Active centers, bimetallic number

According to isotopic and infrared data [lA], the formation of hydrocarbons from CO and H2 on ruthenium occurs via the dissociative adsorption of CO. The dissociation seems to proceed on multiple active centers, the number of atoms (n) of which can be estimated on the basis of the assumption of homogeneous composition and equal surface and bulk compositions of bimetallic particles from the formula [22,23]... 

A.3. Possible reasons for nonadditive properties of (Ru+Pd)/Si09 catalysts. According to the ideas developed by Ponec and Sachtler [22,23] in the theory of bimetallic catalysts containing alloy species, the analysis of catalytic properties should take into account, first, the effect of dilution of surface metal atoms by those of less active metals, which leads eventually to a decrease in concentration of multiple active centers for the reaction. In a number of cases, it is also necessary to consider the ligand effect, that is, the presence of the second metal in the nearest environment of a given atom, which may affect its electronic, chemisorptive, and, consequently, catalytic properties. For bimetallic particles of more than 10 A in size, the surface composition can differ from that of the bulk. In view of these ideas, we now consider the properties of Ru-Pd catalysts. 

Of special theoretical interest in catalysis by metals is the question of the smallest number of atoms in the active center for a given reaction. For example, for ruthenium, the smallest n amber of atoms in the active center for CO dissociation is equal to 4. The proposed method for obtaining bimetallic catalysts opens up additional possibilities for experimental study of this problem by changing nuclearity of a cluster supported on a metal carrier. 

The dioxygen reduction site of the key respiratory enzyme, cytochrome c oxidase [E.C. 1.9.3.1], is a bimetallic catalytic center comprised of a heme iron adjacent to a Type 2 mononuclear copper center see Cytochrome Oxidase. The recent solution of the X-ray crystal structure of this enzyme revealed an entirely unanticipated covalent modification of the protein structure, a cross-link between a histidine and tyrosine side chain (23) within the active site (Figure 2). This extraordinary posttranslational modification has been confirmed by peptide mapping and mass spectrometry, and has been detected as a conserved element in cytochrome c oxidases isolated from organisms ranging from bacteria to cows. The role of the cross-linked structure in the function of cytochrome c oxidase is still controversial. A number of possibilities have been suggested, including... 

The catalyst complex of the TiCls/AlRs system essentially acts as a template for the successive orientation and isotactic placement of the incoming monomer units. Though a number of structures have been proposed for the active species, they fall into either of two general categories monometallic and bimetallic, depending on the number of metal centers. The two types can be illustrated by the structures (I) and (II) for the active species from titanium chloride (TiCU or TiCls) and alkylaluminum (AIR3 or AIR2CI). 

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