From nondissociative to dissociative adsorption of benzene-thiol on Au(111): A density functional theory study

The adsorption of the benzene-thiol (C ₆H ₅SH) molecule on an Au(111) surface was investigated using the density functional theory method. Unlike prior studies that assume the dissociation of C ₆H ₅SH to C ₆H ₅S + H and the subsequent chemisorption of C ₆H ₅S/Au, the present computational work first clarifies the sites and energetics of both the nondissociative molecular adsorption and the dissociative adsorption and then charts the dissociative chemisorption pathways and transition states. The calculations took into account of the reaction steps in these processes, steps including S-H cleavage, C ₆H ₅S/Au formation, H/Au diffusion, and H ₂ desorption. The thoroughness of this approach yields the discovery of a molecular nondissociative chemisorption state with the S atom sitting on top of a gold atom. This state is stable at room temperature as its adsorption energy amounts to 0.28 eV. Its direct molecular dissociation to form C ₆H ₅S/Au and H/Au is barred by an activation barrier of 0.58 eV, and the dissociation is endothermic. However, the presence of neighboring H/Au can assist the dissociative reaction to form C ₆H ₅S/Au and gaseous H ₂ by lowering the dissociation activation barrier from 0.58 to 0.35 eV. More importantly, the dissociation changes from endothermic to exothermic, and it can proceed with mild heating. Finally, this work also gives adequate simulation results to interpret the surface configurations of the nondissociative molecular adsorption of C ₆H ₅SH/Au and dissociative adsorption C ₆H ₅S/Au, which were previously observed by STM experiments.