Hierarchically Porous Nitrogen-Doped Carbon with High Conductivity for Rapid and Efficient Cr(VI) Reduction

Carbon-based materials derived from metal-organic frameworks typically exhibit microporous structures and low conductivity, which significantly limit their catalytic activity. Herein, an effective strategy to prepare dodecahedral hierarchical porous nitrogen-doped carbon-based composites (d-PNC) by using ZIF-8 encapsulated with ionic liquid as pyrolysis precursors for efficient Cr(VI) reduction is developed. The encapsulated ionic liquid helps to precisely regulate the hierarchically porous structure in d-PNC. This hierarchically porous structure not only creates a favorable reaction microenvironment, facilitating the mass transfer of Cr species and their interaction with active sites, but also enhancing the conductivity of d-PNC and consequently accelerating the electron transfer of •CO₂ ^- radicals to Cr species, thereby speeding up the reduction process of Cr(VI). Additionally, with the calcination temperature increasing, the content of defective C increases, and N species progressively transforms into graphitic-center N (N3). Density functional theory calculations reveal that the defective C active center substantially decreases the free energy change of the rate-determining step (from Cr(IV) to Cr(III)) through the synergistic effect of N3. Given these outstanding characteristics, the optimized d-PNC material can completely reduce Cr(VI) (333.3 mg g^-1) in an oxalic acid solution within 2 min, outperforming its counterparts without a hierarchical structure and those calcined at significantly lower temperatures.