Transition metal clusters mechanism

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

After Faraday s seminal report on the preparation of transition metal clusters in the presence of stabilizing agents in 1857 [31], Turkevich [19-21] heralded the first reproducible protocol for the preparation of metal colloids and the mechanism proposed by him for the stepwise formation of nanoclusters based on nucleation, growth, and agglomeration [19] is still valid but for some refinement based on additional information available from modem analytical techniques and data from thermodynamic and kinetic experiments [32-41], Agglomeration of zero-valent nuclei in the seed or, alternatively, collisions of already formed nuclei with reduced metal atoms are now considered the most plausible mechanism for seed formation. Figure 3.1 illustrates the proposed mechanism [42],... 

The complete conversion of C2H2 to ethylidyne requires the presence of surface hydrogen atoms and proceeds rapidly only at 350 K. By comparison with reported reaction mechanisms on related transition metal clusters it seems likely that vinylidene... 

The accurate quantum mechanical first-principles description of all interactions within a transition-metal cluster represented as a collection of electrons and atomic nuclei is a prerequisite for understanding and predicting such properties. The standard semi-classical theory of the quantum mechanics of electrons and atomic nuclei interacting via electromagnetic waves, i.e., described by Maxwell electrodynamics, turns out to be the theory sufficient to describe all such interactions (21). In semi-classical theory, the motion of the elementary particles of chemistry, i.e., of electrons and nuclei, is described quantum mechanically, while their electromagnetic interactions are described by classical electric and magnetic fields, E and B, often represented in terms of the non-redundant four components of the 4-potential, namely the scalar potential and the vector potential A. 

The well-proven computational technique of choice for dealing with transition metal clusters is Density-Functional Theory (DFT) [17]. The main advantage ofDFT over other quantum mechanical methods is that it allows for an integrated treatment of electron correlation effects combined with relatively high computational speed. Both of these are important for transition metal clusters with their high number of... 

Mechanism for H2 Dissociation on Transition Metal Clusters and Surfaces... 

The calculations present here among the first to give serious consideration to the competing effects of spin coupling and electron delocalization in polynuclear transition metal clusters. The qualitative features support the "double exchange" model put forth to describe experimental spectra (7), although we do not postulate a particular mechanism in these calculations--the results arise solely from analysis of computed total energies for various states. 

The electronic state calculations of transition metal clusters have been carried out to study the basic electronic properties of these elements by the use of DV-Xa molecular orbital method. It is found that the covalent bonding between neighboring atoms, namely the short range chemical interaction is very important to determine the valence band structure of transition element. The spin polarization in the transition metal cluster has been investigated and the mechanism of the magnetic interaction between the atomic spins has been interpreted by means of the spin polarized molecular orbital description. For heavy elements like 5d transition metals, the relativistic effects are found to be very important even in the valence electronic state. 

Several studies have demonstrated the ability to observe a complete catalytic cycle in the gas-phase. Wallace and Whetten, and Woste and coworkers combined gas-phase experiments and theoretical calculations to elucidate the fuU catalytic cycle of CO oxidation including intermediate reaction steps [27-29]. Schwarz et al. have also demonstrated a full gas-phase catalytic cycle for the oxidation of CO in the presence of cationic platinum oxide clusters [30]. Furthermore, Armentrout and co-workers have studied the energetics of the individual steps in the overall catalytic cycles and produced a wealth of information on the thermochemistry, structure, and bond energies of transition metal clusters [31]. Clearly, the ability to probe the active sites and intermediates of complex catalytic reactions through gas-phase ion-molecule studies has yielded significant insight into the mechanisms of condensed-phase catalytic processes. 

This chapter will walk through the various forms these catalytic resins take. The catalysts covered in this review fall into three classes, (i) transition metals covalently bonded to the polymer support through an organometallic bond, (ii) transition metals coordinated to the polymer support, typically in ionic form and (iii) transition metal clusters that are formed by precipitating metals into nanoparticles within the polymeric framework. Additionally, this chapter covers the synthetically useful and industrially practiced reactions catalyzed by transition metals loaded onto organic supports and comments on the mechanisms and reusability aspects of the processes [1]. 

The mechanism of trimerisation of acetylene on extended surfaces is thought to be similar to that observed in homogeneous catalysis using transition-metal cluster compounds.A stepwise mechanism, shown below is thought to occur ... 

As stated in the previous section, dynamic behaviour involving intramolecular rearrangements of the metal skeletons of Group 11 metal heteronuclear clusters is relatively common, in marked contrast to the situation observed for almost all other transition metal clusters, which have metal frameworks that are stereochemically rigid in solution. The mechanisms of these metal core rearrangements are, therefore, of considerable interest. 

Fig. 2.58. Comparison of mechanisms of hydrogenation and halogenation [Deeming AJ (1980) In Johnson BFG (ed) Transition metal clusters. John Wiley Chichester, p 391]...

Table 11.12 Representative studies of the processes and mechanisms in compiex graphene systems invoiving transition metal clusters.

In recent years there has been an explosive growth in the study of transition metal cluster compounds. Many factors have contributed to this growth -- the development of preparative and separative procedures, the expectation that cluster compounds mimic catalytic metal surfaces and so may provide insights into catalytic mechanisms (as well as, potentially, being catalysts themselves), the development of new techniques by which metal surfaces - and species absorbed onto them -may be studied, the recognition that novel bonding patterns may exist within them, and their involvement in bioinorganic chemistry, particularly as electron sinks and sources. 

The experimental data and arguments by Trassatti [25] show that at the PZC, the water dipole contribution to the potential drop across the interface is relatively small, varying from about 0 V for An to about 0.2 V for In and Cd. For transition metals, values as high as 0.4 V are suggested. The basic idea of water clusters on the electrode surface dissociating as the electric field is increased has also been supported by in situ Fourier transfomr infrared (FTIR) studies [26], and this model also underlies more recent statistical mechanical studies [27]. 

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