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 Cr(6+) to benign Cr(3+) under mild and eco-friendly conditions. This article describes a facile fabrication of nitrogen doped carbon (N@C-g-C(3)N(4)) as a metal-free catalyst for Cr(6+) reduction using lignin as a carbon source and g-C(3)N(4) nanosheets as a nitrogen source. The structural properties of the N@C-g-C(3)N(4) 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 m(2) g(−1)), mesopore size (17.3 nm) and defects (I(D)/I(G) = 0.97) of N@C-g-C(3)N(4), which contribute to its excellent catalytic activity in HCOOH-mediated reduction of Cr(6+) to Cr(3+) 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 Cr(6+) and HCOOH, catalyst loading, pH, temperature, and foreign ions. Particularly, the N@C-g-C(3)N(4) catalyst shows superior stability and renewability with little loss of activity (≥95%) after 8 months storage and five repeated uses. Furthermore, N@C-g-C(3)N(4) can be applied in other hydrogenation reactions involving K(3)[Fe(CN)(6)], 4-NP and BPA using NaBH(4) as a hydrogen donor, and the removal of organic dyes. These findings illustrate that N@C-g-C(3)N(4) as a metal-free catalyst is effective, versatile and eco-friendly for the reduction of Cr(6+) from contaminated environments.