Research on ammonia emissions from three-way catalytic converters based on small sample test and vehicle test

Abstract

In this paper, a combination of catalyst sample evaluation and vehicle test is used to deeply study the formation mechanism of ammonia in the process of three-way catalytic reaction, and further explore the influence of catalyst formulation and aging on ammonia emissions. The catalytic sample test shows that CO reacts with terminal hydroxyl and bridging hydroxyl on the surface of the catalyst to generate H2 at low temperature, which then reduces NO to generate NH3. At high temperatures, CO reacts with water to generate H2, or hydrocarbon compounds in exhaust react with steam to generate hydrogen, and then H2 reacts with nitrogen oxides to generate NH3. On the one hand, the presence of water vapor can be prompted catalytic hydroxylation of materials and promote the reaction of the hydroxyl and bridging hydroxyl to improve the selectivity of NH3, on the other hand, as the competitive adsorption of H2O molecules and NO on the catalyst surface inhibits the reduction reaction between NH3 and NO, the consumption of NH3 molecules is reduced, and more NH3 vaporizes from the catalyst surface to the gas phase. The combination of Pd/Rh can effectively reduce the NH3 generation compared with the single Pd formulation. Ammonia emission can be effectively reduced by precisely controlling the air-fuel ratio of the engine and combining it with the catalytic converter which optimizes the ratio of precious metals.