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

OP-I-24Θ DZ centersdeactivationr D1r X centersZr XX centersformationdeactivationΘ Z Θr FXr D2ΘD(2)Such a «qualitative» deactivation leads to the volcano–shape nonlinear dependences ofactivity vs. time. Nevertheless, the mechanisms and intrinsic kinetics of each of threeprocesses in scheme (2) can be linear.Interaction in adsorbed layerMost of nonlinear deactivation mechanisms can be reduced to binary interactions in theadsorbed layer, because of a very low probability for more than two species to interact on thesurface. In this case, only three types of nonlinear binary interactions can be formulated:Double-center adsorptionA PZ Z → Z Z1 molecule of precursorpoisons 2 active centers2 Z → 2 PZSurface dimerizationA B P PZ + Z → Z+ Z2 molecules of precursorpoisons 1 active center2 Z → 1 PZSurface disproportionationA B P CZ + Z → Z + Z1 molecule of precursorpoisons 1 active center2 Z → 1 PZZ – active center, AZ, BZ – precursors of poison or coke, PZ – blocked center.General deactivation equations for these schemes take following nonlinear forms [1]:dao o 2 dao o− = wD( ΘiΘ j ) a , − = 2 wD ( ΘiΘ j ) a a(3)dtdtDiffusion of ions in catalyst bulkThe mobility of ions and atoms in solid catalysts is widely observed and can influence thecatalytic reaction significantly. It concerns the mobility (diffusion) of oxygen ions in metaloxides [3], of chlorine in metal chlorides [4], etc. Since ions diffusion in bulk is caused byreaction on the surface, it affects the state of surface, and consequently affects the reactiondynamics. Because the capacity of bulk is much higher than that of the surface, ions mobilityincreases the working period of the catalyst (decreasing its deactivation). It makes acceptablethe non-stationary cyclic regime of the process with reaction and catalyst recovery periods.Such a regime provides usually higher selectivity than the stationary one in oxidationprocesses. In other processes it can increase the «catalyst life», or «poison resistance».References1. Ostrovskii N.M. – Catalyst deactivation kinetics (in Russian) – Moskva: Nauka, 2001, 334 p.2. Ostrovskii N.M. – Chem. Eng. Journ., 2006, v. 120, No. 1-2, p. 73-82.3. Ostrovskii N.M., Reshetnikov S.I. – Chem. Eng. Journ., 2005, v. 107, No. 1-3, p. 141-146.4. Ostrovskii N.M., Kenig F. – Chem. Eng. Journ., 2005, v. 107, No. 1-3, p. 73-77.72