Interaction of hydrogen with copper-containing oxide catalysts: V. Structural transformations in copper chromite during reduction-reoxidation

High-precision X-ray powder diffraction and differential dissolution method s are used to show that copper chromite with a structure of tetragonal spin el is recovered under certain conditions upon the reoxidation of the system of phases formed by the reduction of the initial copper chromite, cation-d eficient spinel stabilized by hydrogen and Cu-0 particles epitaxially bound to its surface at 250-350 degreesC. However, because of destruction of the initial surface layer at the initial moment of reduction, equilibrium cann ot be complete: reoxidized spinel is nonstoichiometric and contains some la ttice oxygen-bound protons, and the copper oxide is formed on the surface i n some amount. The difference in the phase transition of copper chromite du ring reduction-reoxidation from the reversible polymorphic transitions char acteristic of copper chromite during temperature variations is analyzed.