The exploration of pattern formation by reaction-diffusion systems in complex bounded domains has begun only recently. While theoretical and numerical information points to a strong interaction between patterns and boundaries, experiments are rare and for heterogeneous catalytic reactions practically nonexistent. By constructing (using microlithography) catalytic surfaces of arbitrary shape acid size, we are able to study this interaction for the catalytic oxidation of CO on Pt(110). Experiments along these lines shed light on issues such as anisotropic diffusion and the behavior of individual defects. In addition, certain geometries give rise to patterns that have not been observed on the untreated catalyst and bring to light surface mechanisms that have no analog in homogeneous reaction-diffusion systems. Simple domains of controlled size constitute paradigms that make the comparisons between theory and experiment more fruitful, as we demonstrate through modeling and simulation of such examples. This approach opens the way for systematically probing certain aspects of pattern formation unique to heterogeneous catalysis.