Low-Temperature CO Adsorption on Ag+/SiO2 and Ag−ZSM-5: An FTIR Study

Abstract

Carbon monoxide is adsorbed on Ag+ cations of Ag+/SiO2 and Ag−ZSM-5 samples via coordination bonds having predominantly σ-character, with the π-character being weak. The support strongly affects the strength of the Ag−CO bond. Thus, the carbonyls on Ag+/SiO2 are characterized by ν(CO) at 2169 cm-1 and are easily decomposed by evacuation even at low temperature, which reveals weak σ-bonding. On the contrary, the Ag+−CO species formed on Ag−ZSM-5 absorb at 2192 cm-1 and are highly resistant toward evacuation even at ambient temperature, indicating a relatively strong σ-bond. The Ag+ cations in Ag−ZSM-5 are able to coordinate a second CO molecule, thus forming dicarbonyls at low temperature. The angle between both CO ligands is near 180°, and as a result, the dicarbonyl species gives rise to a single C−O stretching mode at 2186 cm-1. These conclusions are confirmed by adsorption of 12CO−13CO isotopic mixtures.