Cyclic Redox Research Articles

Investigating the Origin of Enhanced C2+ Selectivity in Oxide-/Hydroxide-Derived Copper Electrodes during CO2 Electroreduction.

Oxide-/hydroxide-derived copper electrodes exhibit excellent selectivity toward C2+ products during the electrocatalytic CO2 reduction reaction (CO2RR). However, the origin of such enhanced selectivity remains controversial. Here, we prepared two Cu-based electrodes with mixed oxidation states, namely, HQ-Cu (containing Cu, Cu2O, CuO) and AN-Cu (containing Cu, Cu(OH)2). We extracted an ultrathin specimen from the electrodes using a focused ion beam to investigate the distribution and evolution of various Cu species by electron microscopy and electron energy loss spectroscopy. We found that at the steady stage of the CO2RR, the electrodes have all been reduced to Cu0, regardless of the initial states, suggesting that the high C2+ selectivities are not associated with specific oxidation states of Cu. We verified this conclusion by control experiments in which HQ-Cu and AN-Cu were pretreated to fully reduce oxides/hydroxides to Cu0, and the pretreated electrodes showed even higher C2+ selectivity compared with their unpretreated counterparts. We observed that the oxide/hydroxide crystals in HQ-Cu and AN-Cu were fragmented into nanosized irregular Cu grains under the applied negative potentials. Such a fragmentation process, which is the consequence of an oxidation-reduction cycle and does not occur in electropolished Cu, not only built an intricate network of grain boundaries but also exposed a variety of high-index facets. These two features greatly facilitated the C-C coupling, thus accounting for the enhanced C2+ selectivity. Our work demonstrates that the use of advanced characterization techniques enables investigating the structural and chemical states of electrodes in unprecedented detail to gain new insights into a widely studied system.

Sulfur Transformation Behavior of Inorganic Sulfur-Containing Compounds in Chemical Looping Combustion

Chemical looping combustion (CLC) is a promising technology for fuel conversion with inherent carbon capture. Because sulfur is an integral part of coal, the existence of sulfur contaminants in coal may degrade the purity of CO₂ and deteriorate the performance of the oxygen carrier in the CLC process. To uncouple the complex behavior of inorganic sulfur in coal, sulfur release of three inorganic sulfur-containing compounds (FeS₂, FeSO₄, and CaSO₄) mixed with activated carbon during the temperature-programmed process and the CLC process using iron ore as an oxygen carrier were investigated in a batch fluidized bed reactor at 950 °C. The results showed that, in the temperature-programmed process, the initial decomposition temperatures of FeS₂, FeSO₄, and CaSO₄ were about 400, 500, and 650 °C, respectively. During the reduction and oxidation stages of CLC, SO₂ was the main sulfur-containing gaseous compound. The sulfur stability of CaSO₄ was higher than those of FeS₂ and FeSO₄. The fractions of sulfur converted to gaseous species in the reduction stage (XS,g,ᵣₑd) of FeS₂ and FeSO₄ reached 90.38 and 87.42%, respectively, within 20.5 min, while the corresponding value was 53.97% for CaSO₄. During 5 cyclic redox experiments at 950 °C, the fraction of sulfur converted to gas species was almost maintained stable, while the energy-dispersive X-ray spectroscopy analysis showed the accumulation of sulfur after the cyclic reaction.

CO Methanation on Mesoporous Ni–VOx/FDU‐12 Catalyst: Effects of the VOx Promoter on Low‐Temperature Activity

Herein, a series of Ni–VOx/FDU‐12 catalysts are prepared by an incipient impregnation method with FDU‐12 (cubic, Fm‐3m) as the support. The samples are characterized by nitrogen adsorption, X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, H2‐temperature programmed reduction, and X‐ray photoelectron spectroscopy. Among all catalysts, Ni–VOx/FDU‐12 with NiO of 10 wt% and V2O5 of 2 wt% (10N2VF) show the best catalytic performance, which reaches maximum CO conversion and CH4 yield of 99.4% and 89%, respectively, at 325 °C. The lifetime test is conducted at a constant temperature of 450 °C for 100 h at 0.1 MPa and a high weight hourly space velocity (WHSV) of 60 000 mL g−1 h−1. The 10N2VF exhibit high antisintering and coking properties. The addition of the vanadium promoter improve Ni dispersion and change Ni electron density, which enhance the low‐temperature ability for CO dissociation via the weakening of CO linkage. In contrast, a large number of oxygen vacancies formed by the oxidation–reduction cycle of V5+/V4+/V3+ can enhance the dissociation of the by‐product CO2 and generate oxygen intermediates, which prevent carbon deposition on the surface of Ni particles.

MoO3/Al2O3 catalysts for chemical-looping oxidative dehydrogenation of ethane.

MoO3/γ-Al2O3 catalysts containing 0.3-3 monolayer (ML) equivalents of MoO3 were prepared, characterized, and tested for ethane oxidative dehydrogenation (ODH) in cyclic redox and co-feed modes. Submonolayer catalysts contain highly dispersed (2D) polymolybdate structures; a complete monolayer and bulk Al2(MoO4)3 are present at >1ML loadings. High ethylene selectivity (>90%) in chemical looping (CL) ODH correlates with Mo+VI to Mo+V reduction; COx selectivity is <10% under these conditions. Mo+V and Mo+IV species trigger CH4 production resulting in much higher conversion albeit with <20% selectivity. In CL-ODH, submonolayer catalysts exhibit ethylene selectivities that decrease linearly from 96% at near-zero conversion to 70% at 45% conversion. >1ML catalysts provide higher conversions albeit with 10%-18% lower selectivity and greater selectivity loss with increasing conversion. In co-feed mode, ethylene selectivity drops to <50% at 46% conversion for a 0.6ML catalyst, but selectivity is virtually unaltered for a 3ML catalyst. We infer that at <1ML loadings, small domain size and strong Mo-O-Al bonds decrease 2D polymolybdate reducibility and enhance ethylene selectivity in CL-ODH.

Enhancement of highly-concentrated hydrogen productivity in chemical looping steam methane reforming using Fe-substituted LaCoO3

This work addresses chemical looping steam methane reforming (CL-SMR) for producing highly-concentrated hydrogen and syngas without any separation unit. The effect of transition metal substitution on the CL-SMR were investigated with LaCoO3 and B-site substituted LaCo0.6B0.4O3 (B = Fe, Mn, Ni). The cyclic CH4-H2O redox reaction revealed that Fe and Mn-substituted carriers maintained high steam regeneration, while LaCoO3 was not fully oxidized by steam. In particular, the Fe-substituted carrier provided the highest extent of steam regeneration, hydrogen purity and enhanced selective oxidation of CH4 to syngas, which was reflected in the 92% CO selectivity and 99.3% hydrogen purity with a produced amount of 2.22 mmol H2/gcat at a relatively low temperature of 700 °C. H2-TPR showed that the CO selectivity increased due to the lowering of the lattice oxygen transfer to the surface vacancies by the Fe substitution. In-situ XPS and O2-TPD measurements revealed that the substitution enhances the adsorption and the decomposition of hydroxide ion on the surface and the oxygen mobility from the surface to the lattice, which facilitates steam splitting and the regeneration of lattice oxygen. LaCo0.6Fe0.4O3 was selected as the optimal oxygen carrier for the low-temperature CL-SMR process, which demonstrated the highest syngas selectivity and hydrogen purity with the enhanced cyclic stability.

The activation and conversion of carbon dioxide on the surface of zirconia-promoted ceria oxides

This study reports the conversion of carbon dioxide (CO2) by ceria (CeO2)-zirconia (ZrO2) composites with various ZrO2 contents (0, 20, 50, 80 and 100 %). The modification introduced by ZrO2 altered the structural properties of the ceria samples, affecting their reducibility and subsequent activity to convert CO2. The characterisation results showed a clear relationship between the structural properties of the materials and the activity for CO2 conversion. The addition of ZrO2 was effective to improve a close interaction between CeO2 and ZrO2 and induced the formation of structural defects that further promoted oxygen transport and facilitated the creation of oxygen vacancies that were critical for CO2 conversion. All samples investigated were found to be effective in converting CO2 into carbon monoxide (CO) at 450 °C, and the addition of ZrO2 was also found to be effective in improving the thermal resistance and stabilising the crystalline structure of the samples especially when the ZrO2 content was > 50 %. When the content of ZrO2 controlled between 50 % and 80 %, the onset temperature for CO2 conversion was found to be 285 °C, which was about 80 °C lower than that required for the sample without ZrO2 modification. Thus, modifying CeO2 by ZrO2 was effective to prevent both the textural and structural alteration of the samples under a high temperature environment as well as cyclic redox operations, resulting in a constant and high degree of CO2 conversion into CO.

Copper and cobalt co-doped ferrites as effective agents for chemical looping CO2 splitting

The sintering of oxygen carrier materials can be circumvented by operating chemical looping at relatively low temperatures (<750 °C). However, low temperature has a degrading effect on the redox kinetics of the materials and thereby limited efficiencies. In the current work, we report several Co, Cu co-doped spinel-structured ferrites as effective redox materials in the chemical looping scheme for CO2 thermo splitting at 650 °C. Cu0.2Co0.8Fe2O4 presents high average CO2 splitting rate of ~144.9 μmol.g−1.min−1 and CO yield of ~9.0 mmol. g−1 with satisfactory stability over cyclic redox operations. The improved performance can be attributed to the improved reduction of iron oxides by Cu, Co co-dopant, and the reversible phase change of the reduced sample in the CO2 splitting stage. Additionally, increasing Cu doping amount leads to the formation of insufficiently active impurities which is unfavorable for the CO2 thermo-splitting. The synergy in this work can be helpful for the design of the redox materials at moderate temperatures.

Redox Cycling Driven Transformation of Layered Manganese Oxides to Tunnel Structures.

Mn oxides are among the most ubiquitous minerals on Earth and play critical roles in numerous elemental cycles in biotic/abiotic loops as the key redox center. Yet, it has long puzzled geochemists why the laboratory synthesis of todorokite, a tunnel-structured Mn oxide, is extremely difficult while it is the dominant form over other tunneled phases in low-temperature natural environments. This study employs a novel electrochemical method to mimic the cyclic redox reactions occurring over long geological time scales in an accelerated manner. The results revealed that the kinetics and electron flux of the cyclic redox reaction are key to the layer-to-tunnel structure transformation of Mn oxides, provided new insights for natural biotic and abiotic redox reactions, and explained the dominance of todorokite in nature.

The Role of ERO1α in Modulating Cancer Progression and Immune Escape.

Endoplasmic reticulum oxidoreductin-1 alpha (ERO1α) was originally shown to be an endoplasmic reticulum (ER) resident protein undergoing oxidative cycles in concert with protein disulfide isomerase (PDI) to promote proper protein folding and to maintain homeostasis within the ER. ERO1α belongs to the flavoprotein family containing a flavin adenine dinucleotide utilized in transferring of electrons during oxidation-reduction cycles. This family is used to maintain redox potentials and protein homeostasis within the ER. ERO1α’s location and function has since been shown to exist beyond the ER. Originally thought to exist solely in the ER, it has since been found to exist in the golgi apparatus, as well as in exosomes purified from patient samples. Besides aiding in protein folding of transmembrane and secretory proteins in conjunction with PDI, ERO1α is also known for formation of de novo disulfide bridges. Public databases, such as the Cancer Genome Atlas (TCGA) and The Protein Atlas, reveal ERO1α as a poor prognostic marker in multiple disease settings. Recent evidence indicates that ERO1α expression in tumor cells is a critical determinant of metastasis. However, the impact of increased ERO1α expression in tumor cells extends into the tumor microenvironment. Secretory proteins requiring ERO1α expression for proper folding have been implicated as being involved in immune escape through promotion of upregulation of programmed death ligand-1 (PD-L1) and stimulation of polymorphonuclear myeloid derived suppressor cells (PMN-MDSC’s) via secretion of granulocytic colony stimulating factor (G-CSF). Hereby, ERO1α plays a pivotal role in cancer progression and potentially immune escape; making ERO1α an emerging attractive putative target for the treatment of cancer.

Dry reforming of methane catalysed by molten metal alloys

Dry reforming of methane usually affords low-quality syngas with equimolar amounts of CO and H. Here we report the high conversion of CH and CO to syngas and solid carbon through simultaneous pyrolysis and dry reforming of methane in a bubble column reactor using a molten metal alloy catalyst (65:35 mol% Ni:In). The H to CO ratio can be increased above 1:1 using feed ratios of CH:CO greater than 1:1 to produce stoichiometric solid carbon as a co-product that is separable from the molten metal. A coupled reduction–oxidation cycle is carried out in which CO is reduced by a liquid metal species (for example, In) and methane is partially oxidized to syngas by the metal oxide intermediate (for example, InO), regenerating the native metal. Moreover, the H:CO product ratio can be easily controlled by adjusting the CH:CO feed ratio, temperature, and residence time in the reactor.

A reconstructed porous copper surface promotes selectivity and efficiency toward C2 products by electrocatalytic CO2 reduction.

Electrocatalytic synthesis of multicarbon (C2+) products from CO2 reduction suffers from poor selectivity and low energy efficiency. Herein, a facile oxidation–reduction cycling method is adopted to reconstruct the Cu electrode surface with the help of halide anions. The surface composed of entangled Cu nanowires with hierarchical pores is synthesized in the presence of I−, exhibiting a C2 faradaic efficiency (FE) of 80% at −1.09 V vs. RHE. A partial current density of 21 mA cm−2 is achieved with a C2 half-cell power conversion efficiency (PCE) of 39% on this electrode. Such high selective C2 production is found to mainly originate from CO intermediate enrichment inside hierarchical pores rather than the surface lattice effect of the Cu electrode.

Cyclic redox scheme towards shale gas reforming: a review and perspectives

Alkanes are potential precursors to many value-added chemicals such as olefins and other petrochemicals.

Electrochemical SERS and SOERS in a single experiment: A new methodology for quantitative analysis

In the present work, a new methodology which combines two different phenomena to enhance the Raman signal is used to resolve a mixture of two compounds with similar molecular structures. The use of Raman spectroelectrochemistry (Raman-SEC) allows us to collect simultaneously, with high time-resolution, the enhancement of the Raman signal of the compounds present in a sample during the electrochemical oxidation-reduction cycle (ORC) of a silver screen-printed electrode. During such ORC two different phenomena appears depending on the stage of the electrochemical modification of the silver substrate, which are known as electrochemical surface enhanced Raman scattering (EC-SERS) and electrochemical surface oxidation enhanced Raman scattering (EC-SOERS). This work is a proof of concept that demonstrates the advantage of using EC-SOERS and EC-SERS in a single experiment to resolve mixtures of similar molecules such as vitamin B3, which components are nicotinic acid and nicotinamide. Although the interaction between analytes and substrates influence a univariate calibration, the trilinear character of Raman-SEC makes possible to deconvolve such interactions and provide a good calibration curve for both, nicotinic acid and nicotinamide.

Improved Catalytic Performance of Ethane Dehydrogenation in the Presence of CO2 over Zr-Promoted Cr/SiO2.

The Zr-, Ce-, Sr-, and Sn-promoted Cr/SiO2 catalysts were prepared by the incipient wetness impregnation method, then characterized by N2 adsorption/desorption, X-ray diffraction, X-ray photoelectron spectroscopy, H2-TPR, CO2-TPD, UV–vis, high-angle annular dark-field imaging–scanning transmission electron microscopy—energy dispersive X-ray spectroscopy elemental mapping, Raman, and thermogravimetric techniques to study the structural evolution under the preparation/reaction conditions, and applied to catalyze ethane oxidative dehydrogenation with CO2. The results suggested that the Cr6+ species were indispensable for activating the dehydrogenation reaction thanks to the oxidation–reduction cycle between Cr6+ and Cr3+. The type of the promotor also affected significantly the ability of replenishing the lattice oxygen through the disassociation of CO2, leading to the different catalytic performances. The Zr-promoted sample had the best performance in converting the reactants as well as the catalytic stability.

In Situ AFM Imaging of Platinum Electrode Surface during Oxidation–Reduction Cycles in Alkaline Electrolyte

The development of energy conversion systems depends strongly on our fundamental understanding of the electrochemical interface of the electrocatalyst. Here, we study the changes in the surface morphology of a platinum polycrystalline electrode during oxidation-reduction cycles in a wide potential window (0.05-2.0 V) in sodium hydroxide by in situ atomic force microscopy. Platinum nanoparticles are observed on the surface after cycling due to the redeposition of dissolved platinum ions. The influence of scan rate on platinum redeposition is studied by applying asymmetric potential sweep programs. The negative-going scan rate is the key factor here, as it controls the reduction of platinum oxide, as well as the redeposition of platinum. Moreover, it is found that chloride ions in the electrolyte impede the redeposition by complexing with platinum. This investigation enables us to reveal the surface roughening processes on platinum electrode in alkaline electrolyte and assists in understanding the fundamentals of the stability of platinum-containing energy conversion systems.

Near 100% CO selectivity in nanoscaled iron-based oxygen carriers for chemical looping methane partial oxidation

Chemical looping methane partial oxidation provides an energy and cost effective route for methane utilization. However, there is considerable CO2 co-production in current chemical looping systems, rendering a decreased productivity in value-added fuels or chemicals. In this work, we demonstrate that the co-production of CO2 can be dramatically suppressed in methane partial oxidation reactions using iron oxide nanoparticles embedded in mesoporous silica matrix. We experimentally obtain near 100% CO selectivity in a cyclic redox system at 750–935 °C, which is a significantly lower temperature range than in conventional oxygen carrier systems. Density functional theory calculations elucidate the origins for such selectivity and show that low-coordinated lattice oxygen atoms on the surface of nanoparticles significantly promote Fe–O bond cleavage and CO formation. We envision that embedded nanostructured oxygen carriers have the potential to serve as a general materials platform for redox reactions with nanomaterials at high temperatures.

Orbitally driven redox fluctuations during Cretaceous Oceanic Anoxic Event 2 (OAE2) revealed by a new magnetic proxy

Recent studies of the Cretaceous sedimentary records have documented that rapid redox changes between euxinic (sulfidic) and ferruginous conditions occurred during the perturbations to the global carbon cycle associated with oceanic anoxic events. This observation has major implications for the dynamics of and feedbacks on marine primary productivity and organic carbon burial during these events. The high-resolution rock magnetic study of a significantly expanded OAE2 interval in southern Tibet, presented here, provides further insight into the redox dynamics during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) (~94 Ma). The ratio of hard isothermal remanent magnetization to magnetic susceptibility, HIRM/χ, is used as a proxy for bottom-water redox conditions and shows repeated fluctuations between relatively oxic and dysoxic/anoxic conditions in the eastern Tethys of the Southern Hemisphere during OAE2. Spectral analysis of a log (HIRM/χ) time series reveals cyclical variations at orbital frequencies and the subtle cyclic redox changes are found to occur within the obliquity frequency band. Moreover, variability in redox conditions at the onset of OAE2 correlates well with that recently documented for the OAE2 interval at Tarfaya, Morocco, implying that the cyclic redox variations spanned both hemispheres. Notably, the presence of obliquity signals, which dominantly influence high-latitude climate processes, in the tropics and subtropics of both the Northern and Southern Hemispheres further confirms previously hypothesized more vigorous meridional overturning circulation (MOC) during OAE2 and implies the interhemispheric/global nature of this major shift in oceanic circulation.

Atomic-Scale Identification of the Electrochemical Roughening of Platinum.

Electrode degradation under oxidizing conditions is a major drawback for large-scale applications of platinum electrocatalysts. Subjecting Pt(111) to oxidation–reduction cycles is known to lead to the growth of nanoislands. We study this phenomenon using a combination of simultaneous in situ electrochemical scanning tunneling microscopy and cyclic voltammetry. Here, we present a detailed analysis of the formed islands, deriving the (evolution of the) average island growth shape. From the island shapes, we determine the densities of atomic-scale defect sites, e.g., steps and facets, which show an excellent correlation with the different voltammetric hydrogen adsorption peaks. Based on this combination of electrochemical scanning tunneling microscopy (EC-STM) and CV data, we derive a detailed atomistic picture of the nanoisland evolution during potential cycling, delivering new insights into the initial stages of platinum electrode degradation.

Excellent low-temperature NH3-SCR NO removal performance and enhanced H2O resistance by Ce addition over the Cu0.02Fe0.2CeyTi1-yOx (y= 0.1, 0.2, 0.3) catalysts.

In the present work, a serial of Cu0.02Fe0.2CeyTi1-yOx catalysts are prepared by sol-gel method and applied for NH3-SCR of NO, meanwhile Cu0.02Fe0.2Ce0.2Ti0.8Ox shows good low-temperature NH3-SCR performance with/without water and an outstanding water resistance. The bulk structure, redox ability, surface acidity and surface species of Cu0.02Fe0.2CeyTi1-yOx are measured and discussed by series of characterization in details to illuminate the reasons for the good low-temperature activity and water resistance. The Ce modification can tune the surface acidic distribution, improve the surface oxygen content and surface oxidation reduction cycle (Ce4++Fe2+ ↔ Ce3++Fe3+), which contribute the good activity. In addition, the effect of water on NH3-SCR performance over Cu0.02Fe0.2TiOx and Cu0.02Fe0·2Ce0·2Ti0.8Ox are investigated emphatically by in situ DRIFTS.

Solid State Synthesis and Reversible Oxygen Capacity of Li/Mg Overstoichiometric Solid Solutions Based on the Spinel MgMnO3 – δ

Series of spinel solid solutions with a specified cation composition, Mg1 +xMnO3 – δ and, for the first time, MgLixMnO3 – δ, potential materials of catalytic systems for oxidation reactions such as oxidative conversion of methane and chemical looping combustion process were prepared by solid-state syntheses from mechanocomposites obtained by milling of ultra-high purity precursors (MgO, Mn2O3, MnO2, and Li2CO3). Their homogeneity region was found to be up to 1573 K at p(O2) = 0.21 atm. For both series of solid solutions, the synthesis from Mn2O3 and MnO2 in air was investigated by the TG-DSC method and the oxygen content in the resulting materials was determined (TG-DSC, titration). The products of extensive reduction of spinels Mg1 +xMnO3 – δ and MgLixMnO3–δ with an Ar + 4.8% H2 mixture were investigated. Parameters of conversion of the reduction products to the initial single-phase spinels on treatment with air were determined. The obtained materials showed good reproducibility of reversible oxygen capacity over 20 reduction–oxidation cycles.