MOF-derived CoN/N-C@SiO2 yolk-shell nanoreactor with dual active sites for highly efficient catalytic advanced oxidation processes

Optimizing the structure and composition of the yolk-shell nanoreactors are one of the effective approaches to enhance their performance in heterogeneous catalysis, yet challenging. Here, we report a facile route to synthesize a highly efficient and recyclable yolk-shell CoN/N-C@SiO2 nanoreactor with dual active sites by the direct nitridation of MOF@SiO2 precursor. The CoN yolks (similar to 50-70 nm) as active sites were encapsulated by the hydrophilic SiO2 shells (similar to 30 nm), and some small dots (similar to 5-10 nm) was sandwiched between void spaces. In-situ formation of N-doped carbon layer not only served as active sites but also improved electron transfer. The yolk-shell nanoreactors were applied for catalytic tetracycline (TC) degradation by activation of peroxymonosulfate (PMS), and in the optimal case, it degraded>similar to 95% TC within 30 min in a wide pH value range of 2.02-9.94. The quenching experiments and EPR measurements further revealed that synergistic effect of the radicals and nonradical in PMS activation, O-2(center dot-), (OH)-O-center dot, SO4 center dot- and O-1(2) were involved in TC degradation. The improved catalytic performances could be ascribed to the following two aspects: (1) the hydrophilic SiO2 shell and the microenvironment formed by the yolk-shell structure not only enhance catalytic stability but also provide driving force to improve reaction rate (structural modulation); (2) CoN core and N doped carbon layer as dual active sites can achieve the synergistic effect of radicals and non-radicals in PMS activation (composition modulation). Moreover, possible degradation pathways of TC were proposed through the identification of intermediates using LC-MS/MS techniques. This research highlights the critical role of structure and component optimization in construction of advanced oxidation process systems and provides the possibility of utilizing MOF derivatives in the field of electrocatalysis, lithium battery and supercapacitor etc.