Rate Constant Decreases Research Articles

Synthesis of Bis-β-Diketonate Lanthanide Complexes with an Azobenzene Bridge and Studies of Their Reversible Photo/Thermal Isomerization Properties.

The ligand, bis-β-diketone with an azobenzene bridge (4,4′-(4,4,4-trifluoro-1,3-butanedione)azobenzene, H2L), was prepared for the synthesis of a series of dinuclear lanthanide complexes with the formula [Ln2L3(DMSO)4] (Ln = Eu3+, Gd3+, Tb3+, and DMSO = dimethyl sulfoxide). X-ray crystallographic analysis reveals that the three complexes are triple-stranded dinuclear structures formed by three bis-β-diketonate ligands with two lanthanide ions (Ln3+). The trans-to-cis photoisomerization rates of the azobenzene group of the three [Ln2L3(DMSO)4] complexes in ethanol and acetonitrile solutions are similar to those of the pure H2L ligand and other azobenzene-containing mononuclear lanthanide complexes, but the trans-to-cis quantum yields (Φt→c = 10–3) are 1 order of magnitude smaller. The first-order rate constant for the cis-to-trans thermal isomerization at 50 °C of the H2L ligand is similar to those of azobenzene derivatives, while those for the [Ln2L3(DMSO)4] complexes (kiso = 10–4 s–1) are higher than those of the mononuclear azobenzene-containing lanthanide complexes. Furthermore, as the lanthanide ionic radius becomes smaller from Eu3+ to Gd3+ to Tb3+, the thermal isomerization rate constant decreases and the half-life increases. All these results are proposed to arise from the rigidity at both ends of the azo group by coordination to the dinuclear lanthanide ions and the different isomerization mechanisms. These are the first examples of bis-β-diketonate dinuclear lanthanide complexes with an azobenzene bridge and help illustrate the mechanism of azobenzene isomerization.

Modeling of CO<sub>2</sub>-Activated Adsorption on Chitosan Hydrogel for Dye Removal in Aqueous Solution

This work focuses on adsorption kinetics and isotherm modelling in CO2-activated chitosan. Removal efficiency and adsorption capacity were measured for three anionic dyes: Brilliant Blue FCF (BBF), Congo Red (CR), Orange II (O-II) and one cationic dye: Crystal Violet (CV). While the highest removal efficiencies of BBF, O-II and CR were 97%, 91% and 48%, it was found that very low adsorption capacity occurred in the case of CV due to the repulsion between the cationic groups of CV and that of CO2-activated chitosan. In terms of adsorption kinetics, pseudo-first-order and pseudo-second-order models were applied. The adsorption in the CO2-activated system was found to follow the pseudo-first-order model. The activation of adsorption by CO2 was found to lead to a higher adsorption rate constant and a higher adsorption capacity due to the protonation of chitosan’s amino groups. Interestingly, as the temperature of the CO2-activated system increased, the pseudo-first-order model predicted a decrease in adsorption rate constant, k1. This is thought to be a result of lower CO2 dissolution into the aqueous solution, which leads to a slower protonation process of chitosan’s adsorption sites. The chemical structure of the dye species affects the adsorption kinetics in the CO2-activated system. There are four charged functional groups on the CR molecule and three on the BBF molecule, which allow faster alignment onto adsorption sites compared with O-II molecules with only one charged group. In terms of adsorption isotherm, BBF adsorption was found to follow the Langmuir model in the CO2-activated system.

Degradation and mineralization of violet-3B dye using C-N-codoped TiO<sub>2</sub> photocatalyst

The present study investigated the photodegradation of synthetic organic dye; violet-3B, without and with the addition of C-N-codoped TiO<sub>2</sub> catalyst using a visible halogen-lamp as a light source. The catalyst was synthesized by using a peroxo sol-gel method with free-organic solvent. The effects of initial dye concentration, catalyst dosage, and pH solution on the photodegradation of violet-3B were examined. The efficiency of the photodegradation process for violet-3B dye was higher at neutral to less acidic pH. The kinetics reaction rate of photodegradation of violet-3B dye with the addition of C-N-codoped TiO<sub>2</sub> followed pseudo-first order kinetics represented by the Langmuir-Hinshelwood model, and increasing the initial concentration of dyes decreased rate constants of photodegradation. Photodegradation of 5 mg L<sup>-1</sup> violet-3B dye achieved 96% color removal within 240 min of irradiation in the presence of C-N-codoped TiO<sub>2</sub> catalyst, and approximately 44% TOC was removed as a result of the mineralization.

Temporal trends of <sup>137</sup>Cs activity concentration in pond waters in the vicinity of Fukushima Dai-ichi nuclear power plant

Abstract. Closed and semi-closed water bodies, such as lakes and ponds, are important water resources in Fukushima area and they are the most sensitive environments to radioactive contamination after the Fukushima Dai-ichi nuclear power plant accident. Wakiyama et al. (2017) investigated 137Cs activity in water and bottom sediment in four ponds; Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ), and Kashiramori (KM), within 10 km zone from the FDNPP during 2015–2016. This study follows up their observation to address longer time trends of 137Cs activity concentration in pond waters and to show speciation of 137Cs in soil and bottom sediment. Mean total 137Cs activity concentration in water ranged from 2.5 to 29 Bq L−1. There was not found steady trend in the activity concentration of total and particulate 137Cs for four ponds. The concentration of dissolved 137Cs was usually low in winter during the entire observation period in all four ponds. A tendency to a decrease in the 137Cs activity concentration in suspended sediments was found for four ponds and the decreasing rate constants, including radiological decay, on SU, FS, IZ and KM were 0.33, 0.53, 0.29 and 0.25 yr−1, respectively. The results of sequential extractions of soil and bottom sediment samples showed higher proportion of bioavailable 137Cs, i.e., exchangeable and organic bound 137Cs, in bottom sediment than in the soil.

Do CSIA data from aquifers inform on natural degradation of chlorinated ethenes in aquitards?

Back-diffusion of chlorinated ethenes (CEs) from low-permeability layers (LPLs) causes contaminant persistence long after the primary spill zones have disappeared. Naturally occurring degradation in LPLs lowers remediation time frames, but its assessment through sediment sampling is prohibitive in conventional remediation projects. Scenario simulations were performed with a reactive transport model (PHT3D in FloPy) accounting for isotope effects associated with degradation, sorption, and diffusion, to evaluate the potential of CSIA data from aquifers in assessing degradation in aquitards. The model simulated a trichloroethylene (TCE) DNAPL and its pollution plume within an aquifer-aquitard-aquifer system. Sequential reductive dechlorination to ethene and sorption were uniform in the aquitard and did not occur in the aquifer. After 10 years of loading the aquitard through diffusion from the plume, subsequent source removal triggered release of TCE by back-diffusion. In the upper aquifer, during the loading phase, δ13C-TCE was slightly enriched (up to 2‰) due to diffusion effects stimulated by degradation in the aquitard. In the upper aquifer, during the release phase, (i) source removal triggered a huge δ13C increase especially for higher CEs, (ii) moreover, downstream decreasing isotope ratios (caused by downgradient later onset of the release phase) with temporal increasing isotope ratios reflect aquitard degradation (as opposed to downstream increasing and temporally constant isotope ratios in reactive aquifers), and (iii) the carbon isotope mass balance (CIMB) enriched up to 4‰ as lower CEs (more depleted, less sorbing) have been transported deeper into the aquitard. Thus, enriched CIMB does not indicate oxidative transformation in this system. The CIMB enrichment enhanced with more sorption and lower aquitard thickness. Thin aquitards are quicker flushed from lower CEs leading to faster CIMB enrichment over time. CIMB enrichment is smaller or nearly absent when daughter products accumulate. Aquifer CSIA patterns indicative of aquitard degradation were similar in case of linear decreasing rate constants but contrasted with previous simulations assuming a thin bioactive zone. The Rayleigh equation systematically underestimates the extent of TCE degradation in aquifer samples especially during the loading phase and for conditions leading to long remediation time frames (low groundwater flow velocity, thicker aquitards, strong sorption in the aquitard). The Rayleigh equation provides a good and useful picture on aquitard degradation during the release phase throughout the sensitivity analysis. This modelling study provides a framework on how aquifer CSIA data can inform on the occurrence of aquitard degradation and its pitfalls.

Pollution-induced slowdown of coarse woody debris decomposition differs between two coniferous tree species

Boreal forests store a large portion of the planet’s terrestrial carbon. A significant portion of this carbon is stored in coarse woody debris (CWD). Industrial pollution greatly inhibits organic matter decomposition and thus enhances carbon sequestration in the soil. However, little is known about the decomposition of CWD in polluted areas. In this work, by means of dendrochronological cross-dating, we determined the death dates of 90 downed Siberian spruce (Picea obovata Ledeb.) and Siberian fir (Abies sibirica Ledeb.) logs in the latest stages of decay in an undisturbed boreal forest and in two industrially polluted forests with 10-fold and more than 100-fold higher copper content in the soil. We found that, in the unpolluted area, the mean halftime of fir and spruce decomposition was 26 years and 23 years, respectively. In polluted areas, this time increased by approximately 16 years for fir and 5 years for spruce. Based on an exponential decay model, pollution caused a 16–60% decrease in the wood decomposition rate constant. Copper concentrations in CWD were similar between tree species and were about 10–20 times lower than in the surrounding soil. These values are comparable with the concentrations tolerated by fungi in laboratory tests, indicating that heavy metal excess cannot be considered a primary inhibitor of wood decomposer activity. The greater pollution-induced delay in the decomposition of fir trees, which are lighter and smaller in size than spruce trees, suggests that dead tree leaning is an important factor in the wood decomposition slowdown in polluted areas where stand density increases due to intensive forest regeneration because of the reduced competition of tree seedlings with toxically inhibited herbaceous vegetation.We also evaluated dynamic probing as a rapid, low-cost, and nondestructive surrogate of wood density measuring. Dynamic probing performed in the field on heavily decayed CWD explained about 40% of wood density variability and about 70% of variability when taking into account the gravimetrically measured wood moisture content. With negligible modifications, this method can be applied in the field monitoring of wood density loss. For the purpose of between-study comparisons, we recommend transforming the widely used measure of penetration depth to specific resistance to penetration and encourage the elaboration of reference tables that relate specific resistance to penetration and wood density for different tree species across distinct ecological zones.

Intermediate temperature corrosion behaviour of Fe-12Cr-6Al-2Mo-0.2Si-0.03Y alloy (C26M) at 300–600 °C

Iron-chromium-aluminium alloys are of interest to the nuclear materials community due to their resistance to high temperature steam oxidation and good mechanical properties under irradiation. The present work investigates oxide formation on Fe-12Cr-6Al-2Mo-0.2Si-0.03Y alloy at temperatures relevant to light water reactor cladding operation following extended aging to assess growth kinetics, chemical composition, and oxide microstructure. Oxide growth follows a logarithmic time dependence. When the oxidization temperature is 400 °C or below, the oxide is amorphous. At 500 °C or above, crystalline α-Al2O3 oxide film develops and the correlated logarithmic rate constant decreases significantly, indicating enhanced oxidation resistance of the formed oxide film.

Structure-Emission Property Relationships in Cyclometalated Pt(II) β-Diketonate Complexes.

Extending the ligand π-system of phosphorescent (C∧C*) or (C∧N) cyclometalated platinum(II) β-diketonate complexes can lead to large and seemingly abrupt variations of the photophysical properties such as triplet quantum yields and phosphorescence lifetimes. Quantum chemical studies using methods including elements from density functional theory (DFT) and multireference configuration interaction (MRCI) as well as spin-orbit coupling (SOC) provide a rationale for these observations. In the Franck-Condon region, the first excited singlet states (S1) of these complexes are characterized by mixed metal-to-ligand charge-transfer (MLCT) and ligand-centered (LC) excitations. With increasing extension of the effective π-system, the lowest-lying triplet state yields more and more LC character, thus leading to a decrease of the phosphorescence rate constant. The ability to undergo efficient intersystem crossing from S1 to T1 is not diminished as the S1 state largely retains its character. In the N-heterocyclic carbene (NHC) complexes investigated here, at least two triplet states are found energetically below the S1 state. Out-of-plane distortion enhances the probability for nonradiative decay of the triplet population. In the smaller compounds emitting in the violet or blue spectral region, the phosphorescent state is separated from the lowest-lying dark metal-centered (MC) triplet state by a small barrier only, explaining their experimentally observed low photoluminescence quantum yields in liquid solution. The semiempirical DFT/MRCI-R2018 Hamiltonian employed in our studies proves well-suited for investigating the absorption and emission properties of these platinum(II) complexes. Generally, good agreement is observed between our calculated data and the experimental findings.

Understanding the Effects of Coordination and Self-Assembly on an Emissive Phenothiazine.

The local environment surrounding luminophores can significantly influence their photophysical properties. Herein, we report the self-assembly of a highly emissive platinum(II)-based metallacage. In order to accommodate the connectivity of the platinum(II) building block used in the self-assembly process, the luminophore-containing building block adopts a highly twisted geometry relative to its free form, leading to the emergence of an emissive transition with a radiative rate constant an order of magnitude higher than that of the free luminophore. This increased rate constant is the primary driver for the 10-fold increase in quantum yield from 4.2% to 40%. Model complexes with platinum or methyl groups bound to the nitrogen were synthesized. These complexes had lower quantum yields (10% and non-emissive, respectively) due mainly to decreases in radiative rate constants. Computational studies were conducted and indicated that the excited state of the ensembles, as well as the model complexes, is a result of charge transfer to the pyridyl groups, in contrast to the free luminophore, which involves the diphenyl sulfone moiety. The differences in quantum yields can be explained by a twist in the chromophore upon coordination of platinum or methylation on the pyridyl group, leading to intersystem crossing to a triplet state. This state then becomes more emissive with the addition of platinum, which increases the radiative rate constant via the heavy atom effect. The formation of a metallacage also decreases the non-radiative rate constant by inhibiting the intramolecular motions of the incorporated luminophore.

Influence of the mean volumetric temperature of a thermoelement on reliability indicators and the dynamics of a cooler

The paper deals with the influence of the mean volumetric temperature of a thermo-element branch on the basic parameters, reliability indicators and the dynamics of operation of thermo-electric cooler under different temperature changes at the assigned heat load, geometry of branches of thermo-elements for specific current operation modes. It was shown that the mean temperature of the thermo-element, which is a reference point when calculating the energy performance of the thermoelectric cooler, can be used only for calculations in the stationary operation mode. Its use in the dynamic mode leads to significant errors. It was proven that the mean volumetric temperature of a thermo-electric branch can be such a reference point for a dynamic mode. The ratio for assessment of the mean volumetric temperature depending on the relative operating current was determined. The relationships between the mean volumetric temperature of a thermo-element, the time required to enter a stationary mode, the required number of thermo-elements, the differences between the mean volumetric and the mean temperature, and a cooling coefficient, depending on the relative working current, were analyzed. It was shown that at an increase in the mean volumetric temperature at the assigned current operation mode and a temperature difference, exceeding 40K, the magnitude of operating current, the number of thermo-elements, consumption power, failure rate and time constant decrease and cooling factor increases. The time for entering a stationary mode during the transition from the mode of minimum failure rate to the mode of maximum cooling capacity decreases by 5 %, while the failure rate increases by 16 %. The practical significance of the conducted research is both to improve the quality of design of coolers and to select the necessary modes of thermo-electric system for ensuring thermal modes of electronic equipment depending on the importance of dynamic or reliability criteria of control

Removal of metronidazole antibiotic pharmaceutical from aqueous solution using TiO2/Fe2O3/GO photocatalyst: Experimental study on the effects of mineral salts

A TiO2/Fe2O3/GO photocatalyst is synthesized via the sol-gel method and characterized by X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), FT-IR, Brunauer-Emmett-Teller (BET), and Ultraviolet-Visible Diffuse Reflection Spectroscopy (UV-Vis DRS) analyses. Metronidazole (MET) concentration (10-20 mg/L), photocatalyst concentration (0.5-1.5 g/L), irradiation time (60-120 min), and initial pH (4-6) are investigated through response surface methodology (RSM), and the optimal process conditions are determined. The removal efficiency of MET with the TiO2/Fe2O3/GO photocatalyst is 97% under optimal conditions: a pollutant concentration of 10 mg/L, the irradiation time of 120 min, photocatalyst concentration of 1 g/L, and pH of 5. The influence of mineral salts concentrations (50-800 mg/L), including NaCl, Na2SO4, NaHCO3, KCl, MgSO4, and CaCl2, are examined at the initial pH of 5, photocatalyst concentration of 1 g/L, and pollutant concentration of 20 mg/L. According to the results, the reaction rate constant decreases with an increase in mineral salts concentrations up to 800 mg/L, especially with Na2SO4 (42.43% deactivation) and also with MgSO4 (38.08%) and NaHCO3 (37.73%), under the same operational conditions. The effects of mineral salts such as NaCl and KCl on the reaction rate constant for the contaminant removal efficiency have a downward trend until these salts reach a 200 mg/L concentration, and then they experience an upward trend.

A steady state system for in vitro evaluation of steroidogenic pathway dynamics: Application for CYP11B1, CYP11B2 and CYP17 inhibitors

Disorders featuring dysregulated adrenal steroidogenesis, such as primary aldosteronism, can benefit from targeted therapies. The aldosterone and cortisol producing enzymes, aldosterone synthase (CYP11B2) and 11-beta-hydroxylase (CYP11B1), share 93% homology requiring selective drugs for pharmacological treatment. Herein, we introduce an effective in vitro assay for evaluation of steroidogenic enzyme kinetics based on intracellular flux calculations. H295RA cells were cultured in chambers under constant medium flow. Four hourly samples were collected (control samples), followed by collections over an additional four hours after treatment with fadrozole (10 nM), metyrapone (10 μM), SI_191 (5 nM), a novel CYP11B2 inhibitor or SI_254 (100 nM), a newly synthesized 17-alpha-hydroxylase/17,20-lyase inhibitor. Mass spectrometric measurements of multiple steroids combined with linear system computational modeling facilitated calculation of intracellular fluxes and changes in rate constants at different steroidogenic pathway steps, enabling selectivity of drugs for those steps to be evaluated. While treatment with fadrozole, metyrapone and SI_191 all reduced fluxes of aldosterone, corticosterone and cortisol production, treatment with SI_254 led to increased flux through the mineralocorticoid pathway and reduced production of steroids downstream of 17-alpha-hydroxylase/17,20-lyase. Drug-induced decreases in rate constants revealed higher selectivity of SI_191 compared to other drugs for CYP11B2 over CYP11B1, this reflecting additional inhibitory actions of SI_191 on catalytic steps of CYP11B2 downstream from the initial 11-beta-hydroxlase step. By culturing cells under perfusion the described system provides a realistic model for simple and rapid calculations of intracellular fluxes and changes in rate constants, thereby offering a robust procedure for investigating drug or other effects at specific steps of steroidogenesis.

Decrease in Swelling Capacity of Pine Wood Modified with Aminoborates

Upon direct contact of wood with water, a hydrophilic hydroxylated substrate actively absorbs water molecules due to the capillary-porous structure of the wood. Since water is characterized by high dielectric permittivity, sorption of water is usually accompanied by the swelling of the wood. It leads to the deterioration of the physical and the mechanical properties of the wood composite and, when the surface is covered with a film of paintwork materials while the wood is not subjected to an appropriate treatment, the pressure of the constrained swelling can cause destruction of the substrate. We have established in our work that the modification of the pine wood with the aqueous solutions of aminoborates leads to a decrease in the equilibrium degree of swelling, with 50 % modifier solutions being the most effective in this respect. In this study, the swelling rate of the substrate was found to be inversely proportional to the concentration of the modifier — with an increase of the concentration of the modifiers the swelling rate was markedly reduced due to the rate constants decrease. The results obtained allow us to speak of a decrease in swelling capacity of pine wood modified with aminoborates.

Kinetic analysis of liquid phase photocatalysis and photolysis: A frequent disguise!

Abstract Studies of phenol photocatalyzed oxidation and of organic dye photolyzed or photocatalyzed destruction which explored variable initial concentrations, Co, often assumed pseudo first order behavior, and fitted data to a semilog plot of concentration, C(t) vs time, and found that the apparent first order rate constant decreases with increasing Co. We show from multiple literature examples that this behavior often disguises a true zero order initial rate of reaction. We recommend a re-analysis of existing literature reporting first order rate constants which vary with initial reactant concentration, Co.

Role of various factors affecting the photochemical treatment of N-nitrosamines related to CO2 capture

ABSTRACT Post-combustion CO2 capture using amine solvents is the most feasible method of reducing anthropogenic CO2 emissions, which are the largest contributor to global warming. The formation of carcinogenic N-nitrosamines (i.e. by-products) can hinder the industrial application of this technology. In this study, the effects of direct UV photolysis (N-nitrosamine concentration and amines) and advanced oxidation processes (UV/H2O2 and UV/O3) on the three specific N-nitrosamines that are commonplace in amine-based CO2 capture (i.e. N-nitrosodiethylamine (NDEA), N-nitrosodiethanolamine (NDELA), and N-nitrosomorpholine (NMOR)) were examined. A significant decrease in the photodegradation rate constants was observed for NDEA (1.02 × 100 to 2.94 × 10−1 min−1), NDELA (1.52 × 100 to 3.32 × 10−1 min−1), and NMOR (1.93 × 100 to 2.20 × 10−1 min−1) as their concentrations increased within 1–50 mg/L. This is the first report of a significant increase in the degradation rate constants of N-nitrosamine with an increase in amine concentrations (i.e. monoethanolamine, diethanolamine, and morpholine) within 10–200 mM. The photodegradation rate constants increased as the molar ratio of H2O2 to N-nitrosamine increased to 20, but then decreased at molar ratios beyond this. O3 had a negligible effect on the photodegradation of N-nitrosamines.

Theoretical Study on the Reaction Mechanism and Kinetics of Criegee Intermediate CH2OO with Acrolein.

The detailed reaction mechanism and kinetics of Criegee intermediate CH2OO with acrolein were investigated. CH2OO may add to the C═O or C═C double bond of acrolein to form a five-membered ring adducts, and it may also insert the terminal oxygen atom or insert itself into the C-H bond of acrolein. The addition reactions are more favorable in energy than the insertion reactions. The master equation calculation show that the most competitive reaction channel is the 1,3-cycloaddition of CH2OO across the C═O double bond forming the secondary ozonide (SOZ). The lowest energy pathway for SOZ decomposition involves the formation of the singlet biradical intermediate by ring fission, the H-shift isomerization and the dissociation to products. The calculated overall rate constant decreases as the temperature increases from 200 to 500 K, and at 298 K, it is 4.31 × 10-12 cm3 molecule-1 s-1. The branching ratio of collisionally stabilized SOZ increases with the increase of pressure. At low pressure, some of SOZ decompose to HCOOH + acrolein or HCHO + acrylic acid. The pressure dependence of this reaction is in agreement with the previous theoretical and experimental observations for the reaction of CH2OO with acetaldehyde.

Self-Inhibitory Electron Transfer of the Co(III)/Co(II)-Complex Redox Couple at Pristine Carbon Electrode.

Applications of conducting carbon materials for highly efficient electrochemical energy devices require a greater fundamental understanding of heterogeneous electron-transfer (ET) mechanisms. This task, however, is highly challenging experimentally, because an adsorbing carbon surface may easily conceal its intrinsic reactivity through adventitious contamination. Herein, we employ nanoscale scanning electrochemical microscopy (SECM) and cyclic voltammetry to gain new insights into the interplay between heterogeneous ET and adsorption of a Co(III)/Co(II)-complex redox couple at the contamination-free surface of electron-beam-deposited carbon (eC). Specifically, we investigate the redox couple of tris(1,10-phenanthroline)cobalt(II), Co(phen)32+, as a promising mediator for dye-sensitized solar cells and redox flow batteries. A pristine eC surface overlaid with KCl is prepared in vacuum, protected from contamination in air, and exposed to an ultrapure aqueous solution of Co(phen)32+ by the dissolution of the protective KCl layer. We employ SECM-based nanogap voltammetry to quantitatively demonstrate that Co(phen)32+ is adsorbed on the pristine eC surface to electrostatically self-inhibit outer-sphere ET of nonadsorbed Co(phen)33+ and Co(phen)32+. Strong electrostatic repulsion among Co(phen)32+ adsorbates is also demonstrated by SECM-based nanogap voltammetry and cyclic voltammetry. Quantitatively, self-inhibitory ET is characterized by a linear decrease in the standard rate constant of Co(phen)32+ oxidation with a higher surface concentration of Co(phen)32+ at the formal potential. This unique relationship is consistent not with the Frumkin model of double layer effects, but with the Amatore model of partially blocked electrodes as extended for self-inhibitory ET. Significantly, the complicated coupling of electron transfer and surface adsorption is resolved by combining nanoscale and macroscale voltammetric methods.

Electrochemical Behavior of Niobium Complexes in Chloride-Fluoride Melts with Addition of Alkaline Cations

The goal of the present research was the study of Mg2+, Ca2+, Sr2+ and Ва2+ (Me2+) strongly polarizing cations influence on the standard rate constants of charge transfer (ks ) for the redox couple Nb(V)/Nb(IV) in NaCl-KCl(equimol.)-K2NbF7 (I) and NaCl-KCl(equimol.)-NaF(10 wt.%)-K2NbF7 (II) melts. Electrochemical studies were carried out in the temperature range of 973-1173 K by cyclic voltammetry. Voltammetric curves were recorded at a glassy carbon electrode vs. a glassy carbon quasi-reference electrode. The sweep rate (v) varied from 0.4 up to 2.0 V s-1. The electrochemical redox process: Nb(V) + e- ↔ Nb(IV) (1) was classified as quasi-reversible at a sweep rate ν>0.5 V s-1. The values of k s were calculated by using the Nicholson’s method [1]. It was shown that k s increase with increasing temperature. It is due to increase in the number of particles capable of overcoming the potential barrier for the electron transfer process. Addition of alkaline earth metal cations resulted in increasing of ks to the certain ratio of Me2+/Nb(V) for the all alkaline earth metal cations due to substitution of Na+ and K+ cations by Me2+ in the second coordination sphere of niobium complexes that leads to decreasing of niobium fluoride complexes strength. Further addition brought to some decrease of the standard rate constants because the viscosity of melts increasing which brings to decrease of the diffusion coefficients. The temperature dependences of molten salt systems with addition of strongly polarizing cations were determined and the activation energies of charge transfer were calculated. Values of activation energy for systems with strongly polarizing cations are considerably less than activation energies of the initial systems. The standard rate constants in molten salt system (I) were higher than in the melt (II) due to lower viscosity. It was determined that in the melt (I) ks a little bit increased with increasing scan rate probably due to the secondary electrode process which accompanied redox process (1). In the system with sodium fluoride (II) the redox process (1) occurs at more negative potentials that allow eliminating the secondary process.

Electrochemical Behavior of Niobium Complexes in Chloride-Fluoride Melts with Addition of Alkaline Cations

The standard rate constants of charge transfer (k s) for the redox couple Nb(V)/Nb(IV) in NaCl-KCl(equimol.)-NaF(10 wt.%)-K2NbF7 (I) and NaCl-KCl(equimol.)-K2NbF7 (II) melts with addition of alkaline earth metal cations were determined. It was found that addition of alkaline earth metal cations resulted in increasing of k s to the certain ratio of Me2+/Nb(V) for the all alkaline earth metal cations due to substitution of Na+ and K+ cations by Me2+ in the second coordination sphere of niobium complexes that leads to decreasing of niobium fluoride complexes strength. Further addition brought to some decrease of the standard rate constants because the viscosity of melts increasing which brings to decrease of the diffusion coefficients. It was determined that in the melt (II) k s a little bit increased with increasing scan rate probably due to the secondary electrode process.

Effects of Coadsorbed Water on the Heterogeneous Photochemistry of Nitrates Adsorbed on TiO2.

Nitric acid, a well-known sink of NO x gases in the atmosphere, has been found to be photoactive while adsorbed on tropospheric particles. When adsorbed onto semiconductive metal oxides, nitrate's photochemical degradation can be interpreted as a photocatalytic process. Yet, the photolysis of nitrate ions on the surface of aerosols can also be initiated by changes in the symmetry of the ion upon adsorption. In this study, we use quantum chemistry to model the vibrational spectra of adsorbed nitrate on TiO2, a semiconductor component of atmospheric aerosols, and determine the kinetics of the heterogeneous photochemical degradation of nitrate under simulated solar light. Frequencies and geometry calculations suggest that the symmetry of chemisorbed nitrate ion depends strongly on coadsorbed water, with water changing the reactive surface of TiO2. Upon irradiation, surface nitrate undergoes photolysis to yield nitrogen-containing gaseous products including NO2, NO, HONO, and N2O, in proportions that depend on relative humidity (RH). In addition, the heterogeneous photochemistry rate constant decreases an order of magnitude, from (5.7 ± 0.1) × 10-4 s-1 on a dry surface to (7.1 ± 0.8) × 10-5 s-1 when nitrate is coadsorbed with water above monolayer coverage. Little is known about the roles of coadsorbed water on the heterogeneous photochemistry of nitrates on TiO2, along with its impact on the chemical balance of the atmosphere. This work discusses the roles of water in the photolysis of surface nitrates on TiO2 and the concomitant renoxification of the atmosphere.