Modeling metal adsorption at amorphous silica: Gold atoms and dimers as example

For Au and Au2 species at three types of defects as adsorption sites (non-bridging oxygen centers, silanolate groups, E′ centers), we carried out periodic supercell DFT calculations to compare two models of amorphous silica, derived from the walls of MCM-41 or from edingtonite. For atomic adsorption, both types of models afforded similar adsorption geometries and binding energies. Striking differences were found for Au2 adsorption, where the structural characteristics of edingtonite automatically lead to simultaneous interactions of adsorbates with neighboring sites, whereas the MCM models allowed the structural isolation of defects, hence are more flexible when one wants to describe varying defect densities.