Mass spectral studies of thermal decomposition of metal nitrates

Residual gas analysis and static secondary ion mass spectrometry are used in an attempt to elucidate the process responsible for the appearance of metal oxides in the gas phase during low temperature nitrate decomposition of Pb, Cu, Cd and Ag nitrates in vacuo. Observed signals for MO, MNO(3)(+) together with M(2)(+) species in the gas phase during the decomposition of some of the metal nitrates indicate that a physical expulsion mechanism is probably responsible for the low temperature production of these metal-containing species. The ''gasification mechanism'' provided by L'vov states that metal nitrates decompose in a single mechanistic step to product MO(g), O-2 and NO2 and that the temperature of the decomposition is predictable from thermodynamics. The observation of unexplained gas phase species, the absence of O-2 during CuO+ production, and the lack of agreement between activation energies and appearance temperatures fail to support the gasification model. Instead, it is suggested that during the decomposition, which is governed by the kinetics of the process, the evolution of the gaseous products, such as O-2 and NO2, carry the observed metal-containing species into the vapor phase when the liquid state is present on the surface (e.g. AgNO3, Cu(NO3)(2) or Cd(NO3)(2)). The crystal rearrangement of the solid (e.g. conversion of Pb(NO3)(2) to PbO) and subsequent loss of integrity of the crystal lattice results in the dislocation of the metal-containing species, which are swept from the surface during the O-2 and NO2 evolution.