Catalyst from binary metal carbonyl

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

The reactions between the Group VI metal hexacarbonyls, representing relatively simple binary carbonyl complexes, and a variety of oxides is illustrative and also derives interest from the active alkene metathesis catalysts provided [20,21]. The initial interaction of Mo(C0)5 with silica has been monitored by infrared [22] and Raman [23] spectroscopies. The complex has been shown to be physisorbed in a similar environment to that adopted in a polar solvent and to have its symmetry lowered. At 44°C, in vacuo, this species is converted to a chemically bound carbonyl which itself is apparently decarbonylated. This is complete by 100 C, giving rise to an active propene metathesis catalyst [22]. Although no C-0 infrared absorptions are evident, only 3 CO groups per metal atom have been released [21], but the nature of the molybdentam in this yellow material is unclear. Some of the metal is EPR active, and this signal indicates a Mo site in an axially symmetric site, possibly square pyramidal [24]. The proportion of Mo increases to a maximum with an activation temperature of 200°C [25]. At that... 

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