The Value of Neutrons II. Thermal Decomposition

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Thermal Decomposition of Cobalt Acetate Hydrate

This study by Grimes and Fitch using the ILL neutron source (J. Mater. Chem. 1, 461 (1991)) is a nice example of how one can extract additional information concerning specific parts of a structure by selective deuteration. The study was of the decomposition of cobalt acetate hydrate (Co(CH₃CO₂)₂.4H₂O) into cobalt oxide, CoO, as it is heated up from room temperature to 600°C. The initial starting powder could be made in 4 alternative forms:

The in-situ neutron diffraction patterns obtained in these 4 cases were very different as seen in the next figure in which, for visual comparison purposes, the maximum heights of each sequence have been made similar:

click on image to enlarge (186 kB)

The large background in three of these is due to the presence of hydrogen and therefore the drop in these backgrounds as the temperature rises (i.e. the step-like appearance) represents the loss of hydrogens from either the acetate group, CH₃CO₂, or the water, H₂O, or both. Even by just looking at these pattern sequences one can make some qualitative observations regarding the decomposition process, for example consider:

• why does the first case have two background steps, the next two cases have one step, and the last case have no steps?
• at what temperatures is the water lost?
• at what temperatures are the acetate groups breaking up (i.e. losing hydrogen)?
• why do the peak:background ratios change so markedly between the four cases?

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