Charge separation and molecule activation promoted by Pd/MIL-125-NH2 hybrid structures for selective oxidation reactions

While heterogeneous photocatalysis is regarded as a sustainable strategy to achieve organic reactions for the production of high-value-added organics, photocatalytic efficiency is generally limited by poor charge separation, dull surface catalytic activity and the consequently caused insufficient generation of active species. Herein, the hybrid structures of metal-organic frameworks (MOFs) incorporated with Pd ultrasmall nanocrystals are synthesized, and their photocatalytic performance is evaluated by the oxidative coupling of benzylamine as a model reaction. The photocatalytic activity of the Pd/MIL-125-NH2 hybrid material with an optimal Pd loading is greatly superior to that of the pristine MIL-125-NH2, achieving a high benzylamine conversion rate of 3136 mu mol h(-1) g(cat)(-1) at 94.08% conversion. The enhanced charge transfer and separation in Pd/MIL-125-NH2 are verified by photoluminescence spectroscopy and photoelectrochemical experiments. Photogenerated reactive oxygen species are probed by 3,3 ',5,5 '-tetramethylbenzidine (TMB) as an indicator and the reactive intermediates in the photocatalytic systems are identified by in situ ESR detection. The results confirm that the optimized Pd/MIL-125-NH2 hybrid catalyst possesses higher ability for molecule activation and that both superoxide radical (O-2(-)) and singlet oxygen (O-1(2)) are the dominant oxidative species in the present photocatalytic system. Therefore, the dual advantages of Pd/MIL-125-NH2 hybrid structures for promoting the photogenerated charge transfer/separation and providing highly catalytic sites for molecule activation contribute to higher photocatalytic activity.