Acetylene cyclotrimerization on supported size-selected Pd-n clusters (1 <= n <= 30): one atom is enough!

We studied the cyclotrimerization of acetylene on size-selected Pd, cluster s (1 less than or equal to n less than or equal to 30) supported on thin Mg O(100) films by thermal desorption and Fourier transform infrared spectrosc opy. Surprisingly, the production of benzene is already observed on a singl e palladium atom at low temperature (300 K). Using density functional theor y (DFT) calculations we show that free inert Pd atoms are activated by char ge transfer from defect sites of the MgO substrate upon deposition. For lar ger clusters (7 less than or equal to n less than or equal to 30) benzene i s additionally produced at a temperature of 430 K and our results suggest t he existence of a critical ensemble of seven palladium atoms for this high- temperature reaction mechanism.