Microstructural Characterization of Catalytic Copper- and Chromium-Containing Alumina Aerogels
Internal combustion engines produce carbon monoxide, hydrocarbons, and nitrogen oxides as exhaust, and these gases can react to produce smog, which has negative health effects. Gasoline-powered vehicles 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. The effects of heat-treatment, exposure to simulated automotive exhaust, and acid slurrying on the microstructure of copper-alumina and chromium-alumina catalytic aerogels were studied. 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. Characterization was performed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (XRD), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). Clusters over 30 nm in diameter (typical of aerogels) were identified via AFM in all aerogels examined. XRD results showed that heat-treated Cu-Al aerogel contains copper aluminate spinel crystal structure, and catalytic testing with humidity gives rise to CuO, as well. As-prepared chromium-alumina aerogels were largely amorphous. Following heat-treatment, eskolaite crystal structure was identified with XRD, and supported by SEM/EDX micrographs clearly showing chromium- and oxygen-containing hexagonal microcrystals embedded in the aerogel matrix.