Single-layer MoS2 nanoclusters were synthesized on a Au substrate as a model system for the hydrotreating catalyst and studied by atomically resolved scanning tunneling microscopy (STM) in order to achieve atomic-scale insight into the interactions with hydrogen and thiophene (C4H4S). Surprisingly, STM images show that thiophene molecules can adsorb and react on the fully sulfided edges of triangular single-layer MoS2 nanoclusters. We associate this unusual behavior with the presence of special brim sites exhibiting a metallic character. The STM images reveal that these sites exist only at the regions immediately adjacent to the edges of the MoS2 nanoclusters, and from density-functional theory such sites are found to be associated with one-dimensional electronic edge states. The full sulfur-saturated sites are from STM images found to be capable of adsorbing thiophene, and when thiophene and hydrogen reactants are coadsorbed here, a reaction path is revealed which leads to partial hydrogenation of the thiophene followed by C-S bond activation and ring opening of thiophene molecules. This may be regarded as important first steps in the hydrodesulfurization of thiophene. The metallic brim sites are suggested to be important for other hydrotreating reactions over MoS2-based catalysts, and the properties of the brim sites directly explain why hydrogenation reactions of aromatics are not severely inhibited by H2S. The presence of brim sites in MoS, nanoclusters also explains previous structure-activity relations and observations regarding steric effects and the influence of stacking of MoS? on the reactivity and selectivity. (C) 2004 Elsevier Inc. All rights reserved.