Superstructures formed by the ordering of vacancies in a selective oxidation catalyst: grossly defective CaMnO 3

Abstract

Certain ternary metal oxides release oxygen at their surfaces when they function as catalysts for the selective oxidation of hydrocarbons. In so doing, they become non-stoichiometric, often quite markedly so (up to x = 0.5 in AMO 3- x ). In perovskite related phases of this kind, oxygen from the gas phase is readily taken up by the non-stoichiometric catalyst; and migration of oxide ions in the defective solid is rapid, thus sustaining the selective catalysis. Taking CaMnO 3 , which has the perovskite (i. e. CaTiO 3 ) structure, as a model catalyst and rendering it non-stoichiometric by reduction (CaMnO 3 (s) ↔ CaMnO 3- x (s) + ½ x O 2 (g)) we have used selected area electron diffraction, high resolution electron microscopy (in conjunction with computed images) to prove, directly, the nature of the ordering of the vacancies in the non-stoichiometric catalyst in the range 0 < x < 0.5. In all of the compositions of the five distinct (grossly non-stoichiometric) solids studied by us (CaMnO 2.8 , CaMnO 2.75 , CaMnO 2.667 , CaMnO 2.556 and CaMnO 2.5 ) oxygen vacancies are found to be ordered in such a manner as to preserve most of the structural features of the parent stoichiometric perovskite, a fact which itself suggests that this is a fundamental feature of the mode of action of these unusual catalysts. All the ordered structures are based on interconnected MnO 6 octahedra and MnO 5 square pyramids, the proportion of the latter increasing, and of the former decreasing, in proceeding from the CaMnO 3 to the CaMnO 2.5 end members. All the structures that we have discovered may be pictured as superlattice repeats of the parent (undistorted) perovskite, but the super­lattice mesh is sometimes rotated (by an angle R) in the (001) plane. Thus: for CaMnO 2.8 the structure may be symbolized √5√5 R26.5°; for CaMnO 2.75 , we have clear evidence for √2×2√2 R45° and √2×4√2 R45° and strong indications that two other types of structure, each of which can be symbolized √2×4√2 R45°, probably exist; for CaMnO 2.667 , we have identified two ordered structures both symbolized √2×3√2 R45°; for CaMnO 2.556 there is probably a structure 3√2×3√2 R45°; and for CaMnO 2.5 we have identified four structures, √2√2 R45°, √2×3√2 R45°, and √2×4√2 R45° and, the most widely occurring in our preparations, the √2×2√2 R45° structure. The implications of our results are discussed in the light of other defective perovskite-based systems that may now be expected to exhibit such behaviour. The work also sheds some light on the question of clarifying which oxide systems are likely to display catalytic activity.