*2.4. Evaluation of Catalytic Performance*

The catalytic reduction of 4-nitrophenol (4-NP) using NaBH4 was chosen as the model reaction for investigating the catalytic performance of magnetic carbon-doped CuO/Fe3O4 nanocomposite catalysts. Typically, catalytic reduction of a 30-mL 4-NP solution (1 mmol/L) was carried out in a beaker (100 mL) by continuously stirring at room temperature. Upon the addition of NaBH4 (10 mM) into the 4-NP solution, its color changed immediately from light yellow to dark yellow due to the formation of 4-nitrophenolate ions (formed from the high alkalinity of NaBH4) [37]. Then, the dark-yellow color faded with time (due to the conversion of 4-NP to 4-aminophenol (4-AP)) after the addition of carbon-doped CuO/Fe3O4 nanocomposite catalyst (5 mg) (Scheme 1). UV–Vis adsorption spectra were recorded using a UV–Vis spectrophotometer in the range of 250–550 nm. When the reaction completed, the catalyst was easily separated from the solution using an external magnet due to its good magnetic performance.

**Scheme 1.** Catalytic reduction of 4-nitrophenol to 4-aminophenol.

During the degradation process of organic dyes, 30 mL of dye solution (10 mg/L) and NaBH4 (10 mM) were mixed in a beaker (100 mL) by continuously stirring at room temperature. Then, 5 mg of carbon-doped CuO/Fe3O4 nanocomposite catalyst was added to the reaction system, and the catalytic process was monitored using the UV–Vis spectrophotometer. When the reaction ended, the catalyst was readily separated from the solution using an external magnet.
