Electrospun Nanofiber Metal Oxides for Reactive Sorption and Catalysis
- Author(s): Alshafei, Faisal
- Advisor(s): Simonetti, Dante A
- et al.
Electrospun metal oxides is a new class of materials that have demonstrated auspicious potential and have been used in a wide range of applications. In this work, various smooth, continuous, and defect-controlled metal-polymer nanofibers were synthesized via electrospinning with diameters ranging from approximately 50 to 600 nm, and subsequently thermally treated to decompose the polymer (PVP or PEO) and form highly porous, fibrous metal (Cu-, Ni-, Mg-, and Ca-) oxide nanostructures. In the first part of this thesis, parameters that influence the electrospinning process were systematically investigated for PVP-Cu(NO3)2 systems. Both solution properties (polymer/metal concentration, polymer molecular weight, and solvent identity) and processing conditions (applied voltage, tip-of-needle to collector distance, extrusion rate, and humidity) were varied to probe the effect of these electrospinning factors on fiber quality prior to thermal treatment. The data collected demonstrated that factors that do not directly and strongly influence viscosity, conductivity and solvent evaporation (e.g., applied voltage, extrusion rate, and tip-of-needle to collecting plate distance) do not have substantial effects on fiber diameter and morphology. Subsequent thermal treatment of the electrospun nanofibers and choice of metal, however, were found to markedly impact the morphology of the formed fiber oxides (e.g., string-like structures or segmented particles).
In the second part of this thesis, electrospun fiber metal oxide materials were tested in two main applications (high temperature CO2 removal and low-temperature H2S removal) and their performance was compared to materials prepared via traditional synthesis routes (e.g., sol-gel, co-precipitation, hydrothermal treatment, etc.) In the first application, CaO-based materials were tested as potential sorbents in sorption enhanced steam methane reforming (SE-SMR) to capture CO2 and shift the reaction towards producing more hydrogen. The electrospun CaO-nanofibers, when reacted with CO2, achieved complete conversion to CaCO3 and had an initial CO2 sorption capacity of 0.79gCO2/gsorbent at 873 K and 923 K (highest of all materials tested), as the macro-porosity imparted by the electrospinning process improved the CO2 diffusion through the CaCO3 product layers. Furthermore, when these electrospun sorbents were added to a commercial catalyst and tested in SE-SMR conditions, they had three to four times longer breakthrough times than CaO sorbents derived from natural sources (e.g., CaO-marble). To further improve the stability of CaO-based sorbents, chemical doping of Ca-supports with Mg, Al, Y, La, Zn, Er, Ga, Li, Nd, In, and Co was combined with electrospinning to yield mixed oxide materials with high sorption capacities (~0.4-0.7 gCO2/gsorbent) and improved durability (up to 17 cycles). It was demonstrated that metals that have high Tammann temperatures were effective at reducing sintering and CaO particle agglomeration by acting as spacers, thus, retaining the sorbent’s initial sorption capacity upon repeated cycling.
In the second application, CuO nanofibers with varying diameters (~70-650 nm) were prepared from two polymers (PEO and PVP) and reacted with H2S at ambient conditions to form CuS. The results from this study demonstrated that the sulfur removal capacity of CuO materials, whether prepared via electrospinning, hydrothermal treatment, sol-gel or co-precipitation, was strongly dependent on crystallite size (a linear relationship was established between CuO removal capacity and crystallite size and held true for all CuO materials with crystallites between 5-26 nm) and CuO purity (i.e., presence of residual carbon on the surface of the oxide). Indeed, properties such as surface area, pore volume and morphology (e.g., flowerlike, fiber-like, belt-like, etc.) were found to have an insignificant impact on removal capacity. This work offers fundamental insights into the design of multifunctional and highly porous metal oxide nanofibers for sorptive and catalytic applications.