Characterization of the sites involved in the adsorption of CO on ZrO2 and ZnO/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 mechanism of formation of the formate species which can be considered as the intermediate adsorbed species in the methanol synthesis from CO/H-2, The adsorption 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 ZrO2 and 2183 cm(-1) on ZnO/ZrO2, It is shown that this adsorption follows the Langmuir's model on both solids. At higher temperatures (T>400 K) formats species are detected characterized on ZrO2 by IR bands at 2967, 2882, 1576 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 interpreted 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 adsorbed on the cationic site M-1, followed by a reaction with an OH group to produce 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. All rights reserved.