IDENTIFICATION OF THE ADSORPTION MODE OF THIOPHENE ON SULFIDED MO CATALYSTS

The adsorption and reactions of thiophene (C4H4S) On sulfided Mo/Al2O3 catalysts have been investigated over wide ranges of pressure (10(-9) -5 x 10(2) Torr) and temperature (140-693 K) using infrared (IR) spect roscopy and temperature-programmed desorption (TPD). When dosed at 190 K, thiophene adsorbs molecularly onto sites located on MoS2-like stru ctures and on uncovered alumina regions of sulfided Mo/Al2O3 catalysts as determined by IR spectroscopy. Thiophene is weakly chemisorbed to the catalyst surface and desorbs in a single peak with a maximum rate of desorption at 243 K. Based upon interpretation of IR spectra, the a dsorption mode of thiophene on the sulfided Mo portion of the catalyst surface has been determined to be eta(1)(S), with thiophene bonded to coordinately unsaturated (cus) Modelta+ sites located on the edge pla nes of MoS2-like structures. The saturation coverage of thiophene on M oS2-like structures of a sulfided 17.6 wt % Mo/Al2O3 catalyst is estim ated to be C4H4S/Mo = 0.074. Thiophene is observed to become reactive on sulfided Mo/Al2O3 catalysts only in the presence of gas-phase hydro gen and at high temperatures (i.e., 693 K). Temperature-programmed des orption in ultrahigh vacuum following heating of a sulfided 9.4 wt % M o/ Al2O3 catalyst to 693 K in a thiophene/H-2 mixture results in desor ption of C-4 hydrocarbons, H2S, and H-2 from the catalyst surface.