INFRARED SPECTROSCOPIC STUDY OF THE ADSORPTION AND REACTIONS OF CO2 ON K-MODIFIED RH/SIO2

Adsorption of CO2 on K-free Rh/SiO2 at 140 K produced only a strong band at 2340 cm-1 characteristic of weakly bonded CO2. There was no carbonate formation and CO2 desorbed completely from this surface below 250 K without any detectable dissociation. Co-adsorption of H2 + CO2 on Rh/SiO2 led to the formation of CO bands at 2050 and 2015 cm-1, indicating the dissociation of CO2. Formate bands were not observed. Adsorption of CO2 on K-modified Rh/SiO2 produced an intense band at 1695-1675 cm-1 and a weak band at 1325-1360 cm-1, which are attributed to the formation of CO2- anions bonded to K+ ions. Spectral changes observed above 200 K suggest that this species is transformed into bidentate carbonate characterized by absorption bands at 1660, 1525, and 1360 cm-1. As the same features were observed for Rh-free samples, it was concluded that both species are coordinated to K+-sites on the silica surface. CO2- reacted with hydrogen on (K + Rh)/SiO2 to give formate at temperatures as low as 150 K. No formate was produced on K/SiO2 in the absence of Rh. © 1990.