(139f) Use of Catalytic Micro-Reactor to Improve Selectivity of Gas Sensors :

The development of gas micro-reactors able to work at high temperature is an important current objective. There are several requests for catalytic reactions of mixtures of gases. We have recently work in this sense in order to produce a micro-reactor able to work at high temperature up to 500 or 600°C. We have firstly focussed this development on a micro-production of hydrogen using the catalytic reaction of deshydrogenation of MCH into toluene. For this goal, it was necessary to use a catalyst (platinum supported on alumina). The objective of the present paper is to use this micro-reactor for gas analysis in the sense of Micro-TAS development. The micro-reactor is associated to a conventional gas micro-sensor (semi-conductor type using electrical properties of tin oxide). As the main defect of such gas sensor is its lack of selectivity, it is interesting to place a micro-reactor before the sensor in order to have a pre-conditioning action on the gas mixture. The catalytic reactor can be used to change the composition of the gas mixture and so to improve the selectivity of the device. A Silicon micro-reactor has been developed with micro-technologies. Channels are etched in silicon substrates with classical deep reactive ion etching process. These channels of 100mm depth are structured with pillars to increase the surface and thus improve the overall reaction rate compare to a channel without pillar. The structured microreactor is molecularly bonded with a silicon cover to form an airtight devices. In order to work at high temperatures, it was necessary to include in this system some heaters to have an autonomous microreactor. Thick film resistances of platinum have been deposited by the screen printing and used as microheaters. The tests of microheaters are realised using the infrared thermographic camera. Special gas connexions able to work at 400 or 500°C have been specially developed with metallic tubes (300 microns diameter) and sealing process with ceramic material. The second part of the study concerns the deposition of the catalyst directly in the closed silicon channels by fluidic process. Firstly, the alumina is deposited by wash-coat associated to a vacuum step and a sintering process. The platinum is then deposited by fluidic impregnation. Physical and characterisations of the catalyst in the channels are presented (DRX, MEB, TEM, XPS, AFM?). The catalytic activity of the micro-reactor is also evaluated on a special catalytic bench developed for such very low flows. The bench is also equipped with gas analyses and especially a mass spectrometer in order to measure to gas composition before and after the micro-reactor. Finally, the micro-reactor is used in association with a gas sensor. The SnO2 sensor is placed in a small cell after the micro-reactor. As such sensor presents response both to CO and CH4, the micro-reactor is used to transform CO into CO2 (no response with SnO2). As platinum has no action on CH4 at medium temperature, the device become selective to detect CH4 in a mixture of CO + CH4. The influence of the temperature is also evaluated for such selective responses. On the base of such feasibility study, new types of pre-processing of gas mixtures are currently investigated for more complex gas detection, for example in NOx detection. Another application is also under investigation for the development of a pre-concentrator for gas analysis. For this objective, the micro-reactor must be filled with absorbent materials. Current work are performed for the moment the achieved such objectives.