Density Functional Theory Study On Adsorption Mechanism Of Monocyclic Aromatics On Transition Metal Catalysts

The adsorption of aromatics on transition metal surfaces is the starting point to a wide range of applications,including the tuning of a self-assembled monolayer,organic-based devices(e.g.,organic light-emitting diodes,organic photovoltaic cells,and organic field effect transistors),and catalytic upgrading of lignin-derived bio-oils(e.g.,hydrodeoxygenation of phenolics).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(vd W)corrections to the calculated adsorption energies can be remarkably sensitive to the relative molecular polarizability of aromatics,that is,as the relative molecular polarizability of aromatic hydrocarbons increases,van der Waals(vd W)correction increases,and the calculated adsorption energies using the opt B88-vd W 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.By incorporating the coupling of five states of gas-phase benzene to the d-states of metals,our model-predicted establish intrinsic mechanism for analyzing the bond strength between monocyclic aromatic hydrocarbons and transition metal surfaces.Our results show that adsorption energies agree reasonably well with the calculated results using generalized gradient approximation-Perdew-Burke-Ernzerhof functional resulting in a nearly linear diagonal line with ～0.18 e V of underestimation,which is however greatly underestimated by ～0.88 e V relative to opt B88-vd Wcalculated adsorption energies,suggesting that the vd Wdispersion contributions to adsorption energies are not captured by our model.In order to compare with a simple two-level model describing the coupling of CO and metal d valence states in the literature,we also establish a two-level model that considers only the highest occupied molecular orbital and the lowest unoccupied molecular orbital,show that the model prediction of monocyclic aromatics considers that five orbitals is better than consider only the highest occupied molecular orbital and the lowest unoccupied molecular orbital,which is determined by the complexity of the orbital of the gas phase benzene.Simulated scanning tunneling microscopy images have provided the atom-resolved aromatics/metal surface morphology,we find the similar pattern of the six-member ring of adsorbed BZs,which exhibit only the subtle variation in spot brightness,depending on the metals.In addition,except Cu(111),the two points forming monodentate σ-type binding are much brighter than the other four bidentate π-type binding with the shortest binding distance.