Tag Archives: mechanism

Oxidation of Silver Cyanide Ag(CN)2 by the OH Radical: From Ab Initio Calculation to Molecular Simulation and to Experiment

by Céline Léonard, Frédéric Le Quéré, Daniel Adjei, Sergey A. Denisov, Mehran Mostafavi and Pierre Archirel

J. Phys. Chem. A 2020, 124, 51, 10787–10798; doi.org/10.1021/acs.jpca.0c08038

We investigate the oxidation of silver cyanide  in water by the OH radical in order to compare this complex with the free cation Ag+ and to measure the influence of the ligands. High-level ab initio calculations of the model species  enable the calibration of molecular simulations and the prediction of the oxidized species:  and its absorption spectrum, with an intense band at 292 nm and a weaker one at 390 nm. Pulse radiolysis measurements of the oxidation of  by the OH radical in water yields a transient species with a broad, intense band at 290 nm and a weaker band at 410 nm at short times after the pulse and a blue shift of the spectrum at longer times. The prediction of the simulations, that the oxidized complex  is formed, is confirmed by thermochemistry. Our calculations also suggest that the formation of the OH-adduct is possible only in very basic solution and that the blue shift observed at long times after the pulse is due to disproportionation of the oxidized complex. We also perform molecular simulations of the oxidation of free Ag+ cations by the OH radical. The results are compared to that of the literature and to the results obtained with the  complex.

Mechanisms of metal nanoparticles nucleation and growth studied by radiolysis

by J.Belloni, J.-L.Marignier, M.Mostafavi

Radiation Physics and Chemistry 2020, 169, 107952; doi.org/10.1016/j.radphyschem.2018.08.001

The ultradivided matter is used for long in various applications, for example in colloids, inks and paints, cosmetics, stained glasses, catalysts, photographic emulsions, … But the progressive need of nanoparticles for various miniaturized devices and the different approaches for the synthesis have suddenly increased.

All of the bottom-up synthesis methods from a diluted precursor to metal nanoparticles imply several steps: a reduction reaction of ionic precursors by electron transfer, inducing the nucleation of atoms then the growth of the seeds into particles, more or less inhibited by stabilizers. The final size, shape, structure and dispersity of the particles strongly depend on the thermodynamics and the kinetics of these steps. The interaction of high energy radiation with the solvent provides, quantitatively and homogeneously distributed in the bulk, strong electron donors (solvated electrons, reducing radicals) which reduce metal ions as precursors into atoms. The radiation chemistry, on one hand in the steady state regime with an accurate knowledge of the yields of all the radiolytic products, and on the other hand in the pulse regime giving access to time-resolved data, constitutes a unique tool to elucidate the detailed mechanisms and to provide the keys of really controlling these processes in view of various applications.

Pulse radiolysis study on the reactivity of NO3˙ radical toward uranous(iv), hydrazinium nitrate and hydroxyl ammonium nitrate at room temperature and at 45 °C

by R. Musat, J. L. Marignier, C. Le Naour,  S. Denisov,  L. Venault, Ph. Moisy  and  M. Mostafavi

Phys. Chem. Chem. Phys., 2020, 22, 5188-5197; https://doi.org/10.1039/C9CP07034F

Concentrated nitric acid solutions subjected to radiation produce radicals of extreme importance in the reprocessing of spent nuclear fuel. Knowledge of the different rate constants of the reactions involved in this chemistry is needed to improve the efficiency of the process and to define safe operating practices. Pulse radiolysis measurements are performed to find the rate constant of the reaction between NO3˙ radicals and U(IV) in highly concentrated nitrate solution. The optimal stabilization conditions toward thermal oxidation are defined for the considered solutions at room temperature and at 45 °C by adding anti-nitrous agents such as hydrazinium nitrate (HN) and hydroxyl ammonium nitrate (HAN). The decay of the NO3˙ radical is monitored and its reaction rates with HN, HAN and U(IV) are found to be 1.3 × 105, 1.5 × 107 and 1.6 × 106 M−1 s−1 at room temperature. The latter value is more than 10 times lower than the one currently used in numerical codes for simulation of the long-term radiolytic degradation associated with the reprocessing and storage of spent nuclear waste. At 45 °C, conditions similar to the reprocessing of spent fuel, the values of the rate constants of NO3˙ radical toward HN, HAN and U(IV) increase and are found to be 2.6 × 105, 2.9 × 107 and 9.3 × 106 M−1 s−1.

Mechanism of (SCN)2·– Formation and Decay in Neutral and Basic KSCN Solution under Irradiation from a Pico- to Microsecond Range

by Furong Wang, Pascal Pernot, Jean-Louis Marignier, Pierre Archirel and Mehran Mostafavi

J. Phys. Chem. B 2019, 123, 6599–6608; doi.org/10.1021/acs.jpcb.9b05560

The detailed mechanism of the reaction between SCN and the OH· radical and the formation of the dimer radical (SCN)2·– are studied by picosecond pulse radiolysis. First, concentrated SCN solutions are used to observe directly the formation and decay of SCNOH·– in neutral and basic solutions. Then, the spectro-kinetic data, constituting a large matrix of data of the absorbance at different times and different wavelengths, obtained by pulse radiolysis measurements with a streak camera, in neutral and basic SCN solutions, are analyzed simultaneously. Data analysis allowed us to deduce the absorption spectra of different radicals with their extinction coefficient and also to determine the rate constants of different reactions involved in the formation and decay of (SCN)2·–. Molecular simulations of the absorption spectra of the different species were also performed. The absorption spectrum of the radical SCN· is determined and is found to be different than that reported previously. It does not present a Gaussian shape centered at 330 nm; the absorption around 310 and 380 nm is not negligible. In addition, in a solution at pH 13, it is found that the (SCN)2·– radical is paired with an alkaline cation, inducing a blueshift of the absorption band compared to the free (SCN)2·–. Finally, the presence of K+ cations catalyzes the disproportionation reaction of (SCN)2·– and affects the kinetics.

Decoding the Three-Pronged Mechanism of NO3 Radical Formation in HNO3 Solutions at 22 and 80 °C Using Picosecond Pulse Radiolysis

by Raluca Musat, Sergey A. Denisov, Jean-Louis Marignier and Mehran Mostafavi

J. Phys. Chem. B 2018, 122, 2121–2129; doi.org/10.1021/acs.jpcb.7b12702

With nitric acid (HNO3) being at the core of nuclear technology through actinides separation and extraction processes, achieving a complete characterization of the complex processes involving concentrated HNO3 solutions under ionizing radiation equates bringing efficiency and safety into their operation. In this work, the three mechanisms contributing to the formation of nitrate radicals (NO3) in concentrated nitric acid were investigated by measuring the radiolytic yield of NO3 in HNO3 solutions (0.5–23.5 M) at room (22.5 °C) and elevated (80 °C) temperatures on time scales spanning from picosecond to microsecond by pulse radiolysis measurements. We conclude that the formation yield of NO3, just after the 7 ps electron pulse, is due to the direct effect and to the ultrafast electron transfer reaction between NO3 and the water cation radical, H2O•+. The absolute formation yield of NO3 radicals due to the direct effect, GNO3dir, is found to be (3.4 ± 0.1) × 10–7 mol·J–1, irrespective of the concentration and temperature. On longer time scales, >1 ns, an additional contribution to NO3 formation from the reaction between OH radicals and undissociated HNO3 is observed. The rate constant of this reaction, which is activation-controlled, was determined to be (5.3 ± 0.2) × 107 M–1·s–1 for 22.5 °C, reaching a value of (1.1 ± 0.2) × 108 M–1·s–1 at 80 °C.

Reaction mechanisms in swelling clays under ionizing radiation: influence of the water amount and of the nature of the clay mineral

by M. Lainé, E. Balan, T. Allard, E. Paineau, P. Jeunesse, M. Mostafavi, J.-L. Robert and S. Le Caër

RSC Adv.
, 2017,7, 526-534; doi.org/10.1039/C6RA24861F

We have studied the H2 production under ionizing radiation of water confined in synthetic saponite and montmorillonite as a function of the relative humidity. The H2 radiolytic yields in the dry systems are very similar to that measured in a non-swelling clay mineral. They are 2–3 times higher with one water layer in the interlayer space, evidencing very efficient energy transfers and efficient recombination reactions due to a high confinement. With two water layers, the H2 yields decrease as compared to the previous case, but remain higher than in bulk water, proving that recombination reactions of hydrogen atoms are less efficient. Electron paramagnetic resonance measurements evidence that reactivity changes significantly with the number of water layers. Saponite and montmorillonite give similar results, showing that reactivity is driven by the amount of water and that the details of the clay structure play a less important role. Lastly, the behavior of natural vs. synthetic swelling clays is discussed. The presence of impurities, even in small quantities, significantly alters energy transfers and has a positive implication for the geological nuclear waste management.

Ultra-fast charge migration competes with proton transfer in the early chemistry of H2+

by Furong Wang, Uli Schmidhammer, Aurélien de La Landea and Mehran Mostafavi

Phys. Chem. Chem. Phys., 2017, 19, 2894-2899; doi.org/10.1039/C6CP07013B

Oxidation by the ultra-short lived radical cation of water, H2+, can potentially take place at the interface of water and numerous heterogeneous systems involved in radiation therapy, energy and environmental industries. The oxidation processes induced by H2+ can be mimicked in highly concentrated solutions where the nearest neighbors of H2+ may be molecules other than water. The reactivity of H2+ and D2+ is probed in hydrogenated and deuterated sulfuric acid solutions of various concentrations. The oxidized solute, sulfate radical, is observed at 7 ps and remarkably higher yields are found in deuterated solutions. The isotopic effects reveal the competition between two ultrafast reactions: proton transfer toward H2O (D2O) and electron transfer from HSO4 to H2+ (D2+). Density functional theory simulations decipher the electron transfer mechanism: it proceeds via sub-femtosecond charge migration and is not affected by isotopic substitution. This work definitively demonstrates why direct oxidation triggered by H2+ can be competitive with proton transfer.

Role of PF6 in the radiolytical and electrochemical degradation of propylene carbonate solutions

by Daniel Ortiz, Isabel Jimenez, Gordon Solène Legand, Vincent Dauvois, Jean-Pierre Baltaze, Jean-Louis Marignier, Jean-Frédéric Martin, Jacqueline Belloni, Mehran Mostafavi, Sophie Le Caër

Journal of Power Sources 2016, 326, 15, 285-295; doi.org/10.1016/j.jpowsour.2016.06.122

The behavior under irradiation of neat propylene carbonate (PC), a co-solvent usually used in Li-ion batteries (LIB), and also of Li salt solutions is investigated. The decomposition of neat PC is studied using radiolysis in the pulse and steady state regime and is assigned to the ultrafast formation, in the reducing channel, of the radical anion PC by electron attachment, followed by the ring cleavage, leading to CO. In the oxidative channel, the PC(-H) radical is formed, generating CO2. The CO2 and CO yields are both close to the ionization yield of PC. The CO2 and CO productions in LiClO4, LiBF4 and LiN(CF3)2(SO2)2 solutions are similar as in neat PC. In contrast, in LiPF6/PC a strong impact on PC degradation is measured with a doubling of the CO2 yield due to the high reactivity of the electron towards PF6 observed in the picosecond range. A small number of oxide phosphine molecules are detected among the various products of the irradiated solutions, suggesting that most of them, observed in carbonate mixtures used in LIBs, arise from linear rather than from cyclical molecules. The similarity between the degradation by radiolysis or electrolysis highlights the interest of radiolysis as an accelerated aging method.