Synthesis and Characterization of Copper-Nanoparticle-Containing Silica Aerogel Prepared via Rapid Supercritical Extraction for Applications in Three-Way Catalysis

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.