(429f) Transient Interplay of Catalytic Cycle and Deactivation

The dynamic behavior of the complex catalytic gas-solid reaction accompanied by catalyst deactivation, reversible and irreversible, was studied. In this case, two “small parameters” determine the temporal properties of the system:

• the small parameter caused by the difference between number of catalyst active sites and number of gaseous molecules
• the small parameter caused by the difference between the deactivation parameters and parameters of the main catalytic cycle

Different scenarios of transient interplay between the main cycle relaxation and deactivation dynamics are presented. Using the semi-phenomenological three-factor kinetic equation of catalyst deactivation previously obtained, different domains of the quasi-steady-state behavior have been distinguished. The developed approach is applied to the classical two-step mechanism by Temkin-Boudart accompanied by irreversible deactivation for typical kinetic devices and industrial reactions.

In the typical situation, there was found the relationship for the total reactant consumption during the complex catalytic reaction over the deactivated catalyst (the case of small conversions):

A_lim = lim_{t→∞} A(t) = N_s (1 - (1/K)) (k_app/k_irr)

where A(t) is the integral reactant consumption per the catalyst unit (mol cm^-2 or mol g_cat^-1) at time t; A_lim is the total reactant consumption per the catalyst unit (mol cm^-2 or mol g_cat^-1); N_s is the total number of active sites (mol cm^-2 or mol g_cat^-1) of the fresh catalyst; K is the apparent equilibrium constant of the main catalytic reaction); k_app is an apparent parameter (s^-1) of the main catalytic cycle; k_irr is an apparent parameter (s^-1) of the irreversible deactivation. Therefore, the total consumption of the reactant is proportional to the number of active sites of the fresh catalyst multiplied by the ratio of the apparent parameters of the main cycle and the deactivation process. This ratio (‘turnover number’) illustrates the competition between the main catalyst cycle and deactivation process for some active intermediate.