Titania-based alloys

The second major difference between ceria- and titania-supported systems is related to the re-oxidation conditions allowing to recover them from decorated or alloyed states. By analogy with the well established conditions for regenerating M/TiOj catalysts from the SMSl state, re-oxidadon treatments at 773 K, and even lower temperatures, have often been applied to ceria-based systems. However, as shown by the HREM studies reported in section 4.3.3.2, this reference temperature does not allow the reversion of the above effects. Consequently, if the catalyst is recovered from a deactivated state, it should be interpreted as a proof of the absence of significant decoration or alloying in the catalyst. 

In the traditional enamelling methods, the adhesion is aided by the ground coat enamel and by adhesive oxides which arc responsible for galvanic corrosion of the metal and thus for coarsening the surface. More recent advances in technology allow direct application of the cover coat enamel onto the metal surface. This method is possible in the case of titania enamels and requires steel of special composition, either alloyed with titanium or carbon-free. Another possibility is based on the deposition of Ni on the metal surface. It is also necessary to adjust the slip composition and the coating thickness suitably the fired enamel thickness is only 0.1 —0.2 mm, compared with the usual 0.4 mm. 

The modification of palladium-based catalysts by addition of various promoters and additives, usually metals or metal oxides, has been investigated. Studies have shown improved catalytic performance for the total oxidation of light alkanes, usually leading to higher conversions and lower deactivation. The reason for this promotion is still under discussion, since the metal oxide additives alone usually show relatively low activity for alkane oxidation over the range of reaction temperatures. Alloying phenomena, modification of the properties of the support, modification of the PdO particle size, variations in the Pd oxidation states or an enhanced reduction-reoxidation cycle are considered as the most likely factors for the enhancement of activity. For example, if Pd/Al203 catalysts are modified with titania, a... 

Figure 8.3 shows transmission electron micrographs of selected regions of the anodic film located near the surface (Fig. 8.3a), within the film (Fig. 8.3b), and adjacent to the alloy/film interface (Fig. 8.3c). The anodic film reveals a porous morphology, with pores confined within alumina cells the barrier layer is evident beneath the pore base together with the scalloped alloy/film interface. The cell and pore diameters are about 30 and 10 nm, respectively, with a barrier layer thickness of 11 nm. Titania nanoparticles are distributed in a thin, outer layer of several tens of nanometres thickness the particles have diameters up to 10 nm. No particles are evident in the porous skeleton of the anodic film.

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