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Abstract

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 oscillatory 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 and oxygen. Regions covered by CO had a higher work function, they therefore appeared darker than oxygen-covered areas. The temporal oscillations 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 spirals were formed, while the temporal CO2 reaction rate remained constant. The effect of the elongation of the structures was attributed to two different reaction-diffusion rate processes along and across the [110] troughs. The damping of large amplitude temporal oscillations synchronized by the gas phase was accompanied by a gradual development of structural patterns (in this case target patterns). The oscillations of the pacemakers responsible for these target patterns were, for the most part, out of phase