Cation Distribution in Cu(Cr_(2 - x)Al_x)O_4 and Cu(Fe_(2 - x)Al_x)O_4 According to Neutron-Diffraction Studies and Their Catalytic Properties in the Water-Gas Shift Reaction

摘要

The neutron structural analysis of Cu(Cr_(2 - x)Al_x)O_4 and Cu(Fe_(2 - x)Al_x)O_4 (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)O_4: 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 Al~(3+) ions substitute Cr~(3+) ions, a number of Cu~(2+) ions move from tetrahedral spinel sites to octahedral ones: the degree of inversion δ changes from 0 for CuCr_2O_4 to ≈0.4 for CuAl_2O_4. In the Cu(Fe_(2 - x)Al_x)O_4 system, the cubic spinel forms at all x, except x = 0. The degree of inversion δ varies from 1 for CuFe_2O_4 to ≈0.4 for CuAl_2O_4 as Al~(3+) ions substitute Fe~(3+) ions. The change in the activation energy of the water-gas shift reaction correlates with the inversion of Cu-containing spinels.