Hollow Hierarchical Pd/HNC Nanoreactor as a High-Performance Catalyst for CO₂ Hydrogenation to Formate

The catalytic CO2 hydrogenation to formic acid/formate represents an attractive atom-economic reaction in pursuit of carbon neutrality. The metal/nitrogen-doped carbon (M/NC) nanomaterials have emerged as favorable catalysts for CO2 hydrogenation, yet the rational design of novel M/NC catalysts remains a significant challenge. In this study, we report a facile melamine (MA)-assisted co-pyrolyzed route for constructing a Pd-embedded hierarchical nitrogen-doped carbon (Pd/HNC) nanoreactor derived from Pd@ZIF-8/MA precursors. Transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDX), and N-2 physisorption reveal that the Pd/HNC nanoreactor possesses a three-dimensional hollow and hierarchical porous nanoarchitecture, making it a promising high-performance heterogeneous catalyst for CO2 hydrogenation. The Pd/HNC catalyst exhibits enhanced performance, achieving nearly 1.5-fold higher activity compared to the Pd/NC catalyst evolved from Pd@ZIF-8 precursors. Detailed investigations into various reaction parameters further highlight the exceptional activity of the Pd/HNC catalyst, achieving an impressive turnover number (TON) of 242 under the optimized conditions. Elemental analysis and CO2 temperature-programmed desorption (CO2-TPD) unveil that the incorporation of melamine results in a higher nitrogen-doped level and enhanced CO2 adsorption capacity for the Pd/HNC catalyst, thus reducing the activation energy barrier of CO2 hydrogenation. Furthermore, the Pd/HNC catalyst demonstrates good reusability and stability upon multiple successive catalytic cycles. A plausible reaction pathway for CO2 hydrogenation to formate over the Pd/HNC catalyst is proposed. This work presents an effective strategy for the fabrication of metal-organic frameworks (MOFs)-derived hollow hierarchical nanoreactors for various catalytic applications.