Synthesis and Characterization of Copper-Nanoparticle-Containing Silica Aerogel Prepared via Rapid Supercritical Extract
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Synthesis and Characterization of Copper-Nanoparticle-Containing Silica Aerogel Prepared via Rapid Supercritical Extraction for Applications in Three-Way Catalysis Ann M. Anderson1, Elizabeth A. Donlon1, Adam A. Forti1, Vinicius P. Silva1, Bradford A. Bruno1, Mary K. Carroll2 1 Department of Mechanical Engineering, 2Department of Chemistry Union College, Schenectady, NY 12302 USA
ABSTRACT Copper-alumina and copper-silica aerogels formed by impregnation of a copper(II) salt into an alumina or silica wet gel before supercritical extraction have been found to contain copper in multiple oxidation states: Cu0, Cu+1 and Cu+2. These aerogels are effective at catalyzing the reduction of NO and the oxidation of HCs and CO under conditions similar to those found in automotive three way catalysts. In this work we have developed a preparation method incorporating Cu0, Cu+1 and Cu+2 nanoparticles directly into silica aerogels. Nanoparticles in the form of (a) Cu0 nanorods (100 nm diameter, 10-20 μm length); (b) Cu+1 nanoparticles (350 nm diameter); and (c) Cu+2 nanoparticles (25-55 nm diameter) were added (0.5-15% by weight) to separate precursor mixtures consisting of tetramethyl orthosilicate, methanol, water and ammonia. These precursor mixtures were then processed using a rapid supercritical extraction (RSCE) method to form aerogels. The resulting aerogels show evidence of nanoparticles dispersed throughout the silica aerogel structure. Addition of Cu+1 and Cu+2 nanoparticles decreases the surface area of the aerogels significantly. X-Ray diffraction shows that regardless of initial oxidation state of the nanoparticles, crystalline Cu0 is detected after RSCE processing to 290 ˚C. Following heat treatment at 700 °C, crystalline Cu+2 is detected. The copper containing silica aerogels are found to be catalytically active with light-off temperatures (50% conversion) for NO and CO at 400 °C in three-way catalytic applications. INTRODUCTION A three-way catalytic converter (TWC) oxidizes carbon monoxide and unburnt hydrocarbons to carbon dioxide and water and reduces nitrogen oxides (NOx) to nitrogen gas. These devices are employed in automobiles to convert exhaust gas pollutants. Catalytic converters are typically formed from an alumina washcoat supported on a honeycomb structure. Catalysis is promoted through the use of one or more platinum group metals (PGM) in the washcoat; however, PGMs are rare, expensive and damaging to the environment when mined. Our work has focused on the use of catalytic aerogels in place of PGMs [1-6]. Aerogels are highly porous, lightweight, thermally stable and can be catalytically active. A number of investigators have studied their application in catalysis applications [7]. Due to their high porosity and low density, aerogels have high surface areas, which may promote catalytic activity. In previous work we fabricated catalytically active copper silica aerogels using an impregnation method with a copper(II) salt [6] and found that Cu0, Cu+1 and Cu+2 are present in the resulting aerogel. In this stu
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