The effect of palladium clusters (Pdn, n =2–8) on mechanisms of acetylene hydrogenation: A DFT study.

The mechanisms of acetylene hydrogenation on palladium clusters (Pdn, 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 Pdn cluster from the reactant acetylene to the product ethane. One of the pathways undergoes through two intermediates, the vinyl (Pdn(H)⋯CH＝CH2) and ethene (Pdn⋯CH2一CH2) to form the ethane, and the other goes along vinylidene (Pdn(2H)⋯C＝CH2), ethylidyne Pdn(2H)⋯C＝CH3) and ethylidene (Pdn(2H)⋯CH＝CH3) to ethane. Those intermediates in the two pathways can convert into each other which make the reaction profile complicated. The value of n in Pdn cluster can directly affect the reaction pathway: when n ≤4, the acetylene hydrogenation reaction will proceed via the pathway of Pdn(2H)⋯CH ≡ CH → Pdn(H)⋯CH＝CH2 → Pdn⋯CH2 CH2 → Pdn(2H)⋯CH2 CH2 to form ethane. However, when n >4, the reaction choose the following pathway: Pdn(2H)⋯CH ≡ CH → Pdn(H)⋯CH＝CH2 → Pdn⋯CH一CH3 → Pdn (2H)⋯CH一CH3. In addition, the value of their turnover frequency (TOF) for the ethylene formation catalyzed by Pdn cluster is larger than that for ethane, which indicates that the catalytic cycles in the formation of ethylene is efficient. [ABSTRACT FROM AUTHOR]