Nanoclusters open fascinating opportunities for quantum engineering because quantum-size effects become dominant in determining catalytic, − optical, electronic, and magnetic6 properties. We succeeded in the controlled production of low-energy and high-flux monodispersed cluster beams, which allow for a systematic study of their reactivity after deposition onto a chemically inert substrate. We investigated the catalytic reaction that is the oxidation of CO on platinum and observed a distinct atom by atom size dependency for monodispersed platinum clusters on thin MgO(100) films. These results clearly show that the efficiency of a heterogeneous catalytic reaction can be tuned by the judicious choice of particle size.

We studied the cyclotrimerization of acetylene on size-selected Pd n clusters (1 e n e 30) supported on thin MgO(100) films by thermal desorption and Fourier transform infrared spectroscopy. Surprisingly, the production of benzene is already observed on a single palladium atom at low temperature (300 K). Using density functional theory (DFT) calculations we show that free inert Pd atoms are activated by charge transfer from defect sites of the MgO substrate upon deposition. For larger clusters (7 e n e 30) benzene is additionally produced at a temperature of 430 K and our results suggest the existence of a critical ensemble of seven palladium atoms for this high-temperature reaction mechanism.

Particles of nanometer size (nanoparticles) supported on well-characterized oxide surfaces are of particular interest to model the high complexity of real catalysts to answer questions such as the role of intrinsic size effects and the influence of the support. , Model systems so far consisted of size-distributed nanoparticles deposited on oxide substrates, − which do not allow an unambiguous determination of the cluster's chemical nature. Here, we report on the size-dependent chemical reactivity of nickel clusters, size selected and deposited with low energy (0.2 eV/atom) on thin MgO(100) films. Monodispersed Ni30 clusters show a higher reactivity for CO dissociation than Ni11 and Ni20. In particular, Ni30 clusters are extremely reactive and dissociate up to 10 CO molecules at temperatures below 280 K. Our results demonstrate that such small, supported clusters are unique for catalytic reactions not only due to their high surface-to-volume ratio but essentially because of the distinctive properties of different cluster sizes.