Reduction of saturated surface compounds

By the pretreatment with oxygen at high temperatures the surface compoimds contain the metal in a high oxidation state and are coordinatively saturated. By reduction the situation is changed. Oxidation and coordination numbers drop drastically, the metal forms a c.u.s. For instance the surface chromate(VI) anchored via two oxygens to the support and saturated by two other oxygens is reduced to a surface chromium(II) with only two links to the support and four vacant coordination sites  

How about the other metals In the case of molybdenum a simultaneous irradiation with UV light is necessary to get the Mo(VI) attacked by CO [26, 27]. Even then, the average oxidation number reached was not lower than 2.7 (oxygen titration [27]). Spectroscopic evidence shows that the product contains Mo(IV) and Mo(II), formed by the reduction of octahedral and tetrahedral Mo(Vl) [28]. Vanadium(V) requires a very long time and higher temperatures (600 °C) for the reduction by CO [19, 40] (slow de-aggregation ). For iron and copper see Section 18.4.2. 

To prepare the catalytic centres for a later reaction of olefins with the c.u.s., hydrocarbons (alkanes, alkenes) may be used as reducing agents. Here the extent of the reduction is usually lower. Cr reaches oxidation numbers near three [14]. The great variety of organic re- 

---