Adsorption of α,β-Unsaturated Aldehydes on Pt(111) and Pt-Sn Alloys: II. Crotonaldehyde

By employing high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED), we have studied the adsorption and thermal decomposition of crotonaldehyde (2-butenal) on Pt(111) as well as Pt3Sn/Pt(111) and Pt2Sn/Pt(111) surface alloys. In order to understand the adsorption structures and the vibrational properties, an extensive theoretical study of the possible adsorption geometries and vibrational spectra on the three considered Surfaces has been carried out using density functional theory (DFT). A careful analysis of the variety of possible adsorption configurations of crotonaldehyde allowed structural identification by correlating their vibrational fingerprints with the measured HREELS peaks. The mixed phases of crotonaldehyde formed on the model catalysts turned out to be even more complex than those we found in previous studies for prenal. The set of stable configurations identified by combination of HREELS with DFT consists of eta(2), eta(3), and eta(4) flat adsorption structures, which exhibit adsorption energies from -69 to -80 kJ/mol. Nonetheless, the thermal decomposition measured with TPD and the general adsorption behavior show similarities to prenal and acrolein. Starting from a strongly adsorbed state on Pt(111) at low temperatures, crotonaldehyde decomposes at temperatures close to 300 K. On the surface alloys, a high-coverage phase with structures of low hapticity such as eta(2)-di sigma(CC) is measured at low temperatures (similar to 160 K), and a low-coverage situation of high hapticity eta(2), eta(3), and eta(4) configurations is formed at higher temperatures (similar to 200 K). On Pt3Sn/Pt(111) two energetically competitive eta(1)-top-E-(s)trans-OSn forms cannot be excluded at low temperatures.