Infrared spectra of chemisorbed CO on Rh/γ–Al2O3: Site distributions and molecular mobility

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Abstract
Chemisorption of CO on a 0.92 wt. % Rh/γ-Al2O3 catalyst gave three species of adsorbed CO molecules: Species RhCO, at about 2000 cm−1; species Rh(CO)2, showing a doublet with peak frequencies at 2100 and 2030 cm−1; and a bridged species Rh2CO, leading to a very broad band peaked between 1850 and 1900 cm−1. The thermostability of these adsorbed species is in the sequence Rh2CO≳RhCO≳Rh(CO)2. Only the doublet band of Rh(CO)2 and the very broad band of the bridged species Rh2CO were found on well dispersed Rh metal in its δ phase. The band contours of the RhCO and Rh(CO)2 species were Fourier transformed into the time domain. The result showed that the configuration RhCO was spread over an approximately four times wider distribution of adsorption sites (surface–adsorbed CO interactions) than that of the Rh(CO)2 species. Furthermore, it was shown that the CO molecules adsorbed as RhCO species are ’’floppy,’’ undergoing some type of fast (probably librational) motion with a characteristic time of 0.1×10−12 sec. On the contrary, the CO molecules adsorbed as RH(CO)2 obey a slow modulation regime, with a characteristic time of about 3×10−12 sec. It is pointed out that this greatly different behavior may be of significance with regard to the selectivity and activity of the different catalytic sites as well as to the directional properties of the metal–carbon bond orbitals.