SPATIAL SELF-ORGANIZATION DURING CATALYTIC REACTION OF CO AND OXYGEN ON A PD(110) SURFACE

The formation of spatial patterns during the catalytic oxidation of CO on a Pd(110) surface has been studied using a photoelectron emission microscope (PEEM). The reaction exhibited both steady state and oscill atory reaction rates over a wide pressure range of reactants ( 10(-6) < P-o2 < 10(-1) torr). The coupling of reaction and surface diffusion of reactants resulted in the formation of spatial structures that were made visible due to differences in the work function of adsorbed CO a nd oxygen. Regions covered by CO had a higher work function, they ther efore appeared darker than oxygen-covered areas. The temporal oscillat ions in the CO2 reaction rate were correlated with alternate switching between the high CO-covered and high oxygen-covered phases. However, in some cases spatial patterns such as waves, target patterns and spir als were formed, while the temporal CO2 reaction rate remained constan t. The effect of the elongation of the structures was attributed to tw o different reaction-diffusion rate processes along and across the [1( 1) over bar0$] troughs. The damping of large amplitude temporal oscill ations synchronized by the gas phase was accompanied by a gradual deve lopment of structural patterns (in this case target patterns). The osc illations of the pacemakers responsible for these target patterns were , for the most part, out of phase.