Engineering local nitrogen coordination environments of Palladium subnanometric clusters in metal-organic frameworks for efficient hydrogenation.

Subnanometric clusters (SCs) bridge the gap between single-atom catalysts and nanoparticles by combining high atomic utilization with cooperative multi-atom effects. However, stabilizing low-coordinated SCs while maintaining accessible active sites remains challenging. Here, we introduce pyrazole-3,5-dicarboxylic acid (PZDC), pyridine-3,5-dicarboxylic acid (PDC), and pyrrole-3,5-dicarboxylic acid (PPy) as secondary ligands in metal-organic frameworks to regulate the local nitrogen (N) coordination environment of Pd SCs (∼0.6 nm). Specifically, PZDC provides a chemically differentiated pyrazolic dual-N environment containing formally pyridinic-like and pyrrolic-like N sites. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) and X-ray absorption spectroscopy (XAS) confirm the formation of low-coordinated Pd clusters containing PdN and PdPd interactions, while CO diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS) reveals a distinctive adsorption environment characterized by geminal dicarbonyl species and strongly suppressed bridge-bonded CO adsorption. Within this catalyst series, the PZDC-functionalized material exhibits the highest activity and tetrahydrocyclopentadiene (THDCPD) selectivity in dicyclopentadiene (DCPD) hydrogenation. These results demonstrate that modification of the local N environment can regulate the average coordination structure, adsorption behavior, and catalytic properties of MOF-supported Pd SCs.