Thermal decomposition of metal nitrates in air and hydrogen environments

The decomposition of metal nitrates in air has been systematically studied by thermogravimetry. Observed temperatures of decomposition (T-d) have been inversely correlated to the charge densities (CD) of the metal cations. Due to a back-donation of electronic cloud from the nitrate to an unfilled d-orbital of transition and noble metals, their nitrates generally exhibited lower T(d)s (<700 K) than those of the base metals (>850 K). The thermal stability/reducibility of metal nitrates in an hydrogen atmosphere has also been studied by temperature-programmed reduction (TPR). Observed reduction temperatures (T-r) for nitrates of the base metals and the noble metals are lower than their T-d, i.e., T-r < T-d. The lowering of T-r might be attributed to a spillover of hydrogen to a nitrate moiety through heterolytic (ionic) and homolytic (atomic) dissociation of hydrogen on the respective base and noble metals. The stoichiometry of hydrogen consumption, quantitatively measured from TPR, varied with the group of metal cations. According to the stoichiometry, the end product in the TPR reduction was NH3 (N-H2/N-NO3- ∼4.4) and N-2 (N-H2/N-NO3- ∼2.4) for nitrates of the noble and base metals, respectively. The T(r)s for nitrates of the transition metals are often ∼20 K higher than their T(d)s, and the ratio N-H2/N-NO3- varies widely between 0.7 and 3.2. Their reduction may be triggered by thermal decomposition.