Spillover titania reduction

Spillover has also been shown to induce the reduction of sulfides, specifically silver sulfide (131,132). In the studies by Fleisch and Abermann (132), electron microscopy was used to follow the decrease of the area of a silver sulfide island as a function of time (see Fig. 10). Baker investigated these phenomena in relation to the spillover-induced reduction of titania (giving rise to SMSI) (95). The phenomena surrounding spillover are not distinct but reflect diverse extents of interaction. 

When the support contains cations that are not easily reducible (i.e. those of Al, Si, Mg, Zr), hydrogen spillover occurs above 573 K without observable chemical reaction (Class A). However if it contains ferric ions as impurities, as is often the case with alumina, reduction to ferrous ion is detectable by EPR and if it contains sulfate ion, as may be the case with titania, reduction of the precursor with hydrogen automatically generates hydrogen sulfide which poisons the metal (Class B). If deuterium is used in place of hydrogen, support hydroxyls... 

Therefore, at least on titania, transition metals promote the spillover of hydrogen to the support this is a necessary step in the reduction of the support (and hence modification of the global solid s catalytic properties). In other words, hydrogen spillover is a prerequisite in each of these recently recognized metal-support interactions (SMSI and IFMSI). Evidently these very specific metal-support interactions are, from the point of view of the spillover phenomena, merely the reduction of more or less easily reducible metal oxides, as mentioned in the preceding subsection. 

The CO-H2 synthesis properties of metal/titania catalysts have been found in several studies to be essentially unaffected by the temperature of reduction, in contrast to the strong effect this factor has on chemisorption properties. This problem has focused attention on the special nature of the metal-titania contact perimeter. Reduction of titania, undoubtedly through hydrogen spillover, begins there. It is important to note that Ti J+ cations are produced by reduction temperatures as low as 473K, as shown by 02 adsorption/H20 decomposition measurements (36) or by temperature programmed reduction. In the latter study, the amount of-TiJ+ produced at temperatures below 503K was equivalent to a TiJ+/Pt atom ratio of 0.6 (37). 

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