Nitrogen-doped lignin-derived carbon for catalytic reduction of hexavalent chromium via HCOOH-mediated hydrogenation

It is highly desirable to explore efficient catalysts for reducing toxic Cr⁶⁺ to benign Cr³⁺ under mild and eco-friendly conditions. This article describes a facile fabrication of nitrogen doped carbon (N@C-g-C₃N₄) as a metal-free catalyst for Cr⁶⁺ reduction using lignin as a carbon source and g-C₃N₄ nanosheets as a nitrogen source. The structural properties of the N@C-g-C₃N₄ catalyst are characterized by TEM, HR-TEM, XRD, TGA, Raman, EDS-mapping, XPS and BET techniques. The summation of these analyses sheds light on the high surface area (903 m² g^-1), mesopore size (17.3 nm) and defects (I _D/I _G = 0.97) of N@C-g-C₃N₄, which contribute to its excellent catalytic activity in HCOOH-mediated reduction of Cr⁶⁺ to Cr³⁺ with high rate constant (2.98 min^-1) and turnover frequency (2.21 mol_K2Cr2O7 g_catalyst ^-1 min^-1) and complete degradation (100%) within 5 min. The catalytic performance of the catalyst reveals that the reduction activity is significantly dependent on the concentration of Cr⁶⁺ and HCOOH, catalyst loading, pH, temperature, and foreign ions. Particularly, the N@C-g-C₃N₄ catalyst shows superior stability and renewability with little loss of activity (≥95%) after 8 months storage and five repeated uses. Furthermore, N@C-g-C₃N₄ can be applied in other hydrogenation reactions involving K₃[Fe(CN)₆], 4-NP and BPA using NaBH₄ as a hydrogen donor, and the removal of organic dyes. These findings illustrate that N@C-g-C₃N₄ as a metal-free catalyst is effective, versatile and eco-friendly for the reduction of Cr⁶⁺ from contaminated environments.