Density functional theory (DFT) calculations are performed to compare the dissociation of NH_x (x = 1-3) species on the Pt{100}, Pt{111}, and Pt{211} surface. Pt{211} is a stepped surface, and Pt{100} consists of square arranged surface atoms. Both surfaces are less compact than Pt{111}. The question is addressed whether the Pt{100} and the Pt{211} surface promote the dissociation of NHx species by lowering the activation barriers with respect to Pt{111}. The NH dissociation reaction is promoted on Pt{100} but not on Pt{211}. The NH₂ dissociation reaction is neither promoted by Pt{100} nor Pt{211}. The dissociation of NH₃ is also not promoted and turns out to be a two-step reaction on the platinum surfaces. Atop-bonded NH₂ is intermediate and equally stable on the three surfaces. The nature of the transition states, which is late in the formation and early in the rearrangement of this atop-bonded NH₂, makes the barriers almost independent of the surface topology. Because the reaction energies of the NH_x dissociation reactions do depend on the surface topology the findings are unexpected, but they are consistent with an experimentally found moderate structure sensitivity of the ammonia decomposition on platinum.