Recent developments in hydrogels containing copper and palladium for the catalytic reduction/degradation of organic pollutants

Abstract

The elimination of toxic and hazardous contaminants from different environmental media has become a global challenge, causing researchers to focus on the treatment of pollutants. Accordingly, the elimination of inorganic and organic pollutants using sustainable, effective, and low-cost heterogeneous catalysts is considered as one of the most essential routes for this aim. Thus, many efforts have been devoted to the synthesis of novel compounds and improving their catalytic performance. Recently, palladium- and copper-based hydrogels have been used as catalysts for reduction, degradation, and decomposition reactions because they have significant features such as high mechanical strength, thermal stability, and high surface area. Herein, we summarize the progress achieved in this field, including the various methods for the synthesis of copper- and palladium-based hydrogel catalysts and their applications for environmental remediation. Moreover, palladium- and copper-based hydrogel catalysts, which have certain advantages, including high catalytic ability, reusability, easy work-up, and simple synthesis, are proposed as a new group of effective catalysts.

Scheme 1. Different methods employed for environmental remediation.

Scheme 2. Preparation of Cu NPs inside p(AN-co-MA) and p(AN-co-AAc) particles.

Scheme 3. Schematic of the creation of Cu NPs inside p(AMPS) hydrogel network and their use for the reduction of 4-NP to 4-AP.

Scheme 4. Bulk p(VPA) hydrogel crosslinking by p(EGDA).

Fig. 1. Preparation of the blended poly(AAc10–SSS15–GO0.01) composite hydrogel via a crosslinking process based on gamma radiation. This figure has been reproduced from ref. with permission from John Wiley & Sons Inc. Copyright 5316550528241.

Scheme 5. Synthesis of p(AAc-co-AAm)–Cu.

Fig. 2. (a) Formation of Cu₂O/Cu/rGO@CN photocatalysts using sodium alginate hydrogel as a template. SEM images of (b) Cu₂O/Cu@CN, (c) Cu₂O/Cu/rGO@CN-4 and (d) rGO. This figure has been reproduced from ref. with permission from Elsevier. Copyright 5316550031989.

Scheme 6. Chemical reduction of 4-NP and MB.

Scheme 7. Degradation of 2-NP, 4-NP and CB by Cu/CS–CMM catalyst.

Scheme 8. (A) Preparation of CS/F–Cu film and hydrogel nanocomposites and (B) proposed mechanism for the reduction of Cu²⁺ to Cu⁰.

Scheme 9. Reduction of 4-nitrophenol by CS-PAmCu-NP catalyst.

Scheme 10. Reduction of 4-nitrophenol by Cu@GLA–PEI–CA.

Scheme 11. Reducing methyl orange and Congo red by GL–CuO hydrogel nanocomposite catalyst.

Fig. 3. Preparation of Cu⁰/Alg–CNB catalytic beads and their catalytic utilization. This figure has been reproduced from ref. with permission from Elsevier. Copyright 5316550823396.

Scheme 12. Reduction of 4-NP in the presence of N–C/Cu/N–C.

Scheme 13. Proposed degradation path for MO utilizing Cu₂O/TiO₂/CNF/rGH.

Scheme 14. Reduction of 4-NP in the presence of 3D Pd–CNT–GH.

Fig. 4. Procedure for the preparation of the 3D Pd/MoS₂–rGO composite hydrogel. This figure has been reproduced from ref. with permission from Elsevier. Copyright 5316560270510.

Scheme 15. Reduction of 4-NP in the presence of rGSs/Fe₃O₄–Pd.

Scheme 16. Possible mechanism for the formation of PPy hydrogel.

Fig. 5. Schematic showing the formation of the hybrid NP–NC gel: (a) metal ions penetrate the NC gel, (b) metal ions interact with the silanol groups on the clay surface, and (c) metal NPs are formed by the ascorbic-acid reduction of ions, which are subsequently trapped near the clay surface.

Fig. 6. Preparation and catalytic application of PEI–Ag/PEI–Pd composites and PEI hydrogel.

Fig. 7. Postulated stabilization mechanism for Pd NPs immobilized on hydrogels. This figure has been reproduced from ref. with permission from Elsevier. Copyright 5318331364158.

Fig. 8. Catalytic performances and process for the synthesis of rGO-based composite hydrogels. This figure has been reproduced from ref. with permission from Elsevier. Copyright 5318330689362.

Scheme 17. Synthesis of cellulosic hydrogel anchoring 1,1,3,3-tetramethylguanidine poly-ionic liquid moiety.

Scheme 18. Reduction of 4-NP to 4-AP applying Pd NP-embedded alginate hydrogel.

Scheme 19. Reduction of 4-NP to 4-AP applying Pd NPs. This figure has been reproduced from ref. with permission from Elsevier. Copyright 5316560975161.

Fig. 9. Synthesis of Pd Ni–N@C. This figure has been reproduced from ref. with permission from Elsevier. Copyright 5357550496279.