Iron ternary oxides

The Fe-Ca-0 system is of obvious interest to the steel industry. Reduction of the three calcium iron(III) oxides 2 CaO Fe203, CaO Fe203 and CaO 2 Fe203 gives rise to several mixed-valence ternary compounds belonging to the CaFe204-Fei x0 tie... 

As this review is intended to illustrate, the interplay between metal and oxygen leads to a richness of reactivity that is reflected in the surface structure of oxides. Much of this richness can be rationalised as varying proportions of ionic and covalent character in the metal-oxygen bonding, and is manifest in a variety of non-stoichiometric surfeces. We therefore focus on the prototypical transition metal oxide smface rutile Ti(>2 (1 1 0). This is contrasted with computational results for one of the most widely-studied p-block oxide surfaces - corundum Al2O3-(0 0 0 1) - and we refer also to computational surface studies on oxides of ruthenium, iron, vanadium, tin and silver, as well as ternary oxides. 

As a rule, present-day catalysts are based on iron oxide Fe203, although, at the outset, the pronounced tendency of this compound to lead to elemental iron, which favors dealkylation and coke formation reactions, caused it to be discarded in favor of other systems such as ternary oxide mixes, such as ZnO, A1203 and CuO (IG Farbenl, or more recently the attempt to employ a combination of V2Os and A1,03, first alone, and then in the presence of alkali metals (Dow, CC1 Catalysts and Chemicals Inc a United Catalysts subsidiary). Gradually, the good activity of iron oxide was exploited by... 

The economic significance of iron has resulted in great interest over the years in the products of its oxidation, and more recently in the vast ranges of ternary oxides such as spinel ferrites which have been commercially exploited in their own right. 

The following subdivisions will be considered (i) binary oxides (ii) spinel oxides (iii) other ternary oxides (iv) iron(IV) oxides (v) chalcogenides (vi) silicate minerals and (vii) lunar samples. 

In this section the results on high-spin iron(II) systems are presented before those on iron(III). The latter dominate, and are ordered approximately as follows the major structural classes of Fe203/M203 solid solutions, MFe03 perovskites, MFe03 orthoferrites, M3FesOi2 garnets and other iron(III) oxides approximately in the periodic table classification of the second metal. Any quaternary oxides are included with the most appropriate ternary system. 

The quadrupole splitting of the Fe + ion in the ternary oxide FeSb204 decreases from 2-88 mm s at 50 K to 1-02 mm s at 817 K as a result of thermal population of the tzg levels [97]. The iron is in a site of 6-coordination with three mutually perpendicular twofold axes, and it is possible to describe the three lowest states as yz)>, xzy, and primarily although the... 

Kusama H, Wang N, Miseki Y, Sayama K (2010) Combinatorial search for iron/titanium-based ternary oxides with a visible-light response. J Comb Chem 12 356-362... 

The other ternary oxides also occur as spinel superstructures such as CaFe204 (ferrite) or potassium iron oxides which contain the spinel structure as a building block. These complex structures of the beta alumina type consist of the formal composition (K2O x 11 Fc203) and can be described as a layer structure with unit cells of spinel linked by layers of potassium ions octahedrally coordinated to the (4 -h 4) oxygen ions of two adjacent spinel units. 

In a subsequent study, Ertl et al reported on the overall surface composition of a reduced catalyst which had a bulk composition similar to catalyst 1 in the present study. The surface composition of the two samples is, however, vastly different. In particular, the iron content is lower than the iron content of the present sample, in the unreduced state. The levels of potassium and aluminum are, on the other hand, much larger. It was pointed out that the surface composition as determined was consistent with earlier chemisorption experiments, and led to the conclusion that ca 40% of the accessible surface should be covered with potassium oxide. If the potassium and aluminum oxides were indeed present, as suggested, in the form of surface films, then the bulk of the catalyst would consist predominantly of metallic iron. If a sampling depth of 2-4 nm for XPS is assumed, then the analysis presented in Ref. 19 would represent a catalyst which is largely covered by a multilayer of promoter oxides. Consequently, a very much smaller fraction than 60% of the surface would consist of free iron. In accordance with the SEM and Auger map images the activation procedure of Ertl et led to a surface of elemental iron, which is covered almost entirely by a layer of promoter oxides that contain occasional crystals of embedded ternary oxides. 

The oxide-layer network is present essentially in the bulk of the catalyst. The significant amount of surface iron oxides found in some samples of the present study can be attributed in part to a thick layer (=incomplete reduction) of network oxides and must be ascribed in part to individual segregated oxide crystals. These segregates, which have been identified by electron microscopy as well as by differential charging in the XPS (see, e.g.. Fig. 2.42), may be binary or ternary oxides. Whatever their structure, they play the role of spectator species, which have been formed as an undesirable consequence of an inappropriate choice of reduction conditions. 

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