Defect chemistry and catalysis of Sr(Ti,Fe)O3-δ perovskite

R. Zohourian, R. Merkle

Proton-conducting acceptor-doped perovskites have three carriers: oxygen vacancies  , protons , and electron holes . Their concentrations are determined by two reactions describing water and oxygen uptake:

       

Alternatively, proton uptake can occur by a redox reaction as expense of holes:

        

(this reaction is a linear combination of reactions (1) and (2), i.e. not independent). Under conditions leading to a high  and low  concentration (high pO2, and/or redox-active perovskites), this is the dominating mode of proton uptake.

Because in the three carrier system only all three carriers together have to fulfil the electroneutrality conditions, an increase of pH2O can lead to complex stoichiometry relaxation phenomena. At sufficiently high hole concentration, "two-fold" relaxation is observed, which also comprises a characteristic non-monotonic change of the hole concentration.

Qualitatively, in first approximation this can be viewed as fast uptake of protons at expense of holes, followed by a slower uptake of oxygen again compensated by hole migration. The overall reaction is essentially water uptake, but it proceeds via an over-reduced intermediate state of the sample. A derivation of exact analytical expressions and numerical examples are given on the publication below.

Fig. 1: (a) Electronic conductivity (b) oxygen exchange rate constant of Sr(Ti1-xFex)O3-d as function of Fe content.

In the use of Sr(Ti1-xFex)O3-δ for oxidation catalysis (e.g. CO + 1/2 O2 → CO2), the competition of two branches determines the concentration of surface oxygen species: the "blue" branch of oxygen adsorption and dissociation, and the "red" branch of oxygen consumption by a reducing molecule (e.g. CO), Fig. 2a. If the oxygen supply is slower than consumption (in presence of highly reactive CO), a steady state with increased oxygen deficiency d in the whole perovskite grain develops under the reaction conditions (red curves in Fig. 2b). Presence of less reactive CH4 does not change the oxygen stoichiometry compared to the pure O2/N2 mixture (blue and green curves in Fig. 2b).

Fig. 2: (a) Competing reaction paths for surface atomic oxygen O* (b) Perovskite's oxygen stoichiometry 3-δ under reaction conditions with CO (red) or CH4 (blue), measured by thermogravimetry.

Publications:

  • D. Poetzsch, R. Merkle, and J. Maier
    Solution of the three-carrier problem: A natural explanation for surprising observations Annual Report MPI-FKF 2014
  • D. Poetzsch, R. Merkle, and J. Maier 
    Stoichiometry Variation in Materials with Three Mobile Carriers—Thermodynamics and Transport Kinetics Exemplified for Protons, Oxygen Vacancies, and Holes 
    Advanced Functional Materials 25(10), 1542–1557 (2015). DOI: 10.1002/adfm.201402212
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