This invention discloses a durable catalyst for internal combustion engine or power plant exhaust gas purification which is comprised of a zeolite as the support and Fe (II) base-exchanged cation as the active component for the NO.sub.x selective catalytic reduction (SCR). It has stable activity with up to 20% H.sub.2 O and up to 250 ppm SO.sub.2 and combinations thereof over many hours in the exhaust stream. It is resistant to temperature excursions up to about 1,000.degree. C. The method of producing this catalyst preferably comprises ion-exchanging the zeolite with Fe (II) in an aqueous solution of iron (II) oxalate, iron (II) citrate or iron (II) formate, or a mixture of oxalic acid, citric acid/salts, formic acid/salts, 2,3,4,5,6-pentahydroxyl caproic acid/salts and iron (II) salts, for example, sulfate, nitrate, chloride, carboxylate, chloride acetate, phosphate and carbonate, in the pH range from 3 to 13 under anaerobic conditions. The catalyst produced in this way shows no noticeable deactivation in its activity for reducing NO.sub.x to N.sub.2 under accelerated aging conditions (up to about 20% H.sub.2 O and up to about 250 ppm SO.sub.2 present in the mixture of NO, HC (iso-butane), and O.sub.2) for at least 2,500 hours which meets the Government standard. A method of using this catalyst is also disclosed. Methane can be used satisfactorily as the reducing agent but in the temperature range of greater than 600.degree. C. but less than 750.degree. C.