Catalytic Processes on Free Metal Clusters

Free clusters are ideal model systems to probe the influence of their intrinsic, size-dependent properties on the catalytic activity due to the lack of any support interactions. Free clusters are prepared from cluster sources [20] and only very low densities are obtained. They are highly unstable under normal conditions and, even under UHV conditions, exothermal catalytic reactions may lead to fragmentation without the presence of a buffer gas. Thus, free clusters may not become relevant for industrial applications. Nevertheless, they are important vehicles to gain a fundamental understanding of nanocatalysis. 

Let us now turn our attention to Au2 where a full catalytic cycle could unambiguously be observed by measuring the kinetics of the process at different 

02 Pa p(CO) = 0.03 Pa p(He) = 1.0 Pa. Open symbols represent the normalized experimental data. The solid lines are obtained by fitting the integrated rate equations of the catalytic reaction cycle (see text) to the experimental data. (C) Five optimized structures (a)-(e) ofAu2C03, with bond lengths in A. The relative stability of these structures is 

40% for n 4 so only a few collisions would be required for complete conversion. From this it was concluded that at near room temperatures the gas-phase metal cluster anions are better catalysts than the supported catalysts used in current technology for automotive catalytic converters. These need to be heated to high temperatures on platinum surfaces, temperatures of 400-500 K are typically required for oxidation of CO [45]. 

Both examples revealed unique and size-dependent catalytic properties of very small metal clusters. As mentioned in the introduction, gas phase clusters will probably never be important in real catalysis, however, such small clusters can be stabilized on support materials so they may become relevant for future applications. 

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