(128b) In-Situ Cast Micro Reactor Catalyst Supports for Methanol Reforming

Due to its high energy density and low reforming temperature, methanol is an ideal choice for fueling portable proton-exchange membrane (PEM) fuel cells. It has been shown that CuO/ZnO/Al2O3¬ catalysts are able to produce hydrogen while maintaining a high selectivity to CO2 at relatively low temperatures (230°C), keeping CO from poisoning the catalyst at the PEM anode. For a 20W fuel cell system operating at a 25% overall efficiency, about 3g of catalyst would be needed for the reformer operating isothermally at 230°C. Portability requires compact dimensions and reliable operation. A variety of approaches have been reported in the recent literature including catalyst-coated annular-flow micro reactors [1] and powdered catalyst packed micro reactors [2]. We have previously shown the feasibility of coating non-porous, micro-channel surfaces with a commercial catalyst and reported that there is an inherent limit on the one-pass coating thickness which is related to the physical properties of the coating fluid, the reactor geometry, and the coating procedure [3, 4]. While wall-coated reactors do operate isothermally with low-pressure drop, they suffer from low volumetric catalyst loadings. In-situ cast monoliths as catalyst supports in micro-channels are an attractive alternative, offering high loading potential and porosity. Monolithic supports have been reported for various materials, including porous alumina, silicon carbide and mesoporous carbon [5,6,7]. In this study, we report the fabrication and characterization of silica monolith catalyst supports cast in-situ in 250-530 micron diameter capillary reactors. Monolithic silica columns were first described for liquid chromatography applications [5], but we have successfully applied the methods to fabricate micro reactors suitable for methanol reforming. The relative ease of silica monolith fabrication in micro-channels and good control of parameters such as porosity and domain size (defined as the combined size of a through-pore and a skeleton) make this an attractive approach for studying monolith/catalyst performance. A typical silica monolith cross-section showing thee through-pore and skeleton is shown in Figure 1. The silica monoliths were prepared from tetramethyloxysilane (TMOS) using poly-ethylene glycol (PEG) as the template material. Porosity and domain size are controlled by the ratio of TMOS to PEG. A TMOS/PEG/urea gel is prepared and injected into fused-silica capillary tube with subsequent aging, mesopore formation and heat treatment to decompose the organic moieties to form the silica monolith. At this point, catalyst can be loaded by either washcoating or, as in our study, repeated injection and drying of a prepared solution of palladium nanoparticles. Our presentation will describe in detail the following: ? Preparation of silica monoliths with controlled domain size and porosity in micro channels. ? Critical micro reactor parameters: porosity, monolith uniformity, total pressure drop. ? Catalyst loading by the impregnation method using prepared nanoparticle solutions. ? Performance of the reforming micro-reactor. ? Extensions of this method to monolith supports using other materials. Figure 1: In-situ cast silica monolith in a 530µ capillary at 2000x magnification showing the through-pore and skeleton detail for a particular preparation. This work has been funded by the U. S. Army Research Laboratory under the Collaborative Technology Alliance Program, Cooperative Agreement DAAD19-01-2-0010 References 1. Yu X, Tu ST, Wang Z, Qi Y Chem. Eng. J 116, (2006) 2. Yoshida K, Tanaka S, Hiraki H, Esahi M Jrn. Micromechanics MicroEng., 16 (2006) 3. Conant T, Karim A, Rogers S, Samms S, Randolph G, Datye A Chem. Eng. Sc. 61, 17 (2006). 4. Karim, A, Bravo J, Gorm D, Conant T, Datye A Catal. Today 110, 86 (2005). 5. Ganley JC, Reichmann KL, Seebauer EG, Masel RI J. Cat, 227 (2004) 6. Christian M, Kenis P, Lab Chip, 6 (2006) 7. Chang H, Joo SH, Pak C J. Mater. Chem., 17 (2007) 8. Hara T, Kobayashi H, Ikegami T, Nakanishi K, Tanaka N Anal. Chem. 78 (2006)