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The UV (3.8 eV) photolysis of atomically-dispersed Rh I (CO) 2 species supported on an Al 2 O 3 surface in the presence of N 2 at 175 K caused the replacement of CO by N 2 ligands. Therefore dinitrogen chemisorption effectively probes the coordinatively unsaturated Rh(CO) species generated during photodecomposition of Rh I (CO) 2 / Al 2 O 3 . Two infrared bands, observed at 2234 and 2048 cm -1 , are attributed respectively to ν N2 and ν CO in Rh(N 2 )(CO), and a band at 2188 cm -1 is assigned to Rh(N 2 ) 2 surface species. The assignments are based on frequencies and the relative rates of spectral development for the two N 2 -containing species. The Rh(N 2 ) 2 species on Al 2 O 3 exhibits identical N-N stretching modes to a similar species produced by matrix isolation methods. It is shown that both the Rh-CO bond and the Rh-N 2 bond may be broken by photolysis.

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The ultraviolet (3.8 eV) photolysis of atomically dispersed RhI(13C18O)2 species supported on an Al2O3 surface in the presence of CO2 at 256 K has been studied by infrared spectroscopy (FTIR). Carbon dioxide is activated on photochemically produced RhI(13C18O) sites to produce various isotopically labeled rhodium gem-dicarbonyl species. The two major products of CO2 activation exhibit infrared bands at 2077 and 1958 cm-1 assigned to RhI(12C16O)(13C18O) species and bands at 2036 cm-1 and near 1958 cm-1 assigned to RhI(13C16O)(13C18O). The infrared band assignments for the isotopic rhodium gem-dicarbonyl species are supported by a thermally driven isotopic exchange experiment with 13C16O(g) and by a comparison to the frequencies predicted with frequency calculations based on known force constants. Two dissociation pathways are proposed for CO2 coordinated on RhI(13C18O) sites. Path I involves direct dissociation of the C−O bond in CO2 and Path II involves oxygen atom exchange between CO2 and CO ligands, followed by C−O bond dissociation in the CO2 ligand which was produced by the O exchange process. The lack of formation of oxidized rhodium carbonyl species during CO2 activation suggests that CO2 temporarily coordinates to RhI(CO) with η1 bonding. This work demonstrates the first steps of a potential low-temperature route for the conversion of CO2 into other molecules on catalyst sites using ultraviolet light as an energy source.

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 ligandbased 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.

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