An infrared study of the physisorption and reaction of molybdenum hexacarbonyl on γ-alumina

The infrared spectra of molybdenum hexacarbonyl adsorbed on two types of γ-alumina have been determined after initial adsorption at 25°C and after activation in flowing helium at 100°C to produce Mo(CO)3ads. Nearly fully hydroxylated alumina (HA), partially dehydroxylated alumina (PDA, pretreated at 450–500°C), and nearly fully dehydroxylated alumina (DA, pretreated at 1000°C) were employed. Reaction between physisorbed Mo(CO)6 and the surface of alumina commences at 25°C or slightly above. It is proposed that surface O2− (σ-O−) or OH− (σ-OH) add to the carbon atom of one CO ligand with consequent labilization of other CO ligands for displacement by σ-O− or σ-OH. An infrared absorption band for Mo(CO)3ads was observed on HA, PDA, and DA at near 1580 cm−1 but only weakly with HA. On HA it is proposed that Mo(CO)3ads is primarily (σ-OH)3Mo(CO)3, and on DA, species like (σ-O)3Mo(CO)2(C00-σ). On PDA, (σ-O)3 would be replaced by (σ-O)2(σ-OH) or (σ-O)(σ-OH)2.