Maximize coordinately unsaturated metal sites via removal of compact MOF external surface: Enhanced Fenton-like performance inside yolk@shell nanoreactor

Abstract

Catalytic property of metal–organic frameworks (MOF) in peroxymonosulfate (PMS)-based Fenton-like reaction was closely related to the number of exposed metal sites. Therefore, coordinately unsaturated metal sites (CUMSs) engineering became an effective method for enhancing activity. Usually, the exterior of MOF was very compact due to the Ostwald ripening, leading to difficulty in CUMS production. In this work, ZIF-67 was employed as template for yolk@shell nanoreactor via only two steps. During the Co2SiO4 shell coverage, ZIF-67 exterior was etched by H2O, resulting in a gap between Co2SiO4 shell and residual ZIF-67. CUMSs were created, and the loose interior was directly exposed. In subsequent heat-treatment, ligands were decomposed, leading to the further increased of CUMSs number. Mechanism study indicating that the external surface removal played a key role for maximize the CUMSs yield, since their amount in yolk@shell nanoreactor was 2.2 times higher than the counterparts without external surface removal. Benefiting from the plenty of CUMSs and enhanced Lewis acidity, 80.5 % of PMS were effectively activated to reactive oxygen species (ROSs), and 90.1 % of metronidazole were degraded within 8.0 min, much higher than the reference catalysts. The underlying mechanism, including ROSs generated on CUMSs, Lewis acid-base reaction between PMS and CUMSs, and confinement effect were carefully studied. Moreover, although plenty of CUMSs were exposed, the metal leaching was strictly suppressed due to the Co2SiO4 shell protection. This work provided a feasible way to create more CUMSs via removal of compact MOF external surface.