The catalyst excited state is a stronger reductant because its highest energy electron has been excited to an even higher energy state through the photoexcitation process.
In many cases, this electron transfer takes place with a stoichiometric two-electron reductant or oxidant, although in some cases this step has also been implemented to activate a second reagent.
A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and is adsorbed onto a catalyst.