Energetics of hydrocarbon adsorption on model catalysts

The energetics of hydrocarbon adsorption on model catalysts have been studied using a new single crystal adsorption calorimeter (SCAC) for benzene and cyclohexene adsorption on Pt(111) and using temperature programmed desorption (TPD) for propene adsorption on gold islands on TiO₂(110). A new SCAC specifically constructed for the determination of the heats of adsorption (ΔH_ads) consists of a new effusive molecular beam for low vapor pressure molecules, a new bakeable LiTaO₃ sensor for the calibration of the calorimeter in situ, a very thin (1 μm) single crystal metal sample and a very sensitive pyroelectric detector which is brought into physical contact with the very thin single crystal metal sample for measurement of ΔH_ads. Using this new SCAC, ΔH_ads and sticking probability (S) as a function of the fractional coverage of the adsorbate (&thetas;) are measured for the adsorption of benzene and cyclohexene on Pt(111). ΔH _ads extrapolated to &thetas; = 0 for benzene on Pt(111) was found to be 164 kJ/mole. S extrapolated to &thetas; = 0 for benzene and cyclohexene on Pt(111) was found to be 0.9 and 0.853, respectively. The shape of S as a function of &thetas; for both benzene and cyclohexene shows precursor mediated sticking. The adsorption of propene on rutile TiO₂(110) and on gold islands dispersed on TiO₂(110) [Au/TiO₂(110)], both at 120 K, has been studied using TPD, x-ray photoelectron spectroscopy (XPS) and He+ low energy ion scattering spectroscopy (LEIS). Propene adsorbs on both TiO₂(110) and Au/TiO₂(110), with desorption peak temperatures at low coverage of ∼190 and ∼240 K, respectively. The 190 K peak, seen also without Au, is due to propene bound to bare Ti sites. The 240 K peak is attributed to propene adsorbed to Ti sites at the edges of gold islands. A TPD feature at ∼150 K which is more prominent at higher Au coverages and higher propene doses, is due to propene bound only to metallic Au islands.