Chemisorption measure the rate and activation energy of adsorption

8 Chemisorption measure the rate and activation energy of adsorption 

The measurement of rates and activation energy of chemisorption is very important for catalytic mechanism and its applied research. It is clear that when the adsorption is the reaction determining step of catalytic reaction, the rate of adsorption process will determine the rate of the reaction and activity of catalysts. Therefore, activation energy and heat of adsorption can reflect the nature of the active site and distinguish the t3rpe of active site imder certain conditions. 

There are two major chemisorption patterns. One is non-activated chemisorption, which can happen at low temperatures. The adsorption rate is very fast, and does not require any activation energy. For example, H2 can be quickly adsorbed in Ni, Pt and Pd at 77 K. Another pattern is activated chemisorption, which is referred as activated adsorption, which requires a higher temperature. It is characterized by the activation energy need, just as the real chemical reaction. 

Studies on the adsorption kinetics have shown that the adsorption activation energy also increases with the increase of the surface coverage. As a result, the concept of adsorption activation energy as adsorption heat is referred to the given surface coverage, which is represented by the Eq. (7.81) 

The rate constant k can be calculated by the relationship between the amount and the time of adsorption, which have many t5rpes of expression. On the uniform surface, it can use Langmuir rate equation  

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