A REACTIVE PHASE-DIAGRAM OF CO OXIDATION ON PD(110) - STEADY AND OSCILLATORY STATES

The steady and oscillatory regions of the CO oxidation reaction on the Pd (110) surface have been determined as a function of externally controlled parameters (flow rate, CO and oxygen partial pressures, temperature) over a wide range. At constant sample temperature and flow rate, the experiments yield a characteristic cross-shaped phase diagram separating regions of monostability, bistability, and oscillatory behavior. The existence of a cross-shaped phase diagram indicates the operation of a slow feedback process, which could be traced back to the (experimentally verified) formation and retarded removal of subsurface oxygen during the reaction. The diagram reflects one of the first well-defined oscillatory systems in heterogeneous catalysis and may provide a general basis for mechanistic studies and models of oscillatory surface reactions.