Adsorbate Mobility During Catalytic Reactions

Heterogeneous catalysis is by nature a surface phenomenon. An efficient catalytic system under reactive conditions demands a delicate energetic balance. For a catalytic reaction to occur, reactant molecules must adsorb on a surface, react, and 

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Molecular Studies of the Mobility of Surface Metal Atoms and Adsorbates During Catalytic Reactions... 

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. 

When catalytic processes are employed, complex molecules (such as those that may be found in the original asphaltene fraction) or those formed during the process, are not sufficiently mobile (or are too strongly adsorbed by the catalyst) to be saturated by hydrogenation. The chemistry of the thermal reactions of some of these constituents (Chapter 4) dictates that certain reactions, once initiated, cannot be reversed and proceed to completion. Coke is the eventual product. These deposits deactivate the catalyst sites and eventually interfere with the hydroprocess. 

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