Kinetic Aspects of the Electrochemical Reduction of Uranyl in HCl Solutions

The interfacial mechanism of uranyl electroreduction at Au-electrodes in HCl solutions was discussed on the light of systematic studies of cyclovoltammetry, normal pulse voltammetry, UV-Vis spectroscopy and published electroanalytical research. Voltammetric waves were numerically deconvoluted on the basis of a reaction model consisting of a first mass-controlled, quasi-reversible first electron transfer and a subsequent reduction of interfacial U(V) intermediate species with adsorption of generated U(IV) products. The dependence of the kinetic parameters on [HCl] indicates an electron transfer following an inner-sphere type mechanism assisted by electrosorption of chloride ligands. The interfacial accumulation of chloride exerts a strong electrostatic repulsion of complexed uranyl and a consequent edged drop of electron transfer rates at [HCl] ∽ 0.5 mol l-1. The electron transfer steps are followed by a chemical desorption reaction of the unstable tetravalent uranyl in U(H2O)9 4+ species. It is shown that the numerical reproduction of voltammetric waves suits as a method for calculating kinetic parameters in multi-steps electrochemical reactions.

Ammonium Chloride (NH4Cl)—Ammonia (NH3): Sorption Characteristics for Heat Pump Applications

In a resorption heat pump, the adsorption and desorption reaction of ammonium chloride (NH4Cl) with ammonia (NH3) is of interest as a Low Temperature Salt (LTS). Reviewing previously published NH4Cl-NH3 equilibrium lines, ammonium chloride appears to offer useable working temperatures (50–70 °C) in the 10–15 bar pressure range during the adsorption reaction, and provides beneficial working conditions for the desorption reaction, when compared with alternative LTS candidates at atmospheric pressure. The NH4Cl-NH3 adsorption and desorption reactions, using a NH4Cl composite salt, have been evaluated under dynamic ‘real-world’ conditions in a Large Temperature Jump (LTJ) experimental testing rig; although there are concerns with mass transfer characteristics, the salt exhibits no hysteresis between the adsorption and desorption reactions, contrary to previous literature. The experimentally obtained equilibrium line values for the reaction enthalpy and entropy are 29,835 J/mol and 207 J/(mol∙K), respectively. Using a semi-empirical model, the NH4Cl composite salt has been successfully characterised, enabling the prediction of salt reaction behaviour. The model constants, A and n, identified are 4.5 and 5 for adsorption and 5 and 4 for desorption, with an overall salt active fraction (applicable to both reactions) of 0.98. Overall, the working equilibrium line and the dynamic performance of ammonium chloride has been investigated and the applicability of NH4Cl as a LTS for a resorption heat pump determined.

Mechanistic Insights into the Conversion of Dimethyl Ether over ZSM-5 Catalysts: A Combined Temperature Programmed Surface Reaction and Microkinetic Modelling Study

Rates of adsorption, desorption, and surface reaction of dimethyl ether (DME) to olefins over fresh and working ZSM-5 catalysts of different Si/Al ratios (36 and 135) have been decoupled using a combination of temperature programmed surface reaction experiments and microkinetic modelling. Transient reactor performance was simulated by solving coupled 1D non-linear partial differential equations accounting for elementary steps occurring during the induction period based on the methoxymethyl mechanism on the zeolite catalyst, and axial dispersion and convection in the reactor. Propylene is the major olefin formed and scaling relations between activation energies of DME desorption and barriers of formation of methoxymethyl and methyl propenyl ether are observed. Six ensembles of sites are observed with a maximum of three adsorption/desorption sites and three adsorption/desorption/reaction sites. Barriers are generally higher over working catalysts than fresh catalysts. Activation energies of propylene formation of ca. 200 kJ mol^-1 are obtained corroborating direct mechanistic proposals.

Mechanistic Insights into the Conversion of Dimethyl Ether over ZSM-5 Catalysts: A Combined Temperature Programmed Surface Reaction and Microkinetic Modelling Study

Rates of adsorption, desorption, and surface reaction of dimethyl ether (DME) to olefins over fresh and working ZSM-5 catalysts of different Si/Al ratios (36 and 135) have been decoupled using a combination of temperature programmed surface reaction experiments and microkinetic modelling. Transient reactor performance was simulated by solving coupled 1D non-linear partial differential equations accounting for elementary steps occurring during the induction period based on the methoxymethyl mechanism on the zeolite catalyst, and axial dispersion and convection in the reactor. Propylene is the major olefin formed and scaling relations between activation energies of DME desorption and barriers of formation of methoxymethyl and methyl propenyl ether are observed. Six ensembles of sites are observed with a maximum of three adsorption/desorption sites and three adsorption/desorption/reaction sites. Barriers are generally higher over working catalysts than fresh catalysts. Activation energies of propylene formation of ca. 200 kJ mol^-1 are obtained corroborating direct mechanistic proposals.

A quantitative multiscale perspective on primary olefin formation from methanol

Our quantitative multi-scale perspective on the formation of the first C–C bond decouples the adsorption, desorption, reaction, and mobility of species and provides new insights that could guide rational catalyst design.

Effects of Ball Milling and TiF3 Addition on the Dehydrogenation Temperature of Ca(BH4)2 Polymorphs

The changes introduced by both ball milling and the addition of small amounts of TiF3 in the kinetics of the hydrogen desorption of three different Ca(BH4)2 polymorphs (α, β and γ) have been systematically investigated. The samples with different polymorphic contents, before and after the addition of TiF3, were characterized by powder X-ray diffraction and vibrational spectroscopy. The hydrogen desorption reaction pathways were monitored by differential scanning calorimetry. The hydrogen desorption of Ca(BH4)2 depends strongly on the amount of coexistent α, β and γ polymorphs as well as additional ball milling and added TiF3 to the sample. The addition of TiF3 increased the hydrogen desorption rate without significant dissociation of the fluoride. The combination of an α-Ca(BH4)2 rich sample with 10 mol% of TiF3 and 8 h of milling led to up to 27 °C decrease of the hydrogen desorption peak temperature.

Desorption reaction in MgH2 studied with in situ μ+SR

To study the mechanism determining the desorption temperature of hydrogen storage materials, we have measured muon spin rotation and relaxation (μ+SR) in MgH2 together with the pressure in the sample space.

On the possibility of spontaneous chemomechanical oscillations in adsorptive porous media

We derive general conditions for the emergence of sustained chemomechanical oscillations from a non-oscillatory adsorption/desorption reaction in a gas/solid porous medium. The oscillations arise from the nonlinear response of the solid matrix to the loading of the adsorbed species. More particularly, we prove that, in order for oscillations to occur, adsorption of the gas must in general cause a swelling of the solid matrix. We also investigate the prototypical case of Langmuir kinetics both numerically and analytically. This article is part of the theme issue ‘Dissipative structures in matter out of equilibrium: from chemistry, photonics and biology (part 2)’.