US Patent 8225597 System for reducing NOx in exhaust

A system and method for NOx reduction is described, with a catalytic unit including a first zeolite catalyst with a first NOx conversion performance in a first temperature range and a second NOx conversion performance, lower than said first NOx conversion performance, in a second temperature range. The catalytic unit also comprises a second zeolite catalyst with a third NOx conversion performance, lower than said first NOx conversion performance, in the first temperature range and a fourth NOx conversion performance, higher than said second and third NOx conversion performances in the second temperature range, said first temperature range being higher than said second temperature range. The system further includes a controller configured to adjust an amount of reducing agent added to the NOx reducing system responsive to a temperature of the catalytic unit.