Highly porous ceramic wash coats for applications in micro reaction technology

By initiating the Fraunhofer Alliance for Modular Micro Reaction Systems FAMOS six Fraunhofer Institutes are working together to develop a modular, software-supported micro reaction system with interchangeable components, which is as universally applicable as possible. The micro reaction toolkit will contain the major operational units of a processing plant all in the form of standardized micro fluidic components and available in different materials (for instance silicon, metals, plastics, and ceramics) for use in different reaction environments. By using hot moulding technique in metallic tools as a near net shape technique or micromachining of dry pressed green parts various micro fluidic components in different ceramic materials are fabricated. One example is a ceramic reaction module with an optical window (sapphire) and interchangeable inlays. The coating of multi- channel plates with porous ceramic layers consisting of nanosized powders which can be included in the cavity of the module and used for catalyst screening experiments is described here. The micro reaction module consisting of a hexagonal component with a rectangular cavity (18 x 24 x 3 mm(exp 3)) and a top with an optical window such as the multi channel plates with parallel rectangular channels (150 x 150 micron(exp 2)) were produced by hot moulding of thermoplastic alumina slurry in metallic tools. Up to three plates can be stacked in one cavity. Before inserting the channel plates into the reaction modules, the micro channels were coated with nanosized (TiO2 p25 by Degussa, average particle size 21 nm) and fine-grained powders (alumina TM-DAR by Ta Mai, average particle size 200 nm). respectively. For that purpose, suspensions of the initial powders were prepared (solid contents 25 wt.- %) and stabilized by HNO3 in the case of TM-DAR and by Dolapix (Zschimmer & Schwarz) in the case of p25. Mowiol (Hoechst) as a temporary binder was used to adjust the viscosity of the suspensions and to influence thickness and flexibility of the coating layer. The channel plates were coated up to three times with a spin coating device (500 to 1000 min(exp -1)) and air dried afterwards. The layers were sintered at 850 deg C (p25) and 1000 deg C (TM-DAR), resp. and characterised by FESEM (Field Emission Scanning Electron Microscopy). For investigating the ability of the porous layers to deposit catalysts a multi channel plate coated with TM-DAR was infiltrated by an aqueous (NH4)2PdCl4- solution. The result of this deposition was investigated by FESEM and EDX (Energy-Dispersive X-Ray), and evidence of palladium distributed in the porous alumina was done by both methods. With both initial powders, titania p25 and alumina TM-DAR porous coatings of the channel surface could be achieved. Because of the very low pore volume of the thin coatings no pore size distribution was measurable. The thickness of the titania layer is less than 4 microns.