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 con trolled parameters (flow rate, CO and oxygen partial pressures, temper ature) over a wide range. At constant sample temperature and flow rate , the experiments yield a characteristic cross-shaped phase diagram se parating regions of monostability, bistability, and oscillatory behavi or. The existence of a cross-shaped phase diagram indicates the operat ion of a slow feedback process, which could be traced back to the (exp erimentally verified) formation and retarded removal of subsurface oxy gen during the reaction. The diagram reflects one of the first well-de fined oscillatory systems in heterogeneous catalysis and may provide a general basis for mechanistic studies and models of oscillatory surfa ce reactions.