Cobalt-alumina sol gels: Effects of heat treatment on structure and catalytic ability

Cobalt-alumina (Co-Al) aerogel catalysts show potential as less expensive, sustainable alternatives to the platinum-group-metal catalysts used in three-way catalytic converters. Effects of heat treatment on the structure of Co-Al xerogels and aerogels and catalytic performance of Co-Al aerogels were investigated. Alumina gels were prepared by epoxide-assisted sol-gel synthesis, impregnated with ~ 3% cobalt, and either processed via rapid supercritical extraction to yield aerogels, or dried under ambient conditions to form xerogels. These materials were calcined at temperatures ranging from 400 °C to 1100 °C and characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, powder X-ray diffraction, thermogravimetric analysis and Brunauer-Emmett-Teller gas adsorption. When the aerogels and xerogels were heated to ~ 500 °C, crystalline portions of the alumina support underwent the expected phase transformation from boehmite to γ-alumina. Aerogels resisted further changes in samples heated to 1100 °C, whereas γ-alumina in the xerogels converted to α-alumina by 1100 °C. Heat-treated aerogels had high thermal stability, evidenced by their maintaining a surface area of 400 (± 30) m2/g after significant heating and resisting catalytically inhibiting θ/α-alumina phase changes. Color changes observed following heating are consistent with changes in cobalt-containing species within the materials. Preliminary catalytic tests showed that pre-heating Co-Al aerogels to 750 °C improves catalytic performance.