Application of temperature-programmed reduction to the characterization of anionic clays

Temperature-programmed reduction (TPR) has been used for characterization of anionic clays containing reducible cations. Results are reported on the reducibility properties of several anionic clays with the hydrotalcite or takovite structure containing transition metal cations, and of the products (usually a mixture of rock-salt-type oxides and spinels) thereof obtained by calcination. The systems studied include those possessing two reducible cations (e.g., Co2+ and Fe3+) and also others with only a reducible species (e.g., Ni2+ and Al3+). Carbonate, the most common interlayer anion in these compounds, is removed by decomposition before being reduced to C or CHx, as concluded from X-ray diffraction analysis of the residue after reduction. Reduction of the isolated cations in the brucite-like structure of the anionic clay (or in the decomposition products) proceeds at appreciable lower temperatures than that required for reduction of bulk spinels and other oxides formed upon calcination at high temperatures.