The La2Mo2O9 and La2(MoO4)3 powders were synthesized using a solid-state reaction method and used to prepare dense ceramics. X-ray photoelectron spectroscopy was used to study the chemical ...composition and charge numbers of the elements in the subsurface area of dense ceramics of lanthanum molybdates. The spectra were measured under an ultra-high vacuum of 7 × 10−11 atm at 30 °C and 600 °C, and under an oxygen atmosphere at 2 × 10−3 atm at 600 °C and 825 °C. High resolution spectra for La 3d, Mo 3d and O 1s states were obtained and analyzed. The kinetics of oxygen exchange were considered in the framework of a two-step model including the consecutive steps of dissociative adsorption and the incorporation of oxygen. The oxygen adsorption (ra) and incorporation (ri) rates were calculated. Correlations between the oxide surface defect chemistry and the rates of individual oxygen-exchange steps were discussed.
Oxygen surface exchange kinetics of La
0.7
A
0.3
MnO
3±δ
has been studied by the isotope exchange method with gas phase equilibration using a static circulation experimental rig at the temperature of ...850 °C and oxygen pressure of 1 kPa. Oxides of La
0.7
A
0.3
MnO
3±δ
with different dopant at one level of doping were considered. Rates of elementary acts of the oxygen exchange were found to change differently. So, the oxygen dissociative adsorption rate increased, the oxygen incorporation rate decreased, and the increase in the cation radius did not influence the interphase exchange rate. These trends are associated with the AO segregation processes at the surface and the appropriate mechanism of oxygen reduction is considered.
Graphical Abstract
The oxygen dissociative adsorption rate increased, the incorporation rate decreased, and the increase in the cation radius did not influence the interphase exchange rate. These trends are associated with the AO segregation processes at the surface.
The interaction between gas-phase oxygen and LSM–YSZ (LSM is La0.6Sr0.4MnO3±δ; YSZ is ZrO2·Y2O3 electrolyte) composite catalytic materials was studied by the in situ method based on isotope ...equilibration with the gas phase at T = 600–850 °C and pO2 = 0.2–1.3 kPa. We made an attempt to develop mathematical criteria to prove the appearance of the spillover effect migration of adsorbed particles from the active phase to triple-phase boundary (TPB) from the experimental data. The heterogeneous exchange rate (r H) and the three exchange types (r 0, r 1, r 2) were calculated, where 0, 1, and 2 differ in the number of surface oxide atoms participating during one elementary exchange act. The heterogeneous exchange rate for lanthanum–strontium manganite was 2.5–3 orders higher than the heterogeneous exchange rate for the zirconia-yttria electrolyte over the investigated temperature range. The exchange mechanism of composites LSM–YSZ was different from the LSM and YSZ oxides due to the fact that the ratios between the rates of the three exchange types of composites were also different from the individual phases. An original model was proposed for describing the kinetic dependences taking into account the oxygen exchange at LSM and YSZ individual oxides, at a TPB, and exchange between oxygen forms in an adsorption layer and at a TPB. It was shown, that at 600 °C, the exchange occurred without a significant effect of the exchange on TPB (r TPB). The exchange on TPB became considerable with increasing temperature, so that at a temperature of 800 °C and higher, the rate-determining stage was the exchange between the forms of oxygen in the adsorption layer and on TPB, corresponding to the spillover effect. The phenomenon of the oxygen spillover effect on LSM–YSZ composites has been considered.
The La
2
Mo
2
O
9
and La
2
(MoO
4
)
3
powders were synthesized using a solid-state reaction method and used to prepare dense ceramics. X-ray photoelectron spectroscopy was used to study the chemical ...composition and charge numbers of the elements in the subsurface area of dense ceramics of lanthanum molybdates. The spectra were measured under an ultra-high vacuum of 7 × 10
−11
atm at 30 °C and 600 °C, and under an oxygen atmosphere at 2 × 10
−3
atm at 600 °C and 825 °C. High resolution spectra for La 3d, Mo 3d and O 1s states were obtained and analyzed. The kinetics of oxygen exchange were considered in the framework of a two-step model including the consecutive steps of dissociative adsorption and the incorporation of oxygen. The oxygen adsorption (
r
a
) and incorporation (
r
i
) rates were calculated. Correlations between the oxide surface defect chemistry and the rates of individual oxygen-exchange steps were discussed.
The chemical state of the near-surface La
2
Mo
2
O
9
and La
2
(MoO
4
)
3
ceramic layers differs from the bulk state, which has a significant influence on the interaction kinetics of the gas-phase oxygen and the surface of the oxide material.
The La 2 Mo 2 O 9 and La 2 (MoO 4 ) 3 powders were synthesized using a solid-state reaction method and used to prepare dense ceramics. X-ray photoelectron spectroscopy was used to study the chemical ...composition and charge numbers of the elements in the subsurface area of dense ceramics of lanthanum molybdates. The spectra were measured under an ultra-high vacuum of 7 × 10 −11 atm at 30 °C and 600 °C, and under an oxygen atmosphere at 2 × 10 −3 atm at 600 °C and 825 °C. High resolution spectra for La 3d, Mo 3d and O 1s states were obtained and analyzed. The kinetics of oxygen exchange were considered in the framework of a two-step model including the consecutive steps of dissociative adsorption and the incorporation of oxygen. The oxygen adsorption ( r a ) and incorporation ( r i ) rates were calculated. Correlations between the oxide surface defect chemistry and the rates of individual oxygen-exchange steps were discussed.
Oxygen surface exchange kinetics of La.sub.0.7A.sub.0.3MnO.sub.3±delta has been studied by the isotope exchange method with gas phase equilibration using a static circulation experimental rig at the ...temperature of 850 °C and oxygen pressure of 1 kPa. Oxides of La.sub.0.7A.sub.0.3MnO.sub.3±delta with different dopant at one level of doping were considered. Rates of elementary acts of the oxygen exchange were found to change differently. So, the oxygen dissociative adsorption rate increased, the oxygen incorporation rate decreased, and the increase in the cation radius did not influence the interphase exchange rate. These trends are associated with the AO segregation processes at the surface and the appropriate mechanism of oxygen reduction is considered. Graphical The oxygen dissociative adsorption rate increased, the incorporation rate decreased, and the increase in the cation radius did not influence the interphase exchange rate. These trends are associated with the AO segregation processes at the surface.
Oxygen surface exchange kinetics of La.sub.0.7A.sub.0.3MnO.sub.3±delta has been studied by the isotope exchange method with gas phase equilibration using a static circulation experimental rig at the ...temperature of 850 °C and oxygen pressure of 1 kPa. Oxides of La.sub.0.7A.sub.0.3MnO.sub.3±delta with different dopant at one level of doping were considered. Rates of elementary acts of the oxygen exchange were found to change differently. So, the oxygen dissociative adsorption rate increased, the oxygen incorporation rate decreased, and the increase in the cation radius did not influence the interphase exchange rate. These trends are associated with the AO segregation processes at the surface and the appropriate mechanism of oxygen reduction is considered.
