Possibility of chemisorption for benzene on stepped surface of coinage metals

Abstract

It is well known that the flat, ideal (111) surface of coinage metals Au, Ag, and Cu interact weakly, through physisorption, with most molecules. We study and compare benzene adsorption on perfect and stepped surfaces of coinage metals using Density Functional Theory (DFT) to answer the question of whether stepped surfaces can lead to stronger binding for benzene. Our calculation shows an increase in adsorption energy on stepped surfaces compared to perfect (111) surfaces using PBE functional, indicating their more reactive nature. The stronger adsorption signifies the possibility of a chemical bond on the stepped surface of Cu and Au, whereas no such bonds are observed on Ag. Analysis based on adsorption height and electronic structure of benzene/Cu (221) system further strengthen this idea. The role of vdW forces in the adsorption process is also examined, using optimized vdW-DF functionals. The optimized vdW functionals predict almost the same energy on flat and stepped surfaces at adsorption sites where the chemical bonds are not present. But on those adsorption sites with a weak chemical contribution, the stepped surface predicts larger adsorption energy than on a perfect (111) surface. The results of our calculations indicate that on perfect and defective coinage metal surfaces, the binding of benzene occurs primarily through physisorption. However, on stepped surfaces of Cu and Au, weak chemisorption also contributes towards the binding.