The first two steps of methane dissociation on Rh(111) have been investigated using density-functional theory, focusing on the dependence of the catalyst's reactivity on the atomic coordination of the active metal site. We find that, although the barrier for the dehydrogenation of methane (CH4 --> CH3 + H) decreases as expected with the coordination of the binding site, the dehydrogenation of methyl (CH3 --> CH2 + H) is hindered at an ad-atom defect, where the first reaction is instead most favored. Our findings indicate that, if it were possible to let the dissociation occur selectively at ad-atom defects, the reaction could be blocked after the first dehydrogenation step, a result of high potential interest for many dream reactions such as, for example, the direct conversion of methane to methanol.