| The sub-molten solt (SMS), a strong alkaline solution with high chemicalreactivity and high ionic activity， is a new reaction media that is able to convert thereferactory amphoteric mineral efficiently and cleanly under relatively mild condition.This new reaction media provides a new approach for efficient utilization of resourceand energy while significantly decreasing the instrustrial waste discharge. Previousstudies suggest that the high reactivity of the SMS is due to the existence of reactiveoxygen species (ROS) in the media, and consequently, the fundamental understandingthe ROS formation mechanisms as well the transformation principles is critical for themanipulating the reaction thermodynamics and kinetics in SMS. In this dissertation,electrochemical methods were used to investigate the formation and the existing statesof ROS in alkaline media, and the influence of media concentration, temperature, andoxygen partial pressure were systematically examined. Some innovative progresseshave been obtained as follws:(1) The media-modulated scheme of different ROS formation in alkaline mediahas been obtained by cyclic voltammetry (CV) method. It is observed that during theelectrochemical reduction of oxygen in alkaline media, ROS (HO2-or O2-) can beproduced as reaction intermidiates. The formation of ROS can be promoted byadjusting the soluation alkalinity, so that the solution mass transfer properties can beenchanced, beneficial for the increase of the oxygen solubility and diffusivity.Furthermore, the effect of alkali metal cation on ROS formation and regulation hasbeen revealed, and it is found out that KOH solutions are superior with respect to thepromotion of ROS formation in comparison with NaOH solutions, mainly because thesolubility and diffusivity of oxygen in KOH solutions are much higher due to theimproved mass transfer properties.(2) The oxygen reduction reaction (ORR) pathway in alkali solutions was furtherinvestigated via rotating ring-disk electrode (RRDE) method. It is found out that4electrons pathway is dominating at dilute solutions (less than2mol·L-1), and anaverage number of electrons transferred between1and2in a moderate alkaliconcentration (under6mol·L-1).1electron reaction process at more concentrated(higher than8mol·L-1) KOH solutions. Further, the ROS formation is dependent on the electrode potential, O2-or HO2-as the primary product at low overpotential andOH-at higher potential. The kinetics parameter of ORR in alkali solutions, such asTafel plots,·n i0, rate constants and activation energy were also obtained.(3) The effect of oxygen partial pressure on ROS formation, the oxygen pressurehas been observed to have a significant influence on the ORR, substantiallypromoting the ORR due to the significant increase of oxygen solubility in thesolutions. The Henry’s constant of oxygen in KOH solutions of differentconcentrations were obtained. By examining the CVcurves of the ORR with theaddition of H2O2, the ORR mechanism and the existence of HO2-intermediate werefurther validated.(4) By using CV and RRDE methods, it is found that temperature has exhibitobvious dual effects with respect to the ORR as evidenced by the presence of anoptimal temperature for the ORR in a given alkaline solution. And the observation isdiscussed in terms of the oxygen reaction activity, oxygen solubility and diffusioncoefficient. And the activation energy of ORR at different electrode potentials indifferent KOH concentrations is obtained.(5) The formation and transfer regulation of different ROS in alkaline media havebeen obtained. SMS has been confirmed to be an ideal carrier for the ROS, which is theorigin of chemical intensifying effect. The ROS formation can be manipulated bycontrolling the media alkaline concentration, media temperature. The reactive activity ofthe SMS media can be enhanced by increasing the oxygen solubility and utilizingelectrochemical strengthening methods. |
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