Atomic-level-controlled catalysts

As excellent candidates for design at the atomic or molecular level, heteropoly catalysts have proven to be of value in fundamental studies as well as practical applications. But it is also true that much remains to be done. Efforts to establish methodologies for design of practical catalysts are still under way. The acid strength and acid site density can be controlled quite well both in solution and in the solid state, but the redox properties in the solid state are much less well understood because of the lack of sufficient thermal stability of mixed-metal (mixed-addenda) heteropolyanions. The acid strengths of some solid heteropolyacids have been suggested to reach the range of superacids, but they... 

To optimize the use of the amorphous sodium titanate powders as catalyst substrates, it is important to fully characterize the ion-exchange properties of the material. Further, the solution properties of the active metal to be loaded onto the support will be an important parameter in the control of the adsorption process. For example, exposure of sodium titanate to a nickel salt solution does not guarantee that nickel will be loaded onto the sodium titanate, or that the nickel, if loaded, will be dispersed on an atomic level. Sodium titanate only behaves as a cation exchange material under certain pH conditions. The solution pH also influences the hydrolysis and speciation of dissolved nickel ions (3), which can form large polymeric clusters or colloidal particles which are not adsorbed by the sodium titanate via a simple ion-exchange process. 

The catalyst preparation process is highly specific to generate a catalyst for a specific reaction and also with specific selectivity. A large body of literature describes such procedures/recipes. Control of the nature (crystal type, size) of the active species on the atomic level is considered to be an important factor. Thus, atoms of a given metal can exhibit different behavior since these have different properties (energy levels) at different locations in the crystallite. The multielement catalysts widely used in current practice are much more complicated since atoms of the different metals that... 

A proper kinetic description of a catalytic reaction must not only follow the formation and conversion of individual intermediates, but should also include the fimdamental steps that control the regeneration of the catalyst after each catalytic turnover. Both the catalyst sites and the surface intermediates are part of the catalytic cycle which must turn over in order for the reaction to remain catalytic. The competition between the kinetics for surface reaction and desorption steps leads to the Sabatier principle which indicates that the overall catalytic reaction rate is maximized for an optimal interaction between the substrate molecule and the catalyst surface. At an atomic level, this implies that bonds within the substrate molecule are broken whereas bonds between the substrate and the catalyst are made during the course of reaction. Similarly, as the bonds between the substrate and the surface are broken, bonds within the substrate are formed. The catalyst system regenerates itself through the desorption of products, and the self repair and reorganization of the active site and its environment after each catalytic cycle. 

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