The Ammonia Decomposition Mechanism On The Nickel Supported Monolayer Surfaces And The Catalyst Design

NH3 is a hydrogen- rich poisonous compound. It is very important to transform NH3 into N2 and H2 to reduce the environmental pollution and the hazard to human health.In addition,the generated H2 can be used as a high purity source for fuel cell.There are many ways to the transformation and usage of NH3,in which the catalytic decomposition of NH3 is currently the focus of research.The main research in this field include two parts.The first is the designing of catalyst,the second is the NH3 decomposition mechanism.In this thesis,we present two primary investigation.This article is based on the ensity functional theory and periodical slab model.In the first phase,we present designed Pt-supported Ni monolayer surface and the results of the research about the electronic structure,the NH3 decomposition mechanism.In the second phase,a Ti C supported Ni-monolayer surfaces is designed and studied as a catalyst for NH3 decomposition.The main conclusion are summarized as follows:(1) The electronic structure of Pt-supported Ni monolayer surface Ni@Pt(111) is calculated with Ni(111) and Pt(111) surface.The results indicate that the relexation behaviour of the supported Ni-monolayer is evidently different from the pure metals.The surface stretching tension due to the lattice mismatch leads to the localization of Ni d state.The interaction between top layer and the subsurface in Ni@Pt(111) is stronger than these in Ni(111) and Pt(111) surfaces.The Ni adsorption energy on Pt(111) is larger than the metal atom adsorption energy of Ni(111) and Pt(111) surfaces,which is helpful to the adsorption of NH3.The surface workfunction of Ni@Pt(111) is larger than Ni(111) and Pt(111)surfaces,which facilitate the adsorption of electronic doner compounds.Therefore,the workfunction can be used as a descriptor to guide the design of bimetallic catalysts.(2) On Ni@Pt(111) surface, NH3 adsorbed on the top site through the mixing of 3a1 orbital and the Ni s and dz2 orbital. NH2 adsorbed on the bridge position through the mixing of 3a1 and b2 orbital with the Ni s and dxz orbital.NH prefers the fcc site due to the mixing of 2a1 and 3a1+a2 orbital with the Ni s and dxz.N and H prefers the fcc site. Stepwise dehydrogenation of NH3 on the surface is calculated,The N-H bond is easy brokened in NH2 and hard scission in NH.NH is the main surface intermediate.The N-N recombination,possesses the maximum adsorption energy,is the reaction-limited step.(3) The detailed and possible decomposition mechanism of NH3 on Ni@Pt(111) surface is studied.Except for the stepwise dehydrogenation,The other decomposition mechanism studied include mainly the following four paths:NH3+NHâ†'NH2+NH2â†'N2H4â†'N2H3+Hâ†'N2H2+2Hâ†'N2H+3Hâ†'N2+4HNH3+Nâ†'NH2+NHâ†'N2H3â†'N2H2+Hâ†'N2H+2Hâ†'N2+3HNH+NHâ†'N2H2â†'N2H+Hâ†'N2+2HNH+Nâ†'N2Hâ†'N2+H The results indicate the recombination of NH2 and NH to N2H3 and the stepwise dehydrogenation is plausible for NH3 decomposition.(4) The Ti C supported Ni monolayer surface is designed and used to decompose the NH3..The results show whether on C-terminated or Ti- terminated Ti C(111) surfaces(labeled as Ti C-C(111) and Ti C-Ti(111)respectively),Ni prefers the fcc site on Ti C-C(111) and Ti C-Ti(111), NH3 and NH2 perefer the top and bridge sites respectively,NH adsorbs at hcp site on Ni@Ti C-Ti(111) and fcc site on Ni@Ti C-C(111)respectively.The catalytic performance of Ni@Ti C-Ti(111) for NH3 stepwise dehydrogenation is superior to Ni@Ti C-C(111) surface. Ni@Ti C-Ti(111) can be used as a economic catalyst for NH3 decomposition.