Size and composition dependence in CO oxidation reaction on small free gold, silver, and binary silver-gold cluster anions

Gas phase kinetics experiments with mass-selected gold, silver, and binary silver-gold cluster ions in a radio frequency (rf)-ion trap reactor are presented. For the catalytic CO combustion reaction, the adsorption of molecular oxygen onto the free clusters is identified as the first reaction step. A comparison of the measured O2 adsorption reaction rate constants for the investigated AgnAum− (n + m = 1, 2, 3) clusters reveals a pronounced size and composition dependence. Favorable activation of the oxygen molecular bond is only expected in the case of Au2−. In the reaction of the gold dimer with O2 and CO, a coadsorption complex is identified at cryogenic temperatures as decisive reaction intermediate in the observed CO oxidation reaction. Through detailed reaction kinetics measurements in combination with first-principles calculations, a comprehensive understanding of the molecular details of the catalytic reaction cycle emerges. The obtained data also permit the determination of the CO2 formation rate and the catalytic turn-over-frequency in the rf-ion trap reactor. In contrast to gold, odd size silver cluster anions are found to adsorb two oxygen molecules. These AgnO4− complexes are proposed to be key intermediates in oxidation reactions with silver clusters involved, and first experimental indications for catalytic activity of selected cluster sizes are presented.