Formation of brownmillerite type calcium ferrite (Ca 2 Fe 2 O 5 ) and catalytic properties in propylene combustion
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Catalysis Letters Vol. 110, Nos. 1–2, August 2006 ( 2006) DOI: 10.1007/s10562-006-0104-0
Formation of brownmillerite type calcium ferrite (Ca2Fe2O5) and catalytic properties in propylene combustion Daisuke Hirabayashi,a,* Takeshi Yoshikawa,a Kazuhiro Mochizuki,a Kenzi Suzuki,a and Yoichi Sakaib a EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan Department of Chemistry, Daido Institute of Technology, Nagoya 457-8530, Japan
b
Received 15 February 2006; accepted 1 June 2006
Several types of calcium ferrite base catalysts (Ca/Fe = 0.33–3) for propylene (C3H6) combustion were prepared. Calcium ferrite catalyst with brownmillerite crystal structure provided catalytic activity for C3H6 combustion in the temperature range of 250–450 C. The brownmillerite phase (Ca2Fe2O5) was responsible for the formation of oxygen adspecies (O2)) in the surface layer below 450 C. KEY WORDS: brownmillerite; calcium ferrites; superoxide.
1. Introduction In recent years, the treatment of volatile organic compounds (VOCs) has attracted scientists’ attention from various fields. In particular, catalytic combustion in the presence of oxidation catalysts has been shown to be a promising method for the destruction of the toxic chemicals [1,2]. Noble metal based catalysts, such as platinum and palladium have been regarded as a desirable, due to high activity for the destruction of polluting materials [3–9]. The several transition-metal based catalysts (e.g. CoO, CuO) shows relatively high activity, but a large consumption of these metals would be desirable environmentally due to the toxicity. Perovskite type oxides (figure 1a) are also candidates for VOCs combustion and are possible alternative oxidation catalysts and catalysts supports [10–13]. The structure permits accommodation of a wide variety of metal cations of different valences and has an unusual capacity to support a number of different types of defects. Depending on the composition, these structures provide important properties in the catalytic oxidation such as high electronic electron–hole and oxide-ion conductivity and the capacity of the conductive oxide ions. The oxide ions can be formed by the oxygenexchange reaction between oxygen and the oxygen vacancies in the crystal lattice: 1 O2 ðgÞ þ VO , Ox þ 2h ; 2
*To whom correspondence should be addressed. E-mail: [email protected]
ð1Þ
where VOÆÆ represents oxygen vacancy included in the structure, Ox oxide ion at normal lattice site, h electron hole. Thus, it was considered that the defects in the perovskite structures strongly affect the formation of the conductive oxide ions as well as their catalytic activities. Catalytic combustion of C3H6 and other VOCs over perovskite-type oxides has been studied for decades, however there has been little attention on brownmillerite-type calcium ferrite for the same purpose. The brownmillerite structure (Ca2Fe2O5, figure 1b) has an orthorhombic crystal, which is composed of a threedimensional framework of corner-sharing FeO6 octahedron and FeO4 tetrahedr
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