Novel Zn4In2S7 Loaded Mg/N Doped CQDs Composites with Improved Visible-Light Photocatalytic Properties for Depolymerization of Sodium Lignosulfonate

Photocatalysis purifies industrial paper wastewater by selectively breaking bonding bonds in lignin. To address the shortcomings of single photocatalysts such as narrow photoresponsive region, high photogenerated carrier complexation rate, and low photocatalytic activity. In this paper, Mg and N doped carbon quantum dots (Mg/N doped CQDs) were prepared by hydrothermal method. Then Zn₄In₂S₇ series catalysts with different loadings (0, 0.3, 0.6, 1.2 wt %) of Mg/N doped CQDs (denoted as ZIS 350 °C, ZIS-3 350 °C, ZIS-6 350 °C, ZIS-12 350 °C) were constructed based on condensation reflux and inert gas calcination method. The catalyst possesses highly efficient visible light-catalyzed depolymerization of sodium lignosulfonate (SLS), an industrial waste material. From the BET curve, the specific surface area of ZIS-6 350 °C was the largest. The composite of Mg/N doped CQDs reduced the band gap of Zn₄In₂S₇ while promoting the separation and transfer of photogenerated electrons/holes, as confirmed by DRS, Mott-Schottky, transient photocurrent and EIS analyses. From the photocatalytic activity test, the photocatalytic depolymerization of lignin by ZIS-6 350 °C was increased by 21.2% compared with that of ZIS 350 °C, and the reactions all follow the pseudo first order kinetic model. A possible photocatalytic mechanism was proposed based on the active species capture experiments.

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