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The surface chemistry of thiophene (C 4 H 4 S) on sulfided Cu/Al 2 O 3 , Mo/Al 2 O 3 , and Rh/Al 2 O 3 catalysts has been investigated under well-defined conditions using infrared (IR) spectroscopy and temperature programmed desorption (TPD). The results of these experiments have been found to correlate nicely with CO chemisorption measurements of site densities and thiophene hydrodesulfurization (HDS) activities measured in a flow reactor system. In agreement with the literature, the catalysts were found to have dramatically different thiophene HDS activities increasing in the following order: sulfided Cu < sulfided Mo < sulfided Rh. The trend of HDS activities is mirrored in similar trends for the site densities and turnover frequencies for the sulfide catalysts. IR spectra of adsorbed thiophene on the sulfided Cu, Mo, and Rh catalysts at low temperatures show that the amount of thiophene adsorbed on cus metal sites of the sulfide catalysts correlates with the CO chemisorption estimates of site densities. Thiophene reactivity on the catalysts differs significantly, with reaction in thiophene/H 2 mixtures observed on the sulfided Mo and Rh catalysts annealed at 300 and 500 K, respectively, and not at all on a sulfided Cu catalyst annealed up to 700 K. Strongly adsorbed species produced on the sulfided Cu, Mo, and Rh catalysts during the annealing sequence were subjected to TPD experiments that yielded C 4 hydrocarbons and H 2 S, with the predominant C 4 product being butenes. IR and TPD measurements of 1,3-butadiene and 1-butene on sulfided Rh/Al 2 O 3 catalysts, both in the pure gas and in gas/H 2 mixtures, provide evidence for the tentative assignment of strongly adsorbed species produced by cleavage of C-S bonds in thiophene to be a σ-bonded allyl species with C 4 H 7 stoichiometry. † Part of the special issue "Gabor Somorjai Festschrift".

Interpretation and comparison of the vibrational spectra of organometallic complexes containing thiophene ligands with IR spectra of adsorbed thiophene on sulfided Mo/Al2O3 catalysts have permitted a definitive assignment of the adsorption mode of the surface-bonded thiophene. Infrared and Raman spectra were acquired for three organometallic complexes:  (η5-C5D5)Re(CO)2(η(S)-C4H4S), (η5-C4H4S)Cr(CO)3, and [(η5-C4H4S)Mn(CO)3]BF4. The vibrational properties of η1(S)- and η5-coordinated thiophene were further investigated through normal-mode analyses of the IR spectra of thiophene coordinated in (η5-C4H4S)Cr(CO)3 and (η5-C5D5)Re(CO)2(η1(S)-C4H4S). Perturbations among the force constants of a gas-phase thiophene model, consistent with the structure of η1(S)- and η5-coordinated thiophene ligands, gave rise to respective thiophene force fields that allowed for the accurate determination of the IR spectra of thiophene coordinated in the Cr and Re complexes. Spectral shifts observed for the IR spectrum of the η1(S)-coordinated thiophene ligand, with respect to the IR spectrum of free thiophene, are similar to those observed for analogous bands of thiophene adsorbed at the surface of sulfided Mo/Al2O3 catalysts. Furthermore, perturbations among the force constants of η1(S)-coordinated thiophene, necessary to model such shifts, indicate that the hydrocarbon backbone of thiophene is strengthened upon η1(S) adsorption, whereas the C−S bonds are significantly weakened. These bonding changes, along with the assignment of the adsorption mode of thiophene, suggest an HDS mechanism in which the initial steps are η1(S) adsorption of thiophene followed by subsequent cleavage of one of the C−S bonds.

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