On the thermal decomposition of zinc hydroxide nitrate, Zn5(OH)8(NO3)2⋅2H2O

Abstract

The layered basic hydroxide Zn5(OH)8(NO3)2⋅2H2O can be thermally decomposed to ZnO via a series of intermediary compounds. Application of in situ X-ray diffraction to dry powder samples reveals three reactions: formation of anhydrous Zn5(OH)8(NO3)2, then de-hydroxylation to Zn3(OH)4(NO3)2 and, finally, decomposition of the latter to ZnO. In contrast, thermal analysis and mass spectroscopy of the evolved volatiles suggests that four reactions take place. Whereas de-hydroxylation reactions only produce H2O, there is also a distinctive pulse of NOx and O2 at the end of the sequence of reactions. The evidence points to the formation of an intermediate, poorly crystalline phase with a stoichiometry of [Zn(OH)2-x]⋅[NO3]x (1 ​< ​x ​< ​2) during the final stages of the reaction sequence. Isothermal calcination of Zn5(OH)8(NO3)2⋅2H2O at 120 ​°C showed that the anhydrous Zn5(OH)8(NO3)2 compound is unstable, rehydrating very rapidly on cooling or decomposing within 6 or 7 ​h at 120 ​°C to Zn3(OH)4(NO3)2 (at a rate of 1.33 ​× ​10-4 s-1). Zn3(OH)4(NO3)2 itself decomposes slowly to ZnO at 120 ​°C, but the process is slower (5.33 ​× ​10-6 s-1) and there was still considerable Zn3(OH)4(NO3)2 present even after 140 ​h. The mixtures of Zn3(OH)4(NO3)2 and ZnO prepared by calcination are unstable under ambient conditions and react with moisture to reform Zn5(OH)8(NO3)2⋅2H2O.