Electronic Excitation of RhI(CO)₂ Species on Al₂O₃ Surfaces:  Photodissociation of the RhI−CO Bond

The photoexcitation of Rh I (CO) 2 /Al 2 O 3 has been studied by infrared and ultraviolet absorption spectroscopy. A photodissociation energy threshold between 2.8 and 2.9 eV, for the Rh I −CO bond of the Rh I (CO) 2 species, has been determined by monitoring the depletion of the CO moiety, leading to the production of a highly active surface site. A Rh I −CO photodissociation cross section of 3.6 × 10 -20 cm 2 photon -1 and a quantum efficiency of 0.01 [Rh I −CO dissociation] photon -1 has been estimated for the initial stages of Rh I (CO) 2 photodepletion. Three absorption bands, observed at 3.3, 3.9, and 4.6 eV in the ultraviolet absorption spectra, are assigned to electronic transitions from filled Rh d-character molecular orbitals to d-character and/or ligand-based antibonding orbitals. Photodissociation of the Rh I −CO bond occurs from the direct excitation of Rh I (CO) 2 species. During later stages of photolysis, the role of depletion of Rh I (CO) 2 decreases dramatically and this effect may be due to the slow diffusion of CO through the pores of the Al 2 O 3 support.