Catalytically active species, identification

The identification of catalytically active species in FTS is of fundamental importance, as an improved understanding could enable the development of catalysts with increased activity and selectivity. In cobalt- and ruthenium-catalysed FTS, metallic cobalt and ruthenium function as active catalysts. However, in iron-catalysed FTS there are several distinct species generated during the reaction. Due to the lower, or similar, activation energy for iron carbide formation in comparison to carbon monoxide hydrogenation, iron-carbide formation is typically observed in FTS. The formation of several iron-carbide phases have been observed -Fe2C/8 -Fe2.2C (hexagonal... 

If the tetra- and tripodal Ti structures and the titanium oxo species derived from these structures in the presence of ROOH (R = H, alkyl) are involved as active sites and reaction intermediates, the next step beyond their identification is to seek correlations between the structure and concentrations of these titanium oxo species and catalytic activity and selectivity. Clerici and Ingallina (204) were the first to propose the Ti(02H) group as the active site of alkene epoxidation by... 

First steps to elucidate the reaction mechanism of PDC were achieved by the investigation of model reactions using ThDP or thiamine [36,37], Besides the identification of C2-ThDP as the catalytic center of the cofactor [36], the mechanism of the ThDP-catalyzed decarboxylation of a-keto acids as well as the formation of acyloins was explained by the formation of a common reaction intermediate, active acetaldehyde . This active species was first identified as HEThDP 7 (Scheme 3) [38,39]. Later studies revealed the a-carbanion/enamine 6 as the most likely candidate for the active acetaldehyde [40 47] (for a comprehensive review see [48]). The relevance of different functional groups in the ThDP-molecule for the enzymatic catalysis was elucidated by site-directed substitutions of the cofactor ThDP by chemical means (for a review see... 

Solution grafting has been the predominant approach for the immobilization of rare-earth metal precatalyst components [288]. The identification of the catalytically active surface species, commonly formed upon interaction with organoaluminum compounds, is difficult and assisted by molecular model complexes. Several types of support materials including magnesium chloride [289], silica [290], and organic (co-)polymers [291,292], were examined both in the gas-phase and the slurry polymerization of 1,3-dienes. 

The detection of enzymes by activity staining is important not only for the direct detection of separated enzymes, but also for the detection of noncatalytic proteins after staining with enzyme-labeled antibodies to the protein of interest. The ability to detect a particular catalytic activity greatly reduces background staining, and allows the unequivocal identification of a particular analyte species in very complex sample mixtures. 

The intermediacy of polynuclear cluster species in catalytic cycles has been unequivocally proved in few occasions only. These occasions are those that use catalyst precursors derived from the face-bridged compound [Ru3(, -H)(/i3-ampy)(CO)9] (38) (Table 1, entries 24, 25, 27, 29-31) and from the heteronuclear cluster [Ru6Pt3( 3-H)(/z-H)3(CO)2i] (58),cases in which the study of the catalytic activity has been accompanied by the identification of intermediate cluster... 

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