Acid and redox properties of mixed oxides prepared by calcination of chromate-containing layered double hydroxides

摘要

Layered double hydroxides (LDHs) with Mg and Al in the layers and carbonate, nitrate or chloride in the interlayer, or with Zn and Al in the layers and chloride in the interlayer, have been prepared by coprecipitation, and have been used as precursors to prepare chromate-containing LDHs. All these systems, as well as those obtained upon their calcination up to 800 degrees C, have been characterised by powder X-ray diffraction, FT-IR and vis-UV spectroscopies, temperature-programmed reduction (TPR), nitrogen adsorption at -196 degrees C for surface texture and porosity assessment, and FT-IR monitoring of pyridine adsorption for surface acidity determination. The results obtained show that the crystallinity of the chromate-containing LDH depends on the precursor used. The layered structure of the Mg, Al systems is stabilised up to 400 degrees C upon incorporation of chromate; however, the Zn,Al-chromate samples collapse between 200 and 300 degrees C, with simultaneous formation of ZnO. Calcination of the samples above 400 degrees C gives rise to a reduction of Cr(VI) to Cr(III), as concluded from vis-UV spectroscopic studies. The TPR profiles show that chromate in ZnAl hydrotalcite is more easily reduced than that incorporated in the magnesium ones. Moderately strong surface Lewis acid sites exist in all samples calcined below 500 degrees C. (c) 2005 Elsevier Inc. All rights reserved.