Core-shell Structured Nanoparticles And Their Application In CO_x-free H₂ Production Via NH₃ Decomposition

The on-site generation of hydrogen is essential for proton-exchange membrane fuel cells (PEMFCs). The hydrogen production directly from carbonaceous compounds such as methanol and methane has its limitations because the COx (X=1,2) side products degrade the cell performance even at extremely low concentrations. Alternatively, the direct generation of COx-free hydrogen through catalytic ammonia decomposition has been considered. Many kinds of catalytic materials have been studied for this purpose. Ru and Ni are found to be the most active among the noble and cheap metals, respectively.Core-shell structured materials have attracted great attentions in recent years because of the unique structural feature and physicochemical properties. Enwrapping a nano-material in a stable shell can enhance the stability of the core material and in addition, cause a change in morphology, size, electron charge, reactivity and functionality of the enwrapped material.The application of NPs in heterogeneous catalysis is highly desirable due to the intrinsic "surface effects". Unfortunately, NPs are unstable and aggregate easily at elevated temperatures. The aim of the present thesis study is to investigate the feasibility of retaining the intrinsic character of NPs by having them encapsulated inside a porous but stable shell, such as SiO2, Al2O3, and MgO.In the present study, the core-shell nanostructures (nano-M@SiO2, Al2O3, MgO;M=Fe, Ni, Ru) were synthesized as the novel catalysts for the production of COx-free H2 through NH3 decomposition. It was found that the core property is closely related to the core constitution and its starting material as well as the relative amount of shell to core, while the shell encapsulation effect (thickness of shell, single or hierarchical enwrapping) is dependent upon the core nature as well as the core-shell interaction. The obtained core-shell catalysts show superior activity and stability to the naked cores or the supported counterparts. The core-shell structured Ru@SiO2 is known to be the most active catalyst for NH3 decomposition.In addition, the doping of La, Ce and other elements to the core-shell structured nano-M@SiO2 (M=Fe, Ni, Ru) has been systematically investigated. Two approaches, namely, pre-impregnation of core metal oxide precursor NPs and post-impregnation of core-shell structures have been adopted for dopant introduction. The former method is more effective for catalyst modification. Acid pre-treatment of core-shell structures can modify the characteristics of core-shell catalysts and improve the introduction of dopant. It was found that the La and Ce doping caused decrease in surface area and that there was a higher fraction of La dopant located in the pores of SiO2 shells. The doped CeO2 and La2O3 species are in crystalline and amorphous state, respectively. The dispersion of Ni cores was enhanced with La and Ce addition. Re-distribution of Ni NPs could occur in the hierarchical SiO2 shells during the H2-reduction and the following reaction. It was also found that the La-and Ce-introduction can notably promote the reduction of NiO cores, and such effect is also related to the dopant content. The La- and Ce-doped nano-Ni@SiO2 catalysts are the most active among the Ni-based catalysts for ammonia decomposition. The promotion effect is related to the changes in morphology and chemical nature of Ni core particles induced by the Ni-La2O3/CeOx interaction. Introduction of La, Ce to nano-Ru@SiO2 does not show the promotion effect on catalyst activity, and doping of Mo、K、Ba elements to the core-shell catalysts shows negative effect on catalyst performance.The core-shell structured NPs can provide the unique environment around cores and function as microcapsular-like reactors in which the reactant molecules can be enriched. The consequence is enhanced adsorption and reaction on the core, and higher catalytic activity. Such effect is significant for the core-shell structured catalysts; this is because although the measured surface metal exposure of cores per unit mass of catalyst is lower than that of the conventional supported metal particles, the core-shell catalysts are notably more active than the supported catalysts. Furthermore, the core-shell catalysts also show better stability than the naked or supported NPs owing to the stable shells that effectively prevent the core NPs from aggregation during reaction.