Boreskov Institute of Catalysis of the Siberian Branch of Russian ...

OP-I-24NONLINEAR PHENOMENA AND MECHANISMSIN CATALYST DEACTIVATIONN.M. OstrovskiiHIPOL a.d., 25250 Odžaci, SerbiaFax: +381-25-464-702, E-mail: n.ostrovski @ hipol.comThere are several phenomena in catalyst deactivation that are considered as nonlinearones. Those are the volcano-shape of catalyst activity vs. time, the changing of selectivityduring deactivation, the drastic drop of catalyst activity after long stable work, etc.As a matter of fact, only some of them are caused by nonlinear mechanisms ofdeactivation. The others are related to the simultaneous action of several linear processes.In this report, four types of nonlinear deactivation kinetics will be analyzed, caused by:1 – combined effect of deactivation and self-regeneration; 2 – simultaneous formation andpoisoning of active centers; 3 – precursors interaction in adsorbed layer; 4 – ions diffusion incatalyst bulk.Deactivation and self-regenerationDeactivation (r D ) and self-regeneration (r R ) are assumed to be slow steps external tocatalytic cycle (r):ReactionDeactivationr DΘ Dr Rr Θ j71−d ad t=wwDjroa − a1−aSS(1)where r, r D , r R are rates of reaction, deactivation and self-regeneration;Θ j is the intermediate (coverage) participating in the rate limiting step of catalytic cycle;Θ D refers to the coverage excluded from the catalytic cycle due to deactivation;r o is the reaction rate on fresh catalyst; a S is the “residual” activity;w j = r j /Θ j , w D = r D /Θ j are weights of rate limiting step and of deactivation step.According to [1, 2] the general equation for such a reversible deactivation has a form (1)in terms of relative activity a = r / r o = 1–Θ D . The «residual» or «stationary» activity (a S ) isachieved when the rate of deactivation (r D ) becomes equal to the rate of self-regeneration (r R ).When a formal fitting of such deactivation experiments is applied, the nonlinear kineticsis usually required da / dt = – k P a n , at n ≥ 2. Therefore, the apparent nonlinear kinetics ofdeactivation can be caused by self-regeneration phenomenon that accompanied the linearintrinsic mechanisms of reaction and deactivation.«Qualitative» deactivationIn some catalytic processes, the formation of active centers and their deactivation occursimultaneously, during the reaction. Schematically it can be presented as a catalytic cycle onZ - type centers (reaction rate r Z ) with slow deactivation (r D ) and slow transformation (r F ) ofZ to X - type centers (reaction rate r X ):