Chemisorptive Properties of the Catalyst

An important step in heterogeneous catalysis is the adsorption and desorption of reactants and products of the reaction. Important information on the mechanism of ammonia synthesis has come from the study of the adsorption of H2, N2 and NH3. The adsorption of H2O and O2 is interesting because of the role of H2O as a poison for NH3 synthesis. 

As mentioned above, not all of the surface of the catalyst is active. The adsorption of other gases, such as CO and CO2, is interesting because of these gases adsorb selectively on the catalyst surface. 

The presence of adsorbed atoms on a metal surface may have consequences for subsequent adsorption of another gas. There are several possible outcomes of this procedure. The preadsorbed atoms may weaken or prevent the subsequent adsorption the strength of adsorption for both species may increase a compound may be formed or the preadsorbed atoms may be displaced to the bulk or to the gas phase. 

While the reactions of the catalyst may be fairly complicated, studies of chemisorption on single crystals has resulted in a detailed understanding of the more important adsorption reactions. 

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Broadly speaking, promoters can be divided into structural promoters and electronic promoters. In the former case, they enhance and stabilize the dispersion of the nanoparticle-dispersed active phase on the catalyst support. In the latter case, they enhance the catalytic properties of the active phase itself. This stems from their ability to modify the chemisorptive properties of the catalyst surface and to significantly affect the chemisorptive bond strength of reactants and intermediates. At the molecular level this is the result of direct ( through the vacuum ) and indirect ( through the metal ) interactions. The term through the vacuum denotes direct electrostatic, Stark type, attractive or repulsive interactions between the adsorbed... 

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