The catalytic reduction of NO with H-2 or CO and the O-2 + H-2 reaction have been investigated at low pressure (p < 10(-3) mbar) on microstructured bimetallic Pt(100)/Rh and Pt(100)/Ti surfaces prepared by lithographic techniques. Photoemission electron microscopy (PEEM) was the spatially resolving technique used. It is shown that diffusional coupling leads to dynamic effects which are size-dependent and thus can be controlled through the design of the surface microstructure. In connection with periodic parameter forcing these dynamic effects can potentially be exploited to improve the yield and selectivity of catalytic reactions.