Surface reactions steric aspects

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

The fifth contribution by M. Putkonen and L. NiinistO presents an overview of Organometallic Precursors for Atomic Layer Deposition (ALD). The key principle of ALD in contrast to CVD is the exclusion of any gas-phase prereaction allowing the thin film growth to be fully controlled by surface reactions and adsorption/desorption kinetics. ALD is thus ideally suited for the growth of ultra-thin layers and atomically abrupt interfaces needed in future nanoelectronic devices. While CVD and ALD have many aspects in common, precursors suitable for ALD generally need to be much more reactive than those used for CVD. Another challenge is to combine low steric demand with very high selectivity of the surface reactions. 

Which catalyst should be chosen for a given reaction will depend upon chemical, steric, and mechanistic factors. The application of Pearson s soft and hard acid-base (SHAB) principle has often proved a valuable qualitative guide as to suitable surface sites for a particular reactant. In fact, certain solids actually owe their catalytic power to attached Bronsted or Lewis acid and base groups as exemplified by weak acid ion exchange resins (Sect. 2.3), alumina (Sect. 3.2), and sometimes charcoals. Steric aspects can be con-... 

It was noted that in the closed form there exists significant steric hindrance for the quinuclidine-iV lone pair to participate in interaction with pyruvate, whereas in the open conformation of the Cnd N-lone pair of quinuclidine is more readily accessible to the reactant. Scheme 5.26. shows the model of interaction of Cnd in the open conformation with methyl pyruvate as calculated by Schwalm et al. for the intermediate complex. Another important aspect is the role of adsorption of the intermediate complex on the Pt active center. In the intermediate complex the pyruvate molecule is bound to the modifier via stabilizing hydrogen bond interactions, N-H—O, between the protonated quinuclidine-N atom and the 0-atom of the alpha-cdx )ony group of pyruvate, or 0-H—N-bond for unprotonated system, such as for reactions in toluene solution. In this case, the H-atom can come from dissociatively adsorbed dihydrogen from the Pt-surface. The same... 

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