טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentShener Yuval
SubjectMorphology Control and Catalytic Activity of Ceramic
Electrospum Nanofibers
DepartmentDepartment of Chemical Engineering
Supervisors Professor Gideon Grader
Dr. Gennady Shter
Full Thesis textFull thesis text - English Version


Abstract

Mats comprised of ceramic one-dimensional nanostructure, such as nanofibers and nanobelts, have shown great potential in a variety of applications due to their high surface area to volume ratio and intrinsic hierarchical porosity. One of the effective methods to produce fibrous ceramic mats is electrospinning (ES). In this method, a high voltage electrical field is used in order to draw nanofibers from a viscous precursor solution. The as-electrospun nanofibers are comprised of polymer and ceramic precursors and are called "green nanofibers". The green nanofibers then undergo thermal treatment to remove organics and to sinter ceramics. These nanofibers are known as 'white nanofibers". 

One of the most important aspects of one-dimensional nanostructures is the morphology, or sub-structure, of the single nanofiber. In this work, we investigated the effect of various ES process parameters on the final morphology of ceramic Fe-Al-O fibers and belts. The precursor for ES included metal acetylacetonates, polymer and solvent. It was found that an increase in weight ratio of acetylacetonates to polymer (PVP) led to the formation of a unique lamellar-like surface. Further increase in this ratio also led to an orientation shift of surface lamellas from radial to axial as well increasing the lamellas width. The formation of lamellas was explained by phase separation caused by melting of Al(acac)3 into local polymer-poor inclusions. The lamellar-like nanofibers displayed surface area 40% greater than smooth nanofibers analogs.

Another factor studied was the combined effect of the green mat thickness and heating rate during thermal treatment on the morphology. It was found that beyond a certain thickness, which depends on heat rate, the morphology transformed from solid nanofibers to hollow nanofibers and flat nanobelts. This transformation was explained as a result of heat accumulation in the thicker samples, which greatly influenced the rate of oxidative decomposition of the organic component in the fibers. The sample thickness from which morphology transformation occurs was found to depend on the acetylacetonate to PVP ratio. The acetylacetonate to PVP ratio also affected the distribution of nanobelts and hollow nanofibers.  

To investigate the catalytic activity of the Fe-Al-O nanofibers in CO2 hydrogenation to hydrocarbons a pilot equipped with controlled feed system, reactor and on-line GC analysis was modified and adapted to investigated process. The combined effect of nanofibers morphology and potassium promoter on surface area loss and CO2 selectivity during catalyst activation (carburization) with a CO:H2:He mixture gas was examined. The lamellar nanofibers displayed a smaller surface area loss in comparison to smooth fibers. This result is consistent with the lamellar fibers' improved morphology. In general, a CO2 hydrogenation test was performed and the conversion of CO2 into hydrocarbons was demonstrated. However, an optimization of the process conditions and promoter impregnation is required to achieve higher conversion and selectivity to C5 hydrocarbons.