Selective oxidation catalyst microstructure

Figure 1.9 Schematic representation of the microstructure of a working oxide catalyst for selective oxidation.

V. Cominos, V. Hessel, C. Hofmann, et at Selective oxidation of carbon monoxide in a hydrogen-rich fuel cell feed using a catalyst coated microstructured reactor. Catal. Today 2005, 110, 140-153. 

Several examples of oxidation reactions, both in the liquid and in the gas phase, have been investigated in microreactors. Often the use of the microstructured device allows a better selectivity to the product of partial oxidation, because of a better temperature control on the catalyst surface (see, for instance, several examples in reviews [61a,b]). Indeed, several gas-phase oxidations can be completed in milliseconds, at significantly high temperatures. 

Investigation of the sol-gel derived bismuih-molybdcnum-titanium xerogel and aerogel mixed oxide as catalyst for the oxidation of butadiene to furan l 14J 15] indicated activities and selectivities comparable to other suitable catalysts. The unique microstructure and good catalytic performance of the bismuth molybdenum oxide particles are attributed to lilania matrix. How ever, these favorable properties are limited to low temperature reaction conditions since both xerogels and aerogels are prone to rapid restructuring at elevated temperatures, which result in the loss of their unique redox properties. 

Foams were proved to be highly suitable as catalytic carrier when low pressure drop is mandatory. In comparison to monoliths, they allow radial mixing of the fluid combined with enhanced heat transfer properties because of the solid continuous phase of the foam structure. Catalytic foams are successfully used for partial oxidation of hydrocarbons, catalytic combustion, and removal of soot from diesel engines [14]. The integration of foam catalysts in combination with microstructured devices was reported by Yu et al. [15]. The authors used metal foams as catalyst support for a microstructured methanol reformer and studied the influence of the foam material on the catalytic selectivity and activity. Moritz et al. [16] constructed a ceramic MSR with an inserted SiC-foam. The electric conductive material can be used as internal heating elements and allows a very rapid heating up to temperatures of 800-1000°C. In addition, heat conductivity of metal or SiC foams avoids axial and radial temperature profiles facilitating isothermal reactor operation. 

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