A 13C NMR study of the adsorbed states of CO on Rh dispersed on Al2O3

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Abstract
The results of nuclear magnetic resonance(NMR)spectroscopy have been analyzed with respect to previous infrared studies of CO adsorbed on Rh dispersed on Al2O3 to quantify the site distribution and to describe the adsorbed state. The 13C NMRspectra account for all the 13CO adsorbed on a 2.2% Rh on Al2O3 substrate. Although the spectra from the different adsorbed states of CO overlap, the line shapes may be separated into two components based on differences in the 13C spin–lattice relaxation times. These two components have been assigned to the 13CO dicarbonyl formed on single Rh atoms and to 13CO adsorbed on Rh rafts. The component attributed to the CO adsorbed on the raft sites is further separated into linear and bridged CO state contributions based on chemical shiftinformation, yielding a quantitative distribution of the three adsorbed states of CO on Rh. The 13CO distribution is used to estimate the molar integrated intensities of the infrared spectrum of 13CO on Rh at high coverage and to determine the degree of dispersion of Rh on the Al2O3. The 13C NMRline shapes of CO adsorbed on Rh are different from the powder pattern of Rh2Cl2(CO)4. It is suggested that the line shape of the dicarbonyl surface species is narrowed to a Lorentzian curve by reorientation at the site and the line shape of CO on the Rh rafts is modulated by exchange between sites on a single raft. The 13C relaxation time distribution provides further evidence for the existence of isolated Rh atoms on the Al2O3 surface.

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