A density functional study of C1–C4 alkyl adsorption on Cu(111)

To better understand the nature of alkyl intermediates often invoked in reactions involving hydrocarbon reactants and products, the adsorption of linear and branched C(1)-C(4) alkyls on Cu(111) at 1/4 ML and 1/9 ML coverages was studied using density functional theory. The adsorption energy and site preference are found to be coverage-dependent, and both direct alkyl-alkyl interactions and changes in the Cu electronic structure play a role in these trends. It was found that methyl strongly prefers the hollow sites, the branched alkyls strongly prefer the top site, and the linear C(2)-C(4) alkyls have weak site preferences that change with coverage. To explain these differences, rationalize alkyl adsorption trends, and predict the binding energy of other alkyls, a simple model was developed in which the binding energy is fit as a linear function of the number of C-Cu and C-H-Cu interactions as well as the C-H bond energy in the corresponding alkane. Site preference can be understood as a compromise between C-Cu interactions and C-H-Cu interactions. Density of states analysis was used to gain a molecular-orbital understanding of the bonding of alkyls to Cu(111).