Experimental Activation Energy Research Articles

Kinetics of the chromium(III)/l‐glutamic acid complexation reaction: Formation, decay, and UV‐vis spectrum of a long‐lived intermediate

AbstractThe kinetics of the aqueous reaction of Cr(III) with either l‐glutamic acid or sodium hydrogen l‐glutamate at pH 2.46‐5.87 have been followed by means of absorbance readings. The rate of formation of the reaction products showed acceleration‐deceleration periods, caused by the accumulation and posterior decay of an intermediate in nonnegligible concentration. A double‐exponential integrated rate law allowed obtaining two rate constants for each absorbance‐time experimental series, associated with the appearance (k1) and decay (k2) of the long‐lived intermediate. An increase of the initial concentrations of either hydrogen l‐glutamate (apparent kinetic orders < 1) or hydroxide (kinetic orders = 1) ions resulted in an increase of both k1 and k2, but addition of an inert electrolyte (KNO3) resulted in opposite effects on k1 (decrease) and k2 (increase). The experimental activation energies were 83 ± 10 (for k1) and 95 ± 5 (for k2) kJ mol−1. The electronic spectrum of the low reactivity detected intermediate resembled more closely to that of the blue/green reactant than that of the violet reaction product. The low number of protons set free by the complexating hydrogen l‐glutamate ligand seems to suggest that some polymerization of the coordinated amino acid (to form a di‐ or tripeptide) might take place. The available experimental data indicate that the coordination of the organic ligand must be preceded by the breakdown of a strong Cr(III)–H2O chemical bond in the slow steps of the mechanism.

OXIDATION KINETICS OF SULFIDE COPPER BEARING PELLETS

This paper studies the results of investigating oxidation kinetics of pellets from copper concentrate produced by Gaisky GOK under conditions of controlled convective heat and mass exchange at the minimum radiation component of heat transfer between heated (450–700 °С) gaseous oxidant (air) and pellets. It was found that as the air consumption increases, the initial oxidation decelerates though theoretically it was proved that the oxidant contact with pellet surface does not limit the process. The paper includes formulas deduced by the author for calculating the period of pellet heating up to temperature of initial oxidation and heated air. It was determined that the higher mechanical strength of pellets decreases the speed and completeness of pellet oxidation. The positive effect of increased oxidant (air) consumption on the oxidation speed due to the lack of external diffusion slow-down is explained by a shorter pellet heating period. Experimental activation energy (Е = 155,1÷337,1 kJ/mole) is typical for kinetic or transition mode at 450–500 °С and pellet strength 718,1 and 932,0 N, and for diffusion mode (E = 33,3÷57,4 kJ/mole) at 500–700 °С. The ratio of efficient diffusion in active layer pores was calculated for an isometric part of the kinetic curve at 600 °C. The indicated part shows Knudsen diffusion.

Evaluation of dry reforming reaction catalysts via computational screening

In this work, a computational screening method introduced in a previous report is employed to determine the optimal operating temperature and reactant feed ratio (CO2/CH4) for catalysts used in the dry reforming reaction of methane aiming for high reactivity and stability. It is found that changes in temperature would affect the rate of the main reaction, coke formation and coke removal, while changes in the feed ratio could modify the boundary dividing coke formation from removal zones. The increase in CO2/CH4 ratio is found to expand the coke removal zone. Four strategies based on stability and reactivity ratings (RT-S and RT-R) of the catalysts are proposed. Type 1, high RT-S and RT-R catalysts benefit from high T and high CO2/CH4 since these promote high rate of reaction and coke removal; Type 2, high RT-S but low RT-R catalysts need the same condition as Type 1 except that the maximum achievable reaction and coke removal rate are lower than Type 1; Type 3, low RT-S but high RT-R catalysts which are located in the coke formation zone should be treated first with high CO2/CH4 to shift them into the coke removal zone before applying the same treatment as in Type 1 by increasing the temperature. If the shift is not possible, the alternative is to operate at low T and low CO2/CH4 to avoid high coke formation; Type 4, low RT-S and RT-R catalysts exhibit the least reactivity and coke-resistance and as a result, the best operating condition that could be used is to operate them at low T and low CO2/CH4 to attain lowest rate of coke formation. The ratings concept extended to the interpretation of experimental data in order to predict the stability of the catalysts is demonstrated on four Pt supported (Pt/SiO2, Pt/TiO2, Pt/Cr2O3 and Pt/ZrO2) and Rh supported catalysts (Rh/TiO2, Rh/Al2O3, Rh/MgO and Rh/SiO2). For these catalysts, experimental apparent activation energies of methane consumption were used as input parameters to predict the rate of coke formation. For the Pt catalysts, the predicted trends of coke formation rate via the ratings concept are in good agreement with the measured carbon deposition rate trend calculated from temperature-programmed hydrogenation: Pt/SiO2 > Pt/Cr2O3 > Pt/TiO2 > Pt/ZrO2. However, in the case of Rh, the measured carbon deposition via the temperature-programmed oxidation could not be well predicted by our extended ratings concept. Thus, the ratings concept is an effective screening tool for reactive and stable DRR catalysts, however further development of the concept via the incorporation of more experimental parameters would make applications to general catalysts achievable.

The missing agostomer in the fluxionality of cyclohexenylmanganese tricarbonyl

Fluxionality of the cyclohexenylmanganese tricarbonyl complex [(C6H9)Mn(CO)3], which has an Mn-H-C 3-center 2-electron agostic interaction, have been analyzed with density functional theory (DFT) computations. The fluxionality of this complex is a classic in organometallic chemistry (as highlighted by its inclusion as a study problem in the textbook by Kegley and Pinhas, Problems and Solutions in Organometallic Chemistry), and we provide new insights into its chemistry. Besides the three previously reported fluxional processes [1,3-endo fast exchange (TS-1), CO ligand equivalence (TS-2), and 1,2-metal migration via Cs η4 diene hydride species (TS-3)], two additional processes were identified: 1,2-agostic isomerization (TS-1-2), and a low-energy hydride transfer process (TS-1-9). This investigation uncovered a previously unidentified agostic interaction in the fluxionality of (C6H9)Mn(CO)3: the 1,2-agostic isomerization leads to a previously uncharacterized “closed” Cs agostomer (structure 2) with a Mn-H distance of 2.04 Å, a Mn-H-C bond angle of 104.3°, and the 1H NMR chemical shift of −4.7 ppm. Energetically-similar agostic isomers have previously been defined as “agostomers”. The variable-temperature 1H NMR spectra were also simulated. Excellent agreement is observed (R2 = 0.9949) when comparing the computed and experimental 1H NMR chemical shifts. Reasonable agreements between the computed activation energies (6.1, 12.9, and 13.8 kcal mol−1) and experimental activation energies (8.3 ± 0.3, 13.1 ± 0.1, and 15.4 ± 0.3 kcal mol−1) were found.

DFT rationalization of the room-temperature luminescence properties of Ru(bpy) 3 2+ and Ru(tpy) 2 2+ : 3MLCT–3MC minimum energy path from NEB calculations and emission spectra from VRES calculations

Extensive experimental data covering 40 years of research are available on Ru(bpy) 3 2+ and Ru(tpy) 2 2+ , which are the archetypes of inorganic photochemistry. The last decade has enabled computational chemists to tackle this topic through density functional theory and to shed some new light on our old friends. For the first time, this theoretical study maps the minimum energy path linking the 3MLCT (metal-to-ligand charge transfer) and the 3MC (metal-centred) states with the nudged elastic band method, also providing the calculation of the corresponding energy barrier. Remarkably, the obtained data are in very good agreement with the experimental activation energies reported from variable-temperature luminescence measurements. Calculation of vibrationally resolved electronic spectra is also in excellent agreement with the experimental emission maximum and bandshape of Ru(bpy) 3 2+ . Additionally, the 3MC–GS minimum energy crossing point was optimized for each complex. The combination of these data rationalizes the room-temperature luminescence of the bpy complex and non-luminescence of the tpy complex.

Phosphate-Catalyzed Succinimide Formation from Asp Residues: A Computational Study of the Mechanism.

Aspartic acid (Asp) residues in proteins and peptides are prone to the non-enzymatic reactions that give biologically uncommon l-β-Asp, d-Asp, and d-β-Asp residues via the cyclic succinimide intermediate (aminosuccinyl residue, Suc). These abnormal Asp residues are known to have relevance to aging and pathologies. Despite being non-enzymatic, the Suc formation is thought to require a catalyst under physiological conditions. In this study, we computationally investigated the mechanism of the Suc formation from Asp residues that were catalyzed by the dihydrogen phosphate ion, H2PO4−. We used Ac–l-Asp–NHMe (Ac = acetyl, NHMe = methylamino) as a model compound. The H2PO4− ion (as a catalyst) and two explicit water molecules (as solvent molecules stabilizing the negative charge) were included in the calculations. All of the calculations were performed by density functional theory with the B3LYP functional. We revealed a phosphate-catalyzed two-step mechanism (cyclization–dehydration) of the Suc formation, where the first step is predicted to be rate-determining. In both steps, the reaction involved a proton relay mediated by the H2PO4− ion. The calculated activation barrier for this mechanism (100.3 kJ mol−1) is in reasonable agreement with an experimental activation energy (107 kJ mol−1) for the Suc formation from an Asp-containing peptide in a phosphate buffer, supporting the catalytic mechanism of the H2PO4− ion that is revealed in this study.

Co-firing of blends of sugarcane bagasse and coal

The interaction effects between sugarcane bagasse and a Brazilian coal during co-firing were investigated by means of thermal decomposition behavior, comparison between theoretical and experimental results, activation energy, and ignition temperature. The blends were prepared in the ratios of 100:0; 75:25; 50:50; 25:75; 0:100 (bagasse/coal). The interaction effect evaluated in this study was related to the interference of the bagasse volatile matter content in the coal thermal decomposition. The thermal decomposition behavior analyses were performed in a thermogravimetric balance, and the apparent activation energy was determined by two different models—model-free and local linear integral isoconversional method—under two different heating rate ranges. The results showed that the high volatile content of the sugarcane bagasse leads to more intense combustion, lower ignition temperature, and more complex reaction mechanism, as compared to coal. When the fuels are blended, there is a temperature anticipation of the events related to the decomposition of the coal portion in the mixture, the reaction rates increase and the ash formation is affected. The kinetic data also suggested that the interaction between both materials may occur and improve the burnout of the blend in relation to the pure coal firing due to the contribution of sugarcane bagasse volatile matter. Nevertheless, the presence of the bagasse did not allow to lower activation energy during the blends devolatilization process.

Experimental and theoretical comparison of Sb, As, and P diffusion mechanisms and doping in CdTe

Fundamental material doping challenges have limited CdTe electro-optical applications. In this work, the As atomistic diffusion mechanisms in CdTe are examined by spatially resolving dopant incorporation in both single-crystalline and polycrystalline CdTe over a range of experimental conditions. Density-functional theory calculations predict experimental activation energies and indicate that As diffuses slowly through the Te sublattice and quickly along GBs similar to Sb. Because of its atomic size and associated defect chemistry, As does not have a fast interstitial diffusion component similar to P. Experiments to incorporate and activate P, As, and Sb in polycrystalline CdTe are conducted to examine if ex situ Group V doping can overcome historic polycrystalline doping limits. The distinct P, As, and Sb diffusion characteristics create different strategies for increasing hole density. Because fast interstitial diffusion is prominent for P, less aggressive diffusion conditions followed by Cd overpressure to relocate the Group V element to the Te lattice site is effective. For larger atoms, slower diffusion through the Te sublattice requires more aggressive diffusion, however further activation is not always necessary. Based on the new physical understanding, we have obtained greater than 1016 cm−3 hole density in polycrystalline CdTe films by As and P diffusion.

Theoretical calculations of homogeneous catalysis in the gas phase: elimination kinetics of 2,2-dimethoxypropane in the presence of HCl, F3CCOOH and CH3COOH

ABSTRACTThe mechanisms of gas-phase elimination kinetics of 2,2-dimethoxypropane in the presence of hydrogen chloride, trifluoroacetic acid and acetic acid were studied using Moller Plesset, ab initio combined method Complete Basis Set (CBS)-QB3 and various density functional theory methods with 6-311G(d,p) and 6-311++G(d,p) basis sets. The M06-2X/6-311++G(d,p) method provided reasonable agreement with the experimental enthalpy and energy of activation. Formation of 2-methoxypropene and methanol products occurs through six-membered cyclic ring transition state (TS) structure. The TS was characterised by single imaginary frequency, and confirmed through intrinsic reaction coordinate (IRC) calculations. The IRC calculations suggest the development of a van der Waal complex between the 2, 2-dimethoxy propane and the acid catalyst, leading to the TS formation. The process of decomposition in the absence of the acid catalyst requires much higher temperature with an energy of activation above 200 kJ/mol. This fact appears to be a consequence of a four-membered cyclic TS-type of mechanism in the non-catalysed reaction. Structural parameters, analyses of natural bond orbital charges and bond orders of the acid-catalysed elimination reactions in this study suggest that the polarisation of the C–O bond, in the direction Cδ+—Oδ−, is rate-determining in the TS. These reactions are non-synchronous concerted polar in nature.

Effect of a Tertiary Butyl Group on Polar Solvation Dynamics in Aqueous Solution: A Computational Approach.

The current computational study investigates the changes in solvation dynamics of water when introducing hydrophobic side chains to the molecular probe N-methyl-6-oxyquinolinium betaine. High-precision transient fluorescence and absorption measurements published in the companion article (10.1021/acs.jpcb.7b05031) revealed an influence of hydrophobic side chain alterations on the observed solvation dynamics of a chromophore in water. As the influence of shape, size, and structure of chromophores on the time-dependent Stokes shift was so far thought to play a role only in slowly rotating solvents compared to the solute or if the hydrogen bonding ability of the solute changes, this finding is quite unexpected. Analysis of the time-dependent Stokes shift obtained from nonequilibrium simulations corroborates experimental retardation factors and activation energies, and indicates that solute motion, namely vibration, is mainly responsible for the observed retardation of solvation dynamics. The faster dynamics around the smaller chromophore is in fact achieved by some normal modes located at the pyridinium part of the chromophore. Rotation also contributes to a very small extent to hydration dynamics, but for small and large derivatives alike. Local residence times furthermore reveal slight retardations in the first solvent shell around the chromophores. The current picture of the solute acting as a passive molecular probe therefore needs to be revised even for solvents like water.

Ionic Conductivity in Ti-Doped KFeO2: Experiment and Mathematical Modeling

The structure peculiarities of K0.9Fe0.9Ti0.1O2 that favor the emergence of a superionic state have been studied using neutron powder diffraction data as a function of temperature. The migration paths in the structure of both undoped and doped potassium ferrite were modeled by topological (tiling) and DFT methods. It is shown that heating of the low-temperature phase leads to increase of the ionic conductivity thanks to widening the migration channels and the appearance of thermally induced cation vacancies. The calculated migration barrier is found to not exceed 0.3 eV/ion in all phases, which is consistent with the experimental data. Doping also increases the ionic conductivity, but up to about 10% of Ti only; then the experimental activation energy even increases. The DFT modeling shows that it can be caused by growth of the regions unavailable for the mobile cations; the regions are formed around the dopant atoms.

(Invited) Probing Dopants in 2H MoS2 Crystals and 2D Layers by Electron Paramagnetic Resonance: Identification and Quantification

Bulk MoS2 (molybdenite) is an abundant soft transition metal dichalcogenide (TMD), consisting of three-layered S-Mo-S sheets of hexagonal structure and strong in-plane ionic-covalent bonding, The layers are held together by van der Waals forces, enabling plain separation (e.g., through exfoliation) of two-dimensional (2D) layers with unique electronic, optical, and mechanical properties [1]. MoS2 is a semiconductor with indirect band gap of ~1.3 eV, turning to a direct gap of 1.8 eV in single layer form [2]. Obviously, to enable full exploitation of the unique properties of atomically-thin MoS2 films, similar to traditional semiconductor technology, stable n and p-type conduction in the TMDs is a key requirement for all intended applications, such as heterojunction bipolar transistors, light emitting diodes, and photodetectors [2]. Yet, the bipolar doping strategy of the TMDs needs to be refined. Customarily, controlling majority charge carriers via doping is realized in conventional semiconductors by covalently bonded impurities. However, as to MoS2, the insight on the ability of robust and tunable ‘conventional’ doping appears limited, in particular regarding p-type dopants. In this respect, the current work deals with multifrequency electron spin resonance (ESR) study of three types of covalently bonded impurity acceptors in p-type 2H MoS2 crystals, where ESR is demonstrated as an exclusive technique for defect identification, quantification, and, outstandingly, first inference of the corresponding electrical activation energy Ea. A first part will deal with p-type geological (geo) 2H MoS2 crystals, where ESR reveals p-type doping predominantly originating from As atoms substituting for S sites in densities (spin S = ½) of ~2.4 ´ 1017 cm-3. Observation of a ‘half field’(g ~ 3.883) signal firmly correlating with the central Zeeman As accepter signal indicates the presence of spin S > ½ As agglomerates, which together with the distinct multicomponent makeup of the Zeeman signal points to manifest non-uniform As doping; only ~13% of the total As response originates from individual decoupled As dopants. From ESR monitoring of the latter vs.T, an activation energy Ea = (0.7 ± 0.2) meV is obtained. This unveils As as a noticeable shallow acceptor, appropriate for realization of efficient and stable p-type doping. Next, in studying p-type synthetic 2H MoS2, we will present results on the first ESR observation of the N impurity dopant. It is emerging as a signal of axial-symmetry characteristic for a hole-type center in MoS2 [3]. The signal, corresponding to a defect density (S = ½) of ~2.3 x 1017 cm-3, is identified as originating from N acceptor dopants, the N atoms substituting for S sites. For the applied magnetic field B//c-axis, the signal is mainly comprised of a 14N hyperfine triplet with, on top, a center line accounting for ~23% of the total signal intensity. The additional observation of a weak half-field signal (g = 3.19) correlating with the main full-field Zeeman response points to the presence of spin S³ 1 N agglomerates. The overall signal properties indicate that only ~ 77% of the N acceptors occur as isolated decoupled dopants. Careful monitoring of the total ESR signal intensity over a broad T range unveils the N dopant as a shallow acceptor, making it a promising candidate for stable substitutional p-type doping in MoS2-based novel nanoelectronic devices. Finally, dealing with one more observation on p-type geo 2H MoS2 crystals, we address a previously unreported ESR signal characterized by g// = 2.102, g2 ~1.998 and g3 ~1.973, and corresponding to a defect density (S = ½) of ~5 ×1014 cm-3. For the applied magnetic field B//c-axis, the response is comprised of a single central asymmetric Zeeman peak at zero-crossing g = 2.102(1), amid a symmetrically positioned hyperfine doublet of 6.6-G splitting. The g pattern points to a defect of lower than axial symmetry, likely orthorhombic (C2v). Based on signal composition analysis, it is ascribed to Pb impurity atoms substituting for Mo, the defect operating as acceptor dopant with estimated thermal activation energy >10 meV. The adduced insight from ESR experiments on covalently-bonded substitutional acceptor dopants may contribute to developing appropriate and tunable p-type doping strategies for few-layer MoS2 application in future high-efficiency electronic nano-devices. The key experimental information provided, i.e., regarding advanced atomic models and a pioneering platform of experimental activation energies, may stimulate theoretical verification through carrying out first-principles calculations. [1] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, Nat. Nanotechnol. 6, 147 (2011) [2] Q.H. Wang, K. Kalantar-Zadeh, A. Kis, J.N. Coleman, and M.S. Strano, Nat. Nanotechnol. 7, 699 (2012) [3] R. S. Title and M. W. Shafer, Phys. Rev. B8, 615 (1973).

Fast Lithium Ion Migration in Room Temperature LiBH4

The defect structure and the Li ion diffusion mechanism of orthorhombic LiBH4 (o-LiBH4) are studied by first-principles calculations to elucidate the Li ion transport in o-LiBH4. Two metastable Li interstitial sites are identified, and the formation energies of the Schottky and Frenkel defect pair are calculated to be 1.2–1.4 eV, the former being slightly easier to form. The energy required to form intrinsic defects is higher than that of hexagonal LiBH4 (h-LiBH4). On the other hand, the migration energy barrier of the Li vacancy or interstitial ranges from 0.1 to 0.3 eV, which is comparable to that of h-LiBH4. Therefore, the higher defect formation energy mainly accounts for the much lower Li ion conductivity of o-LiBH4. The calculated overall activation barrier for the Li ion conduction is in fair agreement with the experimental activation energy. Molecular dynamics simulation demonstrates that both the interstitial and the interstitialcy mechanisms are operative for the Li interstitial diffusion and th...

Transformation of topologically close-packed β-W to body-centered cubic α-W: Comparison of experiments and computations.

The enthalpy and activation energy for the transformation of the metastable form of tungsten, β-W, which has the topologically close-packed A15 structure (space group Pm3¯n), to equilibrium α-W, which is body-centered cubic (A2, space group Im3¯m), was measured using differential scanning calorimetry. The β-W films were 1 μm-thick and were prepared by sputter deposition in argon with a small amount of nitrogen. The transformation enthalpy was measured as -8.3 ± 0.4 kJ/mol (-86 ± 4 meV/atom) and the transformation activation energy as 2.2 ± 0.1 eV. The measured enthalpy was found to agree well with the difference in energies of α and β tungsten computed using density functional theory, which gave a value of -82 meV/atom for the transformation enthalpy. A calculated concerted transformation mechanism with a barrier of 0.4 eV/atom, in which all the atoms in an A15 unit cell transform into A2, was found to be inconsistent with the experimentally measured activation energy for any critical nucleus larger than two A2 unit cells. Larger calculations of eight A15 unit cells spontaneously relax to a mechanism in which part of the supercell first transforms from A15 to A2, creating a phase boundary, before the remaining A15 transforms into the A2 phase. Both calculations indicate that a nucleation and growth mechanism is favored over a concerted transformation. More consistent with the experimental activation energy was that of a calculated local transformation mechanism at the A15-A2 phase boundary, computed as 1.7 eV using molecular dynamics simulations. This calculated phase transformation mechanism involves collective rearrangements of W atoms in the disordered interface separating the A15 and A2 phases.

Quantum chemical approach to atomic manipulation of chlorobenzene on the Si(111)- 7×7 surface: Resonance localization, vibrational activation, and surface dynamics

We present a cluster model to describe the localization of hot charge carriers on the Si(111)-$7\ifmmode\times\else\texttimes\fi{}7$ surface, which leads to (nonlocal) desorption of chlorobenzene molecules in scanning tunneling microscope (STM) manipulation experiments. The localized charge carriers are modeled by a small cluster. By means of quantum chemical calculations, this cluster model explains many experimental findings from STM manipulation. We show that the negative charge is mainly localized in the surface, while the positive one also resides on the molecule. Both resonances boost desorption: In the negative resonance the adatom is elevated; in the positive one the chemisorption bond between the silicon surface adatom and chlorobenzene is broken. We find normal modes promoting desorption matching experimental low-temperature activation energies for electron- and hole-induced desorption.

DC conductivity and Seebeck coefficient of nonstoichiometric MgCuZn ferrites

Abstract Nonstoichiometric series of Mg0.5−xCuxZn0.5Fe1.9O4−δ where x = 0.0, 0.1, 0.15, 0.2 and 0.25 has been synthesized by conventional solid state reaction route. The single phase spinel structure of the double sintered ferrites was confirmed by X-ray diffraction patterns (XRD). The ferrite series was studied in terms of DC electrical conductivity and thermoelectric power in the temperature ranging from room temperature to 300 °C and 400 °C, respectively. It was observed that DC electrical conductivity and Seebeck coefficient α decreased with the increase in x. DC electrical conductivity was found to decrease by about 4 orders. All the compositions showed a negative Seebeck coefficient exhibiting n-type semiconducting nature. From the above experimental results, activation energy and mobility of all the samples were estimated. Small polaron hopping conduction mechanism was suggested for the series of ferrites. Owing to their low conductivity the nonstoichiometric MgCuZn ferrites are the best materials for transformer core and high definition television deflection yokes.

Ab initio calculation of the attempt frequency of oxygen diffusion in pure and samarium doped ceria.

The rate of oxygen ion jumps in a solid oxide depends not only on the activation energy but also on the pre-exponential factor of diffusion. In order to allow a fully ab initio prediction of the oxygen ion conductivity in pure and samarium doped ceria, we calculated the attempt frequency for an oxygen ion jump from first principles combining DFT+U, the NEB method, phonon calculations and the transition state theory. Different definitions of the jump attempt frequency are presented. The equivalence of the Eyring and the Vineyard method is shown without restriction to the Gamma point. Convergence checks of the phonon mesh reveal that the common reduction to the Gamma point is not sufficient to calculate the attempt frequency. Calculations of Sm doped ceria revealed an increase of the prefactor. The attempt frequency for the constant pressure case in quasi-harmonic approximation is larger than the attempt frequency at constant volume in harmonic approximation. The calculated electronic energies, enthalpies and entropies of migration are in agreement with the experimental diffusion coefficients and activation energies.

СОВМЕСТНАЯ ПОВЕРХНОСТНАЯ СЕГРЕГАЦИЯ СЕРЕБРА И ОЛОВА В ТРОЙНЫХ СПЛАВАХ Cu–Ag–Sn

The study of Cu–Ag–Sn ternary alloys by the methods of temperature-programmed desorption (TPD) and statistical modeling is с arried out. The results of the TPD experiment on the combined surface segregation of Ag and Sn atoms in dilute solutions of silver and tin in copper in polycrystalline and liquid states are presented. It was found out that the desorption of particles from the surface of the alloy occurs from regions enriched with this component. In addition, the silver atoms form, in the main, slightly layered islets. Data are also provided for computer simulation of the surface of these alloys by the Monte Carlo method in combination with the embedded atom method (EAM). Using the EAM method, it was possible to avoid a priori assumptions about the composition and structure of the alloy surface. Simulation showed that the binding energy of surface atoms is close to the experimental activation energies of thermal desorption for Cu and Ag atoms. But there is a significant difference for tin atoms. Secondly, in the modeling for the solid state of the Cu 98 Ag 1 Sn 1 alloy, the surface concentrations of Θ Ag ≈ Θ Sn = 30 at.%, which for Ag atoms are close to the results of the TPD experiment. In the melt, the degree of surface segregation is reduced to 8 at.% Ag and 14 at.% Sn. Both the TPD experiment and the simulation give an excess of 10 to 30 times the surface concentrations of Ag and Sn over bulk values, both in the solid and in the liquid state of the alloy. Third, computer models reveal surface structures in the form of zigzag chains of Sn atoms, which alternate with chains of Ag atoms. The profiles of tin concentrations in alloys confirm the hypothesis of a two-layered character of segregation of tin.

Insights on hydride formation over cerium-gallium mixed oxides: A mechanistic study for efficient H2 dissociation

A four-step reaction mechanism is proposed for the H2 dissociation over pure ceria and gallium-promoted mixed oxide materials, in a combined experimental and computational investigation. Two samples of cerium-gallium mixed oxides with Ce/Ga atomic ratios equal to 90/10 and 80/20 were studied by time-resolved diffuse reflectance infrared spectroscopy under H2 (D2) flow at isothermal condition in the range of 523–623K. X-ray photoelectron spectrometry allowed to conclude that only Ce4+ is reduced to Ce3+ (Ga3+ is not reduced), in agreement with density functional theory (DFT) results. The time evolution profiles of gallium hydride (GaH) species, hydroxyl groups (OH) and Ce3+ infrared signals were analyzed and kinetic rate parameters for each step were obtained by mathematical modeling. The values for activation energies were in agreement with those calculated by DFT, for the different elementary pathways. A small activation energy (∼4kcal/mol) was found for H2 dissociation found on Ga⋯OCe sites assuming that the heterolytic cleavage of the HH bond is the rate determining step. On pure ceria, the experimental activation energy is ∼23kcal/mol, showing that the addition of Ga3+ cations boosts the splitting of H2. Interestingly, the reduction step of pure CeO2 surface domains seems to proceed via a CeH/OH pair intermediate, according to DFT calculations. Moreover, 71Ga NMR experiments indicate the possible presence of gallia nanodomains. It is proposed that the generation of Ga⋯OCe sites in the perimeter of such surface gallia nanodomains is responsible for the enhanced reactivity of the mixed materials. The key role of this new type of sites to improve the efficiency of relevant catalytic reactions such as selective alkyne hydrogenation and light alkane dehydrogenation is then analyzed.

Racemization of the Succinimide Intermediate Formed in Proteins and Peptides: A Computational Study of the Mechanism Catalyzed by Dihydrogen Phosphate Ion.

In proteins and peptides, d-aspartic acid (d-Asp) and d-β-Asp residues can be spontaneously formed via racemization of the succinimide intermediate formed from l-Asp and l-asparagine (l-Asn) residues. These biologically uncommon amino acid residues are known to have relevance to aging and pathologies. Although nonenzymatic, the succinimide racemization will not occur without a catalyst at room or biological temperature. In the present study, we computationally investigated the mechanism of succinimide racemization catalyzed by dihydrogen phosphate ion, H2PO4−, by B3LYP/6-31+G(d,p) density functional theory calculations, using a model compound in which an aminosuccinyl (Asu) residue is capped with acetyl (Ace) and NCH3 (Nme) groups on the N- and C-termini, respectively (Ace–Asu–Nme). It was shown that an H2PO4− ion can catalyze the enolization of the Hα–Cα–C=O portion of the Asu residue by acting as a proton-transfer mediator. The resulting complex between the enol form and H2PO4− corresponds to a very flat intermediate region on the potential energy surface lying between the initial reactant complex and its mirror-image geometry. The calculated activation barrier (18.8 kcal·mol−1 after corrections for the zero-point energy and the Gibbs energy of hydration) for the enolization was consistent with the experimental activation energies of Asp racemization.