  Surface catalytic processes produce, under certain conditions, small clusters
of adsorbed atoms or groups, called {\em islands} which, after they have been
formed, move as individual entities. Here we consider the catalytic reduction
of NO with hydrogen on platinum. (i) Using video field ion microscopy, we
observe the dynamic motion of small hydroxyl islands on the Pt(001) plane;
despite changes in their morphology, the islands dimensions are confined to
values corresponding to 10 to 30 Pt atoms suggesting cooperative effects to be
in operation. (ii) We construct an automaton (or lattice Monte-Carlo) model on
the basis of a set of elementary processes governing the microscopic dynamics.
The agreement between the simulation results and the experimental observations
suggests a possible mechanism for the formation and dynamics of hydroxyl
islands.
