Size-effects in the acetylene cyclotrimerization on supported size-selected Pd_n clusters (1≤n≤30)

Motivated by the suggestion that mainly geometric arguments can explain the observed structure sensitivity in the cyclotrimerization of acetylene to benzene on palladium single crystals and on supported palladium particles, we studied this reaction on size-selected Pd_n clusters (1≤n≤30) supported on thin MgO(100) films by thermal desorption spectroscopy. Already on small Pd_n (1≤n≤6), benzene is readily produced, mainly at low temperature (300 K). For larger clusters (7≤n≤30) benzene also desorbs at a temperature of 430 K, indicating that a critical ensemble of seven atoms is necessary for high-temperature cyclotrimerization. Using density functional theory, we attribute this high-temperature desorption of benzene to the presence of three-dimensional clusters.