Characterization of the sites involved in the adsorption of CO on ZrO2 andZnO/ZrO2 methanol synthesis aerogel catalysts

The interaction of CO with two aerogel solids have been investigated by in situ FTIR spectroscopy in order to obtain experimental data on the mechanis m of formation of the formate species which can be considered as the interm ediate adsorbed species in the methanol synthesis from CO/H-2, The adsorpti on of CO on ZrO2 and ZnO/ZrO2 aerogel solids was studied in the temperature range 298-623 K. At temperatures lower than 373 K, CO is mainly reversibly adsorbed on a cationic site M-1, leading to an IR band at 2192 cm(-1) on Z rO2 and 2183 cm(-1) on ZnO/ZrO2, It is shown that this adsorption follows t he Langmuir's model on both solids. At higher temperatures (T>400 K) format s species are detected characterized on ZrO2 by IR bands at 2967, 2882, 157 6 1382 and 1367 cm(-1). It is shown that the formation of:this species: (a) decreases the number of cationic sites M-1 which adsorb CO at temperatures lower than 373 K and (b) creates a new cationic site M-2. This is interpre ted by a mechanism of formation of the formate species involving two sites of the surface: an OH group and a cationic site M-1. The CO is first adsorb ed on the cationic site M-1, followed by a reaction with an OH group to pro duce a formate species. This last species is adsorbed on the cationic site M-1 and a new cationic site M-2 is formed. (C) 1999 Elsevier Science B.V. A ll rights reserved.