Cooperative Chemisorption-Induced Physisorption of CO₂ Molecules by Metal–Organic Chains

Effective CO2 capture and reduction can be achieved through a molecular scale understanding of interaction of CO2 molecules with chemically active sites and the cooperative effects they induce in functional materials. Self-assembled arrays of parallel chains composed of Au adatoms connected by 1,4-phenylene diisocyanide (PDI) linkers decorating Au surfaces exhibit self-catalyzed CO2 capture leading to large scale surface restructuring at 77 K (ACS Nano 2014, 8, 8644-8652). We explore the cooperative interactions among CO2 molecules, Au-PDI chains and Au substrates that are responsible for the self-catalyzed capture by low temperature scanning tunneling microscopy (LT-STM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), temperature-programmed desorption (TPD), and dispersion corrected density functional theory (DFT). Decorating Au surfaces with Au-PDI chains gives the interfacial metal-organic polymer characteristics of both a homogeneous and heterogeneous catalyst. Au-PDI chains activate the normally inert Au surfaces by promoting CO2 chemisorption at the Au adatom sites even at <20 K. The CO2(δ-) species coordinating Au adatoms in-turn seed physisorption of CO2 molecules in highly ordered two-dimensional (2D) clusters, which grow with increasing dose to a full monolayer and, surprisingly, can be imaged with molecular resolution on Au crystal terraces. The dispersion interactions with the substrate force the monolayer to assume a rhombic structure similar to a high-pressure CO2 crystalline solid rather than the cubic dry ice phase. The Au surface supported Au-PDI chains provide a platform for investigating the physical and chemical interactions involved in CO2 capture and reduction.