Energetics of Cyclohexene Adsorption and Reaction on Pt(111) by Low-Temperature Microcalorimetry Ole Lytken, Wanda Lew, Jonathan J. W. Harris, Ebbe K. Vestergaard, J. Michael Gottfried, † and Charles T. Campbell* Department of Chemistry, UniVersity of Washington, Box 351700, Seattle, Washington 98195-1700 Received March 12, 2008; E-mail: campbell@chem.washington.edu Abstract: 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-σ bonded cyclohexene on Pt(111), and the heat of adsorption is well described by a second-order polynomial (130 - 47 θ - 1250 θ 2 ) kJ/mol, yielding a standard enthalpy of formation of di-σ bonded cyclohexene on Pt(111) at low coverages of -135 kJ/mol and a C-Pt σ bond strength of 205 kJ/mol. At 281 K, cyclohexene dehydrogenates upon adsorption, forming adsorbed 2-cyclohexenyl (c-C 6 H 9,a ) and adsorbed hydrogen, and the heat of adsorption is well described by another second-order polynomial (174 - 700 θ + 761 θ 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. 1. Introduction Platinum is used to catalyze a wide range of dehydrogenation reactions. As the simplest examples of catalytic aromatization, the dehydrogenation reactions of cyclohexane and cyclohexene into benzene over platinum catalysts have been the focus of much kinetic and mechanistic study. Since Pt(111) is the most stable platinum single-crystal surface, it has been the main model catalyst used in such studies. Both cyclohexane and cyclohexene adsorb intact on Pt(111) at temperatures below 150 K. 1–7 Above 180 K both cyclohexane and cyclohexene dehydrogenate to form a c-C 6 H 9 intermediate, 1,3,4,7 which above 280 K dehydrogenates further into benzene. 1–5,7 XPS, 5 HREELS, 1,7 and NEXAFS 7 studies reveal that, upon adsorption on Pt(111) at 100 K, cyclohexene exhibits only sp 3 character, suggesting a loss of its double-bond character and the formation of two new σ bonds to the platinum surface; density functional theory (DFT) calculations by Morin et al. 8 corroborate this finding. The c-C 6 H 9 intermediate has been found, using XPS 5 and HREELS, 1,4 to have partially sp 2 character. This intermediate has been sug- gested to be a 2-cyclohexenyl species, 3 formed by removing an allylic hydrogen from adsorbed cyclohexene. This is sup- ported by DFT calculations by Morin et al. who find 2-cyclo- hexenyl to be the most stable c-C 6 H 9 species on Pt(111). 8 Below, we will refer to this c-C 6 H 9 intermediate as the π-allyl. A similar intermediate has been observed for cyclohexene and cyclohex- ane adsorption on Pt(100). 9 Above 280 K the π-allyl dehydro- genates further to adsorbed benzene. At elevated temperatures some benzene desorbs (if at high coverage) and the remainder dehydrogenates further via an intermediate with a 2:1 C/H ratio to eventually form graphitic carbon on Pt(111). 5 A similar dehydrogenation pathway has been reported for the adsorption of cyclohexene on platinum nanoparticles supported on thin- film Al 2 O 3 . 10 The π-allyl species has also been observed during high pressure reactions of cyclohexene with Pt(111). 11,12 The heats of adsorption of reactants, intermediates, and products are important parameters for all catalytic systems. They reveal the thermodynamics of the elementary steps, which directly (or indirectly via Brønsted relations) influence both activity and selectivity. Traditionally, heats of adsorption are measured on single crystals using Temperature Programmed Desorption (TPD) or equilibrium measurements. However at low coverage, most hydrocarbons decompose on Pt(111) before any desorption occurs, and both TPD and equilibrium measure- ments fail since they are based upon reversible adsorption/ desorption. At coverages close to saturation, dehydrogenation is suppressed, since no free sites exist on the surface to accommodate the dehydrogenation products, and desorption † Current address: Lehrstuhl fuer Physikalische Chemie II, Universitaet Erlangen, Egerlandstr. 3, 91058 Erlangen, Germany. (1) Henn, F. C.; Diaz, A. L.; Bussell, M. E.; Hugenschmidt, M. B.; Domagala, M. E.; Campbell, C. T. J. Phys. Chem. 1992, 96, 5965– 5974. (2) Bussell, M. E.; Henn, F. C.; Campbell, C. T. J. Phys. 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