Nanocatalysts for the Degradation of Refractory Pollutants

The rapid development of industrialization has resulted in the excessive emission of hazardous contaminants into our water and air resources, adversely affecting both health and the environment [1]. Consequently, effective pollutant control methods, including adsorption, biological oxidation, chemical oxidation, and incineration, are essential [2]. However, the efficiency of these strategies is often constrained by diffusive mass transport, necessitating external agitation to enhance yields. Simultaneously, the rapid growth of nanotechnology has introduced new dimensions to environmental remediation processes. Due to their nanoscale size, nanoparticles exhibit unique physical and chemical properties, including large surface-area-to-volume ratios and high interfacial reactivity [3]. To date, an increasing number of nanoparticles have been shown to interact specifically with pollutants in water, gas, and soil, offering promising prospects for novel and advanced environmental technologies [4].

Acknowledging this trend, we are pleased to announce the launch of a Special Issue in Catalysts titled “Nanocatalysts for the Degradation of Refractory Pollutants”. This Special Issue aims to highlight recent progress and advances in nanocatalysts and their applications in water treatment and air purification. The issue includes 12 research articles that showcase a broad range of topics related to the application of nanocatalysts in pollutant removal.

Magnetite (Fe₃O₄) is a promising heterogeneous Fenton-like catalyst due to its intrinsic peroxidase-like activity and efficient magnetic separation [5]. To further enhance the catalytic performance of Fe₃O₄ for practical applications, Chen et al. synthesized fluorinated Fe₃O₄ microspheres through the glycothermal method. The fluorination remarkably improved the performance of F-Fe₃O₄ in the degradation of anionic dyes such as orange G and Congo red (contribution 1).

In contribution 2, Kumar et al. immobilized WO₃ and BiOCl on polyaniline (PAn) to construct a heterojunction nanocomposite photocatalyst (BiOCl/WO₃@PAn), which exhibited enhanced catalytic performance compared to the pristine catalysts. The reactive species •O₂⁻ and •OH were identified as the primary species responsible for the mineralization of 2-chlorophenol (2-CP).

In contribution 3, Wang et al. reported the preparation of heterometallic Ni and Pd-incorporated Fe₃O₄ (Ni–Pd/Fe₃O₄) yolk-shelled nanospheres through solvothermal treatment followed by high-temperature annealing. The magnetic properties and yolk-shelled structure, combined with the uniformly dispersed active heterometals within the shell and yolk of Fe₃O₄, endowed Ni–Pd/Fe₃O₄ with superior recyclability and enhanced catalytic performance. This was particularly evident in the degradation of three nitrogen-containing organic dyes: 4-nitrophenol, Congo red, and methyl orange, compared to its monometallic counterparts.

In contribution 4, Haq et al. investigated the treatment of real refinery wastewater using a TiO₂/activated carbon composite through integrated photocatalytic oxidation and adsorption processes. Under optimal conditions, the TiO₂/activated carbon composite caused a 95% decrease in chemical oxygen demand, surpassing the individual adsorption and photocatalytic degradation using pristine activated carbon or TiO₂.

Three-dimensional (3D) printing, one of the most rapidly progressing technologies of the 21st century, is receiving widespread recognition. Three-dimensional-printed catalysts offer the advantages of stability, high reactivity, and recyclability [6]. In contribution 5, Guo et al. developed a hierarchically porous zero-valent copper (3DHP-ZVC) using 3D printing and used it as a catalyst for tetracycline (TC) degradation through Fenton-like processes. The 3DHP-ZVC/H₂O₂ system demonstrated excellent stability in 20 consecutive cycles and decomposed over 93.2% of TC within 60 min, outperforming the homogeneous Cu²⁺/H₂O₂ system.

Using Acacia nilotica seed extract as a stabilizing and capping agent, Taha et al. synthesized manganese dioxide (MnO₂) nanoparticles supported on palm waste. The findings indicated that the MnO₂/palm waste biochar nanocomposite significantly enhanced the removal efficiency of methyl orange compared to bare palm fronds and biochar, showing a 6-fold improvement over the former and a 1.5-fold increase over the latter (contribution 6).

Among the various developed photocatalysts, monoclinic scheelite bismuth vanadate (BiVO₄) stands out due to its cost-effectiveness, environmental friendliness, and chemical stability [7]. However, pristine BiVO₄ faces challenges related to poor charge transport and the rapid recombination of photogenerated electron–hole pairs. To address these issues, Hu et al. incorporated carbon quantum dots (CQDs), a novel member of carbon-based nanomaterials, to facilitate charge separation and elevate the energy level of BiVO₄. As anticipated, the optimal CQDs/BiVO₄ composite exhibited significantly enhanced photocatalytic activity in degrading a typical paraben pollutant—benzyl paraben (contribution 7).

Bi₂WO₆ and BiO_2−x have garnered significant interest in the fields of photocatalysis and wastewater treatment because of their substantial application potential [8]. In contribution 8, Zhang et al. developed a Z-scheme heterojunction Bi₂WO₆/BiO_2−x composite, which could restrain the recombination of photogenerated carriers. Therefore, the composite exhibited outstanding photocatalytic activity towards ciprofloxacin degradation with good stability.

In contribution 9, Li et al. developed metal–organic frameworks (MOFs)-derived nitrogen-doped carbon nanotubes coated with multi-active component CoMoN@NCNT nanocomposites. These nanocomposites demonstrated superior bifunctional activity in both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Due to its remarkable catalytic performance and stability, the catalyst holds promise for application in wastewater treatment.

Carbon nitride (g-C₃N₄) has garnered widespread attention owing to its excellent visible light catalytic activity and eco-friendly characteristics [9]. However, the relationship between the precursor type and the performance of the resultant catalyst deserves further exploration. In contribution 10, Liu et al. investigated the impact of different precursors on the photocatalytic performance of g-C₃N₄. Their findings revealed that the supramolecular polymer precursor, formed by combining melamine/urea or trimeric uric acid, generates a unique one-dimensional nanotube structure of g-C₃N₄, which can expose more catalytic active sites and efficiently degrade norfloxacin in water.

The photocatalytic performance of semiconductors can be enhanced by manipulating their morphology [10]. In contribution 11, Wang et al. synthesized BiOBr sub-microspheres smaller than 1 μm using a solvothermal method and poly(vinylpyrrolidone) regulation. Under simulated sunlight exposure, these BiOBr sub-microspheres demonstrated remarkable photocatalytic performance in the degradation of benzyl alcohol.

Appropriate oxygen vacancy concentration is beneficial for enhancing the photocatalytic depolymerization of sodium lignosulfonate into high-value chemicals. In contribution 12, Wang et al. fabricated nickel-doped WO_3−x nanosheets using a solvothermal strategy. The introduction of Ni dopants can diminish the band gap and increase the oxygen vacancy concentration within the WO_3−x nanosheets, thereby enhancing their photocatalytic performance in the depolymerization of sodium lignosulfonate into valuable chemicals such as vanillic acid, vanillin, and guaiacol.

As guest editors of this Special Issue, we are grateful to the editors of Catalysts, in particular Ms. Carina Liu, Section Managing Editor of MDPI, for her meticulous handling of this Special Issue. We also appreciate the remarkable contributions and efforts of all authors and reviewers who have been instrumental in making this Special Issue possible. We sincerely hope that this Special Issue will serve as a source of inspiration, encouraging researchers in both academia and industry to leverage their multidisciplinary knowledge and experience to develop more effective nanocatalysts for the degradation of refractory pollutants.