Preparation and properties of ceramic foam catalyst supports

Abstract

Ceramic foams are preformed materials used extensively as filters, advanced burners, diffusers and mixers, but catalytic applications are now beginning to appear. These include catalytic solar receivers, partial oxidation, catalytic combustion, and diesel exhaust catalytic filters. The foams are sponge-like porous structures and are made by filling flexible open-cell organic polymer foams with slurries of ceramic particles such as alpha alumina, zirconia, silicon nitride, etc.. The plastic is burned off and the ceramic sintered to yield either a positive or negative replica of the original foam depending on exact loading procedures. Mega-pore openings range from 0.02 to 1.5 mm, apparent densities from 0.5 to 1.5 g cm −3 , porosities from 40 to 85%, and the number of pores per cm from 4 to 30. With appropriate moulding or machining of the plastic foam, the ceramic may be fabricated in any shape or size. Megaporosity enhances turbulence in gases flowing through the foam and produces better mixing with lower pressure drop. The foam may be impregnated with catalytic agents, with or without an appropriate washcoat. Although pressure drop correlations follow the Ergun equation, the high porosity gives much lower pressure drop than equivalent beds of particles. Mass transfer follows standard correlations but turbulent flow is seen at much lower flow rates. Heat transfer is enhanced by the superior conductivity of the web structure. These feature combine to make ceramic foams attractive possibilities for many applications. The only disadvantage is the relatively low strength, a feature which may be controlled in some cases.