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 b enzene on palladium single crystals and on supported palladium particles, w e studied this reaction on size-selected Pd-n clusters (1 less than or equa l to n less than or equal to 30) supported on thin MgO (100) films by therm al desorption spectroscopy. Already on small Pd-n (1 less than or equal to n less than or equal to 6), benzene is readily produced, mainly at low temp erature (300 K). For larger clusters (7 less than or equal to n less than o r equal to 30) benzene also desorbs at a temperature of 430 K, indicating t hat a critical ensemble of seven atoms is necessary for high-temperature cy clotrimerization. Using density functional theory, we attribute this high-t emperature desorption of benzene to the presence of three-dimensional clust ers. (C) 2000 Elsevier Science B.V. All rights reserved.