Highlights  CO2 absorption into aqueous amine blended solutions containing MEA, AMP, N,N-DMEA, and N,N-DEEA  Overall CO2 mass transfer coefficients in blends similar to those in standalone MEA solutions  CO2 absorption and cyclic capacities predicted using chemical equilibrium modelling tool

Piperazine (PZ) is widely recognized as a promising solvent for postcombustion capture (PCC) of carbon dioxide (CO(2)). In view of the highly conflicting data describing the kinetic reactions of CO(2)(aq) in piperazine solutions, the present study focuses on the identification of the chemical mechanism, specifically the kinetic pathways for CO(2)(aq) in piperazine solutions that form the mono- and dicarbamates, using the analysis of stopped-flow spectrophotometric kinetic measurements and (1)H NMR spectroscopic data at 25.0 °C. The complete set of rate and equilibrium constants for the kinetic pathways, including estimations for the protonation constants of the suite of piperazine carbamates/carbamic acids, is reported here using an extended kinetic model which incorporates all possible reactions for CO(2)(aq) in piperazine solutions. From the kinetic data determined in the present study, the reaction of CO(2)(aq) with free PZ was found to be the dominant reactive pathway. The superior reactivity of piperazine is confirmed in the kinetic rate constant determined for the formation of piperazine monocarbamic acid (k(7) = 2.43(3) × 10(4) M(-1) s(-1)), which is within the wide range of published values, making it one of the faster reacting amines. The corresponding equilibrium constant for the formation of the monocarbamic acid, K(7), markedly exceeds that of other monoamines. Kinetic and equilibrium constants for the remaining pathways indicate a minor contribution to the overall kinetics at high pH; however, these pathways may become more significant at higher CO(2) loadings and lower pH values where the concentrations of the reactive species are correspondingly higher.

The time resolved mechanism of electrodeposition and the effect of this changing mechanism on the nucleation and growth of solid manganese dioxide has been investigated in both acidic and neutral electrolytes on the rotating ring disc electrode (RRDE). The fate of the Mn 3+ intermediate is a key feature of this electrodeposition mechanism, the formation of which is dependent on the substrate, which in this case is either platinum, MnO 2 or MnOOH. On the platinum surface, for all electrolytes, soluble Mn 3+ is produced initially. The stability of this soluble Mn 3+ species determines the initial morphology, and rate of change of mechanism for the process. In a neutral electrolyte, nucleation and growth of MnO 2 occurs primarily through the precipitation of a 2D film of MnOOH on the platinum, which rapidly covers the surface. Nucleation in an acidic H 2 SO 4 system occurs primarily via a disproportionation route which forms 3D MnO 2 hemispheroids that cover the substrate slowly. Subsequent growth of MnO 2 in both electrolytes then proceeds via formation of a MnOOH film, which is subsequently oxidized in the solid state to form MnO 2 . MnOOH oxidation to MnO 2 appears kinetically limited, which is overall a limiting factor in the electrodeposition process.

In the current work ultraviolet spectrophotometric titrations at different S(iv) concentrations have been globally analysed using the entire spectral dataset to determine the complete speciation of S(iv) in aqueous solution over a large pH range (from 9.6 to 1). As a result, the dimerisation constant for the formation of disulfite from hydrogen sulfite has been accurately determined. Further, protonated disulfite has been identified and quantified for the first time. In addition, the molar absorptivities of all S(iv) species are also reported over the studied wavelength range.

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