(11e) Fabrication of Ultra-Thin PVP-Metal Oxide Nanofibers Via Electrospinning for Applications in Catalysis and Adsorption

Electrospinning has been recognized as an efficient technique for the synthesis of polymer-based nanofibers and has recently gained accrued attention due in part to a surging interest in nanotechnology. Electrospun nanofibers exhibit exceptionally high surface areas, large length to diameter ratios and effective electronic properties, which makes them promising candidates for catalytic, photocatalytic and sorption applications. In the electrospinning process, a high voltage is used to create an electrically charged jet of a polymer containing solution. Before reaching the collecting plate, the solution jet evaporates and small solid fibers in the micro-, sub-micro-, or nano-range are collected. Although numerous synthesis techniques of different inorganic materials have been reported and published over the past decade, little work has been done to understand the relationship between the electrospinning conditions and the microstructure and properties of the formed fibers.

In this work, sol-gel processing and electrospinning was used to fabricate 1-D inorganic-organic composite nanofibers from solutions containing polyvinylpyrrolidone (PVP) and suitable aqueous precursors of nickel, copper, calcium, magnesium, zinc, and aluminum nitrates or a combination thereof. The primary objective was to synthesize nanofibers that had diameters that were consistent and controllable, defect-free or defect-controllable, and continuous. To achieve this objective, the parameters influencing the transformation of the polymer-containing solution into fibers were investigated, which include (1) the polymer(s) properties such as, molecular weight and solubility; (2) solvent(s) properties such as, viscosity, conductivity, and surface tension; (3) the concentration of polymer(s) and metal precursor(s) in the solution; (4) the ratio of polymer(s) to metal(s) in the solution; (4) voltage and electric potential at the capillary tip; (4) distance between the tip and collecting plate; (5) extrusion rate; and (6) ambient parameters such as, solution temperature, humidity, and air velocity in the electrospinning chamber.

In this work, the electrospun solutions consisted of dissolving different amounts of PVP in ethanol/methanol/water/DMF mixtures and adding different ratios of polymer to metal salt. Electrospinning was conducted at wide range of operational conditions: 10-35 kV, 0.1-5 mL/hr extrusion rate, and 4-20 inch distance between the tip of the needle and collector plate. The formed fibers had diameters that ranged from 50µm to 10nm. The spun fibers were calcined at 500^oC for 2 hours at a ramping rate of 2^oC/min to completely remove the PVP matrix and convert the as-spun fibers to inorganic oxides for further characterization. The composition, physio-chemical properties, and structural morphologies of the PVP-metal nanofibers—before and after calcination—were examined by X-Ray diffraction (XRD), N2-Physisoprition (BET), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) with electron dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) to elucidate information about the influence of synthesis parameters on the properties of the electrospun nanofibers.

The obtained results revealed that the concentration of PVP, ratio of PVP to metal in the polymer solution, and ambient conditions played a pivotal role in influencing the shape and diameter of the nanofibers as well as the formation of beads. For instance, ribbon-like structures were observed at high PVP concentrations while the onset of beads formation was observed at lower PVP content. The effects of extrusion rate, distance between the tip and collecting plate, and voltage were less pronounced, but seemed to strongly affect the range and distribution of the nanofibers’ diameters. When the synthesis parameters and experimental conditions were optimized to yield smooth fibers with diameters that were 10-100nm, several advantageous catalytic and mechanical characteristics were observed which include: (1) a significant increase in the surface area to volume ratio, (2) flexibility in surface functionalities, and (3) superior mechanical performance (e.g., stiffness and tensile strength) compared with other forms of the material.

The synthesized fibers were tested in two applications. In the first, they were tested as sorbents for natural gas purification. Natural gas and oil typically contain a wide range of contaminants and require purification before processing because these contaminants damage process equipment and catalysts and are hazardous to humans and the environment. Metal oxides are extensively used to remove sulfur compounds via chemical reaction; however, their use is limited by incomplete conversion and mass-transfer limited sorption kinetics. Electrospun nanofibers are an attractive class of materials for reactive sorption of sulfur compounds because of their small reactive domains and large intra-fiber void spaces. This work studies the effects of synthesis parameters on the physical properties and reactivity of electrospun nanofibers. We investigate these properties using a combination of advanced characterization techniques and fixed bed sorption experiments.

In another application, the synthesized nanofibers were probed as hybrid catalyst-sorbents for sorption-enhanced steam methane reforming. Supplying distributed hydrogen at a price that is cost competitive with gasoline is the largest barrier to the widespread implementation of hydrogen fuel cell vehicles. While steam reforming of carbon-based feedstocks remains the most economical way to produce hydrogen, the economics of small scale reforming for hydrogen production are poor. An attractive alternative to conventional reforming is sorbent-enhanced steam reforming (SE-SR) using a catalyst-sorbent material that simultaneously adsorbs CO₂ during reforming reactions. Typical SE-SR processes use admixtures of reforming catalysts with mineral oxide powder based adsorbents. The limitations of these materials are incomplete conversion, loss of initial sorbent capacity, and mass-transfer limited sorption rates. Metal-oxide nanofibers, as hybrid catalyst-sorbent materials that place catalytic sites within molecular proximity of CO₂ sorption sites, allow for easy access of CO₂ to sorption sites, and thus these materials have the potential to achieve stoichiometric CO₂capacities without mass transfer restricted sorption rates. Furthermore, metal oxide nanofibers have the potential to be more active and stable catalysts for reforming compared to supported metal clusters because the nanofiber topology will prevent agglomeration of active metal sites and the deposition of carbon, which poisons catalytic sites.

This research effort provides shrewd insight into the design of fine-tuned and defect-free or defect-controllable nanofibers, which could be used in various applications—in particular as hybrid catalyst-sorbents for sorption-enhanced steam methane reforming and as sorbents in natural gas purification.