Redox Catalyst Research Articles

Solid state – Green construction of starch- beaded Fe3O4@Ag nanocomposite as superior redox catalyst

In this present work, a novel approach which had been developed for the green synthesis of starch-decorated Fe3O4 @Ag nanocomposite by using one-step grinding method. The magnetic core shell nanocomposite was designed by magnetite (Fe3O4) surrounded by the starch (maize corn), a biopolymer which act as a matrix for argentum (Ag) nanoparticles. This leads to the core shell structure starch – Fe3O4 @Ag nanocomposite (NC). The morphological and physical properties of the prepared nanocomposite was characterized by FE-SEM, HR-TEM, XPS, VSM, XRD, FT-IR and UV-Vis spectroscopic studies. The average particle size of the nanocomposite was found to be 9–11 nm. In addition, the nanocomposite gains a retentivity magnetisation of 25.50 emu/g makes the NC saveable for reusability. The nanocomposite had a greater stability for more than a month was confirmed with the help of UV-DRS and XRD analysis. The main advantage of the catalyst is temperature independent; During the conversion of benzaldehyde to benzoic acid, the product yield was achieved higher at ambient condition. Furthermore, the NC exposes the outstanding catalytic performance for reductive degradation of various carcinogenic water pollutants such as Methylene Blue (MB) and Methyl Red (MR) within 14 min and 18 min respectively. Moreover, it is also capable for reduction of 4-nitrophenol in an efficient manner. Thus, the NC holds a greater activity towards oxidation and reductive degradation reaction.

Phaeolschidin F, a new symmetrical bis(styrylpyrone) derivative with redox-catalyzing activity from the mushroom Gymnopilus aeruginosus (order Agaricales).

Phaeolschidin F (1) was isolated from fruiting bodies of the bitter and toxic mushroom Gymnopilus aeruginosus. Structure analysis by NMR and MS revealed that 1 is a new symmetrical bis(styrylpyrone). A series of anti-oxidant and pro-oxidant tests characterized that 1 is a redox catalyst having more anti-oxidant and less pro-oxidant activities than quercetin.

Ce-Doped LaMnO3 Redox Catalysts for Chemical Looping Oxidative Dehydrogenation of Ethane

As a novel reaction mode of oxidative dehydrogenation of ethane to ethylene, the chemical looping oxidative dehydrogenation (CL-ODH) of ethane to ethylene has attracted much attention. Instead of using gaseous oxygen, CL-ODH uses lattice oxygen in an oxygen carrier or redox catalyst to facilitate the ODH reaction. In this paper, a perovskite type redox catalyst LaMnO3+δ was used as a substrate, Ce3+ with different proportions was introduced into its A site, and its CL-ODH reaction performance for ethane was studied. The results showed that the ratio of Mn4+/Mn3+ on the surface of Ce-modified samples decreased significantly, and the lattice oxygen species in the bulk phase increased; these were the main reasons for improving ethylene selectivity. La0.7Ce0.3MnO3 showed the best performance during the ODH reaction and showed good stability in twenty redox cycle tests.

Compound-specific stable isotope analysis for characterization of the transformation of γ-HCH induced by biochar

The role of biochar as the redox catalyst in the removal of reductive pollutants from soil and water system has been extensively studied recently, but there is still a lack of qualitative description of its specific mechanisms in redox processes. In this study, the mechanism of biochar in the transformation process of γ-HCH under anoxic condition was revealed by the compound-specific isotope analysis. The concentration and carbon isotopic composition (δ13C) of γ-HCH were detected in the treatments with different initial concentrations of γ-HCH and biochar materials with different redox properties and varied doses. The surface functional groups and electrochemical properties of biochar before and after the reaction were also characterized. The addition amount of biochar could affect the reduction of γ-HCH concentration, which were 59.1%, 34.6% and 22.4% in treatments with the addition of 5%, 1% and 0.2% biochar, respectively. Meanwhile, the δ13C value of γ-HCH also increased from −26.6 ± 0.2‰ to −23.8 ± 0.2‰ with the addition amount of biochar, especially in the treatment with 5% biochar. As evidenced by X-ray diffraction analysis and electrochemical analysis, biochar promoted the adsorption and transformation of γ-HCH simultaneously, and the oxygen-containing functional groups on the surface of biochar played an important role in the redox process. The isotopic fractionation value (εC) of γ-HCH transformation by biochar was first reported as −3.4 ± 0.4‰. The results will enable the quantitative description of the transformation degree of organic pollutants induced by biochar, and provide a new approach for evaluating the in-situ remediation effects of biochar in a complex environment.

Activity and reusability of immobilized gold nanoparticles for the catalysis of both oxidation and reduction reactions

Colloidal gold nanoparticles (AuNP) are well-known to present a catalytic activity. However, they are characterized by certain limitations, as a rather poor stability and lack of both handling and reusability. Their immobilization on surfaces could help to overcome these drawbacks. This work evaluated the impact of the immobilization of AuNP towards their catalytic activity on reduction and oxidation reactions. Colloidal AuNP stabilized with citrate ions were synthesized, characterized, and immobilized by the dip-coating method, the obtained immobilized AuNP (iAuNP) were characterized. The catalytic activity of AuNP and iAuNP was evaluated and compared to each other through the p-nitrophenol reduction reaction. The kinetic of the catalytic activity of iAuNP was slower than colloidal AuNP. The catalytic activity appeared to be impacted by the less available surface of iAuNP. The reduction efficiency of iAuNP was also evaluated with the DPPH•/DPPH2 reduction reaction. The capacity of iAuNP to catalyze the reduction of DPPH• in DPPH2 was shown even after 130 days and 20 reuses of the nanostructured surface on the contrary to non-reusable colloidal AuNP. The oxidation efficiency of iAuNP was measured by the ability to catalyze the auto-oxidation of curcumin (HPLC-UV/vis). For all these experiments, the addition of 4-mercapto-phenol on the surface of iAuNP passivated the surface, decreasing the catalytic activity of the materials. In conclusion, iAuNP are redox catalysts with both anti- and pro-oxidant effects and high reusability over a very long period.

Ferrocene/air double-mediated FeTiO3-photocatalyzed semi-heterogeneous annulation of quinoxalin-2(1H)-ones in EtOH/H2O.

With both ferrocene and air as the redox catalysts, for the first time, the low-cost natural ilmenite (FeTiO3) was successfully used for photocatalytic bond formations. Under the assistance of a traceless H-bond, and HCHO as the methylene reagent, a variety of imidazo[1,5-a]quinoxalinones were semi-heterogeneously photosynthesized in high yields with good functional group compatibility.

Tailoring enzyme strategies and functional groups in biosynthetic pathways.

Covering: 2000 to 2022Secondary metabolites are assembled by drawing off and committing some of the flux of primary metabolic building blocks to sets of enzymes that tailor the maturing scaffold to increase architectural and framework complexity, often balancing hydrophilic and hydrophobic surfaces. In this review we examine the small number of chemical strategies that tailoring enzymes employ in maturation of scaffolds. These strategies depend both on the organic functional groups present at each metabolic stage and one of two tailoring enzyme strategies. Nonoxidative tailoring enzymes typically transfer electrophilic fragments, acyl, alkyl and glycosyl groups, from a small set of thermodynamically activated but kinetically stable core metabolites. Oxidative tailoring enzymes can be oxygen-independent or oxygen-dependent. The oxygen independent oxidoreductases are often reversible nicotinamide-utilizing redox catalysts, flipping the nucleophilicity and electrophilicity of functional groups in pathway intermediates. O2-dependent oxygenases, both mono- and dioxygenases, act by orthogonal, one electron strategies, generating carbon radical species. At sp3 substrate carbons, product alcohols may then behave as nucleophiles for subsequent waves of enzymatic tailoring. At sp2 carbons in olefins, electrophilic epoxides have opposite reactivity and often function as "disappearing groups", opened by intramolecular nucleophiles during metabolite maturation. "Thwarted" oxygenases generate radical intermediates that rearrange internally and are not captured oxygenatively.

Nanozymes with versatile redox capabilities inspired in metalloenzymes.

Metalloenzymes represent exemplary systems in which an organic scaffold combines with a functional inorganic entity, resulting in excellent redox catalysts. Inspired by these natural hybrid biomolecules, biomolecular templates have garnered significant attention for the controlled synthesis of inorganic nanostructures. These nanostructures ultimately benefit from the protection and colloidal stabilization provided by the biomacromolecule. In this study, we have employed this strategy to prepare nanozymes with redox capabilities, utilizing the versatile catalytic profile of Pt-loaded nanomaterials. Thus, we have investigated protein-templated Pt-based nanoclusters of different sizes and compositions, which exhibit remarkable oxidase, catalase, and reductase-like activities. The interplay between the composition and catalytic activity highlighted the size of the nanocluster as the most prominent factor in determining the performance of the nanozymes. Additionally, we have demonstrated the use of protein-templated nanozymes as potential co-catalysts in combination with enzymes for coupled reactions, under both sequential and concurrent one-pot conditions. This study provides valuable insights into nanozyme design and its wide range of applications in the design of complex catalytic systems.

Dual Ni active sites mediated by In to separate ethane activation and oxidation for enhanced ethene production via chemical looping scheme

Chemical looping oxidative dehydrogenation (CL-ODH) opens a unique avenue to achieve highly efficient conversion of alkanes to value-added alkenes. The challenge lies in identical metal-oxo active sites responsible for both alkane activation and oxidation causing either insufficient activity or over-oxidation thus inferior ethene yield and productivity at relatively low temperature (< 650 °C). Herein, we designed a novel redox catalyst by the incorporation of main-group In (6NiInx/HY, x indicates the atomic ratio of In/Ni) which achieved more than 92 % ethene selectivity at 40 % ethane conversion and long-term stability at 600 °C with 0.22 mmol g−1 ethene productivity at 650 °C. This was because Ni2+ Lewis acid sites conducted catalytic dehydrogenation of ethane to ethene and hydrogen, and the Ni-O-In moiety outside HY framework enabled selective oxidation of hydrogen rather than ethene due to well-regulated Ni 3d electron densities leading to the reduction of adsorption for ethene thus decreasing over-oxidation.

Challenges in Elucidating the Free Energy Scheme of the Laccase Catalyzed Reduction of Oxygen.

Artificial redox catalysts are typically limited by unfavorable scaling relations of reaction intermediates leading to a significant overpotential in multi-electron redox reactions such as for example the oxygen reduction reaction (ORR). The multicopper oxidase laccase is able to catalyze the ORR in nature. In particular the high-potential variants show a remarkably low overpotential for the ORR and apparently do not suffer from such unfavorable scaling relations. Although laccases are intensively studied, it is presently unknown why the overpotential for ORR is so low and a clear description regarding the thermodynamics of the catalytic cycle and the underlying design principles is lacking. In order to understand the laccase catalyzed ORR from an electrochemical perspective, elucidation of the free energy scheme would be of high value. This article reviews the energetics of the proposed laccase catalyzed ORR mechanisms based on experimental and computational studies. However, there are still remaining challenges to overcome to elucidate the free energy scheme of laccase. Obtaining thermodynamic data on intermediates is hard or even impossible with analytical techniques. On the other hand, several computational studies have been performed with significantly different parameters and conditions, thus making a direct comparison difficult. For these reasons, a consensus on a clear free energy scheme is still lacking. We anticipate that ultimately conquering these challenges will result in a better understanding of laccase catalyzed ORR and will allow for the design of low overpotential redox catalysts.

Oxidation and Reduction Pathways in the Knowles Hydroamination via a Photoredox‐Catalyzed Radical Reaction**

AbstractSystematic reaction path exploration revealed the entire mechanism of Knowles's light‐promoted catalytic intramolecular hydroamination. Bond formation/cleavage competes with single electron transfer (SET) between the catalyst and substrate. These processes are described by adiabatic processes through transition states in an electronic state and non‐radiative transitions through the seam of crossings (SX) between different electronic states. This study determined the energetically favorable SET path by introducing a practical computational model representing SET as non‐adiabatic transitions via SXs between substrate's potential energy surfaces for different charge states adjusted based on the catalyst's redox potential. Calculations showed that the reduction and proton shuttle process proceeded concertedly. Also, the relative importance of SET paths (giving the product and leading back to the reactant) varies depending on the catalyst's redox potential, affecting the yield.

Oxidation and Reduction Pathways in the Knowles Hydroamination via a Photoredox-Catalyzed Radical Reaction.

Systematic reaction path exploration revealed the entire mechanism of Knowles's light-promoted catalytic intramolecular hydroamination. Bond formation/cleavage competes with single electron transfer (SET) between the catalyst and substrate. These processes are described by adiabatic processes through transition states in an electronic state and non-radiative transitions through the seam of crossings (SX) between different electronic states. This study determined the energetically favorable SET path by introducing a practical computational model representing SET as non-adiabatic transitions via SXs between substrate's potential energy surfaces for different charge states adjusted based on the catalyst's redox potential. Calculations showed that the reduction and proton shuttle process proceeded concertedly. Also, the relative importance of SET paths (giving the product and leading back to the reactant) varies depending on the catalyst's redox potential, affecting the yield.

Photocatalytic Decarboxylative Cyclization of 1,6-Enynes and N-Methacryloyl-2-phenylbenzimidazoles by Potassium Iodide/ Triphenylphosphine Catalysis

AbstractWe describe a photocatalytic decarboxylative cyclization of 1,6-enynes or N-methacryloyl-2-phenylbenzimidazoles for efficient and ecofriendly syntheses of lactams or benzimidazo[2,1-a]isoquinoline-6(5H)-ones, respectively, by the use of KI/PPh3 as a redox catalyst system. In this method, N-hydroxyphthalimide esters were used as alkyl-radical precursors to achieve this decarboxylative cyclization without any dyes, transition-metal catalysts, oxidants, or bases.

Fast Responsive, Reversible Colorimetric Nanoparticle-Hydrogel Complexes for pH Monitoring.

Hydrogels containing redox-sensitive colorimetric nanoparticles (NPs) have been used to sense ambient pH in many fields owing to their simple and fast visualization capabilities. However, real-time pH monitoring still has limitations due to its poor response rate and irreversibility. Herein, we developed a fast responsive colorimetric hydrogel called ferrocene adsorption colorimetric hydrogel (FACH). Ferrocene, an organometallic compound, plays a vital role as an electron transfer mediator (i.e., redox catalyst) within the hydrogel network. FACH shows fast color change performance with high reactivity and penetrability to ambient pH changes. In detail, FACH shows distinct color change within 2 min under various pH conditions from four to eight, with good reliability. The speed for color change of FACH is approximately six times faster than that of previously developed colorimetric hydrogels, suggesting the fastest hydrogel-based colorimetric pH sensor. Furthermore, FACH shows reversibility and repeatability of the redox process, indicating scalable utility as a sustainable pH monitoring platform.

Redox Behavior and ORR Catalytic Activity of Fe-N-C Electrocatalysts in Various Electrolytes and in the Presence of Probe Molecules

Materials in the Fe-N-C family are the most promising platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). Significant progress has been made in recent years in improving both the ORR activity and durability of the Fe-N-C catalysts. However, further improvements are needed, especially in long-term performance durability in hydrogen-air PEFCs, to enable use of these catalysts in practical applications, such as powering light and heavy-duty vehicles. One of the major factors that hinder the further enhancement of the catalysts is the lack of clear understanding of the nature of the active sites in the Fe-N-C catalysts that catalyze the ORR reaction, due to the complexity of the Fe-N-C catalysts composition and structure. The most active Fe-N-C catalysts have been synthesized by pyrolyzing iron salts or other iron-containing compounds with zinc-based zeolitic imidazolate frameworks (ZIFs) and/or phenanthroline (as carbon and nitrogen sources), or by heat treating iron-substituted ZIFs. It has been shown that many synthesis variables, such as type of iron precursors, the metal and carbon-nitrogen macrocycle ratio, the heat treatment temperature, atmosphere, and temperature profile, all affect the properties and structure and thus the activity and stability/durability of the resulting catalysts. In order to further improve the Fe-N-C catalysts, it is critical to gain insights into the origins of ORR activity and the reaction mechanism. In this work, the redox behavior of different Fe-N-C catalysts, synthesized by a variety of techniques mentioned above, and correlation of this redox behavior with ORR catalytic activity in different electrolytes will be discussed and the effect of nitric oxide gas-phase adsorption on the catalyst redox feature and ORR activity will be presented. The implication of these results on the identity or identities of the ORR active sites of the Fe-N-C catalysts will be discussed.This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat 2.0). This work utilized the resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility operated by Argonne National Laboratory for DOE Office and was authored by Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.

Tungstate promoted CaMnO3-based core-shell redox catalysts for efficient chemical looping oxidative coupling of methane

Chemical looping oxidative coupling of methane (CLOCM) provides a promising route for methane conversion to high-valued olefins, in which oxygen carriers serve as oxygen donors and catalysts to facilitate methane activation and C–C coupling reactions. Herein, the perovskite CaMnO3 was used as the oxygen carrier for CLOCM due to the high catalytic activity of Mn, and tungstate was selected to modify CaMnO3 for improving the C2 product selectivity. The results showed that CaMnO3 with 6% Na2WO4 doping exhibited the highest activity at 800 °C with a C2 yield of 15.5% and good redox stability for at least 30 cycles. The coexistence of Na2WO4 with CaMnO3 synergistically controlled the methane conversion and C–C coupling. Further characterizations indicated that the core-shell structure with the inner layer of CaMnO3 and the outer layer of Na2WO4 effectively regulated the oxygen release and transformation. Na2WO4 modification resulted in an obvious decrease of nonselective oxygen species which were prone to the deep oxidation of methane while leading to a great increase of O2− species that could selectively oxidize methane to C2 hydrocarbons. These findings provide guidance for dopant-induced regulation of oxygen vacancies and selective oxygen species in oxygen carriers for improving CLOCM performance.

Effect of metal-doping (Me = Fe, Ce, Sn) on phase composition, structural peculiarities, and CO oxidation catalytic activity of cryptomelane-type MnO2

Manganese oxides attract considerable attention as redox catalysts for abatement of environmental pollutants due to their relatively high activity and low costs contrary to noble metal catalysts. The present work is focused on the effect of cryptomelane-type MnO 2 catalyst modification with Fe 3+ , Ce 3+ /Ce 4+ , Sn 4+ on their phase composition, structural peculiarities, and catalytic activity in CO oxidation. The samples are investigated by XRD, Raman spectroscopy, XPS, XRF, and comprehensive HRTEM, with their catalytic activity being studied under the model dry conditions and simulated real exhaust conditions comprising CO, O 2 , and water balanced by inert. The Mn 4+ substitution in the cryptomelane structure by Me n+ is revealed to result in its monoclinic distortion, with the Sn- and Fe-doped cryptomelane-type MnO 2 formation being accompanied by the formation of SnO 2 and Fe 1−x Mn x O 3 /Fe 1+x Mn 2−x O 4 , respectively. For Ce-doped samples, the K + presence in the cryptomelane tunnels is essential to stabilize the cryptomelane structure and nanorod morphology. The cryptomelane structure stabilized by only Ce n+ cations is collapsed during the sample calcination to form Mn 2−x Ce x O 3 and Mn 3−x Ce x O 4 , while high Ce content does not favor the formation of the cryptomelane-type MnO 2 . The composites based on the Me-doped cryptomelane-type α-MnO 2 , especially those doped with Ce, are shown to be promising candidates to develop effective oxide-based catalysts for CO oxidation. • Mn 4+ substitution by Me n+ results in monoclinic distortion of cryptomelane structure. • K + ions are essential to stabilize cryptomelane structure and nanorod morphology. • Doped cryptomelane-type MnO 2 is promising for designing of new CO oxidation catalysts.

Iron‐Based Imidazolium Salt as Dual Lewis Acid and Redox Catalyst for the Aerobic Synthesis of Quinazolines

AbstractA low transition temperature mixture formed with 1‐butyl‐3‐(methoxycarbonylmethyl)imidazolium chloride and iron(III) chloride has proven to be an efficient catalyst for the synthesis of quinazolines following a sequence of condensation‐cyclization‐oxidation reactions. The protocol is simple and effective for coupling 2‐acylanilines and benzylamines to form nitrogen containing heterocycles with moderate to excellent yields (up to 93 %), being possible to perform the reaction in preparative scale. The functionalized imidazolium salt is crucial for the activation of the reagents under solvent‐free conditions, and the presence of iron in the catalyst mediates the oxidation step with atmospheric oxygen. These features make the presented procedure better, from an environmental point of view, than other processes previously described. For this comparison, different “green” metrics have been used, such as atom economy, stoichiometric factor, reaction mass efficiency, E‐factor, and EcoScale.

Redox oxide@molten salt as a generalized catalyst design strategy for oxidative dehydrogenation of ethane via selective hydrogen combustion

The current study demonstrates a redox oxide @ molten salt core-shell architecture as a generalized redox catalyst design strategy for chemical looping – oxidative dehydrogenation of ethane. 17 combinations of redox active oxides and molten salts were prepared, evaluated, and characterized. X-ray diffraction indicates that the redox oxides and molten salts are fully compatible, forming separate and stable phases. X-ray photoelectron spectroscopy demonstrates that the molten salts aggregate at the redox oxide surface, forming a core-shell structure to block the non-selective sites responsible for COx formation. Up to ∼74 % single-pass olefin yields were achieved using the proposed redox catalyst design strategy. Statistical analyses of the performance data indicate the potential to achieve up to 86.7 % single-pass yield by simply optimizing the operating conditions using the redox catalysts reported in this study. Meanwhile, the generalizability of the catalyst design strategy offers exciting opportunities to further optimize the composition and performance of the redox catalysts for ethane ODH under a chemical looping scheme with significantly reduced energy consumption and CO2 emissions.

Synchronization of Belousov-Zhabotinsky oscillators with electrochemical coupling in a spontaneous process.

A passive electrochemical coupling approach is proposed to induce spontaneous synchronization between chemical oscillators. The coupling exploits the potential difference between a catalyst redox couple in the Belousov-Zhabotinsky (BZ) reaction, without external feedback, to induce surface reactions that impact the kinetics of the bulk system. The effect of coupling in BZ oscillators under batch condition is characterized using phase synchronization measures. Although the frequency of the oscillators decreases nonlinearly over time, by a factor of 2 or more within 100 cycles, the coupling is strong enough to maintain synchronization. In such a highly drifting system, the Gibbs-Shannon entropy of the cyclic phase difference distribution can be used to quantify the coupling effect. We extend the Oregonator BZ model to account for the drifting natural frequencies in batch condition and for electrochemical coupling, and numerical simulations of the effect of acid concentration on synchronization patterns are in agreement with the experiments. Because of the passive nature of coupling, the proposed coupling scheme can open avenues for designing pattern recognition and neuromorphic computation systems using chemical reactions in a spontaneous process.