Covalent organic frameworks for design of ruthenium catalysts with high single-atom site density for CO2 hydrogenation into formic acid

Carbon dioxide is an abundant carbon resource for chemical and fuel synthesis. Formic acid, vital for hydrogen storage, has numerous applications. Covalent organic frameworks are a unique class of materials composed of interconnected organic building blocks through covalent bonds. They possess porosity and functional groups, making them suitable for creating supported metallic catalysts. In this study, we present a strategy that utilizes covalent organic frameworks with diverse structures and chemical compositions to enhance carbon dioxide hydrogenation to formic acid at low temperatures. This enhancement arises from both high density of single-atom ruthenium sites and their intrinsic activity. Operando X-ray absorption and catalytic tests demonstrate that the concentration of nitrogen functional groups affects the intrinsic single-site ruthenium activity, whereas the impact of oxygen-containing groups is minor. Catalyst stability is attributed to the ability of single atoms to resist reduction to metallic state. This strategy has broad applicability for various covalent organic framework-supported single-atom catalysts.