Photocatalytic CO2 Reduction Using Ti3C2Xy (X = Oxo, OH, F, or Cl) MXene–ZrO2: Structure, Electron Transmission, and the Stability

CO 2 photoreduction using a semiconductor-based photocatalyst is a promising option for completing a new carbon–neutral cycle. The short lifetime of charges generated owing to light energy is one of the most critical problems in further improving the performance of semiconductor-based photocatalysts. This study shows the structure, electron transmission, and stability of Ti 3 C 2 X y (X = oxo, OH, F, or Cl) MXene combined with a ZrO 2 photocatalyst. Using H 2 as a reductant, the photocatalytic CO formation rate increased by 6.6 times to 4.6 μmol h –1 g cat –1 using MXene (3.0 wt %)–ZrO 2 compared to that using ZrO 2, and the catalytic route was confirmed using 13 CO 2 to form 13 CO. In clear contrast, using H 2 O (gas) as a reductant, CH 4 was formed as the major product using Ti 3 C 2 X y MXene (5.0 wt %)–ZrO 2 at the rate of 3.9 μmol h –1 g cat –1 . Using 13 CO 2 and H 2 O, 12 CH 4, 12 C 2 H 6, and 12 C 3 H 8 were formed besides H 2 12 CO, demonstrating that the C source was the partial decomposition and hydrogenation of Ti 3 C 2 X y . Using the atomic force and high-resolution electron microscopies, 1.6 nm thick Ti 3 C 2 X y MXene sheets were observed, suggesting ∼3 stacked layers that are consistent with the Ti–C and Ti···Ti interatomic distances of 0.218 and 0.301 nm, respectively, forming a [Ti 6 C] octahedral coordination, and the major component as the X ligand was suggested to be F and OH/oxo, with the temperature increasing by 116 K or higher owing to the absorbed light energy, all based on the extended X-ray absorption fine structure analysis.