Energetics of Cyclohexene Adsorption and Reaction on Pt(111) by Low-Temperature Microcalorimetry

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-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.

DFT-based modeling of benzene hydrogenation on Pt at industrially relevant coverage

“…The initial sticking probability s 0 is taken as 1 for all adsorption steps in this work, to avoid inclusion of data from experimental sources. This is realistic for benzene, with an initial measured sticking probability of 0.97 for adsorption on Pt(1 1 1) [35], but is significantly larger than experimentally observed values for hydrogen (<0.1) [69], cyclohexane (<0.1) [8] and cyclohexene (0.21 [70] to 0.95 [71]). The 1,3-cyclohexadiene initial sticking probability on Pt(1 1 1) could not be found.…”

“…The discrepancy in selectivity between theory and experiment may also be caused by other reasons. Firstly, it can also be due to the assumption in the model that all sticking coefficients are 1, while it is known from experiment that the initial sticking probability for CHE (0.21 [70]-0.95 [71]) is larger than for CHA (<0.1) [8]. The assumed initial sticking probability of 1 could decrease the possibility for CHE to remain on the surface.…”

Kinetic Prefactors of Reactions on Solid Surfaces

“…This derivation gives the same result as the more common kinetic derivation which assumes the desorption rate is proportional instead to θ and S is proportional to (1 − θ), but conceptually it is quite different. When measured carefully, sticking probabilities are often observed to be very close to unity right up to saturation, with the apparent decrease in long-term sticking probability above 90% of saturation coverage actually due to desorption of transiently adsorbed species [29,[60][61][62]. This θ/(1 − θ) factor in the desorption rate expression fully explains their short lifetimes on the surface at such high coverages: new sites or strongly repulsive lateral interactions are not needed to qualitatively explain such observations.…”

Structure and stability of weakly chemisorbed ethene adsorbed on low-index Cu surfaces: performance of density functionals with van der Waals interactions

“…A few experiments have characterized molecular binding geometries, adsorption heights, and adsorption energies using a combination of X-ray standing wave and temperature-programmed desorption (TPD) experiments [46]. However, the accurate determination of binding energies has been very difficult, with TPD experiments being the technique of choice at present, although more advanced techniques based on microcalorimetry and a direct determination of the adsorption energetics have become available more recently [71][72][73].…”