K. Okumura et al. / Surface Science 603 (2009) 2544–2550
2549
At the same time, the following reactions for the production of
N2 and N2O,
ciation of NO is favored. Thus, NO dissociation is considered to be
enhanced by a change of the electronic structure of noble metal. As
the temperature increases, NO dissociation seems to be initiated
leading to high NO conversion rate and the selectivity for N2 for-
mation regardless of metal oxide supports.
2Nads ! N2
ð7Þ
ð8Þ
Nads þ NOads ! N2O
are possible, as explained in temporal analysis of products (TAP)
experiment with Pt/Al2O3 [22] and Pt/SiO2 catalysts [23]. In addi-
tion to above reaction steps (7) and (8), NH3 can react with NO to
produce N2 and N2O according to the following reaction formula,
5. Conclusions
The catalytic properties of thin films composed of noble metals
and metal oxide supports were investigated with regard to NO
reduction by H2. The catalytic performances of Rh and Ir catalysts
were hardly affected by a choice of a metal oxide support. N2 was
the dominant products on those catalysts. In contrast to this, the
transient formation of NH3 on Pd catalysts and Pt catalysts was
higher than that on Ir catalysts and Rh catalysts. It was also noted
that CeO2 and ZrO2 supports were more effective than Al2O3 and
SiO2 supports for Pt and Pd on NO conversion rate and N2 forma-
tion selectivity at relatively low temperatures.
NH3 ! NHx;ads þ H3ꢁx;ads
NOads þ NHx;ads ! N2 þ HxO
NOads þ NHx;ads ! N2O þ Hx;ads
ð9Þ
ð10Þ
ð11Þ
as observed on Pt and Pt–Rh metals in TAP [24,25], Pt single crystal
under an ultra high vacuum (UHV) system with Q-MS [26] and Pt/
Al2O3 with our experimental apparatus [16].
This assortment is consistent with ability to dissociate NO
depending on metals without the effect of any support materials.
There the border line between dissociative adsorption of NO and
molecular adsorption passes between Rh and Pd on the periodic ta-
ble. Our experimental results discriminated that the catalytic prop-
erty of noble metals could be assorted into two groups, i.e. Rh and
Ir group whose own property would mainly dominate the catalytic
performance, and Pd and Pt group whose interaction with metal
oxides supports would clearly contribute to the reaction of NO
with H2. The transient analysis clearly indicated the transient for-
mation of NH3 as well as N2O in the middle of the NO reduction
process especially on Pd catalysts and Pt catalysts. In addition,
NO reduction activity of Pd and Pt was found to be promoted above
that of Rh and Ir, provided that Pd and Pt were supported by CeO2
and ZrO2.
The transient experiments under moderate pressures with NO/
H2 ratio that is close to 1 revealed that the NO dissociation step (2)
was the rate-determining step on Pt/Al2O3 [22]. The extensive
studies on the catalytic NO chemisorption and reactions on single
crystal metal surfaces [13] and NO decomposition on transition
metals on MgAl2O4 support [27] summarizes that Pt surface mainly
adsorbs NO as molecular chemisorption than dissociative chemi-
sorption. The behavior on the Pd surface is in the same manner
as that on the Pt surface. However, the metals such as Rh and Ir
tend to dissociate NO than Pd and Pt.
Fig. 4 indicates that Pt catalysts and Pd catalysts especially on
Al2O3 and SiO2 produced large amount of NH3. In the case of the
conventional technique under atmospheric pressure such as
fixed-bed flow reactors [1], it is usually difficult to observe such
by-products NH3 and N2O because of consecutive reactions relat-
ing those products. As shown in Figs. 4 and 6, the catalytic perfor-
mance of Rh and Ir were hardly affected by a choice of the metal
oxide support. Moreover, the selectivity for N2 formation of Rh cat-
alysts and Ir catalysts were higher than that of Pd catalysts and Pt
catalysts whose selectivity for NH3 formation is relatively high in
our experiments. Therefore, in the case of Rh and Ir catalysts, the
strong NO dissociation ability of those noble metals seems to
dominate NO reduction activity and selectivity without any contri-
bution from metal oxide supports. It had been found that NO
molecule chemisorbed on Rh horizontally leading to the NO disso-
ciation while on Pd vertically on the basis of quantum chemistry
simulation [28].
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