Employing photoelectron emission microscopy (PEEM) as a spatially resolving method the catalytic reduction of NO with CO and H has been investigated on microstructured composite surfaces in the 10 and 10 mbar range. By deposition of Ti and Rh onto a Pt(100) surface, domains of varying size and geometry are created in which the Pt is either surrounded by an inert Ti/TiO layer or by a reactive Rh layer. For the NO + CO reaction the behaviour of pulses in circular and ring-shaped geometries is studied. It is shown that the pattern forming properties of the Pt(100) substrate are significantly altered by size restrictions. In the NO + H reaction, which was investigated on Pt(100)/Rh microstructures, a strong diffusional coupling between the two metallic substrates occurs. This strong coupling presumably originates from highly mobile adsorbed hydrogen. Pattern formation and front nucleation take place preferentially at the Pt/Rh interface thus suggesting that this region exhibits a particularly high catalytic activity.