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

Abstract

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 O 2 adsorption reaction rate constants for the investigated Ag n Au m − ( 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 Au 2 −. In the reaction of the gold dimer with O 2 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 CO 2 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 Ag n O 4 − 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.