The oxygen reduction reaction (ORR) has attracted the attention of researchers due to the fact that it underlies many technologically important processes. However, despite the large volume of research, important fundamental gaps remain in understanding the key stages of the ORR, which is associated with a multi-stage reaction and a large number of factors, which effects on the kinetics. Part of the work was also the study of catalytic reactions for the hydrogen peroxide oxidation/reduction reactions (HPRR/HPOR) at different pH values, which will help better understanding the processes occurring in ORR.
The objects of study were Mn2O3, LaMnO3, MnOOH. Samples were synthesized and characterized by various methods such as BET, scanning electron microscopy (SEM) and X-ray power diffraction. This choice of oxides is due to different structural types of compounds, their different catalytic activity in ORR (activity decreases in the order Mn2O3 > LaMnO3 > MnOOH), as well as different limiting stages of the oxygen reduction mechanism.
Polarization curves for the ORR and HPRR/HPOR for LaMnO3, Mn2O3 and for MnOOH oxides and for Sibunit carbon (considering that the latter is always added to oxides in order to improve the electronic conductivity of electrodes) was obtained. It was demonstrated that for carbon RDE voltammograms shift systematically with pH (40 mV/pH). For Mn2O3 and MnOOH the ORR RDE voltammograms also shift towards higher overpotentials with the decrease of pH, although the behavior is somewhat different from that demonstrated by carbon. Theoretical analysis  of the influence of pH on the rates of heterogeneous electrocatalytic reactions involving transfer of electrons and protons indicates that rates of reactions involving simultaneous electron and proton transfer do not depend on pH, while strong pH dependence suggests separate transfer of a proton and an electron. Thus, experimental data suggest that ORR on carbon likely involves separate steps of an electron and proton transfer, with an intermediate formation of O2-. Similar reaction steps may occur on the studied Mn oxides. It is interesting to note that for the HPRR/HPOR, the behavior of Mn2O3 and MnOOH is significantly different. For the former, RDE curves are nearly independent on pH (the only difference related to the pH dependence of the O2 diffusion coefficient). In contrast, for MnOOH there is a strong effect of pH on polarization curves. The observed difference may be attributed to the fast kinetics of hydrogen peroxide reactions on Mn2O3 (the reaction being limited by diffusion) and slow kinetics of the hydrogen peroxide reactions on MnOOH. Furthermore, considering the above discussion, it is likely that the HPRR/HPOR on MnOOH also follow pathway involving separate electron and proton transfer.
This work was funded by RFBR according to the research project № 20-33-90160.
 M. T. M. Koper, “Theory of multiple proton-electron transfer reactions and its implications for electrocatalysis,” Chem. Sci., (2013), 4, pp. 2710–2723.
|Affiliation of speaker||Lomonosov Moscow State University|
|Publication||Impact Factor journals|