INFRARED-SPECTROSCOPY AND TEMPERATURE-PROGRAMMED DESORPTION STUDY OF ADSORBED THIOPHENE ON GAMMA-AL2O3

The adsorption of thiophene (C4H4S) On gamma-Al2O3 has been investigat ed in ultrahigh vacuum (UHV) using infrared (IR) spectroscopy and temp erature-programmed desorption(TPD). Following thiophene adsorption ont o gamma-Al2O3 at 130 K, TPD reveals two peaks with maximum rates of de sorption at 175 and similar to 220 K. The former peak is assigned to d esorption 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 provid es further evidence that thiophene interacts only weakly with the alum ina support; no decomposition of the thiophene overlayer is observed u pon heating to 600 K under UHV conditions or a partial pressure of thi ophene of 3.0 Torr. Three kinds of adsorbed thiophene species exist on the alumina surface at saturation coverage: one in which thiophene in teracts with hydroxyl groups, presumably via hydrogen bonding, a secon d in which thiophene is coordinated via its sulfur atom to coordinatel y unsaturated Al3+ sites on the surface, and a third species which is present only at high thiophene coverages. The heat of adsorption for t hiophene on gamma-Al2O3 has been determined under equilibrium conditio ns (P-Th = 3.0 Torr) to be Delta H-ads = -28.9 kJ/mol. A direct correl ation has been established between the IR and TPD data, permitting int egrated extinction coefficients to be determined for adsorbed thiophen e in both the monolayer and multilayer coverage regimes. Extinction co efficients in the two coverage regimes are markedly different, undersc oring the need to use care when interpreting the IR spectral intensiti es for adsorbed species. While, as expected, this study has shown that thiophene adsorbs only weakly on gamma-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.