Gordon wrote:

Since the catalytic converter effects burning of unburned fuel in the
exhaust, it might make sense to put one ahead of the turbine in a
turbocompound engine.

The catalytic converter would be annihilated by the high temps and
(supersonic?) blast waves from the exhaust.  We have a hard enough time
keeping the muffler innards intact...

Russell Kent

Pumping modest fresh air pressure into an old (prior to 1986 or so) 13B
thermal reactor will also work. Mazda installed air pumps in those
days for that purpose. It did not get all the HC so along came
the cat.