Multilayer Oxide

In part the parabolic law may also apply to multilayer oxide systems where the cation diffusion coefficient is much higher in the lower oxide tlran in the higher oxide, which, growing as a thin layer, undergoes plastic deformation at high temperatures, thus retaining the overall oxide layer as impervious to enuy of tire gas. 

James SD. 1969. Multilayer oxide films on anodized platinum. J Electrochem Soc 116 1681-1682. 

The results presented in this paper therefore show that V and Mo species supported on alumina can give rise to a catalyst which has a high selectivity for the oxidation of propane to propene and a reasonable selectivity to acrolein and that both species are essential to give the optimal behaviour. Contrary to our previous observations and what observed for bulk catalysts [5], the presence of Nb and W seem to have little effect, perhaps because the methods used here restrict the active phase to a monolayer whereas previously prepared materials may have contained multilayer oxidic species. 

Burke and coworkers [241] have studied the multilayer oxide films grown on silver in base during repetitive potential cycling. It was shown, on the basis of its reduction behavior, that the type of oxide obtained was dependent on the lower limit of the oxide growth cycles. Using limits of 1.03-2.60 V (SHE) the oxide film was assumed to be predominantly Ag20, while at limits 0.7-2.60 V, oxide deposit was assumed to be AgOH. Both types of silver oxides are assumed to be involved in premolecular oxidation and electrocatalysis at silver in base. 

Fig. 18. Multilayered oxides of different stoichiometry growing on a parent metal.

Fig. 19. One of the interior oxide layers labeled i in a sandwich array of multilayered oxides growing by cation interstitial (ci) diffusion illustrated with the relative magnitudes of the positively directed particle currents through layer i and the adjacent layers i — 1 and i + 1.

Thus far, our development has been for any inner layer i of the multilayer oxide. It is now required to examine separately the growth of the two layers Lx and LN which are in contact, respectively, with the parent metal and the gaseous oxygen. The growth of Lx proceeds in exactly the... 

As seen from these data, the loading of parent NaY zeolite with iron oxide via trinuclear Fe carbonyl as a precursor increases greatly the total rate of methanol conversion because of the appearance of oxidative active centers. In contrast, the use of carbonyl complexes of four and more nuclearity resulted in a dramatic drop of MeOH conversion even in comparison to the starting material. It should be noted that all Fe-containing samples have the same amount of iron oxide (about 1 wt %, metal basis), and such an effect can be explained only on assuming the formation of multilayer oxide deposits that cover the outer zeolite surface including its pore mouths. This assumption was verified by measuring the surface area for this set of samples that decreases from about 700 mVg for NaY down to a few tens m /g for three last samples in Table 5. 

Burke LD, ODwyer KJ (1992) Multilayer oxide growth on Pt under potential cycling conditions. Electrochim Acta 37 43-50... 

Mozota J, Conway BE (1983) Surface and bulk processes at oxidized iridium electrodes-1 monolayer stage and transition to reversible multilayer oxide film behavior. Electrochimica Acta 28 1-8... 

Singh Raman et have reeently reported the role of NC structmes in remarkably improving oxidation resistanee. In this eontext, it may be imperative to first have an overview of the role of grain size in development of the protective layer of oxidation resistant oxide (whieh is generally the irmer layer of the multilayered oxide scale). As suggested earlier, for a given combination of... 

---