The catalytic NO+H-2 reaction on Rh(111) has been investigated in the 10(-6)-10(-5) mbar range using photoelectron emission microscopy as spatially resolving method. Without extended oxygen pretreatment the system is excitable, i.e., one observes spiral waves and target patterns. In addition, irregular islands form. The existence range of the patterns has been mapped out for p(NO)=1x10(-6) mbar. After extended oxygen treatments (p(O2)=2x10(-4), T=770 K, t(ox)>24 h) low work function (WF) areas develop upon the collision of pulses which are assigned to subsurface oxygen (sub-O). A WF decrease up to 0.8 eV below the level of the clean surface is found. The long range interaction of pulses leading to sub-O formation below a distance of approximate to100-150 mum is attributed to diffusing atomic hydrogen.