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

## TL;DR

This paper introduces a new carbon-based material that efficiently and quickly reduces toxic Cr(VI) to less harmful Cr(III) using a hierarchical porous structure and nitrogen doping.

## Contribution

A novel strategy to synthesize defect-rich, hierarchical porous nitrogen-doped carbon for rapid Cr(VI) reduction is presented.

## Key findings

- The d-PNC material achieves complete Cr(VI) reduction (333.3 mg g⁻¹) in 2 minutes.
- Hierarchical porosity and nitrogen doping enhance conductivity and electron transfer for faster catalytic reactions.
- Defective carbon and graphitic N3 species synergistically lower the energy barrier for Cr(VI) reduction.

## Abstract

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.

The d‐PNC(1100, 10%) catalyst, synthesized via a zinc‐based ionic liquid self‐sacrificing pore‐forming strategy to create a defect‐rich, hierarchical, nitrogen‐doped porous structure, enhances conductivity, promotes rapid mass transfer and synergistic catalytic activity, driving efficient Cr(VI) reduction in OA solution to generate Cr(III)‐OA complexes.