Calorimetric Measurement of the Heat of Adsorption of Benzene on Pt(111)

Abstract

The heat of adsorption of benzene on clean Pt(111) at 300 K is measured calorimetrically and found to decrease with coverage (θ) as (197 -48θ -83θ 2 ) kJ/mol. Saturation coverage (θ ) 1.0) is 2.3 × 10 14 molecules/cm 2 . Sticking probabilities of benzene on Pt(111) were measured by mass spectrometry, giving an initial value of 0.97 and showing Kisliuk-type behavior with increasing coverage that implies there is a precursor to sticking with a ratio of its hopping rate to its desorption rate of ∼28. Benzene adsorbs transiently on the benzene-saturated surface at 300 K with a trapping probability of 0.90 and a heat of adsorption of 63-73 kJ/mol.

Heat of Adsorption of Naphthalene on Pt(111) Measured by Adsorption Calorimetry

“…The average bond energy is 317/10 kJ/mol ≈ 32 kJ/mol. Within the margins of error of our measurement, this value is identical with the average Pt-C bond energy found for benzene, 3 33 kJ/mol. If we exclude the first pulse, which is probably influenced by adsorption on defect sites, from the extrapolation to zero coverage and use the second-order polynomial fit in eq 3, the initial adsorption heat is reduced to 300 kJ/mol.…”

“…Compared to benzene (V 0 ) 21.8 kJ/mol, ) 6.9 kJ/mol), 3 we find here a higher value for V 0 , which is plausible considering the larger size of the naphthalene molecules. We also find a higher value for , which seems to scale roughly with the number of C atoms.…”

“…Using the gas-phase standard enthalpy of formation of naphthalene, 150.6 kJ/mol, 23 we obtain a value of -166 kJ/mol in the limit of zero coverage (or, with eq 3 under exclusion of the first pulse, -149 kJ/mol). For comparison, benzene on Pt(111) at zero coverage has a standard heat of formation of -114 kJ/mol as can be estimated from the heat of adsorption, 197 kJ/mol, 3 and the gas-phase standard heat of formation, 82.9 kJ/mol. 24 The heat of adsorption can also be employed to calculate the average bond energy between an sp 2 hybridized aromatic C atom in the naphthalene molecule and the Pt(111) surface.…”

“…3,4 Briefly, we employed an ultrahigh vacuum (UHV) chamber equipped with the SCAC, the MBS, and facilities for X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), low-energy ion scattering (LEIS), and low-energy electron diffraction (LEED).…”

“…3,4 Here, we have employed this technique to study the adsorption of naphthalene on a Pt(111) surface.…”