Desorption activation energies, associated with

The classical approach for discussing adsorption states was through Lennard-Jones potential energy diagrams and for their desorption through the application of transition state theory. The essential assumption of this is that the reactants follow a potential energy surface where the products are separated from the reactants by a transition state. The concentration of the activated complex associated with the transition state is assumed to be in equilibrium... 

K on the W(IOO), (310), and (210) planes, respectively (equivalent to desorption activation energies of approximately 314 kJ moP, although adsorption does not apparently occur on the close-packed planes such as W(llO). The high heats of adsorption are associated with high dissociative sticking probabilities of between 0.24 and 0.37 at low coverages. 

If r0 and m are known quantities, the activation energy for desorption may be simply determined from the temperature, Tp, at which the maximum rate of desorption occurs (117). For associatively adsorbed CO the reaction order for desorption may be safely assumed to be one and frequently vo = 1013 sec-1 is assumed to be a reasonable value. If the resulting data for d are compared with values for the isosteric heats of adsorption a (these should be equal since the kinetics of adsorption is nonactivated), very often deviations by several kcal/mol occur (91) that indicate the weakness of this assumption. More sophisticated techniques for analyzing thermal desorption spectra (118-121) allow the independent determination of both parameters, v and d. The results demonstrate that vQ may deviate considerably from 1013 sec-1. For example, for the system CO/Ru(001) Menzel et al. (122) came to the conclusion that v0 may reach values up to 1018 sec-1, whereas a rather small number of 1011 sec-1 was derived by Weinberg et at. (76) for CO desorption from an oxidized Ir(l 10) surface. An additional complication arises from the fact that analysis of thermal desorption spectra on the basis of (4) may yield misleading results if desorption takes place via transition to a precursor state (102). which may be the case for adsorbed CO. 

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