Catalytic conversion of waste CO₂ into value-added fuels and chemicals offers unprecedented opportunities for both environmental protection and economic development. Electrocatalytic CO₂ reduction reaction (CO₂RR) has garnered attention for its ability to efficiently convert CO₂ into clean chemical energy under mild conditions. However, the relatively high energy barrier for *COOH intermediate formation often becomes the determining step, significantly limiting the reaction efficiency.

Inspired by the enzyme catalysis, a research team led by Prof. JIANG Hailong and Prof. JIAO Long from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences developed a novel strategy to stabilize *COOH intermediate and enhance electrochemical CO₂ reduction by constructing and modulating the hydrogen-bonding microenvironment around catalytic sites. The study was published in PNAS.

The researchers co-grafted catalytically active Co(salen) units and proximal pyridyl-substituted alkyl carboxylic acids (X-PyC_n) onto Hf-based metal–organic framework nanosheets (MOFNs) via a post decoration route, affording Co&X-PyC_n/MOFNs (X = o, m or p representing the ortho-, meta-, or para- position of pyridine N relative to alkyl chain; n = 1 or 3 representing the carbon atom number of alkyl chains) materials. 

The Co&X-PyC_n/MOFNs achieved precise control over the spatial positioning of the N atoms in pyridine groups relative to the Co(salen), which provided a novel and facile approach to microenvironment modulation around catalytic sites at atomic scale. Among the catalysts, the optimized Co&p-PyC₃/MOFNs exhibited significantly enhanced catalytic activity and selectivity in electrochemical CO₂ reduction, superior to Co/MOFNs without pyridine unit and other Co&X-PyC_n/MOFNs counterparts.

Furthermore, the researchers observed the in situ reduction of pyridine to pyridinyl radical (PyrH^•) during electrochemical CO₂ reduction, and confirmed that the in situ formed PyrH^• species was the real microenvironment around Co(salen) for enhanced performance. 

Mechanism investigations revealed that PyrH^• could collaborate with trifluoroethanol (TFE) molecules in electrolyte to stabilize the *COOH intermediate by generating *COOH···TFE···PyrH^• triad intermediate via hydrogen-bonding interaction, greatly minimizing reaction energy barrier. This provided a clear picture of the working mode of the microenvironment for performance optimization during the catalysis.

This work unambiguously demonstrates the significance of the microenvironment modulation around catalytic sites for enhancing catalysis.