Adsorption and vibrations of α,β-unsaturated aldehydes on pure pt and Pt-Sn alloy (111) surfaces I.: Prenal

The adsorption and thermal decomposition of the alpha,beta-unsaturated aldehyde prenal (3-methyl-2-butenal) have been studied on Pt(111), the Pt3Sn/Pt(111) and Pt2Sn/Pt(111) surface alloys, and the corresponding terminations of the Pt3Sn(111) bulk alloy by means of high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). By comparing the experimental results with extensive theoretical calculations of the multitude of possible adsorption configurations of prenal using density functional theory (DFT), the adsorption configurations actually present on all model catalysts have been identified. This approach, thus, reveals a new way to identify complex, multifunctional molecules adsorbed on model catalyst surfaces. On Pt(111), prenal is strongly adsorbed and decomposes at approximately 300 K. By the aid of density functional theory (DFT), five flat-lying adsorption structures of eta(2), eta(3), and eta(4) hapticity, which exhibit similar adsorption energies E-ads between -47 and -59 kJ/mol, have been identified on the surface. The adsorption energy of prenal on the considered Pt-Sn alloys is significantly weaker. On the Pt3Sn and the Pt2Sn/Pt(111) surface alloys, the HREEL spectra recorded at 170 K are essentially assigned to two vertical eta(1)-top-(s)-trans configurations (E-ads = -39.1 and -30.8 kJ/mol on Pt3Sn and -33.4 kJ/mol on Pt2Sn) adsorbed atop the protruding Sn atoms. Due to the weak adsorption of these structures, the vibrational frequencies are only slightly perturbed as compared to their corresponding gas-phase values: The primary role of tin is a general weakening of the adsorption of prenal on the alloy surfaces. While on Pt(111), flat adsorption configurations are preferred, alloying with tin induces a drastic change in the adsorption geometries to vertical eta(1)-top forms. On the alloy surfaces, generally an oxygen-tin interaction is required to form competitive adsorption structures at all, whereas a coordination from the aldehydic function to a Pt is hardly stable.