Carbon supported cerium dioxide and ferric oxide photocatalyst containing S-scheme heterojunction for charge transfer regulation and efficient photocatalytic depolymerization of lignite to humic acids.

Photocatalytic depolymerization of lignite to prepare valuable humic acids is an effective way to fulfill the non-energy utilization of the low-ranked lignite. Herein, the ultra-nano cerium dioxide (CeO2) particles were strategically positioned on carbon nanosheets, and the composite was combined with magnetic iron (III) oxide (Fe3O4) nanospheres, forming an S-scheme heterojunction. Through optimizing structure and phase project, the prepared C/CeO2/Fe3O4 heterojunction possessed simultaneously more Ce4+/Ce3+ reversible pairs, stronger light capture ability, higher charge separation efficiency, and more exposed active site density. Moreover, Fe3+/Fe2+ cycle formed a Fenton system. Under ultraviolet light irradiation, the depolymerization effect of the optimized catalyst was remarkable, with depolymerization degree of lignite up to 93.0%, and the yield of humic acids and fulvic acids up to 93.0% and 10.0%, respectively. The catalyst can be recycled for up to 6 cycles without obvious decreasing in efficiency. DFT calculation confirmed the charge transfer from the Fe3O4 to the CeO2 components, proving the proposed S-scheme charge transfer mechanism. Free radical capture (EPR) studies determined that OH and O2- were the main oxidizing substances. This S-scheme heterojunction strategy enables efficient light energy resource valorization of lignite under mild conditions.