Selective oxidative dehydrogenation (ODH) of light alkanes to alkenes over metal oxides usually obeys a redox catalysis cycle, where the release and recovery of lattice oxygen play a key role in catalytic performance. Therefore, it remains a challenge to prepare metal oxides with limited reducibility, such as TiO2, as a dominant active component rather than as a support or modifier in this process. Here, we find a significant increase of ethane ODH performance over a layered TiO2 (M-TiO2) catalyst, which is in situ evolved from a Ti3C2Tx MXene material during the reaction. The reactivity of this M-TiO2 is 4 times higher than that of P25 TiO2, with an excellent ethylene productivity of 15.4 gC2H4 gcat–1 h–1 that outperforms the previously reported catalysts. Experimental characterizations and theoretical calculations reveal the importance of Ti and oxygen vacancy defects over M-TiO2 on the conversion of C2H6. The Ti vacancy sites can stabilize the defective structure and increase the reducibility of lattice oxygen to reduce the activation energy barrier of ethane, while the oxygen vacancy sites facilitate the activation of O2 to recover lattice oxygen.
In this research, two interesting phenomena were reported and analyzed: First, two-dimensional tiny titania nanoplates (4–5 nm lateral size, 0.4–0.5 nm thickness) with rich edge-defects were found to self-assemble into free-standing thin sheets with large lateral size in a freeze-drying process with the aid of linker; Second, when subjected to a moderate hydrogenation treatment, these self-assembled sheets could be crystallized into integral anatase sheets while the edge-defects of the building blocks were maintained on the surface or inside the body of the crystal and the flexible sheet-like shape were also maintained. Based on these observations, a method for scalable preparing free-standing defect-rich TiO 2 with a two-dimensional sheet-like morphology was proposed. The defects (oxygen vacancies) endowed the TiO 2 a grayish-black color and visible-light responsibility. By using the dispersion of these defect-rich anatase sheets, a film could be densely deposited on flexible substrates in a layered structure and showed an efficient performance in acetaldehyde decomposition under the irradiation of visible light. Since it has always been a technical challenge to prepare crystallized visible-light-driven TiO 2 films on plastic substrates with a low heat tolerance, this work may also provide a new approach to this problem.
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