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Thermal Decomposition of Metal Nitrates in Air and Hydrogen Environments

2003; American Chemical Society; Volume: 107; Issue: 4 Linguagem: Inglês

10.1021/jp026961c

1520-6106

S. Yuvaraj, Lin Fan-Yuan, Tsong-Huei Chang, Chuin‐Tih Yeh,

Catalytic Processes in Materials Science

The decomposition of metal nitrates in air has been systematically studied by thermogravimetry. Observed temperatures of decomposition (Td) 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 Tds ( 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 (Tr) for nitrates of the base metals and the noble metals are lower than their Td, i.e., Tr < Td. The lowering of Tr 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 (NH2/NNO3− ∼4.4) and N2 (NH2/NNO3− ∼2.4) for nitrates of the noble and base metals, respectively. The Trs for nitrates of the transition metals are often ∼20 K higher than their Tds, and the ratio NH2/NNO3− varies widely between 0.7 and 3.2. Their reduction may be triggered by thermal decomposition.