Prasad, Ravi Mohan
Polymer-Derived Microporous Ceramics for Membranes and Sensors for High Temperature Hydrogen Purification and Sensing.
[Ph.D. Thesis], (2012)
Polymer-Derived Microporous Ceramics for Membranes and Sensors for High Temperature Hydrogen Purification and Sensing -
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|Item Type:||Ph.D. Thesis|
|Title:||Polymer-Derived Microporous Ceramics for Membranes and Sensors for High Temperature Hydrogen Purification and Sensing|
The growing interest in the use of hydrogen as main fuel has increased the need for pure hydrogen (H2) production and purification. There are several by-products (CO, H2O, CO2) associated with the production of hydrogen which might damage the production rate. Therefore, separation of hydrogen from other gases is an important step in the hydrogen production process. If H2 can be selectively removed from the product side during hydrogen production in membrane reactors, then it would be possible to achieve complete CO conversion in a single-step under high temperature conditions.
The main goal of the present work is the high temperature H2 purification and sensing by applying polymer-derived ceramics. To prove the concept, the microporous SiBCN, Si3N4 and SiCN ceramic membranes have been synthesized by the polymer-pyrolysis route and their performance for the hydrogen separation have been evaluated in tubular membranes as well as in planar chemiresistors.
The synthesis of amorphous SiBCN ceramics has been realized through pyrolysis of poly(organoborosilazanes) in argon. Multilayered amorphous SiBCN/γ-Al2O3/α-Al2O3 membranes with gradient porosity have been realized and assessed with respect to the thermal stability, pore-size distribution and H2/CO permeance. N2-adsorption measurement indicates micropores in the range of 0.68-0.73 nm for three-fold SiBCN/γ-Al2O3/α-Al2O3 membrane. SEM characterization of three-fold SiBCN/γ-Al2O3/α-Al2O3 membrane shows the thickness of SiBCN membrane layer is 2.8 μm; gas permeance measurements of the membrane shows H2/CO selectivity of about 10.5 and the H2 permeance of about 1.05x10-8 mol m-2 s-1 Pa-1. The observed gas permeation properties point out that the transportation of gas molecules through the membrane is governed by both activated and Knudsen diffusion.
The stability and sensing characteristics of SnO2 sensors coated with amorphous microporous SiBCN layers have been studied in oxygen-free atmospheres. The SiBCN layers coated on SnO2 sensors are amorphous, crack-free and microporous. The diameter of micropores (about 0.70 nm) is larger than the kinetic diameter of H2 (0.289 nm) and CO (0.376 nm) molecules, allowing in this way their diffusion towards the bottom SnO2 sensing layer. Transient response characteristics and sensor signals of uncoated SnO2, three-fold and five-fold SiBCN-coated SnO2 sensors exposed to CO (10, 20 and 120 ppm) and H2 (40, 400 and 900 ppm) in nitrogen at 350 and 530 °C are obtained. Uncoated SnO2 sensor is reduced at 530 °C in H2 to tin while SiBCN-coated SnO2 sensors show reversible resistance changes while exposed to CO and H2.
Si3N4-ceramics have been synthesized via a dry ammonia pyrolysis of commercially available polysilazane (KiON HTT 1800). Amorphous microporous-Si3N4 ceramic layers deposited on the top of GaN sensing layer followed by dry ammonia treatment leads to the improved H2 to CO selectivity of Si3N4/GaN sensors in the oxygen-free atmosphere. Transient response of the uncoated-, three-fold Si3N4 coated- and ammonia treated-GaN sensors exposed to CO (10, 20 and 120 ppm) and H2 (40, 400 and 900 ppm) in pure nitrogen at 350 and 530 °C are investigated. The results indicate that uncoated-GaN sensor shows high response towards both CO and H2 whereas for microporous Si3N4 coated- and ammonia treated-GaN gas sensors the sensitivity towards the interfering gas CO is significantly reduced.
High-surface area micro- and mesoporous carbon-rich SiCN ceramics have been obtained by controlled thermolysis of a carbon-rich poly(diphenylsilylcarbodiimide) precursor under argon. The formation of porous SiCN ceramics is due to the carbothermal reaction of amorphous silicon nitride phase with excess carbon, which leads to materials with high specific surface area of about 500-600 m2 g−1. High-resolution Transmisson Electron Microscopy indicates that pores are embedded only in the free carbon phase. The transformation from micro- to mesoporous ceramics after heat treatment between 1600 and 1700 °C, due to the organization of graphene-like free carbon phase, is discussed.
|Place of Publication:||Darmstadt|
|Uncontrolled Keywords:||Microporous, Membrane, Gas sensor, Hydrogen purification and sensing, Polymer-derived ceramics|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 500 Naturwissenschaften
600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften
|Divisions:||Fachbereich Material- und Geowissenschaften
Fachbereich Material- und Geowissenschaften > Materialwissenschaften > Disperse Feststoffe, Dispersive Solids
|Date Deposited:||06 Dec 2012 14:26|
|Last Modified:||07 Dec 2012 12:06|
|Referees:||Riedel, Prof. Dr. Ralf and Roth, Prof. Dr. Christina and Ensinger, Prof. Dr. Wolfgang and Schneider, Prof. Dr. Jörg|
|Refereed:||11 June 2012|