Date of Award
Open Access Dissertation
Molecular layer deposition (MLD), a gas phase deposition technique, was applied to deposit conformal organic-inorganic hybrid coatings by conducting a series of sequential, self-limiting surface reactions on substrates with exquisite thickness control at the sub-nanometer level. Obtained organic-inorganic hybrid coatings can subsequently be converted into porous coatings by removing the organic compound. Potential of functional coatings and membranes prepared by MLD was explored for applications from adsorptive separation, water purification, to gas storage.
We demonstrated a new concept, pore misalignment, to continuously fine tune the molecular-sieving “gate” of 5A zeolite by adjusting the external porous Al2O3 MLD coating thickness. For the first time, small organic molecules with sub-0.01 nm size differences were effectively distinguished by size. As an extension of the pore misalignment concept, a composite zeolite adsorbent was prepared by depositing an ultrathin porous TiO2 coating on 5A zeolite by MLD. This composite adsorbent showed great potential for effective C3H6/C3H8 separation based on both equilibrium and adsorption kinetics differences (approximately 5 times higher ideal adsorption selectivity and 44 times higher diffusivity, compared to uncoated 5A zeolite). MLD coated zeolite (5A and 13X) composite adsorbents were also found to have great potential for CO2 capture from flue gases; greatly enhanced CO2/N2 ideal adsorption selectivity was obtained, while maintaining high CO2 adsorption capacity, by controlling calcination conditions.
Molecular layer deposition was also used as a highly controllable method to prepare TiO2 nanofiltration membranes by depositing microporous TiO2 coating on mesoporous anodic aluminum oxide (AAO) support with excellent control of coating quality, thickness and nanometer-sized membrane pores for water purification. Optimized TiO2 nanofiltration membranes had a pure water permeability as high as ~48 L/(m2∙h∙bar). Salt and dye rejection measurements showed moderate rejection of Na2SO4 (43%) and MgSO4 (35%) and high rejection of methylene blue (~96%). In addition, natural organic matter (NOM) removal testing showed high rejection (~99%) as well as significantly improved antifouling performance and recovery capability.
A novel concept of utilizing nanoporous coatings as effective nano-valves on microporous adsorbents was developed for high capacity natural gas storage at low storage pressure. For the first time, the concept of nano-valved adsorbents capable of sealing high pressure CH4 inside the adsorbents and storing it at low pressure was demonstrated. Traditional natural gas storage tanks are thick and heavy, which makes them expensive to manufacture and highly energy-consuming to carry around. Our design uses unique adsorbent pellets with nano-scale pores surrounded by a coating that functions as a valve to help manage the pressure of the gas and facilitate more efficient storage and transportation. The optimal nano-valved adsorbents comprise of a ~7.5 μm thick MCM-48 mesoporous layer coated on the outer surface of 5A beads. After modification by 3 cycles of MLD, the steady state CH4 storage capacity of MLD-MCM-48-5A adsorbent (loading pressure 50 bar, storage pressure 1 bar) was about 55.8-58.4% (40.7-42.6 V/V) of the maximum capacity of the uncoated 5A beads in three CH4 storage cycles, which is about 200% higher than storage capacity of the uncoated 5A beads at the same storage pressure.
Song, Z.(2016). Ultrathin Microporous Metal Oxide Coatings: Preparation by Molecular Layer Deposition, Characterization And Application. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/3483