Petrol Cat Converters do reduce NOx
Unlike Diesel, modern petrol engines have 3 way catalytic converters which do reduce NOx (2NOx → xO2 + N2).
3 Way addresses Hydrocarbons, CO and NOx
When operated in a closed cloop they operate with the ECU to maintain the petrol engine in the ideal range for the cat to operate most efficiently. Diesel engines currently can't achieve this.
Details from Volkswagen ironically: