THE ROLE OF ADSORBATE-ADSORBATE INTERACTIONS IN THE RATE OSCILLATIONS IN CATALYTIC CO OXIDATION ON PD(110)

The CO+O-2 reaction on Pd(110) exhibits kinetic oscillations above p(O2) approximate to 10(-3) Torr and bistability below this pressure. Based on the reversible formation of subsurface oxygen and the Langmuir Hinshelwood mechanism of catalytic CO oxidation, a mathematical model had been developed which described the occurrence of rate oscillations and most of the qualitative features of the oscillations. This model, however, failed to reproduce the change from bistability to oscillatory behaviour with increasing p(O2). In this paper we demonstrate that by introducing repulsive interactions between COad and O-ad, the subsurface oxygen model correctly reproduces the experimentally determined stability diagram in p(O2), p(CO) parameter space. The effect of the repulsive interactions is to reduce the activation barrier for penetration of chemisorbed oxygen into the subsurface region, thus facilitating the formation of subsurface oxygen at high coverages. For the improved subsurface oxygen model a bifurcation analysis has been conducted in p(O2), p(CO) parameter space. The influence of the constants in the model has been analyzed likewise with bifurcation theory.