Compensation Effect in Temperature Programmed Desorption

The usual derivation of an activation energy from a set of temperature dependent rates as the slope of an Arrhenius plot gives  

However, if the prefactor and the desorption energy depend on coverage, the derivative will take the form  

the activation energy contains coverage-dependent, second-order terras, which are usually ignored. This is only allowed in three cases. The trivial cases are when the kinetic parameters are constant, or when the coverage does not change 

Such behavior is known as the compensation effecf . The important point is that if we ignore the additional term in Eq. (18), we essentially force the kinetic parameters to satisfy Eq. (19) resulting in a correlation between the prefactor and the desorption energy according to the compensation effect  

Finally, although both temperature-programmed desorption and reaction are indispensable techniques in catalysis and surface chemistry, they do have limitations. First, TPD experiments are not performed at equilibrium, since the temperature increases constantly. Secondly, the kinetic parameters change during TPD, due to changes in both temperature and coverage. Thirdly, temperature-dependent surface processes such as diffusion or surface reconstruction may accompany desorption and exert an influence. Hence, the technique should be used judiciously and the derived kinetic data should be treated with care  

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