MOF encapsulation derived slow-release oxygen species to enhance the activity and selectivity of methane selective oxidation: A transient DRIFTs Study

The methane selective oxidation was a “holy grail” reaction. However, peroxidation and low selectivity limited the application. Herein, we combined three Au contents with TiO₂in both encapsulation (xAu@TiO₂) and surface-loaded (xAu/TiO₂) ways by MOF derivation strategy, reported a catalyst 0.5Au@TiO₂exhibited a CH₃OH yield of 32.5 μmol·g^–1·h^–1and a CH₃OH selectivity of 80.6% under catalytic conditions of only CH₄, O₂, and H₂O. Mechanically speaking, the catalytic activity was controlled by both electron-hole separation efficiency and core-shell structure. The interfacial contact between Au nanoparticles and TiO₂in xAu@TiO₂and xAu/TiO₂induced the formation of oxygen vacancies, with 0.5 Au content showing the highest oxygen vacancy concentration. At the same Au content, xAu@TiO₂generated more oxygen vacancies than xAu/TiO₂. The oxygen vacancy acted as an effective electron cold trap, which enhanced the photogenerated carrier separation efficiency and thereby improved the catalytic activity. In-situ DRIFTs revealed that the isolated OH (non-hydrogen bond adsorption) were key species for the methane selective oxidation, playing a role in the activation of CH₄to *CH₃. However, an overabundance of isolated OH led to severe overoxidation. Fortunately, the core-shell structure over xAu@TiO₂provided a slow-release environment for isolated OH through the intermediate state of *OH (hydrogen bond adsorption) to balance the formation rate and consumption rate of isolated OH, doubling the methanol yield and increasing the > 29% selectivity. These results showed a new strategy for the control of the overoxidation rate via a strategy of MOF encapsulation followed by pyrolytic derivation for methane selective oxidation.