While skiing as a young teen, I was introduced to the
wonders of reusable handwarmers, which use supercooled
molten sodium acetate in some sort of exothermic
crystallization reaction to keep your extremities from
turning that nice shade of blue.
Vehicle catalytic converters do not perform
well at all until
they reach a certain light-off temperature, which is well
into the 400 to 600 F range. It takes a few minutes for
the exhaust temperatures at the converter to reach that
temperature, and so a sizable portion of vehicle emissions
control is a function of getting the converter to
temperature as quickly as possible.
I am not a chemist, and am not certain that the
temperatures and available heat energy from a sodium
acetate reaction or a similar one would be that significant
in terms of improving catalytic converter warm-up times,
but I'm really quite curious to find out.