*3.2. Catalytic Reduction*

In this study, we investigated the application of carbon-doped CuO/Fe3O4 composite catalysts for the catalytic reduction of 4-nitrophenol. In Figure 4a, the results are shown for the catalytic reduction of 4-NP to 4-AP in the presence of NaBH4 and carbon-doped CuO/Fe3O4 composite catalysts. The reduction process was followed based on the UV–Vis spectrophotometry. It shows that the absorbance at 400 nm (4-NP) decreased gradually as a function of time, while the absorbance at 290 nm (due to 4-AP) increased, confirming the catalytic reduction of 4-NP to 4-AP [44]. The catalytic reduction was almost complete within 10 min at room temperature, and the color of the solution changed from yellow to colorless. Similar results were also reported in the literature; Bordbar et al. [45] found that, using CuO/ZnO nanocomposites, catalytic reduction of 4-NP to 4-AP (using NaBH4 as the reducing agent) was completed in several minutes.

The kinetics of calcined CMF, CuO, CuO/Fe3O4, and carbon-doped CuO/Fe3O4 composites are shown in Figure 4b. In the absence of catalyst, the reduction of 4-NP by NaBH4 was negligible. In each case, the pseudo-first-order kinetic prevailed. The calcined CMF also had a negligible effect on the reduction of 4-NP. The rate constants (k) for CuO, CuO/Fe3O4, and carbon-doped CuO/Fe3O4 composite samples were 1.3 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−1, 3.6 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−<sup>1</sup> and 6.5 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−1, respectively. In the case of CuO, the catalytic activity was the lowest due to the aggregation of CuO nanoparticles, while the catalytic efficiency of carbon-doped CuO/Fe3O4 composites was much better than that of CuO/Fe3O4, demonstrating that carbon doping is effective for enhancing the catalytic activity of the catalysts.

The catalytic reduction of 4-NP by NaBH4 using metal-oxide nanoparticles (CuO) has two steps [46]: (1) borohydride ions are adsorbed onto the nanoparticle surface, forming active surface-hydrogen, while 4-NP is also adsorbed onto the nanoparticle surface; (2) active hydrogen attacks the positively charged nitrogen in the nitro group of 4-NP, followed by the addition of two hydrogen atoms, producing 4-AP.

**Figure 4.** (**a**) Time-dependent ultraviolet–visible (UV–Vis) absorption spectra of 4-nitrophenol (4-NP) reduced by NaBH4 in the presence of carbon-doped CuO/Fe3O4 composite catalysts; (**b**) the first-order kinetic plot (absorbance at 400 nm, ln(At/A0)) versus reaction time for the reduction of 4-nitrophenol; At and A0 represent the absorbance values of 4-NP at 400 nm at designated time t and t = 0, respectively.

Cationic and anionic organic dyes were chosen to further investigate the catalytic properties of carbon-doped CuO/Fe3O4 composite catalyst. As shown in Figure 5a, for cationic dye (methylene blue), in the presence of NaBH4 and carbon-doped CuO/Fe3O4 composite catalyst, the absorbance at 660 nm (MB) gradually decreased as a function of time; furthermore, the catalytic reduction was completed within 6 min at room temperature (the color of the solution was colorless).

The results from methyl orange (an anionic dye) are shown in Figure 5c. Under otherwise the same conditions, the color change (from orange to colorless) was slower than that for MB (Figure 5a, from blue to colorless). The pseudo-first-order rate law was also valid here (Figure 5d). For the carbon-doped CuO/Fe3O4 composite catalyst, the rate constant (k) was 2.4 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−<sup>1</sup> for MO, while it was 12.9 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−<sup>1</sup> for MB.

We compared the catalytic performance of the carbon-doped CuO/Fe3O4 composite catalyst for the reduction of 4-NP, MB, and MO, with other related ones from the literature (Table 1). As shown, the carbon-doped CuO/Fe3O4 composite catalyst showed much improved results, and the pseudo-first-order rate constant (k) for the nanocomposite catalyst from the present study was indeed consistently higher than that reported in the literature. The improved results may be attributed to the unique original morphologies associated with HKUST-1.

**Table 1.** Comparison of the catalytic performance for the reduction of 4-nitrophenol, methylene blue (MB), and methyl orange (MO) using the carbon-doped CuO/Fe3O4 composite catalyst and other reported catalysts in the literature. NP—nanoparticle.


**Figure 5.** (**a**) Time-dependent UV–Vis absorption spectra of methylene blue (MB) with the carbon-doped CuO/Fe3O4 composite catalyst in the presence of NaBH4; (**b**) the corresponding first-order kinetic plot (absorbance at 660 nm, ln(At/A0)) versus reaction time for the reduction of MB; At and A0 represent the absorbance of MB (660 nm) at designated time t and t = 0, respectively; (**c**) time-dependent UV–Vis absorption spectra of methyl orange (MO) with the carbon-doped CuO/Fe3O4 composite catalyst in the presence of NaBH4; (**d**) the corresponding first-order kinetic plot (absorbance at 460 nm, ln(At/A0)) versus reaction time for the reduction of MO; At and A0 represent the absorbance of MO (460 nm) at designated time t and t = 0, respectively.

From the viewpoint of practical application, the recycling/reuse of the catalyst is of critical importance. In the present study, after the catalytic degradation experiments, the magnetic carbon-doped CuO/Fe3O4 composite catalyst was readily separated from the reaction system using an external magnet. The used catalysts were collected, and rinsed with distilled water several times. After a thorough washing process, the recovered magnetic catalyst was reused in the subsequent run of catalytic reduction of 4-NP under identical conditions, and the same process was repeated five times. The results are shown in Figure 6. The catalytic performance of the magnetic carbon-doped CuO/Fe3O4 composite catalyst decreased only slightly (the 4-NP reduction ratio decreased from 100% to 96%) after five cycles. Similar results were obtained in the reuse/recycling experiments of the as-prepared magnetic carbon-doped CuO/Fe3O4 composite catalyst during the catalytic reduction of MB and MO. Therefore, the as-prepared magnetic carbon-doped CuO/Fe3O4 composite catalyst is a promising system for practical applications.

**Figure 6.** Reduction conversion ratio of 4-NP, MB, and MO after five successive cycles using the carbon-doped CuO/Fe3O4 catalyst.
