Ammonia Dissociation on Pt{100}, Pt{111}, and Pt{211}:  A Comparative Density Functional Theory Study

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 NH x 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.