Efficient Photocatalytic CO<sub>2</sub> Reduction with High Selectivity for Ethanol by Synergistically Coupled MXene-Ceria and the Charge Carrier Dynamics
Rajashree P. Mishra, Madoori Mrinalini, Niharika Kumar, Sweta Bastia, Yatendra S. Chaudhary
Abstract
The primary factors that govern the selectivity and efficacy of CO 2 photoreduction are the degree of activation of CO 2 on the active surface sites of photocatalysts and charge separation/transfer kinetics. In this context, the rational synthesis of heterostructured MXene-coupled CeO 2 -based photocatalysts with different loading concentrations of Ti 3 C 2 MXene via a one-step hydrothermal approach has been undertaken. These photocatalysts exhibit a shift in X-ray diffraction peaks to higher 2θ values and changes in stretching vibrations of 5 wt % Ti 3 C 2 MXene/CeO 2 (5-TC/Ce) that indicate interaction between Ti 3 C 2 MXene and CeO 2 . Moreover, XPS analysis confirms the presence of the Ce 3+ /Ce 4+ states. A sharp band at 2335 cm –1 observed during the CO 2 photoreduction process corresponds to bidentate b-CO 3 2–, which facilitates the adsorption of CO 2 at the surface of the catalyst as revealed by the TPD analysis. Furthermore, the Schryvers test and NMR analysis were undertaken to confirm the formaldehyde intermediate formation during CO 2 photoreduction to C 2 H 5 OH. The decrease in emission intensity, reduced lifetimes (2.68 ns), and lower interfacial resistance, as revealed by PL, TR-PL, and EIS analysis, imply an efficient charge separation and charge transfer in the case of the Ti 3 C 2 MXene/CeO 2 heterojunction. The decrease in the intensity of peaks in the EPR spectrum in the case of 5-TC/Ce further confirms efficient charge transfer kinetics across the interface. The optimized 5-TC/Ce shows CO 2 reduction with a drastically enhanced yield of ethanol on the order of 6127 μmol g –1 at 5 h with 98% selectivity and 7.54% apparent quantum efficiency, which is 6-fold higher than that of ethanol produced by bare CeO 2 . Herein, CeO 2 that acts as a redox couple (Ce 3+ /Ce 4+ ) when coupled with MXene having a metallic nature that reduces the electron transfer resistance is in unison, enabling an enhanced mobilization of electrons. Thereby, the synergistic coupling of Ti 3 C 2 MXene with CeO 2 leads to an efficient photoreduction of CO 2 under visible light illumination.