Described are exhaust gas treatment systems for treatment of a gasoline engine exhaust gas stream. The exhaust gas treatment systems comprise an ammonia generating and hydrocarbon oxidation catalyst, a TWC catalyst, and an ammonia selective catalytic reduction (SCR) catalyst downstream of the TWC catalyst. The ammonia generating and hydrocarbon oxidation catalyst comprises a refractory metal oxide support, a platinum component, and a palladium component. The ammonia generating and hydrocarbon oxidation catalyst is substantially free of ceria and substantially free of NOx storage components. The platinum and palladium components are present in a platinum to palladium ratio of greater than about 1 to 1.