Cation site, regenerative mechanism

The regenerative mechanism fits well with the observed catalytic properties of Fe O. The octahedral iron cations, which undergo rapid electron hopp%g, would appear to be the natural cation sites for the reversible oxidation/reduction required by the mechanism, as will be discussed later. Indeed, assuming the sites to be exponentially distributed with respect to oxygen affinity, a rate expression can be derived which fits measured kinetic data well ( ) Additional support for this mechanism comes from the work of Boreskov et al.(9J who showed that the rate of WGS corresponds... 

It is possible, however, that available pair-sites may be blocked by the adsorption of CO2 or H2O. For example, CO2 may adsorb on these sites, forming bidentate carbonate species. Although such adsorbed species may suppress the regenerative mechanism over anion-cation pair-sites, these species may be important for adsorptive WGS mechanisms. The general conclusion, however, is that the adsorption isotherms for CO and COo and for H2 and H2O are useful for probing the pair-sites necessary Tor WGS. 

This review has focused on recent research directed toward characterization of the active sites for water-gas shift over magnetite-based catalysts. The reaction can be described by a regenerative mechanism wherein gas phase or weakly adsorbed CO reduces anion sites and steam oxidizes the resultant surface oxygen vacancies. Kinetic relaxation techniques indicate this to be a primary pathway. The sites which participate in this reaction comprise only about 10% of the BET monolayer, and these sites can be titrated using CO/CO2 adsorption at 663 K. In contrast, the total cation site density is effectively titrated with NO at 273 K. In fact, the ratio of the extent of CO/CO2 adsorption to the extent of NO adsorption provides a measure of the fraction of the magnetite surface which is active for water-gas shift. 

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