An investigation of the nature and reducibility of Ni-hydroxy-montmorillonites using various methods including temperature-programmed reduction (TPR)

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Abstract
A number of Ni-hydroxy-montmorillonite complexes were prepared by titrating NiCl2solutions with NaOH in the presence of charge-reduced and normal montmorillonite suspensions. These complexes were characterized by chemical analysis, CEC, XRD, IR and temperature-programmed reduction (TPR). TheR(charge-reduced) Ni-hydroxy complexes proved to be merely mechanical mixtures of Ni(OH)2and clay. The reducibility of Ni contained in these samples was not affected by preliminary heat treatments. The nature of theN(normal) Ni-hydroxy montmorillonites depended principally on the amount of Ni actually incorporated into the precipitates. Neither the OH/Ni ratio selected for the preparation or the quantity of Ni initially added completely determined the type of complex formed. TPR patterns of theNsamples generally showed three reduction bands whose respective intensities varied with the quality and the amount of Ni present. One of these reduction bands, occurring at high temperature, measured the extent of incorporation of Ni into the silicate framework during the TPR run. The intensity of this high-temperature band with respect to lower temperature ones was increased by thermal pretreatments. The results emphasize that when TPR patterns of natural Ni-bearing clays are being interpreted it is important to recognize that thermal transformations may occur in the samples caused by the heat supplied during the TPR run itself.