Catalytic abatement of NOx: Chemical and mechanistic aspects

Abstract The chemical and mechanistic aspects of the selective catalytic reduction (SCR) of NO by ammonia and by methane have been investigated. In the classical NH 3 -SCR process, operating near 600 K over vanadia-titania based catalysts, ammonia is activated by coordination over Lewis acid sites and reacts with gas phase or weakly adsorbed NO. The same mechanism occurs for the low-temperature (400–500 K) NH 3 -SCR over Mn and Fe based catalysts. On the contrary, low-temperature NH 3 -SCR over protonic zeolites implies the activation of ammonia as ammonium ions and the previous oxidation of part of NO to NO 2 . The CH 4 -SCR over Co-zeolite catalysts is supposed to imply the activation of NO x in the form of an adsorbed oxidized species, that reacts with the reductant, CH 4 , from the gas phase or activated by adsorption into the zeolite channels. The other catalytic denoxing technologies, like the NO decomposition over Cu-zeolite based catalysts, the N 2 O decomposition over noble or transition metal based catalysts, the storage-reduction on Ba-aluminate based catalysts and the reduction by CO over noble metal based three way catalysts, imply a strong adsorption and activation of NO x over the surface, although activation of hydrocarbons and CO over the noble metals can also be helpful in the last two cases.