Ebbe K. Vestergaard

Observation of all the intermediate steps of a chemical reaction on an oxide surface by scanning tunneling microscopy.

By means of high-resolution scanning tunneling microscopy (STM), we have revealed unprecedented details about the intermediate steps for a surface-catalyzed reaction. Specifically, we studied the oxidation of H adatoms by O(2) molecules on the rutile TiO(2)(110) surface. O(2) adsorbs and successively reacts with the H adatoms, resulting in the formation of water species. Using time-lapsed STM imaging, we have unraveled the individual reaction intermediates of HO(2), H(2)O(2), and H(3)O(2) stoichiometry and the final reaction product-pairs of water molecules, [H(2)O](2). Because of their different appearance and mobility, these four species are discernible in the time-lapsed STM images. The interpretation of the STM results is corroborated by density functional theory calculations. The presented experimental and theoretical results are discussed with respect to previous reports where other reaction mechanisms have been put forward.

Energetics of cyclohexene adsorption and reaction on Pt(111) by low-temperature microcalorimetry.

The heat of adsorption and sticking probability of cyclohexene on Pt(111) were measured as a function of coverage using single-crystal adsorption calorimetry in the temperature range from 100 to 300 K. At 100 K, cyclohexene adsorbs as intact di-sigma bonded cyclohexene on Pt(111), and the heat of adsorption is well described by a second-order polynomial (130 - 47 theta - 1250 theta(2)) kJ/mol, yielding a standard enthalpy of formation of di-sigma bonded cyclohexene on Pt(111) at low coverages of -135 kJ/mol and a C-Pt sigma bond strength of 205 kJ/mol. At 281 K, cyclohexene dehydrogenates upon adsorption, forming adsorbed 2-cyclohexenyl (c-C6H(9,a)) and adsorbed hydrogen, and the heat of adsorption is well described by another second-order polynomial (174 - 700 theta + 761 theta(2)) kJ/mol. This yields a standard enthalpy of formation of adsorbed 2-cyclohexenyl on Pt(111) at a low coverage of -143 kJ/mol. At coverages below 0.10 ML, the sticking probability of cyclohexene on Pt(111) is close to unity (>0.95), independent of temperature.