**1. Introduction**

Recently, metal nanoparticles (NPs) were widely used in the fields of biomedicine and chemical reactions due to their high selectivity and catalytic efficiency [1–3]. Noble-metal nanoparticles (gold, silver, etc.) [4–6] and non-noble-metal nanoparticles (copper, zinc, and their oxides, sulfides, etc.) [7–10] are particularly noticeable. For example, Jiang et al. [11] reported that CuO and Au domains could greatly improve the photocatalytic activity and stability of Cu2O cubes in the photocatalytic degradation of methyl orange (MO). Rodríguez et al. [12] reported that potassium poly(heptazine imide) (PHIK)/Ti-based metal–organic framework (MIL-125-NH2) composites had superior photocatalytic activity in rhodamine B (RhB) degradation under blue-light irradiation. Among the applications, metal nanoparticles can also be used for treating wastewater and drinking water due to their large surface areas and high activity [13,14]. With growing focus on the development

of cost-effective, efficient catalysts, more attention is being paid to non-noble-metal catalysts, such as metal-oxide catalysts.

Metal NPs with nanometer-scale dimensions are unstable and tend to aggregate due to their high surface energy and surface area, which can lead to the loss of catalytic efficiency [15–17]. The concept of immobilization/stabilization of metal NPs onto support/substrates is one of the effective methods to overcome aggregation of NPs [18,19]. There are many types of substrates/matrices that can be used to support metal NPs, such as carbon [20,21], silica [22], metal oxide [23], polymers, etc. [24–26]. Carbon, as a support for metal nanoparticles, provides multiple accessible channels for diffusion/transport to take advantage of the excellent catalytic functionalities of metal nanoparticles [27].

In past decades, metal–organic frameworks (MOFs) attracted much attention due to their porous structures and potential applications in gas storage, molecule separation, chemical sensing, drug delivery, and catalysis [28–30]. Recently, MOF-based derivative catalysts received more attention because MOF-derived materials have advantages in terms of tailorable morphologies, hierarchical porosity, and easy functionalization with other heteroatoms and metal oxides [31,32]. For instance, Ji et al. used ZIF-67 as a precursor to synthesize a Pt@Co3O4 composite to improve the catalytic activity of CO oxidation [33]. Yang et al. reported that ZnO nanoparticles prepared via calcination of MOF-5 in air at 600 ◦C showed excellent photocatalytic degradation of rhodamine B [34]. Niu et al. synthesized a hybrid catalyst consisting of Cu/Cu2O NPs supported on porous carbon for the catalytic reduction of 4-nitrophenol (4-NP) using HKUST-1 as a precursor [27].

The direct pyrolysis/thermolysis treatment of MOFs is a simple and controllable method to prepare various metal oxides in a one-step process. By following it, we can successfully synthesize carbon-doped CuO/Fe3O4 composite catalysts for organic pollutant reduction. Herein, we prepared carbon-doped CuO/Fe3O4 composite catalysts via direct calcination of HKUST-1/Fe3O4/CMF composites under air. We then applied the as-prepared carbon-doped CuO/Fe3O4 composite catalysts for the catalytic reduction of 4-NP. Its catalytic performance, in comparison with a CuO/Fe3O4 composite from HKUST-1/Fe3O4 composite and CuO, is remarkably better, which is attributed to the fact that carbon doping can (1) minimize the aggregation of CuO/Fe3O4, (2) provide high surface-to-volume ratio and chemical stability for the catalyst in contact with the target pollutants, and (3) enhance the catalytic activity of the CuO/Fe3O4 catalyst. Furthermore, the carbon-doped CuO/Fe3O4 composite catalyst has excellent efficiency in the reduction of methylene blue (MB) and methyl orange (MO). The features of the carbon-doped CuO/Fe3O4 composite catalyst are as follows: CuO acts as the effective catalyst, with the doped carbon having the three functions discussed, and the magnetic Fe3O4 supports easy reuse/recycling of the catalyst using a magnet.
