Development of Catalytic Chromia-based Aerogels

Over one billion automobiles are in use around the world, the majority of which employ internal combustion engines. Catalytic converters are used to convert the toxic compounds found in car exhaust -- carbon monoxide, nitrogen oxides (NOx) and hydrocarbons -- to less harmful gases. The typical catalytic converter employs as catalysts expensive raw materials (platinum, palladium and/or rhodium) wash-coated onto an alumina-based ceramic substrate. Aerogel materials have high surface area and thermal stability, properties that make them attractive for catalysis applications. Aerogels made with transition metal oxides are candidates to replace platinum in the catalytic converter. Chromium oxide (chromia) materials have demonstrated catalytic activity in other applications due to favorable redox chemistry, stability and selectivity. In this work, sol-gel synthesis techniques are adapted to a patented rapid supercritical extraction (RSCE) method to fabricate chromia and chromia-based catalytic aerogels. In one example of a co-precursor technique, a mixture (1:5 mole ratio) of chromium(III) to aluminum salt in ethanol is reacted with a weak base or proton scavenger to induce gelation. Following solvent exchanges of the wet gels with absolute ethanol, and RSCE processing yields chromia-alumina aerogels. Observed shrinkage is higher than for copper-containing aerogels fabricated via the same process. Physical characterization via several methods, including XRD and SEM, reveals post-calcination chromia-alumina aerogels contain ?- Cr₂O₃ nanocrystals within the alumina backbone. Catalytic testing is evaluated using an in-house-constructed testbed in which the aerogel materials are exposed to simulated automotive exhaust under temperature conditions that approximate those experienced in a catalytic converter. Chromia-alumina aerogels completely convert HC and CO gases at 400 °C under high air conditions and completely convert NO at 450 °C under low air conditions.

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