NMR study of protonic transport in hydronium beta″ alumina

NMR relaxation time measurements have been made at 20 MHz for proton in polycrystalline hydronium beta″ alumina. Motional narrowing occurs at T > 62;230 K(E_a = 19 kJ mol^?1) in the fully hydrated beta″ alumina and the relaxation behavior is compatible with fast proton transport. The protonic motion in the partially dehydrated beta″ alumina has a high activation energy as 24.5 kJ mol^?1.     

A kinetic study of the thermal decomposition process of potassium metabisulfite: Estimation of distributed reactivity model

The thermal decomposition kinetics of potassium metabisulfite was studied by thermogravimetric (TG) and differential thermogravimetric (DTG) techniques using non-isothermal experiments. The apparent activation energy (E_a) is determined using the differential (Friedman) isoconversional method. The results of the Friedman's isoconversional analysis of the TG data suggests that the investigated decomposition process follows a single-step reaction and the observed apparent activation energy was determined as 122.4±2.1 kJ mol⁻¹. A kinetic rate equation was derived for the decomposition process of potassium metabisulfite with contracting area model, f(α)=2(1−α)^1/2, which is established using the Malek's kinetic procedure. The value of pre-exponential factor (A) is also evaluated and was found to be A=1.37×10¹² min⁻¹. By applying the Miura's procedure the distributed reactivity model (DRM) for investigated decomposition process was established. From the dependence α versus E_a, the experimental distribution curve of apparent activation energies, f(E_a), was estimated. By applying the non-linear least-squares analysis, it was found that the Gaussian distribution model (with distribution parameters E₀=121.3 kJ mol⁻¹ and σ=1.5 kJ mol⁻¹) represents the best reactivity model for describing the investigated process. Using the Miura's method, the A values were estimated at five different heating rates and the average A values are plotted against E_a. The linear relationship between the A and E_a values was established (compensation effect). Also, it was concluded that the E_a values calculated by the Friedman's method and estimated distribution curve, f(E_a), are correct even in the case when the investigated decomposition process occurs through the single-step reaction mechanism.     

Intermolecular potentials for ammonia based on the test particle model and the coupled pair functional method

The intermolecular interaction ΔE in (NH₃)₂ is investigated on the SCF level, with inclusion of correlation effects by means of the CPF method and within the simple test particle model. Whereas the linear hydrogen bonded structure is favoured on the SCF level, ΔE = -7·65 kJ mol^-1, the most stable geometry on the highest level of theory is a cyclic structure, ΔE = -12·96 kJ mol^-1. The minimum is very shallow and allows for appreciable angular motions. The test particle model reproduces the general features of ΔE but shows deviations in details. The computed potentials are used in MD simulations to compute static and dynamic properties of liquid NH₃. Good agreement with available experimental results is obtained throughout.     

Reactive scattering of a supersonic oxygen atom beam O+ICl

Reactive scattering of O atoms with ICl molecules has been studied at an initial translational energy E = 40 kJ mol^-1 using a supersonic beam of O atoms seeded in He and at E = 15 kJ mol^-1 using O atoms seeded in Ne. Velocity distributions of OI product were measured by cross-correlation time-of-flight analysis. Full contour maps of the differential reaction cross-section have been obtained which show peaking almost equally in the forward and backward directions at both initial translational energies. The product translational energy distributions are consistent with a long-lived O-I-Cl collision complex dissociating via a loose transition state. The stability of the O-I-Cl complex is attributed to the low electronegativity of the central I atom compared with the peripheral atoms. This electronegativity ordering rule also determines the stability of the intermediates in the other reactions of oxygen atoms with halogen molecules. The mild peaking of the product angular distributions for O + ICl and IBr indicates that collision complexes have quite modest collision angular momenta L ～ 40 ? corresponding to impact parameters b ～ 1·4 Å and that the angular momentum of the OI molecule in the loose transition state may be approximately half the product orbital angular momentum.     

NMR study of Li diffusion in β-LiAl

Diffusion of a Li atom in an anode material β-LiAl was studied over the composition range of 48.0–50.2 at % Li by the ⁷Li magnetic relaxation times. Dependence of T₁ upon temperature and the resonance frequency was successfully explained by a vacancy diffusion on the Li sublattice involving a distribution of the activation energy for migration, E_a. The central value of E_a was almost independent of the composition (? 13.8 kJ mol^-1), while the width of distribution increased with increase of the number of substituted Li on the Al sublattice. The obtained diffusion constants, extrapolated to 415°C, were larger by a factor of 1.8–3.4 than those obtained by the electrochemical method.     

Density functional study of the chemisorption of O2 on the armchair surface of graphite

The reaction between O₂ and the armchair surface of a model graphite molecule has been studied using density functional calculations at the B3LYP/6-31G(d) level of theory. Both equilibrium and transition state geometries were optimized to provide a fundamental understanding of the energetics and kinetics of the chemisorption, desorption, rearrangement, and migration reactions that contribute to carbon gasification. A small barrier of 18 kJ mol⁻¹ was found for the chemisorption reaction, which is 578 kJ mol⁻¹ exothermic overall, producing a stable quinone. A number of reaction pathways with barriers below 578 kJ mol⁻¹ were characterized. Gasification of carbon occurs as CO, with barriers of 296 and 435 kJ mol⁻¹ for the first and second CO loss, respectively. The stable quinone can also undergo a rearrangement reaction to form two ketene groups, with a barrier of 260 kJ mol⁻¹. If the armchair edge is extended to include an adjacent aromatic ring, the oxide can migrate along the surface. This initially forms a furan-like bridge structure, with a barrier of just 89 kJ mol⁻¹. A further barrier of 383 kJ mol⁻¹ leads to CO desorption from the furan. The furan can also rearrange further with a barrier of 212 kJ mol⁻¹ to form a five-membered lactone, the most stable structure identified on the potential energy surface. Rearrangement and migration reactions, which have not generally been incorporated into carbon gasification models, are shown to be potentially important pathways in carbon oxidation reactions.     

Electron magnetic resonance investigation of gadolinium diffusion in zircon powders

The electron magnetic resonance (EMR) technique was used to investigate the diffusion of gadolinium in zircon (ZrSiO₄) powders. The EMR absorption intensity was measured for several annealing times and three different temperatures of isothermal annealing: 1273, 1323 and 1373 K. The activation energy for diffusion, calculated from the experimental data using a theoretical model based on the Fick equation, was found to be E_A=506±5 kJ mol⁻¹. This value is close to the ones for the diffusion of Gd in UO₂ and CeO₂, but much larger than for the diffusion of gadolinium in a compound with the same crystal structure as zircon, YVO₄. This is attributed to a difference in the relative sizes of the ions involved in the diffusion process.     

Metal/electrolyte surface chemistry: The adsorption of nitric acid and water on Ag(110)

The adsorption of HNO₃/H₂O mixtures on Ag(110) was investigated to learn more about the chemistry of the metal/electrolyte interface. The experiments were performed in ultrahigh vacuum (UHV) using thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED), and electron stimulated desorption ion angular distribution (ESDIAD) over temperatures of 80–650 K and coverages of 0–10 monolayers (ML). As this is the first known study of HNO₃ in UHV, the mass spectrometer cracking pattern for HNO₃ is here reported. HNO₃ adsorbs irreversibly on the clean surface at 80 K and loses its acidic proton to form an adsorbed surface nitrate (NO₃) below 150 K. The saturation amount of adsorbed NO₃ is 0.4 ± 0.1 ML for which adsorption occurs in either a normal or split c(2 × 2) structure. N03 is stable on the surface up to 450 K beyond which it decomposes directly to gaseous NO₂ and NO and adsorbed atomic oxygen. NO₃ decomposition is first order with an activation energy E_a = 151±4 kJ mol⁻¹ and a pre-exponential factor of A = 10^15.4±0.4s⁻¹. NO₃ stabilizes adsorbed H₂O by about 8 kJ mol⁻¹ and is hydrated by as many as three H₂O molecules. Multilayers of HNO₃/H₂O desorb at 150–220 K and show evidence of extensive hydrogen bonding and hydration interactions. No evidence for HNO₃-induced corrosion or other surface damage was detected in any of these experiments.     

Carbon monoxide oxidation over well-defined Pt/ZrO2 model catalysts: Bridging the material gap

Four different Pt/ZrO₂/(C/)SiO₂ model catalysts were prepared by electron beam evaporation. The morphology of these samples was examined before and after the catalytic reaction by Rutherford back-scattering (RBS), transmission electron microscopy (TEM) and grazing-incidence small-angle scattering (GISAXS). The catalytic behavior of such model catalysts was compared to a conventional Pt/ZrO₂ catalyst in the CO oxidation reaction using different oxygen excess (λ = 1 and 2). The so-called material gap was observed: model catalysts were active at higher temperature (620-770 K) and resulted in higher activation energy values (E_a = 77-93 kJ mol⁻¹ at λ = 1 and 129-141 kJ mol⁻¹ at λ = 2) compared to the powdered Pt/ZrO₂ catalyst (370-470 K, E_a = 74-76 kJ mol⁻¹). This material gap is discussed in terms of diffusion limitations, reaction mechanism and apparent compensation effect. Diffusion processes seem to limit the reaction on planar samples in the reactor system that was shown to be appropriate for the evaluation of the catalytic activity of powder samples. Kinetic parameters obeyed the so-called apparent compensation effect, which is discussed in detail. Langmuir-Hinshelwood-type of reaction, between CO_ads and O_ads, was proposed as the rate-determining step in all cases. Pt particles deposited on planar structures can be used for modeling conventional powdered catalysts, even though some limitations must be taken into account.     

Absolute coverage and isostemc heat of adsorption of deuterium on Pt(111) studied by nuclear microanalysis

The absolute coverage (θ) of deuterium adsorbed on Pt(111) in the ranges 180< T<440 K and 5 × 10^?6 < P < 5 × 10^?2 Pa D₂ has been determined by nuclear microanalysis using the D(³He, p)⁴He reaction. From these data, the isosteric heat of adsorption (E_a) has been determined to be 67 ± 7 kJ mol^?1 at θ ? 0.3. This heat of adsorption yields values of the pre-exponential for desorption (10^?5 to 10^?2 cm² atom^?1 s^?1) that lie much closer to the normal range for a second order process than those determined from previous isosteric heat measurements. The E_a versus θ relationship indicates that the adsorbed D atoms are mobile and that there is a repulsive interaction of 6–8 kJ mol^?1 at nearest neighbour distances. At 300 K the coverage decreases to ? 0.05 monolayer (? 8 × 10¹³ D atoms cm^?2) as P→ 0, apparently invalidating a recent model of site exchange in the adsorbed layer.     

Electrical conductivity of BaCe0.9Nd0.1O3−α in H2+H2O+Ar gas mixture

Electroconductivity of BaCe_0.9Nd_0.1O_3−α was studied as a function of the composition of the H₂+H₂O+Ar mixture and temperature in the interval from 873 to 1173 K. It was shown that the electroconductivity was independent of P_H2 (0.97 to 0.10 atm) and P_O2 (10⁻²¹ to 10⁻²⁶ atm), but depended on P_H2O (0.08 to 0.005 atm). A mathematical processing of the P_H2O dependencies of the electroconductivity, which was performed in terms of a classical model of defect formation in high-temperature proton-conducting solid electrolytes, yielded equilibrium constants of the reaction of water dissolution in BaCe_0.9Nd_0.1O_3−α and mobilities of protons and oxygen ions. The temperature dependencies of these quantities were used to determine the mobility activation energies of protons (E_a=34±7 kJ/mole) and oxygen ions (E_a=72±8 kJ/mole), and also the enthalpy (ΔH=−150±25 kJ/mole) and the entropy (ΔS=153±26 kJ/mole·K) of the reaction of water dissolution in BaCe_0.9Nd_0.1O_3−α.     

Reactive scattering of a supersonic oxygen atom beam O + I2

Reactive scattering of O atoms with I₂ molecules has been studied at an initial translational energy E = 43 kJ mol^-1 using a supersonic beam of O atoms seeded in He and E = 18 kJ mol^-1 using O atoms seeded in Ne. Velocity distributions of OI product were measured by cross-correlation time-of-flight analysis. Full contour maps of the differential reaction cross section were obtained which show predominantly rebound scattering at both initial translational energies. Scattering in the forward direction has a product translational energy distribution similar to that predicted for a long-lived collision complex but scattering in the backward direction has a much higher product translational energy. The greater predominance of rebound scattering observed for O + I₂ compared with O + Br₂ may be attributed to the greater exoergicity of the O + I₂ reaction. However, comparison with the O + IBr reaction which exhibits a long-lived collision complex mechanism indicates that the predominance of backward scattering for O + I₂ also arises from diminished forward scattering from larger impact parameter collisions.     

A study of the n-n interaction from a complete experiment of the n + d break-up at 14.5 MeV

Results of the analysis of two complete experiments on ²H(n, nnp) at 14.5±0.1 MeV are given. In the first n-n final-state-interaction (f.s.i.) experiment (neutron lab angles θ₁ = θ₂ = +30°, n-n relative energy E_nn measured between 0 and ≈ 800 keV) about 4400 events were recorded. In the second n-p f.s.i. experiment (neutron lab angles θ₁ = +30°, θ₂ = −80°, and E_np ? 500 keV) about 2860 events were recorded. The analysis has been done by exact solutions of the Faddeev equations with separable potentials: the Amado model was extended using the Yamaguchi potential with charge-independent (a_nm = a_np) and with charge-dependent (a_nn = −16 fm, a_np = −23.7 fm) parameters. The results are: (a) the charge-independent (a_nn = −23.7 fm) model seems to give a better fit for the shape of the n-n f.s.i. peak, but (b) the two-nucleon separable potential calculations disagree by a factor ≈ 2 with the absolute cross sections measured for the f.s.i. peaks.     

Kinetics of the gas‐phase homogeneous unimolecular elimination of selected ethyl esters of 2‐oxo‐carboxylic acids

The gas‐phase elimination kinetics of selected ethyl esters of 2‐oxo‐carboxylic acid have been studied over the temperature range of 270–415 °C and pressures of 37–114 Torr. The reactions are homogeneous, unimolecular, and follow a first‐order rate law in a seasoned static reaction vessel, with an added free radical suppressor toluene. The observed overall and partial rate coefficients are expressed by the following Arrhenius equations:

• Ethyl oxalyl chloride
• log k_overall (s^?1) = (13.22 ± 0.45) ? (179.4 ± 4.9) kJ mol^?1 (2.303 RT)^?1
• Ethyl piperidineglyoxylate
• log k_(CO2) (s^?1) = (12.00 ± 0.30) ? (191.2 ± 3.9) kJ mol^?1 (2.303 RT)^?1
• log k_(CO) (s^?1) = (12.60 ± 0.09) ? (210.7 ± 1.2) kJ mol^?1 (2.303 RT)^?1
• log k_t(overall) (s^?1) = (12.22 ± 0.26) ? (193.4 ± 3.4) kJ mol^?1 (2.303 RT)^?1
• Ethyl benzoyl formate
• log k_(CO2) (s^?1) = (12.89 ± 0.72) ? (203.8 ± 9.0) kJ mol^?1 (2.303 RT)^?1
• log k_(CO) (s^?1) = (13.39 ± 0.31) ? (213.3 ± 3.9) kJ mol^?1 (2.303 RT)^?1
• log k_t(overall) (s^?1) = (13.24 ± 0.60) ? (205.8 ± 7.6) kJ mol^?1 (2.303 RT)^?1

The kinetic and thermodynamic parameters of these reactions, together with those reported in the literature, lead to consider three different mechanistic pathways of elimination. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrical and mechanical properties of proton conducting SrCe0.95Yb0.05O3 − α

Yb³⁺-doped ceramic strontium cerate of exactly the composition SrCe_0.95Yb_0.05O_{3 − α} was prepared, having a relative density of 99.0 (± 0.3%). Great care was taken to obtain homogeneous, carbonate free material. Analysis are made of the X-ray powder diffraction pattern of the as-prepared dense ceramic, resulting in the orthorhombic unit cell parameters a = 6.997(2) Å, b = 12.296(3) Å, c = 8.588(2) Å, Z = 8 and d_x = 5.806(2) g cm⁻³. Bending strength values of the ceramic in non-proton and proton conducting state are found to be 177 and 194 MPa respectively. The ceramic kept under proton conducting conditions for 500 h at 300 °C to 800 °C in a N₂ flow containing 155 mbar water vapour and 245 mbar H₂, have shown to remain chemically and structurally stable. Impedance spectroscopy measurements of the bulk conductivity of the proton conducting ceramic revealed an activation energy of 53.2 kJ mol⁻¹ and a preexponential factor of 359.1 (Ω cm)⁻¹ K. In the non-proton conducting state the ceramic is mainly oxygen ion vacancy conducting, which indicates that charge compensation on substituting Yb⁺³ in SrCeO₃ takes place by oxygen ion vacancies.     

<Superscript>1</Superscript>H Spin–Lattice Relaxation Study of Dynamical Inequivalence of Methyl Groups in Solid 1,2-<Emphasis Type="Italic">O</Emphasis>-(1-Ethylpropylidene)-α-<Emphasis Type="SmallCaps">d</Emphasis>-Glucofuranose

We investigated the dynamics of methyl groups in organic polycrystalline 1,2-O-(1-ethylpropylidene)-α-d-glucofuranose by the proton spin–lattice relaxation method. The temperature and nuclear magnetic resonance Larmor frequency dependence of relaxation time is presented and interpreted in terms of simple possible dynamical model for the reorientation of methyl groups: the random hopping for methyl groups, which are in a, b, and c sites in the crystal. The energy E _a of 13.5 kJ mol⁻¹ for the a-type methyl groups is typical for methyl groups in ethyl groups. In contrast, the b- and c-methyl groups characterized by the lower E _a values of 9.5 and 6.5 kJ mol⁻¹ are located in the crystal structure where the intermolecular interactions significantly influence the potential, leading to a decrease in the total energy.     

Adsorption of SO2 on Li atoms deposited on MgO (1 0 0) surface: DFT calculations

The adsorption of sulfur dioxide molecule (SO₂) on Li atom deposited on the surfaces of metal oxide MgO (1 0 0) on both anionic and defect (F_s-center) sites located on various geometrical defects (terrace, edge and corner) has been studied using density functional theory (DFT) in combination with embedded cluster model. The adsorption energy (E_ads) of SO₂ molecule (S-atom down as well as O-atom down) in different positions on both of O⁻² and F_s sites is considered. The spin density (SD) distribution due to the presence of Li atom is discussed. The geometrical optimizations have been done for the additive materials and MgO substrate surfaces (terrace, edge and corner). The oxygen vacancy formation energies have been evaluated for MgO substrate surfaces. The ionization potential (IP) for defect free and defect containing of the MgO surfaces has been calculated. The adsorption properties of SO₂ are analyzed in terms of the E_ads, the electron donation (basicity), the elongation of S-O bond length and the atomic charges on adsorbed materials. The presence of the Li atom increases the catalytic effect of the anionic O⁻² site of MgO substrate surfaces (converted from physisorption to chemisorption). On the other hand, the presence of the Li atom decreases the catalytic effect of the F_s-site of MgO substrate surfaces. Generally, the SO₂ molecule is strongly adsorbed (chemisorption) on the MgO substrate surfaces containing F_s-center.     

Bounds on charged lepton mixing with exotic charged leptons

We examine the effects of mixing induced light heavy charged lepton neutral currents on the partial wave amplitude for the process l⁺l⁻ →ZZ (withl = e,μ or τ). By imposing the constraints that the amplitude should not exceed the perturbative unitarity limit at high energy (√s = Λ), we obtain bounds on light heavy charged lepton mixing parameter sin²(2θ _L ^a ) where θ _L ^a is the mixing angle of the ordinary charged lepton with its exotic partner. For Λ = 1 TeV, no bound is obtained on sin² (2θ _L ^a ) form _E < 0.69 TeV. However, sin² (2θ _L ^a ) ≤ 1.52×10⁻⁵ form _E = 5 TeV, sin² (2θ _L ^a ) ≤ 2.41 ×10⁻⁷ form _E = 10 TeV. Similarity for Λ = ∞ no bound is obtained on sin² (2θ _L ^a ) for m_E < 1.97 TeV and sin² (2θ _L ^a ) ≤ 0.15 form _E = 5 TeV and sin² (2θ _L ^a ) ≤ 3.88×10^-2 form _E = 10 TeV.     

Homo‐Diels–Alder reaction of a very inactive diene,bicyclo[2,2,1]hepta‐2,5‐diene,with the most active dienophile, 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione. Solvent,temperature, and high pressure influence on the reaction rate

Solvent, temperature, and high pressure influence on the rate constant of homo‐Diels–Alder cycloaddition reactions of the very active hetero‐dienophile, 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione (1), with the very inactive unconjugated diene, bicyclo[2,2,1]hepta‐2,5‐diene (2), and of 1 with some substituted anthracenes have been studied. The rate constants change amounts to about seven orders of magnitude: from 3.95^.10^?3 for reaction (1+2) to 12200 L mol^?1 s^?1 for reaction of 1 with 9,10‐dimethylanthracene (4e) in toluene solution at 298 K. A comparison of the reactivity (ln k₂) and the heat of reactions (?_r‐nH) of maleic anhydride, tetracyanoethylene and of 1 with several dienes has been performed. The heat of reaction (1+2) is ?218 ± 2 kJ mol^?1, of 1 with 9,10‐dimethylanthracene ?117.8 ± 0.7 kJ mol^?1, and of 1 with 9,10‐dimethoxyanthracene ?91.6 ±0.2 kJ mol^?1. From these data, it follows that the exothermicity of reaction (1+2) is higher than that with 1,3‐butadiene. However, the heat of reaction of 9,10‐dimethylanthracene with 1 (?117.8 kJ mol^?1) is nearly the same as that found for the reaction with the structural C=C counterpart, N‐phenylmaleimide (?117.0 kJ mol^?1). Since the energy of the N=N bond is considerably lower (418 kJ/bond) than that of the C=C bond (611 kJ/bond), it was proposed that this difference in the bond energy can generate a lower barrier of activation in the Diels–Alder cycloaddition reaction with 1. Linear correlation (R = 0.94) of the solvent effect on the rate constants of reaction (1+2) and on the heat of solution of 1 has been observed. The ratio of the volume of activation (?V^≠) and the volume of reaction (?V_r‐n) of the homo‐Diels–Alder reaction (1+2) is considered as “normal”: ?V^≠/?V_r‐n = ?25.1/?30.95 = 0.81. Copyright © 2012 John Wiley & Sons, Ltd.