Evaluation of Parameter Consistency in Rate Expressions for Ideal Surfaces

5 Evaluation of Parameter Consistency in Rate Expressions for Ideal Surfaces 

However, having acknowledged this paradox, there exists a set of rules and guidelines that allows assessment of the validity of the application of Langmuirian kinetics, i.e., the assumption of an ideal surface, to a particular set of kinetic data [26,31,32]. Criteria to evaluate whether rate parameters, such as the adsorption equilibrium constants appearing in the denominator of the Langmuir isotherm (and subsequently in the denominator of the rate 

These criteria are listed in Table 6.9, and they are comprised of three strong rules and two guidelines. First, adsorption is invariably exothermic, thus the enthalpy of adsorption is negative, i.e., -AH°j 0(Qad 0). Second, there must be a decrease in entropy after adsorption, including dissociative adsorption of a diatomic molecule [32], thus AS = — S 0, 

2 Unimolecular Surface Reaction or Desorption td = kd[A-S] = LAde-E-/RT0A Guideline LAd 10 site (or molecule) cm s or, alternatively per site, Ad s 10 s  

Danner and T. E. Daubert. Manual for Predicting Chemical Process Design Data , Design Institute for Physical Property Data, AIChE, NY, 1983. 

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