Intermediate Species Research Articles

Impact of Intermediate Species in Simulating Oblique Detonation with Pre-Vaporized N-Decane/air Mixtures Substituting for Kerosene/Air Mixtures

ABSTRACT A renewed two-step n-decane mechanism is presented to improve reliable and robust simulation results, substituting for kerosene. This mechanism demonstrates that effective control of ignition delay time and heat release, based on the Chapman – Jouguet condition, enables successful simulation of oblique detonation waves. Chain reactions influence the overall reaction heat and explosion temperature, especially in simplified mechanisms with few steps. Therefore, carbon monoxide (CO) generation is crucial for balancing the overall reaction heat and reducing species sensitivity to pressure, a factor previously not discussed in simplified models. The computational time for solving detailed chemical kinetics increases exponentially with species count, driving the pursuit for further reduction in kinetic mechanism size. The sensitive interaction between density and temperature during the fuel/air mixture explosion process affects initiation zone formation and cellular structure. Analyzing the distribution of CO can provide insights into structural details. The study considers applying the Rankine – Hugoniot curve to confirm detonation speeds, temperatures, and kinetic energy changes induced by chemical reactions.

Cyanogel-Induced Facile Synthesis of Palladium Hydride for Electrocatalytic Oxygen Reduction.

Palladium hydride (PdHx) is one of the well-known electrocatalytic materials, yet its synthesis is still a challenge through an energy-efficient and straightforward method. Herein, we propose a new and facile cyanogel-assisted synthesis strategy for the preparation of PdH0.649 at a mild environment with NaBH4 as the hydrogen source. Unlike traditional inorganic Pd precursors, the unique Pd-CN-Pd bridge in Pdx[Pd(CN)4]y ⋅ aH2O cyanogel offers more favourable spatial sites for insertion of H atoms. The characteristic three-dimensional backbone of cyanogel also acts as a support scaffold resulting in the interconnected network structure of PdH0.649. Due to the incorporation of H atoms and interconnected network structure, the PdH0.649 achieves a high half-wave potential of 0.932 V, a high onset potential of 1.062 V, and a low activation energy, as well as a long-term lifetime for oxygen reduction reaction. Theoretical calculation demonstrates a downshift of the d-band centre of Pd in PdH0.649 owing to the dominant Pd-H incorporation that weakens the binding energies of the *OH intermediate species. Zn-air batteries (ZAB) based on PdH0.649 exhibits high power density, competitive open circuit voltage, and good stability, exceeding that of commercial Pt black. This work not only opens up a new avenue for the development of high-efficiency Pt-free catalysts but also provides an original approach and insight into the synthesis of PdHx.

Design for all-round PHE/HER/OER sandwich nanostructure catalyst: Ultrathin nanosheet ZnIn2S4-encapsulated rich-Mo edge active site Mo2C-N Schottky junction

The development of efficient multifunctional catalyst remains a huge challenge due to their unsatisfactory oxygen/hydrogen intermediate species adsorption, ineffective kinetics rate, lack of active reaction sites. Herein, rich-Mo edge active sites in the molybdenum carbide (Mo2C) were created through nitrogen doping, which was used as cocatalyst to enhance the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and photocatalytic hydrogen evolution (PHE) of ZnIn2S4 (ZIS). Mo2C-N/ZIS affords a remarkable HER activity with a low overpotential of 74 mV as well as excellent OER activity with a reduced overpotential of 250 mV at 10 mA·cm−2. The formed Schottky junction between Mo2C-N/ZIS are responsible for the enhanced PHE/HER/OER activity, which owns double transfer route beneficial to enhancing separation of photogenerated carriers and reducing bulk/surface recombination of ZIS. Specially, the density functional theory (DFT) calculations demonstrate that Mo2C-N/ZIS Schottky junction with optimal absorption energy for the hydrogen and oxygen intermediates.

PdSnW Ternary Alloy Nanoparticle with Excellent CO Anti‐Poisoning Activity for Boosting Alkaline Ethanol Oxidation Reaction

AbstractEfficient ethanol oxidation reaction (EOR) catalysts with anti‐poisoning ability and high selectivity are in high demand for the widespread application of direct ethanol fuel cells (DEFCs). Here, the PdSnW ternary alloy nanoparticles are synthesized by a facile solvothermal method and exhibit an enhanced EOR catalytic performance. CO stripping experiment, X‐ray photoelectron spectroscopy and in situ Fourier transform infrared are employed to study the correlation between catalyst structure and EOR catalytic performance. The oxyphilic Sn and W not only provide dangling O−H bond, accelerating the subsequent oxidation of adsorbed intermediate CO species, but also moderate the electron state of Pd, enhancing the adsorption of CH3CH2OH, thus resulting in the promotion of C−C bond cleavage and an increased C1 selectivity. As a consequence, PdSnW alloy nanoparticle catalyst exhibit a better stability and a higher mass activity than those of Pd/C, PdSn and PdW, respectively. This work not only offers novel insights to strategically design highly efficient electrocatalysts for ethanol oxidation, but also further clarifies the inherent structure‐activity relationship.

Nursing Home to Emergency Care Transition Form Has Limited Uptake But Improves Documentation

Critical information gaps exist in nursing home–to–emergency department (NH-ED) transfer documentation. Standardization of forms may address these gaps. In a single state, a Continuity of Care Acute Care Transfer (CoC) Form was standardized and mandated to be used for all NH-ED transfers. The objective of this study was to evaluate adoption and effectiveness of the standardized CoC form. We used a random cross-sectional sample of 2019–2022 electronic health record encounter data to determine NH-ED documentation completeness after standardized CoC form implementation. Using patient characteristic adjusted linear and logistic regressions, we examined if CoC form standardization was associated with the number of key elements present on NH-ED transfer documentation and hospital admission, respectively. We then compared documentation completeness (out of 15 key data elements) to previously published pre-implementation data (2015–2016, n = 474). Of the 203 NH-ED transfer visits after CoC standardization (2019–2022), mean patient age was 81.8 years and 41.4% had dementia. Any NH-ED transfer form was present for 80.8% (n = 164) of encounters and 28.6% (n = 58) used the standardized CoC form. In comparison with the 2015–2016 data, there was an increase in documentation for functional baseline (20% to 30%), cognitive baseline (25% to 37%), and reason for transfer (25% to 82%). Post implementation, the use of the standardized CoC form was (1) associated with 2.55 (95% CI, 1.66–3.44) more key data elements documented and (2) not associated with a decreased odds of admission [odds ratio (OR), 1.06; 95% CI, 0.54–2.05] after controlling for confounders. Implementation of a statewide standardized CoC form for NH-ED transfers improved documentation of key elements, yet significant information gaps remain. Implementation evaluation is needed to identify how to achieve greater uptake of the form and improve the quality of information exchange between NHs and EDs.

The dominant influencing factors of desertification and ecological risk changes in Qinghai Area of Qilian Mountains National Park: Climate change or human activity?

Transitional features of desert environments partially determine the risks associated with ecosystems. Influenced by climate change and human activities, the variability and uncertainty of desertification levels and ecological risks in the Qinghai Area of Qilian Mountain National Park (QMNPQA) has become increasingly prominent. As a critical ecological barrier in northwest China, monitoring desertification dynamics and ecological risks is crucial for maintaining ecosystem stability. This study identifies the optimal monitoring model from four constructed desertification monitoring models and analyzes spatiotemporal changes in desertification. The spatial and temporal changes in ecological risks and their primary driving factors were analyzed using methods such as raster overlay calculation, geographic detector, cloud model, and trend analysis. The main conclusions are as follows: The desertification feature spatial model based on GNDVI-Albedo demonstrates better applicability in the study area, with an inversion accuracy of 81.24%. The levels of desertification and ecological risks in QMNPQA exhibit significant spatial heterogeneity, with a gradual decrease observed from northwest to southeast. From 2000 to 2020, there is an overall decreasing trend in desertification levels and ecological risks, with the decreasing trend area accounting for 89.82% and 85.71% respectively, mainly concentrated in the southeastern and northwestern parts of the study area. The proportion of areas with increasing trends is 4.49% and 7.05% respectively, scattered in patches in the central and southern edge areas. Surface temperature (ST), Digital Elevation Map (DEM), and Green normalized difference vegetation index (GNDVI) are the most influential factors determining the spatial distribution of ecological risks in QMNPQA. The effects of management and climatic factors on ecological risks demonstrate a significant antagonistic effect, highlighting the positive contributions of human activities in mitigating the driving effects of climate change on ecological risks. The research results can provide reference for desertification prevention and ecological quality improvement in QMNPQA.

Synthetic chemistry recreates transitional forms in prebiotic membrane evolution

Synthetic chemistry recreates transitional forms in prebiotic membrane evolution

Plasma-assisted NH3/air flame: Simultaneous LIF measurements of O and OH

In the emerging field of plasma-assisted ammonia (NH3) combustion, the evolution of key intermediate species has rarely been reported. This work establishes a simultaneous measurement system of laser-induced fluorescence (LIF) for hydroxyl (OH) and quantitative two-photon-absorption LIF for atomic oxygen (O), to explore the OH and O dynamics in an NH3/air flame affected by a nanosecond (ns) pulsed plasma discharge. Firstly, with the plasma on, the molar fraction of O is quantified to reach 8.7 × 10–3 in the burnt zone, about two orders of magnitude higher than that without plasma. In addition, the OH LIF signal intensity is four times higher, indicating a significant kinetic enhancement. Then, the spatial characteristics of OH and O are discussed and compared, showing remarkable discrepancy. The discrepancy between them indicates that O production is dominated by plasma kinetics, however, the OH production, primarily stemming from reactions between O and NH3/H2O, still depends on parameters associated with combustion kinetics. We further study the temporal dynamics of O and OH. It is concluded that O and OH peaks at 1.75 μs are mainly attributed to the pathway of quenching of the excited species. After that, O and OH start to decay but show significant differences between unburnt and burnt zones, which are characterized by a single-exponential decay and a bi-exponential decay, respectively. In the unburnt zone, the OH decay is much slower than the O decay due to the diverse pathways for OH production. In the burnt zone, the bi-exponential decay of O and OH can essentially be regarded as a process in which the NH3/air reactive system reaches chemical equilibrium. At this stage, the impacts of the excited species from the plasma gradually diminish and combustion kinetics dominates alone.

In-Depth Analysis of the Species and Transformations during Sol Gel-Assisted V2PC Synthesis.

The sol-gel reaction mechanism of 211 MAX phases has proven to be very complex when identifying the intermediate species, chemical processes, and conversions that occur from a mixture of metal salts and gelling agent into a crystalline ternary carbide. With mostly qualitative results in the literature (Cr2GaC, Cr2GeC, and V2GeC), additional analytical techniques, including thermal analysis, powder diffraction, total scattering, and various spectroscopic methods, are necessary to unravel the identity of the chemical compounds and transformations during the reaction. Here, we demonstrate the combination of these techniques to understand the details of the sol-gel synthesis of MAX phase V2PC. The metal phosphate complexes, as well as amorphous/nanocrystalline vanadium phosphate species (V in different oxidation states), are identified at all stages of the reaction and a full schematic of the reaction process is suggested. The early amorphous vanadium species undergo multiple changes of oxidation states while organic species decompose releasing a variety of small molecule gases. Amorphous oxides, analogous to [NH4][VO2][HPO4], V2PO4O, and VO2P2O7 are identified in the dried gel obtained during the early stages of the heating process (300 and 600 °C), respectively. They are carbothermally reduced starting at 900 °C and subsequently react to crystalline V2PC with the excess carbon in the reaction mixture. Through CHN analysis, we obtain an estimate of left-over amorphous carbon in the product which will guide future efforts of minimizing the amount of carbon in sol gel-produced MAX phases which is important for subsequent property studies.

Rare‐Earth‐Metal Alkyl and Aryl Compounds in Organic Synthesis

AbstractRare‐earth‐metal (Ln) alkyl and aryl compounds feature highly reactive, thermally labile [Ln−C] σ‐bonds which result in rapid and violent decomposition in the presence of air and moisture. Nevertheless, such [Ln−C] moieties display important intermediate species in numerous organic transformations. Reagents containing [Ln−C] bonds are deliberately generated in situ like Imamoto‐type reagents Ln(III)Cl3/RLi (routinely at low temperatures) or “heavy” lanthanide‐Grignard compounds RLnX. Samarium(III)‐alkyl species are supposed intermediates of SmI2‐promoted Barbier‐type carbon–carbon coupling reactions. Alkyl/aryl ligand exchange at Ln(III) centers has been identified as the crucial step of lanthanide–halogen‐exchange reactions. In the meantime, several such mixed alkyl/halogenido complexes, devoid of an ancillary ligand, could be isolated and scrutinized with regard to structure and reactivity. More recently, Ln(III)‐alkylidene complexes could be accessed, structurally authenticated and successfully employed in Tebbe‐like olefination reactions of ketones.

Hadronic light-by-light scattering contributions to (g−2)μ from axial-vector and tensor mesons in the holographic soft-wall model

We compute the light axial-vector and tensor meson two-photon transition form factors in the soft-wall holographic model of quantum chromodynamics (QCD) in the flavor-symmetric case. They are used to evaluate the axial-vector and tensor meson contributions to the anomalous magnetic moment of the muon via the hadronic light-by-light scattering process. As expected, these contributions are smaller than the one from pseudoscalar mesons. The result for axial-vector mesons is higher than the value found in other approaches. Published by the American Physical Society 2024

Mullite-like SmMn2O5-Derived Composite Oxide-Supported Ni-Based Catalysts for Hydrogen Production by Auto-Thermal Reforming of Acetic Acid.

The x%Ni/Sm2O3-MnO (x = 0, 10, 15, 20) catalysts derived from SmMn2O5 mullite were prepared by solution combustion and impregnation method; auto-thermal reforming (ATR) of acetic acid (HAc) for hydrogen production was used to explore the metal-support effect induced by Ni loadings on the catalytic reforming activity and product distribution. The 15%Ni/Sm2O3-MnO catalyst exhibited optimal catalytic performance, which can be due to the appropriate Ni loading inducing a strong metal-support interaction to form a stable Ni/Sm2O3-MnO active center, while side reactions, such as methanation and ketonization, were well suppressed. According to characterizations, Sm2O3-MnO mixed oxides derived from SmMn2O5 mullite were formed with oxygen vacancies; nevertheless, loading of Ni metal further promoted the formation of oxygen vacancies, thus enhancing adsorption and activation of oxygen-containing intermediate species and resulting in higher reactivity with HAc conversion near 100% and hydrogen yield at 2.62 mol-H2/mol-HAc.

The asymptotic solution of near-limit chain-branching premixed flames with the Zel’dovich–Liñán two-step mechanism in the linear and fast recombination regime

Near-limit characteristics of chain-branching premixed flames is investigated numerically and asymptotically by employing the Zel’dovich–Liñán two-step mechanism with linear and (sufficiently) fast recombination, for which both chain-branching and -recombination steps take place in a thin reactive layer of O(β−1), with β being the chain-branching Zel’dovich number. The numerical and asymptotic results reveal that the rate-ratio eigenvalue r between the chain branching and recombination steps approaches a minimum critical value of rmin→e as the chain-recombination step becomes infinitely fast. The existence of the finite asymptotic value of r=e indicates that no steadily propagating flame solution can be found for fuel concentrations lower than its critical value, corresponding to the criticality of r=e, thereby explaining the (lean) flammability limit. This intrinsic flammability characteristics with linear recombination is the most outstanding contrast to the quadratic recombination counterpart, for which flammability is achieved only with external heat loss added to the model.Novelty and significance statementNear-limit combustion characteristics of chain-branching flames is investigated numerically and asymptotically by employing the Zel’dovich–Linan two-step chemical kinetics with linear and fast recombination. This work demonstrates that the rate-ratio eigenvalue between the chain-branching and -recombination steps asymptotically approaches a critical minimum value, corresponding to the flammability limit. The existence of flammability limit with the linear recombination is found to be the most significant contrast to the quadratic recombination model, which is incapable of predicting flammability limit. By this study, a formal asymptotic procedure, by which the crossover temperature and lean flammability limit condition can be determined without assuming a steady-state approximation of intermediate species, is established and can be applied to analyze flame structures with multi-step reduced mechanisms.

Evolution of rarity and phylogeny determine above- and belowground biomass in plant-plant interactions.

Rare species are often considered inferior competitors due to occupancy of small ranges, specific habitats, and small local populations. However, the phylogenetic relatedness and rarity level (level 1-7 and common) of interacting species in plant-plant interactions are not often considered when predicting the response of rare plants in a biotic context. We used a common garden of 25 species of Tasmanian Eucalyptus, to differentiate non-additive patterns in the biomass of rare versus common species when grown in mixtures varying in phylogenetic relatedness and rarity. We demonstrate that rare species maintain progressively positive non-additive responses in biomass when interacting with phylogenetically intermediate, less rare and common species. This trend is not reflected in common species that out-performed in monocultures compared to mixtures. These results offer predictability as to how rare species' productivity will respond within various plant-plant interactions. However, species-specific interactions, such as those involving E. globulus, yielded a 97% increase in biomass compared to other species-specific interaction outcomes. These results are important because they suggest that plant rarity may also be shaped by biotic interactions, in addition to the known environmental and population factors normally used to describe rarity. Rare species may utilize potentially facilitative interactions with phylogenetically intermediate and common species to escape the effects of limiting similarity. Biotically mediated increases in rare plant biomass may have subsequent effects on the competitive ability and geographic occurrence of rare species, allowing rare species to persist at low abundance across plant communities. Through the consideration of species rarity and evolutionary history, we can more accurately predict plant-plant interaction dynamics to preserve unique ecosystem functions and fundamentally challenge what it means to be "rare".

Incorporation of Pd Catalyst into Highly Effective Borophene Nanosheet Co-Catalyst for Electrokinetics and Electrochemical Oxygen Reduction Reactions

To improve the performance of the system, it is of great importance to develop efficient catalysts for ethanol (EtOH) electro-oxidation. Pd/B electrocatalyst was synthesized using a sonochemical method. Structural and electrochemical properties of the prepared nanomaterial were investigated using electrochemical and physical techniques such as Raman spectroscopy, electrochemical impedance spectroscopy (EIS), x-ray diffraction (XRD), zetersizer, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and energy-dispersive x-ray spectroscopy (EDS) and cyclic voltammetry (CV). FTIR confirmed all the functional groups of carbon black, Pd/C, borophene, and Pd/B, and the crystallinity was investigated using XRD. EIS showed that Pd/B has a faster charge transfer and, through investigation using CV, Pd/B showed a more negative onset potential and higher current (−0.76 V vs. Ag|AgCl; 0.07 mA) than Pd/C (−0.65 V vs. Ag|AgCl; 0.05 mA), indicating a more catalytic behavior and tolerance of Pd/B. The active sites could be attributed to the addition of borophene. During the anodic sweeping direction of Pd/B electrocatalyst, it was observed that the ratio of backward peak current (Ibwd) to forward peak current (Ifwd), (Ibwd/Ifwd) of in a 2 M of NaOH + 2 M of EtOH is almost equal to (Ibwd/Ifwd) 1 which shows excellent tolerance of Pd/B to poisoning by ethanol intermediate species. The electron transfer rate (Ks) values for Pd/B at 0.1 M, 0.5 M, 1 M, 1.5 M, and 2 M were estimated to be 4.50 × 10−13 s−1, 1.08 × 10−12 s−1, 4.28 × 10−13 s−1, 5.25 × 10−14 s−1 and 9.35 × 10-14 s−1. At 2 M there is a faster electron transfer than at other concentrations which is also evidenced by the obtained diffusion values (D) of the system which were found to be 2.92 × 10−7 cm2 s−1, 4.72 × 10−8 cm2 s−1, 4.82 × 10−8 cm2 s−1, 1.22 × 10−7 cm2 s−1, and 9.12 × 10−8 cm2 s−1. The electrochemically active surface area (ECSA) is strongly related to intrinsic activity, Pd/B (1.85 cm2/mg × 10−5 cm2/mg) denotes the highest Pd-O stripping charge than Pd/C (1.13 cm2/mg × 10−5 cm2/mg).

Interface reconfiguration of MnCO3/Mn3O4 heterostructure enhances the ozone decomposition in the entire humidity range

The catalytic efficiency of manganese-based catalysts in practical ozone decomposition applications is limited by challenging desorption of intermediate oxygen species and competitive adsorption of H2O molecules in humid environments. In this work, MnCO3/Mn3O4 composites with heterogeneous structures were synthesized through a facile one-step strategy. The obtained MnCO3/Mn3O4-1/2 catalyst exhibited high content of oxygen vacancies, fast electron mobility rate and obvious advantages in catalytic ozone decomposition performance at a space velocity of 600 L g−1 h−1 and 95% RH. In-situ DRIFT spectra indicated that the rate-determining steps for ozone decomposition of MnCO3 and Mn3O4 are the reaction of atomic oxygen with ozone to form O22- and desorption of O22-, respectively. MnCO3/Mn3O4 heterogeneous catalyst undergoes reconfiguration under ozone atmosphere, inducing discontinuous MnOx coatings on the MnCO3 surface, which form a potential difference with Mn3O4. MnCO3/Mn3O4 heterogeneous structure modulates the electronic state of active site, and the synergistic effect of MnCO3 and Mn3O4 improves catalytic performance.

Design and synthesis of heteroleptic Ni(II) dipyrrin complexes for electrochemical proton reduction reactions: Cyclic voltammetric and theoretical studies

The effect of nuclearity on electrochemical hydrogen generation using new heteroleptic Ni(II) complexes containing redox-active dipyrrin and dithiocarbamate ligands has been described. Complexes 1–2 have been meticulously characterized by spectroscopic studies (ESI-MS, IR, 1H, 13C NMR, UV–vis) and their structures unambiguously confirmed by X-ray single crystal analyses. Electrocatalytic properties of the complexes toward hydrogen evolution reaction have been investigated by cyclic voltammetric studies in an organic medium in the presence of acetic acid as a weak proton source. Notably, complexes 1 and 2 produce H2 via doubly reduced Ni(II) species i.e. Ni(0) in the presence of acetic acid. Further, these complexes exhibited significant electrocatalytic activity (TOF: 264 (1) and 650 s−1 (2). Controlled potential electrolysis established a minimum Faradaic efficiency of 92 (1) and 96 % (2). Complex 2 exhibited higher turnover frequency relative to 1, while 1 showed lower overpotential (0.35 V) in comparison to 2 (0.45 V). The stability of the complexes and the amount of produced H2 has been investigated by bulk electrolysis study. A tentative mechanism (ECEC; E, electrons and C, chemical steps) and involved intermediate species for the proton reduction reaction for 1 has been established by theoretical studies.

Investigation of Iron Dissolution Mechanism in Acidic Solutions with and without Dissolved CO2—Part I: Electrochemical Impedance Spectroscopy Measurements

Aqueous CO2 corrosion of mild steel is one of the major problems in the oil and gas industry. While current understanding primarily focuses on cathodic reaction mechanisms, less attention has been given to the impact of aqueous CO2 on the anodic iron dissolution reaction. In contrast, the mechanism of iron dissolution in strongly acidic environments has been thoroughly investigated. Among the reaction mechanisms found in the open literature, a multipath mechanism was identified that could explain the iron dissolution in strong acidic sulfate solution; both in terms of steady-state polarization sweeps and impedance data at various pH values and current densities. However, the role of aqueous CO2 in solutions containing chlorides on the mechanism of iron dissolution remained an open question. The present study used electrochemical impedance spectroscopy (EIS) as the main technique, to study the mechanism of iron dissolution in strong acid chloride solution with and without the presence of CO2. Results showed that the presence of chloride ions (0.5 M) decreased the rate of iron dissolution by competing with hydroxide ions to adsorb on the metal surface, forming chloride-containing intermediate species that participate in the iron dissolution reaction. The resulting decrease in the availability of hydroxide intermediates, which are more effective at enhancing the reaction rate compared to chloride-containing intermediates, leads to an overall decrease in the rate of iron dissolution. While the presence of CO2 increases anodic current density, EIS investigation revealed that neither aqueous CO2 nor other carbonic species directly react on the bare metal surface to form adsorbed intermediates involved in the anodic reaction. EIS investigation suggested that aqueous CO2 may induce changes in the chemical composition of adsorbed species, rate constants, and surface coverage, thereby altering the kinetics of the underlying reactions.

Revealing the mechanism of Ce promoter in modulating product distribution of CO2 hydrogenation over Fe-based catalysts

This study elucidates the influence of the Ce promoter on the structure and surface intermediates of Fe-based catalysts and provides insights into the mechanism controlling the distribution of CO2 hydrogenation products. The product distribution revealed a volcano-shaped trend for the selectivity of light olefins and C5+ products as the Ce content increased from 0 to 10 wt%, whereas the selectivity for CH4 shows an inverted volcano-shaped trend. Concurrently, the selectivity for CO gradually increased from 14.4 % to 58.8 %. When the Ce content was 1 wt%, the selectivity for light olefins and C5+ products reached maximum values of 41.4 % and 19.1 %, respectively, and the selectivity for methane decreased to 34.3 %. Characterization by in situ X-ray diffraction (XRD), in situ infrared spectroscopy (IR), and X-ray photoelectron spectroscopy (XPS) techniques demonstrated that the variation in product distribution could be attributed to the altered ratio of Fe3O4 to Fe5C2 species, surface enrichment of the Na promoter, and generation and desorption of surface intermediate species induced by the Ce promoter. These factors affect the rate of the reverse water gas shift reaction (RWGS) and the subsequent Fischer-Tropsch (FT) reaction, thereby modulating the product distribution. Understanding the regulation mechanism of the CO2 hydrogenation product distribution by the Ce promoter is expected to provide valuable insights for guiding the design of catalyst structures.

The current state of transition metal-based electrocatalysts (oxides, alloys, POMs, and MOFs) for oxygen reduction, oxygen evolution, and hydrogen evolution reactions

Climate change is showing its impacts now more than ever. The intense use of fossil fuels and the resulting CO2 emissions are mainly to blame, accentuating the need to develop further the available energy conversion and storage technologies, which are regarded as effective solutions to maximize the use of intermittent renewable energy sources and reduce global CO2 emissions. This work comprehensively overviews the most recent progress and trends in the use of transition metal-based electrocatalysts for three crucial reactions in electrochemical energy conversion and storage, namely, the oxygen evolution (OER), oxygen reduction (ORR), and hydrogen evolution (HER) reactions. By analyzing the state-of-the-art polyoxometalates (POMs) and metal-organic frameworks (MOFs), the performance of these two promising types of materials for OER, ORR, and HER is compared to that of more traditional transition metal oxides and alloy-based electrocatalysts. Both catalytic activity and stability are highly influenced by the adsorption energies of the intermediate species formed in each reaction, which are very sensitive to changes in the microstructure and chemical microenvironment. POMs and MOFs allow these aspects to be easily modified to fine-tune the catalytic performances. Therefore, their chemical tunability and versatility make it possible to tailor such properties to obtain higher electrocatalytic activities, or even to obtain derived materials with more compelling properties towards these reactions.