts, but the low metal content
and the inevitable exposure to ambient air dur-
ing sample handling for the FTIR experiments
may lead to W oxidation.
catalysts. On Pdrg-Al2O3, typical bands as-
y1
Ž
.
signed to linear 2084 cm
and bridge species
.
can be recognized. The pre-
y
1
Ž
1937–1970 cm
dominance of bridged CO indicates that the Pd
sample had a moderate metal dispersion. Upon
W addition, a strong attenuation on the CO
band intensities was observed, a clear evidence
that W was being deposited on the Pd particles;
the main attenuation was observed for multico-
ordinated CO. As the decrease in band intensity
was not accompanied by a shift in frequency,
we conclude that the addition of W does not
lead to a dilution of the surface Pd atoms or
indicate the presence of electronic effects. The
suppression of CO chemisorption demonstrates
that the method of preparation has been success-
ful to obtain W in close contact with Pd. IR
bands due to CO adsorbed on partially reduced
Fig. 3 displays the spectra of CO adsorption
on reduced Pdrg-Al2O3 and Pd–Wrg-Al2O3
w
x
W, as reported by Yan et al. 14 , were not
observed. However it is important to note that
our experiments were carried out with low
loaded samples exposed to much lower NO
pressures.
Fig. 4 illustrates the infrared spectra for NO
adsorption on Pdrg-Al2O3 and Pd–Wrg-
Al2O3. On Pdrg-Al2O3 the sharp peak located
Ž .
a and
Fig. 3. FTIR Spectra of CO adsorbed on Pdrg-Al2O3
1
Ž .
b
at 1732 cmy is assigned to linear NO on Pd
Pd–Wrg-Al2O3

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A.M. Sica et al.rJournal of Molecular Catalysis A: Chemical 137 1999 287–295
293
w
x
15,16 . In addition there is a low intensity
chemisorption of NO relative to that observed
on Pdrg-Al2O3 and Wrg-Al2O3.
1
signal at 1650 cmy attributed to bridged NO.
In the low frequency region, only the band at
1232 cmy1, which was also observed on Wrg-
Al2O3, is revealed.
3.3. NO decomposition
Ž .
In order to determine if the NO Pd–W
When NO was added to Pd–Wrg-Al2O3 the
1
band at 1732 cmy was absent but more rele-
interaction mentioned above plays a beneficial
role in the NO decomposition reaction, the cat-
alytic activity and selectivity of all samples
have been examined at 473 K.
vant features, associated to nitrite and nitrate
species, appeared below 1600 cmy1. The 1232
1
cmy band, identical to that observed on Wrg-
Al2O3 and Pdrg-Al2O3, has been previously
Fig. 5 shows that Pdrg-Al2O3 and Wrg-
Al2O3 catalysts exhibit a comparable behaviour
as a function of time. The initial activity was
very high but it was followed by a deactivation
period of about 100 min. It can be seen that the
Wrg-Al2O3 sample is slightly more active. On
the other hand the behaviour of Pd–Wrg-Al2O3
was quite different; it showed 100% conversion
of NO for at least 80 min and then deactivates
like the other samples. This enhanced activity is
attributed to a close Pd–W interaction; it should
be observed that the behaviour of Pd–Wrg-
Al2O3 cannot be predicted from the combined
effect of Pdrg-Al2O3 and Wrg-Al2O3.
assigned to bridge nitrite species on g-Al2O3
w
x
17,18 . Additional bands can be seen at 1315
1
cmy1, 1470 cmy and 1560 cmy1. The one at
1470 cmy is tentatively assigned to a linearly
1
w
x
coordinated nitrite ion 18 . Those that are broad
1
and intense, at 1315 cmy and 1560 cmy1, have
w
x
been ascribed to nitrate species 17,18 . First it
is important to state that these bands cannot be
removed by evacuation or heating. Since they
were not observed when Pdrg-Al2O3 or Wrg-
Al2O3 were exposed to NO, the results indicate
a promotional effect due to both Pd and W. If
the Pd particles are covered to a certain extent
by W, the close proximity of both metals facili-
tates the partial reduction of W during the hy-
drogen treatment at 573 K. We suspect that
these Pd–W particles induce the dissociation of
NO. Probably some oxygen atoms lead to W
oxidation while others remain on the Pd surface
and react with NO to form NO2, which in turn
could be responsible for the strong nitrate bands
found on g-Al2O3.
It is generally accepted that the catalytic de-
composition of NO occurs on reduced metals
w
x
20 . The reaction mechanism involves the ad-
sorption of NO on the metal surface followed
by dissociation into nitrogen and oxygen, and
This interpretation is in accordance with re-
w
x
cent results. Hoost et al. 16 have shown that
adsorption of NO on oxidized Pdrg-Al2O3 en-
hanced the bands due to nitrite and nitrate
species, suggesting the possibility of NO oxida-
tion. Another relevant study is that of Centi et
w
x
al. 19 ; they found that NO adsorption on g-
Al2O3, at 3008C, revealed bands quite similar to
those shown in Fig. 4. Consequently the results
of the FTIR study also suggest that the prepara-
tion of Pd–Wrg-Al2O3, using a modified hex-
acarbonyl precursor, leads to a specific Pd–W
interaction and to sites that modify the
Fig. 5. Activity and selectivity for NO decomposition at 473 K:
` Pdrg-Al2O3; v Wrg-Al2O3; I Pd–Wrg-Al2O3. For clar-
ity the selectivity of Pdrg-Al2O3 has been omitted.

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)
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A.M. Sica et al.rJournal of Molecular Catalysis A: Chemical 137 1999 287–295
the recombination of adsorbed nitrogen atoms to
form N2. However the oxygen produced re-
mains strongly adsorbed, inhibiting further ad-
sorption of NO and consequently the catalysts
deactivate. Depending on the reaction condi-
tions the tightly bound oxygen may even lead to
oxidation of the metal. Another important reac-
tion pathway on metal surfaces is that of ad-
sorbed NO with adsorbed nitrogen atoms to
form N2O. However the essential role of the
catalyst surface is the dissociation of NO. Evi-
dence for molecular adsorption of NO on Pd has
These results confirm the conclusion of the
FTIR study in the sense that the preparation of
Pd–Wrg-Al2O3 by reaction of modified tung-
sten hexacarbonyl with Pdrg-Al2O3, leads to a
Pd–W interaction that stabilizes W in a partially
reduced state. Moreover the catalytic behaviour
of the Wrg-Al2O3 also indicates the existence
of partially reduced W species on the alumina
support. Consequently the preparation method
described above offers a convenient route to
obtain bimetallic catalysts with metal–metal in-
teractions. Although supported Pt–Mo and
Pt–W catalysts with a proven metal–metal in-
teraction have already been prepared using
w
x
been provided by FTIR studies 15,16 and con-
firmed here. On the other hand the dissociation
of chemisorbed NO to form N2 and N2O on Pd
has been demonstrated by TPD experiments.
Because oxygen desorption is not observed the
formation of subsurface oxygen and even disso-
lution of oxygen into the bulk was suggested
w
x
organometallic precursors 20,23 the present
study is the first to consider the reactivity of a
reduced metal surface with hexacarbonyl com-
pounds. The preparation method has been suc-
cessful to control the oxidation state of W but
the inconvenience of using PPh3 ligands is the
insertion of P. In addition the decrease in sur-
face acidity may have an adverse effect for NOx
reduction with hydrocarbons. A recent publica-
w
x
21,22 .
The method of preparation described above
leads to partially reduced W atoms that are
close to Pd8 sites and this interaction facilitates
the fixation of oxygen and becomes responsible
for the period of high activity observed on the
Pd–Wrg-Al2O3 catalysts. However it is impor-
tant to emphasize that W deposited on the Al2O3
surface exhibits a high initial activity by itself,
and this is a clear indication of the existence of
partially reduced species even in the absence of
Pd. Evolution of O2 was never detected which
suggests that this decomposition product is in
fact retained in the catalysts surface or leads to
metal oxidation.
At high conversion values the main nitrogen
containing product for all samples was N2; the
formation of N2O was observed to increase
along with catalyst deactivation. The N2 mass
balance based on the amounts of N2O and N2
formed, and unreacted NO, gave approximate
results, due to the low NO content in the feed
mixture and the inevitable chromatographic er-
rors. Consequently we cannot discard the forma-
tion of small amounts of NO2, from NO oxida-
tion, and then of nitrite or nitrate species on the
alumina surface.
w
x
tion 24 emphasizes the beneficial role of WO3
to increase the acidity and the catalytic activity
of g-Al2O3 for NOx reduction. We are planning
to address this subject in a future study.
4. Conclusions
It has been demonstrated that the photochem-
ical reaction of W hexacarbonyl with PPh3 in
the presence of Pdrg-Al2O3 allows the adsorp-
tion of mono and disubstituted carbonyl species
on the palladium–alumina surfaces. The Pd–
Wrg-Al2O3 catalyst obtained after thermal de-
composition and reduction stabilizes W in a
partially reduced state that, to a certain extent, is
in close association with the Pd atoms. FTIR
analysis of adsorbed NH3 demonstrated that the
method of preparation practically suppresses the
Lewis acidity of the alumina support. In addi-
tion FTIR studies have shown that the resulting
Pd–W interaction modifies the chemisorptive
properties for NO adsorption with respect to

(
)
A.M. Sica et al.rJournal of Molecular Catalysis A: Chemical 137 1999 287–295
295
w x
Ž .
A. Kazusaka, R.F. Howe, J. Catal. 63 1980 447.
I.M. Baibich, J.H.Z. Dos Santos, F.C. Stedile, V.E. San-
8
w x
9
those of the monometallic Pd and W catalysts.
Consequently the bimetallic catalyst exhibits an
enhanced activity for NO decomposition.
Ž
.
tarosa, I.J.R. Baumvol, Polyhedron 16 1997 1937.
w
w
x
Ž
.
10 A. Zechina, C.O. Arean, Catal. Rev. Sci. Eng. 35 1993 261.
x
11 A.A. Davydov, in: Infrared Spectroscopy of Adsorbed Species
on the Surface of Transition Metal Oxides, Chap. 1, Wiley,
New York, 1990, p. 29.
Acknowledgements
w
w
x
x
Ž
.
12 G.R. Cairns, R.J. Cross, D. Stirling, J. Catal. 166 1997 89.
13 R.I. Masel, E. Umbach, J.C. Fuggle, D. Menzel, Surf. Sci. 79
This work was supported in part by CON-
Ž
.
1979 26.
Ž
.
Ž
.
w
x
Ž
.
ICET Argentina , UNS Argentina , CNPq
14 Y. Yan, Q. Xin, S. Jiang, X. Guo, J. Catal. 131 1991 234.
w
x
Ž
.
.
15 C.M. Grill, R.D. Gonzalez, J. Am. Chem. Soc. 84 1980
Ž
.
Ž
.
Brazil and FAPERGS Brazil .
878.
w
x
Ž
16 T.E. Hoost, K. Otto, K.A. Laframboise, J. Catal. 155 1995
303.
References
w
w
x
Ž
.
17 F. Solymosi, J. Rasko, J. Catal. 62 1980 253.
x
18 A.A. Davydov, in: Infrared Spectroscopy of Adsorbed Species
on the Surface of Transition Metal Oxides, Chap. 1, Wiley,
New York, 1990, p. 54.
w x
1
H. Muraki, H. Shinjoh, Y. Fujitani, IEC. Prod. Res. Dev. 25
Ž
.
1996 419.
w x
2
w x
3
H. Muraki, SAE Paper 910842, 1991.
K.M. Adams, H.S. Ghandi, Ind. Eng. Chem. Prod. Res. Dev.
w
x
19 G. Centi, S. Perathoner, D. Biglino, E. Giamello, J. Catal.
Ž
.
151 1995 75.
Ž
.
22 1983 207.
w
x
20 S. Kawi, O. Alexeev, M. Shelef, B.C. Gates, J. Phys. Chem.
w x
4
I. Halasz, A. Brenner, M. Shelef, K.Y. Simon, Appl. Catal.
Ž
.
99 1995 6926.
Ž
.
A 82 1992 51.
w
w
x
x
Ž
.
21 X. Xu, D. Wayne Goodman, Catal. Lett. 24 1994 31.
22 H. Conrad, G. Ertl, J. Kuppers, E.E. Latta, Surf. Sci. 65
w x
5
Y.J. Mergler, A. van Aalst, J. van Delft, B.E. Nieuwenhuys,
J. Catal. 161 1996 310.
Ž
.
Ž
.
1977 245.
w x
Ž
.
6
w x
7
M. Sato, Surf. Sci. 95 1980 269.
H.K. Plummer, S. Shinozaki, K.M. Adams, H.S. Ghandi, J.
Mol. Catal. 20 1983 251.
w
w
x
x
Ž
.
23 O. Alexeev, M. Shelef, B.C. Gates, J. Catal. 164 1996 1.
24 R.J. Kudla, S. Subramanian, M.S. Chattha, T.E. Hoost, Ind.
Ž
.
Ž
.
Eng. Chem. Res. 35 1996 4394.