Metal-support interaction: A theoretical approach

Metal support effects in catalysis are poorly understood, but are important because they have been shown to profoundly affect catalytic rates and selectivity. Density functional calculations have been done to model the isolated single metal atom system rhodium gem-dicarbonyl an silica for which there is experiment infrared data The systems were modeled by calculations for RhCO, Rh4CO, (CO)RhOSiH3, (CO)(2)RhOSM3 and fragments thereof. Very good agreement was found between calculated and experimental infrared frequencies. Support effects include an upward shift of the CO frequency and charge transfer from Ph to the support. Per qualitative ideas the Ph charge, C-O frequency and pi backbonding correlate. The second CO on a supported Rh(1) atom is almost as strongly bonded as the first and the binding energy of Rh to the support is not affected by the adsorption of the CO molecules. Further the adsorption of a single CO on isolated Rh(1) could contribute to an observed infrared band in the 2040 to 2070 cm(-1) region which is normally assigned to CO on Ph clusters.