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Opponent: Professor Aboubakr Abdullah, Qatar University
Supervisor: Professor Joydeep Dutta, Tillämpad fysik
Over the past decades, mesoporous silica has emerged as a new material with interesting physical and chemical properties, such as high surface-to-volume ratio, stability, and the feasibility of surface functionalization, amongst others. Mesoporous silica provides an opportunity to improve conventional commercial catalyst supports, and sorbent due to its low-cost synthesis. In this research field, investigations on various materials prepared by different techniques as potential support have been reported. The main objective of the studies was to produce highly stable mesoporous materials for energy and environmental applications using simple and low-cost synthesis.
In this thesis, we utilized the sol-gel and self-templating processes in synthesizing three types of highly porous silica nanomaterials, mainly aerogels (SiO2 AG), xerogel (SiO2 XG), and nanosheets (SiO2 NS). SiO2 AG and SiO2 XG were synthesized using the sol-gel method, while SiO2 NS was synthesized using the soft-templating hydrothermal technique. SiO2 AG, SiO2, XG, and SiO2 NS are explored as catalyst support for low-temperature carbon monoxide (CO) oxidation and compared to commercial Fumed Silica. The synthesized materials are investigated as metal support materials (M/SiO2) for environmental applications such as CO oxidation and carbon dioxide (CO2) adsorption. The morphology of the synthesized materials was investigated experimentally using several characterization techniques such as X-ray Diffraction (XRD), X-ray Photo-electron Spectroscopy (XPS), Transmission Electron Microscopy (TEM), Thermo-Gravimetry-Differential Scanning Calorimetry (TGA-DSC), gas adsorption by Brunauer–Emmett–Teller (BET) method, Fourier Transform Infra-Red spectroscopy (FTIR), UV-Vis spectroscopy, and Temperature Programmed Reduction (TPR) for understanding the surface chemistry of supports. The M/SiO2 was subjected to a detailed study of the morphology before, during, and after the low-temperature CO oxidation reaction under various pretreatment and reaction parameters for a better understanding of the chemical and physical changes occurring on the catalyst. It was found that the catalyst structure, surface area, and morphology of M/SiO2 play a crucial role in the catalytic performance and stability under different reaction conditions compared to unsupported metal catalysts by preventing agglomeration of metal nanoparticles inside pores and aiding in a better dispersion of active sites to boost the mass heat transfer in the silica mesopores. The catalyst pretreatment conditions can affect CO conversion efficiency at low light-off temperatures (Tig) because of their effect on surface area, particle size, and size distribution of metal nanoparticles, which have a substantial effect on diffusion and mass transport of reactants (CO, O2) and products (CO2) and active sites accessibility. The lowest Tig ~ 195 ℃ and 65 ℃ for aerogel-supported palladium (Pd/a-SiO2) and aerogel-supported silver (Ag/a-SiO2) catalysts treated in the CO/O2 mixture, respectively. Moreover, the effect of reaction conditions on the catalytic CO oxidation was studied. The intrinsic apparent activation energy (Ea) and the number of active sites were calculated experimentally from the Kinetics of CO oxidation and fitted using Arrhenius plots, Ea for Pd/a-SiO2 ~ 87.6 kJmole-1. In this dissertation, we have also investigated the conversion hysteresis effect during carbon monoxide (CO) oxidation on metal/silica (M/SiO2) as a function of different pre-treatment and reaction conditions. The hysteresis behavior has been explored on Pd/SiO2, which showed a normal hysteresis due to the increased stability of the active sites. In contrast, the Ag/a-SiO2 showed an inverse counter-clockwise CO oxidation hysteresis. Cyclic and long-term stabilities of the catalysts were investigated, where Pd/a-SiO2 showed good stability for four consecutive cycles and long-term stability for ~ 27 hrs.
Furthermore, Pd/SiO2 was investigated for CO2 adsorptionunder dry and humid conditions. The adsorption isotherms under variant temperatures and pressures were studied experimentally by evaluating the CO2 gas adsorption onto Pd/a-SiO2 at low and moderate temperatures and pressures and fitted theoretically using the Langmuir fitting model. The material showed the highest equilibrium adsorption capacity of CO2 at low temperatures (3.25 molekg-1 at 16 ℃ and 1 MPa). Moreover, the moisture effect was investigated and found to play a role in reducing CO oxidation efficiency and increasing CO2 adsorption.
The main objective of the studies was to produce highly stable mesoporous-supported Catalysts for multi-energy and environmental applications using simple and low-cost synthesis.