HYDROGEN GENERATION FROM HYDROUS HYDRAZINE DECOMPOSITION OVER SOLUTION COMBUSTION SYNTHESIZED NICKEL-BASED CATALYSTS KangWooram 2019 <div>Hydrous hydrazine (N<sub>2</sub>H<sub>4</sub>·H<sub>2</sub>O) is a promising hydrogen carrier for convenient storage and transportation owing to its high hydrogen content (8.0 wt%), low material cost and stable liquid state at ambient temperature. Particularly, generation of only nitrogen as byproduct, in addition to hydrogen, thus obviating the need for on-board collection system for recycling, ability to generate hydrogen at moderate temperatures (20-80 °C) which correspond to the operating temperature of a proton exchange membrane fuel cell (PEMFC), and easy recharging using current infrastructure of liquid fuels make hydrous hydrazine a promising hydrogen source for fuel cell electric vehicles (FCEVs). Since hydrogen can be generated from catalytic hydrazine decomposition, the development of active, selective and cost-effective catalysts, which enhance the complete decomposition (N<sub>2</sub>H<sub>4</sub> → N<sub>2</sub>+2H<sub>2</sub>) and simultaneously suppress the incomplete decomposition (3N<sub>2</sub>H<sub>4</sub> → 4NH<sub>3</sub>+N<sub>2</sub>), remains a significant challenge.</div><div>In this dissertation, CeO<sub>2</sub> powders and various Ni-based catalysts for hydrous hydrazine decomposition were prepared using solution combustion synthesis (SCS) technique and investigated. SCS is a widely employed technique to synthesize nanoscale materials such as oxides, metals, alloys and sulfides, owing to its simplicity, low cost of precursors, energy- and time-efficiency. In addition, product properties can be effectively tailored by adjusting various synthesis parameters which affect the combustion process.</div><div>The first and second parts of this work (Chapters 2 and 3) are devoted to investigating the correlation between the synthesis parameters, combustion characteristics and properties of the resulting powder. A series of CeO<sub>2</sub>, which is a widely used material for various catalytic applications and a promising catalyst support for hydrous hydrazine decomposition, and Ni/CeO<sub>2</sub> nanopowders as model catalysts for the target reaction were synthesized using conventional SCS technique. This demonstrated that crystallite size, surface property and concentration of defects in CeO<sub>2</sub> structure which strongly influence the catalytic performance, can be effectively controlled by varying the synthesis parameters such as metal precursor (oxidizer) type, reducing agent (fuel), fuel-to-oxidizer ratio and amount of gas generating agent. The tailored CeO<sub>2</sub> powder exhibited small CeO<sub>2</sub> crystallite size (7.9 nm) and high surface area (88 m<sup>2</sup>/g), which is the highest value among all prior reported SCS-derived CeO<sub>2</sub> powders. The Ni/CeO<sub>2</sub> catalysts synthesized with 6 wt% Ni loading, hydrous hydrazine fuel and fuel-to-oxidizer ratio of 2 showed 100% selectivity for hydrogen generation and the highest activity (34.0 h<sup>-1</sup> at 50 ºC) among all prior reported catalysts containing Ni alone for hydrous hydrazine decomposition. This superior performance of the Ni/CeO<sub>2</sub> catalyst is attributed to small Ni particle size, large pore size and moderate defect concentration.</div><div>As the next step, SCS technique was used to develop more efficient and cost-effective catalysts for hydrous hydrazine decomposition. In the third part (Chapter 4), noble-metal-free NiCu/CeO<sub>2</sub> catalysts were synthesized and investigated. The characterization results indicated that the addition of Cu to Ni/CeO<sub>2</sub> exhibits a synergistic effect to generate significant amounts of defects in the CeO<sub>2</sub> structure which promotes catalytic activity. The 13 wt% Ni<sub>0.5</sub>Cu<sub>0.5</sub>/CeO<sub>2</sub> catalysts showed 100% H<sub>2</sub> selectivity and 5.4-fold higher activity (112 h<sup>-1</sup> at 50 ºC) as compared to the 13 wt% Ni/CeO<sub>2</sub> (20.7 h<sup>-1</sup>). This performance is also superior to that of most reported non-noble metal catalysts and is even comparable to several noble metal-based catalysts. In the fourth part (Chapter 5), low Pt loading NiPt/CeO<sub>2</sub> catalysts were studied. The modified SCS technique was developed and applied to prepare NiPt/CeO<sub>2</sub> catalysts, that overcomes the typical problem of conventional SCS which leads to deficiency of Pt at catalyst surface due to the diffusion of Pt into bulk CeO<sub>2</sub>. The Ni<sub>0.6</sub>Pt<sub>0.4</sub>/CeO<sub>2</sub> catalysts with 1 wt% Pt loading exhibited high activity (1017 h<sup>-1</sup> at 50 ºC) along with 100% H<sub>2</sub> selectivity owing to the optimum composition of NiPt alloy, high metal dispersion and a large amount of CeO<sub>2</sub> defects. Its activity is higher than most of the reported NiPt-based catalysts which typically contain high Pt loading (3.6-42 wt%).</div><div>Next, the intrinsic kinetics of hydrous hydrazine decomposition over the NiPt/CeO<sub>2</sub> catalysts, which are necessary for efficient design and optimization of the hydrous hydrazine-based hydrogen generator system, were investigated (Chapter 6). From the experimental data obtained at different reaction temperatures, the intrinsic kinetic model based on the Langmuir-Hinshelwood mechanism was established. The developed model</div><div>provides good predictions with the experimental data, especially over a wide range of initial reactant concentration, describing well the variation of reaction order from low to</div><div>high reactant concentration.</div><div>Finally, the conclusions of the dissertation and recommendations for future work are summarized in Chapter 7.</div>