Theoretical Studies On The Reaction Mechanism Of NH₃ Catalytic Decomposition And Oxidation On Ir Surfaces

NH₃ is one of atmospheric pollutants which will do harm to the humankinds and the environments, effective and economical steps must be taken to remove or to convert it into friendly or innocuous substances. Hitherto, there are many techniques to be proposed, of which catalytic decomposition and oxidation of NH₃ are the main approches to deal with this noxious substance. High purity H₂ can be obtained by decomposing NH₃, which will benefit to the hrdrogen economy. Selectively to oxidize NH₃ to N₂ and H₂O will eleminate its damage to the nature in which we live. To thoroughly understand all the process underneath, the reaction mechanisms for NH₃ cataltic decomposition and oxidation are technologically and scientifically important and indispensable. As a catalyst, Ir fills the bill to deompose or oxidise NH₃ and transsform it into N₂ and H₂ or H₂O, finding the reaction path at the atomic level is plausible for understanding NH₃ chemistry on the Ir surfaces.In this thesis, catalytic decomposition and oxidation behavior of NH₃ on Ir surfaces have been investigated by using density functional theory and the slab model, the main findings are:(1) On Ir(100) surface, NH₃ is found to predominately bind on the top position through the mixing of the 3a1 orbital and the Ir 5dz2 state, while the mixing of 3a1 and 1b1 orbitals of NH₂ with Ir 5dyz state is responsible for NH₂ anchoring on the bridge site, and the recombination of 3a1 and 1a2 orbitals of NH with the Ir 5dz2 explains the NH sitting on the four fold hollow site. In the stepwise dehydrogenation processes, the activation energies for N-H bond breakage in each step are very closes one another. The energy barrier of the first N-H bond breaking is very close to the NH₃ adsorption energy, indicating the competition between desorption and decomposition of NH₃, NH₂ is at the lowest bottom in the reaction path, signifying the dificulty both for its decomposing to NH+H and recombining with H to form NH₃, NH₂ should be the most stable surface species, its accumulation on the surface will be furture blocking NH₃ decompositon and make the first N-H bond breaking to be the rate-limiting step.(2) On Ir(100) surface, the reaction path, i.e. stepwise dehydrogenation and N-N recombination, is not the unique way for NH₃ decomposition. The combination of the decomposed species such like NH₂, NH and N and then dehydrogenation to form N₂ are also plausible, which inludes the following steps: (3) On Ir(110) surface, the most stable adsorption position for NH₃ is the top site, and the perferred adsorption sites for NH_x (x =1, 2), N, H are all short-bridge site. In NH₃ decomposition process, NH is found to lie down on vally bottom in the reaction path, signifying that NH should be the most stable species on this surface. Much effort should be paid for N-N recombination, implicating that the formtion of N₂ is the rate-determining step. Energy barrier analysis indicates that the geometric deformation of the reactants, the interaction between reactants and surface and the interactions among the species in transition state constitue the main part of the energy barrier while the binding to the surface of the species in transition state is the main factor to reduce the activation energy.(4) On Ir(111) surface, the adsorption mechanism of NH₃ is similar to that on Ir(100) and Ir(110), d-orbitals coorperation for Ir is expected since the lone pair of NH₃ repels the electrons on Ir d states due to the Pauli repausion. NH₂ is found to adsorb on the bridge position through the interactions among the 3a1 and b2 orbitals with, b1 orbital has a weak effect with Ir 5dxz. NH favors the fcc site, the mixing of 1a2 with the Ir 5dxz and 5dyz orbital are observed. More effort should be paid to break the N-H bond of NH₃ than to desorb the NH₃, implying that NH₃ would rather desorb than to decompose. Furthermore, N-N recombination barrier is the highest of all the process considered in this paper. Therefore, NH₃ would rather molecularly adsorb and desorb than to decompose to the derived species as that on Ir(100) and Ir(110).(5) On Ir(111) surface, the preferred co-adsorption state of NH_x(x=0, 1, 2, 3) with O, OH, or H₂O is calculated. The favored co-adsorption site pairs are the top position for OH, H₂O, O perfer on the fcc site. Our results show that the co-adsorption of NH_x with O or OH will enhance the binding to the surface each other, while the co-adsorption of the NH_x with H₂O will decrease their adsorption on the surface. The preadsorbed O and OH will promote the N-H bond cleavage of NH_x (x = 1,2,3), especially the OH will greatly reduce the energy barrier for N-H bond scission.