Transition Metal Based Materials As Active And Stable Catalysts For CO_x-Free Hydrogen Production From Ammonia

There is widespread consensus that fossil fuel reserves, especially oil reserves, will be exhausted to a large extent in the course of the current century, possibly leading to shortages relatively soon. For many years, hydrogen has been discussed as one potential solution due to its cleanly, high-efficiency, storable and transportable properties. Up to now, there are still many problems hinders the broad applications of hydrogen and fuel cells although scientists have gained a series of important breakthroughs. Especially providing cheap and practical hydrogen source is one of the key technologies that have been paid more attentions. On the other hand, ammonia has been considered as an excellent hydrogen carrier due to its unique properties. Ammonia provides a high hydrogen storage capacity and also a high energy density compared with traditional carbonaceous materials. In addition, ammonia can be easily liquefied under moderate conditions, getting transportation and storage more readily handled. The decomposition of ammonia is an efficient way to generate COx-free hydrogen. Raising interest in ammonia decomposition is motivated by the necessity for highly pure hydrogen for proton exchange membrane fuel cells because CO, even in small amounts, would be poisonous to fuel cells.Hence, the effective release of hydrogen from ammonia via catalytic decomposition is of both fundamental and practical importance. Currently, the research on ammonia decomposition catalyst is mainly focus on noble metals and transition metal catalysts. It has been known that noble metals generally exhibit high activity for catalytic ammonia decomposition. However, ammonia decomposition over various supported noble metals has the disadvantage of high costs, poor temperature stability, and difficulties for large-scale applications. Transition metal catalysts also exhibit good performance for ammonia decomposition. Nevertheless, most of the presently studied transition metal catalysts are supported materials, which limit the amount of active species that can be loaded, and thus result in a low activity for ammonia decomposition. Moreover, in the previous literatures, researchers put more emphasis on the selection and preparation of catalyst systems, while that on the identification of active species during reaction process was inadequate.In this dissertation, novel ammonia decomposition catalysts based on transition metals were designed and synthesized. Substance with good thermal stability was used to stabilize the active nanostructures and transition metal catalysts with high active species content were synthesized by facile methods. The catalytic behavior of these transition metal based composites was investigated. Our aim is to establish the dependence of catalytic activity and stability on the relative content of stabilizing material and active component, element distribution and microstructures. In order to understand the catalysis mechanism and further guide the design of ammonia decomposition catalysts, in situ XRD measurements were performed to monitor phase transformations during the reaction conditions and thus the active species of the catalysts was identified. We believe our research is helpful to expand the new design and/or synthesis routes towards highly active and thermally stable catalytic materials on other heterogeneous catalysis reactions. There are three main experimental studies in this dissertation as follows:1. Transition metal (Fe, Co, and Ni) nanoparticles dispersed in an alumina matrix as catalysts for NH3 decomposition have been synthesized by a facile co-precipitation method. The fresh and used catalysts were characterized by various techniques including powder X-ray diffraction (XRD), N2 adsorption/desorption, and transmission electron microscopy (TEM). Also, temperature-programmed reduction by hydrogen (H2-TPR) combining in situ XRD was performed to investigate the reducibility of the studied catalysts. For the ammonia decomposition reaction,88% conversion of ammonia can be realized at the reaction temperature as low as 600℃ using a space velocity of 72000 cm3 gcat-1 h-1 NH3 during a long term (72 h) catalysis test without any observable deactivation. The small amount of alumina (low to 10 at%) can act as the matrix in which the catalytically active transition metal nanoparticles were stabilized. Thus, the agglomeration of active transition metals under reaction conditions was significantly suppressed and the high activity of catalysts was maintained.2. Novel Co-Al nanocomposite catalysts with different cobalt amounts have been successfully synthesized via a one-pot evaporation-induced self-assembly method. A small amount of Al (ca.10 at.%) can significantly improve the stability of the Co catalyst via suppressing agglomeration. The as-obtained catalysts displayed very high activity and stability for the NH3 decomposition reaction at a space velocity as high as 36000 cm3 gca-1h-1. About 37 mmol gcat-min-1 constant H2 production rate was maintained even after 120 h at 600℃ without any deactivation of the 90CoAl catalyst. The in situ XRD experiments showed that the stepwise reduction of the cobalt aluminum spinel (Co,Al)(Co,Al)2O4 to CoO and metallic Co is significantly influenced by the Co content. With increasing cobalt content, the starting temperature of the reduction of (Co,Al)(Co,Al)2O4 to CoO is decreased by almost 100℃. From the combination of the in situ XRD measurements with the catalytic tests, metallic Co (cubic phase) was identified to be most probably the active phase for the decomposition of ammonia; also contributions of CoO to the catalytic activity cannot be excluded. However, growth of the crystallites during long term stability test shows that the activity is also negatively influenced by crystal growth. To our knowledge, this is the first study by in situ XRD to identify the active phase in Co-based catalysts on the ammonia decomposition reaction.3. Carbon nanosheets embedded with cobalt and aluminum species have been synthesized by a one-step liquid crystal templating method. The nanosheets structure were obtained by exfoliating layered carbon intermediate which originated from carbonization of lamellar liquid crystals self-assembled by block copolymer molecules. The carbon nanosheet was high graphited with aluminum species highly dispersed in it and the uniform-sized cobalt nanoparticles homogeneously embedded in the carbon and aluminum matrix. This structure can effectively prevent the active Co species from sintering and guarantee the obtained nanocomposite highly active and stable catalyzing ammonia decomposition. The Co-Al@C catalyst realized complete NH3 decomposition at 500℃ with a GHSV of 12000 cm3 gcat-1h-1 and 96% conversion at 600℃ with a GHSV of 76000 cm3 gcat-1h-1. The high-temperature stability of the catalyst was also outstanding with no observable deactivation during a long term (144 h) catalysis test. The activity of this Co-Al@C catalyst was the best reported catalytic result of transition metal based catalysts for ammonia decomposition, and even comparable to some Ru-based catalysts. Combined the facile, inexpensive and scalable synthetic method, we believe that the Co-Al@C nanocomposite in this work can not only as a prospective candidate for replacing noble metals in the application of COx-free H2 production from ammonia but also be applied to many other promising applications in future.