Dyes are used in various industries as coloring agents. The discharge of dyes, specifically synthetic dyes, in wastewater represents a serious environmental problem and causes public health concerns. The implementation of regulations for wastewater discharge has forced research towards either the development of new processes or the improvement of available techniques to attain efficient degradation of dyes. Catalytic oxidation is one of the advanced oxidation processes (AOPs), based on the active radicals produced during the reaction in the presence of a catalyst. This paper reviews the problems of dyes and hydroxyl radical-based oxidation processes, including Fenton's process, non-iron metal catalysts, and the application of thin metal catalyst-coated tubular reactors in detail. In addition, the sulfate radical-based catalytic oxidation technique has also been described. This study also includes the effects of various operating parameters such as pH, temperature, the concentration of the oxidant, the initial concentration of dyes, and reaction time on the catalytic decomposition of dyes. Moreover, this paper analyzes the recent studies on catalytic oxidation processes. From the present study, it can be concluded that catalytic oxidation processes are very active and environmentally friendly methods for dye removal.

Catalytic Oxidation Process for the Degradation of Synthetic Dyes: An Overview.

A novel green synthesis of silver nanoparticles and their catalytic action in reduction of Methylene Blue dye

Supported metal nanoparticles, M-NPs, are of great scientific and economic interest as they encompass application in chemical manufacturing, oil refining and environmental catalysis. Oxidation and hydrogenation reactions are among the major reactions catalyzed by supported M-NPs. Although supported M-NPs are preferable due to their easy recovery and reuse, there are still some practical issues regarding their catalytic activity and deactivation. This review highlights the general features of supported M-NPs as catalysts with particular attention to copper, gold, platinum, palladium, ruthenium, silver, cobalt and nickel and their catalytic evaluation in various reactions. The catalytic performance of noble M-NPs has been explored extensively in various selective oxidation and hydrogenation reactions. In general, noble metals are expensive and sensitive to poisons. Despite their significant merits and potential (easily available, comparatively inexpensive and less sensitive to poisons), catalysis by base M-NPs is relatively less explored. Therefore, activity of base M-NPs can be improved, and still, there is potential for such catalysts.

Review of supported metal nanoparticles: synthesis methodologies, advantages and application as catalysts

Among various approaches synthesizing metal nanoparticles and tiny clusters, a template method using dendrimers has significant advantages over other chemical approaches with respect to their synthetic precision and the scalability. A dendrimer of polydentate ligands assembles metal ions or salts into the interior allowing production of metal nanoparticles in the dendrimer. The dendrimer-encapsulated nanoparticles (DENs) exhibit unique and remarkable catalytic properties depending on the size and elemental formula. Recent advances in dendrimer chemistry even enabled the atom precise synthesis of subnanometer metal clusters that have been impossible to prepare by wet chemical methods. In addition, not only for the synthesis of metal nanoparticles and clusters, the dendrimer itself can also provide the modulation of activity and selectivity in the catalysis. In this review, we summarized the most relevant research in which the dendrimer was employed as the template, modulator, or stabilizer for nanoparticle synthesis for catalytic applications.

New Horizon of Nanoparticle and Cluster Catalysis with Dendrimers

Selective reduction of Cu2+ with simultaneous degradation of tetracycline by the dual channels ion imprinted POPD-CoFe2O4 heterojunction photocatalyst.

The catalytic oxidation of methylene blue was investigated in the presence of dendrimer generation five terminated NH2 encapsulated silver (AgDENs), and gold (AuDENs) nanoparticles as catalyst. The synthesis process of encapsulated metal nanoparticles were monitored by UV/vis spectrophotometry and their average diameters determined by transmission electron microscopy analysis, respectively. The kinetics was monitored using UV/vis spectrophotometry, under pseudo-first order conditions, resulting in observed rate constants kobs. Langmuir–Hinshelwood approach was applied to model the kinetic process. The adsorption of methylene blue on the catalyst surface is related to the type of metal nanoparticle present, and presents the rate-determining step.

Catalytic oxidation of methylene blue by dendrimer encapsulated silver and gold nanoparticles

Effective Catalytic Reduction of Methyl Orange Catalyzed by the Encapsulated Random Alloy Palladium‐Gold Nanoparticles Dendrimer.

We present a study on the synthesis of narrowly dispersed silver and gold nanoparticles using generation five amino-terminated poly(amidoamine) dendrimers as a template. UV–vis spectrophotometry was performed to monitor the synthesis process and to characterize the metal nanoparticles. Infrared (IR) spectroscopy, and transmission electron microscopy (TEM) were also used to characterize the metal nanoparticles. A catalytic oxidation of morin (quercetin) in the presence of hydrogen peroxide was performed as a model reaction to evaluate the metal nanoparticle activities. Ultra high performance liquid chromatography analysis was performed to identify the reaction products. The kinetic data obtained were modeled to the Langmuir–Hinshelwood equation. The encapsulated silver and gold nanoparticles show high activity which confirm the passive interaction with the dendrimer.

Synthesis of narrowly dispersed silver and gold nanoparticles and their catalytic evaluation for morin oxidation

Well-dispersed, random alloy, palladium-gold nanoparticles (2.66 ± 0.51 nm) were immobilized onto several reducible mesoporous transition metal oxide materials. The composites (palladium-gold nanoparticles immobilized onto mesoporous transition metal oxide (PdAu–MTMO)) were characterized through several analytical methods such as UV–vis spectroscopy, BET, XRD, FT-IR, ICP-OES, TEM and TPR analyses. Catalytic oxidation of morin (quercetin) was performed as a model reaction in the presence of hydrogen peroxide to investigate the synergic catalytic activity of the composite. Silica was used as inert support to isolate the catalytic activity of the metal nanoparticles (32.69 ± 9.93 kJ mol−2). Synergistic interaction of PdAu–MTMO was mechanically described according to Langmuir-Hinshelwood and Mars-van Krevelen approaches. The TOF of PdAu–Co3O4 (6073.23 ± 85.01 s−1 mol−1) was considerably larger than that of random alloy nanoparticles (PdAu–SiO2 (25.71 ± 2.35 s−1 mol−1)). The Arrhenius-type plot was constructed to determine the synergistic activity of the composite, where PdAu–Co3O4 described the best synergistic interaction.

Random alloy nanoparticles of Pd and Au immobilized on reducible metal oxides and their catalytic investigation

Generation six hydroxyl terminated PAMAM dendrimer was used as template for the synthesis of several palladium-gold random alloy nanoparticles. The synthesis process was monitored using UV–spectroscopy. The microscopy image of the alloy nanoparticles was captured by transmission electron microscopy and their average size was compared to the theoretical value. Catalytic oxidation of morin by hydrogen peroxide was performed to investigate the activity of the alloy nanoparticles. The Langmuir–Hinshelwood approach was used to conduct a full kinetic evaluation of the catalytic process. The catalytic activity exhibited by the alloy nanoparticles demonstrates a proportional increase with the relative palladium amount. The encapsulated alloy and mono metal nanoparticles remained narrowly dispersed after the reaction.

Ali K. Ilunga

Papers

Catalytic oxidation of methylene blue by dendrimer encapsulated silver and gold nanoparticles

Effective Catalytic Reduction of Methyl Orange Catalyzed by the Encapsulated Random Alloy Palladium‐Gold Nanoparticles Dendrimer.

Synthesis of narrowly dispersed silver and gold nanoparticles and their catalytic evaluation for morin oxidation

Random alloy nanoparticles of Pd and Au immobilized on reducible metal oxides and their catalytic investigation

Catalytic and kinetic investigation of the encapsulated random alloy (Pdn-Au110-n) nanoparticles