Reductants are used. Ammonia, for one, via urea for example. Hydroxides as well, but the engine will be short lived (it's been the obvious example proposed by the same people that brought you solutions based on assumptions of spherical cows in a vacuum).Controlling NOx formation can be a simple matter by using some pre-oxidized hydrogen to change the chemistry and thermal properties within the combustion chamber.
Water injection has one practical NOx lowering purpose: to cool the charge to reduce the rate of NOx formation and salvage some of the excess enthalpy in the process.
One would be compelled to expend perhaps a million portions of terrestrial H2 in a storied attempt to harvest a single portion of H2 from the sun.Well, if we can harvest the hydrogen from the sun, we would deplete our O2 somewhat and raise our H2O. If we take the the H2 from the H20 in the ocean, it remains same level