ISOTOPIC EXCHANGE BETWEEN HYDROGEN FROM THE GAS PHASE AND PROTON-CONDUCTING OXIDES: THEORY AND EXPERIMENT

Abstract

The interaction of gaseous hydrogen isotopes from the gas phase with proton-conducting oxides with the perovskite structure La 1−x Sr x ScO 3− α (x = 0; 0.04) was studied by means of the hydrogen isotope exchange with gas phase equilibration in the temperature range T = 300−800 ºC and in hydrogen pressure range pH 2 = 2−20 mbar. A novel kinetic model for the hydrogen isotope exchange experimental data treatment taking into account the isotopic effects was developed. This model was implemented for the obtained experimental results. The heterogeneous exchange rates of hydrogen isotopes with investigated oxides La 1−x Sr x ScO 3− α (x = 0; 0.04) were calculated. The mole fractions of hydrogen isotopes were determined for the investigated materials. It was found that deuterium uptake is higher in comparison with protium, whereas the deuterium surface exchange coefficient for the proton-conducting oxide La 0.96 Sr 0.04 ScO 3– α is smaller in comparison with the protium surface exchange coefficient. The thermodynamic isotope effect can be caused by the difference of energy of zero-point oscillations between OH- and OD-defects and molecular H 2 and D 2 . The kinetic isotope effect can be explained by the different strength of OH and OD bonds. The rate determining stage of hydrogen exchange is shown to be the process of exchange between the forms of hydrogen in the gas phase and in the adsorption layer of the proton-conducting oxide (the stage of dissociative hydrogen adsorption). For the first time, a new statistical criterion is proposed that allows dividing the observed surface inhomogeneities caused by not only the natural surface roughness but also the presence of different isotopes of hydrogen (protium and deuterium) with different binding energies on a solid surface. The activity of the investigated proton-conducting oxides with respect to the hydrogen heterogeneous exchange is comparable to the heterogeneous hydrogen exchange activity for oxides based on cerates and zirconates of alkaline earth metals. High catalytic activity with respect to the process of hydrogen exchange from the gas phase in reducing atmospheres allows us to consider the proton-conducting oxides based on the lanthanum scandates as the very promising electrolytes for numerous electrochemical applications.