Active site species

The polymerization activity of a titanium catalyst increases with an increasing molar ratio of MAO to Ti. The amount of cationic Ti(III) species measured by ESR also increases with increasing ratio of MAO to Ti. This suggests that MAO acts as a reducing agent for Ti(IV) to Ti(III). Cationic Ti(III) might be an active species in the synthesis of SPS. Newman and Malanga [38] synthesized Cp Ti(OMe)2 via reduction of Cp Ti(OMe)3 and found that the catalytic activity of this complex with smaller amounts of MAO is almost the same as that of Cp Ti(OMe)3. 

The polydispersity (Mw/Mn) produced by the titanocene catalyst with MAO is always found to be about 2.0. These results show that there is only one active site and that chain transfer occurs during the polymerization. The numbers and the kind of active sites were clarified by the stopped flow method. Without a chain transfer reaction, the polymerization proceeds like a living polymerization and is an easy to analyze reaction. / -Methylstyrene (PMS) was used in this experiment to avoid plugging during polymerization due to crystallization. The 

By the addition of hydrogen to PMS, the number of chains increased without any broadening of the molecular weight distribution. Hydrogen did not act as a chain transfer reagent but the number of polymer chains showed that almost all Ti compounds turned to active sites. After the polymerization with hydrogen, ethylmethylbenzene was observed as a by-product. The amount of ethylmethyl-benzene is almost equimolar to the hydrogen added. These results showed that almost all of the titanium compounds turned to active sites, but about half of the active sites become dormant by some type of coordination with 

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There are two distinctly different mechanisms for a surface reaction between two species [8], for example toluene (T) and an active surface species ( ). In the Langmuir-Hinshelwood (LH) mechanism, reaction occurs between toluene emd the active surface species when both are adsorbed on the catalyst surface. If this step is the slow initiation step, the rate is proportional to the product of the coverages of toluene and the active site species ... 

The main advantage of the reaction is that alkylation is possible at sufficiently low temperature, where methanol shows mild activity. Nonetheless, the alkylation activity with MeOH and DMC followed a similar trend with respect to the catalyst composition indicating the participation of same active sites/species of catalysts irrespective of methylating agent. 

Despite the fact that carbonic anhydrase was the first zinc metalloenzyme identified1233 and a good deal is known of its structure, there is still controversy about the nature of the various active-site species and the detailed mechanisms of their action. In particular, the identity of the group with a pXa of 7 that is involved in the mechanism, and the stereochemistry around the zinc ion during catalysis, are still in dispute. The various mechanisms proposed assume either ionization of a histidine imidazole group (bound or not to the zinc) and nucleophilic attack on C02 by the coordinated imidazolate anion,1273,1274 or ionization of the Znn-coordinated water and nucleophilic attack on C02 by OH. 1271 Many papers on this problem have appeared recently and the extensive literature is the subject of the several review articles referred to above. 

These metallocene/aluminoxane catalysts are considered "single site" catalysts and therefore produce PE with veiy narrow polydispersily index (close to Stockmayer s distribution where the polydispeisity index is 2) and narrow chemical composition distribution, CCD. They ate usefiil in producing HDPE, and LLDPE. However, one drawback to the metallocene/aluminoxane catalysts is that significant quantities of aluminoxane by-products must be removed after production, due to the high Al/Zr ratios required for optimum performance. The primary role of the aluminoxane is in formation of the active site species. (The initial stracture in the catalytic cycle shown in Figure 5.) The series of complexation and fast alkylation... 

The unique composition of drugs [17-20] assures a presence in their composition of an iodine complex, whose structure is unique and differs from the structure of iodine complexes in other known dmgs. In the active site species the molecular iodine interacts with the bio-organic ligands (polypeptides) and lithium halogenide and exhibits acceptor properties with respect to polypeptides and donor properties in relation to lithium halide. The of existence such iodine complex is the probable cause of the low toxicity of the drugs. 

The other debate in the non-PGM catalyst community is whether or not there is a second active site species at play. Hydrogen peroxide is commonly produced as an intermediate or ORR by-product. Serov et al. [29] based on an observed loading dependence on RDE activity showed that the ORR occurs by a 2 x 2 electron reduction mechanism on catalysts prepared by heat-treating a mixture of iron chloride, polyethyleneimine, and carbon black. This unveils the rather complicated mechanism of oxygen reduction, as the generated hydrogen peroxide could very... 

The active site (MMS) redox response, which apparently occurs in a fast, quasi-reversible manner, is usually of low magnitude and overlaps with, and is not easily distinguished from, the capacitive double layer response. Eurthermore, when an electrocatalytic process takes place, mediated by the active site species, the catalytic process usually dominates the dc response, i.e., the redox behavior of the active site is not at all obvious. In such circumstances advantage may be taken of ac voltammetry techniques to investigate the vital role of the active sites at the interface [8]. Large amplitude Fourier transformed ac (LAFT-ac) voltammetry has been explored, at both a theoretical and experimental level [9-11], by Bond and co-workers, and the application of this technique to the study of gold surfaces in aqueous acid and base was described recently [12]. 

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