The mechanisms of acetylene hydrogenation on palladium clusters (Pd-n n=2-8) are researched by using the B3PW91/GENECP method of density functional theory. The calculation results indicate that there are two possible pathways for the hydrogenation reaction on Pd-n, cluster from the reactant acetylene to the product ethane. One of the pathways undergoes through two intermediates, the vinyl (Pd-n(H)center dot center dot center dot CH=CH2) and ethene (Pd-n center dot center dot center dot CH2-CH2) to form the ethane, and the other goes along vinylidene (Pd-n(2H)center dot center dot center dot C=CH2), ethylidyne (Pd-n(2H)center dot center dot center dot C=CH3) and ethylidene (Pd-n(2H)center dot center dot center dot CH=H-3) to ethane. Those intermediates in the two pathways can convert into each other which make the reaction profile complicated. The value of n in Pd-n cluster can directly affect the reaction pathway: when n <= 4, the acetylene hydrogenation reaction will proceed via the pathway of Pd-n(2H)center dot center dot center dot CH CH -> Pd-n(H)center dot center dot center dot CH=CH2 -> Pd-n center dot center dot center dot CH2-CH2 -> Pd-n(2H)center dot center dot center dot CH2-CH2 to form ethane. However, when n>4, the reaction choose the following pathway: Pd-n(2H)center dot center dot center dot CH CH -> Pd-n(H)center dot center dot center dot CH=CH2 -> Pd-n center dot center dot center dot CH-CH3 -> Pd-n(2H)center dot center dot center dot CH-CH3. In addition, the value of their turnover frequency (TOF) for the ethylene formation catalyzed by Pd-n cluster is larger than that for ethane, which indicates that the catalytic cycles in the formation of ethylene is efficient. (C) 2012 Elsevier B.V. All rights reserved.