A system is provided for the on-board production of reductants. The system comprises a fuel tank adapted to directly or indirectly supply a first fuel stream and a second fuel stream. An engine is in fluid communication with the fuel tank, and is configured to receive the first fuel stream and create an exhaust stream. The system further includes an emission treatment unit to treat the exhaust stream. A fuel conversion unit is configured to receive the second fuel stream, and also receive a stream comprising oxygen to partially oxidize at least a portion of the second fuel stream thereby forming reductants. In addition, the fuel conversion unit is configured to supply a reductant stream comprising the reductants to the exhaust stream. The invention further provides a method for the on-board production of reductants including supplying a first fuel stream to an engine, wherein the engine is configured to create an exhaust stream. A second fuel stream and a stream comprising oxygen are supplied to a fuel conversion unit. At least a portion of the second fuel stream is partially oxidized in the fuel conversion unit to form reductants, and a reductant stream comprising the reductants is supplied to the exhaust stream. The selective catalytic reduction of NOx present in the exhaust stream is performed.