Adsorption of Monocyclic Aromatics on Transition Metal Surfaces: Insight into Variation of Binding Strength from First-Principles

Understanding the adsorption of monocyclic aromatics on transition metal surfaces is of great interest to both fundamental and applied research. Herein, using density functional theory, we report a systematic study on the binding mechanism of four monocyclic aromatic compounds (benzene, toluene, phenol, and m-cresol) on 3d metal surfaces [Fe(110), Co(111), Ni(111), and Cu(111)] and 4d and 5d noble metal surfaces [Ru(0001), Rh(111), Pd(111), and Pt(111)]. Our results show that van der Waals (vdW) corrections to the calculated adsorption energies can be remarkably sensitive to the relative molecular polarizability of aromatics, and the calculated adsorption energies using the optB88-vdW functional agree well with the experimental results. The role of functional groups at the phenyl ring is less significant in enhancing the adsorption strength compared to the phenyl ring itself, which contributes most to the electronic interactions with the surface metal atoms. We have analyzed the origin of both electronic and geometric effects on the variation of binding strength of monocyclic aromatics adsorbed on metal surfaces. By incorporating the coupling of five states of gas-phase benzene to the d-states of metals, our model-predicted adsorption energies agree reasonably well with the calculated results using generalized gradient approximation-Perdew–Burke–Ernzerhof functional. Simulated scanning tunneling microscopy images have provided the atom-resolved aromatics/metal surface morphology and the visual support for differentiating σ- and π-type bindings.