Abstract
Ph.D. (Chemistry)
This work reports on the synthesis, characterization, and the catalytic activity of mesoporous metal oxides (MMOs) of manganese (Mn), cobalt (Co), and cerium (Ce). The hard-template, often referred to as nanocasting, and the soft-template approaches were used to successfully synthesize these mesoporous metal oxides. In the hard-template synthesis, KIT-6 (a typical silica material) was used as a template, while for the soft-template synthesis P-123 was used as the surfactant to template the synthesis of mesoporous metal oxides. Also reported herein is the synthesis of dendrimer-encapsulated monometallic and bimetallic nanoparticles (DENs). Bimetallic gold (Au) and palladium (Pd) nanoalloys were synthesized inside the cavities of poly(amidoamine) (PAMAM) dendrimer (AuPd-DENs). Furthermore, monometallic platinum (Pt) dendrimer-encapsulated nanoparticles were successfully synthesized using PAMAM dendrimer (Pt-DENs). In order to achieve stable heterogeneous catalytic systems containing nanoparticles, the synthesized DENs were immobilized on the as-synthesized MMOs.
Prior to catalytic evaluation, all the catalysts were fully characterized using various techniques. Mesoporous metal oxides were characterized using nitrogen sorption measurements (BET) for physicochemical properties, powder X-ray diffraction (p-XRD) for phase identification, temperature-programmed reduction/oxidation (TPR/TPO) and desorption (TPD) for surface redox properties and nature of active sites, respectively, transmission electron spectroscopy (TEM) for porosity identification, and X-ray photoelectron spectroscopy (XPS) for elemental composition and oxidation states. While the synthesis of dendrimer-encapsulated nanoparticles was monitored using Ultraviolet-Visible spectroscopy (UV-Vis) and transmission electron spectroscopy (TEM) was used to check the morphology of the nanoparticles.
The hard-templated mesoporous MnO2 was used as a catalyst to study kinetics of morin oxidation as a model reaction. Two different kinetic models were used to elucidate the surface reaction, namely, Langmuir-Hinshelwood and Mars-van Krevelen models. The Langmuir-Hinshelwood model which assumes adsorption of both the substrate and the oxidant onto the catalyst surface appeared to describe the kinetics of morin oxidation better than the Mars-van Krevelen kinetic model which assumes participation of the catalyst’s lattice oxygen in the oxidation of the substrate...