Catalysis with small free noble metal clusters

In general, reaction mechanisms in heterogeneous catalysis are difficult to reveal, because of the complexity of the reaction system. Our approach employs mass-selected metal clusters as well-defined model reaction centers. Chemical reactions on these clusters are investigated in a temperature controlled radio frequency (rf)-ion trap setup. Product ion concentrations as a function of storage time enable the determination of reaction kinetics and reaction mechanisms of free clusters. As an example, the CO combustion reaction on small gold clusters is studied. It is known that supported gold clusters with few atoms up to nm size exhibit relevant catalytic properties. Our ion trap measurements reveal catalytic activity already for negatively charged gold dimers. When the reaction kinetics are investigated as a function of temperature, an intermediate product with CO and O2 coadsorbed can be isolated. In contrast, small atomic silver clusters have not been found relevant for catalytic oxidation processes so far. Our investigations show for the first time evidence for a strongly size dependent catalytic activity of Agn^-.