Lattice oxygen reactivity effect

The studies performed over promoted manganese molybdate catalysts have shown significant changes in catalytic behavior due to presence of the promoter. The preliminary results suggest that the pronounced differences observed in selectivity and activity may be related to the effect of the promoter cations on the reactivity of the lattice oxygen and the availability of adsorbed oxygen. 

This paper is a summary of our current understanding of this system. In particular, we will be discussing the observations in terms of selectivity with respect to the availability of reactive lattice oxygen. The organization of the paper is as follows. First, the general features of the reaction scheme for alkane oxidation on vanadate catalysts will be presented. This is followed by a discussion of results on the effect of ease of removal of oxygen from the lattice on the selectivity, and then a discussion on the importance of the atomic arrangement of the active sites. 

Use of Halide Ions to Improve Selectivity. Earlier work has claimed that enhanced selectivities for alkene oxidation can be achieved by the inclusion of electronegative elements such as S, Se, or halogens. This has been reviewed elsewhere. " More recent work has demonstrated substantial improvements in selectivity for propene (25—70%) and isobutene (35—80%) oxidation when either chloride or bromide is present. Both elements are added to the catalyst in the form of trace levels of organo-halide in the process gas stream. The selectivity increase is the result of a decrease in the rate of complete oxidation rather than an increase in the partial oxidation rate. Since the reaction is first order in oxygen pressure and zero order with respect to alkene in the presence and absence of halide, the reaction mechanism is probably similar in both cases. In the light of Anshits recent work, the effect of the halide is presumably to reduce the relative number and/or reactivity of surface lattice oxygen species and thus reduce the amount of irreversibly adsorbed alkene. 

The remaining studies on oxidation of metals have been performed mainly with the aim of looking into the corrosion properties of materials. Thus, in 1971 it was found that the implantation of boron generated passivation in copper The oxidation rate of zircaloy4 in oxygenated water at 300°C was suppressed by ion bom-bardment Implantation of reactive ions such as O and chemically inactive ones such as Ar and Xe yielded the same retardation of the oxidation, suggesting that the lattice damage was the main reason for this effect. 

It is very interesting that Lennard-Jones 51) predicted in 1928 that lattice parameters near and perpendicular to the surface should be smaller as compared to those within the crystal lattice, but unfortunately this was not confirmed by experiment (52). Indeed truly clean surfaces are hard to obtain, as the highly reactive surface area tends to react with constituents of the atmosphere such as oxygen or water. Under normal conditions many surfaces are covered by oxides or hydroxides, by which the effect of bond contraction at and beneath the surface area is greatly reduced or cancelled. 

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