The invention provides devices and methods for generating H.sub.2 and CO in an O.sub.2 containing gas stream. The invention also provides devices and methods for removal of NO.sub.X from an O.sub.2 containing gas stream, particularly the oxygen-rich exhaust stream from a lean-burning engine, such as a diesel engine. The invention includes a fuel processor that efficiently converts added hydrocarbon fuel to a reducing mixture of H.sub.2 and CO. The added fuel may be a portion of the onboard fuel on a vehicle. The H.sub.2 and CO are incorporated into the exhaust stream and reacted over a selective lean NO.sub.X catalyst to convert NO.sub.X to N.sub.2. thereby providing an efficient means of NO.sub.X emission control.