Thermodynamics and kinetics of CO and benzene adsorption on Pt(111) studied with pulsed molecular beams and microcalorimetry

The adsorption and desorption of the system CO/Pt(111) and C6H6/Pt(111) at 300 K has been investigated with a pulsed molecular beam method in combination with a microcalorimeter For benzene the sticking probability has been measured in dependence of the coverage 0 For coverages theta>0 8 transient adsorption is observed From an analysis of the time dependence of the molecular beam pulses the rate constant for desorption is determined to be 5 6 s(-1) With a precursor mediated kinetic adsorption model this allows to obtain also the hopping rate constant of 95 5 s(-1) The measured adsorption enthalpies could be best described by (199 77 theta 51 theta(2)) kJ/mol in good agreement with the literature values For CO on Pt(111) also transient adsorption has been observed for theta>0 95 at 300 K The kinetic analysis yields rate constants for desorption and hopping of 20 s(-1) and 51 s(-1) respectively The heats of adsorption show a linear dependence On coverage (131 - 38 theta) ki/mol between 0 <=theta <= 0 3 which is consistent with the desorption data from the literature For higher coverage (up to theta = 0 9ML) a slope of -63 kJ/mol describes the decrease of the differential heat of adsorption best This result is only compatible with desorption experiments if the pre-exponential factor decreases strongly at higher coverage We found good agreement with recent quantum chemical calculations made for (theta = 0 5ML) (C) 2010 Elsevier B V All rights reserved