Low-temperature CO adsorption on Ag+/SiO2 and Ag-ZSM-5:: An FTIR study

Carbon monoxide is adsorbed on Ag+ cations of Ag+/SiO2 and Ag-ZSM-5 samples via coordination bonds having predominantly sigma-character, with the pi-character being weak. The support strongly affects the strength of the Ag-CO bond. Thus, the carbonyls on Ag+/SiO2 are characterized by nu(CO) at 2169 cm(-1) and are easily decomposed by evacuation even at low temperature, which reveals weak sigma-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 alpha-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 degrees, 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 (CO)-C-12-(CO)-C-13 isotopic mixtures.