Thermal reactions of inorganic hydroxy-compounds under applied electric fields, III

Abstract

The effect of electric fields on the thermal dehydration of amorphous hydrated iron(III) oxide under oxidizing, inert and reducing atmospheres was studied by X-ray diffractometry. In oxidizing and inert atmospheres, the transformation to c~-Fe203 was enhanced by electric fields, especially at the negative electrode. The crystallite size of the product was also greatest at the negative electrode. Both results are explained in terms of the migration of protons to the negative electrode, where they subsequently form water which acts as a nucleating agent for the crystalline phase. In reducing atmospheres the formation of Fe3Oa and FeO at the expense of Fe203 is facilitated by electric fields, particularly at the negative electrode. Possible reaction mechanisms are considered, and the role of protons in stabilizing defect-spinel intermediates and products is discussed.In previous papers [1,2] it was reported that the dehydroxylation of compounds such as Mg(OH)2 and kaolinite (A12Si2Os(OH)~) occurs at lower temperatures and with decreased activation energies when carried out under the influence of d.c. electric fields. Similar effects have been observed in studies on the dehydration of pseudoboehmite (a poorly crystalline hydrous aluminium oxyhydroxide), in which both the reaction rate and crystallite size of the product are increased under the influence of d.c. electric fields [3]. On the other hand, electrolysis was found to have little or no effect on the dehydroxylation of crystalline AI(OH)z [2 ]. The question therefore arises as to whether the crystallinity of the reactant hydroxide is an important factor in determining the role of applied electric fields in dehydration and dehydroxylation reactions. A study of the effect of electrolysis on the thermal decomposition of a fully amorphous hydrated material would provide useful additional information on this matter.A further interesting question which has not so far been addressed in electrolysis studies concerns the behaviour of variable-valence compounds in the vicinity of the electrodes, in particular, whether oxidation or reduction might be initiated or facilitated by the electric field.One compound whose study could provide information on both these questions is amorphous iron III hydroxide. Previous workers [4] have reported that this compound dehydrates progressively to form amorphous Fe20 a which exothermi, cally crystallizes in the a-form at about 270 ~ under reduced pressure [5], or 430 ~