Fabrication and characterization of cerium- and copper-containing aerogels

Aerogels have unusual characteristics such as high surface area, low density, and high thermal conductivity, which render them attractive for applications in heterogeneous catalysis. In this study, cerium-​copper-​silica (Ce-​Cu-​Si), cerium-​copper-​alumina (Ce-​Cu-​Al), and copper-alumina (Cu-Al) aerogels were prepared and characterized. Silica wet gels were fabricated through a tetraethyl orthosilicate (TEOS) based recipe, using oxalic acid to catalyze hydrolysis and ammonia to catalyze gelation. Alumina wet gels were fabricated from aluminum chloride hexahydrate through an epoxide-​assisted reaction. These silica and alumina gels were impregnated with cerium(III) and/or copper(II) during an ethanol solvent-​exchange step, with molar ratios of 75:25, 50:50 and 25:75 Ce:Cu employed for the mixed-metal aerogels and varying amounts of copper(II) for the Cu-Al gels. The resulting gels were then processed via rapid supercritical extraction to yield aerogels. The aerogels were heat-treated at 800°C for 24 h and characterized using a variety of physical and spectroscopic methods. Bulk densities are in the range of 0.083 to 0.098 g​/mL. The observed color variations of the gels during fabrication and heat treatment show changes in ligand structure and speciation of Ce and Cu. The aerogels are predominantly amorphous; however, XRD identified microcrystalline regions within the aerogel matrixes. Ce-​Cu-​Si aerogels include small peaks attributable to CuO and several peaks that indicate the presence of Ce(IV)​.  XRD plots for Ce-​Cu-​Al aerogels demonstrate the presence of gamma-​alumina and Ce(IV)​. The aerogels in this study were subjected to catalytic characterization using an in-lab-​constructed catalytic testbed. Aerogels were exposed to a mixture of propane, NO and CO and the conversion was measured. These aerogels show promise as three-​way catalysts.