Design and synthesis of nanocatalysts enabling hydrogen storage, release and electrocatalysis
Creators: name: Koh, Katherine
Publisher: name: No Publisher Supplied
Year: 2017

description: In light of the given limited available energy resources and the constant increase in energy consumption and demand worldwide, the development of sustainable alternative energy sources has never become more vital. Among the conceivable renewable energy choices, hydrogen has been considered as one of the most promising energy carrier especially in the fuel-cell technology. Based on the concept of “hydrogen economy”, as coined by Bockris in 1970,1 efficient storage, large-scale production and convenient distribution of hydrogen are of the utmost importance for successful utilization of this energy carrier. One of the most promising strategies to achieve the sustainable hydrogen carriers is through the hydrogenation/dehydrogenation interconversion reactions involving substances such as formic acid (H2 + CO2   HCOOH) or formate (HCO2‾ + H2O   HCO3‾ + H2).2 Other reactions that are also important for fuel cell applications are the hydrazine oxidation (HOR), oxygen reduction reaction (ORR), or hydrogen evolution reaction (HER). In this thesis, I reported the design and synthesis of various nanocatalysts for several important fuel cell applications including formic acid dehydrogenation, formate/bicarbonate reversible cycle, and hydrazine oxidation reaction (HOR).   In chapter 2, I have developed a facile synthetic route to amine-functionalized nanoporous silica-supported ultrasmall Pd nanoparticles (Pd/SBA-15-Amine) that are proven to be highly active catalysts for formic acid dehydrogenation, producing hydrogen at ambient temperature with a high turn-over-frequency (TOF). The TOF values reported for the materials therein are among the highest TOFs ever reported for the reaction. I have also shown that the catalyst could be easily recyclable multiple times, without losing their catalytic activity. The catalyst may, therefore, contribute to some of the solutions of our current renewable energy and sustainability challenges (by enabling the so-called hydrogen economy).  In chapter 3, I have synthesized new types of palladium nanoparticles (Pd NPs) supported on amine-functionalized SBA-15, which have high catalytic activity for formic acid dehydrogenation. In this case, I have also demonstrated the synthesis of SBA-15 mesoporous silica materials grafted with three different amine groups (primary, secondary, and tertiary amine) and the interactions between the Pd NPs and the grafted amine groups to create favorable synergistic catalytic effects toward the reaction. The effects of the different amine types, their grafted density on the chemical and catalytic activities of the supported Pd NPs in formic acid dehydrogenation are then thoroughly investigated using various state-of-the-art characterization techniques. The study has also allowed some understanding of structure-catalytic activity relationship of such catalytic materials.  In chapter 4, the formate and bicarbonate reversible reactions are discussed. Those reversible cycle can be used to store, release and allow hydrogen (H2) to serve as an effective energy carrier in energy systems such as fuel cells. However, to feasibly utilize these reactions for renewable energy applications, efficient catalysts are necessary to promote the formate-bicarbonate reversible reactions. I have reported the synthesis of novel polyaniline (PANI)-derived mesoporous carbon-supported Pd NPs that can efficiently catalyze these reversible reactions. The resulting nanomaterials has been shown efficiently catalyze both reactions, i.e., the dehydrogenation of formate (HCO2‾ + H2O → H2 + HCO3‾) and the hydrogenation of bicarbonate (H2 + HCO3‾ → H2O + HCO2‾). The study further revealed that having an optimum density of N dopant species in the catalysts could improve Pd’s catalytic activity toward both reactions. Among the different materials studied here, the one synthesized at 800 °C with relatively high amount of colloidal silica templates gave the best catalytic activity and these TOF and TON values are among the highest reported for heterogeneous catalysts for these reversible reactions so far.  Lastly, nitrogen and oxygen co-doped metal-free, rice-derived mesoporous carbons (RDMCs) have been successfully synthesized by a combination of three synthetic processes: i) a low temperature hydrothermal treatment (HTC), ii) followed by a high pyrolysis temperature in presence of colloidal silica templates and iii) finally removal of the silica templates from the carbonized products. The obtained mesoporous carbons effectively electro-catalyzed the hydrazine oxidation reaction (HOR) with negative onset and/or peak potentials and high peak current densities, and show long-term stability. By optimizing the synthetic parameters, such as the amount of colloidal silica templates and pyrolysis temperatures used for the synthesis, RDMCs possessing the high electrocatalytic performances have been obtained. It has also been found that the catalytic activities of the materials would depend on the BET surface area and amount of dopants in the materials. The material pyrolyzed at 800 °C along with hydrothermal reaction with moderate silica amount, in particular, gave the best activity toward hydrazine electrooxidation.     Reference  1. Bockris, J. O. Science 1972, 176, 1323-1323.  2. Grasemann, M.; Laurenczy, G. Energy Environ. Sci. 2012, 5, 8171-8181.