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The adsorption geometries of acrolein, crotonaldehyde, methylcrotonaldehyde (prenal), and cinnamaldehyde on Pt and Pd surfaces have been studied by means of semiempirical extended Hckel calculations: Pt(111), Pt(100), Pt(110), and Pd(111) surfaces, and steps on Pt(111) have been compared. Depending on the face and the substituents of the organic molecule, the adsorption geometries are totally different: a di-σ form is preferred on Pt(111), a planar η4 one on Pd(111) and Pt(100), and a π_CC one on Pt(110) and on the step. These preferred adsorption modes allow one to explain the selectivity observed during the hydrogenation of prenal on these well-defined surfaces. The results obtained in this work also allow one to understand the behavior of different metal catalysts towards the hydrogenation of the α-β unsaturated aldehydes.