The catalytic O-2+H-2 reaction on Rh(1 1 1) has been investigated in the 10(-6)-10(-5) mbar range using photoelectron emission microscopy as spatially resolving method. While the reaction without pretreatment of the sample displays simple bistable behavior, we find that after extended pre-oxidation of the sample (p(O2) = 2 x 10(-4) mbar, T = 770 K, t(OX) > 24 h), low work function (LWF) areas develop dynamically in the collision of reaction fronts. The LWF areas have been assigned to subsurface oxygen. We present a simple three-variable model which reproduces the formation of LWF areas in the collision of reaction fronts.