Abstract: In accordance with one embodiment of the present invention, a catalyst system is provided. The catalyst system includes a catalyst bed that comprises a plurality of catalyst segments arranged such that an exhaust flow passes along a longitudinal axis of the catalyst system through the plurality of catalyst segments from a first one of the plurality of catalyst segments through a last one of the plurality of catalyst segments, each of the plurality of catalyst segments comprising a plurality of catalytic cells, wherein a concentration of catalytic material decreases from the first one of the plurality of catalyst segments through the last one of the plurality of catalyst segments.

Abstract: A method comprises injecting a reductant stream comprising a hydrocarbon reductant and water into an exhaust chamber comprising a nitrogen oxide-containing exhaust stream; contacting a composition comprising the reductant stream and exhaust stream with a catalyst; and decreasing a concentration of the nitrogen oxide in the composition comprising the reductant stream and exhaust stream. An exhaust stream treatment system, comprises an exhaust chamber comprising a catalyst; an exhaust stream comprising a nitrogen oxide; and a reductant stream comprising a hydrocarbon reductant and water.