Ammonia dissociation on Pt{100} Pt{111}, and Pt{211}: A comparative density functional theory study

D. functional theory (DFT) calcns. are performed to compare the dissocn. of NHx (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 dissocn. of NHx species by lowering the activation barriers with respect to Pt{111}. The NH dissocn. reaction is promoted on Pt{100} but not on Pt{211}. The NH2 dissocn. reaction is neither promoted by Pt{100} nor Pt{211}. The dissocn. 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 topol. Because the reaction energies of the NHx dissocn. reactions do depend on the surface topol. the findings are unexpected, but they are consistent with an exptl. found moderate structure sensitivity of the ammonia decompn. on platinum