Microstructured bimetallic surfaces prepared by optical lithography allow us to study dynamic effects under low pressure conditions with spatially resolving in situ techniques such as photoelectron emission microscopy (PEEM) and scanning photoelectron microscopy (SPEM). Reactions of the automotive catalytic converter on microstructured Pt/Rh surfaces have been investigated. The following effects have been observed: macroscopic size effects in reactivity due to diffusional coupling, front nucleation at Pt/Rh domain boundaries and the formation of Stationary concentration patterns due to reversible adsorbate-induced segregation in bimetallic alloyed films. The experiments are complemented by simulations of the reactions for composite media aimed at "microchemical engineering" of catalytic surfaces.