Through-the-metal interactions

Indirect ( through the metal ) interaction due to the redistribution of electrons in the metal. In this case an electropositive promoter decreases the work function of the surface and this in turn weakens the chemisorptive bond of electropositive (electron donor) adsorbates and strengthens the chemisorptive bond of electronegative (electron acceptor) adsorbates. 

The second idea to consider is the very popular but very hypothetical through-the-metal interaction, induced by electron transfer, which interaction is supposed to have one of the following forms ... 

A massive electron transfer between the metal particles and the supports (or promoters) and the penetration of an electric field into the metal are thus not realistic ideas on the through-the-metal interaction. However, there is one mechanism for such an interaction which is well supported by the quantum theory of chemisorption when a covalent chemisorption bond is formed, it causes periodic variation (with the distance) in the chemisorption bond strength in its environment. At the nearest site a repulsion is felt, on the next-nearest an attraction, etc. [46a]. However, it is important to realize how strong this interaction is. A realistic estimate, based on observations of the field ion emission images, shows that these interactions are comparable in their strength to the physical (condensation) van der Waals forces [46b]. 

Broadly speaking, promoters can be divided into structural promoters and electronic promoters. In the former case, they enhance and stabilize the dispersion of the nanoparticle-dispersed active phase on the catalyst support. In the latter case, they enhance the catalytic properties of the active phase itself. This stems from their ability to modify the chemisorptive properties of the catalyst surface and to significantly affect the chemisorptive bond strength of reactants and intermediates. At the molecular level this is the result of direct ( through the vacuum ) and indirect ( through the metal ) interactions. The term through the vacuum denotes direct electrostatic, Stark type, attractive or repulsive interactions between the adsorbed... 

Equally important is the observation that, as Figure 25 shows, the chemisorptive binding energy-work function behavior can be described almost quantitatively by taking into account only the electrostatic (Stark) interactions, i.e., only the through the vacuum electrostatic interactions in the double layer and neglecting the through the metal interactions, i.e., the redistribution of electron states near the Fermi level of the cluster. This is an important result as it provides... 

The binding strength of adsorbates is affected due to both direct electrostatic interactions in the effective double layer (i.e., through-the-vacuum interactions), and through-the-metal interactions. ... 

Decompositions of crystalline mixed hydroxides to mixed oxides often occur at temperatures lower than those required to produce the same phases through the direct interaction of metal oxides. This route thus offers an attractive approach for the preparation of catalysts of high area and activity [1147]. Detailed kinetic investigations comparable with those for the dehydroxylations of a number of pure hydroxides (Sect. 2.1) are not, however, available. 

Consequently one of the key experimental observations of electrochemical promotion obtains a firm theoretical quantum mechanical confirmation The binding energy of electron acceptors (such as O) decreases (increases) with increasing (decreasing) work function in a linear fashion and this is primarily due to repulsive (attractive) dipole-dipole interactions between O and coadsorbed negative (positive) ionically bonded species. These interactions are primarily through the vacuum and to a lesser extent through the metal . 

Equation (6.20) and the semiquantitative trends it conveys, can be rationalized not only on the basis of lateral coadsorbate interactions (section 4.5.9.2) and rigorous quantum mechanical calculations on clusters89 (which have shown that 80% of the repulsive O2 - O interaction is indeed an electrostatic (Stark) through-the-vacuum interaction) but also by considering the band structure of a transition metal (Fig. 6.14) and the changes induced by varying O (or EF) on the chemisorption of a molecule such as CO which exhibits both electron acceptor and electron donor characteristics. This example has been adapted from some rigorous recent quantum mechanical calculations of Koper and van Santen.98... 

In the case of conventional (i.e. non-superconductive) metals, the inter pretation of the results of experimental studies on electron tunneling through the "metal-insulator metal junction becomes possible within the framework of the simplest model of non-interacting electrons. Here, the interaction between electrons and between electrons and phonons both inside the barrier and in the bulk of metals are neglected. 

A metal can be considered as a fixed lattice of positive ions permeated by a gas of free electrons. Positive ions are the atomic cores, while the electrons are the valence electrons. For example, copper has a configuration (electronic structure) ls22s22p63s23p63dl04sl (superscripts designate number of electrons in the orbit) with one valence electron (4s). The atomic core of Cu+ is the configuration given above, less the one valence electron 4s1. The free electrons form an electron gas in the metal and move nearly freely through the volume of the metal. Each metal atom contributes its valence electrons to the electron gas in the metal. Interactions between the free electrons and the metal ions makes a large contribution to the metallic bond. 

To overcome this problem, we have modified a commercial ion gun to generate a diffuse fast-atom beam [116, 117]. The ion beam neutralizer shown in Figure 7 consists of a multi-hole metal plate through which the primary ions pass. The ions are neutralized by the ion/surface interactions that occur as the beam passes through the metal aperatures and by charge-exchange reactions that occur within the gun assembly. A repeller grid is used to remove the residual ions from the neutralized beam. 

The most widespread method of synthesis of complex compounds of pseudohalide ions is through the immediate interaction of ligands with their corresponding metal salts (4.1) [44] ... 

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