OP-II-3

OP-I-6TRENDS IN BISTABILITY DOMAINS OF CO OXIDATION ONTRANSITION METALS CALCULATED FROM FIRST PRINCIPLESSheintuch M., German E., Nekhamkina O.Dept. of Chemical Engineering, Technion-I.I.T., Haifa, 32000cermsll@tx.technion.ac.ilA century of research into CO oxidation revealed a plethora of uniquephenomena, but still many questions remain open in deriving a predictive kineticmodel for the most popular catalysts. The main qualitative phenomena associatedwith this and similar reactions are the negative-order kinetics, multiple solutions(bistability), oscillatory behavior and spatiotemporal patterns. It is important,therefore, to suggest a methodology for finding its kinetic parameters and overall ratefrom first principles, when possible.We consider two models of CO oxidation on group VIII metals: both include COadsorption and desorption and surface reaction:k1k3CO + σ ↔ COs; COs+ Os⎯⎯→ CO2+ 2k−1σ(1)While for oxygen adsorption we use either a two-step indirect (a) or direct (b) model:k2 Mktk22+ σ ↔ 2s 2s + σ ↔ sb2+ σ ↔ s (2)k−2 Mk− t = 0 k−2= 0(a) O O ; O 2 O ; ( ) O 2 2 OThe first model can be reduced approximately to the second one which is betteramenable to analytical treatment and predicts a rate (R CO ) vs. CO partial pressure(P CO ) diagram as a clockwise hysteresis loop (Fig. 1). Bistabilty has been observedand modelled for many catalytic reactions on either single crystals of Pt, Pd, Ir underhigh vacuum conditions using molecular beam or in atmospheric-pressure studies onpolycrystalline Pd and Pt.In the present study we employ our previous results for the elementary steps rateconstants of the models described above. Three of them, the rate constants of COoxidation (k 3 ), of O 2 dissociative adsorption (k t ) and of CO diffusion constant (k A,dif )were calculated using the analytical approach searching for the saddle point (SP) onsemi-empirical adiabatic potential energy surface (APES)[1-4]; other rate constantswere estimated using the standard theory of rate processes. The APES isconstructed from two diadiabatic surfaces U i and U f characterizing the potential47