(348e) Hydrolysis of Calcium Bromide: Immobilization of the Reactant on a High Surface Area Support

Thermochemical cycles involving calcium and bromine are
considered to be one of the two most promising family of routes for producing
hydrogen by splitting water. The two primary reactions in these cycles are
gas-solid heterogeneous reactions involving cycling between the oxide and
bromide salts of calcium as shown below:

CaBr₂ + H₂O
→ 2HBr + CaO

CaO + Br₂
→ CaBr₂ + ½ O₂

The hydrogen bromide formed in the first reaction needs to
be converted to bromine to complete the cycle. This can be achieved through
another set of reactions involving oxide and bromide salts of iron (the UT-3 cycle)
or electrolytic/plasma-chemical splitting of HBr (Ca-Br cycle). One of the main
challenges to the development of these cycles is the loss of reactive surface
of the solid reactant as it undergoes repeated transformations between the
bromide and oxide forms having significantly different molecular volumes. It is
proposed to conduct the reactions by coating a high-surface area substrate with
thin layer of calcium bromide to minimize the negative effects of the sintering
and maintain a large surface area for reaction. A high-surface area monolithic
ceramic substrate was used in these studies. Special techniques were developed
to coat the substrate with calcium bromide, an extremely hygroscopic substance.
The substrate was characterized prior to- and after- coating through electron
microscopy. Experiments were then conducted to determine the effect of the ceramic
substrate surface on the reaction and to quantify resultant surface defects.  The
results of the study indicate that it is possible to obtain high conversions of
calcium bromide immobilized on the support, and this technique has potential to
counteract the loss of reactive surface in the thermochemical cycle.