On the thermal decomposition of zinc hydroxidenitrate, Zn5(OH)8(NO3)2⋅2H2O
- Elsevier BV
- Publication Type:
- Journal Article
- Journal of Solid State Chemistry, 2020, 286
- Issue Date:
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© 2020 Elsevier Inc. 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.
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