Internal combustion engines produce carbon monoxide, hydrocarbons, and nitrogen oxides as exhaust, and these gases can react to produce photochemical smog, which is detrimental to human (and other organism) health. Virtually all gasoline-powered vehicles on the road today are equipped with catalytic converters that transform this exhaust into more benign gases. Catalysis is performed by platinum, palladium, and rhodium; however, the process of mining these metals damages the environment. Metal-containing aerogels are a promising alternative thanks to the unique properties of aerogels: high specific surface area (resulting from their high nanoporosity) and resistance to heat allow for good performance in heterogeneous catalysis. In the present work, the microstructures of copper-alumina and chromium-alumina catalytic aerogels were characterized. These aerogels were synthesized using an epoxide-assisted ring-opening approach with aluminum chloride hexahydrate and nitrate salts of copper(II) and chromium(III) as the gel precursors. Solvent removal was performed with a patented rapid supercritical extraction (RSCE) method. The effects of heat-treatment and exposure to simulated automotive exhaust on the chemical and microcrystalline structure of the catalytic aerogels were studied. Characterization was performed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (XRD), and atomic force microscopy (AFM). Clusters over 30 nm in diameter (typical of aerogels) were identified via AFM in all the copper- and chromium-alumina aerogels. A largely amorphous structure was observed with SEM for copper-alumina aerogels that had been heat-treated to 800 C, slurried in aqueous acid, and then dried; however, after these samples had undergone catalytic testing, copper- and oxygen-containing microparticles were observed via EDS. As-prepared chromium-alumina aerogels were largely amorphous. Following heat-treatment, eskolaite crystal structure was identified with XRD, and supported by SEM/EDS micrographs clearly showing chromium- and oxygen-containing hexagonal microcrystals embedded in the aerogel matrix.

Microstructural characterization of catalytic copper- and chromium-containing alumina aerogels

ACS Spring Meeting 2021

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