Unsaturated Ni<sup>II</sup> Centers Mediated the Coordination Activation of Benzylamine for Enhancing Photocatalytic Activity over Ultrathin Ni MOF-74 Nanosheets
Taikang Wu, Yingzhang Shi, Zhiwen Wang, Cheng Liu, Jinhong Bi, Yan Yu, Ling Wu
Abstract
Creating accessible unsaturated active sites in metal–organic frameworks (MOFs) holds great promise for developing highly efficient catalysts. Herein, ultrathin Ni MOF-74 nanosheets (NMNs) with high-density coordinatively unsaturated NiII centers are prepared as a photocatalyst. The results of in situ ATR–IR, Raman, UV–vis DRS, and XPS suggest that abundant NiII centers can act as the active sites for boosting benzylamine (BA) activation via forming −Ni–NH2– coordination intermediates. The generation of coordination intermediates assists the transfer of photo-generated holes to BA molecules for producing BA cation free radicals, better impelling the breaking of N–H bonds and the photooxidation of BA molecules. The photo-generated electrons further activate O2 molecules to O2•– radicals for triggering the reaction. The experiments reveal that the coordination activation of BA molecules may be a rate-determining step on NMNs rather than the adsorption and activation of O2 molecules. Moreover, NMNs possess a better ability for the separation of photo-generated carriers in comparison with bulk Ni MOF-74 (NMBs). As a result, NMNs achieve a kinetic rate constant of 0.538 h–1 for the photocatalytic oxidative coupling of BA under visible light, about 50 times higher than that of NMBs (0.0011 h–1). Finally, a probable synergetic catalytic mechanism with coordination activation and photocatalysis is discussed on a molecular level. This study not only highlights the importance of coordination activation for heterogeneous photocatalysis but also affords an inspiration for building ultrathin MOF nanosheets.