Catalysts For Ammonia Decomposition: Structure Manipulation And Reaction Mechanism

Ammonia decomposition has been widely applied to the decomposition of NH3 arising from biomass gasification or coal gasification, and the production of hydrogen as clean energy or reducing atmosphere in chemical, steel, glass and electronic industries. Recently, it has been attracted an increasing level of attention due to its use as a COx-free source of hydrogen for proton exchange membrane fuel cells (PEMFCs). However, non-catalytic ammonia decomposition is difficult to take place because of its extremely high activation energy (ca.1172 kJ·mol-1). Hence, an efficient catalyst is crucial to the facile decomposition of ammonia to produce hydrogen. In this thesis, in order to explore the structure-activity relationship, the structure manipulation of Ru-, Ni-, Fe- and CoMo-based catalysts and the reaction mechanism have been studied by combining experimental methods with density functional theory (DFT). In the context, the following aspects were studied as:(1) Ammonia decomposition over nanocarbon supported Ru catalysts. The microscopic properties of nanocarbon supports, such as the microstructures, the surface oxygen-containing groups and the surface defects, have proved to impact on Ru catalytic activity greatly. However, Ru catalytic activity seems unlikely to be affected by the degree of graphitization and the interaction between nanocarbon supports and Ru. Ru catalysts supported on CNFs oxidized by H2O2 and HNO3 exhibit higher activities, because the treated CNFs are abundant of the electron donating groups, the surface defects and the unsaturated carbon atoms. Ammonia decomposition over Ru/CNFs is proved to be a structure sensitive reaction. The highest activity is found over a catalyst with a Ru particle size of～2.2 nm, which just falls into the optimal range of Ru B5 active sites. The activation energy is 185±5 kJ·mol-1, which is close to the energy barrier at Ru step sites (195 kJ·mol-1), and much lower than that at the Ru terrace sites (278 kJ·mol-1). This reaction is assumed to occur at Ru step sites.(2) Ammonia decomposition over Ni-based catalysts. Activity of different Ni surfaces takes on an order of Ni(110)>Ni(111)>Ni(211)>Ni(100), indicating that Ni B5 sites are not active. Nitrogen recombination reactions on Ni(100) and Ni(211) show high activation energies, resulting in N atoms blocking the 4-fold hollow sites of Ni(100) and Ni(211). Structure sensitivity for Ni-based catalysts results from relative quantity of different Ni crystal surfaces. Catalytic activity of Ni-based catalysts is determined by the ratio of Ni terrace sites to Ni step sites. On that basis, Ni-CNFs catalyst with (110) crystal orientation is prepared by a CCVD method using a Ni/Al2O3 catalyst and CH4 as a carbon source, which shows higher ammonia decomposition activity and temperature stability.(3) Tuning the morphologies of Fe particles on carbon nanofibers for ammonia decomposition. For the Fe-CNFs catalyst, the Fe particles on top of CNFs have a wide distribution of particle size and a mean size of 141.4 nm, while for the Fe-PCNFs catalyst, the Fe particles on top of PCNFs have a narrow distribution of particle size and a mean size of 84.5 nm. The Fe particles on top of PCNFs have a polyhedron shape and more facets accessible to the reactant molecules, in contrast with the Fe particles with a cone shape embedded on CNFs. The Fe-PCNFs catalyst shows higher activity than the Fe-CNFs catalyst. Ammonia decomposition over Fe-PCNFs catalysts is proved to be a structure sensitive reaction. The highest activity is found over a catalyst with a Fe particle size of～39.3 nm.(4) The effect of carbon on Fe crystal orientation. For clean Fe surfaces, the relative stability has the order of Fe(110)>Fe(100)>Fe(211)>Fe(111), while when the carbon coverage on Fe surfaces reaches 1/12, the relative stability has the order of Fe(111)>Fe(211)>Fe(110)>Fe(100), suggesting that carbon can enhance the stability of Fe active surfaces. For the first time, Fe particles on top of PCNFs are found to be a Fe(C) superstructure with carbon substituting Fe atom located in body-centered of a-Fe, and are composed of Fe(111), Fe(110) and Fe(C)(001) crystal faces. This indicates a remarkable crystal orientation of Fe. Fe-PCNFs catalysts have higher activities than commercial catalysts and the reported Fe-based catalysts, which are ascribed to that of crystal orientation of Fe particles and the effect of carbon of Fe(C) superstructure on Fe-PCNFs.(5) Ammonia decomposition over Co-Mo bimetallic catalysts. Two kinds of CoMo-based catalysts, CoMoI/γ-Al2O3 and CoMoII/γ-Al2O3, are prepared using one component (Co(en)3MoO4 (ethylenediamine, en)) and bi-component (Co(NO3)2 and (NH4)6Mo7O24) as the Co-Mo precursors, respectively. The CoMoI/γ-Al2O3 catalyst shows higher activity than the CoMoII/γ-Al2O3 catalyst, which is due to a higher alloying extent and more reducible active components in CoMoI/γ-Al2O3. Moreover, the activities of MCM-41 supported Co-Mo bimetallic catalysts are sensitive to the Co/Mo molar ratio, and the Co7Mo3/MCM-41 catalyst presents the highest activity.