Bimetallic structures

Kucera, V. and Mattson, E., Atmospheric Corrosion of Bimetallic Structures, ex Atmospheric Corrosion, 561, J. Wiley and Sons, (1982)... 

EXAFS evidence for bimetallic structure on PtRu/MgO after decarbonylation... 

These kinetic and stereochemical results give us a direct evidence for the bimetallic structure of the active center in which alkylaluminum components are involved as important ligands. 

The value of KM decreases with increasing electronwithdrawing capability of the aluminum component, i.e. with decreasing electron density at the vanadium induced by the aluminum component bonded to the vanadium in the bimetallic structure of the active center. This result seems to suggest that electron back-donation from a filled vanadium d orbital to the empty propylene jc obital (it-bonding) is the main factor in determining the vanadium-propylene interaction. 

Fig. 1 Side view of slab models of various bimetallic structures often used in computational studies. In each case, the bottom layers of the material are defined using the structure of a specified bulk material. The number of surface and bulk layers varies in different studies, (a) In the sandwich structure the surface is one component, often the same component as the bulk material and the second layer is another component. This structure is often used to determine ligand effects, (b) The pseudomorphic monolayer structure combines strain and ligand effects in one structure by placing a second component on top of a bulk material, (c) The near surface alloy combines strain, ligand and ensemble effects in one structure by considering an alloy film defined by just a few atomic layers on top of an ordered bulk material.

M=Rh, Pd, or Pt) bondings. The highly dispersed RhFe and PdFe bimetallic ensembles were found to retain metal compositions fairly similar to those of the precursors. They still existed preferentially (> 85%) in the Fe state even after H2 reduction at 400°C and CO + Hj reaction at 250°C. The results imply that Fe in RhFe bimetallic structures acts not only as an anchor to fix the Rh, Pd, and Pt particles at the cluster-support interface, but also to provide M—Fe —OSi= species (M=Rh, Pd, Pt, and Ir) which are highly... 

The active groups that polymerization proceeds by iniertion Into the Al-OR bonds is clearly demonstrated by the fact that linear chains produced contain (after hydrolysis) one OR and one OH end-group. Furthermore, the direct relationship between the catalytic properties and a y-oxo-bridged multinuclear bimetallic structure has been ascertained by using oxo-alkoxldes obtained from hydrolysis of Meerwein complexes AI2(OR)g (OR)2... 

The term bimetallic was introduced by Sinfelt to account for the fact that a catalyst may contain a multitude of phases containing the active metallic components.22 Of these many phases, a characteristic one is the binary alloy. The term alloy can describe a broad range of situations from well-defined phases or solid solutions to surface alloys in cases where bulk alloys are not thermodynamically favoured but a clearly defined surface local arrangement is obtained. Note that the novel core-shell bimetallic structures are included in this catch-all term. A historical overview of the properties of alloys in connection with catalysis has been published by Ponec.23 At present, a... 

When atoms of both the additive metal and the catalyst metal form a bimetallic nanocluster composed of the two elements, it is considered to have a variety of bimetallic structures. These have been discussed in the previous section and shown in Fig. 3.18. Here, some examples of the catalytic reactions in which bimetallic nanoclusters have higher activity than the corresponding monometallic nanoclusters are described. 

Other Attempts to Switch the Flow Using Bimetallic Structures... 

The cyclotrimerisation of electron-rich alkynes is also efficiently catalysed by the bimetallic indenyl complex 4, which contains Cr(0) and Rh(l) coordinated in a transoid fashion across the indenyl ring (Scheme 3) [17]. Using catalyst 4, quantitative conversion of methyl propiolate to a mixture of the 1,3,5- and 1,2,4-substituted benzene products is achieved. Of particular note is the fact that 4 can be recovered at the end of the reaction, suggesting that the bimetallic structure remains intact during catalysis. In contrast, the monometallic Rh(l) catalyst... 

No matter what function the bimetallic catalyst fulfils, two key trends of catalyst design emerge from the above examples (1) a relatively constrained catalyst structure is often essential to ensure a beneficial interaction of both metals this most likely helps overcome the entropic penalty associated with creating an ordered bimetallic transition state during the catalytic cycle (2) for catalysts that do not contain any prearranged metal-metal bonding or bridging atom interactions, a relatively short intermetallic distance also appears necessary. Clearly, for many of the catalysts described here, much work remains to tmderstand exactly how the bimetallic structure enhances the catalysts performance. 

As shown in fig. 13, the logarithm of the rate of RNA hydrolysis steeply increases with increasing pH, up to pH 8, and then reaches a plateau. The slope in the steep region is greater than 2. The experimental points fairly fit the theoretical line (the solid line) which shows the equilibrium concentration of [Nd 2(OH)2]". The other species never satisfy the experimental results. Undoubtedly, this bimetallic cluster is the active species for the RNA hydrolysis. In order to hydrolyze RNA efiiciently, the bimetallic structure is essential, as is the case in DNA hydrolysis. The concentration of this active species is strongly dependent on pH, since it is accompanied by the release of two protons. Because of this, the rate of RNA hydrolysis by lanthanide(III) ions is so highly pH dependent. 

Humbert MP, Mtaming CA, Chen JG (2010) Replacing bulk Pt in Pt-Ni-Pt bimetallic structures with tungsten monocarbide (WC) Hydrogen adsorption and eyelohexene hydrogenation on Pt-Ni-WC. J Catal 271 132-139... 

Bimetallic switches (Fig. 18.20) are widely used in on-off temperature control systems. If two metal strips with different coefficients of thermal expansion are bonded together while both strips are at the same temperature, the bimetallic structure will bend when the temperature is changed. Although these devices are often called thermal cutouts, implying that they are used in normally closed switches, they can be fabricated in either normally closed or normally open configurations. The bimetallic elements can also be fabricated in coil or helical configurations to extend the range of motion due to thermal expansion. 

The American company IC-Sensors offers a thermomechanically driven microvalve (with 2/2-way functionality) for gases (Fig. 6.113) [315]. The valve consists of an elastically suspended valve reed and a rigid valve base with a valve opening. The valve reed can be put into motion using the thermomechanical (bimetallic) effect. The temperature of the bimetallic structure of aluminium and silicon is used to control the actuator force. The normally closed valves are designed for a maximum pressure of 10... 200kPa. Gas flows of up to 0.11/min are achieved with a driving power of 300 mW. 

P(C6H4SMe-2)2CH2CH2P(C6H4SMe-2)2 (tfttppe) results in a bridged bimetallic structure [ (CO) 3M(jj-mtppe)M(C0) 3] which has been established by X-ray diffraction studies for M = A... 

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