Oxidized Pd clusters

In the first H2 exposure, the CN of the Pd—O(N) bond decreased up to about lOmin, accompanied by an increase in the CN of the Pd Pd bond. The growth ofthe Pd—Pd bond stopped when the CN of Pd—Pd reached 5.1. A small Pd—O bond appeared on exposure to 02, as can be seen at the beginning of the second exposure to H2. In the second exposure, the partially oxidized Pd clusters were quickly reduced on exposure to H2, in less than 2 min the CN reached 6.7, which was larger than after the first process. In the third and fourth runs, a similar change in the CN was observed, where the ultimate CN of Pd—Pd reached 7.8 and 8.7 after the third and fourth runs, respectively. The progressive increase in the CN with the number of H2 exposure cycles suggests that stepwise growth of Pd clusters occurred in the H-U SY support. 

The aqueous solution layer that forms at the metal interface can ultimately provide a medium for the dissolution of Pd ions or oxidized Pd clusters into the supported liquid layer where they can then act as homogeneous catalysts. As was discussed earlier, the acetoxylation of ethylene can be carried out over various Pda,OAcj, clusters where alkali metal acetates are typically used as promoters. DFT calculations were carried out on both the Pd2(OAc)2 and Pd3(OAc)e clusters in order to examine the paths that control the solution-phase chemistry. The Pd3(OAc)e cluster is the most stable structure but is known experimentally to react to form the Pd2(OAc)2 dimer and monomer complexes in the presence of alkali metal acetates. The reaction proceeds by the dissociative adsorption of acetic acid to form acetate ligands. Elthylene subsequently inserts into a Pd-acetate bond. The cation is then reduced by the reaction to form the neutral Pd°. The reaction is analogous to the Wacker reaction in which ethylene is oxidized over Pd + to form acetaldehyde. Pd° is subsequently reoxidized by oxygen to form pd2+[35,36,44]... 

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