Adsorption on gold: Effects on surface morphology and reactivity

Computational tools are used in conjunction with experiments to understand adsorption and reactions on gold surfaces. We focus on systems important for oxidation, a class of reactions gold can perform quite readily. Experimental results have illustrated that the structure of gold is sensitive to adsorbate type and coverage, temperature, and many other factors. We find the adsorption of atomic oxygen and chlorine can significantly affect the morphology of the surface, either by favorably interacting with defects on the surface or by incorporating gold into the adsorbate layer, a process that is energetically favorable at higher coverages. The adsorbate-gold interaction becomes more covalent upon gold incorporation, which lowers the partial negative charge on the adsorbate, allowing for closer packing on the surface. The temperature dependence on the oxygen-gold interaction motivated the use of ab-initio molecular dynamics to model the dynamic surface modification during adsorption of atomic oxygen. We successfully match calculated vibrational spectra with experimental high-resolution electron energy loss spectroscopy results and suggest chemisorbed oxygen (oxygen bound in a locally flat three-fold coordination site) as the reactive species for oxidation on the surface. Since reactivity is closely related to surface morphology we studied the reaction of propene with atomic oxygen on different defect covered gold surfaces. Reaction barriers for allylic hydrogen abstraction and oxametallacycle formation can differ by at least a factor of two and different pathways can be selected by modifying the surface structure. Furthermore, we find the correct pathway for phenol formation on cyclohexene and use ab-initio molecular dynamics to model the oxygen-covered gold surface for the oxidation of CO. We qualitatively reproduce experimental trends in reactivity and gain insight into the mechanism of CO oxidation on the oxygen-covered gold surface, confirming our previous result that chemisorbed oxygen is the reactive species on the surface.