Heterogeneity molecular-scale

In many supported catalytic systems, it is nearly impossible to determine either the specific species, responsible for the observed catalytic activity, or the mechanistic pathway of the reaction. Using a defined SAM system in which careful molecular design is followed by controlled deposition into a solid-supported catalyst of known morphology, surface coverage, mode of binding and molecular orientation, allows direct correlation of an observed catalytic activity with the structure on the molecular scale. SAM and LB-systems allow detailed and meaningful studies of established surface bound catalysts to understand their behavior in heterogeneous... 

A major portion of this chapter is concerned with physical adsorption, particularly from a global thermodynamic point of view. This is followed by a molecular-scale examination of crystalline surfaces and a brief discussion of chemisorption and its relevance to heterogeneous catalysis. 

The innate complexity of practical catalytic systems has lead to trial and error procedures as the common approach for the design of new and more proficient catalysts. Unfortunately, this approach is far from being efficient and does not permit to reach a deep insight into the chemical nature of the catalytic processes. The consequence of this difficulty is a rather limited knowledge about the molecular mechanisms of heterogeneous catalysis. To provide information about catalysis on a molecular scale, surface science experiments on extremely well controlled conditions have been designed and resulted in a new research field in its own. However, even under these extremely controlled conditions it is still very difficult, almost impossible, to obtain precise information about the molecular mechanisms that underlie catalytic processes without an unbiased theoretical guide. The development of new and sophisticated experimental techniques that enable resolution at... 

Solid state NMR offers several advantages for the investigation of filled rubbers since molecular properties of elastomer chains can be measured selectively by NMR e>q)eriments. The method is very sensitive to the molecular scale heterogeneity in a sample. The network structure which is composed of chemical, physical and topological junctions can also be andyzed by NMR relaxation experiments [11,12,14,15],... 

The microemulsion method utilizes a water/oil/surfactant system to construct a micro reactor, in which NCs could be s)mthesized. The microemulsions have a wide range of applications from oil recovery fo fhe s)mfhesis of nanoparticles. Microemulsion is a system of water, oil, and surfactant, and it is an optically isotropic and thermod3mamically stable solution. At molecular scale, the microemulsion is heterogeneous with an internal structure either of nanospherical monosized droplefs (micelles or reverse micelles) or a bicontinuous phase, depending on the given temperature as well as the ratio of its constituents (Eriksson et al., 2004). The small droplets could be utilized as microreactors in order to s)mthesize the fine NCs in a controllable way. 

In passing, Israelachvili and Geel) proposed to average / (l + cosa ) rather than cosaj, as being applicable to heterogeneity on a near-molecular scale but Johnson and Dettre contested their derivation and applicability ). 

The increased selectivity and much milder experimental conditions required for high conversions make molecular catalysts compete with heterogeneous ones in many chemical processes. The application of traditional molecular catalysis in large-scale reactions such as HDS and HDN, however, is not possible. In order to overcome this drawback, many research efforts are being directed toward the heteroge-nization of molecular catalysts [21]. Successful applications of heterogenized molecular catalysts in several large-volume reactions have already been obtained [21]. 

For purposes of clarity, it is perhaps instructive to describe the distinction between a solution and dispersion. The term solution refers to a complete solvent shell around an atom, molecule, or a single polymeric chain on the molecular scale. It refers to a single phase and a homogeneous solution. The term dispersion means that particles larger than Inm are surrounded by and embedded in a medium. Thus, dispersions are not homogeneous, but consist of at least two phases. This heterogeneous system contains a pure solvent phase with no dissolved molecules and the pure dispersed phase. Both phases approach each other at the interfaces. 

The rotating-frame spin-lattice relaxation time for protons, Tip,( H), was measured indirectly from CPMAS/DD NMR to probe possible molecular scales of heterogeneity in the miscible poly(benzyl methacrylate)/poly(ethylene oxide) blend over the whole composition range. ... 

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