Mixed-metal system

Mixed-Metal Systems. Mixed-metal systems, where a zirconium alkyl is formed and the alkyl group transferred to another metal, are a new apphcation of the hydrozirconation reaction. These systems offer the advantages of the easy formation of the Zr—alkyl as well as the versatiUty of alkyl—metal reagents. For example, Cp2ZrRCl (R = alkyl or alkenyl) reacts with AICI3 to give an Al—alkyl species which may then be acylated with... 

Alter the chemistry of the common fluid to render it less conductive and/or less corrosive. Generally, water corrosivity increases with an increase in temperature and oxygen content and a decrease in pH. Inhibitors may he effective. Note that in mixed-metal systems, higher dosages of inhibitors may be required than would be necessary in single-metal systems in the same environment. 

In a mixed metal system the protection potential to be adopted is that of the least noble metal, i.e. if the heat exchanger channel had been iron and also required protection, the relevant protection potential would have been —0-85 V vs. CU/CUSO4 i.e. the [>otential relating to steel. 

The protection potential refers to the potential at which experience shows corrosion of a metal will cease. Different materials have different protection potentials (Table 10.6). Occasionally a less negative protection potential will be specified because some degree of corrosion is permissible. It should be noted that in a mixed metal system the protection potential for the most base metal is adopted. 

These reactions have been studied in detail for materials such as silica, and understanding of reaction mechanisms, as well as of the role of the precursor and catalyst (acid or base), has been well documented.63,64 Similar studies have been carried out in other material systems, most notably, lead zirconate titanate [Pb(Zr,Ti)03 PZT].52,65-68 For multicomponent (mixed-metal) systems such as those noted, prehydrolysis of less reactive alkoxides is sometimes employed to improve solution compositional uniformity. Other synthetic strategies to achieve molecular level mixing of reagents have also been employed. Here, synthesis of mixed-metal alkoxides has been a focus of investigators.40-42 A key point is to restrict the amount of water and to control how it is added to form solubalizable precursor species, rather than to induce precipitation.1,52,69,70... 

In addition to modification of surfaces by non-metals, the catalytic properties of metals can also be altered greatly by the addition of a second transition metaP ". Interest in bimetallic catalysts has arisen steadily over the years because of the commercial success of these systems. This success results from an enhanced ability to control the catalytic activity and selectivity by tailoring the catalyst composition . A long-standing question regarding such bimetallic systems is the nature of the properties of the mixed-metal system which give rise to its enhanced catalytic performance relative to either of its individual metal components. These enhanced properties (improved stability, selectivity and/or activity) can be accounted for by one or more of several possibilities. First, the addition of one metal to a second may lead to an electronic modification of either or both of the metal constituents. This... 

Other recent reports have also indicated that mixed-metal systems, particularly those containing combinations of ruthenium and rhodium complexes, can provide effective catalysts for the production of ethylene glycol or its carboxylic acid esters (5 9). However, the systems described in this paper are the first in which it has been demonstrated that composite ruthenium-rhodium catalysts, in which rhodium comprises only a minor proportion of the total metallic component, can match, in terms of both activity and selectivity, the previously documented behavior (J ) of mono-metallic rhodium catalysts containing significantly higher concentrations of rhodium. Some details of the chemistry of these bimetallic promoted catalysts are described here. 

More recently, the same investigators (14) reported a convenient modihcation of the 1,3-elimination process using a bis(chloromethylether) with a mixed-metal system (Scheme 4.11). 

A variety of routes have been employed for the synthesis of hetero-metallic clusters containing Fe(NO)S fragments, and examples of such systems have been described in which the second metal is V, Mo, Co, Ni, and Pt it seems probable, given the variety of potential synthetic approaches to such mixed metal systems, that many more examples await discovery. 

In all cases the complexes were strongly luminescent at room temperature, in contrast to the precursor homonuclear derivatives, and at an energy that was tunable, displaying a site-selective excitation that depended on the Au Ag molar ratio, lying between the emission bands of the pure Ag and Au compounds. For instance, while the homonuclear derivative La[Ag(CN)2]3 emitted at 77 K at 345 and 470 nm (exc. 310 nm) and the gold complex La[Au (CN)2]3 at 431 and 493 nm (exc 310 nm), the mixed metal systems tended toward Ag or Au peak positions, depending on loading. 

In all cases, the real situation involves simultaneous competition for the binding sites by many metals. While the majority of studies have focused on the complexation of only one metal ion, a few recent publications have considered multiple-metal equilibria (Cabaniss, 1992 Cao et al., 1995 Einax and Kunze, 1996 Mota et al., 1996 Norden et al., 1997 Takahashi et al., 1997). The fact that different metal ions do not experience the same apparent heterogeneity does pose challenges for modelling mixed metal systems (Tipping, 1993 Benedetti et al., 1995 1996 Kinniburgh et al., 1996). 

Processes of this kind have some synthetic utility, since they permit exchange of silicon-containing groups from the readily prepared Si-Co compounds to other metal centres. The partial exchange of groups on silanediyl derivatives also provides a route to mixed-metal systems ... 

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