Formation of active sites for the NO + CO reaction over palladium catalysts supported on mesoporous silica

Article abstract of DOI:10.1246/bcsj.78.192

The catalytic activities and structural changes of Pd containing mesoporous silica (Pd/MCM-41) catalysts for the NO + CO reaction were investigated. Pd/MCM-41 after calcination in air showed a very high catalytic activity for the NO + CO reaction because Pd metal particles with an appropriate size for the reaction were formed by the reaction gases.

Full text of DOI:10.1246/bcsj.78.192

192 Short Articles
Bull. Chem. Soc. Jpn., 78, 192–194 (2005)
(200) 1.81 nm]. A broad peak at angle 2ꢀ of 33.9ꢁ, correspond-
ing to palladium oxide (tetragonal PdO) particle, was observed
in the XRD pattern of Pd/MCM-41 after calcination at 673 K.
Broad peaks at angles 2ꢀ of 40.1ꢁ and 46.7ꢁ, corresponding to
palladium metal particles, were vanishingly observed. This re-
sult suggested that Pd/MCM-41 formed predominantly small
PdO particles before the reaction. Fourier-transformed k3-
weighted EXAFS function for the Pd/MCM-41 catalyst before
the NO þ CO reaction is shown in Fig. 1 by the dotted line.
Three peaks appeared at 0.17, 0.25, and 0.31 nm (phase shift
uncorrected). The peak at 0.17 nm was assigned to Pd–O con-
tributions, and the other two peaks at 0.25 and 0.31 nm were
assigned to two kinds of Pd–Pd contributions. These three
peaks were derived from PdO particles.14 This result indicated
that the PdO particles existed in Pd/MCM-41 before the NO þ
CO reaction, corresponding to the XRD pattern.
Formation of Active Sites for the
NO þ CO Reaction over Palladium
Catalysts Supported on Mesoporous
Silica
Aritomo Yamaguchi,1 Takamitsu Hayashi,1
ꢀ;1;2
Kenichi Oyaizu,2 and Makoto Yuasa
1Department of Pure and Applied Chemistry,
Faculty of Science and Technology,
Tokyo University of Science, Noda 278-8510
2Institute of Colloid and Interface Science,
Tokyo University of Science, Tokyo 162-8601
We investigated the catalytic activity for the NO þ CO re-
action with the catalyst Pd/MCM-41 after calcination at 673
K (open-circle plots in Fig. 2). Although NO was reduced to
N2 and N2O, in the temperature range of this study the conver-
sion to N2O was less than 20%. The conversion in the figure
represents the percentage of the decrement of NO. NO conver-
Received August 2, 2004; E-mail: yuasa@rs.noda.tus.ac.jp
The catalytic activities and structural changes of Pd
containing mesoporous silica (Pd/MCM-41) catalysts for
the NO þ CO reaction were investigated. Pd/MCM-41 after
calcination in air showed a very high catalytic activity for the
NO þ CO reaction because Pd metal particles with an appro-
priate size for the reaction were formed by the reaction gases.
12
Pd-Pd
10
8
Pd-O
Pd-Pd
6
4
2
0
Periodic mesoporous silicas have been of great interest since
the discovery of mesoporous materials,1–3 and they are prom-
ising materials for catalyst support because of their high sur-
face area and uniform pore dimensions.4–8 The removal of
three toxic compounds, such as NO, CO, and hydrocarbons,
in the exhaust gases from internal combustion engines is an
important subject in order to reduce air pollution. Three-way
catalysts (TWCs) have been developed to eliminate the three
toxic compounds, and TWCs are typically based on Pt þ Rh
or Pd þ Rh. Pd particles attract much attention of many re-
searchers due to the high catalytic activity for the NO þ CO
reaction (2NO þ 2CO ! N2 þ 2CO2)9,10 to simultaneously
eliminate two compounds, NO and CO. However, Pd support-
ed on mesoporous silicas have not been applied to NO reduc-
tion. The NO þ CO reaction was found to be sensitive to Pd
surface structures.11,12 The Pd(111) surface showed higher ac-
tivity of the reaction than the Pd(110) and Pd(100) surfaces. In
addition, the reaction was supposed to be sensitive to the Pd
particle size, although the effective particle size for the active
site of the NO þ CO reaction has been controversial.11,12 Re-
cently, Nishihata et al. reported that Pd species in Pd-perov-
skite were self-regenerated for automotive emissions.13 The
self-regeneration of the Pd species for the reaction becomes
an interesting topic in the field of catalysis. Here, we report
on the formation of Pd particles with a suitable size for the
NO þ CO reaction from Pd species supported on mesoporous
silica.
0
1
2
3
4
5
6
R / 10-1 nm
Fig. 1. Fourier-transformed k3-weighted EXAFS functions
for Pd/MCM-41 before (dotted line) and after (solid line)
NO þ CO reaction at 773 K.
100
80
60
40
20
0
400
500
600
700
800
Reaction Temperature / K
Fig. 2. NO conversion for the NO þ CO reaction against
reaction temperature. : Pd/MCM-41 catalyst after calci-
nation at 673 K; : Pd/MCM-41 catalyst after calcination
at 673 K followed by reduction with H2; : Pd/SiO2;
Pd/Al2O3.
The XRD pattern in the 2ꢀ range from 2ꢁ to 7ꢁ has revealed
that the prepared catalyst, Pd/MCM-41, retained its hexagonal
structure of mesopore [XRD (100) 3.57 nm, (110) 2.09 nm,
:
Published on the web January 13, 2005; DOI 10.1246/bcsj.78.192

Bull. Chem. Soc. Jpn., 78, No. 1 (2005)
Ó 2005 The Chemical Society of Japan
193
(c)
(b)
(a)
50 nm
20 nm
100 nm
Fig. 3. TEM images of Pd/MCM-41 (a) before and (b) after the NO þ CO reaction, and (c) Pd/Al2O3 after the reaction. Arrows
indicate Pd metal particles.
sion in the effluent gas through the Pd/MCM-41 catalyst in-
creased at ca. 473 K. On the other hand, those of reference
|FT|
samples, such as Pd/SiO2 and Pd/Al2O3, increased at higher
temperature, as plotted in Fig. 2 by the closed circle and
square, respectively. This result indicated that this Pd/MCM-
10
8
6
4
2
0
700
650
600
550
500
450
41 catalyst had higher catalytic activity for NO reduction by
CO than the reference samples. Two sharp peaks at angles
2ꢀ of 40.1ꢁ and 46.7ꢁ, corresponding to Pd metal particles,
were observed in the XRD pattern of Pd/MCM-41 after the
NO þ CO reaction at 773 K. The averaged size of the Pd metal
particles was 17.1 nm, derived from Scherrer’s formula.15 The
averaged size of the Pd metal particles for Pd/SiO2 and Pd/
Al2O3 after the reaction was more than 40 nm, calculated from
the results of XRD measurements. Fourier-transformed k3-
weighted EXAFS function for Pd/MCM-41 catalyst after the
NO þ CO reaction is shown in Fig. 1 by the solid line. The
large peak at 0.25 nm was assigned to the Pd–Pd contributions
of the Pd metal particles. The coordination number (CN) and
interatomic distance of the Pd–Pd bond were 10:7 ꢂ 0:7 and
0:273 ꢂ 0:001 nm, respectively. The CN of Pd metal particles
having a diameter of 17.1 nm should be larger than 10.7, which
suggested that a small part of the Pd atoms may not form Pd
metal particles. The XRD and EXAFS results demonstrated
that small PdO particles which existed before the NO þ CO re-
action were reduced to make larger Pd metal particles during
the reaction. It seems reasonable to suppose that the PdO spe-
cies before the reaction locates in the mesopore, but the Pd
species go to the outer surfaces during the reaction to form
large metal particles. The Pd metal particles had high catalytic
activity for the NO þ CO reaction, as indicated by the fact that
NO reduction occurred at 473 K. The formation of Pd metal
particles with a suitable size for the reaction from PdO species
occurred during the reaction. The Pd/MCM-41 catalyst after
the NO þ CO reaction had the same temperature profile of cat-
alytic activity for the reaction once again. This result also in-
dicated that the size of the Pd metal particles formed after the
reaction was suitable for the reaction. We measured TEM im-
ages of Pd/MCM-41 before and after the NO þ CO reaction,
and Pd/Al2O3 after the reaction, as shown in Fig. 3. The Pd
species were not clearly observed in the TEM photograph of
Fig. 3(a) for Pd/MCM-41 before the reaction, because the
PdO species in the sample were suggested to be dispersed in
the mesopore of MCM-41. After the reaction, large Pd metal
0
1
Temperature / K
2
400
350
300
250
3
4
R / 10-1 nm
5
Fig. 4. Fourier transforms calculated from EXAFS spectra
of the Pd K-edge during the temperature-programmed re-
duction of Pd/MCM-41 in H2 from 293 K to 673 K.
particles (indicated by arrows) were observed in Pd/MCM-
41 and Pd/Al2O3, as shown in Figs. 3(b) and (c), respectively,
of which the size agreed with the size calculated from the XRD
results. TEM images of Figs. 3(a) and (b) support that the PdO
species before the reaction locates in the mesopore, and the Pd
species go to the outer surfaces during the reaction to form the
large metal particles.
Pd/MCM-41 calcined at 673 K in air was reduced in a flow
of H2 (3%)/He (97%) (50 cm3 minꢃ1) at 673 K for 2 h in order
to prepare the pre-reduced Pd/MCM-41 catalyst. The reduc-
tion process of Pd/MCM-41 during temperature-programmed
reduction (293–673 K) was studied by EXAFS spectra. The
acquisition time of each spectrum was 4 min. After back-
ground subtraction, k3-weighted EXAFS functions were Four-
ier transformed into R-space, as shown in Fig. 4. Phase-shift
correction was not carried out in these spectra. A Pd–O peak
was observed at 0.17 nm for the calcined Pd/MCM-41. During
the reduction in H2, the intensity of the peak decreased at 373
K, and a Pd–Pd peak at 0.25 nm increased. The CN and inter-
atomic distance of the Pd–Pd bond for the pre-reduced catalyst
were 10:6 ꢂ 0:7 and 0:273 ꢂ 0:001 nm, respectively. This
result revealed that PdO particles in MCM-41 were reduced
to Pd metal particles in a flow of H2, and that the particle size
was same as that of the Pd/MCM-41 catalyst after the NO þ
CO reaction. We also measured NO conversion of the NO þ
CO reaction with this pre-reduced Pd/MCM-41 catalyst (open
square plots in Fig. 2). The pre-reduced Pd/MCM-41 catalyst
had very high catalytic activity for the NO þ CO reaction, and

194 A. Yamaguchi et al.
Bull. Chem. Soc. Jpn., 78, No. 1 (2005)
amplitude reduction factor was 0.88 for Pd–Pd16 and the Debye–
Waller factor used in the fitting was calculated from EXAFS spec-
tra at Pd K-edge of Pd foil. TEM images were measured using a
Hitachi H-7650 electron microscope with a cold field-emission
gun operated at 120 kV.
had almost the same catalytic activity as the Pd/MCM-41 cat-
alyst without the H2 treatment. This result indicated that the
active sites were Pd metal particles, and that PdO particles be-
fore the reaction without the H2 treatment were reduced during
the reaction by CO at the reaction temperature. Pd metal par-
ticles of the appropriate size for the NO þ CO reaction were
generated by the reduction of Pd/MCM-41 by H2 before the
reaction. In the case of Pd/MCM-41 without the H2 treatment,
Pd metal particles of approximately the same size were formed
during the NO þ CO reaction.
We thank Mr. Taku Ogura, Prof. Hideki Sakai, and Prof.
Masahiko Abe of Tokyo University of Science, for measuring
the TEM.
In conclusion, we reveal that Pd/MCM-41 catalyst has the
high catalytic activity of NO reduction from low temperature
because Pd metal particles with an appropriate size for the
NO þ CO reaction were formed by the reaction gases.
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