No abstract

The adsorption of thiophene (C4H4S) on γ-Al2O3 has been investigated in ultrahigh vacuum (UHV) using infrared (IR) spectroscopy and temperature-programmed desorption (TPD). Following thiophene adsorption onto γ-Al2O3 at 130 K, TPD reveals two peaks with maximum rates of desorption at 175 and ∼220 K. The former peak is assigned to desorption of multilayer thiophene while the latter peak is assigned to desorption of weakly chemisorbed thiophene from the alumina surface. IR spectroscopy of adsorbed thiophene at submonolayer coverages provides further evidence that thiophene interacts only weakly with the alumina support; no decomposition of the thiophene overlayer is observed upon heating to 600 K under UHV conditions or a partial pressure of thiophene of 3.0 Torr. Three kinds of adsorbed thiophene species exist on the alumina surface at saturation coverage:  one in which thiophene interacts with hydroxyl groups, presumably via hydrogen bonding, a second in which thiophene is coordinated via its sulfur atom to coordinately unsaturated Al3+ sites on the surface, and a third species which is present only at high thiophene coverages. The heat of adsorption for thiophene on γ-Al2O3 has been determined under equilibrium conditions (P Th = 3.0 Torr) to be ΔH ads = −28.9 kJ/mol. A direct correlation has been established between the IR and TPD data, permitting integrated extinction coefficients to be determined for adsorbed thiophene in both the monolayer and multilayer coverage regimes. Extinction coefficients in the two coverage regimes are markedly different, underscoring the need to use care when interpreting the IR spectral intensities for adsorbed species. While, as expected, this study has shown that thiophene adsorbs only weakly on γ-Al2O3, more importantly it has shown that the combined IR−TPD methods can be used to determine both the thiophene coverage and the mode of bonding with the surface.