Preparation and characterization of copper-containing alumina and silica aerogels for catalytic applications

Abstract

Catalytic, copper-impregnated alumina and silica aerogels were prepared. Alumina gels were prepared from a solution of aluminum chloride via an epoxide-assisted synthesis. Silica gels were fabricated from tetramethyl orthosilicate using a base-catalyzed approach to the hydrolysis and polycondensation reactions. Copper was introduced into the alumina and silica gels through exposure of the wet gel to a solution of copper(II) nitrate during a solvent-exchange step prior to aerogel formation via rapid supercritical extraction. Undoped silica and alumina aerogels were fabricated for comparison. A barrage of physical characterization methods were employed to analyze the aerogels as-prepared, following heat-treatment and following catalytic testing. These include bulk density, Brunauer-Emmett-Teller surface area, Barrett-Joyner-Halenda pore distribution, infrared spectroscopy, X-ray diffraction, and scanning electron microscopy with energy-dispersive X-ray spectroscopy. As-prepared copper-silica aerogels have density 0.11 g/cm3, surface area 780 m2/g, and 9-nm average pore diameter. As-prepared copper-alumina aerogels have density 0.09–0.11 g/cm3, surface area 430 m2/g, and 23-nm average pore diameter. Calcining to 800 °C results in 10% lower surface area and average pore size 22 nm for copper-silica aerogels, 70% lower surface area for copper–alumina aerogels and, in both types of materials, yields microcrystalline CuO. A catalytic testbed was employed to assess the suitability of the copper–alumina and copper–silica aerogels as three-way catalysts for eventual application in automotive pollution mitigation. Both copper–silica and copper–alumina aerogels performed as three-way catalysts.