Co-Adsorption of O2 and CO on Au2- : Infrared Photodissociation Spectroscopy and Theoretical Study of [Au2 O2 (CO)n ]- (n=2-6).

ABSTRACT

The co-adsorption of O2 and CO on anionic sites of gold species is considered as a crucial step in the catalytic CO oxidation on gold catalysts. In this regard, the [Au2 O2 (CO)n ]- (n=2-6) complexes were prepared by using a laser vaporization supersonic ion source and were studied by using infrared photodissociation spectroscopy in the gas phase. All the [Au2 O2 (CO)n ]- (n=2-6) complexes were characterized to have a core structure involving one CO and one O2 molecule co-adsorbed on Au2- with the other CO molecules physically tagged around. The CO stretching frequency of the [Au2 O2 (CO)]- core ion is observed around ν˜ =2032-2042 cm-1 , which is about 200 cm-1 higher than that in [Au2 (CO)2 ]- . This frequency difference and the analyses based on density functional calculations provide direct evidence for the synergy effect of the chemically adsorbed O2 and CO. The low lying structures with carbonate group were not observed experimentally because of high formation barriers. The structures and the stability (i.e., the inertness in a sense) of the co-adsorbed O2 and CO on Au2- may have relevance to the elementary reaction steps on real gold catalysts.