STRUCTURAL ALTERATIONS OF HIGHLY DISPERSED RH/TIO2 CATALYST UPON CO ADSORPTION AND DESORPTION INVESTIGATED BY INFRARED-SPECTROSCOPY

Structural alterations of highly dispersed Rh/TiO2 Catalyst upon CO adsorption and desorption was investigated by Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy. It was found that, during CO adsorption and desorption, the inter-conversions between the three adsorbed species, namely linear Rh0CO at ca. 2070 cm-1, bridged Rh2(0)CO at 1865-1900 cm-1 and gem dicarbonyl Rh(I)(CO), at 2096 and 2032 cm-1, involve some unstable intermediate species. The CO adsorption at 300 K, resulted in the disruption of rhodium crystallites to isolated rhodium sites, and was found to proceed via a bridged Rh2ICO species (at ca. 2005 cm-1) as intermediate. The decarbonylation of supported RhI(CO)2 at elevated temperatures, which leads to the agglomeration of rhodium sites, takes place involving Rh2ICO and Rh(I)CO (at ca. 2087 cm-1) as intermediates. In addition to alteration of the dispersive state of rhodium, the structure of the TiO2 carrier was also observed to be changed during the adsorption of CO at 300 K, as manifested by the enhancement or depletion of surface hydroxyl groups on the carrier. The presence of hydrogen significantly promoted the decarbonylation of RhI(CO)2, via a process which did not involve Rh2ICO and Rh(I)CO as intermediates. While no chemical reaction between the gem dicarbonyl species and hydrogen was observed in the temperature range of 300-450 K, dissociation of the C-O bond and formation of hydrocarbon fragments, mainly originating from the bridged adsorbed CO, already occurred at temperatures as low as 325 K.