High Activation Enthalpy Research Articles

A one-dimensional model for Pt degradation and precipitation in proton exchange membrane fuel cell considering Pt nucleation, particle size growth, and band formation

Reducing Pt catalysts amount without sacrificing the performance and durability is critical for the commercialization of proton exchange membrane fuel cells (PEMFCs). Pt degradation in the cathode catalyst layer (CCL) decreases the electrochemically active surface area (ECSA), and Pt precipitation, especially Pt band formation, has a significant effect on proton exchange membrane (PEM) stability, resulting in severe PEMFC degradation during long-term operation. In this study, a one-dimensional model is developed focusing on Pt catalyst degradation and precipitation, including Ostwald-ripening, mass loss, precipitation in the PEM from nucleation, particle size growth, to Pt band formation. The model accuracy is comprehensively validated against the corresponding experimental data, including ECSA loss and particle size distribution (PSD) evolution in the CCL under different operating conditions (temperature, relative humidity, and loading mode), as well as the precipitated Pt size distribution characteristics and local potential in the PEM. Using this model, the Pt band formation process is comprehensively analyzed, and the effects of operating conditions on Pt degradation and Pt band formation are investigated in detail. It is found that the nucleation stage is brief, and then the significant inhomogeneity at the size growth stage causes Pt band formation. In addition, the high activation enthalpy of dissolution under low humidity can effectively inhibit Pt degradation, and Ostwald-ripening is the mean cause of Pt degradation, which is likely to occur during the rapid voltage change period.

Influence of crystallographic anisotropy on the electrical conductivity of apatite at high temperatures and high pressures

Abstract The electrical conductivity of apatite single crystals along three main crystalline directions was measured in situ using a YJ-3000t multi-anvil apparatus and a combined system consisting of the impedance/gain-phase analyzer (Solartron 1260) and dielectric interface (Solartron 1296) at 973–1373 K and 1.0–3.0 GPa. The obtained results indicate that the relationship between the electrical conductivity and temperature conforms to the Arrhenius relation. At 2.0 GPa, the electrical conductivity of apatite with relatively high activation enthalpies of 1.92–2.24 eV shows a significant anisotropy with an extremely high anisotropic degree (τ = ~8–16) value. For a given [001] crystallographic orientation, the electrical conductivity of apatite slightly decreases with increasing pressure, and its corresponding activation energy and activation volume of charge carriers are 2.05 ± 0.06 eV and 9.31 ± 0.98 cm3/mol, respectively. All of these observed anomalously high activation enthalpy and positive activation volume values suggest that the main conduction mechanism is related to the monovalent fluorine anion at high temperature and high pressure. Furthermore, three representative petrological average schemes, including the parallel, Hashin-Shtrikman upper bound, and average models were selected to establish the functional relation for the electrical conductivity of the phlogopite-apatite-peridotite rock system along with the volume percentages of apatite ranging from 1 to 10% at 973–1373 K and 2.0 GPa. For a typical Hashin-Shtrikman upper bound model, the electrical conductivity-depth profile for peridotite containing the 10% volume percentage of apatite was successfully constructed in conjunction with our acquired anisotropic electrical conductivity results and available temperature gradient data (11.6 and 27.6 K/km) at depths of 20–90 km. Although the presence of apatite in peridotite cannot explain the high-conductivity anomalies in western Junggar of Xinjiang autonomous region, it may provide a reasonable constraint on those of representative apatite-rich areas.

Lithium ion transport in water-containing Li(SCN) over a wide compositional range: From water doping to hydration

This work explores the Li+ cation transport mechanisms in the system Li(SCN) · x H2O, complementary to our previous studies on crystal structures and hydration properties of lithium thiocyanate Li(SCN). Anhydrous Li(SCN) is a Schottky defective material with lithium vacancies VLi' being the dominant mobile defect. Two water incorporation mechanisms are identified: (i) At very low concentrations H2O acts as a dopant and forms H2OSCN• defects, constituting a substitutive incorporation mechanism. Doping with water not only increases the concentration of VLi' in Li(SCN), but also the vacancy mobility, making H2O-doping a special case. A complete quantitative defect chemical model is derived, including enthalpies and entropies of defect formation, association, and migration. (ii) Higher water activities induce chemical and structural changes in Li(SCN) · x H2O, i.e. hydration by reactive incorporation occurs. Single-phases as well as composites are investigated for their Li+ ion transport mechanisms. In case of x ≤ 1, defect associates determine the conductivity resulting in exceptionally high activation enthalpies. Further hydration to 1 < x < 2 creates composites of solid mono- and dihydrates, in which the low-melting diyhdrate can form percolating pathways and increase the conductivity to values as high as 7 × 10−4 S/cm at room temperature. Further increase of x leads to complete liquefaction. The entire range of x in the Li(SCN) – H2O system was studied regarding ion transport properties, identifying the individual mechanisms and temperature dependences. The results of this work deepen the understanding of hydration effects on solid ion conductors in solid state ionics, and help to elucidate transport phenomena in salt – solvent systems.

High temperature ionic and electronic conductivity in MgPSZ

The high temperature ionic (σi) and n-type electronic conductivity (σn) of 7.5 mol% MgO-doped partially stabilized ZrO2 (7.5MgPSZ) were determined using 4 probe total conductivity (in air, 800–1600 °C), and oxygen concentration cell open circuit voltage measurements (1200–1600 °C). The latter involved two distinct cell configurations (Al or Cr-based electrodes versus air). The σn values of 4.78 × 10−4 S.cm−1 at 1600 °C (extrapolated to pO2 = 1 atm) and activation energy of 424.0 kJ.mol−1 are hereby reported for the first time. The observed ionic conductivity of 0.60 S.cm−1 at 1600 °C and activation enthalpy of 119.5 kJ.mol−1 for the 1400–1600 °C temperature range are both within upper ranges reported for other PSZ materials. This high activation enthalpy suggests a strong defect association between oxide-ion vacancies and acceptor dopant, exceeding typical values observed with trivalent dopants. The reasons for wide literature data ranges on ionic transport are discussed taking into consideration the variable phase content, phase composition, and potential non-equilibrium conditions observed in experiments. The specific case of hysteresis in conductivity versus temperature curves is analyzed in detail. Complementary structural and microstructural characterization, besides thermal expansion behavior, are used to support the analysis of electrochemical performance.

Ordering processes and atomic defects in FeCo

Abstract The present knowledge on self-diffusion and lattice vacancies in near-stoichiometric body-centred cubic FeCo alloys is critically reviewed and related to the establishment of structural order in the alloy after quenching from elevated temperatures. The available data may be consistently explained in terms of the so-called six-jump cycle mechanism of Huntington, Elcock and McCormick, plus the additional concept of ‘pseudo-selfdiffusivity’. It is shown that this leads to an understanding of the observed combinations of high activation enthalpy and large pre-exponential factor in the Fe self-diffusivity on the one hand, and of a lower activation enthalpy and ‘normal’ pre-exponential factor in the Co self-diffusion on the other hand. For some vacancy properties numerical estimates are given. The potential of muon spin rotation measurements for filling existing gaps in our knowledge is emphasized. A critical discussion of the “500 °C anomaly” shows that the explanations attempted in the literature are unsatisfactory. It is proposed that the tetragonality of the compounds FeCo3 and Fe3Co predicted by ab-initio calculations of Díaz-Ortiz et al. may lead to a metastable pattern of antiphase domains on a very small scale, the dissolution of which may cause the anomaly.

Oxygen Diffusion in Platinum Electrodes: A Molecular Dynamics Study of the Role of Extended Defects

AbstractPlatinum serves as a model electrode in solid‐state electrochemistry and as the inert electrode in redox‐based resistive random‐access memory (ReRAM) technology. Experimental work has proposed that oxygen may diffuse faster along platinum's extended defects, but quantitative, unambiguous transport data do not exist. In this study, the diffusion of oxygen atoms in crystalline platinum and along its extended defects is studied as a function of temperature by means of molecular dynamics (MD) simulations with the ReaxFF interatomic potentials. The MD simulations indicate that platinum vacancies trap oxygen atoms, inhibiting their diffusion through the platinum lattice and leading to a high activation enthalpy of diffusion of around 3 eV. This picture of trapping is supported by static density‐functional‐theory calculations. MD simulations of selected dislocations and selected grain boundaries indicate that oxygen diffusion is much faster along these extended defects than through the Pt lattice at temperatures below 1400 K, exhibiting a much lower activation enthalpy of ≈0.7 eV for all extended defects examined. Producing specific electrode microstructures with controlled densities and types of extended defects thus offers a new avenue to improve the performance of ReRAM devices and to prevent device failure.

Cubic Octa-Carbon: Quantum-Chemical Design of Molecular Structure and Potential Way of Its Synthesis from Cubane.

Quantum-chemical calculation of most important parameters of molecular and electronic structures of octa-carbon C8 having cubic form (bond lengths, bond and torsion angles) using CCSD(T)/QZVP and DFT B3PW91/QZVP methods, has been carried out. NBO analysis data and HOMO/LUMO images for this compound are presented, too. Good agreement was found between the structural data obtained using the above two quantum-chemical methods and, also, with corresponding experimental data. Also, the standard thermodynamic parameters of formation of cubic C8 considered here, and namely standard enthalpy ΔfH0(298K), entropy Sf0(298K) and Gibbs’ energy ΔfG0(298K) of formation for this compound were calculated. By using this data, a theoretically possible variant of the synthesis of this compound by dehydrogenation of cubane C8H8 is considered, and the thermodynamic characteristics of each of the four stages of this process have been calculated. It is noted that each of the four stages of this process is characterized by a very high (about 500 kJ/mol) enthalpy of activation, as a result of that, for their realization within a sufficiently short time, the use of appropriate catalysts is necessary.

Role of a Multivalent Ion-Solvent Interaction on Restricted Mg2+ Diffusion in Dimethoxyethane Electrolytes.

The diffusion behavior of Mg2+ in electrolytes is not as readily accessible as that from Li+ or Na+ utilizing PFG NMR, due to the low sensitivity, poor resolution, and rapid relaxation encountered when attempting 25Mg NMR. In MgTFSI2/DME solutions, "bound" DME (coordinating to Mg2+) and "free" DME (bulk) are distinguishable from 1H NMR. With the exchange rates between them obtained from 2D 1H EXSY NMR, we can extract the self-diffusivities of free DME and bound DME (which are equal to that of Mg2+) before the exchange occurs using PFG diffusion NMR measurements coupled with analytical formulas describing diffusion under two-site exchange. The high activation enthalpy for exhange (65-70 kJ/mol) can be explained by the structural change of bound DME as evidenced by its reduced C-H bond length. Comparison of the diffusion behaviors of Mg2+, TFSI-, DME, and Li+ reveals a relative restriction to Mg2+ diffusion that is caused by the long-range interaction between Mg2+ and solvent molecules, especially those with suppressed motions at high concentrations and low temperatures.

Unveiling the Substituent Effects in the Stereochemistry of [3+2] Cycloaddition Reactions of Aryl‐ and Alkyldiazomethylphosphonates with Norbornadiene within a MEDT Perspective

AbstractThe [3+2] cycloaddition (32CA) reactions of norbornadiene with aryl and alkyl substituted diazomethylphosphonates (SDAPs) have been studied within the molecular electron density theory (MEDT). Electron localization function (ELF) shows the presence of apseudoradicalC3 center at the alkyl SDAPs, classifying them aspseudo(mono)radicalspecies, while the aryl SDAP is classified as a zwitterionic one, owing to the delocalization of the C3 electron density on the C3‐P bond. These 32CA reactions present high activation enthalpies in dichloromethane, between 17.3 and 21.6 kcal mol−1, being strongly exothermic by more than 33.8 kcal mol−1. While these 32CA reactions present a totalsynfacial selectivity, theendo/exostereoselectivity depends on the steric hindrance caused by the bulk substituents present in these SDAPs along the approach to norbornadiene. Interestingly the phenyl group provokes a lesser hindrance than that of the small methyl group, justifying the 95 % ofexostereoselectivity experimentally observed. ELF and the AIM (Atoms‐in‐Molecules) analyses account for the bonding changes along these 32CA reactions. The energy cost for the rupture of the norbornadiene C−C and that of the SDAPs N−N double bonds are the major contributors to the activation energies of these non‐polar 32CA reactions.

Closing in on the heat‐activation mechanisms of TRPV channels

The sensation of heat is mediated by cation-permeable ion channels of the thermoTRP kind. Among these, some of the principal heat-activated ion channels are formed by the TRPV1–4 members of the TRPV subfamily (Latorre et al. 2009). The mechanism by which these membrane proteins convert thermal energy to a channel-opening conformational change is still being intensely investigated and for the most part is not well understood. The availability of high-resolution structures provided by the cryo-EM revolution has made it easier to propose structure–function hypotheses to study heat activation. However, high quality functional studies based in electrophysiology are still the benchmark to prove mechanisms for ion channels. Quantifying the contribution of a channel region to the heat-absorbing step is a difficult task. So far, a good experimental approximation is the estimation of the activation enthalpy associated with the opening of the channel, measured from the steepness of the channel open probability as a function of temperature. In previous seminal work (Yao et al. 2011), the laboratory of Feng Qin identified a so-called membrane proximal domain (MPD), located between the first transmembrane domain and the N-terminal ankyrin repeat domains in all TRPV channels. The authors produced evidence which suggests that the MPD is a main determinant of the activation enthalpy in TRPV1–4 thermoTRP channels. Importantly, the suggestion that this region is involved in a heat-absorbing conformational change was supported by careful measurement of the apparent enthalpy. In the present work, Liu and Qin (2021) further dissect the contribution of the components of the MPD and of individual amino acid residues within the MPD to the apparent enthalpy of activation in TRPV1 and TRPV2 channels. These important new results begin to paint a detailed picture of the fine-tuning involved in channel activation by temperature. TRPV2 channels are the thermoTRP channels with the highest enthalpy of activation –more than double the activation enthalpy of TRPV1 – and are also activated by very high temperatures (>50°C compared to >40°C for TRPV1). Interestingly, the activation of TRPV2 is also different from TRPV1 in that it shows marked hysteresis. TRPV2 channels activate steeply after initial temperature stimuli that do not activate a large fraction of current. Regardless of these differences, Liu and Qin make use of a unique heat-activation technique that relies on rapid infrared laser activation of currents and of chimeric constructions in which parts of the MPD region are swapped between both types of channels to successfully identify a helix–turn–helix motif comprising 10 residues in TRPV2 and 11 residues in TRPV1 that is part of the MPD. Transplantation of this motif between the two channels is enough to produce chimaeras that activate with the enthalpy of activation of the donor channel. Further mutagenesis experiments identified single residues that are implicated in the high temperature dependence of activation of TRPV2. Of particular interest is a serine present in TRPV1 and absent in TRPV2 and TRPV3, another high temperature-activated channel. Reintroduction of the serine in the 10-residue motif of TRPV2 reduced the apparent enthalpy of activation to values comparable to TRPV1. The fact that a single serine is capable of such a large effect suggests that this serine drastically modulates the local structure of the motif or its local interactions or both. According to cryo-EM structures of TRPV2 (Zubcevic et al. 2016), this helix–turn–helix motif is poised to interact with the S2–S3 intracellular loop and the functionally important TRP domain helix. What is the function of the helix–turn–helix in the complete gating cycle of thermoTRP channels? One possibility is that it is part of a protein domain involved in heat absorption, a heat sensor of sorts. As discussed by the authors, this possibility is unlikely due to its small size and the fact that it is generally thought that a large number of interactions are needed to produce high temperature sensitivity. Another possible function for this motif is as a coupling region, capable of transmitting a heat-induced conformational change initiated in other regions of the channel to the activation gate formed by the S6 transmembrane segments. Careful functional experiments such as the ones reported in the present work as well as detailed structural determination of activated and deactivated states will be needed in the future in order to fully understand heat activation in TRPV channels. Meanwhile, these experiments remind us of the immense usefulness of careful electrophysiology. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. None declared. Sole author. Funded by grant DGAPA-PAPIIT-UNAM No. IN215621.

Oxygen transport in single-crystal LaAlO3 substrates

The rates of oxygen incorporation into, and oxygen diffusion in, cubic, nominally undoped ${\mathrm{LaAlO}}_{3}$ were investigated by means of isotope exchange experiments and subsequent time-of-flight secondary ion mass spectrometry analysis. $^{18}\mathrm{O}$ diffusion profiles were introduced into single-crystal samples by annealing in isotope-enriched molecular oxygen ($^{18}\mathrm{O}_{2}$) at various temperatures in the range $873\phantom{\rule{4.pt}{0ex}}\text{K}\ensuremath{\le}T\ensuremath{\le}1173\phantom{\rule{4.pt}{0ex}}\text{K}$ at an oxygen activity of $a{\mathrm{O}}_{2}=0.2$ or at various oxygen activities $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\ensuremath{\le}a{\mathrm{O}}_{2}\ensuremath{\le}1$ at $T=983\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. All measured isotope profiles consisted of a sharp drop at the surface, attributed to hindered diffusion through a depletion space-charge zone, followed by a shallow decrease, attributed to faster diffusion in a uniform bulk phase. A numerical description of an entire isotope profile with a single solution of the diffusion equation yielded the tracer diffusion coefficient ${D}^{*}$, the surface exchange coefficient ${k}^{*}$, and the surface space-charge potential ${\mathrm{\ensuremath{\Phi}}}_{0}$. The relatively high activation enthalpy of oxygen diffusion of $\mathrm{\ensuremath{\Delta}}{H}_{{D}^{*}}=(0.90\ifmmode\pm\else\textpm\fi{}0.06)\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ was attributed to oxygen vacancies interacting with cation vacancies. The activation enthalpy for oxygen surface exchange was surprisingly low ($\mathrm{\ensuremath{\Delta}}{H}_{{k}^{*}}&lt;3\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) in view of the large band gap of ${\mathrm{LaAlO}}_{3}$ ($\ensuremath{\approx}\phantom{\rule{0.28em}{0ex}}5.6\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$).

Ligand Exchange Reaction in [Co4O4]-Cobalt Cubane: A Versatile Strategy Towards the Preparation of Cobalt Cubane-based Functional Small Molecules and Polymeric Materials

Herein, we report the mechanistic and application study of ligand exchange reaction in Co4O4-cobalt cubane, a well-known water oxidation catalyst (WOC). The ligand exchange reaction involves the instantaneous dissociation of attached pyridine ligand and replacement of target ligand in an appropriate solvent. The kinetic and thermodynamic parameters were quantified for the ligand exchange reaction under mild conditions. Activation parameters revealed that the ligand exchange mechanism follows dissociative (D) or interchange-dissociative (Id) pathway in case of Co4O4, as it is evident by the high activation enthalpy (ΔH‡) and the positive activation entropy (ΔS‡) values. The effects of ligand derivatization, reaction temperature and type of the solvent on the exchange kinetics and the product purity were also investigated in detail. Furthermore, our theoretical studies disclosed the most probable ligand exchange pathway in case of Co4O4. Finally, this methodology was successfully applied to the preparation of Co4O4-containning functional poly-4vinylpyridine (P4VP) and cross-linked heterogeneous polymeric catalysts. The present mechanistic study clearly demonstrates that the ligand exchange method is an efficient and versatile method for the preparation of Co4O4-based functional materials.

Infinite dilution in doped ceria and high activation energies

Oxygen ion diffusion determines the performance of materials in energy conversion, energy storage and catalysis. For nominally pure cerium oxide, experiments measure high activation enthalpies while calculations predict low activation enthalpies. Moreover, for doped oxides, e.g. doped ceria, experiments show a high activation enthalpy for both pure ceria and for high dopant fractions, leading to a minimum in activation enthalpy for small dopant fractions. While for high dopant fractions the increase in activation enthalpy is correlated with the association of oxygen vacancies and dopant ions, which are both created by doping, the minimum in activation enthalpy is assumed in the literature to be related to the maximum in ionic conductivity at similar dopant fractions. In this study, density functional theory (DFT) calculations and Kinetic Monte Carlo (KMC) simulations are combined in order to calculate the ionic conductivity and activation enthalpy in doped oxides. We show that the experimental ionic conductivity and activation energy in nominally pure cerium oxide is dominated by impurities. We resolve the discrepancy between activation enthalpies of nominally pure oxides in experiments as opposed to calculations. This will lead to a more comprehensive understanding of the oxygen ion conductivity and its underlying atomistic mechanisms. Moreover, such a focus will be of great benefit to the future development of sustainable and efficient materials.

H‐D Interdiffusion in Single‐Crystal Olivine: Implications for Electrical Conductivity in the Upper Mantle

AbstractKnowledge of water content and distribution in the Earth's mantle is critical to understanding the geochemical evolution and geodynamic processes of the Earth, since water can incorporate into nominally anhydrous minerals at high pressure and dramatically affect the chemical and physical properties of mantle minerals. Hydrogen diffusion controls the transport of water and electrical conductivity in the deep Earth but is not fully understood for olivine, the most abundant mineral in the upper mantle. Here we present new hydrogen self‐diffusion coefficients determined from interdiffusion in H‐ and D‐doped olivine single‐crystal couples at the upper mantle conditions (3–13 GPa and 1,000–1,300 K). Present activation enthalpy for hydrogen migration is significant smaller than previous work determined within a limited measured temperature range. Parallel interdiffusion experiments with diversified water concentrations demonstrated that hydrogen diffusivity strongly accelerated by the water content in olivine. The geometric average diffusion coefficient on olivine is showed as a function of temperature and water content: . Combined with the Nernst‐Einstein relation, the present results can constrain the contribution of water to the electrical conductivity on olivine. It suggests that in situ conductivity measurements on hydrous olivine at low temperatures (&lt;1,000 K) produced too low activation enthalpy to extrapolate to the higher temperatures. Comparison with previous results by conductivity measurements on single‐crystal olivine suggests that the literature data except for Dai and Karato (2014) might overestimate water effect on conductivity because of heterogeneity of synthetic single crystals. Because of a change of dominant hydrogen diffusion mechanism at high temperature, this study suggests that the modeling of mantle conductivity with a high activation enthalpy from diffusion data is more trustworthy. Considering a reevaluated activation enthalpy on hydrogen diffusion and water solubility in olivine, comparisons between present conductivity model and geophysical observations suggest that hydration of olivine cannot account for extremely high conductive values (10−2–10−1 S/m) observed in the oceanic asthenosphere.

The Mysticism of Pericyclic Reactions: A Contemporary Rationalisation of Organic Reactivity Based on Electron Density Analysis

The molecular mechanism of the five most representative “pericyclic reactions” has been studied by applying molecular electron density theory (MEDT). The different phases into which the reaction paths are topologically divided can be regrouped into four well‐characterised steps: 1) the rupture of the C–C double bonds, 2) the formation of pseudoradical centres at the interacting carbons, 3) the formation of new C–C single bonds and 4) the formation of new C–C double bonds in the final products. Consequently, the bonding changes in these reactions are neither concerted nor cyclic. As the transition‐state structures are located at the end of the large first group of phases or in the narrow second group, the high activation enthalpies found in these reactions are mainly associated with the rupture of C–C double bonds. The present MEDT study makes it possible to rule out the definition of “pericyclic reactions”, made by Woodward and Hoffmann in 1969, in which “all first order changes in bonding relationships take place in concert on a closed curve”.

Understanding the Intramolecular Diels‐Alder Reactions of N‐Substituted N‐Allyl‐Furfurylamines: An MEDT Study

AbstractThe intramolecular Diels‐Alder (IMDA) reactions of N‐allyl‐furfurylamine (1a) and N‐trityl‐allyl‐furfurylamine (1b), were studied within the molecular electron density theory (MEDT) using density functional theory method [B3LYP/6‐31G(d)]. In spite of the high activation enthalpies, the low unfavourable activation entropies associated to these intramolecular processes permit these IMDA reactions to take place. The IMDA reaction of 1a is thermodynamically unfavourable. The presence of the bulky −CPh3 group in the amine nitrogen atom that destabilises the extended conformation of 1b turns the process into an exergonic one. This behaviour does not only affect the thermochemistry of the reaction, but also the kinetic parameters, thus accelerating the reaction. Electron localisation function topological analysis of the C−C single bond formation along the IMDA reaction of 1a shows a bonding pattern similar to non‐polar intermolecular Diels‐Alder reactions. The present MEDT study explains the experimental results; although the steric buttress is able to change the direction of these reversible IMDA reactions, this change is only possible due to the aromatic nature of the furanyl diene system.

Comparison of Cu and Pt point-contact electrodes on proton conducting BaZr0.7Ce0.2Y0.1O3−δ

The hydrogen oxidation kinetics of Cu and Pt point contact-electrodes on proton conducting BaZr0.7Ce0.2Y0.1O3−δ (BZCY72) were studied over a range of temperatures and hydrogen pressures using impedance spectroscopy. Characteristic capacitances were used to identify process contributions as charge and mass transfer, then Langmuir adsorption theory and Butler-Volmer charge transfer formalism were used to propose a hydrogen oxidation model to describe the experimental data. The charge transfer hydrogen pressure dependencies were pH23/4 for Cu, attributed to a high occupancy of adsorbed oxygen at three-phase boundary sites in the water-vapor containing atmosphere, and pH2-1/4 for Pt, corresponding to a hydrogen saturated interface. The Cu and Pt point contact electrodes exhibit similar temperature dependencies for charge transfer, with activation enthalpies of 0.82 and 0.93eV, and pre-exponentials of approximately 160 and 1000 Ω−1cm−1, respectively. Mass transfer dominated the total polarization resistance of both metal point contact electrodes, exhibiting a pH21/2 hydrogen pressure dependency. The activation enthalpies are 1.21eV for Cu and 0.73eV for Pt, reflecting the higher catalytic activity generally expected for Pt. Pre-exponentials for mass transfer of approximately 10,000 Ω−1cm−1 for Cu and 2.6 Ω−1cm−1 for Pt, indicate that Cu utilizes a much larger active interface. Cu may be a suitable candidate electrode material for use in carbonaceous atmospheres, but the results of this work indicates that it suffers from a high activation enthalpy for mass transfer in the hydrogen oxidation reaction.

Understanding the reaction mechanism of the Lewis acid (MgBr2)-catalysed [3+2] cycloaddition reaction between C-methoxycarbonyl nitrone and 2-propen-1-ol: a DFT study

The mechanism of the non-catalysed and the MgBr2-catalysed [3+2] cycloaddition (32CA) reactions between C-methoxycarbonyl nitrone and 2-propen-1-ol has been theoretically investigated within the molecular electron density theory using DFT methods at the B3LYP/6-31G(d) computational level. Analysis of DFT reactivity indices allows explaining the role of the MgBr2 Lewis acid in the catalysed 32CA reaction. The 32CA reaction between C-methoxycarbonyl nitrone and 2-propen-1-ol takes place with a relative high activation enthalpy, 13.5 kcal mol−1, as a consequence of the non-polar character of this zw-type 32CA reaction. Coordination of the MgBr2 LA to C-methoxycarbonyl nitrone accelerates the corresponding zw-type 32CA reaction by taking place through a polar mechanism and with lower activation enthalpy, 8.5 kcal mol−1. Both 32CA reactions, which take place through a one-step mechanism, are completely meta regioselective and present low exo stereoselectivity, which increases in the catalysed process. Energy and non-covalent interaction analyses at the transition-state structures indicate that the formation of an intramolecular H–Br hydrogen bond in the catalysed process could be responsible for the exo selectivity experimentally observed.

Computational Study of Oxygen Diffusion along a[100] Dislocations in the Perovskite Oxide SrTiO3.

We used classical molecular-dynamics simulations to study the atomistic structure of, and the diffusion of oxygen ions along, the periodic array of edge dislocations comprising a symmetrical 6.0° [100] tilt grain boundary in SrTiO3. The results indicate that, at elevated temperatures, the two types of dislocation core (TiO2-type and SrO-type) that make up the boundary are stable and that oxygen-deficient cores maintain their dissociated structures. They also confirm that oxygen vacancies prefer to reside at the cores rather than in the bulk. Tracer diffusion coefficients of oxygen were obtained for oxygen-deficient bulk and grain-boundary simulation cells at temperatures in the range of 1000 ≤ T/K ≤ 2300. Calculated values of the oxygen-vacancy diffusion coefficient for the bulk phase agree extremely well with published experimental data. Tracer diffusion coefficients obtained for the grain-boundary cell are, in comparison to those for the bulk, lower in magnitude and have a higher activation enthalpy, indicating that, relative to the bulk, the migration of oxygen ions along a[100] dislocation cores in SrTiO3 is hindered. These results provide further support for the decoupled model of filament formation in resistively switching SrTiO3.

The Contribution of the Activation Entropy to the Gas-Phase Stability of Modified Nucleic Acid Duplexes.

Tricyclo-DNA (tcDNA) is a sugar-modified analogue of DNA currently tested for the treatment of Duchenne muscular dystrophy in an antisense approach. Tandem mass spectrometry plays a key role in modern medical diagnostics and has become a widespread technique for the structure elucidation and quantification of antisense oligonucleotides. Herein, mechanistic aspects of the fragmentation of tcDNA are discussed, which lay the basis for reliable sequencing and quantification of the antisense oligonucleotide. Excellent selectivity of tcDNA for complementary RNA is demonstrated in direct competition experiments. Moreover, the kinetic stability and fragmentation pattern of matched and mismatched tcDNA heteroduplexes were investigated and compared with non-modified DNA and RNA duplexes. Although the separation of the constituting strands is the entropy-favored fragmentation pathway of all nucleic acid duplexes, it was found to be only a minor pathway of tcDNA duplexes. The modified hybrid duplexes preferentially undergo neutral base loss and backbone cleavage. This difference is due to the low activation entropy for the stranddissociation of modified duplexes that arises from the conformational constraint of the tc-sugar-moiety. The low activation entropy results in a relatively high free activation enthalpy for the dissociation comparable to the freeactivation enthalpy of the alternative reaction pathway, the release of a nucleobase. The gas-phase behavior of tcDNA duplexes illustrates the impact of the activation entropy on the fragmentation kinetics and suggests that tandem mass spectrometric experiments are not suited to determine the relative stability of different types of nucleic acid duplexes. Graphical Abstract ᅟ.