From Nondissociative to Dissociative Adsorption of Benzene-thiol on Au(111): A Density Functional Theory Study

The adsorption of the benzene-thiol (C6H5SH) molecule on an Au(111) surface was investigated using the density functional theory method. Unlike prior studies that assume the dissociation of C6H5SH to C6H5S + H and the subsequent chemisorption of C6H5S/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, C6H5S/Au formation, H/Au diffusion, and H2 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 C6H5S/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 C6H5S/Au and gaseous H2 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 C6H5SH/Au and dissociative adsorption C6H5S/Au, which were previously observed by STM experiments.