A comparative study on perovskite-type mixed oxide catalysts A′xA1 − xBO3 − λ (A′ = Ca, Sr, A = La, B = Mn, Fe, Co) for NH3 oxidation

Three series of samples having the stoichiometry A′xA1 − xBO3 − λ (x = 0 − 1, B = Mn, Fe, Co) were prepared and used as catalysts for NH3 oxidation. It was found that even at x = 0 or x = 1 the compositions of the catalysts were nonstoichiometric. The nonstoichiometric amount of oxygen, λ, with which the crystal structure, defects in the solid, reactivity with reactant oxygen, and catalytic activity could be correlated, was a function of x. A single-phase, solid solution exists in the composition range from x = 0 to 0.4. A miscibility gap appears at about A′A = 1(x = 0.4) and then an A′BO3 phase is found at A′A = 1. This two-phase region extends until the appearance of the cubic phase at x = 1.0. Doping with lower valence cations (A′) in the case of ABO3 results either in an increase in the oxidation state of B or in the formation of oxygen vacancies. In the case of Mn, both λ and the concentration of Mn4+ depend linearly upon x, but in the case of Co, due to the instability of Co4+ toward reduction by O2−, only λ increases. The case of Fe is situated between the above two. By ED, EM, and Mossbauer investigation, a vacancy-ordering in the ferrite system was observed. XPS, TPD, and EPR measurements gave results suggesting the possibility of the formation of O−2 or O−. The adsorbing capacity of catalyst surface to oxygen depends closely on λ. The catalytic activity of A′xA1−xBO3−λ mixed oxides in the NH3 oxidation in general could be attributed to the extent of the redox reaction 2 B4+ + O2−coagMn 2 B3+ + 12O2(g) + Vo. The Mn and Co systems are just two extreme cases. The dependence of the activity of Fecontaining mixed oxides on their redox potential was confirmed by TPR and 18O-isotopic exchange Study.