Adsorption and Vibrations of alpha,beta-Unsaturated Aldehydes on Pt(111) and Pt-Sn Alloy (111) Surfaces. 3. Adsorption Energy vs Adsorption Strength

作者：Haubrich J^*; Loffreda D; Delbecq F; Sautet P; Jugnet Y; Becker C; Wandelt K

来源：Journal of Physical Chemistry C, 2010, 114(2): 1073-1084.

DOI：10.1021/jp908353p

Recently we presented high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD) studies on the adsorption of prenal (3-methyl-2-butenal) and crotonaldehyde (2-butenal) on Pt(111) and on ultrathin Pt3Sn/Pt(111) and Pt2Sn/Pt(111) surface alloys. Vibrational studies employing HREELS have been analyzed with the help of extensive density functional theory (DFF) calculations of the energetic, structural. and vibrational properties of a large set of stable adsorption configurations. We were able to identify various adsorbate Structures ranging from low hapticity eta(1)-top and eta(2)-di sigma(CC) to flat eta(4) types present in mixed phases below room temperature oil these substrates. Completing this study, we now analyze common features and differences between the two molecules on the three surfaces. The key finding is that the adsorption energy turns out to be all unsatisfying measure of the interaction strength Of Such complex molecules on surfaces. For several identified Structures lower desorption temperatures and smaller adsorption energies from DFF corroborated a weaker adsorption strength; however, vibrational and geometric properties obtained consistently from experiments and theory pointed to similarly strong molecule-surface bonding. We will show here that this phenomenon can only be rationalized by clearly differentiating between adsorption energy and interaction energy. Since the adsorption process includes significant relaxations of surface metal atoms and adsorbed molecules to optimize their covalent interactions, there is a deviation between the adsorption strength, measured by the interaction energy, and the final adsorption energy where the endothermic energetic costs of both relaxation processes have to be taken into account. Calculating these properties from DFT, we can show that adsorption energies may indeed decrease while the overall interaction energy of a molecule with a surface can, nonetheless, increase. The adsorption energy is hence not a good, unambiguous measure of molecule-surface interactions.

• 出版日期2010-1-21