I'd recently learned of emissions-reduction kits fitted to diesel systems which use ammonia to reduce NOx emissions.

I believe that's Selective catalytic reduction:

<https://en.wikipedia.org/wiki/Selective_catalytic_reduction>

The process uses "diesel exhaust fluid" (not to be confused with blinker fluid), sold as "AdBlue" in Europe, possibly elsewhere:

<https://en.wikipedia.org/wiki/Diesel_exhaust_fluid>