Development Of Highly Efficient Non-Noble Catalysts For Ammonia Decomposition And Structure-Performance Relationship

Catalytic decomposition of ammonia has long been actively studied in academia and industry, because of not only being the model system for the fundamental understanding in catalysis, but also having the wide (potential) applications in energy and environmental industries. A volcano-type relationship between N binding energy of the catalyst and ammonia decomposition activity would encourage us to develop low-cost alternatives or catalysts with the activities near to or even exceeding noble Ru with the optimal N binding energy. Tuning the shape, crystal facet, crystal phase and/or composition of metal particles could be an efficient method to change the electronic and structural properties and as a result obtain the optimal N binding energies, aiming for the synthesis of highly efficient catalysts. In this thesis, we focused on the study of effects of parameters of catalytic chemical vapor deposition methods on the manipulation of the morphology of Ni and Fe particles, and investigated the effects of catalyst precursor type, calcination atmosphere, nitridation temperature on the performance of Co-Mo bimetallic catalysts for ammonia decomposition in order to correlated the relationship between the catalyst structure and the catalytic performance.(1) Effects of carbon source on Ni particle morphology and their catalytic performance. Ni particles at the tips of CNFs were obviously influenced by the type of carbon source:no obvious Ni particles can be observed when using C2H4as carbon source; Ni particles are at the tips of CNFs with double cone-polyhedral shape and pear shape when using CH4and CO as the carbon soure, respectively. Owing to the more exposed active facets, polyhedral-shpaed Ni catalysts are more active then pear-shaped Ni catalysts even having larger particle size.(2) Effects of partial pressure of CH4on Ni particle morphology. Effects of partial pressure of CH4on the morphology of Ni particles were studied by fixing the carbon souce, i.e., CH4. The reshaping mechanism of Ni paricles during CCVD process was also investigated by combining DFT calculations. The results showed that high partial pressure of CH4lead to the formation of Ni particles with polyhedral shape and more exposed active facets (e.g.,(111) while favor the formation of Ni particles with pear shape. The extremely high matching degree between the Ni(111) and graphene layer may mainly attributed to the more exposed (111) facet of polyhedral Ni catalysts, though Ni(110) is more favorable to form as the increase of C coverage on the surface of Ni particle.(3) Ammonia decomposition over structured Fe catalsyts. The structured Fe-CNFs/CMFs catalysts were prepared using Fe/CMFs as the catalyst. The effects of partial pressure of CO on the morphology of Fe particles at the tips of CNFs were studied. It is found that the particle size of Fe particles decreased as the growth time of CNFs increased. Fe particles were semi-spherical with crystal phase of Fe3C when the growth time was6h. Fe particles were polyhedral-shaped with crystal phase of a-Fe(C) when under synthetic condition of CO/H2=20/80and growth time of6h. Furthermore, by using DFT calculations, we studied the effect of C coverage on the reconstruction of Fe facets, and the mechanism of ammonia decomposition over Fe facets incorporated by surface or subsurface carbon. The results showed that the adsorbed C can induce the reconstruction of Fe facets. The ratio of active facets in the bulk increased with the C coverage. In addition, surface or subsurface carbon incorporated on the active Fe facets can lower the activation energy of N recombinative desorption, and thus enhance the ammonia decomposition activity.(4) Ammonia decomposition over unsupported Co-Mo bimetallic catalysts. The effect of calcination atmosphere and nitridation temperature of unsupported Co-Mo bimetallic catalysts prepared by direct calcination of Co(en)3MoO4on their catalytic performance of ammonia decomposition were studied. It is indicated that the calcination atmosphere greatly influences the microstructures and the crystal phase compositions of the calcined samples. Due to higher surface area and more active species (e.g., Co3Mo3N), the CoMo-Air-R exhibits higher stable activity and undergoes a shorter induction period than CoMo-Ar-R. High nitridation temperature was found to largely increase NH3conversion.(5) Ammonia decomposition over supported Co-Mo bimetallic catalysts. The effects of metal precursor types, calcination atmosphere and calcination temperature on catalytic performance of supported Co-Mo bimetallic catalysts were studied. It is demonstrated that Co-Mo bimetallic catalysts prepared using mono-component Co(en)3MoO4showed better synergistic effect than those prepared using bi-component:Co(NO3)2and (NH4)6Mo7O24) as the precursor. Moreover, Co-Mo bimetallic catalysts prepared using Ar as the calcination atmosphere and at high calcination temperature exhibited higher ammonia decomposition activity, which may be due to the more active Co3Mo3N on the surface.