AVS 52nd International Symposium
    Surface Science Monday Sessions
       Session SS1-MoA

Paper SS1-MoA7
First Principles Study of Factors Controlling the Rate of Ammonia Decomposition on Ni and Pd Surfaces

Monday, October 31, 2005, 4:00 pm, Room 202

Session: Catalysis for the Hydrogen Economy
Presenter: S. Stolbov, Kansas State University
Authors: S. Stolbov, Kansas State University
T.S. Rahman, Kansas State University
Correspondent: Click to Email
Using the plane wave pseudopotential method within the density functional theory with the generalized gradient approximation for exchange and correlation potential, we have calculated adsorption energies (Ead), diffusion barriers and the first dissociation barriers (E1) for NH3 on Ni(111), Pd(111), Ni(211), and Pd(211). While the top site is found to be preferred for NH3 adsorption on both Ni(111) and Pd(111), the calculated diffusion barrier is substantially higher for Pd(111) than for Ni(111). We also find that during the first dissociation step (NH3 => NH2 +H) on both surfaces NH2 moves from the top site to the nearest hollow site, while on the stepped surface (211) it moves from the initial top site at the step edge to the bridge site in the same step chain. H is found to occupy the hollow sites for all four surfaces. For the reaction on Ni(111), E1 is found to be 0.23 eV higher than Ead, while at the step of Ni(211), E1 and Ead are almost equal. This suggests that the molecule would rather desorb than dissociate on Ni(111), and dissociate on the stepped surface. On the other hand in the case of Pd surfaces, we find that the dissociation barrier is much higher than Ead, even for the stepped surface. This may explain why ammonia decomposition rate on Pd is much lower than that on Ni.