Catalytic Processes Eley-Rideal Mechanism

For heterogeneous catalytic processes in which solid surface sites ( ) play a crucial role, an analogous treatment is applied. Let us consider a linear two-step mechanism  

FIGURE 2.3 Schematic illustration of a catalytic process A + B (Eley-Rideal mechanism). 

After defining the vectors = [A B C] and a = [ A ], the generation rates become 

Equation 2.39 reveals an interesting fact r = —fA = 0, which implies that the two equations contain the same information. To solve the concentrations of the intermediates ( and A ) as a function of the bulk-phase components, the total balance of the active sites is used  

The concentrations are inserted into the equation system, which becomes 

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The second step is the interaction of the external reactant R2 with intermediate Ki by the Eley-Rideal mechanism. The stationary rate of this catalytic stepwise process is... 

It is also possible that one of the reactants, say B in the above reaction, is not adsorbed. In such a mechanism (known as the Eley-Rideal mechanism), we simple use pg or [5] for B (and not Og). While the LHHW mechanism requires the adsorption of all reactants on the surface, the Eley-Rideal mechanism proceeds with one adsorbed reactant and one gas phase species. Depending on the interaction between the adsorbate and the adsorbent, one of the species may be so weakly bound to the surface that it is essentially not adsorbed. Furthermore, some of the reactions may proceed via a nonadsorbed intermediate. In addition to catalytic reaction kinetics, the Eley-Rideal mechanism is frequently encountered during the crystal growth processes. 

Finally, the most critical issue in all DFT calculations of catalytic processes is the identification of an active site for a specific reaction. This is particularly true for hydrogenation processes involving parahydrogen since, as already mentioned, extended metal surfaces may be inactive in terms of PHIP generation. It is possible that PHIP effects may be produced predominantly or exclusively on certain low-dimensional active sites that are able to significantly reduce the mobility of chemisorbed hydrogens or even perform addition of physisorbed H2 to a substrate (e.g., Eley-Rideal mechanism). These may include low-dimensional defects, kinks, edges, and corners of nanoparticles, metal-support interface, individual metal atoms or ions on the surface of a support, and so on. 

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