Heterogeneous catalysis defined

In discussing the mechanism of eliminations over solids, the nomenclature which has been developed for homogeneous reactions will be used. Therefore the basic mechanisms of olefin formation have first to be outlined and their meaning in heterogeneous catalysis defined. 

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

The reader must have already identified some of the basic concepts which play a key role in understanding the electrochemical activation of heterogeneous catalysis catalysis, electrocatalysis, promotion, electrochemical promotion, spillover, backspillover. It is therefore quite important to define these terms unambiguously so that their meaning is clearly determined throughout this book. 

Before deriving the rate equations, we first need to think about the dimensions of the rates. As heterogeneous catalysis involves reactants and products in the three-dimensional space of gases or liquids, but with intermediates on a two-dimensional surface we cannot simply use concentrations as in the case of uncatalyzed reactions. Our choice throughout this book will be to express the macroscopic rate of a catalytic reaction in moles per unit of time. In addition, we will use the microscopic concept of turnover frequency, defined as the number of molecules converted per active site and per unit of time. The macroscopic rate can be seen as a characteristic activity per weight or per volume unit of catalyst in all its complexity with regard to shape, composition, etc., whereas the turnover frequency is a measure of the intrinsic activity of a catalytic site. 

For the cross-metathesis of functionalised alkenes the ill-defined classical catalyst systems currently offer very few advantages (cost and heterogeneous catalysis) over the more functional group tolerant Schrock and Grubbs alkylidene... 

In fact, the chemical industry often favors heterogeneous catalysis, which is also more than a century old (Sabatier was probably one of its real fathers), despite its so-far empirical nature. The development of better catalysts in heterogeneous catalysis has always relied on empirical improvement since it has been difficult to characterize active sites on the surfaces, as the so-called active sites are usually small in number(s). Presently, the number of accepted elementary steps (as defined above) is stiU Hmited to a few examples, mostly demonstrated by means of surface science [1-3] and the predictive approach, based on molecular concepts. 

The surface basicity of a solid catalyst can be defined in a way analogous to that applied to conventional bases. Thus, a surface Lewis base site is one that is able to donate an electron pair to an adsorbed molecule. If we take the definition of surface basicity in a more general way, it could be said that the active surface corresponds to sites with relatively high local electron densities. This general definition will include not only Lewis basicity but also single electron donor sites. We emphasize that the literature of heterogeneous catalysis often reports that both single-electron and electron-pair donor sites exist on basic catalysts. 

How do you define an energetic spot and how does it act in heterogeneous catalysis Is there any characteristic difference between a protein and a crystal in this respect ... 

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