Applied Catalysis B: Environmental 96 (2010) 370–378
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Applied Catalysis B: Environmental
journal homepage: www.elsevier.com/locate/apcatb
On the importance of the catalyst redox properties in the N
2
O decomposition
over alumina and ceria supported Rh, Pd and Pt
S. Parres-Esclapez, M.J. Illán-Gómez, C. Salinas-Martínez de Lecea, A. Bueno-López
∗
Inorganic Chemistry Department, University of Alicante, Ap. 99, E-03080 Alicante, Spain
article info
Article history:
Received 21 December 2009
Received in revised form 21 February 2010
Accepted 24 February 2010
Available online 3 March 2010
Keywords:
N2O decomposition
Noble metal catalyst
Ceria support
Cerium–praseodymium
Cerium–lanthanum
abstract
Rh, Pd and Pt have been supported on -Al
2
O
3
, pure CeO
2
and La- or Pr-doped CeO
2
, and these catalysts
have been tested for N
2
O decomposition. The effect of CO and O
2
in the feed has been studied. The
characterisation techniques used were Raman spectroscopy, XRD, N
2
adsorption at -196
◦
C, H
2
-TPR and
TEM. The catalytic activity for N
2
O decomposition of the noble metals follows the trend Rh > Pd > Pt,
and the support affects significantly the activity. For CeO
2
-containing catalyst, a relationship between
N
2
O decomposition capacity and H
2
reduction of ceria has been found, the easier is the reduction the
higher is the catalytic activity. The rate-limiting step of the N
2
O decomposition mechanism over noble
metal/ceria catalysts seems to be the reduction of the catalytic active sites. For Rh catalysts, ceria supports
are involved actively in the decomposition of N
2
O, and all the ceria-based supports improve the catalytic
activity of Rh with regard to -Al
2
O
3
due to the redox properties of ceria. The Pd catalysts with pure and
doped ceria support showed similar activity, this being higher than that of Pd/-Al
2
O
3
. Pt/CeO
2
is the
most active catalyst among those of Pt, but ceria doping by La or Pr has a negative effect on the activity.
The most active catalyst among those prepared in this study is Rh/CeO
2
(Pr).
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
N
2
O is the major source of NO
x
in the stratosphere and con-
tributes to the greenhouse effect and global warming. In the last
decade, the relevance of N
2
O abatement has been reflected in the
literature [1–5]. Some anthropogenic sources of N
2
O are nitric acid
plants, fossil fuels and biomass combustion and land cultivation
[1]. In addition, certain catalytic converters introduced for gas pol-
lution control in vehicles also contribute to N
2
O emission. Three
Way Catalysts (TWC) used in gasoline-powered engines for CO,
hydrocarbons (HC) and NO
x
removal yield N
2
O as by-product, and
the undesired formation of N
2
O has been related with the TWC
aging. TWC combine noble metals, mainly Pt, Pd and/or Rh, with
CeO
2
-based materials [6,7] to promote the noble metal dispersion,
increase the thermal stability of the Al
2
O
3
support, promote the
water gas shift (WGS) and steam reforming reactions, favour the
catalytic activity at the interfacial metal-support sites, promote CO
removal and store and release oxygen under, respectively, lean and
rich conditions.
The main reactions occurring in a TWC are
CO + (1/2)O
2
→ CO
2
(1)
∗
Corresponding author. Tel.: +34 965 90 34 00x2226; fax: +34 965 90 34 54.
E-mail address: agus@ua.es (A. Bueno-López).
C
x
H
y
+ (x + y/4)O
2
→ xCO
2
+ (y/2)H
2
O (2)
NO + CO → CO
2
+ (1/2)N
2
(3)
(2x + y/2)NO + C
x
H
y
→ xCO
2
+ (y/2)H
2
O + (x + y/4)N
2
(4)
Reactions (1) and (2) are catalysed by Pt and Pd, whereas Rh
is mainly involved in NO
x
reductions reactions ((3) and (4)). The
reduction of NO with CO can yield N
2
O instead of N
2
:
2NO + CO → N
2
O + CO
2
(5)
Cant et al. [8] studied the formation of N
2
O in model Rh,
Pd and Pt catalysts under a simulated gasoline exhaust with
CO + NO + C
3
H
6
+C
3
H
8
+H
2
+O
2
. Rh yielded the highest amount
of N
2
O, with maximum production between 250 and 400
◦
C. Pt
yielded N
2
O in a narrow range of temperatures around 400
◦
C, and
Pd produced N
2
O in two temperature ranges, below 200
◦
C and
above 300
◦
C.
Other catalytic converters that yield N
2
O, which are under
development for Diesel vehicles [9], are those based on Pt catalysts
for the Selective Catalytic Reduction (SCR) of NO
x
with hydrocar-
bons. The pollutants emitted by diesel engines are soot, NO
x
, CO
and HC [10]. The concentrations of CO and HC in diesel exhausts
are significantly lower than those emitted by a gasoline engine,
NO
x
and soot being the main pollutants. Diesel engines operate
with O
2
excess, which results in high levels of O
2
in the exhaust
(10–20%). The high concentration of O
2
hinders NO
x
removal, since
NO
x
reduction cannot be performed with the reducing species
0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2010.02.034