Infrared spectroscopic study of the adsorption and reactions of CO 2 on K-modified Rh/SiO 2

Abstract

Adsorption of CO 2 on K-free Rh/SiO 2 at 140 K produced only a strong band at 2340 cm −1 characteristic of weakly bonded CO 2. There was no carbonate formation and CO 2 desorbed completely from this surface below 250 K without any detectable dissociation. Co-adsorption of H 2 + CO 2 on Rh/SiO 2 led to the formation of CO bands at 2050 and 2015 cm −1, indicating the dissociation of CO 2. Formate bands were not observed. Adsorption of CO 2 on K-modified Rh/SiO 2 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 CO 2 − 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. CO 2 − reacted with hydrogen on (K + Rh)/SiO 2 to give formate at temperatures as low as 150 K. No formate was produced on K/SiO 2 in the absence of Rh.