Vibrational spectroscopy of CO adsorbed on supported ultra-small transition metal particles and single metal atoms

Ultra-small rhodium, palladium and iridium particles with minimum average sizes of five atoms have been grown on a thin, well-ordered alumina film at temperatures of 90 K and below. Scanning tunneling microscopy (STM) served to characterize the morphology of these deposits. In the infrared spectra of adsorbed carbon monoxide, characteristic features were observed by infrared reflection absorption spectroscopy (IRAS). These originate from CO adsorbed on single metal atoms and extremely small aggregates, most likely dimers or trimers. In this way, we have detected the atomic nuclei of heterogeneous rhodium nucleation at 90 K. Located at oxide point defects, these atoms form rhodium dicarbonyl species with a symmetric stretch frequency of 2117 cm−1, while for iridium dicarbonyl species a frequency of 2107 cm−1 was found. At 60 K, rhodium aggregates also grow elsewhere on the oxide film. Comparing the nucleation behaviour at low temperatures, we find an increasing metal–oxide interaction strength in the order Pd<Rh<Ir.