Modification of Electronic Structure

Zeolite supported metal clusters are important catalysts. The zeolite matrix not only imposes steric constraints on reacting molecules (shape selective catalysis) and provides addic sites, but it also apparently affects the electronic properties of the encaged metal clusters. 

The modification of the electronic and catalytic properties of metal clusters in zeolites has been reviewed by Gallezot. [108] He showed that in addition to changes in the electronic structure of metal clusters as a result of intrinsic size effects, the electronic structure of a metal cluster can also be modified by the cluster environment, for example, by electron transfer from the metal dusters to electron acceptor sites in the zeolite lattice. He considered Lewis add sites, Br0nsted add sites, and multivalent cations to be potential electron acceptors. Electron defident clusters appear to be resistant to poisoning by sulfur, which is advantageous in catalytic applications. The issues of duster size effects and electronic effects are complex and continue to be vigorously debated. 

Metals catalyze many reactions of practical importance, and the most conunon form of an industrial metal catalyst is a supported metal consisting of dusters or particles on a high surface area support such as a metal oxide. The support may also be a zeolite as in the Pd supported in faujasite catalyst used for hydrocracking. For the most part, zeolite supported metals are structurally complex and are beyond the scope of this chapter. They have been reviewed elsewhere. [105] 

Examples of shape selective catalysis for each dass are well illustrated with the addic zeolites, but there are far fewer examples of such catalysis with metal containing zeolites. Reactant shape selectivity is illustrated by the early work of 

Weisz et al., [188] who used Pt dusters in zeolite A for selective catalysis of the hydrogenation (or the oxidation) of straight chain hydrocarbons in mixtures with branched hydrocarbons. 

---

Returning for a moment to the description of bonding inside the crystal, " those d-orbitals whose interactions are responsible for bonding nearest neighbours (viz. the t2g family) will form a band which is broader than that formed by the 6g family, since interactions between next-nearest neighbours are less strong. Extending this concept to surface atoms, we see on the (KX)) surface for example that the absence of atoms above the plane means that the overlap of dxz and d y orbitals has decreased and their band is narrowed, while the dyz orbitals in the surface plane are unaffected, and their band remains broader. Similar but smaller effects will occur with the Cg and 5-orbitals. The modification of electronic structure of atoms at steps and kinks is then easily rationalised, and the story will be resumed in Chapter 2, where other concepts developed in the context of small metal particles will be considered. 

The k C bonding mode includes most of the reported work on ylides. However, modifications of the structure of the yhde could be advantageous, in particular the introduction of additional donor atoms to form chelate hgands. The combination of the pure a-donor properties of the ylide with those of the auxiliary donor atom could be used for tuning the steric and electronic properties of yhde complexes. There are reports of useful C,P- and C,C-chelates, which will be detailed here. 

Since the electronic properties of solids depend on the crystal structure, the transition from the crystalline to the amorphous state is expected to result in some modification of electronic (and surface) properties. Amorphous materials have first been used in catalysis [558-560] where some evidence for higher activity has been obtained [561]. In particular, hydrogenation reactions are catalyzed by this class of materials [562]. Studies on the H recombination reaction are also available [563]. However, the evidence that the amorphous state is really the origin of enhanced catalytic activity is not completely clear [562, 564]. These materials have the peculiarity that their surface is relatively homogeneous for a solid and in particular it is free from grain boundaries [565, 566]. Therefore, they have been suggested [562] as ideal model surfaces for studying elementary catalytic reactions, since they can be prepared with controlled electronic properties and controlled dispersion. Nevertheless, many prob-... 

Both formal analysis and computational developments associated with DFT can be carried over intact to nDFT. For example, the exact two-particle ground-state density, no(x), can be determined through a constrained search [34] for that many-particle, properly symmetrized or antisymmetrized wave function, with symmetry imposed with respect to ordinary particles, which yields n0 and also minimizes the many-particle energy, T + Vpp, where Vpp denotes the interparticle interaction in two-particle space. Essentially any method developed within a single-particle application of DFT for the study of electronic structure can, with appropriate technical modifications, be extended to two-, or rc-particle states. The use of multiple-scattering theory to calculate fully correlated two-particle densities in solids will be given in a future publication. 

Thus, the ion irradiation leads to the modification of the structure of electronic states in the energy gap of HOMO-LUMO. In this case the electron spectra of all traps both dimeric, and impurity, are affected, that indicates about the changes in the atomic structure of fullerenes as a result of their radiation destruction and about the accumulation of radiation defects in a crystal lattice. The effects of radiation exposure indicated affect also distribution of electron energy in the valence and vacant bands, that specifies the transformation of optical interband transitions. 

Fig. 14 shows further modifications of the structure of retinal, compounds 47-60, which however, yielded negative results, i.e., these structures, due to unfavorable steric and/or electronic interactions with the binding site did not form rhodopsin analogs. 

J. P. Quirk, Some physico-chemical aspects of soil structural stability—A review, in Modification of Soil Structure (W. W. Emerson, R. D. Bond, and A. R. Dexter, eds.) Wiley, Chichester, U.K., 1978. Y. Chen and A. Banin, Scanning electron microscope (SEM) observations of soil structure changes induced by sodium-calcium exchange in relation to hydraulic conductivity, Soil Sci. 120 428 (1975). Y. Chen, J. Tarchitzky, J. Brouwer, J. Morin, and A. Banin, Scanning electron microscope observations on soil crusts and their formation, Soil Sci. 130 49 (1980). 

---