Confined micro- and nanoreactors are of growing interest, not only for Fenton technologies, but for many other chemical reaction systems because it is expected to improve conversion rates due to the high local concentrations of reactants. According to several studies , ,  and , the catalysts confined in the inner space of nanoreactors or microreactors display enhanced catalytic performance due to the protection of the active sites, which improves the catalytic efficiency. However, only a few works have been published on this topic, overall for the application in Fenton-type oxidation. Cui et al.  synthesized a yolk-shell structured Fe2O3@mesoporous-SiO2 nanoreactor through a simple polymeric carbon-assisted method, which allows tuning the void space size. First, the authors studied the CHIR-98014 of methylene blue using the bare Fe2O3, which showed relatively low activity, achieving only 20% discoloration after 7 h. The mesoporous shell itself led to 30% methylene blue removal during the first hour due to adsorption but not further removal was observed beyond that time. In contrast, the yolk-shell structured Fe2O3@mesoporous-SiO2 nanoreactor was quite active in Fenton oxidation of methylene blue (0.5 g L−1 Fe2O3, 50 mg L−1 methylene blue, 18 g L−1 H2O2, 25 °C and pH 5.7). The activity was related to the size of the void space, increasing from 70 to 90% as the void space was increased from 16 to 40 nm. With the latter, complete conversion was achieved although high reaction times were required (10 h). More recently, Zeng et al.  have developed a yolk–shell nanoreactor with a Fe3O4 core and Fe3O4/C shell, improving previous results due to the presence of magnetite, which is positive for both the activity and recovery of the catalyst. The performance of this nanoreactor exceeded significantly that of the new MNPs under the same operating conditions (0.5 g L−1 catalyst, 200 mg L−1 4-chlorophenol, 0.68 g L−1 H2O2, 25 °C and pH 4). In this sense, while complete conversion of 4-chlorophenol was achieved in 1 h, only 28% was reached using bare MNPs. The authors showed that due to the outermost carbon layer and high-magnetization properties, the nanoreactor can be re-used several times with limited iron leaching, thus maintaining its activity almost unchanged.