Copper- and silver-zirconia aerogels: Preparation, structural properties and catalytic behavior in methanol synthesis from carbon dioxide

Copper- and silver-zirconia aerogels containing 10 at% IB metal were prepared from tetra-n-butoxy zirconium(IV) and IB metal acetates using the solution sol-gel method and ensuing high-temperature (HT) and low-temperature (LT) supercritical drying, respectively. The influence of preparation parameters and calcination on the structural and catalytic properties of the aerogels for the synthesis of methanol from carbon dioxide and hydrogen was investigated. After calcination in air at 573 K, the catalysts had BET surface areas in the range of 100-143 m(2) . g(-1) (Cu/ZrO2) and 77-125 m(2) . g(-1) (Ag/ZrO2), respectively. Due to the reductive alcoholic atmosphere during high-temperature supercritical drying, metallic copper and silver existed in all raw HT-aerogels. The mean size of the copper crystallites was 30 nm. The silver crystallite size for the HT-aerogel prepared with nitric acid was 10 nm, whereas for samples prepared with acetic acid it was 5-7 nm. Calcination in air at 573 K led to the formation of highly dispersed amorphous copper oxide and silver. Comparing the catalytic behavior of the calcined copper-zirconia aerogels with corresponding xerogels prepared by coprecipitation revealed highest activity for the LT-aerogel, whereas the HT-aerogels were least active. In contrast, similar catalytic behavior was observed for the differently dried silver-zirconia samples. Generally, CO2-conversion of the copper-zirconia aerogels was markedly higher than that of the corresponding silver-zirconia aerogels, whereas methanol selectivity was similar. (C) 1998 Academic Press.