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Title Heterogeneous Catalysis in Micro-systems
Author Thybo, Susanne (Department of Chemical Engineering, Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark)
Supervisor Johannessen, Tue (The Aerosol Laboratory, Department of Chemical Engineering, Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark)
Quaade, Ulrich (Department of Physics, Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark)
Institution Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark
Thesis level Master's thesis
Year 2003
Abstract Chemical microsystems are systems that behave as continuous flow systems and have dimensions in sub-millimeter range, an effect that improves heat and mass transfer properties and enables better process control. Chemical microsystems are turned into catalytic microreactors when wall coated with thin films of catalytic material(s) or packed with catalytic particles. This study treats the subject of heterogeneous catalysis in microsystems, the aim being to investigate the prospects of using flame deposition as a technique for incorporating catalysts in microsystems fabricated by IC technology fabrication techniques. Flame deposition is based on flame spray pyrolysis, a technique that enables preparation of supported catalysts with a large surface area and a long thermal stability. The deposition is obtained by placing a cooled sample in the warm flame reactor, thus the catalytic particles in the gas deposit on the colder surface and create a porous coat. The incorporation in microsystems requires adhesion and deposition in a desired confined area in order to avoid plugs of loose material in the channels and enable sealing of the microstructure into which the catalyst has been deposited. To investigate the direct flame deposition as a technique for catalyst incorporation a series of microsystems are designed, fabricated and tested. The thesis contains a literature review on catalytic microsystems and technique used for catalytic incorporation and a description of the development of an experimental technique for obtaining adhesion and confinement of flame deposited material. With this foundation the study describes the step-by-step development of microsystems in which catalysts can be directly deposited. This includes the development of an appropriate design, a fabrication procedure, and an experimental setup in which the microsystem can be interfaced, tested and characterized. Finally, design optimizations are suggested. Among other things adhesion properties of the coated material depends on substrates and the surface properties of the substrates on which the catalyst coat is deposited and adhesion is good on e.g. silicon surfaces of which the native silicon oxide layer has been removed. A confined coat can be obtained with different proposed approaches. One of these are a lift-off approach where catalyst is deposited on a wafer with a photoresist mask, which following can be dissolved in acetone. However the technique is believed to remove the large surface area properties of the remaining catalyst coat. The confinement of the catalyst in the developed microsystem is therefore obtained by the use of a shadow mask, containing a hole that only enables coating within the microstructure. The microfabricated silicon structures are minimum 300 µm deep and contain a reaction zone that is 15 mm long and 1500 µm wide, inlet and outlet channels that are 100 µm wide and minimum 5 mm long. Composition in the gas is determined by a mass spectrometer, which is connected directly to the microsystem in the outlet channel before the gas exhaust the system. The deposited catalyst is coated on the bottom and side walls of the reaction zone, creating a coat thickness up to 100 µm. The design resulted in thermal diffusion effects creating large gradients of heavy gas components at the inlet to the microsystem, an effect that resulted in transients in the concentrations when changes in flow rates were applied. CO conversion over 1wt% Au/TiO2 resulted in conversions up to 20% and reactions rates between 60 and 90 µmol/(gAu· s), experimental results similar to previously measured for direct deposited flame catalyst. The thesis is based on collaboration between the Interdisciplinary Research Center for Catalysis (ICAT) and the Micro- and Nanotechnolgy Research Center (MIC) at The Technical University of Denmark (DTU).
Imprint DK-2800 Kgs. Lyngby, Denmark
Pages 50
Admin Creation date: 2006-06-22    Update date: 2012-12-13    Source: dtu    ID: 41137    Original MXD