Investigation of structural changes in mixed-metal catalytic aerogels at elevated temperatures

Aerogels possess physical properties that make them appealing for applications in catalysis. They have a low density as well as high surface area and porosity. Alumina-based aerogels also have high thermal stability. Most catalytic converters currently in use in automobiles employ precious metals such as platinum, palladium, and rhodium as 'three-way' catalysts to oxidize carbon monoxide (CO) and unburned hydrocarbons (HCs) and reduce nitrogen oxides (NOx). These precious metals are expensive due to their low abundance in the earth's crust. Aerogels containing more abundant catalytic metals such as copper or nickel could be a less expensive alternative to the modern solution. In prior work, copper-, nickel-, and cobalt-containing silica and alumina aerogels have demonstrated three-way catalytic ability. These mixed-metal catalytic materials are readily fabricated from metal salts using an epoxide-assisted sol-gel synthetic approach with subsequent processing via a patented rapid supercritical extraction (RSCE) method to yield aerogels. Although the aerogels are predominantly amorphous, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and powder X-ray diffraction (XRD) analyses of the catalytic aerogels demonstrate convincingly that the materials have transition-metal-containing microcrystalline components. In order to develop a fundamental understanding of catalytic aerogel materials, it is necessary to identify the forms that are present at the elevated temperatures employed in catalytic converters. In this presentation, a comparison of powder XRD patterns of mixed-metal RSCE aerogels is made. Differences observed in the XRD patterns and SEM images for these materials as-prepared, following calcination (up to 800 ˚C), and at temperature (up to 800 ˚C) provide convincing evidence of the chemical and structural changes that have occurred. Through the use of powder x-ray diffraction (XRD) with thermal control, the microcrystalline components that are present at elevated temperatures can be identified.