The Pt and Ir surfaces are known to provide charactersitic superstructures by changing the atomic arrangement although these metals have only one crystalline form in the bulk. So far, such structural rearrangement has been accepted as a two dimensional phase change. In this review, it is discussed from a view of morphological change of the crystal. Metal surface has a potentiality for the synthesis of novel compounds, and in fact some novel compounds have been prepared on these metal surfaces by performing catalytic reactions. Surface nitride on Pd (100) is an example of a novel compound which is prepared by a catalytic reaction of NO and H2. The nitride has a c (2×2) strucrture and is a reactive intermediate giving ammonia. A hexagonal OH overlayer grown on Ni (100) surface during the reaction of H2 and O2 is also a novel compound, which is an intermediate for water formation reaction. These are the first examples showing that the intermediates have ordered structures, and this fact might contradict the reaction mechanisms based on random reaction of adsorbed species.
