Surface reactivity tuning

B.l. Microcrystal Surface Area A Means for Tuning Surface Reactivity... 

Properties of supported catalysts by bimetallic substrates depend on the changes in geometry of the catalyst material by the strain of the substrate. Using a bimetallic substrate multiphes the possibilities to tune the catalyst to specific requirements. The chemistry of the nanosized overlayer is affected by the different orbital overlaps of atoms from the catalyst cluster and those from the substrate. Additionally, small supported metallic islands show low coordination and reduced near-neighbor distances thus their chemical properties are different with respect to those of flat surfaces. " Reactivity of several bimetallics were also studied by Balbuena et al., including bimetallics systems . Norskov et al. found several relations for the bimetallic systems considering local and nonlocal effects have also been reported. ... 

These discrete, soluble multimetal oxides are characterized by a formidable structural variety that can be achieved by tuning reaction parameters (such as concentrations and/or stoichiometric ratio of the reagents, temperature and pH), thus yielding nanosized complexes with a different shape, charge density and surface reactivity. When considering isostructural POMs, in particular, such properties can be controlled at the molecular level by changing their constituent elements (other transition metals or the heteroatom X). 

However, several aspects should be considered in the development of new catalytic systems. Thus, in addition to an extensive knowledge of the nature of active sites, the multifunctionality (redox, acid-base, etc.) of catalysts should be finely tuned. Moreover, the stability of well-defined crystalline structures and the effect of promoters will also be important factors to be considered. But, in addition to these, the alkane feed, the reactivity of olefinic intermediates, and the stability of partial oxidation products are key aspects to take into account. In this way, modification of the catalyst surface reactivity by chemisorbed species in the oxidation of alkanes is lower than that observed for the oxidation of olefins, which can facilitate a lower volume of undesired products in the case of alkane-based processes. 

Based on insight gained by Fig. 6.33 we can start to design surfaces that are fine tuned towards the desired chemisorption bond strength and reactivity, as we shall see in the following. 

These studies indicate that the charge transfer at the metal-oxide interface alters the electronic structure of the metal thin film, which in turn affects the adsorption of molecules to these surfaces. Understanding the effect that an oxide support has on molecular adsorption can give insight into how local environmental factors control the reactivity at the metal surface, presenting new avenues for tuning the properties of metal thin films and nanoparticles. Coupled with the knowledge of how particle size and shape modify the metal s electronic properties, these results can be used to predict how local structure and environment influence the reactivity at the metal surface. 

Due to the pyramidalization of the C atoms and the rigid cage structure of Cjq the outer convex surface is very reactive towards addition reactions but at the same time the inner concave surface is inert (chemical Faraday cage). This allows the encapsulation, observation and tuning of the wavefimction of extremely reactive species that otherwise would immediately form covalent bonds with the outer surface. 

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