Three-way catalyst-induced formation of ammonia—velocity- and acceleration-dependent emission factors

Ammonia (NH 3) is classified as a toxic air pollutant but its release from vehicles is not regulated. Herein we report on the efficiency of the catalytic reduction of nitrogen monoxide (NO) and the selectivity towards NH 3. Chemical ionization mass spectrometry (CIMS) has been applied to monitor NH 3 and NO emissions at a time resolution of 2 s. At real world driving, intense, catalyst-induced NH 3 formation was detected for a gasoline-fueled passenger car equipped with a Pd/Rh-based three-way-catalyst (TWC). Post-catalyst NH 3 emissions strongly depend on velocity and acceleration and varied by two orders of magnitude from 1 to 170 mg km −1. For most vehicle conditions, tail-pipe NH 3 emissions exceeded those of NO. Excellent NO conversion above 95% was noticed as soon as catalyst light-off occurred. Post-catalyst NO emissions were lowest when NH 3 formation was most intense and vice versa. This complementary behavior indicates that a TWC can be operated in a way that either NH 3 or NO emissions dominate. The NH 3/NO mixing ratio was mainly influenced by the air-to-fuel ratio. At fuel-rich combustion ( λ 1 ). Catalyst temperature effected the selectivity of the DeNOx process. Highest NH 3 selectivity up to 0.45 was found when operating the catalyst below 280 °C. Above this temperature, the selectivity was reduced to 0.02–0.05. The obtained results highlight those parameters, influencing the NH 3 output of a TWC vehicle at real world driving.