Subsequent Reoxidation Research Articles

Operando spectroscopic studies on redox mechanism for CO2 hydrogenation to CO on In2O3 catalysts

The catalytic activities of In2O3 catalysts with different surface area for both cyclic unsteady-state (transient) and steady-state reverse water–gas shift (RWGS) reactions were systemically investigated. The initial CO formation rates during CO2-oxidation of the H2-reduced In2O3 catalysts were close to the CO formation rates in steady-state RWGS conditions at 325 °C, suggesting a redox mechanism for the steady-state RWGS reaction over the In2O3 catalysts. Transient kinetics for the In3+/In+ redox and products (H2O, CO) formation during the cyclic H2-reduction and CO2-oxidation of In2O3 under periodic feeding of H2 ↔ CO2 were studied by time-resolved operando UV–vis and In K-edge X-ray absorption experiments at 325 °C. During the H2-reduction, the surface In3+-O species was reduced to produce H2O and In+-□ (□: oxygen vacancy). Subsequent re-oxidation of the In+-□ by CO2 gave CO and the In3+-O species. The transient CO formation rates were close to the consumption rates of In+-□ under CO2, providing a quantitative evidence on the redox mechanism for unsteady-state RWGS reaction over In2O3. These results indicate that the unsteady-state and steady-state RWGS reactions are primary driven by the In3+/In+ redox mechanism. Kinetic results show that the reoxidation step is the rate-limiting step in the steady-state RWGS reaction.

Effect of metal cation coaddition in phosphovanadomolybdate anion catholytes on the redox flow polymer electrolyte fuel cell performance

Redox flow polymer electrolyte fuel cells (RFFCs) employing heteropolyanions (HPAs) as cathode redox mediators enable continuous power generation through the electrochemical reduction of the HPAs at the carbon cathode and subsequent reoxidation of the reduced-form HPAs in the cathode tank. Currently, effective reoxidation methods are required to achieve both high performance and long-term stability. Heteropolyanions such as α–[PW12O40]3– and α–[SiVW11O40]5– have increased reoxidation rates in the presence of radical species. In this study, fuel cell performance is investigated by adding Fe3+ and Cu2+ to the aqueous solution of [PV3Mo9O40]6–, which is used as a catholyte in RFFCs to increase the concentration of radicals in the system. Current–voltage measurements show that the added metal cations in the catholyte increase the power density. A temporary improvement in cell performance is observed during a prolonged current passage test.

Adsorptive transfer cyclic square-wave voltammetry and HPLC with tandem electrochemical detection – Two novel methods for determination of chili peppers pungency

The pungency of chili peppers, the most popular hot spice used worldwide, is caused by capsaicinoids (CPDs), the content of which can vary greatly due to varietal differences and growing conditions. For the first time, a novel simple method for the fast determination of CPDs in chili peppers and chili products was developed based on adsorptive transfer cyclic square-wave voltammetry (AdTCSWV), using adsorption of lipophilic CPDs on an unmodified glassy carbon electrode surface from methanolic extracts of chili pepper samples. The CSWV is based on short oxidation of adsorbed CPDs to quinoid products, and their subsequent reduction and re-oxidation to provide specific analytical signals with a linear range from 0.05 to 1.00 mg L−1. This principle was also implemented in tandem coulometric and amperometric detection of CPDs after HPLC separation. The two-step electrochemical detection provides increased selectivity for CPDs in case of CPDs co-elution with other electrochemically oxidizable components that cannot be reversibly reduced.

Hydrogen reduction of iron ore pellets: A surface study using ambient pressure X-ray photoelectron spectroscopy

Using Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS), this study investigates the reduction behavior of iron oxides in direct reduction (DRI) and blast furnace (BF) pellets. H2, CO, and a H2–CO mixture are used as reducing agents at 650 °C. The investigation aimed to elucidate variations in the rate of reduction over time and under different conditions. Additionally, contour plots are generated to visualize the X-ray photoelectron peak intensity variations as a function of time. Furthermore, phase stability diagrams based on Fe–O–C and Fe–O–H systems are employed to enhance the understanding of reduction behavior. Results revealed that an increased gas flow rate significantly accelerated the reduction rate due to enhanced gas diffusion, while elevated pressure facilitated the reduction of wüstite to metallic iron. Notably, the DRI pellet achieves around 90% metallization degree reduction with hydrogen, but the introduction of carbon monoxide into the reducing gas prevented the reduction of the DRI pellet. In the case of BF pellet reduction, approximately 20% metallization degree is observed using H2–CO (50:50), yet subsequent reoxidation of the reduced iron to wüstite and magnetite occurred. Further investigation identified a significant increase in the partial pressure of H2O and CO2, particularly within the surface porosities, as the underlying cause of this reoxidation phenomenon.

An enzyme cascade biosensor based on multiwalled carbon nanotube-RuO2 nanocomposite for selective amperometric determination of lactose in milk samples

An enzyme-based electrochemical biosensor was fabricated for the sensitive determination of lactose. The utilized enzyme cascade system is composed of β-galactosidase (β-Gal) and glucose oxidase (GOx). The ruthenium(IV) oxide (RuO2) presents in MWCNT-RuO2 nanocomposite immobilized on the glassy carbon electrode acts as an electrochemical mediator, resembling a second-generation enzyme biosensor. The functional mechanism of the biosensor was discussed, explaining the chemical oxidation of H2O2, the final product of the enzymatic reaction, by RuO2 and subsequent reoxidation of generated Ru to RuO2 at the electrode surface. This shifts the oxidation of H2O2 to a lower potential magnitude of +0.40 V, enhancing the biosensor’s selectivity. The analytical figures of merit were verified for the developed lactose biosensor through repetitive measurements. The precision of the lactose biosensor was ensured with good reproducibility (RSD % = 2.68) and repeatability (RSD % = 4.12). The selectivity of the biosensor towards various saccharides and ionic species potentially present in milk samples was investigated, and no notable interference effect was detected. Moreover, the accuracy of the lactose biosensor was tested by analyzing spiked samples and a semi-skimmed milk (SS-milk) sample with certified lactose values. The limit of detection (LOD) and limit of quantification (LOQ) were calculated to be (0.036 mM) and (0.121 mM), respectively. Thanks to the short response time of the fabricated lactose biosensor, it was transferred to a screen-printed carbon electrode (SPCE) and successfully employed in flow injection analysis (FIA).

Reoxidation dynamics of the methylene blue reduced by laser-synthesized silver nanoparticle

The catalytic reduction of methylene blue (MB) by metal nanoparticles and subsequent reoxidation is widely recognized and often referred to as a "redox reaction." The reduction mechanism of MB has been extensively studied and is well understood but its reoxidation has received little attention. Here we report an experimental study on the reoxidation dynamics of MB reduced by silver nanoparticles (Ag NPs). Ag NPs synthesized using the Pulsed Laser Ablation (PLAL) technique are used to reduce MB in the presence of the reducing agent NaBH4. The reduction/reoxidation dynamics are monitored using UV−Vis spectroscopy through the decrease/increase of characteristic MB absorbance peak and color switching between methylene blue and leucomethylene (LMB). The influence of the various volumetric concentrations of Ag NPs and NaBH4 on reoxidization dynamics of reduced MB is investigated. The reoxidation of reduced MB exhibited almost linear growth over time for different concentrations of both Ag NPs and NABH4 used in the reduction process. Higher NaBH4 concentrations appeared to decelerate the reoxidation process, while the concentration of Ag NPs had no discernible impact on the reoxidation rate. The reduction of reoxidized MB after adding additional amounts of Ag NPs and NaBH4 revealed that Ag NPs remained stable in the reaction medium but NaBH4 deteriorated. Therefore, the deterioration of the NaBH4 is identified as the major cause of the reoxidation of the reduced MB. A series of reduction-reoxidation experiments using chemically synthesized Ag NPs indicated that MB reoxidation is not limited to laser-synthesized Ag NPs.

Investigation of the structure-forming role of dietary fibre components in gluten-free cereal- and pseudocereal-based food matrices

The aim of this work was to investigate the structure-forming ability of cereal β-glucan (BG) and arabinoxylan (AX) in gluten-free (GF) dough and slurry matrices. 2, 5 or 8%, w/w BG was dosed to white millet and buckwheat flours and its effect on the mixing and pasting properties was examined. Furthermore, AX “incorporation” into dough structure and possible interaction with millet and buckwheat proteins were investigated by using reduction and subsequent re-oxidation based on our previous work.BG addition increased consistency and changed mixing properties in both millet and buckwheat doughs by forming viscous gel and aggregates. However, its impact on the pasting properties of the two types of GF flours differed in tendency. Similarly, a different behaviour of millet and buckwheat matrices was found in case of AX incorporation, explained by their distinct macromolecular (especially protein) composition. SDS-PAGE results referred to interactions between AX molecules and non-gluten proteins, however the changes in free ferulic acid contents did not confirm this. Although, white flours of millet and buckwheat are simpler matrices than wholemeals, they might be still too complex to examine the effect of a specific component, especially in a redox system. Thus, development of model gluten-free dough systems is a further goal of this work.Research works focusing on AX or BG functionality in gluten-free matrices are rare. The current work might contribute to a better understanding of dietary fibre functionality in dough systems with non-gluten proteins, as well as to develop healthier gluten-free products of higher fibre content.

Avian cryptochrome 4 binds superoxide

Flavin-binding cryptochromes are blue-light sensitive photoreceptors that have been implicated with magnetoreception in some species. The photocycle involves an intra-protein photo-reduction of the flavin cofactor, generating a magnetosensitive radical pair, and its subsequent re-oxidation. Superoxide (O2•−) is generated in the re-oxidation with molecular oxygen. The resulting O2•−-containing radical pairs have also been hypothesised to underpin various magnetosensitive traits, but due to fast spin relaxation when tumbling in solution would require immobilisation. We here describe our insights in the binding of superoxide to cryptochrome 4 from C. livia based on extensive all-atom molecular dynamics studies and density-functional theory calculations. The positively charged “crypt” region that leads to the flavin binding pocket transiently binds O2•− at 5 flexible binding sites centred on arginine residues. Typical binding times amounted to tens of nanoseconds, but exceptional binding events extended to several hundreds of nanoseconds and slowed the rotational diffusion, thereby realising rotational correlation times as large as 1 ns. The binding sites are particularly efficient in scavenging superoxide escaping from a putative generation site close to the flavin-cofactor, possibly implying a functional relevance. We discuss our findings in view of a potential magnetosensitivity of biological flavin semiquinone/superoxide radical pairs.

Magnetic activated carbon from spent coffee grounds: iron-catalyzed CO2 activation mechanism and adsorption of antibiotic lomefloxacin from aqueous medium.

The facile fabrication of low-cost adsorbents possessing high removal efficiency and convenient separation property is an urgent need for water treatment. Herein, magnetic activated carbon was synthesized from spent coffee grounds (SCG) by Fe-catalyzed CO2 activation at 800°C for 90min, and magnetization and pore formation were simultaneously achieved during heat treatment. The sample was characterized by N2 adsorption-desorption, XRD, VSM, SEM, and FTIR. Batch adsorption experiments were conducted using lomefloxacin (LMO) as the probing pollutant. Preparation mechanism was revealed by TG-FTIR and XRD. Experimental results showed that Fe3O4 derived from Fe species can be reduced to Fe by carbon at high temperatures, followed by subsequent reoxidation to Fe3O4 by CO2, and the redox cycle between Fe and Fe3O4 favored the formation of pores. The promotion effects of Fe species on CO2 activation can be quantitatively reflected by the yield of CO as the signature gaseous product, and the suitable activation temperate range was determined to be 675 to 985°C. The BET surface area, total pore volume, and saturated magnetization value of the product were 586 m2g-1, 0.327 cm3g-1, and 11.59 emu g-1, respectively. The Langmuir model was applicable for the adsorption isotherm data for LMO with the maximum adsorption capacity of 95mgg-1, and thermodynamic analysis revealed that the adsorption process was endothermic and spontaneous. This study demonstrated that Fe-catalyzed CO2 activation was an effective method of converting SCG into magnetic separable adsorbent for LMO removal from aqueous medium.

Promotion of Humic Acid Transformation by Abiotic and Biotic Fe Redox Cycling in Nontronite.

The redox activity of Fe-bearing minerals is coupled with the transformation of organic matter (OM) in redox dynamic environments, but the underlying mechanism remains unclear. In this work, a Fe redox cycling experiment of nontronite (NAu-2), an Fe-rich smectite, was performed via combined abiotic and biotic methods, and the accompanying transformation of humic acid (HA) as a representative OM was investigated. Chemical reduction and subsequent abiotic reoxidation of NAu-2 produced abundant hydroxyl radicals (thereafter termed as ·OH) that effectively transformed the chemical and molecular composition of HA. More importantly, transformed HA served as a more premium electron donor/carbon source to couple with subsequent biological reduction of Fe(III) in reoxidized NAu-2 by Geobacter sulfurreducens, a model Fe-reducing bacterium. Destruction of aromatic structures and formation of carboxylates were mechanisms responsible for transforming HA into an energetically more bioavailable substrate. Relative to unaltered HA, transformed HA increased the extent of the bioreduction by 105%, and Fe(III) reduction was coupled with oxidation and even mineralization of transformed HA, resulting in bleached HA and formation of microbial products and cell debris. ·OH transformation slightly decreased the electron shuttling capacity of HA in bioreduction. Our results provide a mechanistic explanation for rapid OM mineralization driven by Fe redox cycling in redox-fluctuating environments.

Multi-metal contaminant mobilizations by natural colloids and nanoparticles in paddy soils during reduction and reoxidation

Naturally-occurring colloids and nanoparticles are crucial in transporting heavy metal contaminants in soil-water systems. However, information on particle-bound metals’ size distribution and elemental composition in paddy soils under redox-fluctuation is scarce. Here, we investigated the mobilization of Cu, Cd, and Pb-containing nanoparticles and colloids in four contaminated soils with distinctive geochemical properties during reduction and subsequent re-oxidation. Using AF4-UV-ICP-MS and STEM-EDS, we observed that particle-bound metals were primarily associated with two sizes ranges: 0.3−40 kDa (F1) and 130 kDa−450 nm (F2), which mainly consisted of organic matter (OM), iron hydroxide and clay minerals. Cu and Pb were more likely bound to colloid than Cd. Colloidal Cu, Pb and Cd accounted for averages of 83.2%, 72.4% and 19.8% of their total concentration in solution (<0.45 µm) during soil reduction, and decreased during soil re-oxidation. This proportion was also positively correlated with aqueous pH and DOC but negatively correlated with Eh. Further quantitative analysis demonstrated that Cu/Cd positively correlated with OM at nanometric scale (F1). This study provides quantitative insights into the size, composition and abundance of polymetallic pollutant-carrying particles in paddy soils during redox fluctuation, and highlights the importance of nanometric interactions between OM and toxic cationic metals for their release.

Mobilization of Colloid- and Nanoparticle-Bound Arsenic in Contaminated Paddy Soils during Reduction and Reoxidation.

The association of arsenic (As) with colloidal particles could facilitate its transport to adjacent water systems or alter its availability in soil-rice systems. However, little is known about the size distribution and composition of particle-bound As in paddy soils, particularly under changing redox conditions. Here, we incubated four As-contaminated paddy soils with distinctive geochemical properties to study the mobilization of particle-bound As during soil reduction and subsequent reoxidation. Using transmission electron microscopy-energy dispersive spectroscopy and asymmetric flow field-flow fractionation, we identified organic matter (OM)-stabilized colloidal Fe, most likely in the form of (oxy)hydroxide-clay composite, as the main arsenic carriers. Specifically, colloidal As was mainly associated with two size fractions of 0.3-40 and >130 kDa. Soil reduction facilitated the release of As from both fractions, whereas reoxidation caused their rapid sedimentation, coinciding with solution Fe variations. Further quantitative analysis demonstrated that As concentrations positively correlated with both Fe and OM concentrations at nanometric scales (0.3-40 kDa) in all studied soils during reduction and reoxidation, yet the correlations are pH-dependent. This study provides a quantitative and size-resolved understanding of particle-bound As in paddy soils, highlighting the importance of nanometric Fe-OM-As interactions in paddy As geochemical cycling.

Nitriding model for zirconium based fuel cladding in severe accident codes

A model has been developed to describe the nitriding of partially oxidized zirconium based cladding during an air ingress sequence when the reaction has become starved of oxidant (oxygen and/or steam), and the subsequent re-oxidation of nitride following of restoration of coolant. Key aspects of the model are the estimation of oxygen-stabilised alpha zirconium, α-Zr(O), formed during pre-oxidation and its reaction with the nitrogen. Nitriding of metallic Zr is much slower than α-Zr(O), and plays a comparatively minor role. The model is based on data from separate-effects tests comprised pre-oxidation, nitriding in the absence of oxidant, and re-oxidation in the absence of nitrogen, which were used to derive the kinetic parameters for the main reaction processes. Developmental assessment was performed using the test results, demonstrating favourable agreement for the main reaction signatures. Independent assessment against Integral Test data is underway.

Thermal Aging of Rh/ZrO2–CeO2 Three-Way Catalysts under Dynamic Lean/Rich Perturbation Accelerates Deactivation via an Encapsulation Mechanism

A Rh/ZrO2–CeO2 (Rh/ZC) three-way catalyst was exposed to high-temperature exhaust gas mixtures fluctuating between stoichiometric (S), oxidizing (fuel-lean, L), and reducing (fuel-rich, R) compositions. The catalyst deactivation during thermal aging at 1000 °C for 40 h under a dynamic SLR cycle condition (S: 25 s, L: 2.5 s, and R: 2.5 s) was more severe than that under static conditions (S, L, or R). Chemisorption, transmission electron microscopy, and X-ray photoelectron spectroscopy showed that the total encapsulation of Rh particles with a ZC overlayer caused physical blockage and suppressed catalytic activity. This deactivation mode of the SLR-aged catalyst was characterized by the Rh particle size (ca. 17 nm) as small as that of the R-aged catalyst (ca. 15 nm in size), which preserved the highest activity. On the other hand, their CO chemisorption capacities differed by 50-fold. Almost complete encapsulation occurred under a dynamic SLR cycle condition but not under the reducing (R) and other static conditions (S and L). Furthermore, post-treatment in air at 1000 °C did not recover the catalyst from the encapsulation state. This result was in contrast to the well-known strong metal–support interaction-induced decoration or encapsulation effects of metal catalysts supported on CeO2-based oxides, which occur under a strongly reducing atmosphere at high temperatures but disappear after subsequent reoxidation. The encapsulation under a dynamic SLR cycle condition suggests that the migration of ZC components to overcoat and embed Rh particles is activated by repeated oxygen release and storage near the metal–support interface.

Tracking the Seconds of a Clock Reaction: A Multiparametric Experimental Study on the Catalytic Reduction of Methylene Blue

The catalytic reduction and subsequent reoxidation of methylene blue (MB) is a well-known reaction, commonly referred to as a “clock reaction”. Specifically, the reduction process is accompanied by a color change from intense blue to clear and can be readily monitored by UV–vis spectroscopy via the decrease in absorbance at λmax = 664 nm, providing facile access to valuable reaction kinetics. Unsurprisingly, the reduction of methylene blue has become a widely used probe for heterogeneous catalyst reactivity. Yet, despite its broad utility, the mechanism of reduction is not well-understood. Herein, we report an experimental study on the mechanism of MB reduction using Pt/C nanoparticles. Through a multiparametric study incorporating in situ probing of reaction kinetics, pH, dissolved oxygen, and variable catalyst and reductant loading, the multiple factors impacting the reaction are disentangled. We demonstrate that the reaction steps that limit the reduction of MB are the H2 evolution and reoxidation sequences. We limit these competitive reactions by optimizing reductant concentration and catalyst loading, N2 purging, and restriction of O2 redissolution. This achieves a highly competitive activity parameter of 25,740 min–1 gPt–1 L.

Investigating the Role of Silver Oxidation State on the Thermodynamic Interactions between Xenon and Silver-Functionalized Zeolites

The molecular level understanding of the strong adsorption of Xe to silver-modified zeolites remains elusive. Here, we probe the effect of silver oxidation state on the thermodynamics of Xe sorption in silver-functionalized zeolites by measuring the enthalpies of adsorption after various treatments using inverse gas chromatography (IGC). The enthalpy of adsorption was measured for silver-functionalized chabazites (AgCHA) before and after hydrogen reduction and subsequent reoxidation. The sorption enthalpy (ΔH) for AgCHA was 35.2 kJ/mol, which decreased to 25.8 kJ/mol with hydrogen reduction. After reoxidation (O2-AgCHA), 95% of the binding strength was restored. Hydrogen reduction of the base chabazite (CHA) did not influence Xe adsorption. Henry’s law constant for Xe adsorption increased in the order AgCHA > O2-AgCHA > H2-AgCHA > CHA. A decrease in enthalpy and Henry’s constant with silver reduction and increase with reoxidation suggest that ionic silver is playing a role in Xe binding. The effect of reduction and reoxidation on the zeolite microstructure was analyzed using surface area analysis, powder X-ray diffraction (p-XRD), scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS), and X-ray photoelectron spectroscopy (XPS). These results lay the groundwork for better material design of strong noble gas adsorbents.

Synthesis of Sterically Shielded Nitroxides Using the Reaction of Nitrones with Alkynylmagnesium Bromides.

Sterically shielded nitroxides, which demonstrate high resistance to bioreduction, are the spin labels of choice for structural studies inside living cells using pulsed EPR and functional MRI and EPRI in vivo. To prepare new sterically shielded nitroxides, a reaction of cyclic nitrones, including various 1-pyrroline-1-oxides, 2,5-dihydroimidazole-3-oxide and 4H-imidazole-3-oxide with alkynylmagnesium bromide wereused. The reaction gave corresponding nitroxides with an alkynyl group adjacent to the N-O moiety. The hydrogenation of resulting 2-ethynyl-substituted nitroxides with subsequent re-oxidation of the N-OH group produced the corresponding sterically shielded tetraalkylnitroxides of pyrrolidine, imidazolidine and 2,5-dihydroimidazole series. EPR studies revealed large additional couplings up to 4 G in the spectra of pyrrolidine and imidazolidine nitroxides with substituents in 3- and/or 4-positions of the ring.

Laser beam brazing of aluminum alloys in XHV-adequate atmosphere with surface deoxidation by ns-pulsed laser radiation

Laser beam brazing is an established manufacturing process due to its low heat input and esthetically appealing seams. However, brazing of materials with high oxygen affinity, such as aluminum alloys, requires the removal of surface oxides prior to the brazing process, commonly through the application of chemical fluxes that may be harmful to the environment and to health. The approach presented here dispenses with the use of fluxes and involves oxide layer removal by means of ns-pulsed laser radiation within an atmosphere that is adequate to an extreme high vacuum (XHV) in regard to the oxygen content. By doping the process gas with monosilane (SiH4), an oxygen content equivalent to an extreme high vacuum with an oxygen partial pressure below 10−20 mbar is realized. Hence, a subsequent reoxidation is actively prevented so that wetting of the base material by the filler material and consequent diffusion processes are enabled. The wetting angle between filler material and material is used to evaluate the effectiveness of laser-based deoxidation under an XHV-adequate atmosphere.

PEG-Supported Hypervalent Iodine Reagent for Sulfonamide Synthesis

AbstractA reusable and recoverable PEG-supported hypervalent iodine reagent is reported. This hypervalent iodine reagent, immobilized in a soluble polymer support, was easily prepared in six steps from PEG-OH 2000. This reagent was successfully applied in a mild sulfonamide synthesis using a sulfinate salt and an amine in up to 95% yield. The use of the soluble polymer allowed a facile workup procedure and reaction monitoring, by simple precipitation and filtration of the reagent, with an easy recovery and subsequent re-oxidation and reuse. This approach greatly improves the sustainability of this sulfonamide synthesis method, avoiding frequent preparation of reagents and generation of waste during the purification steps.

Benchmarking of oxygen evolution catalysts on porous nickel supports

Summary Active and inexpensive oxygen evolution reaction (OER) electrocatalysts are needed for energy-efficient electrolysis applications. Objective comparison between OER catalysts has been blurred by the use of different supports and methods to evaluate performance. Here, we selected nine highly active transition-metal-based catalysts and described their synthesis, using a porous nickel foam and a new Ni-based dendritic material as the supports. We designed a standardized protocol to characterize and compare the catalysts in terms of structure, activity, density of active sites, and stability. NiFeSe- and CoFeSe-derived oxides showed the highest activities on our dendritic support, with low overpotentials of η100 ≈ 247 mV at 100 mA cm–2 in 1 M KOH. Stability evaluation showed no surface leaching for 8 h of electrolysis. This work highlights the most active anode materials and provides an easy way to increase the geometric current density of a catalyst by tuning the porosity of its support.