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Published: 2009

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ISBN-13: 9781109386523

Catalytic kinetics and thermal management in fabricated microreactors were studied for the design of distributed energy and portable power production systems. Specifically, kinetically relevant experimental data was generated for the following chemistries: preferential oxidation (PROX) of CO in excess H 2, water-gas shift (WGS), reverse water-gas shift (RWGS), and H 2, CO, syngas, CH 4, C 2 H 6, and C 3 H 8 oxidation over a supported Pt/Al 2 O 3 catalyst. The effect of wall material properties and reactor configuration was also determined through the modeling, design, fabrication, and experimentation of microcombustors for integration with thermoelectrics and enhancement of thermal stability from heat recirculation. CO oxidation over Pt was found to be structure sensitive, as the observed turnover frequency (TOF) rate increased with larger Pt crystallite sizes. A multisite, microkinetic model (containing reaction and diffusion steps) developed using density functional theory (DFT) energy barriers and thermodynamically consistent preexponentials for terraces (Pt(111)) and steps (Pt(211)) also predicts this trend. An excessive fraction of H 2 was shown to enhance and inhibit CO oxidation at low and high temperature, respectively. By increasing the CO:O 2 ratio in the presence of excess H 2, CO conversions above the equilibrium value were observed and rationalized with a microkinetic model. WGS and RWGS experiments were performed at high temperatures (where RWGS is favorable) and positive order kinetics were observed for H 2 O and H 2 in WGS and RWGS, respectively. In the catalytic combustion of syngas mixtures (1:1 and 1:3 for coal gas and methane reformate, respectively), high CO selectivities were observed at low temperatures. CO and H 2 catalytic combustion experiments were also performed for comparison purposes. H 2 catalytic oxidation was strongly inhibited by the presence of CO. Hysteresis was also observed at high H 2 conversions and is discussed. Kinetic parameters were estimated for lean CH 4, C 2 H 6, and C 3 H 8 catalytic combustion. The relative activity was observed to be C 3 H 8> C 2 H 6> CH 4 and the catalytic combustion of small alkanes over Pt/Al 2 O 3 was found to follow a homologous series. Thermal management of an integrated thermoelectric/single channel, catalytic microcombustor was studied using H 2, CH 3 OH, and C 3 H 8 fuels. Electrical power generation (maximum 0.65 W) with a thermal efficiency up to ~ 1.1% was measured. Thermal management strategies, such as heat recirculation, were exploited with fabricated microreactors designed via computational fluid dynamics (CFD) for C 3 H 8 combustion. It was shown through both experiments and simulation that catalytic heat recirculation burners have similar stability to single channel burners in the limit of highly conductive walls. In contrast, for low conductivity walls, heat recirculation proved to be effective at increasing combustion stability relative to single channel burners.