Magnetite pair-sites

The idea of competitive adsorption on pair-sites has also been used to describe the interaction of Ho and H2O with magnetite ( ). When isotherms were collected using M2/H2O mixtures following the same approach as discussed above, it was found that the data fit a model where H2 adsorbed dissociatively and H2O adsorbed associative-ly, with both species competing for pair-sites. These studies were conducted at water-gas shift reaction temperatures (e.g., 650 K) and as for the adsorption of CO and CO2, only a fraction of the magnetite surface was capable of adsorbing H2 and H2O. 

The above ideas that anion-cation pair sites are the surface sites for CO and CO2 adsorption on magnetite was verified directly by Udovic and Dumesic (43 ). These authors prepared films of magnetite on polycrystalline iron foils and varied the oxidation state of the surface by vacuum-annealing at different temperatures. In short, it was shown by Auger electron spectroscopy and X-ray photo-... 

A simple model for the Verwey transition has been proposed (Honig, Spalek Gopalan, 1990) octahedral sites in magnetite were represented by a site pair, with a ground energy state (an electron trapped), a first excited state (the electron resonating between the two components of the site pair) and a second excited state (two electrons in the site pair). An important characteristic of this model was that the Verwey transition was driven by the coulomb repulsive interaction between electrons in the site pair. 

However, deviations from this ideal ratio are often observed. The main reason is that magnetite may partly be oxidized by replacing Fe + by Fe " and introducing vacancies. Because the electron hopping occurs in pairs, this oxidation does not result in another intermediate Fe valence on the octahedral sites, but in a decrease of the Fe " component and the appearing of a B-site Fe " sextet for which the hyperfine parameters do not appreciably differ from those of the A-site sextet. Together with the introduced vacancies, a decrease of the Fe + B-site sextet area and an apparent increase of that of the A-site sextet are observed. Therefore, one cannot speak anymore about one A-site and one B-site sextet—a mistake that is... 

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