The chemisorption of SO2 on the Cu/Au(1 1 1) surface: Interplay between ensemble and electronic effects

Ion-scattering spectroscopy (ISS), synchrotron-based high-resolution photoemission spectroscopy (PES) and first-principles density-functional (DF) calculations were used to study the adsorption of SO2 on Cu/Au(1 1 1). ISS experiments for Cu/Au(1 1 1) clearly demonstrate that Cu intermixes with the Au(1 1 1) substrate at 300 K or higher temperature, but a substantial fraction of the Cu atoms stays on the Au(1 1 1) surface when the deposition is done at 100 K. When 1 ML of Cu was pre-deposited onto Au(1 1 1) at 300 K, the Cu/Au(1 1 1) surface exhibited a negligible reactivity towards SO2 very similar to that of clean Au(1 1 1). However, when Cu was pre-deposited at 100 K, the Cu/Au(1 1 1) system bonded SO2 well and the adsorbate remained on the surface up to temperatures above 300 K. DFT calculations for Cu/Au(1 1 1) show electronic perturbations in the Cu overlayer that should enhance their chemical reactivity, but the Cu atoms were more stable when penetrating into the substrate rather than sitting on the surface. Calculations for SO2 adsorption on Cu/Au(1 1 1) surfaces with different θCu showed that the value of θCu determines the binding energy of SO2. In general, the more Cu atoms in the surface, the stronger the bonding energy towards SO2. Cu/Au(1 1 1) behaves differently from pure Cu. A temperature increase does not lead to S–O bond cleavage on the bimetallic system. Instead, the Cu atoms in the surface migrate into Au, accompanied by SO2 desorption. Ensembles of the active sites necessary for the dissociation of SO2 are not available. For Cu/Au(1 1 1), ensemble effects clearly overcome electronic effects. The Cu/Au(1 1 1) interface illustrates how effective ensemble effects can be for the prevention of the corrosion of alloys by SO2.