A New Structural Transformation Mechanism in Catalytic Oxides

Abstract

During dynamic reduction of vanadyl pyrophosphate using in situ electron microscopy and diffraction under controlled reaction conditions, recurrent dislocation of atoms is observed, which leads to the formation of extended defects by a glide shear mechanism. Ordering of the glide shear defects leads to a new structure by transforming the orthorhombic vanadyl pyrophosphate into an anion-deficient tetragonal structure. These defects are formed close to the surface and the nature of the defects is such that they accommodate the misfit between the reduced surface layers containing anion vacancies and the underlying unreduced bulk. The glide shear planar defects (GS) essentially preserve anion vacancies and do not lead to a lattice collapse, and are distinct from the well known crystallographic shear planes (CS, which eliminate anion vacancies leading to lattice collapse). In important complex oxides such as vanadyl pyrophosphates, and in a variety of model ReO3- and V2O5-based oxides used as catalysts, my in situ studies suggest that glide shear is the most effective defect mechanism by which the catalysts accommodate nonstoichiometry and continue to operate in partial oxidation reactions. Anion vacancy formation resulting from the oxide reduction is the driving force for the generation of glide misfit defects and their ordering can give rise to new phases or structures in oxides. The studies have important implications in oxide catalysis and, more generally, in oxide crystallography.