Cation distribution in Cu(Cr_2–x Al _x )O₄ and Cu(Fe_2–x Al _x )O₄ according to neutron-diffraction studies and their catalytic properties in the water-gas shift reaction

Abstract

The neutron structural analysis of Cu(Cr_{2 − x} Al _x )O₄ and Cu(Fe_{2 − x} Al _x )O₄ (0 ≤ x ≤ 2) nanopowders is performed. The samples are prepared by the thermal decomposition of mixed hydroxy compounds at 900°C. Different spinel phases are shown to form in Cu(Cr_{2 − x} Al _x )O₄: a tetragonally distorted phase when x ≤ 1.0, a cubic phase when x > 1.25, and a mixture of both phases when 1 < x < 1.25. As Al³⁺ ions substitute Cr³⁺ ions, a number of Cu²⁺ ions move from tetrahedral spinel sites to octahedral ones: the degree of inversion δ changes from 0 for CuCr₂O₄ to ≈0.4 for CuAl₂O₄. In the Cu(Fe_{2 − x} Al _x )O₄ system, the cubic spinel forms at all x, except x = 0. The degree of inversion δ varies from 1 for CuFe₂O₄ to ≈0.4 for CuAl₂O₄ as Al³⁺ ions substitute Fe³⁺ ions. The change in the activation energy of the water-gas shift reaction correlates with the inversion of Cu-containing spinels.