A process is described that removes by oxidation the excess ammonia (NH.sub.3) gas from flue gases that have been subjected to selective catalytic reduction (SCR) of oxides of nitrogen (NO.sub.x) by ammonia injection. The inventive process incorporates a secondary catalyst of precious metal and vanadia on a metal oxide such as titania to catalyze the oxidation of both ammonia and carbon monoxide (CO) while maintaining sulfur dioxide (SO.sub.2) levels.