A selective catalytic reduction (SCR) system includes an on-board ammonia generation system that produces nitrogen from air and hydrogen from a source of a hydrogen-containing compound, and generates an ammonia product from the nitrogen and hydrogen to provide the ammonia product into an exhaust from a NO.sub.x generator to reduce the NO.sub.x in the exhaust. Oxygen from one or both of the nitrogen generator and the hydrogen generation cell can be supplied to the NO.sub.x generator for cleaner combustion or to a particulate filter for cleaning the filter. H.sub.2O from the NO.sub.x generator can at least partially provide a water source for the hydrogen generation cell.