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Acetaldehyde on Pt(111) and Pt/Sn(111): a DFT study of the adsorption structures and of the vibrational spectra.

The relative stability of the eta1mu1 (atop) and eta2mu2 (di-sigma) geometries of acetaldehyde are compared on Pt(111) and on two PtSn alloys ((2 x 2) and (square root(3) x square root(3))R30 degrees) by means of density functional theory (DFT) calculations. At low coverage on Pt (1/9 ML), the two forms are equivalent in energy, with eta1mu1 being slightly more stable. At high coverage (1/4 and 1/3 ML), eta2mu2 is less competitive and acetaldehyde is adsorbed through the aldehydic hydrogen. The evolution of the adsorption energy with the coverage and the apparition of the structure adsorbed through the aldehydic hydrogen are explained by the existence of attractive dipole-dipole interactions. On PtSn, only the eta1mu1 geometry is stable with an adsorption energy equal to that on Pt, in agreement with temperature-programmed desorption (TPD) experiments. The calculated vibrational spectra allow us to conclude that the experimental spectrum corresponds to a mixture of eta1mu1 (majority) and eta2mu2 (minority) structures on Pt and to only eta1mu1 on PtSn. The various interactions and the relative stability of the species on Pt and PtSn are explained by the density of states (DOS) curves.