Mechanism of Selective Hydrogenation of α,β-Unsaturated Aldehydes on Silver Catalysts: A Density Functional Study

Supported silver catalysts exhibit a remarkably high selectivity in the industrially important hydrogenation of alpha,beta-unsaturated aldehydes to unsaturated alcohols. We carried out density functional calculations to clarify factors that affect the catalytic function of silver in hydrogenating unsaturated aldehydes. We examined the activity and the selectivity of model silver catalysts for acrolein, the simplest, yet most difficult unsaturated aldehyde to be selectively hydrogenated. We focused on describing bulky catalyst particles, represented by sites on extended silver surfaces, on the regular clean Ag(110) surface and the surface O-sub/Ag(111) with subsurface oxygen centers. On Ag(110) our results imply propanal, the undesired saturated aldehyde, to be the main product. In contrast, the calculations suggest a very high selectivity of O-sub/Ag(111) for the corresponding unsaturated alcohol, allyl alcohol, although the activity of this system is lower than that of clean silver. At valiance with Pt(111), where the selectivity to allyl alcohol is strongly reduced by the hindered desorption of the latter, allyl alcohol and propanal products are predicted to desorb easily from both Ag(110) and O-sub/Ag(111) at common reaction temperatures. We also analyzed inherent limitations for all accurate description of the chemical regioselectivity by contemporary computational methods.