Low Activation Energy Barrier Research Articles

Molecular simulations of microscopic mechanism of the effects of chain length on stereocomplex formation in polymer blends

By means of dynamic Monte Carlo simulations, the effects of chain length on the formation of stereocomplex crystallites (SCs) in polymer blend systems with different chain lengths have been studied. The decrease in chain length can lead to the increase of final SC contents. Three factors contribute to the enhanced SC formation in short-chain systems, including miscibility between polymers of different types, chain mobility and nucleation mode. The systems with shorter chains exhibit higher miscibility between polymers of different types and stronger chain mobility. In this condition, polymer chains have more chance to meet other chains of a different type in their neighbouring regions, and in the meantime the stronger chain mobility leads to lower diffusion activation energy and smaller nucleation energy barrier for the SC formation. In addition, the nucleation mode of intermolecular packing alignment dominates in the systems with short chains, and thus more chains of different types choose to pack alternatively during crystallization. Due to the effects of these factors, the systems with shorter chains exhibit higher final SC contents.

Temperature-dependent lithium diffusion in phographene: Insights from molecular dynamics simulation

It is noteworthy to elucidate the underlying atomistic insights of next-generation battery electrode materials to overcome the existing constraints associated with its rapid progress. By employing classical molecular dynamics (MD) simulations, we have studied the temperature dependent structural, thermo-mechanical, and Lithium diffusion properties of α- and β-phographene (PhoG) for Lithium-ion battery (LIB) anode applications. Our results show that at 300K both the PhoGs possess negative thermal expansion coefficient and is expounded as proof of anharmonicity present in it due to the existence of pliable bending modes in the out-of-plane direction. The computed ultrahigh stiffness of PhoG helps to prevent the acute lattice expansion issue upon Li intercalation. The study also brings out that Li atom could freely diffuse on the surface of the PhoGs, and thus a fast Li diffusivity and superior conductivity is observed. The calculated Li diffusion activation energies (<0.20eV) of these membranes are lower than many of the typical for Li-based graphitic anode with a Li diffusion coefficient of 10−10–10−12cm2s−1. In this regard, the excellent structural and thermo-mechanical stability, and low activation energy barrier in PhoGs assures its application as an anode material in high-performance LIBs.

Facile migration of potassium ions in a ternary P3-type K0.5[Mn0.8Fe0.1Ni0.1]O2 cathode in rechargeable potassium batteries

A ternary P3-type K0.5[Mn0.8Fe0.1Ni0.1]O2 material is introduced, herein, as a promising cathode material for K ion batteries. The disadvantages associated with Mn-based layered cathode materials – structural degradation and capacity deterioration – are overcome by the partial replacement of Mn3+ with Fe3+ and Ni2+, which tends to increase the average oxidation state of Mn to 3.75+. First-principles calculation predicts the sequence of redox pairs from Mn3+/4+, Fe3+/4+, and Ni2+/3+ with increasing the operating voltage to 3.9 V K0.5[Mn0.8Fe0.1Ni0.1]O2 exhibits a high reversible discharge capacity (~120 mAh g−1) and excellent structural integrity over 300 cycles (74% capacity retention) between 1.5 – 3.9 V at 50 mA g−1. This outstanding performance of K0.5[Mn0.8Fe0.1Ni0.1]O2 is attributed to the slight structural variation (~4.1%) of its P3–O3 phase transition predicted by the first principles calculation. Surprisingly, the obtained capacity reaches 76 mAh g−1 at a rate of 2.5 A g−1, in which the facile migration of K ions is explained by the low activation energy barrier of ~438 meV predicted by the nudged elastic band (NEB) method.

Symmetry-Adapted Perturbation Theory Decomposition of the Reaction Force: Insights into Substituent Effects Involved in Hemiacetal Formation Mechanisms.

The decomposition of the reaction force based on symmetry-adapted perturbation theory (SAPT) has been proposed. This approach was used to investigate the substituent effects along the reaction coordinate pathway for the hemiacetal formation mechanism between methanol and substituted aldehydes of the form CX3CHO (X = H, F, Cl, and Br), providing a quantitative evaluation of the reaction-driving and reaction-retarding force components. Our results highlight the importance of more favorable electrostatic and induction effects in the reactions involving halogenated aldehydes that leads to lower activation energy barriers. These substituent effects are further elucidated by applying the functional-group partition of symmetry-adapted perturbation theory (F-SAPT). The results show that the reaction is largely driven by favorable direct noncovalent interactions between the CX3 group on the aldehyde and the OH group on methanol.

Na-Ions Diffusion Impacts Supercapacitor Performance for Amaryllis-like NiCo2O4 Nanostructures.

NiCo2O4 nanomaterials with exceptional electrochemical performances are synthesized via a simple and low-cost method. The synthesized nanostructures exhibit a high specific surface area of 121.52 m2 g-1 and excellent specific capacitance of 2498.49 F g-1 at a current density of 2 A g-1, an energy density of 79 Wh kg-1, and power density of 3570 W kg-1. The remarkable cycling stability of 92.61% retention after 5000 cycles demonstrates that NiCo2O4 nanomaterials have a potential for practical application in energy storage devices. The Na+ ion diffusion (by VASP) affirms a low activation energy barrier for Na ion intercalations onto the electrode material, illuminating excellent electrochemical performances.

Synthesis of Structurally Diverse Substituted Aziridinyl Glycoconjugates via Base-Mediated One-Pot Post-Ugi Cyclization.

The base-promoted intramolecular cyclization of Ugi-azide adduct has been demonstrated for the synthesis of highly substituted aziridinyl glycoconjugates in one pot. The reactions are scalable and efficient and have an operationally simple broad substrate scope. To gain insight into the mechanism of aziridine formation, DFT and control experiments show that the cyclization of the aziridine glycoconjugate pathway was preferred, as it proceeds with a low activation energy barrier (0.57 kcal mol-1), which supports our experimental observation.

DFT study of inter-ring haptotropic rearrangement in CpRu+ complexes of polycyclic aromatic ligands

Inter-ring haptotropic rearrangements (IRHRs) of different types are well-known phenomena in organometallic and catalytic chemistry. So far, they are reported for transition metal complexes with carbo- and heterocyclic polyaromatic hydrocarbons (PAH) of small and medium size. Here, we report DFT studies of RuCp+ shifts between neighboring six-membered rings (η6⇌ η6-IRHR) on an extra-large PAH as a model for graphene and compare it to naphthalene. Our calculations predict that η6⇌ η6-IRHRs proceed with much lower activation energy barrier of rearrangement in the case of the RuCp+ complex of η6-graphene model.

Nitric oxide oxidation on warped nanographene (C80H30): a DFT study

The possible use of the recently synthesized warped nanographene C80H30 for NO oxidation by O2 molecule has been investigated using density functional theory. The reaction starts with the adsorption and dissociation of O2 molecule on the central pentagon of C80H30 with the activation energies of 24.2–26.6 kcal/mol depending on the active sites. Then, the dissociated O atoms readily oxidize NO to NO2 twice. The first NO oxidation occurs with barrierless, while the second NO oxidation requires a small energy barrier of 16.0 kcal/mol. The low activation energy barrier pathway indicates high catalytic activity of this nanographene for NO oxidation. Charge analysis reveals that such high catalytic activity of nanographene is attributed to the charge transfer from the saddle-shaped C80H30 to the dissociated O atoms which makes it reactive to NO molecule. Desorption of NO2 product, which is the rate-limiting step of NO oxidation in some catalysts, is easily occurred in this nanographene (less than 2 kcal/mol), indicating the prevention of catalyst poisoning. This study suggests that C80H30 nanographene is a promising catalyst for NO removal in ambient condition.

A novel thermal-tuning Fe-based amorphous alloy for automatically recycled methylene blue degradation

The comprehensive utilization of the excellent magnetic properties and advanced catalytic ability of Fe-based amorphous alloys (FAAs) is reported for the first time. A novel Fe63Cr5Nb4Y6B22 (at%) amorphous alloy that has ferromagnetic to paramagnetic transition near room temperature and good catalytic performance for methylene blue degradation is successfully synthesized. The alloy is applicable for automatically recycled dye degradation with its thermal-tuning magnetic behavior. When an external dye wastewater is applied (high temperature), the FAAs are paramagnetic and disperse into water to participate in degradation. When the wastewater has been degraded and the temperature drops to room temperature, the FAAs change to ferromagnetic and the remediated water can be released. This alloy exhibits a low thermal activation energy barrier of 21.8 kJ/mol due to the reduced atomic bonding forces with Cr addition, and has excellent catalytic performance at high temperature (323–343 K) and in acidic solution (pH = 3). The excellent chemical properties are mainly attributed to the heterogeneous structure consisting of local Fe-rich and Fe-poor atomic clusters. The present findings not only provide a new, highly efficient and low cost commercial method for wastewater treatments, but also shed light on the comprehensive utilization of the magnetic and chemical properties of FAAs.

Assessing Substitution Effects on Surface Chemistry by in Situ Ambient Pressure X-ray Photoelectron Spectroscopy on Perovskite Thin Films, BaCe xZr0.9- xY0.1O2.95 ( x = 0; 0.2; 0.9).

Performance of proton-solid oxide fuel cells (H+-SOFC) is governed by ion transport through solid/gas interfaces. Major breakthroughs are then intrinsically linked to a detailed understanding of how parameters tailoring bulk proton conductivity affect surface chemistry in situ, at an early stage. In this work, we studied proton and oxygen transport at the interface between H+-SOFC electrolyte BaCe xZr0.9- xY0.1O2.95 ( x = 0; 0.2; 0.9) thin films and the gas (100 mTorr of H2O and O2) by using synchrotron-based ambient pressure X-ray photoelectron spectroscopy at operating temperature (>400 °C). We developed highly textured BaCe xZr0.9- xY0.1O2.95 epitaxial thin films, which exhibit high level of in-plane proton conductivity, that is, up to 0.08 S cm-1 at 500 °C for x = 0.9. Upon applying 100 mTorr water partial pressure above 300 °C, major changes are observed only in the O 1s and Y 3d core level spectra, with a clear Zr/Ce ratio dependency. OH- formation is favored by Ce content while initiated near Y. Hydration is also associated with surface secondary phase growth comprising oxygen-under-coordinated yttrium and/or yttrium hydroxide. With BaCe0.2Zr0.7Y0.1O2.95, high levels of ionic conductivities and chemical stability are obtained as a result of the optimized surface reaction kinetics, with low activation energy barrier for proton transport while restraining formation of OH-/SO42- adsorb species.

Synergistic CO2 Mediation and Photocatalysis for α-Alkylation of Primary Aliphatic Amines

Recent work published by Rovis, Schoenebeck, and co-workers in Nature Chemistry describes a synergistic CO2 mediation and photocatalysis strategy for challenging α-alkylation of primary aliphatic amines by using quinuclidine as a hydrogen atom transfer (HAT) catalyst for γ-lactam synthesis. Electrostatic attraction between the in-situ-generated carbamate and quinuclidinium radical cation is critical for the selectivity.

A New Defect‐Rich CoGa Layered Double Hydroxide as Efficient and Stable Oxygen Evolution Electrocatalyst

AbstractDeveloping highly efficient and earth‐abundant electrocatalysts is urgent for oxygen evolution reaction (OER), and layered double hydroxides (LDHs) show great potentials as advanced electrocatalysts toward OER. However, the catalytic activity of LDHs is normally limited by their poor electronic conductivity and low electrochemical active areas. Herein, a novel sputtering–painting–alloying–dealloying strategy is reported to synthesize CoGa LDH anchored on stainless steel mesh (CoGa LDH/stainless steel mesh, SSM), in which abundant oxygen vacancies are introduced into the ultrathin CoGa LDH nanosheets. Owing to the unique painting‐like operation, the nanosheet‐decorated CoGa LDH/SSM electrode can be fabricated on a large scale (8 × 8 cm2 for instance), and possesses excellent flexibility and self‐supported feature. Free of any conducting/binder agents, the CoGa LDH/SSM electrode shows excellent electrocatalytic activity toward OER in an alkaline solution (low overpotential of 258 mV @ 10 mA cm−2, small Tafel slope of 34.1 mV dec−1, long‐term stability of 70 h), which is superior to that of state‐of‐the‐art Co‐based electrocatalysts. More importantly, the electrode needs only an overpotential of 198 mV to reach 10 mA cm−2 at 335 K. The OER performance of CoGa LDH/SSM can be rationalized by numerous oxygen vacancies, ultrathin nanosheet morphology, self‐supported nature, and low activation energy barrier.

Composition Screening of Lithium- and Sodium-Rich Anti-Perovskites for Fast-Conducting Solid Electrolytes

Li- and Na-rich anti-perovskites are receiving increasing attention as high-performance solid electrolytes for applications within all-solid-state batteries. The defect chemistry and ionic transport in a wide range of Li3–xNaxOCl1–yBry compositions are examined using advanced materials modeling techniques. Our calculations indicate that alkali-halide Schottky defect pairs are the dominant type of intrinsic disorder in these materials. Li-ion conductivity is shown to significantly exceed Na-ion conductivity, with the highest conductivities found for Li3OCl and Li3OBr. The effect of Cl/Br mixing on conductivity is shown to be small but could be used to fine-tune the activation energy. Both low conductivities and high activation energy barriers are found for the mixed Li/Na systems, which suggests that they would make poor solid electrolytes. The results presented here will aid the future optimization of anti-perovskite materials for solid electrolyte applications.

Insights into Interfacial Synergistic Catalysis over Ni@TiO2- x Catalyst toward Water-Gas Shift Reaction.

The mechanism on interfacial synergistic catalysis for supported metal catalysts has long been explored and investigated in several important heterogeneous catalytic processes (e.g., water-gas shift (WGS) reaction). The modulation of metal-support interactions imposes a substantial influence on activity and selectivity of catalytic reaction, as a result of the geometric/electronic structure of interfacial sites. Although great efforts have validated the key role of interfacial sites in WGS over metal catalysts supported on reducible oxides, direct evidence at the atomic level is lacking and the mechanism of interfacial synergistic catalysis is still ambiguous. Herein, Ni nanoparticles supported on TiO2- x (denoted as Ni@TiO2- x) were fabricated via a structure topotactic transformation of NiTi-layered double hydroxide (NiTi-LDHs) precursor, which showed excellent catalytic performance for WGS reaction. In situ microscopy was carried out to reveal the partially encapsulated structure of Ni@TiO2- x catalyst. A combination study including in situ and operando EXAFS, in situ DRIFTS spectra combined with TPSR measurements substantiates a new redox mechanism based on interfacial synergistic catalysis. Notably, interfacial Ni species (electron-enriched Niδ- site) participates in the dissociation of H2O molecule to generate H2, accompanied by the oxidation of Niδ--O v-Ti3+ (O v: oxygen vacancy) to Niδ+-O-Ti4+ structure. Density functional theory calculations further verify that the interfacial sites of Ni@TiO2- x catalyst serve as the optimal active site with the lowest activation energy barrier (∼0.35 eV) for water dissociation. This work provides a fundamental understanding on interfacial synergistic catalysis toward WGS reaction, which is constructive for the rational design and fabrication of high activity heterogeneous catalysts.

Revisiting the alluaudite NaMnFe2(PO4)3 sodium insertion material: Structural, diffusional and electrochemical insights

Among the gamut of sodium battery insertion materials, NaMnFe2(PO4)3 was reported as the first alluaudite framework compound albeit with poor electrochemical activity [Chem. Mater. 22 (2010) 5554]. We hereby report auto-combustion synthesis of carbon coated NaMnFe2(PO4)3 alluaudite and its Na+ diffusion, ionic conductivity and electrochemical activity synergizing experiments with bond valence site energy (BVSE) modeling. It registered a 2.8 V redox activity with a reversible capacity of ∼60 mAh g−1 with good cycling stability. BVSE calculations revealed an exceptionally low one-dimensional migration barrier of 0.31 eV for Na-ion diffusion. It was in sync with the low activation energy barrier of 0.162 eV derived from ac impedance spectroscopy. NaMnFe2(PO4)3 alluaudite cathode was found to have conductivity value of 0.5×10−6 S cm−1 at room temperature. Among the PO4-based sodium battery insertion materials, NaMnFe2(PO4)3 alluaudite shows excellent ionic conductivity with very low Na+ migration barrier. It can lead to the realization of superior reversible capacity in this alluaudite cathode comparable to LiFePO4.

Oxygen Evolution Reaction Kinetic Barriers on Nitrogen-Doped Carbon Nanotubes.

We investigate kinetic barriers for the oxygen evolution reaction (OER) on singly and doubly nitrogen-doped single-walled carbon nanotubes (NCNTs) using the climbing image nudged elastic band method with solvent effects represented by a 45-water-molecule droplet. The studied sites were chosen based on a previous study of the same systems utilizing a thermodynamic model which ignored both solvent effects and kinetic barriers. According to that model, the two studied sites, one on a singly nitrogen-doped CNT and the other on a doubly doped CNT, were approximately equally suitable for OER. For the four-step OER process, however, our reaction barrier calculations showed a clear difference in the rate-determining *OOH formation step between the two systems, with barrier heights differing by more than 0.4 eV. Thus, the simple thermodynamic model may alone be insufficient for identifying optimal OER sites. Of the remaining three reaction steps, the two H2O forming ones were found to be barrierless in all cases. We also performed solvent-free barrier calculations on NCNTs and undoped CNTs. Substantial differences were observed in the energies of the intermediates when the solvent was present. In general, the observed low activation energy barriers for these reactions corroborate both experimental and theoretical findings of the utility of NCNTs for OER catalysis.

Theoretical insights on the C-C bond reductive elimination from Co(III) center

Abstract The density-functional theory (DFT) calculations using B3LYP functional were performed to inspect the mechanism of the reductive elimination of ethane from a cobalt (III) dimethyl complex, (PMe3)3Co(CH3)2I. Three different pathways, i.e., radical mechanism (path A), concerted C-C bond formation (path B) and α-hydride elimination (path C) were studied for the reductive elimination reaction. A PMe3 ligand of the hexa-coordinated cobalt complex dissociates to form penta-coordinated complex. The penta-coordinated complex undergoes reductive elimination reaction. The calculated Gibbs free energy for the formation of methyl radical is 27.6 kcal/mol. Path B involving concerted C-C bond formation shows the activation energy barrier of 12.8 kcal/mol for the reductive elimination reaction. The calculated activation energy barrier for α-hydride elimination mechanism is 53.1 kcal/mol. Path B shows the lowest activation energy barrier of 12.8 kcal/mol, however, this value is much lower than the experimentally determined activation energy (25.0 kcal/mol). With MN12SX functional, the activation energy value improves to 20.8 kcal/mol which is close to the experimental value.

Theoretical studies on copper-catalyzed arylation of nitrogen heterocycles from benzenediazonium acetate under ligand-free conditions

The density functional theory (DFT) method was used to investigate the mechanism of ligand-free copper-catalyzed arylation of nitrogen heterocycles from benzenediazonium acetate. Two possible mechanisms, single electron transfer and oxidative addition/reductive elimination reaction mechanism were investigated for two possible Cu(I) complexes in the reaction solution. Complex 1 has an acetate and a pyrazole ligand coordinated to Cu(I), and complex 2 has two pyrazole ligands coordinated to Cu(I). Both Cu(I) complexes have sufficiently low activation energy barriers for oxidative addition/reductive elimination reactions. Activation energy barrier for single electron transfer reaction is significantly higher than oxidative addition/reductive elimination reaction.

Flexible proton-responsive ligand-based Mn(i) complexes for CO2 hydrogenation: a DFT study.

A series of flexible proton-responsive group substituted N^N-bidentate ligand based Mn(i) complexes have been studied for base free CO2 hydrogenation. We show here that proton responsive ligands play a critical role in base free CO2 hydrogenation. Our calculated reaction free energy barrier values for heterolytic H2 cleavage show that such flexible proton-responsive ligands require a very low activation energy (∼3 kcal mol-1) barrier. Such flexible proton responsive ligands improve the strong dihydrogen (HH) bonding, which in turn improves heterolytic H2 cleavage - an important step for base free CO2 hydrogenation. Therefore, we believe that such flexible proton responsive ligand-substituted metal complexes can be promising for base free CO2 hydrogenation reactions.

Na1.5La1.5TeO6: Na+ conduction in a novel Na-rich double perovskite.

Increasing demand for lithium batteries for automotive applications, coupled with the necessity to move to large-scale energy storage systems, is driving a push towards new technologies and has seen Na-ion batteries emerge as a leading alternative to Li-ion. Amongst these, all solid-state configurations represent a promising route to achieving higher energy densities and increased safety. Remaining challenges include the need for Na+ solid electrolytes with the requisite ionic conductivities crucial for use in a solid-state cell. Here, we present the novel Na-rich double perovskite, Na1.5La1.5TeO6. The transport properties, explored at the macroscopic and local level, reveal a low activation energy barrier for Na+ diffusion and great promise for use as an electrolyte for all solid-state Na-batteries.