Effective Activation Barrier Research Articles

Ligand-Coordinated Pt Single-Atom catalyst facilitates Support-Assisted Water-Gas shift reaction

The water–gas shift (WGS) reaction (CO+H2O→CO2+H2,ΔH=-41.1kJ/mol) is an essential process in the production of hydrogen and has grown significantly in usage over time. In this study, we further explore a novel strategy that can create single-atom catalysts (SACs) from metal-ligand single-sites on high surface area oxide supports for catalyzing the WGS reaction. The metal–ligand approach results in high metal loading, exhibits an ultimate dispersion of metal species and maximizes atom efficiency. 1,10-phenanthroline-5,6-dione (PDO) was chosen as the ligand for its oxidative potential for stabilizing Pt metal cations via two different binding pockets. The single-atom nature of the Pt-ligand is characterized with extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and transmission electron microscopy (TEM) techniques. The catalytic activity is evaluated for the WGS reaction, revealing that Pt-ligand SACs supported on defective TiO2 show higher inherent catalytic activity than Pt NPs and a significantly lower activation energy. This characteristic is generally desirable for the redox mechanism. Density functional theory (DFT) calculations reveal that metal–ligand SACs allow CO interaction with oxygen atoms from the TiO2 surface. The redox mechanism of the WGS reaction demonstrates a lower effective activation barrier than the associative mechanism for Pt-ligand SAC. Moreover, the metal–ligand strategy, according to DFT results, facilitates the reaction redox mechanism by reducing the CO binding energy and promoting CO2 and H2 formation process.

Strain-dependence of Te interstitial diffusion in CdTe

While the dominant defects which control non-radiative recombination and long-range interstitial diffusion in CdTe correspond to Cd vacancies and Te anti-sites, the short-range diffusion of Te and Se interstitials between these defects is also of interest, since they both play a role in defect passivation. In addition, since CdTe thin films are typically polycrystalline and may also involve interfaces with materials with different lattice constants, the effects of strain are also of interest. Here we present the results of molecular dynamics (MD) simulations of Te interstitial diffusion in zincblende CdTe for values of the triaxial strain ranging from −2% (compressive) strain to +2.8% (tensile) strain. By carrying out MD simulations of Te interstitial diffusion over a range of temperatures, and then carrying out Arrhenius fits, we have determined the effective activation barrier Ea and prefactor D 0 for each value of the global strain. We find that both Ea and D 0 exhibit non-monotonic behavior, increasing with both compressive and tensile strain. We also present an analysis of the key diffusion pathways for 3 different values of the strain which explains the non-monotonic strain dependence obtained in our simulations. Our results also indicate that in each case, the diffusion of interstitial Te involves a variety of concerted events with a wide range of activation barriers.

Multiscale modeling of dislocation-mediated plasticity of refractory high entropy alloys

Refractory high entropy alloys (RHEAs) have drawn growing attention due to their remarkable strength retention at high temperatures. Understanding dislocation mobility is vital for optimizing high-temperature properties and ambient temperature ductility of RHEAs. Nevertheless, fundamental questions persist regarding the variability of dislocation motion in the rugged energy landscape and the effective activation barrier for specific mechanisms, such as kink-pair nucleation and kink migration. Here we perform systematic atomistic simulations and conduct statistical analysis to obtain the effective activation barriers for the mechanisms underlying various types of dislocation motion in a typical RHEA, NbMoTaW. Moreover, a stochastic line tension model is developed to calculate the activation barrier with substantially reduced computational costs. By incorporating the effective activation barriers into the crystal plasticity model, a multiscale simulation framework for predicting the mechanical properties of RHEAs is established. The ambient temperature yield strength of NbMoTaW is well-predicted by the kink-pair nucleation mechanism of screw dislocations, while the strengthening originating from screw dislocations does not predominate at high temperatures. Our work provides a robust foundation for atomistic studies of effective dislocation behaviors in random solution solids, elucidating the intricate relationship between microscopic mechanisms and macroscopic properties.

Nitroderivatives of N-pyrazolyltetrazoles: Thermal decomposition and combustion

Thermal decomposition and combustion of a series of novel promising nitro and amino substituted energetic pyrazolyltetrazoles were studied using a number of complementary experimental (thermogravimetry, differential scanning calorimetry, manometry, microthermocouple measurements in a combustion wave, and HPLC) and theoretical (quantum chemical calculations) techniques. According to the DSC and manometry data, the N-(pyrazolyl)tetrazoles turned out to be less thermally stable than the C-(pyrazolyl)tetrazole: the activation energies of their melt-phase decomposition are 117–127 and 140 kJ mol−1, respectively. The kinetics of decomposition in the solid phase is more complicated due to a strong influence of the effect of sub-melting and possible azido-tetrazole isomerization. The reliable CCSD(T)-F12 quantum chemical calculations provided mechanistic insight into the thermolysis of the compounds studied. All species decompose via the ring-opening reaction yielding a transient azide intermediate followed by the N2 elimination. The effective activation barriers of the latter reactions are in reasonable agreement with the melt-phase experiments for N-pyrazolyltetrazoles. On the basis of the HPLC detected products, we also outlined the reactions leading to the stable decomposition products observed. All compounds studied showed a self-sustained fast-burning character. Their typical burning rates are 70−80 mm s−1 at 10 MPa, which is as much as two times higher than the burning rate of a powerful explosive CL-20 under the same conditions. Using the pressure dependence of the surface temperature, we also determined the vapor pressures of (nitropyrazolyl)tetrazoles studied. The boiling points of the latter species are higher than the boiling point of hexogen. The melt-phase kinetic parameters also agree well with the condensed-phase-controlled burning rate model. The low activation energies of the leading combustion reactions are responsible for low pressure exponents of these compounds varying in the range of 0.5 – 0.75.

Thermal Stability of Bis-Tetrazole and Bis-Triazole Derivatives with Long Catenated Nitrogen Chains: Quantitative Insights from High-Level Quantum Chemical Calculations.

Azobis tetrazole and triazole derivatives containing long catenated nitrogen atom chains are of great interest as promising green energetic materials. However, these compounds often exhibit poor thermal stability and high impact sensitivity. Kinetics and mechanism of the primary decomposition reactions are directly related to these issues. In the present work, with the aid of highly accurate CCSD(T)-F12 quantum chemical calculations, we obtained reliable bond dissociation energies and activation barriers of thermolysis reactions for a number of N-rich heterocycles. We studied all existing 1,1'-azobistetrazoles containing an N10 chain, their counterparts with the 5,5'-bridging pattern, and the species with hydrazo- and azoxy-bridges, which are often present energetic moieties. The N8-containing azobistriazole was considered as well. For all compounds studied, the radical decomposition channel was found to be kinetically unfavorable. All species decompose via the ring-opening reaction yielding a transient azide (or diazo) intermediate followed by the N2 elimination. In the case of azobistetrazole derivatives, the calculated effective activation barriers of decomposition are ∼26-33 kcal mol-1, which is notably lower than that of tetrazole (∼40 kcal mol-1). This fact agrees well with the low thermal stability and high impact sensitivities of the former species. The activation barriers of the N2 elimination were found to be almost the same for the azobis compounds and the parent tetrazole, and the effective decomposition barrier is determined by the thermodynamics of the tetrazole-azide rearrangement. In comparison with 1,1'-azobistetrazole, the hydrazo-bridged compound is more stable kinetically due to the lack of pi-conjugation in the azide intermediate. In turn, the azoxy-bridged compounds are entirely unstable due to tremendous azide stabilization by the hydrogen bond formation. In general, the 5,5'-bridged species are more thermally stable than their 1,1'-counterparts due to a much higher barrier of the N2 elimination. Apart from this, the highly accurate gas-phase formation enthalpies were calculated at the W1-F12 level of theory for all species studied.

Catalysis at Metal/Oxide Interfaces: Density Functional Theory and Microkinetic Modeling of Water Gas Shift at Pt/MgO Boundaries

The impact of metal/oxide interfaces on the catalytic properties of oxide-supported metal nanoparticles is a topic of longstanding interest in the field of heterogeneous catalysis. The significance of the metal/oxide interaction has been shown to vary according to both the inherent reactivity of the metal nanoparticle and the properties of the oxide support, with effects such as the metal d-band center, the nanoparticle shape, and the reducibility of the oxide believed to contribute to the overall system reactivity. In recent years, the water gas shift (WGS) reaction, wherein carbon monoxide and water are converted to carbon dioxide and hydrogen, has emerged as a model chemistry to probe the molecular-level details of how catalysis can be promoted in such environments, and this reaction is the focus of the present contribution. Using a combination of periodic Density Functional Theory calculations and microkinetic modeling, we present a comprehensive analysis of the WGS mechanism at the interface between a quasi-one dimensional platinum nanowire and an irreducible MgO support. The nanowire is lattice matched to the MgO support to remove spurious strain at the metal/oxide interface, and reactions both on the nanowire and at the three-phase boundary itself are considered in the mechanistic analysis. Additionally, to elucidate the consequences of adsorbate–adsorbate interactions on the WGS chemistry, an ab-initio thermodynamic analysis of CO coverage is performed, and the impact of the higher coverage CO states on the reaction chemistry is explicitly evaluated. These results are combined with detailed calculations of adsorbate entropies and dual-site microkinetic modeling to determine the kinetically significant features of the WGS reaction network which are subsequently, validated through experimental measurements of apparent reaction orders and activation barrier. The analysis demonstrates the important role that the metal/oxide interface plays in the reaction, with the water dissociation step being facile at the interface compared to the pure metal or oxide surfaces. Further, explicit consideration of CO interactions with other adsorbates at the metal/oxide interface is found to be central to correctly determining reaction mechanisms, rate determining steps, reaction orders, and effective activation barriers. These results are captured in a closed-form Langmuir–Hinshelwood model, derived from a simplified version of the complete microkinetic analysis, which reveals, among other results, that the celebrated carboxyl mechanism of Mavrikakis and coworkers is the governing pathway when accounting for reaction-relevant CO coverages.

Dynamic surface properties of aqueous media imparted by a sugar-modified organosiloxane surfactant

A novel tetrasiloxane-tailed surfactant bearing oligo(ethylene oxide) methyl ether and sugar moieties (TGA-2) has been evaluated. The dynamic surface tension (DST) and spreading properties imparted to water by TGA-2 have been investigated. DST parameters, namely effective diffusion coefficient, activation barrier, and contact angle have been calculated and analyzed. A general diffusion mechanism of surfactant TGA-2 has been delineated. DST analyses yielded significantly different long-term (DL) and short-term (DS) diffusion coefficients, implying that the adsorption process involves a mixed diffusion-kinetic mechanism. Adsorption activation energies (Ea) of the surfactant TGA-2 have been measured as 9.14 kJ/mol at 4 × 10−5 M and 19.37 kJ/mol at 4 × 10−4 M. These small Ea values suggest that the tetrasiloxane group is favorable for adsorption. A polytetrafluoroethylene (PTFE) membrane was chosen as a solid substrate to demonstrate the spreading ability imparted by TGA-2. At a TGA-2 concentration of 0.5% in water, the contact angle decreased from 57° to 7.8° in 60 s, such that the droplets became almost flat.

Dynamic Description of the Catalytic Cycle of Malate Enzyme: Stereoselective Recognition of Substrate, Chemical Reaction, and Ligand Release.

In protein engineering, investigations of catalytic cycle facilitate rational design of enzymes. In the present work, deeper analysis on the catalytic cycle of malate enzyme (EC 1.1.1.40), an enzyme involved in cancer metabolic and fatty acid synthesis, was performed. In substrate binding, stereoselective recognition of a substrate originates from distance and angle difference between two chiral substrates and Mn2+ as well as monodentate or coplanar ion reaction with Arg165. In catalytic transformation, the activation barrier for the hydride transfer of d-malate is 20.28 kcal/mol higher than that for l-malate. The activation barrier for β-decarboxylation of oxaloacetate is about 4.59 kcal/mol higher than the activation barrier for the hydride transfer of l-malate. The effective activation barrier is 16.44 kcal/mol, which is in close agreement with the value derived from the application of transition-state theory and the Eyring equation to kcat. In ligand release, l/d-malate needs to overcome a higher barrier than pyruvate to break all bonds in parallel and then to escape from the binding pocket. Leu167 and Asn421 comprise a swinging gate to control the product release. The more open gate is possibly required in the direction of pyruvate to l-malate. Our studies are focused on extending structural knowledge regarding the malate enzyme and provided a powerful strategy for future experimental investigations.

Decomposition Pathways of Titanium Isopropoxide Ti(OiPr)4: New Insights from UV-Photodissociation Experiments and Quantum Chemical Calculations.

The UV-photodissociation at 266 nm of a widely used TiO2 precursor, titanium tetraisopropoxide (Ti(OiPr)4, TTIP), was studied under molecular-beam conditions. Using the MS-TOF technique, atomic titanium and titanium(II) oxide (TiO) were detected among the most abundant photofragments. Experimental results were rationalized with the aid of quantum chemical calculations (DLPNO-CCSD(T) and DFT). Contrary to the existing data in the literature, the new four-centered acetone-elimination reaction was found to be the primary decomposition process of TTIP. According to computational results, the effective activation barrier of this channel was ∼49 kcal/mol, which was ∼13 kcal/mol lower than that of the competing propylene elimination. The former process, followed by the dissociative loss of an H atom, was a dominating channel of TTIP unimolecular decay. The sequential loss of isopropoxy moieties via these two-step processes was supposed to produce the experimentally observed titanium atoms. In turn, the combination of these reactions with propylene elimination can lead to another detected species, TiO. These results indicate that the existing mechanisms of TTIP thermal and photoinitiated decomposition in the chemical-vapor deposition (CVD) of titanium dioxide should be reconsidered.

Importance of metal-oxide interfaces in heterogeneous catalysis: A combined DFT, microkinetic, and experimental study of water-gas shift on Au/MgO

The water-gas shift (WGS) reaction is central to a spectrum of industrially important catalytic processes, ranging from the manufacture of hydrogen to the processing of biomass-derived feedstocks. Recently, oxide-supported Au catalysts have attracted attention for low-temperature WGS reactions, but the role of the Au/oxide interface in promoting this chemistry remains under debate. In this contribution, we combine periodic Density Functional Theory (DFT) calculations, detailed microkinetic modeling, and rigorous kinetic measurements to elucidate the impact of this interface on the molecular-level features of WGS chemistry of a gold nanowire supported on a MgO(100) substrate. The results demonstrate that the barrier to activate water, which is prohibitively high (∼2eV) on a clean Au(111) surface, is decreased to essentially zero at the Au/MgO interface. From the DFT-calculated energetics, a dual-site microkinetic model of the Au/MgO interface is constructed. Rate and thermodynamic control analysis demonstrate both the high degree of kinetic control of COOH formation at the interface and a strong influence of competitive adsorption between CO and H. A procedure to refine the microkinetic predictions by iterative replacement of the energies of kinetically sensitive steps with a higher accuracy hybrid HSE06 prediction is introduced, and the determined effective activation barriers and reaction orders agree well with the results of detailed kinetic measurements on Au nanoparticles on MgO substrates. The results clearly show the critical role that the metal/oxide interface plays in WGS catalysis, and the approach introduced to predict kinetics at the metal/oxide interface should be applicable to a variety of catalytic processes on oxide-supported metal nanoparticles.

Understanding the viscosity of supercooled liquids and the glass transition through molecular simulations

We discuss a potential energy landscape (PEL) approach to calculate the characteristically sluggish viscous behaviour of supercooled liquids. This phenomenon is central to the theoretical understanding of the liquid-to-glass transition, a long-standing challenge in non-equilibrium statistical mechanics of amorphous states of matter. Experimentally, the shear viscosity of supercooled glass-forming liquids shows strong variations with the quench temperature. In particular, two types of behaviour are observed, Arrhenius and highly super-Arrhenius. Conceptually, the molecular description of the structural and dynamical processes underpinning the viscosity behaviour has become a topic of interest because of general implications for the kinetics of slow relaxation in the glassy state. We regard the supercooled liquid as an assembly of interacting particles undergoing thermal fluctuations such that the system evolves in space and time by crossing a series of potential energy barriers. To find these barriers and their corresponding atomic configurations, we apply an energy landscape sampling algorithm that maps out the evolution trajectories in the form of alternating sequences of local energy minima and saddle points. The energy sequences and the atomic coordinates constitute a body of atomic-scale data which we then process using two distinct methods. One is based on an effective activation barrier extracted from the transition-state pathway data, and the other is based on the linear response theory of statistical mechanics. The former is heuristic by being more physically transparent, while the latter is theoretically more rigorous. Through these two complementary calculations, an understanding of the temperature variations of shear viscosities of supercooled liquids, as well as the nature of fragile and strong behaviour of the glass transition, emerges. Our calculation provides a molecular-level account of the viscosities of supercooled liquids in a unified and consistent manner without invoking ad hoc assumptions. Relative to the nature of the glass transition, the usefulness of the PEL perspective is demonstrated, along with the concept of crossover between strong and fragile behaviour. In terms of advancing atomistic simulation capability, we believe the time-scale limitations of traditional molecular dynamics may be significantly extended through the use of metadynamics algorithms for sampling transition-state pathways.

Self-learning kinetic Monte Carlo simulations of Al diffusion in Mg

Vacancy-mediated diffusion of an Al atom in the pure Mg matrix is studied using the atomistic, on-lattice self-learning kinetic Monte Carlo (SLKMC) method. Activation barriers for vacancy-Mg and vacancy-Al atom exchange processes are calculated on the fly using the climbing image nudged-elastic-band method and binary Mg–Al modified embedded-atom method interatomic potential. Diffusivities of an Al atom obtained from SLKMC simulations show the same behavior as observed in experimental and theoretical studies available in the literature; that is, an Al atom diffuses faster within the basal plane than along the c-axis. Although the effective activation barriers for an Al atom diffusion from SLKMC simulations are close to experimental and theoretical values, the effective prefactors are lower than those obtained from experiments. We present all the possible vacancy-Mg and vacancy-Al atom exchange processes and their activation barriers identified in SLKMC simulations. A simple mapping scheme to map an HCP lattice onto a simple cubic lattice is described, which enables simulation of the HCP lattice using the on-lattice framework. We also present the pattern recognition scheme which is used in SLKMC simulations to identify the local Al atom configuration around a vacancy.

Rotation assisted diffusion of water trimers on Pd{111}

Diffusion barriers for a cluster of three water molecules on Pd{111} have been estimated from ab-initio Density Functional Theory. A model for the diffusion of a cluster of three water molecules (trimer) based in rotations yields a simple explanation of why the cluster can diffuse faster than a single water molecule by a factor ≈102 [1]. This model is based on the differences between the adsorption geometry for the three molecules forming the trimer. One member interacts strongly with the surface and sits closer to the surface (d) while the other two interact weakly and stay at a larger separation from the surface (u). The trimer rotates nearly freely around the axis determined by the d-like monomer. Translations of the whole trimer imply breaking the strong interaction of the d-like molecule with the surface with a high energy cost. Alternatively, thermal fluctuations can exchange the position of the molecule sitting closer to the surface with a lower energetic cost. Rotations around different axis yield a diffusion mechanism where the strong interaction is maintained along the diffusion path, therefore lowering the effective activation barrier.

Synergistic effect of aryl heterocyclic aluminum phosphate and sodium laurate on non-isothermal crystallization kinetics of isotactic polypropylene

Kinetics of non-isothermal crystallization of isotactic polypropylene (iPP)/nucleating agent blends was studied for the nucleation mechanism. Avrami, Jeziorny, and correctional Friedman methods were applied to analyze the non-isothermal crystallization of the iPP/aryl heterocylic aluminum phosphate/sodium laurate (iPP/AHP-Al/L-Na) blends. For a given cooling rate of 10°C min−1, the onset temperature ( To) and crystallization temperature ( Tc) of the iPP/AHP-Al/L-Na blends increased by 4.19°C and 3.57°C, respectively, compared to iPP/L-Na. The To and Tc of iPP/AHP-Al/L-Na blends increased by 2.13°C and 1.79°C, respectively, compared to iPP/AHP-Al, when the AHP-Al mass fraction reached 1.2‰. The effective activation barrier (Δ E) for non-isothermal crystallization was the least when the AHP-Al mass fraction was 0.8‰. The value of the initial slope of the exotherm increased with the addition of AHP-Al monotonically. All these findings indicated that AHP-Al could improve the initial crystallization rate and also that the addition of AHP-Al/L-Na reduced the Δ E for non-isothermal crystallization. It is speculated that an ion exchange reaction between AHP-Al and L-Na occurs during the mixing process and could be possible that this results in the generation of a more efficient nucleating agent.

Thermally‐activated post‐growth dewetting of fullerene C60 on mica

Understanding and controlling the growth and stability of molecular thin films on solid surfaces is necessary to develop nanomaterials with well‐defined physical properties. As a prominent model system in organic electronics, we investigate the post‐growth dewetting kinetics of the fullerene C60 on mica with real‐time and in situ X‐ray scattering. After layer‐by‐layer growth of C60, we find a thermally‐activated post‐growth dewetting, where the smooth C60‐layer breaks up into islands. This clearly shows that growth is kinetically limited before the system moves over an activation barrier into an energetically favored configuration. From the temperature‐dependent dewetting kinetics we find an effective activation barrier of 0.33 eV, which describes both the temperature‐dependent macroscopic changes in the surface morphology and the microscopic processes of inter‐ and intralayer diffusion during dewetting. magnified image(© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

The Reaction Mechanism with Free Energy Barriers for Electrochemical Dihydrogen Evolution on MoS2.

We report density functional theory (M06L) calculations including Poisson-Boltzmann solvation to determine the reaction pathways and barriers for the hydrogen evolution reaction (HER) on MoS2, using both a periodic two-dimensional slab and a Mo10S21 cluster model. We find that the HER mechanism involves protonation of the electron rich molybdenum hydride site (Volmer-Heyrovsky mechanism), leading to a calculated free energy barrier of 17.9 kcal/mol, in good agreement with the barrier of 19.9 kcal/mol estimated from the experimental turnover frequency. Hydronium protonation of the hydride on the Mo site is 21.3 kcal/mol more favorable than protonation of the hydrogen on the S site because the electrons localized on the Mo-H bond are readily transferred to form dihydrogen with hydronium. We predict the Volmer-Tafel mechanism in which hydrogen atoms bound to molybdenum and sulfur sites recombine to form H2 has a barrier of 22.6 kcal/mol. Starting with hydrogen atoms on adjacent sulfur atoms, the Volmer-Tafel mechanism goes instead through the M-H + S-H pathway. In discussions of metal chalcogenide HER catalysis, the S-H bond energy has been proposed as the critical parameter. However, we find that the sulfur-hydrogen species is not an important intermediate since the free energy of this species does not play a direct role in determining the effective activation barrier. Rather we suggest that the kinetic barrier should be used as a descriptor for reactivity, rather than the equilibrium thermodynamics. This is supported by the agreement between the calculated barrier and the experimental turnover frequency. These results suggest that to design a more reactive catalyst from edge exposed MoS2, one should focus on lowering the reaction barrier between the metal hydride and a proton from the hydronium in solution.

Heterospin single-molecule magnet behaviors after irradiation of polymorphous 2:2 cyclic diazo-cobalt(II) complexes

Three kinds of crystals (crystals, α, β, and γ) of 2:2 cyclic cobalt complexes formulating as [Co(Br-hfpip)2(D2py2)]2 X: X=2CH2Cl2 (1dα), 4CH2Cl2 (1dβ), and 4CH2Cl2·4EtOH (1dγ) and Br-hfpip=1,1,1,5,5,5-hexafluoro-4-(4-bromophenylimino)-2-pentanonate, were obtained and were determined to be polymorphous by X-ray crystallography, having the space groups of P1¯ for 1dα and 1dβ, and P21/n for 1dγ. In three polymorphs, the local coordination structures of cobalt complex units in the molecule were the same; compressed octahedral. The molecular structure for 1dα was different from those for 1dβ and 1dγ. The dihedral angles between the X–Y plane and the pyridine ring were 43° and 44° for 1dα and 70–80° for 1dβ and 1dγ. In the crystal packings, 1dα had a short contact between the bromo substituent in hfpip ligand and the carbon at the β position of the pyridine ring in D2py2, while 1dβ and 1dγ had no short contact between the magnetically significant atoms. The powder X-ray diffraction (PXRD) patterns for the crushed samples of three polymorphs indicated that the crystal structures were changed by loosing crystal solvent and 1dα and 1dβ were microcrystals (1dα′ and 1dβ′), while 1dγ became amorphous (1dγ′). In dc and ac magnetic measurements after irradiation of crushed samples (1dα′, 1dβ′, and 1dγ′), the photoproducts, 1cα′, 1cβ′, and 1cγ′ exhibited slow magnetic relaxation and magnetic hysteresis. 1cα′ showed a single-molecule magnet (SMM) behavior with the effective activation barrier, Ueff/kB=98K and coercive force, Hc=2.9kOe at 1.9K affected by the intermolecular antiferromagnetic interaction. 1cβ′ showed the magnetic behavior with Ueff/kB=134K and Hc=13kOe at 1.9K typical of heterospin SMM. In 1cγ′, weak χmol″ signals with frequency dependency and the temperature dependent hysteresis loop; Hc=1.1kOe at 1.9K, were observed.

Computational study of the catalytic olefination reaction

Catalytic olefination of hydrazones was computed to proceed with effective activation barrier of 19.9 kcal mol –1 . Initially the catalyst- assisted abstraction of Cl – anion from CCl 4 occurs that is accompanied by simultaneous C–C bond formation; then proton is eliminated by ammonia present in the reaction mixture yielding a neutral intermediate; the second CuCl-assisted abstraction of Cl – anion from CCl 3 group is followed by synchronous reaction that affords the olefination product and releases N 2 together with CuCl.HCl.

Magnetic Properties of 1:2 Mixed Cobalt(II) Salicylaldehyde Schiff-Base Complexes with Pyridine Ligands Carrying High-Spin Carbenes (Scar = 2/2, 4/2, 6/2, and 8/2) in Dilute Frozen Solutions: Role of Organic Spin in Heterospin Single-Molecule Magnets

The 1:2 mixtures of Co(p-tolsal)2, p-tolsal = N-p-tolylsalicylideniminato, and diazo-pyridine ligands, DXpy; X = 1, 2, 3l, 3b, and 4, in MTHF solutions were irradiated at cryogenic temperature to form the corresponding 1:2 cobalt-carbene complexes Co(p-tolsal)2(CXpy)2, with Stotal = 5/2, 9/2, 13/2, 13/2, and 17/2, respectively. The resulting Co(p-tolsal)2(CXpy)2, X = 1, 2, 3l, 3b, and 4, showed magnetic behaviors characteristic of heterospin single-molecule magnets with effective activation barriers, Ueff/kB, of 40, 65, 73, 72, and 74 K, for reorientation of the magnetic moment and temperature-dependent hysteresis loops with a coercive force, Hc, of ∼0, 6.2, 10, 6.5, and 9.0 kOe at 1.9 K, respectively. The relaxation times, τQ, due to a quantum tunneling of magnetization (QTM) were estimated to be 1.6 s for Co(p-tolsal)2(C1py)2, ∼2.0 × 10(3) s for Co(p-tolsal)2(C2py)2, and >10(5) s for Co(p-tolsal)2(CXpy)2; X = 3b, 3l, and 4. In heterospin complexes, organic spins, carbenes interacted with the cobalt ion to suppress the QTM pathway, and the τQ value increased with increasing the Stotal values.

Electrochemical Double Layers in Electrochemical Energy Storage and Conversion: A Multiscale Simulation Study

The nanometric liquid/solid electrochemical interface is the pivotal scale behind the operation principles of low-temperature batteries and fuel cells. It involves ionic transport by diffusion and electro-migration in the electrolyte, surface multi-step REDOX reactions, among other mechanisms. Developing an understanding of its functional role is thus crucial to help on determining the limiting factors of batteries and fuel cells performance and stability [1-3].A very significant amount of work has been carried out since more than 100 years to understand the structure of such interfaces in electrochemical systems (also known as electrochemical double layers -EDL-) but under ideal conditions (equilibrium conditions, absence of REDOX reactions): this is not representative of the non-equilibrium conditions typically found in electrochemical power generators. In this work a new non-equilibrium EDL multiscale model is presented, able to capture the following features of relevance on designing high performant and stable electrolytes in batteries and fuel cells [4]:- the impact of solvent composition, additives and lithium salt concentrations onto the charge-discharge curves in lithium ion and lithium air batteries (LIBs and LABs), and thus discriminating between thermodynamic and kinetic effects;- the impact of the platinum-based catalysts oxidation state onto the structure of the EDL, the ionomer morphology and the effectiveness of the Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells (PEMFCs).The model is supported on an extension of the so-called "Poisson-Nernst-Planck approach" and accounts for solvent orientational polarization effects, finite ion size effects and extended Transition State Theory. REDOX reactions occuring on the surface of the active particles (LIBs and LABs) and catalysts (PEMFCs) are described through an elementary kinetic approach parametrized with DFT-calculated activation energies [5]. The influence of the coverage onto the effective activation barriers is evaluated through on lattice Kinetic Monte Carlo simulations within the Variable Step Size Method (VSSM) describing surface diffusion, reactions between adspecies and adsorption/desorption.Some application cases of this model are discussed: for example, the model reproduces nicely well complex phenomena such as the irreversible Nafion reduction/oxidation peaks observed in cyclic voltammetry experiments in [6] (PEMFC), and it predicts how solvent composition impacts polarization and lithium ion transport properties (LIB/LAB). Finally, a cell device multiscale model integrating these EDL models is presented [7], and the EDL impact onto the overall cell performance dynamics (as function of the applied potentiodynamic and galvanodynamic conditions in PEMFCs, and cycling rate for batteries) is studied. Acknowledgements. Prof. Dominiquer Larcher and Dr. Abdelouahab El Kharbachi (LRCS) are gratefully acknowledged for their experiments devoted to our models validation.