Catalysis Today 59 (2000) 347–363
Catalytic behavior of La–Sr–Ce–Fe–O mixed oxidic/perovskitic systems
for the NO+CO and NO+CH
4
+O
2
(lean-NO
x
) reactions
V.C. Belessi
a
, C.N. Costa
b
, T.V. Bakas
c
, T. Anastasiadou
b
,
P.J. Pomonis
a,∗
, A.M. Efstathiou
b,1
a
Department of Chemistry, University of Ioannina, Ioannina 45110, Greece
b
Department of Chemistry, University of Cyprus, PO Box 20537, CY 1678 Nicosia, Cyprus
c
Department of Physics, University of Ioannina, Ioannina 45110, Greece
Abstract
Mixed oxides of the general formula La
0.5
Sr
x
Ce
y
FeO
z
were prepared by using the nitrate method and characterized by
XRD and Mössbauer techniques. The crystal phases detected were perovskites LaFeO
3
and SrFeO
3−x
and oxides -Fe
2
O
3
and
CeO
2
depending on x and y values. The low surface area ceramic materials have been tested for the NO+CO and NO+CH
4
+O
2
(“lean-NO
x
”) reactions in the temperature range 250–550
◦
C. A noticeable enhancement in NO conversion was achieved by
the substitution of La
3+
cation at A-site with divalent Sr
+2
and tetravalent Ce
+4
cations. Comparison of the activity of the
present and other perovskite-type materials has pointed out that the ability of the La
0.5
Sr
x
Ce
y
FeO
z
materials to reduce NO by
CO or by CH
4
under “lean-NO
x
” conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover
frequencies (TOFs, s
−1
) at 300
◦
C (based on NO chemisorption) revealed values comparable to Rh/-Al
2
O
3
catalyst. This
is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was
found that there is a direct correlation between the percentage of crystal phases containing iron in La
0.5
Sr
x
Ce
y
FeO
z
solids
and their catalytic activity. O
2
TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic
activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic
defects). Additionally, a remarkable oscillatory behavior during O
2
TPD studies was observed for the La
0.5
Sr
0.2
Ce
0.3
FeO
z
and La
0.5
Sr
0.5
FeO
z
solids. ©2000 Elsevier Science B.V. All rights reserved.
Keywords: Perovskites; NO reduction; Lean-NO
x
;O
2
TPD; NO TPD; Oscillations
1. Introduction
The development of new catalysts for the selective
reduction of NO
x
emissions from both stationary and
mobile sources has gained an increasing interest in the
last few years. This is due to the need for replacing
∗
Corresponding author. Fax: +30651-44836.
E-mail addresses: ppomonis@cc.uoi.gr (P.J. Pomonis),
efstath@ucy.ac.cy (A.M. Efstathiou)
1
Co-corresponding author. Fax:+3572-339060.
the NH
3
-SCR process with possibly an HC-SCR pro-
cess (hydrocarbon-selective catalytic reduction) and
demands for improvement of fuel economy in inter-
nal combustion engines with low CO
2
emissions. The
latter has pushed the car industry towards “lean-burn”
engine technology and other strategies for automotive
“lean-NO
x
” emission control [1–5].
Noble metals (especially Pt) have proven to be
promising components of a “lean-NO
x
” catalyst with
particular applications in diesel internal combustion
engines (low-temperature NO
x
reduction in strongly
0920-5861/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved.
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