Kinetic and catalytic characterization of lanthanum ferrites and manganites for gas sensor and fuel cell applications

Lanthanum ferrites and manganites have been shown to be excellent catalysts for use as sensing electrodes for potentiometric gas sensors to detect CO, CO2, and NOx gases and for use as cathode materials for solid oxide fuel cells (SOFC) to reduce oxygen. However, these materials have complex surfaces that are not well understood. For further optimization, more understanding of the surface behavior is needed.;NOx adsorption behavior on LaFeO3 (LFO) and LaMnO 3+delta (LMO) was characterized using temperature controlled methods and mass spectrometry. Temperature program desorption revealed decomposition of complex surface species formation when NO or NO2 was adsorbed on LFO and LMO. LFO exhibited higher adsorption capacity for NOx species than LMO and was shown to be more active for NOx surface conversion. Both effects were attributed to the different B-site cations, with iron in LFO in the 3+ valence state, and manganese in LMO in the 3+ and 4+ valence states. Results from diffuse reflectance infrared spectroscopy were used to identify specific nitrite and nitrate species that are formed on the surfaces of LFO and LMO at room temperature. Temperature programmed reaction revealed a complex NO2 decomposition mechanism to NO and O2 for LFO and LMO in which the formation of nitrite and nitrate species serve as intermediates below ∼600°C. NOx sensing mechanisms were considered and predicted based on the types and quantities of surface species formed.;A novel approach using isotope exchange was developed called isothermal isotope exchange (IIE) to study the oxygen reduction behavior on lanthanum ferrites and manganites. Two common kinetic parameters have been used to characterize the ability of a material to exchange oxygen: the tracer diffusion coefficient (D*) and the surface exchange coefficient (k*). D* is a measure of the bulk diffusivity while k* is a measure of surface exchange. Both aspects are involved in the oxygen reduction reaction (ORR), but in particular k* can be difficult to measure. Techniques used to measure k* currently require dense, thick samples that restrict testing to the diffusion controlled regime. IIE has the capability of testing powder materials which allows for accurate measurement of k* in the surface exchange controlled regime.;(La0.6Sr0.4)(Co0.2Fe0.8)O 3-delta (LSCF) was shown to have low activation behavior of k* between 500-800°C indicating the surface is catalytically unchanged within the temperature range. On the other hand k* for (La0.8Sr0.2)MnO 3 (LSM) was observed to increase with decreasing temperature. This behavior is consistent with a precursor-mediated mechanism in which there is no energy barrier for chemisorbed dissociative adsorption. Composite cathodes were tested and results showed that when a pure electronic conductor is combined with a pure ionic conductor, k* increases to higher values than either of the pure materials alone. k* for different A- and B-site stoichiometries of lanthanum ferrites, cobaltites, and manganites were also measured. The mixed ionic and electronic conductors were shown to exhibit higher k* values than the pure electronic conductors indicating the importance of having both electrons and oxygen vacancies present for ORR These results provide guidance for improving k* of SOFC cathodes and suggestions for optimization were made.