Synthesis and characterisation of La
1Àx
Ca
x
FeO
3
perovskite-type oxide
catalysts for total oxidation of volatile organic compounds
Bibiana P. Barbero
a,1
, Julio Andrade Gamboa
b
, Luis E. Cadu
´
s
a,
*
a
Instituto de Investigaciones en Tecnologı
´
a Quı
´
mica (INTEQUI), UNSL, CONICET, Casilla de Correo 290, 5700 San Luis, Argentina
b
Comisio
´
n de Energı
´
a Ato
´
mica (CNEA), 8400 San Carlos de Bariloche, Rı
´
o Negro, Argentina
Received 21 May 2005; received in revised form 28 September 2005; accepted 21 November 2005
Available online 2 February 2006
Abstract
La
1Àx
Ca
x
FeO
3
perovskite-type oxides with x = 0, 0.2 and 0.4 were prepared by the citrate method and characterised by means of X-ray
diffraction (XRD), X-ray fluorescence (XRF), surface area measurement BET, X-ray photoelectron spectroscopy (XPS), Fourier transformed
infrared spectroscopy (FT-IR), laser Raman spectroscopy (LRS), oxygen temperature-programmed desorption (O
2
-TPD) and temperature-
programmed reduction (TPR). The citrate method shows to be simple and appropriate to obtain single phases avoiding segregation and/or
contamination. Moreover, controlling the calcination temperature, specific surface areas adequate for catalysts to be used in oxidation reactions are
achieved. The structure refinement by using the Rietveld method indicates that the partial calcium substitution modifies the orthorhombic structure
of the LaFeO
3
perovskite towards a less distorted one. From XRF and XPS, a slight surface enrichment in lanthanum and calcium was detected.
XRD, FT-IR and TPR results indicated that the electronic debalance caused by the partial substitution for La
3+
by Ca
2+
is compensated by an
oxidation state increase of a part of Fe
3+
to Fe
4+
.O
2
-TPD results revealed that at a substitution level higher than x = 0.2, oxygen vacancies are also
formed to preserve the electroneutrality. Finally, an improvement of the catalytic activity in propane and ethanol combustion was observed on the
substituted perovskites. Correlating this with the characterisation results, the active sites would be associated to the Fe
4+
ions.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Lanthanum iron perovskite; Calcium; XRD; XRF; XPS; FT-IR; Raman spectroscopy; O
2
-TPD; TPR; Rietveld refinement; Propane combustion;
Ethanol combustion
1. Introduction
An increasing interest has been shown in catalytic
combustion processes during the last decades since they are
a convenient way for emission prevention (the control of
nitrogen oxides NO
x
and unburned hydrocarbons in heat and
power generation plants) as well as clean-up (volatile organic
compounds removal, automobile exhaust converters) [1].
Perovskite-type oxides display prominent catalytic activities
in many fields such as the total oxidation of methane and of
volatile organic compounds [2]. This activity, coupled with a
high thermal stability, postulates to pervoskite-type oxides as
potential catalysts in substitution of very active noble metals such
as Pt and Pd, which are more expensive and do not resist
operating at high temperatures [1–3]. Perovskite oxides have
general formula ABO
3
, where the 12-coordinated A sites may be
occupied by rare-earth, alkaline-earth, alkali or other large ions
and the 6-coordinated B sites are usually filled with transition
metal cations. A large number of metallic cations can occupy the
A and the B sites. Furthermore, the great stability of the
perovskite framework allows partial substitution at the A sites
and/or the B sites modifying the catalytic, redox and structural
properties. The substitution at A site with ions having lower
valence can allow the formation of structural defects such as
anionic or cationic vacancies and/or a change in the oxidation
state of the transition metal cation to maintain the electro-
neutrality of the compound. When the oxidation state of B cation
increases, the relative ease of the redox process generates larger
quantities of available oxygen at low temperature and the overall
oxidation activity enhances. Moreover, the oxygen vacancies
favour the catalytic activity in oxidation reaction because they
increase the lattice oxygen mobility.
www.elsevier.com/locate/apcatb
Applied Catalysis B: Environmental 65 (2006) 21–30
* Corresponding author. Fax: +54 2652 426711.
E-mail addresses: bbarbero@unsl.edu.ar (B.P. Barbero),
lcadus@unsl.edu.ar (L.E. Cadu
´
s).
1
Fax: +54 2652 426711.
0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2005.11.018