NANOSTRUCTURED MATERIALS FOR SOLAR HYDROGEN GENERATION

Abstract

Hydrogen can be considered a nonpolluting and inexhaustible energy carrier for the future. However, hydrogen is not readily available for use as a fuel. It exists in bound form with other elements (e.g. water, hydrocarbons) and as such energy is required to abstract molecular hydrogen from various feedstocks. Solar energy due to its abundance and low cost is being considered as the energy source for environmentally safe hydrogen generation.

This dissertation focuses on the development and characterization of nano-structured materials for solar thermochemical hydrogen generation, on the principle that concentrated solar radiation can be employed as the high-temperature energy source for driving an endothermic hydrogen generation process. The reaction mechanism and kinetics of different solar thermochemical processes using those nano-structured materials as reactants or catalysts were investigated. The experimental works in this dissertation can be divided into two main areas. The first area is to study the properties and reactivity of in-situ generated Zn nanocrystals (NCs) for solar thermochemical Zn/ZnO water splitting cycle for hydrogen production. The particle size-resolved kinetics of Zn NCs oxidation, evaporation, and hydrolysis were studied using a tandem ion-mobility method in which the first mobility characterization size selects the NCs, whereas the second mobility characterization measures changes in mass resulting from a chemical reaction of the NCs. The second part of the dissertation is concentrated on the investigation of in-situ generated nano-sized metal particles as catalysts in liquid hydrocarbon decomposition process for hydrogen generation. Catalytic decomposition of liquid fuels (n-octane, iso-octane, 1-octene, toluene and methylcyclohexane) was achieved in a continuous tubular aerosol reactor as a model for the solar initiated production of hydrogen, and easily separable CO free carbonaceous aerosol product. The effects of fuel molecular structure and catalyst concentration on the overall hydrogen yield were studied. Using the similar aerosol catalysis idea, ignition of liquid fuels catalyzed by in-situ generated metal nanoparticles was investigated. The morphological change of catalyst particles during fuel ignition process and the catalytic ignition mechanism are discussed.