Energy Barrier Eb Research Articles

Enhanced reactivity over Ce2(SO4)3/MgO with NiO modification for chemical looping partial oxidation of methane

Chemical looping partial oxidation of methane to syngas is a promising process. Thermodynamic and kinetic analysis both forecast the NiO/Ce2(SO4)3–MgO is a prospective oxygen carrier (OC) to this process, which enables with 100% syngas selectivity, 2.15H2/CO ratio, excellent methane conversion and stable cycling characteristic. Compared with Ce2(SO4)3, the methane conversion rate rise by 73.19% because of the excellent lattice oxygen availability, less carbon deposit, and large specific surface area. The distinctive pore structure also provides a favorable reaction environment. Through the density functional theory, Ni doping can lower the energy barrier (Eb), promote lattice oxygen activation and diffusion, and the oxygen vacancy (Ov) can further promote these processes. CH3 → CH2 + H (TS2) is the rate-limiting step. Compared with Ce2(SO4)3, the Ni doping can reduce Eb by 16.8%, and the Ov can further reduce it by 43.0%. It does not change when the Ov concentration is more than 1.96%.

Computational study on the adsorption and mechanisms of the hydrogenation of N-Cbz-protected glycine, alanine, valine, and phenylalanine on palladium, Pd (1 0 0), surface

Materials Studio (MS) is powerful software (with various useful tools) that is designed for simulating and modeling of different molecules and materials in computational chemistry, cheminformatics, bioinformatics, and etc. Density functional theory (DFT), with periodic slab model, is one of the most applied theoretical approaches for understanding surface-adsorbate interaction strength and reactivity. In this study, DFT method have been used via Materials Studio software to study the adsorption (at T = 298.15 K, 308.15 K, and 318.15 K) and hydrogenation (at constant temperature T = 298.15 K) of four series of N-Cbz-protected amino acids (Cbz-aa's include N-Cbz-l-phenylalanine, Cbz-pheala, N-Cbz-l-valine, Cbz-val, N-Cbz-l-alanine, Cbz-ala, and N-Cbz-l-glycine, Cbz-gly) on palladium, Pd (1 0 0), surface. During calculations of this study, a cell with periodic boundary conditions was constructed and each Cbz-aa was located and studied in this cell. For calculations of the hydrogenation of Cbz-aa's on Pd (1 0 0) surface at constant temperature, Materials Studio QMERA (combined QM/MM methods) has been employed. For calculations of the adsorption of Cbz-aa's on Pd (1 0 0) surface at different temperatures, the Materials Studio Sorption module has been applied. In the adsorption processes, average adsorption isosteric heat (Qst) and average total energy or overall system energies (Etotal) were extracted. In the hydrogenation processes, the energy of reaction (ΔH) and energy barrier (Eb) were obtained. In this study, three different mechanisms (mechanism 1: M1, mechanism 2: M2, and mechanism 3: M3) were suggested for the hydrogenation of each Cbz-aa on Pd (1 0 0) surface. We suggested these mechanisms based on the sites (atoms) that are probable for adding two hydrogen atoms (from H2) as well as the strength of bonds that should be broken for adding two hydrogen atoms (from H2). For these three mechanisms, the energy of reaction (ΔH), transition state, and energy barrier (Eb) were calculated. The obtained values of ΔH show that mechanism 3 (M3) provides the most facile mechanism to perform hydrogenation of Cbz-aa's on Pd (1 0 0) surface. Further, the negative sign of ΔH for M3 shows that mechanism 3 is an exothermic reaction in the case of all our studied Cbz-aa's. Hydrogenation and adsorption data show that Qst very slowly decreases and Etotal very slowly increases when temperature increases. Also, molecular weight and stereochemistry of Cbz-aa's molecules are two important factors that affect the values of Qst, Etotal, Eb, and ΔH.

Mechanistic study of chemical looping partial oxidation of methane over Ce2(SO4)3/MgO with CoO modification

Chemical looping partial oxidation of methane (CLPOM) to syngas provides an exceptional opportunity for methane utilization. However, it is still urgent to obtain efficient oxygen carrier (OC) and clarify its microscopic mechanism. In this work, CoO and MgO modified Ce2(SO4)3 OC are optimized based on various preparation methods and different CoO and Ce2(SO4)3 doping amount. Considering the conversion rate (XCH4), selectivities of CO (SCO) and H2 (SH2), H2/CO molar ratio, when the doping amount of CoO, Ce2(SO4)3 and MgO is 20 wt%, 30 wt% and 50 wt% respectively, the OC prepared by mechanical ball milling method has a better thermodynamic and kinetic properties in chemical looping reaction. Compared with literature, this OC has a larger XCH4. In multiple cycles, XCH4 is still higher than 90%, SCO and SH2 remain above 95%. Density functional theory calculation reveal that the Co doping reduces the energy barrier (Eb) of rate-limiting step by 24.3% and lowers the migration Eb of lattice oxygen by 78.2%. The OC with oxygen vacancy can further reduce the Eb of rate-limiting step, but the Eb basically remains unchanged when the oxygen vacancy (Ov) concentration is greater than 1.96%. Because of the high Eb of CO2 formation and low Eb of syngas formation, the syngas demonstrates high selectivity and does not produce CO2 in experiment. Meanwhile, the doping of CoO and MgO reduces carbon deposition, refines the nano-particles of OC, increases the specific surface area (SSA), and enhances resistance to sintering and aggregation. The unique pore structure also provides a favorable microenvironment for the reaction. These findings demonstrate that CoO and MgO modified Ce2(SO4)3 OC have the potential for application in CLPOM, and these basic insights can also be applied to other chemical looping systems.

Polarization switching pathways of ferroelectric Zr-doped HfO2 based on the first-principles calculation

Based on the first principles calculation, the mechanisms of polarization switching behavior in ferroelectric Zr-doped HfO2 are investigated. Seven switching pathways, divided into two categories by the identified orientation of polarization switching and value, are analyzed based on atomic migration and energy barrier. The effects of Zr dopant on switching energy barrier (Eb) and spontaneous polarization (Ps) are analyzed as well. In one of the categories, two pathways with tetragonal-like transition states show low energy barriers and can be further minimized with higher Zr dopant proportion, which originates from the stabilizing effect of Zr dopant on the tetragonal phase (T, P42/nmc). Especially, in the two tetragonal-like pathways, a distorted tetragonal-like transient state (T′, Pbcn) resulting from distinct atomic displacement is transformed to a highly symmetric T-phase along with the incorporation of Zr, elucidating this pathway as energy favorable as the regular T-pathway. This work provides an atomic insight for ferroelectric switching behavior and predicts the probable ferroelectric switching pathway in Zr-doped HfO2 films.

Decoupling the Kinetic Essence of Iron‐Based Anodes through Anionic Modulation for Rational Potassium‐Ion Battery Design

AbstractPotassium‐ion batteries (PIBs) have favorable characteristics in terms of cell voltage and cost efficiency, making them a promising technology for grid‐scale energy storage. The rational design of suitable electrode materials on a theoretical basis, aiming at high power and energy density, is of paramount importance to bring this battery technology to the practical market. In this paper, a series of iron‐based compounds with different non‐metal anions are selectively synthesized to investigate the nature of kinetic differences induced by anionic modulation. A combination of experimental characterization and theoretical calculation reveals that iron phosphide, with its moderate adsorption energy (Ea) and lowest diffusion barrier (Eb), exhibits the best cycling and rate properties at low electrochemical polarization, which is related to the narrow Δd‐p band center gap that facilitates ion transfer. In addition, the optimization of the electrolyte formula results in the carbon‐supported iron phosphide anode running stably over 2000 cycles at 0.5 A g−1 and exhibiting a high rate capacity of 81.1 mAh g−1 at 2 A g−1. The superior electrochemical properties are attributed to the robust KF‐rich solid electrolyte interphase formed by the highly compatible KFSI in ethylene carbonate (EC)/diethyl carbonate (DEC) configuration.

Design strategy of polyamine for CO2 absorption guided by a novel descriptor of hydrogen bond strength

Mixed amine solutions have been proposed as a promising method for improving the efficiency of CO2 adsorption. However, the process of experimentally screening the performance of different amine pairings can be labor-intensive. Here, a novel descriptor was proposed for the proton transfer energy barrier (Eb) to accelerate this process. The Eb of the proton transfer process was calculated using density functional theory (DFT) for 42 mixed systems. The independent gradient model based on Hirshfeld partition (IGMH) found the N–H⋯N type hydrogen bond (HB) is the dominant factor for the interactions. Based on the Atoms-In-Molecules (AIM) approach, the stronger the hydrogen bond, the lower the Eb of the proton transfer process. Three descriptors were identified (electron density (ρ-BCP), energy density (E(r)), and the electron localization function (ELF)) for linking the energy barrier to the strength of HBs. Surprisingly, this descriptor can also be applicable to tri-mixed amine systems, with a high square of the correlation coefficient (R2 > 0.8). Using this descriptor, the Eb of the proton transfer process for the quaternary mixed amines system was successfully predicted. We successfully build the relationship between the hydrogen bond strength and the reaction energy barrier, providing theoretical guidance for the design of mixed amines.

Unveiling the synergistic effect between the metallic phase and bridging S species over MoS2 for highly efficient nitrogen fixation

Electrocatalytic nitrogen reduction reaction (NRR) is considered an appealing approach towards sustainable NH3 production but still undergoes serious challenges with unsatisfactory catalytic performance, which hinders its large-scale application. In this work, the S-rich 1T‐MoS2 with an ultrahigh 1T phase content and S-enrichment has been successfully synthesized and firstly verified as an exceptional NRR catalyst with high activity and selectivity. The optimized MoS2.30 catalyst exhibits a high NH3 yield rate (98.30±1.63 μg h–1 mg–1cat.) and FE (23.10±0.38%), surpassing nearly all the reported MoS2-based NRR catalysts and the overwhelming majority of advanced NRR catalysts, demonstrating the encouraging role-playing of the synergistic effect between 1T phase and bridging S22– species on NRR performances. DFT calculations reveal that the high properties empowered by MoS2.30 are on account of the synergistic effect that induces the enhanced NRR activity via the strengthened N2 adsorption and reduced energy barrier (Eb) as well as the improved NRR selectivity through the optimized competitive adsorption and reduced energy barrier against the hydrogen evolution reaction (HER). This work fabricates a MoS2-based electrocatalyst for highly efficient NRR and proves an effective strategy to improve the catalytic performance, which is worth being extended to other catalytic reactions.

Nucleation and manipulation of single skyrmions using spin-polarized currents in antiferromagnetic skyrmion-based racetrack memories

In this work, an ultrafast nucleation of an isolated anti-ferromagnetic (AFM) skyrmion was reported in an AFM layer with DMi strengths of 0.47-0.32 mathrm{mJ}/{mathrm{m}}^{2} using spin-transfer torque by locally injecting pure spin currents into magnetic tracks. Besides, we revealed the key advantages of AFM skyrmion-based racetrack memories by comparing the motion of AFM and FM skyrmions driven by spin–orbit torques (SOTs) for different skyrmion sizes along racetrack memories with various notch sizes. Our results indicate that for AFM skyrmion, the skyrmion Hall effect does not exist during the skyrmion motion, therefore at small skyrmion sizes, we succeeded to overcome the repulsive forces developed in the notch area for low and large SOTs. The obtained findings were carefully analyzed by computing the variation of energy barriers associated with the notch for different skyrmion sizes using minimum energy path (MEP) calculations. We showed that the larger the skyrmion size, the harder it is to shrink the skyrmion in the notch which produces a high energy barrier (Eb) for large skyrmion sizes. Moreover, as the notch size increases, the skyrmion size shrinks further, and hence Eb increases proportionally. Nevertheless, we proved that AFM skyrmions are more efficient and flexible than FM skyrmions against boundary forces.

Insight into the elemental mercury immobilization mechanism with carbon and sulfur over the mackinawite (FeS) surface via density functional theory

Elemental mercury (Hg0) adsorption behaviors and mechanisms on the low-Miller index mackinawite surface (FeS(001)-S surface), with the engagement of carbon or sulfur, were investigated via DFT-D2 theoretical calculations. The simulation results show that the most favorable adsorption site for the C and Hg atoms was the lower-S site on the pristine surface, while S atom preferred the upper-S site. C atom could cause charge transfer and inter-atomic charge redistribution during adsorption. The carbon atom performs as a linker to connect the substrate and the Hg atom in the most stable mercury adsorption geometry onto the carbon-doped surface. Hg0 is physically adsorbed over the sulfur-doped surface. The HgS molecule chemically attached to the surface at the upper-S site via the Hg atom to form a S-Hg-S structure, and its generation process follows Langmuir-Hinshelwood Mechanism. Firstly, the Hg and S atoms were pre-adsorbed on the FeS(001)-S surface. Then the system needed to overcome an activation energy barrier (Eb) of 89.82 kJ·mol−1. It was an endothermic process in which two separate atoms generated HgS*. However, C atom will impede the reaction as occupying the adsorption sites of mercury and sulfur.

Enabling Fast Na+ Transfer Kinetics in the Whole-Voltage-Region of Hard-Carbon Anodes for Ultrahigh-Rate Sodium Storage.

Efficient electrode materials, that combine high power and high energy, are the crucial requisites of sodium-ion batteries (SIBs), which have unwrapped new possibilities in the areas of grid-scale energy storage. Hard carbons (HCs) are considered as the leading candidate anode materials for SIBs, however, the primary challenge of slow charge-transfer kinetics at the low potential region (<0.1V) remains unresolved till date, and the underlying structure-performance correlation is under debate. Herein, ultrafast sodium storage in the whole-voltage-region (0.01-2V), with the Na+ diffusion coefficient enhanced by 2 orders of magnitude (≈10-7 cm2 s-1 ) through rationally deploying the physical parameters of HCs using a ZnO-assisted bulk etching strategy is reported. It is unveiled that the Na+ adsorption energy (Ea ) and diffusion barrier (Eb ) are in a positive and negative linear relationship with the carbon p-band center, respectively, and balance of Ea and Eb is critical in enhancing the charge-storage kinetics. The charge-storage mechanism in HCs is evidenced through comprehensive in(ex) situ techniques. The as prepared HCs microspheres deliver a record high rate performance of 107 mAh g-1 @ 50 A g-1 and unprecedented electrochemical performance at extremely low temperature (426 mAh g-1 @ -40 °C).

Auger-Assisted Electron Transfer between Adjacent Quantum Wells in Two-Dimensional Layered Perovskites

Two-dimensional (2D) layered perovskites are naturally formed multiple quantum-well (QW) materials, holding great promise for applications in many optoelectronic devices. However, the further use of 2D layered perovskites in some devices is limited by the lack of QW-to-QW carrier transport/transfer due to the energy barrier formed by the insulating ligands between QWs. Herein, we report an Auger-assisted electron transfer between adjacent QWs in (CmH2m+1NH3)2PbI4 2D perovskites particularly with m = 12 and 18, where the electron energy barrier (Eb) is similar to the QW band gap energy (Eg). This Auger-assisted QW-to-QW electron transfer mechanism is established by the observation of a long-lived and derivative-like transient absorption feature, which is a signature of the quantum confined Stark effect induced by the electron-hole separation (thus an internal electric field) between different QW layers. Our finding provides a new guideline to design 2D perovskites with an optically tunable QW-to-QW charge transport property, advancing their applications in optoelectronics and optical modulations.

H2O splitting on Run/TiO2(101) surface: Lowered energy barrier due to charge transfer at interface

Metal-support interactions have been explored as an effective method to improve the photocatalytic activity in heterogeneous catalysis. In this study, first-principles calculations confirmed that H2O can be split on Ru8 cluster decorated anatase TiO2(101) surface (denoted as Ru8/TiO2(101)), whose adsorption energy (Eads) and energy barrier (Eb) are lower than H2O on isolated Ru8 cluster, due to significant charge transfer from Ru8 cluster to TiO2(101). Compared with Ru8/TiO2(101), single Ru atom decorated TiO2(101) has higher Eads with H2O, manifesting better catalytic property. The similar dissociation process of H2O on isolated Ru8 cluster and Ru8/TiO2(101) indicated that we can study the molecule/cluster in detail firstly, and then apply it to molecule/cluster/surface, which will save much computing resource.

Cassie–Wenzel wetting transition on nanostructured superhydrophobic surfaces induced by surface acoustic waves

We report irreversible Cassie–Wenzel wetting transition on a nanostructured superhydrophobic surface employing surface acoustic wave (SAW) vibration. The transition is achieved upon penetration of the liquid into the nanogrooves driven by the inertial energy of the drop imparted by the SAW. However, the filling up of nanopores imposes an energy barrier (Eb) to the transition, which requires the displacement of the initial solid–air interface inside the pores with a solid–liquid interface. We unravel that the relative magnitudes of the input acoustic energy (Eac), and this energy barrier, hence, dictate the occurrence of the wetting transition, with the irreversibility in the transition, therefore, being explained from energy minimization of the system following the transition. In addition, observing the dynamics of the wetting front allowed the different regimes of the wetting transition process to be identified.

Effect of Thermal Fluctuation Field on the Magnetization Switching by Spin-Transfer Torque

In this work, the effect of thermal fluctuation field on the magnetization dynamics in Co1.5Fe1.5Ge alloy free layer is reviewed in a spin-valve structure with in-plane magnetization using the stochastic Landau-Lifshitz-Gilbert-Slonczewski (s-LLGS) equation taking into account the thermally assisted switching. The ultimate goal is to systematically study the heating effect on the magnetization reversal by simulating the magnetization dynamics in different thicknesses under very low current densities of about 106–105 A/cm2 in the presence of the thermal effect by applying different temperature values between T = 50 and T = 200 K. Results showed that the magnetization has different reversal processes among the large and small temperature values due to the competition between the thermal energy Et and magnetic energy barrier Eb, which allows the magnetization to trigger switching for specific temperature values. Consequently, at a temperature increase of about 200 K, we have proved that the magnetization reversal in Co1.5Fe1.5Ge alloy free layer is possible under a low current density of J =− 3.5 × 106 A/cm2. Additionally, we carefully examined the thermal impact on the threshold current density and switching time through analyzing the magnetization behavior for each temperature value, and as a result, we succeeded to reduce the critical current density in Co1.5Fe1.5Ge/Ag/Co1.5Fe1.5Ge structure from Jc = − 5.7 × 106 A/cm2 to Jc = − 9.5 × 105 A/cm2 for a thickness of 1 nm.

Discovery of Oxide-Ion Conductors with a New Crystal Structure, BaSc2–xAxSi3O10–x/2 (A = Mg, Ca) by Screening Sc-Containing Oxides through the Bond-Valence Method and Experiments

We have discovered oxide-ion conductors with a new crystal structure by combining the bond-valence (BV) method and experiments. In the present work, the BV-based energy barrier Eb for oxide-ion migration was calculated for 123 kinds of Sc-containing oxides as a screening process. We found that the monoclinic BaGd2Si3O10-type barium scandium silicate BaSc2Si3O10 has a relatively low Eb, indicating that it is potentially a new oxide-ion conductor. BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A = Mg, Ca) were prepared by solid-state reactions. Rietveld analyses of X-ray powder diffraction data of these samples were successfully performed using the monoclinic P21/m BaGd2Si3O10-type structure. Lattice volume V of BaSc1.9Mg0.1Si3O9.95 was smaller than V of BaSc2Si3O10 due to the substitution of smaller sized Mg2+ for Sc3+, while the V of BaSc1.9Ca0.1Si3O9.95 was larger than V of BaSc2Si3O10 due to the substitution of larger sized Ca2+ for Sc3+, indicating the formation of solid solutions. Electrical conductivity and ul...

Roles of sliding-induced defects and dissociated water molecules on low friction of graphene

Sliding contact experiments and first-principles calculations were performed to elucidate the roles of structural defects and water dissociative adsorption process on the tribo-chemical mechanisms responsible for low friction of graphene. Sliding friction tests conducted in ambient air and under a dry N2 atmosphere showed that in both cases a high running-in coefficient of friction (COF) occurred initially but a low steady-state COF was reached only when the sliding was continued in air with moisture. Density functional theory (DFT) calculations indicated that the energy barrier (Eb) for dissociative adsorption of H2O was significantly lower in case of reconstructed graphene with a monovacancy compared to pristine graphene. Cross-sectional transmission electron microscopy of graphene transferred to the counterface revealed a partly amorphous structure incorporating damaged graphene layers with d-spacings larger than that of the original layers. DFT calculations on the reconstructed bilayer AB graphene systems revealed an increase of d-spacing due to the chemisorption of H, O, and OH at the vacancy sites and a reduction in the interlayer binding energy (EB) between the bilayer graphene interfaces compared to pristine graphene. Thus, sliding induced defects facilitated dissociative adsorption of water molecules and reduced COF of graphene for sliding tests under ambient and humid environments but not under an inert atmosphere.

Design of a Low-Voltage Analog-to-Digital Converter Using Voltage-Controlled Stochastic Switching of Low Barrier Nanomagnets

The inherent stochasticity in many nano-scale devices makes them prospective candidates for low-power computations. Such devices have been demonstrated to exhibit probabilistic switching between two stable states to achieve stochastic behavior. Recently, superparamagnetic nanomagnets (having low energy barrier EB $\sim$ 1kT) have shown promise of achieving stochastic switching at GHz rates, with very low currents. On the other hand, voltage-controlled switching of nanomagnets through the Magneto-electric (ME) effect has shown further improvements in energy efficiency. In this simulation paper, we first analyze the stochastic switching characteristics of such super-paramagnetic nanomagnets in a voltage-controlled spintronic device. We study the influence of external bias on the switching behavior. Subsequently, we show that our proposed device leverages the voltage controlled stochasticity in performing low-voltage 8-bit analog to digital conversions. This eliminates the need for comparators, unlike the Complementary Metal-Oxide Semiconductor (CMOS)-based flash Analog-to-Digital converters (ADC). This device allows for a simple and compact design which can potentially be applied in implementing sensors which desire low voltage conversions.

Asymmetric and Speed-Dependent Capillary Force Hysteresis and Relaxation of a Suddenly Stopped Moving Contact Line.

We report on direct atomic-force-microscope measurements of capillary force hysteresis (CFH) and relaxation of a circular moving contact line (CL) formed on a long micron-sized hydrophobic fiber intersecting a water-air interface. The measured CFH and CL relaxation show a strong asymmetric speed dependence in the advancing and receding directions. A unified model based on force-assisted barrier crossing is utilized to find the underlying energy barrier Eb and size λ associated with the defects on the fiber surface. The experiment demonstrates that the pinning (relaxation) and depinning dynamics of the CL can be described by a common microscopic framework, and the advancing and receding CLs are influenced by two different sets of relatively wetting and nonwetting defects on the fiber surface.

Theoretical study of the ammonia nitridation rate on an Fe (100) surface: a combined density functional theory and kinetic Monte Carlo study.

Ammonia (NH3) nitridation on an Fe surface was studied by combining density functional theory (DFT) and kinetic Monte Carlo (kMC) calculations. A DFT calculation was performed to obtain the energy barriers (Eb) of the relevant elementary processes. The full mechanism of the exact reaction path was divided into five steps (adsorption, dissociation, surface migration, penetration, and diffusion) on an Fe (100) surface pre-covered with nitrogen. The energy barrier (Eb) depended on the N surface coverage. The DFT results were subsequently employed as a database for the kMC simulations. We then evaluated the NH3 nitridation rate on the N pre-covered Fe surface. To determine the conditions necessary for a rapid NH3 nitridation rate, the eight reaction events were considered in the kMC simulations: adsorption, desorption, dissociation, reverse dissociation, surface migration, penetration, reverse penetration, and diffusion. This study provides a real-time-scale simulation of NH3 nitridation influenced by nitrogen surface coverage that allowed us to theoretically determine a nitrogen coverage (0.56 ML) suitable for rapid NH3 nitridation. In this way, we were able to reveal the coverage dependence of the nitridation reaction using the combined DFT and kMC simulations.

Determination of 2D distribution of magnetic nanoparticles derived from magnetization curves

This paper refers to determination of magnetic nanoparticles distribution as a function of both magnetic moment µ and energy barrier EB. The original analysis consists in a numerical analysis of experimental M(H) curves determined at two different temperatures and is based on the Stoner–Wohlfarth model. As it was shown, the applied “simulated annealing (SA)” calculation algorithm with additional entropy maximum condition allows characteristic distribution profiles to be determined. Moreover, from the temperature shift of the profiles, the real positions of their homological points in µ–EB space can be calculated. It should be emphasized that the novel approach does not require any preliminary assumption of a distribution shape and number of components. The proposed magnetic characterization was tested with the use of computer-generated as well as experimental M(H) curves. The obtained results confirm the efficiency of the method and its usefulness in analysis of hard magnetic powder systems.