Heterogeneous Catalysis for Environmental Remediation

The intensive human activities in chemical industry and environmental purification urge the development of advanced protocols for green production and waste management. In environmental science, developing highly efficient and environmentally-friendly catalytic materials and systems are very favourable approaches to green chemical synthesis and remediation of contaminated air, soil, and wastewater. Therefore, unveiling the relationship between material structure/chemistry and performances in heterogeneous catalysis would provide valuable guidance for rational catalyst design as well as addressing the challenges in potential applications in environmental science. Here, we dedicate this special issue to showcasing the recent progress in fabrication and evaluation of state-of-the-art carbon/metal catalysts for green chemistry, photocatalysis, advanced oxidation processes (AOPs), and other applications in environmental technologies.

Advanced oxidative processes have been demonstrated as a powerful technique for activating superoxides producing oxidative species (free radicals) for complete degradation of organic pollutants in aqueous systems. Wang et al. [1] synthesized magnetic carbon supported manganese oxides (Fe₃O₄/C/Mn), which could effectively activate peroxymonosulfate (PMS) for phenol mineralization. The redox Mn⁴⁺/Mn³⁺ couple is the catalytic site for radical generation and the magnetic Fe₃O₄ counterpart not only serves as a support but also results in easy separation of the catalyst from the water by an external magnetic field. Zhu et al. [2] developed a Co-Fe alloy catalyst which outperformed CoFe₂O₄ for triggering PMS to evolve sulfate radicals, while the formation of Co-Fe nitride crystallites significantly improved the stability in the aqueous oxidative environment. Chen et al. [3] reported a Ce-Mg/Al₂O₃/ozone system that exhibited great oxidative efficiency for decomposition of resistant petroleum organic wastes from the petroleum refinery industry.

Chemical synthesis usually requires a green and robust catalyst to transfer hydrocarbons to target products with desired conversion efficiency, selectivity, and stability. Zhao et al. [4] synthesized a Cu-g-C₃N₄/activated-carbon composites to replace the toxic mercury-based catalysts for acetylene hydrochlorination which yielded a high conversion of acetylene and great selectivity of vinyl chloride. Meanwhile, the catalyst maintained superb stability in resistance to coke deposition. Lin et al. [5] discovered that sulphated tin ion-exchanged montmorillonite (SO₄²⁻/Sn-MMT), with both Brønsted and Lewis acid sites, could catalytically convert xylose and xylan into furfural. Chung et al. [6] revealed that the acid strength and porous structure of microporous zeolites could be manipulated to achieve selective glucose conversion to decyl glucoside.

Carbon monoxide (CO) and nitrogen oxides (NO_x) generated from industrial production and human activities are hazardous gases that would cause severe air pollution. The nanocomposites such as mesoporous CuO-TiO nanotubes (Zedan et al. [7]) and CuO nanorods-reduced graphene oxide (Wang et al. [8]) were developed for catalytic oxidation of CO to CO₂ at low temperatures. Di and co-workers [9] discovered that the thermal activation atmosphere dramatically impacted the catalytic activity of CuBTC MOF for CO oxidation. Besides, mixed metal oxides of Fe-W-Ce (Stahl et al. [10]) and V₂O₅-WO₃/TiO₂ (Qi et al. [11]) could be utilized for selectively converting NO_x with NH₃ into harmless N₂ and water. The reduction of nitrous oxide (N₂O) and oxidative dehydrogenation of ethane to ethylene could be simultaneously achieved on Cr/Al₂O₃ (Zhang et al. [12]). It was also discovered that N₂O could be directly decomposed on Cu-Zn/ZuAl₂O₄ (Zeng et al. [13]) and Cu-Zn/γ-Al₂O₃ (Zhang et al. [14]).

Developing photocatalysts for efficient utilization of solar energy would contribute to a sustainable future for the human race. Truppi et al. [15] conducted a comprehensive review of the recent progress in novel TiO₂-based nanocomposites as visible-light-driven photocatalysts for versatile environmental applications. The mesoporous TiO₂/SiO₂ composites from a biotemplating method (Yan et al. [16]) and TiO₂-impregnated porous silica tubes (Hayashi et al. [17]) have been demonstrated as outstanding photocatalysts for dye purification under UV irradiation. Two/three-unit hybrid nanomaterials of MoS₂/TiO₂ nanobelts (Liu et al. [18]), three-dimensional WO₃-TiO₂ nanoflowers (Lee et al. [19]) and TiO₂/RGO/Ag (Tian et al. [20]) were constructed for photocatalytic Cr(VI) reduction, photo-oxidation of toxic aromatic volatile compounds, and photodegradation of methylene blue, respectively. The superior photocatalytic activity of the composites compared with the single compounds was due to the enhanced light absorption, improved charge separation efficiency, and optimized band structure of the semiconductors. Photocatalysts beyond TiO₂ were also explored in this special issue. Shu et al. [21] reported that immobilized ZnO/Vis could be applied for decomposition of orange G in wastewater. Meng et al. [22] prepared ZnCr layered double hydroxides (LDHs) with salen-metal complex (M = Co or Ni) intercalation which exhibited much better photocatalytic activity than traditional LDHs. Additionally, layered perovskite K₂La₂Ti₃O₁₀ was modified with a Cu²⁺ iron-exchange (Pang et al. [23]) for mineralization of chlorobenzene in the presence of CO₂ under simulated solar light irradiation.

Overall, this special issue covers state-of-the-art heterogeneous catalysis for applications in environmental science which would contribute to addressing technical problems for material design as well as underpinning the mechanistic insights of environmental catalysis. The guest editors would like to express their appreciation for the professional assistance from the editorial team and for the excellent research findings from all the authors which made this issue a great success.