Amorphous matters: Heterogeneity and defects of nanopore silica surfaces enhance CO[formula omitted] adsorption

Abstract

Despite its importance for many applications (such as catalysis and structure ripening), adsorption on amorphous silica remains poorly understood as most studies focus on crystalline surfaces. Here, we investigate the adsorption of carbon dioxide, CO2, on hydroxylated silica nanopores by means of molecular dynamics (MD) simulations that compare amorphous and crystalline surfaces. We find that adsorption onto amorphous surfaces is enhanced by the heterogeneity of the structure and by the presence of under-coordinated defects (non-bridging oxygen and threefold coordinated silicon). We identify two classes of adsorption features: (i) a continuous region characterized by an intermediate density of adsorbed CO2 molecules, which is generated by weak interactions with hydroxyl groups and forms a network that allows molecules to diffuse through the surface adsorption layer; (ii) high-density cage-like features (embedded into the network and induced by the presence of defects) in which CO2 molecules form multiple short- and long-lived bonds with surrounding atoms and remain adsorbed for a longer time. The rich variety of adsorption features enhances physisorption and leads to longer mean adsorption time than onto the crystalline surfaces. The adsorption time onto amorphous surfaces deviates from the exponential distribution observed in silica crystals and displays a characteristic fat tail. Our findings support the experimental discovery of the potential of defected silica surfaces for CO2 adsorption and subsequent catalytic conversion to methane, suggesting the possibility of designing and manufacturing specific adsorption features for tailored heterogeneous catalysis.